Bulletin of the Seismological Society of America, Vol. 97, No. 1A, pp. S25–S42, January 2007, doi: 10.1785/0120050626

Seismicity Associated with the – Earthquake of 26 December 2004 by James W. Dewey, George Choy, Bruce Presgrave, Stuart Sipkin, Arthur C. Tarr, Harley Benz, Paul Earle, and David Wald

Abstract The U.S. Geological Survey/National Earthquake Information Center (USGS/NEIC) had computed origins for 5000 earthquakes in the Sumatra–Andaman Islands region in the first 36 weeks after the Sumatra–Andaman Islands mainshock

of 26 December 2004. The cataloging of earthquakes of mb (USGS) 5.1 and larger is essentially complete for the time period except for the first half-day following the 26 December mainshock, a period of about two hours following the Nias earthquake of 28 March 2005, and occasionally during the swarm of 26–30 Jan- Ն uary 2005. Moderate and larger (mb 5.5) aftershocks are absent from most of the deep interplate thrust faults of the segments of the Sumatra–Andaman Islands sub- duction zone on which the 26 December mainshock occurred, which probably reflects nearly complete release of elastic strain on the seismogenic interplate-thrust during the mainshock. An exceptional thrust- source offshore of Banda Aceh may rep- resent a segment of the interplate thrust that was bypassed during the mainshock. The 26 December mainshock triggered a high level of aftershock activity near the axis of the and the leading edge of the overthrust Burma plate. Much near-trench activity is intraplate activity within the subducting plate, but some shallow-focus, near-trench, reverse-fault earthquakes may represent an unusual seis- mogenic release of interplate compressional stress near the tip of the overriding plate. The interplate-thrust Nias earthquake of 28 March 2005, in contrast to the 26 De- cember aftershock sequence, was followed by many interplate-thrust aftershocks along the length of its inferred rupture zone.

Introduction Hypocenters, magnitudes, and focal mechanisms rou- a shallowly dipping, multisegment plate boundary (Banerjee tinely determined by the U.S. Geological Survey/National et al., 2005; Bilham et al., 2005; Lay et al., 2005; Tsai et Earthquake Information Center (USGS/NEIC) have played an al., 2005). In the plate nomenclature of Figure 1, the 26 De- important role in shaping geophysical understanding of the cember earthquake (Mw 9.1) (Park et al., 2005) involved great Sumatra–Andaman Islands earthquake of 26 December primarily the underthrusting of the Ninety East–Sumatra or- 2004 and its associated seismicity. In this article, we elab- ogen (India and Australia plates) beneath the Burma plate. orate on strengths and limitations of the USGS/NEIC data that Coseismic rupture occurred from Simeulue north through should be appreciated by other users of the data. We con- the Andaman Islands, over a distance of about 1200 km and centrate our effort on USGS/NEIC detection and calculation over a time of about 500 sec (deGroot-Hedlin, 2005; Guil- of hypocenters, on the USGS body-wave moment-tensors bert et al., 2005; Ishii et al., 2005; Kru¨ger and Ohrnberger, produced by the methodology of Sipkin (1982), and on the 2005; Tolstoy and Bohnenstiehl, 2005). In addition to in- seismic energies and apparent-stress values calculated with tense aftershock activity on, or in the close neighborhood of, the methodology of Choy and Boatwright (1995). the interplate-thrust separating the India/Australia plate from In addition to summarizing the characteristics of the the Burma plate, the mainshock triggered intense aftershock routine USGS/NEIC data, we will reconsider these data in activity on the ridge/transform boundary just east of the Nic- light of a consensus that has emerged on the broadscale char- obar Islands that separates the Burma plate from the Sunda acteristics of the Sumatra–Andaman Islands sequence. Ar- plate (Figs. 1 and 2). A second great thrust-fault earthquake ticles published soon after the great Sumatra–Andaman Is- (Harvard CMT magnitude [Mw (HRV)] 8.6) occurred on 28 lands earthquake of 26 December 2004 have established that March 2005, on the segment of the Sumatra zone the earthquake occurred as predominantly thrust faulting on immediately to the south of the 26 December 2004 rupture.

S25 S26 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

Figure 1. Plate tectonic boundaries and nomenclature for the region of Sumatra and the Andaman Islands. Plate names are those of Bird (2003). The boundary between the India and Australia plate is here approximated by a dashed line (Bird, 2003) that lies in the interior of a broad plate boundary region that Bird terms the “Ninety East– Sumatra Orogen.” The Burma plate extends south at least to northern Sumatra, but we have not attempted to represent the plate’s southern boundary. The axis of the Sumatra Trench is taken from Bird (2003). The Sumatran fault is taken from Sieh and Nata- widjaja (2000). Other faults in the Sumatra–Andaman Islands backarc north of the equator are taken from Pubellier et al. (2003).

Still farther to the south, the segment of the Sumatra sub- framework of the foregoing general characteristics of the duction zone that ruptured in the great earthquake of 1833 Sumatra–Andaman Islands earthquakes, we explore impli- has remained largely quiescent in the year following the cations of USGS/NEIC computed locations and source pa- 26 December 2004 earthquake and remains a region of high rameters for more complete understanding of the mainshock concern for a future great underthrust earthquake (Newcomb rupture process and the seismotectonics of the Sumatra– and McCann, 1987; Zachariasen et al., 1999). From the Andaman Islands subduction zone. Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S27

Figure 2. Seismicity of the Sumatra–Andaman Islands region, as defined by USGS/ NEIC epicenters. Red epicenters, earthquakes occurring in 1 January 2000–25 Decem- ber 2004; yellow epicenters, earthquakes occurring in 26 December 2004–27 March 2005; green epicenters, earthquakes occurring in 28 March 2005–31 December 2005. Red circles are plotted over yellow circles, which are in turn plotted over green circles. Large yellow and green stars denote epicenters of mainshocks of 26 December 2004 and 28 March 2005, respectively. Black triangles are volcanoes (Smithsonian Institu- tion, 2005). Major faults and plate boundaries are from Figure 1.

Preliminary Determination of Epicenters (PDE) Hypocenters and Magnitudes of the automatically determined data, add new data, and re- compute the origin. The subsequent “first reviewed solution” The initial origin (hypocenter and magnitude) for a large is the first origin that is posted on USGS websites. The pre- non-U.S. earthquake detected at the USGS/NEIC is usually ferred origin of the earthquake will, however, change with determined automatically from computer-detected phase ar- time as new data are interpreted at the USGS/NEIC or re- rivals and amplitudes. The automatically determined origin ceived from cooperating networks: the USGS/NEIC receives is reviewed by an analyst, who will commonly change some arrival-time data and origin estimates from many regional S28 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald seismographic networks and from the Reviewed Event Bul- letin of the International Data Centre of the United Nations Comprehensive Nuclear-Test-Ban Treaty Organization. The first reviewed solution and subsequent early refinements to the epicenter are collectively known as the Quick Epicenter Determination (QED) for the earthquake. Iterations on the USGS/NEIC origin cease about six weeks after the earth- quake’s occurrence with the electronic publication of the Preliminary Determination of Epicenters (PDE) catalog for the week of the earthquake. The QED and PDE origins may be accessed electronically at sites given in the Data Sources section of this article. As noted in that section, some of the data sources provide assessments of the reliability of the earthquake epicenter and hypocenter and an indication of the type of data or assumptions used to constrain focal depth. In this article, we primarily consider earthquakes that occurred through 2. September 2005, the date of the last PDE origin published at the time we began this study. Within the region bounded by latitudes 10Њ S and 17Њ N and longitudes 87Њ E and 107Њ E, the USGS/NEIC located 5000 earthquakes. Of these, 1143 were located with a precision such that their Figure 3. Change of USGS/NEIC final PDE epicen- 90% confidence ellipses on epicentral coordinates had semi- ter with respect to the first reviewed solution epicenter axes of 10 km or less, and of these highest quality epicenters, for the subset of Sumatra–Andaman Islands after- 562 were associated with focal depths calculated from depth shocks that occurred from 26 December 2004 through phases. Figure 2 also contains QED epicenters for the last 2 September 2005 and for which USGS body-wave four months of 2005 that are not included in other figures. moment tensor solutions were computed. The median distance between first reviewed epicenter and final Because many consumers of USGS/NEIC data make use PDE epicenter is 5.7 km. of early QED hypocenters, it is important to comment on the stability of the USGS hypocenters throughout the process of updating from the earliest QED to the final PDE. Figure 3 biases similar to those of the EHB epicenters since the two illustrates that refinement of the QED and PDE solutions in sets of epicenters are computed with much of the same data, the iterative USGS/NEIC procedure rather commonly re- using the same travel-time model. Biases affecting teleseis- sulted in epicentral changes of 10 km or more for earth- mically determined epicenters from other parts of the quakes of the Sumatra–Andaman Islands sequence. The Sumatra–Andaman Islands region may, however, differ. events represented in Figure 3 were very well recorded at Widiwijayanti et al. (1996) report evidence of a 20-km east- teleseismic distances, so Figure 3 probably understates the ward bias in the PDE epicenter of a large earthquake on the size of changes that may occur between the first reviewed southern Sumatran fault in 1994. solution and the final USGS/NEIC origin for small, poorly recorded, shocks. The formal estimates of precision that accompany QED Completeness of PDE Registration of Earthquakes in and PDE hypocenters in some USGS/NEIC catalogs and bul- the Sumatra–Andaman Islands Subduction Zone, letins (see Data Sources) do not account for the possibility 1964–Mid-2005 of bias in hypocenter locations that would result from The completeness of PDE registration of earthquakes source-station velocities different than those of the AK135 has improved about 0.5 magnitude units since 1964, with model (Kennett et al., 1995) that is used at the USGS/NEIC the greatest part of this improvement being associated with to calculate hypocenters. Biases of several tens of kilometers the PDE’s incorporation of arrival-time data produced by the would not be surprising based on experience worldwide. United Nations Comprehensive Nuclear-Test-Ban Treaty Engdahl et al. (2007) have compared four epicenters recom- Organization’s International Monitoring System in 1995. puted by the method of Engdahl et al. (1998) (hereafter re- Figure 4 shows the distribution of PDE short-period mag- ferred to as EHB epicenters) with epicenters for the same nitudes (mb) of Sumatra–Andaman Islands earthquakes as a earthquakes located by Araki et al. (2006) using data from function of time for three time-periods since 1964. an ocean-bottom seismograph network that was deployed In Figure 4, we also plot a running mean of computed above the interplate thrust of the 2004 earthquake. The EHB mb: at the time corresponding to the ith earthquake, we plot K i 98מi 99מ סii epicenters tend to be southwest of the epicenters of Araki et the valuemean100 (m b) mean(m b, m, b,m b) , i al. (2006), but by less than 15 km. We think it likely that wheremb denotes the PDE mb of the ith earthquake. For the PDE epicenters from the same region would experience many geographic regions and time periods, the distribution Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S29

Figure 4. Temporal distribution of PDE short-period magnitudes (mb) of Sumatra– Andaman Islands earthquakes: (a) 1964 - 2 September 2005; (b) 16 December 2004– 14 May 2005; (c) 26 December 2004–1 January 2005. The 100-point running mean of mb is plotted at the time of the last earthquake that contributes to the mean, so that the running mean produces a delayed indication of changes in the magnitude–frequency distribution. Earthquakes considered in this figure are those occurring in the region bounded by latitudes 10Њ S and 17Њ N and longitudes 87Њ E and 107Њ E. S30 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

of recorded magnitudes is such that the mean value of mb is Following the mainshock of 26 December 2004, fluc- i within a few tenths of a magnitude unit of the mb value above tuations inmean100 (m b) and, correspondingly, the threshold which earthquakes from the region are essentially com- of completeness (Figs. 4b,c) represent three effects. First, in pletely recorded. This empirical result depends on a balanc- the immediate aftermath of the largest shocks (26 December ing of the intrinsic increase in the number of earthquakes 2004 and 28 March 2005) and during episodes of particu- with decreasing magnitude (described by the b-value of the larly intense seismicity (Andaman Sea swarm of late January regional Gutenberg–Richter magnitude–frequency relation) 2005), earthquakes of a size that would normally have been with the decrease in earthquake detectability as a function easily detected and located were lost in the codas of previous of decreasing magnitude below the threshold of complete- shocks. Second, the USGS/NEIC procedure is dependent on ness. From an incremental magnitude versus frequency plot input from the United Nations Comprehensive Nuclear-Test- for a region (e.g., Fig. 5), one may visually evaluate the Ban Treaty Organization’s Reviewed Event Bulletin (REB) extent to which the mean magnitude is likely to approximate in order to catalog smaller teleseisms from the Sumatra re- i the threshold of completeness. Our use ofmean100 (m b) as an gion. At the times that the USGS/NEIC cataloged the earth- indicator of catalog completeness is similar to Wald et al.’s quakes represented in Figure 4, the IDC had not published (1998; p. 533) use of the mode of the magnitude distribution REBs for the periods 28 December 2004–6 January 2005 and as a “quick and dirty indicator of the detection threshold.” 30 January–08 February 2005. The unavailability of REB For the Sumatra–Andaman Islands region in 1964– data raised the threshold of completeness for the PDE for the i 1978,mean100 (m b) fluctuated between 5.2 and 4.9 (Fig. 4a), two 10-day periods. Third, there is evidence of a “little vari- with a mean mb for the whole period (777 earthquakes) of able factor” (e.g., Freedman, 1966) in the extent to which 5.05. A magnitude (mb)–frequency plot for the period 1964– different USGS/NEIC analysts interpreted data from small 1978 suggests that the threshold of completeness for the en- teleseismically recorded aftershocks during the early part of tire period is about 5.2 (Fig. 5a). For the Sumatra–Andaman the 26 December aftershock sequence. Individual days of i Islands region in 2000–25 December 2004, mean100 (m b) the aftershock sequence were typically handled by single fluctuated between 4.5 and 4.8 (also Fig. 4a), with a mean USGS/NEIC analysts. In the intensely active early days of the mb for the whole period (1468 earthquakes) of 4.63. The aftershock sequence, analysts were told to concentrate their magnitude (mb)–frequency plot for the same time interval efforts on aftershocks whose mb might approach 5 or greater suggests that the threshold of completeness is about 4.7 and to pick up significantly smaller aftershocks as time al- (Fig. 5b). lowed. For the period December 28–January 01 (Fig. 4c)

Figure 5. Cumulative and incremental magnitude (mb)–frequency plots for (a) 1964–1978; (b) 1 January 2000–25 December 2005; (c) 20 days in 2004 and 2005 for which the REB of the International Data Centre (IDC) was not published in sufficient time for REB data to be included in the USGS/NEIC data stream. We estimate the c threshold of completeness(mb) , by eye, as the magnitude below which numbers of earthquakes fall-off from a line with b-value 1.25 (Frohlich and Davis, 1993) that passes through N for mb 5.2–5.5. Earthquakes considered in this figure are those occurring in the region bounded by latitudes 10Њ S and 17Њ N and longitudes 87Њ E and 107Њ E. Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S31 there is a day-to-day variability in the smallest aftershock of the moment tensors are generally similar, though some located, which must reflect how individual analysts handled differences exist due to the different types of data used at the process of selecting events to process. the USGS/NEIC and at Harvard (Sipkin, 1986). Because most

For the Sumatra–Andaman Islands region, the mean mb of the teleseismic body-wave data come from ray paths with for a time window appears to be about 0.1 magnitude unit steep takeoff angles, strikes of steeply dipping planes in the less than the magnitude corresponding to essentially com- USGS moment tensor solutions tend to be better resolved plete registration of earthquakes (Fig. 5). We think it likely than strikes of shallowly dipping planes. Focal depths should that the USGS/NEIC record of aftershocks to the 26 Decem- generally be well resolved by the USGS body-wave moment ber earthquake is significantly incomplete at mb 5.1 for the tensor inversion, with the exception of large events whose i first half-day after the mainshock.Mean100 (m b) , after reach- source time functions are longer than the time intervals sepa- ing a peak of 5.3 after the mainshock, doesn’t subside to a rating the direct P phase and reflected phases. value of 5.0 until 16 hr after the mainshock. Otherwise, Fig- Figure 6 shows epicenters of shallow-focus (depth Ͻ ure 4 would suggest completeness at mb 5.1 and above at a 70 km) earthquakes in the region of the Sumatra– resolution corresponding to the time window spanned by Andaman Islands earthquake, 26 December 2004 to 2 Sep- 100 events. We think that the record of aftershocks is in fact tember 2005, for which USGS moment tensors were deter- missing shocks of mb 5.1 in the first two hours following the mined. Focal mechanisms are represented according to their Nias earthquake of 28 March 2005, but that the effect is i attenuated inmean100 (m b) (Fig. 4b) because the 100-point mean in the immediate aftermath of the March mainshock was taken over a time window of more than 5 hr. A few mb 5.1 and larger shocks of the Andaman Sea swarm of 26–30 January could be lost in the codas of immediately preceding shocks of the same swarm. For other time periods, the record of aftershock seismicity should be substantially complete for mb 5.1 and larger. A plot of magnitude versus frequency for the 20 days for which there were no REB suggests that the threshold of completeness for those days was about mb 5.0 i (Fig. 5c), but values ofmean100 (m b) suggest that the thresh- old may be more like 5.1 for subintervals within the 20 days (Fig. 4c). For time intervals other than those just identified i as problematical, the values ofmean100 (m b) suggest a level of completeness of mb (PDE) of about 4.6 (e.g., Fig. 4b).

USGS Body-Wave Moment Tensors of the Sumatra– Andaman Islands Mainshock/Aftershock Sequence

The USGS/NEIC routinely determines moment tensors of earthquakes larger than about 5.5 using the body-wave moment tensor methodology of Sipkin (1982). The USGS body-wave moment tensors are available electronically through sites that are described in Data Sources. The USGS moment tensors are based on inversion of composite P waveforms consisting of P, pP, and sP, and are independent of the moment tensors produced by Harvard University us- ing the centroid moment tensor methodology (Dziewonski et al., 1980; Global CMT Project, 2006), which are based substantially on later-arriving signals. Moments computed Figure 6. USGS body-wave moment tensor solu- by the two methodologies are on average very similar for tions for earthquakes of depth less than 70 km in the northern Sumatra–Andaman Islands region, 26 De- earthquakes smaller than about 7.3; the Harvard CMT mo- cember 2004 through 2 September 2005. Contours of ments tend to be systematically larger than the USGS mo- maximum mainshock displacement enclose regions ment for larger earthquakes (Sipkin, 1986). We attribute the of Ն5m slip in model III of Ammon et al. (2005). systematic differences in moments of largest earthquakes to OBA, Offshore Banda Aceh source. The Sumatran the shorter data time windows used in the body-wave mo- fault is taken from Sieh and Natawidjaja (2000), and the deformation front and offshore fault south of ment tensor method, which result in the USGS procedure the equator are generalized from the same source. The sampling less of the long source time functions of the largest deformation front and offshore faults north of the earthquakes than the Harvard CMT procedure. Orientations equator are taken from Pubellier et al. (2003). S32 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald type of faulting: normal-fault mechanisms are those for Another interesting feature of Figure 6 is the number of which the T axis has a plunge Յ45Њ and the P axis has plunge moderately large aftershocks near the trench axis, among Ͼ45Њ; reverse-fault mechanisms are those for which the T which are reverse, normal, and strike-slip earthquakes. The axis has plunge Ͼ45Њ and the P axis has plunge Յ45Њ; strike- diversity of faulting types is inconsistent with the global ten- slip mechanisms are those for which both T and P axes have dency for near-trench, intraplate aftershocks following great plunge Յ45Њ. We have also distinguished reverse-fault thrust-fault earthquakes to be predominantly extensional mechanisms whose moment tensors (though not necessarily events within the oceanic plate underlying the aseismic thin their locations) are consistent with shallow interplate thrust edge of the accretionary wedge on the overriding plate faulting. These interplate-thrust-consistent mechanisms are (Christensen and Ruff, 1988). We will discuss these events defined with respect to the four interplate-thrust segments further in subsequent sections. whose orientations are given in Table 1 and that are plotted in map view in Figures 9, 10, and 11. The interplate-thrust- Estimates of Apparent Stress for Large Aftershocks consistent mechanisms are those reverse-fault mechanisms for which the azimuth of the slip vector for motion on the The USGS/NEIC determines radiated energy ES and en- most shallowly dipping plane is directed to the north or east ergy magnitude Me for many earthquakes larger than 5.5 and is within 20Њ of being perpendicular to the strike of the using the methodology of Boatwright and Choy (1986). Table 1 segment within which the epicenter lies. A noteworthy feature of Figure 6 is the scarcity along much of the 26 December rupture zone of moderate and large shocks (those of a size that USGS body-wave moment tensors could be determined for them) whose locations and focal mechanisms are consistent with their being interplate- thrust earthquakes. There are some probable interplate thrust aftershocks at the southern end of the 26 December rupture, and there is an intense, 150-km-long zone of likely inter- plate-thrust earthquakes offshore of Banda Aceh, which we will call the Offshore Banda Aceh source. Elsewhere along the 26 December rupture, there are only isolated events whose locations and mechanisms are consistent with their occurring on a shallowly east- or northeast-dipping . Under the assumption that interplate-thrust aftershock activity would be intense on unruptured sections of the in- terplate thrust that were adjacent to the mainshock rupture surface (e.g., Aki, 1979; King, 1983), we infer from the pau- city of interplate-thrust consistent aftershocks in most of the 26 December aftershock zone that the mainshock ruptured completely through the seismogenic zone along most of its length. Throughgoing rupture of most of the seismogenic zone is also implied by geodetic observations (Chlieh et al., 2007).

Table 1 Orientation of Interplate Thrust Segments of the Sumatra– Andaman Arc, Used for Defining “Interplate-Thrust Consistent” Mechanisms and for Construction of Cross Sections of Figure 12 Figure 7. Apparent stresses determined for earth- quakes of depth less than 70 km by the methodology Segment Name Strike Dip of Choy and Boatwright (1995), 26 December 2004– Њ Њ 2 September 2005. Symbols are differentiated by the Andaman Islands 315 12 level of apparent stress and by focal mechanism as 340Њ 15Њ determined by USGS body-wave moment tensor Northwest Simeulue 10Њ 17.5Њ Њ Њ method: rev, reverse-fault mechanism, including Nias Siberut 320 12 shallowly dipping thrust-fault mechanisms; norm, Orientations of the northwest Simeulue, Nicobar Islands, and Andaman normal-fault mechanism; ss, strike-slip fault mecha- Islands segments are from model III of Ammon et al. (2005). Strike and nism. Deformation front and faults are as in Figure 6. dip of the Nias–Siberut segment is that of a typical shallowly northeast- Dashed line AAЈ defines the plane of projection for dipping nodal plane of Harvard CMT solutions (Harvard Seismology, 2006) Figure 8 and also separates the near-trench and fore- for the 28 March 2005 mainshock and aftershocks. arc earthquakes of that figure. Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S33

Figure 8. Distribution of apparent stresses for reverse-fault earthquakes of Figure 7, plotted as a function of distance along line AAЈ of Figure 7. Apparent stresses of the mainshocks of 26 December 2004 and 28 March 2005 and the Siberut earthquake of April 2005 are identified with the year/month/day of those events. Shocks identified as forearc shocks are those with epicenters between 60 km and 220 km from the trench axis. The near-trench reverse-fault shocks of Figure 8 all have epicenters within 60 km of the trench axis.

ס From the ES, apparent stress sa can be computed as sa are associated with strike-slip earthquakes that, we shall ar- lES/M0, where l is rigidity at the source and M0 is seismic gue below, occurred in the oceanic lithosphere of the Ninety moment. The ES values are available online at sites listed in East–Sumatra orogen. This is consistent with a global trend Data Sources. For the purpose of estimating apparent stress for oceanic, strike-slip intraplate earthquakes to have high we use M0 determined by the Harvard CMT methodology apparent stress (Choy and McGarr, 2002). (Dziewonski et al., 1980; Global CMT Project, 2006) and l In subsequent sections of this article we will be particu- that is implied for the earthquake focal depth by the AK135 larly concerned with the genesis of reverse-fault and thrust- velocity model (Kennett et al., 1995). The focal depth used fault earthquakes within the Sumatra–Andaman Islands af- in computing sa is that measured from the broadband wave- tershock sequence. Figure 8 shows the along-arc distribution forms (Choy and Dewey, 1988). of apparent stresses for these earthquakes. The forearc Among shallow-focus Sumatra–Andaman Islands after- shocks of Figure 8 occur at a position within the subduction shocks whose seismic energies and apparent stresses could zone similar to that in which great interplate thrust-fault be determined by the methodology of Boatwright and Choy earthquakes occur worldwide (Byrne et al., 1988; Wells et (1986), the vast majority have apparent stresses less than al., 2003). Most of these shocks are interplate-thrust consis- 1 MPa (Fig. 7). Due to the exceptionally long duration of tent by the convention of Figure 6, although a few of the the body waves (ϳ 500 sec), the seismic energy and apparent forearc reverse-fault shocks have USGS body-wave moment stress of the 26 December 2004 mainshock could not be tensor focal mechanisms whose northeast-dipping nodal calculated with the methodology routinely used at the USGS. plates dip more steeply than 45Њ. The apparent stresses for Choy and Boatwright (2007) develop a methodology spe- the forearc reverse faults have a mean of 0.4 MPa and a cifically for the 26 December mainshock and estimate a ra- median of 0.3 MPa, which compare well with the average apparent stress of 0.3 MPa found by Choy and Boatwright 1010 ן 3.7 ס J. Assuming l 1017 ן diated energy of 1.4 22 ן ס Pa and using the Harvard M0 4.0 10 N m, this yields (1995) for interplate thrust-fault earthquakes worldwide. The an apparent stress of 0.13 MPa for the 26 December main- five near-trench reverse-fault earthquakes of Figure 8 occur shock. The highest apparent stresses represented in Figure 7 in a tectonic environment that, worldwide, does not com- S34 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

Figure 9. Seismicity of the northwest Simeulue Figure 10. Seismicity of the Nicobar Islands and segment of the Sumatra–Andaman Islands arc, 26 De- Andaman Islands segments of the Sumatra–Andaman cember 2004–2 September 2005. The cross section in Islands arc, 26 December 2004–2 September 2005. Figure 12c is based on data within the box; the dashed Cross sections of Figures 12a and 12b are based on line corresponds to the 0-km point of the cross sec- data within the boxes; the dashed lines correspond to tion. The contours of maximum mainshock slip en- the 0-km points of the cross-sections. The deforma- close regions having slip greater than 5 m in model tion front and faults are as in Figure 6. III of Ammon et al. (2005). Events located with depth phases and without depth phases had epicenters for which the 90% confidence ellipses had semiaxes of shock (Fig. 2). The Nias–Siberut segment (Fig. 11) became 10 km or less in length. The deformation front and faults are as in Figure 6. OBA, Offshore Banda Aceh highly active with the earthquake of 28 March 2005. source. Cross sections for all segments are plotted in Figure 12. The projections of the plate interface have dips indicated in Table 1. They are positioned so as to emerge at the defor- monly produce reverse-fault earthquakes (see Nicobar Is- mation front of the Sumatra–Andaman Islands accretionary lands and Andaman Islands Segments of the 26 December wedge, as mapped by Pubellier et al. (2003). Rupture Zone and the Andaman Sea). They have apparent Focal depths of most PDE (depth phase) events are stresses that are average or below average with respect to based on interpreted pP phases read on short-period wave- the global mean for interplate thrust faults. forms and interpreted with the AK135 velocity model (Ken- nett et al., 1995). In regions beneath the Sunda Trench near Seismicity following the 26 December Mainshock the deformation front, where water depths may be as large as 5 km, the phase pwP may be misinterpreted as pP on Seismicity in the 9 months following the great 26 De- short-period records (Choy and Engdahl, 1987; Engdahl et cember 2004 earthquake occurred densely in an approxi- al., 1998), leading to overestimates of focal depth in PDE mately 2000-km-long zone of the shallow Sumatra subduc- (depth phase) events by up to 15 km. Because there is a well- tion zone. For purposes of discussion and data presentation, recognized mechanism by which the near-trench PDE (depth we subdivide the region of most intense activity into four phase) hypocenters might be biased too deep, we base our segments (Table 1). The northwest Simeulue (Fig. 9), Nic- subsequent discussion of near-trench seismotectonics pri- obar (Fig. 10), and Andaman (Fig. 10) segments were all marily on focal depths determined by the body-wave mo- active in the immediate aftermath of the 26 December main- ment tensor methodology. Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S35

The Northwest Simeulue Segment of the 26 December Rupture Zone The mainshock of 26 December nucleated in the north- west Simeulue segment (Fig. 9). The USGS body-wave mo- ment tensor procedure yielded a focal depth of 7 km for the mainshock. The finite source dimensions and the extended source time function of the mainshock, source properties that are not accounted for in the USGS procedure, make this focal depth too unreliable to influence tectonic speculation, and we have not plotted the mainshock hypocenter in the cross section, Figure 12c, that corresponds to Figure 9. We have plotted the epicenter of the mainshock at the 0-km depth of the section. If the mainshock nucleated on a plate interface that has an average dip of 12Њ from the deformation front to the hypocenter, the hypocenter would be at a depth of about 36 km. Interplate thrust events at the southern end of the 26 December aftershock zone presumably occurred on the un- ruptured interplate-thrust fault that is adjacent to the south end of the mainshock rupture, where stresses favorable to interplate thrusting were increased rather than decreased by the occurrence of the mainshock. USGS body-wave moment tensor depths for most of these earthquakes place them close to a thrust interface that dips 12Њ northeastward from the trench (Fig. 12c) The Offshore Banda Aceh source has the characteristics Figure 11. Seismicity of the Nias–Siberut seg- expected of an interplate thrust source. Its distance from the ment of the Sumatra–Andaman Islands arc, 26 De- deformation front at the Sunda trench (about 200 km) and cember 2004–2 September 2005. The cross section in the focal depths of its shocks (those determined by the body- Figure 12d is based on data within the box; the dashed wave moment tensor procedure have average depths of about line corresponds to the 0-km point of the cross sec- tion. The 1833 rupture zone is as plotted by Zachar- 40 km) are consistent with its lying on an interface that has iasen et al. (1999). The deformation front and faults an average dip of about 10Њ from the deformation front to are as in Figure 6. the source region, slightly shallower than the 12Њ dip as- sumed for the interplate thrust in Figure 12c. Assuming that the Offshore Banda Aceh source does represent northeast- In Figure 12, events with body-wave moment-tensor directed underthrusting, body-wave moment tensor focal mechanisms are plotted at centroid depths computed by mechanisms of the source events imply that the plate inter- Њ matching observed waveforms to theoretical waveforms, face locally dips about 30 , which in turn implies a steep- again using the AK135 model. As an independent assess- ening of the subducted slab with depth. A steepening of the ment of the body-wave moment tensor focal depths, we dip of the subducted slab in the vicinity of the Offshore checked them against broadband focal depths (Choy and Banda Aceh source is also demonstrated effectively with the Dewey, 1988) for a set of 44 Sumatra–Andaman Islands hypocenter data and cross-section projections of Engdahl et earthquakes for which both USGS/NEIC body-wave moment al. (2007). tensors and USGS/NEIC radiated energies had been deter- The strength of the Offshore Banda Aceh source sug- mined. The two sets of focal depths agree to within 2 km, gests that, in contrast to most of the Sumatra–Andaman Is- in the mean and median, with a standard deviation of 5 km. lands interplate thrust north of the 26 December epicenter, The generally good agreement of the moment tensor depths strain on the interplate thrust offshore of Banda Aceh in- with the broadband focal depths notwithstanding, we ac- creased rather than decreased as a result of the 26 December knowledge the possibility that some of the moment tensor mainshock. The mainshock rupture of 26 December may depths could be biased by unusual conditions such as those have passed to the west of the region of the Offshore Banda found near the trench axis. The USGS/NEIC body-wave mo- Aceh source, leaving the interplate thrust offshore of Banda ment tensor methodology does not explicitly account for the Aceh unbroken but in close proximity to a segment of the presence of the near-trench water layer, for example, or for thrust that experienced very large displacements. This ex- the effect of seafloor topography interacting with the water planation is consistent with model III of Ammon et al. layer (e.g., Wiens, 1989). (2005), which has low displacement at the site of the Off- S36 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

Figure 12. Sections of aftershock seismicity across the Sumatra–Andaman Arc. (a) Andaman Islands (Fig. 10); (b) Nicobar Islands (Fig. 10); (c) northwest Simeulue (Fig. 9); (d) Nias and Siberut (Fig. 11). Hypocenters are plotted from within the boxes shown in Figures 9–11. Hypocenter symbols are as in Figures 9–11, but PDE hypo- centers are plotted only when constrained by depth phases. OBA source, Offshore Banda Aceh source. Inclined planes have dips of segments in Table 1, and their updip depths correspond to the local trench depths. The earthquakes of 26 January, 28 March, and 10 April all occurred in 2005 and are discussed in the text. Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S37 shore Banda Aceh source and a zone of very high displace- trench aftershocks for which reverse-fault mechanisms were ment just updip of the source (Fig. 9). We note that the determined by the USGS body-wave moment tensor meth- Offshore Banda Aceh source lies downdip of a change in odology; the focal mechanisms are represented in Figure 13. the trend of the Sumatra trench and beneath a change in trend The near-trench regions of most subduction zones are not of faults in the overlying forearc (Fig. 9) and speculate that characterized by reverse-fault earthquakes. Shallowly dip- a distortion of the plate interface at depth, corresponding to ping plate interfaces just landward of trench axes are com- the change of trends of the surface features, may have cre- monly aseismic, even in major aftershock sequences (Byrne ated a barrier to mainshock rupture propagation along the et al., 1988). In subduction zones that do have near-trench deep plate interface. The Offshore Banda Aceh subset of the reverse-fault earthquakes, these earthquakes typically occur forearc shocks has a large range of apparent stresses (Fig. 8). below the neutral plane of the bending subducting litho- Following the reasoning of Choy and Kirby (2004), this may sphere and below the earthquakes induced by extensional imply that the Offshore Banda Aceh source is producing stress in the bent plate (Chapple and Forsyth, 1979). Body- both events occurring on relatively low-strength preexisting wave moment tensor focal depths of most reverse-fault faults and events occurring as the result of breakage of new earthquakes in the northwest Simeulue segment (Fig. 9) are or immature faults. deep enough that these shocks might be attributable to com- Aftershock activity in the northwest Simeulue segment pressive stresses below the neutral plane of a bending slab is high near the updip edges of the zones of maximum dis- (Fig. 12c), but most of the near-trench reverse-fault after- placement in model III of Ammon et al. (2005) (Fig. 9). The shocks in the Nicobar and Andaman Islands segments of the focal depths of most near-trench shocks in the northwest Sumatra–Andaman aftershock zone (Fig. 10) have shallower Simeulue segment, including most of the depths determined body-wave moment tensor focal depths than most of the from the body-wave inversion method, would place them normal-fault earthquakes and they occur very near the in- well below an inclined thrust that dips at about 12Њ from the terface between the subducting plate and the overriding plate deformation front at the tip of the accretionary wedge to the (Fig. 12a,b). We also view the average or slightly below epicentral region of the 26 December mainshock (Fig. 12c). average apparent stresses of the near-trench reverse-fault A notable exception is the aftershock of 26 January 2005 earthquakes (Fig. 8) as being more suggestive of rupture of (discussed in the next section). relatively weak near-surface lithosphere than they are sug- gestive of rupture of competent oceanic lithosphere below the neutral plane of a bent plate. Nicobar Islands and Andaman Islands Segments of the The close spatial association of near-trench reverse-fault 26 December Rupture Zone and the Andaman Sea aftershocks with the deformation front at the toe of the ac- The Nicobar Islands segment also experienced substan- cretionary wedge (Fig. 13) suggests that the aftershocks are tial near-trench activity that was concentrated near the updip a seismogenic response to interplate compressional stress edge of the zone of maximum thrust faulting (model III, near the tip of the overriding plate. The hypocenter of the Ammon et al., 2005) in the 26 December mainshock earthquake of 26 January 2005, 22:00 coordinated universal (Fig. 10). Normal-fault, strike-slip, and reverse-fault mech- time (UTC), is quite close to the inferred position of the anisms are present among aftershock focal mechanisms. The interplate thrust (Fig. 12c, 13), and the shock’s focal mech- occurrence of aftershocks with normal-fault mechanisms anism is consistent with an interplate-thrust origin (Fig. 13). trenchward of the source of a great underthrust earthquake Bilek (2006) determines a long source duration for this is consistent with a pattern observed worldwide; near-trench earthquake, which is also consistent with a shallow-focus, intraplate earthquakes in subducting slabs are ultimately due interplate-thrust mechanism. Focal mechanisms of most of to plate-bending extensional stresses and they are facilitated the shallow-focus reverse-fault near-trench events of the by the reduction of compressional strain associated with the Nicobar Islands segment, however, are not consistent with downdip occurrence of the thrust-fault earthquake (e.g., slip on a shallowly arcward-dipping interplate thrust (Fig. Stauder, 1968; Chapple and Forsyth, 1979; Christensen and 13). In light of possible epicenter biases of several tens of Ruff, 1988). The near-trench strike-slip aftershocks probably kilometers (see PDE Hypocenters and Magnitudes), and con- occurred in the subducting Ninety East–Sumatra orogen of sidering uncertainty in the configuration of the interplate de- the India/Australia plates. Strike-slip earthquakes occur else- collement, we think it is possible that some of the focal where in the Ninety East–Sumatra orogen (Stein and Okal, mechanisms with steeply arcward-dipping nodal planes rep- 1978; Bergman and Solomon, 1985), and coupling of intra- resent slip on splay faults that branch upward from the in- plate strike-slip and interplate-thrust faulting in the Sumatra terplate thrust several tens of kilometers landward of the subduction zone near 5Њ S (Fig. 2) is documented by Aber- deformation front. This origin for the near-trench reverse- crombie et al. (2003). fault aftershocks would not conflict with the hypothesis (e.g., The near-trench reverse-fault earthquakes have faulting Seno and Hirata, 2007) that the large-displacement 26 De- mechanisms that are unusual worldwide for near-trench cember coseismic rupture extended to the deformation front environments that are associated with shallowly dipping in the latitudes of the near-trench reverse-fault aftershocks. interface-thrust zones. Table 2 lists hypocenters of near- Alternatively, large horizontal compressional stress near the S38 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

Table 2 tip of the overriding plate, such as would cause reverse slip Near-Trench Aftershocks to the Earthquake of 26 December on faults of diverse orientation, might result from the 26 2004 for Which Reverse-Fault Focal Mechanisms Have Been December coseismic rupture terminating at shallow depth Determined. downdip of the deformation front, as is postulated for some Date Origin Time Depth sections of the 26 December rupture by Chlieh et al. (2007). (yy/mm/dd) (UTC) Latitude Longitude (km) Mw Coseismic failure of the deeper compressional interface cou- 2004/12/31 022400.5 7.12 92.53 20 6.0 pled with nonfailure of the shallow interface would redis- 2004/12/31 120457.5 6.20 92.91 11 6.0 tribute interplate compressional stress to shallower depths 2005/01/01 222813.8 7.19 92.76 9 5.5 and might thereby trigger reverse slip on faults other than 2005/01/02 153556.7 6.36 92.78 26 6.1 the interface decollement. This alternative origin could ac- 2005/01/04 191449.1 10.55 91.72 8 5.7 2005/01/05 053238.5 3.52 93.65 22 5.5 count for shallow-focus near-trench reverse-fault earth- 2005/01/14 213816.7 3.13 93.92 23 5.6 quakes situated slightly beneath or seaward of the shallow 2005/01/26 220042.6 2.69 94.6 5 6.1 interplate thrust. We do not regard either of the previous 2005/01/29 061043.8 3.32 93.70 11 5.5 explanations for the association of shallow-focus seismicity 2005/02/22 171200.6 10.82 91.77 12 5.5 with the deformation front as accounting for near-trench 2005/05/04 023241.2 9.29 91.68 10 5.4 2005/06/13 195952.7 2.74 94.16 33 5.8 reverse-fault earthquakes with focal depths of 20 km or more 2005/07/05 213618.5 3.95 93.42 20 5.6 (Fig. 13). Additional hypotheses are clearly needed. 2005/08/03 234347.2 6.72 92.66 8 5.5 Aftershocks beneath the Andaman Sea east of the Nic- The focal mechanisms of these shocks are plotted in Figure 13. Depths obar Islands occurred on the /spreading center are those determined with the body-wave moment tensor methodology. system that forms the boundary of the Burma plate with the (Curray et al., 1979; Guzma´n-Speziale and Ni, 1996), and these earthquakes are characterized by normal- fault and strike-slip focal mechanisms. The seismic swarm of 26–30 January 2005 on the transform fault/spreading cen- ter system near 8Њ N, 94Њ E produced over 600 earthquakes that were located by the USGS/NEIC: Twenty-one of these

shocks had magnitudes [Mw(HRV)] of 5.0 or greater, and the largest had Mw(HRV) 5.9. The Burma plate–Sunda plate boundary beneath the Andaman Sea has had a history of seismic swarms. For example, the swarm of 5–23 July 1984 (Fig. 14a) produced 58 shocks that were located by the Ն USGS/NEIC, of which five had Ms 5.0. At the resolution of USGS/NEIC epicenters that have formal 90% uncertainties of 10 km or less (Fig. 14), the swarm of 26–30 January did not constitute an extension of aftershock activity to a previously quiescent part of the Burma plate–Sunda plate boundary but rather an intensifi- cation of activity within a region that was already producing aftershocks. The transform fault/spreading center system be- tween 6.5Њ N and 10.5Њ N, including the region of the swarm of 26–30 January 2005, became active in the first hours after the 26 December mainshock and produced many aftershocks prior to 26 January (Fig. 14b). Following 30 January, most seismicity on the Burma plate–Sunda plate boundary contin- ued to be concentrated between 6.5Њ N and 10.5Њ N (Fig. 14c).

Nias–Siberut Segment of the Sumatra Subduction Zone, Including Source Regions of the 28 March 2005 Figure 13. USGS/NEIC body-wave moment tensor mechanisms and focal depths for regions near the toe and 10 April 2005 Earthquakes of the northwest Simeulue and Nicobar segments of In contrast to aftershock activity triggered by the main- the Sumatra arc. Table 2 lists the earthquakes for shock of 26 December 2004, seismicity associated with the which mechanisms and depths are plotted. The de- formation front of the Sumatra–Andaman Islands ac- Mw(HRV) 8.6 Nias earthquake of 28 March 2005 was gen- cretionary wedge is as mapped by Pubellier et al. erally similar to aftershock sequences of great interplate- (2003). thrust earthquakes worldwide. Aftershocks with locations Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S39 and mechanisms consistent with thrust faulting on the plate U.S. Geological Survey’s National Earthquake Information interface occurred along the length of the inferred rupture Center. Our conclusions on the data themselves are as fol- zone at distances of more than 40 km from the deformation lows: front at the toe of the overriding plate (Figs. 11, 12d). The The USGS/NEIC has computed hypocenters and mag- region within 40 km of the deformation front was largely nitudes for a huge number of earthquakes of the Sumatra– quiescent; the lack of aftershock activity accords with the Andaman Islands sequence that began with the magnitude hypothesis that strain release at the toe of the overriding plate [Mw(HRV)] 9.0 earthquake of 26 December 2004. The num- is usually accommodated by aseismic slip (Byrne et al., ber of events (5000) processed for this sequence alone 1988). through 2 September 2005 is about equal to the total number

The Siberut earthquake of 10 April 2005, Mw(HRV) 6.7, of earthquakes published annually in the PDE in the early occurred within the inferred rupture zone of the great 1833 1970s. Sumatra earthquake (Fig. 11) (Newcomb and McCann, Among USGS/NEIC data that are available electronically 1987; Rivera et al., 2002) near the projected position of a over the Internet, there are parameters associated with in- plane dipping inland from the steep inner wall of the trench dividual seismic events that permit selection of subsets of at an angle of 12Њ (Fig. 12d). The focal mechanism of the hypocenters that are most likely to be reliably located. For 10 April earthquake implies reverse faulting, but the north- this study, we have based a number of conclusions on the east-dipping plane has a dip of about 60Њ, which is not con- subset of hypocenters whose 90% epicenter ellipses have sistent with slip on a gently dipping interplate thrust. The semiaxes of 10 km or less in length and whose focal depths apparent stress of the 10 April earthquake is also somewhat were constrained by use of depth phases. These events con- high with respect to interplate-thrust earthquakes worldwide stitute about 11% of all hypocenters located by the USGS/ (Fig. 8). Taken together, the location, focal mechanism and NEIC. apparent-stress observations suggest that the 10 April earth- The routine USGS/NEIC hypocenters and confidence in- quake was either an intraplate earthquake occurring close to, tervals don’t account for some widely recognized sources of but not on, the plate interface or a splay fault rooted on the hypocentral error, such as biases due to lateral variations in plate-interface decollement. seismic-wave velocity or misinterpretation of the PwP phase Under the hypothesis that great fault earthquakes tend as a pP phase. The 11% of hypocenters that we consider to nucleate near locations marked by previous clusters of most likely to be well located may still be affected by these seismicity within generally quiescent seismic gaps (Kana- sources of hypocentral error. mori, 1977), the 10 April earthquake and its aftershocks are Since the 1960s and early 1970s there has been a re- of scientific interest as possibly marking the nucleation point duction of about half an mb magnitude unit in the threshold of a future repetition of the 1833 Sumatra earthquake. The of completeness for the registration of routine seismicity in

26 December 2004 Sumatra mainshock is, in fact, an ex- the Sumatra–Andaman Islands region, from mb(USGS) about ample of a great earthquake that nucleated within a previ- 5.2 to mb(USGS) about 4.6. Within the 2004–2005 Sumatra– ously active segment of a subduction zone and propagated Andaman Islands aftershock sequence, however, there were to sections of the subduction zone interface that had been fluctuations in the level of completeness due to the loss of quiescent prior to the mainshock (Fig. 2); (Deshon et al., events in the coda of large earthquakes, due to USGS/NEIC 2005). The practical implications of the occurrence of the dependence on the REB of the United Nations Comprehen- 10 April earthquake are not clear. The source region of the sive Nuclear-Test-Ban Treaty Organization for data on small 1833 Sumatra earthquake is already recognized as the po- Sumatra-region teleseisms, and due to a variability in the tential site of a great underthrust earthquake in upcoming performance of different USGS/NEIC analysts. Outside of pe- decades (Newcomb and McCann, 1987; Zachariasen et al., riods of about half a day following the 26 December 2004 1999), and the occurrence of the 10 April earthquake does mainshock, a period of about 2 hr following the 28 March not provide a clear indication of when a future great under- 2005 earthquake, and possibly short periods during the An- thrust earthquake might occur. The shallow subduction zone daman Sea swarm of 26–30 January 2005, the cataloging of southeast of Siberut had experienced seismicity in the de- aftershock activity should be complete down to mb(USGS) cades prior to the 10 April 2005 earthquake, including many 5.1. Excluding periods of most intense activity and exclud- shocks having focal mechanisms consistent with interplate ing a 20-day period for which data were not available from thrusting (see figure 1 of Rivera et al. [2002]). The 10 April the REB, the threshold of completeness should be about shock therefore does not represent the emergence of seis- mb(USGS) 4.6. micity in a segment of the 1833 rupture zone that had been USGS body-wave moment tensor solutions of Sumatra– quiescent prior to the earthquake of 26 December 2004. Andaman Islands earthquakes allow identification of differ- ent modes of strain release within the sequence and provide Discussion estimates of focal depth for larger aftershocks that are based on waveform modeling. In the Data Sources section of this article we provide Apparent stress estimates for the Sumatra–Andaman Is- Internet addresses for accessing seismological data of the lands are consistent with trends seen in global studies. High- S40 J. W. Dewey, G. Choy, B. Presgrave, S. Sipkin, A. C. Tarr, H. Benz, P. Earle, and D. Wald

Figure 14. Occurrence of earthquakes in the central 26 December aftershock zone, before and after the Andaman Sea swarm of 26–30 January 2005. Deformation front and faults are from Pubellier et al. (2003). est stresses (Ͼ1.0 MPa) are associated with strike-slip earth- mainshock rupture. Near-trench normal-fault and strike- quakes of the subducting oceanic plate. Most reverse-fault slip earthquakes probably represent intraplate seismicity earthquakes have apparent stresses less than 0.5 MPa. within the Ninety East–Sumatra orogen of the India and Our interpretation of the characteristics of the Sumatra– Australia plates, and some of the deeper near-trench Andaman Islands sequence proper are guided by the hy- reverse-fault earthquakes may also correspond to release pothesis that aftershock activity will tend to be most intense of stresses generated by plate bending. A group of on the margins of the coseismic rupture zone, within still shallow-focus, near-trench, reverse-fault earthquakes stressed or newly stressed brittle crust that can fail seismo- represents a type of seismic activity that is not common genically, rather than within the interiors of the fault plane in subduction zones and may correspond to an unusual that produced maximum moment release in the mainshock. seismogenic response to interplate convergence near the Some models of coseismic moment release (e.g., model III tip of the overriding plate. of Ammon et al. [2005] and model G-M9.15 of Chlieh et 4. The extraordinary Andaman Sea swarm of 26–30 January al. [2007]), taken together with USGS/NEIC aftershock lo- 2005 occurred on a section of the transform fault/spread- cations, are consistent with this hypothesis. From this per- ing center system of the Burma plate–Sunda plate bound- spective, our conclusions on the mechanism of the Sumatra– ary. At a regional scale, it constituted a burst of activity Andaman Islands sequence are as follows: within a section of the plate boundary that was already producing aftershocks and that would continue to pro- 1. The absence of interplate-thrust events along most of the duce aftershocks following the swarm. downdip edge of the 26 December 2004 mainshock rup- 5. The intense interplate-thrust aftershock activity following ture implies that the rupture extended completely through the 28 March 2005 earthquake contrasts with the lack of the downdip part of the seismogenic interplate thrust of such activity along much of the deep 26 December rup- the Sumatra–Andaman Islands subduction zone for most ture and suggests that the rupture zone of the 28 March of the length of the mainshock. 2005 earthquake, as large as it was, either did not break 2. An exceptional, 150-km-long zone of intense interplate- completely through the seismogenic zone or was rapidly thrust activity offshore of Banda Aceh may represent a restressed from neighboring, currently stressed segments segment of the deep, seismogenic, interplate thrust that of the interplate thrust. was bypassed by the mainshock fault rupture 6. The Siberut earthquake of 10 April and its aftershocks 3. Intense aftershock activity occurred near the Sunda are of unusual interest because they occurred in the mid- trench and the deformation front updip of the inferred dle of the rupture zone of the great Sumatra earthquake Seismicity Associated with the Sumatra–Andaman Islands Earthquake of 26 December 2004 S41

of 1833. The location and focal mechanism of the 10 database of USGS body-wave moment tensors from 1980 April mainshock imply that it occurred close to, but not and radiated energy estimates from November 1986 is up- on, the shallowly dipping interplate thrust. Although dated about a year after the occurrence of an earthquake and much of the 1833 rupture zone has been quiescent in is available at http://neic.usgs.gov/neis/sopar/ (last accessed recent decades, the region of the 10 April shock had ex- May 2006). Broadband focal depths associated with the ra- perienced appreciable earthquake activity in the decades diated energy estimates are available from author G. L. Choy before 26 December 2004. upon request.

Data Sources Acknowledgments We acknowledge helpful reviews by Bob Engdahl, Art McGarr, Steve The first reviewed solution and subsequent QED origins Hartzell, and Dan McNamara. within seven days of the earthquake are posted at http:// earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes _all.php/ (last accessed May 2006). References The QED and final PDE origins after seven days follow- ing the earthquake are available at http://neic.usgs.gov/neis/ Abercrombie, R. E., M. Antolik, and G. Ekstro¨m (2003). The 2000 Mw 7.9 epic/ (last accessed May 2006). The compressed file format earthquakes south of Sumatra: deformation in the India-Australia plate, J. Geophys. Res. 108, no. B1, 2018, doi 10.1029/2001 is the most data-rich output format: in addition to providing JB000674, 6:1–6:16. hypocenter and several magnitude types, this format also Aki, K. (1979). Characterization of barriers on an earthquake fault, J. Geo- provides a qualitative assessment of the reliability of the phys. Res. 84, 6140–6148. earthquake epicenter and an indication of the type of data or Ammon, C. J., C. Ji, H. K. Thio, D. Robinson, S. Ni, V. Hjorleifsdottir, H. assumptions used to constrain focal depth. Kanamori, T. Lay, S. Das, D. Helmberger, G. Ichinose, J. Polet, and D. Wald (2005). Rupture process of the 2004 Sumatra–Andaman 90% confidence ellipses on epicenter coordinates, used earthquake, Science 308, 1133–1139. to select PDE epicenters and hypocenters that are plotted in Araki, E., M. Shinohara, K. Obana, T. Yamada, Y. Kaneda, T. Kanazawa, Figures 9–12 and Figure 14, were calculated from 90% con- and K. Suyehiro (2006). Aftershock distribution of the 2004 Decem- fidence ellipsoids on hypocentral coordinates that are con- ber 26 Sumatran–Andaman earthquake from ocean bottom seismo- tained in machine-readable Earthquake Data Reports that are graphic observation, Earth Planet Space 57, 1–7. Banerjee, P., F. F. Pollitz, and R. Bu¨rgmann (2005). 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