interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

temporal changes of subsurface velocities during strong shaking as seen from seismic interferometry 1 we apply a deconvolution method to a strong motion data set recorded at the surface and in boreholes in northeast honshu japan we try to characterize the nonlinear effects of the subsurface soil during strong shaking and show the change of the subsurface velocity structure during the shaking the deconvolved waveforms reflect the subsurface velocity structure and their horizontal and vertical components correspond to s and p wave respectively traveling from the borehole to the ground surface the strong motion records with smaller values of peak acceleration do not include significant nonlinear effects so the deconvolved waveforms of the observed accelerations can be well simulated by the program shake91 for high acceleration motions during the shaking of two separate earthquakes large reductions of near surface velocities are seen in results for the 2008 iwate miyagi nairiku earthquake the large high frequency ground motions over 4g at one near source station caused a nonlinear response of the soil and the reduction of the average shear wave velocity reached 24 this corresponds to a stiffness change of over 75 the soil properties and the stiffness coefficient which changed during the shaking did not fully recover after the shaking leaving a static change

sh propagator matrix and qs estimates from borehole and surface recorded earthquake data summary a new method is proposed for the solving of elastic and anelastic parameters of a medium between two seismometers one in a borehole and one at the surface this allows us to retrieve equivalent quality factors of the materials and at the same time gives wave velocities as well as reflection and transmission coefficients in a layered structure the study of local site effects shows that the quality factor of shallow materials can be completely masked by resonance effects and near surface amplification our method is based on the sh propagator matrix in the time domain this separates the effects of resonance and amplification in the elastic structure from those of attenuation due either to scattering or physical attenuation and we find that the elastic problem is decoupled from the anelastic one after a synthetic test we applied the method to seismic records made with a borehole surface instrument operated in the southern rhine graben valley on the french german border due to high seismic noise at the surface we were only able to resolve a one layered mean 500 m deep structure between the borehole and surface instrument an apparent qs of 40 is found between 1 and 5 hz a frequency band where classical methods failed this is in close agreement with qs at frequencies larger than 15 hz obtained from the classical method based on the slope of spectral ratios

test of seismic hazard map from 500 years of recorded intensity data in japan abstract maximum seismic intensity maps for japan are constructed using the recorded intensity data from 1498 to 2007 and are used to test the probabilistic seismic hazard map pshm by the headquarters for earthquake research promotion japan the historical intensity maps are compared with the hazard maps of probable maximum seismic intensity for a 475 yr return period assuming a poisson distribution 10 in 50 yrs we look at cases that include all events only subduction zone earthquakes and all events excluding subduction zone earthquakes the megathrust earthquakes in the subduction zones produce large bands of high intensities along the pacific coast side while onshore crustal earthquakes create a patchy distribution of large intensities over all of japan the maximum recorded intensity map for the past 500 yrs and the maximum predicted intensity map for the return period from the pshm are very similar for the cases including all events and the subduction zone earthquakes while there is poor correlation for the third category that includes mostly onshore crustal earthquakes if we consider only the amount of area not the specific locations the recorded intensity map and the pshm using the maximum case have a high degree of correlation for japanese meteorological agency jma intensity higher than 4 for all of the cases statistically the present hazard maps for japan seem to agree with the past intensity distributions and can be regarded as appropriate hazard maps even though there may be strong dependencies on uncertain model parameters for the pshms

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

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

field investigations of the threshold of grain motion by ocean waves and currents abstract instrumented tripods were deployed on three occasions at continental shelf sites twice in the united states december 1978 and march 1979 and once in australia december 1979 a total of 37 days of data were collected data included measurements of current speed and direction 100 cm off the seabed mean bottom pressure and pressure fluctuations water turbidity with nephelometer or transmissometer and photographs of the seabed bottom sediment samples were also collected prior to each deployment these data were analyzed to estimate the vector averaged velocity u100 tides gravity waves near bottom sediment concentration bed configuration and bottom sediment texture during the sampling period sediment resuspension occurred frequently as a result of oscillatory currents due to surface gravity waves the field data have been used to evaluate several existing relationships for predicting the threshold of grain motion under oscillatory flow conditions the first method is an evaluation of the shields 1936 entrainment function for unidirectional flows in which the boundary shear stress is computed using the wave friction factor of jonsson 1966 the second is an equation presented by komar and miller 1973 1975 that is based on the laboratory investigations of bagnold 1946 and manohar 1955 the third method computes the boundary shear stress as a nonlinear combination of stresses due to waves and currents as proposed by grant and madsen 1979 and further developed in this paper the results show that the shields diagram in which the shields entrainment function for unidirectional currents is plotted with respect to a grain reynolds number adequately predicts the threshold of grain motion on the continental shelf although the results of all three computational methods were within the scatter of results of the laboratory studies upon which the predictive relationships were based the wave current model of grant and madsen is preferred because it includes the influence of currents in the computation of boundary shear stress

influence of a sedimentary basin infilling description on the 2 d p sv wave propagation using linear and non linear constitutive models numerical modelling is a useful tool to understand the role of different parameters site geometry impedance contrast material properties constitutive model input motion governing site effects we focus here on the 2 d p sv seismic wave propagation in a simple shaped asymmetric model we consider two ways of describing the sedimentary infilling properties the basin is either composed of distinct homogeneous geological layers or is described through properties progressively changing as a function of depth smooth model in order to study the wave propagation in the two basins with similar soil properties p and s velocities of the layered basin are deduced from the smooth basin ones making both models compatible in a kinematic point of view p and s velocities globally increase with depth following epri non linear properties are considered as constant in layers for the two basins we assess the influence of the model choice on the wave propagation while taking into account linear and non linear constitutive models we test the influence of the input motion by propagating two different input motions the first being a simple impulsive one and the second a real complex accelerogram we find that transfer functions computed in each basin are quite similar irrespective of the soil constitutive behaviour and the input motion moreover we show that the maximum shear strain spatial distribution depends on the input motion frequency content and maximum values depend on the input motion strength and complexity maximum shear strains are generally higher for superficial layers than for deeper ones we show that the highest shear strain values are located above discontinuities where velocities are lower and the soil is more non linear than in the underlying layer regardless of the input motion indeed in a layered basin the shear strain is higher above interfaces for all the soil constitutive models that were used in the smooth visco elastoplastic model a kind of layering appears absent in a viscoelastic model due to the different non linear properties in each layer moreover no matter what the model structure and the input motion used maximum shear strain levels are higher in the visco elastoplastic soil than in the viscoelastic one such a numerical study helps to understand the non linearity location and therefore the consequences of a model choice when performing a site specific study

monitoring a building using deconvolution interferometry i earthquake data analysis abstract for health monitoring of a building we need to separate the response of the building to an earthquake from the imprint of soil structure coupling and from wave propagation below the base of the building seismic interferometry based on deconvolution where we deconvolve the wave fields recorded at different floors is a technique to extract this building response and thus estimate velocity of the wave that propagates inside the building deconvolution interferometry also allows us to estimate the damping factor of the building compared with other interferometry techniques such as cross correlation and cross coherence interferometry deconvolution interferometry is the most suitable technique to monitor a building using earthquake records for deconvolution interferometry we deconvolve the wave fields recorded at all levels with the waves recorded at a target receiver inside the building this receiver behaves as a virtual source and we retrieve the response of a cut off building a short building that is cut off at the virtual source because the cut off building is independent from the structure below the virtual source the technique might be useful for estimating local structure and local damage we apply deconvolution interferometry to 17 earthquakes recorded during two weeks at a building in fukushima japan and estimate time lapse changes in velocity and normal mode frequency as shown in a previous study the change in velocity correlates with the change in normal mode frequency we compute the velocities from both traveling waves and the fundamental mode using coda wave interferometry these velocities have a negative correlation with the maximum acceleration of the observed earthquake records

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

damage detection of a building caused by the 2011 tohoku oki earthquake with seismic interferometry we apply seismic interferometry based on deconvolution to earthquake data observed in a building at tokyo metropolitan university to extract building response and estimate velocities and frequencies of traveling waves inside the building the building is a 10 story reinforced concrete structure in tokyo japan the earthquake data were recorded 2 months before 6 months after and 16 months after the m w 9 0 tohoku oki earthquake 11 march 2011 the observation period of each acquisition was 20 30 days we analyze more than 300 earthquakes recorded in these time periods and we find significant reduction 20 in wave velocities and fundamental mode frequencies after the tohoku oki earthquake because deconvolution interferometry separates the response of the building from the effect of complicated wave propagation outside the building we interpret that this reduction is mostly related to the building we can also estimate frequencies of the fundamental mode from observed data and these frequencies contain the responses of building and the ray paths from hypocenters therefore as we find from the data the frequencies estimated from observed data are generally lower than those from deconvolved waveforms due to the effects of wave propagation although the velocities in the building decrease 20 after the tohoku oki earthquake we cannot find significant reduction in the s wave velocities at the near surface depth shallower than 100 m this fact indicates the decrease in velocities is mainly related to the building and hence with deconvolution interferometry we can detect damages to the building caused by the tohoku oki earthquake

gently north dipping median tectonic line mtl revealed by recent seismic reflection studies southwest japan abstract the median tectonic line mtl with a length of more 1000 km is the most significant fault in japan it juxtaposes the high p t sambagawa metamorphic rocks against the low p t metamorphic rocks of the ryoke belt the mtl was probably formed in the cretaceous with many subsequent reactivations the western segment of the mtl is still active with an almost pure right lateral sense of motion although a great amount of geological information on the mtl has been accumulated information about the subsurface especially the deep seated structure of the mtl is still insufficient it has been generally assumed that the mtl is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion recently new geophysical data have suggested that the mtl dips gently northward at depth we have acquired a complementary set of geophysical profiles seismic reflection and refraction gravity and mt across the mtl in east shikoku our results confirm that the mtl dips northward at about 30 to 40 degrees from the surface to about 5 km depth where it becomes listric this fault geometry more reasonably explains the reactivation history of the mtl the motion has occurred on a listric type fault in so called oblique or lateral ramp manner

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

feasibility of borehole ambient noise interferometry for permanent reservoir monitoring the analysis of seismic ambient noise acquired during temporary or permanent microseismic monitoring campaigns e g improved enhanced oil recovery monitoring surveillance of induced seismicity is potentially well suited for time lapse studies based on seismic interferometry no additional data acquisition required ambient noise processing can be automatized to a high degree and seismic interferometry is very sensitive to small medium changes thus there is an opportunity for detection and monitoring of velocity variations in a reservoir at negligible additional cost and effort data and results are presented from an ambient noise interferometry study applied to two wells in a producing oil field in romania borehole microseismic monitoring on three component geophones was performed for four weeks concurrent with a water flooding phase for improved oil recovery from a reservoir in ca 1 km depth both low frequency 2 hz 50 hz p and s waves propagating through the vertical borehole arrays were reconstructed from ambient noise by the virtual source method the obtained interferograms clearly indicate an origin of the ambient seismic energy from above the arrays thus suggesting surface activities as sources it is shown that ambient noise from time periods as short as 30 seconds is sufficient to obtain robust interferograms sonic log data confirm that the vertical and horizontal components comprise first arrivals of p wave and s waves respectively the consistency and high quality of the interferograms throughout the entire observation period further indicate that the high frequency part up to 100 hz represents the scattered wave field the temporal variation of apparent velocities based on first arrival times partly correlates with the water injection rate and occurrence of microseismic events it is concluded that borehole ambient noise interferometry in production settings is a potentially useful method for permanent reservoir monitoring due to its high sensitivity and robustness

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

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

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

high resolution lithospheric imaging with seismic interferometry in recent years there has been an increase in the deployment of relatively dense arrays of seismic stations the availability of spatially densely sampled global and regional seismic data has stimulated the adoption of industry style imaging algorithms applied to converted and scattered wave energy from distant earthquakes leading to relatively high resolution images of the lower crust and upper mantle we use seismic interferometry to extract reflection responses from the coda of transmitted energy from distant earthquakes in theory higher resolution images can be obtained when migrating reflections obtained with seismic interferometry rather than with conversions traditionally used in lithospheric imaging methods moreover reflection data allow the straightforward application of algorithms previously developed in exploration seismology in particular the availability of reflection data allows us to extract from it a velocity model using standard multichannel data processing methods however the success of our approach relies mainly on a favourable distribution of earthquakes in this paper we investigate how the quality of the reflection response obtained with interferometry is influenced by the distribution of earthquakes and the complexity of the transmitted wavefields our analysis shows that a reasonable reflection response could be extracted if 1 the array is approximately aligned with an active zone of earthquakes 2 different phase responses are used to gather adequate angular illumination of the array and 3 the illumination directions are properly accounted for during processing we illustrate our analysis using a synthetic data set with similar illumination and source side reverberation characteristics as field data recorded during the 2000 2001 laramie broad band experiment finally we apply our method to the laramie data retrieving reflection data we extract a 2 d velocity model from the reflections and use this model to migrate the data on the final reflectivity image we observe a discontinuity in the reflections we interpret this discontinuity as the cheyenne belt a suture zone between archean and proterozoic terranes

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

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

fluvial networks of the iberian peninsula a chronological framework abstract knowledge of the evolution of spanish fluvial networks has improved during recent years as more river systems have been studied and more geochronological data has become available however the chronological framework is a major issue as the range of applications is limited by methodological constraints and spatial coverage is sparse integration of absolute dating methods with biostratigraphy and palaeomagnetism permits the recent evolution of these river systems to be reviewed the timing of incision from the late neogene to the present varies between the major iberian fluvial systems depending on the substrata and tectonic settings early pleistocene and older fluvial sequences in the core areas of the iberian peninsula provide a more extensive record of fluvial evolution and are better preserved than the terrace flights in the coastal lowlands middle pleistocene sequences are well developed in most of the major river systems in iberia particularly those of the tajo guadalquivir and aguas river and frequently represent the principal climatic cycles of that period although tectonic and sea level effects can also be seen for late pleistocene to holocene times the scheme becomes more complex our review suggests that each river system has responded differently to local and regional climate control glacial and periglacial processes in headwaters in high mountain areas glacio eustatic sea level changes and local and regional tectonic patterns

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

seismic interferometry of teleseicmic s wave coda for retrieval of body waves an application to the philippine sea slab underneath the japanese islands summary the reconstruction of surface waves from the cross correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches however the reconstruction of body waves has not been extensively studied in this study we present a method for extracting body waves that is direct p and s waves and reflected waves from the philippine sea slab we use the cross correlation of the wavefield generated by the teleseismic s waves observed by hi net tiltmeters with a passband of 0 07 0 5 hz to enhance the contribution of the s coda we reduced the source time function and the deterministic phases ss scs and ss and the surface wave in s coda the records of the processed s coda observed at every station pair are cross correlated for each teleseismic event the cross correlation functions ccfs of different earthquakes are stacked to retrieve wavefields propagating between the station pairs as a result direct p s and the reflected waves could be extracted when the stacked ccfs are aligned as a function of distance of separation between two stations the gradients of the traveltime curves for the direct p and s waves are approximately 5 7 and 3k m s 1 respectively these velocities correspond to the seismic velocities of the crust or the p wave velocity of the uppermost mantle in order to enhance the reflected waves we searched for the reflection points by assuming that the later phases in the ccfs are ss reflections and map the amplitudes onto the depth sections as a result the negative phases dipping to the north can be traced right below the region of hypocentre distribution these negative phases probably correspond to the oceanic moho within the philippine sea slab we also show that the oceanic moho can also be traced by assuming pp reflections these results indicate that the ccfs plausibly contain information regarding both p and s waves propagating between the two receivers and are capable of detecting reflected phases in addition to the direct waves

bayesian approach for detecting dynamically triggered very low frequency earthquakes in the nankai subduction zone and application to the 2016 mw 5 9 off kii peninsula earthquake japan