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1 Elsevier Editorial System(tm) for Earth and Planetary Science Letters Manuscript Draft Manuscript Number: EPSL-D R2 Title: Structural analysis of aftershock sequences from the Great Sumatran Earthquakes Article Type: Regular Article Section/Category: Frontiers Keywords: Sumatra; Andaman Sea; Great Earthquakes; CMT solutions; structural analysis; geodynamics; collapse; tectonics Corresponding Author: Professor Gordon Lister, Corresponding Author's Institution: First Author: Gordon Lister, PhD Order of Authors: Gordon Lister, PhD; Brian Kennett, PhD; Marnie Forster, PhD; Simon Richards, PhD Manuscript Region of Origin: Abstract: The Global Centroid Moment Tensor Database now records 771 solutions for aftershocks from the Great Sumatran Earthquakes of The geometry of these aftershocks has been analyzed, with surprising results. In the south, the aftershock sequence was dominated by thrusts and reverse faults that were kinematically coordinated with the main shocks. The movement directions were on average towards ~220, close to the direction of relative plate movement. These aftershocks define fault planes with attitudes consistent with Coulomb-Mohr failure due to horizontal compression of the crust above the main rupture. In the north, however, two competing movement patterns appear to have been operative, even on seismic timescales. Motion consistent with the effects of relative plate movement is reflected in right-lateral strike-slip parallel to northern extensions of the Sumatra fault, and on Andaman Sea transforms. Similarly, normal faulting that took place on Andaman Sea 'spreading centres' can be related to the effects of relative plate motion. But some motions are not compatible with the effects of relative plate movement, e.g., arc-parallel normal faults that allowed displacement towards the west, or left-lateral strike-slip faults parallel to spreading ridges in the Andaman Sea. This suggests that the initial rupture decoupled the over-riding lithosphere from the underlying gently dipping subducting slab, and thereby allowed a gravity-driven westward surge of extending Indonesian continental crust above a weak basal detachment.

2 Cover Letter INSTITUTE OF ADVANCED STUDIES RESEARCH SCHOOL OF EARTH SCIENCES Canberra ACT 0200 Australia Professor Gordon Lister Tel: Fax: gordon.lister@anu.edu.au 4 th March, 2007 Re: Structural analysis of aftershock sequences from the Great Sumatran Earthquakes Gordon Lister, Brian Kennett, Marnie Forster and Simon Richards Dear Rob, Please find our resubmitted manuscript, as above. Also included is a list of possible reviewers. Bill Kidd (geologist/structure tectonics) Bill Kidd <wkidd@atmos.albany.edu> Simon Klemperer (geophysicist/geodynamicist) Simon Klemperer <sklemp@stanford.edu> John Nabelek (geophysicist/seismologist) John Nabelek <nabelek@coas.oregonstate.edu> Joseph Curray <jcurray@ucsd.edu> Stephen Cox (structural geologist/petrophysicist) Stephen Cox <Stephen.Cox@anu.edu.au> John Platt (structural geologist/tectonicist) John Platt <john.platt@usc.edu> Peter Betts (structural geologist) Peter Betts <Peter.Betts@sci.monash.edu.au> Peter Cawood (structural geologist/tectonicist) Peter Cawood <pcawood@cyllene.uwa.edu.au> James Jackson (geologist/geophysicist) James Jackson <jaj2@esc.cam.ac.uk> all the best gordon

3 * Manuscript Structural analysis of aftershock sequences from the Great Sumatran Earthquakes Gordon Lister, Brian Kennett, Marnie Forster and Simon Richards Research School of Earth Sciences, The Australian National University, Canberra, Australia. Abstract The Global Centroid Moment Tensor Database now records 771 solutions for aftershocks from the Great Sumatran Earthquakes of The geometry of these aftershocks has been analyzed, with surprising results. In the south, the aftershock sequence was dominated by thrusts and reverse faults that were kinematically coordinated with the main shocks. The movement directions were on average towards ~220, close to the direction of relative plate movement. These aftershocks define fault planes with attitudes consistent with Coulomb-Mohr failure due to horizontal compression of the crust above the main rupture. In the north, however, two competing movement patterns appear to have been operative, even on seismic timescales. Motion consistent with the effects of relative plate movement is reflected in right-lateral strike-slip parallel to northern extensions of the Sumatra fault, and on Andaman Sea transforms. Similarly, normal faulting that took place on Andaman Sea spreading centres can be related to the effects of relative plate motion. But some motions are not compatible with the effects of relative plate movement, e.g., arc-parallel normal faults that allowed displacement towards the west, or left-lateral strike-slip faults parallel to spreading ridges in the Andaman Sea. This suggests that the initial rupture decoupled the over-riding lithosphere from the underlying gently dipping subducting slab, and thereby allowed a gravity-driven westward surge of extending Indonesian continental crust above a weak basal detachment. Keywords: Sumatra; Andaman Sea; Great Earthquakes; CMT solutions; structural analysis; geodynamics; collapse; tectonics

4 2 1.0 Introduction This study was commenced shortly after the events of December 2004, with the aim to apply modern structural analysis to the analysis of the tectonic evolution of the region. We benefited from the detailed analysis provided by Ishii et al. (2005) and Lay et al. (2005). The focus of our effort was to determine whether or not the geometry of aftershock sequences can be analyzed using standard techniques employed by structural geologists. To help achieve this goal a computer program (equakes) was written (using the C++ language and compiled using Xcode in a MacOSX environment). This program allows analysis of data that can be downloaded from the Centroid Moment Tensor (CMT) database, initially at Harvard, but now relocated. A description of the Global CMT project can be found at Data initially analyzed involved 250 aftershocks from the Harvard CMT database, including the CMT solutions for the two Great Sumatran Earthquakes of 2004 and As the result of ongoing effort by the Global CMT project this paper is now able to report the analysis of 771 aftershocks, from 2004/12/26 until 2006/05/31. Compared with the results of the initial sample more definite spatio-temporal patterns have begun to emerge, and distinct clusters are now evident in the data that were not there before (e.g., the 2006 Andaman normal fault, shown in cluster B, Fig. 1). The equakes computer program ( allows automatic classification of the different fault geometries implicit in the two conjugate solutions listed for each centroid moment tensor. This is possible because the Centroid Moment Tensor (Fig. 2) provides information as to the symmetry and geometry of energy release during an earthquake. The fault planes that can be identified are conjugate in that: a) the pole to one fault plane is the slip line (or movement direction) of the other; b) the sense of shear on

5 3 one fault plane is conjugate to the sense of shear on the other. This means that if one solution is a reverse fault (with movement up the dip of the fault plane) then the conjugate solution will be a thrust, with a lower angle of dip, but with movement again up the dip of the fault plane. Similarly if one solution is a high-angle normal fault (with movement down the dip of a steeply dipping plane) then the conjugate solution will represent a low-angle normal fault, with movement down the dip of a more gently inclined plane. The rake of the slip line on the fault plane allows distinction between a pure normal or reverse fault (rake in the range 90±20 ) and a strike-slip fault (rake in the range 0±20 or 180±20 ). Strikeslip faults can be similarly recognized, and classified as either right- or left-lateral, depending on the specified data. Strike-slip faults are complex to analyze in that oblique slip on a vertical fault offers a conjugate solution to horizontal slip on a dipping plane. Nevertheless, by providing some limits, it is possible to readily classify fault type by noting the rake, and the fault plane dip. To ensure that there is no ambiguity, strike-slip faults were classified as faults planes that are more steeply dipping than 70 and with a slip direction that has a rake less than ±20 from a horizontal slip line. The locations for these earthquake hypocentres were superimposed on an image derived from the NOAA topographic data [ This image (Fig. 3a) shows that aftershocks fall into distinct spatial groups, with thrusts dominating in the south, and with normal faults dominating in the north. Strike-slip faults also cluster in the north, and on the Indian plate itself. Of the 771 quakes analyzed, a total of 285 were normal faults, 103 were strike-slip faults and 383 were reverse faults or thrusts. 2.0 equakes and Stereoplots Structural analysis requires more than a pictorial beachball representation of the centroid moment tensor, so the equakes computer program was written to allow analysis of the data on a stereoplot. A stereographic projection plots a point with trend φ and plunge ρ as a point

6 4 with azimuth φ and distant from the centre of the projection by r = R tan (ρ/2), where R is the radius of the projection circle. Stereographic projections are particularly useful in that they preserve subtended angles, and thereby facilitate geometric analysis. The first issue that we addressed was whether the aftershock sequences were kinematically coordinated, i.e. with a common relative movement direction. In structural analysis it is often so that lineations that record relative movement on faults or shear zones are roughly parallel, while at the same time there can be considerable divergence in terms of the orientation of the different planes on which movement takes place. If the fault systems are kinematically coordinated (as described above) there is much that can be inferred in respect to the dynamics of the processes that drive their movement. Structural geologists generally use the lower hemisphere to project dip, and dip direction of planes, or the plunge and trend of lineations. In this case however we are not so restricted because we have more information. The CMT data provide the actual direction of movement of the upper-plate of the fault, incorporating data as to the sense of movement. To ensure we utilize this additional information we adopt a new convention, using the upper hemisphere projection to portray data in respect to thrusts and reverse faults, while the lower hemisphere projection is used for high- and low-angle normal faults, as well as for strikeslip faults. It is evident from the diagrams that result (Fig. 4) that there is a high degree of kinematic coordination for thrust and reverse fault solutions whereas there is more scatter in the orientation distribution of low-angle (LANF) and high-angle (HANF) normal faults. 3.0 Orientation Groups The stereoplots are complicated by the fact that each conjugate solution is represented by two points, one of which represents the pole to the slip plane for one of the two conjugate

7 5 solutions, while the other represents the slip line of the corresponding fault solution. If we consider the conjugate solutions simultaneously there are four points to consider. How can we proceed to resolve ambiguity? Since the solutions are conjugate it is not possible to distinguish one from the other, unless we can introduce additional constraints, for example based on independent knowledge as to the geology. We are helped in this endeavour because the same (or very similar) plots result if we plot the slip lines instead of the fault plane poles. Any deviation from an exact match is the result of small numerical errors that have resulted during the CMT analysis. Each plot therefore can be interpreted in the light that it shows an aggregate of slip lines and/or poles, with each quake represented by two (undifferentiated) orthogonal points. We can potentially resolve remaining ambiguity if we can identify orientation groups in the preferred orientation diagram and use local geology to argue that the associated maxima represent either a cluster of poles to fault planes, or a cluster of slip line orientations. The equakes program was written to facilitate the recognition of spatial, temporal or orientation clusters in the CMT solutions. In most cases a cluster on the map can be selected (Fig. 1) and the solutions examined on the stereoplot. The type of earthquakes can be automatically qualified (normal, reverse, left-lateral or right-lateral strike-slip). Definition of orientation groups in most cases allows a specific criterion to be applied to remove ambiguity between whether the selected group represents actual slip lines, or poles to a fault plane. For example, orientation cluster A (Fig. 1) can be interpreted to represent aftershocks from steeply dipping normal faults in the Indian ocean, with NNE trending slip lines. These faults appear to have fractured the Indian slab parallel to ancient transform faults, and with a geometry that suggests mode III fractures flexing the plate in the area immediately south of the frontal thrust of the subduction zone. Similarly it can be shown that aftershock cluster B represents a spatial cluster parallel to a spreading ridge reported in the Andaman Sea (Raju et al., 2004). This spatial cluster is

8 6 also a well-defined orientation cluster. The data suggest a group of aftershocks on a highangle normal fault (HANF) parallel to the northern margin of the spreading ridge. The interpreted spreading centre may well thus be a rift zone extending NNW-SSE. The Nicobar swarm (cluster C in Fig. 1) is more complex and consists of several fault types, and several orientation groups (Fig. 5). We will analyse the geometry of this cluster in detail, later in the paper. Clusters D and E appear to represent thrust planes that dip gently NE, with a direction of movement towards ~220. Cluster F interestingly enough represents reverse faults, with the same trends, but at steeper angles. Insufficient information is available to allow a decision as to whether this cluster represents NE and/or SW dipping fault planes. The method of orientation groups described above is limited in that it does not allow unambiguous choice except in circumstances where the geology so allows. Nevertheless we proceeded with a complete analysis, cataloguing our choices so that any errors of judgement can be corrected in the light of future work. The choices we made are contained in a file equakessumatrasolutions.txt which is available through the supplementary information stored by Earth and Planetary Science Letters. Slip lines for the solutions chosen are plotted in Figure 3b. To allow full access to our code it is provided open source through the ACcESS MNRF site at from which site the current executable version of equakes can be obtained by downloading a MacOSX disk image. This disk image contains the code, input data, and a text file equakessumatrasolutions.txt that lists the preferred solutions identified in this paper. The solution set can be read by using the Open CMT datastore menu item in the equakes program. If different choices as to which solution is preferred are made by the program operator these can be saved as separate files using menu item Save CMT datastore.

9 7 4.0 Discussion The tectonics of SE Asia can be interpreted in terms of the movement of microplates (Curray, 2005) but there are troubling aspects to such an approach. The frontal thrusts marking the locus of initial subduction of the Indian plate have an arcuate pattern. There is also evidence of arc normal extension. These two observations suggest orogenic collapse and/or subduction hinge roll-back, in this case in a direction almost orthogonal to the direction of relative motion. It is therefore possible that the aftershocks of the Great Sumatran Earthquakes of reflect the effect of two competing movement patterns, one driven by relative plate movement, and the other driven by roll-back of the subducting slabs to the west, and by the accompanying gravitational energy driven collapse of the adjacent orogen and its associated sedimentary basins. It is evident that thrust and reverse fault solutions (Fig. 4a) show a high-degree of kinematic coordination, with movement clustered about a trend toward 220. This is also the same trend as that of the initial movement direction inferred for both of the main ruptures, both of which seem to have rapidly propagated as Mode III fractures, slipping in a direction: a) -8 - > 219 for Sumatra I; and b) -7 -> 220 for Sumatra II. The dip of the thrusts and reverse faults in the aftershock sequence is within the range of orientations expected for Coulomb- Mohr failure within the upper plate, contradicting the asperity model which would require fault orientations similar to that of the initial rupture. It also should be noted that this direction will be close to the direction of relative plate movement. The plate-tectonic approach to SE Asian tectonics also predicts arc-parallel right lateral strike-slip faults, again as observed in the aftershock sequence. The arc-parallel Sumatran Fault steps eastwards across the Andaman Sea spreading ridges (Raju et al., 2004; Curray, 2005) eventually connecting with the Sagaing Fault. In this sense the wrench faults act somewhat in the sense of oceanic transform faults, so that NNW-SSE extension across the

10 8 Andaman Sea spreading ridges (cluster B in Fig. 1) is compatible with a plate tectonic interpretation of the effects of relative movement. Normal faults (Fig. 4b) show considerably less kinematic coordination than do thrusts and reverse faults, but the orientation data is compatible with the existence of arcuate normal faults parallel to the boundary of the over-riding continental crust, with slip lines that cluster around a direction plunging ~40 W. This data suggests that the crust above the initial megathrust was extending in an ~E-W trending direction, compatible with the hypothesis that aftershocks in the northern region reflect the effects of roll-back and/or orogenic collapse. Since the initial megathrust rupture (Ishii et al., 2005) separated the overlying crust from the gently dipping Indian slab beneath, the latter option must be deemed the more likely. The continued sub-seismogenic movement of the northern region westward (Lay et al., 2005; Vigny et al., 2005) after the main rupture had finished propagating is also compatible with this hypothesis. It can therefore be concluded that the aftershock sequence is compatible with two competing patterns of movement in the over-riding seismogenic crust above the initial rupture of the Sumatran megathrust. In the case of thrusts and reverse faults (which dominate in the south of the map area) the direction of relative movement is compatible with failure largely driven by horizontal compressive stress parallel to the direction of relative plate motion. In the case of normal faults (which occur largely in the north of the map area) the movement is compatible with a westward gravity-driven surge of the continental crust, and failure during horizontal extension. The Nicobar aftershock swarm (Curray, 2005) shown in cluster C on Figure 1 is of particular interest because it may represent a direct consequence of these competing movement patterns. The orientation data suggest several orientation groups (Fig. 5). Orientation group C1 is either: a) ~N-S trending right-lateral strike-slip; or b) ~E-W

11 9 trending left-lateral strike-slip. Orientation group C3 is either: a) NNW-SSW trending rightlateral strike slip; or b) ENE-WSW trending left-lateral slip. Orientation group C3 is either: a ) moderately SW to WSW dipping normal faults with right-lateral strike-slip towards ~330 ; or b) steeply dipping normal faults with oblique movement towards ~ C4 is either: a) moderately NW-WNW dipping normal faults with west directed movement; or b) moderately E-dipping normal faults with ESE to SE directed movement. Orientation groups C2 and C3 are really part of the same group - with one orientation merging into the other. They have a common pole (Fig. 5d) allowing definition of a ~ENE- WSW trending fault trace. Orientation groups C4 and C5 define moderately dipping normal faults with ~N-S fault traces. Similarly orientation group C1 defines steep to vertical strikeslip ~N-S trending strike-slip faults with sub-horizontal right-lateral motion. Orientation group C4 defines a single plane (trace dashed on Fig. 5e) with a plethora of different rightlateral oblique slip movement directions. All three orientation groups (C1, C4, C5) are compatible with aftershocks on the right-lateral Sumatran wrench system. In contrast orientation groups C2 and C3 are compatible with aftershocks on the left-lateral strike-slip faults parallel to the ~ENE-WSW traces of the Andaman Sea spreading centre as it steps the Sumatran wrench system towards the Sagaing Fault. Ambiguity in the choice of conjugate solutions disappears if only ~ENE-WSW trending faults traces (orientation groups C1 and C3 on Fig. 5) and ~N-S trending fault traces are allowed. Interestingly clusters on such trends are visible in the map data (Fig. 6) although it should be noted that there is little resolution in the depth data (which was set arbitrarily at 35 km) for most of these hypocentres. According to this model flow of the crust towards the west is accommodated by C2 and C3 left-lateral strike-slip faults, while right-lateral wrenching accommodates relative plate motion (orientation groups C1, C4 and C5). These two competing movement patterns produce geometric incompatibilities in the seismogenic

12 10 middle-crust, and we suggest that it is these incompatibilities that are responsible for the vigour of aftershocks in the Nicobar swarm. It is of interest to note that similarly oriented left-lateral strike-slip faults are found further to the north (Lacassin et al., 1998) and that these also lie on small circles about the eastern syntaxis of the Himalayan orogen. Similarly it is interesting to observe that the orientation of the deviatoric stress axes inferred for the Nicobar swarm (Fig. 5e) has the trend of σ 1 parallel to the direction of motion of the Indian slab, whereas σ 3 is oriented so that it dips ~25 towards the northwest. The map pattern shows that thrusts and reverse faults dominated in the south, while normal faults dominated in the north. This suggests a variation in tectonic mode from overall horizontal shortening in the south, to overall horizontal extension of the crust in the north. This conclusion is of particular interest because it highlights a conundrum (or dilemma) faced by modern-day tectonicians. On the one hand it is possible to describe the geology of this region according to plate tectonic theory (Curray, 2005). Oblique convergence is accommodated by a right lateral wrench fault system (the Sumatran Fault and the Andaman Sea transforms). The Andaman Sea spreading centres accommodate ~NNW-SSE directed extension as the wrench system steps eastward as we travel north. On the other hand, the geology of the region also requires arc-normal extension, due to a combination of roll-back, as the flexure of the Indian slab rolls-back towards the west, or to gravity driven collapse as the crust flows westward over the retreating slab. In the region of Sumatra, on average, gravity-driven westward collapse of the orogen must be taking place at a rate faster than roll-back allows hinge retreat, leading to the present gently-dipping angle of subduction, at least at surficial levels. It is this factor and not the age of the subducting slab that creates the geodynamic setting that requires continual failure of the Sumatran megathrust.

13 11 Overall the aftershock data are consistent with the hypothesis that the ruptures responsible for the Great Sumatran Earthquakes of 2004 and 2005 were gently dipping thrusts that rapidly propagated as Mode III fractures. As these fractures propagated they detached the Indian slab from the over-riding Indonesian crust (Fig. 7). The Indonesian crust then surged southwest and west thereby releasing (and causing the accumulation of) heterogeneous distributions of deviatoric stress in the over-riding Indonesian crust during the aftershock sequence. Aftershocks in this scenario are more the result of this ongoing flow, or afterslip (cf., Hsu et al., 2006) with movement facilitated by the weak underlying detachment defined by the initial rupture of the megathrust. 4.0 Conclusion Switches in tectonic mode as described above are generally recognized at longer time scales. Here we present evidence that suggests such mode switches can take place on seismogenic time scales. In the south reverse faults and thrusts operated with movement directions reflecting the direction of relative plate motion. In the north extending crust moved westward. This implies a surge of the continental crust driven by its own gravitational potential energy, a circumstance made possible by decoupling accomplished by the initial rupture of the basal megathrust. As is the case of the Tibetan Plateau, paraphrasing England and Molnar (2005), the orogen behaves more like a fluid than a plate, here, even at seismogenic timescales. 5.0 References Raju, K. A. K., Ramprasad, T., Rao, P. S., Rao, B. R., Varghese, J New insights into the tectonic evolution of the Andaman basin, northeast Indian Ocean. Earth and Planetary Science Letters 221,

14 12 Curray, J. R., Tectonics and history of the Andaman Sea region. Journal of Asian Earth Sciences 25, Ishii, M., Shearer, P. M., Houston, H., and Vidale, J. E Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, doi: /nature03675 Lay, T., Kanamori, H., Ammon, C. J., Nettles, M., Ward, S. N., Aster, R. C., Beck, S. L., Bilek, S. L., Brudzinski, M. R., Butler, R., DeShon, H. R., Ekström, G., Satake, K. and Sipkin, S The Great Sumatra-Andaman Earthquake of 26 December Science 308, Vigny, C., Simons W. J. F., Abu, S., Bamphenyu, R., Satirapod, C., and Choosakul, N., Subarya, C., Socquet, A., Omar, K., Abidin, H. Z., and Ambrosius, B. A. C., Insight into the 2004 Sumatra Andaman earthquake from GPS measurements in southeast Asia. Nature 436 doi: /nature03937 Lacassin, R., Replumaz. A., and Leloup, P. H., Hairpin river loops and slip-sense inversion on southeast Asian strike-slip faults. Geology 26, Hsu, Y.-J., Simons, M., Avouac, J.-P., Galetzka, J., Sieh, K., Chlieh, M., Natawidjaja, D., Prawirodirdjo, L., and Bock, Y., Frictional afterslip following the 2005 Nias- Simeulue earthquake, Sumatra. Science 312, England, P., and Molnar, P., Late Quaternary to decadal velocity fields in Asia. Journal Of Geophysical Research, 110, B12401 Doi: /2004jb Acknowledgments Research funded by an Australian Research Council Discovery Grant to study the evolution of the Alpine- Himalayan orogenic belt.

15 13 Figure Captions Figure 1. A map showing hypocenters of earthquakes for which the Centroid Moment Tensors were analysed using the equakes computer program. Stereoplots of conjugate solutions are shown for spatial groups identified (except for cluster C). Clusters D, E, F are thrusts and reverse faults with dark circles showing poles to fault planes, and the lighter circles showing slip line directions, on an upper hemisphere stereographic projection. Cluster A similarly shows steeply dipping oblique slip normal faults in the Indian Ocean, and cluster B shows a normal fault, on the lower hemisphere projection. Except for cluster C each spatial cluster shows a high-degree of kinematic coordination. Figure 2. The Centroid Moment Tensor allows the identification of two orthogonal conjugate fault plane solutions. These have the property that the pole of one fault plane is the slip line of the other. The sense of relative displacement on one plane is also the conjugate of the other. This means that if one solution is a normal fault then so is the other. Similarly that if one solution is a reverse fault then so is the other. The equakes program uses this geometry to classify earthquake types. Figure 3. (a) earthquake hypocenters superimposed on an image derived from NOAA topographic data [ ]. The equakes program has been used to identify fault types. Red dots show thrusts or reverse faults. Blue dots show normal faults. Yellow dots show strike-slip faults. Normal faults dominate in the northern region, where slow slip occurred after the earthquake (Vigny et al., 2005). (b) Slip lines are plotted for the solutions chosen in the course of this analysis, illustrating the substantial difference in aftershock behaviour between north and south.

16 14 Figure 4. equakes has been used to distinguish earthquake types for aftershocks in the 12 months subsequent to the 2004 Great Sumatran Earthquake: a) for thrust and reverse faults, on the upper hemisphere of a stereographic projection; b) normal faults, and c) strike-slip faults, on lower hemisphere projections. Green (or dark) dots represent left-lateral faults, while gold (or grey) dots represent rightlateral faults. The Nicobar swarm (cluster C on Fig. 1) has been excluded, as has the 2006 Andaman aftershock swarm (cluster B on Fig. 1). Note the distinct clusters defined by thrusts and reverse fault solutions. Figure 5. The Nicobar aftershock swarm (cluster C on Fig. 1) may result because two competing wrench systems intersect: a) shows all solutions; b) normal faults; and c) strike slip faults, all on lower-hemisphere projections. Orientation groups C1 and C3 merge, and may represent ENE-WSW trending normal faults parallel to the Andaman spreading ridges, but here moving with left-lateral strike-slip and some dip-slip motion. Orientation groups C4 and C2 may represent differently oriented ~N-S trending right-lateral wrench faults accommodating relative plate motion. The geometric incompatibilities produced by these competing movement patterns may be responsible for the vigour of aftershocks in the Nicobar cluster. Figure 6. The Nicobar swarm examined at higher resolution shows alignment of aftershocks along trends as mentioned in the text. N-S trends are parallel to strands of the Andaman Sea transforms, whereas the ENE-WSW trends are parallel to inferred spreading centres, or normal fault tilt blocks. Figure 7. The geodynamic model that best allows explanation for the observed aftershock pattern is westward gravity-driven collapse of the extending Indonesian crust above a low-strength detachment, here represented by the rupture plane of the 2004 Great Sumatran Earthquake.

17 15 Supplemental information equakessumatrasolutions.txt is a TAB-separated text file that can be read by program equakes after data for the Sumatra region has been read from NDK formatted files as provided by the Global CMT project. After reading this file hit the Reload CMT solutions button on the MapWindow and you will see a classified set of earthquake data which is the same as that described in this paper.

18 Figure 01 PSD Click here to download high resolution image

19 Figure 02 PSD Click here to download high resolution image

20 Figure 03a PSD colour Click here to download high resolution image

21 Figure 03b PSD colour Click here to download high resolution image

22 Figure 04 PSD print version Click here to download high resolution image

23 Figure 04 PSD web + PDF version Click here to download high resolution image

24 Figure 05 PSD colour Click here to download high resolution image

25 Figure 06 PSD Click here to download high resolution image

26 Figure 07 PSD Click here to download high resolution image

27 Supplementary material for on-line publication only Click here to download Supplementary material for on-line publication only: equakessumatrasolutions.txt

28 * Revision Notes INSTITUTE OF ADVANCED STUDIES RESEARCH SCHOOL OF EARTH SCIENCES Canberra ACT 0200 Australia Professor Gordon Lister Tel: Fax: gordon.lister@anu.edu.au 4 th March, 2007 Re: Structural analysis of aftershock sequences from the Great Sumatran Earthquakes Gordon Lister, Brian Kennett, Marnie Forster and Simon Richards Dear Rob, Please find our resubmitted manuscript, as above. In the cover letter I included a revised list of possible reviewers. The revision has formatting correct (I hope) and a few minor changes. I added one figure and deleted another. There will be two colour figures in the print version (figs. 3 and 5). all the best gordon