indications of a link between seismotectonics and ch4 release from seeps off costa rica measurements of ch4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the pacific margin off costa rica indicate large scale variations of ch4 release ch4 is emitted from mud extrusions and a slide scar at 1000 2300 m water depth maximum ch4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart tidal and current changes were observed but found to apply only to individual seep sites increased seismic activity connected to the moment magnitude m w 6 4 earthquake offshore costa rica in june 2002 could have had an impact on all seep sites and thereby caused an increase in ch4 emission this is supported by the largest variations of ch4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements even though our data indicate a relation between seismicity and ch4 seepage the relation is not proven and future work is needed to comprehensively test this hypothesis

validation of regional and teleseismic travel time models by relocating ground truth events recently developed 3d global seismic velocity models have demon strated location improvements through independent regional and teleseismic travel time calibration concurrently a large set of high quality ground truth gt events with location accuracies 10 km or better gt0 gt10 has been collected for europe the mediterranean north africa the middle east and western eurasia in this study we demonstrate event location improvements using this new data set by applying the regional and teleseismic model based travel time calibrations independently and jointly besides relocating events using all arrivals a subset of the gt events was also relocated using controlled station geometries generated from a constrained boot strapping technique this approach simulates sparse networks and reduces the effect of correlated errors to ensure valid 90 error ellipse coverage statistics with respect to the gt events we compared event relocations with and without travel time cal ibrations considering statistics of mislocation error ellipse area 90 coverage or igin time bias origin time errors and misfit relocations of over 1200 gt events show that pn and or p calibration reduced mislocation for 60 70 of the events joint regional pn and teleseismic p travel time calibration provided the largest lo cation improvements and achieved approximately gt5 accuracy levels due to cor related errors event locations using large numbers of stations have deficient 90 error ellipse coverage however the coverages derived from the model errors are appropriate for the simulated sparse regional and teleseismic networks our valida tion effort demonstrates that the global model based calibrations of pn and teleseis mic p travel times reduce both location bias and uncertainty over wide areas

ability of a global three dimensional model to locate regional events 1 we assess the ability of a global three dimensional 3d seismic model of the crust and upper mantle cub2 0 to locate seismic events using regional travel time data alone epicentral distances 20 assessments are based on a ground truth gt database comprising nearly 1000 uncommonly well located events which occur in 23 event clusters across eurasia and north africa groomed p and pn arrival time data observed at regional distances from individual events and 1000 empirical phase path anomalies produced from event clusters evidence is presented that supports the conversion of the shear velocities vs in cub2 0 to compressional velocities vp in the upper mantle using the thermoelastic properties of an assumed mantle composition rather than traditional empirical scaling relationships two principal results are presented first this 3d vp model fits the empirical p and pn phase path anomalies very well in both pattern and absolute level second location assessments demonstrate that intrinsic regional location accuracy is 5 km using the 3d vp model and 10 km using the 1d model ak135 these findings highlight the importance of gt databases in assessing regional travel time models and establish that a global 3d vs model together with vp inferred from it provides a solid basis on which to build improvements in regional location capabilities

characterizing earthquake location uncertainty in north america using source receiver reciprocity and usarray abstract the comprehensive nuclear test ban treaty community often uses calibration events with well determined origins to improve absolute locations of nearby seismic events by accounting for the biasing effects of unknown velocity structure but the number of these ground truth events is limited to provide additional constraints source receiver reciprocity allows us to use seismic stations as calibration events with known locations the dense and uniform spacing of the usarray transportable array stations makes them ideal to measure the spatial coherence of mislocation vectors across north america and hence to assess how close calibration events or stations need to be to target events to improve locations for a given region we use a grid search approach for the station relocations using both teleseismic earthquakes and simulated regional events our results show that the mislocation vectors are spatially coherent for scales up to 500 km in many regions but that in some places such as regions that can be associated with strong velocity anomalies in the upper mantle mislocation vectors exhibit large changes over short distances online material figures showing station location and dislocations and travel time residuals

coseimic massive methane release from a submarine mud volcano abstract we report here the discovery of two methane enriched plumes in a water column above the kumano knoll 5 submarine mud volcano kk 5 in nankai trough during 2004 a slight methane enrichment was observed in this region in 2000 and 2010 the deeper of the two plumes spread horizontally at depths near the top of kk 5 whereas a shallow plume spread at a height of 300 m from the top of kk 5 the end member 13 c values of methane in both plumes 46 1 4 8 and 51 4 5 8 in the deep and shallow plumes respectively and methane extracted from sediment in kk 5 52 2 are statistically indistinguishable we conclude that the plumes were created by massive release of methane from kk 5 and that this event probably occurred in response to the large earthquakes m w 7 5 that occurred shortly before our observation in 2004 the discharge rates from submarine mud volcanoes smvs in subduction zones may be highly variable in response to major earthquakes

global event location with full and sparse data sets using three dimensional models of mantle p wave velocity in order to improve on the accuracy of event locations at teleseismic distances it is necessary to adequately correct for lateral variations in structure along the ray paths either through deterministic model based corrections empirical path station corrections or a combination of both approaches in this paper we investigate the ability of current three dimensional models of mantle p wave velocity to accurately locate teleseismic events we test four recently published models two are parameterized in terms of relatively long wavelength spherical harmonic functions up to degree 12 and two are parameterized in terms of blocks of constant velocity which have a dimension of a few hundreds of km these models together with detailed crustal corrections are used to locate a set of 112 global test events consisting of both earthquakes and explosions with p wave travel time data compiled by the international seismological centre isc the results indicate that the supposedly higher resolution block models do not improve the accuracy of teleseismic event locations over the longer wavelength spherical harmonic models for some source locations the block models do not predict the range of observed travel time residuals as well as the longer wavelength models the accuracy of the locations largely varies randomly with geographic position although events in central asia are particularly well located we also tested the effect of reduced data sets on the locations multiple location iterations using 30 p wave travel times indicate that teleseismic events may be located within an area of 1000 km of the true location 66 of the time with only the model based corrections and increasing to 75 if calibration information is available if as few as 8 phases are available then this is possible only 50 of the time further refinement in models and or procedure such as the addition ofp phases azimuth data and consideration of p wave anisotropy may provide further improvement in the teleseismic location of small events

epicentre accuracy based on seismic network criteria summary we establish reliable and conservative estimates for epicentre location accuracy using data that are readily available in published seismic bulletins a large variety of seismic studies rely on catalogues of event locations making proper assessment of location uncertainty critical event location and uncertainty parameters in most global regional and national earthquake catalogues are obtained from traditional linearized inversion methods using a 1 d earth model to predict traveltimes reported catalogue uncertainties are based on the assumption that error processes are gaussian zero mean and uncorrelated unfortunately these assumptions are commonly violated leading to the underestimation of true location uncertainty especially at high confidence levels we find that catalogue location accuracy is most reliably estimated by station geometry we make use of two explosions with exactly known epicentres to develop local network location 0 2 5 accuracy criteria using monte carlo simulations of network geometry we find that local network locations are accurate to within 5 km with a 95 per cent confidence level when the network meets the following criteria 1 there are 10 or more stations all within 250 km 2 an azimuthal gap of less than 110 3 a secondary azimuthal gap of less than 160 and 4 at least one station within 30 km to derive location accuracy criteria for near regional 2 5 10 regional 2 5 20 and teleseismic 28 91 networks we use a large data set of exceptionally well located earthquakes and nuclear explosions beyond local distances we find that the secondary azimuthal gap is sufficient to constrain epicentre accuracy and location error increases when the secondary azimuthal gap exceeds 120 when station coverage meets the criterion of a secondary azimuth gap of less than 120 near regional networks provide 20 km accuracy at the 90 per cent confidence level while regional and teleseismic networks provide 25 km accuracy at the 90 per cent confidence level

two subevents across the japan trench during the 7 december 2012 off tohoku earthquake mw 7 3 inferred from offshore tsunami records a tsunamigenic earthquake with a moment magnitude of 7 3 occurred near the japan trench off tohoku northeast japan on 7 december 2012 operational seismic monitoring inferred that the earthquake was composed of doublet sources of comparable magnitudes the first event was reverse faulting and the second event which occurred 10 20 s later was normal faulting an associated tsunami was observed at offshore stations having an amplitude of 10 1 to 101 cm inverse modeling using the observed tsunami records reveals possible vertical seafloor deformations we can then constrain in terms of the source location that the tsunamigenic earthquake involves two subfaults across the japan trench one is the outer trench reverse faulting as the first event and the second is the inner trench normal faulting the present study shows that tsunami analysis with seismograph analyses effectively constrains the epicenter locations even in the case of complex earthquakes like doublets

assessment of regional distance location calibration using a multiple event location algorithm abstract we test the use of a multiple event seismic location method to improve epicenter accuracy estimates regional arrival time observations of 74 nevada test site explosions with known locations comprise the test data set we investigate epicenter accuracy as a function of the number of events in the multiple event system that are constrained at the known hypocenter calibration the effect of distance between calibration and unconstrained events and the use of velocity models with varying travel time prediction accuracy further we test the utility of using a posteriori travel time residuals to assess location and travel time prediction accuracy we find that constraining one event at the known hypocenter reduces epicenter error for all other events by 58 on average compared to locations produced without constraining events the incremental improvement in epicenter accuracy rapidly diminishes as more hypocenters are constrained and incremental location improvement is minimal when the number of constrained hypocenters exceeds 10 events closest to a constrained event exhibit small location bias distinct epicenter bias occurs when the distance between the calibration event and the relocated event is greater than a few tens of kilometers last we confirm that metrics based on a posteriori travel time residuals are poor indicators of both epicenter accuracy and velocity model based travel time prediction accuracy

global scale p wave tomography optimized for prediction of teleseismic and regional travel times for middle east events 2 tomographic inversion 1 we construct a model of three dimensional p wave velocity structure in the crust and mantle that is global in extent but with detailed upper mantle heterogeneities throughout the greater middle east region fully three dimensional ray tracing is employed to achieve accurate travel time predictions of p and pn arrivals requiring the characterization of irregular and discontinuous boundaries therefore we explicitly represent undulating seismic discontinuities in the crust and upper mantle within a spherical tessellation modeling framework the tessellation based model architecture is hierarchical in that fine node sampling is achieved by recursively subdividing a base level tessellation determining the required node spacing to effectively model a given set of data is problematic given the uneven sampling of seismic data and the differing wavelengths of actual seismic heterogeneity to address this problem we have developed an inversion process called progressive multilevel tessellation inversion pmti that exploits the hierarchical nature of the tessellation based design and allows the data to determine the level of model complexity pmti serves as an alternative to existing multiresolution approaches and robustly images regional trends while allowing localized details to emerge where resolution is sufficient to demonstrate our complete modeling concept we construct a velocity model based on teleseismic p travel time data for global events and regional pn travel time data for events occurring throughout the middle east input data are a product of the statistical procedure called bayesloc that simultaneously models all components of a multievent system including event locations origin times and arrival times described in the myers et al 2011 companion paper the initial tomographic image provides a new glimpse of the complex upper mantle velocity anomalies associated with the convergence of the arabian and indian plates with eurasia more important for event monitoring the model accurately predicts both teleseismic and regional travel times for events occurring within the middle east region

carbon isotopic distribution of methane in deep sea hydrothermal plume myojin knoll caldera izu bonin arc implications for microbial methane oxidation in the oceans and applications to heat flux estimation the concentration and stable carbon isotopic composition 13c of methane have been measured for both seafloor hydrothermal venting fluids and effluent plume waters supplied from the vents at the myojin knoll caldera izu bonin arc in the western north pacific the hydrothermal end member concentrations and 13c of methane show near homogeneity among vents 41 2 mol kg and 16 3 0 8 pdb respectively while those in the effluent plume are stratified in the caldera and vertically exhibit a large variation 2 1 11 nmol kg and 29 0 to 11 3 pdb respectively comparison of concentration and 13c data between vent fluids and plume waters suggest that the methane is not conserved but microbially oxidized along with the plume gradual decrease of diffusive methane flux in proportion to distance from the vent fields supports the occurrence of significant microbial oxidation within the plume assuming steady state emission of methane from the vents 1 kinetic isotope effect due to the microbial oxidation k12 k13 2 methane flux from the vents fvents 3 heat flux from the vents q and 4 average turnover time of hydrothermal methane tall are estimated to be k12 k13 1 005 0 001 fvents 90 340 mol d q 30 110 mw and tall 60 240 d respectively the estimated turnover times however are not uniform within the water column around the vents a turnover time of less than 50 d for methane is one of the shortest values in the pelagic ocean while those at the distant points correspond to values more typical for deep ocean water

salsa3d a tomographic model of compressional wave slowness in the earth s mantle for improved travel time prediction and travel time prediction uncertainty abstract the task of monitoring the earth for nuclear explosions relies heavily on seismic data to detect locate and characterize suspected nuclear tests motivated by the need to locate suspected explosions as accurately and precisely as possible we developed a tomographic model of the compressional wave slowness in the earth s mantle with primary focus on the accuracy and precision of travel time predictions for p and pn ray paths through the model path dependent travel time prediction uncertainties are obtained by computing the full 3d model covariance matrix and then integrating slowness variance and covariance along ray paths from source to receiver path dependent travel time prediction uncertainties reflect the amount of seismic data that was used in tomography with very low values for paths represented by abundant data in the tomographic data set and very high values for paths through portions of the model that were poorly sampled by the tomography data set the pattern of travel time prediction uncertainty is a direct result of the off diagonal terms of the model covariance matrix and underscores the importance of incorporating the full model covariance matrix in the determination of travel time prediction uncertainty the computed pattern of uncertainty differs significantly from that of 1d distance dependent travel time uncertainties computed using traditional methods which are only appropriate for use with travel times computed through 1d velocity models

from craton to rift empirically based ground truth criteria for local events recorded on regional networks region specific empirically based ground truth ebgt criteria used to estimate the epicentral location accuracy of seismic events have been developed for the main ethiopian rift and the tibetan plateau explosions recorded during the ethiopia afar geoscientific lithospheric experiment eagle the international deep profiling of tibet and the himalaya indepth iii experiment provided the necessary gt0 reference events in each case the local crustal structure is well known and handpicked arrival times were available facilitating the establishment of the location accuracy criteria through the stochastic forward modeling of arrival times for epicentral locations in the vicinity of the main ethiopian rift a seismic event is required to be recorded on at least 8 stations within the local graphic 1 crossover distance and to yield a network quality metric of less than 0 43 in order to be classified as ebgt595 gt5 with 95 confidence these criteria were subsequently used to identify 10 new gt5 events with magnitudes greater than 2 1 recorded on the ethiopian broadband seismic experiment ebse network and 24 events with magnitudes greater than 2 4 recorded on the eagle broadband network the criteria for the tibetan plateau are similar to the ethiopia criteria yet slightly less restrictive as the network quality metric needs to be less than 0 45 twenty seven seismic events with magnitudes greater than 2 5 recorded on the indepth iii network were identified as gt5 based on the derived criteria when considered in conjunction with criteria developed previously for the kaapvaal craton in southern africa it is apparent that increasing restrictions on the network quality metric mirror increases in the complexity of geologic structure from craton to plateau to rift 1 embed inline graphic 1 gif

regional travel time uncertainty and seismic location improvement using a three dimensional a priori velocity model we demonstrate our ability to improve regional travel time prediction and seismic event location accuracy using an a priori 3d velocity model of western eurasia and north africa wena1 0 travel time residuals are assessed relative to the iasp91 model for approximately 6000 pg pn and p arrivals from seismic events having 2 epicenter accuracy between 1 km and 25 km ground truth 1 gt1 and gt25 respectively recorded at 39 stations throughout the model region ray paths range in length between 0 and 40 local regional and near teleseismic providing depth sounding that spans the crust and upper mantle the dataset also provides representative geographic sampling across eurasia and north africa including aseismic areas the wena1 0 model markedly improves travel time predictions for most stations with an average variance reduction of 29 for all ray paths from the gt25 events when we consider gt5 and better events alone the variance reduction is 49 for location tests we use 196 geographically distributed gt5 and better events in 134 cases 68 of the events locations are improved and average mislocation is reduced from 24 9 km to 17 7 km we develop a travel time uncertainty model that is used to calculate location coverage ellipses the coverage ellipses for wena1 0 are validated to be representative of epicenter error and are smaller than those for iasp91 by 37 we conclude that a priori models are directly applicable where data coverage limits tomographic and empirical approaches and the development of the uncertainty model enables merging of a priori and data driven approaches using bayesian techniques online material correction surfaces and histograms of travel time residuals for 40 stations

source specific station corrections for regional phases at fennoscandian stations the iaspei91 global travel time curves are used as the default for event location at the prototype international data center pidc in order to improve event location a 1 d baltic travel time model was implemented at the pidc in 1997 for locating events using regional phases from fennoscandian stations where a single model is insufficient for characterizing the regional geology path dependent corrections or source specific station corrections ssscs are more appropriate for event locations we have developed ssscs for regional phases at the fennoscandian stations by interpolating travel times through different 1 d models ssscs for stations nris and spits are also derived given the fact that paths from both stations to high latitude events are within the fennoscandia regionalization as baltic

development and testing of a 3d velocity model for improved event location a case study for the india pakistan region 3d velocity models will play an important role in improving location accuracies of small events within the context of a comprehensive nuclear test monitoring treaty commonly such models are developed using tomographic methods that require the initial velocity model to be a reasonable approximation of the true earth structure in this article we present a method for the development of an a priori model for the crust and upper mantle in the india pakistan region and techniques for evaluating its suitability for use as an initial model in tomographic studies we developed the weston india pakistan 3d velocity model winpak3d by integrating the results of more than 60 previous studies related to crustal velocity structure in the region a well located event that occurred near nilore pakistan on 14 february 1977 provided a basis for testing the model via several different approaches evaluation of the 3d model included 1 cross validation analysis 2 comparison of model determined hypocenters with a known event location and 3 comparison of model based and empirically derived travel time correction surfaces generated for an event within 5 km of the seismic station at nilore the 3d model is demonstrated to improve the regional location of the 1977 event as compared to 1d models the epicenter mislocation using the first arrival phase at 23 regional stations determined by the 3d model was only 6 2 km compared to 13 1 km and 15 4 km for a regional 1d model and the iasp91 model respectively results of extensive testing demonstrate that winpak3d will be a particularly good initial model for tomographic studies as it improves regional location accuracy specifically for sparse networks these results demonstrate that 3d velocity models are essential for improving location accuracy in regions that have complicated crustal geology and structures such 3d models may be a prerequisite for achieving location accuracies as called for by the comprehensive nuclear test ban treaty particularly for small events that are not recorded teleseismically

reply to comment on improving sparse network seismic location with bayesian kriging and teleseismically constrained calibration events by stephen c myers and craig a schultz by a douglas we thank alan douglas douglas 2004 for his thoughtful comments on myers and schultz 2000 in myers and schultz 2000 we improve seismic location accuracy and uncertainty estimates for a sparse regional network by improving travel time predictions and better characterizing residual uncertainties station specific travel time corrections and associated uncertainty estimates are calculated using the bayesian kriging methodology of schultz et al 1998 the bayesian kriging methodology is an empirical approach that exploits the spatial correlation of travel time residuals to interpolate an empirical calibration set of travel time residuals into a minimum variance geographically continuous surface of travel time corrections and an associated uncertainty surface in myers and schultz 2000 we test the bayesian kriging approach by relocating events in the 1991 racha georgia earthquake sequence 42 n 43 e we compare uncalibrated locations based on p wave travel times for the ak135 model kennett et al 1995 and calibrated locations calibrated travel times and uncertainties to benchmark locations that are constrained with a dense local distance network the calibration data set consists of event locations that are constrained by a teleseismic network one goal of the study was to assess the utility of teleseismically constrained events for calibration we find that better travel time prediction for regional paths can be achieved by using the teleseismically constrained calibration events and the improved travel time prediction improves regional network location accuracy a major advantage of bayesian kriging over many other interpolation techniques is the propagation of uncertainty from the calibration data set into a travel time prediction uncertainty surface sources of data set uncertainty include calibration event locations phase observations for calibration events and calibration event spacing we find that coverage ellipses evernden 1969 are representative of observed epicenter accuracy i e confidence level of the coverage ellipse is indicative of the number of occurrences of the benchmark location within the ellipse when the

epicentral location based on rayleigh wave empirical green s functions from ambient seismic noise summary a new method to locate the epicentre of regional seismic events is developed with strengths and limitations complementary to existing location methods this new technique is based on applying empirical green s functions egfs for rayleigh waves between 7 and 15 s period that are determined by cross correlation of ambient noise time series recorded at pairs of seismic receivers the important advantage of this method in comparison with standard procedures based on use of body wave traveltimes is that it does not rely on an earth model rather it is based on interpolating the egfs to arbitrary hypothetical event locations the method is tested by locating well known ground truth crustal events in the western united states magnitude 4 earthquakes in california magnitude 3 earthquakes in utah and a mine collapse as well as locating seismic stations as virtual events in these applications location errors average less than 1 km but are expected to vary with event mechanism and depth numerical simulations show that the method optimizes when source depth is less than 1 km or more than 5 km and when the source mechanism is nearly purely strike slip thrust or normal

a crust and upper mantle model of eurasia and north africa for pn travel time calculation abstract we develop a regional seismic travel time rstt model and method for use in routine seismic analysis the model parameterization is a global tessellation of nodes with a velocity profile at each node interpolation of the velocity profiles generates a 3d crust and laterally variable upper mantle velocity the upper mantle velocity profile at each node is represented as a linear velocity gradient which enables travel time computation in approximately 1 millisecond this computational speed allows the model to be used in routine analyses in operational monitoring systems we refine the model using a tomographic formulation that adjusts the average crustal velocity mantle velocity at the moho and the mantle velocity gradient at each node while the rstt model is inherently global our first rstt tomographic effort covers eurasia and north africa where we have compiled a data set of approximately 600 000 pn arrivals ten percent of the data set is randomly selected and set aside for testing purposes travel time residual variance for the validation data is reduced by 32 based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better epicenter error using 16 pn arrivals is reduced by 46 from 17 3 km ak135 model to 9 3 km rstt model after tomography the median uncertainty ellipse area is reduced by 68 from 3070 km 2 ak135 to 994 km 2 rstt and the number of ellipses with area less than 1000 km 2 which is the area allowed for onsite inspection under the comprehensive nuclear test ban treaty is increased from 0 ak135 to 51 rstt

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

path dependent regional phase travel time corrections for the international monitoring system in north america the software developed for the comprehensive nuclear test ban treaty ctbt international data centre idc in vienna austria uses the iaspei91 seismic travel time tables as default however in reality travel times as a function of source location are different for each station especially at regional distances this is not modeled by a global one dimensional model such as iaspei91 the software therefore allows the application of a suite of travel time corrections for improved travel time prediction and consequently more accurate hypocentral locations to account for path dependency we have developed gridded source specific station corrections ssscs for pn sn pg and lg phases for 22 international monitoring system ims stations in north america in deriving the ssscs and associated modeling errors we used a methodology that had been successfully applied previously for fennoscandia we first compiled regionalized one dimensional travel times for north america then we assigned travel times as weighted sums of the travel times in each region to calculate ssscs validation testing of the ssscs was conducted using two separate sets of north american ground truth gt events with location accuracies within 10 km gt0 gt10 these events were recorded at ims stations between 1995 and 1999 and distributed throughout north america test event location results with ssscs applied show reductions in both mislocations and error ellipses for gt events with location accuracies better than 2 km the median change in location was a 5 km improvement and the median area of the 90th percentile error ellipses was reduced from 2550 km 2 to 910 km 2 using ssscs for both data sets the median improvement is less than 5 km the testing emphasized that gt events with accuracies 5 km or better are crucial for validating ssscs using ssscs for all 51 gt0 gt10 events used in the validation testing the median area of the error ellipses was reduced to less than 1000 km 2 and the error ellipse reductions were achieved without significant degradation of coverage the modeling errors used in this study may be somewhat underestimated however leaving the target goal of error ellipses less than 1000 km 2 still somewhat in question we expect the ssscs event locations and uncertainties can be improved by future refinement of travel times and modeling errors these corrections were implemented in the operational prototype idc system located in arlington virginia in march 2000 and have been delivered to the vienna idc in january 2001

empirically based ground truth criteria for seismic events recorded at local distances on regional networks with application to southern africa wepresentanewapproachtoobtainingempiricallybased eb criteriafor estimating the epicentral location accuracy i e ground truth gt of seismic events recorded at local distances on a regional network the approach has been developed usinga jackknife resampling method appliedto carefully pickedpgphasearrivaltimes for gt reference events from several south african gold mines the events were well recorded locally by southern african seismic experiment sase stations within the archean kaapvaal craton an area of relatively simple crustal structure the region specific criteria obtained specify an ebgt395 level of epicentral accuracy if events are recorded on eight or more stations at distances less than the pg pn crossover 215 km when the stations have a primary azimuthal gap 202 degrees in addition when nine or more stations are used for event location and one of them is within 79 km of the event we find that a focal depth accuracy of 4 km at the 95 confidence level can be obtained and that an accuracy of 6 km can be obtained if eight stations are used this result illustrates that gt criteria commonly applied to global event catalogs can be relaxed if an accurate velocity model and carefully picked phase arrival times are used for event locations consequently it is likely that additional events can be added to gt compilations by developing ebgt criteria for other regional networks and using them to identify candidate gt events for example the ebgt criteria developed in this study when applied to the sase seismicity catalog yields 10 new gt events

global scale p wave tomography optimized for prediction of teleseismic and regional travel times for middle east events 1 data set development 1 we extend the bayesloc seismic multiple event location algorithm for application to global arrival time data sets bayesloc is a formulation of the joint probability distribution spanning multiple event location parameters including hypocenters travel time corrections pick precision and phase labels stochastic priors may be used to constrain any of the bayesloc parameters markov chain monte carlo sampling is used to draw samples from the joint probability distribution and the posteriori samples are summarized to infer conventional location parameters such as the hypocenter the first application of the broad area bayesloc algorithm is to a data set consisting of all well recorded events in the middle east and the most well recorded events with 5 spatial sampling globally this sampling strategy is designed to provide the ray coverage needed to determine lithospheric scale p wave velocity structure in the middle east using the complementary ray geometry provided by regional subhorizontal and teleseismic subvertical raypaths and to determine a consistent albeit lower resolution image of global mantle structure the data set consists of 5401 events and 878 535 p pn pp sp and pcp arrivals recorded at 4606 stations relocated epicenters are an average of 16 km from bulletin locations the data set included events that are known to an accuracy of 1 km a k a gt1 based on nonseismic information the average distance between gt1 epicenters and our relocated epicenters is 5 6 km for arrivals labeled p pn and pcp 92 90 and 96 are properly labeled with probability 0 9 respectively incorrect phase labels are found to be erroneous at rates of 0 6 0 2 1 6 and 2 5 for p pn pcp and depth phases pp and sp respectively labels found to be incorrect but not erroneous were reassigned to another phase label p and pn residual standard deviation with respect to ak135 travel times are dramatically reduced from 3 45 s to 1 01 s the differences between travel time residuals for nearly reciprocal raypaths are significantly reduced from the input event locations suggesting that bayesloc relocation improves data set consistency the reciprocity tests suggest that the dominant contribution to travel time residuals calculated from information provided in global bulletins is location and picks errors not travel time prediction errors due to 3 d structure modeling the whole multiple event system results in accurate locations and an internally consistent data set that is ideal for tomography and other travel time calibration studies simmons et al 2011 companion paper use the bayesloc processed data set to develop a 3 d tomographic image which further reduces residual standard deviation to 0 50 s

on seismological reference models and the perceived nature of heterogeneity a key ingredient in understanding earth structure is the integration of seismological information with results from mineral physics for both the dominant radial dependence of material properties and the 3d structure revealed by seismic tomography in each case an important role is played by reference models for the internal structure of the earth originally constructed to meet seismological needs representations of the travel times of seismic phases are required for the location of seismic events and these have been supplemented by models that match the frequencies of the free oscillation of the earth which have played a major role in characterising seismic sources through the centroid moment tensor derived from long period records parameterised models were introduced to combat the computational demands of inversion and have been extensively used however in such models e g prem the form of the seismic parameters is prescribed to have a functional behaviour such as a cubic based on mathematical convenience rather than physical requirements any 1d models face the challenge of the significant lateral heterogeneity at the top and bottom of the mantle as well as the more subtle variations in between it may well be possible to capture the long wavelength components of 3d heterogeneity but the nature of available data sources means that it will be difficult to achieve comparable p and s definition for the whole mantle in many circumstances reference models are used for comparison with models constructed for different data sets it is then best that e g a cratonic reference model is used to judge results from cratons regionalisation of travel time observations provides a means of constructing such reference models for a broad range of tectonic environments when coupled with suitable path corrections it is possible to make a good account of regional 2d variations it is desirable that reference models used for interpretation of seismic tomography are tied as clearly as possible to mineral physics results with a representation based on the bulk modulus k shear modulus g and density rather than just the p and s wavespeeds comparator models for different mineral compositions and temperature regimes have the potential to aid the interpretation of tomographic images particularly in the separation of the influences of temperature and composition

j362d28 a new joint model of compressional and shear velocity in the earth s mantle summary the discrimination between chemical and thermal heterogeneity in the earth s mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics to answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies we describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes complete waveforms and surface wave dispersion measurements a horizontal tessellation consisting of 362 spherical splines is used to parametrize the model which is approximately equivalent to a spherical harmonic of degree 18 in resolution the model contains peak variations from prem of up to 7 per cent in s velocity and 2 5 per cent in p velocity in the upper mantle these variations decrease to 1 5 and 0 6 per cent respectively at 1000 km depth and reach 2 5 and 1 0 per cent respectively in the d region the rms ratio of s to p velocity perturbations is fairly constant between 2 0 and 2 5 in the lower mantle but a local minimum in this ratio occurs at a depth of approximately 1700 km resolution tests show that the recovery of p and s velocity is not geographically uniform but also show that this amplitude ratio is well resolved between 670 and 2700 km depth we also observe a persistent negative correlation between bulk sound and shear velocity throughout the same region of the mantle in addition the model contains a minimum correlation between p and s velocity between 670 and 1100 km this feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion in which the p and s velocities are perfectly correlated to our data set of p wave traveltimes the power spectra of both p and s velocity heterogeneity are similar although we note a slightly larger dominance of degree two in the spectra for p velocity in the mid mantle where resolution is highest

improving sparse network seismic location with bayesian kriging and teleseismically constrained calibration events monitoring the comprehensive nuclear test ban treaty will require improved seismic location capability for small magnitude events the international monitoring system ims is well suited to locate events that are large enough to be recorded at teleseismic distances however small events are likely to be recorded on a sparse subset of ims stations at regional to upper mantle distances less than 30 and sparse network locations can be strongly effected by travel time errors that result from path specific velocity model inaccuracies in an effort to improve sparse network location capability we outline a procedure that applies empirical corrections to travel times determined with an appropriate velocity model more specifically bayesian kriging and calibration events constrained with a global network are used to estimate epicenter specific travel time corrections for a test sparse network of stations we calculate travel time residuals for the calibration events relative to the ak135 velocity model travel time residuals are assigned to the respective calibration epicenter forming a set of spatially varying travel time correction points the spatial set of correction points is declustered to reduce the dimension of the observations with minimal reduction in accuracy of the travel time corrections we then use the declustered set of calibration points and bayesian kriging to form continuous travel time correction surfaces for each station of the test network the effectiveness of travel time correction surfaces is evaluated by locating with and without corrections a subset of the 1991 racha earthquake sequence caucasus mountains for which we have accurate locations that were independently determined with a dense local network when no travel time correction is applied the mean horizontal distance between the local and test network locations is 42 km and there is a distinct bias in sparse network locations toward the north northwest the mean difference between local and sparse network locations is cut to 13 km when corrections are applied and the bias in location is significantly reduced when calibration events in the racha vicinity are not used to make the correction surfaces there is still a significant improvement in location with mean mislocations of 15 km when corrections are not applied only one of the locally determined locations lies within the associated 90 coverage ellipse determined with the test sparse network however by using travel time corrections and estimates of model uncertainty determined using kriging representative error ellipses are obtained this study demonstrates that kriging correction surfaces based on global network constrained calibration events can improve the ability to accurately locate lower magnitude events while providing representative coverage ellipses

monitoring aseismic tectonic processes via hydrologic responses an analysis of log periodic fluid flow events at the costa rica outer rise a log periodic series of fluid inflow events from the outer rise off the nicoya peninsula costa rica was recorded on a flow meter deployed at the sediment surface similar periodic series have been seen in many failure processes in laboratory and field research including seismic and chemical records preceding earthquakes each cycle has the appearance of an impulse and subsequent decay the hypothesis that the recorded flow pattern is a result of a series of relatively small strain events resulting from aseismic extensional faulting of the outer rise basement is explored and analytical results supporting this hypothesis are presented

improved regional and teleseismic p wave travel time prediction and event location using a global 3d velocity model a global validation dataset of 116 seismic events and 20 977 associated pn and p arrivals is used to assess travel time prediction and event location accuracy for the global scale 3d p wave velocity model called llnl g3dv3 simmons et al 2012 strong regional trends that are observed for ak135 travel time residuals are largely removed when llnl g3dv3 is used for prediction the 25th 75th quantile spread of travel time residuals is reduced by 30 40 at teleseismic distances and the spread is reduced by 60 at regional distances 16 epicenter error decreases when more data are used to constrain event locations until more than 40 arrivals times are used at which point epicenter error reduction tends to plateau median epicenter errors for the ak135 and llnl g3dv3 models plateau at 8 0 and 5 5 km respectively for teleseismic p datasets median epicenter errors for the ak135 and llnl g3dv3 models plateau at 12 0 and 4 0 km respectively for regional pn datasets we demonstrate that spatially correlated travel time residual errors for the ak135 model lead to increasing epicenter error when 40 to 100 pn arrivals are used to constrain the location the effect of correlated error is mitigated by llnl g3dv3 for which epicenter error steadily decreases to 4 km when 100 pn arrivals are used the median area of 0 95 epicenter probability bounds for ak135 and llnl g3dv3 are 1811 and 758 km2 respectively the ak135 ellipses are inflated to achieve the desired rate of true events occurring inside the probability region whereas llnl g3dv3 error ellipses based on empirical residual distributions cover the true location at the expected rate because location bias is minimal

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

late quaternary fluvial dynamics of the jarama river in central spain the jarama river and its catchment provide a valuable natural archive to study and reconstruct past environmental conditions in central spain this region is highly prone to changes in the hydrological cycle under predicted climate warming in particular with respect to aridity periods rainfall variability and the occurrence of extreme flood events this paper presents 15 exposures covering a time span of the last 44 ka which were documented in the field and of which seven exposures were soil chemical analyzed regarding soil texture organic matter as well as carbonate and iron content ages are based on radiocarbon dating on 32 samples latest holocene sediments were found within the channel belt where permanent remobilization of sediments takes place due to the migration of the meandering river course in more distal floodplain positions sediment sequences show a complex fluvial architecture referring to periods of varying character and intensity of alluviation as well as periods of geomorphic stability indicated by soil formation furthermore sedimentation patterns vary along different river sections aggradations of coarse gravels took place between 40 and 18 ka cal bp over the entire valley floor alluviation of fine material was documented between 17 and 16 at 7 5 between 5 1 and 3 3 at 2 8 between 2 1 and 1 5 at 1 0 and around 0 4 ka cal bp until recent times astonishingly in late holocene times between 4 2 and 3 1 ka cal bp aggradations of coarse gravels even in distal floodplain areas overlap with sedimentation of fine material in adjacent river sections pointing to a complex constellation of parameters involved periods of soil formation were detected around 43 at 31 between 16 and 12 6 after 7 5 until 5 1 between 2 8 and 2 1 and at times after 1 5 ka cal bp phases of geomorphic stagnation were found at 38 ka cal bp and for a duration of 200 years at 3 0 ka cal bp and refer to absent geomorphic or pedogenic processes all these patterns are an expression of the interaction between climatic variations tectonic impulses and human influences a comparison with other terrestrial archives enabled us to reconstruct late pleistocene and holocene palaeoenvironmental conditions on a regional scale and furthermore to link specific stages of floodplain development to prevalent influencing variables thus with emphasis on the fluvial system response a model of a cause and effect relationship is presented that concentrates on rapid climate changes long lasting climatic deteriorations and the role of human interventions