S13A-1045
Source Rupture Process of the Solomon Islands Earthquake of April 1, 2007 Inferred from Teleseismic Body Waves
C. Berk Biryol (cbbiryol@email.arizona.edu) and Susan L. Beck (slbeck@email.arizona.edu)
GEOSCIENCES
Department of Geosciences, University of Arizona Gould-Simpson Building, 1040 E. Fourth St., Tucson, AZ 85721-0077
UASCIENCE
3. SOURCE RUPTURE PROCESS
160
180
0
DG
D
-8
2
4
3
5
6
200
220
-20
7
20
0.00
E
ST
OB
AL
TR
MAJO
JNU
P
YOJ
TAOE
TLY
P
P
30
SNZO
RER
PTCN
RER
P
P
RPN
PAF
QSPA
RPN
P
PAF
yr
0c
m/
~1
(2) the least-squares, continuous (point-by-point) waveform inversion method of Hartzell
and Heaton (1983; 1985).
SNAA
We used broadband teleseismic body-wave records from a distance range of 30°-100°
recorded by GSN and other global Networks with data archived at the IRIS DMC. For the
final inversions, we used 15 P-wave and 4 SH-wave records from 19 stations with a
reasonable azimuthal distribution (Figure 2).
P
SNZO
SNAA
P
QSPA
SH
P
FOCAL MECHANISM: GCMT
STATION DISTRIBUTION
Using both techniques we obtained sub-event and slip distribution along fault models with
fixed mechanisms, varying mechanisms and varying slip directions. We compared the
results of analyses with each other in order to see how much we were able to resolve about
the source parameters and rupture process of this event.
Figure 2. Station distribution and recorded waveforms.
Variation in Focal Mechanism
Variation in Depth of Hypocenter
0.8
The results obtained from body-wave inversion are used to model the coulomb stress
changes in the region. We used the COULOMB 3.1 software package for these calculations
(Toda et al., 2005; Lin et al., 2004). We addopted the same physical fault parameters that we
used for the waveform inversion. The resulting stress distribution is analyzed to interpret the
interaction of this fault segment with the nearby fault segments and defined asperities that
were sites of major energy release during past earthquakes.
0.6
Residual Waveform Misfit
(Error)
0.6
0.5
0.4
0.3
0.2
0.1
140
120
S trike= 297
100
fr o
nt
60
re
p tu
Ru
40
Distance (km)
80
30
40
50
GCMT
This
Study
0.0
60
70
80
90
F
2
1
20
0.05
0
20
40
60
80
Time (sec.)
0.1
PTCN
P
R PN
P
PAF
P
21
Mo = 1.6x10 Nm
Mw = 8.1
0.05
S NZO
SH
S NA A
P
0
QS P A
P
20
40
60
Time (sec.)
Color Coded Source Time Functions
Centroid (GCMT)
Main shock
Biggest
aftershock
(Mb=6.6)
6.0 < Mb < 6.5
5.0 < Mb < 6.0
4.5 < Mb < 5.0
Mb < 4.5
Axis of ridges
Rupture
Initiation
Location of
GCMT
Depth (m)
2
-2000
Bathymetry Profile
-4000
30
-6000
20
00
0
2
0
-3 0 0 0
-3
r
g Ce n te
n
i
ead
Woo dlar k S
0
-2 0 0
50
100
Aftershocks in segment 1
-1000
-1000
-2 0 0
0
-3 0 0 0
10
1
idge
Simbo R
0
3
-2
00
Aftershocks in segment 2
0
150
200
Distance (km)
250
300
350
400
Background seismicity of the
past 7 months before the
main shock
The complex character of this event is revealed by the seperate locations of the centroid and
rupture initiation (Figure 1). The results of our analysis also showed two major subevents that
are separated in time by 15-20 seconds (Figure 4E). The first major subevent occurred at the
southern tip of the fault and was followed by a relatively larger subevent to the NW. This
indicates that the rupture advanced to the northwest of the epicenter in a unilateral sense. All
methods and models that we used were able to resolve this feature of this event (Figure 4A, 4B
and 4C). However the fault model with varying rakes did a significantly better job than the
other two models in fitting the observed data (Figure 4D and 4E).
The large discrepancy between seismic moments calculated in this study and that of GCMT
can be attributed to the longer period energy release that cannot be resolved using the limited
frequency bandwidth of the body-waves.
Aftershocks in segment 3
0
-3 0
00
The slip and subevent distributions clearly show that the rupture propagated through the
-12
interface across 3 plates. The seismic character along each of these individual interfaces is
154
152
156
158
160
different (Figure 5). This might be due to the different thermal, tectonic and dynamic characters
Figure 5. Aftershock distribution of the event along the subduction zone together with the of these plates. Therefore we might expect the presence of heterogenities along the subducted
subducting seafloor bathymetry.
portions of these plates.
-1 0
00
155
4. CONCLUSIONS
Our results indicate that the April 1, 2007, earthquake ruptured a 240-km-long sector of the New
Britain - San Cristobal subduction zone with two major subevents located where the edge of the
Australian Plate subducts and where the Woodlark Microplate subducts.
-6
156
157
158
1975A
(M w=7.6)
NB
OJP
1975B
(M w=7.3)
02.01.1974
(M w=7.4)
-7
3
2
Based on the timing and position of the subevents, the rupture started at the interface between
the Australian Plate and overriding Solomon Island Arc and then propagated northwest to
rupture a large asperity located on the Woodlark plate. These two subevents dominated the first
65 seconds of the event within the 120-140 km length of the rupture plane.
SS
1
2
1
SIA
0.4
0.3
0.2
NEIC
This
Study
10
11
0.1
7
8
9
Double-Couple Focal Mechanisms
12
13
14
15
16
17
18
19
20
Depth of Hypocenter (km)
pulse stripping method
point-by-point waveform inversion method
Figure 3. Results of sensitivity analysis for varying focal mechanisms and rupture initiation
depths using both of the techiques mentioned above.
The rupture area of this event partially overlaps the portions of the fault plane that ruptured in
the 1974 and 1975 doublets (Figure 6). Coulomb stress analysis indicates that some patches of
increased stress overlap with the positions of the asperities that ruptured in these past events.
This might be an indication that these asperities are loaded and might be the locations of major
energy release in a future event along this portion of the subduction zone.
0
-1
Being an area with a high rate of plate convergence, such spatially frequent fault plane
heterogenities might be the reason for the occurrence of earthquake doublets in this region.
Thus, this event, with its two energetic subevents, might be a good example to show how
failure of an asperity triggers the failure in the adjacent one given that the spacing of them is
sufficiently small.
0.5
Bars
4
1974a
(M w=7.3)
-8
0.7
Residual Waveform Misfit
(Error)
In order to observe trade-offs among many a priori source parameters, we run several
sensitivity analyses on the data (Figure 3) with some constraints implied by the geometry of
the subduction zone where this event took place. Based on the goodness of the fit between
calculated waveforms and observed body-wave records, the test results favored a fault 240
km x 80 km in area and focal parameters of 305, 35, 80 (strike, dip, rake).
0.7
20
0.1
YS S
SH
TAOE
PTCN
SH
P
Mo= 0.444x10 Nm
Mw= 7.70
AFI
AFI
CHTO
90
POHA
CHTO
P
21
AFI
SH
-4
pr
INCN
POHA
YOJ
COR
YSS
10
Figure 4. A summary of the results of the analyses, together with the correlation of observed and calculated waveforms.
is
e
P
Calculation Depth = 14km
Time (sec.)
T IXI
P
Number of Aftershocks
along profile
SH
TLY
55
T A OE
P
GCMT
QS P A
P
00
COR
60
1
0
0.10
RER
P
S NZO
SH
-2 0
INCN
0.08
65
Y OJ
P
GCMT
Solution
R PN
P
S NA A
P
(1) the pulse stripping method of Kikuchi and Kanamori (1982; 1986; 1992)
220
70
INC N
SH
PTCN
P
-10
TIXI
200
C HT O
P
-2 0 0 0
P
0.06
AFI
SH
Woo dlar k R
TIXI
0.04
TLY
P
PAF
P
2. METHOD & DATA
P
180
75
P OHA
P
T A OE
P
RER
P
-1 0 0
SH
160
80
C OR
P
C HT O
P
We used two different methods in our analysis of source processes of the event. Our aim
here was to analyze different aspects of the event and observe degree of agreement
between reults of each method. These two methods are;
140
P OHA
P
TLY
P
-12
Figure 1. Tectonic map of the study area with focal mechanisms of major doublets and
the April 1, 2007, earthquake. The dashed box outlines the region of aftershocks and
ruptured portion of the megathrust . AU=Australian Plate, WL=Woodlark Microplate,
SIA=Solomon Island Arc, OJP=Ontong Java Plateau, SS=Solomon Sea Microplate,
NB=North Bismarc Microplate, SB=South Bismarc Micoplate
120
J NU
P
Synthetics
Y OJ
P
EN
CH
100
Strike, Dip, Rake
305, 25, 80
Observed data
1
YSS
80
MA J O
P
INC N
SH
-8
JNU
60
85
Moment ( x10 )
5.0<mb<6.0
6.0<mb<6.5
6.5<mb
T IXI
P
3
P
40
90
21
YS S
SH
MA J O
P
-10
2
0 03
0.02
0
20 sec.
1
20 s ec .
-6
MAJO
6
0 .0 9
0. 06
Nm
C OR
P
CR
I
0
Aftershocks
mb<4.5
4.5<mb<5.0
Mo = 0.635x10 Nm
Mw = 7.80
J NU
P
SAN
09
0. 0
21
Strike, Dip, Rake
305, 25, 73
0.
-40
20 s ec .
E
AU
1
-20
Slip Vector
(3.5m)
Slip
SIA
RI
Projected
Centroid (GCMT)
CORRELATION (%)
0
140
0. 03
C
2
-20
120
Maximum Slip
NW
Moment Rate
21
(x10 Nm)
GE
D
I
R
O
B
M
SI
00
100
0
m
0
-1 0 0 00
0
-2
-3 0
80
Hypocenter
ANALYSIS FOR VARYING MECHANISMS
Moment Rate
21
(x10 Nm)
Ri
se
la
rk
kling
to n R
ise
60
2
B
Ridge
imbo
of S
Poc
00
-2 0
0
0
0
-3
40
5
43
20
Subevents
(scaled)
Woodlark Microplate
03
re
Woo dlark Sp
n
a di
e
C
g
20
04/01/2007
(Mw=8.1)
GI
ZO
r
e
t
n
0
3
GCMT
USGS (NEIC)
02/01/1974
(Mw=7.4)
o
-20
3
3
1
-40
N O RTH SOLOMO
N TR
ENC
H
04/01/2007
(mb=6.6)
4
4
Australian Plate
0.
Analysis of coulomb stress changes showed increased stresses on the
neighboring fault segments and the trenchward face of the outer rise.
W
gh
WL
All three models indicated that a major part of the seismic moment is released in
the form of two pulses separated by 15-20 seconds. The second pulse is the
largest one and it is located northwest of the hypocenter, implying a northwestward
directed unilateral rupture.
The results also reveal a complex rupture pattern for this earhquake. We interpret
these complexities as due to fault plane heterogenities. These heterogenities are
also thought to be responsible for the occurence of earthquake doublets in this
region.
ro
u
6
5
5
SE
Solomon
Sea
Microplate
tion
jec
Pro
-200 0
-1 0 0
0
Teleseismic body wave inversion techniques are used in order to investigate the
source properties.
CH
4
-6
07/20/1975B
(Mw=7.3)
4
-20
NW
of Woodlark Rise
ection
Proj
00
A
0
OJP
07/20/1975A
(Mw=7.6)
5
3
AUSTRALIA
01/31/1974
(Mw=7.3)
bir
an
dT
20
Woodlark Microplate
o Ridge
Simb
-3 0
(3) varying focal mechanisms along the faulted region.
Study Area
Australian Plate
of
ion
ject
Pro
SS
SE
FAULT MODEL WITH FIXED THRUST MECHANISM
Solomon
Sea
Microplate
Strike
Dip
BRITA
IN
NEW
TR
EN
FAULT MODEL WITH VARYING RAKES
-4
NB
Tro
(2) varying slip directions over the area of rupture, and
160
07/14/1971
(Mw=8.1)
SB
We investigated the source process of the April 1, 2007, earthquake using three
different faulting models. These are:
(1) fixed thrust mechanism over the entire extend of the fault,
158
07/26/1971
(Mw=8.0)
This region, where 4 plates intersect, is tectonically complex. Along the Solomon
Island convergent margin, the relatively small Woodlark and Solomon plates enter
into the subduction zone side-by-side with the much larger Australia plate (Fig. 1).
On a regional scale, this subduction zone plate boundary is characterized by the
occurrence of large earthquake doublets (a sequence of 2 large events) in 1971,
1974, 1975 and 2000 (Mw > 7.0)
156
of Woodlark Rise
ection
j
o
r
P
154
od
A large earthquake (Mw 8.1) followed by a tsunami took place in the southeast
Pacific along the New Britain subduction zone on April 1, 2007.
152
3
1. INTRODUCTION & SUMMARY
0. 0
THE UNIVERSITY
OF ARIZONA
-2
-3
-9
-4
AU
WL
Biggest Aftershock
mb<4.5
Centroid (GCMT)
4.5<mb<5.5
Epicenter (NEIC)
5.5<mb<6.0
Asperities
(this study)
Asperities from
1974-1975 events
(Xu and Schwartz, 1993)
Figure 6. Coulomb Stress change in the study area and the
locations of past and present asperities.