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

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

pre and post seismic slow slip on the plate boundary off sanriku ne japan associated with three interplate earthquakes as estimated from small repeating earthquake data abstract the spatio temporal variation of quasi static slow slip on the plate boundary before and after three major earthquakes mj 7 1 in 1989 mj 6 9 in 1992 and mj 7 6 in 1994 off sanriku northeastern japan is estimated using small repeating earthquake data the small repeating earthquake sequence employed is considered to correspond to repeated rupture of a small isolated asperity surrounded by a quasi static slipping area and the activity of the source reflects the quasi static slip rate small repeating earthquakes accompanying moderate sized events appear to have been activated prior to the major events under investigation with activity starting 6 days 2 days and 8 months before the occurrence of the 1989 1992 and 1994 events these results indicate that the quasi static slip near the hypocenters of these major earthquakes was accelerated prior to the main shocks causing stress concentration at the asperities responsible for the main shocks this idea is supported by the fact that both the epicenters locations of initial breaks and areas where quasi static slip was accelerated were located on the same side of the main shock asperities migration of quasi static slip acceleration in the off sanriku region was also identified apparent in the aftershock activity of the 1994 event and the earthquake swarms including the 1989 mj 7 1 and 1992 mj 6 9 events the present results reveal that quasi static slip may have played a crucial role in the occurrence of these large earthquakes as well as contributing to aftershock and swarm activity on the plate boundary

envelope inversion analysis for high frequency seismic energy radiation from the 2011 mw 9 0 off the pacific coast of tohoku earthquake abstract i study s wave energy radiation in higher frequencies of 1 8 hz from the 2011 m w 9 0 off the pacific coast of tohoku japan earthquake hereafter called the tohoku oki earthquake based on a seismogram envelope inversion method i first evaluate two medium parameters of scattering attenuation and intrinsic absorption for s waves using spatial and temporal distributions of seismic energy from aftershock records which are necessary for calculating green s functions next applying the envelope inversion method to 27 near field strong motion records from the mainshock i estimate the s wave energy radiation from 40 subfaults on the fault of 500 km 250 km each subfault is allowed to rupture for eight consecutive source time windows rupture velocity and the duration of energy radiation for each time window are determined by a grid search to be 2 5 km s and 8 0 s respectively energy radiated from the entire fault is 5 8 10 16 j for 1 2 hz 4 5 10 16 j for 2 4 hz 1 5 10 16 j for 4 8 hz and totaling 1 2 10 17 j in the 1 8 hz range strong energy emanated twice from greater depths of the off miyagi region corresponding to two peaks seen at the northern stations and energy radiation at greater depths off the fukushima ibaraki border is compatible with a single peak found at the southern stations strong high frequency radiations at greater depths strikingly contrast with large slips at shallower depths estimated in lower frequencies this frequency dependent rupture may accord with structural changes along the dip direction online material snapshots of high frequency energy radiation from the fault plane and results of estimating scattering parameters by the multiple lapse time window analysis in 1 2 and 2 4 hz

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

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

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

monitoring a temporal change of seismic velocity in a volcano application to the 1992 eruption of mt merapi indonesia multiplets i e events with similar waveforms are selected from shallow earthquakes recorded on merapi volcano indonesia before the eruption of february 2nd 1992 two multiplet families are found with their sources close to the summit their seismograms are analyzed using the moving window cross spectrum technique which measures the precise time delay between seismic phases in the entire seismogram for both families of multiplets a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption coda waves are becoming progressively faster up to 1 2 per cent as the time interval to the eruption shortens this observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano the increase in velocity started in may 1991 and was observed until september 1991 4 months before the eruption this velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks

do the north atlantic winds drive the nonseasonal variability of the arctic ocean sea level the gravity recovery and climate experiment grace satellites have observed coherent and nearly uniform nonseasonal fluctuations of bottom pressure throughout the arctic ocean and the nordic seas strong correlation between the nonseasonal grace and satellite altimetry data is found in the nordic and barents seas which suggests a possibility of using the longer altimetry records in these areas as a proxy for the nonseasonal sea level variability over the entire arctic this study identifies the dominant pattern of the nonseasonal atmospheric pressure variability that drives strong zonal wind anomalies over the northeastern north atlantic associated with the nonseasonal sea level anomalies in the nordic seas our results show that wind driven northward ekman transport anomalies in the northeastern north atlantic may induce coherent changes of sea level across the entire arctic ocean

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

passive seismic imaging using microearthquakes we have developed a method of processing seismic signals generated by microearthquakes to image local subsurface structure beneath a recording station this technique uses the autocorrelation of the vertically traveling earthquake signals to generate pseudoreflection seismograms that can be interpreted for subsurface structure processed pseudoreflection data from microearthquakes recorded in the island of hawaii show consistent reflectivity patterns that are interpreted as near surface horizontal features forward modeling of the pseudoreflection data results in a p wave velocity model that shows reasonable agreement with the velocity model derived from a refraction study in the region usable signal to noise ratio is obtained down to 2 s a shear wave velocity model was also generated by applying this technique to horizontal component data

seismic wave velocity changes and stress build up in the crust of the kanto tokai region we show there are large temporal p wave velocity changes in the crust of the kanto tokai region in japan from repeated observations of explosions we find that the variation is the response of the crustal rocks to two components of stress change the tidal stress and the secular stress accumulation the inferred annual rate of fractional velocity change year caused by the stress accumulation in the upper crust is of the order of 10 3 year the results indicate that the build up of stress in the crust can be monitored by precise observations of seismic wave velocities

seismic velocity changes of p and s waves associated with the 2011 tohoku oki earthquake mw 9 0 as inferred from analyses of repeating earthquakes

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

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

separation of site effects and structural focusing in santa monica california a study of high frequency weak motions from earthquakes and blasts recorded during the los angeles region seismic experiment near surface site factors and the effects of deep structural focusing were estimated in the santa monica mountains and santa monica california from a portable array of 75 seismic stations deployed during the los angeles region seismic experiment phase ii larse ii the objective was to examine further the origin of seismic wave amplification in the region of intense damage south of the santa monica fault from the northridge earthquake the analysis used normalized spectral amplitudes in the 4 to 8 and 8 to 12 hz range in direct and coda waves from local earthquakes in santa paula northridge redlands and hector mine coda waves indicated that site factor amplifications are larger south of the santa monica fault relative to the north spectral ratios of direct s waves corrected for site effects show additional amplification south of and adjacent to the santa monica fault attributable to focusing by a deeper structure gao et al 1996 concluded that localized focusing effects contributed to anomalous p and s wave amplification in the santa monica damage zone for northridge aftershocks within a specified range of azimuths in an attempt to reproduce the hypothesized focusing from the northridge earthquake two shots 4000 and 3750 lb were detonated one at pyramid lake a distance of about 69 km to the north northwest of central santa monica and the other near fort tejon a distance of 91 km the azimuth of the shots was chosen to be that expected to give anomalous amplification at these distances steeply incident seismic energy from pg pmp waves are expected to pass through the underground focusing structure and be selectively amplified after the local site factors are removed the waveforms from the fort tejon shot exhibited localized amplification adjacent to and south of the fault 2 3 times larger than that of the surrounding area the effect is less for waves from the pyramid lake shot which could be due to their higher angle of incidence the observations lend support to the argument that deep structural focusing is an important factor in determining azimuth dependent amplification of seismic waves along a basin edge manuscript received 21 may 2001

time lapse changes of p and s wave velocities and shear wave splitting in the first year after the 2011 tohoku earthquake japan shallow subsurface

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

seismic velocity changes in the backarc continental crust after the 2011 mw 9 0 tohoku oki megathrust earthquake the 2011 mw 9 0 tohoku oki megathrust earthquake accompanied coseismic and postseismic displacements around the eastern eurasian continental plate noise cross correlations produced transient seismic waveforms along interstation paths in the korean peninsula we measured the traveltime changes of the fundamental mode rayleigh waves over the range of 0 03 0 08 hz after the megathrust earthquake the temporal seismic velocity changes in the lower crust were assessed from the traveltime changes the traveltimes increased instantly after the megathrust earthquake and were gradually recovered over several hundreds to thousands of days the instant shear wave velocity decreases ranged between 0 731 0 057 and 4 068 0 173 the temporal medium perturbation might be caused by the transient uniaxial tensional stress due to the coseismic and postseismic displacements the medium properties may be recovered by progressive stress field reconstruction

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

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

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

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

pseudoreflection profiling method an efficient complement to cdp method1 a method for pseudoreflection profiling prp is presented by using transmission seismograms this method produces time sections similar to those obtained using the common depth point cdp method pseudoreflection profiles are obtained using the following procedure equivalent transmission responses at sites on soft deposits are derived by wiener filter estimation using seismograms acquired on the sites and on the basement rock these responses are substituted into the claerbout kunetz equation to obtain the pseudoreflection profiles a 3d subsurface model is produced when the prp method is applied to explosion seismograms the time sections thus obtained are consistent with those obtained using the cdp method in a neighbouring area the maximum station interval for obtaining coherent profiles with this method is estimated to be one quarter of the wavelength of the transmitted waves this technique should be used in combination with another method such as the cdp method because it has two weak points the velocity structure cannot be obtained directly and undesirable waves cannot be eliminated through processing