venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

comparison of frequency of occurrence of earthquakes with slip rates from long term seismicity data the cases of gulf of corinth sea of marmara and dead sea fault zone summary in this article through the comparison of knowledge relating to historical earthquakes with the understanding of present day earthquake mechanics and overall gps slip rates in the eastern mediterranean region it has been possible to obtain an idea of how frequently large earthquakes may be expected in some parts of the region it has also been possible to make an assessment from these early events of slip rates over a long period of time for the gulf of corinth in greece the marmara sea in turkey and the dead sea fault system as well as deriving long term magnitude frequency relations for these same regions it has been demonstrated that slip rates calculated from historical data are in general comparable to those calculated from gps measurements and field observations while the size of historical earthquakes and their uncertainty can be quantified this permits a more reliable estimation of the long term hazard the calculation of which is the concern of the engineering seismologist it has also been shown that in most cases large earthquakes are less frequent when they are estimated from long term data sets rather than from the instrumental period making the notion of recurrence time and of hazard assessment questionable this study focuses on some of the few areas in the world for which long term macroseism information exists and which facilitate this kind of analysis

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

locating seismic events in the ctbt context the verification of the comprehensive nuclear test ban treaty ctbt requires the determination of accurate location of seismic events from a fixed network of seismic stations across the globe the requirements of possible on site inspections mean that the goal is to place the location estimate in a zone smaller than 1000 km2that includes the true location because a defined set of stations will be used corrections can be refined to represent the influence of departures from the global reference model iaspei91 the primary stations in the international monitoring scheme ims are mostly seismic arrays and therefore the present location scheme is based on minimisation of a misfit function built from arrival time azimuth and array slowness residuals the effective network will change markedly with the magnitude of the event and as a result regional information has to be integrated into the location process

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

seismic location calibration of the european arctic a crustal velocity model has been developed for fennoscandia the baltic shield and adjacent areas this model represents a simplified average of various models developed for parts of this region we show that p wave travel times calculated with this model provide an excellent fit to observations at the fennoscandian krsc and iris station networks for a set of seismic events with known or very well constrained locations the station event paths cover large parts of western russia and the barents sea thus indicating that this model which we denote the barents model is appropriate for this entire region we show by examples that significant improvements in event location precision can be achieved compared to using the 1aspei model we finally use the barents model to calculate locations of recent small seismic events in the novaya zemlya region of interest in a ctbt monitoring context

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

the 11 october 2010 novaya zemlya earthquake implications for velocity models and regional event location characterizing the seismicity of novaya zemlya and the surrounding arctic seas requires accurate event location estimates low magnitude events in this region are currently observed only by a small number of stations in the european arctic with a large azimuthal gap making the accuracy of regional velocity models all the more important regional travel time calibration is difficult given the scarcity of sufficiently well constrained events on 11 october 2010 a magnitude 4 5 event occurred close to the northern tip of novaya zemlya this event is significant in that it is the first event in this region to have been recorded both on the relatively recent regional networks and arrays and also teleseismically with good azimuthal coverage we examine how well we can constrain the location and origin time using only teleseismic phases using only first teleseismic p arrivals we constrain the epicenter to approximately 76 25 n and 64 75 e but with no depth resolution clear depth phases notably on stations in the southern united states indicate a depth between 9 and 15 km this independent hypocenter and origin time estimate allow evaluation of regional phase travel time prediction using different models the predicted sn travel time appears to cause the greatest variability in regional location estimates the 3d regional seismic travel times models provide excellent pn travel time estimates for barents sea paths but may slightly overestimate sn travel times from this source region a modified regional 1d velocity model is defined which best predicts pn and sn observations at multiple stations up to 15 the significance of the regional travel time models for estimating location is demonstrated for a low magnitude event on or close to the northern island of novaya zemlya in march 2014 recorded with a satisfactory signal to noise ratio at only four stations

migration velocity analysis based on common shot depth migration applied to the seismic data of the caribbean oceanic plateau summary in areas where the geology is characterized by strong vertical and lateral velocity gradients pre stack depth migration will produce the optimum subsurface image an accurate velocity model is required for an accurate pre stack depth migration we developed a velocity analysis based on recursive pre stack depth migration that gives better results than the traditional cmp velocity analysis when applied to data over the caribbean oceanic plateau first the shot records are migrated with a series of constant velocities for each identified geological layer the velocity that best flattens the event in common receiver gathers crg is picked a large number of velocity analyses are run along the seismic line in order to define the reflector geometry for each layer we obtain the depth and interval velocity in order to make a model down to that depth this model is then used as the input for the next layer velocity analysis finally when all the layers have been picked the entire line is depth migrated with the derived model the migration algorithm used is a common shot kirchhoff technique which uses the ray tracing method of vidale 1988 to compute the traveltimes needed to determine the operator shape we applied this migration velocity analysis technique to seismic profiles of the casis cruise to define the deep structure of the caribbean oceanic plateau we imaged a 2500 m deep reflection sub b beneath the top of the plateau b horizon which defines the top of a 4500 4700 m s 1 layer the images obtained with this method will help to define odp sites that will provide information about geochemical and petrological variations of the volcanic basement

dipping basement reflectors along volcanic passive margins new insight using data from the kerguelen plateau multichannel seismic reflection data from the southern kerguelen plateau show many dipping basement reflectors associated with volcanic flows these reflectors are quite similar in their shape to seaward dipping basement reflectors observed along volcanic passive margins on the kerguelen plateau the sources are updip of the basement reflectors in the presently extinct and eroded volcanoes we suggest that the same source reflector geometry may also apply to the seaward dipping basement reflectors observed along passive margins we interpret these reflectors to be the result of volcanism on the passive margin which flowed in all directions into the newly created ocean basin at an early spreading stage

seismic reflection profiles across the southern dead sea basin abstract the main structural and stratigraphic elements that form the southern dead sea basin sdsb which is part of the dead sea transform fault system have been broadly delineated with various degrees of certainty by a number of researchers the recent exchange of seismic data between israel and jordan enable us to fill in some of the dashed lines and eliminate some of the question marks that appear on the previously published geological cross sections two east west seismic time sections one from each side of the international border that dissect the basin have been selected composed and interpreted a remarkable correlation of the three major sedimentary sequences that form the basin fill miocene clastics pliocene salt and pleistocene holocene clastics and evaporites has been established between the lines and hence across the entire width of the sdsb the miocene and pliocene series are cut off in the east by the ghor safi fault that is the equivalent of the western sedom fault thus indicating that all three major structural steps known to exist in the west 1 rim block 2 intermediate block and 3 deep block occurs also in the east and that the sdsb is indeed a full graben the ghor safi fault is the northern extension of the major arava strike slip fault the very thin miocene if at all and lack of pliocene salt in the eastern intermediate block unlike the western one suggests that the ghor safi fault was initiated earlier then the sedom fault salt is present over the entire width of the basin but does not exceed the 900 m that were penetrated by the sedom deep 1 borehole the salt is conformable with the overlying thick pleistocene and except for the sedom diapir no halokinesis phenomena have been recognized to be associated with both units

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

a three dimensional geophysical model of the crust in the barents sea region model construction and basement characterization summary barents50 a new 3 d geophysical model of the crust in the barents sea region has been developed by the university of oslo norsar and the u s geological survey the target region comprises northern norway and finland parts of the kola peninsula and the east european lowlands novaya zemlya the kara sea and franz josef land terminate the region to the east while the norwegian greenland sea marks the western boundary in total 680 1 d seismic velocity profiles were compiled mostly by sampling 2 d seismic velocity transects from seismic refraction profiles seismic reflection data in the western barents sea were further used for density modelling and subsequent density to velocity conversion velocities from these profiles were binned into two sedimentary and three crystalline crustal layers the first step of the compilation comprised the layer wise interpolation of the velocities and thicknesses within the different geological provinces of the study region linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed we therefore used the separately compiled area wide sediment thickness data to adjust the total crystalline crustal thickness according to the total sedimentary thickness where no constraints from 1 d velocity profiles existed the barents50 model is based on an equidistant hexagonal grid with a node spacing of 50 km the p wave velocity model was used for gravity modelling to obtain 3 d density structure a better fit to the observed gravity was achieved using a grid search algorithm which focussed on the density contrast of the sediment basement interface an improvement compared to older geophysical models is the high resolution of 50 km velocity transects through the 3 d model illustrate geological features of the european arctic the possible petrology of the crystalline basement in western and eastern barents sea is discussed on the basis of the observed seismic velocity structure the barents50 model is available at http www norsar no seismology barents3d

the seismic disturbance on 1986 august 1 near novaya zemlya a source of concern summary on 1986 august 1 a small seismic disturbance occurred to the east of a soviet nuclear test site at novaya zemlya reported attempts to identify the nature of the source using only short period seismological data recorded at regional distances were unsuccessful however by using teleseismic p wave data it is possible to identify the source as an earthquake which occurred within the crust at a depth of 24 km the source is shown to have a double couple mechanism with a well constrained fault plane solution the identification as an earthquake is supported by the mb ms evidence

linking late cretaceous to eocene tectonostratigraphy of the san jacinto fold belt of nw colombia with caribbean plateau collision and flat subduction collision with and subduction of an oceanic plateau is a rare and transient process that usually leaves an indirect imprint only through a tectono stratigraphic analysis of pre oligocene sequences in the san jacinto fold belt of northern colombia we show the late cretaceous to eocene tectonic evolution of northwestern south america upon collision and ongoing subduction with the caribbean plate we linked the deposition of four forearc basin sequences to specific collision subduction stages and related their bounding unconformities to major tectonic episodes the upper cretaceous cansona sequence was deposited in a marine forearc setting in which the caribbean plate was being subducted beneath northwestern south america producing contemporaneous magmatism in the present day lower magdalena valley basin coeval strike slip faulting by the romeral wrench fault system accommodated right lateral displacement due to oblique convergence in latest cretaceous times the caribbean plateau collided with south america marking a change to more terrestrially influenced marine environments characteristic of the upper paleocene to lower eocene san cayetano sequence also deposited in a forearc setting with an active volcanic arc a lower to middle eocene angular unconformity at the top of the san cayetano sequence the termination of the activity of the romeral fault system and the cessation of arc magmatism are interpreted to indicate the onset of low angle subduction of the thick and buoyant caribbean plateau beneath south america which occurred between 56 and 43 ma flat subduction of the plateau has continued to the present and would be the main cause of amagmatic post eocene deposition

erratum anatomy of the dead sea transform from lithospheric to microscopic scale rewiews of geophysics 2009 47 rg2002 doi 10 1029 2008rg0002264

unusually deep earthquakes in east africa constraints on the thermo mechanical structure of a continental rift system shudofsky 1985 has established that earthquakes associated with the east african rift system have well constrained focal depths as great as 25 30 km using published heat flow measurements as a guide to the local geotherm we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong these results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning any model of the rifting process in east africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels

travel times and attenuation relations for regional phases in the barents sea region a database containing 45 events in the barents sea region has been compiled and analyzed with the aim of evaluating crustal models travel times and attenuation relations in the context of performing regional detection threshold monitoring of this region the 45 events are mostly located around the circumference of the study area due to the virtually aseismic nature of the barents sea itself regional p n and s n phases were observable for most events in the database while p g and l g phases were only observable for events with raypaths that do not cross the tectonic structures in the barents sea this corroborates a number of previous observations of l g wave blockage within the barents sea three existing velocity models were evaluated with a model having slightly lower s velocities than earlier assumed in the upper mantle giving the overall best fit to the observed arrivals in order to estimate magnitudes short term average sta and spectral amplitude values were calculated in several frequency bands for all phase arrivals in the database there were no significant differences between spectral and sta amplitudes so the latter were used as this parameter is more efficient to calculate in real time processing an inversion was performed in order to determine an attenuation relation specific for this region the resulting magnitudes based on p n p g s n and l g phases gave an internally consistent reasonably stable set of values which can be calibrated towards any existing global or regional scale

locating and modeling regional earthquakes with two stations 1 we developed a new technique caploc to retrieve full source parameters of small seismic events from regional seismograms which include origin time epicenter location depth focal mechanism and moment magnitude despite rather complicated propagation effects at short periods a simple localized one dimensional model can well explain signals of periods 3 10 s if we break the three component records into different segments and allow differential time shifts among them these differential time shifts once established from a calibration process or a well determined tomographic map can be used together with p wave travel times to refine an event s location in this study we tested whether our new method could produce satisfactory results with as few as two stations so that we can improve source estimates of poorly monitored events with sparse waveform data we conducted the test on 28 events in the tibetan plateau the focal mechanisms and locations determined from only two stations agree well with those determined from a whole passcal array in particular our new method produces better locations than international seismological centre with the average mislocation error reduced from 16 to 5 km we also tested whether an event s depth and mechanism can be determined separately from its epicenter relocation in a two step approach we find that the two step approach does not always give the correct solution but the reliability of a solution can be evaluated using a reduced chi square value

sub b layering in the southern caribbean the aruba gap and venezuela basin abstract multichannel seismic data in the aruba gap region near joides dsdp site 153 verify the presence of a deep sub b reflection one multichannel seismic line trends ne sw on and along the edge of beata ridge and passes within 1 km of site 153 and another line runs n s across the entire aruba gap with the drill site 4 km east from its northern end closely spaced velocity analyses indicate the presence of deep primary reflection events and enable calculation of interval velocities between the a b marker horizons deconvolved true amplitude normal incidence profiles sharply delineate the a b marker horizons and indicate the presence of the sub b reflection event on the ne sw line this deep reflector is best described as a diffuse discontinuous zone relatively horizontal lying about 0 8 seconds of two way travel time below the b reflector with an interval velocity of approximately 5 0 km s between horizon b and this reflection the n s line is more complex since the sub b reflection event is masked by a strong internal multiple from the a b interval in the central and western venezuela basin deep primary reflections beneath horizon b are also observed on the northern and western sides of what appears to be a major fault zone this fault zone separates the smooth b and sub b reflectors on the northern and western sides of this fault zone from what appears to be typical oceanic basement the widespread presence of sub b reflections yielding high interval velocities for the section between these events and horizon b suggest that this material is probably igneous in origin

migration velocity analysis accounting for the effects of lateral velocity variations the most common migration velocity analysis algorithms iterative profile migration focusing analysis and stack power analysis are based on restrictive subsurface assumptions that may cause the methods to break down in the presence of lateral velocity variations typically the subsurface is assumed to have either a constant velocity or at most a depth variable velocity recent innovations include the use of traveltime inversion philosophy to invert migration measurements of the curvature as a function of offset exhibited by an event following migration traveltime inversion makes few assumptions regarding the subsurface but is a rather unstable process thus an important question is under what conditions do the traditional methods break down and make the use of traveltime inversion methods mandatory marrying the common migration analysis with tomography results in a set of equations that while useful for generating updates from migration measurements are too complex for answering the above qu

migration of common shot gathers three depth migration methods which operate on common shot data are presented the first migration method maps digitized horizons from the x t domain to the x z domain because this method is based on ray tracing its computation time is short it is suggested for iterative velocity analysis the other two migration methods map the entire common shot gather into a depth section the common shot migration requires calculation of the arrival time of the direct wave from the source to all the depth points and is done through a direct solution of the eikonal equation all three methods are suitable for areas with both lateral and vertical velocity variation

high resolution body wave tomography beneath the svekalapko array i a priori three dimensional crustal model and associated traveltime effects on teleseismic wave fronts summary assessment of contributions from shallow lithosphere to teleseismic wave front distortion is a prerequisite for high resolution regional teleseismic tomography several methods have been proposed in the past for the correction of these effects e g application of station correction terms we propose an approach that is independent of the subsequent inversion and uses the available a priori knowledge of the crustal structure to calculate crustal traveltime effects of teleseismic wave fronts our approach involves the construction of a 3 d crustal model based on controlled source seismology data and calculation of the associated traveltime anomalies for incoming teleseismic wave fronts the model for central fennoscandia shows a maximum crustal thickness of 64 km and includes a high velocity lower crust as derived for parts of the study area by previous authors traveltimes calculated using finite differences for teleseismic waves travelling through this crustal model are compared with those from the standard reference model iasp91 and the residuals are used to correct observed teleseismic arrival times at the svekalapko array to test the performance of this approach in a second part of the study a synthetic traveltime data set is obtained by tracing wave fronts through a mantle structure with known velocity anomalies and the 3 d crustal model this data set is inverted with and without correction for crustal effects the 3 d crustal effects alone with a homogeneous mantle are also inverted and the results show that the crustal effects propagate down to 450 km the comparison of the inversion results demonstrates the need to apply appropriate 3 d crustal corrections in high resolution regional tomography for upper mantle structure beneath the baltic shield

upper crustal structure of the western colombian basin caribbean sea multichannel seismic reflection profiles show that most of the western colombian basin is underlain by a large oceanic plateau the crust of this plateau has a smooth upper surface an oceanic velocity structure and a thickness at least twice that of typical oceanic crust locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments a mounded reflection pattern within acoustic basement at the crest of the plateau mono rise may represent volcanic knolls smooth surfaced basement of the western colombian basin is probably equivalent to the upper cretaceous basement complex below seismic reflection horizon b in the central venezuelan basin and resembles the basements of some cretaceous oceanic plateaus and aseismic rises in the western pacific an area of apparently typical oceanic crust lies adjacent to the colombian basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed reflections within the flank of the colombian basin plateau appear to overlap this crust indicating that the plateau is younger than the oceanic crust oceanic plateau crust extends to panama and costa rica without apparent structural break and may be continuous with the basement complex of southern central america

a probabilistic seismic model for the european arctic 1 the development of three dimensional seismic models for the crust and upper mantle has traditionally focused on finding one model that provides the best fit to the data while observing some regularization constraints in contrast to this the inversion employed here fits the data in a probabilistic sense and thus provides a quantitative measure of model uncertainty our probabilistic model is based on two sources of information 1 prior information which is independent from the data and 2 different geophysical data sets including thickness constraints velocity profiles gravity data surface wave group velocities and regional body wave traveltimes we use a markov chain monte carlo mcmc algorithm to sample models from the prior distribution the set of plausible models and test them against the data to generate the posterior distribution the ensemble of models that fit the data with assigned uncertainties while being computationally more expensive such a probabilistic inversion provides a more complete picture of solution space and allows us to combine various data sets the complex geology of the european arctic encompassing oceanic crust continental shelf regions rift basins and old cratonic crust as well as the nonuniform coverage of the region by data with varying degrees of uncertainty makes it a challenging setting for any imaging technique and therefore an ideal environment for demonstrating the practical advantages of a probabilistic approach maps of depth to basement and depth to moho derived from the posterior distribution are in good agreement with previously published maps and interpretations of the regional tectonic setting the predicted uncertainties which are as important as the absolute values correlate well with the variations in data coverage and quality in the region a practical advantage of our probabilistic model is that it can provide estimates for the uncertainties of observables due to model uncertainties we will demonstrate how this can be used for the formulation of earthquake location algorithms that take model uncertainties into account when estimating location uncertainties

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

crustal structure of the levant basin eastern mediterranean abstract a seismic refraction wide angle reflection experiment was undertaken in the levant basin eastern mediterranean two roughly east west profiles extend from the continental shelf of israel toward the levant basin the northern profile crosses the eratosthenes seamount and the southern profile crosses several distinct magnetic anomalies the marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources the results of this study strongly suggest the existence of oceanic crust under portions of the levant basin and continental crust under the eratosthenes seamount the seismic refraction data also indicate a large sedimentary sequence 10 14 km thick in the levant basin and below the levant continental margin assuming the crust is of cretaceous age this gives a fairly high sedimentation rate the sequence can be divided into several units a prominent unit is the 4 2 km s layer which is probably composed of the messinian evaporites overlying the evaporitic layer are layers composed of plio pleistocene sediments whose velocity is 2 0 km s the refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single layer oceanic crust takes place along the levant margin the transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual the northern profile north of the carmel structure presents a different structure the continental crust is much thinner there and the transition in the crustal structure is more rapid the crustal thinning begins under western galilee and terminates at the continental slope the results of the present study indicate that the levant basin is composed of distinct crustal units and that the levant continental margin is divided into at least two provinces of different crustal structure

sinistral strike slip motion on the dead sea rift confirmation from new magnetic data new aeromagnetic data from jordan provide information about basement structures that were previously unknown large east west magnetic anomalies are truncated by the dead sea rift comparison of these data with aeromagnetic data from west of the rift strongly suggests that these anomalies have undergone slightly more than 100 km of left lateral offset this is the same amount of offset concluded from several studies of surface bedrock geology offset of magnetic anomalies provides confirmation of the total amount of offset but no sense of timing or the polyphase history of motion suggested by other studies

comparison of location procedures the kara sea event of 16 august 1997 various location procedures and velocity models are compared for the ml 3 3 kara sea event of 16 august 1997 this event has been the subject of considerable discussion because of its geographical position and the difficulty in obtaining a reliable focal depth estimate a comprehensive data set was extracted by re reading the records from all available stations these readings have then been used in a sequence of location experiments to examine the effect of using different velocity models to describe the travel times of the phases and also to compare the use of a fully nonlinear scheme shakena sambridge and kennett 2001 and a linearized location algorithm hyposat schweitzer 2001 2002 a standard least squares misfit criterion has been used for direct comparisons between the two methods the results confirm both the importance of s wave information in assessing the depth of regional events and the need to apply a reliable velocity model to place the strongest constraints on the location of the event even with only a limited data set but an adequate velocity model it is possible to find the position of the kara sea event close to the most probable locations however there is then no depth resolution reported error ellipses from standard data centers tend to have relatively small error ellipses with the commonly made assumption that the reading errors and the a posteriori residuals have an unbiased normal distribution such inversion results may indicate an unreasonably high resolution and accuracy of the solution the epicenter estimates for the whole data set using the range of different tech niques agree quite well with some overlap of the estimated confidence regions the observed seismic source was most likely an earthquake in the middle or lower crust at about 10 30 km depth

a mid pleistocene deformation transition in the hula basin northern israel implications for the tectonic evolution of the dead sea fault the dead sea fault dsf plate boundary has accommodated relative sinistral motion between the sinai and arabian plates since the neogene geologically based models point to a long term 5 0 ma n trending relative motion roughly parallel to the central sector of the dsf gps based calculations of present day relative motion between the sinai and arabian plates indicate a northward increasing convergent component across the dsf suggesting a kinematic change over the plio pleistocene period we study the evolving deformation of the hula rhomb shaped graben situated along the central sector of the dsf in order to examine the possible kinematic change the hula graben is widely accepted as a pull apart basin part of a series of basins extending southward we use a 3 d approach combining mapped surface structures with subsurface seismic reflection profiles 173 km and borehole data 38 boreholes gathered here into a single geographic information systems database results indicate that during the mid pleistocene a major tectonic transition modified the structure of the basin fill a subvertical nnw trending left lateral throughgoing strike slip fault developed diagonally across the basin consequently the basin entered a new geodynamic phase where subsidence was controlled by both the basin bordering faults and the diagonal fault while the vertical displacement across the transverse faults is minor synchronous structural changes recorded in additional localities along the dsf attests to a mid pleistocene 1 ma regional tectonic transition that was associated with a northward increasing convergent component across the central and northern sectors of the dsf

structural geometry and evolution of the dead sea jordan rift system as deduced from new subsurface data abstract analysis of deep drilling data and seismic reflection surveys obtained in recent years permit establishment of a tectonic model of the dead sea jordan rift system that is in part conformable with some of the many often controversial theories on this subject but which also introduces new information negating some concepts that are widely accepted in the literature the dead sea jordan rift system is not a tensional graben between two parallel sets of faults but instead can be best defined as a sinistral transform connecting an incipient oceanic ridge the red sea with an upthrusted collision zone the taurus range the shear is caused by the opening of the red sea and the motion of the arabian sub plate away from the african plate the shearing occurs along a largely south north trending slightly arcuate une consisting of a series of en echelon left stepping left lateral strike slip master faults of varying lengths these faults characteristically die out to the north by bending outward northeast and the movement is taken up by the strike slip faults to the left i e to the west where overlap of the two faults occurs very narrow and very deep grabens have evolved in places the width being equal to the depth the narrowness of these grabens is dictated by the closeness of the overlapping segments of the successive strike slip faults this feature points to the probability that the en echelon strike slip faults have a common root zone at great depth these grabens are not typical leaky pull aparts in that they have no gravimetric magnetic or heat flow anomalies nor are they closed on all four sides by large faults their northern ends are characterized by gradually rising graben floors without major diagonal cross faults the dead sea jordan rift system sensu stricto extends over a distance of 420 km from the northern shore of the gulf of elat to the northern margin of the hula valley southward it is connected with the red sea along the gulf of elat transition zone which has elements of strike slip as well as tensional opening a northern transition zone exists between the hula valley and the taurus arc where diminishing strike slip movement is compensated by several underthrust belts the time of the beginning of the movement along the rift can be established as pre middle miocene because mid miocene sediments overlie an upper cretaceous graben floor rifting has continued intermittently to the present volcanism in the northern fifth of the rift is genetically associated with the seismically active tensional golan hauran jebel drouz wadi sirhan tectonic trend which has a north west southeast orientation and not with the north south trending dead sea jordan rift

active tectonics of the turkish iranian plateau 1 this paper examines how active faulting in the turkey iran caucasus region accommodates the arabia eurasia collision and the velocity field observed by gps the overall shortening across the zone is in general spatially separated partitioned into right lateral strike slip in the turkish iranian plateau and thrusting in the greater caucasus a band of counterclockwise rotating nw se right lateral strike slip faults accommodates a nw se gradient in ne directed velocity relative to eurasia between the black and caspian seas a nnw sse band of previously unrecognized oblique normal faults is present on the turkey iran border we estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 2 ma at present rates this implies a reorganization of deformation in the collision zone at that time after the initial collision at 12 ma probably in response to mantle induced dynamic uplift

study of low magnitude seismic events near the novaya zemlya nuclear test site abstract a study of available seismic data shows that all but one of the 42 known underground nuclear explosions at novaya zemlya have been detected and located by stations in the global seismic network during the past 30 years only one seismic event in this area has been unambiguously classified as an earthquake 1 august 1986 m b 4 3 several other small events most of which are thought to be either chemical explosions or aftereffects of nuclear explosions have also been detected since 1990 a network of sensitive regional arrays has been installed in northern europe in preparation for the global seismic monitoring network under a comprehensive nuclear test ban treaty ctbt this regional network has provided a detection capability for novaya zemlya that is shown to be close to m b 2 5 three low magnitude events have been detected and located during this period as discussed in this article 31 december 1992 m b 2 7 13 june 1995 m b 3 5 and 13 january 1996 m b 2 4 to classify the source types of these events has proved very difficult thus even for the m b 3 5 event in 1995 we have been unable to provide a confident classification of the source as either an earthquake or explosion using the available discriminants a study of m b magnitude in different frequency bands shows as expected that the calculation of m b at regional distances needs to take into account source scaling effects at high frequencies thus when comparing a 4 to 8 or 8 to 16 hz filter band to a teleseismic 2 to 4 hz band the smaller events have relatively speaking significantly more high frequency energy up to 0 5 m b units than the larger events this suggests that a p wave spectral magnitude scale might be appropriate the problem of accurately locating small events using a sparse array network is addressed using the 13 january 1996 event which was detected by only two arrays as an illustrative example our analysis demonstrates the importance of using accurately calibrated regional travel time curves and at the same time illustrates how array processing can be used to identify an interfering phase from a local disturbance thereby avoiding location errors due to erroneous phase readings

improved location procedures at the international seismological centre summary the international seismological centre isc is a non governmental non profit organization with the primary mission of producing the definitive account of the earth s seismicity the isc bulletin covers some 50 yr 1960 2011 of seismicity the recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases owing to the ever increasing number of stations worldwide similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the earth resulting in underestimated location uncertainties and for unfavourable network geometries location bias hence the denser and more unbalanced the global seismic station coverage becomes the less defensible is the assumption that is the observations are independent which is made by most location algorithms to address this challenge we have developed a new location algorithm for the isc that accounts for correlated error structure and uses all iaspei standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations in this paper we describe the new isc locator and present validation tests by relocating the ground truth events in the iaspei reference event list as well as by relocating the entire isc bulletin we show that the new isc location algorithm provides small but consistent location improvements considerable improvements in depth determination and significantly more accurate formal uncertainty estimates we demonstrate that the new algorithm through the use of later phases and testing for depth resolution considerably clusters event locations more tightly thus providing an improved view of the seismicity of the earth

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma

the rift like structure and asymmetry of the dead sea fault abstract whereas the dead sea fault is a major continental transform active since ca 13 18 ma ago it has a rift like morphology along its southern part it has been argued that this results from a transtensional component active since 5 ma ago due to a regional plate kinematics change we present the results of 3d laboratory experiments carried out to test this hypothesis and to explore its consequences for the structure and morphology of the dead sea fault we conclude that a two stage tectonic history invoking a first stage of pure strike slip and a second stage marked by the addition of a minor transtensional component is consistent with most of the striking geological and geophysical features of the dead sea fault the structural and morphological asymmetry of the dead sea fault can be explained by a transverse horizontal shear in the ductile lower crust below the transform zone a large scale heating event of the arabian mantle is not required to explain these features of the dead sea fault