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

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

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

seismic and aseismic deformation associated with the 1952 kern county california earthquake and relationship to the quaternary history of the white wolf fault synthesis of geodetic geologic and seismic data from the white wolf fault california indicates that the fault separates an area of late quaternary and continuing rapid uplift in the tehachapi mountains and transverse ranges from even more rapid subsidence in the southern san joaquin valley on july 21 1952 rupture of the white wolf fault produced the ml 7 2 kern county earthquake we used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952 1953 horizontal shear strains to model the measured coseismic vertical displacements with an elastic dislocation model a curved fault trace with decreasing fault depth 27 to 10 km from the surface vertically to the base slip 3 to 1 m and dip 75 to 20 from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record the measured seismic moment the fault plane solution and the pattern of surface rupture two short releveled lines near the 1952 epicenter tilted 4 and 17 rad down to the north from 5 10 years before the earthquake the preseismic tilts differ significantly from ten other surveys of these lines left lateral fault crossing shear strain from 0 2 20 years before the quake was two times greater than both preseismic off fault strains and the post seismic fault crossing strains during the first seven years after the earthquake aseismic deformation was negligible from 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone rising first in the epicentral region and then at the northeast end of the fault this was unaccompanied by any surface fault slip reconstruction of the vertical separation on the white wolf fault from late quaternary and late miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty fold from 0 1 0 2 mm yr to 3 9 mm yr between the past 10 15 m y and the most recent 0 6 1 2 m y we estimate a 170 to 450 yr average recurrence interval for earthquakes on the white wolf fault with slip equivalent to that in 1952 the 1952 earthquake appears to be characteristic of the quaternary record of fault displacement in the increase in white wolf slip toward the san andreas fault the ratio of reverse to lateral slip 1 3 1 and the ratio of vertical fault slip to emergence of the hanging wall block 3 1 the 8500 m deep sedimentary basin on the down thrown block cannot be explained by repeated slip of the white wolf fault in an elastic medium

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

sharpening the tomographic image of the subducting slab below sumatra the andaman islands and burma summary taking advantage of the increased ray coverage due to seismicity following the 2004 december and 2005 march great earthquakes an improved iterative regional global tomographic method was applied to the sumatra andaman and adjacent regions to better constrain the 3 d mantle velocity heterogeneity in the region velocity and hypocentral parameters were iteratively perturbed to sharpen the image of the subducted slab several iterations were performed and the effects of source mislocation were considered in the iterative process an issue usually neglected in global tomography we find that source relocation between iterations increases the amplitudes of slab anomalies and sharpens the definition of slab geometry beyond what can be achieved by a fixed source iterative inversion alone in addition extensive restoration tests of synthetic data were conducted that emphasize enhancements obtained by our iterative process these tests show significant increases in amplitude and decreased smearing of synthetic slab features thus when applied to the real data similar enhancements are inferred in the resulting model which better illustrates the complex slab geometry in the upper mantle and transition zone regions along the sumatra andaman and burma subduction zones

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

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

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

buckle controlled seismogenic faulting in peninsular india as intraplate earthquakes are often not associated with major known faults their location as well as their timing is unpredictable in peninsular india the larger mx5 0 events occur mainly on reverse faults in a series of belts b400 km apart which are aligned roughly normal to the azimuth of convergence between the indian and eurasian plates the location of the belts is controlled largely by the buckling wavelength of the lithosphere and the seismogenic faults do not generate folding and sometimes result from it there is consequently no need to postulate the creation of regularly spaced normal faults in an antecedent extensional phase and the deformation is consistent with a plate driving force such as gravity glide which is unlikely to reverse its polarity and which creates structures that are influenced by plate geometry at the leading edge the thesis is potentially of value to seismic hazard mitigation as it identifies the zones that are most at risk r 2004 published by elsevier ltd

defining the source region of the indian ocean tsunami from gps altimeters tide gauges and tsunami models abstract to understand the role of the co seismic moment magnitude mw 9 1 9 3 sumatra andaman earthquake rupture mechanism on the severity of the indian ocean tsunami we used permanent global positioning system gps data and carried out an analysis of co seismic displacement and tsunami models tsunami modelling validated against independent jason 1 altimetry data and tsunami arrival time data as determined from tide gauges was used to analyse the results of five co seismic slip inversions using gps seismicity and or uplift data in this way we determined the most likely slip distribution characterized by slip maxima of 20 m in the south and 20 m in the north we used both the distribution and temporal evolution of the co seismic slip as derived from the gps data we show that the 9 min propagation time of the rupture led to constructive interference of waves radiating first from the south and minutes later from the north strengthening the tsunami in southern india sri lanka and thailand we conclude that the incorporation of permanent real time gps stations would represent a valuable component of future tsunami warning systems

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

seismic b value anomalies in the sumatran region seismotectonic implications abstract the spatial variations in the seismic b value seismic moment bouguer gravity change and fault plane solution are estimated in the sumatran region based on homogeneous seismicity data from june 2005 to march 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 sumatran earthquake and the 2012 east indian ocean earthquakes based on changes in the seismicity rate this time period is divided into five intervals with different earthquake distributions and faulting styles the seismic b values faulting mechanisms and seismic moment release values are mapped throughout the sumatran region and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite grace gravity data the b values vary from 0 45 to 1 59 and an inverse correlation is observed between the b values and convergence rates low b values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate the lowest b values are located at a depth of approximately 28 km in both regions which indicates a homogeneous fault system between them the areas with low b values are associated with megathrust and strike slip mechanisms while the area with the lowest b values is dominated by a megathrust mechanism seismically active regions have low b values associated with a high stress accumulation and high seismic moment release low b value areas are located mainly along the andaman sunda trench and andaman nicobar sumatra forearc temporal variations in the seismicity distribution and faulting style are recognized with temporal variations in the b values in each period relationships exist between the epicenter of the 2004 event and the location of the maximum bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum bouguer gravity change in 2004 furthermore a zone of continuous 2 year positive gravity change is located near the nicobar islands the 2004 event boost the 2012 events and intraplate seismicity via stress transfer the seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

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

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

classification of deep sea fine grained sediments abstract most deep sea sediments contain one or more biogenic components and one dominant nonbiogenic component usually clay or silty clay we present a descriptive classification scheme in which deep sea fine grained sediments are placed within a three component system of calcareous biogenic siliceous biogenic and nonbiogenic components in a three step procedure the user assesses whether the dominant component is biogenic or nonbiogenic whether the dominant biogenic component is siliceous or calcareous and what the relative abundances of the biogenic components are within limits of 10 25 and 50 the terminology proposed is that commonly used by many sedimentologists with some refinements and greater precision in the use of terms

source model of the 2009 mw 7 6 padang intraslab earthquake and its effect on the sunda megathrust summary we investigate the source of the mw 7 6 padang earthquake by inverting three component global positioning system gps data and broad band regional seismic displacement waveforms the earthquake involved oblique reverse slip either on an e w south dipping plane or on a n s west dipping plane finite fault inversions indicate that the rupture primarily propagated downdip and southwest from the hypocentre with a scalar seismic moment between 3 4 and 3 7 10 20 nm mw 7 62 7 65 analysis of the seismic and geodetic data do not allow for unique identification of the causative focal plane aftershock patterns strongly suggest the e w plane was the causative focal plane but aligned geological structures in the downgoing plate favour the n s plane in either case this unusually large intraslab earthquake has moved closer to failure the deepest portion of the overlying megathrust which last ruptured during a great earthquake more than two centuries ago and is late in its seismic cycle

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

velocity gradients in the northern indian ocean inferred from earthquake moment tensors and relative plate velocities the region in the northern indian ocean between the central indian ridge the sumatra trench and 15 s to 9 n exhibits high seismicity along with basement and sediment deformation this area represents a diffuse plate boundary between the distinct and separate portions of the indian and australian plates we invert the spatial distribution of horizontal strain rates in this diffuse plate boundary zone to obtain the horizontal velocity field associated with the moment release from 61 earthquakes that have occurred over the past 80 years in the inversion the three dimensional rotation vector function w is determined at all corner points i e knot points of a grid such that the predicted strain rate values match the observed strain rates within the grid areas in a least squares sense the rotation vector of australia relative to india defined by the earthquake data alone is 10 1 s 9 1 81 5 e 5 4 0 1 0 06 m y 1 sigma error which is slightly different in location from the expected vector estimated using seafloor magnetic anomalies and transform fault trends 3 2 s 75 1 e 0 288 m y demets et al 1994a b by minimizing the rate of work within a continuous viscous medium that accommodates australia s motion relative to india demets et al 1994a we determine the complete horizontal velocity gradient tensor within the zone of diffuse deformation the viscosity tensor within all of the deforming areas can be specified when all the regions are free to strain in any direction and have the same viscosity uniform isotropic viscosity the predicted strain pattern is either pure extension or pure compression which is in contrast to the style of earthquake deformation east of chagos where both thrust and strike slip mechanisms are observed however when the isotropic viscosity of regions within the wharton basin is increased relative to the isotropic viscosity levels of areas along the ninetyeast ridge the plate motion is accommodated by combined thrust and strike slip styles of strain along the ninetyeast ridge and within the eastern central indian basin this predicted strain that accommodates plate motion is in agreement both in direction and style with the earthquake related strains in these regions although most of the observed style and direction of strain can be explained by lithospheric strength differences between the wharton basin and ninetyeast ridge areas alone the presence of strike slip faulting within the wharton basin requires a preexisting fabric that favors strike slip faulting there as a strain mechanism that accommodates in part the motion of australia relative to india to first order the direction of principal strain rates the style of strain and the spatial distribution of strain rates from the earthquakes are in accord with the accommodation of the expected plate motion whereas across the plate boundary zone the rates from 80 years of seismicity average out to at most 60 of the total strain rate

evidence of pervasive trans tensional deformation in the northwestern wharton basin in the 2012 earthquakes rupture area abstract the wharton basin in the indian ocean is one of the most extensively deforming ocean basins as confirmed by the occurrence of several very large earthquakes starting from january 12 2012 with mw 7 2 followed by the great earthquakes of april 11 2012 with mw 8 6 and mw 8 2 although the mw 7 2 and mw 8 2 earthquakes seem to have ruptured the re activated n s striking fracture zones the largest event mw 8 6 required the rupturing of several faults oblique to each other in a very complex manner in order to understand the nature of deformation in these earthquakes rupture zones we recently acquired 90 000 km2 of bathymetry 11 400 km of sub bottom profiling gravity and magnetic data covering the rupture areas of the 2012 earthquakes east of the ninety east ridge in the northwestern wharton basin these new data reveal six n8 e striking re activated fracture zones f5b f6a f6b f7a f7b and f8 where the fracture zone f6a can be followed for over 400 km and seems to be most active the epicenters of the mw 8 6 and mw 8 2 earthquakes lie on the fracture zones f6a and f7b respectively the newly observed fracture f5b in the east is short and has an extensional basin at its southern tip the fracture zone f8 defines the eastern boundary of the ninety east ridge the presence of en echelon faults and pull apart basins indicate left lateral motion along these fracture zones in between these fracture zones we observe pervasive 290 striking right lateral shear zones at 4 8 km intervals one of which has cut through a seamount that might have ruptured during the mw 8 6 earthquake we also observe another n20 e striking left lateral shear zones in the vicinity of f7b and f8 which is coincident with the strike of one of the nodal planes of the mw 8 6 focal mechanism these n20 e striking shear zones are interpreted as r riedel shears and the n290 e striking shear zones as r riedel shears these shear zones are formed by a series of n335 e striking en echelon normal faults our data also show the presence of n65 e striking thrust faults east of the ninety east ridge orthogonal to the regional principal direction of compression furthermore extensive bending related faulting is also observed close to the sumatra trench with normal faults also striking at n335 e similar to the normal faults that form the shear zones normal faults with a similar orientation are also present at the southern tip of f5b we explain all these observations with a single coherent model of deformation in the wharton basin where a dominant part of the regional nw se compressional stress is accommodated along the n8 e re activated fracture zones and the rest is distributed along shear zones thrust and normal faults between these fracture zones the thrust and normal faults are orthogonal to each other and define the direction of principal compressive and extensive stresses in the region whereas the two shear zone systems form a conjugate pair

fault reactivation in the central indian ocean and the rheology of oceanic lithosphere the intraplate deformation in the central indian ocean basin is a well known example of a deviation from an axiom of plate tectonics that of rigid plates with deformation concentrated at plate boundaries here we present multichannel seismic reflection profiles which show that high angle reverse faults in the sediments of the central indian ocean extend through the crust and possibly into the uppermost mantle the dip of these faults which we believe result from the reactivation of pre existing faults formed at the spreading centre is 40 in the basement which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres this style of deformation coupled with the observation of large earthquakes in the mantle lithosphere indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle ductile transition and propagate through the crust

a complete structural model and kinematic history for distributed deformation in the wharton basin abstract the equatorial eastern indian ocean hosts a diffuse plate boundary where widespread deformation accommodates the relative motion between the indian australian and capricorn sub plates we integrate iodp expedition 362 borehole data which for the first time provides an accurate ground truthed chronostratigraphy of the sedimentary sequence east of the ninety east ridge ner with 2d seismic reflection profiles and multibeam bathymetry to assess the styles of faulting between the ner and the sunda subduction zone timing of activity and comparison with physical and rheological properties we identify four distinct fault sets east of the ner in the northern wharton basin n s 350 010 orientated faults associated with the n s fracture zones formed at the now extinct wharton spreading centre are still active and have been continuously active since at least 10 ma nne and wnw trending fault fabrics develop between the fracture zones the orientations and likely sense of displacement on these three sets of faults defines a riedel shear system responding to nne ssw left lateral strike slip activity at depth demonstrated by the recent 2012 great intraplate earthquakes we also find evidence of ne sw reverse faults similar in style to e w reverse faults observed west of the ner where reverse faulting is more dominant we show that the activity of this strike slip system increased ca 7 9 ma contemporaneous with reverse faulting and intraplate deformation west of the ner

active faulting on the ninetyeast ridge and its relation to deformation of the indo australian plate 1 the 4500 km long ninetyeast ridge ner in the northeastern indian ocean crosses a broad zone of deformation where the indo australian plate is fracturing into three smaller plates india capricorn australia separated by diffuse boundaries whose extents are poorly defined new multichannel seismic reflection profiles image active faults along the entire length of the ner and show spatial changes in the style of deformation along the ridge the northern ner 0 n 5 n displays transpressional motion along wnw ese faults observed fault patterns confirm strike slip motion at the western extent of the april 2012 wharton basin earthquake swarm in the central ner 5 s 8 s deformation on wnw ese trending thrust faults implies nearly n s compression an abrupt change in fault style occurs between 8 and 11 s with modest extension characterizing the southern ner 11 s 27 s although extension is dominant narrow zones of faults with strike slip or compressional character also occur in the southern ner suggesting a complex combination of fault motions at all sites active faulting is controlled by the reactivation of original spreading center formed normal faults implying that deformation is opportunistic and focused along existing zones of weakness even when original fault trend is oblique to the direction of relative plate motion observed faulting can be interpreted as india australia deformation in the northern ner and capricorn australia deformation in the southern ner the india capricorn boundary is directly adjacent to the northern ner and this juxtaposition combined with a different style of faulting to the east of the ner imply that the ridge is a tectonic boundary

buckling of the oceanic lithosphere from geophysical data and experiments two major hypotheses have been advanced for the formation of the long wavelength 100 300 km undulations of oceanic basement and overlying sediments developed in the central indian ocean basin whole layer folding buckling and local thickening inverse boudinage using appropriately scaled two layer analogue models for the oceanic lithosphere comprising a brittle layer above a ductile layer we show that buckling of the entire brittle layer is likely to be the mode of deformation however the lithosphere asthenosphere boundary remains undisturbed we find a relationship between the thickness of the brittle layer and the wavelength of folding such that the wavelength is 7 times the brittle layer thickness

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

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

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

a 100 year average recurrence interval for the san andreas fault at wrightwood california evidence for five large earthquakes during the past five centuries along the san andreas fault zone 70 kilometers northeast of los angeles california indicates that the average recurrence interval and the temporal variability are significantly smaller than previously thought rapid sedimentation during the past 5000 years in a 150 meter wide structural depression has produced a greater than 21 meter thick sequence of debris flow and stream deposits interbedded with more than 50 datable peat layers fault scarps colluvial wedges fissure infills upward termination of ruptures and tilted and folded deposits above listric faults provide evidence for large earthquakes that occurred in a d 1857 1812 and about 1700 1610 and 1470

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

upper mantle rheology from grace and gps postseismic deformation after the 2004 sumatra andaman earthquake 1 mantle rheology is one of the essential yet least understood material properties of our planet controlling the dynamic processes inside the earth s mantle and the earth s response to various forces with the advent of grace satellite gravity measurements of mass displacements associated with many processes are now available in the case of mass displacements related to postseismic deformation these data may provide new constraints on the mantle rheology we consider the postseismic deformation due to the m w 9 2 sumatra 26 december 2004 and m w 8 7 nias 28 march 2005 earthquakes applying wavelet analyses to enhance those local signals in the grace time varying geoids up to september 2007 we detect a clear postseismic gravity signal we supplement these gravity variations with gps measurements of postseismic crustal displacements to constrain postseismic relaxation processes throughout the upper mantle the observed gps displacements and gravity variations are well explained by a model of visco elastic relaxation plus a small amount of afterslip at the downdip extension of the coseismically ruptured fault planes our model uses a 60 km thick elastic layer above a viscoelastic asthenosphere with burgers body rheology the mantle below depth 220 km has a maxwell rheology assuming a low transient viscosity in the 60 220 km depth range the grace data are best explained by a constant steady state viscosity throughout the ductile portion of the upper mantle e g 60 660 km this suggests that the localization of relatively low viscosity in the asthenosphere is chiefly in the transient viscosity rather than the steady state viscosity we find a 8 10 18 pa s mantle viscosity in the 220 660 km depth range this may indicate a transient response of the upper mantle to the high amount of stress released by the earthquakes to fit the remaining misfit to the grace data larger at the smaller spatial scales cumulative afterslip of about 75 cm at depth should be added over the period spanned by the grace models it produces only small crustal displacements our results confirm that satellite gravity data are an essential complement to ground geodetic and geophysical networks in order to understand the seismic cycle and the earth s inner structure

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

indian ocean actively deforms according to plate tectonics plates are rigid and deform only at narrow boundaries but studies have shown that intraplate deformation can occur in continental plates in her perspective deplus discusses recent evidence for active deformation in the indian ocean offering a rare opportunity to study intraplate deformation of oceanic lithosphere

estimating prehistoric earthquake magnitude from point measurements of surface rupture abstract we have developed a method for estimating the magnitude of prehistoric earthquakes using displacement data that usually can be collected from paleoseismic investigations this method is necessary because essentially all current magnitude estimates for prehistoric events rely on determining the total length of coseismic surface rupture which is rarely measurable or rely on segmentation scenarios for which uncertainties cannot be quantified although surface rupture length is a better predictor of magnitude than displacement for historic earthquakes paleoseismic investigations are better at providing measurements of the amount of displacement at a site along a fault the key to our method is to incorporate the variability in displacement observed in 14 modern events which allows a formal uncertainty in magnitude to be assigned to prehistoric ruptures we show how multiple measurements along a preserved fraction of a rupture can be combined to reduce the uncertainty in the estimate of magnitude our analysis shows that uncertainty asymptotically approaches the natural variability of ruptures so 5 to 10 displacement measurements are sufficient to characterize paleomagnitude we conclude that sampling of scarps with lengths of even 10 of the original rupture can provide magnitude values that reasonably estimate the earthquake tests of the method using randomly sampled data from the 1992 m w 7 3 landers and 1954 m s 6 8 dixie valley earthquakes provide close approximations of the actual magnitudes

burma plate motion 1 plate motion in the indo burmese arc andaman sumatra region of burma plate is poorly resolved this is mainly due to lack of relevant data and complex tectonics of the region we analyze 1 azimuths of coseismic displacements due to the 2004 sumatra andaman and 2005 nias earthquakes 2 estimates of interseismic deformation in the indo burmese arc andaman sumatra and sagaing fault regions all based on gps measurements 3 long term plate motion rates across sumatra fault system sagaing fault and andaman sea from geomorphological and other geophysical studies and 4 the earthquake focal mechanisms in the region we suggest that the ssw motion of sunda plate with respect to indian plate may be partitioned into the dextral strike slip motion across the sagaing fault in the north and sumatra fault system in south in the back arc region and the arc normal motion across the sumatra subduction zone which becomes oblique in andaman and southern indo burmese arc region and dextral in the northern indo burmese arc region of the fore arc under the rigid plate approximation we estimate a pole for india burma plate pair at 27 1 n 82 1 1 e with an angular velocity of 0 845 0 12 ma and for burma sunda at 22 3 1 1 n 109 3 2 5 e with an angular velocity of 0 67 0 12 ma thus the plate motion in the northern and southern regions of burma plate namely the indo burmese arc and andaman sumatra arc may be explained by a single pole and does not require a boundary between the two

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

enhanced intraplate seismicity along continental margins some causes and consequences abstract in australia much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression shmax intriguingly along the margin side of one such zone a near optimally oriented crustal scale structure the darling fault has no recorded historical seismicity to explain these enigmatic features we explore how the compositional and thermal structuring of continental passive margins may lead to variations in mechanical response to imposed forcing using forward modelling we show how lateral heat flow across such margins can produce increases in moho temperatures by several 10s of degrees relative to the continental interior extending up to 200 km inboard of the ocean continent transition provided the step in thermal lithosphere thickness into the continent is greater than 30 km for smaller lithospheric steps moho temperatures beneath continental margins will be cooler than beneath interiors our mechanical models show that such thermal effects can elevate strain rates along the margin by several factors relative to continental interiors when subject to far field forcing in the shallow seismogenic realm the strain rate maximum is concentrated at around 100 150 km inboard of the ocean to continent lithospheric step providing a plausible explanation for the pattern of seismicity observed in some of australia s marginal seismic zones this interpretation requires that strength changes associated with moho temperature variations of several 10s of degrees are sufficient to override the reactivation potential of near optimally oriented crustal penetrative structures with thermal processes providing an important switch controlling reactivation potential of existing structures it also provides an insight into the mechanism of formation of continental ribbons during rifting of thermally mature margins