the 2005 and 2010 earthquakes on the sumatra andaman trench evidence for post 2004 megathrust intraplate rejuvenation great earthquakes on oblique subduction zones accommodate slip on strike slip faults on the overriding and subducting plates leading to increased intraplate seismicity after the 2004 sumatra earthquake there was an increase in activity in the wharton basin wb which is considered to be part of a diffuse plate boundary here the intraplate deformation occurs in response to northwest southeast directed compression as opposed to the north northeast south southwest directed plate motion along the sumatra andaman subduction zone earthquakes here mostly occur by left lateral strike slip on north south n s trending fossil fractures although ruptures on east west planes are not uncommon here we discuss the 24 july 2005 m w 7 2 and the 12 june 2010 m w 7 5 earthquakes both of which were left lateral strike slip events sourced in the northern wb our source models for these events based on teleseismic waveform inversion favor slip on the n s fossil fracture systems consistent with previous studies in the wb although the former sourced at similar to 16 km depth was not tsunamigenic the latter similar to 34 km triggered a small tsunami possibly due to its oblique slip at similar to 5 km depth we use conceptual models of plate deformation to suggest that both these events have resulted from post 2004 redistribution of stresses and rejuvenation of intraslab fracture systems within the wb an actively deforming oceanic intraplate region

simultaneous rupture along two conjugate planes of the wharton basin earthquake analysis of broadband teleseismic data shows that the 18 june 2000 wharton basin earthquake a moment magnitude 7 8 intraplate event in the region of diffuse deformation separating the indian and australian plates consisted of two subevents that simultaneously ruptured two near conjugate planes this mode of rupture accommodates shortening by a mechanism different from that previously known elsewhere in the region the larger subevent occurred on a fossil fracture zone with a relatively high stress drop of about 20 megapascals showing that large stresses can accumulate in regions of distributed deformation

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

comparison of dynamic flow models for the central aleutian and tonga kermadec subduction zones using three dimensional 3 d instantaneous dynamic models of the central aleutian subduction zone we show that a low viscosity low density deltarho 10 kg m3 region in the wedge is needed to match observations of topography the geoid and the stress state in the slab and overriding plate previous models of the tonga kermadec subduction zone also require a low viscosity low density deltarho 20 kg m3 region in the wedge a low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down dip tension to down dip compression if the low viscosity region is large as in the tonga kermadec models then the decoupling may also cause the overriding plate to go into extension these are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones

mw 8 6 sumatran earthquake of 11 april 2012 rare seaward expression of oblique subduction the magnitude 8 6 and 8 2 earthquakes off northwestern sumatra on 11 april 2012 generated small tsunami waves that were recorded by stations around the indian ocean combining differential travel time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench parallel component the oblique plate convergence indicates that 20 50 m of trench parallel displacement could have accumulated since the last megathrust earthquake only part of which has been taken up by the great sumatran fault this suggests that the remaining trench parallel motion was released during the magnitude 8 6 earthquake on 11 april 2012 within the subducting plate the magnitude 8 6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the sumatra andaman earthquake in 2004

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

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

neotectonics of the nanga parbat syntaxis pakistan and crustal stacking in the northwest himalayas the northwest termination of the himalayan arc is marked by two large antiformal structures termed syntaxes and a nw se zone of intermediate depth earthquakes which have yielded both strike slip and thrust sense fault plane solutions this contribution presents new structural data from the nanga parbat area the northern syntaxis one of the fastest rising portions of the orogen 7 mm yr 1 our studies show uplift related to syntaxis growth to be accommodated by structures located along its western margin fault kinematics in the southern part of the margin indicate nw directed thrusting along a shear zone and high level cataclastic faults termed the liachar thrust zone this carries amphibolite facies basement rocks out onto quaternary sediments further north along the western margin active faults are dominantly dextral strike slip oriented north south e g the shahbatot fault overprinting relationships suggest that the strike slip fault zone has migrated southwards into the nw directed thrust zone these relationships are consistent with the northwest termination of the arcuate himalayan thrust belt at a lateral tip generating folding and radial thrust directions faulting patterns at nanga parbat suggest that this tip zone has migrated southwards active faulting is now concentrated in the northwest continuation of the hazara southern syntaxis along a seismogenic strike slip and thrust zone these deep level tips lie on the crustal stacking thrusts which cut through the higher level sse directed thrusts of pakistan

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

strike slip earthquakes in the oceanic lithosphere observations of exceptionally high apparent stress summary the radiated energies e s and seismic moments m0 for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses a e s m0 where is the modulus of rigidity the globally averaged a for shallow earthquakes in all tectonic environments and seismic regions is 0 3 mpa however the subset of 49 earthquakes with the highest apparent stresses a greater than about 5 0 mpa is dominated almost exclusively by strike slip earthquakes that occur in oceanic environments these earthquakes are all located in the depth range 7 29 km in the upper mantle of the young oceanic lithosphere many of these events occur near plate boundary triple junctions where there appear to be high rates of intraplate deformation indeed the small rapidly deforming gorda plate accounts for 10 of the 49 high a events the depth distribution of a which shows peak values somewhat greater than 25 mpa in the depth range 20 25 km suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere a recently proposed envelope for apparent stress derived by taking 6 per cent of the strength inferred from laboratory experiments for young less than 30 ma deforming oceanic lithosphere agrees well with the upper bound envelope of apparent stresses over the depth range 5 30 km the corresponding depth dependent shear strength for young oceanic lithosphere attains a peak value of about 575 mpa at a depth of 21 km and then diminishes rapidly as the depth increases in addition to their high apparent stresses which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10 30 km our set of high a earthquakes show other features that constrain the nature of the forces that cause interplate motion first our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of a for the two types secondly many of the intraplate events have focal mechanisms with the t axes that are normal to the nearest ridge crest or subduction zone and p axes that are normal to the proximate transform fault these observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high a earthquakes inside oceanic plates extant transform boundaries may be misaligned with current plate motion to accommodate current plate motion the pre existing plate boundaries would have to be subjected to large horizontal transform push forces a notable example of this is the triple junction near which the second large aftershock of the 1992 april cape mendocino california sequence occurred alternatively subduction zone resistance may be enhanced by the collision of a buoyant lithosphere a process that also markedly increases the horizontal stress a notable example of this is the aleutian trench near which large events occurred in the gulf of alaska in late 1987 and the 1998 march balleny sea m 8 2 earthquake within the antarctic plate

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

en echelon and orthogonal fault ruptures of the 11 april 2012 great intraplate earthquakes the magnitude 8 7 earthquake that occurred off the coast of the sumatra subduction zone on 11 april 2012 is shown to have had an extraordinarily complex four fault rupture these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates on 11 april 2012 two of the largest strike slip earthquakes ever recorded at magnitudes of 8 7 and 8 2 occurred in the northeastern indian ocean a few hundred kilometres off the coast of sumatra three groups now report the analysis of seismic data from the days and months before and after these events as well as the events themselves matthias delescluse and co authors show that these earthquakes are part of the ongoing boost of intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes they conclude that the australian plate driven by slab pull forces at the sunda trench is gradually detaching from the indian plate han yue and colleagues show that the 11 april event involved a complex four fault rupture lasting several minutes followed two hours later by a magnitude 8 2 aftershock these great ruptures on a lattice of strike slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the indian and australian plates fred pollitz and colleagues show that in the six days following 11 april the global rate of remote earthquakes with magnitudes greater than 5 5 increased nearly fivefold and events up to magnitude 7 seem to have been triggered the unprecedented delayed triggering power of this earthquake may arise from its strike slip source geometry or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure the indo australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary from an environment of continental collision to an island arc subduction zone1 2 on 11 april 2012 one of the largest strike slip earthquakes ever recorded seismic moment magnitude mw 8 7 occurred about 100 200 kilometres southwest of the sumatra subduction zone occurrence of great intraplate strike slip faulting located seaward of a subduction zone is unusual it results from northwest southeast compression within the plate caused by the india eurasia continental collision to the northwest together with northeast southwest extension associated with slab pull stresses as the plate underthrusts sumatra to the northeast here we use seismic wave analyses to reveal that the 11 april 2012 event had an extraordinarily complex four fault rupture lasting about 160 seconds and was followed approximately two hours later by a great mw 8 2 aftershock the mainshock rupture initially expanded bilaterally with large slip 20 30 metres on a right lateral strike slip fault trending west northwest to east southeast wnw ese and then bilateral rupture was triggered on an orthogonal left lateral strike slip fault trending north northeast to south southwest nne ssw that crosses the first fault this was followed by westward rupture on a second wnw ese strike slip fault offset about 150 kilometres towards the southwest from the first fault finally rupture was triggered on another en echelon wnw ese fault about 330 kilometres west of the epicentre crossing the ninetyeast ridge the great aftershock with an epicentre located 185 kilometres to the ssw of the mainshock epicentre ruptured bilaterally on a nne ssw fault the complex faulting limits our resolution of the slip distribution these great ruptures on a lattice of strike slip faults that extend through the crust and a further 30 40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates

intraplate deformation and closure of the australia antarctica africa plate circuit to determine the current motion between the australian antarctic and african plates and to test whether this plate circuit obeys closure all plate motion data available along the southeast southwest and central indian ridges are analyzed and reduced to 67 spreading rates 38 transform fault azimuths and 135 earthquake slip vectors carlsberg ridge data were excluded because they record india africa not australia africa motion new data include 10 new transform azimuths along the southwest indian ridge many slip vectors and a dense aeromagnetic survey along the southeast indian ridge all published and many unpublished magnetic profiles are modeled to determine rates consistently over a 3 m y time averaging interval for three reasons 1 magnetic profiles from the southeast and central indian ridges suggest recent spreading rate changes 2 some published rates differ for identical profiles and 3 prior studies have not used identical criteria and time scales to determine spreading rates the new rates differ from published rates by as much as 5 mm yr transform fault azimuths are estimated from bathymetry seasat altimetric data epicenter distributions and offsets of magnetic lineations azimuths of plate motion are also estimated from the horizontal projections of slip vectors from the centroid moment tensor solutions and other focal mechanism studies our new australia antarctica and australia africa euler vectors differ from all prior euler vectors at the 95 confidence level along the southeast and central indian ridges our model gives rates differing by 4 7 mm yr from those of prior models from a systematic analysis of the plate motion data we find no evidence for a nubia somalia antarctica triple junction along the southwest indian ridge we also find no evidence for a triple junction previously proposed to be at 80 e along the southeast indian ridge any deformation within this seismically active region of the australian plate adds up to less than a few millimeters per year along each boundary data are fit well by a single euler vector except for a 5 systematic misfit to azimuthal data along the eastern southeast indian ridge near a seismically active region of the australian plate south of tasmania although seismicity and the azimuthal misfit suggest the australian plate is deforming measurably possibly by distributed deformation or by the westward motion of a small microplate southeast of tasmania the significance of the misfit is marginal and is small enough that systematic errors may cause it these alternative explanations could be distinguished by surveying these transform faults with modern seafloor mapping techniques prior studies found that enforcing plate circuit closure causes systematic misfits to rates along the southeast indian ridge here we find that enforcing closure causes no systematic misfits to any data moreover only insignificant nonclosure is found by an f ratio test which numerical experiments suggest could detect deformation exceeding 2 mm yr we conclude that indian ocean plate circuit nonclosure and the deformation it suggests are much smaller than thought before the absence of significant nonclosure argues against the usefulness of a model of deformation distributed throughout an indo australian plate but favors a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial indian ocean between the central indian ridge and the sumatra trench

hydrological responses of the chalk aquifer to the regional climatic signal piezometric variability was investigated using monthly time series from 40 piezometers in different areas across the upper normandy region over the 1968 2009 period the piezometers were classified according to their hydrological behaviour this paper aimed at characterising the dominant modes explaining piezometric variability and at investigating the causes of this variability significant components in the piezometric signals were identified the annual cycle and an interannual mode 4 12 years with different variances for each observation borehole group the influence of the morphostructural organisation of the upper normandy chalk plateau on the modalities of the water level variability in the chalk aquifer was defined the relative contributions and standard deviations of each component found in the signal time series of the groundwater level was estimated the highest values were recorded in the northern study area and clay with flints and aquifer thicknesses influence the transfer on an annual scale while the loess thickness influences the transfer on a multi year scale

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

the june 2000 mw 7 9 earthquakes south of sumatra deformation in the india australia plate 1 two large mw 7 9 earthquakes occurred on 4 and 18 june 2000 south of sumatra beneath the indian ocean both earthquakes were predominantly left lateral strike slip on vertical n s trending faults that we interpret to be reactivated fracture zones the 4 june enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake the first strike slip subevent within the subducting plate triggered a thrust subevent on the plate interface which comprised at least 35 of the total moment and ruptured se away from the 1833 earthquake the 18 june earthquake in the wharton basin is one of the largest shallow strike slip faulting earthquakes ever recorded a small second subevent with reverse slip is required to fit the body waves the orientation of both subevents in our preferred model is consistent with the current stress field in the region both the june 2000 earthquakes are consistent with recent models of distributed deformation in the india australia composite plate the occurrence of the enggano earthquake implies that the stress field within the indian plate continues to a depth of 50 km in the subducting slab the purely strike slip source model of the wharton basin earthquake obtained by robinson et al 2001 matches the p waves very poorly and fits the s waves no better than our preferred model the strike slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the enggano earthquake this difference is consistent with previous observations of oceanic and subduction earthquakes

analysis of satellite gravity and bathymetry data over ninety east ridge variation in the compensation mechanism and implication for emplacement process 1 we investigate the mode of compensation emplacement history and deep density structure of the ninety east ridge indian ocean using spectral analyses and forward modeling of satellite gravity and bathymetry data we find that the northern 0 10 n and the southern 20 30 s parts of the ridge are flexurally compensated with an effective elastic thickness 15 km whereas the central part 0 20 s is locally compensated furthermore we find that for a part of central block 10 20 s over osborn knoll the compensation depth is unreasonably very high 30 40 km therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles these results suggest that the northern and southern parts of ninety east ridge were emplaced off to a ridge axis compared to the central one which might have been emplaced on or near a spreading center locally compensated large topography thick underplated crust in the central part near osborn knoll might result from an interaction of a hot spot with the extinct wharton spreading ridge

the 2012 mw 8 6 wharton basin sequence a cascade of great earthquakes generated by near orthogonal young oceanic mantle faults we improve constraints on the slip distribution and geometry of faults involved in the complex multisegment mw 8 6 april 2012 wharton basin earthquake sequence by joint inversion of high rate gps data from the sumatran gps array sugar teleseismic observations source time functions from broadband surface waves and far field static gps displacements this sequence occurred under the indian ocean 400 km offshore sumatra the events are extraordinary for their unprecedented rupture of multiple cross faults deep slip large strike slip magnitude and potential role in the formation of a discrete plate boundary between the indian and australian plates the sugar recorded static displacements of up to 22 cm along with time varying arrivals from the complex faulting which indicate that the majority of moment release was on young wnw trending right lateral faults counter to initial expectations that an old lithospheric nne trending fracture zone played the primary role the new faults are optimally oriented to accommodate the present day stress field not only was the greatest moment released on the younger faults but it was these that sustained very deep slip and high stress drop 20 mpa the rupture may have extended to depths of up to 60 km suggesting that the oceanic lithosphere in the northern wharton basin may be cold and strong enough to sustain brittle failure at such depths alternatively the rupture may have occurred with an alternative weakening mechanism such as thermal runaway

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

kinematic constraints on distributed lithospheric deformation in the equatorial indian ocean from present motion between the australian and indian plates from an expanded accurate and up to date data set comprising 110 spreading rates 46 transform azimuths and 151 earthquake slip vectors from the indian ocean and gulf of aden we determine a new rigid plate model describing the motion since 3 ma between india and australia the euler vector 0 313 m y about 5 s 78 e lies near the middle of the equatorial diffuse plate boundary dividing the indian from the australian plate and predicts a rate of north south shortening along 85 e of 4 3 mm yr only 30 as fast as predicted by our prior model the new model also predicts north south extension of 6 2 mm yr at 68 e along the western segment of the diffuse plate boundary where our prior model predicted north south contraction using data only along the carlsberg and central indian ridges and no other plate boundaries we show that plate motion data cannot be fit by a single euler vector however the data are well fit by two euler vectors when an east west striking india australia plate boundary is assumed to intersect the central indian ridge near the equator the best location along the central indian ridge for this triple junction is 6 s 3 s with 95 confidence limits of 9 s 4 n just west of a region of intense seismicity the sense of deformation as recorded in earthquake focal mechanisms reverse faults mapped with reflection seismic data and undulations in basement topography surface gravity and the geoid agrees well with the north south extension predicted in the western part and the north south shortening predicted in the eastern part of the india australia boundary the predicted rate of north south shortening between 79 e and 86 e is consistent with the rate of shortening inferred from observed faulting and folding shortening taken up by faulting is 6 to 100 times greater than that taken up by the spectacular basement folds little of the rapid north south shortening predicted east of 86 e is taken up by crustal thickening instead lithosphere is transported northeastward toward the sumatra trench through strike slip faulting and possible clockwise rotation the failure of the convergent segment of the diffuse plate boundary to form a subducting trench provides some new observations that must be satisfied by models for the initiation of subduction a model of separate rigid indian and australian plates divided by a diffuse plate boundary appears to be valid and useful because the predictions of the model agree with independent data and because the predicted velocity of relative motion is statistically significant and comparable with that across other plate boundaries

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

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

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

lateral variation in slab orientation beneath toba caldera northern sumatra the investigator fracture zone ifz subducts beneath toba caldera the earth s largest quaternary caldera in northern sumatra suggesting a possible relationship between them locations of sub crustal earthquakes based on arrival times of p and s waves at a seismograph network surrounding toba reveal the geometry of the subducted slab and the ifz beneath toba a vertical tear of less than 20 km in the slab across the ifz as previously suggested cannot be ruled out but the large scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench the slab shape is probably a response to the trench curvature can explain the change in trend of the volcanic arc near toba and may cause shallowing of the forearc basin near nias island the decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle possibly resulting in lateral compressive stress in the slab the high rate of seismicity along the subducted investigator fracture zone that intersects the slab obliquely to its plunge direction is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab

motion between the indian capricorn and somalian plates since 20 ma implications for the timing and magnitude of distributed lithospheric deformation in the equatorial indian ocean summary approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the carlsberg ridge and central indian ridge are used to estimate the rotations of the indian and capricorn plates relative to the somalian plate for 20 distinct points in time since 20 ma the data are further used to place limits on the locations of the northern edge of the rigid capricorn plate and of the southern edge of the rigid indian plate along the central indian ridge data south of and including fracture zone n the fracture zone immediately south of the vema fracture zone which intersects the central indian ridge near 10 s are well fit assuming rigid capricorn and somalian plates while data north of fracture zone n are not in agreement with prior results data north of fracture zone h which intersects the central indian ridge near 3 2 s are well fit assuming rigid indian and somalian plates while data south of and including fracture zone h are not resulting in a smaller rigid indian plate and a wider diffuse oceanic plate boundary than found before the data are consistent with capricorn somalia motion about a fixed pole since 8 ma but require rotation about a pole 15 farther away from the central indian ridge from 20 to 8 ma the post 8 ma pole also indicates capricorn somalia displacement directions that are 7 clockwise of those indicated by the pre 8 ma stage pole in contrast india somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 ma india somalia motion has changed little during the past 20 myr nonetheless astronomically calibrated ages for reversals younger than 12 9 ma allow resolution of the following small but significant changes in spreading rate india somalia spreading slowed from 31 to 28 mm yr 1 near 7 9 ma and later sped up to 31 mm yr 1 near 3 6 ma capricorn somalia spreading slowed from 40 to 36 mm yr 1 near 11 0 ma later sped up to 38 mm yr 1 near 5 1 ma and further sped up to 40 mm yr 1 near 2 6 ma the motion between the indian and capricorn plates is estimated by differencing india somalia and capricorn somalia rotations which differ significantly for all 20 pairs of reconstructions india has rotated relative to the capricorn plate since at least 20 ma if about a pole located near 4 s 75 e the rate of rotation was slow 0 11 0 01 myr 1 95 per cent confidence limits from 20 to 8 ma but increased to 0 28 0 01 myr 1 95 per cent confidence limits at 8 ma the onset of more rapid rotation coincides within uncertainty with the inferred onset at 7 8 ma of widespread thrust faulting in the central indian basin and with the hypothesized attainment of maximum elevation and initiation of collapse of the tibetan plateau at 8 ma the plate kinematic data are consistent with steady india capricorn motion since 8 ma and provide no evidence for previously hypothesized episodic motions during that interval the convergence since 8 ma between the indian and capricorn plates significantly exceeds by 13 to 20 km the convergence estimated from three north south marine seismic profiles in the central indian basin where and how the additional convergence was accommodated is unclear

the relationship of the seismic source and subduction zone structure for the 2004 december 26 sumatra andaman earthquake abstract studies of the 2004 december 26 sumatra andaman earthquake indicate that the rupture and subsequent down dip slip of the mw 9 3 event propagated with variable speed as it progressed northwards the transitions between different behaviour for the source correspond to changes in the physical properties of the subducted slab along the sumatra andaman arc revealed by multi wavespeed seismic tomography the transition from rapid slip to dominantly slower slip near 6 5 n and the end of the main zone of tsunamigenic source at 10 n lies in each case just to the south of localised features where changes in the physical properties of the slab are indicated from the ratio of shear wavespeed to bulk sound speed these features are associated mainly with bulk modulus variations the changing morphology of the subduction zone is associated with changes in physical properties that are manifested in the tomographic images and have modified the character of the slip along the 1500 km long arc segment that ruptured during this great earthquake

fault reactivation and selective abandonment in the oceanic lithosphere 1 normal and transform faults originally formed at a spreading centre can be reactivated in diffuse plate boundary zones and in areas of lithospheric flexure such as at peripheral bulges to subduction zones using new seismic reflection profiles and modeling we investigate how the original oceanic fabric is reactivated in the simple case of fault perpendicular compression in the central indian basin well oriented normal paleofaults were reactivated with reverse motion at the very onset of deformation 9 ma but only a small subset remained active past 7 ma suggesting that most of the densely spaced small offset faults were abandoned while deformation localized onto fewer faults with larger spacing we find a similar evolution using a 2d finite element code of lithospheric shortening using a pseudoplastic rheology weak zones 3 km spaced and 30 40 weaker than the surrounding material are introduced to simulate the fabric formed at the ridge axis we show that reactivation and selective abandonment require strain weakening followed by strain rate weakening once a maturation threshold is reached a maturation fault slip of less than 50 m is needed to produce a fault network similar to that in the central indian ocean

historical seismicity and implications for diffuse plate convergence in the northeast indian ocean large historical earthquakes provide constraints on the geographical extent and kinematics of the slow diffuse plate boundary extending from the central indian ridge to the sumatra trench the fault parameters of pre 1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms surface wave spectral amplitudes and polarity and polarization constraints and location uncertainties are determined using a bootstrap resampling method the march 9 1928 ninetyeast ridge m 7 7 event shows left lateral strike slip faulting along a plane approximately coincident with the orientation of the ninetyeast ridge two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault the 1928 waveforms can also be fit by a thrust faulting solution with ene striking fault planes but a variety of evidence including the recent strike slip earthquakes at the same location suggest the strike slip solution these earthquakes are located along the western scarp of the ridge and do not represent reactivation of the ninetyeast fracture zone to the east of the edifice which was a major plate boundary in the early cenozoic the january 23 1949 m 6 75 and march 22 1955 m 7 0 wharton basin earthquakes although only 350 km apart show different strike slip and thrust faulting mechanisms all earthquakes show a regionally consistent pattern of horizontal compressional stress which changes from nw se in the wharton basin to n s in the central indian basin the compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure indicating little change in stress orientation since the onset of folding seismic deformation along the diffuse boundary consists of sinistral strike slip faulting associated with n s features such as the ninetyeast ridge and the 86 e fracture zone and thrust or reverse faulting in regions between fracture zones assuming transform motion the approximate slip rate along the northern ninetyeast ridge calculated from the summed moments of earthquakes is 2 8 mm yr this value is an order of magnitude less than previous estimates and suggests that at most only a small fraction of the current relative plate motion 7 mm yr occurs along the ridge the geographic distribution of seismicity suggests that the wharton basin east of the ninetyeast ridge is nearly as seismically active as the central indian basin to the west this result along with observed long wavelength gravity anomalies in the wharton basin indicates that the india australia plate boundary intersects the sumatra trench along a diffusely deforming region several thousand kilometers long rather than along a discrete transform zone

intraplate shortening in the central indian ocean determined from a 2100 km long north south deep seismic reflection profile although intraplate deformation is commonly observed on the continents there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary the central indian ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere reflecting an exceptionally high stress level however the strain pattern derived from indirect and incomplete observations remains poorly known a more complete data set was obtained during the phedre cruise 1991 by means of deep seismic reflection profiling one 2100 km long profile runs along the 81 e meridian from lat 14 s to the coast of sri lanka and provides us with the first long cross section through the region of observed crustal level faults fault analysis offers new estimates of the amount of shortening which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry the contribution of the seismically resolvable faults amounts to 22 37 km of shortening or 2 5 4 3 this is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

april 2012 intra oceanic seismicity off sumatra boosted by the banda aceh megathrust the two earthquakes of respective magnitudes 8 6 and 8 2 that occurred off the coast of the sumatra subduction zone on 11 april 2012 are shown to be part of a continuing boost of the intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes on 11 april 2012 two of the largest strike slip earthquakes ever recorded at magnitudes of 8 7 and 8 2 occurred in the northeastern indian ocean a few hundred kilometres off the coast of sumatra three groups now report the analysis of seismic data from the days and months before and after these events as well as the events themselves matthias delescluse and co authors show that these earthquakes are part of the ongoing boost of intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes they conclude that the australian plate driven by slab pull forces at the sunda trench is gradually detaching from the indian plate han yue and colleagues show that the 11 april event involved a complex four fault rupture lasting several minutes followed two hours later by a magnitude 8 2 aftershock these great ruptures on a lattice of strike slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the indian and australian plates fred pollitz and colleagues show that in the six days following 11 april the global rate of remote earthquakes with magnitudes greater than 5 5 increased nearly fivefold and events up to magnitude 7 seem to have been triggered the unprecedented delayed triggering power of this earthquake may arise from its strike slip source geometry or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure large earthquakes nucleate at tectonic plate boundaries and their occurrence within a plate s interior remains rare and poorly documented especially offshore the two large earthquakes that struck the northeastern indian ocean on 11 april 2012 are an exception they are the largest strike slip events reported in historical times1 2 and triggered large aftershocks worldwide3 yet they occurred within an intra oceanic setting along the fossil fabric of the extinct wharton basin rather than on a discrete plate boundary4 5 6 7 8 here we show that the 11 april 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes subsequent to a stress transfer process recognized at other subduction zones9 10 using coulomb stress change calculations we show that the coseismic slips of the aceh and nias earthquakes can promote oceanic left lateral strike slip earthquakes on pre existing meridian aligned fault planes we further show that persistent viscous relaxation in the asthenospheric mantle several years after the aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events on a short timescale the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore on a longer geological timescale the australian plate driven by slab pull forces at the sunda trench is detaching from the indian plate which is subjected to resisting forces at the himalayan front6 8 11

seismicity associated with the sumatra andaman islands earthquake of 26 december 2004 the u s geological survey national earthquake information center usgs neic had computed origins for 5000 earthquakes in the sumatra andaman islands region in the first 36 weeks after the sumatra andaman islands mainshock of 26 december 2004 the cataloging of earthquakes of m b usgs 5 1 and larger is essentially complete for the time period except for the first half day following the 26 december mainshock a period of about two hours following the nias earthquake of 28 march 2005 and occasionally during the andaman sea swarm of 26 30 january 2005 moderate and larger m b 5 5 aftershocks are absent from most of the deep interplate thrust faults of the segments of the sumatra andaman islands subduction zone on which the 26 december mainshock occurred which probably reflects nearly complete release of elastic strain on the seismogenic interplate thrust during the mainshock an exceptional thrust fault source offshore of banda aceh may represent a segment of the interplate thrust that was bypassed during the mainshock the 26 december mainshock triggered a high level of aftershock activity near the axis of the sunda trench and the leading edge of the overthrust burma plate much near trench activity is intraplate activity within the subducting plate but some shallow focus near trench reverse fault earthquakes may represent an unusual seismogenic release of interplate compressional stress near the tip of the overriding plate the interplate thrust nias earthquake of 28 march 2005 in contrast to the 26 december aftershock sequence was followed by many interplate thrust aftershocks along the length of its inferred rupture zone

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

the tectonic framework of the sumatran subduction zone the great aceh andaman earthquake of december 26 2004 and its tragic consequences brought the sumatran region and its active tectonics into the world s focus the plate tectonic setting of sumatra has been as it is today for tens of millions of years and catastrophic geologic events have likely been plentiful the immaturity of our understanding of great earthquakes and other types of geologic hazards contributed to the surprise regarding the location of the 2004 earthquake the timing however is probably best understood simply in terms of the inevitability of the infrequent events that shape the course of geologic progress our best hope is to improve understanding of the processes involved and decrease our vulnerability to them

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

the june 2010 nicobar earthquake fault reactivation on the subducting oceanic plate the similar to 1300 km long rupture zone of the 2004 andaman sumatra megathrust earthquake continues to generate a mix of thrust normal and strike slip faulting events the 12 june 2010 m w 7 5 event on the subducting plate is the most recent large earthquake on the nicobar segment the left lateral faulting mechanism of this event is unusual for the outer rise region considering the stress transfer processes that follow great underthrusting earthquakes another earthquake m w 7 2 with a similar mechanism occurred very close to this event on 24 july 2005 these earthquakes and most of their aftershocks on the subducting plate were generated by left lateral strike slip faulting on north northeast south southwest oriented near vertical faults in response to north northwest south southeast directed compression pre 2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern post 2004 left lateral faulting on the subducting oceanic plate clusters between 5 degrees n and 9 degrees n where the 90 degrees e ridge impinges the trench axis our study observes that the subducting plate off the sumatra and nicobar segments behaves similarly to a chip of the india australia plate deforming in response to a generally northwest southeast oriented compression an aspect that must be factored into the plate deformation models

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

space geodetic test of kinematic models for the indo australian composite plate the existence of a distinct capricorn component plate within the composite indo australian plate has previously been questioned if there is no capricorn plate the global positioning system site at bangalore on the indian plate is predicted to move relative to the australian plate at a velocity of 10 mm yr 1 if there is a distinct capricorn plate bangalore is predicted to move relative to the australian plate at a velocity of 17 mm yr 1 space geodetic data from australia and india are accurate enough to provide quantitative tests of this prediction and to discriminate between alternative composite plate models the data best fit a composite plate model that assumes the existence of a distinct capricorn component plate moving relative to both the indian and australian component plates

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

source area and rupture parameters of the 31 december 1881 mw 7 9 car nicobar earthquake estimated from tsunamis recorded in the bay of bengal 1 on the morning of 31 december 1881 a submarine earthquake beneath the andaman islands generated a tsunami with a maximum crest height of 0 8 m that was recorded by eight tide gauges surrounding the bay of bengal since the earthquake occurred 8 years before the construction of the world s first teleseismic recording seismometer little has been known about its rupture parameters or location waveform and amplitude modeling of the tsunami indicate that it was generated by a mw 7 9 0 1 rupture on the india andaman plate boundary resulting in 10 60 cm of uplift of the island of car nicobar the rupture consisted of two segments the northern 40 km long segment is separated from the southern 150 km long segment by a 100 km region corresponding to the westward projection of the west andaman spreading center the main rupture occurred between 8 5 n and 10 n with a total area of 150 km 60 km dipping 20 e with a mean slip of 2 7 m the recurrence time for 1881 type events is estimated to be 114 200 years on the basis of inferred gps convergence rates and inferred plate closure vectors although slip partitioning in the region may extend this estimate by as much as 30

ages and relative sizes of pre 2004 tsunamis in the bay of bengal inferred from geologic evidence in the andaman and nicobar islands 1 geologic evidence along the northern part of the 2004 aceh andaman rupture suggests that this region generated as many as five tsunamis in the prior 2000 years we identify this evidence by drawing analogy with geologic records of land level change and the tsunami in 2004 from the andaman and nicobar islands a n these analogs include subsided mangrove swamps uplifted coral terraces liquefaction and organic soils coated by sand and coral rubble the pre 2004 evidence varies in potency and materials dated provide limiting ages on inferred tsunamis the earliest tsunamis occurred between the second and sixth centuries a d evidenced by coral debris of the southern car nicobar island a subsequent tsunami probably in the range a d 770 1040 is inferred from deposits both in a n and on the indian subcontinent it is the strongest candidate for a 2004 caliber earthquake in the past 2000 years a n also contain tsunami deposits from a d 1250 to 1450 that probably match those previously reported from sumatra and thailand and which likely date to the 1390s or 1450s if correlated with well dated coral uplift offshore sumatra thus age data from a n suggest that within the uncertainties in estimating relative sizes of paleo earthquakes and tsunamis the 1000 year interval can be divided in half by the earthquake or earthquakes of a d 1250 1450 of magnitude 8 0 and consequent tsunamis unlike the transoceanic tsunamis generated by full or partial rupture of the subduction interface the a n geology further provides evidence for the smaller sized historical tsunamis of 1762 and 1881 which may have been damaging locally

the april 2012 indian ocean earthquakes seismotectonic context and implications for their mechanisms the 11 april 2012 earthquakes m w 8 6 and m w 8 2 were sourced within the northern wharton basin in the northeastern part of the indo australian diffuse plate boundary this unusually active oceanic intraplate region has generated many large earthquakes in the past most of which are believed to have occurred by strike slip motion triggered by the nw se oriented compressional stresses acting across the indian and australian plates in the aftermath of the 2004 megathrust earthquake along the nearby sunda trench increased seismicity in the northern wharton basin is attributed to the stress transfer from the sumatra andaman plate boundary models proposed for the april 2012 earthquakes differ somewhat in details but partly attribute their complex rupture to the reactivation of pre existing structures these structures include previously mapped n s trending fracture zones within the northern wharton basin and e w lineations across the ninetyeast ridge in this paper we review the regional tectonics and past seismicity on the indo australian plate in order to understand the seismotectonic setting of the april 2012 indian ocean earthquakes c 2014 elsevier b v all rights reserved

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved