double beam imaging mapping lower mantle heterogeneities using combinations of source and receiver arrays we present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays the method is based on the single scattering assumption and is closely related to the double beam method of kruger et al 1993 1995 1996 in that it exploits amplitude delay time slowness and azimuth information in two arrays simultaneously a crucial step in the method is the application of static time corrections for a chosen reference phase here pcp one type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase this allows the determination of the spatial origin of coherent phases in the p coda in a complementary approach we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three dimensional 3 d grid the scattering strength is expressed as the resulting beam power beam amplitude or semblance in a time window determined from the theoretical delay time with respect to the reference phase while likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering double beam migration provides information about the scattering strength the method is applied to nuclear explosion sources in eastern kazakhstan recorded at the yellowknife array in northern canada we identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the p coda these anomalies range in depth from the core mantle boundary cmb up to 500 km into the lowermost mantle and are mainly located under the eurasian side of the arctic the joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the cenozoic and mesozoic subduction of the pacific plate and the kula plate the determination of the nature of such 3 d anomalies necessitates further array studies with better azimuthal coverage

analysis of asymmetric multipathing with a generalization of the double beam method abstract an array method integrating source and receiver array beamforming is presented the method is a generalization of previous work kruger et al 1993 to independent variation of slowness and azimuth in both the source and the receiver array compared to the analysis capabilities of a single array the resolution is enhanced and asymmetric scattering configurations and multipathing can be studied the generalized double beam method dbm is applied to a data set consisting of nuclear explosions detonated in eastern kazakhstan and recorded at the yellowknife array in northern canada the multipathing found for pcp and pdp is interpreted in terms of lateral inhomogeneity in the lowermost mantle below severnaya zemlya indications for rayleigh to p wave scattering observed for the shagan river and degelen subsites of the former main test site of the ussr in eastern kazakhstan can be related to the structure in the vicinity of the two subsites

modeling of seismic guided waves at the dead sea transform 1 on several recordings of linear seismometer arrays crossing the arava fault af in the middle east we see prominent wave trains emerging from in fault explosions which we interpret as waves being guided by a fault zone related low velocity layer the af is located in the arava valley and is considered the principal active fault of the mainly n s striking dead sea transform system in this section observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations they exhibit large amplitudes and are almost monochromatic we model them by a two dimensional 2 d analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces a hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space resolution is investigated by synthetic tests the observations are adequately fit by models with a narrow only 3 12 m wide waveguide with s wave velocity reduced by 10 60 of the surrounding rock we relate this vertical low velocity layer with the damage zone of the af since the location of receivers observing and of shots generating the guided waves respectively match with the surface trace of the fault the thickness of the damage zone of the af at least at shallow depths seems to be much smaller than in other major fault zones this could be due to less total slip on this fault

reflections from a distant subduction zone accurate models of the distribution of heterogeneities in the earth s mantle especially of subduction zones are crucial for our understanding of the composition and dynamics of the earth here we present broad band bb seismograms with p waves reflected at the subduction zone under southern japan at about 84 distance from german regional seismic network the earthquakes occurred in the mariana islands region at 100 to 105 distance array techniques allow the separation of this wave with an asymmetric path p1p2 from reflections and conversions under the source the receivers and the midpoint between the source and the receivers our observation demonstrates that subduction zones can act as strong seismic reflectors and can be mapped with a depth resolution of 20km with bb arrays at teleseismic distances the analysis of such data will help to derive a more detailed picture of the structure of the earth s mantle

crustal scattering at the ktb from a combined microearthquake and receiver analysis summary in december of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the ktb main borehole oberpfalz germany these events were recorded with a temporal seismic network at the surface out of the complete data set a cluster of five events recorded at four mini arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the ktb for this purpose the double beam method dbm kruger et al 1993 1995 1996 and the double beam imaging method scherbaum kruger weber 1997 were extended to curved wave fronts to drop the restriction of plane wave propagation this technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close by mini arrays the results of the array analysis show that the composition of the p coda is mainly avected by the site location of the arrays near surface and deeper crustal scattering contribute in a very complicated pattern furthermore with the present data set it was possible to identify reflections from the top of the erbendorf body this is a very pronounced arrival in most of the recorded traces in one of the arrays its amplitudes are even greater than the direct p phases five to eight coherent phases could be identified by the mini arrays using only these phases synthetic p coda traces were constructed which only contain the coherent part of the observed wavefield by subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the p coda of up to 37 per cent this leads to the conclusion that a large amount of the p coda at the ktb can be modelled by a simple deterministic single scattering model using a small number of individual scatterers

stress shadows determined from a phase dynamical measure of historic seismicity the pattern informatics pi technique tiampo et al 2002 is founded on the premise that changes in the seismicity rate are a proxy for changes in the underlying stress these stress changes are correlated over large spatial regions and can be quantified using a phase dynamical analysis of the changes in historic seismicity rates this new approach to the study of seismicity quantifies its local and regional space time patterns and identifies regions of local quiescence or activation here we study those local changes in an attempt to objectively quantify short term stress shadow regions we determine that while persistent stress shadows are detectable with this method they do not occur uniformly throughout the spatio temporal region

internal structure of the dead sea leaky transform rift in relation to plate kinematics abstract the structures along the dead sea transform rift are related to the motions of the sinai and arabia plates which border it and to the irregularities of their boundaries the total slip was 105 km left lateral but the present structures were formed mainly during the last 40 km of slip which probably occurred in the plio pleistocene along the southern half of the transform the strike slip motion takes place on en echelon faults this produces rhomb shaped grabens or pull aparts which are sometimes composite and in which there is local crustal separation thus much of the transform is leaky these structures occur in a morpho tectonic rift valley delimited by normal faults which express a small component of transverse extension along a few segments the shape of the transform is such that lateral motion produces local transverse compression the geometric relations of the structures along the transform define an eulerian pole of relative plate motions at 32 8 n 22 6 e 0 5 the older motion was somewhat different and is described by a pole located about 5 west of the above then the component of transverse extension and crustal separation was much smaller than now while local transverse compression was more important the northern half of the dead sea transform has an irregular shape and the bordering plates did not remain rigid as lateral motion continued here transverse compression is often important

tectonic evolution of northern wadi araba jordan abstract in the area east of northern wadi araba several phases of tectonic fracturing can be distinguished the oldest is of precambrian age and is represented by a system of dikes a second pre cretaceous tectonic phase is represented by a distinct joint system which is restricted to the cambrian sediments and has not cut the cretaceous sediments the major tectonic movements which lead to the formation of the dead sea rift produced faults and joint systems which cut all rock sequences the major deformation of the area started in the early tertiary when the accumulation of the se nw directed compressive stresses produced fold structures which are older than the rift movements as a result of this stress a conjugate system of fractures was produced ese trending right lateral antithetic faults and sse trending left lateral synthetic faults a major group of tensional faults and fractures trending 130 was produced due to this compression the major displacement along the rift followed the formation of the above mentioned faults these fault systems dissected the area into different blocks as the compression continued the faults and the blocks between them were rotated relative to each other two phases of rotational movements were recorded contemporaneous with the two phases of the rift formation during its northward movement the arabian plate came into collision with the sinai palestine plate in the dahal area in northern wadi araba various indications prove that the wadi araba fault is still active and the northward displacement of the arabian plate along this fault continues in recent time

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

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

slip rate on the dead sea transform fault in northern araba valley jordan the araba valley lies between the southern tip of the dead sea and the gulf of aqaba this depression blanketed with alluvial and lacustrine deposits is cut along its entire length by the dead sea fault in many places the fault is well defined by scarps and evidence for left lateral strike slip faulting is abundant the slip rate on the fault can be constrained from dated geomorphic features displaced by the fault a large fan at the mouth of wadi dahal has been displaced by about 500 m since the bulk of the fanglomerates were deposited 77 140 kyr ago as dated from cosmogenic isotope analysis 10 be in chert of pebbles collected on the fan surface and from the age of transgressive lacustrine sediments capping the fan holocene alluvial surfaces are also clearly offset by correlation with similar surfaces along the dead sea lake margin we propose a chronology for their emplacement taken together our observations suggest an average slip rate over the late pleistocene of between 2 and 6 mm yr 1 with a preferred value of 4 mm yr 1 this slip rate is shown to be consistent with other constraints on the kinematics of the arabian plate assuming a rotation rate of about 0 396 myr 1 around a pole at 31 1 n 26 7 e relative to africa

geophysical images of the dead sea transform in jordan reveal an impermeable barrier for fluid flow 1 high resolutionseismictomography and magneto telluric mt soundings of the shallow crust show strong changes in material properties across the dead sea transform fault dst in the arava valley in jordan 2d inversion results of the mt data indicate that the dst is associated with a strong lateral conductivity contrast of a highly conductive layer at a depth of approximately 1 5 km cut off at a position coinciding with the surface trace of the dst at the same location we observe a sharp increase of p wave velocities from 5 km s to the east the high velocities in the east probably reflect precambrian rocks while the high electrical conductivity west of the dst is attributed to saline fluids within the sedimentary filling in this sense the dst appears to act as an impermeable barrier between two different rock formations such a localized fluid barrier is consistent with models of fault zone evolution but has so far not been imaged by geophysical methods the situation at the dst is remarkably different from active segments of the san andreas fault which typically show a conductive fault core acting as a fluid conduit

three dimensional imaging of steeply dipping structure near the san andreas fault parkfield california shot gathers from the parkfield california deep crustal seismic reflection line recorded in 1977 by cocorp reveal coherent events having horizontal to reverse moveouts these events were migrated using a multioffset three dimensional kirchhoff summation method this method is a ray equation back projection inversion of the acoustic wave field which is valid under the born wkbj and far field assumptions migration of full wave acoustic synthetics having the same limitations in geometric coverage as the cocorp survey demonstrates the utility of the imaging process the images obtained from back projection of the survey data suggest that the gold hill fault carries ultramafic rocks from the surface to 3 km depth at a dip greater than 45 degrees where it joins the san andreas fault which may cut through more homogeneous materials at shallow depths to the southwest a 2 km tertiary sedimentary section appears to terminate against a near vertical fault the zone between this fault and the san andreas may be floored at 3 km by flat lying ultramafics lateral velocity inhomogeneities are not accounted for in the migration but in this case do not seriously hinder the reconstruction of reflectors

the distribution of igneous rocks beneath mont megantic the easternmost monteregian as revealed by gravity at the surface mont megantic the easternmost and most imposing of the monteregian hills is composed of 77 5 leucogranite and syenite and 22 5 gabbro nonetheless the intrusion is characterized by a major positive bouguer anomaly of 20 mgal a computed three dimensional gravity adumbration is consonant with the intrusion having the form of a vertical prismatic column with a flat base at a depth of 10 km the base appears to coincide with the unconformity atop precambrian basement rocks gabbro composes 96 of the adumbration leucogranite and syenite make up the remainder and are confined to an uppermost layer no more than about 1 km thick the subvolcanic texture of the granite and presence of miarolitic cavities at megantic show the shallow level of the intrusion the present surface perhaps having been no deeper than 1 or 2 km this is the same as the depth of emplacement elucidated at mont royal at the opposite end of the monteregian chain which in turn points to uniform uplift and erosion sin

conductivity structure of the san andreas fault parkfield revisited 1 a previous interpretation of a high resolution magnetotelluric profile across the san andreas fault at parkfield attributed an anomalously conductive region just west of the fault and extending to depths of 3 km to fractured rock saturated with brines laboratory measured electrical resistivities of samples of sedimentary rocks known to border the fault to the east are comparable to resistivities of the anomalous zone we suggest that the anomalous region results from conductive sedimentary rocks in a plunging syncline that is adjacent to the fault rather than caused by fractured rock if correct the current surface trace of the san andreas fault at middle mountain does not form the boundary between the salinian block and the north american plate instead that boundary is 1 km west and collocated with current seismicity

lithospheric studies based on array analysis of p coda and microseisms abstract a simple coherence measure the semblance coefficient has been used to sample the lithosphere beneath the norsar array in terms of its ability to produce coherent seismic energy the data set is comprised of two samples of teleseismic p coda waves south of honshu and hindu kush events and two samples of microseismic noise data with duration of about one hour each concerning the constitution of the earth structure there are two possibilities for coherent energy to be present in noise like wavefields the first is tied to scattering effects and the second may be brought about by seismic emission effects the emissive component represents the seismic response of the medium to variations in the background stress field which may arise from various sources passing seismic waves body or surface free oscillations of the earth tides an abnormal level of heat flow or geodynamic processes in general the presence of a well known source of seismic signal hunderfossen dam in the vicinity of norsar area provided an opportunity to test the data processing algorithm results obtained for two independent microseismic data samples point to the same location in close vicinity to the dam for p coda data the signal energy from the upper mantle areas as indicated by the maximum of semblance measure values at the depth around 116 km reaches about 3 8 and 5 of the total p coda energy for honshu and hindu kush events respectively for microseismic noise data this value is about 1 at a depth around 100 km comparison with the results of holographic studies supports the assumption that in the case of microseismic data the detected signal energy is of emissive nature while for p coda data we need more coda samples to make a sound judgement about the nature scattering or emission of the detected signals

evidence for normal and inhomogeneous lowermost mantle and core mantle boundary structure under the arctic and northern canada summary seismic p waves from a total of about 200 nuclear explosions in the usa the former ussr and china observed at 10 arrays and four networks in europe canada and the usa are used to analyse the structure at the base of the mantle and the core mantle boundary cmb the simple waveforms and well controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration a new array technique double beamforming kruger et al 1993 integrating both concepts is applied which increases the slowness resolution considerably a total of 56 source receiver combinations i e reflection points in the lower mantle or on the cmb could be analysed in five regions anomalous arrivals pdp with slowness and arrival times between those of p and pcp are observed one of these five areas svalbard region shows short period pcp p amplitude ratios which are about three times higher than those predicted by standard earth models in the severnaya zemlya region where pdp and pcp precursors were observed previously kruger et al 1993 pcp shows azimuth deviations of up to 10 for some other regions deviations of the pcp waveform from the direct p waveform are also observed these anomalous phases and the pcp waveform distortions cannot be explained with standard 1 d earth models they are probably produced by inhomogeneities in the lowermost mantle the observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges the first is the cmb and its immediate vicinity of a few tens of kilometres the second region is the depth range between about 200 and 300 km above the cmb maps of the north pole region giving the distributions of inhomogeneities in the lower mantle and on the cmb are presented these maps show evidence of strong heterogeneity of the d boundary layer and possibly also of the cmb in the same area

imaging the dead sea transform with scattered seismic waves summary with controlled seismic sources and specifically designed receiver arrays we image a subvertical boundary between two lithological blocks at the arava fault af in the middle east the af is the main strike slip fault of the dead sea transform dst in the segment between the dead sea and the red sea our imaging migration method is based on array beamforming and coherence analysis of p to p scattered seismic phases we use a 1 d background velocity model and the direct p arrival as a reference phase careful resolution testing is necessary because the target volume is irregularly sampled by rays a spread function describing energy dispersion at localized point scatterers and synthetic calculations for large planar structures provides estimates of the resolution of the images we resolve a 7 km long steeply dipping reflector offset roughly 1 km from the surface trace of the af the reflector can be imaged from about 1 km down to 4 km depth previous and ongoing studies in this region have shown a strong contrast across the fault low seismic velocities and electrical resistivities to the west and high velocities and resistivities to the east of it we therefore suggest that the imaged reflector marks the contrast between young sedimentary fill in the west and precambrian rocks in the east if correct the boundary between the two blocks is offset about 1 km east of the current surface trace of the af

resolution of prominent crustal scatterers near the noress small aperture array summary our goal is to identify prominent scatterers near seismic arrays so that the secondary phases they generate can be anticipated and recognized as being of local not teleseismic origin toward this end we have developed a technique to scan seismic coda recorded by arrays for phases generated locally by scattering of teleseismic energy from large topographical and crustal heterogeneities in this paper we test the technique by using the noress small aperture array in southern norway to image free surface secondary r sources excited by incoming teleseismic p waves in essence our technique suppresses the teleseismic energy and migrates the residual array records to image coherent local sources to assess the azimuthal and radial resolution that the noress array provides we have analysed point source synthetics a preliminary analysis of recorded data provides an initial view of the scatterers in the vicinity of noress and identifies limitations of imaging using the full coda of individual events single events can be used to image scatterers but more robust images with higher azimuthal resolution can be obtained by using a broadly distributed suite of events radial resolution of scatterers can be enhanced by deconvolution a composite image generated by simultaneously considering a suite of 31 deconvolved teleseisms indicates that two secondary r sources are present at or near the free surface near noress one 27 km to the south west of the array appears to be related to dramatic topographic relief at the south west edge of lake mjosa another roughly 10 km to the east of the array is judged by a bootstrapresampling analysis to be only marginally significant this apparent source does not coincide with obvious topography and is likely buried although the time delay that separates the scattered and parent phases depends somewhat on the angle of approach of the incoming waves in general these secondary sources are contributing wave trains to the seismic coda that begin 4 and 11 s respectively after the primary onset

steep dip seismic imaging of the shallow san andreas fault near parkfield seismic reflection and refraction images illuminate the san andreas fault to a depth of 1 kilometer the prestack depth migrated reflection image contains near vertical reflections aligned with the active fault trace the fault is vertical in the upper 0 5 kilometer then dips about 70 to the southwest to at least 1 kilometer subsurface this dip reconciles the difference between the computed locations of earthquakes and the surface fault trace the seismic velocity cross section shows strong lateral variations relatively low velocity 10 to 30 high electrical conductivity and low density indicate a 1 kilometer wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake