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

hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

the crustal structure of southern baffin bay implications from a seismic refraction experiment baffin bay represents the northern extension of the extinct rift system in the labrador sea while the extent of oceanic crust and magnetic spreading anomalies are well constrained in the labrador sea no magnetic spreading anomalies have yet been identified in baffin bay thus the nature and evolution of the baffin bay crust remain uncertain to clearly characterize the crust in southern baffin bay 42 ocean bottom seismographs were deployed along a 710 km long seismic refraction line from baffin island to greenland multichannel seismic reflection gravity and magnetic anomaly data were recorded along the same transect using forward modelling and inversion of observed traveltimes from dense airgun shots a p wave velocity model was obtained the detailed morphology of the basement was constrained using the seismic reflection data a 2 d density model supports and complements the p wave modelling sediments of up to 6 km in thickness with p wave velocities of 1 8 4 0 km s 1 are imaged in the centre of baffin bay oceanic crust underlies at least 305 km of the profile the oceanic crust is 7 5 km thick on average and is modelled as three layers oceanic layer 2 ranges in p wave velocity from 4 8 6 4 km s 1 and is divided into basalts and dykes oceanic layer 3 displays p wave velocities of 6 4 7 2 km s 1 the greenland continental crust is up to 25 km thick along the line and divided into an upper middle and lower crust with p wave velocities from 5 3 7 0 km s 1 the upper and middle continental crust thin over a 120 km wide continent ocean transi tion zone we classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data the profile did not reach continental crust on the baffin island margin which implies a transition zone of 150 km length at most the new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern baffin bay

the crustal structure of the southern nain and makkovik provinces of labrador derived from refraction seismic data data from a refraction seismic profile parallel to the coast of labrador canada were used to determine the crustal structure across the boundary of the nain and makkovik provinces and to look for evidence for an offshore continuation of the mesoproterozoic nain plutonic suite nps seven seismometers recorded airgun shots along the 283 km long line p and s wave velocity models were developed from forward and inverse modeling of travel times the velocity model distinguishes three distinct zones in the saglek block of the nain province the crust is divided into three layers with p wave velocities between 5 8 and 6 9 km s farther to the south upper crustal velocities increase to 6 3 6 5 km s and the poisson s ratio increases from 0 24 to 0 27 this zone correlates with a gravity low that is interpreted to outline the offshore continuation of the nps the upper crustal velocities are intermediate between anorthositic and granitoid rock samples collected from the nps a lower crustal reflector is li

tectonically buried continent ocean boundary ouachita mountains arkansas a seismic reflection sequence observed at depth beneath the south flank of the benton uplift in arkansas is interpreted to mark the early paleozoic continent ocean boundary the sequence is similar in both external and internal geometry to wedge shaped features commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins and interpreted as layered volcanic sequences it is suggested that slope and rise sediments now exposed in the benton uplift were thrust in a thin skinned fashion over the edge of the north american continent during initial stages of the carboniferous ouachita orogeny but that later uplift and slight northward translation of the continental margin basement occurred in a more thick skinned fashion preserving the boundary wedge intact beneath the earlier thrust sheets

tomographic determination of velocity and depth in laterally varying media estimation of reflector depth and seismic velocity from seismic reflection data can be formulated as a general inverse problem the method used to solve this problem is similar to tomographic techniques in medical diagnosis and we refer to it as seismic reflection tomography seismic tomography is formulated as an iterative gauss newton algorithm that produces a velocity depth model which minimizes the difference between traveltimes generated by tracing rays through the model and traveltimes measured from the data the input to the process consists of traveltimes measured from selected events on unstacked seismic data and a first guess velocity depth model usually this first guess model has velocities which are laterally constant and is usually based on nearby well information and or an analysis of the stacked section the final model generated by the tomographic method yields traveltimes from ray tracing which differ from the measured values in recorded data by approximately 5 ms root mean square the i

seismic imaging of complex structures by non linear traveltime inversion of dense wide angle data application to a thrust belt summary we have used a dense wide angle data set to test a two step procedure for the separate inversion of first arrival and reflection traveltimes data were collected in a complex thrust belt environment southern italy along a 14 km line with closely spaced sources 60 m and receivers 90 m we have applied a fully non linear tomographic technique specially designed to image complex structures to over 6400 first arrival traveltimes in order to determine a detailed velocity model a bi cubic spline velocity model parametrization is used the inversion strategy follows a multiscale approach and employs a non linear velocity optimization scheme the tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion aimed at imaging a target upper crust reflector the interface inversion method is also based on a multiscale approach and uses a non linear technique for model parameters interface position nodes optimization we have applied the interface inversion method to over 1600 reflection traveltimes of a target event picked both in the near and in the wide angle offset range the retrieved interface is well resolved in the central part of the model where ray coverage mainly includes clear post critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays the velocity and interface models thus determined are consistent with vertical seismic profiling data and correlate well with the geometry of known geological structures this study shows that the used inversion approach is efficient for target orientated investigations in complex geological environments

seismic velocity models for the denali fault zone along the richardson highway alaska crustal scale seismic velocity models across the denali fault zone along the richardson highway show a 50 km thick crust a near vertical fault trace and a 5 km wide damage zone associated with the fault near trans alaska pipeline pump station 10 which provided the closest strong ground motion recordings of the 2002 denali fault earthquake we compare models derived from seismic reflection and refraction surveys acquired in 1986 and 1987 to laboratory measurements of seismic velocities for typical metamorphic rocks exposed along the profiles our model for the 1986 seismic reflection profile indicates a 5 km wide low velocity zone in the upper 1 km of the denali fault zone which we interpret as fault gouge deeper refractions from our 1987 line image a 40 km wide 5 km deep low velocity zone along the denali fault and nearby associated fault strands which we attribute to a composite damage zone along several strands of the denali fault zone and to the obliquity of the seismic line to the fault zone our velocity model and other geophysical data indicate a nearly vertical denali fault zone to a depth of 30 km aftershocks of the 2002 denali fault earthquake and our velocity model provide evidence for a flower structure along the fault zone consisting of faults dipping toward and truncated by the denali fault wide angle reflections indicate that the crustal thickness beneath the denali fault is transitional between the 60 km thick crust beneath the alaska range to the south and the extended 30 km thick crust of the yukon tanana terrane to the north online material tables of locations for the tact 1986 and tact 1987 lines

a crustal model for northern melville bay baffin bay the interpretation of seismic refraction and gravity data acquired in 2010 gives new insights into the crustal structure of the west greenland coast and the adjacent deep central baffin bay basin underneath melville bay the depth of the moho varies between 26 and 17 km stretched continental crust with a thickness of 25 to 14 km and deep sedimentary basins are present in this area the deep melville bay graben contains an up to 11 km thick infill of consolidated and unconsolidated sediments with velocities of 1 6 to 4 9 km s seaward at the 60 km wide transition between oceanic and stretched continental crust a mount shaped magmatic structure is observed which most likely formed prior to the initial formation of oceanic crust the up to 4 km high magmatic structure is underlain by a 2 km thick and 50 km wide high velocity lower crust more to the west in the oceanic part of the baffin bay basin we identify a two layered 3 5 to 6 km thin igneous oceanic crust with increasing thickness toward the shelf beneath the oceanic crust the depth of the moho ranges between 11 5 and 13 5 km in the western part of the profile oceanic layer 3 is unusually thin 1 5 km a possible explanation for the thin crust is accretion due to slow spreading although the basement is notably smooth compared to the basement of other regions formed by ultra slow spreading the oceanic crust is underlain by partly serpentinized upper mantle with velocities of 7 6 to 7 8 km s

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

the bent prolongation of the 85 e ridge south of 5 n fact or fiction abstract the enigmatic 85 e ridge crosses the bay of bengal from north to south its strong gravity anomaly low is associated with cretaceous hotspot volcanism south of 5 n the gravity low bends into a sw ne orientation and continues as far sw as the afanasy nikitin seamounts this change has been interpreted to represent a bent hotspot track we report new constraints on the crustal structure and genesis beneath the sw trending gravity low based on new refraction seismic reflection seismic and shipborne gravity data our findings show that the crustal structure across the gravity low does not differ significantly from the adjacent 4 5 to 7 km thick oceanic crust no basement ridge significant crustal thickening or magmatic underplating were identified we found no evidence for a southern prolongation of the 85 e ridge instead our p wave velocity and density models reveal the gravity low to express a flexural basin which is a result of widespread mid miocene to recent lithospheric deformation in the indian ocean the 25 mgal negative gravity anomaly is therefore not related to the passage of the indian plate over a mantle plume casting doubt on the possibility that volcanism at the afanasy nikitin seamounts might be related to the same plume as the northern 85 e ridge the northern 85 e ridge may have been generated at the northern prolongation of the 86 e fracture zone

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

investigating the architecture of the paganica fault 2009 mw 6 1 earthquake central italy by integrating high resolution multiscale refraction tomography and detailed geological mapping this article has been accepted for publication in geophysical journal internationa 2017 published by oxford university press on behalf of the royal astronomical society all rights reserved

an image of the columbia plateau from inversion of high resolution seismic data the authors use a method of traveltime inversion of high resolution seismic data to provide the first reliable images of internal details of the columbia river basalt group crbg the subsurface basalt sediment interface and the deeper sediment basement interface velocity structure within the basalts delineated on the order of 1 km horizontally and 0 2 km vertically is constrained to within plus minus 0 1 km s for most of the seismic profile over 5 000 observed traveltimes fit their model with an rms error of 0 018 s the maximum depth of penetration of the basalt diving waves truncated by underlying low velocity sediments provides a reliable estimate of the depth to the base of the basalt which agrees with well log measurements to within 0 05 km 165 ft the authors use image blurring calculated from the resolution matrix to estimate the aspect ratio of images velocity anomaly widths to true widths for velocity features within the basalt from their calculations of image blurring they interpret low velocity zones lvz within the basalts at boylston mountain and the whiskey dick anticline to have widths of 4 5 and 3 km respectively within the upper 1 5 km of the model at greater depth the widths of these imaged lvzs thin tomore approximately 2 km or less they interpret these linear subparallel low velocity zones imaged adjacent to anticlines of the yakima fold belt to be brecciated fault zones these fault zones dip to the south at angles between 15 to 45 degrees less

crustal structure of central tibet as derived from project indepth wide angle seismic data summary in the summer of 1998 project indepth recorded a 400 km long nnw sse wide angle seismic profile in central tibet from the lhasa terrane across the banggong nujiang suture bns at about 89 5 e and into the qiangtang terrane analysis of the p wave data reveals that 1 the crustal thickness is 65 5 km beneath the line 2 there is no 20 km step in the moho in the vicinity of the bns as has been suggested to exist along strike to the east based on prior fan profiling 3 a thick high velocity lower crustal layer is evident along the length of the profile 20 35 km thick 6 5 7 3 km s 1 and 4 in contrast to the southern lhasa terrane there is no obvious evidence of a mid crustal low velocity layer in the p wave data although the data do not negate the possibility of such a layer of modest proportions combining the results from the indepth iii wide angle profile with other seismic results allows a cross section of moho depths to be constructed across tibet this cross section shows that crustal thickness tends to decrease from south to north with values of 70 80 km south of the middle of the lhasa terrane 60 70 km in the northern part of the lhasa terrane and the qiangtang terrane and less than 60 km in the qaidam basin the overall northward thinning of the crust evident in the combined seismic observations coupled with the essentially uniform surface elevation of the plateau south of the qaidam basin is supportive of the inference that northern tibet until the qaidam basin is underlain by somewhat thinner crust which is isostatically supported by relatively low density hot upper mantle with respect to southern tibet

ultrashallow seismic imaging of the causative fault of the 1980 m6 9 southern italy earthquake by pre stack depth migration of dense wide aperture data 1 a two step imaging procedure including pre stack depth migration psdm and non linear multiscale refraction tomography was applied to dense wide aperture data with the aim of imaging the causative fault of the 1980 m6 9 irpinia normal faulting earthquake in a very complex geologic environment psdm is often ineffective for ultrashallow imaging 100 m of depth and less of laterally heterogeneous media because of the difficulty in estimating a correct velocity model for migration dense wide aperture profiling allowed us to build accurate velocity models across the fault zone by multiscale tomography and to record wide angle reflections from steep reflectors psdm provided better imaging with respect to conventional post stack depth migration and improved definition of fault geometry and apparent cumulative displacement results indicate that this imaging strategy can be very effective for near surface fault detection and characterization fault location and geometry are in agreement with paleoseismic data from two nearby trenches the estimated vertical fault throw is only 29 38 m this value combined with the vertical slip rate determined by trench data suggests a young age 97 127 kyr of fault inception

crustal structure of the eratosthenes seamount cyprus and s turkey from an amphibian wide angle seismic profile abstract in 2010 project cococo incipient continent continent collision recorded a 650 km long amphibian n s wide angle seismic profile extending from the anatolian plateau across southern turkey and cyprus to just south of the eratosthenes seamount esm the aim of the project is to reveal the impact of the transition from subduction to continent continent collision of africa with anatolia arrival picking finite differences ray tracing and inversion of the offshore and on offshore data produced a tomographic model across southern cyprus the accretionary wedge and the esm the main features of this model are 1 crustal p velocities predominantly lower than 6 5 km s beneath the esm 2 crustal thickness between 28 and 37 km 3 an upper crustal reflection at 5 km depth beneath the esm 4 the likely presence of oceanic crust south of the esm and a transform margin north of it and 5 a 12 km thick ophiolite sequence on cyprus land shots on turkey also recorded on cyprus gravity data and geological and previous seismic investigations allow to derive a layered velocity model beneath anatolia and the northern part of cyprus the main features of this model are 1 moho depths of 38 45 km beneath the anatolian plateau 2 an upper and lower crust with large lateral changes in velocity and thickness 3 a north dipping subducting plate below cyprus with a steepening of the dip angle of the plate at about 45 km depth thus the wide angle seismic and gravity data provide detailed insights into the 2 d geometry and velocity structures associated with the cyprus arc collision zone finally integrated analysis of the geophysics and geology allows a comprehensive interpretation of the crustal structure related to the collision process

quantitative imaging of complex structures from dense wide aperture seismic data by multiscale traveltime and waveform inversions a case study an integrated multiscale seismic imaging flow is applied to dense onshore wideaperture seismic data recorded in a complex geological setting thrust belt an initial p wave velocity macromodel is first developed by first arrival traveltime tomography this model is used as an initial guess for subsequent full waveform tomography which leads to greatly improved spatial resolution of the p wave velocity model however the application of full waveform tomography to the high frequency part of the source bandwidth is difficult due to the non linearity of this kind of method moreover it is computationally expensive at high frequencies since a finitedifference method is used to model the wave propagation hence full waveform tomography was complemented by asymptotic prestack depth migration to process the full source bandwidth and develop a sharp image of the short wavelengths the final traveltime tomography model and two smoothed versions of the final full waveform tomography model were used as a macromodel for the prestack depth migration in this study wide aperture multifold seismic data are used after specific preprocessing of the data 16 frequency components ranging from 5 4 hz to 20 hz were inverted in cascade by the full waveform tomography algorithm the full waveform tomography successfully imaged sw dipping structures previously identified as highresistivity bodies the relevance of the full waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling vsp log available on the profile the prestack depth migrated images inferred from the traveltime and the smoothed full waveform tomography macromodels are shown to be on the whole consistent with the final full waveform tomography model a more detailed analysis based on common image gather computations and local comparison with the vsp log revealed that the most accurate migrated sections are those obtained from the full waveform tomography macromodels a resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wideaperture components of the data allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged

the oceanic crustal structure at the extinct slow to ultraslow labrador sea spreading center two seismic refraction lines were acquired along and across the extinct labrador sea spreading center during the seismic investigations off greenland newfoundland and labrador 2009 cruise we derived two p wave velocity models using both forward modeling rayinvr and traveltime tomography inversion tomo2d with good ray coverage down to the mantle slow spreading paleocene oceanic crust has a thickness of 5 km while the eocene crust created by ultraslow spreading is as thin as 3 5 km the upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading with a velocity drop of 0 5 to 1 km s when compared to paleocene upper crustal velocities 5 2 6 0 km s the overall crustal structure is similar to active ultraslow spreading centers like the mohns ridge or the south west indian ridge with lower crustal velocities of 6 0 7 0 km s an oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites 7 3 7 9 km s found 1 5 km below the basement the second ridge parallel line also shows extremely thin crust in the extinct axial valley where 8 km s mantle velocity is imaged just 1 5 km below the basement this thin crust is interpreted as crust formed by ultraslow spreading which was thinned by tectonic extension

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

tectonic evolution of southern baffin bay and davis strait results from a seismic refraction transect between canada and greenland 1 wide angle reflection refraction seismic data were acquired on a 450 km long transect in southern baffin bay extending from baffin island to greenland dense air gun shots were recorded on 22 ocean bottom seismometers apwave velocity model was developed from forward and inverse modeling of the observed travel times beneath the baffin island shelf a three layered continental crust is observed with velocities of 5 5 to 6 9 km s typical for transform margins there is a sharp transition between continental and oceanic crust off baffin island 7 km thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map beneath the deep baffin bay basin 9 km thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone the thicker than normal oceanic crust indicates an ample magma supply possibly related to melt extracted from a mantle plume seaward of the greenland continental crust 20 km thick igneous crust 6 3 to 7 3 km s is encountered in a 25 km wide zone interpreted as a leaky transform fault that can be correlated southward through davis strait the igneous crust is bounded by a 20 km wide basin to the west underlain by 4 km thick crust of unknown affinity this structure is probably associated with transform movements a high velocity lower crustal layer 7 1 km s of 8 km thickness is indicated beneath the greenland crust and can be correlated into the adjacent thick igneous crust both the thick igneous and greenland crust are covered by up to 4 km of paleogene volcanics 5 2 to 5 7 km s

seismic imaging of the eastern algerian margin off jijel integrating wide angle seismic modelling and multichannel seismic pre stack depth migration this study presents the results of a deep seismic survey across the north algerian margin based on the combination of 2 d multichannel and wide angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north south line extending 180 km off jijel into the algerian offshore basin and 25 land stations deployed along a 100 km long line cutting through the lesser kabylia and the tellian thrust belt the final model obtained using forward modelling of the wide angle data and pre stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin the sedimentary layers in the algerian basin are 3 75 km thick to the north and up to 4 5 5 km thick at the foot of the margin they are characterized by seismic velocities from 1 9 to 3 8 km s 1 messinian salt formations are about 1 km thick in the study area and are modelled and imaged using a velocity between 3 7 and 3 8 km s 1 the crust in the deep sea basin is about 4 5 km thick and of oceanic origin presenting two distinct layers with a high gradient upper crust 4 7 6 1 km s 1 and a low gradient lower crust 6 2 7 1 km s 1 the upper mantle velocity is constrained to 7 9 km s 1 the ocean continent transition zone is very narrow between 15 and 20 km wide the continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance the continental crust presents steep and asymmetric upper and lower crustal geometry possibly due to either asymmetric rifting of the margin an underplated body or flow of lower crustal material towards the ocean basin present time deformation as imaged from three additional seismic profiles is characterized by an interplay of gravity driven mobile salt creep and active thrusting at the foot of the tectonically inverted algerian margin

seismic images of the brooks range fold and thrust belt arctic alaska from an integrated seismic reflection refraction experiment abstract we describe results of an integrated seismic reflection refraction experiment across the brooks range and flanking geologic provinces in arctic alaska the seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile shot records show continuous moho reflections from 0 180 km offset as well as numerous upper and mid crustal wide angle events single and low fold near vertical incidence common midpoint cmp reflection images show complex upper and middle crustal structure across the range from the unmetamorphosed endicott mountains allochthon ema in the north to the metamorphic belts in the south lower crustal and moho reflections are visible across the entire reflection profile travel time inversion of pmp arrivals shows that the moho at 33 km depth beneath the north slope foothills deepens abruptly beneath the ema to a maximum of 46 km and then shallows southward to 35 km at the southern edge of the range two zones of upper and middle crustal reflections underlie the northern brooks range above 12 15 km depth the upper zone interpreted as the base of the ema lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central brooks range where the base of the ema outcrops above the metasedimentary rocks exposed in the doonerak window we interpret the base of the lower zone at 12 km depth to be from carbonate rocks above the master detachment upon which the brooks range formed the seismic data suggest that the master detachment is connected to the faults in the ema by several ramps in the highly metamorphosed terranes south of the doonerak window the cmp section shows numerous south dipping events which we interpret as a crustal scale duplex involving the doonerak window rocks the basal detachment reflections can be traced approximately 100 km and dip southward from about 10 12 km near the range front to 14 18 km beneath the doonerak window to 26 28 km beneath the metamorphic belts in the central brooks range the section documents middle and lower crustal involvement in the formation of the brooks range

paleozoic continent ocean transition in the ouachita mountains imaged from passcal wide angle seismic reflection refraction data a wide angle reflection reflection experiment sponsored by the program for array seismic studies of the continental lithosphere passcal was conducted in the ouachita mountains area of southwestern arkansas and northwestern louisiana this experimental employed 400 state of the art seismic recorders and overlapped the southern one third of the cocorp consortium for continental reflection profiling deep seismic reflection profile in the area a wide variety of data processing and interpretation techniques was employed to derive an earth model from these data the model depicts a preserved early paleozoic continental margin buried beneath allochthonous paleozoic strata and younger sedimentary rocks the southern part of the model indicates the presence of oceanic or highly extended continental crust overlain by about 15 km of mostly paleozoic sedimentary rock these results are consistent with little if any shortening of crystalline continental crust during the ouachita orogeny

inversion for interface structure using teleseismic traveltime residuals summary an inversion method is presented for the reconstruction of interface geometry between two or more crustal layers from teleseismic traveltime residuals the method is applied to 2 d models consisting of continuous interfaces separating constant velocity layers the forward problem of determining ray paths and traveltimes between incident wave fronts below the structure and receivers located on the earth s surface is solved by an eycient and robust shooting method a conjugate gradient method is employed to solve the inverse problem of minimizing a least squares type objective function based on the diverence between observed and calculated traveltimes teleseismic data do not accurately constrain average vertical structure so a priori information in the form of layer velocities and average layer thicknesses is required synthetic tests show that the method can be used to reconstruct interface geometry accurately even in the presence of data noise tests also show that if layer velocities and initial interface positions are poorly chosen lateral structure is still recoverable the inversion method was applied to previously published teleseismic data recorded by an in line array of portable seismographs that traversed the northern margin of the musgrave block central australia the solution based on interface parametrization is consistent with models given by other studies that used the same data but diverent methods most notably the standard tomographic approach that inverts for velocity rather than interface structure

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

wide angle seismic imaging of a mesoproterozoic anorthosite complex the nain plutonic suite in labrador canada the mesoproterozoic nain plutonic suite nps of labrador canada one of the largest anorogenic plutonic terranes was studied by a refraction wide angle seismic experiment four ocean bottom seismometers and 18 land stations were deployed along a 330 km profile and recorded air gun shots from the easternmost 160 km with the nps located in the center of the line at the suture of the nain and churchill provinces p and s wave velocity models were developed by forward modeling of travel times and amplitudes upper and middle crustal p wave velocities outside and beneath the nps range from 5 9 to 6 5 km s lower crustal p wave velocities range from 6 55 to 7 0 km s within the anorthositic rocks velocities are as high as 6 8 km s and reflections define the base of the nps to be 8 km deep in the se churchill province and 11 km in the nain province a variation that may be the result of lateral density changes within the country rocks or the anorthosites the total crustal thickness is 39 km west of the nps but is only 32 34 km beneath the nps some 5 km less than nain province crust distal from the nps the inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism the poisson s ratios are 0 275 within the anorthosite plutons 0 27 in the upper and middle crust and 0 285 in the lower crust these values are some 0 03 higher than in the archean nain crust distal to the nps indicating a higher plagioclase content at all crustal levels as result of the plutonism we postulate that a crustal root similar to the root observed farther north in the torngat orogen was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the nain churchill boundary

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

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

multiscale imaging of complex structures from multifold wide aperture seismic data by frequency domain full waveform tomography application to a thrust belt summary an application of full waveform tomography to dense onshore wide aperture seismic data recorded in a complex geological setting thrust belt is presented the waveform modelling and tomography are implemented in the frequency domain the modelling part is solved with a finite difference method applied to the visco acoustic wave equation the inversion is based on a local gradient method only the p wave velocity is involved in the inversion the inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies this defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images the linearized waveform tomography requires an accurate starting velocity model that has been developed by first arrival traveltime tomography after specific pre processing of the data 16 frequency components ranging between 5 4 and 20 hz were inverted ten iterations were computed per frequency component leading to 160 tomographic models the waveform tomography has successfully imaged southwest dipping structures previously identified from other geophysical data as being associated with high resistivity bodies the relevance of the tomographic images is locally demonstrated by comparison of a velocity depth function extracted from the waveform tomography models with a coincident vertical seismic profiling vsp log available on the profile moreover comparison between observed and synthetic seismograms computed in the starting traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide angle reflections from southwest dipping geological structures this study demonstrates that the combination of first arrival traveltime and frequency domain full waveform tomographies applied to dense wide aperture seismic data is a promising approach to quantitative imaging of complex geological structures indeed wide aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography this is critical because the building of the macromodel remains an open question when only near vertical reflection data are considered

depth and morphology of reflectors from the non linear inversion of arrival times and waveform semblance data part ii modelling and interpretation of real data acquired in southern apennines italy in order to retrieve a 2d background velocity model and to retrieve the geometry and depth of shallow crustal reflectors in the southern apennines thrust belt a separate inversion of first arrival traveltimes and reflected waveforms was performed data were collected during an active seismic experiment in 1999 by enterprise oil italiana and eni agip using a global offset acquisition geometry a total of 284 on land shots were recorded by 201 receivers deployed on an 18 km line oriented sw ne in the val d agri region southern apennines italy the two step procedure allows for the retrieval of a reliable velocity model by using a non linear tomographic inversion and reflected waveform semblance data inversion the tomographic model shows that the p wave velocity field varies vertically from approximately 3 km s to 6 km s within 4 km from the earth s surface moreover at a distance of approximately 11 km along the profile there is an abrupt increase in the velocity field in this zone indeed an ascent from 2 km depth to 0 km above sea level of the 5 2 km s iso velocity contour can be noted the retrieved velocity can be associated with plio pleistocene clastic deposits outcropping in the basin zone and with mesozoic limestone deposits the inversion of waveform semblance data shows that a p to p reflector is retrieved at a depth of approximately 2 km this interface is deeper in the north eastern part of the profile where it reaches 3 km depth and can be associated with a limestone horizon

inversion for crustal structure using reflections from the passcal ouachita experiment an interface inversion has been tested and applied to wide angle reflection data from the 1986 passcal ouachita experiment an interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion velocity depth curves derived for shot points 14 19 using the tail sum method are similar to an average one dimensional velocity model from forward modeling using travel time correlations from southern profiles 14 19 the similarity of these velocity depth curves positioned over the southern half of the passcal experiment suggests that crustal structure in this region is approximately laterally homogeneous lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications a two dimensional velocity structure was derived by using the average one dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces the final inversion model is found to be consistent with previous refraction interpretations south of the ouachita orogenic trend and concurrent interpretations of the passcal data set inversion results for the central and southern portion of the passcal profile indicate a depth of 10 12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km a lower crustal layer with an average thickness of 12 km and a moho depth of approximately 29 5 km are also determined interface depths are in agreement with a normal moveout stack of the passcal data set in particular the shallowing of the moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the paleozoic continental margin geophysical studies of the modern atlantic continental passive margin provide the simplest comparison to the crustal structure derived here the lower crustal layer found south of the shallowing of the moho to 30 km beneath the passcal profile is analogous in thickness and position to rift stage lower crust however other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships

diagenesis of radiaxial fibrous calcites in a subunconformity shallow burial setting upper triassic and liassic northern calcareous alps austria multiple generations i iv of radiaxial fibrous calcite rfc cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in upper triassic steinplatte and basal liassic hirlatz carbonates in the northern calcareous alps of austria stratigraphic petrographic and geochemical evidence suggest that rfc precipitated as high mg calcite from marine fluids in the shallow burial 180m environment estimated original 18o and 13c isotopic compositions of late triassic and early jurassic marine waters are 1 88 to 2 21 and 2 37 to 2 89 pdb respectively the rfc subsequently was altered to low mg calcite by meteoric or mixed meteoric marine fluids introduced during periods of subaerial exposure rfc cement generations i iv in steinplatte reef slope beds and i ii in steinplatte reef and lagoon and hirlatz samples retained near original marine oxygen and carbon isotopic compositions 1 88 and 2 89 pdb 2 11 and 2 37 pdb respectively and petrographic identity during the conversion to low mg calcite in contrast rfc generations iii and iv in the hirlatz and upper steinplatte were intensely leached and partly replaced by coarse blocky calcites of meteoric origin with attendant modification of original crystal fabrics and isotopic compositions 4 65 to 5 22 pdb 18o 2 54 to 3 14 pdb 13c these differences in diagenetic behaviour are related to contrasting processes of fluid rock systems and original microfabric variations in the rfc cements the karst affected phreatic marine burial environment of rfc formation and subsequent diagenetic alteration inferred for these rocks have not been previously documented in other ancient rocks

geophysical evidence for the extent of crustal types and the type of margin along a profile in the northeastern baffin bay investigating the crust of northern baffin bay provides valuable indications for the still debated evolution of this area the crust of the southern melville bay is examined based on wide angle seismic and gravity data the resulting p wave velocity density and geological models give insights into the crustal structure a stretched and rifted continental crust underneath southern melville bay is up to 30 km thick with crustal velocities ranging between 5 5 and 6 9 km s the deep melville bay graben contains a 9 km thick infill with velocities of 4 to 5 2 km s in its lowermost part west of the melville bay ridge a 80 km wide and partly only 5 km thick continent ocean transition cot is present west of the cot up to 5 km thick sedimentary layers cover a 4 3 to 7 km thick two layered oceanic crust the upper oceanic layer 2 has velocities of 5 2 to 6 0 km s the oceanic layer 3 has been modeled with rather low velocities of 6 3 to 6 9 km s low velocities of 7 8 km s characterize the probably serpentinized upper mantle underneath the thin crust the serpentinized upper mantle and low thickness of the oceanic crust are another indication for slow or ultraslow spreading during the formation of the oceanic part of the baffin bay by comparing our results on the crustal structure with other wide angle seismic profiles recently published differences in the geometry and structure of the crust and the overlying sedimentary cover are revealed moreover the type of margin and the extent of crustal types in the melville bay area are discussed

tomo3d 3 d joint refraction and reflection traveltime tomography parallel code for active source seismic data synthetic test we present a new 3 d traveltime tomography code tomo3d for the modelling of active source seismic data that uses the arrival times of both refracted and reflected seismic phases to derive the velocity distribution and the geometry of reflecting boundaries in the subsurface this code is based on its popular 2 d version tomo2d from which it inherited the methods to solve the forward and inverse problems the traveltime calculations are done using a hybrid ray tracing technique combining the graph and bending methods the lsqr algorithm is used to perform the iterative regularized inversion to improve the initial velocity and depth models in order to cope with an increased computational demand due to the incorporation of the third dimension the forward problem solver which takes most of the run time 90 per cent in the test presented here has been parallelized with a combination of multi processing and message passing interface standards this parallelization distributes the ray tracing and traveltime calculations among available computational resources the code s performance is illustrated with a realistic synthetic example including a checkerboard anomaly and two reflectors which simulates the geometry of a subduction zone the code is designed to invert for a single reflector at a time a data driven layer stripping strategy is proposed for cases involving multiple reflectors and it is tested for the successive inversion of the two reflectors layers are bound by consecutive reflectors and an initial velocity model for each inversion step incorporates the results from previous steps this strategy poses simpler inversion problems at each step allowing the recovery of strong velocity discontinuities that would otherwise be smoothened

detection of seismic anisotropy using ocean bottom seismometers a case study from the northern headwall of the storegga slide summary azimuthal seismic anisotropy has been identified from the analysis of s waves generated by p to s mode conversion in the pleistocene sediments that form the northern headwall of the storegga slide which were investigated with a seismic experiment employing a seabed array of ocean bottom seismometers and a grid of airgun shots the principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components after the application of moveout to show the variations in amplitude phase and cumulative traveltime effects of s waves ultimately providing information that identified the fast and slow s wave polarization orientations particle motion analysis was used to corroborate the results and provide further information on the magnitudes of cumulative s wave splitting a 2 d ray traced inversion of the traveltimes of pre critical p and ps arrivals provided a velocity model from which the variation with depth of vp vs and anisotropy could be compared with lithological and stratigraphic data from a borehole at the centre of the obs array increased anisotropic response was observed to be coincident with high velocity units which have high mica but low water content and are interpreted to be of glacial origin the analysis of azimuthal seismic anisotropy shows clear evidence for horizontal transverse isotropy or an orthorhombic symmetry the distribution in orientations of the fast plane of symmetry is broadly bimodal e w and ne sw across the obs array the e w group showed correlation with the headwalls of old buried slides and other faults visible within coherency attributes calculated from an accompanying 3 d seismic data set and with the strike of some of the headwalls of slides shown in multibeam bathymetry however the pattern of headwall fractures shown in the bathymetry is complicated and reticulate and the ne sw orientation is also well represented we infer that the cause of the anisotropy is the presence of vertical to sub vertical fluid filled fractures and micro cracks partially held open by high pore fluid pressure the fracture orientations are controlled primarily by the present day gravitationally induced down slope stress which is mediated by the heterogeneous nature of sub surface causing local changes in the orientation of the principal stresses at the margins of incipient or failed slides the fractures if connected are likely to increase vertical permeability within the sediment column significantly and influence the distribution of gas hydrate within the strata

nonlinear refraction and reflection travel time tomography we develop a rapid nonlinear travel time tomography method that simultaneously inverts refraction and reflection travel times on a regular velocity grid for travel time and ray path calculations we apply a wave front method employing graph theory the first arrival refraction travel times are calculated on the basis of cell velocities and the later refraction and reflection travel times are computed using both cell velocities and given interfaces we solve a regularized nonlinear inverse problem a laplacian operator is applied to regularize the model parameters cell slownesses and reflector geometry so that the inverse problem is valid for a continuum the travel times are also regularized such that we invert travel time curves rather than travel time points a conjugate gradient method is applied to minimize the nonlinear objective function after obtaining a solution we perform nonlinear monte carlo inversions for uncertainty analysis and compute the posterior model covariance in numerical experiments we demonstrate that combining the first arrival refraction travel times with later reflection travel times can better reconstruct the velocity field as well as the reflector geometry this combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust we apply this approach to model the crustal structure of the california borderland using ocean bottom seismometer and land data collected during the los angeles region seismic experiment along two marine survey lines details of our image include a high velocity zone under the catalina ridge but a smooth gradient zone between catalina ridge and san clemente ridge the moho depth is about 22 km with lateral variations

large explosive source wide recording aperture seismic profiling on the columbia plateau washington clear subsurface seismic images have been obtained at low cost on the columbia plateau washington the columbia plateau is perhaps the most notorious of all bad data areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield this degradation was mitigated in this study via a large explosive source wide recording aperture shooting method the shooting method emphasizes the wide angle portion of the wavefield where fermat s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces the basalt diving wave normally discarded in standard common midpoint cmp seismic profiling can be used to image basalt velocity structure via traveltime inversion maximum depth penetration of the diving wave tightly constrains basalt sediment interface depth an arrival observed only at shot receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion

crustal and uppermost mantle velocity structure of northern eurasia along the profile quartz the velocity structure of the crust and uppermost mantle beneath the dss profile quartz in northwestern russia is determined using a linearized seismic tomography scheme the analysis is performed on the northern 2200 km of the profile spanning from the kola peninsula the timan pechora basin and the uralian fold belt to the west siberian basin refracted and reflected phases recorded from 28 chemical and two nuclear shot points are analyzed the inversion technique allows determination of the p wave velocity structure and estimation of the depth of the crust mantle discontinuity resolution analysis shows that the upper crustal structure and sections of the lower crust and uppermost mantle are well resolved the modeled velocity and crustal thickness anomalies are confirmed by the resulting structure of the resolution matrix poorer ray coverage in parts of the lower crust and most sections of the uppermost mantle limits their resolvable structures a thick over 10 km high velocity block vp 7 0 km sec in the lower crust of the precambrian beneath the kola peninsula crustal thinning and a wide range of compressional velocities in the sedimentary basin structures and a significant crustal root beneath the uralian fold belt 42 to 45 km moho depth with high lower crustal velocities exceeding 7 0 km sec were identified in the tectonic structure of northwestern eurasia

seismic study of the transform rifted margin in davis strait between baffin island canada and greenland what happens when a plume meets a transform 1 the davis strait transform margin was studied using a 630 km long wide angle reflection refraction seismic transect extending from se baffin island to greenland dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line a p wave velocity model was developed from forward and inverse modeling of the wide angle data and incorporation of coincident deep multichannel reflection seismic data off baffin island in the saglek basin 7 to 11 km thick two layered continental crust 5 8 6 6 km s is observed off greenland continental crust is divided into three layers 5 4 6 8 km s with a maximum thickness of 20 km farther offshore greenland the crust thins to 7 12 km and the lower crust disappears between the continental blocks a 140 km wide zone with oceanic crust layer 2 is 5 4 6 2 km s and layer 3 is 6 7 7 0 km s is located the western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors the eastern part is associated with the ungava fault zone ufz the major transform fault in davis strait the ufz thus acted as leaky transform fault during phases of transtension southward flow of material from the iceland plume created a 4 to 8 km thick underplated layer 7 4 km s beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust plume related paleogene volcanism is indicated by an up to 4 km thick layer 4 3 5 8 km s with basalts and interbedded sediments that can be traced from se baffin island 400 km toward the east

crustal structure of the grenville province in southeastern labrador from refraction seismic data evidence for a high velocity lower crustal wedge the crustal structure of the eastern grenville and makkovik provinces was determined using two onshore offshore refraction seismic lines of the lithoprobe eastern canadian shield onshore offshore transect ecsoot a gravity high in the hawke river terrane correlates with increased p wave velocities in the upper 30 km of the crust 6 2 6 7 km s in the upper and middle crust and 6 9 7 1 km s below which we interpret as structure inherited from the labradorian orogen velocities in the adjacent groswater bay terrane are 6 0 6 55 km s in the upper and middle crust and 6 6 6 95 km s in the lower crust the entire grenville crust is underlain by a 15 20 km thick high velocity lower crustal hvlc wedge consisting of an upper layer 7 1 7 4 km s and a lower layer 7 6 7 8 km s the hvlc wedge is interpreted as an underplated layer formed during iapetan rifting this interpretation is based on the correlation with the 615 ma long range dykes onshore and the eastward termination of the wedge at the cartwright

crustal structure of the southern dead sea basin derived from project desire wide angle seismic data summary as part of the dead sea integrated research project desire a 235 km long seismic wideangle reflection refraction wrr profile was completed in spring 2006 across the dead sea transform dst in the region of the southern dead sea basin dsb the dst with a total of about 107 km multi stage left lateral shear since about 18 ma ago accommodates the movement between the arabian and african plates it connects the spreading centre in the red sea with the taurus collision zone in turkey over a length of about 1 100 km with a sedimentary infill of about 10 km in places the southern dsb is the largest pull apart basin along the dst and one of the largest pull apart basins on earth the wrr measurements comprised 11 shots recorded by 200 three component and 400 one component instruments spaced 300 m to 1 2 km apart along the whole length of the e w trending profile models of the p wave velocity structure derived from the wrr data show that the sedimentary infill associated with the formation of the southern dsb is about 8 5 km thick beneath the profile with around an additional 2 km of older sediments the depth to the seismic basement beneath the southern dsb is about 11 km below sea level beneath the profile seismic refraction data from an earlier experiment suggest that the seismic basement continues to deepen to a maximum depth of about 14 km about 10 km south of the desire profile in contrast the interfaces below about 20 km depth including the top of the lower crust and the moho probably show less than 3 km variation in depth beneath the profile as it crosses the southern dsb thus the dead sea pull apart basin may be essentially an upper crustal feature with upper crustal extension associated with the left lateral motion along the dst the boundary between the upper and lower crust at about 20 km depth might act as a decoupling zone below this boundary the two plates move past each other in what is essentially a shearing motion thermo mechanical modelling of the dsb supports such a scenario as the desire seismic profile crosses the dst about 100 km north of where the desert seismic profile crosses the dst it has been possible to construct a crustal cross section of the region before the 107 km left lateral shear on the dst occurred

crustal structure of the eastern dabie shan interpreted from deep reflection and shallow tomographic data abstract a 20 km long seismic line characterises the crustal reflection pattern of the easternmost dabie shan the archetypal ultrahigh pressure uhp orogen of eastern china the weak to non reflective upper crust 5 s two way travel time twt 15 km depth is interpreted to comprise uhp rocks thrust over lithologically similar but non uhp crust the tectonic contact although not imaged as a distinct reflector is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust thus the seismic pattern of the upper crust implies that mafic oceanic crust does not constitute a significant proportion the middle to lower crust 5 10 s twt 15 33 km depth probably represents cratonal yangtze basement unaffected by the uhp metamorphism the prominent lowermost reflectors 10 12 s twt 33 40 km depth are interpreted to trace the moho excluding the presence of a crustal root inherited from the uhp orogeny a tomographic p wave velocity model for the uppermost crust

full wavefield inversion of wide aperture sh and love wave data summary reverse time linearized inversion is implemented for synthetic wide aperture sh and love wave data including multiple reflections to estimate the s wave velocity in a two dimensionally inhomogeneous medium complete wide aperture wavefields in which triplications pre and post critical reflections and surface waves love waves are present can be imaged by reverse time inversion to estimate the sh velocity distribution the algorithm operates on common source gathers and involves cross correlation of the source and recorded wavefields to define the steepest descent direction to update the velocity at each point on a finite difference grid effects of noise can be reduced by stacking information from different sources

seismic evidence for plume derived volcanism during formation of the continental margin in southern davis strait and northern labrador sea the crustal structure in the southern davis strait and the adjacent ocean continent transition zone in ne labrador sea was determined along a 185 km long refraction wide angle reflection seismic transect to study the impact of the iceland mantle plume to this region a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by ocean bottom seismographs a coincident industry multichannel reflection seismic profile was used to guide the modelling as reflectivity could be identified down to moho the model displays a marked lateral change of velocity structure the sedimentary cover velocities 1 8 3 9 km s 1 is up to 4 km thick in the north and thins to 1 km in the south the segment of the line within southern davis strait is interpreted to be of continental character with a two layered 13 km thick crust with p wave velocities of 5 6 5 8 and 6 4 6 7 km s 1 in the upper and lower crust respectively the crust is underlain by a 2 to 4 km thick high velocity layer 7 5 km s 1 this layer we interpret as underplated material related to the iceland plume the southern segment of the line in labrador sea displays a 2 km thick layer with a velocity of 4 5 km s 1 this layer can be correlated to a well about 100 km to the west of the line where palaeocene basalts and interbedded sediments were drilled underneath is a 12 km thick crust with a 2 km thick upper layer 5 8 6 6 km s 1 and a 10 km thick lower layer 6 8 7 2 km s 1 this crust is interpreted to be of oceanic character s wave modelling yields a poisson s ratio of 0 28 for the lower crust compatible with a gabbroic composition the igneous crust is 5 km thicker than normal oceanic crust we suggest that the increased magma production was created by buoyancy driving flow

application of wave field continuation to the inversion of refraction data three examples of the inversion of refraction data by downward continuation illustrate the applicability of the method to field data the first example is a refraction profile from the mojave desert california these data are spatially aliased and contain clear evidence of lateral inhomogeneity the inversion in this case produces a broken image in the slowness depth domain due to the lateral inhomogeneity but a useful average velocity model is still obtained the second example is a shallow marine reflection profile here the truncation effects due to the finite horizontal aperture of the recording cable produce artifacts in the slowness depth domain the velocity model is however distinct from these artifacts and the presence of strong precritical reflections aids in the inversion the third example is another shallow marine reflection profile the inversion of these data illustrate the utilization of constraints provided by multiples as well as primary arrivals

the 17 may 2012 m4 8 earthquake near timpson east texas an event possibly triggered by fluid injection this study summarizes our investigation of the 17 may 2012 mw rmt4 8 earthquake near timpson texas the largest earthquake recorded historically in eastern texas to identify preshocks and aftershocks of the 17 may event we examined the arrivals recorded at nacogdoches natx 30 km from the 17 may epicenter at nearby usarray transportable array stations and at eight temporary stations deployed between 26 may 2012 and mid 2013 at natx we identified seven preshocks the earliest occurring in april 2008 reliably located aftershocks recorded by the temporary stations lie along a 6 km long nw se linear trend corresponding to a previously mapped basement fault that extends across the highest intensity mmi vii region of the 17 may main shock earthquakes in this sequence are relatively shallow with focal depths ranging from 1 6 to 4 6 km evidence supporting these depths include hypocentral locations of exceptionally well recorded aftershocks s p intervals at the nearest stations and comparisons of synthetics and observed seismograms within 3 km of the linear trend of aftershock activity there are two class ii injection disposal wells injecting at 1 9 km depth beginning in august 2006 and february 2007 with injection rates averaging 42 750 m3 mo and 15 600 m3 mo respectively several observations support the hypothesis that fluid injection triggered the timpson sequence well located epicenters are situated near a mapped basement fault and near high volume injection wells focal depths are at or below the depths of injection and the earliest preshock april 2008 occurred after the onset of injection in 2006

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

seismic imaging of upper crustal structure using travel times from the passcal ouachita experiment a first arrival travel time inversion applied to the 1986 passcal ouachita experiment has been used to image upper crustal structure twenty one shot points every 10 km were recorded along two 100 km long segments with 400 seismographs on each segment a splined velocity model parameterization allowed for the inversion of laterally inhomogeneous models but not velocity discontinuities over 800 travel times were used in the inversion with a variable data covariance weighting the multiple source nature of the experiment design allowed the use of reciprocity relationships among 21 shot points to enhance data correlation a succession of models with velocities specified at 30 90 171 and 221 nodes were inverted for whose horizontal node spacing decreased from 100 to 12 5 km in the final 221 node model vertical node spacings of 3 0 and 1 5 km were used the results of the inversions indicate changes in depth of isovelocity lines which are interpreted as triassic fault structure based on nearby well data the final rms travel time residual is 0 045 s resolution using first arrivals deteriorates rapidly below 9 km depth three distinct velocity gradients can be correlated with geologic units surface exposed cretaceous and jurassic sediments with velocities from 3 0 tomore 3 5 km s extend to 1 5 km depth triassic rift fill sediments which do not outcrop have inverted velocities ranging from 3 5 to 4 75 km s between depths of 1 5 3 0 km paleozoic sediments are imaged by a decreased velocity gradient below a depth of 3 0 km lack of resolution at depths greater than 9 km necessitates detailed modeling of wide angle reflected phases to further constrain deeper crustal structure less

crustal structure of the paleozoic kunlun orogeny from an active source seismic profile between moba and guide in east tibet china abstract we herein present a new seismic refraction wide angle reflection profile that crosses the songpan ganzi terrane the animaqing suture zone and the eastern kunlun mountains comprised of the south kunlun and middle kunlun blocks separated by the middle kunlun fault the profile is 380 km long and extends from moba to guide in eastern tibet the crustal thickness is about 62 km under the songpan ganzi terrane 62 64 km under the south kunlun and 60 km under the middle kunlun block the songpan ganzi flysch seems to be present up to a depth of 15 km south of the animaqing suture zone and up to a depth of 10 km in the middle kunlun block with thicknesses elsewhere that depend on assumptions about the likely lithologies the profile exhibits clear lateral variations both in the upper and lower crust which are indicative of different crustal blocks juxtaposed by the kunlun fault system whether or not the songpan ganzi flysch was originally deposited on oceanic crust at the longitude of our profile 100 e it is now underlain by continental crust and the presence of continental crust beneath the songpan ganzi terrane and of a continental arc under the south kunlun block suggest paleozoic continent continent arc collision in the eastern kunlun mountains comparison of crustal velocity columns from all wide angle seismic profiles across the eastern kunlun mountains indicates a remarkable west to east change in the moho topography across the kunlun fault system 15 20 km moho step at 95 e but only 2 5 km along our profile at 100 e lower crustal thickness of the kunlun terranes is rather uniform about 35 km from 80 95 e which suggests that similar thrust thickening processes have played a role where the qaidam basin abuts the kunlun fault but thins to 20 25 km at 100 e east of the qaidam basin the increased crustal thickness from 93 to 98 e compared to that at 100 e may be due to the differences in the thickness of the crust of the two plates before their collision and or largely achieved by thickening of the lower crust perhaps indicating a crustal flow mechanism operating more strongly in the western region

high resolution densely spaced wide aperture seismic profiling as a tool to aid seismic hazard assessment of fault bounded intramontane basins application to vallo di diano southern italy abstract seismic hazards in extensional regions are mostly posed by high angle normal faults and deep hanging wall basins that can promote significant ground motion amplifications characterized by strong lateral velocity variations these structures represent challenging targets for seismic exploration in this paper we show that prestack depth migration of high resolution seismic reflection data collected with nonconventional dense wide aperture arrays can effectively assist deterministic seismic hazard assessment of fault bounded basins our high resolution 3 2 km long seismic profile was acquired along vallo di diano the largest fault bounded intramontane basin in the southern apennines seismic belt italy the imaging strategy combines prestack depth migration with an iterative velocity building technique that utilizes results of multiscale refraction tomography in particular migration of steeply dipping reflections is essential in imaging the complex morphology of the basement rocks beneath the basin by combining seismic reflection and v p tomography results we were able to evaluate 1 the structure of the vallo di diano basin along a representative transect from near surface to basement depths 2 the shallow depth geometry of the vallo di diano fault system which bounds the basin to the east 3 the structure of the sediment basement interface and 4 the velocity structure of basement and sediments within the basin these findings are valuable both for understanding the tectonic evolution of the vallo di diano and for developing earthquake shaking scenario studies

crustal structure of the flemish cap continental margin eastern canada an analysis of a seismic refraction profile summary the crustal structure of the ne flemish cap margin was determined along a 460 km long refraction wide angle reflection seismic transect flame line to define the thickness structure and composition of the crust and uppermost mantle along the line a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers a coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to moho the model displays a sediment cover of up to 3 6 km thick subdivided into three layers with velocities of 1 8 1 9 km s 1 2 8 3 1 km s 1 and 4 7 4 8 km s 1 for the western part of the flame line over flemish cap the p wave velocity model displays an up to 32 km thick three layer continental crust the continental crust has velocities of 5 8 6 1 km s 1 6 3 6 45 km s 1 and 6 65 6 85 km s 1 and thicknesses of about 5 km 7 km and 20 km in the upper middle and lower layers respectively the thick continental crust thins to a two layer 6 km thick crust upper layer is 5 55 6 0 km s 1 and the layer below is 6 65 6 8 km s 1 over a distance of 45 km s wave velocities are determined in the upper layer of the thick continental crust over flemish cap and the transition zone by assigning poisson s ratios in the p wave velocity model comparison of calculated to observed arrival times gives a poisson s ratio of 0 27 in the upper layer and 0 28 in the layer below which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone the thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7 5 7 9 km s 1 we interpret this layer as partially serpentinized mantle which is consistent with observationsfromthenewfoundlandmargintothesouth theserpentinizedmantleterminates 30 km seaward of the thick continental crust at the seaward most end of the thin continental crust a prominent ridge feature is observed the seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust the reflection seismic data show a high relief basement from the ridge feature and seaward the flame line crosses magnetic anomaly 34 and extends another 50 km seaward well into oceanic crust the ridge is flanked seaward by a two layer oceanic crust the upper layer layer 2 has velocities of 4 8 5 0 km s 1 for the landward most 35 km of oceanic crust and 4 8 6 2 km s 1 for the seaward 60 km the average thickness of layer 2 is 2 km the lower layer layer 3 has velocities of 6 7 7 2 km s 1 and a thickness of 3 5 km the velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature

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

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

shallow high resolution geophysical investigation along the western segment of the victoria lines fault island of malta abstract the victoria lines fault island of malta is a 15 km long and n260 striking segmented normal fault system which is probably inactive since the late pliocene in the westernmost part the fomm ir rih segment displays comparable geologic throw and escarpment height 150 170 m moreover its hangingwall hosts thin patches of middle pleistocene clastic continental deposits red beds which are poorly preserved elsewhere we acquired two seismic transects by collecting ambient vibration recordings processed by using horizontal to vertical spectral ratios complemented by one high resolution 2 d refraction tomography survey crossing this fault where it is locally covered by red beds and recent colluvial deposits we found a resonance peak at 1 0 hz in the hangingwall block whereas clear peaks in the range 5 0 10 0 hz appear when approaching the subsurface fault and we relate them to the fractured bedrock within the fault zone the best fit tomographic model shows a relatively high vp shallow body vp 2200 2400 m s that we relate to the weathered top of the miocene upper coralline limestone fm bounded on both sides by low vp regions 230 m s above the weathered top bedrock our results depict a clear seismic signature of the victoria lines fault characterized by low seismic velocity and high amplification of ground motion we hypothesize that during the middle pleistocene faulting may have affected the basal part of the red beds so that this part of the investigated complex fault system may be considered inactive since 0 6 myr ago

crustal structure in an onshore offshore transitional zone near hong kong northern south china sea to study the crustal structure beneath the onshore offshore transitional zone a wide angle onshore offshore seismic experiment was carried out in northern south china sea near hong kong using large volume airgun sources at sea and seismic stations on land the crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the dangan islands based on the characteristics of pg and multiple pg and the crustal structure beneath the sedimentary layer is relatively simple the moho depth is about 25 28 km along the profile and the p wave velocity increases gradually with depth the velocities in the upper crust range from 5 5 to 6 4 km s while that in the lower crust is 6 4 6 9 km s it also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75 90 km distance which suggests that the littoral fault zone lfz exists beneath the onshore offshore transitional zone the magnetism anomalies bouguer gravity anomalies and active seismic zone along the coastline imply the lfz is a main tectonic fault in the onshore offshore area combined with two previously published profiles in the continental south china l g profile and in the northern margin of south china sea obs1993 respectively we constructed a land sea super cross section about 1000 km long the results show the onshore offshore transitional zone is a border separating the unstretched and the stretched continental crust the low velocity layer lvl in the middle crust was imaged along l g profile however the high velocity layer hvl in the lower crust was detected along obs 1993 by analyzing the mechanisms of the lvl in the middle crust and hvl in the base of crust we believe the crustal structures had distinctly different attributes in the continental south china and in the northern scs which indicates that the lfz could be the boundary fault between them c 2009 elsevier ltd all rights reserved

slant stack velocity analysis of shot point 16 from the 1986 passcal ouachita experiment a x p slant stack algorithm is applied to shot point 16 of the 1986 passcal ouachita experiment to determine the optimal me dimensional velocity structure several enhancements to a standard slant stack algorithm are used to image a one dimensional velocity fimction from the shot point gathered data because only postcritical reflections and refractions are needed to image a velocity function significant muting of precritical energy is perfonned which reduces the effect of noise in calculating the stack for data sets with long offsets scaling the data by a factor equal to or greater than offset distance compensates for the effects of geometrical spreading and attenuation this enhances imaging of the deeper regions of the velocity model and balances amplitudes across the shot point gather finally ray parameter depth images can have difficulty imaging higher velocities since for higher velocities a small change in ray parameter results in a large increment in velocity this problem is overcome by slant stacking in equal increments of velocity instead of ray parameter the enhanced slant stack algorithm is applied to shot point 16 of the 1986 passcal ouachita data set which sufficienfiy satisfies the assumption of lateral homogeneity based on previous studies of the region other velocity models are found to be consistent with the envelope of velocity fimcfion imaged by the slant stack method maximum amplitudes on the x p image are used as a constraint to delineate a more detailed velocity model a seven layer velocity model is derived for shot point 16 which is found to be similar to those of others who have studied the area the top three layers are sedhnents associated with the gulf coastal plain followed by a thick layer derived from sedimentation associated with the closing of the proto gulf of mexico and the associated ouachita orogeny the determination of a more detailed velocity function from the envelope suggests somewhat higher velocities in the lower crustal layer than previous studies however given the one dimensional assumptions of the method as well as the signal to noise ratio of the data the velocity envelope is the most important constraint on the velocity models and tectonic interpretation provided by this analysis

the appalachian ouachita connection paleozoic orogenic belt at the southern margin of north america late paleozoic orogenic belt structures exposed in the appalachian mountains of alabama and in the ouachita mountains of arkansas plunge from opposite directions beneath postorogenic mesozoic cenozoic strata in the mississippi embayment of the gulf coastal plain figure 1 the paleozoic stratigraphic sequence and details of structural style in the ouachita outcrops contrast strongly with those in the appalachian outcrops furthermore straight line projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the coastal plain concealed by the coastal plain cover are answers to questions such as the following are the orogenic belts continuous discontinuous offset or intersecting how are along strike changes in stratigraphy and structural style expressed what is the sequence of tectonic evolution early attempts to interpret the subsurface relationships between the appalachian and ouachita structures were of necessity based on the first few scattered wells that were drilled through coastal plain strata into paleozoic rocks subsequent studies have enjoyed the availability of progressively more numerous deep wells and a growing accumulation of geophysical data various stages in the evolution of thought were outlined by king 1950 1961 1975 and by thomas 1973 1976 this review summarizes available subsurface data in a pre mesozoic paleogeologic

least squares inversion of spatial seismic refraction data a method is demonstrated for recording and inverting in line and broadside seismic refraction and reflection data to obtain three dimensional structure and velocity information this technique of spatial seismic refraction recording produces a superabundance of common ray intersections at the target horizon the full power of the linear least squares inversion method may be used in the overconstrained case a large number of recording instruments are required to perform the experiment efficiently but this has the advantage of reducing the number of sources and difficult source corrections a damped least squares inversion method has been found to give rapid convergence even in the case of complex faulted models using noisy data the spatial seismic refraction recording method has been applied in a suboptimal experiment over the williston basin in southern saskatchewan the crustal structure consists of several normally faulted blocks defined by aeromagnetic anomalies over an area notable for its thick crust local seismicity and a linear conductive anomaly

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position