alignment of near surface inclusions and appropriate crack geometries for geothermal hot dry rock experiments ubiquitous splitting of seismic shear waves indicates that most rocks in the upper half of the crust are pervaded by stress aligned fluid filled inclusions called eda cracks these inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures at depths where the overburden stress is sufficiently large typically below a few hundred metres this minimum stress is usually horizontal so that the eda cracks and hydraulic fractures are typically aligned vertically striking parallel or subparallel to the direction of maximum compression this is confirmed by the polarizations of the split shear waves along raypaths at depth in the crust at the free surface however the vertical stress is zero or approximately zero and cracks and hydraulic fractures at shallow depths in intact rock tend to be horizontal thus the directions of minimum stress and the orientations of hydraulic fractures are likely to swing through 90 near the surface of the earth since the behaviour of cracks and stress is often crucial to drilling operations the rotation of the crack and stress geometry near surface has important implications particularly for optimizing hydrocarbon production and geothermal reservoir management consequently evidence gained from experiments for example in hot dry rock geothermal heat extraction in inappropriate crack geometries at shallow depths may not be valid when applied to other crack and stress geometries at depth in hot rock

auto correlation analysis of coda waves from local earthquakes for detecting temporal changes in shallow subsurface structures the 2011 tohoku oki japan earthquake for monitoring temporal changes in subsurface structures i propose to use auto correlation functions of coda waves from local earthquakes recorded at surface receivers which probably contain more body waves than surface waves use of coda waves requires earthquakes resulting in decreased time resolution for monitoring nonetheless it may be possible to monitor subsurface structures in sufficient time resolutions in regions with high seismicity in studying the 2011 tohoku oki japan earthquake mw 9 0 for which velocity changes have been previously reported i try to validate the method kik net stations in northern honshu are used in this analysis for each moderate earthquake normalized auto correlation functions of surface records are stacked with respect to time windows in the s wave coda aligning the stacked normalized auto correlation functions with time i search for changes in phases arrival times the phases at lag times of 1 s are studied because changes at shallow depths are focused temporal variations in the arrival times are measured at the stations based on the stretching method clear phase delays are found to be associated with the mainshock and to gradually recover with time the amounts of the phase delays are 10 on average with the maximum of about 50 at some stations the deconvolution analysis using surface and subsurface records at the same stations is conducted for validation the results show the phase delays from the deconvolution analysis are slightly smaller than those from the auto correlation analysis which implies that the phases on the auto correlations are caused by larger velocity changes at shallower depths the auto correlation analysis seems to have an accuracy of about several percent which is much larger than methods using earthquake doublets and borehole array data so this analysis might be applicable in detecting larger changes in spite of these disadvantages this analysis is still attractive because it can be applied to many records on the surface in regions where no boreholes are available

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

local deformations around volcanoes associated with the 2011 off the pacific coast of tohoku earthquake 1 an mw 9 0 great earthquake struck northeast japan on 11 march 2011 causing crustal deformation of 1 to 2 m in volcanic areas we applied synthetic aperture radar interferometry using phased array type l band synthetic aperture radar on the alos satellite to investigate local deformation around volcanoes associated with the earthquake we estimated the fault slip distribution along the trough based on obtained interferograms and crustal deformation from geonet and the estimated fault model explained the observed coseismic deformations well the residual suggested local deformation around the akita komagatake kurikoma zao azuma and nasu volcanoes their deformations were obtained from independent synthetic aperture radar pairs additionally deformations in the kurikoma zao and azuma volcanoes were confirmed by geonet data no geonet stations are located in the deformation areas of other volcanoes these facts indicate that the obtained local deformations must be actual deformations rather than noise our hypothesis is that coseismic extensional deformation concentrates in the soft medium under a volcano e g magma and its surrounding rock and that this deformation has caused local deformation with subsidence to validate this hypothesis we carried out numerical experiments using the finite element method for a soft medium under a volcano distributions of observed local deformations stretch in the direction of the minor principal axis of strain and such distributions were obtained in this experiment indicating that the hypothesis is plausible conceivably such a concentration of coseismic deformation in a magma chamber may induce a volcanic eruption

vp vs ratios in the upper crust of the southern korean peninsula and their correlations with seismic and geophysical properties abstract the p and s velocity ratio v p v s is a seismic property that is convertible to poisson s ratio a widely analyzed physical property of elastic materials the lateral variation in the v p v s ratios of the upper crust of the southern korean peninsula is investigated based on the travel times of p and s waves of local events with epicentral distances of 50 km or less and focal depths of 25 km or less the dense seismic networks in the korean peninsula allow us to investigate the regional variation of v p v s ratios these v p v s ratios are estimated to be 1 60 1 91 with an average of 1 69 the v p v s ratios are as high as 1 73 1 91 in the gyeongsang basin jeju island and offshore region of the eastern okcheon belt but have relatively low values of 1 64 1 72 in precambrian massif regions the stability of the v p v s ratios is tested with randomly resampled travel time data the results of which suggest nearly constant v p v s ratios with increasing depth in the upper crust the influences of plausible errors on origin times and arrival times are quantified to verify the v p v s estimates the v p v s ratios are correlated with geological and tectonic structures comparisons with known seismic and geophysical properties suggest that structural features observed on the surface may extend at least to the mantle lid

seismic waveform attributes before and after the loma prieta earthquake scattering change near the earthquake and temporal recovery during 1987 1995 several clusters of nearly identical seismic events multiplets occurred near the loma prieta source region these multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 loma prieta main shock for seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency post loma prieta events have reduced coherencies with pre loma prieta events in a spatially limited region close to the loma prieta hypocenter while other paths remain nearly unaffected these changes gradually recover within a time interval of 5 years after the loma prieta earthquake a possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the loma prieta event postseismic relaxation processes such as crack healing fluid diffusion or after deformations lead to progressive closure of these cracks with time after the main shock thus the scattering properties of the local crust approach the pre main shock state

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

prevalence of viscoelastic relaxation after the 2011 tohoku oki earthquake seafloor global positioning system observations immediately after the great 2011 tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than just afterslip on the fault as is commonly assumed this paper presents revised estimates of the afterslip following the 2011 tohoku oki earthquake based on seafloor gps observations made immediately after the earthquake tianhaozhe sun et al obtain unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than elastic afterslip on the fault as is commonly assumed they conclude that models which erroneously assume an elastic earth substantially overestimate afterslip downdip of the rupture zone and underestimate afterslip updip after a large subduction earthquake crustal deformation continues to occur with a complex pattern of evolution1 this postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip afterslip or relocking of different parts of the fault2 3 4 5 6 when postseismic geodetic observations are used to study earth s rheology and fault behaviour it is commonly assumed that short term a few years deformation near the rupture zone is caused mainly by afterslip and that viscoelasticity is important only for longer term deformation6 7 however it is difficult to test the validity of this assumption against conventional geodetic data here we show that new seafloor gps global positioning system observations immediately after the great tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation these data reveal fast landward motion of the trench area opposing the seaward motion of gps sites on land using numerical models of transient viscoelastic mantle rheology we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake a process previously unknown because of the lack of near field observations our findings indicate that previous models assuming an elastic earth will have substantially overestimated afterslip downdip of the rupture zone and underestimated afterslip updip of the rupture zone our knowledge of fault friction based on these estimates therefore needs to be revised

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

azimuthal anisotropy of rayleigh waves in the crust in southern tohoku area japan to reveal seismic anisotropy within the crust in the southern tohoku area japan we analyze the azimuthal anisotropy of rayleigh waves in the frequency range of 40 65 mhz rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014 the frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust 1 north south fast velocity direction around the upper crust and 2 trench normal fast velocity direction in the deeper crust we also conduct a time lapse analysis of rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 tohoku oki earthquake we demonstrate that the azimuthal location of teleseismic events influences the accuracy of time lapse analysis moreover a decrease of azimuthal anisotropic ratio in time lapse results might be related to changes in the velocity structure associated with release of crustal stress during the tohoku oki earthquake

stress before and after the 2011 great tohoku oki earthquake and induced earthquakes in inland areas of eastern japan 1 stress fields in inland areas of eastern japan before and after the tohoku oki earthquake were estimated by inverting focal mechanism data before the earthquake s1 axis was oriented ew in tohoku but nw se in kanto chubu the stress fields changed after the earthquake in northern tohoku and in southeastern tohoku near iwaki city where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake this indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 mpa the stress field did not change in central tohoku even though the stresses loaded after the earthquake had nearly reversed orientations which indicates that the differential stress magnitudes there were significantly larger than 1 mpa in kanto chubu stresses having nearly the same orientations as the background stresses were loaded after the earthquake and the stress fields did not change as expected this may have caused very high induced seismicities in kanto chubu citation yoshida k a hasegawa t okada t iinuma y ito and y asano 2012 stress before and after the 2011 great tohoku oki earthquake and induced earthquakes in inland areas of eastern japan geophys res lett 39 l03302

fluid driven seismicity activation in northern nagano region after the 2011 m9 0 tohoku oki earthquake the dynamic triggering of earthquakes is well documented however the underlying physical mechanisms remain obscure here we analyze the seismicity in northern nagano central japan following the tohoku oki quake until the occurrence 13 h later of an mw6 2 local earthquake we use waveform detection techniques to identify 17 times more earthquakes than those in the japan meteorological agency catalog the activation of seismicity in the epicentral region of the mw6 2 event is weak and delayed culminating with the occurrence of the moderate shock preceded by two small foreshocks the seismicity activation to the south is shallower abundant and starts during the passage of tohoku oki surface waves of high dynamic stresses the early activation occurs in areas of relatively high near surface fluid temperature indicating that the dynamic triggering is likely caused by excitation of geothermal crustal fluids the mw6 2 northern nagano earthquake might have been delay triggered by fluid migration from a deep source

trench parallel crustal anisotropy along the trench in the fore arc region of japan in northeastern japan the pacific plate is descending beneath the north american plate it is generally understood that trench normal principal stress is dominant in the crust along the japanese island arc because the stress field is controlled by the force of the subducting slab observations of shear wave splitting using crustal earthquakes reveal a marked lateral variation in fast polarization direction with distance from the trench trench normal and trench parallel fast polarization directions are observed on the back arc and fore arc sides respectively in this study two dimensional finite element modeling with subducting slab was conducted to investigate the interseismic stress field during earthquake cycles taking into account linear viscoelasticity in the model trench normal compression is found to dominate in the island arc region however an extensional field appears in the shallow upper crust of the fore arc region during earthquake cycles the trench parallel crustal anisotropy can be explained by this extensional field

evidence from shear wave splitting for the restriction of seismic anisotropy to the upper crust the existence of anistropy in the propagation speed of seismic waves through the earth s crust has been attributed to the non random alignment of microcracks in the crust1 for sparsely distributed parallel vertical cracks perturbation theory predicts the splitting of a shear wave into two orthogonally polarized waves2 4 here we present extremely clear evidence for shear wave splitting in three component seismograms from crustal and sub crustal microearthquakes recorded in the shikoku district of japan quantitative analysis of travel time differences between split shear waves leads to the conclusion that the observed anisotropy is limited to the upper crust shallower than 10 15 km this model is consistent with recent results from reflection seismology5 and implies that microcrack density or orientation changes from the upper to the lower crust

healing microstructures of experimental and natural fault gouge 1 the healing of fault gouge was studied by examining microstructures of naturally and experimentally produced granitoid fault rock we performed deformation experiments on intact granitoid rock samples at t 300 500 c pc 500 mpa and 1 2 10 4 1 3 10 7 s 1 with 0 2 wt h2o added healing experiments were carried out on deformed samples at t 200 500 c pc 500 mpa for 4 h to 14 days under hydrostatic and nonhydrostatic conditions the grain size distributions gsd of the deformed samples were quantified using the d value slope of log frequency log radius of the gsd for quartz and feldspar fault gouge healing causes a decrease in the d value from 2 0 to 1 5 the time dependence of the d decrease is described by a hydrostatic healing law of the form d d t df a e t the results of the laboratory experiments were compared to three natural fault systems 1 nojima fault zone japan 2 fault zones in the black forest germany and 3 orobic thrust italian alps natural and experimental gouges have similar d values healing is only observed in monomineralic aggregates polymineralic i e mixed fault gouges retain their high d value after extended healing times because grain growth is inhibited healing under nonhydrostatic conditions is more rapid than hydrostatic healing the low strain rates which were measured during nonhydrostatic healing are temperature dependent and suggest that diffusive mass transfer processes take place during deformation thus fault rocks at upper to midcrustal depth may deform by combined cataclasis and diffusive mass transfer

changes in seismic activity following the 2011 tohoku oki earthquake effects of pore fluid pressure abstract we examined the effects of pore fluid pressure on seismicity changes following the 2011 off the pacific coast of tohoku tohoku oki earthquake through the analysis of aftershocks based on the coulomb failure criterion background tectonic stress fields in northeast japan are generally characterized by e w compression after the tohoku oki earthquake as expected from decrease in the coulomb failure function shear stress 0 4 normal compressive stress seismicity in the upper crust of northeast japan decreased except some restricted regions where we observed many aftershocks with unfavourable focal mechanisms to the background stress fields by mapping the focal mechanisms of aftershocks on the 3 d mohr diagram region by region we confirmed that the aftershocks occurred on optimally oriented faults in some regions but on misoriented faults in other regions the aftershocks on optimally oriented faults indicate the increase in regional ambient fluid pressure caused by the flow of over pressurized fluid from a deep reservoir on the other hand the aftershocks on misoriented faults which cannot be attributed to coseismic stress rotation indicate the increase in fault confined fluid pressure relative to the ambient fluid pressure

damage to the shallow landers fault from the nearby hector mine earthquake crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes which allows for the relative motion between adjacent crustal blocks although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present day crustal stresses and fault motions to geological structures formed in the past fault zone damage and healing have been documented quantitatively in only a few cases we have been monitoring the healing of damage on the shallow johnson valley fault after its rupture in the 1992 magnitude 7 3 landers earthquake and here we report that this healing was interrupted in 1999 by the magnitude 7 1 hector mine earthquake rupture which occurred 20 30 km away the hector mine earthquake both strongly shook and permanently strained the johnson valley fault adding damage discernible as a temporary reversal of the healing process the fault has since resumed the trend of strength recovery that it showed after the landers earthquake these observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking and may be involved in friction reduction during faulting

rupture process of the 2011 tohoku oki earthquake and absolute elastic strain release 1 on 11 march 2011 the tohoku oki earthquake in eastern japan and the devastating tsunami that followed it caused severe damage and numerous deaths to clarify the rupture process of the earthquake we inverted teleseismic p wave data applying a novel formulation that takes into account the uncertainty of green s function which has been a major error source in waveform inversion the estimated seismic moment is 5 7 1022 nm mw 9 1 associated with a fault rupture 440 km long and 180 km wide along the plate interface the source process is characterized by asymmetric bilateral rupture propagation but we also found continuous slips up dip from the hypocenter which led to a large maximum slip 50 m long slip duration 90 s and a large stress drop 20 mpa the long slip duration large stress drop extensional normal faulting aftershocks in a previously compressional stress regime and low angle normal slips at approximately the depth of the plate interface suggest that the earthquake released roughly all of the accumulated elastic strain on the plate interface owing to exceptional weakening of the fault the stress accumulated on the plate interface was about 20 mpa near the trench and 0 10 mpa in the down dip source region

quaternary sea level history and the origin of the northernmost coastal aeolianites in the americas channel islands national park california usa abstract along most of the pacific coast of north america sand dunes are dominantly silicate rich on the california channel islands however dunes are carbonate rich due to high productivity offshore and a lack of dilution by silicate minerals older sands on the channel islands contain enough carbonate to be cemented into aeolianite several generations of carbonate aeolianites are present on the california channel islands and represent the northernmost quaternary coastal aeolianites on the pacific coast of north america the oldest aeolianites on the islands may date to the early pleistocene and thus far have only been found on santa cruz island aeolianites with well developed soils are found on both san miguel island and santa rosa island and likely date to the middle pleistocene the youngest and best dated aeolianites are located on san miguel island and santa rosa island these sediments were deposited during the late pleistocene following the emergence of marine terraces that date to the last interglacial complex 120 000 yr to 80 000 yr based on radiocarbon and luminescence dating the ages of these units correspond in time with marine isotope stages mis 4 3 and 2 sea level was significantly lower than present during all three time periods reconstruction of insular paleogeography indicates that large areas to the north and northwest of the islands would have been exposed at these times providing a ready source of carbonate rich skeletal sands these findings differ from a previously held concept that carbonate aeolianites are dominantly an interglacial phenomenon forming during high stands of sea in contrast our results are consistent with the findings of other investigators of the past decade who have reported evidence of glacial age and interstadial age aeolianites on coastlines of australia and south africa they are also consistent with observations made by darwin regarding the origin of aeolianites on the island of st helena in the south atlantic ocean more than a century and a half ago

long term evolution of intraplate seismicity in stress shadows after a megathrust abstract megathrusts produce large permanent lithospheric displacements as well as strong transient ground shaking up to regional distances the influence of the 2011 m9 0 tohoku oki earthquake on the seismicity in stable intraplate regions around the korean peninsula is investigated the differential lateral displacements by the megathrust build transient radial tension field over the backarc lithosphere constructing stress shadows for all types of faults in optimal orientation over the wide backarc region including the southern korean peninsula a characteristic seismic velocity decrease after the megathrust supports the medium relaxation the number of earthquakes with magnitudes greater than or equal to 2 5 was increased by 61 after the megathrust the earthquakes occurred episodically a series of unusual earthquake swarms and a temporal cluster of moderate size earthquakes were observed in the yellow sea region the significant seismicity increase since the megathrust may be due to the fluid diffusion during the transient tension field and the pore fluid pressure increase during the ambient compressional stress field recovery by tectonic loading the induced seismicity may continue until the ambient stress field is fully recovered

remote triggered seismicity caused by the 2011 m9 0 tohoku oki japan earthquake 1 seismic waves from large earthquakes have been shown to trigger seismicity large distances from a mainshock and this is termed remotely or dynamically triggered seismicity we performed a global search for seismicity potentially triggered by the seismic waves from the 2011 m9 0 tohoku oki japan earthquake using seismograms from global seismic networks and an event catalog we search for earthquakes and tremors instantaneously triggered during the passing of the seismic waves as well as for statistically significant changes in local and global seismic rates after the passing of the waves for earthquakes we find potential cases of instantaneous triggering in the united states russia china ecuador and mexico while for tremors we find evidence for triggering in taiwan armenia cuba and the united states in addition we observed a potential case of delayed triggering of larger magnitude earthquakes including a m5 2 in baja california mexico

propagation of an earthquake triggering front from the 2011 tohoku oki earthquake 1 increases in seismicity have been widely observed at varying distances from the source area following large earthquakes the increased number of earthquakes are usually called aftershocks if the area is within a rupture length of the mainshock and called remotely triggered events if they are well beyond that distance these earthquakes can be explained as being induced by static and or dynamic stress changes due to the mainshock however clear observations of dynamic triggering have been inadequate to differentiate between the two mechanisms this study shows that early post seismic events triggered by the 2011 mw 9 0 tohoku oki earthquake systematically propagated over japan in a southwestern direction associated with the strong seismic waves from the source the propagation front was consistent with the arrivals of large amplitude surface waves traveling at 3 1 to 3 3 km s and extending to a distance of 1 350 km there were no observations of triggered earthquakes in the northern direction dynamic stress changes toward the north were comparable to or smaller than those necessary for triggering in the southwestern direction static stress changes were one to two orders smaller than dynamic stress changes at remote distance indicating that static stress was not the main mechanism of the triggering furthermore the dynamic stress strain changes play an important role for remote triggering if the value is more than 500 kpa in stress or 10 6 in strain

ambient noise based monitoring of seismic velocity changes associated with the 2014 mw 6 0 south napa earthquake we perform an ambient noise based monitoring to explore temporal variations of crustal seismic velocities before during and after the 24 august 2014 mw 6 0 south napa earthquake a velocity drop of about 0 08 is observed immediately after the south napa earthquake spatial variability of the velocity reduction is most correlated with the pattern of the peak ground velocity of the south napa mainshock which suggests that fracture damage in rocks induced by the dynamic strain is likely responsible for the coseismic velocity change about 50 of the velocity reduction is recovered at the first 50 days following the south napa mainshock this postseismic velocity recovery may suggest a healing process of damaged rocks

mapping pressurized volcanic fluids from induced crustal seismic velocity drops volcanic eruptions are caused by the release of pressure that has accumulated due to hot volcanic fluids at depth here we show that the extent of the regions affected by pressurized fluids can be imaged through the measurement of their response to transient stress perturbations we used records of seismic noise from the japanese hi net seismic network to measure the crustal seismic velocity changes below volcanic regions caused by the 2011 moment magnitude m w 9 0 tohoku oki earthquake we interpret coseismic crustal seismic velocity reductions as related to the mechanical weakening of the pressurized crust by the dynamic stress associated with the seismic waves we suggest therefore that mapping seismic velocity susceptibility to dynamic stress perturbations can be used for the imaging and characterization of volcanic systems

trench normal variation in observed seafloor displacements associated with the 2011 tohoku oki earthquake 1 seafloor geodetic measurements using the gps acoustic technique just above the rupture area of the 2011 tohoku oki earthquake revealed strong trench normal variation in the horizontal crustal displacement associated with the earthquake one set of observations at a site only 50 km away from the trench yielded the most trench ward data ever reported with an accuracy better than 1 m it showed a coseismic displacement of up to 31 m whereas another set of observations 100 km away from the trench showed a 15 m displacement these horizontal data as well as vertical data from pressure gauges and tsunami observations strongly indicate that the seismic slip reached the trench they are also likely to contribute quantitatively to further investigations that reveal the coseismic behavior of the frontal wedge of the overriding plate in the source region along the japan trench

afterslip and viscoelastic relaxation following the 2011 tohoku oki earthquake mw9 0 inferred from inland gps and seafloor gps acoustic data we simultaneously estimate 2 5 years of afterslip and viscoelastic relaxation as well as coseismic slip for the 2011 tohoku oki earthquake displacements at inland gps and seafloor gps acoustic stations are inverted using viscoelastic green s functions for a model with an upper elastic layer and lower viscoelastic substrate the result shows that afterslip is isolated from the rupture area and possibly asperities of historical earthquakes and has almost decayed by 10 september 2013 2 5 years after the main shock the inversion result also suggests that observed landward postseismic displacements at the seafloor gps acoustic stations are caused by the viscoelastic relaxation whereas trenchward displacements at inland stations are mainly an elastic response to afterslip

azimuthal seismic anisotropy in the upper crust of the japanese islands induced by the 2011 tohoku oki megathrust earthquake

monitoring seismic velocity change caused by the 2011 tohoku oki earthquake using ambient noise records 1 we estimated the changes in seismic velocity in the southern tohoku district of japan during the six month period centered on the 11 march 2011 tohoku oki earthquake using scattered waves retrieved by autocorrelation of ambient seismic noise the estimated velocity decrease after the earthquake and after two large aftershocks in the study area was as great as 1 5 in the area nearest to the mainshock the velocity changes displayed gradual healing the spatial distribution of the velocity change showed a correlation with both the changes in static strain derived from gps records and the peak particle velocity experienced during the three earthquakes derived from strong motion records therefore our results show that velocity changes possibly contain information from deep in the crust bearing on coseismic stress release in addition to shallower effects due to strong ground motion

time lapse change in anisotropy in japan s near surface after the 2011 tohoku oki earthquake 1 we apply seismic interferometry to strong motion records to detect the near surface i e an upper few hundred meters deep change in anisotropy caused by the mw9 0 tohoku oki earthquake on 11 march 2011 we show that the earthquake increased the difference between fast and slow shear wave velocities arising from shear wave splitting in most parts of northeastern japan but it did not significantly change fast shear wave polarization directions in the near surface through monitoring of anisotropy and shear wave velocity we find that the changes in anisotropy and velocity partially recover with time they are however still different from the pre event values after nine months the comparison of the spatial distribution between changes in anisotropy and velocity indicates the changes in anisotropy and velocity are generally correlated especially in the northeastern honshu the main island in japan the change in the largest principal stress direction weakly correlates with the change in anisotropy

imaging laterally varying regional heterogeneities from seismic coda using a source array analysis abstract seismic coda is composed of scattered waves originated from various kinds of heterogeneities whose locations are not confined in the great circle ray path direction between the source and receiver we devise a technique to image laterally varying regional heterogeneities using single station seismograms for clustered events we analyze regional seismic records of the borovoye seismic station for the balapan nuclear explosions in kazakhstan scattered waves that arrive coherently over the source arrays are extracted by directional beamformings the locations of scatterers responsible for the scattered waves are mapped from the beamforming direction travel time and slowness equivalently phase velocity the illuminated locations of heterogeneities are highly correlated with the structural variations in surface topography crustal thickness and sedimentary thickness the influence of the structural variations on seismic waves is quantified in terms of scattering intensity and quality factor the scattering properties are observed to vary with phase due to the difference in frequency content and phase velocity the proposed technique appears to be useful for a study of active tectonic regions with limited monitoring stations

dynamic lithospheric response to megathrust and precursory seismicity features of megathrust abstract temporal variations of seismic properties in inner trench regions before and after three megathrusts with magnitudes greater than or equal to 8 8 since 2004 are investigated to understand the nature of megathrust earthquakes the seismicity was increased significantly and the fault type compositions changed after megathrusts the seismicity displays characteristic fault type dependent distribution on rupture planes the postseismic thrustal events were populated around the down dip rupture margins due to the concentration of shear stress after coseismic ruptures normal faulting earthquakes increased after the megathrusts particularly in shallow depth regions with large slips which may be associated with lithospheric rebound and development of splay faults the earthquake occurrence rate b value displays a characteristic slip dependent feature the earthquake occurrence rates decrease with the slip amount of forthcoming megathrust which may be caused by continuous accumulation of plate driven stress and tectonic loading around the future rupture planes on the slab surface the slip dependency of earthquake occurrence rates is enhanced with time until the occurrence of megathrust the level of seismicity after megathrust is inversely proportional to that before megathrust yielding comparable average seismicity over the rupture zone it was also observed that the dynamic lithospheric response is highly correlated with slip distribution on the rupture plane the tension axes of the normal faulting earthquakes for 100 days after the tohoku oki earthquake focus to an apparent pole suggesting a radial viscoelastic deformation of lithosphere the temporal changes of slip dependent b values fit well with an exponential function suggesting an exponential increase of normal stress on the locked region with time until the occurrence of the megathrust this observation suggests that the temporal variation of slip dependent b values may be useful for prediction of forthcoming megathrusts at least several tens years beforehand

volcanic subsidence triggered by the 2011 tohoku earthquake in japan the 2011 tohoku earthquake caused high levels of crustal deformation in japan analysis of satellite radar and gps data show that the earthquake caused nearby volcanic regions to subside instantaneously creating elliptical depressions that are parallel to the direction of quake induced crustal extension

fine scale crustal magnetization variations and segmentation of the east pacific rise 9 10 9 50 n sea surface scalar magnetic field measurements collected over the east pacific rise near 9 30 n and within the brunhes chron have been inverted for magnetization of the upper crust a close track spacing 1 3 km and careful corrections for sources of background noise make this data set particularly suitable for investigating fine scale crustal magnetization a well defined axial magnetization high can be traced throughout the survey area but varies considerably along axis marked changes in the amplitude of axial magnetization are often accompanied by offsets in its along axis linearity e g at 9 25 9 37 and 9 46 n on the basis of comparisons with seismic reflection data and geochemical measurements on basalts dredged from this rise area we argue that the along axis variation in the axial magnetic high is the combined result of variations in the thickness of the magnetic source layer and in the magnetization of the layer caused by variations in the iron and titanium abundances in the extrusive upper crust these two explanations are probably coupled in that a thicker source layer arises from a deeper axial magma chamber which is more prone to the production of evolved magmas rich in iron and titanium because crustal magnetization represents a time integrated signal averaged over a substantial volume of rock the observation of offsets in the linearity of the axial magnetization high implies that such features which we suggest mark boundaries between distinct magmatic systems within the axial crust are stable on timescales of at least several tens of thousands of years a distinct pattern of short wavelength 5 km magnetization anomalies parallel to the rise axis to the north of 9 25 n correlates well with short reversed subchrons within the brunhes chron although independent observations are needed to confirm this explanation the correlation offers the potential that the determination of fine scale magnetization anomalies may prove useful for investigating temporal details of crustal evolution at fast spreading ridges