direct evidence of active deformation in the eastern indian oceanic plate conventional plate tectonics theory postulates that plates only deform on their boundaries to the contrary there is ample evidence of intraplate deformation in the equatorial indian ocean west of the ninetyeast aseismic ridge prior to this study no direct evidence of deforma tion east of the ninetyeast ridge was available we present the results of a multipurpose geo physical cruise showing that intraplate deformation also occurs in this area long at least 1000 km left lateral north south strike slip faults are active and reactivate fossil fracture zones this style of deformation is strikingly different from the east west folds and reverse faults that affect the region west of the ninetyeast ridge contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation west of the ninetyeast ridge there is a continent continent collision and east of the ridge oceanic lithosphere subducts along the sumatra trench the ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas

splay fault activity revealed by aftershocks of the 2010 mw 8 8 maule earthquake central chile splay faults large thrust faults emerging from the plate boundary to the seafloor in subduction zones are considered to enhance tsunami generation by transferring slip from the very shallow dip of the megathrust onto steeper faults thus increasing vertical displacement of the seafloor these structures are predominantly found offshore and are therefore difficult to detect in seismicity studies as most seismometer stations are located onshore the mw moment magnitude 8 8 maule earthquake on 27 february 2010 affected 500 km of the central chilean margin in response to this event a network of 30 ocean bottom seismometers was deployed for a 3 month period north of the main shock where the highest coseismic slip rates were detected and combined with land station data providing onshore as well as offshore coverage of the northern part of the rupture area the aftershock seismicity in the northern part of the survey area reveals for the first time a well resolved seismically active splay fault in the submarine forearc application of critical taper theory analysis suggests that in the northernmost part of the rupture zone coseismic slip likely propagated along the splay fault and not the subduction thrust fault while in the southern part it propagated along the subduction thrust fault and not the splay fault the possibility of splay faults being activated in some segments of the rupture zone but not others should be considered when modeling slip distributions

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

evidence of active backthrusting at the ne margin of mentawai islands sw sumatra summary the indo australian plate subducts obliquely beneath the sunda plate leading to a slip partitioning into pure thrust and strike slip motion just in the last 5 yr three pure thrust earthquakes of mw 8 4 have occurred along this subduction interface the great sumatra fault traversing the sumatra continental block takes up a significant part of the strike slip motion but the mentawai fault bounding the ne margin of mentawai islands has been suggested to accommodate a part of the strike slip motion although the great sumatra fault is active no seismicity has been observed along the mentawai fault using a combination of high resolution seismic reflection and bathymetry data here we show that the mentawai fault seems to be characterized by active sw dipping backthrusts the presence of recent steeply dipping thrust earthquakes suggests that these faults should be active combined with results from north in 2004 earthquake region and south of this study area our results suggest that backthrusting should play an important role in forearc evolution sw of sumatra we also observed several mass wasting sites at ne margin of the mentawai islands which could be erosional features or landslides triggered by earthquake activities localized uplift along the steeply dipping backthrusts at the ne margin of mentawai islands in the fully locked region could pose serious seismic and tsunami risks to the sw coast of sumatra in the future

palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the late archaean witwatersrand basin oxygen deficient atmosphere or hydrothermal alteration petrographic and sulphur isotope studies support the long held contention that rounded grains of pyrite in siliciclastic sequences of the late archaean witwatersrand supergroup originated as placer grains the grains areconcentrated at sites where detrital heavy minerals are abundant within quartz pebble conglomerates and quartzose sandstones depositional sites with abundant pyrite are 1 within the matrix of bar type clast supported conglomerates 2 on scoured or winnowed surfaces and 3 on stratification planes the grains are internally compact or porous with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source rocks during erosion and sediment transport a large range in textures reflects source rock lithologies with known varieties linked to sedimentary hosted diagenetic pyrite volcanic hosted massive sulphide deposits and hydrothermal pyrite laser ablation sulphur isotope analysis of pyrite reveals a broader range in 3 4 s values 5 3 to 6 7 than that of previously reported conventional bulk grain analyses 1 to 4 rounded pyrite from the steyn reef has significant variation in 3 4 s values 4 7 to 6 7 that establishes heterogeneous sulphur compositions with even adjacent grains having diverse isotopic signatures the heterogeneity supports a placer origin for rounded pyrite euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted 3 4 s values 0 5 to 2 5 are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration the placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere its distribution in three sequences witwatersrand supergroup ventersdorp contact reef and black reef and in other sequences not linked to witwatersrand type au u ore deposits implies deposition of redox sensitive detrital heavy minerals during the late archaean consequently rounded grains of detrital pyrite are strong indicators of an oxygen poor atmosphere while not confirming a placer origin for gold in witwatersrand au u ore deposits the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization

limits of the seismogenic zone in the epicentral region of the 26 december 2004 great sumatra andaman earthquake results from seismic refraction and wide angle reflection surveys and thermal modeling the 26 december 2004 sumatra earthquake mw 9 1 initiated around 30 km depth and ruptured 1300 km of the indo australian sunda plate boundary during the sumatra obs ocean bottom seismometer survey a wide angle seismic profile was acquired across the epicentral region a seismic velocity model was obtained from combined travel time tomography and forward modeling together with reflection seismic data from the seacause ii cruise the deep structure of the source region of the great earthquake is revealed four to five kilometers of sediments overlie the oceanic crust at the trench and the subducting slab can be imaged down to a depth of 35 km we find a crystalline backstop 120 km from the trench axis below the fore arc basin a high velocity zone at the lower landward limit of the ray covered domain at 22 km depth marks a shallow continental moho 170 km from the trench the deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault assuming 100 150 c as its updip limit the seismogenic zone is predicted to begin 5 30 km from the trench the downdip limit of the 2004 rupture as inferred from aftershocks is within the 350 450 c temperature range but this limit is 210 250 km from the trench axis and is much deeper than the fore arc moho the deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate

india and sunda plates motion and deformation along their boundary in myanmar determined by gps using a regional gps data set including 190 stations in asia from nepal to eastern indonesia and spanning 11 years we update the present day relative motion between the indian and sundaland plates and discuss the deformation taking place between them in myanmar revisiting measurements acquired on the main boundary thrust in nepal it appears that points in southern nepal exhibit negligible deformation with respect to mainland india including these points using a longer time span than previous studies and making an accurate geodetic mapping in the newest reference frame allows us to refine the present day indian motion our results confirm that the current motion of india is slower than predicted by the nuvel 1a model and in addition our india eurasia motion is significantly 5 mm yr slower than previous geodetic determinations this new indian motion combined with a refined determination of the sundaland motion gives way to a relative india sunda angular velocity of 20 2 n 26 1 e 0 370 myr in itrf2000 predicting a relative motion of 35 mm yr oriented n10 at the latitude of myanmar there the sagaing fault accommodates only 18 mm yr of right lateral strike slip only half of the shear component of motion we present two models addressing how and where the remaining deformation may occur a first model of distributed deformation implies convergence on the arakan subduction the northern continuation of the now famous sumatra andaman trench and wrench faulting in the arakan wedge the second model uses localized deformation where deformation observed west of the sagaing fault is entirely due to elastic loading on a faster and oblique arakan subduction 23 mm yr this latter model predicts that a major earthquake of mw 8 5 may occur every century on this segment of the subduction

possible splay fault slip during the 1946 nankai earthquake this paper considers seafloor deformation associated with the 1946 nankai earthquake which occurred in the nankai trough subduction zone off the coast of southwest japan we note that recent high resolution determinations of the slip distribution assumed to be confined to the plate boundary result in a pronounced maximum in seafloor displacement midway between the coast of shikoku and the trough axis we show that this feature could be due to slip on a splay fault rather than the plate boundary such a model for the western part of the slip distribution is able to account for the main features of the tsunamigenic seafloor deformation obtained in previous studies with a maximum slip consistent with the plate convergent rate and history of earthquake recurrence at the nankai trough

spatial aftershock distribution of the 26 december 2004 great sumatra andaman earthquake in the northern sumatra area 1 in july 2005 about 6 months after the 2004 great sumatra andaman earthquake 20 ocean bottom seismometers were deployed in the northern sumatra area more than 1000 events were identified during the 12 day recording period after relocation the seismicity shows different patterns on each side of the lower splay fault east of this feature and beneath the aceh basin and fore arc the deep earthquakes outline the subduction megathrust fault west of it the aftershocks distribution is strongly influenced by the active n s oceanic fracture zones of the subducted plate two n s trending clusters of 10 to 50 km deep earthquakes observed below the lower wedge are interpreted as reactivated oceanic fracture zones the postseismic activity suggests a significant influence of the n s active fracture zones of the oceanic plate on the toe of the wedge explaining the dextral wrenching of wedge thrusts in the morphology the megathrust fault does not extend to the trench but outcrops east of these active oceanic fractures and seems to merge upward along the lower splay fault where shallow earthquakes are observed

seismic images of the sliver strike slip fault and back thrust in the andaman nicobar region 1 the sliver strike slip great sumatra fault gsf traverses mainland sumatra from the sunda strait in the southeast to banda aceh in the northwest and defines the present day plate boundary between the sunda plate in the north and the burmese sliver plate in the south it has been well studied on mainland sumatra but poorly north of banda aceh in the andaman sea here we present deep seismic reflection images along the northward extension of the gsf over 700 km until it joins the andaman sea spreading centre and we interpret these images in the light of earthquake gravity and bathymetry data we find that the gsf has two strands between banda aceh and nicobar island a transpression in the south and a deep narrow active rift system in the north dotted with volcanoes in the center suggesting that the volcanic arc is coincident with rifting farther north of nicobar island an active strike slip fault the andaman nicobar fault cuts through a rifted deep basin until its intersection with the andaman sea spreading centre the volcanic arc lies just east of the rift basin the western margin of this basin seems to be a rifted continental margin tilted westward and flooring the andaman nicobar fore arc basin the andaman nicobar fore arc basin is bounded in the west by back thrusts similar to the west andaman and mentawai faults the cluster of seismicity after the 2004 great andaman sumatra earthquake just north of nicobar island coincides with the intersection of two strike slip fault systems

structural architecture of oceanic plateau subduction offshore eastern java and the potential implications for geohazards the region offshore eastern java represents one of the few places where the early stage of oceanic plateau subduction is occurring we study the little investigated roo rise oceanic plateau on the indian plate subducting beneath eurasia the presence of the abnormal bathymetric features entering the trench has a strong effect on the evolution of the subduction system and causes additional challenges on the assessment of geohazard risks we present integrated results of a refraction wide angle reflection tomography gravity modelling and multichannel reflection seismic imaging using data acquired in 2006 south of java near 113 e the composite structural model reveals the previously unresolved deep geometry of the oceanic plateau and the subduction zone the oceanic plateau crust is on average 15 km thick and covers an area of about 100 000 km2 within our profile the roo rise crustal thickness ranges between 18 and 12 km the upper oceanic crust shows high degree of fracturing suggesting heavy faulting the forearc crust has an average thickness of 14 km with a sharp increase to 33 km towards java as revealed by gravity modelling the complex geometry of the backstop suggests two possible models for the structural formation within this segment of the margin either accumulation of the roo rise crustal fragments above the backstop or alternatively uplift of the backstop caused by basal accumulation of crustal fragments the subducting plateau is affecting the stress field within the accretionary complex and the backstop edge which favours the initiation of large potentially tsunamogenic earthquakes such as the 1994 mw 7 8 tsunamogenic event

neotectonics and the 2004 and 2005 earthquake sequences at sumatra coseismic and interseismic deformation of the outer arc islands of nias and simeulue associated with the great indonesian earthquakes of 2004 12 26 and 2005 3 28 is generally ascribed to elastic stress storage and release on the interface between the indian and eurasian plates at the java trench however the nature of the associated shallow seismicity and the pattern of holocene deformation of the islands suggest that there has also been substantial activity on imbricate faults within the sediment prism behind the trench the distinction between the two kinds of seismicity is of practical as well as theoretical significance because in contrast with the megathrust outer arc faulting is not subject to appreciable postseismic relaxation and is moreover well placed to generate tsunami

26th december 2004 great sumatra andaman earthquake co seismic and post seismic motions in northern sumatra trench parallel thrust faults verging both landward and seaward were mapped in the portion of wedge located between northern sumatra and the indian indonesian boundary the spatial aftershocks distribution of the 26th december 2004 earthquake shows that the post seismic motion is partitioned along two thrust faults the lower and median thrust faults the latter being right laterally offset by a n s lower plate fracture zone located along the 93 6 n meridian between february 2005 and august 2005 the upper plate aftershock activity shifted from southeast of this fracture zone to northwest of it suggesting that the lower plate left lateral motion along the fracture zone may have induced a shift of the upper plate post seismic activity along the median thrust fault based on swath bathymetric and 3 5 khz data co seismic deformations were weak close to the trench joint seismic geodetic determination of slip distribution and time arrivals and heights of tsunami waves suggest that the co seismic slip was maximum along a portion of the upper thrust fault located north of the tuba ridge suggesting that the upper thrust fault might be a splay fault originated at the interplate fault plane as the upper thrust fault is steeper than the slab the vertical motion of the adjacent outer arc and overlying water is much larger compared to the one resulting from slip on the megathrust alone increasing tsunamogenic effects

structural geology and the seismotectonics of the 2004 great sumatran earthquake y the authors acknowledge funding support from the australian research council discovery project dp120103554 a unified model for the closure dynamics of ancient tethys constrained by geodesy structural geology argon geochronology and tectonic reconstruction and linkage project lp130100134 where to find giant porphyry and epithermal gold and copper deposits the research was also supported by the satellites seismometers and mass spectrometers initiative within the research school of earth sciences at anu bob engdahl is thanked for his support during earlier versions and for providing data

gold and sulphide minerals in tertiary quartz pebble conglomerate gold placers southland new zealand abstract auriferous quartz pebble conglomerates qpc formed during tertiary sedimentary recycling in the waimumu district southland new zealand these sediments contain fine grained gold of detrital origin with abundant surface textures and gold forms associated with authigenic gold remobilisation most authigenic gold contains no detectable silver and occurs as overgrowths on detrital au ag and au ag hg alloys that contain up to 13 wt ag and 9 wt hg fine grained au ag and au ag hg alloys are compositionally heterogeneous exhibiting both well defined silver depleted and silver enriched rims rare coarse au ag alloy is intergrown with quartz and is homogenous discrete grains of authigenic porous sheet like gold occur in carbonaceous mudstone within a qpc sequence some qpc contain abundant sulphide minerals some of these sulphides pyrite and arsenopyrite are of long distance detrital origin presumably from the otago schist whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin derived from the erosion of qpc and underlying tertiary sediments there has also been authigenic deposition of sulphide minerals in the qpc themselves these diagenetic sulphides include framboidal and anhedral marcasite and framboidal and euhedral pyrite sulphur isotope data for the sulphide minerals range from 45 to 18 relative to vcdt sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from 9 to 1 and are most likely derived from the otago schist to the north both framboidal and anhedral marcasite have lower values both the gold and diagenetic sulphides from the belle brook qpc are compositionally similar to gold and sulphides from archaean qpc porous sheet like authigenic gold is morphologically similar to gold associated with carbonaceous material in the witwatersrand in addition southland marcasite textures resemble the rounded and banded pyrite in witwatersrand qpc placers there is abundant evidence from these tertiary qpc in southern new zealand for sedimentary transport of sulphide minerals and post depositional sulphide mineralisation in the surficial environment despite an oxygen rich atmosphere these young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers many of these textures are similar to those commonly ascribed to metamorphic processes in archaean auriferous qpc

structural evolution of connecting splay duplexes and their implications for critical taper an example based on geometry and kinematics of the canyon range culmination sevier belt central utah abstract critical taper in an evolving orogenic wedge can be maintained by structural thickening in the back of the wedge this is often achieved by folding and uplift of pre existing large internal thrust sheets by the continued growth and or reactivation of duplexes in the hinterland the growth of a blind imbricate complex off a lower thrust provides another mechanism for folding a higher thrust individual splays in a blind imbricate complex may have fault propagation folds and or fault bend folds associated with them if the splays reach the overlying thrust they serve as connecting splays between the lower thrust floor and the upper thrust roof thereby forming a connecting splay duplex csd this type of duplex geometry requires periodic reactivation of the pre existing roof thrust the canyon range thrust in central utah is an internal thrust that is folded into an overturned antiform synform pair and overrides its own synorogenic conglomerates the same conglomerates are infolded into the core of the synform and preserve critical timing information regarding the history of folding and thrust reactivation the folding is caused by the development of a footwall antiformal stack in the core of the canyon range antiformal culmination the geometry and timing relationships suggest that the structure grew as a csd regional timing relationships suggest that the growth of the csd and reactivation of the thrust took place during the later part of the evolution of the orogenic wedge and may have helped the wedge maintain critical taper during thrusting in the external fold and thrust belt

lessons learned from the 2004 sumatra andaman megathrust rupture the 2004 sumatra andaman earthquake has been extensively studied because of its great size and devastating consequences large amounts of highquality seismic geodetic and geologic data have led to a number of proposed models for its length duration fault geometry rupture velocity and slip history the latest of these models vary in their details but now largely agree in their large scale features which include significant coseismic slip along the entire 1300 to 1500 km rupture the bulk of which occurred fast enough to radiate seismic waves the earthquake s enormous size has challenged conventional processing approaches and stimulated the development of new analysis and inversion methods including multiple source inversions highfrequency body wave imaging and satellite observations of tsunami heights and gravity changes the sumatra megathrust earthquake was the largest in 40 years and is by far the best documented but it does not seem fundamentally different in its properties from other large subduction zone earthquakes

seismicity gravity anomalies and lithospheric structure of the andaman arc ne indian ocean abstract the andaman arc in the northeastern indian ocean defines nearly 1100 km long active plate margin between the india and burma plates where an oblique benioff zone develops down to 200 km depth several east trending seismologic sections taken across the andaman benioff zone abz are presented here to detail the subduction zone geometry in a 3 d perspective the slab gravity anomaly computed from the 3 d abz configuration is a smooth long wavelength and symmetric gravity high of 85 mgal amplitude centering to the immediate east of the nicobar island where a prominent gravity high follows the nicobar deep the slab residual gravity anomaly srga and mantle bouguer anomaly mba maps prepared for the andaman plate margin bring out a double peaked srga low in the range of 150 to 240 mgal and a wider cum larger mba low having the amplitude of 280 to 315 mgal demarcating the andaman arc trench system the gravity models provide evidences for structural control in propagating the rupture within the lithosphere the plate margin configuration below the andaman arc is sliced by the west andaman fault waf as well as by a set of sympathetic faults of various proportions often cutting across the fore arc sediment package some of these fore arc thrust faults clearly give rise to considerably high post seismic activity but the seismic incidence along the waf further east is comparatively much less particularly in the north although the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults tectonic correlation of the gravity derived models presented here tends to favour the presence of oceanic crust below the andaman nicobar outer arc ridge

a tribute to george plafker abstract in a long and distinguished career george plafker made fundamental advances in understanding of megathrust tectonics tsunami generation paleoseismology crustal neotectonics and alaskan geology chiefly by means of geological field observations george discovered that giant earthquakes result from tens of meters of seismic slip on subduction megathrusts and he did this before the theory of plate tectonics had become a paradigm the discovery was founded on george s comprehensive mapping of land level changes in the aftermath of the 1964 earthquake in alaska and on his follow up mapping in 1968 in the region of the 1960 earthquakes in chile the mapping showed paired parallel belts of coseismic uplift largely offshore and coseismic subsidence mostly onshore a pattern now familiar as the initial condition assumed in simulations of subduction zone tsunamis george recognized moreover that splay faulting can play a major role in tsunami generation and he also distinguished carefully between tectonic and landslide sources for the multiple tsunamis that accounted for nearly all the fatalities associated with the 1964 alaska earthquake george s classic monographs on the 1964 earthquake include findings on subduction zone paleoseismology that he soon extended to include stratigraphic evidence for cyclic vertical deformation at the copper river delta as well as recurrent uplift evidenced by flights of marine terraces at middleton island as a geologist of earthquakes george also clarified the tectonics and hazards of crustal faulting in alaska california and other areas worldwide all the while george was mapping bedrock geology in alaska where he contributed importantly to today s understanding of how terranes were accreted and modified especially important was his documentation of the origin movement subduction and collision of the yakutat terrane in southern alaska

what 2 d multichannel seismic and multibeam bathymetric data tell us about the north sumatra wedge structure and coseismic response recent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world s major subduction margins our study area covers the entire fore arc from northwest of northern sumatra to west of simeulue island the southern portion of the 2004 tsunamigenic earthquake rupture zone the accretionary prism width is up to 180 km water depths between 4 5 km near the sunda trench and 1 km on fore arc high the wedge consists of a steep outer slope 5 12 a plateau 100 120 km wide with anticlinal folds spaced 2 15 km apart and a steep inner slope adjacent to the aceh basin analysis of seismic profiles and bathymetry reveal three main structural zones consistent along strike from the trench landward 1 predominantly landward vergent folds 2 mixed vergent folds and 3 predominantly seaward vergent folds this paper uses those zones to propose a geometry of an underlying rigid backstop this backstop is seaward dipping and extends from under the aceh basin to beneath the mixed vergence zone a dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism indurated accreted sediments form the landward vergence zone along with the possible dynamic backstop beneath the outer wedge and the rigid backstop in the inner wedge all behave as a solid block coseismically this block allows great earthquake rupture to propagate farther seaward toward the sunda trench with resultant hazardous tsunamigenic potential

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

tsunami wave analysis and possibility of splay fault rupture during the 2004 indian ocean earthquake the 2004 indian ocean tsunami was observed by two satellites close in space and time that traversed the indian ocean 2 h after the sumatra andaman earthquake but which observed different tsunami lead wave morphologies the earlier satellite jason 1 recorded a lead wave with two peaks of similar amplitude and wavelength while the later satellite topex poseidon recorded a lead wave with only one longer wavelength uplift to resolve this disparity we examine the travel paths of long wavelength waves over the seafloor bathymetry waves traveling from the margin will traverse significantly different paths to arrive at the two satellite transects the result is that the satellites are sensitive to different parts of the margin jason 1 is highly sensitive to the margin in the area of the epicenter while topex is sensitive to a more northerly section by developing solutions of the ocean gravity wave equations accounting for dispersion we show that the double peak of the jason 1 satellite observations are consistent with coseismic rupture of a splay fault of limited along strike extent located north of simeulue island the doubly peaked morphology can be reproduced with co activation of the subduction zone interface and the splay fault which creates a seafloor uplift pattern with two distinct areas of uplift the jason 1 satellite is sensitive to a splay fault in this portion of the margin whereas the topex satellite would not be significantly affected by this uplift pattern by back projecting satellite observation points to the margin we constrain the location of the proposed splay fault and find that it correlates with a bathymetric high the aftershock locations uplift of corals on simeulue island and a fault scarp on pulau salaut besar are also consistent with the activation of a splay fault in the area delimited by the back projection our work also shows that it is critical to fully capture gravity wave dispersion in order to represent features of the lead wave profile that may not be as well characterized by the shallow water long wavelength model it is also necessary to account for dispersion so as to precisely assess wavefront travel times this leads us to conclude that the rupture must have reached very near to the trench and propagated with an updip rupture velocity of order 2 0 km s or more

teleseismic double difference relocation of earthquakes along the sumatra andaman subduction zone using a 3 d model 1 we have extended the double difference seismic tomography method to teleseismic distances with 3 d ray tracing conducted through nested regional global velocity models and applied the method to relocate seismicity from the sumatra andaman region before and after the great earthquakes of 2004 and 2005 we tested the algorithm s accuracy using both independent local data and an alternate relocation method and found good agreement between the results the use of depth phases differential times and a realistic 3 d velocity model improves the accuracy and precision of epicenters and focal depths systematically shifting them perpendicular to the trench and shallower respectively the relocations refine the location of the megathrust and other faults the patterns of aftershocks and their relation to slip during the two great earthquakes in addition the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs including an arcuate narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the investigator fracture zone when viewed in conjunction with tomography results the geometry and structural features of the subduction zone are revealed in unprecedented detail

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

geological structure of the offshore sumatra forearc region estimated from high resolution mcs reflection survey abstract to investigate detailed fault distributions and shallow geological structure offshore northwestern sumatra we obtained high resolution multi channel seismic mcs reflection data around the sunda trench trench slope and forearc high regions offshore northwestern sumatra in general trench parallel anticlinal ridges are distributed from trench slope region to forearc high region two kinds of different vergence systems are characterized in the sumatra forearc region landward vergence is dominant in the lower trench slope region and seaward vergence is dominant in the forearc high region moreover piggyback or slope basins are recognized between anticlinal ridges deformation in the uppermost part of these basins that is referred to recent deformation in this paper can be identified not only along major thrusts but also between major thrusts and the lower trench slope suggesting these are related to recently active faulting several but the largest number of such deformation are distributed along a major thrust located in the middle of the forearc high region whereas few are done along other major thrusts

study on co seismic energy losses from hypocenter to ocean bottom for sumatra earthquake 2004 using 3 d crustal deformation model our co seismic grace gravity data level 2 rl 05 data product gx og 2 gsm for sumatra earthquake 2004 is obtained by differencing monthly gravity field average for november 2004 from that of january 2005 and band pass filtering 17 30 degrees and orders in spectral domain here we propose an 11 layered 3 d thrust fault gravity model based on several co seismic rupture models in literature previously we have covered the 3 d modelling details and its inferences like slip rate seismic moment momentum etc in our published literature further we extend the inferences through our model for this case study here we have estimated the layer wise energy distribution by undertaking two types energy loss one is spherical spreading and second absorption with constrained by literature we have computed layer wise energy loss equivalent energy differential pressure slip rate ultimate slip and work done the computed differential pressure and work done for sumatra earthquake 2004 are 1 7552 108 n m2 and 1 657 1018 j respectively we also estimated the absorption coefficient calculated absorption coefficients from our model to honour the slip rate of sumatra earthquake 2004 the differential pressure is estimated for ocean bottom and sea level surface the volumetric analysis is also provided for entire 3 d body layer wise using excess mass of our model the computed differential pressure indeed corresponds to an area pulse at ocean bottom that led to a tsunami generation

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

seismic evidence for broken oceanic crust in the 2004 sumatra earthquake epicentral region the 2004 sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust this observation coupled with the hypocentres of aftershocks suggests that the megathrust the interface between the indo australian plate and the sunda plate currently lies in the oceanic mantle

the great sumatra andaman earthquakes imaging the boundary between the ruptures of the great 2004 and 2005 earthquakes segmentation along convergent margins controls earthquake magnitude and location but the physical causes of segment boundaries and their impact on earthquake rupture dynamics are still poorly understood one aspect of the 2004 and 2005 great sumatra andaman earthquakes is their abrupt termination along a common boundary this has led to speculation on the nature of the boundary its origin and why it was not breached for the first time the boundary has been imaged and with newly acquired marine geophysical data we demonstrate that a ridge on the subducting indo australian oceanic crust may exert a control on margin segmentation this suggests a lower plate influence on margin structure particularly its segmentation the ridge is masked by the sedimentary cover in the trench its most likely trend is nne ssw it is interpreted as a fracture zone on the subducting oceanic plate a ramp or tear along the eastern flank of the subducting fracture zone beneath simeulue island may be considered as an intensification factor in terms of rupture propagation barrier

subduction system variability across the segment boundary of the 2004 2005 sumatra megathrust earthquakes subduction zone earthquakes are known to create segmented patches of co seismic rupture along strike of a margin offshore sumatra repeated rupture occurred within segments bounded by permanent barriers whose origin however is still not fully understood in this study we image the structural variations across the rupture segment boundary between the mw 9 1 december 26 2004 and the mw 8 6 march 28 2005 sumatra earthquakes a set of collocated reflection and wide angle seismic profiles are available on both sides of the segment boundary located offshore simeulue island we present the results of the seismic tomography modeling of wide angle ocean bottom data enhanced with mcs data and gravity modeling for the southern 2005 segment of the margin and compare it to the published model for the 2004 northern segment our study reveals principal differences in the structure of the subduction system north and south of the segment boundary attributed to the subduction of 96 e fracture zone the key differences include a change in the crustal thickness of the oceanic plate a decrease in the amount of sediment in the trench as well as variations in the morphology and volume of the accretionary prism these differences suggest that the 96 e fracture zone acts as an efficient barrier in the trench parallel sediment transport as well as a divider between oceanic crustal blocks of different structure the variability of seismic behavior is caused by the distinct changes in the morphology of the subduction complex across the boundary related to the difference in the sediment supply

enhanced gps inversion technique applied to the 2004 sumatra earthquake and tsunami 1 since the devastating earthquake and tsunami in 2004 offshore sumatra many source models have been put forward recent studies clearly show that modern gps processing could achieve high resolving power for slip in near real time which is crucial for determining tsunami initial conditions provided accurate gps processing and inversion here we propose an inversion technique with improved representation of the subduction zone geometry and physically justified boundary conditions we show that the discrepancy between the inversion of near and far field gps data for the 2004 event which is often explained by postseismic slip can be eliminated by using our inversion method and iasp91 earth model inverted source models including versions with splay faulting are shown to be consistent with satellite altimetry data of offshore tsunami wave height suggesting that displacement at the splay fault might have been present but was likely a second order process

active deformation across the sumatran forearc over the december 2004 mw9 2 rupture a 220 km long single channel seismic reflection profile crosses the northern sumatra margin and presumed rupture zone of the december 2004 m w 9 2 tsunamigenic earthquake and images active deformation across the forearc at the largest wavelength tens of kilometers the forearc surface is defined by a steep 55 km wide outer slope a 110 km wide upper slope forming a broad depression between two forearc highs and a 25 km wide steep inner slope between the landward high and forearc basin superimposed on these prism wide variations are anticlinal ridges spaced 13 km apart the inner and outer slopes are characterized by landward and seaward fold vergence respectively between anticlines growth strata deposited in slope basins are folded at 2 3 km wavelengths these small folds deform the seafloor and increase in amplitude with depth verging toward anticlinal hinges we suggest that long wavelength variations are consistent with variations in strength across the forearc the 13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior growth strata geometries indicate ongoing deformation within individual basins our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 m w 9 2 event peeling up shallower and less competent trench fill deforming the toe and the upper slope of the forearc and producing seabottom uplift responsible for the tsunami

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

are rupture zone limits of great subduction earthquakes controlled by upper plate structures evidence from multichannel seismic reflection data acquired across the northern ecuador southwest colombia margin 1 subduction of the nazca plate beneath the ecuador colombia margin has produced four megathrust earthquakes during the last century the 500 km long rupture zone of the 1906 mw 8 8 event was partially reactivated by three thrust events in 1942 mw 7 8 1958 mw 7 7 and 1979 mw 8 2 whose rupture zones abut one another multichannel seismic reflection and bathymetric data acquired during the sisteur cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones the paleogene neogene jama quininde and esmeraldas crustal faults define a 200 km long margin crustal block that coincides with the 1942 earthquake rupture zone subduction of the buoyant carnegie ridge is inferred to partially lock the plate interface along central ecuador however coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge we report on a newly identified manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones during the earthquake cycle the fault is associated with high stress concentration on the plate interface an outer basement high which bounds the margin seaward of the 1958 rupture zone may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami we propose a model of weak transverse faults which reduce coupling between adjacent margin segments together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake

structure and kinematics of the indo burmese wedge recent and fast growth of the outer wedge 1 the northern sunda subduction zone offshore burma and the associated indo burmese wedge mark the active eastern boundary of the burma platelet jammed between the india plate and the sunda plate the aim of this paper is to provide a structural and kinematics analysis of the indo burmese wedge based on seismic reflection geodetic and geological field data we found out that the indo burmese wedge is the place of diffuse strain partitioning right lateral shearing in the innermost part and e w shortening in the outermost part in the outer wedge thick skinned deformation overprints thin skinned deformation it can be explained by the required preservation of the critical taper after the fast westward propagation of the outer wedge above a very efficient clayey decollement layer the thick skinned deformation is here characterized by major right lateral strike slip faults the kaladan fault and the chittagong coastal fault and yet a typical internal deformation of the indo burmese wedge we suggest that the westward migration of internal right lateral shear deformation allows preservation of the strain partitioning ratios between internal n s trending right lateral shearing and external e w shortening some seismic lines show that the outer wedge deformation is not older than 2 ma its propagation could have been enhanced by the large amount of sediments filling up the sylhet flexural basin formed after the pliocene tectonic uplift of the shillong plateau

seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 great sumatra earthquake rupture zone abstract in subduction zones the plate interface megathrust is typically poorly imaged at depths 12 km however its precise geometry and nature as well as the positions of updip and downdip limits of the seismogenic zone are important elements to understand the generation of megathrust earthquakes using deep marine seismic reflection and refraction data we observed discontinuous reflections off the top of the subducting oceanic crust down to 60 km depth in the 2004 great sumatra andaman earthquake rupture zone we find that the top of the downgoing plate does not dip gently into the subduction zone but instead displays a staircase geometry with three successive 5 15 km vertical steps spaced 50 km apart micro earthquake data indicate that most of the seismicity lies below this interface suggesting that the oceanic plate is deforming actively along part of the profile we also image a second reflector located 8 10 km below the top of the oceanic crust the forward modelling of the gravity data along the profile supports the presence of a high density material above this reflector the presence of a staircase shape for the top of the oceanic crust together with constraints from gravity data and earthquake data require that the megathrust goes through this second reflector this leads us to conclude that the megathrust is at least partly located in the oceanic mantle and that underplating of oceanic crust beneath the wedge and underplating of upper mantle beneath the forearc basin are taking place in this region

plate boundary deformation associated with the great sumatra andaman earthquake the sumatra andaman earthquake of 26 december 2004 is the first giant earthquake moment magnitude m w 9 0 to have occurred since the advent of modern space based geodesy and broadband seismology it therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events here we report estimates of the ground displacement associated with this event using near field global positioning system gps surveys in northwestern sumatra combined with in situ and remote observations of the vertical motion of coral reefs these data show that the earthquake was generated by rupture of the sunda subduction megathrust over a distance of 1 500 kilometres and a width of 150 kilometres megathrust slip exceeded 20 metres offshore northern sumatra mostly at depths shallower than 30 kilometres comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1 5 months following the 500 second long seismic rupture both seismic and aseismic slip before our re occupation of gps sites occurred on the shallow portion of the megathrust where the large aceh tsunami originated slip tapers off abruptly along strike beneath simeulue island at the southeastern edge of the rupture where the earthquake nucleated and where an m w 7 2 earthquake occurred in late 2002 this edge also abuts the northern limit of slip in the 28 march 2005 m w 8 7 nias simeulue earthquake

lower plate structure and upper plate deformational segmentation at the sunda banda arc transition indonesia the sunda banda arc transition at the eastern termination of the sunda margin indonesia represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation neighboring margin segments display a high degree of structural diversity of the incoming plate transition from an oceanic to a continental lower plate presence absence of an oceanic plateau variability of subducting seafloor morphology as well as a wide range of corresponding fore arc structures including a large sedimentary basin and an accretionary prism outer arc high of variable size and shape here we present results of a combined analysis of seismic wide angle refraction multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of lombok 116 e and sumba 119 e on the incoming plate the results reveal a 8 6 9 0 km thick oceanic crust which is progressively faulted and altered when approaching the trench where upper mantle velocities are reduced to 7 5 km s the outer arc high located between the trench and the fore arc basin is characterized by sedimentary type velocities vp 5 5 km s down to the top of the subducting slab 13 km depth the oceanic slab can be traced over 70 100 km distance beneath the fore arc a shallow serpentinized mantle wedge at 16 km depth offshore lombok is absent offshore sumba where our models reveal the transition to the collisional regime farther to the east and to the sumba block in the north our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern sunda margin

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

seafloor morphology of the sumatran subduction zone surface rupture during megathrust earthquakes high resolution multibeam bathymetry data from the sumatran subduction zone reveal the regional and local morphology including small scale fault related features and landslides that may be linked to earthquakes in the recent geological past the accretionary prism is steeply sloped and pervasively eroded with evidence of unusual landward vergence seaward fault dip of the frontal thrusts small scale 5 100 m height fault scarps folds and troughs are common along the seaward edge of the frontal thrust at the deformation front a model of back thrust fault slip or bending moment folding during plate boundary slip such as during the 2004 m9 2 megathrust earthquake can explain the position of these features on the seaward fold limb of a seaward dipping thrust we infer that in major sumatran and other similar settings plate boundary earthquakes coseismic surface rupture may occur at the prism toe

hanging wall deformation of a seismogenic megasplay fault in an accretionary prism the nobeoka thrust in southwestern japan the structure and occurrence of deformation within the hanging wall of the nobeoka thrust in kyushu japan was investigated to understand the dynamic aspects of splay faulting in relation to seismic events from field observations hanging wall is suggested to have undergone four phases of deformation the first phase involved horizontal shortening as documented by folding and thrusting followed by a phase of vertical loading shown by the development of horizontal slaty cleavages pressure solution and cleavage parallel mineral vein precipitation a third phase involved shearing and deformation along cleavage restricted to near the nobeoka thrust while the fourth phase produced widespread brittle fracturing associated with the development of pseudotachylyte bearing faults and tension crack filling veins high angle to cleavage these four phases can be explained as follows during the inter seismic period an extensionally stable taper was maintained in the inner wedge of the accretionary prism by dominant vertical loading 1 in combination with a lesser amount of horizontal compression 2 related to the locking of the mega thrust elastic strain energy in the hanging wall of the inner wedge was co seismically released by slip on the mega thrust and horizontal shortening in the outer wedge associated with dynamic ductile weakening of the fault plane this sudden release of elastic strain caused brittle fracturing with 1 at a high angle to the shear surface of the nobeoka thrust most of the displacement resulting from deformation of the footwall

coseismic slip distributions of the 26 december 2004 sumatra andaman and 28 march 2005 nias earthquakes from gps static offsets static offsets produced by the 26 december 2004 m 9 sumatra andaman earthquake as measured by global positioning system gps reveal a large amount of slip along the entire 1300 km long rupture most seismic slip inversions place little slip on the andaman segment whereas both near field and far field gps offsets demand large slip on the andaman segment we compile available datasets of the static offset to render a more detailed picture of the static slip distribution we construct geodetic offsets such that postearthquake positions of continuous gps sites are reckoned to a time 1 day after the earthquake and campaign gps sites are similarly corrected for postseismic motions the newly revised slip distribution m w 9 22 reveals substantial segmentation of slip along the andaman islands with the southern quarter slipping 15 m in unison with the adjacent nicobar and northern sumatran segments of length 700 km we infer a small excess of geodetic moment relative to the seismic moment a similar compilation of gps offsets from the 28 march 2005 nias earthquake is well explained with dip slip averaging several meters mw 8 66 distributed primarily at depths greater than 20 km online material offset estimation procedure comparison of results to other stud ies and simpler solutions and tables of coseismic offsets

structure evolution and tectonic activity of the eastern sunda forearc indonesia from marine seismic investigations forearc structures of the eastern sunda arc are studied by new multichannel reflection seismic profiling we image a high along strike variability of the subducting oceanic plate the interface between subducting and overriding plate the accretionary wedge the outer arc high and forearc basins we highlight ongoing tectonic activity of the entire outer arc high active out of sequence thrust faults connecting the plate interface with the seafloor slope basins showing tilted sedimentary sequences on the outer arc high vertical displacement of young seafloor sediments and tilted sedimentary sequences in the lombok forearc basin while frontal accretion plays a minor role the growth of the outer arc high is mainly attributed to oceanic sediments and crustal fragments which are attached to the base of the upper plate and recycled within the forearc we image ongoing large scale duplex formation of the oceanic crust the incoming oceanic crust is dissected by normal faulting into 5 10 km wide blocks within a 50 70 km wide belt seaward of the deep sea trench these blocks determine the geometry and evolution of duplexes attached to the base of the overriding plate landward of the trench long lasting and ongoing subsidence of the lombok basin is documented by distinct seismic sequences in the lombok basin we image mud diapirs fed from deeply buried sediments which may have been mobilized by rising fluids we propose a wrench fault system in the eastern lombok forearc basin that decouples the subduction regime of the sunda arc from the continent island arc collision regime of the western banda arc the observed tectonic activity of the entire forearc system reflects a high earthquake and tsunami hazard similar to the western part of the sunda arc

structural evolution and strike slip tectonics off north western sumatra abstract based on new multi channel seismic data swath bathymetry and sediment echosounder data we present a model for the interaction between strike slip faulting and forearc basin evolution off north western sumatra between 2 n and 7 n we examined seismic sequences and sea floor morphology of the simeulue and aceh forearc basins and the adjacent outer arc high we found that strike slip faulting has controlled the forearc basin evolution since the late miocene the mentawai fault zone extends up to the north of simeulue island and was most probably connected farther northwards to the sumatran fault zone until the end of the miocene since then this northern branch jumped westwards initiating the west andaman fault in the aceh area the connection to the mentawai fault zone is a left hand step over in this transpressional setting the tuba ridge developed we found a right lateral strike slip fault running from the conjunction of the west andaman fault and the tuba ridge in ssw direction crossing the outer arc high as a result extrusion formed a marginal basin north of simeulue island which is tilted eastwards by uplift along a thrust fault in the west the shift of strike slip movement in the aceh segment is accompanied by a relocation of the depocenter of the aceh basin to the northwest forming one major neogene unconformity the simeulue basin bears two major neogene unconformities documenting that differences in subsidence evolution along the northern sumatran margin are linked to both forearc evolution related to subduction processes and to deformation along major strike slip faults

geodetic strain observations and return period of the strongest earthquakes of a given seismic source zone we present the basis for a method for estimating the return period of large and medium earthquakes that is independent of current deterministic and probabilistic approaches the two standard techniques of seismic hazard assessment probabilistic seismic hazard assessment psha and deterministic seismic hazard assessment dsha suffer from limited knowledge of seismic prehistory a further weakness of psha is its requirement of homogeneous seismic activity within a seismic zone moreover psha and dsha were developed for seismically active areas and thus cannot reliably be used in areas of medium and low activity in this paper we propose the combined use of geodetic strain rate data and the seismic moment data set determined for past seismic events this combination represents a new and independent approach to estimation of future seismic activity using a modified version of kostrov s phys solid earth 1 23 40 1974 equation and the catalogue of seismic moments the minimum return period of the strongest earthquakes of a source area is estimated

crustal deformation and submarine canyon incision in a meso cenozoic first order transfer zone sw iberia north atlantic ocean using a dense grid of high quality 2d seismic profiles dredge and outcrop data the offshore prolongation of a first order transfer zone the messejana plasencia fault zone mpfz is explained within the context of oblique rifting between southwest iberia newfoundland and west tethys the offshore mpfz is shown to comprise a 5 10 km wide region of deformation oblique to the continental margin reactivated in the mesozoic as part of a wider transcurrent domain the sao vicente sub basin here the geometry of faults and strata denotes the generation of a pull apart basin during the jurassic early cretaceous in contrast its late cretaceous to cenozoic evolution favours left lateral transpression during the counter clockwise rotation and eastward migration of iberia towards its present position erosion was subsequently enhanced in the sao vicente sub basin due to the reactivation of syn rift structures by documenting the evolution of the sao vicente sub basin and adjacent mpfz this work demonstrates 1 the temporal and spatial scales in which first order transfer zones accommodate crustal movements during continental rifting and subsequent inversion episodes 2 the generation of an extensive region of strain accommodation in the vicinity of the mpfz an observation with profound implications to future palaeogeographic reconstructions of the north atlantic ocean and 3 that the sao vicente canyon the physiographic expression of the mpfz incised the margin as early as the latest cretaceous paleocene synchronously with the onset of tectonic uplift in southwest iberia in such a setting the sao vicente sub basin and mpfz formed important by pass corridors for sediment sourced from proximal areas of the margin at present the mpfz comprises a complex releasing restraining bend accommodating important vertical and horizontal movements in southwest iberia based on earthquake data from similar transfer zones the mpfz should be able to generate large magnitude earthquakes and potentially destructive tsunamis

morphology and tectonics of the andaman forearc northeastern indian ocean summary the andaman sea has developed as the result of highly oblique subduction at the western sunda trench leading to partitioning of convergence into trench perpendicular and trench parallel components and the formation of a northward moving sliver plate to accommodate the trench parallel motion the andaman forearc contains structures resulting from both components of motion the main elements of the forearc are the accretionary prism and outerarc ridge a series of forearc basins and major n s faults the accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting indian plate the western outer slope of the accretionary prism is very steep rising to depths of 1500 2000 m within a distance of 30 km there is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism the outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to 1000 m and widths of 5 15 km resulting from ongoing deformation of the sediments the inner portion is smoother with lower slopes and forms a strong backstop the width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10 n and 11 30 n it remains at about 40 km to 14 40 n north of there the inner trench wall becomes a single steep slope up to the myanmar shelf the eastern edge of the outerarc ridge is fault bounded and north of the nicobar islands a forearc basin is located immediately to the east a deep gravity low with very steep gradients lies directly over the forearc basin the west andaman fault waf and or the seulimeum strand of the sumatra fault system form the boundary between the burma and sunda plates south of the andaman spreading centre the waf is the most prominent morphologic feature of the andaman sea and divides the sea into a shallow forearc and a deeper backarc region the diligent fault runs through the forearc basin east of little andaman island although it has the general appearance of a normal fault multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the eastern margin fault this could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east west extension

witwatersrand gold particle chemistry matches model of metamorphosed hydrothermally altered placer deposits analyses of gold particles from the ventersdorp contact reef in the klerksdorp and carletonville goldfields of the late archaean witwatersrand basin reveal a wide range in au ag and hg concentrations of 80 9 to 92 9 6 0 to 17 6 and 0 6 to 5 8 wt respectively however individual gold particles are generally homogeneous this observation is predicted by modelling of ag and hg diffusion through gold at the peak metamorphic temperature of about 300 c the only exception is gold in hydrothermal quartz veins the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids as a consequence of the 2023 ma vredefort meteorite impact event inter particle homogenisation on a hand sample scale was generally not achieved the mean gold composition for the same reef in different mines kilometre scale can vary drastically these results confirm the model of a metamorphosed and hydrothermally altered placer deposit furthermore they indicate that in spite of a significant post depositional modification of the composition of individual gold particles the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles

breaking a subduction termination from top to bottom the large 2016 kaikōura earthquake new zealand abstract subduction systems globally terminate allowing plate motion to be transferred from the oceanic megathrust onto continental and or oceanic transform faults the mechanism of this kinematic transition over earthquake timescales is however poorly understood due to a lack of relevant data here we study the 2016 mw 7 8 kaik ura earthquake in new zealand the first large instrumentally recorded earthquake across a subduction termination to investigate this transfer mechanism in detail we find that the kaik ura earthquake unlike standard subduction earthquakes globally involved a predominance 80 of coseismic slip on upper plate faults and minor triggered slip on the underlying oceanic subduction thrust in the months following the earthquake the subduction thrust accommodated most of the earthquake s afterslip down dip of its co seismic rupture zone this top down strain release mechanism is in accord with local geological geodetic and historical seismicity data which suggest that the bulk of plate convergence 75 is accommodated in the upper plate we suggest therefore that this alternative strain release mechanism which is distinct from standard plate boundary earthquakes is characteristic of subduction terminations and results in the majority of seismic tsunami hazard being on steep near surface faults

contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone an example from the nobeoka thrust drilling project 1 we examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the nobeoka thrust drilling project which provide a high resolution transect of properties across the main fault zone the footwall cataclasite exhibits higher averages of neutron porosity 7 6 and lower values of electric resistivity 232 m compared to the hanging wall phyllite 4 8 453 m this clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation despite the contrast observed between the hanging wall and footwall in macroscopic scale the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using archie s law suggesting the similarities in microstructures and mineralogy in this low porosity range above the main fault core of the nobeoka thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow archie s law damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the nankai trough but with much thicker width whereas the footwall damage zone is more extensive in the nobeoka thrust splay faults may exhibit strong deformation in the hanging wall in the early stage and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault the damage effect may eventually be enlarged in the footwall

insight into the 2004 sumatra andaman earthquake from gps measurements in southeast asia data collected at 60 global positioning system gps sites in southeast asia show the crustal deformation caused by the 26 december 2004 sumatra andaman earthquake at an unprecedented large scale small but significant co seismic jumps are clearly detected more than 3 000 km from the earthquake epicentre the nearest sites still more than 400 km away show displacements of 10 cm or more here we show that the rupture plane for this earthquake must have been at least 1 000 km long and that non homogeneous slip is required to fit the large displacement gradients revealed by the gps measurements our kinematic analysis of the gps recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre it also provides evidence that the rupture propagated northward sufficiently fast for stations in northern thailand to have reached their final positions less than 10 min after the earthquake hence ruling out the hypothesis of a silent slow aseismic rupture data culled from gps stations in indonesia malaysia singapore thailand india and china have answered more questions relating to the 26 december sumatra andaman megathrust earthquake permanent displacements were seen 3 000 km away final displacements occurred within 10 minutes of the earthquake ruling out slow aseismic slip on the fault

a mega splay fault system and tsunami hazard in the southern ryukyu subduction zone abstract in april 1771 a subduction earthquake generated a great tsunami that struck the south ryukyu islands and killed 12 000 people whereas its mechanism is still enigmatic nakata and kawana 1995 nakamura 2006 matsumoto et al 2009 in this paper we show its probable source on a mega splay fault system existing along the southern ryukyu forearc analyses of deep multi channel seismic reflection profiles indicate that the mega splay fault system is rising from the summit of a 1 km high ridge situated at a 5 landward dipping plate interface an outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge the inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures the mega splay fault system is parallel to the ryukyu trench east of 125 5 e and is estimated to be 450 km long the origin of this south ryukyu mega splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench parallel luzon okinawa fracture zone in contrast no similar splay fault is found west of 125 5 e where the oblique subduction has produced large shear zones along the south ryukyu forearc we infer that a thrust earthquake linked to the mega splay fault system is responsible for the south ryukyu tsunami however another possible scenario of generating a large tsunami affecting the south ryukyu islands is that the subducted ridge in the western end of the mega splay fault system nucleated a large earthquake and simultaneously triggered the 100 km long e w trending strike slip fault west of 125 5 e and induced a southward dipping tsunami genic subsidence in any case after a quiescence of 241 yr a large earthquake and tsunami is anticipated in the south ryukyu forearc in the near future

anatomy of the western java plate interface from depth migrated seismic images newly pre stack depth migrated seismic images resolve the structural details of the western java forearc and plate interface the structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles approximately 2 3 of the trench sediment fill is detached and incorporated into frontal prism imbricates while the floor sequence is underthrust beneath the decollement western java however differs markedly from margins such as nankai or barbados where a uniform continuous decollement reflector has been imaged in our study area the plate interface reveals a spatially irregular nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment the underthrust sediment is associated with a low velocity zone as determined from wide angle data active underplating is not resolved but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high our profile is located 100 km west of the 2006 java tsunami earthquake the heterogeneous decollement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred the alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution our seismic images resolve a steeply dipping splay fault which originates at the decollement and terminates at the sea floor and which potentially contributes to tsunami generation during co seismic activity

seismic imaging of forearc backthrusts at northern sumatra subduction zone forearc tectonics at accretionary convergent margins has variously been studied using analogue and numerical modelling techniques numerous geophysical investigations have targeted the subsurface structure of active forearc settings at convergent margins however several critical details of the structure mode of tectonic evolution and the role forearcs play in the subduction seismic cycle remain to be further understood especially for large accretionary margins in this study we present a high resolution deep seismic reflection image of the northern sumatran subduction forearc near the 2004 december 26 sumatra earthquake epicentral region the profile clearly demarcates the backthrust branches at the seaward edge of the aceh forearc basin along which the inner forearc continues to evolve sharp bathymetric features at the seafloor suggest that the imaged backthrusts are active coincident wide angle seismic tomographic image of the sumatra forearc allows us to image the geometry of the seaward dipping backstop buttress with which the imaged backthrust branches are associated the presence of forearc backthrusting confirms model predictions for the development of backthrusting over seaward dipping backstops the west andaman fault at the seaward edge of aceh basin appears to be a shallow tributary of the backthrust and sheds light on the complex deformation of the forearc uplifting along the backthrust branches may explain the presence of forearc islands observed all along sumatran margin and help further constrain the tectonic models for their evolution moreover if these backthrusts slip coseismically they would contribute to tsunamigenesis and seismic risk in the region

teleseismic relocation and assessment of seismicity 1918 2005 in the region of the 2004 mw 9 0 sumatra andaman and 2005 mw 8 6 nias island great earthquakes the m w 9 0 2004 sumatra andaman islands and m w 8 6 nias island great earthquake sequences have generated over 5000 catalog reported earthquakes along 1700 km of the sumatra andaman and western sunda regions studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths approximately 3650 teleseismically well recorded earthquakes occurring in this region during the period 1918 2005 are relocated with special attention to focal depth reduced uncertainties of epicenters and depths in the region on the order of 15 and 10 km respectively foster interpretation of focal mechanism data and provide additional details about the subducting indian and australian plates the revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near simeulue island and a steepening in slab dip from south to north the downdip width of the aftershock zone of the 2004 m w 9 0 earthquake varies from 200 km at its northern end to 275 km at its southern end and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone outer rise and near trench normal and strike slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes earthquake swarms triggered along the andaman backarc spreading center both north of sumatra and near siberut island 100 km south of the nias island aftershock sequence illustrate the complex and variable nature of seismicity following these great earthquakes

constructing forearc architecture over megathrust seismic cycles geological snapshots from the maule earthquake region chile we present new field structural data from the chilean coastal cordillera located above the northern and central parts of the interplate contact ruptured by the a d 2010 m w 8 8 maule earthquake the northern study area contains the northwest striking pichilemu normal fault an intraplate structure reactivated after the megathrust event by crustal earthquakes up to m w 7 0 the structural style of this region is dominated by kilometer scale normal faults that have been active at least throughout the quaternary the orientations of these main faults define three structural systems 1 northeast and 2 northwest striking margin oblique faults and 3 north to north northeast striking margin parallel faults from north to south these three systems vary in their predominant occurrence starting with bimodal orientations of groups 1 and 2 followed by predominantly single north to north northeast orientations of group 3 reverse faults coexist in time and space with the normal structures but are scarce and display variable apparently random orientations the shallow crustal normal faults including the pichilemu fault show a persistent kinematic history probably spanning thousands of subduction seismic cycles though historically smaller in magnitude than those of the triggered normal faults interseismic forearc thrust events were recorded above the rupture area prior to the maule earthquake the quaternary reverse faults identified in our study regions may be preserving interseismic slow strain rate permanent deformation signature in the structural grain analogous observations along the a d 2011 tohoku earthquake rupture in japan imply that such a link between the short and long term deformation patterns of the forearc is not exclusive of the maule earthquake region

use of anisotropy of magnetic susceptibility ams in the study of tsunami deposits application to the 2004 deposits on the eastern coast of banda aceh north sumatra indonesia abstract we studied emplacement dynamics of the december 26 2004 tsunami deposits in artificial shallow ponds and in shallow lagoons along the northern coast of sumatra in banda aceh area successive tsunami waves inundated topographic lows which acted as sediment traps and favored preservation of deposits individual waves led to the deposition of fining upwards sedimentary sequences this multiple normal grading deposit with a thickness ranging from 2 to 80 cm constitutes a site of major interest for the study of the deposits emplaced by a tsunami wave train the main interest lies in the characteristics of the deposits that can be correlated to the behavior of the waves attested by numerous testimonies in this setting we aimed at testing the applicability of the ams anisotropy of magnetic susceptibility technique as a proxy of unconsolidated sediment fabric the combination of this proxy and traditional sedimentary analysis helped reconstructing the hydrodynamic conditions particularly the flow direction prevailing during sedimentation we applied the ams technique on eight sections stretching inland from 1 5 km to 2 2 km from the pre tsunami coastline grain size and ams data from these deposits provide similar and partially complementary information traction dominates in the basal parts of the deposits and occurs mainly for the basal layers of the sequences whereas settling dominates for the upper layers of these sequences the vertical evolution of the ams parameters within the deposits is related to the arrival of the successive waves with variations in the traction and decantation processes during deposition variations of current strength during sedimentation are related to the wave s momentum and to the progressive infill of topographic depressions the vertical change of the fifth coarsest percentile vs the median grain sizes of the deposits suggests strong variations of the energy of the currents and confirms the evolution of the ams parameters in the present study the fabric of the deposits which developed exclusively during the uprush phase of the cycle also allows the reconstruction of flood directions that are both normal and parallel to the shoreline with variations related to flow dynamics mean grain size and local topographic control due to the absence of backwash in between the waves and low velocity of the final backwash information about the sedimentation conditions of the tsunami deposits appears to be better preserved in topographic depressions in areas located remote from the seashore the ams proxy appears to be a promising tool for further studies of recent and paleo tsunami deposits

deformation of the northern sumatra accretionary prism from high resolution seismic reflection profiles and rov observations abstract following the 2004 great sumatran andaman m w 9 2 earthquake high resolution seismic reflection multibeam bathymetric and remotely operated vehicle data were acquired to investigate the tectonic framework of the rupture zone and search for evidence of seafloor and near surface displacement three distinct regions off northern sumatra were investigated 1 a portion of the sunda trench 2 the adjacent frontal deformation zone and 3 the seaward flank of the aceh forearc basin a thick 1 5 km sediment section within the sunda trench shows evidence of shallow normal faulting possibly representing early stages of assimilation into the accretionary wedge the frontal deformation zone consists of ridges of predominantly landward verging thrust folds seaward verging backthrust faults at or near the base of the steep slope commonly reach the seafloor we do not observe a single laterally extensive structural offset at the deformation front that might be interpreted as contributing to the 2004 tsunami rather a series of small offset tens of metres faults were noted across this broad zone of the frontal accretionary wedge the western boundary of the aceh forearc basin is the west andaman strike slip fault juxtaposing the accretionary complex s forearc high with basin fill sediments neither the seismic nor rov data show evidence of recent seafloor displacement along the fault trace basin infill demonstrates consistent along strike patterns of tilting and seaward subsidence during sedimentation while modern fill is flat lying and coherent across the entire basin intercalated chaotic layers interpreted as mass transport deposits may record a history of seismicity but recent examples of such deposits were not observed on the modern seafloor either seaward of the deformation front or in the aceh basin lack of any evidence of faulting offset or disruption of sediments within aceh basin suggests that there was little impact of the 2004 earthquake in this area distributed faults throughout the frontal deformation zone combined with observations of landward verging folds at the deformation front folding within piggy back basin sediments and lack of evidence of disruption along the west andaman fault zone and within the forearc basin all support strain partitioning across the margin a proposed strong wedge interior may act as a backstop during major thrust events constraining deformation to the frontal deformation zone and the slope apron tsunami generation in response to the 2004 event did not result from surficial displacements along a single fault or narrow fault zone at the toe of the deformation front but was more likely a result of vertical displacement across the entire outer forearc

the size and duration of the sumatra andaman earthquake from far field static offsets the 26 december 2004 sumatra earthquake produced static offsets at continuously operating gps stations at distances of up to 4500 kilometers from the epicenter we used these displacements to model the earthquake and include consideration of the earth s shape and depth varying rigidity the results imply that the average slip was 5 meters along the full length of the rupture including the approximately 650 kilometer long andaman segment comparison of the source derived from the far field static offsets with seismically derived estimates suggests that 25 to 35 of the total moment release occurred at periods greater than 1 hour taking into consideration the strong dip dependence of moment estimates the magnitude of the earthquake did not exceed mw 9 2

seismic stratigraphy and subsidence evolution of the northwest sumatra fore arc basin abstract new seismic reflection data collected with the indonesian rv baruna jaya iii under a french indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern sumatra within the sumatra fore arc domain the direction of subduction of the indian oceanic crust below the sunda arc is normal in front of java and oblique along sumatra as a result parts of the sumatra margin are moving northwards along two large strike slip faults parallel to the subduction trench the sumatra fault in sumatra and the mentawai fault at sea which separates the accretionary prism from the fore arc basin we describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore arc domain after widespread uplift and erosion during the paleogene the fore arc basin becomes subsident during the miocene as evidenced by shelf deposits organized in two transgressive regressive sequences 2a 2b produced by tectonics and eustatism this is interpreted as the result of a limited e w extension linked to the incipient play of the sumatra and mentawai faults in pliocene quaternary times the fore arc basin is segmented into several sub basins aceh simeulue and nias basins by compressional zones or by strike slip faults the subsidence rate increased strongly producing sequence 3 with two or four subunits for the pliocene and sequence 4 for the quaternary during this period local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore arc basin the sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone

persistent elastic behavior above a megathrust rupture patch nias island west sumatra we quantify fore arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible elevated coral microatolls paleoreef flats and chenier plains show that the sumatran outer arc island of nias has experienced a complex pattern of relatively slow long term uplift and subsidence during the holocene epoch this same island rose up to 2 9 m during the mw 8 7 sunda megathrust rupture in 2005 the mismatch between the 2005 and holocene uplift patterns along with the overall low rates of holocene deformation reflects the dominance of elastic strain accumulation and release along this section of the sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long term plate convergence the fraction of 2005 uplift that will be retained permanently is generally 4 for sites that experienced more than 0 25 m of coseismic uplift average uplift rates since the mid holocene range from 1 5 to 0 2 mm a and are highest on the eastern coast of nias where coseismic uplift was nearly zero in 2005 the pattern of long term uplift and subsidence is consistent with slow deformation of nias along closely spaced folds in the north and trenchward dipping back thrusts in the southeast low holocene tectonic uplift rates provide for excellent geomorphic and stratigraphic preservation of the mid holocene relative sea level high which was under way by 7 3 ka and persisted until 2 ka

the updip seismic aseismic transition of the sumatra megathrust illuminated by aftershocks of the 2004 aceh andaman and 2005 nias events great subduction earthquakes exhibit segmentation both within the rupture of individual events and in the long term history of the margin the 2004 december 26 aceh andaman and 2005 march 28 nias event in northern sumatra are two of the largest earthquakes in recent years with both co and post seismic displacements constrained in unprecedented detail using aftershock locations from a temporary seismic array in the boundary region between both events and waveform modelling of large aftershocks we demonstrate that the vast majority of aftershocks in the study region occur on the plate interface within a narrow band 20 km seaward of the outer arc high comparing the seismicity distribution to the co and post seismic displacements we infer that the seismic band marks the transition between the seismogenic zone and stable sliding the location of the band and therefore the transition appears to be correlated with the 500 m bathymetry contour this close correspondence is disrupted at the boundary between the two great earthquakes where the transition to seismogenic behaviour occurs further landward by 25 km to the west of simeulue where seafloor bathymetry throughout the forearc is deeper than 500 m the seismic band terminates abruptly and the focus of aftershock activity is found near the trench the seismic efficiency of afterslip varies dramatically along strike the segment below the banyak islands in the gap between the two main asperities of the nias earthquake accommodates a much larger proportion of afterslip seismically