the q structure of the upper mantle constraints from rayleigh wave amplitudes 1 the three dimensional shear attenuation or q structure of the upper mantle is still poorly understood despite the increasing accuracy and sophistication of velocity models of the same part of the earth here we present a set of three dimensional q models of the upper mantle based on amplitude measurement of minor and major arc rayleigh waves in the period range 70 170 s the models show that areas of low attenuation underlie continents to a depth of around 300 km and areas of high attenuation are associated with oceans and plate boundaries particularly in the top 200 km below 350 km the sensitivity of the data to attenuation structure decreases rapidly it is well known that the amplitudes of surface waves can be strongly influenced by factors other than attenuation in particular focusing due to elastic structure and errors in source mechanisms we consider the effects of focusing by discussing the implications of the results of selby and woodhouse 2000 comparisons with focusing predictions using phase velocity maps allow us to identify features due to focusing in maps of attenuation distribution the influence of the source on the observed attenuation distribution is investigated by including a source term in the model inversion we find that successfully combining the data at each frequency into one three dimensional model requires that this source term must vary with frequency suggesting that a simple correction to the scalar moment of each event cannot explain the observations although the effects of focusing and the source can be significant we find that the observed attenuation pattern is robust up to degree 8 however shorter wavelength structure can be strongly influenced by other controls finally we use one of the models to predict the degree 2 attenuation pattern for 250 s rayleigh waves the observed pattern suggests that existing normal mode observations may be explainable in terms of structure above 350 km depth

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

three dimensional p wave velocity image under the carpathian arc abstract an inversion of p wave travel time residuals from selected earthquakes in the distance range 30 98 to two seismic station networks was used to model p wave velocity anomalies down to 250 km depth in the first inversion experiment a region between 43 5 47 5 n and 21 29 e was modelled using 35 seismic stations while in the second one a region between 44 47 n and 25 29 e was modelled using 19 seismic stations the 4 layer block model of the first inversion offers 19 reduction in residual variance while the 5 layer block model of the second one offers 26 reduction the rest being explained by noise and smaller scale heterogeneities the obtained velocity anomalies correlate remarkably well with the gravity anomalies and with the tectonic model for the vrancea region of fuchs et al 1979

inversion of reflection seismic amplitude data for interface geometry summary reflection seismic tomography using only traveltime data may be unable to resolve the ambiguity caused by trade off between reflector position and velocity anomaly the inclusion of amplitude data in the inversion may help to resolve this problem because the amplitudes and traveltimes are sensitive to different features of the model therefore providing us with more accurate information about underground structures and velocity distribution the amplitude of a reflected seismic wave is determined partly by the reflection coefficients and partly by the curvature of the reflector the latter causes the spherical divergence of the seismic rays to be modified at the reflection point focused or defocused and can be represented using a simplified analytical expression we show using geologically relevant synthetic models that the information contained in amplitude versus offset data here excluding traveltime data suffices to constrain accurately the geometry of an arbitrary 2 d reflector separating constant velocity layers the most effective inversion method is a subspace gradient algorithm using a model parametrization in which the interface is described as a discrete fourier series with fixed upper and lower bounds on the wavenumber model parameters are allocated to separate subspaces first on the basis of different physical dimensionality we also found that declaring separate subspaces for those parameters defining short intermediate and long wavelength components of the interface geometry based on the magnitude of singular values of the frechet derivative matrix is very effective in accelerating convergence and obtaining a more accurate solution the inversion is robust with respect to data errors and poor initial estimates

seismic tomography in boreholes summary synthetic seismic tomographic data sets for cross hole and hole to surface arrays are computed and inverted inversion imaging is performed through an iterative projection solution to a matrix equation the orientation as well as the position of an anomaly relative to the source and recorder arrays affects both the rate of convergence of the solution and the resolution obtained data density and distribution determine the physical validity of the image obtained at convergence borehole tomography is apparently a viable method for imaging inter hole structure

multiarrival wavefront tracking and its applications 1 multivalued traveltimes have traditionally not been used in seismic imaging with only a handful of notable exceptions in the field of exploration geophysics for studies at local and regional scales e g local earthquake teleseismic tomography the focus has largely been on first arrivals with numerous ray and grid based schemes developed for their calculation however later arrivals often contribute to the length and shape of a recorded wave train particularly in regions of complex geology these arrivals are likely to contain additional information about seismic structure as their two point path differs from that of the first arrival in particular they are more amenable to sampling regions of lower velocity in this work the wavefront construction principle is used as the basis of a new scheme for computing multivalued traveltimes that arise from smooth variations in both velocity structure and interface geometry the idea is to represent the wavefront as a set of points in reduced phase space and use local ray tracing and interpolation to advance the wavefront in a series of time steps the scheme is robust in the presence of strong velocity heterogeneity and interface curvature with phases comprising multiple reflections refractions and triplications successfully tracked outside the field of exploration seismology wavefront construction techniques are rarely used yet they hold great potential for addressing problems in other areas of seismology this paper demonstrates the viability of the new wavefront construction scheme by applying it to a range of scenarios including multiarrival body and surface wave tomography teleseismic receiver function prediction using gaussian beams and the tracking of global phases such as pcp

simultaneous inversion of active and passive source datasets for 3 d seismic structure with application to tasmania 1 refraction wide angle reflection and teleseismic arrival time data are combined in a simultaneous inversion for 3 d lithospheric p wavespeed and moho geometry beneath tasmania southeast australia a new iterative non linear tomography scheme which incorporates a novel grid based method of traveltime prediction is used to obtain the solution model synthetic resolution tests demonstrate that the crucial issue of trade off between interface depth and layer velocity is satisfactorily resolved in contrast to previous results from separate inversions of the active and passive source datasets the new images reveal a zone of elevated wavespeed beneath the cambrian mt read volcanics and indicate that both crustal thinning and elevated wavespeeds occur beneath northeast tasmania which supports the case for the existence of a prior passive margin otherwise most major inferences from the previous studies including evidence for remnant subduction and crustal shortening are supported by the new results

inversion of seismic refraction and wide angle reflection traveltimes for three dimensional layered crustal structure summary we present a method for the determination of crustal structure by simultaneous inversion of seismic refraction and wide angle reflection traveltimes for 3 d interface geometry and layer velocity crustal structure is represented by layers in which velocity varies linearly with depth separated by smooth interfaces with a cubic b spline parametrization lateral variations in structure are therefore represented by variations in interface depth only the model parametrization we have chosen means that ray paths consist of piecewise circular arc segments for which analytic expressions of trajectory and traveltime are calculated the two point problem of finding the first arrival ray path and traveltime of a specified phase between a given source and receiver is solved using a shooting technique a subspace inversion method is used to solve the inverse problem which is formulated as a non linear optimization problem in which we seek to minimize an objective function that consists of a data residual term and a regularization term before performing the inversion each data pick must be assigned as a refraction or reflection from a particular layer or interface since our method represents structure in terms of interfaces fewer parameters would generally be used in a reconstruction compared to an equivalent 3 d variable velocity inversion the method is well suited to wide angle surveys that consist of many sources and relatively few receivers or vice versa such as marine shot lines used in conjunction with land based receivers data coverage in this kind of survey is often sparse and especially if near offset ray paths are unavailable highly variable a 3 d synthetic test with an array consisting of eight sources lying within a three sided square of 79 receivers is described the test model consists of a three interface structure that includes a layer pinch out and the synthetic data set comprises 987 refraction and 930 reflection travel times contaminated with 75 ms of data noise six iterations of an 18 d subspace method demonstrate that the method can produce an accurate reconstruction that satisfies the data from a 1 d starting model we also find that estimates of a posteriori model covariance and resolution obtained from linear theory are useful in analysing solution reliability despite the non linear nature of the problem application of the method to data collected as part of the 1995 tasgo project in tasmania shows that the method can satisfy 1345 refraction and reflection traveltime picks with a geologically reasonable and robust 254 parameter three interface model the inversion results indicate that the moho beneath nw tasmania varies in depth from 27 km near the coast to 37 km near central tasmania with the major increase in depth occurring across the arthur lineament

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

global maps of rayleigh wave attenuation for periods between 40 and 150 seconds studies of seismic attenuation must account for the large amplitude deviations caused by elastic focusing of energy in a new approach we jointly invert phase and amplitude measurements of 19 000 minor arc rayleigh waves between periods of 40 and 150 seconds the simultaneous inversion ensures that attenuation and phase velocity are mutually consistent because the phase and focusing term of amplitude are modelled using a common elastic model at the shortest periods the maps show a good correlation between attenuation and phase velocity suggesting a common cause in the uppermost mantle most probably thermal in origin this correlation is lost at longer periods the main signal beyond periods of 100 seconds is a strongly attenuating circum pacific zone and a pronounced ring of high attenuation around africa this feature seems reliable in our attenuation maps but not correlated to an equivalent structure in phase velocity we thus favour scattering of wave energy on large size structures as a possible cause

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

non linear body wave teleseismic tomography along the tor array summary the tor project investigates the lithosphere asthenosphere structure under the tornquist zone between denmark and sweden around 150 seismic stations 108 short period 28 broadband and 14 other permanent stations were employed in a rectangular array along a 900 km long by 100 km wide strip across the tornquist zone from july 1996 to august 1997 the results obtained based on a non linear teleseismic tomography algorithm reveal significant p velocity variations up to 5 per cent along the tor array from northern germany to southern sweden distinct lithospheric blocks are also recognized in the inversion results two inverse methods singular value decomposition and a quadratic programming method were implemented in order to investigate whether or not the lithospheric blocks and major boundaries in the inversion are required by the data or are artefacts of the inversion according to the results the lithosphere under northern germany is thin but reaches to intermediate thicknesses of about 120 km in the tornquist zone area farther north in the baltic shield more than 200 km of continental lithosphere is recognized in the model abrupt lateral p velocity changes maximum 5 per cent are seen at both the southern and northern sides of the tornquist zone

high and low resolution images of the earth s mantle implications of different approaches to tomographic modeling recently published images of the earth s mantle are characterized by a nominal resolution much higher than that used in previous studies where substantially different techniques were employed the agreement between such high resolution and low resolution images often seems very poor in an attempt to determine the reason for this discrepancy we analyze how the choice of inversion algorithm exact or iterative regularization norm or roughness minimization and parameterization spherical harmonics up to a variable degree blocks affects a tomographic model we also investigate the effects of the varying density of the data coverage on the final solution in our experiments we employ two seismic data sets rayleigh wave phase velocity at 75 s period and p wave travel times we construct a new model of p velocity in the mantle bdp98 based on the international seismological center bulletins 1964 1992 we use our findings in an evaluation of recent mantle models including our own focusing on similarities and discrepancies between models of different nominal resolution in all the models the long wavelength component is the most stable however consistent high resolution details probably corresponding to features of the real earth are also seen in general we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem rather than from actual errors or approximations

global and regional surface wave inversions a spherical spline parameterization a surface wave phase velocity inversion scheme based upon a compactly supported cubic b spline basis on a triangular grid of knots with approximately equal inter knot spacing is implemented to perform global regional and variable resolution inversions numerical results show that the inverted phase velocity maps based upon this method are similar to those obtained using a global spherical harmonic representation nonlinear inversions incorporating ray refraction are feasible based upon the spherical spline model parameterization and the resulting maps are only slightly different from those based upon the great circle approximation a multiresolution analysis is introduced to obtain finer resolution in regions with good ray coverage and lower resolution in regions with poor coverage

geophysical modeling via simultaneous joint inversion of seismic gravity and electromagnetic data application to prestack depth imaging geophysical modeling of multiple geophysical parameters can take place reliably through the simultaneous joint inversion of the different geophysical domains the desired objective is the derivation of a model expressing different geophysical properties of the earth but at the same time showing consistency among the parameters this concept generally referred to as data integration is widespread in geophysics but the way of achieving it is not obvious several authors have proposed different approaches where data interpretation and experience all contributed to the task the result is that no general workflows could be defined that left ample margins for uncertainties and personal judgment

simultaneous velocity and interface tomography of normal incidence and wide aperture seismic traveltime data summary a tomographic inversion technique that inverts traveltimes to obtain a model of the subsurface in terms of velocities and interfaces is presented it uses a combination of refraction wide angle reflection and normal incidence data it simultaneously inverts for velocities and interface depths and it is able to quantify the errors and trade offs in the final model the technique uses an iterative linearized approach to the non linear traveltime inversion problem the subsurface is represented as a set of layers separated by interfaces across which the velocity may be discontinuous within each layer the velocity varies in two dimensions and has a continuous first derivative rays are traced in this medium using a technique based on ray perturbation theory and two point ray tracing is avoided by interpolating the traveltimes to the receivers from a roughly equidistant fan of rays the calculated traveltimes are inverted by simultaneously minimizing the misfit between the data and calculated traveltimes and the roughness of the model this smoothing regularization stabilizes the solution of the inverse problem in practice the first iterations are performed with a high level of smoothing as the inversion proceeds the level of smoothing is gradually reduced until the traveltime residual is at the estimated level of noise in the data at this point a minimum feature solution is obtained which should contain only those features discernible over the noise the technique is tested on a synthetic data set demonstrating its accuracy and stability and also illustrating the desirability of including a large number of different ray types in an inversion

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

inversion of travel time data for earthquake locations and three dimensional velocity structure in the eureka valley area eastern california we develop an earthquake travel time inversion methodology suitable for determining three dimensional velocity structure and fault plane orientation for an area with little a priori information using a cascaded combination of a nonlinear simulated annealing optimization and linearized inversion we investigate local three dimensional compressional velocity structure and estimate the orientation of a fault plane in the eureka valley area of eastern california by inverting travel time data from a moderate earthquake sequence we inverted travel time picks at a 28 permanent and 8 portable stations from a m6 1 mainshock and a few hundred aftershocks for p wave velocity and hypocentral coordinates using the velocity model obtained by the nonlinear optimization as an initial model for linearized inversion we relocated the hypocenters and further fine tuned the model the relocated hypocenters define a north northwest trending fault dipping steeply westwards the final crustal velocity model features a low velocity trend along the strike of the eureka valley and a high velocity block southwest of the valley compared with a fully linearized inversion our scheme demonstrates initial model independence and the potential of not being trapped in local minima

the estuarine geochemistry of rare earth elements and indium in the chao phraya river thailand abstract a new filtration method using a 0 04 m hollow fiber filter was applied to the river estuarine and coastal waters in the chao phraya estuary for geochemical investigation the filtered waters were analyzed for all the lanthanides y and in by using inductively coupled plasma mass spectrometry icpms the dissolved concentrations of rare earth elements rees are significantly lower than those reported previously for other rivers presumably because of effective removal of river colloids by the ultra filtration the variation of dissolved rees in the estuary is dependent on the season the light rees vary considerably in the low salinity s 3 show almost conservative trends being consistent with some of the previous works europium is strongly enriched in the river and estuarine waters compared to the south china sea waters thus the ree source of the chao phraya river must be fractionated and modified in entering to the south china sea dissolved in and ce in the high salinity s 20 25 zone of the estuary are lower than those of the offshore waters and therefore the dissolved flux of the chao phraya river cannot account for the higher concentrations of dissolved in and ce in the surface waters of the south china sea the negative ce anomaly is progressively developed with increasing salinity being consistent with continued oxidation of ce iii to ce iv in the estuary fractionation of the light to heavy rees seems to take place whereas the y ho fractionation is unclear in the estuarine mixing zone

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

two dimensional velocity models from wide angle seismic data by wavefield inversion summary modern wide angle surveys are often multi fold and multi channel with densely sampled source and receiver spacings such closely spaced data are potentially amenable to multi channel techniques involving wavefield propagation methods such as those commonly used in reflection data processing however the wide angle configuration requires techniques capable of handling very general wave types including those not commonly used in reflection seismology this is a situation analogous to that faced in cross borehole seismics where similar wave types are also recorded in a real cross borehole example we compare pre stack migration traveltime tomography and wavefield inversion we find that wavefield inversion produces images that are quantitative in velocity as are the tomograms but are of significantly higher resolution the wavefield inversion results have a resolution comparable to that of the qualitative pre stack migration images we seek to extend this novel development to the larger scale problem of crustal imaging an essential element of the approach we adopt is its formulation entirely within the temporal frequency domain this has three principal advantages 1 we can choose to decimate the data by selecting only a limited number of frequency components to invert thus making inversion of data from large numbers of source positions feasible 2 we can mitigate the notorious non linearity of the seismic inverse problem by progressing from low frequency components in the data to high frequency components and 3 we can include in the model any arbitrary frequency dependence of inelastic attenuation factors q and indeed solve for the spatial distribution of q an initial synthetic test with an anomaly located within the middle crust yields a velocity image with the correct structural features of the anomaly and the correct magnitude of velocity anomaly this is related to the fact that the reconstruction is obtained from forward scattered waves under these conditions the method thus behaves much like tomography a second test with a deeper more extensive anomaly yields an image with the correct velocity polarity and the correct location but with a deficiency in low and high wavenumbers in this case this is because the reconstruction is obtained from backscattered waves under these conditions the method behaves not like tomography but like migration a more extensive test based on a large wide angle survey in south eastern california and western arizona demonstrates a real potential for high resolution imaging of crustal structures although our results are limited by the acoustic approximation and by the relatively low frequencies that we can model today the images are sufficiently encouraging to warrant future research the problem of local minima in the objective function is the most significant practical problem with our method but we propose that appropriate layer stripping methods can handle this problem

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

simultaneous inversion for lateral velocity variations and hypocenters in the yellowstone region using earthquake and refraction data a three dimensional upper crustal p wave velocity structure of the yellowstone region was determined by simultaneous inversion using pg refracted arrival times and earthquake generated direct p wave arrival times the velocity structure was modeled by a two layer block configuration with the velocity of the surface layer fixed the inversion was performed on a 14 4 km thick second layer 5 6 blocks using an optimal block size of 21 2 18 3 km and incorporated the method of separation of parameters results show volumes of higher velocities approximately 6 3 km s north of the yellowstone caldera that correlate with gravity anomaly highs and are interpreted to be associated with near surface crystalline basement the upper crust beneath the caldera showed an average velocity of 5 8 km s with prominent low velocity zones of 4 9 and 5 2 km s in the northeast and southwest portions of the caldera respectively the 4 9 km s low velocity layer in the northeast corner of the caldera coincides with a localized 20 mgal bouguer gravity low and the largest area of hydrothermal activity in yellowstone national park relocating earthquakes with the new velocity model showed a systematic migration of epicenters radially toward the caldera by an average of 1 2 km relative to locations using a homogeneous velocity model

tomographic inversion for the seismic velocity structure beneath northern new england using seismic refraction data the seismic velocity structure of the crust and the upper mantle beneath northern new england was investigated from the travel time data of the 1984 maine seismic refraction profile and the 1988 ontario new york new england seismic refraction profile seismic refraction experiments by using tomographic inversion techniques strong lateral seismic velocity variations were found in the upper 15 km of the crust and these variations are correlated with regional geologic and tectonic structures the grenville basement in the adirondack mountains of eastern new york is characterized by an unusually high p wave velocity of 6 7 km s in the upper 15 km of the crust this high velocity basement extends eastward below western vermont to a depth between 15 and 20 km from the surface more typical upper crustal p wave velocities of 5 6 to 6 4 km s were generally found in central and southern maine and in new hampshire and the anisotropic upper crustal zone west of penobscot bay in maine was imaged by locally slower velocities the correlation of the inversion velocity structure with the tectonic features of northern new england shows that the appalachian mountains in northern new england may extend tectonically to at least 20 km deep in the crust the velocity distribution at 20 km depth shows three northeast elongate juxtaposed belts which correspond to the connecticut valley gaspe synclinorium the bronson hill anticlinorium and the kearsarge central maine synclinorium respectively the connecticut valley gaspe and kearsarge central maine synclinoria have relatively high velocities from 6 94 to 7 12 km s while the bronson hill anticlinorium has relatively low velocities from 6 52 to 6 75 km s in the lower crust another low velocity area of 6 52 km s at 20 km depth is imaged beneath the white mountain batholith in central new hampshire and may be a lower crustal expression of this batholith upper mantle velocity structures were obtained for the grenville basement the appalachians in northern new england and the avalon terrane in southeastern maine from the long range seismic refraction data generally the older grenville lithosphere about 1100 ma in quebec has higher velocity in the upper mantle than does the younger avalon terrane no older than 650 ma in the southeastern part of northern new england the data suggest that the velocity of the upper mantle beneath the old north american craton increases stepwise with observable velocity discontinuities in depth while that beneath the avalon terrane does not increase through the first 40 km of the upper mantle the data analysis also shows that the upper mantle suture between the grenville basement and the avalon terrane occurs in a narrow belt beneath the appalachian mountains of central maine and new hampshire the did of the moho discontinuity beneath northern new england is modeled as nearly flat as 0 7 in the n57 w direction and as steep as 1 2 beneath the grenville basement

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea

automatic boundary extraction from magnetic field data using triangular meshes boundary extraction is a collective term that we use for the process of extracting the locations of faults lineaments and lateral boundaries between geologic units using geophysical observations such as measurements of the magnetic field the process typically begins with a preprocessing stage where the data are transformed to enhance the visual clarity of pertinent features and hence improve the interpretability of the data the majority of the existing methods are based on raster grid enhancement techniques and the boundaries are extracted as a series of points or line segments in contrast we set out a methodology for boundary extraction from magnetic data in which we represent the transformed data as a surface in 3d using a mesh of triangular facets after initializing the mesh we modify the node locations such that the mesh smoothly represents the transformed data and that facet edges are aligned with features in the data that approximate the horizontal locations of subsurface boundaries to illustrate our boundary extraction algorithm we first apply it to a synthetic data set we then apply it to identify boundaries in a magnetic data set from the mcfaulds lake area in ontario canada the extracted boundaries are in agreement with known boundaries and several of the regions that are completely enclosed by extracted boundaries coincide with regions of known mineralization

on the potential of recording earthquakes for global seismic tomography by low cost autonomous instruments in the oceans 1 we describe the development and testing of an autonomous device designed to revolutionize earth structure determination via global seismic tomography by detecting earthquakes at teleseismic distances in the oceans one prototype mermaid short for mobile earthquake recording in marine areas by independent divers was constructed and tested at sea the instrument combines two readily available relatively low cost but state of the art components a sounding oceanographic lagrangian observer or solo float and an off the shelf hydrophone with custom built data logging hardware we report on the development of efficient wavelet based algorithms for the detection and discrimination of seismic events and analyze three time series of acoustic pressure collected at a depth of 700 m in pilot experiments conducted offshore san diego ca in these tests over 120 hours of data were gathered and five earthquakes of which one was teleseismic were recorded and identified quantitative estimates based on these results suggest that instruments of the mermaid type may collect up to a hundred tomographically useful teleseismic events per year the final design will also incorporate a global positioning system receiver onboard signal processing software optimized for low power chips and high throughput satellite communication equipment for telemetered data transfer with these improvements we hope to realize our vision of a global array of autonomous floating sensors for whole earth seismic tomography

adaptive mesh seismic tomography based on tetrahedral and voronoi diagrams application to parkfield california 1 we have developed an adaptive mesh seismic tomography method based on tetrahedral and voronoi diagrams to automatically match the inversion mesh to the data distribution two different irregular mesh interpolation methods linear and natural neighbor are evaluated a synthetic test shows that the adaptive mesh seismic tomography method recovers the true synthetic model well in both the linear and natural neighbor interpolation cases use of more accurate differential data is helpful in removing artifacts in the model that result from the noisier absolute data application of the adaptive mesh scheme to parkfield california results in much more uniform sampling of the model as measured by the derivative weight sum on the optimized irregular inversion mesh compared to a regular grid the across strike cross sections resulting from using either just the absolute data or both the absolute and differential data show a clear velocity contrast at the san andreas fault in the linear and natural neighbor interpolation cases the velocity model obtained using natural neighbor interpolation is smoother than that from linear interpolation including the differential data in the inversion yields more clustered event locations and removes a velocity reversal to the southwest of the fault which is likely caused by noise in the absolute data