systematic analysis of crustal anisotropy along the karadere duzce branch of the north anatolian fault s u m m a r y we perform a systematic analysis of crustal anisotropy along and around the karadere duzce branch of the north anatolian fault naf which ruptured during the 1999 mw 7 4 izmit and mw 7 1 duzce earthquakes a method consisting of an iterative grid search for the best shear wave splitting parameters in sliding time windows is applied to 22 000 measurements recorded in the 6 month period after the izmit main shock based on objective criteria 6600 measurements are assigned high quality and used for further detailed analysis most stations near the rupture zone have fast polarization directions that are parallel to and change with the nearby fault strike the average delay times for ray paths that propagate along the rupture zone are larger than for the other paths these results suggest the existence of an approximately 1 km broad zone around the karadere duzce branch with fault parallel cracks or shear fabric however some fault zone fz stations record bimodal or scattered polarization directions while stations near large structural complexities e g branching and offsets show average fast polarization directions that are almost perpendicular to the local fault strike the average fast polarization directions from ray paths that propagate inside the almacik block south of the karadere duzce branch are neither parallel to the local fault strike nor to the expected regional maximum compressive stress direction the large overall spatial variations of the results imply that multiple structures and mechanisms contribute to the observed crustal anisotropy in our study area most stations do not exhibit a clear dependency of shear wave splitting delay time with increasing depth and hypocentral distance indicating that the anisotropy is confined primarily to the top 3 4 km of the crust using the observed average delay time at fz stations and assumed propagation distance of 3 5 km we estimate the apparent crack density in the damaged shallow fz rock to be approximately 7 per cent

high frequency borehole seismograms recorded in the san jacinto fault zone southern california part 1 polarizations abstract two borehole seismometer arrays knw bh and pfo bh have been established in the southern california batholith region of the san jacinto fault zone by the u s geological survey the sites are within 0 4 km of anza network surface stations and have three component seismometers deployed at 300 m depth at 150 m depth and at the surface downhole horizontal seismometers can be oriented to an accuracy of about 5 using regional and near regional initial p wave particle motions shear waves recorded downhole at the knw bh indicate that the strong alignment of initial s wave particle motions previously observed at the surface knw anza site knw az is not generated in the near surface weathered layer the knw bh surface instrument which sits atop a highly weathered zone displays a significantly different 20 initial s wave polarization direction from that observed downhole and at knw az which is bolted to an outcrop although downhole initial shear wave particle motion directions are consistent with a shear wave splitting hypothesis observations of orthogonally polarized slow shear waves are generally elusive even in seismograms recorded at 300 m a cross correlation measure of the apparent relative velocities of s fast and s slow horizontally polarized s waves suggests shallow shear wave anisotropy consistent with the observed initial s wave particle motion direction of 2 3 1 7 per cent between 300 and 150 m and 7 5 3 5 per cent between 150 and 0 m

shear wave splitting and crustal anisotropy in the shillong mikir plateau of northeast india seismic anisotropy of crust beneath the shillong mikir plateau and the surrounding regions of northeast india have been investigated with the help of splitting analysis of s wave of local earthquakes we estimate a total 83 pairs of splitting parameters and t from 67 local shallow focus earthquakes depth 32 km recorded by the 10 broadband seismological stations operated in the study region the results show delay times ranging from 0 02 to 0 2 s which correspond to anisotropy up to 4 suggesting significant strength of anisotropy in the study region fast polarization direction in the shillong plateau shows mostly nw se trend in the western part and ne sw trend in the northern part s near kopili fault kf follows nw se trend at most of the stations in the study region is consistent with the local stress orientation suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field on the other hand anisotropy observed near the kf is due to aligned macroscopic fracture related to strike slip movement in the fault zone

multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the tibetan plateau and tectonic implications of the haiyuan fault abstract the haiyuan fault is a large strike slip fault in the northeastern ne margin of the tibetan plateau using new data recorded by a temporary seismic array this study obtains crustal seismic anisotropy measurements in different scales across the haiyuan fault by analyzing splitting of shear waves from local earthquakes parameters of shear wave splitting sws indicate clear zoning characteristics in seismic anisotropy in the upper crust the study area is divided into two different anisotropic subzones by the haiyuan fault the dominant orientation of fast polarizations is nne or ne north of the faults and wnw to ew south of the fault this pattern of spatial variation indicates the controlling impact of haiyuan fault to observed seismic anisotropy with a zone of dozens of kilometers from the haiyuan fault the dominant orientations of fast polarizations are wnw generally coincidental with the strike of the haiyuan fault except for several stations it indicates that the influence of the fault on stress could reach to about 10 km from the fault distance spatial pattern of anisotropy indicates multiple effects by stress faults and local tectonics it suggests different anisotropic mechanisms including stress induced anisotropy and tectonic or structural anisotropy the dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics the normalized time delays are higher around the haiyuan fault than those away from the fault suggesting stronger crustal seismic anisotropy in a narrow belt around the strike slip fault zone the sws parameters suggest that the haiyuan fault is the actual crustal boundary of ne tibetan plateau about 200 km north of the reported boundary of the tibetan plateau block combining with other studies in the crust and the lithosphere this paper infers possible two layer crustal anisotropy beneath the yinchuan graben north of the haiyuan fault the dominant orientation of fast polarizations are nne or ne in the upper crust probably originated from crack induced anisotropy but become wnw or nw in the middle lower crust possibly originated from deformation anisotropy

automatic measurement of shear wave splitting and applications to time varying anisotropy at mount ruapehu volcano new zealand 1 we present an automatic shear wave splitting measurement tool for local earthquakes with the sole manual step of choosing an s arrival time we apply the technique to three data sets recorded on mount ruapehu volcano in new zealand that have previously been determined to have fast polarizations that vary in time and with earthquake depth the technique uses an eigenvalue minimization technique applied over multiple measurement windows the dominant period of each waveform sets minimum and maximum window lengths cluster analysis determines the best solution among all the windows and quality grading criteria assess the results automatically when the same filters are used for events determined to be high quality from manual studies the automatic technique returns virtually identical results scatter increases when the automatic technique chooses the best filter but the average automatic results remain consistent with the manual results when the automatic technique is used on sets that include data previously judged as poor quality some stations yield distributions of fast polarizations that include peaks that were not present in previously published results the difference may stem from two factors automatic grading lets through some measurements that independent analysts consider poor quality but also unconscious bias in the manual selection process may downgrade measurements that do not fit expectations nonetheless the new objective analysis confirms changes in the average fast polarizations between 1994 and 2002 and between shallow and deep events therefore this new technique is valuable for objective assessment of anisotropy and its variation in time

shear wave splitting beneath southern korea and its tectonic implication abstract seismic anisotropy beneath the southern korean peninsula skp was investigated based on the scs waves of deep focused earthquakes recorded at 35 broadband stations the most striking feature of the observed splitting patterns is the preference of the fast directions nw se that are nearly parallel to the direction of the absolute plate motion in the region however the splitting patterns over the region show significant variation in splitting parameters indicating a complex anisotropic structure variations of the splitting directions and dissimilarity in the source domains of basaltic volcanisms suggest that the asthenospheric mantle flow since at least the late cenozoic cannot explain the seismic anisotropy beneath the region comparison to shear wave splitting measurements from eastern china revealed that the nw se fast direction of splitting measurements in the skp is close to that in the north china block ncb while the ne sw fast direction might be related to that in the south china block scb the shallow mantle lithosphere beneath the skp retains the fossil anisotropy amalgamated prior to the late paleozoic before the collision between the ncb and scb and the anisotropic structure was not completely realigned by the major orogenic events during the late paleozoic to mesozoic eras

first detection of transient luminous events associated with winter thunderstorms in the eastern mediterranean 1 during the winter of 2005 6 optical ground based observations were performed to detect transient luminous events tles above winter thunderstorms in the vicinity of israel and the eastern coastline of the mediterranean sea two sites were used the tel aviv university tau campus in tel aviv 32 5n 34 5e and the wise astronomical observatory in the negev desert 30 6n 34 76e we also conducted elf and vlf electromagnetic measurements from the tau array in southern israel in 4 different winter storms we detected 30 events 26 sprites and 4 elves the detection ranges varied from 250 to 450 km sprites were found to be produced exclusively by cgs that can be traced to active cells with a vertical dimension of 5 7 km and cloud top temperature 40 c embedded in a much larger matrix of stratiform cloudiness this configuration closely resembles the conditions for winter sprites in the hokuriku region of japan

mapping stress and structurally controlled crustal shear velocity anisotropy in california we present shear velocity anisotropy data from crustal earthquakes in california and demonstrate that it is often possible to discriminate structural anisotropy polarization of the shear waves along the fabric of major active faults from stress induced anisotropy polarization parallel to the maximum horizontal compressive stress stress directions from seismic stations located near but not on the san andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault in contrast seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust inducing shear wave polarizations parallel to the fault plane

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

shear wave splitting of the 2009 l aquila seismic sequence fluid saturated microcracks and crustal fractures in the abruzzi region central apennines italy

seismic anisotropy and its relation with crust structure and stress field in the reggio emilia region northern italy summary shear wave splitting is measured at 14 seismic stations in the reggio emilia region above local background seismicity and two sequences of seismic events the good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume it is about 60 km 2 wide and located between 6 and 11 km depth inside mesozoic age carbonate rocks the splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the pliocene quaternary alluvium above the mesozoic layer the fast polarization directions n30 e are approximately parallel to the maximum horizontal stress 1 is ssw nne in the region and also parallel to the strike of the main structural features in the reggio emilia area the size of the delay times suggests about 4 5 per cent shear wave velocity anisotropy these parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid saturated microcracks aligned by the active stress field we cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry this symmetry is commonly explained by parallel stress aligned microcracks

anisotropy repeating earthquakes and seismicity associated with the 2008 eruption of okmok volcano alaska 1 we use shear wave splitting sws analysis and double difference relocation to examine temporal variations in seismic properties prior to and accompanying magmatic activity associated with the 2008 eruption of okmok volcano alaska using bispectrum cross correlation a multiplet of 25 earthquakes is identified spanning five years leading up to the eruption each event having first motions compatible with a normal fault striking ne sw cross correlation differential times are used to relocate earthquakes occurring between january 2003 and february 2009 the bulk of the seismicity prior to the onset of the eruption on 12 july 2008 occurred southwest of the caldera beneath a geothermal field earthquakes associated with the onset of the eruption occurred beneath the northern portion of the caldera and started as deep as 13 km subsequent earthquakes occurred predominantly at 3 km depth coinciding with the depth at which the magma body has been modeled using geodetic data automated sws analysis of the okmok catalog reveals radial polarization outside the caldera and a northwest southeast polarization within we interpret these polarizations in terms of a magma reservoir near the center of the caldera which we model with a mogi point source sws analysis using the same input processing parameters for each event in the multiplet reveals no temporal changes in anisotropy over the duration of the multiplet suggesting either a short term or small increase in stress just before the eruption that was not detected by gps or eruption triggering by a mechanism other than a change of stress in the system

seismic anisotropy time variations at mt etna this article has been accepted for publication in geophysical journal international the authors 2020 published by oxford university press on behalf of the royal astronomical society all rights reserved uploaded in accordance with the publisher s self archiving policy

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

stress induced seismic azimuthal anisotropy in the upper crust across the north west shelf australia seismic azimuthal anisotropy saa is observed in many areas of the earth and fast polarized directions have been mapped using earthquake surface waves and teleseismic s wave splitting over large regional and tectonic scales higher resolution petroleum exploration data including 3 d seismic surveys and dipole shear logs often exhibit azimuthal anisotropy however we are unaware of any published studies mapping saa from exploration scale data to study large scale regional and tectonic saa trends the physical mechanisms causing saa in earthquake data are a subject of great interest comparing regional trends to the higher resolution exploration trends may help understand these mechanisms we present a saa analysis using seismic exploration data across the north west shelf nws of australia we map the fast polarization directions from 34 dipole shear logs and two 3 d seismic surveys and compare them to in situ maximum horizontal stress orientations h our results show that the and h trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology these results suggest that differential horizontal stress is the likely mechanism causing saa at a wide range of spatial scales on the nws of australia we also show that saa is not observed at exploration scales in some areas of the nws and propose a lithology dependent stress sensitivity model in which saa is strongest in clean unconsolidated quartz dominated sandstones and weaker or nonexistent in well consolidated cemented rocks and shale dominated sediments

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

continental crust anisotropy measurements from tectonic tremor in cascadia we present new observations of crustal anisotropy in the southern cascadia forearc from tectonic tremor the abundance of tremor activity in oregon and northern california during slow slip events offers an enormous amount of information with which to measure and analyze anisotropy in the upper brittle continental crust to accomplish this we performed analyses of wave polarization and shear wave splitting of tectonic tremor signals using three component broadband seismic stations the splitting times range between 0 11 and 0 32 s and are consistent with typical values observed in the continental crust fast polarization azimuths are in general margin parallel and trend n s which parallels the azimuths of the maximum compressive stresses observed in this region this pattern is likely to be controlled by the stress field comparatively the anisotropic structure of fast directions observed in the northern section of the cascadia margin is oblique with respect to the southern section of cascadia which in general trends e w and is mainly controlled by active faulting and geological structures source distribution analysis using a bivariate normal distribution that expresses the distribution of tremors in a preferred direction shows that in northern california and oregon the population of tremors tends to distribute parallel to fast polarization azimuths and maximum compressive stresses suggesting that both tremor propagation and anisotropy are influenced by the stress field results show that the anisotropy reflects an active tectonic process that involves the northward movement of the oregon block which is rotating as a rigid body in northern cascadia previous results of anisotropy show that the crust is undergoing a shortening process due to velocity differences between the oregon block and the north america plate which is moving more slowly with respect to the oregon block making it clash against vancouver island

crustal radial anisotropy beneath cameroon from ambient noise tomography abstract to increase the understanding of crustal deformation and crustal flow patterns due to tectonic processes in cameroon we study the lateral variability of the crustal isotropic velocity and radial anisotropy estimated using ambient noise tomography ant rayleigh and love wave noise correlation functions ncfs were retrieved from the cross correlation of seismic ambient noise data recorded in cameroon and phase velocities at periods of 8 to 30 s were measured to perform surface wave tomography joint inversion of rayleigh and love wave data for isotropic velocity models could not fit the observed dispersions simultaneously we attribute the love rayleigh discrepancy to the presence of radial anisotropy in the crust and estimated its magnitude our 3 d radial anisotropic model reveals the spatial variation of strong to weak positive vsh vsv and negative vsv vsh radial anisotropy in the crust we observe negative radial anisotropy in the upper crust that is associated mainly with the location of a previously reported mantle plume the anisotropy could be attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material a strong positive radial anisotropy is centered at the location of an inferred boundary between the congo craton and the oubanguides belt that might be related to the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone the middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow induced alignment of anisotropic minerals that crystallized during magma intrusion the magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust the imaged patterns of the isotropic velocity and radial anisotropy are consistent with previous studies and agree with regional tectonics

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

stress directions and shear wave anisotropy observations from local earthquakes in southeastern sicily italy the spatial distribution of 414 earthquakes 1 0 m l 4 6 recorded from 1994 to 2002 in southeastern sicily italy has been analyzed the seismicity generally coincides with mapped plio quaternary faults including the north north west south southeast striking offshore fault system which is the most important tectonic structure of the area for the best located events we computed 70 focal mechanisms by combining p wave polarities with s wave polarizations a predom inance of strike slip and normal faults was observed focal mechanisms were then inverted for stress tensor parameters by using the algorithm of gephart and forsyth the results highlighted a region governed mainly by a north northwest south south east to northwest southeast compressional stress regime moreover anisotropy anal ysis of shear waves showed a polarization of fast s waves coherently aligned with this stress direction a finer scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis r1 the eastern sector displays a nearly horizontal r1 trending northwest southeast the central sector is affected by a low dip north northwest south southeast r1 whereas in the western sector a north northwest south southeast oriented r1 with a higher dip angle was detected finally the comparison of the spatial distribution of seismicity occurring during 1994 2002 with locations of previous instrumental earthquakes and larger m 5 0 historical events showed that the seismicity patterns are persistent

shear wave splitting vp vs and gps during a time of enhanced activity at aso caldera kyushu 1 we performed shear wave splitting analyses and calculated vp vs ratios on local earthquakes recorded at seven stations around aso volcano japan between 2001 and 2008 the mean vp vs of 1 61 0 01 from all stations is among the lowest average values found anywhere in the world a temporal change of shear wave splitting fast polarization is observed at one station additionally spatial variations of shear wave splitting delay times and vp vs ratios show a correlation at three stations we suggest that stress controlled anisotropy dominates at these stations gps baselines and 2d strain reveal extension in the caldera in 2003 and compression before and after 2003 strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003 baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low vp vs ratios and the observed compression in the caldera we propose that a small volume of magma intruded into the sill in 2003 caused extension at the surface and triggered ash eruptions between 2003 and 2005 associated localized migration of gases in overpressured cracks with aspect ratios potentially 0 5 in the upper crust with porosities 0 1 could be the source of changes in seismic anisotropy and explain the slight variations observed in vp vs

upper crustal anisotropy of the conjugate strike slip fault zone in central tibet analyzed using local earthquakes and shear wave splittingupper crustal anisotropy of the conjugate strike slip fault zone

shear wave splitting and crustal anisotropy in the eastern ladakh karakoram zone northwest himalaya abstract seismic anisotropy of the crust beneath the eastern ladakh karakoram zone has been studied by shear wave splitting analysis of s waves of local earthquakes and p to s or ps converted phases originated at the crust mantle boundary the splitting parameters and t derived from s wave of local earthquakes with shallow focal depths reveal complex nature of anisotropy with nw se and ne oriented fast polarization directions fpd in the upper 22 km of the crust the observed anisotropy in the upper crust may be attributed to combined effects of existing tectonic features as well as regional tectonic stress the maximum delay time of fast and slow waves in the upper crust is 0 3 s the ps splitting analysis shows more consistent fpds compared to s wave splitting the fpds are parallel or sub parallel to the karakoram fault kf and other nw se trending tectonic features existing in the region the strength of anisotropy estimated for the whole crust is higher maximum delay time t 0 75 s in comparison to the upper crust this indicates that the dominant source of anisotropy in the trans himalayan crust is confined within the middle and lower crustal depths the predominant nw se trending fpds consistently observed in the upper crust as well as in the middle and lower crust near the kf zone support the fact that the kf is a crustal scale fault which extends at least up to the lower crust dextral shearing of the kf creates shear fabric and preferential alignment of mineral grains along the strike of the fault resulting in the observed fpds a similar observation in the indus suture zone isz also suggests crustal scale deformation owing to the india asia collision

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

crustal seismic anisotropy beneath shillong plateau assam valley in north east india shear wave splitting analysis using local earthquakes abstract we present crustal anisotropy estimates constrained by shear wave splitting sws analysis using local earthquakes in the shillong plateau and assam valley area north east india ne india region splitting parameters are determined using an automated cross correlation cc method we located 330 earthquakes recorded by 17 broadband seismic stations during 2001 2014 in the study area out of these 330 events seismograms of 163 events are selected for the sws analysis relatively small average delay times 0 039 0 084 s indicate existence of moderate crack density in the crust below the study area it is found that fast polarization directions vary from station to station depending on the regional stress system as well as geological conditions the spatial pattern of crustal anisotropy in the area is controlled mostly by tectonic movement of the indian plate towards ne presence of several e w and n s trending active faults in the area also play an important role on the observed pattern of crustal anisotropy

activation of optimally and unfavourably oriented faults in a uniform local stress field during the 2011 prague oklahoma sequence the orientations of faults activated relative to the local principal stress directions can provide insights into the role of pore pressure changes in induced earthquake sequences here we examine the 2011 m 5 7 prague earthquake sequence that was induced by nearby wastewater disposal we estimate the local principal compressive stress direction near the rupture as inferred from shear wave splitting measurements at spatial resolutions as small as 750 m we find that the dominant azimuth observed is parallel to previous estimates of the regional compressive stress with some secondary azimuths oriented subparallel to the strike of the major fault structures from an extended catalogue we map ten distinct fault segments activated during the sequence that exhibit a wide array of orientations we assess whether the five near vertical fault planes are optimally oriented to fail in the determined stress field we find that only two of the fault planes including the m 5 7 main shock fault are optimally oriented both the m 4 8 foreshock and m 4 8 aftershock occur on fault planes that deviate 20 29 from the optimal orientation for slip our results confirm that induced event sequences can occur on faults not optimally oriented for failure in the local stress field the results suggest elevated pore fluid pressures likely induced failure along several of the faults activated in the 2011 prague sequence

crustal shear wave anisotropy in southern hawaii spatial and temporal analysis split shear wave arrivals are analyzed in seismograms from local earthquakes in southern hawaii recorded at five temporary arrays and one permanent network station we identify split shear wave arrivals by their orthogonally polarized pulses linear particle motions and similar waveforms and estimate the delay time for the slow shear wave arrival s 2 using a waveform cross correlation method consistent leading shear wave polarizations were measured at the majority of our stations comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source mechanism agreement between fast shear wave s 1 polarizations and independently estimated directions of the maximum horizontal compressive stress h for the ainapo and punaluu gulch arrays leads us to conclude that the predominant source of the observed anisotropy for these two areas is stress aligned cracks consistent with the extensive dilatancy anisotropy eda hypothesis two distinct s 1 polarization directions were observed over distances less than 1 km for the bird park and south flank arrays s 1 polarizations parallel to the ne striking kaoiki pali fault system for the bird park array combined with a nonhorizontal maximum principal stress 1 for the south flank region suggest stress induced cracks aligned by nearby faulting as a source for the observed anisotropy large station to station variations in s 1 polarization and the relationship between delay time and event depth for arrays in the kaoiki and south flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive average delay times for the five arrays vary from about 100 to 230 ms with standard deviations of the order of 30 ms estimated anisotropic velocity variations and crack densities exceeding 10 indicate that the upper crust of southern hawaii is highly fractured a search for possible temporal changes in delay time associated with the 1983 kaoiki main shock m l 6 6 at a station near the epicenter finds no evidence for change

seismic anisotropy in the shallow crust of the loma prieta segment of the san andreas fault system we study seismic anisotropy beneath the santa cruz mountains using digital recordings of s waves from 60 aftershocks of the 1989 loma prieta earthquake m 7 0 recorded at 18 local stations three component particle motion covariance matrix decomposition combined with a cross correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves results from this procedure are compared with those obtained through visual inspection of horizontal particle motions inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of s to p converted phases from our study uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20 and 10 ms respectively about half of the s wave records examined show clear evidence of shear wave splitting two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events 52 of the polarizations are parallel to the strike of the san andreas fault nw se and 24 are parallel to the probable direction of local maximum horizontal compressive stress ne sw the fault parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary the fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the san andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy eda where parallel alignment of fluid filled fractures produces the anisotropy the predominance of fault parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results the splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth this suggests that the seismic anisotropy is no deeper than 2 km the depth of the shallowest earthquakes analyzed for an anisotropic zone 2 km thick the average velocity anisotropy is about 5 in our study region

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

seismic and electrical anisotropy in the mornos delta gulf of corinth greece and its relationship with gps strain measurements shear wave splitting studies on seismo grams from local earthquakes at the site of the mornos delta gulf of corinth greece has revealed a clear seismic anisotropy with a fast s polarization striking n55 25 o magneto telluric soundings in the frequency range 0 01 to 100 hz on the delta showed a clear electric anisotropy with a n55 o 10 o direction for the highest frequency this anisotropy spans through the whole layer of sediments about 1 km thick comparison of the 1966 1972 triangulations and the 1991 1995 gps positions of geodetic points in and around the delta showed rapid extension strain 2x10 s year in the direction n340o 30 o perpendicular to the fast s and the highest conductivity directions suggestive of a causal relationship between these observations we thus propose that this strain controls the two reported anisotropies by the formation and maintaining of fluid filled steeply dipping antithetic faults and fractures and fluid filled vertical cracks all striking n55 o 4 20 o the source of strain is likely to be active normal faulting near and under the delta as independently evidenced by the recent discovery of nearby offshore faults striking about n60 o significantly different from the dominant e w strike of the major faults of the gulf

s wave polarization inversion of the 15 october 1979 23 19 imperial valley aftershock evidence for anisotropy and a simple source mechanism s polarizations from the 16 near source strong motion stations of the 15 october 1979 23 19 imperial valley aftershock have been analyzed for constraining the source mechanism they show a significant delay of the east component relative to the north component at most stations assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments where the rays are dominantly vertical the measured fast s polarizations are mainly oriented between n0 e and n25 e and show a mean delay of 0 25 s between slow and fast s this is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it with a mean fluid filled crack density of 0 1 to 0 2 the records once corrected for the splitting effect show s polarizations stable in time which have been used to estimate the fault mechanism parameters this is done by applying a new inversion method which is based on a model probability estimate the error function is related to the polarization misfits and the associated joint probability function of the data and parameter is computed in the whole model space the inversion of the corrected s polarizations provides a unique and well constrained solution for the imperial valley aftershock mechanism strike 145 5 dip 85 10 slip 155 15 which is compatible with the available p polarities and is quite similar to the main shock mechanism

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

early ordovician granites from the south qiangtang terrane northern tibet implications for the early paleozoic tectonic evolution along the gondwanan proto tethyan margin abstract early ordovician granites occur in the gemuri and bensong co areas of the south qiangtang terrane northern tibet and la icp ms zircon u pb dating reveals that the magmatism took place between 486 and 480 ma the geochemical features of these early ordovician granites are comparable to those of high k calc alkaline s type granites they exhibit negative values of zircon ehf t 8 5 to 1 7 and whole rock end t 8 1 to 0 5 as well as old zircon hf model ages tcdm 1349 1992 ma and whole rock nd model ages tcdm 1145 1804 ma these early ordovician granites can be attributed to partial melting of the paleoproterozoic metasedimentary rocks of the south qiangtang terrane while minor amounts of mantle derived components were introduced into their magma sources taking into account previous data we propose that the early ordovician granites developed in a post collisional tectonic setting and that they were related to the collisional accretion of asian microcontinental fragments such as the south china north china tarim and qaidam kunlun terranes that took place as a result of oceanic subduction along the gondwana proto tethyan margin during the early paleozoic

shear wave anisotropy in the erzincan basin and its relationship with crustal strain the analysis of s wave polarization from a large number of aftershocks of the 1992 erzincan earthquake has revealed a clear s wave splitting which we interpret in terms of seismic anisotropy in and around the erzincan basin and which concerns at least the first few kilometers of the crust in the sedimentary basin our interpretation of the origin of the anisotropy is the alignment of cracks due to the stress field extensive dilatancy anisotropy eda model within the deep sediments and possibly below in the mean direction n335 this is rotated by 20 from the compressive stress direction n355 deduced from other aftershock studies this difference may either be explained by uncertainties in the stress direction or by a stress distortion related to the activity of some nearby active faults in the outcropping basement to the south the fast s direction is n315 significantly different from the present regional stress direction which implies that the anisotropy is controlled either by a possible though yet unreported rock foliation or by cracks related to a local strongly distorted stress field following the preferred hypothesis of stress rotations and of the eda model we use a two dimensional model to calculate the direction of the elastic stress resulting from the combined activity of the adjacent segments on the north anatolian fault east to the basin nae and within the basin nab with a similar stress release the resulting trajectories of the maximal compressive stress in the shallow crust are mainly controlled by the nae segment owing to its much greater length and correctly fit the inferred crack directions in the bedrock sites as well as in the basin sites provided that the stress release on the segments is of the order of the regional deviatoric stress amplitude stored in the shallow crust 1 km in depth in this model the stress and crack orientation inferred from shear wave anisotropy can be explained by some slip deficit on the nab segment with respect to the nae segment which is independently suggested by the historical earthquakes in the area studies of the crack induced anisotropy in the shallow crust can thus bring important information on the spatial and possibly temporal variations of crustal strain and stress near active faults in particular near the limits of the fault segments marked by jogs steps or bends and might constrain the relative potential of adjacent segments for future earthquake ruptures

fracture related wavefield polarization and seismic anisotropy across the greendale fault we investigate seismic signatures of fracturing in a newly ruptured strike slip fault by determining the wavefield polarization in the new zealand canterbury plains area and across the greendale fault which was responsible for the 3 september 2010 darfield mw 7 1 earthquake previous studies suggested that fractured rocks in fault damage zones cause directional amplification and ground motion polarization in the fracture perpendicular direction as an effect of stiffness anisotropy and cause velocity anisotropy with shear wave velocity larger in the fracture parallel component an array of 14 stations was installed following the darfield earthquake we assess polarization both in the frequency and time domains through the individual station horizontal to vertical spectral ratio and covariance matrix analysis respectively and compare the results to previously reported anisotropy measurements from shear wave splitting stations installed in the canterbury plains have an amplification peak between 0 1 and 0 3 hz for both earthquakes and ambient noise we relate the amplification to the resonance of a considerable thickness c 1 km of soft sediments lying over the metamorphic bedrock analysis of seismic events revealed the existence of another peak in amplification between 2 and 5 hz at two on fault stations which was not visible in the noise analysis in contrast to the lower frequency peak the ones between 2 and 5 hz are more strongly anisotropic attaining amplitudes up to a factor of 4 in the n52 direction to interpret this effect we model the fracture pattern in the fault damage zone produced by the fault kinematics we conclude that the horizontal polarization is orthogonal to extensional fractures which predominate in the shallow layers 2 km expected in n101 direction on the basis of the model thus different fracture distribution at different depths may explain different results for amplification compared to anisotropy we propose polarization amplification analysis as a complementary method to s wave splitting analysis polarization analysis is rapidly computed and robust and it can be applied to either earthquakes or ambient noise recordings giving useful information about the predominant fracture patterns at various depths

structure related and stress induced shear wave velocity anisotropy observations from microearthquakes near the calaveras fault in central california we attempt to distinguish between structure related and stress induced shear wave velocity anisotropy in the upper crust using microearthquakes m 2 6 that occurred before and after the tres pinos california earthquake m 5 5 of 26 january 1986 the tres pinos earthquake occurred on the quien sabe fault a northwest southeast trending right lateral strike slip fault approximately 10 km east of the calaveras fault in central california digital three component seismograms are available from 1984 to 1992 from the quien sabe ranch station hqr located only 7 3 km north northeast of the epicenter of the main event we studied polarization directions and delay times using a method that combines cross correlation techniques and particle motion plots to enhance the significance of our interpretations we reduced uncertainties related to the hypocenter location of the earthquakes by source relocation based on joint hypocenter determination and cross correlation in the study area we found two zones with different but stable polarization directions around the quien sabe fault system shear wave polarizations of over 30 microearthquakes clearly indicate that the slow shear wave direction is normal to the northwest southeast trend of this vertical right lateral strike slip fault thus velocity anisotropy appears to be caused by the fault system itself however anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different in this zone the fast and slow shear polarization directions are correlative with the maximum and minimum horizontal stress directions respectively thus in this case it appears the crustal velocity anisotropy is controlled by the state of stress in the crust the observation of these two polarization directions at one station suggests that the shallow crust below the station hqr does not influence shear wave polarization directions

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

foreshock sequence of the 1992 landers california earthquake and its implications for earthquake nucleation the june 28 1992 landers california earthquake mw 7 3 was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events in this study we examine the geometry and temporal development of the foreshocks using high precision locations based on cross correlation of waveforms recorded at nearby stations by aligning waveforms rather than trying to obtain travel time picks for each event independently we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals we perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically with the improved locations the geometry of the foreshock sequence becomes clear the landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane the nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step this geometry is confirmed by the focal mechanisms of the foreshock sequence which are right lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault and are rotated clockwise for foreshocks that occur within the step the extent of the foreshock sequence is approximately 1 km both vertically and horizontally modeling of the coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other this result is particularly clear for the mw 4 4 immediate foreshock since stress transfer in the sequence appears not to have played a significant role in its development we infer an underlying aseismic nucleation process probably aseismic creep other studies have shown that earthquake nucleation may be controlled by fault zone irregularities this appears to be true in the case of the landers earthquake although the size of the irregularity is so small that it is not detectable by standard location techniques

stress and structure induced shear wave anisotropy along the 1999 izmit rupture northwest turkey abstract we perform a shear wave splitting analysis using seismograms of aftershocks following the 1999 m w 7 4 izmit earthquake to delineate between stress and structure controlled anisotropic mechanisms in the upper crust beneath the northwest north anatolian fault zone nafz the splitting analysis is performed on direct shear wave arrivals using a cross correlation technique to calculate the fast shear wave polarization and the delay time between fast and slow shear wave arrivals five stations located both within 1 km the nafz yield a combined 333 quality shear wave splitting measurements the shear wave splitting measurements indicate that it is occasionally possible to delineate between the two anisotropic mechanisms stations greater than 1 km from the nafz tend to have dominant fast shear wave polarizations parallel or subparallel to the regional strike of the maximum horizontal compressive stress which suggests a stress controlled anisotropic mechanism the single station within 1 km of the nafz yields a bimodal dominant fast shear wave polarization that is both parallel and perpendicular to the strike of the nearby mudurnu fault we interpret the fault parallel mode as representing a structure controlled anisotropic mechanism from the mudurnu fault the average delay time for all earthquakes is 0 038 0 031 s and anisotropic intervals do not appear to extend below 8 km the fast shear wave polarizations and delay times suggest a heterogeneous anisotropic distribution within the recording array and that the physical properties of the upper crust vary between the nafz and surrounding area

seismic anisotropy around the gulf of corinth greece deduced from three component seismograms of local earthquakes and its relationship with crustal strain several thousand three component seismograms from local earthquakes recorded during two field experiments in august 1991 and november 1992 in the gulf of corinth have been analyzed to detect shear wave splitting after a first selection of the events located in the s window of the considered stations a second very strict selection of the records is applied in order to avoid the effect of scattered or converted phases which can mimic the behavior of shear wave splitting two main directions of fast s wave polarization have been detected one oriented n105 e n120 e the other n55 e n75 e the first one is perpendicular to the main direction of extension of the gulf provided by focal mechanism global positioning system measurements and tectonic studies and is thus consistent with the extensive dilatancy anisotropy eda model the second direction is subparallel to the direction of the active normal fault closest to the sites this suggests a local control of the anisotropy by these active faults either by a local rotation of the total stress field in which case the eda model may still explain the anisotropy or by the existence of a specific microstructure or macrostructure generated by the long term fault activity set of secondary fault planes parallel to the major one in which case the anisotropy direction would be significantly rotated from the stress direction about 50 the anisotropic signature does not seem to be affected by the geology of the site pre tertiary limestone and pleistocene sediments except for a station located on the thick plio quaternary deposits of a delta where the time delay is significantly larger

observation of shallow anisotropy on local earthquake records at the garner valley southern california downhole array abstract we study very shallow s wave anisotropy effects on local earthquake seismograms recorded by a vertical array of accelerometers at garner valley southern california using seismic events recorded by sensors buried at depth in the crystalline bedrock 220 m in weathered crystalline rocks 22 m and in sediments 15 6 and 0 m we analyze the s wave vertical travel times and polarizations at different depths hodograms plotted for horizontal components at s onset exhibit a clear linear and constant polarization independent of both earthquake location and mechanism this alignment varies with depth and is n 325 13 at 220 m while n 25 20 at 22 15 6 and 0 m a for the deep propagation 220 m the s wave direction of polarization at 220 m is identical to the observation obtained on a crystalline outcrop at the knw usgs stations 7 km away studies conducted on this site concluded to a rock fabric anisotropy due to the alignment of minerals and or microcracks along the lineation direction b for the shallow s fast s slow travel time delays roughly correspond to half the dominant period of the signal and the subsurface linear polarization is interpreted as a degenerated ellipse the north 20 anisotropy fast axis coincides with the maximum horizontal compressive stress associated with the san andreas fault system the shallow anisotropy extension corresponds to slow velocities associated with altered granite it suggests either a stress induced anisotropy due to pore crack deformation in an altered e g hydrothermally medium or the superposition of two crystalline units having different lineation directions these data show that in presence of slow 0 to 200 m subsurface layers shallow anisotropy strongly affects the earthquake surface records in the usual frequency range below 30 hz used in short period seismology

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation