the origin of teleseismic pn waves multiple crustal scattering of upper mantle whispering gallery phases 1 teleseismic pn arrivals with an extensive coda are observed to offsets beyond 3000 km along the peaceful nuclear explosion seismic profiles quartz and ruby which were recorded in the western part of the former soviet union we interpret these arrivals as multiply reflected sub moho refractions that travel over large distances due to a positive vertical upper mantle velocity gradient which is characteristic for the study area analysis of the observed data shows that the teleseismic pn and its coda contain significant amounts of energy at all frequencies between 0 and 10 hz our modeling results show that crustal scattering may fully explain the teleseismic pn coda contrary to another published model with upper mantle heterogeneity in the 35 130 km depth range these conclusions are based on two dimensional viscoelastic finite difference seismic wave field simulations in 2000 km long and 250 km deep models of the crust and upper mantle the computationally demanding calculations are facilitated by the use of supercomputer systems our preferred model of crustal scattering is consistent with the results of other high resolution wide angle and normal incidence seismic investigations of the crust and upper mantle they show pronounced reflectivity of the lower crust above an almost transparent uppermost mantle down to 80 100 km depth our model includes a high vertical velocity gradient in the upper mantle based on the results from seismic refraction studies we suggest that the teleseismic pn wave travels as an upper mantle whispering gallery phase and that the origin of its long coda is crustal scattering

array analysis of seismograms from explosive sources evidence for surface waves scattered at the main topographical features in this article we analyze the seismic wavefield produced by blasts fired at mt vesuvius for a deep seismic sounding experiment and recorded by a dense short period three component seismic array seismograms from five explosions occurring at the bottom of wells located at different distances and azimuth with respect to the array site have been analyzed with the zero lag cross correlation technique applied to both vertical and horizontal components in the frequency range 3 7 hz the aim is to analyze the portion of the seismograms long after the first p onset in order to obtain the slowness and backazimuth distribution of correlated secondary phases polarization analysis in time domain carried out using the array covariance matrix technique has also been applied to the well correlated phases to determine their wave composition results show that the correlated phases detected in the coda of the seismogram shots are composed almost entirely of surface waves location of the scatterers shows a high concentration of the scatterers in and around the volcanic edifice of mt vesuvius confirming that topographical irregularities play an important role in the generation of the high frequency scattering phenomena

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

teleseismic pn arrivals influence of mantle velocity gradient and crustal scattering teleseismic p n phases with a long high amplitude coda are observed to offsets 3000 km along the peaceful nuclear explosion seismic profiles quartz and ruby for the high frequency 5 hz part of the quartz data which is shown here coda lengths of more than 10 s can be visually observed in the seismic sections we interpret the teleseismic p n arrivals as multiple sub moho refractions whispering gallery phases which travel over large distances due to a positive upper mantle velocity gradient the coda of these arrivals is explained by lower crustal scattering of the whispering gallery phases from the mantle the modelling is based on calculation of reflectivity and 2 d visco elastic finite difference synthetic seismograms our models are in agreement with other deep seismic data from continental areas which typically show a reflective lower crust and an almost transparent uppermost mantle the lower crustal heterogeneity is described by von karman distribution with a standard deviation of 0 5 km s 1 for the p wave velocity and correlation lengths of 2 4 km horizontally and 0 6 km vertically the positive vertical upper mantle velocity gradient of 0 004 km s 1 km 1 which is characteristic for profile quartz forms a waveguide which gives rise to the teleseismic p n it is not necessary to include upper mantle heterogeneity in order to explain the characteristics of this arrival

spatial distribution of coda scatterers in the crust around two active volcanoes and one active fault system in central japan inversion analysis of coda envelope abstract coda waves observed by local seismographic networks were analysed by using the inversion method presented by nishigami 1991 to estimate spatial distribution of coda scatterers in the crust first deviations of the decay curve of observed coda envelope away from a reference curve were measured for lapse times greater than about 1 5 2 0 times the s wave travel times we assume this kind of deviation is mainly caused by the non uniform distribution of scatterers in the crust next the 3 d distribution of relative scattering strength in the crust was obtained by solving the observational equation where single isotropic scattering and a spherical radiation from the source are assumed and the observational data is given by the deviation of coda envelope stated above this method was applied to two volcanic regions and one active fault region in the central part of japan in each analysis the target region with dimension of 50 160 km in horizontal and 30 80 km in depth was divided into about 1500 2000 blocks with one side of 2 10 km and 600 900 waveform traces from about 100 250 events observed at 7 38 stations were used the lapse time of coda waves analysed was taken to be as short as possible to reduce the multiple scattering effect it was in the shortest case about 3 13 s for the ontake region the result of inversion analyses showed several significant features strong scatterers were found at shallow part 0 20 km of the crust around the fukui earthquake fault a low seismicity area between active faults showed a correlation with the weak scattering area the detailed distribution of scatterers was also revealed around two active volcanoes mt ontake and mt nikko shirane the vertical distribution of strong scatterers just below the volcano from 0 to 7 km in depth and another horizontal one at a depth of 7 km were estimated in the ontake region an inclined distribution of scatterers was estimated at depths from 0 to 20 km just below mt nikko shirane the location of scatterers around these active volcanoes agreed well with that of the s wave reflectors in the crust obtained by the nmo correction analysis frequency dependence of scatterer distribution was also examined for the ontake region the horizontal distribution of scatterers at 7 km depth showed a similar pattern for frequencies between 5 and 20 hz while other smaller scale scatterers appeared to have frequency dependence

geologic applications of seismic scattering abstract once disregarded as noise scattered seismic waves are finding increasing application in subsurface imaging this sea change is driven by the increasing density and quality of seismic recordings and advances in waveform modeling which together are allowing seismologists to exploit their unique properties in addition to extensive application in the energy exploration industry seismic scattering is now used to characterize heterogeneity in the lower continental crust and subcrustal lithosphere to examine the relationship between crustal structure and seismogenesis and to probe the plumbing of active volcanoes in each application the study of seismic scattering brings wavelength scale structure into sharper focus and characterizes the short scale length fabric of geology

a review of numerical experiments on seismic wave scattering this paper reviews applications of the finite difference and finite element methods to the study of seismic wave scattering in both simple and complex velocity models these numerical simulations have improved our understanding of seismic scattering in portions of the earth where there is significant lateral heterogeneity such as the crust the methods propagate complete seismic wavefields through highly complex media and include multiply scattered waves and converted phases e g p tosv sv top body wave to surface wave the numerical methods have been especially useful in cases of moderate and strong scattering in complex media where multiple scattering becomes important progress has been made with numerical methods in understanding how near surface low velocity basin structures scatter surface waves and vertically incident body waves the numerical methods have proven useful in evaluating scattering of surface waves and body waves from topography of both the free surface and interfaces buried at depth numerical studies have demonstrated the importance of conversions from body waves to surface waves andvice versa when lateral heterogeneities and topographic relief are present in the uppermost crust recently several investigations have applied numerical methods to study seismic wave propagation in velocity models which vary randomly in space this stochastic approach seeks to understand the effects of small scale complexity in the earth which cannot be resolved deterministically these experiments have quantified the relationships between the statistical properties of the random heterogeneity and the measurable properties of high frequency 1 hz seismograms these simulations have been applied to the study of many features observed in actual high frequency seismic waves including the amplitude and time decay of seismic coda the apparent attenuation from scattering the dispersion of waveforms and the travel time and waveform variations across arrays of receivers

storm tracks in the southern hemisphere abstract from 1979 to 1989 global european centre for medium range weather forecasts ecmwf analyses of many meteorological variables important in baroclinic storms have been bandpass filtered to highlight fluctuations on 2 to 8 day time scales in order to illuminate the relationships among the variables in baroclinic storm track regions because of the greater zonal symmetry in the southern hemisphere sh versus the northern hemisphere nh it is possible to examine these relationships over a zonal sector without being dominated by local effects associated with jet stream entrance and exit regions of the storm tracks the bandpassed variance of geopotential height is used to define the storm track and its meridional profile has a pronounced midlatitude maximum in the sh the locations of the jet stream and the variance maxima of meridional and zonal velocity components vorticity vertical motion specific humidity and temperature all bear a distinctive relationship to the storm track but the lat

the relation of crustal scattering to seismicity in southern california dense regional array recordings of 377 teleseismic events are used to map geographic variability in upper crustal p to s scattering in southern california scattered wave energy is sensitive to short wavelength heterogeneity and maps of scattering potential offer a powerful complement to travel time tomography in characterizing seismic crustal heterogeneity the scale length of resolved scattering variability is commensurate with the scale lengths of surface fault trace length and microseismicity variations and both mapped faults and seismicity are concentrated in regions of strong scattering fault proximal scattering averaged across strike of predominately strike slip fault zones is highly correlated with microseismicity levels and is predictive of the pattern of microseismicity variation but not of absolute scale north south profiles through the transverse ranges reveal coincident strong gradients in microseismicity and scattering whereas mapped segment bounds along the san andreas fault zone from san francisco to the salton sea coincide with strong along strike gradients in the level of scattering the overall correlation of microseismicity and scattering potential is consistent with structural control of aftershock and background microseismicity production strain energy control of scattering or both prior evidence for fault offset in the pattern of scattering intensity revenaugh and reasoner 1997 favors the former

site amplification from s wave coda in the long valley caldera region california following the work of phillips 1985 we have computed site amplification factors for coda waves at many sites in the long valley region in the eastern sierra nevada we computed ratios of coda amplitudes measured at 15 stations in and around long valley caldera relative to a granitic site mmpm for six frequency bands centered at 1 5 3 0 6 0 9 0 12 0 and 15 0 hz all station sites located within the caldera experienced large ground motion amplification at 1 5 and 3 0 hz ranging between five and 17 times that of the reference site however at higher frequencies these same sites exhibited significantly less amplification than the reference granite site this is attributed to the competing effects of an impedance contrast between the basement rock and caldera fill and very high absorption in the caldera fill at high frequencies station mmlm located on top of a volcanic plug displayed the largest amplitudes of all the sites studied for frequencies between 9 0 and 15 0 hz a dike structure attached to the plug couples the basement rock to the surface at high frequencies the resulting large amplitudes at mmlm are not due to amplification resulting from a strong impedance contrast rather the absorption under this site is very low perhaps lower than at the reference site mmpm outside the caldera another hard rock site located at devil s postpile mdpm generally behaved like the reference site for all frequencies the lowest amplifications observed came from a site outside the caldera mdcm located on thin pyroclastic ash deposits overlying granitic basement this can be attributed to a dominance of absorption over the amplification caused by lower impedance of this layer variations among sites on similar surface geology may be due to small local variations in impedance and absorption under and adjacent to the site the range in the spectral decay parameter between caldera and rock sites are comparable to results of anderson and hough 1984 for sites on alluvium and rock in the san fernando region these surprisingly different amplifications support the need for additional site specific studies amplifications determined in this study for the frequency range 1 5 and 3 0 hz correlate remarkably well with eaton s 1990 residuals for duration magnitude fmag and amplitude magnitude xmag for the usgs northern california seismic array further supporting the use of coda waves in determining site specific amplification

the seismic coda site effects and scattering in alluvial basins studied using aftershocks of the 1986 north palm springs california earthquake as source arrays abstract we use single station recordings of widely distributed earthquake sources to identify heterogeneities in the earth9s crust that scatter energy into the early part of the observed s wave codas we assume that ray theory accurately describes the propagation of body waves from the earthquake sources to the scatterers but we assume no propagation mechanism between the scatterers and the seismic station a grid of hypothetical scatterers is assumed to cover the earth9s surface and the strength of each scatterer is determined by an iterative inversion method our data are recordings of aftershocks of the 1986 north palm springs earthquake recorded at a seismic station in the coachella valley smp and at a station in the morongo valley smc the technique is applied separately to each horizontal component of motion at each station we identify a particular spot at the northwest edge of the quaternary alluvium in the coachella valley that scatters waves into the s coda observed at smp 8 km distant in the valley the geologic structure that causes the strong scattering seems to be the basin edge structure rather than the mountainous terrain 3 km farther west the early coda at smc is dominated by scattering within a few kilometers of smc either in the basin itself or in the morongo valley fault zone this study shows that laterally propagating waves are sometimes observable in the early coda casting doubt on the use of 1 d techniques for estimating site responses the northwest edge of the coachella valley seems to be an especially strong scatterer of incident waves if other basins have similar loci of strong scattering perhaps these places can be identified using microearthquake seismograms and used to predict strong ground motions in the basins

relation of the 1992 landers california earthquake sequence to seismic scattering measurements of crustal scattering for the area surrounding the 1992 landers earthquake sequence obtained from regional array recordings of teleseismic events for the 10 year period before the sequence showed that the slip distribution on faults could be deducible from the preshock elastic structure scattering intensity correlated strongly with the distribution of aftershocks and slip of the moment magnitude mw 7 3 landers main shock mw 6 1 joshua tree and mw 6 2 big bear events which implies that aftershocks and slip are structurally controlled and broadly predictable scattering within the fault zones was directional and consistent with variable along strike alignment of stress induced cracks

reservoir analog studies using multimodel photogrammetry a new tool for the petroleum industry an advanced photogrammetric method called multimodel stereo restitution is a potential new tool for the petroleum industry when outcrop investigations are necessary such as in reservoir analog studies the method is based on very simple field photography techniques allowing the geologist to use his own standard small frame camera e g 24 36 or 60 60 mm formats it can be applied to geological studies of virtually any scale and outcrop mapping is significantly improved in detail accuracy and volume the method is especially useful when investigating poorly accessible exposures on steep mountain faces and canyon walls the use of multimodel photogrammetry is illustrated by a study of upper cretaceous deltaic sediments from the atane formation of west greenland detailed vertical cross sections of all outcrops in the area are measured and in addition horizontal maps of individual sand bodies are compiled on this basis data such as width thickness ratios sinuosity and the shapes of sand bodies and paleochannels crucial in reservoir analog studies are quantified and collected further potential applications of the method in petroleum explorations are discussed true scale mapping of lithologies in large continuous exposures can be used in understanding basin evolution and in seismic modeling close range applications can be used when modeling fault geometries and in studies of individual bed forms clay laminae cemented horizons and diagenetic fronts

time lapse monitoring of rock properties with coda wave interferometry 1 the coda of seismic waves consists of that part of the signal after the directly arriving phases in a finite medium or in one that is strongly heterogeneous the coda is dominated by waves which have repeatedly sampled the medium small changes in a medium which may have no detectable influence on the first arrivals are amplified by this repeated sampling and may thus be detectable in the coda we refer to this use of multiple sampling coda waveforms as coda wave interferometry we have exploited ultrasonic coda waves to monitor time varying rock properties in a laboratory environment we have studied the dependence of velocity on uniaxial stress in berea sandstone the temperature dependence of velocity in granite and in aluminum and the change in velocity due to an increase of water saturation in sandstone there are many other possible applications of coda wave interferometry in geophysics including dam and volcano monitoring time lapse reservoir characterization earthquake relocation and stress monitoring in mining and rock physics

beam stack imaging using a small aperture array we seek to gain a fuller understanding of seismic coda generation in the continental crust by identifying secondary scattering sources illuminated by a distant primary source we have developed a migration technique to scan seismic coda recorded by a small aperture seismic array for phases generated locally by scattering from large heterogeneities or topographic undulations we use a widely distributed suite of seismic events to illuminate the local crustal volume from different directions and produce an image of the local crust stable apparent secondary seismic sources are observed and interpreted as scatterers excited by the primary events

spatial distribution of relative scattering coefficients determined from microearthquake coda a coda envelope inversion technique is developed to image the three dimensional spatial distribution of relative scattering coefficients the technique is applied to the norris lake community georgia earthquake swarm of 1993 the relative scattering coefficient is the ratio of the scattering coefficient to a regionally determined average used to normalize the coda amplitudes the inversion problem was solved using the algebraic reconstruction technique art with the relative scattering coefficient constrained to positive values although coda envelope inver sion is based on the theory for single scattering we show that it may with a correction factor be used to characterize the distribution of relative scattering coefficients when multiple scattering contributes significantly to the coda the coda envelope inversion for the norris lake community earthquake swarm revealed a highly nonuniform distribution of relative scattering coefficients the zones of strong scattering are correlated with the hypocenters and areas of rough topography moreover at shallow depths the relative scattering coefficients are frequency dependent with scattered energy increasing with frequency we interpret this frequency dependence to imply that open fractures less than 250 m in length are responsible for strong scattering in the coda of these shallow earthquakes at greater depths scattering is independent of frequency and probably relates to velocity layering a strong reflecting layer was identified at or below 7 km which is consistent with the thrust plain identified in the georgia cocorp reflection lines the inhomogeneous distribution of scattering strength obtained in this study argues strongly against the observation based as sumption that the scattering structures are uniformly distributed in the crust

inversion of p coda for isotropic scatterers at the yucca flat test site although many studies have addressed the statistical properties of p coda there have been few attempts to analyze coda in a deterministic framework in this analysis 1050 teleseismic p wave seismograms from 32 yucca flat explosions are inverted for the locations of any isotropic point scatterers in the source region that contribute coherent arrivals to the coda a rectangular grid of potential scattering locations is constructed around the source array and the semiblance a measure of waveform ensemble coherence for each point is computed by slant stacking the full set of seismograms at various move out velocities the potential resolution or sensitivity of the method for this data set is tested by constructing a corresponding suite of synthetic seismograms with direct arrivals plus arrivals from various point scatters and calculating the semiblance at each point on the grid the particular source station geometry produces better sensitivity near the array sensitivity is enhanced parallel to the trend of the elongated source array and diminished in the orthogonal direction stacking the data for a velocity of 2 5 km sec yields relatively high semiblance values in the central portion of the test site as well as about 10 km to the ene and moderately high semiblance valuesmore about 10 km to the wnw less

spatial variation of crustal coda q in california coda wave data from california microearthquakes were studied in order to delineate regional fluctuations of apparent crustal attenuation in the band 1 5 to 24 hz apparent attenuation was estimated using a single back scattering model of coda waves the coda wave data were restricted to 30 s following the origin time this insures that crustal effects dominate the results as the backscattered shear waves thought to form the coda would not have had time to penetrate much deeper results indicate a strong variation in apparent crustal attenuation at high frequencies between the franciscan and salinian regions of central california and the long valley area of the sierra nevada although the coda q measurements coincide at 1 5 hz q c 100 at 24 hz there is a factor of four difference between the measurements made in franciscan q c 525 and long valley q c 2100 with the salinian midway between q c 900 these are extremely large variations compared to measures of seismic velocities of comparable resolution demonstrating the exceptional sensitivity of the high frequency coda q measurement to regional geology in addition the frequency trend of the results is opposite to that seen in a compilation of coda q measurements made worldwide by other authors which tend to converge at high and diverge at low frequencies however the worldwide results generally were obtained without limiting the coda lengths and probably reflect upper mantle rather than crustal properties our results match those expected due to scattering in random media represented by von karman autocorrelation functions of orders 1 2 to 1 3 the von karman medium of order 1 3 corresponding to the franciscan coda q measurement contains greater amounts of high wavenumber fluctuations this indicates relatively large medium fluctuations with wavelengths on the order of 100 m in the highly deformed crust associated with the franciscan however the influence of scattering on the coda q measurement is currently a matter of controversy

monitoring rapid temporal change in a volcano with coda wave interferometry 1 multiply scattered waves typically dominate the late part of the seismic coda in local earthquake seismograms small medium changes that have no detectable influence on the first arrivals are amplified by multiple scattering and may thus be readily observed in the coda we exploit this idea using coda wave interferometry to monitor temporal changes at mount erebus volcano antarctica erebus is one of the few volcanoes on earth with a long lived convecting lava lake large exsolved gas bubbles generate impulsive strombolian explosions that provide a repeating seismic source of seismic energy propagating through the strongly scattering geology of the volcano we examined these signals during a particularly active eruptive two month period between december 1999 and february 2000 early seismograms are highly reproducible throughout this period during the first month this is also the case for the coda approximately midway through this period however the seismic coda decorrelates rapidly over a period of several days this indicates a rapid change in the scattering properties of the volcano likely reflecting subtle changes in the near summit magma conduit system that would not be discernible using direct or single scattered seismic wave methods

identification of crustal and upper mantle heterogeneity by modelling of controlled source seismic data abstract high frequency controlled source seismic sections with dense spatial sampling show the existence of heterogeneity at different depth levels of the continental crust and upper mantle our sources of information are the peaceful nuclear explosion pne seismic data sets recorded to large offsets in the former soviet union supplemented by recordings from the north american early rise deep seismic experiment and normal incidence reflection seismic sections collected in northwest europe heterogeneity in the crust and upper mantle can be uniquely identified in reversed high frequency 2 10 hz pne seismic sections collected with dense spatial sampling nominal receiver spacing of 10 15 km out to 4000 km offset we document pronounced seismic scattering from three heterogeneous zones the lower crust from 20 km to 40 km depth an 80 km thick low velocity zone below 100 km depth and the 320 460 km depth interval around the top of the mantle transition zone we calculate the full seismic wavefield in heterogeneous crust mantle models with a two dimensional finite difference algorithm we represent the heterogeneous layers by random fluctuations of the elastic parameters and q values the spatial horizontal and vertical correlation lengths and the standard deviation of the scattering media are constrained by comparison of observed and calculated seismic sections the lower crustal heterogeneity causes a coda to the upper mantle arrivals at all recorded frequencies this coda is a prominent feature for whispering gallery phases teleseismic pn which travel as multiply reflected refractions below the moho to more than 3000 km offset from the pne sources the heterogeneous mantle low velocity zone causes a scattered coda trailing the first arrivals in the 800 1400 km offset range the best fit to the observations along profile kraton in siberia is obtained by an 80 km thick heterogeneous low velocity zone below 100 km depth represented by fluctuations described by a von karman distribution function with a hurst number of 0 5 and spatial correlation lengths of 5 10 km horizontally and 3 5 km vertically scattered arrivals which trail the reflection from the 410 discontinuity and a reflection from a shallower depth of 320 km constrain heterogeneity around the top of the transition zone this heterogeneity is modelled by fluctuations in the 320 460 km depth interval with correlation lengths on the order of 20 40 km by 5 10 km scattering from the two shallower heterogeneous zones improves the description of the phases from around the transition zone

rg observations from four continents inverse and forward modelling experiments summary using short period recordings of fundamental mode rayleigh waves rg from seven arrays on four continents we have explored structural properties in the respective siting areas through inversion of rg phase velocity dispersion characteristics the arrays studied were noress and arcess in norway geress in germany eskdalemuir in scotland yellowknife in canada gauribidanur in india and alice springs in australia the simplest model used consists of one layer over a half space with shear velocities and layer thickness as unknowns densities were held fixed while p velocities were constrained by a constant poisson ratio of 0 25 with one exception eskdalemuir the estimated s velocities were remarkably consistent between the arrays with an average value of 2 9 0 1 kms 1 in the upper layer and 3 55 0 1 kms 1 in the half space however estimated layer thicknesses varied considerably ranging from 0 12 km yellowknife to 1 6 km alice springs inversions were also performed with a simulated gradient layer near the surface model results and the ability to fit the observed velocity dispersion were very similar with the two types of models thus revealing a resolution problem with the limited frequency range available and the lack of higher modes to examine the effect of lateral inhomogeneities in terms of von karman velocity variations and topography on rg propagation 2 d finite difference synthetics were computed in both cases scattering was most pronounced for frequencies above 1 hz and thus explain the lack of rg phase velocity estimates for periods shorter than 0 6 s

f k analysis of noress array and single station data to identify sources of near receiver and near source scattering abstract f k analyses of short period recordings of both u s and soviet underground nuclear explosions at the eskdalemuir eka scotland array indicate arrivals from a local source about 15 km due northwest of the array use of residual seismograms derived by subtracting the beamed record from each array channel confirmed the presence of the same scatterer the secondary seismic source appears to be short period surface waves due to the scattering of incident p waves at a nearby deep valley known as moffat water in agreement with key s 1967 earlier results similar analyses of the noress norway array data from u s and soviet nuclear shots also indicate secondary arrivals suggesting a local near surface scatterer about 25 30 km south west of the array in the region of lake mjosa thus short aperture array data can be useful in identifying and locating sources of near receiver scattering f k analyses of short period recordings of both explosion and earthquake sources at the high frequency noress array indicate secondary arrivals from a near receiver source about 25 30 km southwest of the array f k power difference plots obtained by subtracting after normalization the f k power versus slowness estimates for the initial p window from those for the later windows provided nearly identical results the secondary seismic source appears to be short period surface waves rg generated by the scattering of incident p waves in the region of lake mjosa 27 km sw of noress where there is about 1 km of relief from the bottom of the lake to the top of an adjacent hill

crustal imaging in southern california using earthquake sequences abstract an inexpensive means to further understand the geometry of active faults in southern california arises from the use of aftershock recordings to image crustal structures the advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active source seismic data we show that it is feasible to image fault structure using three dimensional wide angle prestack kirchhoff migration we achieve this with the use of aftershock traces recorded on the short period vertical stations of the southern california seismic network this work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross sections rather than having to associate events with certain faults further it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths we demonstrate the validity of these images as showing reflective structures and the ability to use clipped high gain seismograms as sign bit data to yield valid geometric imaging work with data from the 1991 sierra madre earthquake sequence images the prominent lower crustal reflective zone observed beneath most of the san gabriel mountains by the los angeles region seismic experiment line 1 aftershocks of the 1994 northridge earthquake allow us to image a north dipping structure that may represent the fault plane of a crustal penetrating blind thrust the images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced section reconstructions of shallow crustal profiles and borehole data our results are more consistent with a thick skinned tectonic regime in the vicinity of the northridge earthquake rather than a thin skinned model

mapping shallow heterogeneity with teleseismic p to rg scattered waves over 9000 records of teleseismic p wave coda from the southern california seismic network are used to map lateral variations in p to rg scattering strength and the group velocity of scattered rg building on the results of revenaugh 1995a we use the correlation of scattering strength and topographic roughness as an indicator of model fit to estimate group velocity dispersion over five frequency bands between 0 and 3 hz rg group velocity displays normal dispersion decreasing from 2 7 0 3 km sec for frequencies between 0 and 0 6 hz to a low of 2 2 0 4 km sec for the 2 0 to 3 0 hz band lateral heterogeneity is introduced by assuming rg velocity is a function of local topographic roughness grid search migration analysis yields group velocities of 2 2 and 2 85 km sec for smooth and rough terrain respectively and a 150 m threshold for the local standard deviation of topography distinguishing smooth from rough the map of scattering strength approximates topographic roughness throughout southern california implying that teleseismic p wave coda duration and relative amplitude level depend strongly on near surface 3 km geology

deep crustal heterogeneity along and around the san andreas fault system in central california and its relation to the segmentation the three dimensional distribution of scatterers in the crust along and around the san andreas fault system in central california is estimated using an inversion analysis of coda envelopes from local earthquakes i analyzed 3801 wave traces from 157 events recorded at 140 stations of the northern california seismic network the resulting scatterer distribution shows a correlation with the san gregorio san andreas hayward and calaveras faults these faults seem to be almost vertical from the surface to 15 km depth some of the other scatterers are estimated to be at shallow depths 0 5 km below the diablo range and these may be interpreted as being generated by topographic roughness the depth distribution of scatterers shows relatively stronger scattering in the lower crust at 15 25 km depth especially between the san andreas fault and the hayward calaveras faults this suggests a subhorizontal detachment structure connecting these two faults in the lower crust several clusters of scatterers are located along the san andreas fault at intervals of 20 30 km from south of san francisco to the intersection with the calaveras fault this part of the san andreas fault appears to consist of partially locked segments also 20 30 km long which rupture during m6 7 events and segment boundaries characterized by stronger scattering and stationary microseismicity the segment boundaries delineated by the present analysis correspond with those estimated from the slip distribution of the great 1906 san francisco earthquake and from the fault geometry as reported by the working group on california earthquake probabilities 1990 although the segment boundaries along the san andreas fault in and around the san francisco bay area are still uncertain

using pkikp coda to determine inner core structure 1 synthesis of coda envelopes using single scattering theories 1 previous seismic studies have reported evidence of scattered seismic energy coming from the inner core ic this implies that the ic has lateral variations in structure or composition with a scale length of tens of kilometers in the present study we focus on synthesizing the coda following precritical pkikp and try to determine the location of the heterogeneities that produce this coda using previously reported observations as a guide using a single scattering approximation and ray theory we generate synthetic pkikp coda envelopes from six distinctive places inside the earth within the lower mantle on the source and receiver side along the core mantle boundary on the source and receiver side along the inner core boundary and within the inner core we use two approaches to generate synthetic coda from topography on a boundary surface and one that is appropriate for volumetric scattering in our computations we calculate the arrival time ray parameter and amplitude of the seismic waves and take into account errors in the back azimuth as well as source and receiver effects we find that previously reported spindle shaped or growing coda can only be produced from volumetric heterogeneities located in the shallowest 350 km of the ic however strong trade offs between the different parameters describing the volumetric heterogeneities i e characteristic wavelengths rms velocity or impedance contrast and total volume preclude the determination of a unique model additionally we find that reasonable models of topography at the core mantle boundary can produce large variations of the pkikp amplitude due to focusing and defocusing effects therefore complexity at the inner core boundary is not necessarily required to account for dramatic amplitude variations in the direct pkikp amplitudes

the contribution of topographic scattering to teleseismic coda in southern california given that seismic coda is largely the result of scattering near the receiver a tie to geological complexity i e to crustal heterogeneity is expected but a systematic and predictive understanding of the important aspects of geology is lacking we have developed a technique for mapping the origins of coda energy as recorded by regional networks that provides insight into the proximal generation of seismic coda here the role of topography in the production of surface propagating coda energy both as a scatterer and as a proxy of shallow geologic structure is examined for the portion of the san andreas fault system within the southern california seismic network we find significant correlation of p to rg scattering potential with topographic roughness that extends to relatively subtle topography the correlation peaks at a migration velocity of 2 9 km s confirming the identification of scattered rg energy and offering an alternative means of estimating regional shallow velocity structure