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 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

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

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

sediments from lago di mezzano central italy a record of lateglacial holocene climatic variations and anthropogenic impact microscopic geochemical and pollen analysis of sediment samples of a lateglacial holocene profile from lago di mezzano a maar lake in central italy reveals evidence of significant climatic and human induced environmental changes time control is provided by a combination of varve chronology and radiocarbon dating the well known lateglacial climatic variations a warmer interstadial and the younger dryas cold phase are clearly represented in all the parameters during the interval between 9200 and 5600 cal bp of the holocene climatic optimum annually laminated organic rich diatom gyttja was deposited two periods of diminished total organic carbon are identified within this interval the first one p1 ranges from 8200 to 7800 cal bp while the second p2 is centred around 6500 cal bp during p1 a predominance of diatoms stephanodiscus parvus and s minutulus over other algae represented by the total organic carbon content is observed the timing of this period coincides with the early to mid holocene climatic transition reported from ice cores and lake sediments stager and mayewski 1997 p2 is characterized by a decrease in all biogenic parameters including biogenic opal organic carbon as well as arboreal pollen from 5000 cal bp to date the sediment pattern changes coincide with the mid holocene climatic deterioration in addition to these natural variations human impact has been recorded and recognized from sedimentological features such as turbidites and charcoal as well as from reduced arboreal pollen content two middle bronze age 3700 cal bp and 3300 cal bp etruscan early roman 2500 cal bp longobardic ad 900 and modern settlements since ad 1700 have been distinguished on the basis of these data

simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects we considered the effects of source propagation path and local site conditions on strong ground motion from the loma prieta earthquake in the following steps first the local site effect was removed from the strong motion records applying the weak motion amplification factors estimated from nearby stations of usgs regional network using the coda method then we confirmed the approximate validity of the 1 r law of geometrical spreading factor for the crustal model applicable to the region and we determined the attenuation parameter q f and source spectrum s f empirically from the site effect corrected spectral amplitude both q f and s f are consistent with the results obtained earlier for major california earthquakes we then synthesized the time history of ground acceleration by applying the stochastic simulation technique the agreement between the observed and predicted records was good for duration and spectral content we found however a strong systematic discrepancy with regard to peak acceleration namely for distances less than 50 km the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the franciscan formation sites since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter it is unlikely that the effect is due to the radiation pattern near source structure or topography it appears that the strong difference in the amplification factor between the franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0 1 to 0 3 g if this is due to the nonlinear response of the sediment sites it is in the range expected from geotechnical engineering studies

in situ assessment of the g γ curve for characterizing the nonlinear response of soil application to the garner valley downhole array and the wildlife liquefaction array abstract we analyze the nonlinear and near surface geological effects of two network for earthquake engineering simulation at university of california santa barbara nees ucsb instrumented sites the garner valley downhole array gvda and the wildlife liquefaction array wla the seismic interferometry by deconvolution method is applied to earthquake data recorded by the multisensor vertical array between january 2005 and september 2013 along the cross section local shear wave velocity is extracted by estimating travel time between sensors the s wave velocity profiles are constructed and compared with classical in situ geophysical surveys we show that velocity values change according to the amplitude of the ground motion and we find anisotropy between east west and north south directions at the gvda site the ratio between average peak particle velocity v and local s wave velocity between two boreholes is tested as a deformation proxy using average peak particle acceleration a the a versus curve is used to represent the stress strain curve for observing the site s nonlinear responses under different levels of excitation nonlinearity is observed from quite low shear strain levels 1 10 5 and a classic hyperbolic model is derived proves to be a good deformation proxy finally the shear modulus degradation curves are constructed for each depth and test site and they are similar to previous laboratory measurements or in situ geophysical surveys a simple comparison regarding nonlinear behavior between gvda and wla is performed

a geospatial model for the analysis of time dependent land subsidence induced by reservoir depletion abstract land subsidence due to fluid depletion is an outcome of physical processes operating across a wide range of time and length scales although geomechanical models are crucial to simulate reservoir compaction and predict its long term fate their use across large regions often bears prohibitive computational costs to overcome this obstacle this paper proposes a simplified modelling framework consisting of i a near field numerical solver simulating the coupling between fluid flow and rock deformation with reference to a simplified one dimensional geometry and ii a far field geospatial algorithm mapping ground settlements across a region through the superposition of poroelastic computations at multiple wells the model computes the delay between depletion history and reservoir compaction in proximity of a producing well by assuming basal depletion of a fluid saturated deformable disk while the geertsma solution of nucleus of strain is used to extrapolate the impact of such time varying reservoir compaction around the well this approach has been used to back analyze the spatio temporal progression of subsidence at the groningen gas field the results are compared against measurements collected over 50 year of production at 25 benchmark locations scattered over an area of 900 km2 it is shown that coupled simulations based on average values of rock compressibility and permeability lead to nonlinear trends of subsidence evolution in good agreement with field measurements while uncoupled analyses overpredict settlements by more than 70 lastly synthetic forecasts based on different rates of depletion were provided the results suggest that slower depletion rates lead to lower subsidence at a given time and that residual subsidence may continue to develop for several decades after interruption of production activities

liquefaction case studies of gravelly soils during the 2008 wenchuan earthquake abstract the 2008 wenchuan earthquake caused widespread liquefaction of gravelly soils i e gravelly sands and sandy gravels which phenomena show many differences compared to those of typical sands to study the liquefaction mechanism and triggering evaluation of gravelly soils during this event comprehensive case studies with measurement of shear wave velocity on both liquefied and non liquefied sites were carried out according to a suite of recommended procedures for site investigation field testing and data processing three typical cases were provided to illustrate the distinct characteristics of ground motion liquefaction manifestation soil profiles and field testing and explore the possible mechanisms of liquefaction eighty one well identified case histories were obtained which feature the updated estimation of ground motion parameters from the usgs shakemap and the probabilistic treatment of the datasets the preliminary analyses on liquefaction triggering boundary curves shear wave velocities of these case histories and the applicability of the ishihara criterion 1985 indicate that the gravel content plays a key role in the liquefaction mechanism and the consequences of gravelly soil deposits and the liquefaction evaluation of gravelly sands and sandy gravels should be treated separately

measurement of q 1 for s waves in mudstone at chikura japan comparison of incident and reflected phases in borehole seismograms seismograms from small events in the southern kanto region have been recorded by a seismometer installed in a 732 m deep borehole at chikura observatory japan where mudstone of early pliocene age is found from the ground surface to the bottom of the borehole strong phases occurring 1 5 1 6 sec after the s wave arrival were interpreted as reflections at the ground surface ray path directions of the incident s waves were determined from the minimum principal axis of the three dimensional trajectory ellipsoid which represents the particle motion for a head wave of s phase transverse horizontal sh components were used to estimate the q 1 value for s waves assuming the free surface acted as an 100 reflector for the sh waves and the incident sh phase as an input and the reflected phase as an output we calculated system functions the q 1 value was measured from the transfer characteristics between the incident and the reflected phases in a single seismogram thus no corrections were necessary for source or site including instrument effects if we applied a power law model the following relationship was obtained from the regression analyses of 20 events log 10 q 1 0 52 0 48 log 10 f 1 28 0 22 1 0 f 5 0 hz where f is frequency in hz and error values are the 95 confidence levels of the regression coefficients

coda wave interferometric detection of seismic velocity changes associated with the 1999 m 3 6 event at mt vesuvius 1 we detect seismic wave velocity changes at mt vesuvius using doublets and the coda wave interferometry method the high sensitivity of multiply scattered coda waves to temporal changes in the medium allows us to detect velocity variation smaller than 0 4 we use 17 doublets some of them grouped in families of multiplets spanning january 1996 to december 1999 data show a systematic increase in velocity from 1996 to end september 1999 followed by a rapid drop in velocity this drop immediately precedes a sustained swarm of vt type earthquakes including the 9th october 1999 m 3 6 event the largest in the region since at least 1972 we propose a long term fluid pressurization followed by influx as a possible causative mechanism

significant effects of shallow seismic and stress properties on phase velocities of rayleigh waves up to 20 s seismic data are increasingly used to monitor subsurface velocity changes associated with tectonic and environmental processes that occur in different depth sections to clarify the differences between effects associated with shallow and deep changes of properties we conduct numerical experiments using simple layered models that include low velocities low attenuation coefficients and stress sensitivity of cracked rocks in the shallow crust we find significant phase velocity drops in the period range of 5 20 s when large structural changes occur in the top 1 3 km the apparent velocity changes v v measured from the first part of the synthetic rayleigh waves with a cross correlation based technique show significant velocity drops in the period bands of 5 10 s and 10 20 s that are consistent with reported values of changes at seismogenic depth the results highlight the importance of accounting for low velocities attenuation coefficients and stress sensitivity of parameters in the top 1 3 km in studies aiming to determine the source region of temporal changes analyses using different frequency ranges and calculations of apparent delay times over multiple period bands are essential for resolving the depth range of temporal changes of properties for temporal changes occurring at seismogenic depths the measured v v values at 5 10 s are significantly larger than those of 10 20 s which are not observed for shallow changes

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

refined thresholds for non linear ground motion and temporal changes of site response associated with medium size earthquakes summary we systematically analyse non linear effects and temporal changes of site response associated with medium size earthquakes using seismic data recorded by the japanese strong motion network kik net we apply a sliding window spectral ratio technique to surface and borehole strong motion records at six sites and stack results associated with different earthquakes that produce similar peak ground acceleration pga in some cases we observe a weak coseismic drop in the peak frequency when the pga is as small as 20 30 gal and near instantaneous recovery after the passage of the direct s waves the percentage of drop in the peak frequency starts to increase with increasing pga values we also observe a coseismic drop in the peak spectral ratio for two sites when the pga is larger than 60 gal to more than 100 gal we observe considerably stronger drops of the peak frequencies followed by logarithmic recovery with time the observed weak reductions of peak frequencies with near instantaneous recovery likely reflect non linear response with essentially fixed level of damage while the larger drops followed by logarithmic recovery reflect the generation and then recovery of additional rock damage the results indicate clearly that non linear site response may occur during medium size earthquakes and that the pga threshold for in situ non linear behaviour is lower than the previously thought value of 100 200 gal

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

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

predicting nonlinear site response using spectral acceleration vs pgv vs30 a case history using the volvi test site in this study we analyze the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments the in situ stress strain relationships are derived from accelerometric vertical array recordings at the tst site in volvi thessaloniki greece first the shear wave velocity between two successive sensors was computed by seismic interferometry and strain was computed as the velocity ratio or the relative displacement between sensors the shear wave velocity profile and in situ shear modulus degradation curve with strain were compared with previous studies performed at the tst site finally the stress strain relationships were derived from data recorded at the surface by extending the strain proxy and stress values to the ratio between peak ground velocity and the vs30 parameter used for site classification i e without requiring the accelerometric vertical array our model captures the in situ nonlinear response of the site without consideration of azimuth or distance of the earthquakes in conclusion the acceleration stress values based on the accelerometric response spectra instead of peak ground acceleration compared with the deformation strain proxy provide an effective model of the in situ nonlinear response providing information that can be integrated into ground motion prediction equations

changes in shear moduli of liquefied and nonliquefied soils during the 1995 kobe earthquake and its aftershocks at three vertical array sites acceleration records of the 1995 kobe earthquake and its first after shocks provided by vertical arrays at port island pi sgk and tks sites were used to estimate changes in the shear moduli of subsurface soils caused by strong ground motion shear moduli estimates are based on stress strain relations deter mined for consecutive time intervals during the mainshock and aftershocks we found that in the upper 0 13 m at pi reclaimed fill above the groundwater level the shear modulus decreased by 80 90 of its initial value owing to liquefaction signs of its recovery appeared 3 5 min after the decrease in the intensity of strong motion at depths of 13 18 m reclaimed fill below the water level and 27 32 5 m water saturated sand at pi 0 11 m at sgk and 0 14 m at tks water saturated alluvium the shear modulus decrease was estimated at about 80 50 60 60 70 and 50 respectively recovery began immediately after the decrease in the intensity of strong motion and was completed within a few minutes the results show that the behavior of the upper layers at sites pi and sgk where the highest accelerations were recorded was dynamically unstable so far as the method can be applied to any site where vertical array records and profiling data are available ac cumulation of representative data on nonlinear soil behavior during strong motion for various soil conditions is desirable summarizing and generalizing these data we may be able to predict soil behavior in future earthquakes

coherency characteristics of body waves traveling in a sedimentary layer basement system in the kanto region japan by means of borehole array recordings obtained at six stations from local events we estimated coherency characteristics of body waves that propagate in a sedimentary layer basement system in the kanto region japan the estimated coherence is characterized by an 2 model for p and s waves an important result is that the values of corner frequency become nearly constant 5 6 hz in travel times less than 2 5 sec for sh waves this suggests that bedrock motions recorded in the pre tertiary basement are becoming incoherent in high frequencies also the corner frequency of p waves is approximately twice that for sh waves hence p waves propagate coherently in high frequencies as compared with sh waves

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

nonlinear behavior of soils revealed from the records of the 2000 tottori japan earthquake at stations of the digital strong motion network kik net acceleration records of the tottori earthquake 6 october 2000 pro vided by stations of the digital strong motion network kik net show clear evidence of the nonlinearity of soil response at sites located in near fault zones in this study records of the mainshock of the tottori earthquake are analyzed and stresses and strains induced by the strong motion in the upper 100 or 200 m of soil are recon structed at sites located within 80 km from the fault plane for reconstructing stresses and strains the method is applied which we developed and used previously for studying the response of soils during the 1995 kobe earthquake nonlinear time dependent stress strain relations in the soil layers are estimated based on vertical array records a good agreement between the observed and simulated accelerograms of the tottori earthquake testifies to the validity of the obtained vertical distributions of stresses and strains in the soil layers we also evaluated variations of the shear moduli of the soil layers caused by the strong motion at stations located at different distances from the fault plane changes in the rheological properties of the upper soil layers were found at the stations closest to the fault plane a similarity in stress strain relations describing the behavior of similar soils during the 1995 kobe earth quake and the 2000 tottori earthquake was obtained indicating the possibility of precasting soil behavior in future earthquakes at sites where profiling data are available

the impact of a buried high velocity layer in the seismic site amplification of the city of llolleo chile programa de riesgo sismico prs at the university of chile comision nacional de investigacion cientifica y tecnologica conicyt 2017 21171480 fondo nacional de desarrollo cientifico y tecnologico fondecyt 1170430 advanced mining technology center amtc fb0809 pia conicyt

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

depth sensitivity of seismic coda waves to velocity perturbations in an elastic heterogeneous medium summary numerous monitoring applications make use of seismic coda waves to evaluate velocity changes in the earth this raises the question of the spatial sensitivity of coda wave based measurements here we investigate the depth sensitivity of coda waves to local velocity perturbations using 2 d numerical wavefield simulations we calculate the impulse response at the surface before and after a slight perturbation of the velocity within a thin layer at depth is introduced we perform a parametric analysis of the observed apparent relative velocity changes e obs versus the depth of the thin perturbed layer through the analysis of the decay of e obs we can discriminate two different regimes one for a shallow perturbation and the other for a deep perturbation we interpret the first regime as the footprint of the 1 d depth sensitivity of the fundamental surface wave mode to interpret the second regime we need to model the sensitivity of the multiply scattered body waves in the bulk we show that the depth sensitivity of coda waves can be modelled as a combination of bulk wave sensitivity and surface wave sensitivity the transition between these two regimes is governed by mode conversions due to scattering we indicate the importance of surface waves for the sensitivity of coda waves at shallow depths and at early times in the coda at later times bulk waves clearly dominate the depth sensitivity and offer the possibility of monitoring changes at depths below the sensitivity of the surface waves based on the transition between the two regimes we can discriminate a change that occurs at the surface from a change that occurs at depth this is illustrated for shallow depth perturbations through an example from lunar data

near surface weakening in japan after the 2011 tohoku oki earthquake 1 the magnitude mw 9 0 tohoku oki earthquake on 11 march 2011 was one of the largest in recent history ground motion caused by the seismicity around the time of the main shock was recorded by kik net the strong motion network that covers most of japan by deconvolving waveforms generated by earthquakes that are recorded at the surface and in a borehole at kik net station fksh18 we detect a reduction of shear wave velocity in the upper 100 m of about 10 and a subsequent healing that varies logarithmically with time using all available borehole and surface records of more than 300 earthquakes that occurred between 1 january 2011 and 26 may 2011 we observe a reduction in the shear wave velocity of about 5 in the upper few hundred meters after the tohoku oki earthquake throughout northeastern japan the area of the velocity reduction is about 1 200 km wide which is much wider than earlier studies reporting velocity reductions following other larger earthquakes the reduction of the shear wave velocity is an indication that the shear modulus and hence the shear strength is reduced over a large part of japan