multi fault rupture and successive triggering during the 2012 mw 8 6 sumatra offshore earthquake 1 on april 11 2012 a mw 8 6 strike slip earthquake struck the northern sumatra offshore region and became the largest intraplate event ever recorded there preliminary study results show a remarkably complex rupture pattern on a fault system manifested by north south trending paleo transform faults and east west striking abyssal hill fabric to image the rupture pattern of the earthquake we back projected teleseismic p wave recordings observed at three regional seismic networks eu au and f net our results indicate that the earthquake ruptured on a conjugate fault system composed of two subparallel wnw ese trending faults and a nne ssw striking fault in between for a duration of at least 120 s the rupture started at the wnw end of the first wnw ese trending fault located at the east of the conjugate fault system and propagated toward ese it then jumped to the second nne ssw trending fault located west of the first fault and first extended sswward and 15 s later nneward after its end on the second fault the rupture jumped onto the third wnw ese trending fault and continued wnwward the static coulomb stress changes calculated from the rupture of the first and then the second faults based on simplified uniform slip models suggest that initiation of the rupture on the second fault could be statically and dynamically triggered by the first fault however the third fault was primarily dynamically triggered by s or even love waves radiated from the southern branch of the second fault

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

dichotomy in mode propagation of coseismic ionospheric disturbance observations from 11 april 2012 indian ocean earthquake the ionosphere response to the great intraplate indian ocean earthquake of 11 april 2012 mw 8 6 and its largest aftershock mw 8 2 is analyzed using gps aided total electron content tec measurements data from the dense gps networks sugar sumatran gps array and the permanent andaman nicobar array formed the near field observations at distances 250 1200 km from the epicenter stations such as iisc dgar and few others provided measurements over 2000 km from the epicenter the coseismic ionospheric disturbances cids with a propagation velocity of 930 1262 m s equals the speeds of the shock acoustic waves arrive within 10 18 min after the earthquake occurrence the observed phenomenon of cid splitting into two modes north and south of the epicenter is akin to the well documented effects of anisotropy on wave propagation closer to the epicenter to its south the propagation velocity of cid is 1 km s and farther southeast of the network the velocity reduces to 500 600 m s in contrast toward andaman in the north the cid propagation velocity increases to 2 3 5 km s the zenith angle of the line of sight between the gps receiver and satellite appears to influence the amplitude of the tec fluctuations the anomalous azimuthal variation of the rayleigh wave radiation pattern best explains the observed n s asymmetry of cid

coseismic offsets due to the 11 april 2012 indian ocean earthquakes mw 8 6 and 8 2 derived from gps measurements 1 the 11 april 2012 earthquake mw 8 6 in the indian ocean about 100 km west off the sumatra subduction zone is the largest intraplate strike slip earthquake in the known history two hours later it triggered another great earthquake of mw 8 2 in its vicinity the earthquakes reflect the internal deformation of the diffused plate boundary between india and australia caused by the differential plate motion between them the slip occurred on conjugate planes and the presence of some of them has been reported from the swath bathymetry and satellite magnetic anomalies we estimate coseismic offsets due to these earthquakes at continuous gps sites in the andaman nicobar region and at other international gnss service igs sites around the earthquakes source region the sites on the andaman islands which are about 900 1200 km to the north of the earthquake epicenters experienced predominantly southward coseismic offset of up to 3 cm the nearest site campbell bay on great nicobar island about 500 km to the north of the earthquake documented an ese offset of about 4 cm the coseismic offsets are consistent with the finite fault slip models derived from back projection of the seismic waves recorded by the global networks

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

pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

dynamic earthquake rupture modelled with an unstructured 3 d spectral element method applied to the 2011 m9 tohoku earthquake an important goal of computational seismology is to simulate dynamic earthquake rupture and strong ground motion in realistic models that include crustal heterogeneities and complex fault geometries to accomplish this we incorporate dynamic rupture modelling capabilities in a spectral element solver on unstructured meshes the 3 d open source code specfem3d and employ state of the art software for the generation of unstructured meshes of hexahedral elements these tools provide high flexibility in representing fault systems with complex geometries including faults with branches and non planar faults the domain size is extended with progressive mesh coarsening to maintain an accurate resolution of the static field our implementation of dynamic rupture does not affect the parallel scalability of the code we verify our implementation by comparing our results to those of two finite element codes on benchmark problems including branched faults finally we present a preliminary dynamic rupture model of the 2011 m w 9 0 tohoku earthquake including a non planar plate interface with heterogeneous frictional properties and initial stresses our simulation reproduces qualitatively the depth dependent frequency content of the source and the large slip close to the trench observed for this earthquake

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

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

the 2012 sumatra great earthquake sequence the equatorial indian ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined on 11 april 2012 this region was hit in a couple of hours by two of the largest strike slip earthquakes ever recorded moment magnitudes mw 8 6 and 8 2 broadband seismological observations of the mw 8 6 mainshock indicate a large centroid depth 30 km and remarkable rupture complexity detailed study of the surface wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents to account for these observations we developed a procedure to invert for multiple point source parameters the optimum source model at long period consists of two point sources separated by about 209 km with magnitudes mw 8 5 and 8 3 to explain the remaining discrepancies between predicted and observed surface waves we can refine this model by adding directivity along the wnw ese axis however we do not exclude more complicated models to analyze the mw 8 2 aftershock we removed the perturbation due to large surface wave arrivals of the mw 8 6 mainshock by subtracting the corresponding synthetics computed for the two subevent model analysis of the surface wave amplitudes suggests that the mw 8 2 aftershock had a large centroid depth between 30 km and 40 km this major earthquake sequence brings a new perspective to the seismotectonics of the equatorial indian ocean and reveals active deep lithospheric deformation

ball and socket tectonic rotation during the 2013 mw7 7 balochistan earthquake abstract the september 2013 m w 7 7 balochistan earthquake ruptured a 200 km long segment of the curved hoshab fault in southern pakistan with 10 0 2 m of peak sinistral and 1 7 0 8 m of dip slip this rupture is unusual because the fault dips 60 15 towards the focus of a small circle centered in northwest pakistan and despite a 30 increase in obliquity along strike the ratios of strike and dip slip remain relatively uniform surface displacements and geodetic and teleseismic source inversions quantify a bilateral rupture that propagated rapidly at shallow depths from a transtensional jog near the northern end of the rupture static friction prior to rupture was unusually weak 0 05 and friction may have approached zero during dynamic rupture here we show that the inward dipping hoshab fault defines the northern rim of a structural unit in southeast makran that rotates akin to a 2 d ball and socket joint counter clockwise in response to india s penetration into the eurasian plate this rotation accounts for complexity in the chaman fault system and in principle reduces seismic potential near karachi nonetheless these findings highlight deficiencies in strong ground motion equations and tectonic models that invoke anderson byerlee faulting predictions

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

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

comments on brugger and others 2018 a quantitative comparison of microfossil extraction methods from ice cores in our comments we re evaluate brugger and others 2018 lycopodium eucalyptus double marker approach based on the fact that previous evidence already demonstrated that the batch of eucalyptus tablets used by brugger and others 2018 is not suitable for quantitative comparisons as they are characterized by inconsistent pollen concentration we present clear evidence that the eucalyptus tablets do feature inaccurate pollen concentrations and are therefore improper for all quantitative comparisons of microfossil extraction methods consequently the results of the quantitative and qualitative assessment of different pollen extraction methods from ice samples compiled by brugger and others 2018 are highly questionable due to the use of faulty marker tablets

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

tectonics of the ninetyeast ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge 1 analysis of new and existing geophysical data for the central indian and wharton basins of the indian ocean were used to understand the formation and evolution of the ninetyeast ridge ner especially its relationship to the kerguelen hot spot and the wharton spreading ridge satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking n5 e one of these at 89 e crosses the n10 e trending ner impacting the ner morphology from 77 to 43 ma the ner lengthened at a rate of 118 km myr twice that of the 48 58 km myr accretion rate of adjacent oceanic crust this difference can be explained by southward jumps of the wharton spreading ridge toward the hot spot which transferred portions of crust from the antarctic plate to the indian plate lengthening the ner magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the ner especially two leaving observable 65 and 42 ma fossil spreading ridges in contrast complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the ner comparison of the ner dates and magnetic anomaly ages implies that the hot spot first emplaced ner volcanoes on the indian plate at a distance from the wharton ridge but as the northward drifting spreading ridge approached the hot spot the two interacted keeping later ner volcanism near the spreading ridge crest by spreading center jumps

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

structural style of intra plate deformation central indian ocean basin evidence for the role of fracture zones abstract the structural style of the intraplate deformation developed in the central indian ocean basin was investigated in an area 78 e 82 e 0 s 6 s to the west of the afanasy nikitin seamount using an integrated data set of seismic reflection profiles from edinburgh university and lamont doherty geological observatory the study area contains two fracture zones which strike 005 e to 010 e with oceanic lithosphere age range 65 78 ma b p younging westwards across them no evidence for recent fault activity in the oceanic basement along the fracture zones could be detected in this area although the close association between intraplate earthquakes and fracture zones elsewhere suggests reactivation of the fracture zones at upper mantle depths in a left lateral strike slip sense a statistical study was carried out into the first and second orders of deformation long wavelength basement undulations and high angle reverse faults respectivelyand the relationships between them the orientations of the axes of the undulations vary from 065 e to 085 e while the high angle faults strike consistently at 090 e to 100 e the results of this analysis suggest that the high angle faults are the result of the reactivation of two sets of pre existing spreading centre normal faults one set originally facing towards the spreading centre and the other facing away furthermore although the long wavelength undulations are not fault generated the high angle faults have modified the basement topography causing the accentuation of some of the basement highs the observation that the undulations are not fault generated is consistent with them being of flexural origin including buckling of the crust and or lithosphere basement undulations are clearly discontinuous across fracture zones and the facing direction of faulting is also offset this discontinuity the orientation of the axes of the undulations the presence of other strong oblique basement trendsand information from regional earthquake studies suggest that the deformation resulted from not only n s compression as a result of the continental collision between india and asia but also left lateral strike slip along fracture zones caused by the difference in resistance to plate motion between the continental collision to the north and subduction at the sunda arc to the northeast

a spatial random field model to characterize complexity in earthquake slip 1 finite fault source inversions reveal the spatial complexity of earthquake slip over the fault plane we develop a stochastic characterization of earthquake slip complexity based on published finite source rupture models in which we model the distribution of slip as a spatial random field the model most consistent with the data follows a von karman autocorrelation function acf for which the correlation lengths a increase with source dimension for earthquakes with large fault aspect ratios we observe substantial differences of the correlation length in the along strike a x and downdip a z directions increasing correlation length with increasing magnitude can be understood using concepts of dynamic rupture propagation the power spectrum of the slip distribution can also be well described with a power law decay i e a fractal distribution in which the fractal dimension d remains scale invariant with a median value d 2 29 0 23 while the comer wave number k c which is inversely proportional to source size decreases with earthquake magnitude accounting for larger slip patches for large magnitude events our stochastic slip model can be used to generate realizations of scenario earthquakes for near source ground motion simulations

stress triggering of the great indian ocean strike slip earthquakes in a diffuse plate boundary zone 1 on april 11 2012 two great magnitude 8 earthquakes occurred within a two hour period off the west coast of northern sumatra indonesia in the broadly distributed india australia plate boundary zone the magnitude 8 6 mainshock holds the distinction of being both the largest instrumentally recorded strike slip earthquake and the largest earthquake away from a recognized plate boundary fault the mainshock involved sequential ruptures of multiple fault planes oriented nearly perpendicular to each other the adjacent 2004 megathrust earthquake statically loaded the northern wharton basin oceanic lithosphere on both of the 2012 mainshock fault plane orientations and greatly enhanced the rate of earthquake activity in the region for a year viscoelastic relaxation of the asthenosphere following the 2004 and 2005 megathrust earthquakes continued to positively stress the offshore region correlating with the locations of later strike slip earthquakes including two magnitude 7 and the 2012 magnitude 8 earthquakes

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

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

supershear rupture on multiple faults for the mw 8 6 off northern sumatra indonesia earthquake of april 11 2012 1 we perform a back projection method to image the rupture propagation and short period energy release of the 2012 off northern sumatra earthquake mw8 6 using hi net data recorded in japan the results show a complex pattern of four conjugate faults over about 180 sec there is a striking correspondence between the lengths and orientations of our rupture pattern with the distribution of aftershocks each of the first three stages of the rupture corresponds to a clear lineation in the aftershocks with lengths of 200 to 400 km rupture speeds for several of the fault segments were very high about 5 km s and exceed the local s wave velocity this is the first example of an oceanic earthquake with supershear rupture speed

interpretation of earthquake epicenter and cmt centroid locations in terms of rupture length and direction abstract a comparison is made between hypocenter and centroid estimates of shallow earthquakes to investigate whether the difference between these two quantities contains useful information on fault rupture length and direction an initial comparison of international seismological centre isc and harvard centroid moment tensor cmt catalog locations suggests that for earthquakes below mw 6 5 the difference in location is dominated by location errors whilst larger earthquakes appear to show the effect of increasing spatial rupture however for this data set combined location errors greater than 25 km are observed for a majority of earthquakes imposing a limit on interpretation we use these constraints to investigate this approach using a consistent earth model in the re analysis of eight previously well studied events new hypocenter estimates based on teleseismic p wave arrivals from the isc bulletin and a 3 d velocity model of the mantle s p12 wm13 are compared with new centroid locations obtained using the same 3 d model and waveform data from international deployment of accelerometers ida global digital seismic network gdsn and re calibrated high gain long period hglp stations the distance between our new hypocenter and centroid locations is shown to provide reliable information on both the rupture length and direction the rupture lengths and azimuths generally agree with the most recent previous results from other methods and provide an absolute estimate of the location of the rupture

magnetic anomalies layered intrusions and mars 1 studies of remanence controlled magnetic anomalies on earth provide possibilities to interpret the nature of crustal rocks that cause the large remanent anomalies on mars what types of conditions on earth can create large remanent magnetic anomalies such an anomaly extending for 20 km centered over a norite layer in the bjerkreim sokndal bks intrusion shows a minimum 13000 nt below background in the helicopter survey modeling of the anomaly requires a natural remanent magnetization nrm value of 24 a m similar to values measured in norite samples and to values invoked to explain the anomalies on mars preliminary magnetic assessment considers the roles of hemo ilmenite magnetite and oxide exsolution in clino and orthopyroxene and high temperature ductilely induced lattice preferred orientation

slow slip acceleration beneath andaman islands triggered by the 11 april 2012 indian ocean earthquakes the m w 8 6 and 8 2 strike slip earthquakes that struck the northeast indian ocean on 11 april 2012 resulted in coseismic deformation both at near and distant sites the slip distribution deduced using seismic wave analysis for the orthogonal faults that ruptured during these earthquakes is sufficient to predict the coseismic displacements at the global positioning system gps sites such as ntus palk and cusv but fall short at four continuous sites in the andaman islands region slip modeling for times prior to the events suggests that the lower portion of the thrust fault beneath the andaman islands has been slipping at least at the rate of 40 cm yr in response to the 2004 sumatra andaman coseismic stress change modeling of gps displacements suggests that the en echelon and orthogonal fault ruptures of the 2012 intraplate oceanic earthquakes could have possibly accelerated the ongoing slow slip along the lower portion of the thrust fault beneath the islands with a month long slip of 4 10 cm the misfit to the coseismic gps displacements along the andaman islands could be improved with a better source model assuming that no local process contributed to this anomaly

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

large fault fabric of the ninetyeast ridge implies near spreading ridge formation 1 ninetyeast ridge ner is a linear volcanic ridge in the indian ocean thought to have formed by hotspot volcanism on the northward drifting indian plate geological data from the ridge are sparse so its tectonic evolution is poorly known we studied satellite derived gravity data seismic reflection profiles and multibeam bathymetry to examine ner structure gravity data show that the ridge displays a series of nearly e w trending lineations with average spacing 0 4 45 km in seismic and bathymetry data these lineations correlate with horsts and grabens that probably formed near the time of ridge emplacement from their extensional nature and trends we infer that these faulted structures formed near the spreading ridge that separated the indian and antarctic plates and their ubiquity implies the hotspot was never far from this spreading ridge

real time forecasting of the april 11 2012 sumatra tsunami the april 11 2012 magnitude 8 6 earthquake off the northern coast of sumatra generated a tsunami that was recorded at sea level stations as far as 4800 km from the epicenter and at four ocean bottom pressure sensors darts in the indian ocean the governments of india indonesia sri lanka thailand and maldives issued tsunami warnings for their coastlines the united states pacific tsunami warning center ptwc issued an indian ocean wide tsunami watch bulletin in its role as an interim service provider for the region using an experimental real time tsunami forecast model rift ptwc produced a series of tsunami forecasts during the event that were based on rapidly derived earthquake parameters including initial location and mwp magnitude estimates and the w phase centroid moment tensor solutions w phase cmts obtained at ptwc and at the u s geological survey usgs we discuss the real time forecast methodology and how successive real time tsunami forecasts using the latest w phase cmt solutions improved the accuracy of the forecast

the 11 april 2012 east indian ocean earthquake triggered large aftershocks worldwide although strong remote aftershocks are exceedingly rare their rate increased fivefold during the six days following the 2012 east indian ocean earthquake perhaps as a result of the strike slip nature of the 2012 event or a build up of close to failure nucleation sites

pressure temperature estimates of the lizardite antigorite transition in high pressure serpentinites serpentine minerals in natural samples are dominated by lizardite and antigorite in spite of numerous petrological experiments the stability fields of these species remain poorly constrained this paper presents the petrological observations and the raman spectroscopy and xrd analyses of natural serpentinites from the alpine paleo accretionary wedge serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub greenschist p 20 kbar t 460 c along a subduction geothermal gradient we used the observed mineral assemblage in natural serpentinite along with the tmax estimated by raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures we show that below 300 c lizardite and locally chrysotile are the dominant species in the mesh texture between 320 and 390 c lizardite is progressively replaced by antigorite at the grain boundaries through dissolution precipitation processes in the presence of sio2 enriched fluids and through a solid state transition in the cores of the lizardite mesh above 390 c under high grade blueschist to eclogite facies conditions antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 c

pervasive deformation of an oceanic plate and relationship to large mw 8 intraplate earthquakes the northern wharton basin indian ocean large magnitude intraplate earthquakes within the ocean basins are not well understood the mw 8 6 and mw 8 2 strike slip intraplate earthquakes on 11 april 2012 while clearly occurring in the equatorial indian ocean diffuse plate boundary zone are a case in point with disagreement on the nature of the focal mechanisms and the faults that ruptured we use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern wharton basin to demonstrate pervasive brittle deformation between the ninetyeast ridge and the sunda subduction zone in addition to evidence of recent strike slip deformation along approximately north south trending fossil fracture zones we identify a new type of deformation structure in the indian ocean conjugate riedel shears limited to the sediment section and oriented oblique to the north south fracture zones the riedel shears developed in the miocene at a similar time to the onset of diffuse deformation in the central indian ocean however left lateral strike slip reactivation of existing fracture zones started earlier in the paleocene to early eocene and compartmentalizes the wharton basin modeled rupture during the 11 april 2012 intraplate earthquakes is consistent with the location of two reactivated closely spaced approximately north south trending fracture zones however we find no evidence for wnw ese trending faults in the shallow crust which is at variance with most of the earthquake fault models

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

on the transient response of serpentine antigorite gouge to stepwise changes in slip velocity under high temperature conditions 1 shear sliding tests were conducted on serpentine antigorite gouge to understand the rheology of serpentine bearing faults the experiments were carried out using a constant confining pressure 100 mpa a constant pore water pressure 30 mpa and a range of temperatures from room temperature to 600 c the transient response in frictional behavior following stepwise changes in the slip velocity were documented at each temperature slip rates varied between 0 0115 and 11 5 m s both the general level of frictional strength and the transient responses changed drastically at around 450 c as the temperature increased from 400 c to 450 c the strength of antigorite rose sharply the transient response also indicated a change in the mode of deformation from flow type behavior at temperatures below 400 c to frictional behavior stick slip at temperatures above 450 c 500 c although only a limited volume of serpentine was involved in the dehydration reaction x ray diffraction analyses and scanning electron microscopy observations showed that forsterite had nucleated in the experimental products at the higher temperatures that were associated with frictional behavior submicron sized streaky forsterite masses in shear localized zones may be evidence of shear induced dehydration that caused strengthening and embrittlement of the gouge although antigorite rheology is complicated the subsequent change in friction coefficient per order of magnitude change in sliding velocity increased with both increasing temperature and decreasing velocity implying that a possible flow mechanism of intragranular deformation became activated

link between plate fabric hydration and subduction zone seismicity in alaska subduction carries water into the earth where it can influence seismicity analysis of the structure of the alaskan subduction zone suggests fluid delivery is influenced by faults in the oceanic plate that formed at the mid ocean ridge

modeling the 2012 wharton basin earthquakes off sumatra complete lithospheric failure a sequence of large strike slip earthquakes occurred west of sunda trench beneath the wharton basin first reports indicate that the main shock was extremely complex involving three to four subevents m w 8 with a maze of aftershocks we investigate slip models of the two largest earthquakes by joint inversion of regional and teleseismic waveform data using the m w 7 2 foreshock we developed hybrid green s functions for the regional stations to approximate the mixture of oceanic and continental paths the main shock fault geometry is defined based on the back projection results point source mechanisms aftershock distribution and fine tune of grid searches the fault system contains three faults labeled f1 89 289 for dip strike f2 74 20 and f3 60 310 the inversion indicates that the main rupture consisted of a cascade of high stress drop asperities up to 30 mpa extending as deep as 50 km the rupture propagated smoothly from one fault to the next f1 f2 and f3 in sequence with rupture velocities of 2 0 2 5 km s the whole process lasted about 200 s and the major moment release 70 took place on the n s oriented f2 the m w 8 2 aftershock happened about 2 h later on a n s oriented fault with a relatively short duration 60 s and also ruptured as deep as 50 km the slip distributions suggest that the earthquake sequence was part of a broad left lateral shear zone between the australian and indian plates and ruptured the whole lithosphere these earthquakes apparently reactivated existing fracture zones and were probably triggered by unclamping of the great sumatran earthquake of 2004

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

coseismic compression dilatation and viscoelastic uplift subsidence following the 2012 indian ocean earthquakes quantified from satellite gravity observations the 2012 indian ocean earthquake sequence mw 8 6 8 2 is a rare example of great strike slip earthquakes in an intraoceanic setting with over a decade of gravity recovery and climate experiment grace data we were able to measure and model the unanticipated large coseismic and postseismic gravity changes of these events using the approach of normal mode decomposition and spatial localization we computed the gravity changes corresponding to five moment tensor components our analysis revealed that the gravity changes are produced predominantly by coseismic compression and dilatation within the oceanic crust and upper mantle and by postseismic vertical motion our results suggest that the postseismic positive gravity and the postseismic uplift measured with gps within the coseismic compressional quadrant are best fit by ongoing uplift associated with viscoelastic mantle relaxation our study demonstrates that the grace data are suitable for analyzing strike slip earthquakes as small as mw 8 2 with the noise characteristics of this region

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

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 2012 mw 8 6 sumatra earthquake evidence of westward sequential seismic ruptures associated to the reactivation of a n s ocean fabric 1 the 11 april 2012 mw 8 6 earthquake offshore sumatra is the largest of the rare great intraplate earthquakes of the instrumental era this major strike slip event occurred in the diffuse zone of deformation that accommodates differential rotation between indian and australian plates we perform a back projection analysis calibrated with well located aftershocks of short period teleseismic p waves recorded by the european array to image the rupture process during the mainshock in complement a love wave analysis is conducted for tracking azimuthal change in the apparent global source duration due to the source spatio temporal extent the combined analysis reveals a complex rupture pattern characterized by three main episodes of energy release the latest being located 370 km west of the epicenter on the ninety east ridge with a delay of 120 s we interpret the 11 april 2012 mw 8 6 offshore sumatra earthquake as a complex westward propagating sequence of dynamically triggered strike slip fault ruptures associated to the reactivation of the inherited nne striking sea floor fabric the dynamic triggering mechanism could result from the interaction between transient surface wave stress perturbations and fluids

ductile to brittle transition in thermally stable antigorite gouge at mantle pressures general shear experiments on antigorite rich serpentinite show a transition from ductile distributed to brittle localized deformation with increasing temperature from 300 c to 500 c at confining pressures from 1 to 2 gpa the coefficient of friction associated with slip along fractures decreases from 0 23 to 0 07 with an increase in temperature from 300 c to 500 c velocity stepping experiments exhibit a positive rate dependence as parameterized by a b values that decrease modestly with increasing temperature from 0 015 at 300 c to 0 01 at 500 c fractures contain fine grained foliated antigorite and there is no evidence of dehydration all samples have a moderate foliation and show microstructural evidence for both plastic and brittle deformation mechanisms under certain conditions the transition to brittle deformation at high pressures and temperatures in antigorite might generate earthquakes which could explain the occurrence of some intermediate depth seismicity within subduction zones in serpentinized regions that are too cold to induce dehydration

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

earthquake in a maze compressional rupture branching during the 2012 mw 8 6 sumatra earthquake seismological observations of the 2012 moment magnitude 8 6 sumatra earthquake reveal unprecedented complexity of dynamic rupture the surprisingly large magnitude results from the combination of deep extent high stress drop and rupture of multiple faults back projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system with relatively slow rupture speed the east southeast west northwest ruptures add a new dimension to the seismotectonics of the wharton basin which was previously thought to be controlled by north south strike slip faulting the rupture turned twice into the compressive quadrant against the preferred branching direction predicted by dynamic coulomb stress calculations orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure insensitive strength of the deep oceanic lithosphere

the 2012 mw 8 6 indian ocean earthquake deep nucleation on a listric like fault abstract the 2012 mw 8 6 indian ocean earthquake the largest instrumentally recorded strike slip event has been described as a westward rupture on orthogonal faults that are part of the original oceanic fabric of the region reported estimates of its depth vary from 10 to 45 km while the maximum depth of its inferred rupture extends to 50 60 km the complexity of the earthquake is undeniable both in terms of fault geometry and rupture propagation with debates on the geometry of the causative faults results from recent seismic surveys enable a fresh look at the earthquake fault geometry nucleation depths and lithospheric rheology in the present study these new findings are incorporated into 22 forward models using a revised velocity structure and n s geometries that flatten with depth the rupture initiation is modeled in the uppermost mantle 25 30 km as well as in the upper mantle at 45 km the latter of which is 15 20 km below the commonly accepted values of maximum depth for frictional failure our study shows that the westward directionality of wave propagation is achieved for strike slip failure on a listric like fault in contrast to subvertical geometries as previously suggested this propagation is persistent for a strike slip failure only for events nucleating at upper mantle depths 45 km rather than for those set to originate at depths between 25 and 30 km however for a dip slip failure the westward propagation is found to be consistent at both nucleation depths on a listric like fault this study highlights a deep nucleation within the lithospheric mantle for the 2012 mw 8 6 earthquake that could be due to strain localization induced by fluid rock interaction and or mineral transformation crystallization near the 680 c isotherm at 45 km depth 1 4 gpa which correlates with deep serpentinization on faults

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

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