noise based monitoring and imaging of aseismic transient deformation induced by the 2006 basel reservoir stimulation abstractwe have analyzed the time dependent properties of the ambient seismic wavefield between 0 1 and 8 hz to detect resolve monitor and image the deformation induced by the water injection associated with the stimulation of the 2006 deep heat mining project in the city of basel switzerland the application of passive methods allowed the detection of an aseismic transient of approximately 35 days duration that began with the onset of the reservoir stimulation peak deformation was reached some 15 days after the bleed off and after the induced seismicity ceased we resolved a significant increase in seismic velocities and a simultaneous decorrelation of the noise correlation coda waveforms the wavefield properties implied that the material response was monitored mainly in the sedimentary layer 2 5 km above the stimulated volume that was approximately 4 5 km deep we inverted the velocity change and decorrelation data to estimate the spatial distribution of the medium changes the resulting imag

evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

temporal changes of the crustal structure associated with the m6 1 earthquake on september 3 1998 and the volcanic activity of mount iwate japan we detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a m6 1 shallow earthquake phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the m6 1 earthquake and mount lwate decreased 0 3 1 0 during the three months since lager decreases of the seismic velocity were observed at the region close to the focal area and a volcanic pressure source we conclude that stress change in the upper crust is a most plausible candidate to cause the velocity changes among other candidates we further apply a coherence analysis to the observed seismic coda to locate regions where short wavelength heterogeneity temporally changed the result indicates a possibility of temporal changes of the heterogeneity beneath the volcano where a magmatic fluid intrusion is inferred

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

velocity change in the zone of a moderate mw 5 0 earthquake revealed by autocorrelations of ambient noise and by event spectra a moderate mw 5 0 earthquake occurred near mogul nevada just west of reno nevada on 26 april 2008 this mainshock was surrounded by notable foreshock and aftershock sequences due to the long foreshock sequence the area was well instrumented at the time of the mainshock we investigated the foreshock and aftershock sequences for evidence of velocity changes in the structure immediate to the hypocenter and above it in the area of observed strong ground motion using autocorrelations of the time periods with nearly continuous foreshocks and aftershocks we detected a nearly 1 negative change in velocity on recordings of station mogl which was approximately over the hypocenter of the mainshock we also observed from these recordings a shift in the spectral peaks to lower frequency following the mainshock again indicative of a velocity decrease due to the different spatial sampling of the two methods the effects could not be attributed to the same subsurface volume however both results are strong evidence for coseismic velocity changes accompanying an earthquake which is very much smaller than those for which previous velocity changes have been reported we hypothesize that these changes can be observed for even smaller earthquakes given a fortuitous placement of observing stations

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

postseismic fault healing on the rupture zone of the 1999 m 7 1 hector mine california earthquake we probed the rupture zone of the october 1999 m 7 1 hector mine earthquake using repeated near surface explosions in october 2000 and november 2001 three dense linear seismic arrays were deployed across the north and south lavic lake faults llfs that broke to the surface in the mainshock and across the bullion fault bf that experienced minor slip in that event two explosions each year were detonated in the rupture zone one on the middle and one on the south llf we found that p and s velocities of fault zone rocks increased by 0 7 1 4 and 0 5 1 0 between 2000 and 2001 respectively in contrast the velocities for p and s waves in surrounding rocks increased much less this trend indicates that the hector mine rupture zone has been healing by strengthening after the main shock most likely due to the closure of cracks that opened during the 1999 earth quake the observed fault zone strength recovery is consistent with an apparent crack density decrease of 1 5 within the rupture zone the ratio of travel time decrease for p to s waves was 0 72 consistent with partially fluid filled cracks near the fault zone this restrengthening is similar to that observed after the 1992 m 7 4 landers earthquake which occurred 25 km to the west li and vidale 2001 we also find that the velocity increase with time varies from one fault segment to another at the hector mine rupture zone we see greater changes on the llfs than on the bf and the greatest change is on the middle llf at shallow depth we tentatively conclude that greater damage was inflicted and thus greater healing is observed in regions with larger slip in the mainshock

dynamics of piton de la fournaise volcano observed by passive image interferometry with multiple references abstract activity of piton de la fournaise pdf volcano in la reunion island modifies the seismic velocities within the edifice using the 2010 and 2011 data from a network of 21 seismic stations in the vicinity of pdf changes of seismic velocities are investigated using passive image interferometry i e interferometry of seismic noise correlations as noise correlations change significantly over time in response to volcanic activity a method is presented that allows us to measure continuous long term velocity changes with high and constant accuracy by using multiple periods as reference a long term velocity increase is found that averages about 0 25 per year this trend is superimposed by short term changes that exhibit a clear connection with summit seismo tectonic earthquakes indicating the effect of volcanic activity characteristic signatures of velocity changes are identified for post eruptive periods of deflation that show an increase of velocity associated with subsidence observed by gps periods of pre eruptive inflation are characterized by decreasing velocity seismic crises can be associated with either increasing or decreasing velocity depending on whether the magma movement leads to deflation due to an eruption emptying the shallow plumbing system or to inflation caused by a non eruptive intrusion with a simple assumption about the spatial sensitivity of the measurements both processes are found to have the strongest effect in the central summit area of the volcano which also shows the strongest surface displacements during the time investigated here we do not observe a dependence of the velocity change on the location of the erupting fissures instead the distribution of changes for the three inflation periods and the two eruptions are similar indicating that the velocity changes observed here reflect the dynamics of a shallow magma reservoir rather than the effect of the eruption at the surface

s coda and rayleigh waves from a decade of repeating earthquakes reveal discordant temporal velocity changes since the 2004 sumatra earthquake temporal changes in the subsurface seismic velocity structure reflect the physical processes that modulate the properties of the media through which seismic waves propagate these processes such a

travel time monitoring with vibroseis abstract the general failure of searches for precursory seismic travel time variations associated with strike slip earthquakes in california has led to this investigation into the feasibility of using a controlled source seismic method to improve significantly the precision of travel time measurements and to investigate the nature of any detected travel time changes travel times have been measured over a period of several years at sites south of hollister california along the seismically active creeping zone of the san andreas fault using a single channel vibroseis system with real time on site data processing at a site near bear valley 7 km from the fault no variations in the travel time of a deep crustal reflection were observed that could be associated with local earthquakes however significant variations 0 5 to 2 5 msec of first arrival travel times observed near the cienega winery may have been associated with a series of nearby earthquakes and with a creep event major sources of measurement error have been identified in source variations and in rainfall induced variations in near surface properties the former limits the precision of the deep reflection measurements to about 0 05 per cent of the travel time and the first arrival measurements to about 0 1 per cent of the travel time the second effect is apparent as seasonal oscillations in travel time of as much as 15 to 20 msec and also in wavelet amplitude and waveform giving an implied travel time accuracy of about 0 2 per cent for the deep reflection measurements and about 1 per cent for first arrivals while these noise levels are disappointing they can be reduced significantly by improved field procedures ongoing experiments are testing such procedures

time lapse changes of seismic velocity in the shallow ground caused by strong ground motion shock of the 2000 western tottori earthquake japan as revealed from coda deconvolution analysis abstract a large earthquake shock often drops the seismic velocity of the shallow ground however it is not clear whether the dropped velocity recovers shortly after the earthquake shock or not the purpose of this article is to report the time lapse changes of seismic velocity in the shallow ground after the 2000 western tottori earthquake japan we deconvolve the coda record of small earthquakes registered on the ground surface by that registered at the 100 m depth in a borehole at a station that experienced a strong shock from the mainshock because coda waves are mostly composed of randomly scattered s waves deconvolution of the two coda records enables us to obtain a robust image of the ground structure assuming that the shear modulus was reduced at the depth of 0 11 m we estimate the shear modulus change in each time period after the mainshock by fitting synthetic coda deconvolution to the observed one from 1 to 16 hz as a result the shear modulus dropped to 52 of the value obtained before the mainshock a few minutes after the strong earthquake shock this caused a decrease in the s wave velocity of 30 and an increase in s wave travel time of 17 msec the shear modulus continued to recover for over 1 yr following the logarithm of the lapse time it recovered to 69 83 87 and 97 of the value obtained before the mainshock in the periods of 0 to 1 week 1 week to 1 month 1 month to 1 yr and 1 to 4 yr after the mainshock respectively

shear wave velocity changes induced by earthquakes and rainfall at the rotokawa and ngatamariki geothermal fields taupō volcanic zone new zealand

coseismic velocity change associated with the 2011 van earthquake m7 1 crustal response to a major event monitoring coseismic velocity changes is a major challenge since the earth crust has to be uniformly sampled at preseismic coseismic and postseismic stages using repeating active or natural sources here we investigate the crustal response to the 2011 van turkey earthquake using ambient noise which provides the best possible temporal resolution combined recordings from the nearest five broadband stations are analyzed for a time period of 6 months framing the main shock we observe a coseismic velocity decrease of up to 0 76 in the vicinity of the main shock in the frequency range of 0 05 0 3 hz the velocity drop is largest at close proximity to the earthquake hypocenter and decreases systematically with distance we also find a correlation between coseismic velocity decrease and the amount of coseismic slip on the rupture plane the observed velocity drop shows the drastic response of the brittle crust in response to a major earthquake

travertines in the northern hula valley israel this study describes the field and petrographic relationships of a widespread deposit of pleistocene travertines in the northern hula valley northern israel the travertines interfinger with conglomerate deposits and basaltic lava flows field relationships and radiometric dating indicate that the travertines accumulated intermittently over the past million years and their formation virtually stopped 25 000 years ago the travertines are characteristically highly porous some of the pores as well as some spar filled voids preserve the shapes of stalks and leaves the abundance of plant material suggests that photosynthesis rather than bacterial or abiogenic processes was the main mechanism which induced carbonate precipitation the reasons for travertine accumulation in the past and for the cessation of its formation today are ascribed to differences in the palaeogeographic setting in the past water flow is viewed as having been mostly sluggish and a widespread and shallow sheet of water was formed the lush vegetation combined with the relatively long residence time of the water in the area led to increased efficiency of the calcium carbonate precipitation today by contrast water flows rapidly in gorges precipitating only a small fraction of its load of dissolved calcium carbonate the conjectured change in hydrological conditions is ascribed to rejuvenation of faulting activity in the late quaternary

spatio temporal changes in seismic velocity associated with the 2000 activity of miyakejima volcano as inferred from cross correlation analyses of ambient noise abstract miyakejima volcano japan showed strong and interesting volcanic activities in 2000 with the occurrence of dike intrusions caldera formation and a large amount of gas emission to understand temporal changes of the volcanic structure associated with this activity we apply ambient noise correlation analyses to the seismic records of five seismic stations at miyakejima volcano from july 1999 to december 2002 we calculate cross correlation functions ccfs for eight available station pairs at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz and retrieve the rayleigh waves propagating with a group velocity of about 1 0 km s comparing ccfs obtained for each day with the reference ccfs that are calculated from a few months of data we estimate seismic velocity changes before and after the 2000 activity results at the two frequency bands show seismic velocity increase and decrease shallow structure at the volcanic edifice on the flanks experienced a seismic velocity increase up to 3 3 on the other hand the regions located close to the collapsed caldera show a seismic velocity decrease down to 2 3 to understand the mechanisms that may introduce these seismic velocity increase and decrease we examine several possible mechanisms of the velocity changes stress changes due to volcanic pressure sources and caldera formation and topographic changes by using the deflation sources previously determined from gps data we calculate the stresses at depths of 0 km and 1 km at the volcano the results suggest that the seismic velocity increases observed at seismic paths on the volcanic flank are explained by the compression caused by the deflation sources associated with magma activity in 2000 stress sensitivities of the seismic velocity changes are estimated to be 1 5 10 2 to 2 2 10 2 mpa 1 and 4 6 10 3 to 2 4 10 2 mpa 1 at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz respectively which are in the range of the previously reported results our three dimensional numerical calculations on seismic wave propagation show that the seismic velocity decreases observed at seismic paths crossing the collapsed caldera can be explained by topographic changes introduced by a caldera we further suggest that the seismic velocity decreases detected around the regions close to the caldera may originate from the dilated regions cause by formation of the caldera

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

cross correlation analysis of long term ambient seismic noise recordings in the caribbean netherlands to monitor the volcanoes on saba and st eustatius

seismic velocity changes in the epicentral region of the 2008 wenchuan earthquake measured from three component ambient noise correlation techniques 1 we investigate temporal changes of seismic velocity in the epicentral region of the 12 may 2008 mw 7 9 wenchuan earthquake using three component continuous waveforms recorded by a seven station small aperture array we use an ambient noise cross correlation technique to compute the empirical green s function between station pairs from august 2004 to september 2011 our results show no obvious precursory change immediately before the main shock clear coseismic reduction of seismic velocity of up to 0 2 and initial postseismic recovery followed by a long lived velocity reduction the coseismic and postseismic velocity changes are most prominent in the period band of 2 4 s approximate depth of 1 4 km and the velocity changes are smaller in other period bands the seismic velocity in the period band of 1 2 s i e top 2 km correlates well with the water level change of the zipingpu reservoir the observed temporal changes likely reflect damage and healing processes with possible permanent deformation in the upper crust associated with the wenchuan main shock

source array analysis of coda waves near the 1989 loma prieta california mainshock implications for the mechanism of coseismic velocity changes coda waves are often considered to be generated by backscattering of primary waves from randomly distributed heterogeneities in the crust however modeling and experimental work indicates that much of the coda may be due to scattering near the receiver knowing where the coda is generated is important for several reasons coda waves have been used to characterize site amplification scattering and attenuation throughout the crust and velocity changes associated with large earthquakes however a lack of knowledge of where coda waves are generated can lead to ambiguous interpretations in this study we analyze 26 s of coda waves recorded at up to 78 stations using slowness stacking on two source arrays and find that at nearly all stations regardless of distance from the source nearly all the arrivals are within one hemisphere most are strongly clustered near the direct arrival and the clusters are in general symmetrically disposed about the array axis fault plane this finding suggests that the coda consists primarily of waves scattered near the stations rather than waves scattered throughout the crust this result holds even at stations very close to the sources where the amount of coda examined is about 6 times the direct s travel time and it suggests that much of the coseismic velocity decrease associated with the 1989 loma prieta earthquake ellsworth et al 1992 occurred in the shallow crust near the stations one possible mechanism for inducing such a change is a decrease in the mean stress from the mainshock slip however on the basis of static stress modeling we find that this mechanism would produce a velocity increase in the region where velocity was observed to decrease we conclude that the velocity decrease was more likely caused by crack opening or crack coalescence due to strong shaking from the mainshock

in situ measurements of elastic wave velocity in a mine and the effects of water and stress on their variation abstract in situ wave velocity measurements were carried out in the ikuno mine of japan to detect the travel time variation of less than 0 5 s over distances of about 11 m using ultrasonic waves the measurements were conducted continuously every 90 min over an 8 day period ultrasonic waves emitted by a generator assembled from lead zirconite titanite discs were received by accelerometers with frequency responses up to 16 khz we found both long term systematic and short term periodic variations in the observed travel time and examined the correlation between the short term periodic variations and earth tides and also the relationship between the long term variations reaching 7 5 s corresponding to a velocity change of 0 3 and atmospheric pressure and rainfall the stress sensitivity of the in situ rock caused by opening and closure of dry cracks was considered insufficient to solely explain the observed semidiurnal variations reaching a range of 2 5 s 0 1 the semidiurnal variations were then found to be explainable by changes in water fraction of less than 0 5 in cracks and joints to study the physical properties of the crust it will be useful to perform such a continuous high precision observation in mines as well as between boreholes

seismological studies at parkfield iv variations in controlled source waveform parameters and their correlation with seismicity 1987 to 1995 since june 1987 at parkfield california the 10 station borehole network of three component sensors has been illuminated 52 times using a shear wave vibrator in three orientations at up to eight source points in a search for temporal changes in elastic wave p and s velocities anisotropy or attenuation the monitoring interval includes the beginning and end of a severe 3 yr drought and four earthquake sequences two of which produced the only a level alerts to date in the parkfield prediction experiment a comprehensive study of the entire data set reveals a progressive travel time advance in the coda of s waves propagating in a localized region southeast of middle mountain the anomalous wave field exhibits high apparent velocities suggesting deep penetration of the fault zone although similar changes are not seen in waveforms from repeating similar microearthquakes accompanying the changes in travel time were systematic variations in spectral content and polarization of the same segments of the wave field these variations correlate well in time and space with significant features of seismicity fault creep and water levels at parkfield a preferred mechanism for the phenomenon is changing hydrologic conditions along the affected stretch of the fault zone possibly deformation induced that perturb the shallow propagating s coda in the upper few hundred meters of section

an attempt to detect temporal variations of crustal structure in the source area of the 2006 wen an earthquake in north china abstract lei et al 2008 revealed a low velocity low v anomaly in the lower crust under the source area of the 4 july 2006 wen an earthquake m 5 1 in north china in this work we tried to investigate the temporal variations of the crustal structure by using a number of p and pmp moho reflected wave arrival times recorded by 107 digital seismic stations from earthquakes that occurred separately in 2002 2003 2004 and 2005 2006 to determine p wave velocity structures in and around the source area of the wen an earthquake in different periods our results show that tomographic images inferred from the data sets in different years are all dominated by a low v anomaly in the lower crust under the wen an source area however there exist some differences in the p wave velocity image in the wen an source area inferred from the p and p pmp data sets suggesting that the pmp data have improved the tomographic images in the middle and lower crust but the results from the 2005 2006 p and p pmp data sets all show a relatively lager increase of the low v anomaly under the wen an source area in the amplitude and extent as compared with those from the 2003 and 2004 p and p pmp data sets incorporating the previous results if this low v anomaly may indicate the existence of fluids then our results suggest that the occurrence of the wen an earthquake is not only related to the long term influence of fluids that decrease the effective normal stress on the fault plane but also closely associated with the drastic increase of such influence however this study is just an experimental work and the results are still preliminary because the resolution scale of the present tomographic model is much larger than the wen an source area and our extensive tests show that different samplings of seismic rays from different data sets have affected the details of the tomographic images suggesting that the present sparse data coverage can hardly detect reliably any temporal variations of the velocity anomalies in the wen an source area in future studies it is necessary to improve the resolution of crustal tomography to the size of the rupture zone and utilize identical seismic ray paths from the same pairs of sources and receivers in order to detect any temporal variations of the crust structure in the source area of a large earthquake

temporal change of phase velocity beneath mt asama japan inferred from coda wave interferometry 1 we estimated the temporal changes of phase velocity of rayleigh waves extracted from cross correlations of s coda waves recorded at 12 stations around mt asama japan first we extracted a rayleigh wave by taking cross correlations of s coda waves for 315 regional earthquakes between october 2005 and february 2009 the dispersion curve of the rayleigh wave was measured and compared with the one extracted from 18 days of ambient seismic noise we found that both dispersion curves are consistent with each other demonstrating the dominance of the fundamental rayleigh waves we then divided the entire time period into sub periods each of which consists of 80 earthquakes to measure the temporal changes at frequencies between 0 3 and 0 6 hz the result shows the reduction of phase velocity by 1 5 and the subsequent recovery before the eruption of mt asama in 2008

coda wave interferometry for estimating nonlinear behavior in seismic velocity in coda wave interferometry one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time with this technique we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions this technique can be used in warning mode to detect the presence of temporal changes in the medium or in diagnostic mode where the temporal change in the medium is quantified

coseismic and postseismic velocity changes measured by repeating earthquakes 1 repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the earth s crust since source and path effects are common to all earthquakes in a repeating earthquake sequence multiplet differences in their waveforms can be attributed to changes in the characteristics of the medium we have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 loma prieta and 1984 morgan hill california earthquakes postmain shock events reveal delays of phases in the early s wave coda of as much as 0 2 s relative to premain shock events the delay amounts to a path averaged coseismic velocity decrease of about 1 5 for p waves and 3 5 for s waves since most of the multiplets are aftershocks and follow omori s law we have excellent temporal sampling in the immediate postmain shock period we find that the amplitude of the velocity decrease decays logarithmically in time following the main shock in some cases it returns to the premain shock values while in others it does not similar results are obtained for the morgan hill main shock because the fractional change in s wave velocity is greater than the fractional change in p wave velocity it suggests that the opening or connection of fluid filled fractures is the underlying cause the magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change our results suggest that the changes are predominantly near the stations and shallow but we cannot exclude the possibility that changes occur at greater depth as well if the variations are shallow we may be detecting the lingering effects of nonlinearity during main shock strong ground motion if the variations are deep it suggests that pore pressures at seismogenic depths are high which would likely play a key role in the earthquake process

time lapse monitoring of seismic velocity associated with 2011 shinmoe dake eruption using seismic interferometry an extended kalman filter approach seismic interferometry is a powerful tool to monitor the seismic velocity change associated with volcanic eruptions for the monitoring changes in seismic velocity with environmental origins such

depth of rock damage from strong seismic ground motions near the 2004 parkfield mainshock changes in the s p delay of repeating earthquakes near parkfield california after strong shaking in the 2004 mainshock occur at surface stations but not at borehole stations this result indicates that rock damage occurs mainly in the upper few tens of meters in fractured rock with a low seismic velocity in addition changes in coda primary delay are comparable to changes in s p delay this observation along with the lack of s p delay changes at borehole stations yields a simple model for the coda direct waves pass through the shallow damaged layer once coda waves scatter or refract in the deep undamaged subsurface and hence also pass through the subsurface just once the coda however contains some brief reverberations within the shallow subsurface where the additional path length scales to the thickness of the shallow layer the coda primary delay measured by correlation includes both the zero delay change of direct coda and the increased delay of reverberating coda published changes in rayleigh wave group velocity before and after the parkfield mainshock are compatible with their main cause being from the shallow velocity changes inferred from repeating earthquakes but have little spatial resolution stacking of earthquake seismograms resolved surface reflections at one shallow 63 m deep borehole station ghib a change in p wave travel time body of 8 msec was marginally resolved and is compatible with the repeating earthquake s p delay changes autocorrelation passive seismology resolved 32 msec p wave reverberation delay change above the 251 m deep borehole station ccrb

seismic evidence for rock damage and healing on the san andreas fault associated with the 2004 m 6 0 parkfield earthquake we deployed a dense linear array of 45 seismometers across and along the san andreas fault near parkfield a week after the m 6 0 parkfield earthquake on 28 september 2004 to record fault zone seismic waves generated by aftershocks and explosions seismic stations and explosions were co sited with our previous exper iment conducted in 2002 the data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 m 6 0 mainshock show 1 0 1 5 decreases in seismic wave velocity within an 200 m wide zone along the fault strike and smaller changes 0 2 0 5 beyond this zone most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 m 6 0 earthquake the width of the damage zone characterized by larger velocity changes is consistent with the low velocity waveguide model on the san andreas fault near parkfield that we derived from fault zone trapped waves li et al 2004 the damage zone is not symmetric but extends farther on the southwest side of the main fault trace waveform cross correlations for repeated aftershocks in 21 clusters with a total of 130 events located at different depths and distances from the array site show 0 7 1 1 increases in s wave velocity within the fault zone in 3 months starting a week after the earthquake the velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time most likely due to the closure of cracks opened during the mainshock we estimate that the net decrease in seismic velocities within the fault zone was at least 2 5 caused by the 2004 m 6 0 parkfield earthquake the healing rate was largest in the earlier stage of the postmainshock healing process the magnitude of fault healing varies along the rupture zone being slightly larger for the healing beneath middle mountain correlating well with an area of large mapped slip the fault healing is most promi nent at depths above 7 km

seismic velocity reductions caused by the 2003 tokachi oki earthquake 1 we use four repeating earthquake sequences located near hokkaido to identify velocity changes caused by the mw 8 0 2003 tokachi oki earthquake using a moving window cross correlation technique we identify delays in the arrival time of seismic waves that accumulate linearly with time into the seismogram this behavior is indicative of multiple scattering within a medium where the seismic velocity has been reduced for all of our earthquake receiver geometries we find evidence of significant velocity reductions close to the receiver the correlation of the size of the velocity reductions with both strong shaking and site characteristics suggests that these velocity reductions are caused by damage to near surface materials created by nonlinear strong ground motion for earthquake receiver geometries where the seismic waves cross the tokachi oki rupture zone we identify particularly large reductions in velocity as a result of the earthquake for these geometries we believe that the rupture zone of the tokachi oki earthquake or the shallow crust above it represents a second region where seismic velocities are reduced as a result of the main shock

a comparison of methods to estimate seismic phase delays numerical examples for coda wave interferometry time shift estimation between arrivals in two seismic traces before and after a velocity perturbation is a crucial step in many seismic methods the accuracy of the estimated velocity perturbation location and amplitude depend on this time shift windowed cross correlation and trace stretching are two techniques commonly used to estimate local time shifts in seismic signals in the work presented here we implement dynamic time warping dtw to estimate the warping function a vector of local time shifts that globally minimizes the misfit between two seismic traces we compare all three methods using acoustic numerical experiments we show that dtw is comparable to or better than the other two methods when the velocity perturbation is homogeneous and the signal to noise ratio is high when the signal to noise ratio is low we find that dtw and windowed cross correlation are more accurate than the stretching method finally we show that the dtw algorithm has good time resolution when identifying small differences in the seismic traces for a model with an isolated velocity perturbation these results impact current methods that utilize not only time shifts between multiply scattered waves but also amplitude and decoherence measurements

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

coseismic velocity change in and around the focal region of the 2008 iwate miyagi nairiku earthquake 1 the 2008 m7 2 iwate miyagi nairiku earthquake occurred in ne japan where dense seismic networks exist using the data from these networks we determined the coseismic seismic velocity change associated with this earthquake by two different methods ambient noise interferometry and vertical array interferometry of coda wave the purpose of this article is to reveal the spatial distribution and the cause of the velocity change by integrating these two approaches ambient noise interferometry revealed a coseismic velocity decrease of rayleigh wave by 0 1 0 5 at 0 25 0 5 hz we also estimated the spatial distribution of the velocity change by a tomographic inversion the velocity decrease was distributed in and around the focal area in the second method we applied cross correlation analysis to coda waves observed by the kik net vertical array we detected a shear velocity decrease of approximately 5 in shallow layers up to a few hundred meters depth quantitative comparison of the two results reveals that the 5 shear velocity decrease in shallow layers can explain the 0 1 0 5 decrease of rayleigh wave velocity the distribution of the velocity decrease is similar to that of the strong ground motion and the static stress change however a significant velocity increase is not observed in the compression area where a velocity increase is expected based on the observations we consider that the primary factor affecting the velocity change is damage in shallow layers due to strong motion the effect of the static stress change might be masked by the larger effect of the strong motion

observations of changes in waveform character induced by the 1999 m w7 6 chi chi earthquake 1 we observe changes in the waveforms of repeating earthquakes in eastern taiwan following the 1999 mw7 6 chi chi earthquake while their recurrence intervals appear to be unaffected there is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the chi chi event these changes are limited to stations in and paths that cross the 70 100 km region surrounding the chi chi source area the area where seismic intensity and co seismic surface displacements were largest this suggests that damage at the near surface is responsible for the observed waveform changes delays are largest in the late s wave coda reaching approximately 120 ms this corresponds to a path averagedswave velocity reduction of approximately 1 there is also evidence that damage in the fault zone caused changes in waveform character at sites in the footwall where source receiver paths propagate either along or across the rupture the reduction in waveform similarity persists through the most recent repeating event in our study november 15 2007 indicating that the subsurface damage induced by the chi chi earthquake did not fully heal within the first 8 years following the chi chi earthquake

depth extent of damage zones around the central calaveras fault from waveform analysis of repeating earthquakes summary we systematically investigate spatial variations of temporal changes and depth extent of damage zones along the calaveras fault that ruptured during the 1984 morgan hill earthquake by the waveform analysis of 333 sets of repeating earthquakes we use a sliding window waveform cross correlation technique to measure traveltime changes in waveforms generated by each repeating cluster we find clear traveltime delays in the s and early s coda waves for events immediately after the morgan hill main shock the amplitudes of the time delays decrease logarithmically with time since the main shock indicating a time dependent recovery healing process following the abrupt coseismic temporal changes the largest temporal changes are observed at station cco that is the closest to the rupture zone of the morgan hill main shock the time delays at this station are larger for clusters in the top 6 km and decrease systematically at larger depth in comparison the time delays observed at other five stations are much smaller and do not show clear relationship with hypocentral depth we suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near surface layers while the temporal changes at station cco are likely associated with the damage zone around the calaveras fault that is well developed in the top few kilometres of the upper crust our results are consistent with the inference of a widespread damage and non linearity in the near surface layers associated with strong ground motions of nearby large earthquakes and localized damages and flower type structures around active faults based on previous studies of fault zone structures and recent 3 d numerical simulations

nonlinear soil response a reality geotechnical models consistently indicate that the stress strain relationship of soils is nonlinear and hysteretic especially at shear strains larger than 10 5 to 10 4 nonlinear effects such as an increase in damping and reduction in shear wave velocity as excitation strength increases are commonly recognized in the dynamic loading of soils on the other hand these effects are usually ignored in seismological models of ground motion prediction because of the lack of compelling corroborative evidence from strong motion observations the situation is being changed by recently obtained data explicit evidence of strong motion deamplification accompanied by changes in resonant frequencies are found in the data from the 1985 michoacan mexico and the 1989 loma prieta california earthquakes the events recorded by the vertical and surface accelerograph arrays in taiwan as well as a number of other events throughout the world evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of 100 to 200 gal nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils the findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong motion seismology

estimating near surface shear wave velocities in japan by applying seismic interferometry to kik net data 1 we estimate shear wave velocities in the shallow subsurface throughout japan by applying seismic interferometry to the data recorded with kik net a strong motion network in japan each kik net station has two receivers one receiver on the surface and the other in a borehole by using seismic interferometry we extract the shear wave that propagates between these two receivers applying this method to earthquake recorded data at all kik net stations from 2000 to 2010 and measuring the arrival time of these shear waves we analyze monthly and annual averages of the near surface shear wave velocity all over japan shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data the estimated shear wave velocities of each year are stable for the niigata region we observe a velocity reduction caused by major earthquakes for stations on soft rock the measured shear wave velocity varies with the seasons and we show negative correlation between the shear wave velocities and precipitation we also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization this allows us to estimate the polarization with the fast shear wave velocity throughout japan for the data recorded at the stations built on hard rock sites the fast shear wave polarization directions correlate with the direction of the plate motion

temporal changes of seismic velocity associated with the 2006 mw 6 1 taitung earthquake in an arc continent collision suture zone 1 to detect temporal changes of elastic properties associated with the 2006 mw 6 1 taitung earthquake in southeast taiwan whereby the collision between the luzon arc riding on the philippine sea plate and the asian margin is taking place we construct the green s functions from auto and cross correlation function acf and ccf of continuous ambient noise between stations time lapse changes in the retrieved coda arrivals are estimated for monitoring spatiotemporal variations of seismic velocities around the ruptured fault zones to the south of the main shock epicenter where the earthquake may have ruptured along two perpendicularly intersecting fault planes resulting in intense coseismic slip and widely dispersed aftershocks the acfs 2 8 hz at nearby stations reveal a large sudden velocity drop of over 1 after the event occurrence the ccfs 0 1 0 9 hz also show moderate reduction about 0 1 for the interstation paths traveling through the southeast quadrant of the focal sphere which has experienced the maximum peak ground acceleration and dilatational strain change the intense seismic shaking combined with the rupture induced damage near the junction of the two faults is the most plausible cause for such localized but significant coseismic velocity reduction the response of groundwater levels to precipitation is in phase correlated with velocity variations over 3 years of investigation unlike the postseismic surface displacements gradually relaxed the velocity remained slow until october 2006 due to the typhoon induced heavy rains between may and september

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

seismic velocity changes caused by the earth tide ambient noise correlation analyses of small array data we examine seismic velocity changes due to the earth tide by conducting cross correlation function ccf analyses of ambient seismic noise recorded at a small array composed of seven seismometers in northeastern japan we calculate ccfs for the dilatational and contractional episodes that are predicted from theoretical tidal volumetric strains ccfs of the two episodes are highly correlated but tiny differences are found in their phases the phase differences are explained by seismic velocity changes of 0 19 0 06 at 1 2 hz which are interpreted to be caused by opening closure of cracks or pores in the shallow subsurface due to the tidal strain strain sensitivities of the seismic velocity changes are estimated to be 6 9 104 strain 1 which are almost consistent with those reported in previous studies using artificial sources

seasonal variations of seismic velocities in the san jacinto fault area observed with ambient seismic noise s u m m a r y we observe seasonal seismic wave speed changes dv v in the san jacinto fault area and investigate several likely source mechanisms velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations we study the interrelation between dv v records frequency dependent seismic noise properties and nearby environmental data of wind speed rain ground water level barometric pressure and atmospheric temperature the results indicate peak to peak seasonal velocity variations of 0 2 per cent in the 0 5 2 hz frequency range likely associated with genuine changes of rock properties rather than changes in the noise field phase measurements between dv v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations the other considered environmental effects produce secondary variations that are superimposed on the thermal based changes more detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth

clock errors in land and ocean bottom seismograms high accuracy estimates from multiple component noise cross correlations many applications in seismology rely on the accurate absolute timing of seismograms however both seismological land stations and ocean bottom seismometers obss can be affected by clock errors which cause the absolute timing of seismograms to deviate from a highly accurate reference time signal usually provided by gps satellites timing problems can occur in land stations when synchronization with a gps signal is temporarily or permanently lost this can give rise to complicated time dependent clock drifts relative to gps time due to varying environmental conditions seismometers at the ocean bottom cannot receive gps satellite signals but operate in more stable ambient conditions than land stations the standard protocol is to synchronize an obs with a gps signal immediately before deployment and after recovery the measured timing deviation called skew is assumed to have accumulated linearly over the deployment interval an assumption that is plausible but usually not verifiable in recent years cross correlations of ambient microseismic noise have been put to use for correcting timing errors but have been limited to interstation distances of at most a few tens of kilometres without reducing the temporal resolution we apply noise cross correlations to the evaluation of clock errors in four broad band land stations and 53 wideband and broad band obss which were installed on and around the island of la reunion in the western indian ocean during the rhum rum reunion hotspot and upper mantle reunions unterer mantel experiment we correlate all three seismic components plus a hydrophone channel in obs stations daily cross correlation functions are derived for intermediate distances similar to 20 km for land to land station pairs stable 10 d stacks are obtained for very large interstation distances up to 300 km for land to obs and obs to obs configurations averaging over multiple station pairs and up to 16 component pairs per station improves the accuracy of the method by a factor of four compared to the single channel approaches of prior studies the timing accuracy of our method is estimated to be similar to 20 ms standard deviation or one sample at a sampling rate of 50 hz in land stations nonlinear clock drifts and clock jumps of up to 6 min are detected and successfully corrected for 52 out of 53 obss we successfully obtain drift functions over time which validate the common assumption of linear clock drift skew values that were available for 29 of these obss are consistent with our independent estimates within their observational error bars for 23 obss that lacked skew measurements linear obs clock drifts range between 0 2 and 8 8 ms d 1 in addition to linear drift three obss are affected by clock jumps of similar to 1 s probably indicating a missing sample problem that would otherwise have gone undetected thus we demonstrate the routine feasibility of high accuracy clock corrections in land and obss over a wide range of interstation distances

sudden drop of seismic velocity after the 2004 mw 6 6 mid niigata earthquake japan observed with passive image interferometry 1 passive image interferometry pii is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the earth first the elastic green s tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period second the constructed seismograms of different time periods are treated as earthquake multiplets and small time shifts in their coda are used to invert a relative change in mean shear wave velocity when applied to the source region of the 2004 mw 6 6 mid niigata earthquake japan centroid depth 5 km we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid niigata earthquake using noise in two frequency bands 0 1 0 5 hz and 2 8 hz we find similar amplitudes for the velocity drop which indicates that changes are not restricted to the shallow subsurface possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake a nonlinear site response in the shallow subsurface layer due to strong ground motion or structural weakening due to the creation of new fractures in the source area of the earthquake

assessment of resolution and accuracy of the moving window cross spectral technique for monitoring crustal temporal variations using ambient seismic noise summary temporal variations in the elastic behaviour of the earth s crust can be monitored through the analysis of the earth s seismic response and its evolution with time this kind of analysis is particularly interesting when combined with the reconstruction of seismic green s functions from the cross correlation of ambient seismic noise which circumvents the limitations imposed by a dependence on the occurrence of seismic events in fact because seismic noise is recorded continuously and does not depend on earthquake sources these cross correlation functions can be considered analogously to records from continuously repeating doublet sources placed at each station and can be used to extract observations of variations in seismic velocities these variations however are typically very small of the order of 0 1 per cent such accuracy can be only achieved through the analysis of the full reconstructed waveforms including later scattered arrivals we focus on the method known as moving window cross spectral analysis that has the advantage of operating in the frequency domain where the bandwidth of coherent signal in the correlation function can be clearly defined we investigate the sensitivity of this method by applying it to microseismic noise cross correlations which have been perturbed by small synthetic velocity variations and which have been randomly contaminated we propose threshold signal to noise ratios above which these perturbations can be reliably observed such values are a proxy for cross correlation convergence and so can be used as a guideline when determining the length of microseismic noise records that are required before they can be used for monitoring with the moving window cross spectral technique

temporal variations of seismic velocity at paradox valley colorado using passive image interferometry in our recent article ugalde et al 2014 we applied the passive image interferometry technique of sens schonfelder and wegler 2006 to monitor seismic velocity changes in paradox valley southwestern colorado for this purpose we computed the autocorrelation

strong seismic shaking of randomly prestressed brittle rocks rock damage and nonlinear attenuation strong seismic waves produce frequent episodes of transient oscillating dynamic stress in the shallow subsurface within seismically active areas dynamic stress drives slip on a network of preexisting small fracture planes and leaves residual stresses once shaking ceases the residual stresses act as prestresses during the next strong earthquake local failure occurs on fractures at low dynamic stresses where the prestresses are favorably aligned while most of the bulk rock remains intact hence nonlinear attenuation of strong seismic waves commences at dynamic stresses well below those for pervasive coulomb failure consideration of the fractal distribution of prestresses indicates that nonlinear attenuation increases rapidly with dynamic stress vertically propagating shear waves provide simple scaling relationships for quick application 1 nonlinear attenuation is concentrated near the quarter wavelength depth 2 the coulomb stress ratio dynamic lithostatic stress above the quarter wavelength depth scales with the sustained dynamic acceleration in g s rate and state friction thus provides estimates of the maximum sustained acceleration from coulomb stress ratio at failure 1 for regolith that has repeatedly failed 1 5 for shallow intact sandstone and 2 for shallow intact granite a potentially observable effect occurs with shallow brittle regolith on hillslopes where gravity causes net downslope creep during strong shaking analogous crack network behavior with prestress occurs in the damage zone surrounding the rupture zone of major faults pulverized rock forms at shallow depths when rock is repeatedly deformed in strong shaking to just beyond the elastic limit but strain does not localize into a crack network

temporal change in site response caused by earthquake strong motion as revealed from coda spectral ratio measurement 1 the spectral ratios of coda waves of local earthquakes have been often used as measures of relative amplification factors of different sites applying this method to coda waves registered by seismometers installed on the surface and at the bottom of a borehole we succeeded in stably measuring the temporal change in site response associated with the occurrence of a large earthquake strong motion a remarkable drop of coda spectral ratio and a shift of the peak frequency were observed during strong shake at two sites by the 2000 western tottori earthquake and at a site by the 2003 tokachi oki earthquake in japan the reduction of the peak frequency reached 30 70 at all the sites after that the peak frequency logarithmically recovered to the value before the strong motions for a few years at two sites whereas the other one quickly recovered in a few tens of minutes

nonlinear strong ground motion in the ml 5 4 chittenden earthquake evidence that preexisting damage increases susceptibility to further damage 1 we use 55 repeating earthquake sequences located near the 1989 mw 6 9 loma prieta earthquake to identify time dependent velocity changes in the shallow crust in addition to large delays caused by the loma prieta mainshock the ml 5 4 chittenden earthquake an aftershock of loma prieta caused direct s wave delays of up to 6ms and s coda delays exceeding 15ms we attribute the delays to cracks formed or opened during the strong shaking of the chittenden earthquake the same mechanism believed responsible for the delays observed following loma prieta the magnitude of the delays caused by chittenden strongly correlate with those caused by loma prieta this suggests that rocks recently damaged by nonlinear strong ground motion are particularly susceptible to further damage until they are completely healed therefore we expect that the onset of nonlinearity will occur at substantially lower ground motion for large aftershocks than would otherwise be anticipated

generalised receiver functions and seismic interferometry abstract classical seismological receiver functions are correlational or deconvolutional combinations of vertical and horizontal component seismometer recordings of earthquake waves that focus information on near receiver subsurface earth structure and properties we show that seismic interferometry can be thought of as a generalisation of receiver functions analysis to cases where recordings at pairs of receivers are considered simultaneously and where either the same or different component recordings are combined further seismic interferometry uses any of deconvolution convolution and cross correlation and energy from either impulsive or random noise sources we show both how receiver functions can logically be extended to a new convolutional form and that the now little used correlational form of receiver functions contains more intuitive information than previously realised seismic interferometry has provided other extraordinary extensions to seismologists arsenal passive noise recordings can be converted into seismograms from virtual imagined earthquakes that in turn can be used to image the real earth active sources e g earthquakes or man made sources can be redatumed into new virtual sources elsewhere or can be converted into virtual sensors seismometers that record seismograms from other real earthquakes man made sources or noise sources that occur either in the future or in the past and the ability to construct virtual sources and sensors at desired times and locations rather than having to wait for earthquake sources that occur at uncontrollable locations promises more repeatable monitoring of changes in earth subsurface properties over time indeed so called coda wave interferometry offers unprecedented accuracy in detecting such changes finally existing theoretical extensions to other regimes such as electromagnetic electrokinetic and diffusive energy propagation may lead to future revolutions in other domains of science

coseismic and postseismic elastic wave velocity variations caused by the 2008 iwate miyagi nairiku earthquake japan 1 we analyze coseismic and postseismic velocity variations caused by the june 13 2008 iwate miyagi nairiku earthquake mw 6 9 using passive image interferometry pii seismic noise is correlated in order to reconstruct the green s function between two sensors shear wave velocity changes are determined by relating the coda parts of the daily green s functions to a long term reference green s function our study extends from january 2008 to august 2010 and includes the correlations for 190 station pairs in three different frequency ranges from 0 125 to 1 0 hz we show that combining the 9 different component cross correlation functions stabilizes the velocity change estimation and increases analysis resolution the observed velocity change curves can be fitted by model time series consisting of a coseismic velocity drop followed by logarithmic postseismic recovery and seasonal velocity variations the coseismic velocity drops are stronger at higher frequencies and are concentrated in the southern part of the fault zone a tomography algorithm was developed to reproject the observed velocity variations of the different sensor pairs onto the single sensors the depth distribution of coseismic changes was modeled for the three stations with the largest velocity drops at two stations the coseismic velocity changes are located in the upper several hundred meters the third station shows indications for deeper changes in the order of kilometers postseismic recovery takes significantly longer than the analyzed two year period seasonal velocity variations with periods of one year are observed at all analyzed frequencies for most station pairs

comprehensive observation and modeling of earthquake and temperature related seismic velocity changes in northern chile with passive image interferometry we report on earthquake and temperature related velocity changes in high frequency autocorrelations of ambient noise data from seismic stations of the integrated plate boundary observatory chile project in northern chile daily autocorrelation functions are analyzed over a period of 5 years with passive image interferometry a short term velocity drop recovering after several days to weeks is observed for the mw 7 7 tocopilla earthquake at most stations at the two stations pb05 and patcx we observe a long term velocity decrease recovering over the course of around 2 years while station pb05 is located in the rupture area of the tocopilla earthquake this is not the case for station patcx station patcx is situated in an area influenced by salt sediment in the vicinity of salar grande and presents a superior sensitivity to ground acceleration and periodic surface induced changes due to this high sensitivity we observe a velocity response of several regional earthquakes at patcx and we can show for the first time a linear relationship between the amplitude of velocity drops and peak ground acceleration for data from a single station this relationship does not hold true when comparing different stations due to the different sensitivity of the station environments furthermore we observe periodic annual velocity changes at patcx analyzing data at a temporal resolution below 1 day we are able to identify changes with a period of 24 h too the characteristics of the seismic velocity with annual and daily periods indicate an atmospheric origin of the velocity changes that we confirm with a model based on thermally induced stress this comprehensive model explains the lag time dependence of the temperature related seismic velocity changes involving the distribution of temperature fluctuations the relationship between temperature stress and velocity change plus autocorrelation sensitivity kernels

increase of shear wave velocity before the 1998 eruption of merapi volcano indonesia 1 we infer temporal changes in the elastic properties of the edifice of merapi volcano java indonesia before its eruption in 1998 by analyzing multiply scattered elastic waves excited by a repeatable controlled seismic source a pre eruptive increase of shear wave velocity which correlates well with pre eruptive seismicity and dome growth is revealed the method can be used as a pressure gauge for pressure changes inside of volcanoes because increasing pressures in rocks are known to cause proportionally increasing elastic wave velocities

temporal change in shear velocity and polarization anisotropy related to the 2011 m9 0 tohoku oki earthquake examined using kik net vertical array data 1 the m9 0 tohoku oki earthquake took place off ne japan which is covered by a dense seismic network in particular kik net observed the strong motions of the main shock and numerous aftershocks both on the ground and at the bottom of boreholes at a depth of a few hundred meters we applied cross correlation analysis to the coda part of a seismogram observed by kik net and estimated not only the temporal change in shear velocity but also the change in polarization anisotropy between the bottom of the borehole and the ground the shear velocity was suddenly reduced by 5 to 10 after the tohoku oki earthquake over a wide area of ne japan which is consistent with the results of a previous study although some stations also reveal a change in polarization anisotropy the polarization anisotropy change is smaller than the velocity change which may suggest that the static stress change due to the coseismic slip on the pacific plate is not large enough to change the direction of polarization anisotropy and or deviatoric stress in the land area of ne japan

location of largest earthquake slip and fast rupture controlled by along strike change in fault structural maturity due to fault growth earthquake slip distributions are asymmetric along strike but the reasons for the asymmetry are unknown we address this question by establishing empirical relations between earthquake slip profiles and fault properties we analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults in particular on the directions of their long term lengthening we find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long term fault propagating tips i e on the most mature half of the broken fault sections meanwhile slip decreased linearly over most of the rupture length in the direction of long term fault propagation i e of decreasing structural maturity along strike we suggest that this earthquake slip asymmetry is governed by along strike changes in fault properties including fault zone compliance and fault strength induced by the evolution of off fault damage fault segmentation and fault planarity with increasing structural maturity we also find higher rupture speeds in more mature rupture sections consistent with predicted effects of low velocity damage zones on rupture dynamics since the direction s of long term fault propagation can be determined from geological evidence it might be possible to anticipate in which direction earthquake slip once nucleated may increase accelerate and possibly lead to a large earthquake our results could thus contribute to earthquake hazard assessment and earthquake early warning

groundwater variations from autocorrelation and receiver functions 1 receiver functions at iu hkt station 2 seismic velocity perturbation data from ambient noise autocorrelation 3 synthetic receiver functions at iu hkt station

constraining depth range of s wave velocity decrease after large earthquakes near parkfield california we use noise correlation and surface wave inversion to measure the s wave velocity changes at different depths near parkfield california after the 2003 san simeon and 2004 parkfield earthquakes we process continuous seismic recordings from 13 stations to obtain the noise cross correlation functions and measure the rayleigh wave phase velocity changes over six frequency bands we then invert the rayleigh wave phase velocity changes using a series of sensitivity kernels to obtain the s wave velocity changes at different depths our results indicate that the s wave velocity decreases caused by the san simeon earthquake are relatively small similar to 0 02 and access depths of at least 2 3 km the s wave velocity decreases caused by the parkfield earthquake are larger similar to 0 2 and access depths of at least 1 2 km our observations can be best explained by material damage and healing resulting mainly from the dynamic stress perturbations of the two large earthquakes

time lapse seismic noise correlation tomography at valhall we show that a reliable and statistically significant group velocity time lapse difference between 2004 and 2010 can be retrieved from ambient seismic noise in an offshore hydrocarbon exploitation setting we performed a direct comparison of scholte wave group velocity images obtained using regularized tomography we characterize the expected variation in group velocity images from the 2004 or 2010 recordings that result from fluctuations in the cross correlations by looking at cross correlations of portions of the recordings we prove that the time lapse difference is statistically significant the time lapse group velocity image from ambient noise data shows strong similarities with a time lapse phase velocity map obtained from controlled source data the most striking features are a northern and a southern group velocity increase due to compaction and subsidence as a result of reservoir production

monitoring crustal changes at volcanoes by seismic noise interferometry mt etna case of study abstract in this work we analysed the seismic noise recorded at mt etna by 18 stations during the interval 2007 2015 in the frequency band 0 1 0 3 hz chosen to avoid contamination from volcanic tremor variations in time of medium seismic velocity in the range 0 8 to 0 8 were found mostly affecting the stations located on the volcano summit and flanks based on the investigated frequency content the v v changes took place from the surface to a depth of 4 5 6 5 km to identify the source mechanism of the observed medium changes the variations were quantitatively compared by wavelet transform coherence with volcano tectonic and meteorological parameters a significant relationship with meteorological parameters with seasonal periodicity especially air temperature and snow loading was found probably caused by thermo elastic strain and increasing decreasing surface loading cycles moreover a sharp medium velocity decrease taking place in mid december 2009 and clearly time related to the largest volcano tectonic strain release phenomenon of the investigated period was also found such a velocity decrease was interpreted as resulting from ascent of fluids and gas exsolution taking place at the same time as the volcano tectonic swarm