scale dependence of seismic energy to moment ratio for strike slip earthquakes in japan we analyzed four pairs of a large m w 6 and a small m w 3 5 to 4 shallow strike slip earthquakes to investigate the scale dependence of the seismic energy to moment ratio an important macroscopic parameter which reflects the basic physical process of seismic slip these earthquakes occurred in the south western part of japan and high quality close in records epicentral distance 50 km are available for both the small and large earthquakes the paired events have almost the same focal mechanism and hypocenter location we used the spectral ratio of the paired events in order to remove the effects of attenuation along the wave propagation path and the station site response we then estimated the seismic energy from the source spectra estimated from the spectral ratio the energy to moment ratio increases with the earthquake size this scale dependence is very similar to that found earlier for earthquakes in southern california

source parameters and fmax in kameng region of arunachal lesser himalaya abstract a data set of 79 local events 0 7 mw 3 7 occurred during february 2003 to may 2003 collected by a temporary network deployed in kameng region of arunachal lesser himalaya have been analyzed to study the source parameters and fmax in this study brune model that yield a fall off of two beyond corner frequency along with high frequency diminution factor for frequencies greater than fmax represented by a butterworth high cut filter boore 1983 has been considered the software eqk src para kumar et al 2012 has been used to estimate the spectral parameters namely low frequency displacement spectral levels 0 corner frequency fc above which spectrum decays with a rate of two the high cut frequency fmax above which the spectrum again decays and the rate of decay n above fmax these spectral parameters are used to estimate source parameters viz seismic moments source dimensions and stress drops and to develop scaling laws for the region seismic moments vary from 1 42 1017 dyne cm to 4 23 1021 dyne cm the source radii vary from 88 7 m to 931 5 m for 28 events stress drops are less than 1 bar and 51 events have stress drops between 1 bar and 40 bars a scaling relation m0 dyne cm 2 1022 f c 3 34 has been derived for earthquakes having seismic moments greater than 1 5 1019 dyne cm the estimated values of fmax values by and large conform to the worldwide observations dependence of fmax on source sizes focal depths epicentral distances and recording sites has been studied on the basis of comparative dependency of fc and fmax the fmax and fc show almost similar dependency to seismic moments which shows fmax is also due to source process and is independent of epicentral distances and focal depths at different recording sites the observed values of fmax show consistent increase with seismic moment this reflects that the source is the main controlling factor rather than recording site conditions for the observed variation of fmax in the kameng region

source parameters and scaling relations for small earthquakes in the matsushiro region southwest honshu japan abstract using near field broadband data from a suite of crustal earthquakes which occurred in a small homogeneous source region we examine the scaling relationships among source parameters for earthquakes in the critical 1 5 to 4 5 magnitude range we take special precautions to insure that our conclusions are not biased by local structure seismic moment fault radius and stress drop relations were determined for 32 small shallow strike slip earthquakes 1 6 m l 4 3 in the matsushiro region of southwest honshu japan spectral and cepstral analyses of short period arrivals exhibit structure interpreted as resulting from reverberations within thin low velocity surface layers below the recording site furthermore calculated crustal transfer functions for sh predict a peak over the range of 2 to 4 hz this type of site response can bias estimates of spectral parameters particularly corner or characteristic frequencies for simple sources such a bias can be reduced by filtering in the cepstral domain the effect of cepstral filtering on corner frequency determinations is generally less than 20 per cent and tends to decrease them particularly for larger events where the structurally induced spectral enhancement is at frequencies above the source related corner frequencies estimates of the seismic moment brune stress drop and fault radius for each event were calculated from sh displacement amplitude spectra the seismic moments range from 10 18 to 10 22 dyne cm the stress drops range from 0 008 to 16 bars the relationship between log stress drop and log moment is approximately linear with a slope of 1 up to a moment of 2 10 21 dyne cm but for higher moments the slope is not significantly different from zero fault radii range from 0 3 to 1 2 km there is a physical scatter in radius for any given moment up to a factor of 1 5 for moments less than 2 10 21 dyne cm there is no significant trend in the radius as a function of moment but for higher moments the radius increases with moment the main conclusion is that the stress drop is the dominant scaling factor for moments less than 10 21 dyne cm for higher moment the source dimension becomes the controlling factor

source characteristics of the nw himalaya and its adjoining region geodynamical implications abstract in this study we propose a novel interpretation of source parameters estimated using brune model for the nw himalaya and its adjoining region analysis of 157 events 2 2 mw 4 9 resulted in the scaling relationship between seismic moment m0 and corner frequency fc for the region as m0 8 1022 fc 2 89 which is in good unison to the garhwal and the kumaon himalaya with analogous seismogenic potential variability of kappa 0 01 s 0 08 s with varying number of events 2 5 m 5 0 at different depths is indicative of nature and extent of structural heterogeneity beneath the source zone which controls the propensity of background seismicity that provides enough testimony to complex geodynamical set up of the nw himalaya and its adjoining region source radii of source zone varying from 148 m to 975 m with static stress drops calculated for a given constant stress of 0 1 bar 1 bar 10 bar and 100 bar correspond to an average stress drop varied from 0 4 bar to 99 9 bar seismic moments of events vary from 2 64 1019 dyne cm to 2 32 1023 dyne cm with increase of the source radii we infer that variability in stress drop for seismic moment mo is attributed to different seismic sources of varying strengths associated with different structural heterogeneities stress drop of the events has good correspondence with source depth having the linear relationship of shear stress regime in the brittle crust with stress drop and source depth suggesting the presence of intricate subsurface seismogenic sources beneath the nw himalaya and its adjoining region

occurrence of low stress drop earthquakes in the garhwal himalaya region abstract the digital seismic data recorded by a telemetered seismic array has been used to compute seismic moments and stress drops of local earthquakes in the garhwal himalaya region based on brune s model of the earthquake source the seismic moments for 18 shallow focus earthquakes range from 7 10 18 dyn cm to 6 23 10 21 dyn cm these earthquakes are found to have very low stress drops less than 1 bar for seven events and between 1 bar and 10 bars for another seven events the maximum stress drop is found to be 38 bars for an earthquake of magnitude 4 2 with focal depth 15 km the shallow focus events having low stress drops in this region indicate that the crust has low strength to withstand the accumulated strain energy

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

non double couple mechanism of moderate earthquakes occurred in lower siang region of arunachal himalaya evidence of factors affecting non dc moment tensor solutions of 104 earthquakes which were observed at local distances in lower siang region of arunachal himalaya have been estimated using isola code the magnitude range of analyzed earthquakes lies between 1 8 mw and 5 3 mw out of 104 earthquakes only 32 events are having good data the signal noise ratio of these 32 events is greater than 6 with magnitude greater than 2 5 all possible sources of non dc such as noise present in the data depth of the source low azimuthal coverage of the event and non dc as a part of real earthquake source process are examined the study reveals that non dc is highly dependent on the source depth as comparatively shallow events shows high clvd the clvd is also highly affected by the noise present in the data another factor is the magnitude of the event the high magnitude event shows quite high dc so the source mechanisms of high magnitude events are double couple dc

estimation and applicability of attenuation characteristics for source parameters and scaling relations in the garhwal kumaun himalaya region india abstract source parameters of the small to moderate earthquakes are significant for understanding the dynamic rupture process the scaling relations of the earthquakes and for assessment of seismic hazard potential of a region in this study the source parameters were determined for 58 small to moderate size earthquakes 3 0 m w 5 0 occurred during 2007 2015 in the garhwal kumaun region the estimated shear wave quality factor q f values for each station at different frequencies have been applied to eliminate any bias in the determination of source parameters the q f values have been estimated by using coda wave normalization method in the frequency range 1 5 16 hz a frequency dependent s wave quality factor relation is obtained as q f 152 9 7 f 0 82 0 005 by fitting a power law frequency dependence model for the estimated values over the whole study region the spectral low frequency spectral level and corner frequency and source static stress drop seismic moment apparent stress and radiated energy parameters are obtained assuming 2 source model the displacement spectra are corrected for estimated frequency dependent attenuation site effect using spectral decay parameter kappa the frequency resolution limit was resolved by quantifying the bias in corner frequencies stress drop and radiated energy estimates due to finite bandwidth effect the data of the region shows shallow focused earthquakes with low stress drop the estimation of zuniga parameter e suggests the partial stress drop mechanism in the region the observed low stress drop and apparent stress can be explained by partial stress drop and low effective stress model presence of subsurface fluid at seismogenic depth certainly manipulates the dynamics of the region however the limited event selection may strongly bias the scaling relation even after taking as much as possible precaution in considering effects of finite bandwidth attenuation and site corrections although the scaling can be improved further with the integration of large dataset of microearthquakes and use of a stable and robust approach

retrieval of super virtual refraction by cross correlation topography and severe variations of near surface layers lead to travel time perturbations for the events in seismic exploration usually these perturbations could be estimated and eliminated by refraction technology the virtual refraction method is a relatively new technique for retrieval of refraction information from seismic records contaminated by noise based on the virtual refraction this paper proposes supervirtual refraction interferometry by cross correlation to retrieve refraction wavefields by summing the cross correlation of raw refraction wavefields and virtual refraction wavefields over all receivers located outside the retrieved source and receiver pair this method can enhance refraction signal gradually as the source receiver offset decreases for further enhancement of refracted waves a scheme of hybrid virtual refraction wavefields is applied by stacking of correlation type and convolution type super virtual refractions our new method does not need any information about the near surface velocity model which can solve the problem of directly unmeasured virtual refraction energy from the virtual source at the surface and extend the acquisition aperture to its maximum extent in raw seismic records it can also reduce random noise influence in raw seismic records effectively and improve refracted waves signal to noise ratio by a factor proportional to the square root of the number of receivers positioned at stationary phase points based on the improvement of virtual refraction s signal to noise ratio using results from synthetic and field data we show that our new method is effective to retrieve refraction information from raw seismic records and improve the accuracy of first arrival picks

finite difference modeling experiments for seismic interferometry in passive seismic interferometry new reflection data can be retrieved by crosscorrelating recorded noise data the quality of the retrieved reflection data is among others dependent on the duration and number of passive sources present during the recording time the source distribution and the source strength to investigate these relations we set up several numerical modeling studies to carry out the modeling in a feasible time we design a finite difference algorithm for the simulation of long duration passive seismic measurements of band limited noise signatures in the subsurface novel features of the algorithm include the modeling of thousands of randomly placed sources during one modeling run the modeling experiments explore the dependency relation between the retrieved reflections and source signature length source positions number of sources and source amplitude variations from these experiments we observed that the positions of the passive sources and the length of the source signals are of direct influence on the quality of the retrieved reflections random amplitude variations among source signals do not seem to have a big impact on the retrieved reflections

seismic refraction interferometry with a semblance analysis on the crosscorrelation gather crosscorrelating wavefields recorded at two receivers to produce data as if one receiver was a source is commonly referred to as seismic interferometry or the virtual source method an artifact in seismic interferometry related to critically refracted waves allowed us to estimate the velocity in the refracting layer in addition we devised a new semblance analysis on the crosscorrelation of reflection and refraction energy to robustly estimate the depth and velocity of the slow layer tested with a numerical example and field data from the boise hydrogeophysical research site

application of super virtual seismic refraction interferometry to enhance first arrivals a case study from saudi arabia complex near surface anomalies are one of the main onshore challenges facing seismic data processors refraction tomography is becoming a common technology to estimate an accurate near surface velocity model this process involves picking the first arrivals of refracted waves one of the main challenges with refraction tomography is the low signal to noise ratio characterizing the first break waveform arrivals especially for the far offset receivers this is especially evident in data recorded using reflection acquisition geometry this low signal to noise ratio is caused by signal attenuation due to geometrical spreading of the seismic wavefield near surface generated noise and amplitude absorption super virtual refraction interferometry improves the quality of the first break picks by enhancing the amplitude of the refracted waves and attenuating the amplitude of the random noise

on a plane wave based crosscorrelation type seismic interferometry we explored a new approach to retrieve virtual seismic responses from crosscorrelating acquired seismic data in the plane wave domain using this method slant stacking is first performed over shot or receiver locations of observed seismic data to produce plane wave transformed gathers crosscorrelation is then performed by selecting traces with the same ray parameters from different shot or receiver locations of the plane wave gathers unlike traditional crosscorrelation type time space domain interferometry where full range of ray parameters is used for each survey location this method directly selects common ray parameters to cancel overlapping raypaths this approach can be used to retrieve reflections in the presence of dispersive waves and to select certain ranges of ray parameters with directional wave paths for retrieval it can avoid spurious arrivals in supervirtual interferometry when unwanted arrivals such as reflections break the requirement of conventional interferometry in addition computation time can be saved with this approach because plane wave transform usually results in a reduction of the original data volume we demonstrate this method with synthetic and ocean bottom seismometer data examples

electrical and magnetometric fields in a layered earth containing buried electrodes the number of analytical magnetometric resistivity mmr results available for basic earth geometries is limited compared to that of electrical resistivity methods which is unfortunate since mmr has advantages for certain classes of problems this paper extends the list of mmr results by deriving the response for the homogeneous half space and the multilayered earth both are calculated for arbitrary source and receiver positions we show how the result for the layered earth reduces to that of the half space when there is only one layer necessary procedures for successful numerical evaluation of results for the layered case are given sample borehole logs of electric and magnetic fields for an example of a two layer model illustrate an advantage of the magnetic field measurement namely that it is sensitive to the position of layer boundaries rather than to the position of transmitter electrodes the algorithm is also applied to interpretation of mmr field data from two boreholes drilled near massive s

metalliferous mining geophysics state of the art after a decade in the new millennium mining exploration was very active during the first decade of the twenty first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration development came from different sources instrumentation improvements new numerical algorithms and cross fertilization with the seismic industry in gravity gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration in potential field methods in general numerous techniques have been developed for automatic interpretation and 3d inversion schemes came into frequent use these inversions will have even greater use when geologic constraints can be applied easily in airborne electromagnetic em methods the development of time domain helicopter em systems changed the industry in parallel improvements in em modeling and interpretation occurred in particular the strengths and weaknesses of the various algorithms became better understood simpler imaging schemes came into standard use whereas layered inversion seldom is used in the mining industry today improvements in ground em methods were associated with the development of squid technology and distributed acquisition systems the latter also impacted ground induced polarization ip methods developments in borehole geophysics for mining and exploration were numerous borehole logging to measure physical properties received significant interest perhaps one reason for that interest was the desire to develop links between geophysical and geologic results which also is a topic of great importance to mining geologists and geophysicists

seismic interferometry reconstructing the earth s reflection response in1968 jonclaerboutshowedthatthereflectionresponseofa 1dacousticmediumcanbereconstructedbyautocorrelatingthe transmission response since then several authors have derived relationships for reconstructing green s functions at the surface using crosscorrelations of noise recordings that were taken at thesurfaceandthatderivedfromsubsurfacesources foracousticmedia wereviewrelationsbetweenthereflection response and the transmission response in 3d inhomogeneous lossless media these relations are derived from a one way wavefield reciprocity theorem we use modeling results to show howtoreconstructthereflectionresponseinthepresenceoftransientsubsurfacesourceswithdistinctexcitationtimes aswellas inthepresenceofsimultaneouslyactingnoisesourcesinthesubsurface weshowthatthequalityofreconstructedreflectionsdepends on the distribution of the subsurface sources for a situation with enough subsurface sources that is for a distribution that illuminates the subsurface area of interest from nearly all directions the reconstructed reflection responses and the migrated depth image exhibit all the reflection events and the subsurface structures of interest respectively with only a few subsurface sources that is with insufficient illumination the reconstructed reflection responses are noisy and can even become kinematically incorrect at the same time however the depth image which was obtained from their migration still shows clearly all the illuminated subsurface structures at their correct positions for the elastic case we review a relationship between the reflection green s functions and the transmission green s functionsderivedfromatwo waywavefieldreciprocitytheorem usingmodelingexamples weshowhowtoreconstructthedifferent components of the particle velocity observed at the surface and resulting from a surface traction source this reconstruciton is achievedusingcrosscorrelationsofparticlevelocitycomponents measured at the surface and resulting from separate p and s wavesourcesinthesubsurface

source receiver interferometric redatuming using sparse buried receivers to address complex near surface environments a case study of seismic imaging quality and time lapse repeatability

virtual shear source makes shear waves with air guns we demonstrate a novel application of the virtual source method to create shear wave sources at the location of buried geophones these virtual downhole sources excite shear waves with a different radiation pattern than known sources they can be useful in various shear wave applications here we focus on the virtual shear check shot to generate accurate shear velocity profiles in offshore environments using typical acquisition for marine walkaway vertical seismic profiling vsp the virtual source method is applied to walkaway vsp data to obtain new traces resembling seismograms acquired with downhole seismic sources at geophone locations thus bypassing any overburden complexity the virtual sources can be synthesized to radiate predominantly shear waves by collecting converted wave energy scattered throughout the overburden we illustrate the concept in a synthetic layered model and demonstrate the method by estimating accurate p and s wave velocity profiles below salt using a walkaway vsp from the deepwater gulf of mexico

a seismic hazard map of india and adjacent areas abstract we have produced a probabilistic seismic hazard map showing peak ground accelerations in rock for india and neighboring areas having a 10 probability of being exceeded in 50 years seismogenic zones were identified on the basis of historical seismicity seismotectonics and geology of the region procedures for reducing the incompleteness of earthquake catalogs were followed before estimating recurrence parameters an eastern united states acceleration attenuation relationship was employed after it was found that intensity attenuation for the indian region and the eastern united states was similar the largest probabilistic accelerations are obtained in the seismotectonic belts of kirthar hindukush himalaya arakan yoma and the shillong massif where values of over 70 g have been calculated

seismic imaging of the olduvai basin tanzania abstract a 5 6 km long line of refraction and reflection seismic data spanning the pliocene pleistocene fill of the olduvai basin tanzania is presented the line is oriented along a northwest southeast profile through the position of olduvai gorge coring project ogcp borehole 2a our aims are to 1 delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement 2 map synsedimentary normal faults and trace individual strata at depth and 3 provide context for the sequence observed in ogcp cores results with refraction tomography and poststack migration show that the maximum basin depth is around 405 m 25 m in the deepest portion which quadruples the thickness of the basin fill previously known from outcrops variations in seismic velocities show the positions of lower density lake claystones and higher density well cemented sedimentary sequences the bed i basalt lava is a prominent marker in the refraction seismic results bottom most sediments are dated to 2 2 ma near where borehole 2a bottoms out at the depth of 245 m however the seismic line shows that the basin fill reaches a maximum stratigraphic thickness of around 380 m deep at borehole 2a in the western basin where the subsidence was greatest this further suggests that potential hominin palaeoenvironments were available and preserved within the basin fill possibly as far back as around 4 ma applying a temporal extrapolation using the average sediment accretion rate

a comparison of receiver technologies in borehole mmr and em surveys a series of downhole magnetometric resistivity dhmmr and downhole electromagnetic dhem surveys were conducted near broken hill new south wales australia and at zinkgruvan sweden to determine how probe and receiver equipment choices affect the amount of noise visible in borehole mmr and em data noise analyses performed on the data using the standard deviation to gauge the relative noise levels between different probes and receiver systems indicate that high noise levels in mmr data result primarily from the use of a three component em probe which has a reduced effective area and hence a higher noise floor compared with a single component em probe high noise attributable to cultural sources such as nearby power lines in either mmr or em data can be reduced through the use of full waveform multipurpose receiver systems these systems allow for the use of tapered stacking which is a more effective method of eliminating coherent noise associated with power line transients than the boxcar stacking method used by traditional receiver systems

using supervirtual first arrivals in controlled source hardrock seismic imaging well worth the effort varied applications of seismic interferometry have arisen in the last decade however the potential of the method to improve reflection seismic processing in hardrock environments has not explicitly been regarded therefore in this paper we investigate the potential of retrieving the first arrivals originally hindered by high noise level in the exploitation of controlled source data acquired over the iron oxide apatite rich deposit at grangesberg sweden and its mining induced structures the supervirtual first arrivals retrieved using interferometry methodologies allowed first breaks to be picked more extensively than in the original data revised static corrections significantly improved the linearity of the first arrivals and continuity of reflections in the source gathers especially reflections considerably enhanced in the source gathers stacked constructively in the final section comparison with geological data supported by traveltime forward modelling indicates that these reflections represent the unmined part of the deposit other reflections at shallower depth are interpreted as anthropogenic faults possibly located at lithological contacts tomographic inversion was also run using the augmented traveltime data the greater resolution and penetration of this new tomographic image allowed better bridging with the results of the reflection seismic section velocity anomalies depict the presence of mining induced structures and a potential brewery fault which is believed to put at risks an urban area even though the potential of first arrival retrieval seems to be case dependent this study illustrates that interferometry may substantially improve the accuracy of static corrections and subsequent stack for hardrock imaging as well as in the penetration and resolution of traveltime tomograms

spatial distribution of radiated seismic energy from earthquakes in taiwan and surrounding regions abstract the radiated energy during earthquakes is one of the essential characteristics that have a significant impact on human lives the study of the released energy during earthquakes and their distribution may provide a detailed knowledge about the driving forces in the present work the earthquakes having magnitude ml between 1 98 and 7 3 that occurred between 1994 and 2017 in and around taiwan between the latitudes 21 5 n 25 5 n and longitudes 119 e 123 e are used to study the spatial distribution of energy in taiwan the maximum reported focal depth for the used earthquakes is 320 km ml is converted to energy with necessary correction in the existing conversion formula using a correction factor based on the energy of two earthquakes that occurred in taiwan it is found that the distribution of earthquake numbers and energy is not uniform in particular 98 of the events occurred within 100 km depth while the remaining 2 were located at deeper depths most of the events about 65 of the total earthquakes are confined to the upper 20 km depth the earthquakes occurring to a depth of 100 km contribute about 88 of the energy in and around taiwan only a few earthquakes occurring beyond 100 km depth contribute to around 12 of total released energy the highest energy release is attributed to the eastern subduction along the ryukyu trench our results show that the lower crust and upper mantle may play an essential role in energy distribution though most of the earthquakes have occurred in the upper crust therefore the temporal and spatial distributions of seismic energy release will be further studied to reveal the characteristics of the seismogenic zone in the future

theory of supervirtual refraction interferometry summary inverting for the subsurface velocity distribution by refraction traveltime tomography is a well accepted imaging method by both the exploration and earthquake seismology communities a significant drawback however is that the recorded traces become noisier with increasing offset from the source position and so accurate picking of traveltimes in far offset traces is often prevented to enhance the signal to noise ratio snr of the far offset traces we present the theory of supervirtual refraction interferometry where the snr of far offset head wave arrivals can be theoretically increased by a factor proportional to here n is the number of receiver or source positions associated with the recording and generation of the head wave arrival there are two steps to this methodology correlation and summation of the data to generate traces with virtual head wave arrivals followed by the convolution of the data with the virtual traces to create traces with supervirtual head wave arrivals this method is valid for any medium that generates head wave arrivals recorded by the geophones results with both synthetic traces and field data demonstrate the feasibility of this method there are at least four significant benefits of supervirtual interferometry 1 an enhanced snr of far offset traces so the first arrival traveltimes of the noisy far offset traces can be more reliably picked to extend the useful aperture of the data 2 the snr of head waves in a trace that arrive later than the first arrival can be enhanced for accurate traveltime picking and subsequent inversion by later arrival traveltime tomography 3 common receiver pair gathers can be analysed to detect the presence of diving waves in the first arrivals which can be used to assess the nature of the refracting boundary and 4 the source statics term is eliminated in the correlation operations so that the timing of the virtual traces is independent of the source excitation time this suggests the possibility of applying this method to earthquake data recorded by receivers that are inline with the refraction paths and source locations

a modified delay time method for statics estimation with the virtual refraction topography and near surface heterogeneities lead to traveltime perturbations in surface land seismic experiments usually these perturbations are estimated and removed prior to further processing of the data a common technique to estimate these perturbations is the delay time method we have developed the modified delay time method wherein we isolate the arrival times of the virtual refraction and estimate receiver side delay times the virtual refraction is a spurious arrival found in wavefields estimated by seismic interferometry the new method removes the source term from the delay time equation is more robust in the presence of noise and extends the lateral aperture compared to the conventional delay time method we tested this in an elastic 2d numerical example where we estimated the receiver delay times above a horizontal refractor taking advantage of reciprocity of the wave equation and rearranging the common shot gathers into common receiver gathers isolated source delay times could also be obtained

enhanced refractor imaging by supervirtual interferometry refraction surveys are a well established method of imaging subsurface velocities both in terms of the deep crustal structure at global scales and in the shallow near surface these surveys generally involve deploying an array of receivers on the surface or water bottom and recording arrivals from a seismic source initiated at or near the surface

measurements of spectral similarity for microearthquakes in western nagano japan 1 we use p wave spectral ratios to estimate the seismic energy of 23 microearthquakes mw 0 9 2 7 recorded on a dense network of surface and borehole stations in western nagano japan these events were recorded at 45 surface stations and 2 borehole stations the data set is unique in that most events have high signal to noise ratio at several surface stations and at the two borehole stations the redundancy provided by the large number of stations recording these events allows us to assess the uncertainty of our results we find that the seismic energy to moment er m0 ratios from p waves for the events in our data set are reliable that they vary between 1 10 6 and 1 10 4 and that smaller events tend to have smaller values of er m0 because they are relatively deficient in radiated energy above the corner frequency we observe two spectral effects a variation in corner frequency which accounts for most of the variation in er m0 and steeper spectral falloff rates which contribute to the small values of er m0 for some of the smaller events our estimates of moment and corner frequency suggest that m0 fc 3 e 0 and this implies that either the static stress drop or rupture velocity or both change with earthquake size

spectral scaling and seismic efficiency for earthquakes in northeast india we examine the scaling of spectral properties of 122 local earthquakes 2 6 that is proportional to stress drop independent of any particular fault geometry and by considering the measurement bias for the events used in this study the value of c r is close to 2 0 a value expected for circular crack model and self similar source scaling however the relatively large uncertainties due to earthquake selection bias and recording bandwidth limitation suggest caution in drawing conclusions from these findings the events are grossly characterized by low and almost constant value of radiation efficiency and positive overshoot corresponding to an increase in relative dynamic strength suggesting the importance of energy dissipation during the rupture process although the scaling of radiated energy and stress drop as obtained here takes into account as much as possible the effects of finite bandwidth attenuation and site corrections the limited event selection may strongly bias it an improvement over the present scaling is suggested with an improved seismic network in the region and also with the use of a stable empirical green s function approach for source parameter estimation 1 embed inline graphic 1 gif

seismic source parameters for microearthquakes of the granada basin southern spain abstract ninety five microearthquakes of the granada basin southern spain with duration magnitudes ranging from 1 3 to 3 5 moment magnitude from 0 9 to 2 5 have been spectrally analyzed with digital recordings from ten stations of the andalusian seismic network the coda q quality factor qc assumed to be frequency dependent was used instead of q for the path correction and the q adopted was equal to 9 4 qc the k parameter for site attenuation correction had values in the range of 0 02 0 05 fourier displacement spectra were calculated for p and s waves and analyzed with respect to the source model of brune 1970 1971 spectral parameters were automatically determined by the method of snoke 1987 the seismic moment ranged from 1017 to 1020 dyne cm with average m0 p to m0 s ratio of 1 02 and the source radii spanned from 0 14 to 0 40 km with average r p to r s ratio of 1 3 the seismic energy values ranged from 1 103 to 9 107 j with p to s energy ratio from 1 to 10 for about 85 of the events whereas the static stress drop varied from 0 02 to 2 6 bar the scaling relation between the seismic moment and the stress drop indicates a decrease in stress drop with decreasing seismic moment apparent stress is linearly correlated with stress drop and have on average similar values and this points out to a partial stress drop

increasing the number and signal to noise ratio of obs traces with supervirtual refraction interferometry and free surface multiples summary the theory of supervirtual interferometry is modified so that free surface related multiple refractions can be used to enhance the signal to noise ratio snr of primary refraction events by a factor proportional to ns where ns is the number of post critical sources for a specified refraction multiple we also show that refraction multiples can be transformed into primary refraction events recorded at virtual hydrophones located between the actual hydrophones thus data recorded by a coarse sampling of ocean bottom seismic obs stations can be transformed in principle into a virtual survey with p times more obs stations where p is the order of the visible free surface related multiple refractions the key assumption is that the refraction arrivals are those of head waves not pure diving waves the effectiveness of this method is validated with both synthetic obs data and an obs data set recorded offshore from taiwan results show the successful reconstruction of far offset traces out to a source receiver offset of 120km the primary supervirtual traces increase the number of pickable first arrivals from approximately 1600 to more than 3100 for a subset of the obs data set where the source is only on one side of the recording stations in addition the head waves associated with the first order free surface refraction multiples allow for the creation of six new common receiver gathers recorded at virtual obs station located about half way between the actual obs stations this doubles the number of obs stations compared to the original survey and increases the total number of pickable traces from approximately 1600 to more than 6200 in summary our results with the obs data demonstrate that refraction interferometry can sometimes more than quadruple the number of usable traces increase the source receiver offsets fill in the receiver line with a denser distribution of obs stations and provide more reliable picking of first arrivals apotentialliabilityofthismethodisthatlong offsetrefractionarrivalsextractedbyinterferometry might not necessarily be head waves from deeper refraction interfaces the extracted arrivals might be from a shallower interface and so only supply redundant information about that portion of the subsurface nevertheless our tomography example shows the value of these arrivals in reducing artefacts and increasing resolution in the tomogram

extracting earth s elastic wave response from noise measurements recent research has shown that noise can be turned from a nuisance into a useful seismic source in seismology and other fields in science and engineering the estimation of the system response from noise measurements has proven to be a powerful technique to convey the essence of the method we first treat the simplest case of a homogeneous medium to show how noise measurements can be used to estimate waves that propagate between sensors we provide an overview of physics research dating back more than 100 years showing that random field fluctuations contain information about the system response this principle has found extensive use in surface wave seismology but can also be applied to the estimation of body waves because noise provides continuous illumination of the subsurface the extracted response is ideally suited for time lapse monitoring we present examples of time lapse monitoring as applied to the softening of soil after the 2011 tohoku oki earthquake the detection of a precursor to a landslide and temporal changes in the lunar soil

scaling law between corner frequency and seismic moment of microearthquakes is the breakdown of the cube law a nature of earthquakes 1 the corner frequency fc and the seismic moment m0 of microearthquakes local magnitude from 1 3 to 1 3 are estimated from the records of sensors located at an 1800 m deep borehole at the nojima fault central japan the digital waveforms of each event are recorded with two different sampling rates 10 khz and 100 hz the source parameters estimated from high sampling data satisfy a relationship of m0 fc 3 corresponding to a constant stress drop law while those calculated from low sampling data follow a relationship of m0 fc 4 a lack of high frequency components in the 100 hz sampling data seem to be obvious indicating that the value of fc and m0 are incorrectly estimated and the relationship of m0 fc 4 is an artifact caused by the limited frequency band of these data the scaling relationship of m0 fc 3 is thus also valid for microearthquakes and the breakdown of the constant stress drop law reported in the literature is not well founded

simulation of strong ground motion for an mw 7 0 earthquake beneath the bhutan himalaya in ne india and its trans boundary seismic hazard implications abstract we simulate the strong ground motion for an expected major earthquake mw 7 0 beneath the bhutan himalayan region with an empirical green s function method using waveforms recorded from the 2009 bhutan mainshock mw 6 1 and its largest aftershock mw 5 1 fault orientation and location of the simulated event extend the length of the fault plane determined from the 2009 bhutan mainshock and aftershock along its strike the simulated pga values are compared with the results derived from a ground motion prediction equation gmpe for the himalayan region and it is found that different levels of accelerations are associated with different rupture initiation points on the fault plane it is observed that the ne indian region is capable of generating peak ground acceleration pga in exceedance to 121 cm s2 for simulated earthquake mw 7 0 the maximum impacts of shaking will be on the sites located near the rupture initiation points that are poised to generate higher values of ground acceleration to validate our simulation we also estimate the extent of rupture directivity of the simulated earthquake with respect to four initiation points indicating that higher value of pga and shaking duration exist either to south or to south west azimuths from the imitated locations which are corroborated with respective geology of the sites this study suggests that areas of maximum ground shaking would occur in the vicinity of the source initiation where possibility of relatively stronger earthquake hazards does exist which in turn requires attention for adoption of earthquake risk mitigation plans in view of impacts of trans boundary earthquakes in the region

the virtual refraction useful spurious energy in seismic interferometry seismic interferometry is rapidly becoming an established technique to recover the green s function between receivers but practical limitations in the source energy distribution inevitably lead to spurious energy in the results instead of attempting to suppress all such energy we use a spurious wave associated with the crosscorrelation of refracted energy at both receivers to infer estimates of subsurface parameters we named this spurious event the virtual refraction illustrated by a numerical two layer example we found that the slope of the virtual refraction defines the velocity of the faster medium and that the stationary phase point in the correlation gather provides the critical offset with the associated critical time derived from the real shot record this approach includes all of the necessary information to estimate wave speeds and interface depth without the need of inferences from other wave types

an overview on the seismic zonation and microzonation studies in india abstract the present study presents a review on the progressive development of the seismic zonation map of india both from official agencies and also from independent individual studies the zonation map have been modified and updated regularly with the occurrence of major destructive earthquakes over the years in the indian subcontinent with the addition of new data this study discusses the criteria chosen for the progressive zonation and the major earthquakes that were responsible for retrospection of the earlier published maps the seismic zonation maps of india have also been prepared by various independent workers by adopting different approaches to achieve the purpose of the zonation despite the endeavors from various sources to provide a solution for the problem of earthquake hazards in india there were many limitations on the zonation map as it gives the picture at a regional scale mostly on the bedrock level without addressing the local site conditions but nevertheless the seismic zonation map gives basic guidelines for any region to know the hazard scenario and if any city or urban population is under threat from seismic point of view further site specific seismic microzonation may be carried out in the international scenario the global seismic hazard assessment program gshap in 1999 prepared a hazard map for world in terms of peak ground acceleration pga with a 10 probability of exceedance in 50 years but it turned out to be an underestimation of the hazard parameter when compared with the observed pga to tackle the problem of seismic hazards there was a need to have a detail study on the local site conditions in terms of its geological geophysical and geotechnical properties with the advent of better instrumentation and knowledge on the mechanics of earthquakes it was possible to identify zones of hazards at a local level and this gives rise to the study of seismic microzonation seismic microzonation work has been carried out in india in some of the strategic important mega cities and industrial build up that has the potential of being damaged from future earthquakes as has been shown in the past though the microzonation map is not the final output map as it can still be updated at later stage with more input data it does provide a more realistic picture on the site specific seismic hazard