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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B03303, doi:10.1029/2006JB004481, 2007
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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 New
Britain trench, Papua New Guinea, to investigate if they are the same for repeated ruptures of the subduction boundary. We determined the slip distributions of two earthquakes (Mw 8) in 1971 using Pdiff waveforms, and an earthquake (Mw 7.9) in 1995 using direct teleseismic P waves. Combining these findings with previous results for two earthquakes (Mw 7.5) in 2000, we compared the source areas and asperity distributions for this region of the New Britain Trench. Our results show that the locations of the asperities for the individual earthquakes did not significantly overlap, although the same portion of the subduction zone seems to have reruptured. This fact supports the idea that asperities are not persistent features when portions of the New Britain subduction 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, the properties and distributions of asperities have been studied for numerous earthquakes. Lay and Kanamori [1980] reported the existence of relatively large, isolated highstress zones on the fault plane and named them ‘‘asperities.’’ During the rupture of large earthquakes, asperities have large slip and release large seismic moment. Ruff and Kanamori [1983] suggested that the maximum earthquake size is related to the asperity distribution on the fault. Accordingly, subduction zones with large earthquakes have very large asperities while those with smaller earthquakes have smaller scattered asperities. Beck and Ruff [1987] indicated that ruptures with multiple asperities trigger larger amounts of moment release in adjacent weaker regions than do single asperity earthquakes. [3] A poorly understood property of asperities is whether the same asperity similarly ruptures when repeated earthquakes reoccur on the same fault. Some studies suggested that an asperity is a fixed structure that ruptures repeatedly [Nagai et al., 2001; Igarashi et al., 2003; Yamanaka and Kikuchi, 2004], while other studies found that the asperities will break in different patterns from one earthquake cycle to another [Thatcher, 1990; Tanioka et al., 1996; Schwartz, 1999; Hirose and Hirahara, 2002]. Studying the repeated behavior of asperity rupture is important for understanding the properties of large earthquakes.
1 Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan.
Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JB004481$09.00
[4] Lay and Kanamori [1980] reported that large, shallow earthquakes in the New Britain and Solomon Island region tend to occur in closely related pairs. They suggested that asperities with homogenous size may be distributed in this region and that the failure of an asperity induces high, rapidly accumulated stress concentrations in adjacent areas. This accumulation can generate a second major event and doublet behavior. This region of Papua New Guinea is one of the most seismically active plate boundaries in the world and over the last 30 years had a relatively large number of M7 – 8 earthquakes. Therefore this region is one of the few places where large earthquakes, that have apparently reruptured the same portion of the subduction zone, can be studied using the global network of seismic stations. In this paper, we use waveform data to compare the asperity distributions of large subduction zone earthquakes that have occurred along the New Britain Trench and reruptured portions 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 Britain Trench, where the Solomon Sea plate subducts beneath the South Bismarck and Pacific plates (Figure 1). The 14 July 1971 event occurred at 0611:28 UTC between New Ireland and Bougainville and was followed by aftershocks that extended in a northwesterly direction for about 200 km. The 26 July 1971 event occurred at 0123:21 UTC, northwest of the 14 July 1971 event. Its aftershocks occurred along the trench in a southwesterly direction for about 250 km. Black and gray curved lines in Figure 2 indicate the aftershock zones of the 14 and 26 July 1971 events, respectively. These large earthquakes produced tsunamis with heights of more than 3 and 6 m on 14 and
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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 Pacific plates.
26 July, respectively (International Seismological Centre catalogue). [6] A large earthquake with a magnitude of Mw 7.9 occurred close to the 14 July 1971 event on 16 August 1995. This event produced a smaller tsunami with a maximum height of 55 cm observed at Rabaul, New Britain. Two more large earthquakes occurred along the New Britain Trench on 16 and 17 November 2000. The first of these events (Mw 7.5) occurred south of New Ireland at 0742:16 UTC on 16 November (designated the 16 November B event by Park and Mori [2007]), and the second (Mw 7.4) occurred off the east coast of New Britain 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 November 2000 (designated the 16 November A event by Park and Mori [2007]) on a transform fault north of the New Britain Trench. [7] These five earthquakes along the New Britain Trench are all thought to be shallow angle thrust events on the subduction interface. Approximate fault areas, inferred from aftershock zones, are shown by rectangles in Figure 2 where the 1995 and 2000 events appear to overlap much of the area of the two 1971 earthquakes. The 1995 and 2000 events might be regarded as repeated rupture of the same fault area that slipped in 1971. To study the rupture distribution on the New Britain subduction zone and to
determine the extent of the overlapping rupture areas, we obtained the slip distributions of the 1971 events and compared them to those in 1995 and 2000. [8] For the five earthquakes studied here, Table 1 lists the source parameters taken from the International Seismological Centre (ISC) and National Earthquake Information Center (NEIC) of the U.S. Geological Survey (USGS) catalogues. Hereafter event identification shown in Table 1 will be used.
2. Slip Distributions of the 1971 Events [9] For the 1971 events, P recordings of the World-Wide Standardized Seismic Network (WWSSN) were used. Since the direct P waves recorded on the paper recording seismographs were frequently saturated during such large earthquakes, we used diffracted P (Pdiff) waveforms to determine the slip distributions for the two large events in 1971. The amplitudes of Pdiff waves, which diffract at the core-mantle boundary, are smaller and clearly recorded on the longperiod vertical components of the WWSSN. Another advantage of using Pdiff waves is a long time window of 3 to 4 min between Pdiff waves and the next arriving PP and PKiKP phases [Ruff and Kanamori, 1983]. [10] Microfilm copies of the paper recordings of Pdiff waveform data recorded on WWSSN long-period seismometers for stations at distance ranges of about 100° and 140°
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Figure 2. Locations of the fault planes and focal mechanisms obtained in our studies of the large earthquakes 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 blue for the 26 July 1971, green for the 16 August 1995, red for the 16 November 2000 B, and pink for the 17 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 of the aftershock zone of the 26 July 1971 event from Lay and Kanamori [1980].
were scanned. The uppermost and lowermost edges of the recorded traces were digitized separately. For both portions a linear trend was removed and interpolated with a sampling interval of 0.05 s. Then the two digitizations were averaged together to obtain waveforms used for the waveform inversions. Figure 3 shows an example where the hand-digitized waveform (Figure 3b) adequately resembles the recorded waveform (Figure 3a). We used 11 stations for the waveform inversions for both events, and Figure 4 shows the station distributions. The azimuthal coverage of stations is not 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 calculating Pdiff wave amplitudes and timing uncertainties of the paper recordings. To obtain the amplitude of the slip distributions, we assumed the total seismic moments obtained by the surface wave analyses of Lay and Kanamori [1980]: 1.2 1028 and 1.8 1028 dyn cm for the 14 and 26 July 1971 events, respectively. [12] To estimate the slip distributions, we used a multiple time window inversion [Hartzell and Heaton, 1983]. In this method, the fault is divided into smaller subfaults
and Green’s functions are calculated for the propagation response from each subfault to each station. A program originally written by Langston and Helmberger [1975] was used to calculate the teleseismic Green’s functions. In calculating the Green’s functions for the Pdiff waves, we assumed that the takeoff angles are the same as for the direct P wave that reaches a distance of 95°. The low-frequency character of the waveforms was simulated using appropriate values 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
Table 1. Source Parameters of Studied Earthquakesa Event 14 26 16 16 17
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Jul 1971 Jul 1971 Aug 1995 Nov 2000 B Nov 2000
a
Origin Time, Depth, UTC Latitude Longitude km Magnitude 0611.28 0123:21 1027:28 0742:16 2101:56
5.52° 4.93° 5.80° 5.23° 5.49°
153.86° 153.18° 154.18° 153.10° 151.78°
43 43 37 30 37
MS 7.9 MS 7.9 Mw 7.6 Mw 7.3 Mw 7.4
Data from ISC for the 1971 events and USGS for other events.
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Figure 3. (a) Example of a microfilm copy of paper record for NOR and (b) the digitized waveform used for the inversion.
waves as a function of distance in Figure 2 of Ruff and Kanamori [1983]. A least squares inversion was then used to determine the slip on each subfault by minimizing the fit between calculated waveforms and data. A smoothing matrix was added to the inversion to increase the stability of the solutions spatially and temporally. After testing, we found values of the smoothing that gave a reasonable tradeoff between the smoothness of the solution and a good spatial resolution. By looking at plots of data variance versus model variance, we choose intermediate values that represent the trade-off between the variance and the size of the solution, where the solution begins to rapidly improve for small changes of the solution length. We used a simplified version of the PREM model for the velocity structure [Dziewonski and Anderson, 1981]. 2.1. The 14 July 1971 Event [13] For the two 1971 events, we assumed fault planes that are consistent with shallow thrust events on the New Britain 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 waveforms (Figure 5b). The fault plane is 200 km length and 112 km wide and reaches a depth of 56 km from the surface. The hypocenter is located on the southeastern part of the fault and at a depth of 36 km. The large slips near the surface can explain the generation of the observed tsunami. The maximum slip was obtained as 13.1 m. The strike, dip and rake angles were determined as 300°, 30° and 60°, respectively (Figure 5b). The source time function (Figure 5c) shows that the seismic moment was released over a period of 64 s. We found the best solution to be for a rupture velocity of 2.8 km/s. [14] We compared our results to those of Lay and Kanamori [1980] and Kikuchi and Fukao [1987] in Table 2. Focal mechanism and depth in our study seem to be more similar to the body wave analyses of Kikuchi and Fukao [1987] than the results of Lay and Kanamori [1980]. Lay and Kanamori [1980] obtained results from surface waves recorded on WWSSN long-period seismographs.
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Figure 4. Station distributions used for the waveform inversions of the two 1971 events. Solid triangles indicate stations used for both events, and open triangles and open inversed triangles indicate stations used for the 14 and 26 July 1971 events, respectively.
They addressed that the strike of 340° is consistent with the trench geometry, which tends to strike slightly more northward near Bougainville Island. [15] According to the slip distribution, large slip occurred around the hypocenter and rupture propagated northwestward. The deeper northwestern portion of the fault has a significant slip that produces a second large peak in the observed seismograms (see Figure 5b). Since synthetic seismograms do not completely fit the second peaks of the observed data, the moment for this area might be underdetermined. When we assumed a larger fault plane to allow slip deeper, no significant slip was found deeper. This possible underestimation of moment might be due to a change of fault geometry. Schwartz et al. [1989], who performed both surface wave analysis and P wave inversions, reported the possibility of a change in the faulting geometry during rupture from a northwest strike to a more northerly strike. This change may be responsible for the difference of strikes in the focal mechanisms of Lay and Kanamori [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 slip distribution of this event has some significant slip at shallow depths with relatively large dip angle. We tested inversion assuming two different dip angles, 20° at 0 – 24 km depth and 30° at 24–56 km depth. The slip patterns are almost the same as the fixed dip case as 30°. So the dip changes do not seem to significantly affect the slip distributions. 2.2. The 26 July 1971 Event [17] The fault plane of the 26 July 1971 event was divided into 20 6 subfaults for setting up the inversion.
Figure 6 shows the slip distribution (Figure 6a) and waveforms (Figure 6b). The fault plane has a length of 200 km and width of 113.6 km and reaches a depth of 48 km from the surface. The hypocenter is located on the northeastern part of the fault and at a depth of 26 km. The strike, dip and rake angles were determined as 240°, 25° and 70°, respectively (Figure 6b). The best solution for rupture velocity was 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 and separated in distance from the hypocenter, and the maximum slip was 14.9 m. This is also shown in the source time function (Figure 6c), which has a total duration of 56 s. The observed tsunami can be explained by the shallow slips of the fault. Comparing our results to other studies (Table 2), as was the case for the previous event focal mechanism is more similar to the results of Kikuchi and Fukao [1987] compared to that of Lay and Kanamori [1980]. The focal depth in this study, however, seems to be slightly shallower than those of other studies.
3. Slip Distributions of the 1995 and 2000 Events [19] To determine the slip distributions for the 16 August 1995 event, we carried out waveform inversions using direct P wave data. Digital P waveforms for stations at distance ranges of about 30° and 90° were obtained from the Incorporated Research Institutions for Seismology (IRIS) data center. Original data were transformed to displacement, using the station instrument files provided by IRIS, and high-passed filtered at 0.01 Hz. The distribution of used 13 stations is shown in Figure 7. The azimuthal coverage is
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Figure 5. Results for the 14 July 1971 event. (a) Slip distribution. Star indicates hypocenter. (b) Fit of synthetic (dotted) and observed (solid) waveforms and focal mechanism; strike 300, dip 30, rake 60. (c) Source time function.
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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 This study Lay and Kanamori [1980] Kikuchi and Fukao [1987] This study Lay and Kanamori [1980] Kikuchi and Fukao [1987] This study USGS HRV
(Strike, Dip, Rake)
Depth, km
Mw
14 Jul 1971 (300, 30, 60) (345, 45, 62)
36 53
8.0
(295, 45, 80)
35
7.8
26 Jul 1971 (240, 25, 70) (240, 40, 55)
26 43
8.1
(235, 30, 45)
40
7.8
37 37
7.9 7.6
46
7.7
16 Aug 1995 (310, 35, 87) (324, 44, 92), (141, 46, 88) (320, 48, 92), (136, 42, 87)
a Rake angle of this study for the 16 August 1995 event is average value for all the subfaults.
relatively good enough although there is only one station toward the southwest. The inversion procedure was the same as for the 1971 earthquakes. The results for the 2000 events are from Park and Mori [2007], which used the same methodology. 3.1. The 16 August 1995 Event [20] We assumed a fault plane to be consistent with a shallow thrust event similar to the 1971 events. For the inversion, the fault plane was divided into 20 5 subfaults. Figure 8 shows the slip distribution (Figure 8a) and waveforms (Figure 8b) for this event. The fault plane has a length of 200 km and width of 70 km and extends from 17 to 57 km in depth. The hypocenter is located on southeastern part of the fault and at a depth of 37 km. The strike and dip angles were determined as 310° and 35°, respectively, and the average value of rake for all subfaults was obtained as 87° (Figure 8b). The source time function (Figure 8c) shows that the seismic moment was released over about 70 sec, which is a longer time than the 1971 events. A rupture velocity of 2.4 km/s was obtained to be the best solution. [21] Large slip areas appeared (Figure 8a) close to the hypocenter and another 40 to 100 km to the northwest of the hypocenter. Large slips on the shallow part of the fault may be responsible for the observed tsunami. Although these shallower slips appear on the edge of the assumed fault plane, we use this slip distribution since we found no indication of shallower slip and it seems to be representative of the source area. When we assumed larger fault plane, even though there was no slip on the shallower part, the rake angles slightly changed. So the rake angles of this event may not be well constrained. However, the locations of the areas of large slip do not change greatly. The largest slip is about 14.5 m. The total moment obtained is 9.1 1027 dyn cm, which is equivalent to Mw 7.9.
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[22] Schwartz [1999] carried out inversions for the source time functions using empirical Green’s function analyses of long-period surface waves and broadband body waves for this event. She suggested that there was a gap in moment release between 20 and 60 km from the epicenter and rather continuous rupture from 60 to 120 km, which fills in a gap between the two asperities of the 14 July 1971 event. This rupture distribution seems consistent with our results although the distances from the epicenter are larger. That may 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 two 2000 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 center of the fault at a depth of 26 km. The strike and dip angles were 240° and 17°, respectively, and the average value of rake for all subfaults was obtained as 70°. The slip distribution has an area of large slip very close to the hypocenter and the largest slip is about 8.5 m. The total moment obtained is 2.5 1027 dyn cm, which is equivalent to Mw 7.5. [25] The fault length of the 17 November 2000 event is 100 km and the hypocenter is located to the southwest from the center of the fault and at a depth of 36 km. The strike and dip angles were determined as 230° and 25°, respectively, and the average value of the rakes was 57°. The slip distribution is fairly simple and has an area of large slip including the hypocenter. The largest slip is about 4 m. The total moment obtained is 1.4 1027 dyn cm, which is equivalent to Mw 7.4. [26] The rake angles of the five earthquakes studied here are in average consistent with the direction of subduction on the New Britain Trench obtained by Tregoning et al. [1998]. However, rake angles of the 16 August 1995 event show some large variations indicating a rather complicated rupture.
4. Relocation of the 1971 Events [27] For studying the possible overlap of the rupture areas of various earthquakes, we need to estimate the location uncertainties in the identified areas of large slip. In the slip distribution from waveform inversions, the resolution of the locations of slip areas is at least the size of one subfault, which is about 10 km. Since the slip distributions are determined using relative times from the first arrival, the absolute location of the slip areas depends on the location of the assumed hypocenter. To have better locations for comparing all the slip areas, we relocated the hypocenters for the 1971 earthquakes, relative to the 16 August 1995 and 16 November 2000 B events. [28] Using P arrival time data at the same set of stations from the ISC catalogue, we calculated the relative azimuths and distances of the epicenter of the 14 July 1971 event compared to the 1995 event and that of the 26 July 1971 event compared to the 16 November 2000 B event, using
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Dt ¼ T0 D cosð80 8ÞP
ð1Þ
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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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Figure 7. Station distribution used for the waveform inversion of the 16 August 1995 event.
where Dt is P arrival time difference, T0 is the time between the two events, D is the distance between the two events, 80 is the azimuth of a line segment connecting the epicenters of the two events, 8 is the azimuth to the station, and P is the ray parameter, assuming the ray parameters from the two events are virtually the same [Wyss and Brune, 1967]. The distance D between the two events and the azimuth 80 of the line segment connecting the epicenters can be obtained by finding the best fits of a linear relationship 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 1995 event were found to be 45.6 km and 309°, respectively. Also, the distance and azimuth of the epicenter of the 26 July 1971 event compared to the 16 November 2000 B event were 32.6 km and 42°, respectively. The relocated epicenters of both 1971 events moved several kilometers to the southeast. Figure 9 plots the relocated epicenters along with the original catalogue epicenters.
5. Resolution of Slip Distributions [30] One of the main purposes of this study is to compare the slip distributions of the large earthquakes occurring in the same region, and particularly to see if the areas of large slip overlap. In order to do this, we need spatial resolution on the order of the size of the asperities, which have dimensions of about 30 to 50 km. We make some qualitative assessments about our inversion results and carry out some numerical tests, to show that we have sufficient spatial resolutions to justify the discussion in section 6 about the degree 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 amount of slip close to the hypocenter. If there is large slip close to
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the hypocenter, the P waves have relatively impulsive waveforms with large amplitudes at the beginning. In contrast, for earthquakes that have a large asperity located more distant from the hypocenter, the waveforms are more complicated and have large amplitudes later in the waveforms. Of course, this is somewhat masked by the depth phases in the recorded data, but the inversion results are able 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 of large slip near the hypocenter. For these events, the locations of the asperities then depend on the accuracy of the hypocenter. As discussed in section 4 we carried out relative relocations and showed that the hypocenters have better accuracy 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 distributions for the fixed rake angles (Figures 10a and 10c) and for the cases allowing rake to be free between 0 and 90° (Figures 10b and 10d). Slip distributions of the 14 July 1971 event (Figures 10a and 10b) both show significant slips surrounding area to the hypocenter. This may be consistent with the first big swing in waveforms (Figure 5b) showing no clear directivity. Farther areas from the hypocenter have smaller slips for both cases. The main area of large slip of the 26 July 1971 event is located about 30– 70 km away from the hypocenter for both cases of fixing (Figure 10c) and allowing free rakes (Figure 10d). The directivity caused by this slip distribution can be recognizable from the shorter pulse duration of waveforms (Figure 6b) to the southwestern stations (PRE, NAI). Since slip distributions for both cases of fixed rakes and allowing free rakes between 0 and 90° show similar patterns, this does not significantly affect the 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 resolution for this area is more critical. To evaluate the accuracy of the slip locations, we performed forward modeling tests of the 26 July 1971 event by shifting the location of the asperity. As shown in Figure 11, we calculated waveforms for asperities 10, 20, and 40 km to the southwest of the original location, and 10, 20, and 40 km to the northeast, which brings the asperity to the position of the large slip for the 16 November 2000 B event. Figure 11a shows calculated waveforms (dotted lines) for the case of 20 km shift to the southwest, compared with the observed data (solid lines). Waveforms for shifts to the southwest show a significant timing difference for the large pulse for stations to the north because of longer times needed for the rupture to propagate to the southwest. When we shift the asperities to the northeast (Figure 11b), the synthetics for station to the north have noticeably narrower widths for the first up and down swings. The waveform example shown in Figure 11b is for an asperity location of the 26 July 1971 event that overlaps the asperity of the 16 November 2000 B event. These calculations show that there are noticeable differences in waveforms corresponding to asperity shifts of 20 to 30 km, which would be resolvable by the inversion process. Figure 11c shows that variance between synthetics and observed data, as a function of asperity location. Assuming systematic errors, such as due to regional structure, are the same for the 26 July 1971 and 16 November 2000 B events,
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Figure 8. Results for the 16 August 1995 event. (a) Slip distribution. Star indicates hypocenter. (b) Fit of synthetic (dotted) and observed (solid) waveforms. Scales are the same for all stations. Focal mechanism; strike 310, dip 35, average rake 87. (c) Source time function.
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Figure 9. Locations of the relocated epicenters of the 1971 events. Solid stars indicate the relocated epicenters and open stars the epicenters from ISC (for the 1971 events) and IRIS (for the 1995 and 2000 events) catalogues.
Figure 11c indicates that the spatial resolution is about 10 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, the 17 November 2000 event has an area of large slip separated slightly from the hypocenter. We did not carry out tests to evaluate the spatial resolution for these asperities, because their exact locations are not important for the discussions in section 6.
6. Discussion [35] Figure 12 shows the horizontal projections of the rupture areas of the five earthquakes in 1971, 1995, and 2000. Contours indicate rupture areas with more than 0.5 m slip, and the same colors used in Figure 2 distinguish the events. The rupture areas of these large earthquakes are generally consistent with normal scaling relations of source areas [i.e., Kanamori and Anderson, 1975] and the aftershock distributions. There are large overlaps in these source areas, which are interpreted to mean that some portions of the subduction zone boundary have reruptured. The areas with more than half of the largest slip are filled and identified 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 we see that both earthquakes have two large asperities that are widely separated. There does not seem to be any large overlap of the rupture areas or asperities for these two earthquakes. Next, we compare the locations of asperities of 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 completely within the source area of the 1971 event. The 26 July 1971 event (light blue area) has one very large asperity and a smaller one to the northeast, while the 16 November 2000 B event (red) has one asperity. Although the entire source area of the 16 November 2000 B event is within the source area of the 26 July 1971 event, the asperities of the two earthquakes do not coincide. Finally, the source area and asperity of the 17 November 2000 event (pink area) are shown to be outside of the source area of the 26 July 1971 event. There does not seem to be any overlap of the entire slip areas for these two events, although the aftershock distributions do overlap. [37] Assuming that the horizontal projections of the slip areas can be used to identify the areas of slip on the subduction zone, in other words, assuming that all the events are occurring on the same fault plane, we can use these results to identify overlapping rupture areas between the various large earthquakes. From the inferred slip distributions of the five earthquakes, we see that the overall source 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 November 2000 B overlap. However, even if there is some spatial overlap, the sizes of the asperities are quite different and it seems difficult to interpret them as representing the same physical 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 when an earthquake repeatedly occurs on a specific fault. Nagai et al. [2001] showed that the 1994 Sanriku-oki, Japan, earth-
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Figure 10
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quake ruptured one of the same asperities of the 1968 Tokachi-oki, Japan, earthquake. They proposed that there exist characteristic (similar and regularly occurring) asperities, where large fault slip occurs and rupture of an individual asperity is usually manifested as a M7 or smaller earthquake. However, sometimes the rupture of several asperities is synchronized producing a M8 earthquake. The sizes (M7 to 8) and reoccurrence intervals (repeated ruptures within about 30 years) of the earthquakes in the Tohoku area are very similar to the earthquakes we studied in the New Britain region, although our conclusions are very different. Igarashi et al. [2003] analyzed small ‘‘similar earthquakes’’ in northeastern Japan along the Japan Trench. These similar earthquakes are supposed to be caused by repeating slips of small asperities surrounding a large asperity. Yamanaka and Kikuchi [2004] compared asperities of large earthquakes with magnitude greater than 7 along the Japan Trench. According to their conclusions, asperities are very strongly coupled but occupy only a fraction of the plate contact area, and the locations of asperities are fixed in space away from the hypocenter. All of these studies suggest that asperities are a material characteristic of the rupture zone, and thus these areas always 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 asperity rupture. Thatcher [1990] investigated recurrence characteristics and spatial distribution of great circum-Pacific earthquakes, whose moment magnitudes (M w) are greater than 7.7. He concluded that individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Tanioka et al. [1996] performed the centroid moment tensor (CMT) inversion using long-period surface waves and joint inversion of geodetic and tsunami data of the 1968 Tokachi-oki, Japan, and 1994 Sanriku-oki, Japan, earthquakes, which are the same events analyzed by Nagai et al. [2001]. They, however, concluded that dominant asperities ruptured by the two earthquakes are different. Schwartz [1999] summarized that recurrent fault slip of large circum-Pacific plate boundary earthquakes do not support characteristic slip models and that dynamic considerations are important. Hirose and Hirahara [2002] performed three-dimensional numerical simulation for rupture process of a large asperity on a subduction zone fault. Their results showed that in the case of an asperity with comparable dimension to the width, the whole area of the asperity can be broken at once and the asperity has a regular periodic seismic cycle. However, in the case of much larger lateral dimensions, the asperity does not break its whole area at once and its slip behavior becomes complex within a certain range. On a smaller scale, the repeating 1966 and 2004 Parkfield earthquakes
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appeared to have quite different slip distributions [Bakun et al., 2005]. [41] For the five large earthquakes along the New Britain subduction zone with apparent rerupture of some parts of the same fault plane after 25 to 30 years, the asperities do not coincide. In the asperity distributions obtained in this study (Figure 12), there is no clear relation between the hypocenter and the large asperities. For some earthquakes the hypocenters are located in the area of asperity, and for others the hypocenters are far from asperity. This is different from the results of Yamanaka and Kikuchi [2004]. Also the sizes of asperities vary among the earthquakes. These facts support the idea that asperities have a noncharacteristic pattern for earthquakes on the New Britain subduction zone. Instead of a model where similar asperities always appear in repeated earthquakes, it may be the case that the asperities are different from one earthquake to another, but the longterm rate of slip averages to be a constant value for many cycles over a large area. [42] Figure 13 is a cartoon describing the idea of noncharacteristic behavior of asperity rupture compared to characteristic behavior. In a characteristic asperity model, the same location of an asperity occurs when a segment along a subduction zone repeatedly ruptures. An earthquake breaks one or more asperities depending on the size of the total rupture area. The areas surrounding the asperities may have small amounts of slip during earthquakes and aseismic slip between earthquakes. The sum of the seismic slip over many earthquakes will show strong concentrations at the asperity locations. In a contrasting model, different asperity locations occur when a segment ruptures in repeated earthquakes and there is a noncharacteristic pattern for the asperity distribution. In this model there is no need for aseismic slip and the total slip over the whole area becomes the same over many earthquake occurrences. [43] One consequence of the characteristic asperity model is that the slip on the asperity controls the recurrence rate of large earthquakes. According to Tregoning et al. [1998], the subduction rate along the New Britain Trench in this area is about 15 cm/yr. Considering maximum slip amounts of the 1971 and 1995 events obtained in this study (about 13 15 m), the recurrence time of a large earthquake with magnitude about 8 in this region would be about 100 years. However, we have shown that source areas can overlap and include regions of large and small slip, so that a simple calculation of recurrence intervals is difficult. On average, the slip rate for a specific portion of the fault should equal the plate rate (assuming 100% coupling); however, the accumulated slip will be the sum of a number of earthquakes with large and small amounts of slip and large earthquakes can occur more often 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 distribution of the 14 July 1971 event when the rake angle is fixed as 60°. (b) Slip distribution of the 14 July 1971 event when the rake angles are allowed to be free between 0 and 90°. (c) Slip distribution of the 26 July 1971 event when the rake angle is fixed as 70°. (d) Slip distribution of the 26 July 1971 event when the rake angles are allowed to be free between 0 and 90°. In either case, slip distributions show similar patterns. 13 of 16
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Figure 11 14 of 16
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Figure 12. Comparison of the rupture areas. Contours indicate rupture areas with more than 0.5 m slip and the areas with more than half of the largest slip are filled, indicating asperities. Different colors are used to distinguish the events (see Figure 2). Stars indicate epicenters.
subduction zones are related to those of negative gravity anomaly and forearc basins. However, there do not seem to be well developed forearc basins along the New Britain Trench, and we could not see any clear correlations between the gravity map [Sandwell and Smith, 1997] and our asperity locations. [45] One interesting observation is that the three larger earthquakes, with magnitudes of about 8 (two events in 1971 and the 1995 event), have two or more large asperities, while the smaller ones with magnitudes of about 7.5 (two events in 2000) have rather simple slip distributions with only one large asperity. This may be related to the interpretation of Nagai et al. [2001], where the rupture of multiple asperities produces larger earthquakes than the rupture of a single asperity.
7. Summary [46] We carried out Pdiff waveform inversions to obtain slip distributions for two great (M 8) earthquakes along the New Britain Trench in 1971. We also performed teleseismic P wave inversions for a large event (M7.9) in 1995. We combined these results with the slip distributions for two large earthquakes (M 7.5) in 2000 from a previous study to compare the slip zones and asperity distributions. The rupture areas of these events have large areas of overlap within the same region of the subduction zone, indicating repeated earthquakes on the subduction zone boundary. For these earthquakes, however, the asperities do not occur in the same locations. These observations support the idea that asperities have noncharacteristic patterns when earthquakes occur 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 B event. (a) Map showing shifted slip locations by 10, 20, and 40 km to the southwest. Light and dark gray asperities are our preferred model for the 26 July 1971 and 16 November 2000 B events, respectively, and open ones show shifted locations of asperities of the 26 July 1971 event. Waveform examples are for the case of 20 km shift. (b) Map showing shifted slip locations by 10, 20, and 40 km on the fault plane to the northeast. Waveform examples are for the case of 20 km shift to the northeast. (c) Variance as a function of asperity location. 15 of 16
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Figure 13. Cartoon of characteristic and noncharacteristic asperity models. In the characteristic asperity model, the same asperities rupture repeatedly, although a particular earthquake may break only one or several asperities depending on the total rupture area. Summing the slip distributions over many earthquakes, there will be a large amount of seismic slip for the asperity locations. In a noncharacteristic model the asperity locations are different from earthquake to earthquake. Over many earthquakes, the sum of the slip averages out to the same value over the total fault area.
zone, and we conclude that asperities are not persistent features. [47] Acknowledgments. We thank Paul Tregoning and two anonymous reviewers for their invaluable comments and suggestions. We used World-Wide Standardized Seismic Network waveform data archived at the Earthquake Research Institute, Tokyo University, and digital waveform data from 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]. kyoto-u.ac.jp)
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