Are asperity patterns persistent? Implication from large earthquakes in Papua New Guinea

Are asperity patterns persistent? Implication from large

earthquakes in Papua New Guinea

Sun-Cheon Park1 and Jim Mori1

Received 3 May 2006; revised 4 October 2006; accepted 30 October 2006; published 8 March 2007.

[1] We studied the distribution of asperities for large recent earthquakes along the NewBritain trench, Papua New Guinea, to investigate if they are the same for repeatedruptures of the subduction boundary. We determined the slip distributions of twoearthquakes (Mw � 8) in 1971 using Pdiff waveforms, and an earthquake (Mw 7.9) in1995 using direct teleseismic P waves. Combining these findings with previous results fortwo earthquakes (Mw � 7.5) in 2000, we compared the source areas and asperitydistributions for this region of the New Britain Trench. Our results show that the locationsof the asperities for the individual earthquakes did not significantly overlap, althoughthe same portion of the subduction zone seems to have reruptured. This fact supportsthe idea that asperities are not persistent features when portions of the New Britainsubduction zone slip in large earthquakes.

Citation: Park, S.-C., and J. Mori (2007), Are asperity patterns persistent? Implication from large earthquakes in Papua New Guinea,

J. Geophys. Res., 112, B03303, doi:10.1029/2006JB004481.

1. Introduction

[2] Since asperity model was proposed in the 1980s, theproperties and distributions of asperities have been studiedfor numerous earthquakes. Lay and Kanamori [1980]reported the existence of relatively large, isolated high-stress zones on the fault plane and named them ‘‘asperities.’’During the rupture of large earthquakes, asperities havelarge slip and release large seismic moment. Ruff andKanamori [1983] suggested that the maximum earthquakesize is related to the asperity distribution on the fault.Accordingly, subduction zones with large earthquakes havevery large asperities while those with smaller earthquakeshave smaller scattered asperities. Beck and Ruff [1987]indicated that ruptures with multiple asperities trigger largeramounts of moment release in adjacent weaker regions thando single asperity earthquakes.[3] A poorly understood property of asperities is whether

the same asperity similarly ruptures when repeated earth-quakes reoccur on the same fault. Some studies suggestedthat an asperity is a fixed structure that ruptures repeatedly[Nagai et al., 2001; Igarashi et al., 2003; Yamanaka andKikuchi, 2004], while other studies found that the asperitieswill break in different patterns from one earthquake cycle toanother [Thatcher, 1990; Tanioka et al., 1996; Schwartz,1999; Hirose and Hirahara, 2002]. Studying the repeatedbehavior of asperity rupture is important for understandingthe properties of large earthquakes.

[4] Lay and Kanamori [1980] reported that large, shallowearthquakes in the New Britain and Solomon Island regiontend to occur in closely related pairs. They suggested thatasperities with homogenous size may be distributed in thisregion and that the failure of an asperity induces high,rapidly accumulated stress concentrations in adjacent areas.This accumulation can generate a second major event anddoublet behavior. This region of Papua New Guinea is oneof the most seismically active plate boundaries in the worldand over the last 30 years had a relatively large number ofM7–8 earthquakes. Therefore this region is one of the fewplaces where large earthquakes, that have apparently rerup-tured the same portion of the subduction zone, can bestudied using the global network of seismic stations. In thispaper, we use waveform data to compare the asperitydistributions of large subduction zone earthquakes that haveoccurred along the New Britain Trench and rerupturedportions of the plate boundary. We use the term ‘‘asperity’’for areas on the fault plane that have relatively large slip(greater than half of the maximum slip).[5] On 14 and 26 July 1971, two large earthquakes with

magnitudes of 7.9 (MS) occurred along the New BritainTrench, where the Solomon Sea plate subducts beneaththe South Bismarck and Pacific plates (Figure 1). The14 July 1971 event occurred at 0611:28 UTC betweenNew Ireland and Bougainville and was followed by after-shocks that extended in a northwesterly direction for about200 km. The 26 July 1971 event occurred at 0123:21 UTC,northwest of the 14 July 1971 event. Its aftershocksoccurred along the trench in a southwesterly direction forabout 250 km. Black and gray curved lines in Figure 2indicate the aftershock zones of the 14 and 26 July 1971events, respectively. These large earthquakes producedtsunamis with heights of more than 3 and 6 m on 14 and

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1Disaster Prevention Research Institute, Kyoto University, Kyoto,Japan.

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26 July, respectively (International Seismological Centrecatalogue).[6] A large earthquake with a magnitude of Mw 7.9

occurred close to the 14 July 1971 event on 16 August1995. This event produced a smaller tsunami with amaximum height of 55 cm observed at Rabaul, NewBritain. Two more large earthquakes occurred along theNew Britain Trench on 16 and 17 November 2000. Thefirst of these events (Mw 7.5) occurred south of NewIreland at 0742:16 UTC on 16 November (designated the16 November B event by Park and Mori [2007]), andthe second (Mw 7.4) occurred off the east coast of NewBritain region at 2101:56 UTC on 17 November (Figure 2).These earthquakes followed the great New Ireland event(Mw 8.2) that occurred at 0454:56 UTC on 16 November2000 (designated the 16 November A event by Park andMori [2007]) on a transform fault north of the NewBritain Trench.[7] These five earthquakes along the New Britain Trench

are all thought to be shallow angle thrust events on thesubduction interface. Approximate fault areas, inferred fromaftershock zones, are shown by rectangles in Figure 2where the 1995 and 2000 events appear to overlap muchof the area of the two 1971 earthquakes. The 1995 and2000 events might be regarded as repeated rupture of thesame fault area that slipped in 1971. To study the rupturedistribution on the New Britain subduction zone and to

determine the extent of the overlapping rupture areas, weobtained the slip distributions of the 1971 events andcompared them to those in 1995 and 2000.[8] For the five earthquakes studied here, Table 1 lists

the source parameters taken from the InternationalSeismological Centre (ISC) and National EarthquakeInformation Center (NEIC) of the U.S. Geological Survey(USGS) catalogues. Hereafter event identification shown inTable 1 will be used.

2. Slip Distributions of the 1971 Events

[9] For the 1971 events, P recordings of the World-WideStandardized Seismic Network (WWSSN) were used. Sincethe direct P waves recorded on the paper recording seismo-graphs were frequently saturated during such large earth-quakes, we used diffracted P (Pdiff) waveforms to determinethe slip distributions for the two large events in 1971. Theamplitudes of Pdiff waves, which diffract at the core-mantleboundary, are smaller and clearly recorded on the long-period vertical components of the WWSSN. Another ad-vantage of using Pdiff waves is a long time window of 3 to4 min between Pdiff waves and the next arriving PP andPKiKP phases [Ruff and Kanamori, 1983].[10] Microfilm copies of the paper recordings of Pdiff

waveform data recorded on WWSSN long-period seismom-eters for stations at distance ranges of about 100� and 140�

Figure 1. Tectonic setting of Papua New Guinea after Tregoning et al. [1998]. In the studied area,indicated with a dotted box, the Solomon Sea plate is subducting beneath the South Bismarck and Pacificplates.

B03303 PARK AND MORI: ARE ASPERITY PATTERNS PERSISTENT?

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were scanned. The uppermost and lowermost edges of therecorded traces were digitized separately. For both portionsa linear trend was removed and interpolated with a samplinginterval of 0.05 s. Then the two digitizations were averagedtogether to obtain waveforms used for the waveform inver-sions. Figure 3 shows an example where the hand-digitizedwaveform (Figure 3b) adequately resembles the recordedwaveform (Figure 3a). We used 11 stations for the wave-form inversions for both events, and Figure 4 shows thestation distributions. The azimuthal coverage of stations isnot optimal because there are no stations toward the south.[11] We used no absolute amplitudes or times of the

observed seismograms to avoid the complexity of calculat-ing Pdiff wave amplitudes and timing uncertainties of thepaper recordings. To obtain the amplitude of the slipdistributions, we assumed the total seismic momentsobtained by the surface wave analyses of Lay and Kanamori[1980]: 1.2 � 1028 and 1.8 � 1028 dyn cm for the 14 and26 July 1971 events, respectively.[12] To estimate the slip distributions, we used a multiple

time window inversion [Hartzell and Heaton, 1983]. Inthis method, the fault is divided into smaller subfaults

and Green’s functions are calculated for the propagationresponse from each subfault to each station. A programoriginally written by Langston and Helmberger [1975] wasused to calculate the teleseismic Green’s functions. Incalculating the Green’s functions for the Pdiff waves, weassumed that the takeoff angles are the same as for the directP wave that reaches a distance of 95�. The low-frequencycharacter of the waveforms was simulated using appropriatevalues of the attenuation operator, t*. The value of 2 for t*was used for the distance range of about 100� to 140�.These values are consistent with the amplitude decay of Pdiff

Figure 2. Locations of the fault planes and focal mechanisms obtained in our studies of the largeearthquakes in 1971, 1995, and 2000. Stars and rectangles indicate epicenters and fault planes,respectively, and different colors are used to distinguish the events: blue for the 14 July 1971, light bluefor the 26 July 1971, green for the 16 August 1995, red for the 16 November 2000 B, and pink for the17 November 2000 events. Focal mechanisms are obtained in our studies [this study; Park and Mori,2007]. Black line indicates the aftershock zone of the 14 July 1971 event, and gray line indicates that ofthe aftershock zone of the 26 July 1971 event from Lay and Kanamori [1980].

Table 1. Source Parameters of Studied Earthquakesa

EventOrigin Time,

UTC Latitude LongitudeDepth,km Magnitude

14 Jul 1971 0611.28 �5.52� 153.86� 43 MS 7.926 Jul 1971 0123:21 �4.93� 153.18� 43 MS 7.916 Aug 1995 1027:28 �5.80� 154.18� 37 Mw 7.616 Nov 2000 B 0742:16 �5.23� 153.10� 30 Mw 7.317 Nov 2000 2101:56 �5.49� 151.78� 37 Mw 7.4

aData from ISC for the 1971 events and USGS for other events.


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waves as a function of distance in Figure 2 of Ruff andKanamori [1983]. A least squares inversion was then usedto determine the slip on each subfault by minimizing the fitbetween calculated waveforms and data. A smoothingmatrix was added to the inversion to increase the stabilityof the solutions spatially and temporally. After testing, wefound values of the smoothing that gave a reasonable trade-off between the smoothness of the solution and a goodspatial resolution. By looking at plots of data varianceversus model variance, we choose intermediate values thatrepresent the trade-off between the variance and the size ofthe solution, where the solution begins to rapidly improvefor small changes of the solution length. We used asimplified version of the PREM model for the velocitystructure [Dziewonski and Anderson, 1981].

2.1. The 14 July 1971 Event

[13] For the two 1971 events, we assumed fault planesthat are consistent with shallow thrust events on the NewBritain subduction zone. The fault plane of the 14 July 1971

event was divided into 20 � 7 subfaults for the inversion.Figure 5 shows the slip distribution (Figure 5a) and wave-forms (Figure 5b). The fault plane is 200 km length and112 km wide and reaches a depth of 56 km from the surface.The hypocenter is located on the southeastern part of thefault and at a depth of 36 km. The large slips near thesurface can explain the generation of the observed tsunami.The maximum slip was obtained as 13.1 m. The strike, dipand rake angles were determined as 300�, 30� and 60�,respectively (Figure 5b). The source time function(Figure 5c) shows that the seismic moment was releasedover a period of 64 s. We found the best solution to be for arupture velocity of 2.8 km/s.[14] We compared our results to those of Lay and

Kanamori [1980] and Kikuchi and Fukao [1987] inTable 2. Focal mechanism and depth in our study seem tobe more similar to the body wave analyses of Kikuchi andFukao [1987] than the results of Lay and Kanamori [1980].Lay and Kanamori [1980] obtained results from surfacewaves recorded on WWSSN long-period seismographs.

Figure 3. (a) Example of a microfilm copy of paper record for NOR and (b) the digitized waveformused for the inversion.


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They addressed that the strike of 340� is consistent with thetrench geometry, which tends to strike slightly more north-ward near Bougainville Island.[15] According to the slip distribution, large slip occurred

around the hypocenter and rupture propagated northwest-ward. The deeper northwestern portion of the fault has asignificant slip that produces a second large peak in theobserved seismograms (see Figure 5b). Since syntheticseismograms do not completely fit the second peaks ofthe observed data, the moment for this area might beunderdetermined. When we assumed a larger fault planeto allow slip deeper, no significant slip was found deeper.This possible underestimation of moment might be due to achange of fault geometry. Schwartz et al. [1989], whoperformed both surface wave analysis and P wave inver-sions, reported the possibility of a change in the faultinggeometry during rupture from a northwest strike to a morenortherly strike. This change may be responsible for thedifference of strikes in the focal mechanisms of Lay andKanamori [1980] and those of Kikuchi and Fukao [1987]and this study.[16] In carrying out the inversion, we typically try a range

of sizes and geometries for the fault plane, and the slipdistribution of this event has some significant slip at shallowdepths with relatively large dip angle. We tested inversionassuming two different dip angles, 20� at 0–24 km depthand 30� at 24–56 km depth. The slip patterns are almost thesame as the fixed dip case as 30�. So the dip changes do notseem to significantly affect the slip distributions.

2.2. The 26 July 1971 Event

[17] The fault plane of the 26 July 1971 event wasdivided into 20 � 6 subfaults for setting up the inversion.

Figure 6 shows the slip distribution (Figure 6a) and wave-forms (Figure 6b). The fault plane has a length of 200 kmand width of 113.6 km and reaches a depth of 48 km fromthe surface. The hypocenter is located on the northeasternpart of the fault and at a depth of 26 km. The strike, dip andrake angles were determined as 240�, 25� and 70�, respec-tively (Figure 6b). The best solution for rupture velocitywas 3.0 km/s.[18] The slip distribution shows roughly one large area of

large slip. The largest slip seems to be delayed in time andseparated in distance from the hypocenter, and the maxi-mum slip was 14.9 m. This is also shown in the source timefunction (Figure 6c), which has a total duration of 56 s. Theobserved tsunami can be explained by the shallow slips ofthe fault. Comparing our results to other studies (Table 2),as was the case for the previous event focal mechanism ismore similar to the results of Kikuchi and Fukao [1987]compared to that of Lay and Kanamori [1980]. The focaldepth in this study, however, seems to be slightly shallowerthan those of other studies.

3. Slip Distributions of the 1995 and 2000Events

[19] To determine the slip distributions for the 16 August1995 event, we carried out waveform inversions usingdirect P wave data. Digital P waveforms for stations atdistance ranges of about 30� and 90� were obtained from theIncorporated Research Institutions for Seismology (IRIS)data center. Original data were transformed to displacement,using the station instrument files provided by IRIS, andhigh-passed filtered at 0.01 Hz. The distribution of used13 stations is shown in Figure 7. The azimuthal coverage is

Figure 4. Station distributions used for the waveform inversions of the two 1971 events. Solid trianglesindicate stations used for both events, and open triangles and open inversed triangles indicate stationsused for the 14 and 26 July 1971 events, respectively.


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Figure 5. Results for the 14 July 1971 event. (a) Slip distribution. Star indicates hypocenter. (b) Fit ofsynthetic (dotted) and observed (solid) waveforms and focal mechanism; strike 300, dip 30, rake 60.(c) Source time function.


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relatively good enough although there is only one stationtoward the southwest. The inversion procedure was thesame as for the 1971 earthquakes. The results for the2000 events are from Park and Mori [2007], which usedthe same methodology.

3.1. The 16 August 1995 Event

[20] We assumed a fault plane to be consistent with ashallow thrust event similar to the 1971 events. For theinversion, the fault plane was divided into 20 � 5 subfaults.Figure 8 shows the slip distribution (Figure 8a) and wave-forms (Figure 8b) for this event. The fault plane has alength of 200 km and width of 70 km and extends from17 to 57 km in depth. The hypocenter is located on south-eastern part of the fault and at a depth of 37 km. Thestrike and dip angles were determined as 310� and 35�,respectively, and the average value of rake for all subfaultswas obtained as 87� (Figure 8b). The source time function(Figure 8c) shows that the seismic moment was releasedover about 70 sec, which is a longer time than the 1971events. A rupture velocity of 2.4 km/s was obtained to bethe best solution.[21] Large slip areas appeared (Figure 8a) close to the

hypocenter and another 40 to 100 km to the northwest of thehypocenter. Large slips on the shallow part of the fault maybe responsible for the observed tsunami. Although theseshallower slips appear on the edge of the assumed faultplane, we use this slip distribution since we found noindication of shallower slip and it seems to be representativeof the source area. When we assumed larger fault plane,even though there was no slip on the shallower part, therake angles slightly changed. So the rake angles of thisevent may not be well constrained. However, the locationsof the areas of large slip do not change greatly. The largestslip is about 14.5 m. The total moment obtained is 9.1 �1027 dyn cm, which is equivalent to Mw 7.9.

[22] Schwartz [1999] carried out inversions for the sourcetime functions using empirical Green’s function analyses oflong-period surface waves and broadband body waves forthis event. She suggested that there was a gap in momentrelease between 20 and 60 km from the epicenter and rathercontinuous rupture from 60 to 120 km, which fills in agap between the two asperities of the 14 July 1971 event.This rupture distribution seems consistent with our resultsalthough the distances from the epicenter are larger. Thatmay be because she assumed a faster rupture velocity,3.0 km/s, compared to the value obtained in this study,2.4 km/s.

3.2. The 2000 Events

[23] We briefly describe the inversion results of the two2000 events obtained by Park and Mori [2007].[24] The 16 November 2000 B event has a 100 km length

fault plane and the hypocenter is located at almost the centerof the fault at a depth of 26 km. The strike and dipangles were 240� and 17�, respectively, and the averagevalue of rake for all subfaults was obtained as 70�. The slipdistribution has an area of large slip very close to thehypocenter and the largest slip is about 8.5 m. The totalmoment obtained is 2.5 � 1027 dyn cm, which is equivalentto Mw 7.5.[25] The fault length of the 17 November 2000 event is

100 km and the hypocenter is located to the southwest fromthe center of the fault and at a depth of 36 km. The strikeand dip angles were determined as 230� and 25�, respec-tively, and the average value of the rakes was 57�. The slipdistribution is fairly simple and has an area of large slipincluding the hypocenter. The largest slip is about 4 m. Thetotal moment obtained is 1.4 � 1027 dyn cm, which isequivalent to Mw 7.4.[26] The rake angles of the five earthquakes studied here

are in average consistent with the direction of subduction onthe New Britain Trench obtained by Tregoning et al. [1998].However, rake angles of the 16 August 1995 eventshow some large variations indicating a rather complicatedrupture.

4. Relocation of the 1971 Events

[27] For studying the possible overlap of the rupture areasof various earthquakes, we need to estimate the locationuncertainties in the identified areas of large slip. In the slipdistribution from waveform inversions, the resolution of thelocations of slip areas is at least the size of one subfault,which is about 10 km. Since the slip distributions aredetermined using relative times from the first arrival, theabsolute location of the slip areas depends on the location ofthe assumed hypocenter. To have better locations for com-paring all the slip areas, we relocated the hypocenters for the1971 earthquakes, relative to the 16 August 1995 and16 November 2000 B events.[28] Using P arrival time data at the same set of stations

from the ISC catalogue, we calculated the relative azimuthsand distances of the epicenter of the 14 July 1971 eventcompared to the 1995 event and that of the 26 July 1971event compared to the 16 November 2000 B event, using

Dt ¼ T0 � D cos 80 � 8ð ÞP ð1Þ

Table 2. Focal Mechanisms and Magnitudes of the Three

Earthquakes Obtained in This Study Compared to the Results of

Lay and Kanamori [1980] and Kikuchi and Fukao [1987] for the

1971 Events and Compared to the USGS Moment Tensor (MT)

and Harvard CMT (HRV) Solutions for the 1995 Eventa

Source (Strike, Dip, Rake) Depth, km Mw

14 Jul 1971This study (300, 30, 60) 36 -Lay and Kanamori[1980]

(345, 45, 62) 53 8.0

Kikuchi and Fukao[1987]

(295, 45, 80) 35 7.8

26 Jul 1971This study (240, 25, 70) 26 -Lay and Kanamori[1980]

(240, 40, 55) 43 8.1

Kikuchi and Fukao[1987]

(235, 30, 45) 40 7.8

16 Aug 1995This study (310, 35, 87) 37 7.9USGS (324, 44, 92),

(141, 46, 88)37 7.6

HRV (320, 48, 92),(136, 42, 87)

46 7.7

aRake angle of this study for the 16 August 1995 event is average valuefor all the subfaults.


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Figure 6. Results for the 26 July 1971 event. Captions are same as Figure 5, except focal mechanism;strike 240, dip 25, rake 70.


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where Dt is P arrival time difference, T0 is the time be-tween the two events, D is the distance between the twoevents, 80 is the azimuth of a line segment connecting theepicenters of the two events, 8 is the azimuth to the station,and P is the ray parameter, assuming the ray parametersfrom the two events are virtually the same [Wyss and Brune,1967]. The distance D between the two events and theazimuth 80 of the line segment connecting the epicenterscan be obtained by finding the best fits of a linear relation-ship between Dt and cos(80 � 8)P.[29] The distance and azimuth of the epicenter of the

14 July 1971 event relative to that of the 16 August 1995event were found to be 45.6 km and 309�, respectively.Also, the distance and azimuth of the epicenter of the 26 July1971 event compared to the 16 November 2000 B eventwere 32.6 km and 42�, respectively. The relocated epi-centers of both 1971 events moved several kilometers tothe southeast. Figure 9 plots the relocated epicenters alongwith the original catalogue epicenters.

5. Resolution of Slip Distributions

[30] One of the main purposes of this study is to comparethe slip distributions of the large earthquakes occurring inthe same region, and particularly to see if the areas of largeslip overlap. In order to do this, we need spatial resolutionon the order of the size of the asperities, which havedimensions of about 30 to 50 km. We make some qualitativeassessments about our inversion results and carry out somenumerical tests, to show that we have sufficient spatialresolutions to justify the discussion in section 6 about thedegree of overlap that exists for the asperities.[31] One aspect of the waveform inversion that is quite

well resolved is determining whether there is a large amountof slip close to the hypocenter. If there is large slip close to

the hypocenter, the P waves have relatively impulsivewaveforms with large amplitudes at the beginning. Incontrast, for earthquakes that have a large asperity locatedmore distant from the hypocenter, the waveforms are morecomplicated and have large amplitudes later in the wave-forms. Of course, this is somewhat masked by the depthphases in the recorded data, but the inversion results areable to resolve this difference. For the 14 July 1971,16 August 1995, and 16 November 2000 B earthquakes,we can fairly confidently conclude that there is an area oflarge slip near the hypocenter. For these events, the loca-tions of the asperities then depend on the accuracy of thehypocenter. As discussed in section 4 we carried out relativerelocations and showed that the hypocenters have betteraccuracy than the 30 to 50 km size of the asperities.[32] For the 1971 events we investigated the effect of

varying rake angle. Figure 10 shows the slip distributionsfor the fixed rake angles (Figures 10a and 10c) and for thecases allowing rake to be free between 0 and 90� (Figures 10band 10d). Slip distributions of the 14 July 1971 event(Figures 10a and 10b) both show significant slips surround-ing area to the hypocenter. This may be consistent with thefirst big swing in waveforms (Figure 5b) showing no cleardirectivity. Farther areas from the hypocenter have smallerslips for both cases. The main area of large slip of the26 July 1971 event is located about 30–70 km away fromthe hypocenter for both cases of fixing (Figure 10c) andallowing free rakes (Figure 10d). The directivity caused bythis slip distribution can be recognizable from the shorterpulse duration of waveforms (Figure 6b) to the southwest-ern stations (PRE, NAI). Since slip distributions for bothcases of fixed rakes and allowing free rakes between 0 and90� show similar patterns, this does not significantly affectthe locations of the area of large slip.[33] Since the asperities of the 26 July 1971 and

16 November 2000 B events are closely located, the reso-lution for this area is more critical. To evaluate the accuracyof the slip locations, we performed forward modeling tests ofthe 26 July 1971 event by shifting the location of theasperity. As shown in Figure 11, we calculated waveformsfor asperities 10, 20, and 40 km to the southwest of theoriginal location, and 10, 20, and 40 km to the northeast,which brings the asperity to the position of the large slip forthe 16 November 2000 B event. Figure 11a shows calculatedwaveforms (dotted lines) for the case of 20 km shift to thesouthwest, compared with the observed data (solid lines).Waveforms for shifts to the southwest show a significanttiming difference for the large pulse for stations to the northbecause of longer times needed for the rupture to propagateto the southwest. When we shift the asperities to thenortheast (Figure 11b), the synthetics for station to the northhave noticeably narrower widths for the first up and downswings. The waveform example shown in Figure 11b is foran asperity location of the 26 July 1971 event that overlapsthe asperity of the 16 November 2000 B event. Thesecalculations show that there are noticeable differences inwaveforms corresponding to asperity shifts of 20 to 30 km,which would be resolvable by the inversion process.Figure 11c shows that variance between synthetics andobserved data, as a function of asperity location. Assumingsystematic errors, such as due to regional structure, are thesame for the 26 July 1971 and 16 November 2000 B events,

Figure 7. Station distribution used for the waveforminversion of the 16 August 1995 event.


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Figure 8. Results for the 16 August 1995 event. (a) Slip distribution. Star indicates hypocenter. (b) Fitof synthetic (dotted) and observed (solid) waveforms. Scales are the same for all stations. Focalmechanism; strike 310, dip 35, average rake 87. (c) Source time function.


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Figure 11c indicates that the spatial resolution is about10 to 20 km, when comparing the two earthquakes.[34] For the 14 July 1971 earthquake there is another area

of large slip located away from the hypocenter. Also, the17 November 2000 event has an area of large slip separatedslightly from the hypocenter. We did not carry out tests toevaluate the spatial resolution for these asperities, becausetheir exact locations are not important for the discussions insection 6.

6. Discussion

[35] Figure 12 shows the horizontal projections of therupture areas of the five earthquakes in 1971, 1995, and2000. Contours indicate rupture areas with more than 0.5 mslip, and the same colors used in Figure 2 distinguish theevents. The rupture areas of these large earthquakes aregenerally consistent with normal scaling relations of sourceareas [i.e., Kanamori and Anderson, 1975] and the after-shock distributions. There are large overlaps in these sourceareas, which are interpreted to mean that some portions ofthe subduction zone boundary have reruptured. The areaswith more than half of the largest slip are filled andidentified as asperities, similar to Yamanaka and Kikuchi[2004].[36] First, we examine the results of the two 1971 events

(blue and light blue slip distributions in Figure 12), and wesee that both earthquakes have two large asperities that arewidely separated. There does not seem to be any largeoverlap of the rupture areas or asperities for these twoearthquakes. Next, we compare the locations of asperitiesof the 14 July 1971 (blue) and 16 August 1995 (green)events, and see that the asperities do not overlap, even

though the source area of the 1995 event is almost com-pletely within the source area of the 1971 event. The 26 July1971 event (light blue area) has one very large asperity anda smaller one to the northeast, while the 16 November 2000B event (red) has one asperity. Although the entire sourcearea of the 16 November 2000 B event is within the sourcearea of the 26 July 1971 event, the asperities of the twoearthquakes do not coincide. Finally, the source area andasperity of the 17 November 2000 event (pink area) areshown to be outside of the source area of the 26 July 1971event. There does not seem to be any overlap of the entireslip areas for these two events, although the aftershockdistributions do overlap.[37] Assuming that the horizontal projections of the slip

areas can be used to identify the areas of slip on thesubduction zone, in other words, assuming that all theevents are occurring on the same fault plane, we can usethese results to identify overlapping rupture areas betweenthe various large earthquakes. From the inferred slip dis-tributions of the five earthquakes, we see that the overallsource areas do overlap; however, the areas of large slip(asperities) do not coincide.[38] There is some question about whether the closely

located asperities for the 26 July 1971 and the 16 November2000 B overlap. However, even if there is some spatialoverlap, the sizes of the asperities are quite different and itseems difficult to interpret them as representing the samephysical structure.[39] Among studies of asperities, Nagai et al. [2001],

Igarashi et al. [2003], and Yamanaka and Kikuchi [2004]suggested that the same asperities rupture repeatedly whenan earthquake repeatedly occurs on a specific fault. Nagai etal. [2001] showed that the 1994 Sanriku-oki, Japan, earth-

Figure 9. Locations of the relocated epicenters of the 1971 events. Solid stars indicate the relocatedepicenters and open stars the epicenters from ISC (for the 1971 events) and IRIS (for the 1995 and 2000events) catalogues.


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Figure

10


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quake ruptured one of the same asperities of the 1968Tokachi-oki, Japan, earthquake. They proposed that thereexist characteristic (similar and regularly occurring) asper-ities, where large fault slip occurs and rupture of anindividual asperity is usually manifested as a M7 or smallerearthquake. However, sometimes the rupture of severalasperities is synchronized producing a M8 earthquake.The sizes (M7 to 8) and reoccurrence intervals (repeatedruptures within about 30 years) of the earthquakes in theTohoku area are very similar to the earthquakes we studiedin the New Britain region, although our conclusions arevery different. Igarashi et al. [2003] analyzed small ‘‘sim-ilar earthquakes’’ in northeastern Japan along the JapanTrench. These similar earthquakes are supposed to becaused by repeating slips of small asperities surrounding alarge asperity. Yamanaka and Kikuchi [2004] comparedasperities of large earthquakes with magnitude greaterthan 7 along the Japan Trench. According to their conclu-sions, asperities are very strongly coupled but occupy only afraction of the plate contact area, and the locations ofasperities are fixed in space away from the hypocenter.All of these studies suggest that asperities are a materialcharacteristic of the rupture zone, and thus these areasalways have large slip when an earthquake occurs.[40] On the other hand, Thatcher [1990], Tanioka et al.

[1996], Schwartz [1999], and Hirose and Hirahara [2002]propose different properties about the behavior of asperityrupture. Thatcher [1990] investigated recurrence character-istics and spatial distribution of great circum-Pacific earth-quakes, whose moment magnitudes (Mw) are greaterthan 7.7. He concluded that individual great earthquakesdiffer significantly from cycle to cycle or rerupture takesplace in a sequence of two or more smaller events. Taniokaet al. [1996] performed the centroid moment tensor (CMT)inversion using long-period surface waves and joint inver-sion of geodetic and tsunami data of the 1968 Tokachi-oki,Japan, and 1994 Sanriku-oki, Japan, earthquakes, which arethe same events analyzed by Nagai et al. [2001]. They,however, concluded that dominant asperities ruptured by thetwo earthquakes are different. Schwartz [1999] summarizedthat recurrent fault slip of large circum-Pacific plate bound-ary earthquakes do not support characteristic slip modelsand that dynamic considerations are important. Hirose andHirahara [2002] performed three-dimensional numericalsimulation for rupture process of a large asperity on asubduction zone fault. Their results showed that in the caseof an asperity with comparable dimension to the width, thewhole area of the asperity can be broken at once and theasperity has a regular periodic seismic cycle. However, inthe case of much larger lateral dimensions, the asperity doesnot break its whole area at once and its slip behaviorbecomes complex within a certain range. On a smallerscale, the repeating 1966 and 2004 Parkfield earthquakes

appeared to have quite different slip distributions [Bakun etal., 2005].[41] For the five large earthquakes along the New Britain

subduction zone with apparent rerupture of some parts ofthe same fault plane after 25 to 30 years, the asperities donot coincide. In the asperity distributions obtained in thisstudy (Figure 12), there is no clear relation between thehypocenter and the large asperities. For some earthquakesthe hypocenters are located in the area of asperity, and forothers the hypocenters are far from asperity. This is differentfrom the results of Yamanaka and Kikuchi [2004]. Also thesizes of asperities vary among the earthquakes. These factssupport the idea that asperities have a noncharacteristicpattern for earthquakes on the New Britain subduction zone.Instead of a model where similar asperities always appear inrepeated earthquakes, it may be the case that the asperitiesare different from one earthquake to another, but the long-term rate of slip averages to be a constant value for manycycles over a large area.[42] Figure 13 is a cartoon describing the idea of non-

characteristic behavior of asperity rupture compared tocharacteristic behavior. In a characteristic asperity model,the same location of an asperity occurs when a segmentalong a subduction zone repeatedly ruptures. An earthquakebreaks one or more asperities depending on the size of thetotal rupture area. The areas surrounding the asperities mayhave small amounts of slip during earthquakes and aseismicslip between earthquakes. The sum of the seismic slip overmany earthquakes will show strong concentrations at theasperity locations. In a contrasting model, different asperitylocations occur when a segment ruptures in repeated earth-quakes and there is a noncharacteristic pattern for theasperity distribution. In this model there is no need foraseismic slip and the total slip over the whole area becomesthe same over many earthquake occurrences.[43] One consequence of the characteristic asperity model

is that the slip on the asperity controls the recurrence rate oflarge earthquakes. According to Tregoning et al. [1998], thesubduction rate along the New Britain Trench in this area isabout 15 cm/yr. Considering maximum slip amounts of the1971 and 1995 events obtained in this study (about 13� 15m),the recurrence time of a large earthquake with magnitudeabout 8 in this region would be about 100 years. However,we have shown that source areas can overlap and includeregions of large and small slip, so that a simple calculation ofrecurrence intervals is difficult. On average, the slip rate for aspecific portion of the fault should equal the plate rate(assuming 100% coupling); however, the accumulated slipwill be the sum of a number of earthquakes with large andsmall amounts of slip and large earthquakes can occur moreoften than in the characteristic asperity model.[44] Song and Simons [2003] and Wells et al. [2003]

reported that locations of asperities of large earthquakes in

Figure 10. Slip distributions of the 1971 events for the cases of fixing and allowing free rake angles. (a) Slip distributionof the 14 July 1971 event when the rake angle is fixed as 60�. (b) Slip distribution of the 14 July 1971 event when the rakeangles are allowed to be free between 0 and 90�. (c) Slip distribution of the 26 July 1971 event when the rake angle is fixedas 70�. (d) Slip distribution of the 26 July 1971 event when the rake angles are allowed to be free between 0 and 90�. Ineither case, slip distributions show similar patterns.


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Figure 11


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subduction zones are related to those of negative gravityanomaly and forearc basins. However, there do not seem tobe well developed forearc basins along the New BritainTrench, and we could not see any clear correlations betweenthe gravity map [Sandwell and Smith, 1997] and ourasperity locations.[45] One interesting observation is that the three larger

earthquakes, with magnitudes of about 8 (two events in1971 and the 1995 event), have two or more large asperities,while the smaller ones with magnitudes of about 7.5 (twoevents in 2000) have rather simple slip distributions withonly one large asperity. This may be related to the interpre-tation of Nagai et al. [2001], where the rupture of multipleasperities produces larger earthquakes than the rupture of asingle asperity.

7. Summary

[46] We carried out Pdiff waveform inversions to obtainslip distributions for two great (M � 8) earthquakes alongthe New Britain Trench in 1971. We also performed tele-seismic P wave inversions for a large event (M7.9) in 1995.We combined these results with the slip distributions fortwo large earthquakes (M � 7.5) in 2000 from a previousstudy to compare the slip zones and asperity distributions.The rupture areas of these events have large areas of overlapwithin the same region of the subduction zone, indicatingrepeated earthquakes on the subduction zone boundary. Forthese earthquakes, however, the asperities do not occur inthe same locations. These observations support the idea thatasperities have noncharacteristic patterns when earthquakesoccur in the same region of the New Britain subduction

Figure 11. Numerical test for the slip locations of the 26 July 1971 event compared with that of the 16 November 2000 Bevent. (a) Map showing shifted slip locations by 10, 20, and 40 km to the southwest. Light and dark gray asperities are ourpreferred model for the 26 July 1971 and 16 November 2000 B events, respectively, and open ones show shifted locationsof asperities of the 26 July 1971 event. Waveform examples are for the case of 20 km shift. (b) Map showing shifted sliplocations by 10, 20, and 40 km on the fault plane to the northeast. Waveform examples are for the case of 20 km shift to thenortheast. (c) Variance as a function of asperity location.

Figure 12. Comparison of the rupture areas. Contours indicate rupture areas with more than 0.5 m slipand the areas with more than half of the largest slip are filled, indicating asperities. Different colors areused to distinguish the events (see Figure 2). Stars indicate epicenters.


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zone, and we conclude that asperities are not persistentfeatures.

[47] Acknowledgments. We thank Paul Tregoning and two anony-mous reviewers for their invaluable comments and suggestions. We usedWorld-Wide Standardized Seismic Network waveform data archived at theEarthquake Research Institute, Tokyo University, and digital waveform datafrom the Incorporated Research Institutions for Seismology data center.Some figures in this paper were made using the General Mapping Tool(GMT) software.

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��J. Mori and S.-C. Park, Disaster Prevention Research Institute, Kyoto

University, Gokasho, Uji, Kyoto, 611-0011, Japan. ([email protected])

Figure 13. Cartoon of characteristic and noncharacteristic asperity models. In the characteristic asperitymodel, the same asperities rupture repeatedly, although a particular earthquake may break only one orseveral asperities depending on the total rupture area. Summing the slip distributions over manyearthquakes, there will be a large amount of seismic slip for the asperity locations. In a noncharacteristicmodel the asperity locations are different from earthquake to earthquake. Over many earthquakes, thesum of the slip averages out to the same value over the total fault area.


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Are asperity patterns persistent? Implication from large earthquakes in Papua New Guinea

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