imaging near surface heterogeneities by natural migration of backscattered surface waves s u m m a r y we present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface this migration method denoted as natural migration uses recorded green s functions along the surface instead of simulated green s functions the key assumptions are that the scattering bodies are within the depth interrogated by the surface waves and the green s functions are recorded with dense receiver sampling along the free surface this natural migration takes into account all orders of multiples mode conversions and non linear effects of surface waves in the data the natural imaging formulae are derived for both active source and ambient noise data and computer simulations show that natural migration can effectively image near surface heterogeneities with typical ambient noise sources and geophone distributions

three dimensional simulations of ground motions in the san bernardino valley california for hypothetical earthquakes on the san andreas fault three dimensional finite difference simulations of elastic waves in the san bernardino valley were performed for two hypothetical earthquakes on the san andreas fault a point source with moment magnitude m5 and an extended rupture with m6 5 a method is presented for incorporating a source with arbitrary focal mechanism in the grid synthetics from the 3 d simulations are compared with those derived from 2 d vertical cross section and 1 d flat layered models the synthetic seismograms from the 3 d and 2 d simulations exhibit large surface waves produced by conversion of incident s waves at the edge of the basin seismograms from the flat layered model do not contain these converted surface waves and underestimate the duration of shaking the seismograms from the 3 d simulations have larger amplitude coda than do the seismograms from the 2 d case because of the presence of off azimuth surface wave arrivals in the 3 d simulations that are not included in the 2 d simulations snapshots of the wavefield of the 3 d simulation show that these off azimuth arrivals represent surface waves reflected from the edges of the basin the anelastic attenuation of the sediments is a key parameter controlling the overall duration of motion some of the coda energy at rock sites near the basin edges represents leakage of surface wave energy out of the basin for the m6 5 earthquake simulation the largest ground velocities occur where surface waves reflected from the edge of the basin interfere constructively with the trapped waves that follow the direct s wave maps of maximum ground velocity are produced for two directions of rupture propagation the largest velocities occur in localized portions of the basin the location of the largest velocities changes with the rupture propagation direction contours of maximum shaking are also dependent on asperity positions and radiation pattern

strong sh to love wave scattering off the southern california continental borderland seismic scattering is commonly observed and results from wave propagation in heterogeneous medium yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging in this study we analyze broadband waveforms recorded by the southern california seismic network and observe strongly scattered love waves following the arrival of teleseismic sh wave these scattered love waves travel approximately in the same azimuthal direction as the incident sh wave at a dominant period of 10 s but at an apparent velocity of 3 6 km s as compared to the 11 km s for the sh wave back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the southern california continental borderland in particular the patton escarpment finite difference simulations using a simplified 2 d bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers the modeling suggests a relatively low shear wave velocity in the continental borderland

ground motion amplification in the santa monica area effects of shallow basin edge structure abstract the 1994 northridge earthquake produced ground motions in the northwest portion of the los angeles basin that were significantly larger than rock site motions observed at locations just north of the basin the santa monica area was hit particularly hard with numerous structures being damaged or destroyed by the strong ground shaking in this region the basin edge geology is controlled by the active strand of the east west striking santa monica fault and virtually all of the structural damage occurred at or south of the fault location we have used 2d finite difference ground motion simulations to investigate the effect of the basin edge structure in amplifying ground response constraints on the basin edge structure come from geologic cross sections geophysical data and seismological observations our simulations indicate that the shallow basin edge structure 1 km deep formed by the active strand of the santa monica fault creates a large amplification in motions immediately south of the fault scarp in very good agreement with mainshock damage patterns recorded ground motions and locations of elevated site response this large amplification results from constructive interference of direct waves with the basin edge generated surface waves and is quite similar to the basin edge effect associated with the 1995 kobe earthquake in addition we find that focusing effects created by the deeper basin structure 3 to 4 km deep cannot explain the large motions observed immediately south of the fault scarp this strongly suggests that the deep basin focusing models proposed by gao et al 1996 and alex and olsen 1998 are not likely explanations of the observed pattern of ground motion amplification in the santa monica area

the dynamic arctic research campaigns over the last decade have yielded a growing stream of data that highlight the dynamic nature of arctic cryosphere and climate change over a range of time scales as a consequence rather than seeing the arctic as a near static environment in which large scale changes occur slowly we now view the arctic as a system that is typified by frequent large and abrupt changes the traditional focus on end members in the system glacial versus interglacial periods has been replaced by a new interest in understanding the patterns and causes of such dynamic change instead of interpreting changes almost exclusively as near linear responses to external forcing e g orbitally forced climate change research is now concentrated on the importance of strong feedback mechanisms that in our palaeo archives often border on chaotic behaviour the last decade of research has revealed the importance of on off switching of ice streams strong feedbacks between sea level and ice sheets spatial and temporal changes in ice shelves and perennial sea ice as well as alterations in ice sheet dynamics caused by shifting centres of mass in multi dome ice sheets recent advances in dating techniques and modelling have improved our understanding of leads and lags that exist in different arctic systems on their interactions and the driving mechanisms of change future arctic research challenges include further emphases on rapid transitions and untangling the feedback mechanisms as well as the time scales they operate on

scattering of teleseismic p waves by the japan trench a significant effect of reverberation in the seawater column abstract we detected a scattered wave train in data from the high sensitivity seismograph network in japan hi net following the arrival of the near vertically incident p wave generated by the 2009 earthquake mw 7 8 off the south island of new zealand the scattered wave train represented predominantly vertical ground motion at a period of 20 to 50 s and with an apparent velocity of 3 5 km s it propagated cylindrically westward through the kanto area of central japan array analysis showed that the scattered wave train developed beneath the pacific ocean near the boso triple junction southeast of the kanto area a 3d finite difference simulation of seismic wave propagation using a high resolution model incorporating subsurface structure topography and bathymetry revealed that the strong scattered waves that were generated along the japan trench and propagated normal to the trench axis represented multiple reverberations of seismic waves between the seafloor and the pacific plate boundary in addition strong reverberation of acoustic waves in the seawater column above the boso triple junction causes elongated scattered waves which reasonably explains our observations

effects of topography on seismic wave propagation an example from northern taiwan topography influences ground motion and in general increases the amplitude of shaking at mountain tops and ridges whereas valleys have reduced ground motions as is observed from data recorded during and after real earthquakes and from numerical simulations however recent publications have focused mainly on the implications for ground motion in the mountainous regions themselves whereas the impact on surrounding low lying areas has received less attention here we develop a new spectral element mesh implementation to accommodate realistic topography as well as the complex shape of the taipei sedimentary basin which is located close to the central mountain range in northern taiwan spectral element numerical simulations indicate that high resolution topography can change peak ground velocity pgv values in mountainous areas by 50 compared to a half space response we further demonstrate that large scale topography can affect the propagation of seismic waves in nearby areas for example if a shallow earthquake occurs in the i lan region of taiwan the central mountain range will significantly scatter the surface waves and will in turn reduce the amplitude of ground motion in the taipei basin however as the hypocenter moves deeper topography scatters body waves which subsequently propagate as surface waves into the basin these waves continue to interact with the basin and the surrounding mountains finally resulting in complex amplification patterns in taipei city with an overall pgv increase of more than 50 for realistic subduction zone earthquake scenarios off the northeast coast of taiwan the effects of topography on ground motion in both the mountains and the taipei basin vary and depend on the rupture process the complex interactions that can occur between mountains and surrounding areas especially sedimentary basins illustrate the fact that topography should be taken into account when assessing seismic hazard

rayleigh wave multipathing along the west coast of north america we have created a movie of surface ground motion for the 3 november 2002 denali fault earthquake based on spectral element simulations using crustal model crust2 0 mantle model s20rts topography and bathymetry model etopo5 and a finite fault slip model the movie features two anomalous wave packets that travel along the west coast of the north american plate following off great circle paths these wave packets are rayleigh waves with dominant periods around 20 sec which are also found in seismograms recorded by the southern cali fornia seismic network one of these packets is the direct surface wave whose group arrival time changes laterally as dictated by the shape of the oregon coast the other packet is a surface wave reflected by a lateral interface underneath the rocky moun tains a linear reflector parallel to the canadian coast offsetting it by a few hundred kilometers can explain its arrival time but the offsetting distance derived from the synthetic seismograms puts the reflector 350 km northeast of the result obtained from the data indicating a need to update the crustal and mantle models in this area online material animation of normalized simulated vertical displacement for the 2002 denali earthquake

a scattered wave image of subduction beneath the transverse ranges over 5600 short period recordings of teleseismic events were used to create deterministic maps of p wave scatterers in the upper mantle beneath southern california between depths of 50 and 200 kilometers the southern flank of the slab subducting beneath the transverse ranges is marked by strong scattering the marked scattering indicates that the edge of the slab is a sharp thermal boundary such a boundary could be produced by slab shearing or small scale convection in the surrounding mantle the northern limb of the slab is not a strong scatterer consistent with thicker lithosphere north of the transverse ranges

anomalous surface waves associated with deep earthquakes generated at an ocean ridge summary broad band seismograms observed at regional distances from deep earthquakes display large amplitude rayleigh and love waves with a predominant period of about 20 s although deep earthquakes in laterally homogeneous structures do not generate such surface waves since the propagation direction of the surface waves inferred from particle motion is approximately radial and the group velocity of the surface waves observed at each station is similar it can be assumed that the surface waves have travelled close to the great circle paths back projection allows us to determine the region in which these surface waves are generated manyof the zones from which the surface waves appear to originate are located close to the norfolk ridge where crustal structure has strong lateral heterogeneity this suggests that the surface waves are generated by the interaction of body waves from the deep earthquakes with this strongly heterogeneous crustal structure in order to investigate the eiect of such heterogeneity in crustal structure on seismic wave elds we have modelled the p sv wave eld using a pseudospectral method the results show that surface waves such as those observed can be generated by ridge structures and indicate that sedimentary layer thickness plays a very important role we suggest that such unexpected surface waves from deep earthquakes may prove useful for delineating heterogeneity in shallow structure

studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array loosely speaking the principle of green s function reciprocity means that the source and receiver positions in a seismic experiment can be exchanged without affecting the observed seismograms consequently the seismograms observed at a single observation location o and caused by a cluster of microearthquakes at locations ei are identical to the time series that would be measured by an array of stress meters emplaced at positions ei recording waves generated by a source acting at o by applying array analysis techniques like slant stacking and frequency wave number analysis to these seismograms we can determine the directions and velocities of the component waves as they travel in the earthquake focal region rather than at the surface we have developed a computationally rapid plane wave decomposition which we have applied to single station recordings of aftershocks of the 1984 morgan hill california earthquake the analysis is applied to data from three seismic stations having considerably different site geologies one is a relatively hard rock station situated on franciscan metamorphics one is within the calaveras fault zone and one is on semiconsolidated sand and gravels we define the early coda to be the part of the coda initiating immediately after the direct s wave and ending at twice the s wave lapse time the character of the s wave and early coda varies from being impulsive at the first station to highly reverberative at the last we examine waves in sequential time windows starting at the s wave and continuing through the early part of the coda at all seismic stations the early coda is dominated by a persistent signal that must be caused by multiple scattering probably within 2 km of each seismic station despite clear station to station differences in the character of the early coda coda q values measured in the late coda greater than twice the s lapse time agree well among stations implying that the mechanisms causing the varying behavior of the early coda do not control the coda decay rate at the stations we have considered coda q values measured on horizontal components of motion agree within a factor of 2 with those measured on vertical components we have not been able to determine the composition of the late coda because of a low signal to noise ratio our analysis technique however is quite appropriate for such a task

global synthetic seismograms using a 2 d finite difference method two dimensional 2 d finite difference fd synthetics which fill the gap between fast 1 d analytic synthetics and time consuming full 3 d synthetics in our ability to model seismograms have been used in many studies we address several issues involving 2 d fd methods in generating global synthetic seismograms these include 1 interfacing point source excitation for earthquakes with 2 d fd methods 2 out of plane spreading corrections and 3 reducing the spherical earth to the flattened models the first issue is tackled using two methods a transparent source box approach and a moment tensor excitation approach where each has its own advantages moreover our source box excitation does not have the late time drift problem that occurred in previous studies the out of plane geometric spreading correction is accounted for by estimating the ray parameter and applying a post simulation filter to 2 d synthetics finally parameters of the earth flattening transformation are discussed and validated the effectiveness of this method is demonstrated by comparing our synthetics with frequency wavenumber summation normal mode and 3 d spectral element synthetics

lithospheric structure across the california continental borderland from receiver functions due to its complex history of deformation the california continental borderland provides an interesting geological setting for studying how the oceanic and continental lithosphere responds to deformation we map variations in present day lithospheric structure across the region using ps and sp receiver functions at permanent stations of the southern california seismic network as well as ocean bottom seismometer obs data gathered by the asthenospheric and lithospheric broadband architecture from the california offshore region experiment albacore which enhances coverage of the borderland and provides first direct constraints on the structure of the pacific plate west of the patton escarpment noisiness of obs data makes strict handpicking and bandpass filtering necessary in order to obtain interpretable receiver functions using h and common conversion point stacking we find pronounced lithospheric differences across structural blocks which we interpret as indicating that the outer borderland has been translated with little to no internal deformation while the inner borderland underwent significant lithospheric thinning most likely related to accommodating the 90 clockwise rotation of the western transverse range block west of the patton escarpment we find that the transition to typical oceanic crustal thickness takes place over a lateral distance of 50 km we detect an oceanic seismic lithosphere asthenosphere transition at 58 km depth west of the patton escarpment consistent with only weak age dependence of the depth to the seismic lithosphere asthenosphere transition sp common conversion point stacks confirm wholesale lithospheric thinning of the inner borderland and suggest the presence of a slab fragment beneath the outer borderland

bizarre geochemistry of komatiites from the crixas greenstone belt brazil komatiite lava flows in the crixas greenstone belt goias brazil have textures and volcanic structures typical of archean komatiites but are geochemically most unusual the flows are porphyritic and massive or layered with spinifex upper parts and olivine cumulate lower parts mgo contents range from 18 to 40 in such lavas only olivine and minor chromite can have crystallized but neither major nor trace elements fall on olivine control lines in mgo variation diagrams cao and sr fall on lines with slopes steeper than olivine control lines sio2 feo na2o k2o and y show little systematic variation zr shows a large variation that does not correlate with mgo and al2o3 decreases markedly with decreasing mgo the aberrant behaviour is highlighted by the ree rare earth elements in spinifex and olivine cumulate layers from three flows in the spinifex layers chondrite normalized ree patterns are hump shaped with maxima at nd or sm la sm n 0 6 gd yb n 1 6 2 3 whereas cumulate zones in the same flows have steadily sloping patterns with lree enriched relative to hree la sm n 1 3 gd yb n 1 4 neither normal magmatic processes acting during emplacement of the komatiites nor thermal erosion and wall rock assimilation can explain these effects and we speculate that elements commonly thought of as immobile e g al zr ree migrated during hydrothermal alteration or metamorphism a pb pb whole rock isochron gave an age of 2 728 140 ma and selected sm nd analyses an apparent isochron age of 2 825 98 ma nd 0 the pb pb age is believed to be the approximate time of emplacement interpretation of the sm nd data is complicated by the evidence of mobility of ree

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

geophysical evidence for miocene extension and mafic magmatic addition in the california continental borderland the california continental borderland is a key region for understanding the effect on crustal structure of tectonic processes associated with the initiation and elongation of the san andreas transform system because the borderland lies near the point where the pacific farallon spreading system first encountered the north american plate margin whereas the central and northern california margins have been extensively investigated by geophysical surveys in recent years the crustal structure of the borderland particularly the outer borderland has received relatively little attention this paper presents three gravity models that stretch from the pacific ocean floor to shore they provide new evidence that the crust of the entire california continental borderland has been extended and infiltrated by mafic melt since early miocene time the gravity models were constructed from interpretations of nearly 750 km of seismic reflection data seismic refraction studies and geologic maps thus minimizing the nonuniqueness of the method the evidence for extension in the models is that the crust of the accretionary complex and forearc basin that make up the borderland is only 22 km thick 35 thinner than comparable crustal sections in central and northern california extension could well have been accommodated by eastward flow of accretionary complex material during middle miocene formation of a metamorphic core complex in the inner borderland the gravity models also show that the accretionary complex is 100 200 kg m 3 more dense than commonly observed to the north this density difference may result from influx of basaltic melts due to mantle upwelling at the rivera triple junction and beneath the highly extended inner borderland the borderland models permit the occurrence of a discrete mafic layer that forms the base of the crust as is seen along the central and northern california coast but do not require such a layer9s presence anywhere except immediately inland of the shelf slope break

localized water reverberation phases and its impact on backprojection images coherent radiators imaged by back projections bp are commonly interpreted as part of the rupture process nevertheless artifacts introduced by structure related phases are rarely discriminated from the rupture process in this study we use a calibration event to discriminate between rupture and structure effects we re examine the waveforms and bp images of the 2012 mw 7 2 indian ocean earthquake and a calibration event mw 6 2 the p wave codas of both events present similar shape with characteristic period of approximately 10 s which are back projected as coherent radiators near the trench s wave bp doesn t image energy radiation near the trench we interpret those coda waves as localized water reverberation phases excited near the trench we perform a 2d waveform modeling using realistic bathymetry model and find that the steep near trench bathymetry traps the acoustic water waves forming localized reverberation phases these waves can be imaged as coherent near trench radiators with similar features as that in the observations we present a set of methodologies to discriminate between the rupture and propagation effects in bp images which can serve as a criterion of subevent identification

seismic determination of elastic anisotropy and mantle flow when deformed many rocks develop anisotropic elastic properties on many seismic records a long period 100 to 250 seconds quasi love wave with elliptical polarization arrives slightly after the love wave but before the rayleigh wave mantle anisotropy is sufficient to explain these observations qualitatively as long as the fast axis of symmetry is approximately horizontal quasi love observations for several propagation paths near pacific ocean subduction zones are consistent with either flow variations in the mantle within or beneath subducting plates or variations in the direction of fossil spreading in older parts of the pacific plate

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

book reviewseismic migration imaging of acoustic energy by wavefield extrapolation a theoretical aspects a j berkhout developments in solid earth geophysics 14a elsevier amsterdam 1982 2nd revised