AN ABSTRACT OF THE THESIS OF
R. Axel Fabrititus for the degree of Master of Science in Geophysics presented on
May 2, 1995. Title: Shear-Wave Anisotropy Across the Cascadia Subduction Zone From a
Linear Seismograph Array.
Abstract anDroved:
Redacted for Privacy
Measurements of shear-wave splitting in the SKS phase have been made at broad
band digital seismic stations of the TORTISS array in Oregon for events during 1993 and
the first quarter of 1994. The average observed splitting parameters, the fast polarization
direction () 700 and the time delay (&) 1.61 sec, are well above average. Worldwide the
largest observed splitting measurements are of the order of 2 sec time delay. According to
gross anisotropic characteristics, the array can be divided in three regions. The first region
extending over the Coast Range and has average splitting values of [72°, 1.34 secj, the
second region extends over the Willamette Valley and the Cascades shows an average
splitting of [66°, 1.74 sec], and the third region, east of the Cascades, has an average
splitting of [790, 1.77 sec]. The splitting measurements within each of the first two regions
show little systematic variation. In the third region, a systematic rotation of 4 from about
70° to 87° from west to east is observed. The observed shear-wave splitting is assumed to
be caused by strain induced lattice preferred orientation of mantle minerals, mainly olivine.
The general trend of the
and öt estimates is most likely due to the absolute plate motion
of the Juan de Fuca plate and the North American plate at the eastern end of the array.
Variations in the -& estimates across the TORTISS array can be explained by variations
in crustal anisotropy; however, only east of the Cascades (third region) the contribution of
cru staT anisotropy could be resolved. A change in dip of the descending slab at about 50
km from the coast might have a contribution to these variations as well. Splitting
measurements for events from 2300 to 280° back-azimuth differ from those from 300° to
350°. These differences can be best explained by deep seated anisotropy along the raypaths
of the northern and southern event group, anywhere between the CMB and the crust
underneath the array.
©Copyright by R. Axel Fabritius
May 2, 1995
All Rights Reserved
Shear-Wave Anisotropy Across the Cascadia Subduction
Zone From a Linear Seismograph Array
R. Axel Fabritius
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Masters of Science
Completed May 2, 1995
Commencement June 1995
Masters of Science thesis of R. Axel Fabritius presented on May 2, 1995
APPROVED:
Redacted for Privacy
MajoProfessor, representing Geophysics
Redacted for Privacy
Dean of College of Oceanic and Atmcphenl Sciences
Redacted for Privacy
Dean of
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader
upon request.
Redacted for Privacy
R. Axel Fabritius, Author
ACKNOWLEDGEMENT
First of all I would like to thank my major professor John Nábëlek for providing me
with the opportunity to work on the TORTISS and INDEPTH II projects. They will be an
unforgettable memory. John's advise throughout my graduate study was always helpful
and his comments and reviews were to the point. I especially enjoyed his easy-going
"non-academic" side while spending time with him travelling, skiing, or digging.
Thanks are also due to Drs. Y. John Chen, Ronald Guenther, R. Bruce Rettig, and
Anne M. Trehu for serving on my committee and making helpful comments and
suggestions regarding my thesis. Drs. Chen, Guenther and Trehu have also contributed to
my graduate education.
I would also like to thank Drs. Bob Duncan, Verne Kuim, Shaul Levi, and Clayton
Paulson who have all contributed to my graduate education.
I really enjoyed the atmosphere among my fellow geophysics students and no, it has
nothing to do with the fact that most of them were german. Thanks to Alex, Beate, Bernd,
Bird, Christof, Ganyuan, Giubiao, Jochen, Kelly, Kurt, Maren, Rugang, Thorsten and
Xiao-Qing and
all
the others who just passed through. I hope they will not forget to slack
off once in a while, even if I am not around to encourage them to do so. I especially like
to thank our geophysics grandfather, Jochen, for endless advice, discussions, and his
patience in all the study related matters and for helping me get through my downs and
times of frustration. My thanks are extended to Xiao-Qing for fruitful discussions and for
showing me how some things on the computer can be done easier in a different way.
Further thanks to Christof who taught me my first steps on the computer; I credit Christof
for the fact that I am now able to distinguish a PC from a SUN workstation.
Additionally I would like to thank Steve Azevedo for introducing me into the mystery
of pearl and for always being there to help me in "logistic matters". Field work with Steve
was always fun.
Many thanks to my dear housemates, past and present, GUnter, Jurgen, Andreas and
Diana for all their support and all the good times we had together. I will always remember
the spontaneous Skat nights with Günter and Jurgen, usually right before exams, and the
"quick" games of pooi with Andreas and Diana. Additional thanks to Diana for reviewing
my thesis.
I especially want to thank my dear M-Friends. Having them around during the last
years made it easy to forget about school and relax. in particular I like to thank Marc for
the great ski trips, Maurice for showing me the "irish way", and finally Marcelino for
introducing me to the "positive vibrations" (thanks man), for all the stuff he has done for
me, and for spending some quality time with me in front of the Bean having café. At this
point I also would like to thank with all my espresso-shaken heart the beanery team for
serving me the thesis essential caffeine.
Thank you to Eric Sandvol for providing me with "the" energy-minimization
program, which speeded up my calculations, let's say, quite a bit.
Last, but certainly not least, I would like to thank my entire family and especially my
parents and my brother. Their endless love and support provided the basis for my
successful studies.
This work was supported by the National Science Foundation grant EAR-9207 181.
TABLE OF CONTENTS
1.0
INTRODUCTION
1
2.0
GEOLOGICAL AND TECTONIC SETTING
5
3.0
EXPERIMENT SETUP
9
12
4.0 THEORY
4.1 Anisotropy and shear-wave splitting
4.2 Methods to investigate shear-wave splitting
12
16
Pariclemotionanalysis
Waveform cross-correlation
16
17
18
4.2.1
4.2.2
4.2.3
Tangential energy minimization
5.0
ANALYTICAL PROCEDURES AND DATA
21
6.0
RESULTS
49
7.0
DISCUSSION
59
7.1 Localization of anisotropy
59
7.1.1 Thickness of the anisotropic layer
7.1.2 Depth of the anisotropic layer
7.2 Relation of anisotropy to tectonic processes
7.3 Slab effect
7.4 Interpretation summary
7.4.1 Coast Range
7.4.2 Willamette Valley and Cascades
7.4.3 East of the Cascades
8.0
59
60
62
64
66
67
67
68
CONCLUSION
70
BIBLIOGRAPHY
72
APPENDICES
75
LIST OF FIGURES
Page
Figure
1.1
Compressional and shear-wave velocites in a monociystal of olivine.
4
2.1
Location map of the Juan the Fuca, Pacific, and North America
Plates system.
7
2.2
Geomorphic divisions of Oregon.
8
3.1
Topographic map showing exact location of TORTISS stations.
10
4.1.1
A rectilinearly polarized shear-wave S entering an anisotropic medium
will split into two rectilinear orthogonal phases S1 and S2.
13
4.1.2
The SKS wave results from a P to SV conversion at the core mantle
boundary.
14
4.1.3
Conventions used to derive the relations giving the radial and
transverse projections of two orthogonal S waves s1(t) and s2(t)
issued from an incident SKS wave s(t).
14
4.2.3.1 Energy minimization method using the Philippine Is!. event
(baz 300.8°) recorded at station A24.
19
5. 1
The S-legs underneath the stations of the SKS raypaths form
cones underneath the stations, if earthquakes from all back-azimuth
are recorded.
24
5.2
Distribution of the events used in this study with respect to Corvallis
25
5.3-13
The following figures show seismograms of all events used in this
study.
26
6.1
White vectors show the average of splitting measurements at
every station. x
52
6.2
54
Vectors showing the average splitting for the Coast Range (first
region, black vectors), Willamette Valley and Cascades (second region,
dark grey vectors) and east of the Cascades (third region, light grey
vectors).
6.3
Comparison of average splitting from northern (white vectors)
southern (black vectors) events.
55
6.4
Vectors showing splitting for different frequency bands for
the Philippine Isl. event.
57
7.3.1
The horizontal plane projections of vectors in the dipping planes.
65
LIST OF TABLES
Page
Table
5.1
Listof events.
23
6.1
Cross-correlation measurements for event: W. CAROLINE ISL.
93/09/26 03:31:19 Mw=6.4 dist=88 .93 baz=283 .4 (1 s-30s)
50
6.2
Weighted average for all events
51
LIST OF APPENDIX FIGURES
Page
Figure
A. 1.1
Anisotropy vector map for the Philippine Isi. event.
77
A. 1.2
Contour plot of energy on the corrected transverse component for
all ®, &) pairs for the Philippine Is!. event.
77
A.2. 1
Anisotropy vector map for the Taiwan event.
83
A.2.2
Contour plot of energy on the corrected transverse component for
all ®, &) pairs for the Taiwan event.
83
A.4. 1
Anisotropy vector map for the Hindu Kush event.
91
A.4.2
Contour plot of energy on the corrected transverse component for
all (4, öt) pairs for the Hindu Kush event.
91
A.5. 1
Anisotropy vector map for the New Ireland event.
98
A.5.2
Contour plot of energy on the corrected transverse component for
all (, &) pairs for the New Ireland event.
98
A.6. I
Anisotropy vector map for the W. Caroline Isi. event.
105
A.6. 1
Contour plot of energy on the corrected transverse component for
all ®, &) pairs for the W. Caroline Is!. event.
105
A.7. 1
Contour plot of energy on the corrected transverse component for
112
all (, öt) pairs for the Xinjiang event.
A.8. 1
Anisotropy vector map for the Eastern New Guinea event.
118
A.8.2
Contour plot of energy on the corrected transverse component for
all (4, &) pairs for the Eastern New Guinea event.
118
A.9. 1
Contour plot of energy on the corrected transverse component for
all (4, &) pairs for the 94 Fiji Isl. event.
125
LIST OF APPENDIX TABLES
Page
Table
A. 1.1
Splitting measurements for the Philippine Isi. event.
76
A.2. 1
Splitting measurements for the Taiwan event.
82
A.3. 1
Splitting measurements for the 93 Fiji Isi. event.
89
A.4. 1
Splitting measurements for the Hindu Kush event.
90
A.5. 1
Splitting measurements for the New Ireland event.
97
A.6. 1
Splitting measurements for the W. Caroline Is!. event.
104
A.7. 1
Splitting measurements for the Xinjiang event.
ill
A.8. 1
Splitting measurements for the Eastern New Guinea event.
117
A.9. I
Splitting measurements for the 94 Fiji Isi. event.
124
1.0 INTRODUCTION
Seismic anisotropy, the phenomenon of a medium showing different velocities for
different propagation directions through the medium, is a material property influenced by
the alignment of minerals. An observational evidence of seismic anisotropy is shear-wave
splitting. If a linearly polarized shear wave propagates through an anisotropic medium it
splits into two orthogonal components propagating at different velocities. Typically shear
wave splitting is characterized by two parameters: the fast (or slow) polarization direction
and the delay time between fast and slow polarization directions.
Shear-wave splitting can be interpreted in terms of tectonic/geologic processes if a
chain of relations between splitting and anisotropy, anisotropy and strain, and between
strain and tectonic/geologic processes are considered (for a detailed discussion see Silver
and Chan, 1991). Previous studies provide evidence that mantle anisotropy is caused by
strain induced lattice preferred orientation of upper mantle minerals (e.g., Nicolas and
Christensen, 1987; Silver and Chan, 1991). The most abundant mineral in the upper mantle
is olivine. For a shear-wave propagating along the b-axis or the c-axis of a olivine crystal
the anisotropy is about 10%, whereas for propagation along the a-axis there is practically
no anisotropy (figure 1.1).
Three principal hypotheses are considered for the origin of the mineral orienting
strain. The first hypothesis is that strain related to the absolute plate motion (APM), orients
the azimuth of the fast polarization direction
parallel to the APM (Leven et al. 1981,
Vinnik et al. 1992). The general applicability of this hypothesis is opposed by Helffrich et
al. (1994) who found almost perpendicular
same plate.
values over 100 km distances within the
2
The second hypothesis is that present crustal stress reflects lithospheric stress that
eventually produces strain induced anisotropy. The conceptual difference to the first
hypothesis is that it does not invoke a particular physical process and rather states that the
origin for crustal stress is also causing anisotropy. In active tectonic areas the dominant
tendency is found that the maximum horizontal stress direction is perpendicular to
.
(e.g.,
Helffrich et aL, 1994; Silver and Chan, 1993).
The third hypothesis is that strain due to the last significant episode of internal
deformation of the continental lithosphere is responsible for the alignment of
(Silver and
Chan, 1991; McNamara et al., personal communication, 1994; Helffrich et al., 1994). This
is interpreted as "fossil" anisotropy in stable continental areas and as reflection of presentday tectonism in active regions.
Crustal anisotropy can have a variety of causes. One possibility is that crustal
anisotropy is a result of preferred alignment of vertically parallel microcracks (Crampin,
1984; Crampin et al., 1984). Another possible origin of crustal anisotropy are foliated
felsic (e.g., gneises, schists) and mafic (e.g., amphibole-bearing gabbros) rocks (Barruol
and Mainprice, 1993). Because the thickness of the crust is generally not more than 40 km,
the crustal contribution to the observed SKS delay time is rather small (i.e., less than 0.3
sec).
Recent studies conducted over short spatial scales indicate that shear-wave splitting
may vary over short distances (Barruol and Souriau, 1995; Helffrich et al., 1994;
Makeyewa et al., 1992; McNamara et al. 199?; Savage and Silver, 1993). Furthermore,
Savage and Silver (1993) found that dividing the contributions to anisotropy into two
layers would satisfy their observations. They attribute the main part of the observed
anisotropy to shallow geologic/tectonic origin, and the smaller part to a lower layer
influenced by APM. Makeyewa et al. (1992) suggest the abrupt change in the fast
polarization direction over a short distance underneath the Tien Shan collisional belt could
be due to a small-scale thermal convection associated with a hot upper mantle.
3
Several suggestions have been made to explain the orientation of
in subduction
zones: "frozen-in" anisotropy within the subducted plate resulting from mineral alignment
prior to subduction (Savage and Silver, 1993), shear strain associated with the subduction
(Shih et al., 1991), or a combination of both (Vinnik and Kind, 1993). Bostock and
Cassidy (1995) suggest that in the southern Canadian Cordillera the mantle flow associated
with the motion of the Juan de Fuca plate, its subduction along the coast, and associated
backarc asthenospheric flow align the fast polarization axis parallel to the flow. For the
northern Canadian Cordillera, however, they suggest that orogen parallel shear deformation
aligns the fast axis parallel to the orogen.
In this thesis shear wave splitting in the Cascadia subduction zone is investigated
using data from the TORTISS (The ORegon Teleseismic Imaging of the Subducting Slab)
experiment, conducted in Oregon during 1993 and 1994. The main purpose of this
experiment was to image the geometry of the subducting Juan de Fuca plate using
teleseismic broadband data. In the case of shear wave splitting, this is the only study from
such large and dense linear array of broadband seismometers. This data set enables the
study of variations in seismic anisotropy over an entire subduction complex and helps to
determine the likely cause for the fast polarization direction in the Cascadia Subduction
Zone.
8.43
798
4.87
4.88
OO1
8.32
'4.42
5.33
4.63
4.64
54LaJ
8.66
t_
9.81tt89I
4.87
I
4.89
b
4.42
o1It4
110
I
5.53 \,.:_-j_-
4.66
8.83'
Figure 1.1 Compressional and shear-wave velocities in a monocrystal of olivine. The main
axes of the crystal are a [100], b [010], and c [001]. (From Babuska, 1991).
5
2.0 GEOLOGICAL AND TECTONIC SETTING
The TORTISS array was designed to image the Juan de Fuca (JdF) plate beneath
Oregon in order to obtain a detailed 2D-picture of the Cascadia Subduction Zone. The JdF
plate, as well as the Explorer plate to the north and the Gorda plate to the south, are small
remnants of the Farallon plate, which once covered much of the eastern Pacific basin
(Atwater, 1970) (figure 2.1). The JdF ridge and Blanco fracture zone constitute the JdF
plate's northwestern and southwestern boundary, respectively. The eastern boundary is
formed by the North American plate, where the JdF plate is being subducted at the
Cascadia subduction zone. According to Wilson (1991), the JdF plate is moving in a
direction of N65°E at a rate of 34.4 mm/year relative to the North American plate and in a
direction of N67.5°E at a rate of 13.8 mm/year relative to a fixed hot spot coordinate
system (values calculated for Corvallis).
The TORTISS array extends over 3 major geologic provinces. The first province is
the Coast Ranges of western Oregon and the Willamette Valley, the subduction zone's
forarc. The second province is the Cascades, the volcanic arc. This province can be
divided in two subprovinces, the Oligocene and Miocene volcanic rocks of the Western
Cascades and the Pliocene to Holocene volcanic rocks of the High Cascades (Sherrod and
Smith, 1989). The third province, the backarc, is represented by the High Lava Plains and
the Blue Mountains (figure 2.2).
In contrast to Washington, Oregon lacks an active Wadatti-Benioff zone, and
therefore little is known about the geometry of the JdF plate beneath Oregon. The most
recent geophysical studies on imaging the subducting slab are the tomography studies by
VanDecar (1991), Harris et al. (1991) and Rasmussen and Humphreys (1988) and a
receiver function study by Owens et al. (1988). For western Washington, Owens et al.
(1988) suggest a shallowly (200) dipping plate. East of the Cascades, VanDecar (1991)
suggests a slab dipping at about 600 and extending to a depth of about 400 km for central
Washington. However, he finds no evidence of deep (below of about 200 km) slab
material beneath southern Washington and northern Oregon. For southern Oregon, Harris
et at. (1991) suggest a steeply dipping (65°) slab beneath the Cascade Range to extending
to at least 200-km depth.
7
1200
1250
1300
LOTTE
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IS
BRITISH
A COLUMBIA
2\
Explorer
50°
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Fault
0
Cobb
4.
0?
Seomounts
WASHINGTON
1
k
Q
Jl.JQfl de
1
Columb'°
R.
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; Fgicc..4
0
P/ate
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PA C/F/C
Mendoc,no
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00
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PLATE
Andreas
\SanFault
San
Fanc
Figure 2.1 Location map of the Juan the Fuca, Pacific, and North America Plates system.
Dashed line along the continental margin is the boundary between North America and Juan
the Fuca Plates. Stars indicate the location of the TORTISS array. (Modified from Duncan
and Kuim, 1989).
GEOMORPHIC
DIVISIONS
of OREGON
,
/
I,
I
*
I
* *
*
* * *
NWny
U
a.
I
'
0
I-
0)
u
o
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'
PLAINS
OWYHEE
UPLAND
c.ct
/
KLAMATH
A%N
MOUNTAINS
-
RANGE
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$L.t,y
.
'
Ufl
Figure 2.2 Geomorphic divisions of Oregon. Stars indicate the location of the TORTISS
array. (Modified from Baldwin, 1976).
3.0 EXPERIMENT SETUP
During the 1993-1994 TORTISS experiment a total of 69 sites was occupied by
portable seismic stations (figure 3.1). The highest number of seismometers was deployed
during the time span of April 1993 until the beginning of December 1993. Fortyfour
portable broadband 3-component seismometers were available for this study (27 Guralp
CMG-3ESP and 17 Streckeisen STS-2). All stations were equipped with REFTEK data
loggers, timed either with GPS or Omega synchronized clocks. The recording was
continuous with 20 samples per second. At the beginning of the experiment, the average
station spacing was 4 km in the western and 8 km in the eastern part of the array. As the
experiment progressed, some stations were moved from the western towards the eastern
part of the array in order to obtain a 4-km spacing over the entire length of the array. From
January 1994 until March 1994, the array was reduced to 15 Streckeisen STS-2
instruments and relocated eastward, with a station spacing of 8 km. The TORTISS
experiment recorded about 130 earthquakes of magnitude 6 and greater.
10
Figure 3.1 Topographic map showing exact location of TORTISS stations. Between
station AOl and A445 station spacing is approximately 4 km and between A445 and A5 15
station spacing is approximately 8 km.
OG6L-
09009
006009009-
009-
0
09L
009
0gt
009
006
090
006
099 L
099L
009L
008 L
096 L
00 6
0966
OOt'Z
0999
MJ
12
4.0 THEORY
4.1 Anisotropy and shear-wave splitting
Anisotropy in the general sense is defined as a variation of a physical property
depending on the direction in which it is measured. In seismology the physical property of
concern is the medium velocity. In this thesis the term "seismic anisotropy" will be used to
describe large-scale (compared to the wavelength of a seismic wave) velocity anisotropy.
The degree of velocity anisotropy of a medium can be described by the coefficient of
anisotropy k:
k=
[(Vmax Vmin)/vmeanl
X 100%
(Birch, 1960)
v = velocity.
One manifestation of seismic anisotropy is shear-wave splitting, or birefringence in
analogy to optics. A rectilinearly polarized shear-wave entering a homogeneous
anisotropic medium will split into two rectilinear orthogonal phases. These two phases
will travel at different velocities. Assuming the anisotropic region is homogeneous over a
significant portion of the ray path, a difference in the travel time of the two phases will be
measurable (figure 4.1.1). Using real data, the most suitable phase to detect shear-wave
splitting on a lithospheric scale is the SKS phase. This phase leaves the source as S wave,
converts into a P wave at the core-mantle boundary (CMB), propagates through the liquid
outer core, then changes back into a S wave at its emergence from the core back into the
mantle (figure 4.1.2). Because SKS results from a P to S conversion at the CMB, it
would be rectilinearly polarized in the vertical plane of propagation for a spherically
isotropic Earth, i.e., it would have no transverse component. However, SKS is often
13
Figure 4.1.1 A rectilinearly polarized shear-wave (S) entering an anisotropic medium will
split into two rectilinear orthogonal phases S1 and S2. These two phases will travel at
different velocities. (From Babuska, 1991).
14
Figure 4.1.2 The SKS wave results from a P to SV conversion at the core mantle
boundary. (From Babuska, 1991).
Transverse
J
epkenter
p..
Axis of symmetry
SKS
ray
Figure 4.1.3 Conventions used to derive the relations giving the radial and transverse
projections of two orthogonal S waves s1(t) and s2(t) issued from an incident SKS wave
s(t). The incident SKS wave s(t) is polarized in the vertical plane and is split into two S
waves while travelling through an anisotropic layer represented by a material with
hexagonal symmetry and a horizontal axis of symmetry. (From Babuska, 1991).
15
observed having a transverse component. Since the SKS wave left the core radially
polarized, the source of the observed transverse energy must be on the receiver side of the
raypath. Anisotropy and aspherical structure are possible explanation, which can be
distinguished with the help of particle motion analysis. Anisotropy will produce an
elliptical particle motion, whereas particle motion caused by an aspherical structure will
remain in most cases predominantly rectilinear. Making the assumption that the incidence
angle of the ray is nearly vertical, one can write:
(4.1)
i (t)=s(t)cos(p
s2(t)=s(t-&)sin(
where:
s(t)
is the radially polarized SKS waveform in
a spherical isotropic Earth
s1(t), s2(t)
- are the projections of the ground motions
onto the fast and slow S-wave polarization
directions
is the azimuth of the fast S-wave polarization
direction with respect to the radial direction
is the time difference between the fast and the
slow arriving components
The rotation of the two split components s 1(t) and s2(t) onto a radial and transverse
coordinate system (figure 4.1.3) yields:
R(t)
(4.2)
s(t)cos2(p + s(t-&)sin2p
T(t) = [(s(t) s(t-&))/2]sin2(p
For small öt compared to the period of s(t), the waveform of the radial component will be
very similar to s(t), the radial waveform in absence of anisotropy, and the waveform of the
transverse component will be approximately proportional to the time derivative of the
radial component [equations (4.2) and figure 4.2.3. 1A]. For (
ØO
and 900 the amplitude
of the transverse component is zero (equation 4.2). Hence, for events with a back-azimuth
identical to the fast or slow polarization direction, no transverse energy is observed. These
events are referred to as non-splitting events.
4.2 Methods to investigate shear-wave splitting
The analysis of shear-wave splitting can be performed with methods such as particle
motion analysis, waveform cross-correlation (Bowman and Ando 1987; McNamara and
Owens 1993) and tangential energy minimization (Silver and Chan 1988, 1991). These
methods are similar in that they attempt to estimate the orientation and the degree of
anisotropy by determining the fast polarization direction and the time delay between fast
and slow direction of the S wave. In this section, these three methods are described.
4.2.1 Particle motion analysis
The polarization anomalies caused by anisotropy are observed as departures from
rectilinearity in particle motion diagrams. The particle motion produced by shear wave
splitting is elliptical and initially polarized in the fast S-wave direction. Resultant particle
motion anomalies are used in a number of techniques (Bowman and Ando, 1987; Silver
and Chan, 1988, 1991; Savage et al., 1989, 1990a; Shih et al., 1989) as the main
diagnostic property of shear-wave splitting.
17
In the particle motion analysis method, the initial particle motion orientation is
measured directly from the horizontal particle motion plot of the radial and transverse
components and taken as the angle () of the fast polarization direction clockwise with
respect to north. The seismograms are then rotated onto a coordinate system defined by
.
In this coordinate system, the time delay (&) between fast and slow direction can be
directly measured, e.g., using cross-correlation. The time window of the seismograms
must be long enough to include both the beginning of the fast and the end of the slow
wavelet. The time lag producing maximum cross-correlation is taken as a measure for &.
To test the accuracy of the estimates of
and öt, the seismograms are shifted by & with
respect to each other and rotated back the radial/tangential coordinate system. If the
estimates of
and & are correct, the "corrected" seismograms' particle motion is
significantly more linear than the original.
4.2.2 Waveform cross-correlation
The waveform cross-correlation method takes advantage of the fact that the
waveforms of the split S phase will be most similar in the fast/slow coordinate system
[equations (4.1); Bowman and Ando, 1987]. To obtain
are rotated in increments from
00
and &, horizontal seismograms
to 180°. At each increment the two seismograms are
shifted with respect to each other (by &) and cross-correlated. The rotation angle and time
shift with the maximum absolute cross-correlation value are taken as estimates of 4 and
4.2.3 Tangential energy minimization
The tangential energy minimization method was originally proposed by Silver and
Chan (1988). Since every P to S converted phase is radially polarized, energy appearing
on the S wavets tangential component is evidence for presence of a nonhomogeneous
structure or anisotropy. Silver and Chan (1988) assume the transverse energy is primarily
due to anisotropy. To obtain
and öt, following steps are performed for all possible
and & pairs: the original N and E component seismograms are rotated through a candidate
and shifted by a candidate & with respect to each other, then rotated into the
radialltangential coordinate system where the tangential energy is calculated; the candidate
rotation angle and time delay that most effectively minimize the energy on the tangential
seismogram are taken as estimate of
4.2.3.1.
and &. The basic steps are displayed in figure
19
Figure 4.2.3.1 Energy minimization method using the Philippine Isi. event (ba.z 300.8°)
recorded at station A24. (A) Radial and tangential component of the recorded SKS phase.
The tangential component is very energetic and its waveform is proportional to the time
derivative of the radial waveform. (B) Seismograms rotated to the fast and slow
polarization directions. In this coordinate system & = 1.7 sec can be directly read of the
diagram and waveforms are most similar. (C) Corrected seismograms in the
radial/tangential coordinate system. The tangential component shows almost no energy. (D)
and (F) show the particle motion plot of the uncorrected and corrected seismograms
respectively. Note the elliptical particle motion in (D) versus the rectilinear, parallel to the
radial component, particle motion in (F). (E) Contour plot of transverse component energy.
The minimum energy for 4 = 69° and t = 1.7 sec is indicated by a star and the 95%
confidence level is shown within the bold contour line.
A
C
- Radial
- Fast
- Slow
- Tangential
0
20
10
30
0
seconds
20
10
Radial
Tangential
30
0
20
10
seconds
30
seconds
D
F
C.)
I
2.0
ci)
E
1.5
I
>
1.0
ci)
0
:: i'
-90 -75 -60 -45 -30 -15 0
I
I
15 30 45 60 75 90
Fast Angle (deg)
Figure 42.3.1
C
21
5.0 ANALYTICAL PROCEDURES AND DATA
Seismograms showing a clear SKS phase are most suitable for shear wave splitting
analysis. The SKS can be observed for earthquakes which occur between 61° to 144°
distances, but the most suitable distance range is from 85° to 1100. At distances closer than
85°, SKS is not well separated from the S phase, and at distances greater than 1100, the
signal amplitude is generally too small. Furthermore, events deeper than 100 km are
desirable in order to avoid complications in the SKS-waveforms caused by near-source
surface reflections. A complete back-azimuth coverage is desirable in order to obtain a
tight web of raypaths underneath a station. Common splitting measurements are probably
caused by common raypaths, whereas variations are probably caused by mutually
exclusive raypaths thus constraining depth and lateral variations of anisotropic regions
more tightly (figure 5.1).
The energy minimization method was chosen for the final analysis. For most events
the splitting measurements were also performed using the cross-correlation method with
very similar results. The energy minimization method was found to be more stable for
events with back-azimuths near the fast or slow polarization directions and the computing
time was shorter. As an example, the splitting results using both techniques for the Western
Caroline Islands event are compared in table 6.1.
All the seismograms used for the calculation of the splitting parameters were
integrated, detrended, demeaned, and bandpass filtered from 1 to 30 sec. A time window
of approximately 40 sec was cut around the SKS arrival and the ends were cosine tapered
(10%). In performing the energy minimization, increments of 1° and 0.05 s for
and &
were chosen. In order to asses the uncertainty of each measurement, and to check for
multiple minima, a contour plot of energy on the corrected tangential component was
22
plotted as a function of all tested 4-öt pairs (appendix A). The resulting 4-& estimate is
shown as a star within a 95% confidence region. The array's configuration (i.e.,
consistency of results for neighboring stations) allows in some cases the incorporation of
measurements with large 95% confidence regions, if the minimum -öt pair or the
signature of the energy contour plot are similar to the estimates from the neighboring
stations, which have well resolved results.
From all the earthquakes recorded with the TORTISS array, only 11 showed SKS
phases suitable for the evaluation of shear wave splitting (table 5.1, figures 5.3-5.13).
Source regions for most of these events are the active subduction zones of the western
Pacific rim. Figure 5.2 shows the back-azimuth coverage provided by these events.
As in other similar studies, it is assumed that all transverse energy is caused by shear-
wave anisotropy. A visual inspection of the seismograms of these earthquakes can by
itself provide an indication on possible velocity anisotropy along the raypaths of the
recorded waves. Transverse energy can be observed for eight events. Theory suggests that
the closer the back-azimuth of an event is to the fast or slow polarization directions, the
smaller should be the amplitude of the signal on the transverse component (equation 4.2).
This can be observed best for the Fiji Isi. events (figure 5.6, figure 5.13) with backazimuths only 50 to 100 from the fast azimuth of polarization, which show very small
amplitudes on the transverse compared to the radial component. Furthermore, for the San
Juan event (figure 5.4), the Xinjiang event (figure 5.10) and the Vanuatu Isl. event (figure
5.12), which have a mutually perpendicular back-azimuths, no SKS arrivals are observed,
indicating the back-azimuths of these events coincide with the fast and slow polarization
directions. Comparing the radial and transverse waveforms for events with significant
transverse component, one observes that the transverse component is indeed a time
derivative of the radial component, as expected for an anisotropic medium (chapter 4.1).
This can be observed very clearly in the seismograms from the Philippine Isi. (figure 5.3)
and Taiwan (figure 5.5) earthquakes.
23
Tabele 5.1 List of events. M* refers to either surface wave, body wave or moment
magnitude, whichever is bigger.
Event ID
Origin
UT
1993.05.18
1993.06.08
1993.08.07
1993.08.07
1993.08.09
1993.09.06
1993.09.26
1993.10.02
1993. 10.13
1994.02.11
1994.03.31
10:19:35
23:17:41
00:00:38
17:53:27
12:42:50
03:55:58
03:31:19
08:42:35
02:06:00
21:17:36
22:40:53
Epicenter
LAT, LON
19.814N 122.418E
31.595S069.222W
26.528N 125.612E
23.940S 179.793W
36.348N 070.840E
04.654S 153.253E
Depth Mag. Dist. Baz
km
186
113
158
560
233
073
10.359N 137.996E 033
38.165N 088.640E 033
05.909S 146.017E 024
18.864S 169.102E 250
21.980S 179.585W 591
M*
6.5
6.4
6.4
6.9
6.9
6.6
6.4
6.3
7.2
7.0
6.5
(0)
92.34
90.68
85.38
85.68
98.06
88.57
88.93
92.72
94.58
89.53
85.05
Comment
(0)
300.79
136.28
303.00
230.25
348.54
262.11
283.40
335.32
266.27
242.83
232.72
Philippine Isl. Reg.
San Juan Province, Arg.
NE. of Taiwan
S. of Fiji Isi.
Hindu Kush Reg., Afgh.
New Ireland Reg.
Western Caroline Isi.
Southern Xinjiang, China
Eastern New Guinea
Vanuatu IsI.
Fiji Isi. Reg.
24
array
IS
S
1'
"k
I
S
- - -
S
-
CORE
- -
Figure 5.1 The S-legs underneath the stations of the SKS raypaths form cones underneath
the stations, if earthquakes from all back-azimuths are recorded. Assuming 100 incidence
angle for SKS phases and a 4 km station spacing the common part of two nearby ray cones
starts at about 22 km depth. Comparison of mutually exclusive areas sampled by two cones
and the splitting results of two stations can constrain the depth of the anisotropic region.
25
Event Distribution with respect to Corvallis
,
Einth Kih
T
4
"WtCaró1.
Isl\
5\V\\\
\\\\\
\
,\
\c\
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I
\4\\\\\\\\\\\\\\
\\\\\\\\\\\\
inea\\\\\\\"\
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\\\
-
us
\\\\\\\)?c,,X\\ \ \ \
\\\\\\\\\\\\X\K\\\\\
,
and\\\\\\\\\\('c.'\\\\\\ \
\\\ \\'.\\\\\\'k\\\\>K\YS\\\\\\\\
\\\\\\\\\\\\\/(\\)S\\\\\\\\\\\. '\
e
I
I
/
\\ \\\'\\\\\\'\\\\X\\\\\\\\\\\\\\\\\
\\\'\\\\\\\V\\\)\\\\..'\"I'Z\\\ \\\
\\
\\\
\\\
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\\\
/
\\\ Vafl1àtVLs1\\\\\\\\\\\\\\\\\\\\\\ \'\\\\\\
\\\\
\\\
\\\\/
\\\\\\\Qc\\\\\\.\\\\\\\\\\\\\\\\'c\\\\ \\\\.\\\
\\\\
\ \\\ \\\\\\\\\\\\\\\\\\\\\\\\_>,,-'\\\\\\
\\
'Fiji Ia1O.pd
Reg.\\\\\\\.\-c-\\\\\\\\\\\ \,.\\\
\\\
'S\
\\\\\
\\
all
r Vlflce\\\\
\\\\\\\\\'4,\\\\\\\\\\\\\\\\\\\\\\\\\\\\S\\\\
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\.\ \\ \\\\\\\\\\\
\\\\\\\\\\
\\\\\\\\\\\\\\s.4\\\\\\\\\\\\\\\\\\\'\\
\\\\\\\\\\
\\\\\\\\\\\\\\\\\\\\\\\\\\\\
\ \\\\\\\\\\\\
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
\\\\\\\\\\\\\
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
5' \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\\\\\\\\\\\\\
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \ \ \.\ \ \ \\ \S \ \ \ \
5'\\\\\\\\\\\\\\\\\\\\\\\\"S\\\\\\\\\\\\\S'\
\\\\\\\\\\\\
\\\',\,\\,'\\'\
Figure 5.2 Distribution of the events used in this study with respect to Corvallis. The map
uses an azimuthal projection. The dashed circles indicate distances of 300, 60°, and 90°
degrees. The dotted pattern indicates the northern and the lined pattern the southern group.
Figures 5.3 5.13 The following figures show seismograms of all events used in this
study. The first arrival in each seismogram is the SKS-wave. Figures with figure numbers
ending in (a) show radial seismograms, and those ending in (b) show transverse
seismograms. Seismograms in (a) have different scales from those in (b).
Distance From The Coast (km)
-NNNN)N)NN)
N)
N)
)N)C)
Cl Co 0 0) -N
0)
0
-4
CD 0 0) -N -N 01 0)
CC) 0
Cl0ClClN)0ClClN)0ClC)N)0ClCl4.N)0ClClN)0ClClN)0ClCl4.N)0Cl0)3
-N
-N
01
Cl
N)
N)
01
Cl
-
Cl
CO
-4
Cl Cl
N)
0
Ho
-N
CD
C',
11
1993/1 38 10:19:35.1
Radial SKSab-wave 1-30 s
19.831N 122.436E 187km M6.3 92.3/300.7 PHILIPPINE ISLANDS REGION.
Figure 5.3a
N)
Distance From The Coast (km)
C)
0
C)
C)
N)
-
0)
-
N) 0
-
C)
01
C)
C)
.
.J
C)
C)
N) 0 C)
(C)
- C) -
C)
4
N)
-' N)
0
N)
C)
- _ _ - - 0) 4- 01 C) C)
C) 4
N) 0 C) C)
J
L
C)
-
-L
N)
N.)
N)
CC) 0 0 N)
0 Cr) C)
N)
N)
.
N)
0)
N)
N)
-
©
N)
-
C)
N)
01
C)
N)
C)
.
N)
-.J
N)
0)
N) C)
N)
N)
0)
C)
C)
4
0) CC) 0
N)
0
-Ho
3
CD
C,)
[!i
1993/138 10:19:35.1
Transverse SKSab-wave 1-30 s
19.831N 122.436E 187km M6.3 92.3/300.7 PHILIPPINE ISLANDS REGION.
Figure 5.3b
Distance From The Coast (km)
-
C C C C)
N) 0)
4
.
.
N) C (0
01 0)
0)
4
N)
C
-
-
(0 C - N)
(0 0) 4 N) 0
C CX)
ND
01
-L
0)
0)
- - - .- - - N) N) N) N) N) N) N) N)
01 0)0) -J (0 (C) C C - N) 0)
01
N) 0 C C)
ND C (0 0) 4 N) C C 0)
.
.
ND
0)
4
N) N)
N)
ND
CX)
CX)
(C) C
'J
N) C C 0)
0)
.
N)
C
-Ho
B
CD
(I)
1993/159 23:17:41 .4
Radial SKSab-wave 1-30 s
31.595S 69.222W 113km M6.4 90.4/1 36.2 SAN JUAN PROVINCE, ARGENTINA
Figure 5.4a
N)
Distance From The Coast (km)
rrc
3
CD
(I)
S.
1993/159 23:17:41 .4
Transverse SKSab-wave 1-30
31.595S 69.222W 113km M6.4 90.4/136.2 SAN JUAN PROVINCE, ARGENTINA
Figure 5.4b
S
U-
Distance From The Coast (km)
01
0
01
C)
N)
4
OD
4
N) 0
4
01
01
0)
01
C)
-
ND
0
01
01
- CC) C C)
4
ND
ND 0
- _ ND
01
-
-
C) 4
01
0)
0)
ND
-
0)
0 CD
J
C)
-
-
01
CC) 0 0 0 0) C)
ND
-
ND
ND
ND
ND
N)
.
ND
0)
ND
ND
4
N) 0 0)
ND
ND
ND
(Dl
C)
-4
0)
4
ND
ND
01
ND
CX)
0 0)
ND
OD
0)
4.
CD 0
ND
0
Ho
2
CD
Cl)
1993/219 00:00:37.5
Radial SKSab-wave 1-30 s
26.528N 125.612E 158km M6.4 85.3/303.0 NORTHEAST OF TAIWAN.
Figure 5.5a
Distance From The Coast (km)
N)
0
-10
2
CD
0)
1993/219 00:00:37.5
Transverse SKSab-wave 1-30 s
26.528N 125.612E 158km M6.4 85.3/303.0 NORTHEAST OF TAIWAN.
Figure 5.5b
I')
N)
4.
Hc
CD
C')
1.
-L
-
-L
..L
L
L
...L
N)
Distance From The Coast (km)
N)
N)
N)
N)
1993/219 17:53:27.3
Radial
23.904S 179.793W 560km M6.9 85.6/230.2 SOUTH OF FIJI ISLANDS.
Figure 5.6a
N)
Distance From The Coast (km)
- - -L
N)
.
.
N) N) N)
N)
N)
N)
N)
N)
N)
N)
N)
N)
N.)
0
-10
3
CD
C')
S.
1993/219 17:53:27.3
Transverse SKSab-wave 1-30 s
23.904S 179.793W 560km M6.9 85.6/230.2 SOUTH OF FIJI ISLANDS.
Figure 5.6b
Distance From The Coast (km)
C)
C)
C) C)
N)
.
0)
.
-P
N) 0 C)
01
C)
C)
-
-4
N)
C) C) CD 0 C)
C)
C)
4
N)
N)
C)
N)
C)
0)
C)
-
01
N)
C) C) -J
C) C) C)
C)
4
CD
N)
C)
C)
N)N)ON)
0 - N) 0)N)N)4 N)
01
CD C)
3
N)
C)
C)
C)
N)
N)N)
4
N)
C)
-J
C)
C)
N)
C)
C)
N)
CD
C)
0)
C)
4
N)
C)
Ic
3
CD
Cl)
S
I.
1993/221 12:42:50.0
Radial sSKS 1-30 s
36.348N 70.840E 233km M6.9 98.4/348.4 HINDU KUSH REGION, AFGHANISTAN.
Figure 5.7a
304
296
288
280
272
264
256
248
240
232
224
216
208
192
- 200
-
184
176
152
160
o 168
I-
120
128
136
E 144
L
(I)
112
20
40
Time(s)
60
(I)
0
Cl)
C,)
U)
0)
(0
I-
I-
L1
z
0
C,
w
I
C,)
2
=
co
co
0)
(0
E
C1)
0(l)
Lw
siZ
0)
0)(0
r
36
N)
C
HC
3
CD
(I)
.
S.
-
-
01 0)
N) C C C 4
N) C)
C C C C -
ND
-
N)N)N)N)N)N
N) 0) 4 01 C C
C (C) C 0 - N) 0)
N) C C C - N) C C C 4 N) C
C C C --
Distance From The Coast (km)
N)
-.J C C (0 C -
N) C C C
Radial S
1993/249 03:55:58.0
4.654S 153.253E 33km M6.6 88.51262.1 NEW IRELAND REGION,
Figure 5.8a
Distance From The Coast (km)
0
Ho
(D
Ci-)
i1
1993/249 03:55:58.0
Transverse SKSab-wave 1-30 s
4.654S 153.253E 33km M6.6 88.5/262.1 NEW IRELAND REGION,
Figure 5.8b
Distance From The Coast (km)
-.
r
c
-
-
01 C)
-
C)
-
CD 0 -L
-
-
-
-
N)
N)
c
.
cu
-
-
C)
C)
-
-' -
r
r
N)
-4 0 (0 Q Q _L
N)
N)
N)
ND
0)
-
'D
-
N)
01
N)
C)
N)
-.4
N)
C)
N)
C)
N)
UD
(0 C)
Hc
B
CD
C')
jI
S.
1993/269 03:31 :18.8
Radial SKSab-wave 1-30 s
l0.359N 137.996E 33km M6.4 88.9/283.4 WESTERN CAROLINE ISLANDS.
Figure 5.9a
Distance From The Coast (km)
C)
C)
C)
C)
N)
.
C)
4
ND 0
.
01
C) C)
C)
-
C) C) CD 0
N) N) 0)
N) 0 01 C)
N) 0 01 C)
-.4
-
01
C)
N) 0
C)
01
.4
C)
C)
.
(0 0 0 -
N) 0 01 C)
N)
4
0)
N) 0
-
C)
CJ1
C)
C)
-
-4
C)
N) 0
N)r)c
C)
01
(C) C
C)
-
N)
0
lo
B
CD
Cl)
1993/269 03:31:18.8
Transverse SKSab-wave 1-30 s
10.359N 137.996E 33km M6.4 88.91283.4 WESTERN CAROLINE ISLANDS.
Figure 5.9b
Distance From The Coast (km)
L
..
-
-L
.L
-L
.
-
.L
N)
N)
N)
N)
N)
N)
N)
N)
N)
N)
N)
N)
N)
0)
N)
0
Ho
2
CD
(I)
S.
1993/275 08:42:35.4
Radial SKSab-wave 1-30 s
38.165N 88.640E 33km M6.O 92.7/335.3 SOUTHERN XINJIANG, CHINA.
Figure 5.lOa
Distance From The Coast (km)
L
C)
N)
0)
-
-
01
C)
-J
C)
C) CD
0 0) C) A N) 0 0) C) A N) 0 0)
C)
C)
-
-
-N)N)N)N)N)N)N)N)N)N)N)N)C)
CD 0 C) A 01
C) C) CD 0
N) 0 0)
N) 0 0) C) A N) 0 0)
A N) 0 0)
N)
N) 0) A 01 C) C)
N) 0 0)
C)
-
-4
C)
C)
-
N) 0)
-
C)
C)
J
C)
A
I
HoA
2
CD
C,)
S.
1993/275 08:42:35.4
Transverse SKSab-wave 1-30 s
38.165N 88.640E 33km M6.0 92.7/335.3 SOUTHERN XINJIANG, CHINA.
Figure 5.lOb
A
N)
N)
C)
HC)
CD
C,)
C)
C) C)
C)
4
N)
4
-
N) 0 C)
0)
01
C)
C)
ND
-1 0)
C)
C) C)
.
(0
0)
N)
N) 0
N)
C)
0)
C)
4
-
01
N)
C) C)
J
C) C) C)
-
C)
C)
C)
C) -
N) 0 0)
CD
Distance From The Coast (km)
C)
-
N)
-
0)
N)
-
C
1993/286 02:06:00.2
Radial S
5.909S 146.017E 24km M6.7 94.5/266.2 EASTERN NEW GUINEA REG
Figure 5.11a
Distance From The Coast (km)
0)
0
0)
N) o
0)
-
N)
oi
.
.
C)
C) C)
C)
-
-
-i
N) 0 C)
C)
4
N)
r'
N) 0 0)
O
C)
a) C)
N) 0 0)
0-i
-
-
C)
C)
4
-N) N)N) N)N) N)N)
(0 0 0 -' N) 0) 4
N) 0 0) C)
N) 0 C)
-
-
N)
N)
N)N) N)
N)
0)
C)
.
N)
0
C)
4
01 0)
-4
C)
C)
C)
(0 0
N)
0
B
CD
C/)
1993/286 02:06:00.2
Transverse SKSab-wave 1-30 s
5.909S 146.017E 24km M6.7 94.5/266.2 EASTERN NEW GUINEA REG., P.N.G.
Figure 5.11b
N)
0
-10
2
CD
(1)
CD
0
CD
0)
4
N) C
CD
0)
4
1994/042 21 :17:36.0
NJ 0
CD
0)
0)
-
-
ND 0
CD
- _ 0)
4-
-
0
N)
CD
N)
0)
ND
-
N)
N)
N)
N) 0
Radial
N)
Distance From The Coast (km)
N) 0 CD
.
-
4
18.864S 169.1 02E 250km M7.0 88.1/241.1 VANUATU ISLANDS.
Figure 5.12a
Distance From The Coast (km)
C)
0
C)
C)
N) 0) A A 01
N) 0 C) C)
-
C)
-
-J
C)
C)
N) 0 C)
(D 0 C)
N) N) 0)
01 C) C)
C) (C C) 0 - N) 0) A C) -J C) C) CD 0
A N-) 0 C) C) A N) 0 C) C) A N) 0 C) C) A N) 0 C) 01
0) A N) 0 0) C) A
-
-.J
N-)
0
HoA
(0
C/)
[S.-
1994/042 21:17:36.0
Transverse SKSab-wave 1-30s
18.864S 169.102E 250km M7.O 88.1/241.1 VANUATU ISLANDS.
Figure 5.12b
Distance From The Coast (km)
C)
0
C)
II
II
-
N) C)
C)
4.
N)
0
II
II
II
-
-
C)
(31
C)
C)
-
-i
C)
0 -
C)
N) 0 C)
C)
4
N)
N) 0
N)
C)
C
4
C)
.
0-i
C)
C)
N) 0 C)
-J
C)
-
N)
N)N)N)N)N)N)N)N)N)N)C)
o
01 C)
4
C) C) CC 0
0 oC) -C) N) 0)
N) 0 0 C)
N) 0 0 C)
-
-
N)
0
-Ho
3
CD
C,)
rii
L1
II
II
1994/090 22:40:53.3
II
II
II
II
II
II
II
II
II
II
Radial SKSab-wave 1-30 s
21.980S 179.585W 591 km M6.5 83.8/230.9 FIJI ISLANDS REGION.
Figure 5.13a
Distance From The Coast (km)
-
C)
0 0 C)
N) 0
.
-
4
01
N) 0 0 C)
C)
4
C)
C)
0 C) -
N) 0 0 C)
4
N)
N) 0
N)
C)
01
-
01 C) C)
-.4
C)
C)
-
N)
C)
-
C)
C)
c
N)
C)
C)
C)
-
C) C)
N)
-
01
N)
-
-
0-1
C)
C)
-.4
CO
C)
-
N)
C)
C)
C)
CO
CD
C)
C)
4
N)
0
Ho,
2
CD
(I)
1994/090 22:40:53.3
Transverse SKSab-wave 1-30 s
21.980S 179.585W 591 km M6.5 83.8/230.9 FIJI ISLANDS REGION.
Figure 5.13b
Co
6.0 RESULTS
The analyzed events are listed in table 5.1. The measurements of shear-wave splitting
for every event are presented in appendix A. Table 6.2 shows the estimates of 4 and öt for
each station obtained as error-weighted averages of all the measurements done. The
averaged results for each station are shown graphically in figure 6.1.
The (-6t estimates are fairly consistent across the array. The arithmetic average of the
weighted averages of all the stations is [70°, 1.61 sec]. Compared to worldwide splitting
measurements, for which 2-sec delay times are about the maximum observed, this result is
well above average. For comparison, delay times obtained from other subduction zones are
not quite as large: e.g., 1.55 sec College, Alaska; 1.00 sec Longmire, Washington (Silver
and Chan, 1991); 1.23 sec Naña, Peru; and 1.48 sec Erimo, Japan (Helffrich et al., 1994).
Simple averaging of the station measurements does not take advantage of the unique
set up of the array. With 4-km station spacing lateral variations in anisotropy can be
charted. Figure 6.1 shows that the array can be roughly divided in three different regions
according to gross anisotropic characteristics. The first region extends over the Coast
Range from station AOl to about A14 (figure 6.1) with an average of [72°, 1.34 sec]. The
delay times found for this region are the lowest in the array, and, compared to the other two
regions, the measurements are less stable (i.e., have larger uncertainties; table 6.2). The
second region extends over the Willamette Valley and the Cascades Range from about
station A 15 to about station A36 with an average value of [66°, 1.74 sec]. This is the region
with the most stable results. The third region is located east of the Cascades from about
station A37 to station AS 15. Reliable splitting measurements were only obtained for
stations A37 to A43 which have an average of [79°, 1.77 sec]. The reason for including
50
Table 6.1 Splitting measurements for event:
W. CAROLINE ISL. 93/09/26 03:31:19 Mw=6.4 dist=88 .93 baz=283 .4 (1 s-30s)
STA refers to the station, parameter () is the fast polarization direction (clockwise with
respect to north), and öt is the delay time in seconds.
(a) Cross-correlation method:
STA
4)
&
A24
A25
A26
A27
A28
A29
A30
77.40
72.40
74.40
77.40
76.40
81.40
83.40
73.40
83.40
79.40
65.40
1.30
1.25
1.30
1.80
1.90
1.55
1.65
1.50
1.70
1.80
1.65
A31
A32
A33
A34
(b) Energy minimization method:
STA
A24
A25
A26
A27
A28
A29
A30
A31
A32
A33
A34
&
4)
68.00
53.00
48.00
56.00
56.00
53.00
54.00
57.00
64.00
73.00
60.00
-i-I-
18.00
+1- 14.00
+1- 13.00
9.00
+18.00
+1+1- 14.00
+1- 15.00
+1- 20.00
+1- 24.00
+1- 26.00
8.00
+1-
1.15
1.25
1.50
1.85
1.90
1.55
1.60
1.50
1.45
1.65
1.60
Tht
0.40
0.35
0.60
0.40
-i-I- 0.35
+1- 0.40
+1- 0.45
+1- 0.65
+1- 0.65
+1- 0.90
-i-I- 0.30
+1+1+1+1-
51
Table 6.2 Weighted average for all events
STA refers to the station, parameter is the fast polarization direction (clockwise with
respect to north), & is the delay time in seconds, G(1) and a& are the la uncertainties, and
#_ev refers to the number of the events used to calculate the weighted average.
STA
AOl
A02
A03
A04
A05
A06
A07
A08
A09
AlO
All
Al2
A13
A14
A15
A16
A17
A18
A19
A20
A21
A22
A23
A24
A25
A26
A27
A28
A29
A30
Mi
A32
A33
A34
A35
A36
A37
A38
A39
A40
A41
A42
A43
4,
69.53
67.01
72.72
75.00
72.00
67.49
61.29
72.27
79.69
76.73
73.96
71.76
74.81
77.88
69.75
66.44
69.09
68.11
67.17
62.74
68.00
64.89
64.93
65.16
65.72
64.85
64.29
64.10
54.80
61.49
69.49
72.46
68.99
65.58
68.59
67.00
73.44
74.80
83.00
75.20
76.02
84.07
86.85
+1+1+1+1+1+1+1+1+1+1+1-
1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1-
i-I+1+1+1+1+1+1+1+1+1+1-
04)
&
5.86
7.94
6.90
9.00
8.00
20.38
43.04
6.29
3.98
7.55
8.41
4.83
4.43
4.78
3.67
14.87
4.03
3.33
2.68
2.59
7.00
1.94
2.61
1.22
1.38
1.10
1.20
1.35
1.00
0.89
1.22
1.44
1.60
+1+1+1+1+1+1+1+1+1+1-
1.51
1-
1.60
1.64
1.64
1.67
1.50
1.64
1.59
1.45
1.78
1.35
1.63
1.63
1.73
1.70
1.73
1.84
1.85
1.57
1.64
1.87
1.96
2.10
1.67
2.21
2.09
1.99
1.96
1.65
1.79
1.73
1.68
+1+1+1+1+1+1+1+1-
1.73
1.69
3.01
2.03
3.65
8.59
11.64
5.82
1.89
1.79
3.72
3.59
7.95
1.73
4.73
25.00
3.97
5.05
6.00
5.60
1.61
(Y&
-i-I-
+1+1+1+1+1+1+1+1+1+1+1+1-
i-I+1+1+1+1+1-
1+1+1+1+1-
0.17
0.34
0.15
0.25
0.25
0.42
0.98
0.17
0.15
0.23
0.30
0.36
0.22
0.28
0.21
0.55
0.18
0.20
0.22
0.20
0.25
0.26
0.20
0.14
0.14
0.20
0.13
0.18
0.26
0.39
0.29
0.27
0.18
0.21
0.23
0.43
0.20
0.22
0.90
0.17
0.21
0.27
0.44
#_ev
4
3
2
1
1
2
2
5
4
3
3
5
5
4
5
4
6
6
5
5
1
4
5
6
6
6
6
5
5
3
4
4
4
5
5
2
5
3
1
5
6
6
4
WEIGHTED AVERAGE OF ALL EVENTS
45 N
3150
3050
2950
2850
2750
2650
2550
2450
2350
2250
2150
2050
1950
1850
1750
1650
1550
1450
1350
1250
1150
1050
44N
950
850
750
650
550
450
350
250
150
50
25
0
124W
123W
122W
Figure 6.1 White vectors show the average of spitting measurements at every station. The
orientation of the vectors corresponds to and the length to &.
121W
Ui
53
stations A435 to A515 into the third region will be discussed later in this chapter.
Compared to the other two regions, where the results within a region are fairly constant,
gradually increases from 73° at station A37 to 87° at station A43 is observed. The averages
for these three regions are displayed in a rose diagram in figure 6.2a.
Correlating splitting measurements with back-azimuth of the earthquakes, one can
divide the events in two groups. The first group covers events with back-azimuths from
1300 to 280° and the second group from 300° to 350° (hereafter called southern and northern
group, respectively; figure 5.2). Rose diagrams for these two groups showing the average
splitting values for the three regions are displayed in figure 6.2b,c. On average, the
northern events show 8° higher
and 0.16 sec higher 6t than the southern events. The
three regions across the array mentioned above are observed for both groups, however, are
less marked for the southern events (figure 6.3). The reason that the three regions are less
marked for the southern events might be the fact that the southern events are less suitable
(i.e., event depth < 50 km, event distance 80° - 90°) for shear-wave splitting measurements
than the northern events, but most likely the reason is the difference in deep seated
anisotropy sampled by northern and southern events.
A visual inspection of the seismograms of events from non-splitting back-azimuths
provides additional constraint for separation of results into northern and southern backazimuths and the three regions along the array. The fact that the non-splitting back-azimuth
of the Xinjiang earthquake, part of the northern events, is 4° larger than the non-splitting
back-azimuth of the Vanuatu Isl. earthquake, part of the southern events, supports the
conclusion that
for northern events is somewhat larger than the one for the southern
events. Gradually increasing amplitudes on the transverse component along the array for
the non-splitting Xinjiang event (figure 5. lOb) eastward from station A37 (i.e., third
region, stations A37-A43), are consistent with the measured gradual increase of
the third region.
within
54
00
sec
00
sec
00
C
sec
Figure 6.2 Vectors showing the average splitting for the Coast Range (first region, black),
Willamette Valley and Cascades (second region, dark grey) and east of the Cascades (third
region, light grey). The orientation of the vectors corresponds to 4 and the length to &.
Figure (a) is the weighted average of all, figure (b) for the "northern", and figure (c) for the
"southern" events.
56
The 1994 Fiji Is!. earthquake (figure 5.13) was recorded only at the eastern end of the
array (A375-A5 15) with a back-azimuth close to the fast polarization direction of the
southern events leading to inconclusive splitting measurements for stations A375 to A475.
For the last three stations at the eastern end of the array (A485, A505, A515; no data were
available for A495), however, stable results were obtained. This suggests that the tendency
of
c
to rotate clockwise from A37 to A43 continues to station AS 15. Therefore the third
region can be expanded up to station AS 15. The only event recorded at the eastemmost part
of the array (A445-A515), suitable for splitting measurements, was the 1994 Fiji Is!.
earthquake. Additional information for this part the array is supplied by the Vanuatu Isl.
event (figure 5.12), which has a non-splitting back-azimuth (243°), suggesting a 4 of
about 63°.
Finally, an event from Argentina (figure 5.4), the only event from the southeast
(back-azimuth 136°) recorded on stations AOl to A43, has been analyzed. Although this
event is close to the slow polarization azimuth, and does not produce a well resolved result,
it suggests
of approximately 55°. This value is compatible with the average result for the
southern events.
In order to obtain constraints on the thickness and depth of the anisotropic layers, the
splitting parameters were calculated for different frequency bands for the Philippine Island
event, which is the event with the most stable measurements. Eight different period bands
were chosen: ls-30s, 3s-30s, 5s-30s, is-lOs, 3s-8s, 3s-7s, 3s-6s and 4s-6s. The splitting
measurements for all bands are shown in figure 6.4. The biggest change (with 4 rotating
anticlockwise) in splitting measurements is obtained while going from 3s-30s band to 5s30s band. Results for is-lOs band show a clockwise rotation compared to the ls-30s band.
The most significant changes occur in the first and the third region of the array. For 3s-8s,
3s-7s, 3s-6s and 4s-6s bands the splitting results are essentially the same, however,
compared to the bands containing periods between lOs and 30s, in the Coast Range and
3600
45 N
3300
3000
2700
2400
2100
1 800
1500
44 N
1200
900
600
300
0
124W
123W
122W
121W
Figure 6.4 Vectors showing splitting results for different frequency bands for the
Philippine Isi. event. Upper array shows results for periods between 5s-30s, 3s-30s, is30s, and 1-lOsec (clockwise from north at every station). Lower array (vectors are
displaced from station locations by 0.2°) shows results for periods between 3s-8s, 3s-7s,
3s-6s, and 4s-6s (clockwise from north at every station).
east of the Cascades,
is rotated clockwise. In summary, the largest frequency-related
changes in splitting are seen at both ends of the array for waves with periods between 3 and
8 seconds. In general, cutting out long periods (lOs-30s) causes 4 to rotate clockwise and
cutting out high frequencies causes
to rotate anticlockwise. The significance of these
observations will be discussed in the next chapter.
59
7.0 DISCUSSION
7.1 Localization of anisotropy
7.1.1 Thickness of the anisotropic layer
Assuming the observed splitting is due to a single anisotropic layer, its thickness can
be estimated by
L= öt vs/k,
where & is the measured delay time, v is the isotropic shear velocity of the layer and k is
the coefficient of anisotropy (Silver and Chan, 1988). To estimate k, we assume that the
anisotropy is caused by lattice preferred orientation of anisotropic minerals, mainly
olivine, which is most abundant in the mantle, and that the long axis [100] of olivine
(figure 1.1) is oriented parallel to the extension direction and the short axis [0101 is
oriented parallel to the maximum shortening direction (Christensen, 1984). In the case of
simple shear, which is an appropriate mode of deformation for the differential motion
along plate boundaries, the [100] axis is contained within the flow plane, and is parallel to
the flow direction (Nicolas and Poirier, 1976). Estimates for the coefficient of anisotropy
in mantle rocks based on calculations from petrofabric strain measurements are around
4%, and are not expected to be higher than 8% (Mainprice and Silver, 1993).
Assuming k=4% and v=4.5 km/sec, the thickness of the anisotropic layer across the
TORTISS array varies from about 112 km. in the Coast Range, to as high as 248 km
underneath station A35. These variations seem to be related to the geology. The Coast
Range shows anisotropic layer thickness values of around 130 km, underneath the
Willamette Valley the thickness increases to about 180 km, and reaches its high (248 km)
underneath the Cascades, to decrease eastward to about 180 km at station A43. However,
the observed delay times, could be caused as well by a half as thick anisotropic layer
assuming k=8%.
One should consider that changes in splitting along the array could be also caused by
two anisotropic layers. According to Savage and Silver (1993), splitting caused by a two-
layer anisotropy shows a it/2 periodicity with back-azimuth. Because the splitting
observations by TORTISS do not display this pattern, a simple two-layer anisotropic
model does not seem to apply here. However, the earthquakes studied here have a rather
poor azimuthal coverage.
7.1.2 Depth of the anisotropic layer
It is generally assumed that the primary source of the observed shear-wave anisotropy
is located in the upper mantle. According to tests done with olivine and perovskite under
high pressure to simulate lower-mantle conditions, the lower mantle (beginning at about
700 km depth) appears to be isotropic (Silver et al., 1993). The contribution of crustal
anisotropy to splitting delay öt was shown to be about 0.3 sec in magnitude, based on
measurements from P to S conversions at the Moho (Silver and Chan, 1991; McNamara et
al., 1989). Barruol and Mainprice (1993) modelled the seismic properties of the crust
using hypothetical polycrystals with typical crustal compositions and commonly measured
petrofabrics and suggest delay times of about 0.1-0.2 s per 10 km of crustal rock. If the
entire observed anisotropy were confined to the crust, assuming the crust is 45-km thick,
the derived k of 18% would be unreasonably high. However, variations in splitting
measurements along the TORTISS array could be explained in part by variations in crustal
anisotropy.
Evidence for shallow crustal anisotropy comes from the examination of the quasi
Fresnell zones in different frequency bands for stations with different observed splitting
measurements. Assuming the distance between two stations showing significantly
different splitting is 16 km, in order to be able to resolve this difference, the distances
from the stations to the anisotropic source regions should differ by at least ?14 (using the
idea of Fresnell zones [Burnett et al., 1958], X is the wavelength). Accordingly, the depth
of the anisotropic source region for ? =4, 8, 12, 16, and 20 km would not be deeper than
3 1.5, 15, 9, 6, and 4 km, respectively. The wavelengths were calculated for v = 4 km/s
and periods of 1, 2, 3, 4, and 5 s, which are the contributing periods responsible for
variations in shear wave splitting along the array found in the frequency tests discussed in
the previous chapter. The above calculation gives only a very rough estimate because the
anisotropic source region is not a point; a shear wave must propagate over sufficient
distance to accumulate observable splitting. The estimate of the depth of the anisotropic
source region could have an error of the order of factor 2. Nevertheless, the above
estimates indicate significant contributions to the shear wave splitting from the crust. The
largest short-wavelength changes in splitting (öt as high as 0.3 sec) occur in the third
region. A change in & of 0.3 sec can be explained by variations of k between 0% and 4%
in a 30 km thick anisotropic layer. Hence, in principle, it is possible to explain most of the
observed variations of splitting along the array by variations in the anisotropy of the crust.
In order to estimate the maximum depth of the anisotropic region which causes the
splitting differences between the first and the second region, the Fresnell zone concept is
applied. Using the average splitting measurement for each region and half the distance
between the centers of the regions (56 1cm), the depth of the source region would be not
62
deeper than 390, 195, 130, 95, 75, 62, 53, 45 km. for ?. = 4, 8, 12, 16, 20, 24, 28, and
32, respectively. This estimate allows for the depth of the source region of the observed
change anywhere within the crust and the upper mantle.
7.2 Relation of anisotropy to tectonic processes
Previous studies provide evidence that mantle anisotropy is caused by strain induced
lattice preferred orientation of upper mantle minerals (e.g., Nicolas and Christensen, 1987;
Silver and Chan, 1991). From the three principal strain directions (shortening,
intermediate, and extension) extension is predicted to be parallel to the [100] axis of
olivine (Nicolas and Poirier, 1976; Christensen, 1984; Nicolas and Christensen, 1987)
This prediction is valid for almost all kinds of finite strain: uniaxial extension and
shortening (pure shear) and simple shear, which is a deformation mode found in the
differential motion between a plate and the underlying mantle or a underlying plate. In the
case of simple shear, the [100] axis is contained within the flow plain parallel to the flow
direction (Nicolas and Poirier, 1976). The same behavior is predicted for
generally parallel to the [100] axis. In order to relate
between stress and strain is needed, because
(1)
since it is
to stress, a constitutive relationship
is primarily related to finite strain (i.e.,
parallel to the axis of the extension direction). The exact dependency is not known, but it
may be reasonably guessed at. The fast polarization direction should be either parallel or
perpendicular to the maximum horizontal stress (maximum compression) direction Y,
depending on the way the stress is generated (Silver and Chan, 1991). If the strains are
produced by a basal shear stress, then
4
should be parallel to the
Yhmax
direction. If the
63
strains result from stresses parallel to the plane of the plate, the fast polarization direction
should be perpendicular to the Yj
direction.
As the origin for strain responsible for the orientation of the fast polarization
direction, the following possibilities should be considered: In the mantle, the relative
(RPM) and absolute plate motion (APM) of the JdF and the North American plates (APM
JdF plate: N76.5°E; APM N. A. plate: N240°E; Gripp and Gordon, 1990) and backarc
corner flow, and, in the crust, the maximum horizontal stress
((Yhmax).
Basal shear strain caused by APM seems to be responsible for the lattice preferred
orientation of mantle minerals. Silver and Chan (1991) analyzed the relation between
and APM in detail, and found that the hypothesis holds only in tectonically stable areas of
fast moving plates. Comparing the APM direction (67.5°) and the RPM direction (65°) of
the JdF plate (with respect to North America), with the average 4 (68.5°) obtained for the
first two regions (west of the Cascades) of the TORTISS array, suggests a possible
relation between plate motion and
.
In this case the strain responsible for the orientation
of the minerals would be related to the mantle flow induced by the motion of the
subducting plate. Comparing the APM of the North American Plate (240°) with
4
(79°)
obtained for the third region, east of the Cascades, a relation between the two parameters
might be possible as well, although not as marked as west of the Cascades. Generally, for
splitting measurements done using longer periods, 5 sec and above,
tends to rotate
anticlockwise, i.e., closer to the APM of the respective plates, implying that deeper seated
anisotropy is more closely related to APM.
For short periods (ls-5s)
4
tends to rotate clockwise, i.e., closer to the east-west
direction, again with largest magnitudes at the ends of the array, where as stated in the
previous chapter the changes in
may have a shallow, crustal origin. Comparing
Yhmax
provided by Zoback et al. (1989), which is about N-S oriented in Oregon, with the fast
polarization direction at the ends of the array (almost E-W), calculated for short periods as
mentioned above, a relation between the two parameters for crustal anisotropy (i.e.,
perpendicular to
Yhmax)
is
seems possible.
East of the Cascades, the fast polarization axes might be aligned by the strain induced
by a possible backarc corner
flow.
Assuming a backarc corner
flow
one would expect a
downward flow somewhere in the vicinity of the Cascades. Since the [100] axis of olivine
aligns with the flow and the angle of incidence of SKS is nearly vertical, splitting due to
such
flow
should be barely detectable. The observation of 2 sec split time delays in the
vicinity of the Cascades argues against the significance of the corner flow mechanism.
7.3 Slab effect
In a subduction zone, the knowledge of how a change in the dip of the slab would
affect the splitting might be useful for the interpretation of the results. The effect of the
change of slab dip can be estimated theoretically by varying the dip angle () of a plane
(figure 7.3.1). The angle between the direction of the [100] axis of a olivine crystal and
the dip direction of a plane in which it is imbedded is p. The angle between the surface
projections of these two directions is a. Increasing the dip angle (), while keeping p
constant, causes an increase in a. For example, assuming p of 20°, an increase in dip of
40° would increase a by 15° (figure 7.3.1). The relation between a and
fast polarization direction, is
= 90°
a.
,
the observed
Furthermore, since the [100] axis of olivine
lays in the subducting slab, the angle between the [1001 axis of olivine and the ray
propagation decreases for steeper slab angles and therefore the delay times should decrease
with increasing dip angle.
65
Figure 7.3.1 The horizontal plane projections (bold) of vectors in the dipping planes. The
relationship between the angles is: tan ix tamp / cosE.
Evidence for a bend in the slab is brought by Nábëlek et al. (1993) in a receiver
function study using data from the TORTISS array, who suggest a change in slab dip
possibly around station A14. Hence, the observed change in the orientation of
between
regions "AO1-A14" and "A15-A36", (chapter 6) could perhaps be explained with a change
in dip of the anisotropic layer. However, the delay times for the second region are higher
than the delay times for the first region, which is in contradiction with our prediction.
Larger delays might be caused by higher straining (i.e., higher anisotropy coefficient)
within the steeper dipping slab, or by overlying anisotropic material which may be
compensating for the decrease in the delay time.
7.4 Interpretation summary
This chapter summarizes the most likely causes for the observed anisotropy along the
TORTISS array. The summary is based on the discussions in the previous chapters and is
presented for each of the regions of the array. Each region will be discussed in terms of
deep (i.e., main, upper mantle) contribution and shallow (i.e., small, crustal) contribution
to anisotropy.
Spitting measurements throughout the array display a back-azimuth dependency.
Therefore the events have been separated in two groups, the northern and the southern,
within which splitting results are roughly consistent. The difference in splitting results for
the two event groups is most likely caused by differences in deep anisotropy, somewhere
between the CMB and the crust underneath the array, because a shallow source would
most likely be sampled by both event groups.
67
7.4.1 Coast Range
The major contribution to the anisotropy in the first region (A01-A14), the Coast
Range, is most likely in the upper mantle. This estimate is based on the thickness of the
required anisotropic layer (130 km) and the "Fresnell depth" of the Coast Range,
considering the Coast Range as a region with uniform anisotropy. The mineral orienting
strain is most likely caused by the APM of the JdF plate (i.e., the motion of the subducting
slab).
Because the changes in the observed splitting are on the order of measurement
uncertainties, it is not possible to say if the crust has a significant contribution to the
observed anisotropy. Nevertheless, because the thickness of the crust in the Coast Range
is about 40 km (Trehu et al., 1994), a crustal contribution to delay times of up to about 0.3
sec is possible.
7.4.2 Willamette Valley and Cascades
The major contribution to the anisotropy in the second region (A15-A36), the
Willamette Valley and the Cascades, is most likely in the upper mantle as well, as
suggested by the "Fresnell depths". The assumed thickness of the anisotropic body is on
average about 180 km. The fast polarization direction appears to be related to the motion of
the subducting slab. The crustal contribution for this region, if any, is not easily
distinguished. Differences in splitting results between stations are very small, and might
be just measurement uncertainties. Because the results are practically indifferent to the
applied frequency bands, "Fresnell depths" are not constrained.
The change in the average 4) from the first region to the second region might be due
to a change in the dip of the slab. However, the delay time is increasing instead of
decreasing as expected from the model simulating the change in anisotropy caused by a dip
change in the slab. A more likely possibility is that the change in the average 4) for the two
regions is due to crustal anisotropy or a combination of both mechanisms. The boundary
between region 1 and 2 does not appear to be clearly correlated with principal crustal
structures of the Coast Range and the Willamette Valley (Trehu et al., 1994). However,
receiver function data suggest that the boundary roughly corresponds to the junction
between the JdF plate's Moho and the North American plate's Moho (Nábëlek et al.,
1993).
7.4.3 East of the Cascades
The main contribution to the observed anisotropy in the third region of the array
(A37-A5 15) comes most likely from the mantle as well. The strain responsible for the
orientation of the minerals might be caused by the APM of the North American plate
(N240°E; Gripp and Gordon, 1990). The fast polarization direction calculated for long
periods (5-30s) is within 100 to the direction of the plate motion. Short period results show
a more E-W tendency, indicating a possible relation of 4) to maximum horizontal stress in
the crust.
The crustal contribution in this region seems to be significantly higher than in the
other two regions. The biggest changes in splitting results from station to station are in this
part of the array. They are most likely caused by short scale crustal variations.
70
8.0 CONCLUSIONS
The results of this thesis show that anisotropy varies across a subduction complex.
The average observed splitting parameters, the fast polarization direction (4)) 700 and the
time delay (&) 1.61 sec, are well above average. Worldwide the largest observed splitting
measurements are of the order of 2 sec time delay. According to gross anisotropic
characteristics, the array can be divided in three regions. The first region extends over the
Coast Range and has average splitting values of [72°, 1.34 sec], the second region extends
over the Willamette Valley and the Cascades with an average splitting of [66°, 1.74 sec]
and the third region is located east of the Cascades with an average splitting of [79°, 1.77
sec]. The splitting results within each of the first two regions show little systematic
variations. In the third region, a systematic rotation of 4) from about 70° to 87° from west
to east is observed. The observed shear-wave splitting is assumed to be caused by strain
induced lattice preferred orientation of mantle minerals, mainly olivine.
The general trend of the
4)
and & estimates is most likely due to the APM of the JdF
plate (i.e., mantle flow induced by the motion of the descending slab) and the North
American plate at the eastern end of the array. Variations in the 4)-6t estimates across the
TORTISS array can be explained by variations in crustal anisotropy; however, only east of
the Cascades (third region) can the contribution of crustal anisotropy be resolved. A
change in dip of the descending slab might contribute to these variations as well. Backazimuth dependent variations can be best explained by deep anisotropy along the raypaths
of the northern and southern event groups, anywhere between the CMB and the crust
underneath the array.
Future work should concentrate on the determination of crustal anisotropy from the P
to S phase conversion at the North American and Juan de Fuca Mohos. This would
71
provide better constraints on the nature of the observed splitting changes across the array.
TORTISS array data can be used for this task. Furthermore, better data between stations
A43 and AS! 5 would be desirable and the investigation of the anisotropy east of station
AS 15 would be interesting.
72
i:iu :i p
(iItit1
:a,i
Atwater, T , Implications of plate tectonics for the Cenozoic tectonic evolution of western
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Leven, J. N., L. Jackson, and A. E. Ringwood, Upper mantle seismic anisotropy and
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74
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75
APPENDICES
76
APPENDIX A. 1
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
PHILIPPINE ISL. REG. 93/05/18 10:19:39 Mw=6.5 dist=92.34° baz=300.79° (1-30s)
Table A. 1.1 Splitting measurements for the Philippine Isl. event.
STA refers to the station, parameter (1) is the fast polarization direction (clockwise with
respect to north), öt is the delay time in seconds, and a4) and a are the la uncertainties.
SFA
AOl
A02
A03
A04
A05
A08
A09
AlO
All
Al2
Al3
A15
A17
Al8
A20
A21
A23
A24
A25
A26
A27
A28
A31
A32
A33
MS
A37
A38
MO
A41
A42
a
4,
73.00
67.00
73.00
75.00
72.00
75.00
80.00
77.00
74.00
72.00
74.00
67.00
74.00
70.00
59.00
68.00
70.00
69.00
69.00
68.00
67.00
68.00
70.00
68.00
69.00
68.00
74.00
75.00
76.00
79.00
83.00
+1-
7.00
+1+1-
8.00
7.00
9.00
8.00
7.00
4.00
8.00
9.00
16.00
7.00
5.00
7.00
5.00
5.00
7.00
6.00
5.00
4.00
6.00
4.00
5.00
6.00
6.00
4.00
4.00
5.00
5.00
5.00
7.00
9.00
+141+1+1+1+1+1+1+1+1+1+1+1+1-
+1+1+1+1+1-
+1+1+1+1+1#1-i-I-
+1+1-
1.30
1.40
1.10
1.20
1.35
1.30
1.45
1.65
1.65
2.10
1.65
1.65
1.65
1.70
1.90
1.35
1.60
1.70
1.65
1.70
1.80
1.85
1.90
1.90
2.00
2.30
1.95
+1-
0.20
0.35
0.15
0.25
.iI-
025
+1-
0.20
0.15
0.25
0.35
0.90
0.25
0.25
0.25
0.25
0.35
0.25
0.25
0.20
0.20
0.30
0.20
0.25
0.30
0.30
0.20
+/+1+1-
+1+1+141-
1.
+1+1+1-
+1+1-i-I-
+1+1+1+1+1-
41+1+1+141-
2.05
1.90
-4-1-
1.85
1.75
-+/-
--1-
+1-
025
0.25
0.25
0.20
0.30
0.35
77
Figure A. 1.1 Anisotropy vector map for the Philippine Isi. event. The vector orientation
corresponds to in (°) clockwise from north, and the size corresponds to the delay time
in seconds.
Figure A. 1.2 Contour plot of energy on the corrected transverse component for all (, St)
pairs for the Philippine Isl. event. The absolute minimum is indicated by a star and the
95% confidence level is shown within the bold contour line.
PHILIPPINE ISL. REG. 93/05/18 10:19:35 Mw=6.5 dist=92.34° baz=300.79° (is -30s)
3600
45 N
3300
3000
2700
2100
1800
1500
44 N
1200
900
600
300
0
124W
Figure A.1.1
123W
122W
121W
i
H
-
:
!FLfJ22J:.
!PcH. '&Pi
t
JL:2J7
1
-I
I
I
-
-
-.
--I
--S
H
i_'
I
-
-
___
--I
--a
H
I
I
-
-
-
-
S
--S
H
-.
I
I
-
E'-
--I
S
--I
.1
A37
A35
A33 930518
0
0
U)
a)
a)
a)
E
E
E
I>'
>.
>
a)
0
a)
a)
a)
a)
a)
a)
a)
a)
0
0
0
0
Fast Angle ( deg)
Fast Angle (deg)
Fast Angle (deg)
A40
A38
0
0
a)
a)
a)
a)
a)
a)
a)
E
E
E
>.
>..
>'
a)
a)
a)
a)
a)
a)
a)
0
oI
Fast Angle (deg)
A42
0
a)
a)
a)
E
Iii
a)
a)
0
Fast Angle (deg)
Figure A.12 Continued
Fast Angle (deg)
I
Fast Angle (deg)
APPENDIX A.2
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
TAiWAN 93/08/07 00:00:37 Mw=6.4 dist=85° baz=303° (ls-30s)
Table A.2. 1 Splitting measurements for the Taiwan event.
STA refers to the station, parameter (1) is the fast polarization direction (clockwise with
and G6t are the icY uncertainties.
respect to north), & is the delay time in seconds, and
STA
AOl
A02
A03
A05
A06
A08
A09
AlO
All
Al2
A13
A14
A15
A16
A17
A18
A19
A20
A22
A23
A24
A25
A26
A27
A28
A29
A30
Mi
A32
A33
A34
MS
A37
A38
A39
A40
A41
A42
A43
&
4)
67.00
62.00
63.00
76.00
72.00
63.00
68.00
76.00
76.00
73.00
73.00
79.00
67.00
72.00
70.00
76.00
65.00
61.00
65.00
69.00
67.00
66.00
67.00
66.00
69.00
72.00
73.00
77.00
68.00
74.00
72.00
74.00
78.00
75.00
83.00
81.00
82.00
87.00
81.00
+1-
+/+1-
+1-
4/4/+1+1-
+/-
4/*1+1+1+1-
4/+/4/4/4/+/4/+1+1+1+1+1-
4/+1-
4/4/4/.
4/+1-
+/+1+1+1+1+1-
20.00
90.00
41.00
90.00
23.00
18.00
90.00
27.00
26.00
23.00
23.00
6.00
16.00
36.00
10.00
42.00
11.00
10.00
10.00
33.00
10.00
12.00
28.00
7.00
12.00
30.00
19.00
90.00
24.00
90.00
13.00
33.00
24.00
20.00
25.00
13.00
24.00
14.00
18.00
1.10
0.95
1.00
0.65
4/-
1.05
1.15
4/-
0.95
1.20
1.20
+1-
1.35
1.40
1.60
1.70
1.70
1.80
1.35
1.40
2.00
1.45
1.40
1.60
1.65
1.70
1.90
1.90
1.75
1.80
1.60
1.90
1.75
1.70
1.85
1.70
1.50
1.65
1.70
1.90
1.85
1.60
-i-I-
4/-
141
+1-
4/4/4/4/+1+1-
+1+1-
4/iI+1-
4/+1-
4/4/4/+1-
4/+1-
4/-
0.50
2.05
1.00
2.35
0.45
0.55
2.05
0.65
0.60
0.55
0.60
0.30
0.65
1.30
0.40
1.50
0.45
0.75
0.40
1.10
0.45
0.60
120
0.35
0.65
1.20
0.85
4/-
1.55
1.05
1.70
*1-
0.45
+1-
4/.
1.15
4/-
0.90
0.55
0.90
+1-
4/4/+1-
4/+1-
030
0.95
0.60
0.60
Figure A.2. 1 Anisotropy vector map for the Taiwan event. The vector orientation
corresponds to in (°) clockwise from north, and the size corresponds to the delay time
in seconds.
Figure A.2.2 Contour plot of energy on the corrected transverse component for all (tv, öt)
pairs for the Taiwan event. The absolute minimum is indicated by a star and the 95%
confidence level is shown within the bold contour line.
NORTHEAST OF TAIWAN 93/08/07 00:00:37 Mw=6.4 dist=85.38° baz=303° (30s highpass)
3600
45 N
3300
3000
2700
2400
2100
1803
1500
44N
1200
900
603
303
0
124W
Figure A.2.1
123W
122W
12VW
I
0,
16
0,
E
A03
A02
3O8O7
"0
a,
I>'
a,
U,
a)
a)
E
E
!k1
!
I-
>
>S
(U
(0
(0
a)
a)
02
I:L
.90 .75 -60 -45
-
.15
0
15 30 45 60 75 00
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A07
A06
A05
3.'
2.1
6.)
a)
a)
U)
(6
a)
a)
a)
E u
E
E
(U
(U
a)
a)
20
I:
>'
a)
LV
>
11
O
.0
9
.90.7590.45.30.150 153049607590
.95.75.00.45.30.150153045607595
.45.30.150 153045007500
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
AlO
A09
I
0)
a)
2.0
A
U,
(I)
07
a)
a)
E
E is
E
I-
>'
(U
(U
a)
a,
0.5
IL
0.0
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A13
Al2
All
C)
a,
(6
a)
a)
E t
a)
a)
>'
>'
20
U,
E
E
)
>'
;1
(0
IC
a)
0)
Fast Angle (deg
Figure A.2.2
0)
Fast Angle (deg
Fast Angle (deg
A15
A14 930807
03
0
a,
a,
a,
C-,
a
0
a,
a
a
E
a,
E
F
E
F
>.
I>.
a
CD
a,
a,
a,
0
0
Fast Angle (deg)
Fast Angle (deg)
A19
S
5.1
25
0
a,
23
a
a,
13
E
*
F
a>
ID
a,
0
051
0_a
-90 -75 .40 .45 .30 .15
.
A20
0
15
30
45
00 75
01
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A22
23
1.2
a,
E
F
C-,
C.)
a
0,
U)
a,
2.0
a,
a,
13
E
E
>
I>S
F
>'
a
a
1.0
a,
0
0,
o
0
0.5
D.0
.95 .75 .40 -45 -30 -15
0
15
30 45
00
75
50
.90 -75 -40 -45 .30 -15
Fast Angle (deg)
0
15
30
45
90 75
90
Fast Angle (deg)
Fast Angle (deg)
3.0 .
A26
A25
A24
3.0
I
231
23
0
a,
20
2.0
a
a,
EisU
F
.
E
F
Oil
a
0
0
-90.75-00-45-30-150 153045007590
Fast Angle (deg)
Figure A.2.2 Continued
75
Fast Angle (deg)
-
as
30
Fast Angle (deg)
A29
A28
A27 930807
C)
C)
2$I
I
24
a
I
a
In
EuI
Ei.s
>'lI
E
>
a
a
.I.0L
I
V
I
-
Fast Angle (deg)
-75
-
A30
.45
-
-is
0
Is
30
45
40 75
ID
Fast Angle (dog)
Fast Angle (deg
A32
A31
2$
Li
2O
a
1.0
'.0
CI)
*
a
a
Els
IL\J
E
L5
a
LO
'-
>.
>-,
a
0$
a
a
'5
SC
-90 -75 -40 -45 -30 -15
0
15
30
45
40
75
a
E 3
45
.5
'5
IL
90 .75 .40 -45 -30 -15
Fast Angle (deg)
AM
A33
0
15
30 45 ao 75 90
Fast Angle (deg)
Fast Angle (deg)
A35
- I
a
CO
a
E
I.-
>'
CO
0)
O
v,.
-90 .75 .40 .45 .30 .15
0
15
30
45
00 75
10 -75 -00 .45 -30 -IS
91
Fast Angle (deg)
0
15
30
45 40 75 90
Fast Angle (deg)
A37
A39
a
CO
ID
E
>.
(0
a
p
Fast Angle (deg)
Figure A.2.2 Continued
Fast Angle (deg)
FiiiiiI1
Fast Angle (deg
[SDJ
- -
- - - Fast Angle (deg
a
a
E
P
a
a
Fast Angle (deg
Figure A.2.2 Continued
- - - -' ---- - - - Fast Angle (deg
- - - -' _I-_
Fast Angle (deg
- -
APPENDIX A.3
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
SOUTH OF FIJI ISLANDS 93/08/07 17:53:27 560 6.9 DIST: 85.68° BAZ: 230.25°
Table A.3. 1 Splitting measurements for the 93 Fiji Is!. event.
STA refers to the station, parameter is the fast polarization direction (clockwise with
respect to north), & is the delay time in seconds, and a4) and a& are the la uncertainties.
(1)
STA
AOl
A02
A05
A06
A08
A09
AlO
All
Al2
A13
A15
A16
A17
A18
A19
A20
A22
A23
A24
A25
A29
A31
A32
A33
A34
A35
A37
A38
A40
A41
A42
A43
64.00
57.00
60.00
59.00
64.00
60.00
64.00
61.00
60.00
62.00
57.00
57.00
75.00
59.00
53.00
43.00
54.00
51.00
48.00
40.00
50.00
56.00
56.00
55.00
59.00
72.00
56.00
62.00
64.00
68.00
59.00
64.00
+1+1+1+1+1+1+1+1+1+1+1+1+1-i-I-i-I-i-I-
+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1-
a4
&
9.00
21.00
4.00
5.00
5.00
52.00
12.00
9.00
14.00
5.00
5.00
6.00
45.00
5.00
4.00
90.00
90.00
90.00
90.00
90.00
89.00
90.00
3.00
11.00
69.00
14.00
4.00
85.00
73.00
19.00
10.00
10.00
2.40
3.00
2.85
2.60
2.30
2.45
1.95
2.05
1.70
1.95
3.00
3.00
0.75
2.45
3.00
3.00
3.00
3.00
3.00
0.40
3.00
2.70
3.00
2.30
0.90
1.35
3.00
1.15
0.65
1.05
3.00
2.10
+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1-i-I-i-I-
+1+1+1+1+1+1+1+1+1+1+1+1-
1.20
2.50
0.60
0.95
0.65
2.00
1.05
1.00
1.30
0.80
1.50
1.90
1.10
0.75
1.30
2.95
2.90
2.95
2.95
2.60
2.85
2.15
1.55
1.80
2.10
0.70
0.85
1.85
2.35
0.80
2.20
1.00
APPENDIX A.4
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
HINDU KUSH 93/08/09 12:42:50 mb=6.3 dist=98° baz=349° (ls-30s)
Table A.4. 1 Splitting measurements for the Hindu Kush event.
STA refers to the station, parameter is the fast polarization direction (clockwise with
respect to north), & is the delay time in seconds, and a and a& are the la uncertainties.
(I)
&
STA
Al2
A13
A14
All
A18
A19
A20
A22
A23
A24
A25
A26
A27
A28
A29
MO
A3l
A32
A33
A34
A35
A37
A41
A42
A43
72.00
73.00
74.00
70.00
70.00
68.00
70.00
65.00
64.00
65.00
66.00
67.00
66.00
63.00
63.00
61.00
72.00
73.00
69.00
67.00
72.00
73.00
84.00
87.00
88.00
+1-4]-
+1-4]-
W-41-4]-
+1-
+/-+14]-
+1-4]-
+1+1+1-i-I-i-I-
+1i-I-
6.00
12.00
65.00
69.00
5.00
3.00
4.00
2.00
3.00
2.00
2.00
4.00
3.00
20.00
45.00
53.00
32.00
2.00
2.00
5.00
9.00
2.00
1.90
1.95
1.90
1.05
1.75
+/-
2.40
2.05
2.00
-i]4]-
1.10
1.40
1.70
1.20
+1-
0.80
0.60
0.65
0.45
0.45
0.30
0.35
0.45
0.50
0.80
+1-
1.05
+1-
1.00
-4]-
-1-1-
2.05
2.15
-4]-
1.95
1.80
1.25
1.05
+1-
0.75
1.90
3.00
2.70
2.10
-4-1-
-4]-
+1+1+1+1-
130
0.95
0.45
0.65
1.85
-i-I-
1.15
-i-I-
0.90
.4]-
-4]-
+1-
90.00
11.00
-i-I-
6.00
3.00
i]-
-4]-
1.85
2.80
3.00
3.00
-if-
+1-
+1-
1.90
1.90
1.25
91
Figure A.4. 1 Anisotropy vector map for the Hindu Kush event. The vector orientation
corresponds to
in seconds.
in (°) clockwise from north, and the size corresponds to the delay time
Figure A.4.2 Contour plot of energy on the corrected transverse component for all (4, t)
pairs for the Hindu Kush event. The absolute minimum is indicated by a star and the 95%
confidence level is shown within the bold contour line.
HINDU KUSH 93/08/09 12:42:50 nth=6.3 dist=98° baz=349° (is - 30s)
3600
450 N
3300
3000
2700
2400
2100
1800
1500
44°N
1200
900
600
300
0
124°W
Figure A.4. 1
123°W
122°W
121°W
93
A03
A02
AOl 930809
C)
0)
In
C)
a)
C)
E
E
a)
'3
I-
>
>
10
a)
a)
a
a)
a
1.)
a)
(I)
a)
E
I-
>.
a)
a)
.
Fast Angle (deg)
A07
C)
a)
a)
0)
E
I
a)
a)
O
5
-
-75
-SI)
-40 -30
IS
0
IS
30
45
60
Fast Angle (deg)
AlO
2.5
C.)
2.0
a)
E '.s
I-
a)
1,0
a
0.5
0.0
Fast Angle (deg)
Figure A.4.2
Fast Angle (deg)
AlS
A14
0
a,
U
a)
a,
a,
a,
a,
E
E
F
I-
>'
a,
a,
a,
0
0
L1)]
Fast Angle (deg)
A18
0
C)
a,
a,
a,
a)
a,
E
E
a)
I-
>'
a,
a,
a,
a)
0
0
-
')
i'j
I .0
i
I____________________
0
a,
a)
E
I-
-
a,
a,
0
0
L,
a)
a)
a,
E
a,
a,
a
I
0a,
a,
a,
E
F
11
a,
a,
0
-90 -75 -60 -45 -90 -15
0
15
90
45 60
Fast Angle (deg)
Figure A4.2 Continued
75
90
A26 930809
U
a)
0,
a)
E
F-
a)
0)
Fast Angle (deg)
A29
3.0
2.5
i
U
2.0
a)
E
1.5
I-
-
a)
10
a
05
0.0
Fast Angle (deg)
-
A32
(3
a)
0)
a)
E
FC,
a)
a
Fast Angle (deg)
A35
C)
a)
Co
a)
E
F-
a)
a)
a
---:
.75 -60 -45 -30 -15
0
15
30
45
60
Fast Angle (deg)
Figure A.4.2 Continued
75
90
Fast Angle (deg)
iI\.
V
W
A40
439 930809
U
C)
a
a
a
a
C.,
a
E
E
F>'
>.
0
0
a
a
a
a
.90 -75 .00 -45 -30 -15
0
15
30
45
90
75
90
Fast Angle ( deg)
Fast Angle (deg)
A43
A42
C)
C.)
a
a
a 2
U)
a
E
a
E
I
F
>
>
a
a
a.
0
0
Ui
oo
.75 -00 -45 .30 .15
Fast Angle (deg)
Figure A.4.2 Continued
0
15
30 45 00 75 90
Fast Angle (deg)
97
APPENDIX A.5
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
NEW IRELAND REGION 93/09/06 03:55:58 Mw=6.6 dist=88.57° baz=262° (ls-30S)
Table A.5. 1 Splitting measurements for the New Ireland event.
STA refers to the station, parameter 4) is the fast polarization direction (clockwise with
and & are the la uncertainties.
respect to north), & is the delay time in seconds, and
&
STA
AOl
A02
A07
A08
A09
AlO
All
Al2
A13
A14
A15
A16
A17
A18
A19
A20
A22
A23
A24
A25
A26
A27
A28
A29
A30
A33
AM
A35
A36
A38
A40
A41
A42
72.00
73.00
69.00
58.00
67.00
71.00
62.00
65.00
66.00
70.00
67.00
70.00
67.00
70.00
67.00
61.00
59.00
55.00
59.00
55.00
53.00
54.00
58.00
64.00
70.00
58.00
49.00
61.00
67.00
71.00
71.00
75.00
74.00
+1-
1+1-W-
+1+1+1+1+1+1+1+1-i-I-
+1+1-
11-1-1-
+1+1+1+1+1-i-I-
-i-I-
+1+1+1+1+1-i-I-
-i-I-
+1-
+1+1-
1.55
1.75
0.85
0.85
+1-
2.15
1.10
1.15
1.75
1.10
1.55
1.50
1.55
1.65
1.55
+1+1+1-
1.55
+1+1+1+1-
0.85
2.10
+1+1-
15.00
90.00
2.15
90.00
55.00
67.00
42.00
58.00
17.00
27.00
90.00
65.00
90.00
8.00
23.00
10.00
8.00
18.00
28.00
9.00
12.00
18.00
7.00
10.00
33.00
77.00
63.00
37.00
58.00
73.00
21.00
90.00
90.00
90.00
1.25
1.80
2.00
2.00
1.50
1.30
1.85
1.80
1.70
2.00
2.10
1.85
1.65
1.25
1.20
1.45
1.35
2.10
1.20
2.10
2.00
1-
-4-!-
+1+1+1+1+1+1+1+1-
1+1+1+1+1-
1+1+1+1+1+1-
130
1.35
1.05
1.50
1.40
1.50
0.90
1.25
1.00
0.70
0.65
0.80
0.50
0.55
0.65
0.35
0.70
1.15
1.55
1.75
0.95
1.45
1.65
1.40
1.80
2.05
1.95
Figure A.5.1 Anisotropy vector map for the New Ireland event. The vector orientation
corresponds to 4 in (°) clockwise from north, and the size corresponds to the delay time
in seconds.
Figure A.5.2 Contour plot of energy on the corrected transverse component for all (, 6t)
pairsfor the New Ireland event. The absolute minimum is indicated by a star and the 95%
confidence level is shown within the bold contour line.
NEW IRELAND REGION 93/09/06 03:55:58 Nw=6.6 dist=88.57° baz=262.1° (30s highpass)
3600
45 N
3300
3000
2700
2400
2100
1800
1500
44N
1200
900
600
300
0
124W
Figure A.5.1
123W
122W
121W
100
AOl 930906
2$
2$
__
1k
0
E 1.01
I
1
o ft
00
2.0
III
i
A03
A02
3.0
3.0
Ii
45
ID
113
E
o
0
WIl
>.-
11(11
1J!III
Ca
A'fflflIl
a)
0.0
0.0
.00.75.00-45.30.150153043007500
.00.75.00-40-30-100103045007500
Fast Angle (dog)
Fast Angle (deg)
Fast Angle (deg)
A07
A06
A05
0
0.)
a,
63
CO
00
(I)
U)
0)
00
E
E
Cs
00
CO
Ca
00
0
a)
E
I
>
0
a)
o
ii
Fast Angle (dog)
Fast Angle (deg)
Fast Angle (dog)
AlO
A09
A08
a.)
0)
5)
CO
C')
00
03
E
E
0$
0
a)
U)
a)
E
I-
I>'
>-.
00
CO
CO
02
0
00
0
0
.90 -IS -Go .45 .30 -IS
0
15
30
45
00
75
a)
90
.90 .75 .00 .45 .30 .15
0
IS
30 45
00
75
90
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
All
Al2
3.0
A13
3r
3.)
2$
0
C-)
00 2J
002.)
2.0
Cl)
U)
0)
ID
Eu
Eu
Ei.s
U2$
>
>S
CO
o
0.5
05
-
0)
0
0.)
02
ox
-90 -15 .00 -45 .30 -15
0
IS
30 45
Fast Angle (deg)
Figure A.5.2
60 75
90
Fast Angle (deg)
Fast Angle (dog)
101
U6
&15
A14 930906
1
0
0)
(0
0)
E
I
(0
I!à
0)
a
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A19
A18
A17
3.0
25
UI2.6
C-,
C)
2.0
25
5,
a,
0)
>,
(0
1.0
-
E
E
E
'.5
15
a
0)
a
05
0.5
0.0,
0.0
.00 75 -60 .45 .30 -15
0
15
30
45 60 75
90
Fast Angle (deg)
A20
3.0
2.5
C)
C,
0)
2.0
(0
II)
0)
E
E
1.6
I-
F-
i1IIlk4:'
I.
i
tfJ1J I
>
(0
1.0
0)
a
0.5
0.0
Fast Angle (deg)
A24
3.0
l
j
2.5
2.0
E
1.6
1.0
-
a
0)
0.5
0.0
Jr
.90 -75 .60 -45 -30 -(5
0
IS 30 45 60 75 90
Fast Angle (deg)
Figure A.5.2 Continued
102
A29
27 930906
3-c
U
C)
C)
a)
63
U)
6)
63
63
23
03
E
E
F:
>,
F-
(5
(5
a)
a)
a
E
>.
>-.
1-c
0-C
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A32
A31
A30
3.0
2.5
2.0
C)
U
(0
(0
a)
a)
a)
E
E
El.5
F:
>'
>,
0.5
00
Jib
-60 .75 -60 .45 -30 -15
0
IS
30
45
60
75
CO
(0
a
a(0
90
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A35
A34
A33
3.0
2.5
C-)
2.0
0)
E 1.5
I-
03
-90.75.00.45.30-IS 0 (53045607590
III'Ii
.9045-60.45-30.150 (53045607590
.90.754045.30.150 153045607590
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A39
A38
A36
3.0
3.0
2.5
2,5
2.0
2.0
Ei.5
E'.s
3.0
2.5
Il
2.0
/
I
It
Ei.s
I
I
F:
,_____//
1.0
-
/
a
i
\__/
0.5
00
53
a)
a
0.5
0.5
0.0
-90.75 .60.45 -30 .55
0
IS
30 45 60 75 90
Fast Angle (deg)
Figure A.52 Continued
1.0
.
a
.90.75-60-45 -30-IS 0
IS
30 45 60 75
Fast Angle (deg)
60
00
.90-7540.40 .30.15 0
15
30 45 66
Fast Angle (deg)
75
90
103
A40 930906
A41
A42
104
APPENDIX A.6
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
W. CAROLINE ISL. 93/09/2603:31:19 Mw=6.4 dist=88.93° baz=283.4° (ls-30s)
Table A.6. 1 Splitting measurements for the W. Caroline Isi. event.
STA refers to the station, parameter is the fast polarization direction (clockwise with
respect to north), & is the delay time in seconds, and Y() and a& are the icT uncertainties.
(1)
&
STA
AOl
A08
A09
Al2
A15
A16
A17
A18
A19
A20
A22
A23
A24
A25
A26
A27
A28
A29
A30
A31
A32
A33
A34
A35
A36
A37
A38
A39
A40
A41
A42
A43
A44
78.00
80.00
84.00
75.00
59.00
57.00
59.00
45.00
69.00
42.00
43.00
69.00
68.00
53.00
48.00
56.00
56.00
53.00
54.00
57.00
64.00
73.00
60.00
67.00
67.00
81.00
71.00
41.00
58.00
73.00
77.00
81.00
69.00
+1+1+1-
4/-i/-
4/-+1-
+1+1+1-
+1+1+1+1+1-
4/4/+1.-
+1+1+1-
+1-
4/4/+1+1+1+1-
#/4/+1-
4/+1-
54.00
36.00
26.00
61.00
21.00
23.00
11.00
13.00
18.00
15.00
40.00
42.00
18.00
14.00
13.00
9.00
8.00
14.00
15.00
20.00
24.00
26.00
8.00
45.00
8.00
6.00
34.00
35.00
14.00
8.00
26.00
41.00
31.00
1.55
*1-
0.95
+1+1-
1.35
0.60
1.55
1.45
1.45
1.45
1.05
1.65
1.00
0.85
1.15
1.25
1.50
1.85
1.90
1.55
1.60
1.50
1.45
1.65
1.60
1.05
2.15
2.35
0.90
1.45
1.45
1.75
1.30
1.55
1.30
i/
4/-
+1-
4/+1-
+1-
+/+1-
4/+/+1+1-
4/+1-
4/+1-
4/+1+14]-
+1-
4/4]-
+1+1-4/-
+1-
4/+1+1-
0.95
1.50
1.10
2.40
0.70
0.80
0.40
0.60
0.45
1.05
1.00
0.75
0.40
0.35
0.60
0.40
0.35
0.40
0.45
0.65
0.65
0.90
0.30
1.80
0.45
0.65
0.85
1.55
0.40
0.40
0.90
1.30
0.90
105
Figure A.6. 1 Anisotropy vector map for the W. Caroline Isi. event. The vector
orientation corresponds to 4 in (°) clockwise from north, and the size corresponds to the
delay time in seconds.
Figure A.6.2 Contour plot of energy on the corrected transverse component for all (, öt)
pairs for the W. Caroline Is!. event. The absolute minimum is indicated by a star and the
95% confidence level is shown within the bold contour line.
W. CAROLINE ISL. 93/09/26 03:31:19 Mw=6.4 dist=88.9° baz=283.4° (is -30s)
3600
45 N
3300
3000
2700
2400
2100
1800
1500
44 N
1200
900
600
300
0
124W
Figure A.6.1
123W
122W
121W
107
A03
A02
t101 930926
C-)
a)
Co
a)
E
P
>,
Co
a)
0
1
:r
-00 .75 .00 .45 .30 .15
0
15
30 45 80 15 50
-
0 .79 .80 .45 .30 .15
A04
0
15
30 45 60
75
9
Fast Angle ( deg)
Fast Angle (deg)
Fast Angle (deg)
A06
05
3.0
IT
25t
0
0
a)
2C
CO
a)
a)
P
I-
Eu
E
CO
a)
15
a)
0
0
01
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A09
A08
A07
3.0
3.0
3.0
2$
2$
2$
2.0
2.0
E1.5
E'.s
E1.5
P
P
P
>
>
0
0
o
00
00
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.75 .90 .45 .30 .15
0
15
30 45
60 75
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90
0
15
30
45
66
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0
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E 1.51
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o.41J
0011
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I
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001
,
.75 -60 -45 .30 .15
0
55
30
45
Fast Angle (deg)
Figure A.6.2
5)5
75
50
IS
30 45 60 75
90
Al2
01
IlI
III
0
3.0
3.0_
2$
>
-75 .60 .45 .30 .15
Fast Angle (deg)
All
AlO
3.0_
.
0.3
-90
90
Fast Angle (deg)
Fast Angle (deg)
E l.5F
U,.,.
0.5
0.5
0.5
P
1.0
.
1.0
.
5.0
.95 .75 .50 .45 .30 .55
0
15
30 45 60 75
Fast Angle (deg)
95)
2$
0
E is
P
>
-
1.0
0
os
00
.90 .75 .5)0 .45 .30 .15
0
IS
30 45
Fast Angle (deg)
60
75
90
A15
A14
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20
II)
E is
I
>
-
-90 -75 .60 .45 -30 -15
2
0
15
1.0
30 45 60 75 9
1I.III.II.i
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
U6
A18
A17
C)
0)
U)
03
E
>
(9
03
-30.75.4(45.30-150
153040007530
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
U9
A22
A20
3.0
25
C)
0)
(15
0)
C)
ri
215
02
E is
E
I-
I-
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(0
0)
c:1
'-I
0.5
0.0
A25
A24
A23
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
3.0
IT1fl
2.5
C-)
215
05
E is
10
Os
015
Fast Angle ( deg)
Figure A.6.2 Continued
Fast Angle ( deg)
Fast Angle (deg)
i09
p.D
32
2
(3
0)
0)20
00
03
03
E
50
05
i1'J
.60
o j530
.90
e
pge (de9
p29
31
p.3O
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3.0
(2
0322
Si)
is5
5$
0)
eg
59
0
0$
.50
deg
1
faSt
(P
03
I-
0
L_--5.00
p34
33
3.0
39
3.0
25
2$
2.0
'p
0)
(deg')
fast
deQ')
1 V1
('29
Si)
29
0$
is5
02
0
0
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0$
2Va
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00
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36
3S
3.0
3.0
2$
22
0)
03
r
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is
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i)
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deg
p.6.2 Cot
110
A40
A39
A38 930926
33
'.5
25
25
(3
a,
65
23
05 20
SC
a,
E
E ii
05
E
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(a
1-3
a,
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F
F
F
ISO
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a,
CD
c
'.5
'3
03
03
P.O
.90 -70 60 -45 -30 -15
0
15 30 45
90
75
90
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A43
A42
A41
33.-.-
2311
C.,
V
a,
In
a,
F
F
115
E
F
-
F
F
>.
Ca
o4
>
'U
a)
0j9L
-00 .75 .60 .45 -30 .15
0
15 30 45 60 75 90
Fast Angle (deg)
A44
as
Cl,
a)
E
I>.
CO
a)
Fast Angle (deg
Figure A.6.2 Continued
-90 .75 .00 -45 -30 -15
0
IC
-
CD
15
30 45
Fast Angle (deg
60
70
91
!..II.Ii
ast Angle (deg)
111
APPENDIX A.7
SPLITTING RESULTS AND ENERGY CONTOUR PLOTS FOR:
XINJIANG, CHINA 93/10/02 08:42:35 M=6.3° dist=92.72° baz=335.32
Table A.7. 1 Splitting measurements for the Xinjiang event.
STA refers to the station, parameter ( is the fast polarization direction (clockwise with
and a& are the 1c uncertainties.
respect to north), öt is the delay time in seconds, and
The baz of this event coincides with the direction of slow polarization leading to a null
measurement.
STA
AOl
A02
A03
A05
A06
A07
A08
A09
AlO
All
Al2
A13
A15
A16
A17
A18
A19
A20
A22
A23
A24
A25
A26
A27
A28
A29
A30
A32
A33
AM
A35
A36
A37
A38
A39
A40
A41
A42
A43
A44
&
4)
-34.00
-19.00
-19.00
-9.00
56.00
68.00
-33.00
-41.00
-39.00
81.00
78.00
81.00
37.00
-18.00
-19.00
83.00
57.00
-16.00
66.00
-34.00
-29.00
75.00
73.00
-36.00
87.00
89.00
-28.00
73.00
82.00
55.00
-85.00
-34.00
-14.00
-73.00
80.00
82.00
83.00
-78.00
80.00
-85.00
+1-
+141#1-
4141+1-
+1+1+141+141+1-i-I-
+1-
1+1-
41+1-
+1+1+1+1+1+1-
-il-
-/+141-
+1+1-
1+1+141+1+1+1+1-
-ii-
2.50
1.30
+1-
1.95
+1-
2.20
2.20
2.95
90.00
4.00
9.00
90.00
90.00
90.00
90.00
61.00
75.00
59.00
72.00
90.00
90.00
90.00
90.00
90.00
90.00
73.00
90.00
77.00
0.50
3.00
2.75
0.80
0.80
3.00
+1-
2.00
90.00
66.00
11.00
66.00
40.00
90.00
80.00
39.00
89.00
21.00
90.00
90.00
33.00
22.00
7.00
66.00
11.00
20.00
18.00
41*1-
1.05
+1-
0.85
+1-
1.05
1.15
1.05
+1-
+1-
1.95
1.30
1.95
1.55
1.75
0.80
3.00
2.45
+1-
2.20
+1+1-
2.40
+1-
1.40
+1-
0.60
2.40
+1-
1.30
+1-
3.00
+1-
1.10
+1-
3.00
0.50
1.50
+1-
1.25
0.60
0.85
3.00
1.65
1.20
2.15
0.90
0.95
2.05
0.90
3.00
2.35
1.05
1.40
1.70
1.40
-i-I-
+1-i-I-
+1+1+1-
i/+1-
+1+1+1-
41-
1.60
1.60
2.40
1.35
1.70
2.95
1.90
0.95
2.50
1.50
0.80
1.85
0.90
2.95
1.40
1.15
0.90
0.45
2.05
+1-
1.05
+1-
0.85
2.05
0.65
41+1+1+1+1+1-
1.95
0.40
1.30
0.80
112
Figure A.7. 1 Contour plot of energy on the corrected transverse component for all (, öt)
pairs for the Xinjiang event. The absolute minimum is indicated by a star and the 95%
confidence level is shown within the bold contour line.The baz of this event coincides
with the direction of slow polarization leading to a null measurement.
113
rn
::i
A03
A02
AOl 931002
11.1
0)
0
(3
5)
20
6)
03
90
0)
5)
5)
E
E
E 15
I-
I
>
>,
I
>,
CO
C0
0)
0
1.0
5)
0
0
0
::
.90 .75 .60 .45 .30 -15
0
15
30 45
60
75
-90.75-60-45-30-150153045907590
90
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A06
A05
A04
3_a
I
2.5
(3
C)
0)
C0
C)
0)
03
E
E is
2.0
2.0
0)
E 1.5
I
I
CO
0)
0
Os
0.5
0.0
.90 .75 .00 .45 -30 -15
0
15
30 45
60
75
90
-90 -75 -60 .45 -30 .15
15
30
45
A09
A08
A07
0
Fast Angle (deg)
Fast Angle ( deg)
Fast Angle (deg)
::riru
0
C)
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0
0
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C')
a)
0)
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E
E i-1
I
I
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(0
CO
03
a,
1.7
0)
0
0
0
ill
I
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
Al2
All
AlO
3.0
2.5
2.0
C)
U
113
0)
U)
61
0
03
E
E
>
I>,
I
(5
CO
0)
0
0
0
0
05
0.3
90
75
90 .45 .30 .15
0
15
30
45
Fast Angle (deg)
Figure A.7.1
60
75
90
Fast Angle (deg)
Fast Angle (deg)
60
75
90
114
A14
A13 931002
a.)
a.)
a.)
a)
a)
U)
U)
U)
a,
U)
09
F
F
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U)
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a,
a,
.90-75.60.45.30.150159045907590
.90-75.60-45-30-150159045907590
Fast Angle (deg)
Fast Angle (deg)
U
Fast Angle (deg)
A18
A17
A16
3.0
3.0
2.5
2.0
2.0
0.5
0.0
90 75 .60 .45
-
U
.15
0
IS
36 45 60 75
90
U)
a,
Eis
E
F:
1.0
ii
6.0
00
.90 .75 .40.45 .30 .15
6
15
90 45
90
75
00
:r'.
.90 .75 .60 .45 .30 .15
0
15
90
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A20
A19
a.,
a)
U)
a,
E
F:
>
U)
.90-75.4045.90-IS 0
Fast Angle (deg)
....
A23
A22
!
15 30454075 95
ii\
.90 .75 -40 -45 .30 .15
0
15 30 45 40 75 90
Figure A.7.1 Continued
-90 .35 .40 .45
90
15 3045 6075 90
Fast Angle (deg)
A24
1 'V
.V \ m!k
Fast Angle (deg)
.90-75 .60 .45 .30.15 0
Fast Angle (deg)
.15
0
15 30
45
Fast Angle (deg)
50 75 90
Fast Angle (deg)
115
3.0
2.5
V
:
-
b
(5
(3
(3
0)
as
U)
U)
((1
0)
as
0)
E
E
0)
>'
(a
(U
as
as
0
as
as
>'
A36
A35
A34
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
2.5
(3
2.0
0)
E
is
I>'
1.0
0)
Os
I!
.90 .75 .90 .45 .30 .)5
Fast Angle (deg)
Figure A.7. 1 Continued
Fast Angle (deg)
0
15
30 4
Fast Angle (deg)
116
A37 931002
A39
38
23
.5
(3
03
U)
23
63
03
E
E 13
a
"p
(0
F
F
F
.0
a
E
>..
66
13
0)
63
iii.
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A42
A41
A40
33
3.0
2D
6)
0,
4
a
E
Ets
E1.5IQ
F
F
F
>.
a
25U.
03
.1O
6)
I
0
0.0
.00 .70 .00 -40 -30 -10
0
15
30
A44
A43
3.0
3X
23
2
0
2X
20
a
a,
Eu
E 13
F
I-
>.
(0
0.5
03
Fast Angle (deg)
Figure Al. 1 Continued
45 60 75 90
Fast Angle (deg)
Fast Angle (deg)
0
.
.9.
ast Angle (deg
0
03
___
OJ
-90
.15 .60 .45 .30 -10
0
15
30 45 60 70
Fast Angle (deg)
90
117
APPENDIX A.8
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
EASTERN NEW GUINEA REGION 93/10/13 02:06 mb=6.5 dist=95° baz=266° (is30s)
Table A.8. 1 Splitting measurements for the Eastern New Guinea event.
STA refers to the station, parameter 4 is the fast polarization direction (clockwise with
and & are the la uncertainties.
respect to north), öt is the delay time in seconds, and
&
STA
AOl
A06
A07
A08
A09
Al2
A13
A14
A15
A16
A17
A18
A19
A20
A21
A22
A23
A24
A25
A26
A27
A28
A29
A32
A34
A35
A37
A38
A40
A41
A42
A43
79.00
51.00
59.00
59.00
70.00
74.00
77.00
76.00
75.00
74.00
70.00
55.00
56.00
50.00
88.00
59.00
60.00
62.00
59.00
56.00
59.00
57.00
51.00
62.00
66.00
63.00
65.00
78.00
77.00
55.00
54.00
62.00
3.00
0.70
+/-
49.00
0.90
+1-
1.75
1.15
1.10
31.00
53.00
13.00
7.00
8.00
6.00
24.00
12.00
24.00
12.00
7.00
90.00
23.00
13.00
7.00
7.00
10.00
0.85
+1-
0.60
1.05
1.65
+1+1-
1.20
1.00
2.10
2.05
2.05
1.40
1.60
+1-
0.90
0.95
0.85
1.15
1.30
1.55
+1+1-
+1-
6.00
+1+1-
44.00
+1+1+1+1+1+1+1-W-
+1+1+1+1-
*1-
*/+1-
+1W+1+1-W-
+1+1-
*/+1-
JW-
*1*141-
8.00
14.00
13.00
52.00
10.00
26.00
8.00
11.00
9.00
29.00
46.00
49.00
+1-
1111-
3.00
+1+1-
1.35
1.70
+1+1-
2.10
1-
1.95
1.75
1.65
+1+1+1+1+1+1+1+1+1-
3.00
1.60
1.10
1.70
1.70
1.75
2.10
1.80
0.95
0.70
0.85
+/+1-
4/+1-
1.20
0.80
0.50
0.40
0.30
0.60
0.80
0.65
0.55
0.45
0.50
0.45
0.60
0.40
1.35
0.60
0.90
0.55
1.20
0.95
0.60
0.95
0.95
118
Figure A.8. I Anisotropy vector map for the Eastern New Guinea event. The vector
orientation corresponds to 4 in (°) clockwise from north, and the size corresponds to the
delay time in seconds.
Figure A.8.2 Contour plot of energy on the corrected transverse component for all (4, öt)
pairs for the Eastern New Guinea event. The absolute minimum is indicated by a star and
the 95% confidence level is shown within the bold contour line.
EASTERN NEW GUINEA REGION 93/10/13 02:06:00 mb=6.5 dist=95° baz=266° (is - 30s)
3600
45 N
3300
3000
2700
2400
2100
1800
1500
44N
1200
900
600
300
0
124W
Figure A.8.1
123W
122W
12VW
120
F03
A02
AOl 931013
3.0
:r
2.5
(.3
(-3
C.)
C)
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2.0
C)
a)
9)
E
E
E 1.5
I-
I-
I-
>
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(C
10
-
1.0
a
a)
a
a
0.5
I
0.0
.15 .00 .45
.lfl
.15
0
IS
30 45 60 75
9
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A06
A05
A04
C.)
C.)
C.)
9)
a)
C)
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Cl)
(I)
9)
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E
E
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0)
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69
0)
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a
a
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Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A09
A08
A07
I
0
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(3
C)
50
9)
(13
II)
E
2.)
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E
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69
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C)
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a)
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0,)
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
Al2
All
10
3.0
2.5
0
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C)
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a)
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Cl)
(13
E
C)
a)
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l\' 'W''i
E
E 1.5
I
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CC
(C
C)
C)
a
a
a
I.30 '75 '60 -45 -30 .15
Fast Angle (deg)
Figure A.8.2
0
15
30 45
Fast Angle ( deg)
60 75
90
-90 .75 -60 -4) -30 -15
0
15
30
45
Fast Angle (deg)
60 75
90
121
A15
A14
30
2.5
0
0
a)
(0
2.0
03
0)
E
E is
F-
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0)
Ce
10
a
0.5
O
0.5
0.0
Fast Angle (deg)
Fast Angle (deg)
A18
A17
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2.5
0
2.0
0
0
0)
in
05
03
0)
a)
0)
E
E
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1.0
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a
0)
.75 .60 .45 .30 .15
IS
30
45
60 75
90
45
60
75
90
60
75
90
A21
A20
iii
!
0
Fast Angle (deg
Fast Angle (deg)
3.0
2.5
0
a)
2.0
In
a)
E to
E
I-
>
5,
a
1.0
a
0)
a
05
00
-90 .75 .60 .45 .30 -15
0
15
30
Fast Angle ( deg)
Fast Angle (deg)
A24
A23
3.0
2.5
0
6)
0
2.0
U)
0)
0)
E is
E
I-
I5,
>
(a
1.0
0)
a)
a
a
0.5
_1.r
4._
00
.90 .75 .60 .45 .30 .15
Fast Angle (deg)
Figure A.8.2 Continued
0
15
30
45
Fast Angle (deg)
122
A27
A26
A25 931013
3.)
2.0
0
0
0.)
5)
U)
0)
2.0
0)
C)
03
E
E
E 1.1
I-
>
-
>
>'
a
(0
I-c
C)
C)
C)
CD
CD
CD
0.2
0.0
.95 .75 .60 .45 .35 .15
60
75
90
30 45 60
75
90
30
45
33
30
23
23
0
0
0.)
a)
U,
2.0
2.0
a)
0)
0)
E 1.5
F-
E
E 1.5
I-
I>'
>..
>-'
-
15
A30
A29
A28
0
Fast Angle (deg)
Fast Angie (deg)
Fast Angie (deg)
a
1.0
1.0
C)
C)
C)
CD
CD
CD
03
0.5
0.0
00
.90 .75 .60 .45 .30 .15
7
15
A34
A32
3.0
3.0
2.5
2.5
2.0
2.0
2.0
E is
E is
E is
11
0
Fast Angle (deg)
Fast Angie (deg)
0
C)
I>'
1.0
0)
C)
CD
CD
CD
0.5
0.5
0.5
0,0
0.0
0,0
5045 -60 .45
.
5
.15
70
45
-90-75-60.45.30-15 0 15)0456075 90
60 75 U 7
Fast Angle (deg)
Fast Angle (deg
Fast Angle (deg)
A37
A36
A35
3.0
2.5
0
0
0.)
a)
0)
0)
6)
2.0
0)
0)
C)
E 1.5
I>,
E
E
F:
>'
I3-.
a
a
1.0
CD
02
C)
CD
CD
0.5
0.0
-90 .75 -60 -45 -90 .15
Fast Angle (deg)
Figure A.8.2 Continued
0
15
30
45
Fast Angle (deg)
60 75 90
Fast Angie (deg)
123
A40
C)
a)
9)
3)
E
I-
>
(3
0)
a
I:L1
0.75-00.4540-150 153045607590
Fast Angle (deg)
Fast Angle (deg)
A43
442
0.)
(0
a)
E
I-
>'
(a
a
II)
Fast Angle ( deg
r U
2.5
0)
E 1.5
F-
>
'C
3)
a
.90 .75 -60 -45 -30 -(5
0
IS
30
45
60
Fast Angle (deg)
Figure A.8.2 Continued
75
90
Fast Angle (deg)
124
APPENDIX A.9
SPLITTING RESULTS, ANISOTROPY VECTOR MAP AND ENERGY CONTOUR
PLOTS FOR:
FIJI ISLANDS REG. 94/03/01 22:40:53 591 6.5 baz=230.9° dist=83.8°
Table A.9. 1 Splitting measurements for the 94 Fiji event.
STA refers to the station, parameter is the fast polarization direction (clockwise with
are the 1c uncertainties.
respect to north), öt is the delay time in seconds, and cvq and
Results suggest that baz of this event is very close to the direction of fast polarization at
A375, but is gradually changing towards the end of the array.
STA
A375
A385
A395
A405
A425
A435
A465
A475
A485
A505
A515
63.00
56.00
63.00
51.00
68.00
62.00
77.00
74.00
67.00
72.00
71.00
18.00
90.00
90.00
90.00
8.00
1- 90.00
+1- 26.00
+1- 12.00
i-I- 18.00
+1- 10.00
+15.00
+1+1+1+1+1-
3.00
3.00
1.45
3.00
2.15
2.25
1.05
2.30
1.70
1.55
1.65
+1+1+1+1+1+1+1-
1+1+1-
1-
2.30
2.95
1.55
2.95
0.85
1.45
0.65
0.95
1.30
0.55
0.35
125
Figure A.9. 1 Contour plot of energy on the corrected transverse component for all (4, öt)
pairs for the 94 Fiji event. The absolute minimum is indicated by a star and the 95%
confidence level is shown within the bold contour line.Results suggest that baz of this
event is very close to the direction of fast polarization at A375 (elongated energy contours
parallel to the time axis), but is gradually changing towards the end of the array.
126
A395
A385
A375 940331
30
3.0
3.0
2.5
2.5
2.5
2.0
2.0
2.0
11.5
E13
E 1.0
______
>.
to
.
.0
1.0
0)
0.5
0.5
::
0.0
0.0
-90-75-60-45-30-150153045607500
-50-75-6045-30.150153045607590
90.7560-45-30-150 153045607530
Fast Angle (deg)
Fast Angle ( deg)
Fast Angle (deg)
A425
A415
A405
3.0
2)
0
0
6)
0
2)
a,
6)
Cl)
0)
03
03
E
E '
E
>
>,
I-
>'
(0
(0
IC
07
03
0)
0.)
0.0
Fast Angle (deg)
Fast Angle (deg)
Fast Angle (deg)
A475
A465
A435
3.0
3.0
2.5
2.5
2.5
U
(3
C-)
2.0
2.0
2.0
0)
03
0)
E is
E is
E '.5
I-
I-
I-
>
1.0
-
6)
1.0
0.5
0.5
0.5
L1I
0.0
VETh.'-
,-
75,
.1I.
I. i----
I
TU - % I
Et
I/I
:l.
i1iR I - ;
____
-:.