JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93,
NO. Bll,
PAGES 13,367-13,389,
NOVEMBER10,
1988
LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATEDWITH SUBDUCTION OF THE
LOUISVILLE
RIDGE
Douglas H. Christensen
GeophysicalInstitute, University of Alaska, Fairbanks
Thorne Lay
Department of GeologicalSciences,University of Michigan, Ann Arbor
Abstract. Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in
the southern Tonga region (22øS-26øS). Five earthquakes with diverse mechanisms and seismic mo-
to in-plate compressionfollowing the great 1977 rupture which relaxed the tensional
ments greater than i x 10•'?dyn cm have occurredin
this region since 1975. These include two outer rise,
one underthrusting, and two intermediate depth
events, all of which occurredin closeproximity to the
subducting ridge. In order to understand better the
Introduction
natureoftheseeventsandto determinetheroleplay-
ed by the Louisville Ridge in the subduction process,
detailed rupture models have been' determined for
each earthquake by body and surface wave analysis.
Three events occurred at shallow depths near the
oblique intersection of the Louisville Ridge and the
Tonga arc. These are the compressional(thrust) outer rise event of October 11, 1975 (Mw-7.4); the tensional (normal) outer rise event of October 10, 1977
(Mw- 7.4); and the subsequentunderthrusting event
on December 19, 1982 (Mw - 7.5), the latter being the
largestdocumented
interplateeventin Tonga. The
intermediate
dept.
h eventsof June2'2,1977(d= 70
kin, Mw=8.1) andApril 13, 1980(d=160 km, Mw=
7.6) both have normal fault mechanisms;however;
the 1977 event has a downdip tenskin axiS, whereas
the 1980 event has a downdip compressionaxis. The
i977 eventis unusualdu•eto its large sizeand the
fact that most intermediate depth •activity in Tonga
involves down•dipcompression.The bodywave radiatiori for the 1977 event indicates concentrated ri•pture in the depth range 70-•90km, near the downdip
projectionof theLouisvilleRid•ge.It appearsthat,the
LouisvilleRidgeperturbsthe regionalstressregime
at both intermediate and shallow depths in the slab.
The ridge appearsto be a buoyant feature that pro-
duceslocallystronginterplate couplingtothe north
ofitsjunctionwith theTongatrench,whichaccounts
for. the 1982 thrust event and the preceding 1975
compressionalouter rise event locatedseawardof the
thrust rupture zone. South of the ridge intersectlob
the plate interface appears to be less strongly coupled: tensional stressesdue to slab pull are communicated to shallow d•pths, resulting in the October
10, 1977 (Mw-7.4), tensionalouterrise event. The
l•cally enhancedinterplate coupling,alongwith a
downdipstressbarrier to the intraplate compressional stressesthat arise from deepresistanceto the slab
subduction, are two effectsof the ridge that appear to
explain the accumulationof in-plate tension in the
region of the June 22, 1977, event. The April 13,
1980, downdip compressionalevent reflects a return
Copyright 1988 by the American GeophysicalUnion.
Paper number 88JB03052.
0148-0227/88/88JB-03052505.00
stress. The unusual
stressregime of the subductingridge and its oblique
trajectory down the plate are thus responsiblefor the
diversity of large earthquake occurrence in the
southern Tonga arc.
The Tonga subductionzone is separatedfrom the
Kermadec subductionzone at about 21øSby the intersectionwith the Louisville Ridge. The Louisville
Ridge is a linear aseisrnicridge complexwhich is several thousandkilometersin lengthandabout100km
in width, with up to 4 km of relief. The ridgeintersectsthe Tonga trench at an Obliqueangle as shown
in Figure 1, altering the trench bathymetry in the
process.The dip of the subductingslab at intermedi-
ate depthschangesfrom70øalongthe Kermadecarc
to 55ø along the TOngaarc. It has beensuggested
that the •subductionof ttie Lou•isvilleRidge influencesbothinterplateandintraplateseismicity
in the
vicinity of the intersecti0n•
however,it is not known
exactly what role the ridge plays [Sykes, 1966; Vogt
et al., 1976; Kelleher and McCann, 1976]. The ridge
may either be a zone of weaknessin the subducting
platethat wouldinhibit lar•gestrainaccumulations
or a buoyant structure which chokesup the subduction process.Subductionof aseismicridges and other
bathymetricstructuressucha•sfracturezoneshas received much attention sincethe seismicpotential for
large thrust eventsin suchregionsis ofteriunkt•own
[e.g., Kelleher et al., 1974; Chuhg and Kanamori,
1978; LeFevre and McNally, 1985]. We will address
the role of the Louisville Ridgein the subductionproCessby examining the seismic characteristics of the
region.
Seismicity patterns in the Tonga arc have been
studied by many authors [e.g., Isacks et al., 1969;
JohnsonandMolnar,1972;Richter,1979;Billington,
1980; Kawakatsu, 1986; Giardini and Woodhouse,
1984; Wyss and Habermann, 1984]. The Tonga region can be characterizedas having large numbers of
small and intermediate
size events in both the shal-
low underthrusting zoneand at all depthswithin the
subducting plate. The intraplate events downdip of
the interplatecontacthavea strongtendencyfor the
compressionalstressaxes to be in the downdip direction, particularly for depths greater than 300 km
[Isacks et al., 1969; Isacks and Molnar, 1971; Fujita
and Kanamori, 1981; Vassiliou, 1984; Vassiliou et
al., 1984; Richter, 1979], leading to the hypothesis
that the subductedplate encounters strong resistance near the 670-km discontinuity and that the
continued sinking of the slab has placed the slab into
compressionall the way to shallow depths. However,
several downdiptensionaleventsin the depth range
13,367
Christensenand Lay: Large Earthquakesin the TongaRegion
13,368
underthrusting event, and two intermediate depth
events (onedowndiptensionaland the other downdip
compressional).Our objectiveis to quantify the nature of these large events, which we infer to be the
most tectonically significant in the region, and to explore their relationshipsto subductionof the Louisville Ridge.
20 ø
6/22/77
-//"-
on/y
"•
Recent Large Events
4/13/80.
2/
In this section we examine
in detail the source
parametersand rupture processes
of the five large
earthquakeswhich occurredin the vicinity of the
r;ii!O/10/77
Louisville Ridge. We will start with the two outer
rise events on October 11, 1975, and October 10,
30 ø
..v.-//..-'
,, •
1977, then consider the December 19, 1982, underthrusting event, and end with the two intermediate
depth events on April 13, 1980, and June 22, 1977.
We will give specialattention to the June 22, 1977
(Mw-8.1), rupture since it is one of the largest
known intraplate events. In this section we will
limit ourselves to discussions of the individual
earth-
quakes. The interaction and relationship between
these events and the role of the Louisville Ridge will
be discussed later.
ZE•-•,•N
DI
4Oø
180 ø
W
I
17O
ø
October 11, 1975, Earthquake
The October 11, 1975, event (Mw-7.4)
is a com-
Fig. 1. Map of the Tonga-Kermadecregion showing
epicentersof five large events associatedwith the
subduction of the Louisville Ridge. The hachured
region representsthe aftershockarea of the December 19, 1982, underthrusting event. The dashedlines
representthe 4000-m bathymetricexpressionof the
pressionalouter rise or trench event (i.e., compressional stressaxis is oriented approximately horizontal and perpendicularto the trench). It occurrednear
the bathymetric trench, north of and adjacentto the
intersection of the Louisville Ridge with the Tonga
Louisville Ridge.
$=0ø; dip • =29ø; rake • =68 ø)was determinedfrom
P wave first motionsby Christensenand Ruff [1988]
and is shown in Figures 2 and 3. The steepwesterly
dipping plane, which is well constrainedby first
motion data, and the location near the bathymetric
trench distinguish this event as an intraplate event
60-300 km have recently been studiedby Kawakatsu
[1986], who suggeststhat these occurrencesare consistent with a double Benioff zone in the Tonga regionproducedby unbendingof the subductedslab.
The seismic history and interplate thrust potential along the Tonga arc are relatively obscure.Several large (M>7) earthquakeshave occurredin the
Tonga region historically [McCann et al., 1979];however, it is not known whether these events are interplate or ir•traplate. Earthquakesnear the northernmost corner of the Tonga arc are usually intraplate
events: Recent large events in this region have all
trench.
The focal mechanism for this event (strike
in the Pacific plate.
The depthof this eventwasdeterminedby Christensen and Ruff [1988] to lie between 0 and 35 km
from detailed analysis of the trade-off betweendepth
and characteristic changesin the sourcetime functions from single station deconvolutions(see Christensen and Ruff [1985] for a description of the tech-
nique). We have further investigatedthe depth of
this event using the simultaneousmultistation itera-
either occurred in the outer rise or can be related to
tive deconvolution scheme of Kikuchi and Kanamori
the tearing of the slab. The occurrenceof large
known underthrusting events is relatively rare in
the Tonga arc. While this region is capableof producingearthquakesas large as magnitude8.0, it is
pose,P waves from eight azimuthally distributed
stations(shownin Figure 3) were simultaneouslydeconvolvedat a seriesof point sourcedepths. Ampli-
especially
in theunderthrusting.zone,
thatthereis a
order that the results be most sensitive to depth (i.e.,
arc [Kanamori, 1977].
tudes. The error reduction (normalized approximation error) versusdepth curve for this event is shown
clear from the small numbers of these large events,
significantcomponentof aseism•csubductionin the
Since 1975, five large earthquakes with moment
magnitudesgreaterthan 7.0 (Mo->1.0x 10•'7dyncm)
have occurredin the southernTonga region. Each of
these events is located in the vicinity of the Louisville Ridge, and eachrepresentsstressaccumulation
in a different environment of the subducting slab.
[1982] and Kikuchi and Fukao [1985]. For this pur-
tude variations
between stations were normalized in
waveshape)rather than randomscatterof the ampli-
by the crosses
in Figure 4. Depthsbetween20 and 35
km producethe best fit to the data, with the lowest
residual error found for a depth of 30 km.
The P waves from the eight stations used in the
depth determination are shownin Figure 3 along
The locations and focal mechanisms of these events
with the deconvolved source time functions
are shown in Figure 2 along with the Louisville
mined by the singlestationdeconvolution
methodof
Ruff and Kanamori [1983] assuminga sourcedepth
of 30 km. The rupture historyof this event is fairly
simple,with onemajorpulseof momentreleasehaving a total duration of about 18 s and an averagemo-
Ridgeand its approximatedownwardextensioninto
the subduction zone. These events have a rema-rkable
diversity of mechanisms,and include two outer rise
events (one compressionaland one tensional), one
deter-
ChristensenandLay: LargeEarthquakesin the TongaRegion
i
i
15,569
I
6/22/77
4/15/80
d=70
km
I0/I
22ø..,.
1/75
d: 160 km
d = 50
km
24 %
12/19/82
10/10/77
B'
d=25
km
26 %
C'
178"
W
176"
I
'
d=10 km 172"
I
I
Fig.2. Blowupofthesouthern
TongaregionfromFigure1. Thehachured
zonerepresents
the
approximate
location
oftheLouisville
Ridge(L.R.)asit intersects
theTongatrenchandissubductedbelowthe Australia-Indiaplate. Lowerhemisphere
equal-areafocalmechanisms
and
depthsareshownforthe fivelargesteventsrecorded
in thisregion.Theseincludetwoouter
rise (or trench)events(October11, 1975,and October10, 1977),oneunderthrustingevent (De-
cember19, 1982),andtwointermediatedepthevents(June22, 1977,andApril 13, 1980). The
linesegments
labeledAA',BB',andCC'arelocations
ofcross
sections
shownin Figure25.
mentfromthe nondiffracted
bodywavesof3.4 x 102?
dyncm.The sourcetimefunctions
donotexhibitany
systematicazimuthalpatternsfromwhichwe canreliably infer any rupture directivity. The moments
determined
from
the deconvolved
source
time
functionsare not very reliable in this casesincethe
moment estimates depend not only on the true moment release of the event, but also on the signal-tonoise ratio in the data. While the stations located in
stableregionsof the P wave radiation pattern (FVM,
OXF, BHP, LPA) have consistentmomentsbetween
1.2 and 2.4 x 102?dyn cm, as well as high signal-tonoiseratios, they are also all slightly diffracted;the
effects of which are not accounted for in the calcu-
lated Green's functions. The nodal stations (SliK,
October 10, 1977, Earthquak e
The October 10, 1977 (Mw-7.4), earthquake is a
tensional outer rise event (tensional axis approxi-
mately horizontal and perpendicularto the trench)
which occurredjust south of the October 11, 1975,
event, in the bathymetric trench near the intersection with the Louisville Ridge. The focalmechanism
for this event ($ - 180ø, $ - 55ø, X- 265ø) shown in
Figures2 and 6 wasdeterminedby Eisslerand Kanamori [1982] from body wave modelingand surface
wave inversionand representsnormal faulting in the
oceanicplate near the trench axis. This event was
studied in detail by Eissler and Kanamori [1982],
who determined a depth of about 20 km and moment
LEM, MUN, ADE) are not diffracted but have low
signal-to-noiseratios and momentswhich scatter
estimatesof 1.0 x 1027dyncmfrom forwardmodeling
of long-periodWorld-Wide Standard Seismograph
waveformsof the nodal stations are not exactly fit by
the synthetic seismograms.
Surface waves recordedby the ultra-long-period
seismometersat Berkeley (BKS) were modeled in
cm from inversion of surface wave data. In order to
widelyfrom0.4 to 8.5 x 10•'?dyncm. In addition,the
order to obtain a more reliable seismic moment. Ob-
served and synthetic Rayleigh waves (R2 and R3)
and Love waves (G3 and G4) are shown in Figure 5.
The syntheticRayleighand Lovewaveswere calculated by the methodof Kanamori and Cipar [1974]
using excitationcoefficients
for a depth of 33 km.
Both the observedand synthetic signals were band-
passfiltered using a three-poleButterworthfilter
with high- and low-frequencyroll-offsat 0.005 and
Network (WWSSN) P wave data, and 1.5 x 10z't dyn
corroborate these results, we conducted additional
bodywave and surfacewave analyses.
To determine the optimum point sourcedepth, we
have performeda simultaneousinversionof eight P
wave observations. The error reduction versus depth
curve from the multistation
iterative deconvolution
methodis shownby the circlesin Figure 4. The best
fit to the data is achievedfor a depth of 10 km. This
depth is slightly shallower than that reported by
Eissler and Kanamori [1982]; however, part of this
inconsistency is due to the different half-space
velocities used in the two studies (6.6 km/s in this
0.001Hz, respectively,
beforebeing.
cross-correlated studyascomparedto 8.0 km/sin the studyby Eissler
to obtain an estimate of the seismic moment.
The
averagesurfacewave momentfrom the four phases
modeledis 1.4 x 102?dyn cm.
and Kanamori [1982]). Individually deconvolved
source time functions and seismograms from the
13,370
Christensen
andLay: LargeEarthquakes
in theTongaRegion
I0/I
1/75
.o: 1.8
••t FVM
A = 101.0
$=
53
SHK
'•
AMo=
0.4
x,027dyn,.c
m
\
•
L••, 0XF
,
•=
56
LEM
BMP
A=
A= 98.8
Mo= 4.7
A= 59.9
Mo= 1.2
A= 96.9
.
I
ADE
I
120s
- v--
A=
40.9
$=
244
-
' -
Fig.3. Lowerhemisphere
equal-area
focalmechanism
fortheOctober
11,1975,compressional
outerriseeventshowing
source
timefunction-seismogram
pairsforeightazimuthally
distrib-
uted stations. The sourcetime functionsfor eachstation (left, hachured)have beenobtained
fromtheobserved
long-period
P waveseismograms
(right,solidtraces)bysinglestationtime
domaindeconvolution
assuming
a depthof30km. Thedashed
traces(right)arethesynthetic
seismograms
forthesource
timefunctionshownat eachstation.Thefocalmechanism
parametersof • =0 ø, • =29 ø,and X=68 øare from Christensenand Ruff [1988]. Solid circlesindicate
compressional
first motions.Stationcode,seismicmoment,epicentraldistance,and station
azimuth are indicated for each station.
shownin Figure 6 for a pointsourcedepthof 10 km.
The sourcetime functionsshowa simplepulseof moment release with a duration of 12 s, no resolvable
directivity, and an averagemomentfrom the nondif-
fractedstationsof 1.0 x 1027dyn cm.
Observed and synthetic surface waves for the
Berkeley (BKS) and Pasadena(PAS) ultra-longperiodrecordingsare shownin Figure 5. Thesegive
an averagemomentof about0.9 x 1027dyn cm. The
syntheticseismograms
werecreatedusingexcitation
coefficients for a depth of 16 km. The Harvard
gion. The focalmechanismfor this earthquake($198ø, 5=22 ø, X=101 ø) given by Dziewonski et al.
[1983]is shownin Figures2 and 7, alongwith eight
deconvolved
sourcetime function-seismogram
pairs.
e
I
.5
.7
.9
-t- I0/I 1/75
ß
centroid moment tensor (CMT) solution [Giardini et
al., 1985] has a momentof 1.0 x 1027dyn cm and a
depthof 23 km for this event. The shallowdepthof
this tensionalouterriseevent(10 km) alongwith the
deeperdepth (20-35 km) for the October11, 1975,
compressionalouter rise event suggestthat these
eventsmay be relatedto stresses
fromthe bendingof
the Pacificplate beneaththe Australia-Indiaplate
[see Chapple and Forsyth, 1979]; however, as we
03
10/10/77
I•' 12/19/82
I
discusslater, these events can also be associatedwith
temporal and spatial changesin the stressesrelated
to coupling in the interplate zone [see Christensen
o
and Ruff, 1983,1988]whichare,in turn, affectedby
Fig. 4. Normalizedapproximationerrorversusdepth
curvesfor the threeeventsindicated.Eachpointrep-
December19, 1982, Earthquake
taneousmultistationiterative deconvolution
of eight
P waves'assuminga point sourceat the indicated
the Louisville Ridge.
resents the residual waveform mismatch for a simul-
On December19, 1982, a large underthrusting
event (Mw-7.5) occurredin the southernTongare-
depth. The eight stations usedfor each event are the
same as thoseshownin Figures 3, 6, and 7.
Christensen and Lay: Large Earthquakes in the Tonga Region
LOVE
RAYLEIGH
April 13, 1980, Earthquake
Mo(IO
•?dyne-cm)
The April 13, 1980, earthquake (Mw=7.6) occurred at about 160 km depth and is located downdip
of the subducted extension of the Louisville Ridge.
The focal mechanism
900
15,371
for this event was determined
by a constrained fault inversion of 256 s period
Rayleigh and Love wave spectra from available
Global Digital SeismographicNetwork (GDSN) and
International Deployment of Accelerometers (IDA)
stations (Figure 8) using the method of Kanamori
and Given [1981]. The inversion was performed by
constraining the steeply dipping plane using first
motion data and inverting for the rake and moment.
Excitation functions for a depth of 165 km were used
based on preliminary modeling of pP and sP phases
in the body waves. The resulting focal parameters of
s
$ = 27ø, • = 75ø, X= 297ø,and Mo=3.3 x 1027dyn cm
are very similar to the Harvard CMT mechanism
($ - 30ø, • - 79ø, ), - 293ø, depth - 166 km, and Mo =
G2
2.8 x 102?dyn cm). Our focalmechanismis shownin
2.7
RI
G4
Fig. 5. Comparisonof observed(solid curves) and synthetic (dashed curves) Rayleigh and Love wave seismograms for the three events indicated. Two Rayleigh and two Love waves recorded at the Berkeley
ultra-long-period station (BKS) are shown for each
event. The R2 wave for the event on October 10, 1977,
is from the Pasadena ultra-long-period station (PAS).
The moments are listed for the fits shown.
A depth of 25 km was assumedin the source time
function deconvolutions.This depth was determined
by simultaneousinversionin the samefashionas for
the outer
rise events.
An
error
reduction
versus
depth curve is plotted by triangles in Figure 4 and
indicates a sourcedepth between 15 and 35 km, with
the best fit to the data obtained for a depth of 25 km.
The locations of the epicenter and the aftershocks
(Figure 1) indicate that the rupture occurredin close
proximity to the subductedextension of the Louisville Ridge. The deconvolvedsourcetime functions
for this event indicate a slightly more complicated
rupture history than for the two outer rise events,
with the main pulse of moment release occurring
about
10-12 s after the initiation
of failure.
The
average moment from the deconvolvedsource time
functionsis 0.9 x 1027dyn cm,with a total durationof
28 s. The source time functions show some coherent
later pulses,but our point sourceGreen's functions
are probablynot very reliable after the first 30 s. Observed and synthetic Rayleigh and Love waves for
the Berkeley ultra-long-periodinstruments (BKS)
are shown in Figure 5 and yield an average moment
of 1.7 x 1027dyn cmusingexcitationcoefficientsfor a
depth of 33 km. The Harvard CMT solution
[Dziewonskiet al., 1983] has a momentof 2.0 x 1027
dyn cm and a depth of 29 km.
Figures 2 and 9. This type of focal mechanism, in
which the compressionalstressaxis is oriented in the
downdip direction, is frequently observed at intermediate and deepdepthsin the Tonga region.
While the depth of this event is fairly well determined because distinct depth phases can be identified in the P wave signals (see Figure 9) and these
are easily modeled by forward and inverse techniques, there is some additional complication in the
signals. Closer examination showsa systematic azimuthal variation in the depth phase (pP and sP)
delay times which leads to an apparent depth discrepancy. In Figure 10 this depth discrepancycan be
easily observed between stations at western and
eastern azimuths by comparisonof observedand synthetic traces. The synthetic seismogramsin Figure
10 were created using the method of Kanamori and
Stewart [1976]. A double pulse sourcetime function
was used in order to match the character
of the direct
P waves, and depths of 170 and 150 km were assumed for the synthetic calculations for the top and
bottom panels, respectively. While the depth phases
are well matched at the western azimuths for a depth
of 170 kin, it is necessaryto decreasethe depth to 150
km to match the depth phases at the eastern azimuths. This delay time discrepancyis between 4 and
7 s, depending on the station. The difference in apparent depth (i.e., depth phase delay times) can be
accountedfor by either a relatively slower velocity in
the overlying wedge (late pP and sP arrivals) for
waves traveling toward the west or a relatively
faster velocity for the direct P waves at western azimuths (which travel downward along the slab).
Either of these explanations is plausible in the Tonga
region.
Numerous studies [e.g., Aggarwal et al., 1972;
Barazangi and Isacks, 1971; Barazangi et al., 1972;
Mitronovas and Isacks, 1971] have shown both exceedingly slow velocities and high attenuation in
portions of the overlying wedge and relatively high
velocities and low attenuation within the downgoing
slab in this region. Of particular interest are the
studies by Aggarwal et al. [1972] and Mitronovas
and Isacks [1971]. Aggarwal et al. [1972] suggest
that P wave velocities in the wedge of material beneath the Lau basin (region between the Lau Ridge
and the Tonga Ridge) may be as low as 7.1 km/s and
that this low-velocity zone extends east only to the
Tonga Ridge, which lies -- 150 km west of the trench.
They also suggest that the wedge of upper mantle
material east of the Tonga Ridge has a velocity which
• 5,572
Christensen and Lay: Large Earthquakes in the Tonga Region
SHK
Mo-
10/10/77
2.7
GUA
LON
Mo=
2.3
x1027dyne-cm
Mo=
0.6
•= 55.0ø
•
O=
1.5
= 59.3
O'
•__•=•46
•= 87.0
•
•
0
RIV
/
•
MO=0.3
•_
•=110
•
,.,
•= 246
•=
•0.0
LPA
=
95.4
ADE
Mo=
•=
1.•
40.•
I
120 s
Fig. 6. Lower hemisphere equal-area focal mechanism for the October 10, 1977, tensional outer rise event showing sourcetime function-seismogrampairs for eight azimuthally distributed
stations. See Figure 3 for further details. A depth of 10 km was assumed in the deconvolutions. The focal mechanism parameters of • = 180ø, 6 = 55ø, and X = 265 ø are from Eissler and
Kanamori [1982]. Open circlesindicate dilatational first motions.
is closer to normal suboceanic velocities (8.2 km/s).
Azimuthal depth phasedelays couldbe accountedfor
if depth phases traveling to the west traversed the
lower-velocity region while depth phasestraveling to
the east traversed the higher-velocity region. Although this may be a factor in the apparent depth
discrepancy, we feel, for reasons which will become
clearer in the next section,that mostof the anomaly
is related to effectsof the deeper slab on the direct P
waves. Mitronovas and Isacks [1971] show evidence
that the P and S velocitiesin the descendingslab are
6-7% higher than those in the surrounding mantle
extending down to the level of the deepest earthquakes. More importantly for our current problem,
they show evidence that P wave residuals for earthquakes at depths less than 200 km recorded at teleseismic distances can be at least 5 s between eastern
and western azimuths because of the effects of the
slab [see Mitronovas and Isacks, 1971, Figure 7].
This number agrees very well and is of the proper
sign with our observeddepth phasediscrepancyof 47 s. Thus, as long as the depth phases are not affected by the same anomalouspaths through the slab
(which they usually should not be), we can explain
attempted because of their short duration. The
average moment from the long-periodbody waves is
6.8 x 1027dyn cm, with a total sourceprocessduration of about 16 s. Once again, this average P wave
moment appearsto be biasedtoward the high side by
the low signal-to-noise ratio at several of the stations.
June 22• 1977, Earthquake
The June 22, 1977, event is the largest of the
events discussedin this paper, with a moment re-
portedby Giardini et al. [1985] of 13.9 x 1027dyn cm
(Mw - 8.0), and it may well be the largest event in the
Tonga region in the historic record. Analysis of 256-s
period Rayleigh and Love wave spectra from available Seismic Research Observatory (SRO) and Abbreviated SRO (ASRO) stations is shown in Figure
11.
The solid curves show the fit to the data of an
Deconvolved source time functions from eight P
waves for the 1980 event are shown in Figure 9 using
depths of 170 km for stations at the western azi-
iterative least squares fault plane inversion [Kanamori and Given, 1981] for the best double-couple
source. Excitation functions for a point sourceat 71
km depth were used. An azimuthally independent
source processtime of 60 s was assumed as well. A
slightly longer source processtime of 84 s may be a
more appropriate boxcar source duration (J. Zhang,
personal communication 1988). The best doublecouple parameters are $ = 185ø, •--79 ø, ),- 266 ø, and
muths and 150 km for stations a,t the eastern azi-
Mo=16.9 x 1027 dyn cm (Mw=8.1).
muths.
strained moment tensor solution, which is not shown,
these time offsets in terms of a slab effect.
Examination
of these source time functions
and complexities in the direct P phasesindicates that
there are two pulses of moment release; however,
more detailed analysis of these subevents was not
An uncon-
gave a similar major double couple, with a minor
double-couple component of only 6%. The dashed
curves show the fit to the data for the major double
Christensen
andLay: LargeEarthquakesin theTongaRegion
13,373
GUA
12/19/82
Mo=
A=
_
ANP
1.6
53.7
.
GOL
Mo=
Mo= 1.5x1027dyne.cm
A=
A=
78.1 ø
1.6
91.,5
PMG
0.8
•
95.7
Mo=
0.3
A=
38.3
112
.• •_._(I)=286
V •,,j•_.
LEM
••
__=269
• ••U V•
I
•
120sJ
o.,
133
TAU
Mo=
0.5
•=
• •
•5.G
vv
Fig.7. Lowerhemisphere
equal-area
focalmechanism
fortheDecember
19,1982,underthrusting
eventshowing
source
timefunction-seismogram
pairsforeightazimuthally
distrib-
utedstations.
SeeFigure3 forfurtherdetails.A depthof25kmwasassumed
in thedeconvolutions.Thefocalmechanism
parameters
of •- 198ø,8-22 ø,andX- 101øarefromDziewonski et al. [1983].
4/I 5/8O
4
I
I
I
--
2
0
I
-2
--
-2
-4
--
-4
G
R
oo
7'.
:
4.
..
ß
5. ß
ß
ß
4.
o
I.
0.0
60
120 180 240 500 560 00
ß
60
120 180 240 500 560
AZIMUTH (deg)
Fig.8. Inversion
of256-speriod
Rayleigh
wave(]eft)andLovewave(right)spectra
forthe
April13,1980,earthquake
recorded
byavailable
ODSNandIDAstations.
Thesolidcurves
showthe•t o•thebestdouble-couple
solution
w•ththesteeplydJpp•nE
plane(½= 2?øand• =
?5ø)held•xed,be•nE
constrained
byMrstmotion
data.Theresu]finE
parameters
•orth•smode] are X- 29?øandMo- 3.3 x • 02?dyn cm.
13,374
Christensen
andLay: LargeEarthquakes
in theTongaRegion
COL
4/15/80
•ø2914i)
3
SEO
MSO
Mo--10.OxlO27dyne.cm
• • Mo5.2
A= 80.2ø .
•
A= 90.3
•= 1o•.•
••
•
• *= 273
•
•
•
•= •.•
*= 133
MUN
I
120s I
Mo=
11.4
A=
58.7
•,=
,
2•
Fig.9. Lowerhemisphere
equal-area
focalmechanism
fortheApril13,1980,downdip
compressional
intermediate
deptheventshowing
source
timefunction-seismogram
pairsforeight
azimuthally
distributed
stations.SeeFigures
3 and6 forfurtherdetails.Depths
of 150and
170kmwereassumed
in thedeconvolutions
forstations
at easternandwestern
azimuths,
re-
spectively.The focalmechanism
parametersof • = 27•, 6= 75•, and • = 297• are discussed
in
the text.
coupleof the Harvard CMT solutionof •= 197ø,
• = 79ø, X=271ø,and Mo= 13.9 x 102?dyn cm. The
two solutionsare very similar. The resultingfocal
mechanism indicates that the tensional stressaxis is
orientedin the downdipdirection,which is unusual
for the Tongaregion.
We have alsoforwardmodeledRayleigh wave
data'from the Pasadenaultra-long-periodinstrument. Observedand syntheticRayleighwaves(R1-
The depthof the June 22, 1977, earthquakeis
controversialin additionto beingproblematic,sowe
will considerit in somedetail. The ruptureextent
hasbeenreportedto extendasshallowas50 km by
Talandierand Okal [1979]fromregional"pP"arrivals on short-period
instrumentsandas deepas 166
km [Silver and Jordan, 1983] basedon aftershocklo-
cations. While the shallowestdepthestimatemay
requirethat the ruptureoccurin the upperplate, the
a rupturethroughthe enR4) areshownin Figure12. Syntheticseismograms deeperestimatesuggests
were calculated by the method of Kanamori and
tire thicknessof the subductingslab. The afterCipar [1974]usingthe Harvard focalmechanismand
shocks,whichwill be discussed
in moredetail later,
excitationfunctionsfor a depth of 62 km. Both the
donot actuallyhelpto definethe ruptureextentand
observed
andsyntheticwavetrainswereband-pass tend to be unrelatedto either possiblefault plane,
filtered using a three-poleButterworth filter with
while the ruptureprocess
is sufficientlycomplexto
high and low roll-off frequenciesof 0.005 and 0.001
make identificationof depthphasesin the raw seisHz, respectively,beforebeingcross-correlated.
The
mogramsvery difficult. The locationreportedby the
National Earthquake Information Service (NEIS)
averagemomentfor the fits shownin Figure 12 is
10.6 x 10•'?dyn cm. The point sourcesynthetics has a depthof 65 km. This depthis alsocloseto the
centroid depth found by the Harvard CMT solution
matchthe data fairly well, whichsuggests
that the
horizontal fault dimension is less than several
(61 km). In the following discussionwe will show
that the depthextent of the bodywave radiation for
hundredkilometers.We will returnto the rupture
this event is extremely localizedfor an event of this
dimensionlater. It hasbeensuggested
that the occurrenceof several downdiptensionaleventsin the
size and is concentratednear the hypocenter at
depths from 70 to 90 km. This result is consistent
Tonga region can be interpreted as evidencefor a
doubleBenioffzoneproduced
by unbendingof the
with the centroidmomenttensorresultand places
subductingslab [Kawakatsu,1986]. However,the
most of the rupture near the surfaceof the subductunusually large size of this event and its location
near the upperedgeof the subducted
plate indicate
that additional factors are associatedwith the accu-
mulationoftensionalstressin thisregion.
ingslabjustbelowthecoupled
interplateregion.
Knowledgeof the vertical depth extent of the
1977eventis very importantfor understanding
the
relationshipof thislargeeventto the stressregimes
Christensenand Lay: Large Earthquakes in the Tonga Region
13,375
4/1 3/8O
DEPTH
PMG
^
170 km
90 o
A: 3 70
'•
,, h•:2860
MSO•A ff,, /•
38o
910
860
SNG
•
J•i /%
280ø
ADE•
[•
243ø
LUB•/•'• ^/•..
54
980
40 o
LPA •
•
0
I
134
o
•
I
I
I
100
I
sec
DEPTH
150 km
37 o
PMG
.A ,!'• /• • 286
o
900
MSO •h /•
860
SNG
•
28O
o
I
40 o
ABE__ /• ,
i!
243
o
/•
380
910
LUB_••
,j,,•__.
980
A
Fig. 10. Comparison
of observed
(solidcurves)andsynthetic(dashedcurves)P waveseismo-
amSfortheApril13,1980,event.Sixseismograms,
threefromeastern
azimuths
andthree
m westernazimuthsare forwardmodeledby syntheticP wavescalculatedat depthsof 170
km (top)and150km (bottom).Thesynthetictraceswerecalculated
usingthemethodofKanamoriandStewart[1976]for a doubletrapezoidalsource.The second
trapezoid(risetime of 2 s
and durationof 4 s) is delayedfromthe first trapezoid(risetime of 3 s and durationof 6 s) by
8.0 s and contains one fourth the moment of the first. The arrows indicate the onset of the most
easilyobservable
depthphase.Epicentraldistanceandstationazimuthareindicatedforeach
station.
in the descendingand overriding plates. We have
determinedthis depth extent through a variety of
means utilizing both single station time domain deconvolutions [Ruff and Kanamori, 1983] and simultaneous multistation
deconvolutions [Kikuchi
and
Kanamori, 1982; Kikuchi and Fukao, 1985]. One
method which we can use to determine the depth of
an event in which the depth phasesare not easily in-
terpretedis to deconvolve
sourcetime functionsfrom
individual seismogramsat a suite of depths and to
then determine the correct depth range by the behavior of the source time functions.
Christensen
and
Ruff [1985] outline a method by which the correct
of the moment relative to a delta function. Christen-
sen and Ruff [ 1985] show that low values of T1/2 and
high values of VMAX indicate the best depth range
for both synthetic and real data cases.
In Figure 13a we have plottedT1/2 and VMAX
versusdepthcurvesfor singlestationdeconvolutions
for the 1977 event. Each curve representsthe average T1/2 and VMAX curve for sevennonnodalstations, with the standard deviation at each source
depthbeingshown.Bothcurvesindicatean optimal
point sourcedepth of about 70 km. There is a
secondaryextremevalue near 140 km which is producedby doublingthe time lags of the depthphases
depth can be determinedby applying a simplicity
relative to a source at 70 km. Since the deconvolved
source time functions for this event indicate a double
Two parameters are introduced, the half-absolute
pulseof momentrelease(seeFigure 14) andbecause
the depthextentof this eventhasbeencontroversial,
we explorethe possibilitythat the secondmoment
rate pulse is located at a substantially different
depththan the first pulse.To testthis possibility,we
criterion
to the deconvolved source time functions.
moment time (T1/2), which measures the concentration of the moment toward the start of the source
time function, and the varimax norm (VMAX [see
Wiggins, 1978]), which measuresthe concentration
576
Christensen and Lay: Large Earthquakes in the Tonga Region
6/22/77
2
-4--
R
"•
0
.5.
JS.•
ß
15.
•
//"'"'
-
:
,\
•
0.. 3.
•;
'• o.
0
/,,,'
/'//
60
,
,,/
i20
i80
--,i
5.
240
500
560
o.
0
60
i20
i80
240
'•'
500
560
AZIMUTH (deg)
Fig. 11. Inversion of 256-s period Rayleigh wave (left) and Love wave (right) spectra for the
June 22, 1977, earthquake recordedby available GDSN stations. The solid curvesshowthe fit
of the best double-couplesolution for the iterative least squaresfault plane inversion, and the
dashedcurvesshow the fit of the major double-couplesolution from the Harvard centroid mo-
ment
tensor
solution
[Giardini
etal.,1985].Theparameters
forthemodels
shown
are• • 185ø,
•-79 ø,X-266 ø,and Mo- 16.9 x 1027dyn cm (solidcurves)and •-197 ø, 5-79 ø, X=271 , and
Mo = 13.9 x 102?dyn cm (dashedcurves).
have performed a similar series of deconvolutions
holding the depth of the first moment rate pulse at 70
km (hypocentral depth) and allowing the Green's
function for the later part of the waveform to have a
depth varying from 50 to 160 km. T1/2 and VMAX
versus depth curves are plotted for the resulting
sourcetime functionsin Figure 13b, again averaging
over the seven individual stations and plotting the
mean
values
and the standard
deviation
at each
depth. It is apparent on the basis of the simplicity
criterion that the preferred depth of the secondpulse
is also closeto 70 km. The rather narrow depth extent of 30 km or less,which is indicated by the depth
curves in Figure 13, is unusual for an event of this
size.
During the review of this paper, a parallel analysis of the June 22, 1977, event by Lundgren and Okal
[this issue]was brought to our attention. Their study
suggests that the body wave radiation is, in fact,
causedby subeventsat very different depths,with an
overall downward rupture propagation. They argue
that rupture initiated near 40 km depth, with a first
source area extending to 60 km and a secondpulse
with lower moment rupturing later near 125 km
depth. This analysis, based on two teleseismic P
waves and two teleseismicSH waves, promptedus to
conduct additional depth resolution analysis of our
larger data set.
Figure 15 displays deconvolved moment rate
functionsfor two representativestationsfor a variety
of two-source depth cases. For each station the
Green's function for the initial pulse was placed at a
depth of either 40, 70, or 100 km. The depth for the
remainderof the seismogramwas varied from 40 to
150 km. All of the moment rate functions exhibit un-
stable reverberations when the sourcedepth for the
later part of the seismogram exceeds80 km. The
simplestmoment rate functionswith the greatest
azimuthal
coherence
are obtained
when
the entire
signal is deconvolvedusing a sourcedepth of 70 km.
If rupture does in fact initiate at 40 km, stable moment rate functions are obtained if the secondary
source depth is near 70 km, consistent with a short
downward rupture. However, relatively stable moment rate functions are also obtained if the rupture
starts at 100 km depth with a secondarysourcedepth
near 40 km, requiring upward rupture, although the
waveform matches are not very good in the case.
Many stations corroborate these results. The summary plots in Figure 16a show that the best average
waveform matches, as measuredby correlation coefficients, are obtained for secondarysourcedepthsof
40 km or 70-80 km for a wide range of initial pulse
sourcedepths. Greater depths do not satisfy the data. In addition, the simplicity criterion provided by
the half moment time also favors depthsof 70-80 km
(Figure 16b). These results refute the modelpresented by Lundgren and Okal [this issue].
Having presented the above evidence that the
body wave moment release occurred over a narrow
vertical extent, it is appropriate to try an additional
depth experiment which explicitly allows for a component of horizontal directivity. In this casewe have
simultaneously deconvolved10 azimuthally distrib-
Christensen and Lay: Large Earthquakes in the Tonga Region
plane, in which case the depth range will automatically be restricted to a narrow range, or the fault
plane is the near-vertical nodal plane and the highfrequency moment release of the event is restricted
to a very narrow strip.
6/22/77
Mo(1027dyne-cm
)
,r,/•
13,377
10.7
Individual
station
deconvolved
source time func-
tions for eight stationsassuminga sourcedepth of 70
km are shown in Figure 14. The two pulses of moment release characteristic
of this event are evident.
The average moment from nondiffracted stations is
about 15 x 1027dyn cm, indicating that most of the
R2 ,.•,•
moment release is being retrieved. Given the large
size of this event, the sourceprocesstime of approximately 60 s, and the two distinct pulses of moment
release, it may be possible to determine the spatial
component of the moment rate history using the
apparent directivity in the source time functions.
There appears to be directivity in the source time
functions in Figure 14, with a shift in the peak of the
first large pulse of about 6 s between the eastern and
western azimuths; however, initial attempts to determine rigorously the spatial locations of the two
pulses were not satisfying. The problem arises from
the lack of relative directivity between the first and
secondpulses. This is demonstrated in Figure 18 for
two stations, which are representative of stations at
the eastern and western azimuths. In Figure 18
,
R3
R4 •
/ • / '• .,'••
I
I
900s
Fig. 12. Comparison of the observed(solid curves)
and synthetic (dashed curves) Rayleigh wave seismograms for the June 22, 1977, event. Observed Rayleigh waves (R1, R2, R3, R4) were recorded at the
Pasadena ultra-long-period station (PAS). Moments
are listed for the fits shown.
(first row) source time functions for station NDI (azimuth -292 ø) and station AAM (azimuth =50 ø) decon-
volved assuming a depth of 70 km show that although there is an apparent directivity of about 6 s
between the truncation of the first moment pulse at
the two stations, the first and secondpulses show no
relative directivity. In other words, the apparent directivity all appears before the first pulse, and the
two main pulsesmust have nearly the same location
(at least in an east-west sense). This offset is com-
uted seismograms using the iterative
method of Kikuchi
inversion
and Fukao [1985] for a ribbon
source (i.e., horizontal line of point sources). Once
again, a seriesof calculationsat different depthswas
performed. While determining depth based on a simplicity criterion for the sourcetime functionsis useful for individual
station deconvolutions (where the
observed and synthetic seismograms are always
nearly perfectly fit), for multistation inversions the
overall fit of the observed and synthetic data for a
commonsourcetime function is the useful depth parameter. The corresponding error reduction versus
depth curve for this event is shown by the crossesin
Figure 17. The best fit to the data is obtained for a
depth of 80 kin. One differencebetween the data sets
used for the multistation
deconvolutions
and the
pletely the result of a time delay of the first major
pulse at the western station relative to the eastern
station. Attempts to determine a spatial rupture history accounting for this offset were not successful
since the inferred directivity of the first pulse requires an unrealistic rupture velocity.
There is a simple explanation for this time shift
independent of the sourceprocess,which we have already discussedin the previous section. An apparent
depth difference between eastern and western azimuths for the April 13, 1980, event was readily observed
in the data
because
of the isolation
of the
depth phases from the direct P waves and the rela-
tively shoFtand simplesourcetime functionfor the
1980 event.
While
we cannot demonstrate
a similar
apparent depth difference directly in the waveforms
single-station deconvolutionsis the inclusion of the
for the 1977event,we candemonstratethat allowing
nodal observations
for an apparent depth difference of 10 or 20 km due to
at western
azimuths
in the multi-
station inversion. Notice that in Figure 17 the range
of depth values for which the residual waveform error is low is very narrow and well defined compared
to the similar depth curves for the smaller events
shown in Figure 4. We concludefrom this that a
depth of about 70-80 km is appropriate for this event.
We will show further evidence later that not only is
this an appropriate body wave centroid depth but
that the vertical extent of the body wave radiation of
this event is extremely narrow for its size. Attempts
to model this event using a large rupture area with a
distributed depth range produce poorer fits to the
data and more complicated source time functions.
The narrow depth range which appears appropriate
for this event
leads to two alternative
conclusions.
Either the fault plane is the near-horizontal nodal
the effects of the fast slab for the western
azimuths
removes the directivity problem and producesa more
internally consistent data set. In Figure 18 (second
and third rows) the two stations have been decon-
volved assuming point sources at 80 and 90 km
depth. While the deconvolvedsourcetime functions
at the eastern
azimuths
become unstable
and more
complicatedfor sourcedepthsgreaterthan 70 km,
the western station yields stable moment rate functions for sourcesas deep as 90 km. Furthermore, for
the western station the "stretched"
Green's functions
for deeper depths tend to remove the interval of low
moment rate before the first pulse. This is particularly apparent becausethe direct P arrival at the
western station is very nodal and the observedwaveforms are dominated by the pP and sP arrivals. Time
13,378
Christensenand Lay: Large Earthquakesin the TongaRegion
T I/• (sec)
(]
40
55
i
I
70
i
I
85
I
VMAX
o
.002
.004 .006
o
.002
.004
•
I
!
I
.008
I
'
b
T I/• (sec)
40
55
70
85
i
i
i
I
VMAX
.006
.008
I
o
I
•
o
o
o
Fig. 13. T1/2and VMAX versusdepthcurvesfor the June 22, 1977,event. (a) Each point representsthe averageT1/2or VMAX value of sevensourcetime functionscalculatedfrom individual station deconvolutions, with the standard deviation indicated by the bar. The best
depthestimatefor eachcase,whichis indicatedby the thick vertical bar, corresponds
to low
valuesofT1/2 or high valuesof VMAX [seeChristensenand Ruff, 1985]. (b) The depthcurves
representaverageT1/2 and VMAX values from sourcetime functionsat the same sevenstationsin Figure 13a;however,the depthsfor the deconvolved
sourcetime functionsare allowed
to vary only for the secondmomentpulse(i.e., the first momentpulseis held at a depthof 70
kin).
lines drawn through the reproductionof the AAM
eastern azimuths.
depth clearly demonstrate a reduction of the apparent directivity relative to NDI, when depthsof 80 and
overall
moment
rate function
obtained
90 km are assumed for the latter
for a source at 70 km
station.
We can further test this hypothesisof azimuthal
variation of the Green's functions by again performing a series of ribbon fault multistation deconvolutions for various depths. This time we will assignthe
stations at western azimuths ",apparent" depths
which are 10, 20, or 30 km deeper than the stations
at eastern azimuths. The resulting waveform mismatch versus depth curves are shown in Figure 17.
The depthslabeled in Figure 17 are referencedto the
The curves for which the stations
at western azimuths are assignedsourcedepths 10 or
20 km deeperthan the eastern azimuths attain lower
residuals
than those in which
the western
stations are at the same depth or 30 km deeperthan
the eastern azimuths. This not only indicates that an
apparent depth difference between eastern and
western stations due to the travel time effects of the
slab is present but also that the apparent difference
is approximately the same as for the 1980 event.
Since the 1977 event is located farther
east than the
1980 event, we do not believe that velocity differencesin the overlying wedge can explain the apparent delays in pP and sP for both the 1980 and 1977
Christensen and Lay: Large Earthquakes in the Tonga Region
15,579
COL
6/22/77
,5,=
.o90.2
••L
SHK
a=
ß=
GOL
75.5 •
.
0.7
/
KOD
46
o=
•.
6.2
ARE
A = 109.•
=
90.4
•
A=
27•
96.2
110
LPA
2.7
••• ,.• •Mo=
•s2
97.6
160s
I
Fig. 14. Lower hemisphere equal-area focal mechanism for the June 22, 1977, downdip tensional intermediate depth event showing sourcetime function-seismogrampairs for eight azimuthally distributed stations. See Figures 3 and 6 for further details. A depth of 70 km was
assumedin the deconvolutions. The focal mechanism parameters of $ = 197ø, $ = 79ø, and ), =
171 ø are from Giardini et al. [1985].
events. The discrepancy due to deep slab effects,
however, should be equally important for both events
and not a factor for the shallower
events which
lie
well to the east of the deepslab extension.
Deconvolved source time function-seismogram
pairs for all observationsof the 1977 event used in
this study are shown in Figure 19. In Figure 19 a
point sourcedepth of 70 km is used for stations at
azimuths between 33øand 132ø,and a depth of 80 km
is assumed
at all other
azimuths.
The deconvolved
source time functions in Figure 19 are very consistent over all azimuths, each showing a similar twopulse character of the moment release. Much of the
east-west directivity has been removed from these
sourcetime functions through the use of the variable
depth assumption which accounts for slab effects.
There is still some directivity observable in the
source time functions
and in the waveforms.
The di-
rectivity is now most apparent at the southern stations (WIN, PRE, BUL), where the second moment
rate pulses occur later in time than at other azi-
muths. The first pulse also appearsto have split into
two pulses at the southern stations. These southern
stations indicate that a rupture toward the north
would be most appropriate. A comparisonbetween
the waveforms at station PRE (to the south) and KBS
(to the north) showsthis effect quite clearly. The two
stations are at similar distances,and not only parallel to the slab (similar slab effects?), but also lie in
similar regionsof the radiation pattern.
If we assumethat an apparent depth increase of
+ 10 km for western azimuths is adequate to account
for the effects of the high-velocity slab, we can attempt to determine more details of the rupture process. Of course, one must be cautious about inter-
preting secondary details of the rupture process
given the limited accuracy of our Green's functions.
Two-dimensional
multistation
inversion
[Kikuchi
and Fukao, 1985] was usedto map the spatiotemporal variation of the moment rate pulses given the assumption that there is a 10-km depth discrepancy
between
eastern
and western
azimuths
in the inver-
sion. Referencesto depth in this discussionwill be
with respectto the eastern stations. Results from the
two-dimensional inversion are shown in Figure 20
for both possiblefault planes. The observedand synthetic seismogramsfor the inversion for plane I (the
steeply dipping fault plane), along with a representative spike train and sourcetime function, are shown
in Figure 21. Either choiceof fault plane yields a residual waveform error of 0.23; thus it is impossible to
determine the actual fault plane on the basis of the
longer-period characteristics of the waveforms that
are matched by this inversion. This is a somewhat
surprising result for such a large event, and in the
next section we will
see that
aftershocks
do not re-
solve this question. The fault plane ambiguity is due
not only to the rather narrow depth range in which
the major displacementsseemto occurbut is also due
to the similar spatial variations in the rupture models that result from selectingeither plane. The effect
of assuming an even larger apparent depth discrep-
15,58o
Christensen and Lay: Large Earthquakes in the Tonga Region
AAM
LPS
•N•T•AL
4O
70
PULSE
I00
DEPTH
40
(kin)
70
I00
4O
5O
6O
E
-•
70
o
I00
•
I10
component of unilateral rupture extending more
than 150 km. Also, given the CMT centroid depth of
61 km and the fact that rupture cannot extend to
much shallower depths without leaving the slab, we
can infer that most of the displacementis probably
constrained to depthsnear the centroid depth.
The moments that we obtain from the body wave
analysis are consistently equal to or larger than the
surface wave moments, suggestingthat the moment
in these two frequency bands was released over the
same area. It should be noted, however, that analysis of radial normal modes by Lundgren and Okal
[this issue] suggests a centroid depth greater than
100 km, while fundamental mode surface wave analysis by Zhang and Lay [1988] gives a centroid depth
of 93 km. While part of the depth differencefrom the
body wave results may be due to differencesin Earth
model (our Green's functionsare for a half-spacewith
a velocity of 6.9 km/s), it doesappear that somelongperiod rupture may have taken place deeperthan the
body wave radiation. This favors the steeply dipping
plane.
One other fairly stable feature of the 1977 event
rupture processis the tendency for the rupture to extend about 60-80 km to the north or northeast
o 120
the result
a.
130
140
150
i
160 sec
Fig. 15. Deconvolved source time functions for
stations AAM (A = 106.5 ø, Az = 50 ø) and LPS (A =
92.4 ø, Az = 75ø). The source time functions shown are
deconvolved from the observed seismograms at stations AAM and LPS using the depth assumptionsfor
the initial and secondmoment pulses as indicated on
the figure.
ancy of 20 km between eastern and western azimuths
is to decrease the observable directivity even more,
thus producing an even smaller rupture area.
The most robust feature of the rupture process
that we can resolve, which holds for either choice of
fault plane, is the relatively small spatial extent over
which the body wave radiation from this great earthquake ruptured. The length of the rupture zone in
the north-south direction is approximately 60-80 km,
while
the width
from
the epicenter (depending on the fault plane). This is
in the east-west
or vertical
direction
(depending on the fault plane used) is substantially
narrower, of the order of 30-40 km.
Wider fault
planes were tried, with depths extending from 40 to
150 km, but the inversion always concentrated the
primary moment release near 70 km. For either
fault plane the depth extent, which is centered near
75 km, does not extend over more than about 30 km,
as was suggestedby the rather tight extrema in the
depth curves. This limited rupture area is alsomanifested in the lack of directivity ,betweenthe two major pulses. The surface wave data are consistentwith
this interpretation in that the point sourcesynthetics
match the PAS data well. In fact, spectralratio analysis of the PAS surface waves preclude any strong
of the waveform
differences
at southern
azimuths (Figure 19, stations PRE, WIN, BUL).
While the rupture front producing the first moment
pulse spread over the rupture area shown in Figure
20 with a rupture velocity of 2.5 knYs or greater; the
secondmoment pulse, which apparently occurredin
the same region, must have initiated after a short delay in the rupture process. The secondpulse may
have further ruptured the region that slipped during
the first moment pulse or perhaps ruptured a region
slightly downdip or east of the first pulse, as suggested by the locations shown in Figure 20. Since we are
not constrained by aftershock locations or tectonic
environment to a single fault plane (such as in the
case of interplate events), perhaps the secondpulse
could have ruptured a secondsubparallel fault. Our
speculations, however, cannot be too extreme given
that a single focal mechanism at a point sourcedepth
seemsto represent the data adequately.
Relationship Between Large Events
and Other Seismicity
Details of the focal parameters and source processesof the five large events in the southern Tonga
region were discussedin the previous section. Each
of these occurredin a unique stressregime which, becauseof the closeproximity of these events, must be
related in spaceand]or time. To summarize these results, the average sourcetime functionsfor these five
events are shown in Figure 22. These averages were
calculated from the eight individually deconvolved
source time functions in Figures 3, 6, 7, 9, and 14.
The source time histories of these events range from
a very simple single pulse to the more complicated
double pulse of the 1977 event. The December 19,
1982, thrust event has a more gradual buildup of moment rate than the three smaller intraplate events,
as well as a longer rupture duration. The June 22,
1977, event is the only event with significant
multiple-source character.
Rupture areas of earthquakes can sometimes be
estimated using the aftershockswhich occurimmediately following the event (days to weeks). One-week
aftershock zones for the three shallow (two outer rise
Christensenand Lay: Large Earthquakesin the TongaRegion
13,381
.97
--
.93
Depth of
First Pulse (km)
.87
ß
4:o
+
5o
ß
6o
ß
70
x
80
-Aß
.83
40
9o
100
60
80
100
120
140
120
140
b) 9o
8O
75
I--
65
60
55
50
45
40
60
DEPTH
80
OF
100
SECOND
PULSE
(km)
Fig. 16. Correlationcoefficientand T1/2 versusdepth curvesfor the June 22, 1977, event. (a)
Each point representsthe average correlation coefficientbetween observedand synthetic seismogramsfrom sevenindividual station deconvolutions.Each curve is calculated using an assumeddepth for the initial momentpulseas indicatedby the symbol. The depth of the second
momentpulseis varied as shownby the horizontal axis. (b) Each point representsthe average
absolute halfmoment
time (T1/2) ofseven source time functions calculated from individual sta-
tion deconvolutions.The symbolsand horizontal axis are the sameas in Figure 16a.
and one underthrusting) events are shownin Figure
23. These aftershockareas are all very closelyassociated with the Louisville Ridge. Using these areas
similar slip estimatesif we assumea rupture velocity
of 3 km/s, and use the sourcedurations of the body
and the associated surface wave moments, we estimate average displacementsof 0.4, 0.4, and 0.3 m for
the October 11, 1975, October 10, 1977, and Decem-
The rupture areas of the intermediate depth
events cannot be similarly constrainedbecauseof a
lack of detectable aftershocks. The April 13, 1980,
event had only three aftershocksin the following 2-
ber 19, 1982, events, respectively. We obtain very
wave time functions to estimate the source areas.
382
Christensen
andLay:LargeEarthquakes
in theTonga
Region
e
.0
.2
.4
.6
.8
I
I
I
I
week period (seeFigure 23, opencircles). If we assume an expanding circular rupture for the 1980
event with a rupture velocity of 3 krrdsand a total
durationof about16 s, we canestimatea displace-
ment of 0.7 m.
Thereare 36 eventswhichfollowedthe June22,
•o
1977, earthquakein the following2 weeks,but most
cannot be considered aftershocks in the normal
sense.Theseevents,whichare shownin Figure23
as the solidcircles,occurthroughoutthe studyregion. Becauseof the small size (mb_<5.4)of these
events,neither their locationsnor their depthsare
wellconstrained.
Nevertheless,
it isclearthatmany,
if not most, of these events do not occur on either
nodal plane. Most of the "aftershocks"are located
east of the epicenterperhapssuggestingthat the
near-horizontal
nodalplaneis the fault plane. How-
ever, a projectionof the horizontalplane towardthe
trenchwouldrequirethat manyof theseaftershocks,
particularly those near the trench, occurred at
Fig. 17. Normalized approximation error versus
depthcurvesfortheJune22, 1977,event.Eachpoint
represents the residual waveform mismatch for a simultaneous multistation iterative deconvolution of
10 P waves assuminga 330-km-long ribbon fault.
The stationsusedare (AAM, ARE, COL,DUG, KBS,
KOD, LPA, LPS,NDI, and SHK). The depthseries
were repeated 4 times assumingan inherent differ-
However,our analysisof the main shockargues
depthsgreaterthan 100 km. A separateclusterof
"aftershocks"
occurred
neartheepicenter
oftheApril
muths of 0 km (crosses),
10 km (circles),20 km (triangles),and 30 km (squares).
48
centerwhich have beenassigneddepthsof 133 and
166 km by the International Seismological
Centre
(ISC) and whichprojecton the deepextensionof the
near vertical fault [see Silver and Jordan, 1983].
against significant body wave moment release at
ence in depth between stations at eastern azimuths
(33 ø< azimuth < 132 ø) and stations at western azi-
20
depthsof up to 100 km in the oceaniclithosphere.
Therearealsotwoeventsnorthandwestoftheepi-
13, 1980, downdip compressionalevent. While it is
not knownwhethertheir hypocenters
are in the up-
sec
NDI
i
AAM
i
15.0"
2
depfh
(km)
106.5
ø
7O
8O
AAM
70 km depth
I
160s
I
Fig.18. geconvolved
source
timefunction-seismogram
pairsforstations
NDI (left,western
azimuth)andAAM (right,easternazimuth).The sourcetime functions
shownweredecon-
volved
fromtheobserved
seismograms
(solid
traces)
using
thedepth
indicated.
Synthetic
seis-
mograms(dashedtraces)calculatedfrom the deconvolved
sourcetime functionsare also
shown
ineach
instance.
Thedeconvolved
moment
pulses
forstation
AAMat70kmdepth
are
reproduced
at thebottom
oftheleft-hand
column.
Timelinesfromthetwopeaks
oftheAAM
moment
pulses
at20and48sareshown
superimposed
ontheNDIsource
timefunctions.
Epicentral distanceand station azimuth are indicatedfor eachstation.
13,383
ChristensenandLay: LargeEarthquakesin theTongaRegion
COL
SCP
N
8.3
A=
90.2
ø•
LON
MUN
Z
Mo=9.7x1027dyne.crn
•o•;
1.7
';"•
,, J
60.2
KOD
N
21.5
.
109.3
84.7
•
MS0
LPS
N
SHL
E
E
5.5
18.2
89.0
ARE
NDI
N
22.0
Z
115.0
1.2
292
•½•.•&
•1•2
:•
.
DUG
N
•'
LPB
NIL
E
16.0
Z
119.3
296
GOL E
LPA
GUA
Z
N
6.6
A• 25.1
50.
,100.
52.9'
':'"',
L•[•,•,..309
46
u
"
90.4
vtJ
'r
TUC
N
WIN
SHK E
Z
5.6
133.1
AAM
Z
1.2
"'•
•18
KBS Z
PRE Z
1.•
12•;94
,••
•,,•
FVM
E
BUL Z
0.8
I
I
160 s
100.2
u'
12.8
"•
,'
131.1
"'•'
½
,,•
6/22/77
Fig.19.Deconvolved
source
timefunction-seismogram
pairsfortheJune22,1977,event.The
sourcetime functionsfor eachstation(left) wereobtainedfromthe observed
long-period
P
waveseismograms
(right,solidtraces)
bysingle-station
timedomain
aleconvolutions.
The
dashed
traces
(right)arethesynthetic
seismograms
forthesource
timefunction
shown
at each
station.Thedataare arrangedin azimuthalorderwith the stationcodes,
seismic
moments,
epicentral
distances,
andstation
azimuths
indicated.
Pwaves
fromazimuths
between
33øand
132øweredeconvolved
assuming
a depthof 70 km. All otherP wavesweredeconvolved
as-
suminga depthof 80 km.
13,384
Christensen and Lay: Large Earthquakes in the Tonga Region
PLANE
DEPTH
I
I00
85
i
I
70
55
I
I
i
+
+
+
9O
(km)
+
PLANE
68
40
71
74
77
by 40 km from the finite sourceinversion, we obtain
an average displacement of 6.2 m and a static stress
drop of --94 bars.
2
80
AlI
-
+
+
:30
0
-:30S
ß
- +
Q
I
DN .30
+
+
+
+
+
-
+
I
I
-15
-30
W-15
UP
DISTANCE
0
15
30
45
E
ALONG DIP (km)
Fig. 20. Spatial and temporal distribution of moment
release for the June 22, 1977, earthquake. Each circle
represents a subevent from the two-dimensionalsimultaneous iterative fault plane inversion. The radius of the circles are proportional to the seismic moment of the individual
subevents.
Inversions
in the source time functions are hachured with a neg-
ative slope, and subeventsrelated to the secondmoment pulsein the sourcetime functionsare hachured
with a positive slope. Later pulsesare left unhachured. The solid lines encompassthe three most significant (largest) subevents released in the first major
moment pulse, while the dashedlines encompassthe
three (plane 1) and two (plane 2) most significant
(largest) subeventsreleased in the secondmajor moment pulse. The time sequencefollowsthe labeling.
Labels a-f for plane i refer to the spike train in Figure
21. The squaresmark the hypocenter.
per or lower plate, the northwest-southeast trend in
locations
match
a trend
found in intermediate
depth events in Figure 24. Several focal mechanisms
have been determined by the Harvard group for the
larger aftershocks in the vicinity of the main shock
which range from normal to thrust fault mechanisms
bearing little resemblanceto the main shock. In general, we feel that the "aftershock" distribution for the
1977 event representsa variety of motionsin the surrounding regions including induced underthrusting
trenchward of the epicenter and downdip compression near the 1980 event.
The aftershocks
thus indi-
cate a readjustment or reloading of portions of the
slab following the 1977 event.
Lacking aftershock constraints, we must rely on
the P wave modeling to estimate the fault area for
the 1977 event. The detailed study of the sourcetime
function of the June 22, 1977, event described in the
previoussectionstronglysuggests
that the kupture
area of the 1977 event is very limited in space. If we
estimate the sourcearea to be approximately 80 km
focal mechanisms
for intermediate
the next section.
Discussion and Conclusion
for both
nodal planes are shown; the steeply dipping plane
($ = 197ø, $ = 79ø)on the left (plane 1) and the shallow
dipping plane ($= 14ø, $-11 ø)on the right (plane 2).
Subevents which are related to the first moment pulse
their
known
depth intraplate events with mb->5.0near the Louisville Ridge [Lay et al., 1988] are shown in Figure 24.
There are 15 downdip compressionalevents in this
region, most located downdip of the 1977 event.
Twelve of the 15 downdip compressionalevents, including the large April 13, 1980, event, followedthe
1977 earthquake. The large slip of the 1977 event
appearsto have had a strongeffecton the slab stress
regime over a large area, given the complexdistribution of events in the following weeks and years. We
suggestthat the occurrenceof the 1977 event had the
effect of loading the downdip region, returning that
region to a downdip compressionalstressstate. It is
interesting to note that Wyss and Habermann [1984]
found a 25-month period of quiescencein the updip
underthrusting zonebeforethe 1977 event, which indicates that the preparatory stagesof the 1977 event
influenced the interplate environment. The actual
rupture of the 1977 event may have affectedthe interplate zoneas well, where mostof the "aftershocks"
occurred;however, this relationship is not completely clear. A discussionof the possiblespatial and temporal correlations in the region will be discussedin
The five large earthquakes in the southern Tonga
region each occurredin responseto a local stressfield
which varies dramatically over distancesof only several hundred
kilometers
or less. The nature
of these
stress regimes may be related to the static stress
field which is governed by the large-scale deformation of the plate; however, it is also possiblethat dynamic stressesrelated to temporal variations in subduction processesinfluence the stress regime in the
adjacent intraplate zones. The term "dynamic" applies only in the sense that the varying regional
stressesassociatedwith the interplate contact zone
proceed on a time scale much shorter than that associated with "static" large-scaleprocesses.
In terms of static stresses,outer rise events can be
related to the bending of the subducting slab [see
Chapple and Forsyth, 1979], in which casecompressional outer rise events should occurat deeper depths
than tensional outer rise events. This depth difference is observed for the October 11, 1975, compressional outer rise event (d--30 km) and the
October 10, 1977, tensional outer rise event (d--10
km), but it is important to note that these events are
at different positions along the arc. Intermediate
depth events can be related to the unbending of the
subducting plate [see Kawakatsu, 1986], in which
casedowndip compressionalevents should be located
closer to the surface of the subducting plate than
downdip tensional events. Given the large sizesand
different depths of the June 22, 1977, downdip tensional event and the April 13, 1980, downdip compressional event, it is not possibleto resolve whether
this is the case. Kawakatsu [1986] suggestedthat a
double Benioff zone does exist in the Tonga region
and that the large downdip tensional event on
June 22, 1977, is related to the unbending of the
plate. However, the dip of the plate changes substantially in this region, so it is difficult to combine
events at different positionsalong the arc. We will
Christensen and Lay: Large Earthquakes in the Tonga Region
24,5
$
I
180s
I
d
292
BUI
131-•'•]? v,•
210
296 VV
Fig. 21. Comparisonbetweenobserved(top) and synthetic (bottom) waveformscorresponding
to the two-dimensional simultaneous inversion discussedin the text and shown in Figure 20
(plane 1). The amplitudesof the observedand synthetic waveformshave true relative scale;
however, the data for diffracted stations have been correctedfor diffracted amplitude effects.
The sourcespike train and moment rate function obtainedfor a station at a distanceof 90øand
an azimuth of 285ø (perpendicular to the fault) are shown in the lower right corner. The six
largest spikeslabeled a-f refer to the locationslabeled a-f in Figure 20 (plane 1). Epicentral
distance and station azimuth
are indicated
for each station.
now consider whether we can explain these events
better as a result of dynamic stressinteractions.
The temporal variation of the stressregime in the
outer rise due to the cyclic behavior of large underthrusting events has been discussedby Christensen
and Ruff [1983, 1988]. In this model, compressional
stress accumulates
in the outer rise oceanward
of a
strongly coupled"locked"interplate region due to the
continued subduction of the adjacent zones. Tensional outer rise events, on the other hand, occur follow-
ing large underthrusting eventsin strongly coupled
subduction zones, or in regions which are weakly
coupled,due to the direct concentrationof tensional
stressin the outer rise from slab pull. At the downdip edgeof strongly coupledzones,the oppositetemporal pattern of stressvariation would be observed,
with downdiptensional stressaccumulatingnear the
strongly coupledzoneprior to a large underthrusting
event, followed by a downdip compressional pulse
after the incremental motions of the underthrusting
event [see Astiz and Kanarnori, 1986; Dmowska and
Lovison, 1988; Dmowska et al., 1988; Spence, 1987;
Lay et al., 1988].
The temporal and spatial correlation of the five
large events to the coupled nature of the interplate
zone and the Louisville Ridge can best be interpreted
by referring to the map in Figure 2 and the crosssections in Figure 25. The spatial correlation of these
large events with the Louisville Ridge is apparent in
the three crosssections,suggestingthat the ridge directly influences the stressin each sourceregion. If
we assume that the Louisville Ridge is a relatively
buoyant structure, we can infer that its subduction
produces strong interplate coupling within the
triangular-shaped wedge north of the ridge intersection with the trench. In Figure 25a, crosssectionAA'
(Figure 2) traverses the arc along the great June 22,
1977, tensional event. If the interplate zone is
strongly coupled above this region as suggestedin
Figure 25, tensional stress may accumulate locally
below the coupled region. In other words, the presenceof the Louisville Ridge perturbs the intermediate depth stress regime by causing a stronger coupling on the regional interplate contact. This may be
related to the temporal quiescencein the interplate
region precedingthe June 22, 1977, event. Note that
the 1982 interplate thrust did not rupture this zone
and that the occurrence of the 1977 event may have
13,386
Christensen
andLay:LargeEarthquakes
in theTongaRegion
J
I
I .•7
I
,I,6/22/77
1979
.I I '"9xlO27dyne'cm
m
-
.E/
• •
, ',
•½
1985
,112/19/82
1985
0.9
22 ø_
1977
24 ø_
4//80
1977
",,",l
i
•
_
0
I
TONGA
1978
"k
1980
•A•.. s
1986 1
26 ø--
120
I
sec
1968
• I0/10/•
1982
1985
I :1975
./
1971 ' •'.:'
1967
: //1977
I
I
•i•. •.
I
i
•ap •iew o• [•e sou[•er•To•a-•or[•em
•er•adec re•o• s•o••
[•e lo•afio•
large e•e•s s•udied• de•ail (s•ars)a•d •eir a•era•e sourcefi•e •u•[io•s. T•e averagesource
•u•cfio•s •or eac• e•e•[ were cal•ula[ed ffo• [•e
e•[ s[afio•ss•ow• • P•ures 3, 6, •, 9, a• 14. •e
•o•e•[ l•dl•a•ed•oreac•e•e• is •e a•era•eo••e
•o•d•rac•ed
s•afio•s use&
I
I
179ø
177ø W
175ø
Fig. 24. Intraplateactivityin the southern
Tonga
regionin additionto the 1982interplatethrustevent
[fromLay et al., 1988]. The approximate
rupture
area of the thrust event is hachured,with the main
shockepicenterbeingindicatedby the circled•star.
Lowerhemisphere
equal-areafocalmechanisms
are
shownfortheintermediate
depthevents.Openstars
indicatedowndiptensionalevents,and solidsl•arsin-
I
dicatedowndip
compressional
events.Opensquares
indicatetensional
outerriseevents,
andsolidsquares
I
6/22/77
indicate compressionalouter rise events. Outer rise
•d:70
•
•cm/yr
km••
tensionaleventsthat occurmorethan 30 yearsafter
an underthrusting
eventareindicated
byopencircles.
--
22o•
Crossesindicateobliquemechanismswhich are nei-
ther simpletensionalnor compressional
type. The
dottedaftershock
zonesare for the largeouterrise
o
4/13/80
events in 1975 and 1977.
•
o
d:•O•m
•
'"• "' lO•1•z•
s
24ø--
temporarilyunloadedthe plate interface. The Louis-
12/19/82
•
ville Ridgemaybea weakregionin theplatewhich
disruptsthecoherence
ofthestressguideat intermediatedepths
in theslab,allowingdowndip
tensional
' • •10/10/77
26•
stressesto accumulatelocally in a slab that is other-
wisein compression
at intermediate
depths.Another
possibility that must be considered in that the
LouisvilleRidgemay actuallybe a "sinker"at inter28ø--
mediate
depths
withphasechanges
givingit higher
densityrelativeto normalsubducting
lithosphere.
In thiscaseit coulddirectlyplaya rolein inducing
tensionalstresses
near the 1977rupturezone. We
havenodirectevidence
tosupport
sucha model.
178ø
I
176ø W
174ø
I
172ø
I
The enhanced
interplatecouplingcausedby a
buoyantLouisvilleRidgecanexplaintheoccurrence
of theDecember
19, 1982,underthrusting
event,in
Fig. 23. Map view of the southernTonga-northern anarcthathasveryfewlargethrusts.Thiscoupling
Kermadec region. Aftershock zones for the three
alsohasthe predictedeffecton the outerrise as seen
shalloweventsonOctober
11,1975,October
10,1977, in Figure 25b where compressional
stressaccumuand December19, 1982,are shownby the hachured latesoceanward
of
the
strongly
coupled
region.The
areas. Aftershocks
for the April 13, 1980,eventand
April 13, 1980,downdipcompressional
eventis not
theJune22,1977,eventareshown
bythe'opencircles necessarilyinfluencedby the perturbed stressreandsolidcircles,
respectively.
Alsoshown
areepicen- gimeat shallowerdepths,sinceits mechanismis contral locations(stars)and focalmechanisms
for the five
sistentwith the normaldowndipcompressional
events.
stressfieldassociated
with the intermediate
depth
Christensenand Lay: LargeEarthquakesin the TongaRegion
200
A
I•0 km
0
I00
TRENCH
A'
O-
•97
I00-
200
-
300-
I
200
I
I00
!
I
km
I(•O
0
B
TRENCH
I
lB'
O-
,,
1975
1982
-..
I00-
km
1980
200-
300-
I
i
I
I
I
I
I
I
I
200
I00
km
0
C
13,387
outer rise event is shown in Figure 25c. In this case,
the Louisville Ridge is located oceanward of the
trench, and the interplate region is not affected by
the buoyancy and is apparently weakly coupled,allowing tensional stressto be transmitted directly to
the outer rise. The strong interplate coupling that
existed just north of the October 10, 1977, event prior
to rupture may have helped concentrate tensional
stressin the adjacent southern zone.
Both the static stress field associatedwith largescale slab deformation and the dynamic regional
stress field associated with interplate coupling can
explain various characteristics of subduction zone
seismicity. In this study we have analyzed five large
events in the Tonga region in closeproximity to the
Louisville Ridge. The large size of the events studied, particularly that of the June 22, 1977, downdip
tensional event, and their spatial correlation with
the subducted trace of the Louisville Ridge suggest
that factors in addition
to those from static deforma-
tion are influencing the stressstate in the intraplate
environment. We infer that the Louisville Ridge is a
buoyant structure, locally enhancing the interplate
coupling and thereby increasing the potential for
large thrust events in the region. This behavior is
similar to that of the Orozco Fracture Zone, site of
large thrust earthquakes in the Middle American
trench, and unlike that inferred for the Tehuantepec
Ridge, which appears to subduct aseismically. The
Louisville Ridge also affects seismicity at intermediate depths through a combination of its enhanced
coupling of the interplate zone and disruption of the
compressional regime characteristic of the Tonga
slab.
Acknowledgments.
We thank Christopher
Lynnes, Susan Schwartz, and Susan Beck for their
comments on the manuscript. Bruce Bolt and Paul
Roberts kindly provided copiesof the BKS and PAS
records, respectively. This research was supported
by National ScienceFoundation grant EAR-8451715
and a Shell Faculty Career Initiation grant to T. Lay.
TRENCH
1977
References
300
Fig. 25. Cross sections (no vertical exaggeration)
AA', BB', and CC' from Figure 2. The position of the
Louisville Ridge (L.R.) for each crosssection is shown
by the hachured area. The five large events are located on the appropriate cross section with the inplate stress axes and the nodal planes for the intraplate events indicated by the arrows and dashed
curves, respectively. Shear motion along the interplate zone is indicated for the 1982 underthrusting
event.
range in the Tonga trench. However the April 13,
1980, event did occur following the great June 22,
1977, event, which, because of its large size, may
have been able to reload the deeper regions. The oc-
Aggarwal, Y.P., M. Barazangi, and B. Isacks, P and S
traveltimes in the Tonga-Fiji region: A zone of
low velocity in the uppermost mantle behind the
Tonga island arc, J. Geophys.Res., 77, 6427-6434,
1972.
Astiz, L., and H. Kanamori, Interplate coupling and
temporal variation of mechanisms of intermediate-depth earthquakes in Chile, Bull. Seismol. Soc. Am., 7_•6,
1614-1622, 1986.
Barazangi, M., and B. Isacks, Lateral variations of
seismic wave attenuation in the upper mantle
above the inclined earthquake zone of the Tonga
is]and arc: Deep anomaly in the upper mantle, J_
Geophys.Res., 76, 8493-8516, 1971.
Barazangi, M., B. Isacks, and J. Oliver, Propagation
of seismic waves through and beneath the lithosphere that descendsunder the Tonga island arc,
J. Geophys.Res., 77, 952-958, 1972.
Billington, S., The morphology and tectonics of
the subducted lithosphere in the Tonga-FijiKermadec region from seismicity and focal mechanism solutions, Ph.D. thesis, 220 pp., Cornell
Univ., Ithaca, N.Y.,
1980.
in the April 13, 1980, sourceregion suggeststhat a
stressinteraction between the two regions doesexist.
Chapple, W.M., and D.W. Forsyth, Earthquakes and
bending of plates at trenches, J. Geophys. Res.,
84, 6729-6749, 1979.
A cross section near the October 10, 1977, tensional
Christensen, D.H., and L.J. Ruff, Outer rise earth-
currence of "aftershocks" of the June 22, 1977, event
Christensenand Lay: LargeEarthquakesin the TongaRegion
13,388
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D. H. Christensen, GeophysicalInstitute, University of Alaska, Fairbanks,AK 99775.
T. Lay, Departmentof GeologicalSciences,University of Michigan, Ann Arbor, M148109.
(Received November 23, 1987;
revised June 10, 1988;
acceptedJanuary 14, 1988.)