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

modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

itrf2008 plate motion model 1 the itrf2008 velocity field is demonstrated to be of higher quality and more precise than past itrf solutions we estimated an absolute tectonic plate motion model made up of 14 major plates using velocities of 206 sites of high geodetic quality far from plate boundaries deformation zones and glacial isostatic adjustment gia regions derived from and consistent with itrf2008 the precision of the estimated model is evaluated to be at the level of 0 3 mm a wrms no gia corrections were applied to site velocities prior to estimating plate rotation poles as our selected sites are outside the fennoscandia regions where the gia models we tested are performing reasonably well and far from gia areas where the models would degrade the fit antarctica and north america our selected velocity field has small origin rate bias components following the three axis x y z respectively 0 41 0 54 0 22 0 64 and 0 41 0 60 95 per cent confidence limits comparing our model to nnr nuvel 1a and the newly available nnr morvel56 we found better agreement with nnr morvel56 than with nnr nuvel 1a for all plates except for australia where we observe an average residual rotation rate of 4 mm a using our selection of sites we found large global x rotation rates between the two models 0 016 ma and between our model and nnr morvel56 of 0 023 ma equivalent to 2 5 mm a at the earth surface

earthquakes beneath the himalayas and tibet evidence for strong lithospheric mantle eleven intracontinental earthquakes with magnitudes ranging from 4 9 to 6 occurred in the mantle beneath the western himalayan syntaxis the western kunlun mountains and southern tibet near xigaze between 1963 and 1999 high resolution seismic waveforms show that some focal depths exceeded 100 kilometers indicating that these earthquakes occurred in the mantle portion of the lithosphere even though the crust has been thickened there the occurrence of earthquakes in the mantle beneath continental regions where the subduction of oceanic lithosphere ceased tens of millions years ago indicates that the mantle lithosphere is sufficiently strong to accumulate elastic strain

optimal fault resolution in geodetic inversion of coseismic data summary with the continued growth in availability of differential interferometry from synthetic aperture radar and gps data space based geodesy has been widely applied to image the coseismic displacement field and to retrieve the static dislocation over the fault plane for almost all the significant earthquakes of the past two decades this is performed by linear data inversion over a set of subfaults generally characterized by a constant and predefined or manually adjusted dimensions in this paper we propose a new algorithm to automatically retrieve an optimized fault subdivision in the linear inversion of coseismic geodetic data the code iteratively keeps the parameter resolution close to a predefined high value we first discuss the rationale supporting ouralgorithmand afteradetaileddescriptionofitsimplementation weanalysetheadvantages of its introduction in the data inversion the algorithm was tested against an exhaustive range of synthetic and real data sets and fault mechanisms among them we present the results for themw 6 2 2009 l aquila central italy earthquake and compare the new and previously published slip distributions showing the disappearance of misleading slip pattern and the increased resolution for shallower zones

crustal and upper mantle responses to lithospheric segmentation in the northern apennines lithospheric tear faults are expected to develop in response to along strike variations in the rates of slab rollback however the exact geometry of such structures and their crustal and upper mantle expressions are still debated we present an analysis of seismic structural and morphological features that possibly represent the expression of lithospheric segmentation in the northern apennines geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern apennines characterized by a change in the spatial distribution of intermediate depth seismicity along strike variations in the pattern of crustal seismicity and a bend in the moho topography the near surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern apennines in the proximity of the livorno sillaro lineament we interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern apennines marking the boundary between domains that underwent contrasting styles of lithospheric deformation which are either associated with different rates of slab rollback or a transition from underplating to retreat we suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures ultimately further tearing along such structures could potentially lead to the occurrence of tear related magmatism and the formation of slab windows

convergence rate across the nepal himalaya and interseismic coupling on the main himalayan thrust implications for seismic hazard we document geodetic strain across the nepal himalaya using gps times series from 30 stations in nepal and southern tibet in addition to previously published campaign gps points and leveling data and determine the pattern of interseismic coupling on the main himalayan thrust fault mht the noise on the daily gps positions is modeled as a combination of white and colored noise in order to infer secular velocities at the stations with consistent uncertainties we then locate the pole of rotation of the indian plate in the itrf 2005 reference frame at longitude 1 34 3 31 latitude 51 4 0 3 with an angular velocity of 0 5029 0 0072 myr the pattern of coupling on the mht is computed on a fault dipping 10 to the north and whose strike roughly follows the arcuate shape of the himalaya the model indicates that the mht is locked from the surface to a distance of approximately 100 km down dip corresponding to a depth of 15 to 20 km in map view the transition zone between the locked portion of the mht and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the himalaya according to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data this transition seems to happen in a zone where the temperature reaches 350 c the convergence between india and south tibet proceeds at a rate of 17 8 0 5 mm yr in central and eastern nepal and 20 5 1 mm yr in western nepal the moment deficit due to locking of the mht in the interseismic period accrues at a rate of 6 6 0 4 10 19 nm yr on the mht underneath nepal for comparison the moment released by the seismicity over the past 500 years including 14 m w 7 earthquakes with moment magnitudes up to 8 5 amounts to only 0 9 10 19 nm yr indicating a large deficit of seismic slip over that period or very infrequent large slow slip events no large slow slip event has been observed however over the 20 years covered by geodetic measurements in the nepal himalaya we discuss the magnitude and return period of m 8 earthquakes required to balance the long term slip budget on the mht

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

seismotectonic model of the sikkim himalaya constraint from microearthquake surveys the seismotectonic model in the sikkim himalaya does not fit well with the proposed steady state or evolutionary models the main boundary thrust mbt is seismogenic and is a mantle reaching fault the earthquakes are not confined to shallower depths 25 km above the plane of detachment as proposed in the models the seismic activity continues from surface to the lower crust 0 45 km to the north of mbt and earthquakes are produced by a thrust mechanism the earthquakes to the east of sikkim in the bhutan himalaya on the other hand are produced along a 200 km long northwest southeast trending lineament by transverse tectonics the seismogenic lineament cuts across the himalayan major thrusts and extends to the goalpara wedge in the southeast the earthquakes occur by strike slip mechanism in the midcrust at a depth range of 10 25 km along this long active lineament

interseismic strain accumulation on the himalayan crustal ramp nepal the departement of mines and geology has been monitoring the seismicity of the central himalayas of nepal since 1985 intense microseismicity and frequent medium size earthquakes ml 4 tend to cluster beneath the topographic front of the higher himalaya this 10 20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data both microseismic and geodetic data indicate strain accumulation on a mid crustal ramp that had been previously inferred from geological and geophysical evidence this ramp connects a flat decollement under the lesser and sub himalaya with a deeper decollement under the higher himalaya and probably acts as a geometric asperity where strain and stress build up during the interseismic period the large himalayan earthquakes could nucleate there and probably activate the whole flat and ramp system up to the blind thrusts of the sub himalaya

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

geophysical constraints on the seismotectonics of the sikkim himalaya abstract the role of transverse lineaments is often invoked in strain partitioning to generate strike slip dominated earthquakes in the sikkim himalaya including the recent m w 6 9 earthquake of 18 september 2011 an integration of seismicity clusters and crustal structures in conjunction with gravity modeling helps to develop a unified seismotectonic model for the sikkim himalaya where transverse lineaments appear to have little role in strain concentration instead deflection of the arc normal slips by rigid eclogitized indian crust in the 40 60 km depth range is the primary source to generate strike slip dominated large magnitude earthquakes the clustering of relatively deep aftershocks and main event near the southern edge of the eclogitized lower crust is a clear manifestation of this deformation this transcurrent deformation also reorients foliation planes in the low velocity block immediately above to inflict anisotropy and promote strike slip dominated moderate magnitude earthquakes in the 20 40 km depth range the low angle northeast dipping detachment in the 10 12 km depth range forms a localized asperity that produces low magnitude earthquakes beneath the lesser himalayan duplex it is surmised that the competent and strong eclogitic layer in the lower crust serves as a repository of high stresses during an earthquake buildup cycle wherein the fluid pressure in the fractured rock matrix above plays a key role in the earthquake generating process

seismicity and one dimensional velocity structure of the himalayan collision zone earthquakes in the crust and upper mantle 1 earthquakes beneath the himalayan collision zone occur at depths between near surface and around 100 km below sea level after relocating earthquakes with two one dimensional 1 d velocity models we found a clear bimodal depth distribution for earthquakes in the himalayas of eastern nepal and the southern tibetan plateau and evidence that some earthquakes originate at upper mantle depths seismicity in nepal shows an accumulation of earthquakes along the front of the himalayan arc with a seismic gap between longitudes 87 3 e and 87 7 e although upper crustal seismicity along the topographic front of the high himalaya is consistent with a region of high strain accumulation associated with convergence on the main himalayan thrust fault microearthquakes do not necessarily occur on this fault instead they concentrate in the hanging wall seismic activity in the sub himalaya and the terai plains is almost exclusively limited to the vicinity of the location of the magnitude 6 5 20 august 1988 udayapur earthquake with most of the earthquakes in the lower crust and the upper mantle clusters of earthquakes in the lesser and high himalayas and south tibet tethyan himalayas mark very well defined zones of seismicity at depths between 50 and 100 km confirming the presence of earthquakes in the upper mantle in the region of continental collision the occurrence of earthquakes at sub moho depths favors the idea that the continental upper mantle deforms by brittle processes

intermediate depth earthquakes beneath the india tibet collision zone we report on three intermediate depth earthquakes in the india tibet collision zone two under the himalayan thrust belt htb and one beneath the indus zangbo suture the mb magnitudes of these three events are from 4 3 to 4 9 and are too small to be well located by conventional means however from modeling their broadband waveforms recorded at near regional distances on a temporary passcal array we can confidently confine the sources to be below the crust between 70 and 80 km deep the existence of these intermediate depth earthquakes in this area suggests relatively low temperatures in the mantle lithosphere the two events under the htb display strike slip mechanisms with some normal faulting component this is quite different from the shallow thrust events typical of the same area the principal p and t axes of all 3 fault plane solutions show roughly ns compression and ew extension consistent with a regional stress field produced by the indenting of the india continent

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

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

geological significance of seismic reflections in collision belts geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well documeted references some of these problems are illustrated by examples coming from various tectonic settings in france and adjacent areas and tentative interpretations are proposed

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

earthquake science shaking faults loose earthquakes often induce aftershocks on other faults but the mechanisms remain elusive an innovative analysis tells us more about the effects of dynamic stresses caused by the passage of seismic waves

earthquake processes of the himalayan collision zone in eastern nepal and the southern tibetan plateau summary focal mechanisms determined from moment tensor inversion and first motion polarities of the himalayan nepal tibet seismic experiment himnt coupled with previously published solutions show the himalayan continental collision zone near eastern nepal is deforming by a variety of styles of deformation these styles include strike slip thrust and normal faulting in the upper and lower crust but mostly strike slip faulting near or below the crust mantle boundary moho one normal faulting earthquake from this experiment accommodates east west extension beneath the main himalayan thrust of the lesser himalaya while three upper crustal normal events on the southern tibetan plateau are consistent with east west extension of the tibetan crust strike slip earthquakes near the himalayan moho at depths 60 km also absorb this continental collision shallow plunging p axes and shallow plunging ew trending t axes proxies for the predominant strain orientations show active shearing at focal depths 60 90 km beneath the high himalaya and southern tibetan plateau beneath the southern tibetan plateau the plunge of the p axes shift from vertical in the upper crust to mostly horizontal near the crust mantle boundary indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate

crustal structure of the darjeeling sikkim himalaya and southern tibet analysis of data from nine temporary broadband seismic stations operated across west bengal and sikkim along with publicly available data from seismographs in the surrounding region provides the first image of the descending indian plate beneath the darjeeling sikkim himalaya the down going indian crust is imaged by receiver function common conversion point stacking using data from 32 sites in combination with more detailed analyses from simultaneous modelling of receiver function data and rayleigh wave group velocity dispersion at 13 stations compared to locations farther south on the indian shield our southernmost station shows evidence for thickened crust beneath the mahanadi rift basin with a possible mafic basal layer north of the mahanadi rift the indian moho is 38 km deep below the archean terranes of the northeast part of the indian shield the moho then dips gently northward beneath the himalayan foreland basin reaching a depth of 44 48 km below the himalayan foothills below sikkim the moho continues to deepen but there are indications of secondary structures in the receiver function image and modelling results suggesting some imbrication of the crust as it flexes downward the crust thickens further beneath the greater himalaya and southern tibet reaching depths of 65 70 km below the southern tibet detachment std below the lhasa terrane north of the std a double discontinuity exists with interfaces at 55 60 km and 80 km depth there is a significant reduction in the average shear wave velocity of the crystalline crust between sites to the south of and on the himalayan foreland basin and sites in the himalaya and to the north below the himalaya and southern tibet the p to s conversion ps has a lower amplitude compared to that observed at sites on the undeformed indian shield this decrease in amplitude of the moho ps phase could arise from a lower impedance contrast across the crust mantle boundary or from scattering due to deformation of the crust and moho a coherent negative arrival beneath the darjeeling sikkim himalaya indicate the presence of a low velocity zone lvz possibly associated with the main himalayan thrust this lvz can be traced beneath the darjeeling sikkim himalaya but disappears beneath the greater himalaya

mantle fault zones beneath the himalayan collision flexure of the continental lithosphere abstract the himalayas and the tibetan plateau are the result of the continental collision between india and eurasia the indian plate underthrusts the himalayan mountains and the southern tibetan plateau recorded seismicity at the himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust mantle boundary the question of whether the near moho earthquakes are in the crust or in the upper mantle has been controversial and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes earthquake locations from several experiments place seismic events in the upper mantle using a finite element model we establish a link between the recorded upper mantle seismicity beneath the himalayan collision zone and flexural bending of the indian lithosphere earthquake locations focal mechanisms and seismic imaging results from the himnt experiment combined with previous constraints on the geometry and deformation of the himalayan collision are used to set up the finite element models of lithospheric loading our purpose is to infer the mechanical state of the lithosphere beneath the himalayas and to evaluate the role of the lithospheric mantle in the support of the loads the pattern of mantle seismicity can be explained by modeling the response of the indian plate to loads corresponding to the weight of the sediments of the ganga basin the himalayan mountains and the southernmost tibetan plateau combined with the effects of a horizontal force per unit length acting upon the lithospheric plate we calculated the steady state stress field in the indian lithosphere where the lithospheric mantle is assumed to be viscoelastic and non newtonian and the asthenosphere is modeled as viscoelastic and newtonian two model suites were tested one with an elastic crust model suite 1 and one with a viscoelastic crust model suite 2 both model suites provide a good fit to the observed patterns of seismicity but model suite 2 is the one that best reproduces the observations high differential stresses concentrate in the upper mantle and predicted principal stress orientations match those inferred from focal mechanisms in the area our models show that beneath the ganga basin and the southernmost himalaya earthquakes at near moho depths do not need to show extension nor is the lower crust required to be weak in order to infer that the uppermost mantle yields by brittle failure even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes mohr coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure

rupture of the indian slab in the 2011 mw 6 9 sikkim himalaya earthquake and its tectonic implications unlike the other himalayan plate boundary segments the eastern nepal to bhutan himalayan region is not known to have generated prominent shallow thrust faulting earthquakes typical of the ongoing convergence this region features strike slip earthquakes over the depth ranges of 40 120 km indicating intraslab deformation here we present for the first time a slip distribution model for the largest recorded intraslab strike slip earthquake in this region the mw 6 9 sikkim event that occurred on 18 september 2011 relying on kinematic source process modeling our results indicate a ne sw trending steeply dipping sinistral source zone within the underthrusting indian slab the rupture propagated radially with a low rupture velocity of 1 7 km s breaking a large asperity of 20 a 20 km 2 with a maximum slip of 1 6 m the rupture nucleated at a depth of 45 km and reached upper mantle depths the computed coseismic stress drop value is 13 6 mpa we suggest that most of the aftershocks occurred on the conjugate plane possibly due to stress triggering stress inversion of focal mechanisms indicates a transpressive stress regime throughout the crust and pure strike slip regime in the upper mantle we observed a unimodal distribution of earthquakes beneath the higher himalaya this indicates a strong brittle indian slab and unravels a scenario of an eventual breakup of the lithosphere the key trigger might be variation in the convergence rates along the himalayan arc 2019 american geophysical union all rights reserved

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

fault size and depth extent of the ecuador earthquake m w 7 8 of 16 april 2016 from teleseismic and tsunami data the april 2016 ecuador mw 7 8 earthquake was the first megathrust tsunamigenic earthquake along the ecuador colombia subduction zone since 1979 mw 8 2 with 200 deaths from tsunami while there was no tsunami damage from the 2016 earthquake small tsunamis were recorded at deep ocean assessment and reporting of tsunami and tide gauges here we designed various fault models with and without shallow slip area and compared the computed teleseismic and tsunami waveforms with the observations while teleseismic inversions were indifferent about inclusion or exclusion of the shallow slip tsunami waveforms strongly favored the slip model without shallow slip our final slip model has a depth range of 15 44 km and its western shallowest limit is located at the distance of 60 km from the trench maximum and average slips were 2 5 and 0 7 m respectively the large slip area was 80 km along strike 60 km along dip in the depth range of 15 35 km

estimation of source parameters of m w 6 9 sikkim earthquake and modeling of ground motions to determine causative fault in this study source parameters of the september 18 2011 m w 6 9 sikkim earthquake were determined using acceleration records these parameters were then used to generate strong motion at a number of sites using the stochastic finite fault modeling technique to constrain the causative fault plane for this earthquake the average values of corner frequency seismic moment stress drop and source radius were 0 12 hz 3 07 1026 dyne cm 115 bars and 9 68 km respectively the fault plane solution showed strike slip movement with two nodal planes oriented along two prominent lineaments in the region the ne oriented kanchendzonga and nw oriented tista lineaments the ground motions were estimated considering both the nodal planes as causative faults and the results in terms of the peak ground accelerations pga and fourier spectra were then compared with the actual recordings we found that the nw se striking nodal plane along the tista lineament may have been the causative fault for the sikkim earthquake as pga estimates are comparable with the observed recordings we also observed that the fourier spectrum is not a good parameter in deciding the causative fault plane

stress field and seismicity in the indian shield effects of the collision between india and eurasia intraplate stresses and intraplate seismicity in the indian subcontinent are strongly affected by the continued convergence between india and eurasia the mean orientation of the maximum horizontal compression in the indian subcontinent is subparallel to the direction of the ridge push at the plate boundary as well as to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone indicating that the intraplate stresses in the subcontinent including the shield area are caused by plate tectonic processes spatial distribution of historic and instrumentally recorded earthquakes indicate that the seismic activity is mostly confined to linear belts while the remaining large area of the shield is stable the available conventional heat flow data and other indicators of heat flow suggest hotter geotherms in the linear belts leading to amplification of stresses in the upper brittle crust many of the faults in these linear belts which happen to be 200 80 m y old are being reactivated either in a strike slip or thrust faulting mode the reactivation mechanisms have been analyzed by taking into consideration the amplification of stresses pore pressures geological history of the faults and their orientation with respect to the contemporaneous stress field the seismicity of the indian shield is explained in terms of these reactivation mechanisms

stress tensor inversion using detailed microearthquake information and stability constraints application to olfus in southwest iceland using detailed microearthquake data we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms the nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well fitting focal mechanisms for each event the range of focal mechanisms significantly improves the quality of the estimated stress tensor relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane the clusters frequently fall on a common fault plane and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane we assume this is the correct fault plane for the event if there is no predefined fault plane we utilize a simple mohr coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism the nodal plane with highest relative instability is chosen as the fault plane differences between the instability and the standard slip angle criterion are investigated the new inversion scheme has been applied to microearthquake data from the olfus area in the vicinity of the southwest iceland triple junction we estimate an oblique strike slip state of stress maximum horizontal stress at n30 e and minimum horizontal stress at n60 w with significant normal faulting influence the instability fault selection criterion predicts very well the orientation of faults mapped by relative localization

spatial relation between main earthquake slip and its aftershock distribution 1 we examine where aftershocks occur relative to the spatial distribution of the main shock slip using data from several recent large earthquakes no universal relation between high and low moment regions and high or low aftershock occurrence or vice versa is found we generally find that few and usually the smaller aftershocks occur in the high slip regions of the fault a notable exception to this being the great 1996 biak indonesia subduction zone earthquake in all cases aftershocks occur on favorably oriented planes of weakness in regions of increased postseismic stress generally they are clustered at both ends of faults but examples where aftershocks occur only at one end or where there is no clustering at the ends are found aftershock clusters are also found at the edge of unbroken barriers and regions of rapid transition from high to low slip within the main fault area we identify examples of geometrical and inhomogeneous barriers and sharp and dull stress concentrations rupture in the main shock is generally found to nucleate in the region of low slip or at the edge of high slip regions the 1996 biak earthquake again being the only exception nucleating in a very high slip region off fault aftershocks are found for all earthquakes in this study and they sometimes rupture the nodal plane conjugate to the main shock fault plane

a refined methodology for stress inversions of earthquake focal mechanisms we develop an improved methodology for reliable high resolution inversions of focal mechanisms to background stress field orientation and stress ratio r in two or three dimensions the earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large scale background stress field the declustered data are discretized with the k means algorithm into groups containing a number of focal mechanisms between a minimum number nmin and 2nmin synthetic tests indicate that nmin 30 provides stable inversion results under different stress regimes and noise conditions when r 0 5 while nmin 45 is needed for r near 0 or 1 additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using a the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor and b the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient the instability criterion is found to provide more accurate inversion results under all tested stress regimes stress ratios and noise conditions the refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups results characterizing neighborhoods of discretized domains merged during the damped inversion provide high resolution information independent of the discretization some aspects of the methodology are illustrated with focal mechanism data from the san jacinto fault zone in southern california

the march 25 1998 antarctic plate earthquake the march 25 1998 antarctic plate earthquake mw 8 1 occurred 250 km from the nearest plate boundary in oceanic lithosphere with an age of 35 55 my analysis of aftershock patterns shows that the earthquake ruptured a fault or series of strike slip fault segments nearly 300 km long the strike of the fault s is nearly perpendicular to the north south trending fossil fracture zones which are the most marked bathymetric features of this region moment release during the mainshock was concentrated in two large subevents clearly visible in the teleseismic body wave waveforms modeling of these body waves using a finite fault source places the first of the two subevents near the point of rupture initiation on the eastern end of the fault with the second large subevent occurring 220 280 km to the west the two pulses of moment release are found to be separated in time by 65 s comparison of the relative s wave amplitudes of the first and second pulses suggests that a rotation of the focal mechanism by 10 occurred between the two subevents

causative source of mw 6 9 sikkim nepal border earthquake of september 2011 gps baseline observations and strain analysis abstract the recent earthquake of mw 6 9 which occurred on september 18 2011 in sikkim nepal border region epicenter 27 72 n 88 06 e depth 20 7 km 68 km nw of the capital city gangtok is the strongest earthquake in the instrumentally recorded history of the region the fault plane solution of this earthquake indicates a strike slip motion however the seismological and geological studies carried out so far after the earthquake could not confirm the causative fault plane in the present study gps observations are used to ascertain causative source in the generation of earthquake and its correlation with the observed seismic data of the region the co seismic displacements recorded by gps show maximum displacement of 11 mm at phodong and 9 mm at taplejung station near the epicenter a simple rigid cross fault model using gps baseline observations was employed to figure out the causative fault plane and seismological characteristic of the region it is inferred that the movement represents the kinematic adjustment of the subsidiary faults as a result of the displacement along the nw se principal plane strain analysis using gps baseline inferred that the region southeast of epicenter has undergone large deformation in addition a significant part of the measured deformation across the surface fault zone for this earthquake can be attributed to post seismic creep

application of grey theory based model to prediction of land subsidence due to engineering environment in shanghai land subsidence is a common geological hazard the long term accumulation of land subsidence in shanghai has caused economic loss to the city since the 1990s the engineering structures have become a new cause of land subsidence many factors affect the process of land subsidence although such a process cannot be explicitly expressed by a mathematical formula it is not a black box whose internal structure parameters and characteristics are unknown therefore the grey theory can be applied to the prediction of land subsidence and provides useful information for the control of land subsidence in this paper a grey model gm gm 1 1 with unequal time intervals was used to predict the subsidence of a high rise building in the lujiazui area of shanghai and the results were compared with the monitored data the prediction of subsidence was also corroborated by laboratory tests and the results were compared with measured data and the predicted data by the adaptive neuro fuzzy inference system anfis it is found that the gm 1 1 with unequal time intervals is accurate and feasible for the prediction of land subsidence

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

rupture processes of the 1987 1988 gulf of alaska earthquake sequence three large earthquakes occurred in the gulf of alaska during 1987 1988 one on november 17 1987 m w 7 2 the second one on november 30 1987 m w 7 8 and the third one on march 6 1988 mw 7 7 these major intraplate strike slip oceanic events occurred along conjugate trends in a region of no previous seismicity body wave modeling places their centroid depths in the upper mantle the depth of the first two events is approximately 20 km and of the third is 15 km as compared to other large events the body wave models for the two largest earthquakes have short rupture lengths 110 km and 40 km respectively and short duration times 36 and 20 s respectively for their magnitude the latter of the two events has a rupture length much shorter than that inferred from aftershock seismicity the short rupture lengths and source durations may reflect differences between the strength of oceanic and continental lithosphere and suggest that events in oceanic lithosphere have a higher moment release per unit area than similar continental events most of the moment release occurred near the epicenter and or regions of apparent structural complexities where seismicity trends intersect

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

the persistence of directivity in small earthquakes abstract we derive a simple inversion of peak ground acceleration pga or peak ground velocity pgv for rupture direction and rupture velocity and then test this inversion on the peak motions obtained from seven 3 5 m 4 1 earthquakes that occurred in two clusters in november 2002 and february 2003 near san ramon california these clusters were located on two orthogonal strike slip faults so that the events share the same approximate focal mechanism but not the same fault plane three earthquakes exhibit strong directivity but the other four earthquakes exhibit relatively weak directivity we use the residual pgas and pgvs from the other six events to determine station corrections for each earthquake the inferred rupture directions unambiguously identify the fault plane for the three earthquakes with strong directivity and for three of the four earthquakes with weak directivity the events with strong directivity have fast rupture velocities 0 63 v 0 87 the events with weak directivity either rupture more slowly 0 17 v 0 35 or bilaterally the simple unilateral inversion cannot distinguish between slow and bilateral ruptures adding a bilateral rupture component degrades the fit of the rupture directions to the fault planes by comparing pgas from the events with strong and weak directivity we show how an up dip rupture in small events can distort the attenuation of peak ground motion with distance when we compare the rupture directions of the earthquakes to the location of aftershocks in the two clusters we find than almost all the aftershocks of the three earthquakes with strong directivity occur within 70 of the direction of rupture

source process of the ms 6 6 udayapur earthquake of nepal india border and its tectonic implication abstract the 1988 udayapur earthquake of nepal india border occurred at longitude 86 6 e and latitude 26 7 n in the vicinity of the rupture area of the great earthquake of 1934 we estimated the focal parameters and slip model of this earthquake by waveform modeling of the teleseismic p waves using a multiple event deconvolution method we analyzed 10 waveform data retrieved from iris gdsn and geoscope we found this earthquake to be a double event earthquake with a time lag of about 8 s between the two successive subevents focal mechanism solution obtained in this study is consistent with already published solutions depth constraints obtained in this study indicates this earthquake to rupture the lower crust of the subducting indian slab the slip model with a fault dimension of 39 km 21 km estimated in this study reveals one main rupture area asperity at the main hypocentral region with a maximum slip of 3 38 m

can an earthquake of mw 9 occur in the himalayan region it has been suggested that all of the earth s subduction zones are capable of producing an m w 9 earthquake such a possibility has also been suggested for the himalayan collisional arc we examined recent and historical earthquakes in the himalayan region and suggest that in the past few hundred years only great earthquakes m w 8 have occurred in the region the largest being the 1950 assam earthquake with m w 8 6 several aseismic ridges segment the himalayan arc and these appear to have limited the extent of rupture in earthquakes of the past 200 years there is also evidence of distributed deformation and out of sequence thrusting where only part of the detachment participates in the accommodation of the convergence we concluded that although it is unlikely that an m w 9 earthquake could occur in the himalaya from the hazard point of view a great earthquake of m w 8 anywhere along the himalayan arc could kill about one million people in the himalaya and adjoining indo ganga plains as an example a scenario developed for a hypothetical m w 8 earthquake occurring in the middle of the night at mandi close to the epicentre of the 1905 kangra earthquake predicts an estimated loss of one million human lives

aftershocks of an m 4 2 earthquake in hawaii and comparison with long term studies of the same volume abstract study of the aftershocks recorded in a 3 hr period after a 4 2 magnitude event on the east rift zone of kilauea volcano hawaii on 12 april 1982 shows that the aftershocks occurred on different planes than the main shock probably as a result of stress redistribution the aftershock locations are probably controlled by preexisting structures this study also suggests that these relatively small aftershocks occurred in the same seismicity patterns as larger events recorded in the same volume over a period of 10 yr slips on most of the aftershocks and the main shock are in the same direction perpendicular to the east rift zone as has been found in studies of other larger earthquakes however fault plane solutions varied more as did the tensional axes and several of the smaller events showed movement in the opposite direction from the main shock and the rest of the aftershocks suggesting some rebound was occurring near the edges of the aftershock zone because ten times as much energy was released in the aftershocks in a narrow linear region as elsewhere and since the main shock epicenter was oceanward of all the aftershocks we suggest that rupture began at the main shock hypocenter and propagated landward implying an almost one dimensional fault for the aftershocks the relationship between moment and magnitude was log m 0 1 18 0 17 m l 17 3 0 17 differences in amplification lead to site differences of up to 0 8 units in local magnitude and 1 5 orders of magnitude in energy release these correlated somewhat with station time corrections in that the stations with the longest delay times also had greatest amplification

active transverse faulting within underthrust indian crust beneath the sikkim himalaya summary deep focus earthquakes within the underthrust indian lower crust beneath the himalaya occur in very specific regions and have distinct source characteristics the study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust indian plate and its influence on the active deformation of the overlying himalayan wedge one of the most significant regions of these deep focus earthquakes is beneath the sikkim and bhutan himalaya we study the source characteristics of the 2011 september 18 mw 6 9 deep focus sikkim main shock and its major aftershocks using global regional and local waveform data we determined the focal mechanism of the main shock using moment tensor inversion of global p and sh waveforms and ascertained the earthquake fault plane using rupture directivity from regional p wave spectra the main shock originated at 53 4 km depth and ruptured at least 20 km thickness of the underthrust indian lower crust faulting occurred on a near vertical dextral strike slip fault oriented nw se strike 127 d ip 81 and rake 167 oblique to the local strike of the himalayan arc the rupture initiated from the se end of the fault and propagated to the northwest the main shock was followed by 20 small to moderate aftershocks mb 3 0 which we relocated using phase arrival times we computed the focal mechanisms of the larger ones mb 3 5 using local waveform inversion we find that all aftershocks originated se of the main shock between depths of 12 and 50 km and have dominantly strike slip mechanisms our results combined with the source mechanisms of earthquakes from previous studies reveals that the entire underthrust indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the sikkim and bhutan himalaya these active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating microplates in northeast india observed from gps geodesy

seismicity in the kingdom of bhutan 1937 2003 evidence for crustal transcurrent deformation we investigate moderate sized historic to recent earthquakes in the bhutan himalaya spanning the years from 1937 to 2003 we find that few moderate sized earthquakes occurred in the region during this time period in order to better characterize the seismicity we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data we use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 m w 6 4 1995 m w 5 4 and 2003 m w 5 4 for the modern events we utilize three techniques for focal mechanism and depth determination regional full waveform time domain modeling of the two moderate m w 5 4 events and surface wave spectral method and body wave depth modeling approach for the m w 6 4 event we find that the first motion and digital data focal mechanisms are mostly strike slip with midcrustal to deep crustal depths although no recent great event has occurred along the main himalayan thrust the indian plate is undergoing significant midcrustal to deep crustal transcurrent deformation likely due to oblique convergence of the indian asian collision in this region unlike oceanic continent oblique subduction strike slip and thrust partitioning occurs throughout the crust not in distinct zones the geological and geophysical nature of the bhutan himalaya appears significantly different than other himalayan regions due to the strike slip nature of many of the events an out of sequence thrust fault and the implications of deformation in the shillong plateau

coseismic and postseismic stress rotations due to great subduction zone earthquakes 1 the three largest recent great subduction zone earthquakes 2011 m9 0 tohoku japan 2010 m8 8 maule chile and 2004 m9 2 sumatra andaman exhibit similar coseismic rotations of the principal stress axes prior to each mainshock the maximum compressive stress axis was shallowly plunging while immediately after the mainshock both the maximum and minimum compressive stress axes plunge at 45 dipping faults can be oriented for either reverse or normal faulting in this post mainshock stress field depending on their dip explaining the observed normal faulting aftershocks without requiring a complete reversal of the stress field the significant stress rotations imply near complete stress drop in the mainshocks with 80 of the pre mainshock stress relieved in the tohoku and maule earthquakes and in the northern part of the sumatra andaman rupture the southern part of the sumatra andaman rupture relieved 60 of the pre mainshock stress the stress axes rotated back rapidly in the months following the tohoku and maule mainshocks and similarly in the southern part of the sumatra andaman rupture a rapid postseismic rotation is possible because the near complete stress drop leaves very little background stress at the beginning of the postseismic reloading in contrast there has been little or no postseismic rotation in the northern part of the sumatra andaman rupture over the 7 years since the mainshock all m 8 0 subduction earthquakes since 1990 with an adequate number of pre and post mainshock events were evaluated and not all show similar coseismic stress rotations deeper earthquakes exhibit smaller coseismic stress rotations likely due to increasing deviatoric stress with depth

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

interseismic coupling on the main himalayan thrust we determine the slip rate and pattern of interseismic coupling on the main himalayan thrust along the entire himalayan arc based on a compilation of geodetic interferometric synthetic aperture radar and microseismicity data we show that convergence is perpendicular to the arc and increases eastwards from 13 3 1 7 mm yr to 21 2 2 0 mm yr these rates are comparable to geological and geomorphic estimates indicating an essentially elastic geodetic surface strain the interseismic uplift rate predicted from the coupling model closely mimics the topography suggesting that a small percentage of the interseismic strain is permanent we find that the fault is fully locked along its complete length over about 100 km width we don t find any resolvable aseismic barrier that could affect the seismic segmentation of the arc and limit the along strike propagation of seismic ruptures the moment deficit builds up at a rate of 15 1 1 10 19 n m yr for the entire length of the himalaya