a detailed seismic anisotropy study during the 2011 2012 unrest period in the santorini volcanic complex abstract the santorini volcanic complex svc is an area in the southern aegean greece which has been characterized by low seismicity rates for the last decades especially in the santorini caldera where they have been very low until 2010 this pattern changed completely in february 2011 when intense microseismic activity was initiated within the caldera during the manual analysis of the events the shear wave splitting phenomenon was observed revealing the existence of an anisotropic upper crust in the svc area a detailed anisotropy study has been conducted using 231 events within the shear wave window that fulfilled the selection criteria the polarization direction of the fast shear wave the time delay between the two split shear waves and the source polarization direction were calculated after visual inspection using both the polarigram and the hodogram representations this procedure applied for eight local stations resulted in the determination of 340 splitting parameters the obtained mean anisotropy directions are not homogeneous revealing a complex regime in the activated area nevertheless these results are explained by the ape model related to the stress sensitive behavior of fluid saturated microcracked rocks a detailed analysis of the temporal evolution of both the time delay and anisotropy direction was carried out the time delays measured in the band 1 window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity as well as to the major m w 5 1 and 5 2 events which took place about 40 km sw of santorini on 26 and 27 january 2012 respectively on the other hand no significant temporal variations or 90 flips of the s fast polarization direction were observed

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

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

shear wave polarization alignments recorded above the kaoiki fault zone hawaii eighty four shear wave records from the seismometer station ain at kaoiki hawaii covering the years 1979 1983 were examined for the presence of shear wave splitting and possible precursory change associated with a mainshock in 1983 time delayed arrivals with different polarizations were detected suggestive of shear wave splitting and the polarizations of the first arriving shear waves were aligned in the direction ese wnw this direction is consistent with the anisotropy being caused by cracks aligned approximately perpendicular to the direction of least principal stress which is uniform over the ray path in an otherwise heterogeneous stress field no time dependent variation of the polarization of the leading split shear wave was observed investigation of temporal variation of time delays between split shear waves was not possible due to the presence of scattered arrivals

an estimate of the absolute stress tensor in kaoiki hawaii the stress tensor orientation was calculated by inversion of 81 fault plane solutions of m 3 5 0 6 earthquakes located in an area 6 by 16 km at depths of 7 2 km this crustal volume is situated on a straight line between the summits of the active volcanoes kilauea and mauna loa the orientation of the greatest principal stress was found to be near horizontal and in the line connecting the two volcanoes this is further evidence supporting the model that magmatic expansion within the feeding conduits of these volcanoes is the source of stress that causes earthquakes in southern hawaii these earthquakes are tectonic earthquakes in the sense that they occur in a brittle elastic crust at distances of a few to several tens of kilometers from the volcanoes and that they are not directly associated with the opening of cracks by intrusions we propose that this model leads to the corollary that the shear stresses responsible for earthquake failure in the shallow hawaiian crust are approximately 3 2 mpa and that the pore pressure of ground water p in the hypocentral volume must be under near lithostatic pressure this result is obtained by arguing that the greatest principal stress 1 is equal to the magma pressure in the volcanic feeder pipe but cannot exceed the least principal stress 3 by more than the tensile strength of the crust because otherwise hydrofracture would occur opening a crack against the least principal stress given the difference between the greatest and least principal stress the fact that the overburden at the 7 km hypocentral depth equals the intermediate principal stress and the ratio r 1 2 1 3 0 4 which is obtained in the inversion of fault plane solutions we estimate that 1 202 mpa 2 200 mpa 3 196 mpa 3 mpa and n 199 mpa with uncertainties on the order of several mpa it follows that for any coefficients of friction larger than 0 15 the pore fluid pressure necessary for faulting on the observed fault planes has to be larger than 90 of the lithostatic pressure these considerations suggest that many earthquakes represent fault ruptures under low ambient shear stresses in the order of the average observed stress drop of 3 mpa

three dimensional imaging of active structures using earthquake aftershocks the northridge thrust california abstract aftershocks of the 1994 northridge earthquake m 6 8 allow us to image the structure beneath the san fernando valley in northwestern los angeles in three dimensions we combine aftershocks and geological data to build an image of the three dimensional 3 d geometry of the previously unrecognized north vergent northridge blind thrust to a depth of 21 km the most striking feature of the imaged fault is mega corrugations oriented parallel to the mean aftershock slip vector with most of the 1994 slip confined to west of the largest corrugation we also image the partially overlying south vergent san fernando thrust which broke to the surface in a complex rupture in 1971 m 7 1 both thrusts produce fault related folding because of either fault propagation or fault bends however this deeper folding is masked by overlying complex deformation in the cover which is one reason why the northridge thrust was not identified until it ruptured in 1994 we use trishear fold modeling based on our 3 d fault geometry to evaluate possible folding due to slip on the northridge thrust as well as its interaction with the overlapping san fernando thrust and with shallow structures in the cover this example illustrates the importance of earthquake data to structural geology and the value of its 3 d integration with surface and near surface geological data

topography reveals seismic hazard the devastating earthquake in the chinese province of sichuan struck an area that was not expected to suffer seismic activity of such magnitude yet topographic analyses of the region indicate active deformation suggesting a way of refining maps of earthquake risk elsewhere

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

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

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

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

differential analysis of coda q using similar microearthquakes in seismic gaps part 1 techniques and application to seismograms recorded in the anza seismic gap abstract a large but conflicting literature exists that suggests that the scattering properties of the earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes to search for temporal variations in coda q in the vicinity of an expected moderate earthquake nucleation region along the san jacinto fault zone pairs and clusters of earthquakes recorded by the anza seismic network between 1982 and 1992 are examined to minimize differences arising from path and source variability only the most similar examples of microearthquake wave fields are intercompared to assess coda q differences between similar microearthquake signals we search for systematic temporal changes in moving window spectral amplitude ratios for a single scattering coda model with an envelope decay function of the form t m e ft qc the relationship between the natural log amplitude spectral ratio and q 1 q 1 c 2 q 1 c 1 is linear i e ln r f t ft q 1 f w f where w f is a time independent relative source term to obtain robust uncertainty estimates of q 1 f and w f we utilize multi taper spectral analysis coupled with a nonparametric monte carlo confidence interval estimation procedure for the most similar events in the anza region coda cross correlation values of approximately 0 7 for 16 sec of coda we find that coda q as a function of time is stable at a typical 1 resolution of only between 25 and 50 of the reference coda q value for the area even in the best constrained frequency bands comparison of similar microearthquake coda signals with both short and long temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda q to obtain estimates of relative coda q that are more precise than this level the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the anza region such earthquakes have been observed to occur in other regions of the san andreas fault system and are analyzed in antolik et al 1996

modeling shear wave splitting due to stress induced anisotropy with an application to mount asama volcano japan we use numerical modeling to investigate the proposed stress based origin for changing anisotropy at mount asama volcano japan stress induced anisotropy occurs when deviatoric stress conditions are applied to rocks which are permeated by microcracks and compliant pore space leading to an anisotropic distribution of open crack features changes to the local stress field around volcanoes can thus affect the anisotropy of the region the 2004 eruption of mount asama volcano coincided with time varying shear wave splitting measurements revealing changes in anisotropy that were attributed to stress changes associated with the eruption to test this assertion we create a model that incorporates knowledge of the volcanic stress ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model anisotropy is calculated in two ways by considering a basic case of having uniform crack density and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress our results show that this approach is sensitive to crack density crack compliance and the regional stress field all of which are poorly constrained parameters in the case of dry crack closure results show that modeled stress conditions produce a much smaller degree of anisotropy than indicated by measurements we propose that the source of anisotropy changes at asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes such as the movement of pore fluid

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

near fault anisotropy following the hector mine earthquake 1 we present anisotropy measurements from shear wave splitting along the hector mine rupture zone six major arrays were deployed in four locations in the year following the m7 1 hector mine earthquake the dense station coverage wide distribution of the arrays and repeated deployments show a clear predominant fast direction and spatial variation of splitting along the fault but no resolvable temporal variations we determined splitting parameters using an automated cross correlation method discarding fast directions with initial source polarizations near crack parallel or perpendicular directions only two of the four array locations give reliable measurements of anisotropy at depth fast directions and delay times are constant across the 1 km wide array length however some spatial variation of splitting is observed along fault strike delay times decrease from north to south with greater splitting in areas of higher slip a change in splitting parameters along fault strike likely reveals the orientation of cracking in the near fault region during a major quake average fast directions are between fault parallel and the regional maximum compressive stress direction we do not see temporal evolution in anisotropy however measured splitting suggests a heterogeneous stress field partially created during rupture that persists over at least a 1 year timescale

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

continuous monitoring of propagation velocity of seismic wave using across 1 temporal variations in the p and s wave travel times were continuously monitored using across which is a newly developed system for utilizing accurately controlled continuous sinusoids for seismic exploration we made an experiment lasting for 15 months at a site near the nojima fault which ruptured during the 1995 kobe earthquake m7 2 the elastic waves emitted from the across vibrators located at the surface were observed with seismometers at 800 m and 1700 m depth the long term variations of about 2 ms for both the p and s waves were observed during the 15 month experiment the major cause of these variations is identified to be that of a near surface property which changes the near source vibration the variations in the deeper structures estimated after the reduction of the near source variation were about 0 5 ms an s wave delay of about 1 ms was detected due to the 2000 western tottori earthquake m6 6

estimates of orientations of stress and strain tensors based on fault plane solutions in the epicentral area of the great hawaiian earthquake of 1868 the 95 best quality fault plane solutions for earthquakes in a 30 by 10 km area near the epicenter of the 1868 hawaiian earthquake m about 7 9 were used for estimating the orientation of the stress and strain tensors most of the earthquakes had magnitudes of ml 3 5 0 6 with nine events in the range 4 2 ml 5 5 although this area hilea is capable of great earthquakes it seldom produces events in the range 5 5 ml 6 5 we found that the directions of the slip and the principal strains are much more uniform in hilea than in the neighboring regions of kaoiki and south kona not varying more than 15 the directions of the stress tensor are poorly constrained because of the uniformity of fault plane solutions therefore one cannot be certain that the stress tensor is uniform in this area but we assume that it is because the strain and slip directions are the stress and strain tensor directions differ significantly from those in the kaoiki and south kona areas the slip directions as well as the plane defined by e1 and e3 are oriented approximately perpendicular to the upper part of mauna loa s southwest rift suggesting that magma intrusions there at a distance of 20 km probably provide the stress for the seismic activity in the hilea area we propose that a tectonic boundary exists near latitude 19 19 separating the kaoiki and hilea areas the stress and strain tensor orientations are significantly different north and south of this boundary and virtually no strike slip earthquakes occur in the hilea area whereas they are the majority of solutions in the kaoiki area the fault plane solutions in hilea have a near horizontal and a near vertical nodal plane in other areas of hawaii the evidence suggests that the near horizontal plane is a zone of weakness provided by buried oceanic sediment which allows slip of the upper crust away from the volcanic centers toward the coasts here however the stress inversion does not lead to a clear choice of preferred fault plane thus it may be that both nodal planes do act as fault planes in this area and that the rupture process in large earthquakes of the 1868 type may be complex involving decollement steeply dipping normal faulting and even strike slip faulting

evidence for shear wave anisotropy in the mantle wedge beneath south central alaska we investigate shear wave polarization and splitting for more than 200 earthquakes with hypocentral depths ranging from 20 to 180 km beneath southcentral alaska data were recorded at three sites stations skn aui and red a preferred polarization parallel to the strike of the wadati benioff zone wbz n25 e is found for a majority of events however some regions show systematic differences in polarization for a subset of nine events we compute focal mechanisms and find no correlation between the theoretical shear wave polarization caused by the fault plane orientation and the observed polarization delays of the slow versus the fast s wave range from less than 0 05 to 0 25 sec for station skn and up to 0 4 sec for stations aui and red a statistically significant correlation of delay with depth is found r 0 78 with the largest delays corresponding to the deepest events this indicates that the source of the anisotropy cannot be located in the crust based on the ray path geometry the source cannot be solely located in the subducting slab either and must therefore originate in the mantle wedge our results for the subduction zone beneath alaska are consistent with similar studies in other subduction zones that describe slab parallel anisotropy in the mantle wedge

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

gps constrained coseismic source and slip distribution of the 2013 mw6 6 lushan china earthquake and its tectonic implications the 20 april 2013 lushan earthquake occurred on the southern section of the longmen shan fault system using gps data from 33 continuous stations we derive a three dimensional coseismic displacement field of the earthquake and invert for the location geometry and slip distribution of the fault rupture our study result indicates that the earthquake occurred on a reverse fault striking n28 e and dipping 43 to the nw with the maximum slip located at 30 292 n 102 943 e and 13 km depth the rupture is dominated by thrust faulting with a slight but still statistically significant sinistral component the seismic moment release is 9 5 1018 n m equivalent to a mw6 6 earthquake our results suggest that at the southern end of the longmen shan fault zone near the triple junction with the xianshuihe and anninghe faults the kinematic deformation field is no longer block like but broadly distributed to accommodate the buttressing effect of deformation around the fault triple junction

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

comparison of orientations of stress and strain tensors based on fault plane solutions in kaoiki hawaii the stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly m 3 5 0 6 located in the 10 km radius kaoiki crustal volume separate inversions for subvolumes containing 20 50 events yielded the same results in several adjacent volumes suggesting that the stress tensor is homogeneous in those parts of the kaoiki area and that the inversion results are stable and meaningful five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20 80 and at confidence levels exceeding 95 the volcano summits of kilauea and mauna loa and their rift systems are identified as the source of stress in the kaoiki crust because the greatest principal stress points to kilauea and mauna loa in addition the strain tensor due to energy released by these 238 earthquakes was computed for the kaoiki area and several subvolumes of it by summing the moment tensors the moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment magnitude relationship a comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike slip faulting in these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes these observations are interpreted to show that in the strike slip regime of the upper part of the crust neither of the nodal planes is preferred for faulting rupture probably occurs along the nw and along the nh striking nodal planes in separate earthquakes subvolumes with more decollement faulting showed significant differences of 30 40 between the principal strain and stress directions in these volumes the near horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions these facts are interpreted to indicate that the decollement plane is weak allowing slip on it even if the principal stresses are inclined at a large angle to it it is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength no well developed fault plane and regimes in which a fault with low frictional strength dominates as a function of time significant rotations of the strain tensor by approximately 45 can be observed which seem to be related to the occurrences of kaoiki mainshocks during three background periods of about 7 years each the average strain tensor showed an orientation typical for predominant decollement faulting while two premainshock periods of 2 5 years each showed an orientation closer to strike slip faulting it is proposed that this pattern may be repeated before the next kaoiki mainshock the strain released seismically is less than the geodetically observed strain by approximately an order of magnitude stress strain fault plane solutions hawaii

seismic anisotropy in the guerrero gap mexico preliminary results three component seismograms of low magnitude earthquakes md 3 recorded at the central station tetitlan of the seismic network located in the guerrero gap mexico show evidence of shear wave splitting the data set contains several months of activity from the installation of the network in 1987 until late 1989 even though the sampling rate is only 75 samples sec and the signal to noise ratio is not very large it was possible to estimate in most cases the polarization direction of the first shear wave as well as shear wave splitting time delays most polarizations show a direction which correlates with the main compressive stress in the area due to subduction of the cocos plate beneath the north american plate two groups of hypocenters can be distinguished a shallow group up to 25 km in depth and a deeper group from 32 to 45 km as there is no difference in the polarization direction pattern of both groups and the time delays do not increase with depth the shear wave splitting is interpreted in terms of an anisotropic layer above 25 km in depth filtering the seismograms to enhance the peak amplitude frequency band does not change the observed polarization directions but helps identify shear wave splitting

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

distribution of attenuation in the kaoiki hawaii source volume estimated by inversion of p wave spectra a new method to simultaneously invert for q structure and source parameters was used on a set of 635 microearthquakes 0 9 m 2 0 in the kaoiki area of southern hawaii approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km the hypocentral depths ranged from o to 14 km with the bulk of the sources in the 7 5 10 5 km range the hypothesis to be tested was that the source volume of the m 6 6 kaoiki main shock of november 16 1983 may be heterogeneous in attenuation distribution we assumed that the observed p wave displacement spectra could be modelled by a source spectrum with an 2 high frequency decay a single layer resonance filler to account for local site resonances and whole path attenuation along the ray path in a next step the attenuation factor q was constrained by tomographically reconstructing the three dimensional q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency the seismic moment m0 and a new q value this process was iterated until the results changed less than 0 1 and were accepted as final the average q was approximately constant and very low 105 q 115 between o and 7 km depth it increased to approximately 160 between 9 and 11 km in the depth ranges were the spatial coverage was good 3 9 km the q values in the nw part of the 1983 aftershock volume were larger by approximately 10 15 compared to those in the se part this 12 contrast correlates with other evidence for heterogeneity the nw region shows high coda q no surface faulting medium seismicity rate and medium precursory quiescence while the se region shows low coda q pervasive faulting high seismicity rate strong precursory quiescence and possibly low b values we conclude that the kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters the new simultaneous inversion technique is well suited to derive detailed q structure and source parameter information from microeaithquake spectra recorded at close epicentral distances

ground ruptures of the 1974 and 1983 kaoiki earthquakes mauna loa volcano hawaii the november 30 1974 ml 5 5 and november 16 1983 ml 6 6 earthquakes generated left stepping en echelon ground cracks within the kaoiki seismic zone on the southeast flank of mauna loa volcano hawaii the general trend of the ruptures n48 55 e parallels a nodal plane of the main shocks focal mechanisms the ruptures themselves consist of short predominantly extension cracks which are up to 20 m long and strike roughly e w 30 50 clockwise from the overall trend of the zones some of the cracks are linked by secondary fractures and rubble breccia to form left stepping crack arrays which are themselves linked to form longer en echelon systems of ground rupture geologic maps and field observations indicate that these features emerge from an underlying strike slip fault and they form a fracture process zone above its tip the maximum displacement measured across cracks in the 1983 rupture zone is 0 5 m trilateration data however suggests that the overall shear displacement was about 1 5 m at depth elastic solutions indicate that a region of significant tensile stress can exist above buried strike slip faults we suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the earth s surface by subsequent growth and linkage of these cracks into the observed arrays we infer that the crack arrays accommodate increased displacement with depth and they merge downward into the parent strike slip fault at an estimated 1 2 km depth where strike slip displacement was probably more or less continuous along the 7 km length of the rupture in the kaoiki region only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years this suggests that the recent 10 year periodicity of moderate magnitude kaoiki strike slip events may not have extended far into the past the tectonic significance of strike slip faulting on mauna loa volcano remains enigmatic

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

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

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

stress aligned cracks in the upper crust of the val d agri region as revealed by shear wave splitting summary shear wave splitting is measured at 19 seismic stations of a temporary network deployed in the val d agri area to record low magnitude seismic activity the splitting results suggest the presence of an anisotropic layer between the surface and 15 km depth i e above the hypocentres the dominant fast polarization direction strikes nw se parallel to the apennines orogen and is approximately parallel to the maximum horizontal stress in the region as well as to major normal faults bordering the val d agri basin the size of the normalized delay times in the study region is about 0 01 s km 1 suggesting 4 5 per cent shear wave velocity anisotropy swva on the south western flank of the basin where most of the seismicity occurs we found larger values of normalized delay times between 0 017 and 0 02 s km 1 these high values suggest a 10 per cent of swva these parameters agree with an interpretation of seismic anisotropy in terms of the extensive dilatancy anisotropy eda model that considers the rock volume pervaded by fluid saturated microcracks aligned by the active stress field anisotropic parameters are consistent with borehole image logs from deep exploration wells in the val d agri oil field that detect pervasive fluid saturated microcracks striking nw se parallel to the maximum horizontal stress in the carbonatic reservoir however we cannot rule out the contribution of aligned macroscopic fractures because the main quaternary normal faults are parallel to the maximum horizontal stress the strong anisotropy and the seismicity concentration testify for active deformation along the sw flank of the basin

on the use of microtremors for hydrocarbon detection the link of spectral anomalies of microtremors to underlying hydrocarbon reservoirs is very controversial as field experiments support both positive and negative opinions and there is not a solid theory supporting this work hypothesis we conducted field tests at different sites with and without oil and gas presence to add new experimental data to the ongoing studies microtremor information may become repeatable and so physically meaningful only when the observation duration exceeds a few days but even in this case factors such as topography and active faults may severely bias the signal ocean waves impinging the coasts provide natural background noise which stands out clearly when the observation time exceeds a dozen days or so in such a way that human noise is stacked out statistically over time microtremors recorded in relatively deep wells may provide useful information about ongoing production in a reservoir and may link well data and seismic surveys as their interferometric analysis can provide information comparable to vertical seismic profiles

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

stress induced upper crustal anisotropy in southern california 1 we use an automated method to analyze shear wave splitting from local earthquakes recorded by the southern california seismic network between 2000 and 2005 the observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression hmax suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern california is regional stress however at other stations fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock splitting delay times range widely within 0 2 s these upper crustal anisotropy observations together with previous studies of sks shear wave splitting surface waves and receiver functions suggest different mechanisms of anisotropy at different depths under southern california anisotropy in the upper crust appears to be in response to the current horizontal maximum compression hmax which differs from the cause of anisotropy in the lower crust and mantle we also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays

shear wave splitting observations in the northern los angeles basin southern california abstract shear wave splitting with 20 to 120 msec time separation is observed on 3 component seismograms from earthquakes occurring at depths of 5 to 18 km in the crystalline basement beneath the los angeles basin shallow events that occur in the basement at the base of the 5 km thick sedimentary section exhibit little splitting while deeper events show progressively greater splitting with depth the polarization direction of the fast shear wave is n s independent of the azimuth of ray path between the event and station we interpret that the shear wave splitting occurs mainly in the crystalline basement and is the result of vertical crustal microcracks aligned in n s direction under the assumption that the alignment of microcracks is due to the subsurface stress regime the results from the shear wave splitting data yield a maximum principal stress direction of n s 15 at depth beneath the northern los angeles basin consistent with results from geological mapping and fault plane solutions ray trace modeling indicates that the observations of shear wave splitting can be explained in terms of an anisotropic basement containing vertical microcracks aligned in the n s direction and with a crack density of 0 04 overlain by a weakly anisotropic sedimentary section with a crack density less than 0 02 during the last 4 yr three major earthquakes occurred in the los angeles basin area they are the m 4 6 and m 4 4 montebello double earthquakes of 1989 the m 5 5 upland earthquake of 1990 and the m 5 8 sierra madre earthquake of 1991 with epicenters of 10 55 and 40 km away from the recorder site scs located in the northern los angeles basin respectively we did not observe significant variations of shear wave splitting at scs before and after the upland and sierra madre earthquakes however we found fairly rapid variations of shear wave splitting following the montebello events the time separation between the split shear waves sharply decreased from 70 msec to less than 20 msec after the mainshock and then returned to the average level gradually this rapid decrease in shear wave splitting may be a temporal variation or a result of location variations of the montebello events that were located approximately where the line singularity is expected

observations of shear wave splitting on the southeast flank of mauna loa volcano hawaii we use a combination of particle motion covariance matrix decomposition particle motion plots and cross correlation to estimate the polarization and split times of shear wave arrivals from local earthquakes in the kaoiki and hilea regions of southern hawaii consistent leading shear wave polarizations and identifiable fast and slow shear wave arrivals were frequently observed average delay times for the four arrays vary from 100 to 230 ms the leading shear wave polarizations at stations in the kaoiki region agree with the direction of the local maximum horizontal compressive stress h determined from focal mechanism studies and field observations of ground rupture for the bird park bp array in the kaoiki region the relationship between delay time and event depth shows evidence for both dominant shallow and weaker pervasive anisotropy the correlation between delays and event depth is much weaker for the ainapo an array than for bp however a smaller slope is apparent conservative estimates of the crack densities for the bp and an arrays are e 12 and e 6 respectively in the hilea region the leading shear wave polarization for the punaluu gulch pg array differs by approximately 30 from the direction of h determined from focal mechanism studies while the waihaka gulch wg array shows no predominant polarization direction no correlation between delay time and event depth was observed for either of the two hilea arrays

on the evolution of seismogenic faults in the longmen shan eastern tibet abstract a fundamental debate exists regarding the geometry and depth extent of seismogenic faults in eastern tibet along the longmen shan geologic and seismic reflection data reveal a belt of low to moderate angle thrust faults some of which may have been activated in devastating earthquakes in 2008 mw 7 9 wenchuan and 2013 mw 6 6 lushan however geologic and geodetic constraints on these ruptures suggest rupture along relatively high angle listric reverse faults here we use a combination of focal mechanisms determined from 276 aftershocks ms 4 0 with well determined waveforms and aftershock distributions from the 2008 event to determine subsurface fault geometry our results imply that seismogenic slip occurred along relatively high angle structures that cross cut low angle imbricate faults imaged in reflection seismic data thus we suggest that current geometric models of seismogenic faults may not fully represent the distribution of subsurface seismic hazard along the heavily populated sichuan basin

seismotectonics of the 2013 lushan mw 6 7 earthquake inversion tectonics in the eastern margin of the tibetan plateau on 20 april 2013 an unexpected mw 6 7 earthquake occurred in lushan county at the southern longmen shan the eastern margin of the tibetan plateau a high resolution seismic reflection profile was combined with near surface geological data earthquake relocation and geodetic measurements and a recent deep artificial seismic reflection profile to identify the active fault and seismotectonics of this earthquake three dimensional imaging of the aftershocks was used to identify two planar faults that together form a y shape f1 and f2 seismic interpretations suggest that fault f1 did not break through the overlying mesozoic and cenozoic rocks and is a typical blind fault geodetic measurements suggest that the coseismic deformation is consistent with the geometry and kinematics of shear fault bend folding deep seismic data indicate the syndepositional nature of fault f1 a preexisting normal fault older than the triassic which underwent positive inversion tectonics during the late cenozoic a thrust fault f3 converges with f1 at a depth of approximately 12 km with an accumulated slip 3 6 km this 2013 lushan earthquake triggered by blind faults is a hidden earthquake blind and reactivated faults increase the potential risk and uncertainty related to earthquakes in the eastern margin of the tibetan plateau

structural geometry of the source region for the 2013 mw 6 6 lushan earthquake implication for earthquake hazard assessment along the longmen shan abstract the 2013 mw 6 6 lushan earthquake occurred in the longmen shan fold and thrust belt sichuan province china near the five year anniversary of the devastating 2008 mw 7 8 wenchuan earthquake to define the fault that generated the 2013 earthquake and its relationship with the beichuan fault which ruptured in the wenchuan earthquake we construct several cross sections and a 3d structural model the sections and models reveal that the main shock of the lushan earthquake occurred on a portion of the range front blind thrust rfbt and that the structural geometry of this fault varies along strike the lushan main shock occurred at a location along the strike of the fault where the geologic shortening and total fault slip are greatest a lateral ramp of the rfbt appears to coincide with the northern limit of aftershocks from the lushan earthquake leading to a 75 km seismic gap between the wenchuan earthquake and the 2013 earthquake sequence although both the wenchuan and lushan earthquakes occurred within the longmen shan fold and thrust belt different faults generated the two events based on this structural characterization and analysis of the aftershocks of the wenchuan and lushan earthquakes we suggest that the lushan earthquake may have been triggered by the 2008 rupture but is best considered as an independent event rather than an aftershock of the wenchuan earthquake the rfbt that generated the lushan earthquake is linked to a detachment that extends into the sichuan basin along the triassic evaporite layer the coulomb stress change simulation suggests that other faults linked to this detachment may have been loaded by the 2008 and 2013 earthquake posing the risk of future earthquakes along the longmen shan and in the densely populated sichuan basin

shear wave anisotropy in the crust around the san andreas fault near parkfield spatial and temporal analysis summary we systematically analysed shear wave splitting sws for seismic data observed at a temporary array and two permanent networks around the san andreas fault saf observatory at depth the purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the saf in this segment and its temporal behaviour in relation to the occurrence of the 2004 parkfield m 6 0 earthquake the dense coverage of the networks the accurate locations of earthquakes and the high resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the saf zone the results show that the primary fast polarization directions pds in the region including the saf zone and the northeast side of the fault are nw se nearly parallel or subparallel to the saf strike some measurements on the southwest side of the fault are oriented to the nne ssw direction approximately parallel to the direction of local maximum horizontal compressive stress there are also a few areas in which the observed fast pds do not fit into this general pattern the strong spatial variations in both the measured fast pds and time delays reveal the extreme complexity of shear wave anisotropy in the area the top 2 3 km of the crust appears to contribute the most to the observed time delays however substantial anisotropy could extend to as deep as 7 8 km in the region the average time delay in the region is about 0 06 s we also analysed temporal patterns of sws parameters in a nearly 4 yr period around the 2004 parkfield main shock based on similar events the results show that there are no appreciable precursory coseismic or post seismic temporal changes of sws in a region near the rupture of an m 6 0 earthquake about 15 km away from its epicentre

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

crustal anisotropy from local observations of shear wave splitting in west bohemia czech republic abstract the results of a systematic study of shear wave splitting observed in the three component digital seismograms of about 400 local microearthquakes that occurred during the 1985 1986 west bohemian earthquake swarm are presented two shear phases polarized nearly perpendicularly in horizontal projection were observed at all five stations of the local seismic network for most of the events the polarization of the faster shear wave is aligned in the wnw ese direction regardless of the mechanism and hypocenter location of the individual events and coincides with the direction of the maximum horizontal compressive tectonic stress in the region these effects are interpreted in terms of effective anisotropy of the upper crust in the region the maximum delay time between the split shear waves is 0 15 sec and corresponds to 6 anisotropy directional variation of the delay time was observed at the nearest station epicentral distance 5 km this variation was compared with functions predicted by theoretical models of cracked media to resolve which model is the most appropriate for crustal anisotropy surprisingly no agreement could be found for hudson9s model of dry or water filled parallel cracks that is widely used by many authors a successful fit was obtained for schoenberg douma9s model of a medium with parallel fractures the fracture normal pointed n31 e and the optimum fracture parameters were e n 0 4 and e t 0 02 the theoretically calculated fractures are aligned parallel to the tectonic stress direction and do not correspond to any real tectonic lines in the region

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

seismic anisotropy of the shallow crust at the juan de fuca ridge microearthquake data recorded on four ocean bottom seismometers are used to study shear wave splitting on the endeavour segment of the juan de fuca ridge the covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records at three of the four sites the results show a remarkably consistent fast direction parallel to the ridge axis the time delays between the fast and the slow waves range from 40 to 200 ms with an average of 90 ms they are not clearly related to earthquake range focal depth or source receiver azimuth the splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge parallel cracks in the shallow crust if we assume that anisotropy is concentrated in the upper 1 2 km the splitting times require a high crack density of 0 1

shear wave splitting and crustal anisotropy at the mid atlantic ridge 35 n 1 shear wave splitting observed in microearthquake data at the axis of the mid atlantic ridge near 35 n has a fast polarization direction that is parallel to the trend of the axial valley the time delays between fast and slow s wave arrivals range from 35 to 180 ms with an average of 90 ms and show no relationship with ray path length source to receiver azimuth or receiver location the anisotropy is attributed to a shallow distribution of vertical fluid filled cracks aligned parallel to the trend of the axial valley joint modeling of the shear wave anisotropy and coincident p wave anisotropy results using recent theoretical models for the elasticity of a porous medium with aligned cracks suggests that the crack distribution that causes the observed p wave anisotropy can account for at most 10 ms of the shear wave delay most of the shear wave delay thus likely accrues within the shallowmost 500 m seismic layer 2a and the percent s wave anisotropy within this highly fissured layer is 8 30 isolated fluid filled cracks at 500 m to 3 km depth that are too thin or too shallow to be detected by the p wave experiment may also contribute to the shear wave delays the joint analysis of p and s wave anisotropy is an important approach for constraining the crack distributions in the upper oceanic crust and is especially suited for seismically active hydrothermal systems at slow and intermediate spreading mid ocean ridges

shear wave splitting detected by using downhole triaxial seismic detector during dilation of artificial subsurface fracture summary a small change in traveltime and shear wave splitting in elastic waves transmitted through a pressurized single fracture has been detected through downhole triaxial cross hole seismic measurements a field experiment was carried out at higashi hachimantai field of tohoku university japan where an artificial single fracture had been created at 369 m by hydraulic fracturing in intact welded tuff p and s waves transmitted through the pressurized single fracture were measured by using a downhole three component seismic detector installed in a borehole near the fracture in our previous studies a small change in traveltime of about 0 1 ms was detected by using the cross spectral moving window technique in this recent study the arrival times of split shear waves and the polarization directions were detected by using the wavelet transform of a three component signal the traveltime difference between the first and second shear waves was 0 05 0 12 ms when the wellhead pressure was increased up to 3 4 mpa and the incident angle of waves into the main fracture was changed from 12 to 20 a fracture model of a single fracture and a microcrack zone in the vicinity of the main fracture was used to explain the shear wave splitting phenomenon the width of the reopening microcrack zone and the crack density were quantitatively evaluated by using the fracture model and shear wave splitting analysis the best fit theoretical curve to the observed data suggested that the width of the reopening zone was 0 8 m and the crack density was 0 39

spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999 m7 4 izmit and m7 1 duzce turkey earthquake sequences summary we analyse spatiotemporal variations of crustal anisotropy along the karadere duzce branch of the north anatolian fault from similar earthquakes in the aftershock regions of the 1999 mw7 4izmit and mw7 1 duzce earthquakes the similar earthquake clusters are identified by performing cross correlation on waveforms generated by 18 000 earthquakes depending on the applied similarity criterion about 4 60 per cent of the events belong to similar event clusters splitting parameters averaged within each cluster show significant variations for slightly different ray paths indicating strong spatial variations of crustal anisotropy in this area we also find clear changes in the spatiotemporal seismicity patterns following the duzce main shock large apparent co seismic changes up to 30 per cent of shear wave splitting delay times are observed across the time of the duzce main shock at stations near the epicentral region however the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity rather than changes in the properties of the anisotropic medium splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the duzce main shock the results do not show systematic precursory changes before the duzce main shock

profile of discontinuities beneath hawaii from s to p converted seismic waves we present new seismological evidence for the depth of depression of the volcano crust and crust mantle moho boundaries and the nature of the volcano crust interface beneath the southeast flank of mauna loa volcano hawaii from a profile of s to p converted waves from deep local earthquakes three component seismic array observations of these converted waves indicate the presence of a low velocity layer lvl at a depth of about 10 km corresponding to the base of the volcanic pile the thickness of the lvl is estimated to be approximately 1 km the lvl and the moho both deepen significantly from southwest to northeast beneath mauna loa s southeast flank the observed polarity reversal of the lvl conversion compared to the moho conversion strongly supports the existence of a buried sediment layer at the volcano ocean crust interface at a depth that is consistent with the depth distribution of thrust earthquakes our results conflict with models of lithosphere flexure at hawaii indicating a depression of the lithosphere greater than that predicted by flexure models the thickness of underlying oceanic crust is observed to be relatively constant so this discrepancy cannot be accounted for by crustal underplating alone

seismic anisotropy of the shallow crust in the umbria marche italy region abstract small magnitude earthquake data recorded during the 1997 umbria marche italy seismic crisis have been used to study the shear wave splitting in the central apennine using the method of diagonalization of the covariance matrix a set of 1234 s waves impinging the earth at an angle less than the critical angle has been selected the cross correlation method was used to estimate the splitting time between the two horizontal s components and the direction of the symmetry axis of the anisotropic structure a further analysis based on visual inspection of corrected traces allowed us to estimate the anisotropic parameters for 884 s waves an average splitting time 32 3 4 ms has been estimated splitting times do not generally show a clear pattern with distance which is an indication of possible spatial variations of seismic anisotropy a minimum 2 km depth of the anisotropic layer is estimated the trend of normalized splitting time vs depth roughly indicates a maximum 4 5 km depth of the anisotropic layer the distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one the stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters a clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found which confirms the spatial variability in the anisotropic properties of the area as inferred by the analysis of splitting times based also on previous studies results are interpreted in terms of the extensive dilatancy anisotropy eda model the estimated crack density is always smaller than the critical coalescence value enforcing the hypothesis that fluid filled separated crack systems control the polarization of s waves

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

seismic anisotropy in the crust at the mid atlantic plate boundary in south west iceland summary seismograms of microearthquakes recorded on stations of the sil network in south western iceland exhibit strong shear wave splitting and are consistent with being caused by aligned parallel cracks in the uppermost crust splitting times of 0 1 0 3 s are observed with the larger values 0 2 0 3 s occurring beneath stations in the highly heated western volcanic zone and the smaller values 0 1 s in the younger less fractured south iceland seismic zone five of the stations have a fast shear wave polarization azimuth of n30 e n40 e7 parallel to the axis of the western volcanic zone i e the mid atlantic ridge in iceland and in the direction of maximum horizontal compression defined by fault dyke and fissure strikes and microearthquake focal mechanisms station gyg in the north western part of the network has the significantly different azimuth of n70 e this is parallel to one of the local strike slip fault trends and is probably due to a heated rock fabric caused by those faults

the 2013 lushan earthquake implications for seismic hazards posed by the range front blind thrust in the sichuan basin china thrust and reverse faults pose significant earthquake hazards in convergent plate margins around the world but have proven difficult to study given the complex nature of their ruptures which often involve multiple along strike and vertically stacked fault segments the 2013 mw 6 6 lushan earthquake exemplified this complexity rupturing a blind thrust fault in the southern longmen shan which border the western sichuan basin in china this event occurred 80 km south of the epicenter of the destructive 2008 mw 7 9 wenchuan earthquake the wenchuan earthquake produced surface ruptures on two parallel fault splays the pengguan and beichuan faults in contrast the lushan earthquake was generated by a ramp in the range front blind thrust rfbt which is in the footwall of the wenchuan rupture we use seismic reflection profiles petroleum wells and relocated seismicity to construct a three dimensional model of this imbricated fault system our model illustrates that the 2013 lushan earthquake ruptured 10 of the rfbt which extends for 250 km along the longmen shan range front and into the western sichuan basin analysis of growth strata in structures above the rfbt fault along strike shows clear evidence of quaternary activity and constrains the middle pleistocene to current slip rate at two locations on the fault single segment and multisegment fault rupture scenarios involving the rfbt suggest the potential for large earthquakes m7 8 that would affect the densely populated western sichuan basin assessing the hazards posed by such complex thrust systems which occur in convergent margins worldwide requires subsurface characterization of fault segments that can be independently associated with geologic and seismologic evidence of fault activity

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less