double ramp on the main himalayan thrust revealed by broadband waveform modeling of the 2015 gorkha earthquake sequence abstract the 2015 m w 7 8 gorkha earthquake sequence that unzipped the lower edge of the main himalayan thrust mht in central nepal provides an exceptional opportunity to understand the fault geometry in this region however the limited number of focal mechanisms and the poor horizontal locations and depths of earthquakes in the global catalog impede us from clearly imaging the ruptured mht in this study we generalized the amplitude amplification factor aaf method to teleseismic distance that allows us to model the teleseismic p waves up to 1 5 hz we used well constrained medium sized earthquakes to establish aaf corrections for teleseismic stations that were later used to invert the high frequency waveforms of other nearby events this new approach enables us to invert the focal mechanisms of some early aftershocks which is challenging by using other long period methods with this method we obtained 12 focal mechanisms more than that in the gcmt catalog we also modeled the high frequency teleseismic p waves and the surface reflection phases pp and sp to precisely constrain the depths of the earthquakes our results indicate that the uncertainty of the depth estimation is as small as 1 2 km finally we refined the horizontal locations of these aftershocks using carefully hand picked arrivals the refined aftershock mechanisms and locations delineate a clear double ramp geometry of the mht with an almost flat decollement sandwiched in between the flat dip 7 degrees portion of the mht is consistent with the coseismic rupture of the mainshock which has a well constrained slip distribution the fault morphology suggests that the ramps both along the up dip and down dip directions play a significant role in stopping the rupture of the 2015 gorkha earthquake our method can be applied to general subduction zone earthquakes and fault geometry studies

the 1999 mw 7 1 hector mine california earthquake sequence complex conjugate strike slip faulting the 1999 m w 7 1 hector mine mainshock showed right lateral strike slip faulting with an initial strike of n6 w and vertical dip the mainshock was preceded within 20 hours by 18 recorded foreshocks of 1 5 m 3 8 within a few kilometers distance of the mainshock hypocenter the aftershocks delineate how the hector mine earthquake ruptured with strike n6 w to the south for a distance of 15 km and possibly to the north for a distance of several kilometers the two largest aftershocks of m 5 9 and m 5 7 occurred near the north and south ends of the first mainshock rupture segment the second segment of rupture starting 15 km to the south away from the mainshock hypocenter delineated by strike slip and thrust faulting aftershocks extends 10 km farther away with a strike of s140 e along the bullion fault the aftershocks also outline an unusual third rupture segment extending from about 5 km south of the hypocenter with a strike of n30 w to n35 w for a distance of 20 km approximately 10 to 25 km farther to the north and west of the mainshock epicenter several clusters form a complex aftershock distribution three dimensional vp and vp vs models of the region exhibit only small regional changes as is typical for the mojave region nonetheless the mainshock rupture started within a region of rapidly varying vp and at least three regions of low vp vs are imaged within the aftershock zone the rate of decay for the hector mine earthquake sequence has been slightly above the mean for both p values and b values in southern california the focal mechanisms of the aftershocks and the state of stress are consistent with strike slip faulting including a component of normal faulting most prominent to the north the orientation of the regional maximum horizontal stress the variation in orientation of the mainshock fault segments by 30 and scattered distribution of aftershocks suggest that the mainshock and aftershock deformation field exhibit volumetric shear deformation accommodated by complex conjugate sets of strike slip faults

3 d rotation of double couple earthquake sources summary we discuss 3 d rotations by which one double couple earthquake source can be rotated into another arbitrary double couple due to the symmetry of double couple sources there are four such rotations an algorithm is obtained in analytical form which is also available as a computer program solving the inverse problem of 3 d rotation of double couple earthquake sources i e for each pair of focal mechanisms or seismic moment tensor solutions the program finds all four rotations which rotate one mechanism into another this algorithm may be used in a wide variety of studies of stress field causing earthquakes investigations of the relationship between the focal mechanisms and the tectonic features of a seismogenic region etc the same inversion algorithm can be used to study the 3 d rotation of any symmetric second rank tensor such as the stress or strain tensor

three dimensional imaging of subsurface structures using resistivity data we have developed a three dimensional inverse scheme for carrying out dc resistivity surveys incorporating complicated topography as well as arbitrary electrode arrays the algorithm is based on the finite element approximation to the forward problem so that the effect of topographic variation on the resistivity data is effectively evaluated and incorporated in the inversion furthermore we have enhanced the resolving power of the inversion using the active constraint balancing method numerical verifications show that a correct earth image can be derived even when complicated topographic variation exists by inverting the real field data acquired at a site for an underground sewage disposal plant we obtained a reasonable image of the subsurface structures which correlates well with the surface geology and drill log data

thermochronology constraints on miocene exhumation in the central range mountains trinidad the central range fault zone is a continental transform that accommodates most of the present day slip between the caribbean and south american plates in trinidad global positioning system data and paleoseismic work suggest that this zone is active today and has been active for at least the past several thousand years the modern fault zone overprints a middle miocene fold and thrust belt therefore the strain recorded in the central range fault zone is the sum of both middle miocene and more recent events thermochronology data from eocene and oligocene sandstones in the central range were collected to evaluate the timing of exhumation driven by crustal shortening and thickening u th he zircon analysis of subhedral zircons collected from eight samples indicated that the burial temperatures of these sedimentary rocks did not exceed 180 c suggesting that these grains record detrital u th he dates apatite fission track aft analysis of 10 samples yielded mixed results with cooling ages ranging from 30 to 15 ma however most sites failed the 2 test suggesting that multiple age cohorts are present pooled aft ages suggest that rocks presently at the surface were exhumed through their aft closure temperature ca 11 18 ma cooling and exhumation thus most likely resulted from middle miocene shortening across the fold and thrust belt in response to early oblique convergence between the caribbean and south america plates post miocene deformation associated with more recent transform tectonics has therefore resulted in more limited 4 km if any exhumation

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

fault geometry rupture dynamics and ground motion from potential earthquakes on the north anatolian fault under the sea of marmara summary using the 3 d finite element method we develop dynamic spontaneous rupture models of earthquakes on the north anatolian fault system in the sea of marmara turkey considering the geometrical complexity of the fault system in this region we find that the earthquake size rupture propagation pattern and ground motion all strongly depend on the interplay between the initial static regional pre stress field and the dynamic stress field radiated by the propagating rupture by testing several nucleation locations we observe that those far from an oblique normal fault stepover segment near istanbul lead to large through going rupture on the entire fault system whereas nucleation locations closer to the stepover segment tend to produce ruptures that die out in the stepover however this pattern can change drastically with only a 10 rotation of the regional stress field our simulations also reveal that while dynamic unclamping near fault bends can produce a new mode of supershear rupture propagation this unclamping has a much smaller effect on the speed of the peak in slip velocity along the fault finally we find that the complex fault geometry leads to a very complex and asymmetric pattern of near fault ground motion including greatly amplified ground motion on the insides of fault bends the ground motion pattern can change significantly with different hypocentres even beyond the typical effects of directivity the results of this study may have implications for seismic hazard in this region for the dynamics and ground motion of geometrically complex faults and for the interpretation of kinematic inverse rupture models

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

superficial simplicity of the 2010 el mayor cucapah earthquake of baja california in mexico the geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation propagation and arrest of seismic ruptures the fault system that runs from southern california into mexico is a simple strike slip boundary the west side of california and mexico moves northwards with respect to the east however the m w 7 2 2010 el mayor cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike slip fault here we use geodetic remote sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake we find that the earthquake produced a straight 120 km long fault trace that cut through the cucapah mountain range and across the colorado river delta however at depth the fault is made up of two different segments connected by a small extensional fault both segments strike n130 e but dip in opposite directions the earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike slip motion we show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves we conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone

relation between continental strike slip earthquake segmentation and thickness of the crust 1 high resolution maps of large continental strike slip earthquake surface ruptures show that they are formed of fault segments these segments are bounded by fault bends step overs or combinations of the two the lowest limit in size for such segments may not be relevant in the understanding of earthquake mechanics as it pertains to the granular properties of fault zones the maximum limit in segment length however is important as it is directly relates to the maximum extent of seismic rupture to measure the length of the segments a new quantitative method based on piecewise linear fitting is developed and is used to automatically retrieve segments from earthquake rupture maps next this approach is tested against a set of ten continental strike slip earthquake ruptures derived from similar high quality maps the test suggests that segments have a maximum length of 18 km independent of regional tectonic setting slip inversions for earthquakes based on seismological and or geodetic data most often are not unique and can show some variability even for one particular event some basic characteristics however such as total moment release or general source geometry seem to persist that are relevant to earthquake mechanics measurements of the maximum horizontal extent of individual slip patches derived from seismic source inversion for strike slip ruptures show that their strike dimension does not increase infinitely with magnitude but instead reaches a maximum value of 25 km these two independent lines of observations complemented by earlier data and analog experiments suggest that it is the thickness of the seismogenic crust that controls the structural scaling of the length of seismic segments and that it is independent of the ultimate size of individual earthquakes

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

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

seismotectonics and fault geometries of the 2019 ridgecrest sequence insight from aftershock moment tensor catalog using 3 d green s functions the 2019 m w 7 1 ridgecrest earthquake occurred on july 6th preceded by the m w 6 4 foreshock on july 4th 2019 these two earthquakes occurred close in space and time with partially overlapping surface ruptures and aftershock patterns raising the question of the relationship between the two events geological surveys and satellite observations provide important constraints on the surface traces of faulting however the subsurface fault geometries which are important for understanding the regional stress field earthquake initiation propagation and termination are not well resolved in this study moment tensor solutions for 256 earthquakes in the 2019 ridgecrest sequence were determined by waveform inversion using 3d velocity model the obtained moment tensor solutions show rotations of the stresses after mainshock indicating the ratio of mainshock stress drop to the background stress to be 0 5 0 9 the obtained moment tensor catalog also facilitates a better understanding of the sub surface fault geometries including 1 splay faults and antithetic faults in the northwest aftershock zone 2 shallow flower structures near the m w 7 1 epicenter 3 sub parallel faults in the southeast aftershock zone the aftershocks studies suggest the very complex surface ruptures near the 2019 ridgecrest m w 7 1 epicenter are near surface features that linked to a simple large through going fault at 5 km depth we also found that the southeastern most aftershocks which are located less than a kilometer from the garlock fault trace have significant different strike directions from that of the garlock fault indicating the central garlock fault remains seismically quiet

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

aftershocks driven by afterslip and fluid pressure sweeping through a fault fracture mesh a variety of physical mechanisms are thought to be responsible for the triggering and spatiotemporal evolution of aftershocks here we analyze a vigorous aftershock sequence and postseismic geodetic strain that occurred in the yuha desert following the 2010 m w 7 2 el mayor cucapah earthquake about 155 000 detected aftershocks occurred in a network of orthogonal faults and exhibit features of two distinct mechanisms for aftershock triggering the earliest aftershocks were likely driven by afterslip that spread away from the main shock with the logarithm of time a later pulse of aftershocks swept again across the yuha desert with square root time dependence and swarm like behavior together with local geological evidence for hydrothermalism these features suggest that the events were driven by fluid diffusion the observations illustrate how multiple driving mechanisms and the underlying fault structure jointly control the evolution of an aftershock sequence

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

computing a large refined catalog of focal mechanisms for southern california 1981 2010 temporal stability of the style of faulting using the method developed by hardebeck and shearer 2002 2003 termed the hash method we calculate focal mechanisms for earthquakes that occurred in the southern california region from 1981 to 2010 when available we use hypocenters refined with differential travel times from waveform cross correlation using both the p wave first motion polarities and the s p amplitude ratios computed from three component seismograms we determine mechanisms for more than 480 000 earthquakes and analyze the statistical features of the whole catalog we filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high quality catalog with approximately 179 000 focal mechanisms as more s p amplitude ratios become available after 2000 the average nodal plane uncertainty decreases significantly compared with mechanisms that include only p wave polarities in general the parameters of the focal mechanisms have been stable during the three decades the dominant style of faulting is high angle strike slip faulting with the most likely p axis centered at n5 e for earthquakes of m 2 5 events using the 210 moment tensor solutions in tape et al 2010 as benchmarks we compare the focal plane rotation angles of common events in the catalog seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty the common events with relatively large rotation angles are either located around the edge of the scsn network or poorly recorded

an evaluation of the scsn moment tensor solutions robustness of the mw magnitude scale style of faulting and automation of the method we have generated moment tensor solutions and moment magnitudes m w for 1700 earthquakes of local magnitude m l 3 0 that occurred from september 1999 to november 2005 in southern california the method is running as an automated real time component of the southern california seismic network scsn with solutions available within 12 min of event nucleation for local events the method can reliably obtain good quality solutions for m w with m l 3 5 and for the moment tensor for events with m l 4 0 the method uses the 1d time domain inverse code tdmt invc software package dreger 2003 the green s functions have been predetermined for various velocity profiles in southern california which are used in the inversion of observed three component broadband waveforms 10 100 sec using data from at least four stations moment tensor solutions have an assigned quality factor dependent on the number of stations in the inversion and the goodness of fit between synthetic and observed data if a minimum quality factor is attained the m l or m w is 5 0 or greater and if the event is in the southern california reporting regions the m w will be the official scsn cisn magnitude the m w from the high quality solutions determined from our method generally correlate very well with reviewed m l except in regions at the perimeter of the network the m w reported here indicates the scsn m l systematically underestimates the magnitude in the mexicali region of baja california mono lakes area coso region and the brawley seismic zone and overestimates the magnitude in the coastal ranges comparisons of the moment tensors determined using this model are made with harvard centroid moment tensors generated for larger earthquakes in the california region and recent 3d models for events in the los angeles region with excellent correlation most of the earthquakes with good quality solutions exhibit strike slip faulting in particular along the major late quaternary strike slip faults thrust faulting on east west striking planes is observed along the southern edge of the transverse ranges while northwest striking thrust faulting is observed in the coastal ranges normal faulting is most common in baja california and southern sierra nevada including the western basin and range region poor quality solutions with unreliable m w are caused by excessive background noise in the waveforms small events m l 4 0 can be affected by ambient noise or teleseisms but larger events can also have unreliable solutions if they follow a recent large regional event

evidence of supershear during the 2018 magnitude 7 5 palu earthquake from space geodesy a magnitude 7 5 earthquake hit the city of palu in sulawesi indonesia on 28 september 2018 at 10 02 43 coordinated universal time it was followed a few minutes later by a 4 7 m high tsunami palu is situated in a narrow pull apart basin surrounded by high mountains of up to 2 000 m altitude this morphology has been created by a releasing bend in the palu koro fault a rapidly moving left lateral strike slip fault here we present observations derived from optical and radar satellite imagery that constrain the ground surface displacements associated with the earthquake in great detail mapping of the main rupture and associated secondary structures shows that the slip initiated on a structurally complex and previously unknown fault to the north extended southwards over 180 km and passed through two major releasing bends the 30 km section of the rupture south of palu city is extremely linear and slightly offset from the mapped geological fault at the surface this part of the rupture accommodates a large and smooth surface slip of 4 7 m with no shallow slip deficit almost no aftershock seismicity was recorded from this section of the fault as these characteristics are similar to those from known supershear segments we conclude that the palu earthquake probably ruptured this segment at supershear velocities the devastating 2018 magnitude 7 5 earthquake in palu indonesia ruptured at supershear speeds according to evidence from space geodesy

active backstop faults in the mentawai region of sumatra indonesia revealed by teleseismic broadband waveform modeling abstract two earthquake sequences that affected the mentawai islands offshore of central sumatra in 2005 mw 6 9 and 2009 mw 6 7 have been highlighted as evidence for active backthrusting of the sumatran accretionary wedge however the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks we refine the locations and focal mechanisms of medium size events mw 4 5 of these two earthquake sequences through broadband waveform modeling in addition to modeling the depth phases for accurate centroid depths we use teleseismic surface wave cross correlation to precisely relocate the relative horizontal locations of the earthquakes the refined catalog shows that the 2005 and 2009 backthrust sequences in mentawai region actually occurred on steeply 60 degrees landward dipping faults masilo fault zone that intersect the sunda megathrust beneath the deepest part of the forearc basin contradicting previous studies that inferred slip on a shallowly seaward dipping backthrust static slip inversion on the newly proposed fault fits the coseismic gps offsets for the 2009 mainshock equally well as previous studies but with a slip distribution more consistent with the mainshock centroid depth 20 km constrained from teleseismic waveform inversion rupture of such steeply dipping reverse faults within the forearc crust is rare along the sumatra java margin we interpret these earthquakes as unsticking of the sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger sunda lithosphere alternatively the reverse faults may have originated as pre miocene normal faults of the extended continental crust of the western sunda margin our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults

structural segmentation controlled the 2015 mw 7 8 gorkha earthquake rupture in nepal the ongoing collision of india with asia is partly accommodated by slip on the main himalayan thrust mht the 25 april 2015 m w 7 8 gorkha earthquake is the most recent major event to rupture the mht which dips gently northward beneath central nepal although the geology of the range has been studied for decades fundamental aspects of its deep structure remain disputed here we develop a structural cross section and a three dimensional geologically informed model of the mht that are consistent with seismic observations from the gorkha earthquake a comparison of our model to a detailed slip inversion data set shows that the slip patch closely matches an ovalshaped gently dipping fault surface bounded on all sides by steeper ramps the gorkha earthquake rupture seems to have been limited by the geometry of that fault segment this is a significant step forward in understanding the deep geometry of the mht and its effect on earthquake nucleation and propagation published models of fault locking do not correlate with the slip patch or our fault model in the vicinity of the earthquake further suggesting that fault geometry was the primary control on this event our result emphasizes the importance of adequately constraining subsurface fault geometry in megathrusts in order to better assess the sizes and locations of future earthquakes

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

complex spatiotemporal evolution of the 2008 mw 4 9 mogul earthquake swarm reno nevada interplay of fluid and faulting after approximately two months of swarm like earthquakes in the mogul neighborhood of west reno nv seismicity rates and event magnitudes increased over several days culminating in an mw 4 9 dextral strike slip earthquake on 26 april 2008 although very shallow the mw 4 9 mainshock had a different sense of slip than locally mapped dip slip surface faults we relocate 7 549 earthquakes calculate 1 082 focal mechanisms and statistically cluster the relocated earthquake catalog to understand the character and interaction of active structures throughout the mogul nv earthquake sequence rapid temporary instrument deployment provides high resolution coverage of microseismicity enabling a detailed analysis of swarm behavior and faulting geometry relocations reveal an internally clustered sequence in which foreshocks evolved on multiple structures surrounding the eventual mainshock rupture the relocated seismicity defines a fault fracture mesh and detailed fault structure from approximately 2 6 km depth on the previously unknown mogul fault that may be an evolving incipient strike slip fault zone the seismicity volume expands before the mainshock consistent with pore pressure diffusion and the aftershock volume is much larger than is typical for an mw 4 9 earthquake we group events into clusters using space time magnitude nearest neighbor distances between events and develop a cluster criterion through randomization of the relocated catalog identified clusters are largely mainshock aftershock sequences without evidence for migration occurring within the diffuse background seismicity the migration rate of the largest foreshock cluster and simultaneous background events is consistent with it having triggered or having been triggered by an aseismic slip event