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

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

joint earthquake source inversions using seismo geodesy and 3 d earth models a joint earthquake source inversion technique is presented that uses insar and long period teleseismic data and for the first time takes 3 d earth structure into account when modelling seismic surface and body waves ten average source parameters moment latitude longitude depth strike dip rake length width and slip are estimated hence the technique is potentially useful for rapid source inversions of moderate magnitude earthquakes using multiple data sets unwrapped interferograms and long period seismic data are jointly inverted for the location fault geometry and seismic moment using a hybrid downhill powell monte carlo algorithm while the insar data are modelled assuming a rectangular dislocation in a homogeneous halfspace seismic data are modelled using the spectral element method for a 3 d earth model the effect of noise and lateral heterogeneity on the inversions is investigated by carrying out realistic synthetic tests for various earthquakes with different faulting mechanisms and magnitude mw 6 0 6 6 synthetic tests highlight the improvement in the constraint of fault geometry strike dip and rake and moment when insar and seismic data are combined tests comparing the effect of using a 1 d or 3 d earth model show that long period surface waves are more sensitive than long period body waves to the change in earth model incorrect source parameters particularly incorrect fault dip angles can compensate for systematic errors in the assumed earth structure leading to an acceptable data fit despite large discrepancies in source parameters three real earthquakes are also investigated eureka valley california 1993 may 17 mw 6 0 aiquile bolivia 1998 february 22 mw 6 6 and zarand iran 2005 may 22 mw 6 5 these events are located in different tectonic environments and show large discrepancies between insar and seismically determined source models despite the 40 50 km discrepancies in location between previous geodetic and seismic estimates for the eureka valley and aiquile earthquakes the seismic data are found to be compatible with the insar location a 30 difference in strike between insar and seismic derived source models is also resolved when taking 3 d earth structure into account in the analysis of the eureka valley earthquake the combination of both insar and seismic data further constrains the dip for the zarand earthquake and in all cases the seismic moment is more robustly constrained in the joint inversions than in the individual data set inversions unmodelled lateral heterogeneities in earth and the models could partly explain some of the observed source parameter discrepancies related to the seismic data the authors 2014 published by oxford university press on behalf of the royal astronomical society

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

geochemical disturbance and paleoenvironmental changes during the early toarcian in nw europe the early toarcian oceanic anoxic event oae is associated with a rapid and severe environmental change whose causes are still a matter of debate a new paleoenvironmental interpretation is proposed based on a multidisciplinary dataset chemical mineralogical and paleontological obtained from a borehole drilled by andra french national radioactive waste management agency in the lower toarcian of the eastern paris basin france during the earlymost toarcian tenuicostatumzone the presence ofwell oxygenated sea water is reflected by an abundant and diverse fauna alongwith paleoredox proxies u al mo al and v al typical of oxidizing conditions associated with predominant mechanical weathering on the continent at the base of the serpentinum zone an important paleoenvironmental change is marked by an initial phase of organic matter enrichment 12 toc during the negative shift of 13c carbon isotope excursion cie euxinic anoxic conditions indicated by cobalt enrichment and the development of framboidal pyrite lead to the disappearance of the benthic and pelagic fauna and alloworganic matter to be preserved in the sediments at this time the lowconcentration of paleoproductivity proxies cd p ni and zn suggests weak chemical weathering on the continent due to a rather arid climate low concentrations of sr and coccoliths are due to an input of fresh water probably coming from the arctic through the laurasian seaway thus the isolation of the paris basin from the tethyan realm and the influx of freshwater favour haline stratification and the preservation of organic matter following the cie oxygen concentration gradually increases up to amaximumflooding surface dated as coevalwith the serpentinum zone elegantulum subzone paleoproductivity proxies indicate a weak primary productivity the top of the lower toarcian is characterized by a second enrichment in organic matter 12 toc due to an important input of nutrients this enhanced primary production generates euxinic anoxic conditions in the sediment and in part of the water column leading to a high enrichment in paleoredox proxies mo u v co and causing the sudden disappearance of the benthic fauna and a progressive decline of the pelagic fauna the metal enrichment is related to intense chemical weathering on the continent due to a more humid climate

satellite radar interferometric map of the coseismic deformation field of the m 6 1 eureka valley california earthquake of may 17 1993 interferometric combination of pairs of synthetic aperture radar sar images acquired by the ers 1 satellite before and after the eureka valley california earthquake of may 17 1993 maps the coseismic deformation field the fringes corresponding to contours of equal change in satellite to ground distance can be enhanced by forming the sum of two of such interferograms with altitudes of ambiguity roughly equal in magnitude but opposite in sign the resulting composite interferogram reveals some 10 cm of change in range indicating a coseismic deepening of the eureka valley graben after unwrapping filtering and decimation the range changes are used to estimate the centroid and focal mechanism these estimates indicate normal faulting on a rupture patch compatible with the distribution of aftershocks and mapped quaternary faults the change in elevation produced by this earthquake resembles the current topography scaled by a factor of 20 000

continental weathering and climatic changes inferred from clay mineralogy and paired carbon isotopes across the early to middle toarcian in the paris basin abstract lower toarcian strata lower jurassic have been extensively studied with a view to understanding the oceanographic climatic and biological processes that drove the earth s system into an oceanic anoxic event oae for this time period the evolution of the european marine seaways is now relatively well constrained owing to multiple geochemical studies but investigations regarding climatic trends in the continental realm remain sparse in the present study we test the clay mineralogy as a continental climate sensitive proxy in the well documented sancerre core southern paris basin we compare variations in the kaolinite content with p co 2 fluctuations derived from paired carbon isotopes 13 c 13 c carb 13 c org taking advantage of the detailed chemostratigraphic palaeoenvironmental and sequence stratigraphy frameworks established for this core material the results indicate a substantial decrease in kaolinite abundance at the end of the pliensbachian which is compatible with a long term diminution in continental weathering and an inferred temperature decrease the early toarcian prior to the carbon cycle perturbation and deposition of black shale facies remained relatively cold with minima in both the proportion of kaolinite and reconstructed p co 2 the mineralogical and geochemical responses across the prominent negative carbon isotope excursion cie itself are not univocal the first of four steps that compose the negative limb of the cie at sancerre is associated with decreased kaolinite and p co 2 and increased carbonate oxygen isotope ratios taken together these trends are compatible with a transient cooling phase immediately preceding the onset of black shale deposition conversely the subsequent steps are marked by substantial enrichment in kaolinite that matches increased osmium isotope ratios measured in yorkshire providing compelling evidence for rapid increases in continental weathering and riverine runoff forced by intensification of greenhouse conditions during the cie relaxation in the intensity of continental weathering as suggested by resumed low kaolinite abundance is seen immediately after the cessation of co 2 input after the fourth step of the cie the interval spanning the upper portion of the early toarcian and the middle toarcian records a subsequent long term increase in the proportion of kaolinite synchronous with significant clay enrichment of the sediment continued greenhouse conditions even after the recovery from the carbon isotope perturbation and regional black shale deposition are likely related to sustained co 2 emission by karoo ferrar volcanism through the considered interval

palaeoclimatic evolution during eocene and its influence on oil shale mineralisation fushun basin china the fushun basin is a small explored coal and oil shale bearing cenozoic fault basin in the liaoning province northeast china the basin mainly consists of eocene swamp to lacustrine deposits of the guchengzi to xilutian formation and contains the biggest opencast oil shale mine in asia this mine has provided an ideal opportunity to undertake palaeoclimate reconstruction in this basin based on a single geological profile and the analyses of 93 samples using various approaches namely field geological observation clay mineralogical and geochemical sr ba sr cu stable c and o isotope analyses all of which were compared with palaeobotanical data the eocene climate of fushun basin evolved from warm temperate to north subtropical and generally changed from warm humid to subhumid semiarid paleoclimatic and geochemical parameters shows that the very warm and humid climate during jijuntun formation increased the initial productivity of lake water and caused a steady stratification of the lake water then caused oxygen lack in the bottom of water productivity of the lake provides the mean origin of organic matters for oil shale formation and steady anoxic environment is beneficial for the conservation of organic matters

interface inversion using broadside seismic refraction data and three dimensional travel time calculations a procedure has been developed to interpret densely sampled broadside seismic refraction data recorded from a large air gun array first arrival travel times are inverted to find the structure on an interface beneath the shot line travel times are calculated for three dimensional velocity models using a rapid finite difference algorithm adapted to allow variable sampling of the model and the determination of rays a simple inversion parameterization eliminates the need for matrix inversion the complete inversion procedure is computationally rapid yet allows the determination of detailed three dimensional structure broadside refraction data recorded in the queen charlotte basin offshore western canada during a multichannel reflection experiment are used to demonstrate the procedure the data are inverted for the basement interface beneath the shot line defining a rapidly varying thickness of sedimentary basin fill the results of the inversion stimulate a reinterpretation of the reflection data and identify a new major basement fault structure out of the plane of the reflection section is determined including the strike of the fault and other nearby features

inversion of seismic reflection traveltimes using a nonlinear optimization scheme we present the use of a nonlinear optimization scheme called generalized simulated annealing to invert seismic reflection times for velocities reflector depths and lengths a finite difference solution of the eikonal equation computes reflection traveltimes through the velocity model and avoids ray tracing we test the optimization scheme on synthetic models and compare it with results from a linearized inversion the synthetic tests illustrate that unlike linear inversion schemes the results obtained by the optimization scheme are independent of the initial model the annealing method has the ability to produce a suite of models that satisfy the data equally well we make use of this property to determine the uncertainties associated with the model parameters obtained synthetic examples demonstrate that allowing the reflector length to vary along with its position helps the optimization process obtain a better solution we put this to use in imaging the garlock fault whose geometry at depth is po

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

continental weathering and redox conditions during the early toarcian oceanic anoxic event in the northwestern tethys insight from the posidonia shale section in the swiss jura mountains abstract the geochemistry and mineralogy of organic rich sediments of the rietheim succession in northern switzerland were studied to evaluate the main impacts of the early toarcian oceanic anoxic event t oae 183 ma ago on the depositional environment in the nw tethys ocean the geochemical data obtained notably 13corg 13ccaco3 rock eval pyrolysis toccff support the hypothesis that the negative c isotope excursion at the onset of the t oae mainly reflects a major perturbation in the global carbon cycle rather than local environmental changes e g basinal restriction productivity rock eval pyrolysis and 13corg 15ntotal crossplots indicate that the organic matter was primarily of marine origin during the t oae and deposited in an epicontinental sea setting detrital proxies notably detritus index siliciclastic balance and ln al2o3 na2o suggest that the elevated pco2 levels and greenhouse climate prevailing during the t oae induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates redox sensitive proxies relative pyrite contents organic and sulfide balances v v ni ratios trace elements enrichment factors trace element toc co variation patterns and toc ptotal molar ratios indicate that the rietheim posidonia shale sediments accumulated under oxic to dysoxic bottom water conditions with more reducing conditions during the t oae anoxic and possibly euxinic these changes in redox conditions were probably induced by substantial thermohaline stratification driven by enhanced freshwater input and the recurrent influx of brackish arctic seawater into the nw tethys through the viking corridor in agreement with previous sedimentological and geochemical evidences from the nw tethys ocean in the studied succession such conditions were perhaps also responsible for sequestration of most of the regenerated p in the deeper water column by adsorption and or co precipitation into authigenic phases overall the major palaeoenvironmental changes documented in this study provide a robust example of earth s possible responses to the rapid onset of extreme greenhouse conditions

continental weathering and palaeoclimatic changes through the onset of the early toarcian oceanic anoxic event in the qiangtang basin eastern tethys abstract the bilong co north section located in the south qiangtang depression northern tibetan plateau provides key information on continental weathering and palaeoclimatic changes through the onset of the early toarcian oceanic anoxic event t oae in the eastern tethys new mineralogical and geochemical data from the bilong co north section suggest that the lower unit of the section corresponds to low chemical index of alteration cia 50 57 and ln al2o3 na2o 0 88 1 16 and high index of compositional variability icv 19 27 indicating a weak weathering and arid climate preceding the onset of the t oae a prominent increase in cia and ln al2o3 na2o values is clearly recorded through the onset of the t oae coinciding with a prominent change in srxs ba and clay fractions suggesting a sharp increase in humidity at the onset of the t oae the palaeoclimatic index of low srxs ba 0 48 4 98 ratios indicates that the climate during the t oae interval was most likely semiarid to semimoist some redox proxies including mo and toc relationships mo u enrichment factors and corg ptotal ratios suggest euxinic conditions during the t oae interval the primary productivity parameters show two main fluctuations within the bilong co north section and their patterns are in good agreement with the observation of palaeoclimatic variation patterns suggesting an influence of palaeoclimate on primary productivity however the palaeoproductivity increase during the t oae in the bilong co north section should mainly be attributed to freshwater inputs rather than solely the result of paleoclimatic change

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

centroid moment tensor solutions for april june 1993 abstract centroid moment tensor solutions are presented for 196 earthquakes that occurred during the second quarter of 1993 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

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

carbonate platform evidence of ocean acidification at the onset of the early toarcian oceanic anoxic event abstract the early toarcian oceanic anoxic event early jurassic 183 myr ago is associated with one of the largest negative carbon isotope excursion cie in the whole phanerozoic 3 7 estimates of the magnitude and rate of co 2 injection in the ocean atmosphere system are compatible with a scenario of ocean acidification many carbonate platforms drowned in the pliensbachian well before the early toarcian event in this paper we test the hypothesis of surface water ocean acidification by presenting data from a resilient carbonate platform the apennine carbonate platform of southern italy the studied sections document a dramatic shift of the carbonate factory from massive biocalcification to chemical precipitation lithiotis bivalves and calcareous algae palaeodasycladus mediterraneus which were the most prolific carbonate producers of pliensbachian carbonate platforms disappear during the first phase of the early toarcian cie before the most depleted values are reached we discuss the local versus supraregional significance of this shift and propose a scenario involving abrupt decline of carbonate saturation forced by co 2 release at the beginning of the early toarcian cie followed by a calcification overshoot driven by the recovery of ocean alkalinity attribution of the demise of carbonate platform hypercalcifiers to ocean acidification is supported by palaeophysiology and reinforced by experimental data on the detrimental effects of ocean acidification on recent shellfishes and calcareous algae

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

tectonic and climate control of oil shale deposition in the upper cretaceous qingshankou formation songliao basin ne china oil shales were deposited in the songliao basin ne china during the upper cretaceous period representing excellent hydrocarbon source rocks high organic matter om contents a predominance of type i kerogen and a low maturity of om in the oil shales are indicated by bulk geochemical parameters and biomarker data a major contribution of aquatic organisms and minor inputs from terrigenous land plants to om input are indicated by n alkane distribution patterns composition of steroids and organic macerals strongly reducing bottom water conditions during the deposition of the oil shale sequences are indicated by low pristane phytane ratios high c14 aryl isoprenoid contents homohopane distribution patterns and high v ni ratios enhanced salinity stratification with mesosaline and alkaline bottom waters during deposition of the oil shales are indicated by high gammacerane index values low mttc ratios high carotene contents low toc s ratios and high sr ba ratios the stratified water column with anoxic conditions in the bottom water enhanced preservation of om moderate input of detrital minerals during the deposition of the oil shale sequences is reflected by titanium concentrations in this study environmental conditions in the paleo lake leading to om accumulation in the sediments are related to sequence stratigraphy governed by climate and tectonics the first member of the qingshankou formation k2qn1 in the songliao basin containing the oil shale sequence encompasses a third order sequence that can be divided into three system tracts transgressive system tract tst highstand system tract hst and regressive system tract rst enrichment of om changed from low values during tst i to high moderate values during tst ii iii and hst i ii low om enrichment occurs during rst i and rst ii therefore the highest enrichment of om in the sediments is related to stages of mid late tst and early hst

first record of the early toarcian oceanic anoxic event from the southern hemisphere neuquen basin argentina abstract the first record of the early toarcian oceanic anoxic event c 183 ma from the southern hemisphere is described from the neuquen basin argentina identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon isotope excursion 13 c org in bulk rock and fossil wood the negative excursion of 6 in bulk organic carbon falling to 31 3 crosses the boundary of the tenuicostatum hoelderi andean ammonite zones equivalent to the tenuicostatum falciferum serpentinum zones of europe these data indicate that the early toarcian oceanic anoxic event was a global phenomenon supplementary material a detailed stratigraphic log chemostratigraphic data and nannofossil data are available at http www geolsoc org uk sup18411

surface displacement of the 17 may 1993 eureka valley california earthquake observed by sar interferometry satellite synthetic aperture radar sar interferometry shows that the magnitude 6 1 eureka valley earthquake of 17 may 1993 produced an elongated subsidence basin oriented north northwest parallel to the trend defined by the aftershock distribution whereas the source mechanism of the earthquake implies a north northeast striking normal fault the 3 millimeter accuracy of the radar observed displacement map over short spatial scales allowed identification of the main surface rupture associated with the event these observations suggest that the rupture began at depth and propagated diagonally upward and southward on a west dipping north northeast fault plane reactivating the largest escarpment in the saline range

hydrothermal venting of greenhouse gases triggering early jurassic global warming abstract the climate change in the toarcian early jurassic was characterized by a major perturbation of the global carbon cycle the event lasted for approximately 200 000 years and was manifested by a global warming of 6 c anoxic conditions in the oceans and extinction of marine species the triggering mechanisms for the perturbation and environmental change are however strongly debated here we present evidence for a rapid formation and transport of greenhouse gases from the deep sedimentary reservoirs in the karoo basin south africa magmatic sills were emplaced during the initial stages of formation of the early jurassic karoo large igneous province and had a profound influence on the fate of light elements in the organic rich sedimentary host rocks total organic carbon contents and vitrinite reflectivity data from contact aureoles around the sills show that organic carbon was lost from the country rocks during heating we present data from a new type of geological structures termed breccia pipes rooted in the aureoles within the shale of the western karoo basin the breccia pipes are cylindrical structures up to 150 meters in diameter and are mainly comprised of brecciated and baked black shale thousands of breccia pipes were formed due to gas pressure build up during metamorphism of the shales resulting in venting of greenhouse gases to the toarcian atmosphere mass balance calculations constrained by new aureole data show that up to 1800 gt of co2 was formed from organic material in the western karoo basin about 15 times this amount of co2 27 400 gt may have formed in the entire basin during the intrusive event u pb dating of zircons from a sill related to many of the pipes demonstrates that the magma was emplaced 182 5 0 4 million years ago this supports a causal relationship between the intrusive volcanism the gas venting and the toarcian global warming

alkali feldspars as a main phosphorus reservoirs in rare metal granites three examples from the bohemian massif czech republic phosphorus rich alkali feldspars were found in three peraluminous highly differentiated albite topaz li mica granites in the w and s parts of the bohemian massif the average contents of p2o5 in k feldspars k i ovy kamen 0 57 wt homolka 0 77 wt and podlesi 0 83 wt are higher than the average contents of p2o5 in albites k i ovy kamen 0 23 wt homolka 0 23 wt and podlesi 0 39 wt the analyses of feldspars indicate that partition coefficient of phosphorus between k feldspar and albite range from 1 5 to 2 5 measured data in k feldspars suggest a statistically significant difference from the al3 p5 2 si4 substitution mechanism for higher phosphorus concentration the p content in k feldspars from the podlesi granite represent the highest p content in natural feldspar known to date up to 2 5 wt of p2o5 it is suggested that all studied granites retained nearly all phosphorus of granitic melts and that their alkali feldspars represent major reservoirs of phosphorus

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

the significance of an early jurassic toarcian carbon isotope excursion in haida gwaii queen charlotte islands british columbia canada abstract during the early toarcian there was a significant disruption in the short term active carbon reservoir as revealed by carbon isotope records which show a broad positive shift that is interrupted by a large 5 7 negative excursion 13 c org carbon isotope excursion co occurs with the deposition of organic rich shales in many areas this perturbation in carbon isotopes is thought to be indicative of severe climate change and marine anoxia the two leading hypotheses as to the cause of this event invoke either global or regional controls here we present carbon isotope data from haida gwaii british columbia canada showing a significant perturbation within a temporally constrained early toarcian succession that was deposited in the northeastern paleo pacific ocean these data reinforce the concept that the short term active carbon reservoir was affected globally and assist with the correlation of ammonite zonal schemes between western north america and europe the 13 c org data show a broad positive shift that is interrupted by a sharp and pronounced negative excursion of 7 8 5 in 13 c wood in the early toarcian kanense zone this negative excursion also coincides with increasing total organic carbon toc from 0 4 to 1 2 these data suggest that the early toarcian carbon isotope perturbation was indeed global and imprinted itself on all active global reservoirs of the exchangeable carbon cycle deep marine shallow marine atmospheric

effects of karoo ferrar volcanism and astronomical cycles on the toarcian oceanic anoxic events early jurassic abstract one of the most profound environmental changes in the mesozoic took place during pliensbachian toarcian early jurassic including oceanic anoxia toarcian oceanic anoxic event t oae the t oae is thought to have been caused by increased atmospheric co 2 triggered by karoo ferrar volcanism this idea however remains debated primarily due to uncertainties in their age constraints of the relevant sedimentary sequences to examine their temporal relationships herein we provide the astronomical time scale of the lower jurassic deep sea bedded chert sequences from the pelagic panthalassa superocean which are exposed in the inuyama area central japan a 405 kyr tuned astrochronology anchored to the end triassic extinction as 201 4 0 2 ma ikeda and tada 2013 allows us to constrain the ages of two black bedded cherts t oae1 and t oae2 the ages of these t oaes overlap u pb ages of karoo volcanic rocks t oae in the european region is also synchronous with the karoo ferrar volcanism based on radiolarian and ammonite biostratigraphic correlation these temporal relationships support the potential impact of karoo ferrar volcanism on t oaes on a global scale on the other hand the onset of t oaes occur at the maxima of 40 kyr 100 kyr and 405 kyr cycles of chert thickness variation the termination of t oaes and the recovery to oxic conditions in pelagic ocean coincide with the minima of 40 kyr 100 kyr and 405 kyr cycles of chert thickness moreover the termination of final black chert and grey chert deposition coincide with the minima of 1800 kyr cycles of chert thickness these temporal relationships imply that orbital scale productivity cycles were important in controlling the onset and termination of t oaes through the carbon cycle dynamics which have been already amplified by karoo ferrar volcanism

stepwise atmospheric carbon isotope excursion during the toarcian oceanic anoxic event early jurassic polish basin abstract during the mesozoic 250 64 ma intervals of about 0 5 myr were subject to severe environmental changes including high sea surface temperature and very low oxygen content of marine water these oceanic anoxic events or oaes occurred simultaneously with profound disturbance to the carbon cycle the carbon isotope anomaly in the early jurassic that marks the toarcian oceanic anoxic event t oae at 182 ma is characterized in marine sections by a series of dramatic steps towards lighter values herein we present new carbon isotope data from terrestrial organic matter phytoclast separates collected through a late pliensbachian middle toarcian coastal and marginal marine succession in the polish basin a setting where hinterland climate and sea level change are well recorded the results show that the shift to light carbon isotope values in the woody organic matter and therefore also in atmospheric carbon dioxide similarly occurred in major steps the steps are here correlated with those identified from marine organic matter where they have previously been attributed to 100 kyr eccentricity forcing of climate the results provide strong support for orbitally and climatically controlled release of isotopically light carbon from gas hydrates into the ocean atmosphere system in a series of rapid bursts additionally a link between the carbon isotope steps and shoreline movements can be demonstrated individual peaks of the negative excursion are mostly associated with facies indicative of sea level rise flooding surfaces however at the same time inferred higher atmospheric carbon dioxide content may be expected to have resulted in increased rainfall and temperature leading to accelerated weathering and erosion and consequently increased sediment supply progradation and regression causing some mismatches between isotope shifts and inferred sea level changes enhanced abundance of megaspores derived from hydrophilic plant groups and marked increase in kaolinite are coincident with the overall development of the negative isotope excursion the combined data suggest that each 100 kyr cycle in carbon isotope values was characterized by increasingly severe palaeoclimatic change culminating in extremely hot and humid conditions co incident with the peak of the final most negative carbon isotope excursion the chemostratigraphic correlation allows very precise dating of the late pliensbachian middle toarcian coastal and marginal marine sedimentary succession in the polish basin

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

late cretaceous campanian carbon isotope events sea level change and correlation of the tethyan and boreal realms abstract carbon stable isotope stratigraphy provides unique insights into environmental change during the campanian the longest 83 5 71 3 ma but least well understood stage of the late cretaceous a new carbon isotope 13c profile for a 500 m thick campanian basal maastrichtian tethyan pelagic hemipelagic section near el kef northern tunisia is calibrated using data from a biostratigraphically well constrained succession at kalaat senan the general shapes of the tunisian 13c reference curve and published tethyan 13c profiles from elles tunisia and bidart sw france and a boreal curve for the trunch borehole eastern england are remarkably similar in all three areas a positive carbon isotope event of 0 2 13c in the mid campanian dated at 78 7 ma and a negative excursion of 0 4 in the upper campanian at 74 8 ma can be correlated between tunisia and england a positive excursion of 0 3 at 83 7 ma spans the santonian campanian boundary these isotope events enable precise inter regional correlations that are consistent with published nannofossil data review of campanian sea level data from north africa the middle east and northern europe indicates that major shifts in 13c profiles coincide with changes in eustatic sea level relatively stable 13c values in the lower campanian and their long term fall through the upper campanian reflect high and then falling eustatic sea levels and increased carbonate production short term 600 kyr positive excursions record greater organic productivity and or organic matter preservation and decreased carbonate fluxes during periods of rapid sea level rise and the drowning of carbonate platforms excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea level fall carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of campanian global biostratigraphic frameworks and improve our understanding of the nature and timing of late cretaceous sea level change

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

a high resolution shallow marine record of the toarcian early jurassic oceanic anoxic event from the east midlands shelf uk the global deposition of organic rich mudrocks a mass extinction and marked geochemical changes in the sedimentary rocks and fossils deposited during the early toarcian indicates a period of extreme environmental change and an oceanic anoxic event this study investigates at a high resolution the environmental and biotic changes that occurred during the event in a shallow marine area using rocks deposited on the east midlands shelf uk we present a new graphic log geochemical data 13corg total organic carbon caco3 total sulphur and total nitrogen and benthic macroinvertebrate ranges from north quarry holwell leicestershire uk comparison of the ammonite ranges between the cleveland basin yorkshire and east midland shelf successions in the uk shows that there is a hiatus on the shelf across the dactylioceras semicelatum cleviceras exaratum subzone boundary the new high resolution geochemical data show that an 5 13corg excursion occurs similar to other lower toarcian sections in the world 13corg shifts a to c are missing in the hiatus but that 13corg shift d is present similar to the cleveland basin the c exaratum subzone of the east midlands shelf succession is dominated by three epifaunal bivalve species but their ranges differ significantly between the successions an increase in the faunal diversity occurs within the upper c exaratum subzone at holwell and other uk sections the biotic data indicate that conditions on the east midlands shelf were more hospitable than in the cleveland basin the bivalve populations on the shelf may have provided a source of new recruits for the basins

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

the northern walker lane refraction experiment pn arrivals and the northern sierra nevada root abstract in may 2002 we collected a new crustal refraction profile from battle mountain nevada across western nevada the reno area lake tahoe and the northern sierra nevada mountains to auburn ca mine blasts and earthquakes were recorded by 199 texan instruments extending across this more than 450 km long transect the use of large mine blasts and the ultra portable texan recorders kept the field costs of this profile to less than us 10 000 the seismic sources at the eastern end were mining blasts at barrick s goldstrike mine the goldstrike mine produced several ripple fired blasts using 8000 44 000 kg of anfo each a daily occurrence first arrivals from the larger goldstrike blasts are obvious to distances of 300 km in the raw records first arrivals from a quarry blast west of the survey near watsonville ca located by the northern california seismic network with a magnitude of 2 2 can be picked across the recording array to distances of 600 km the watsonville blast provides a western source nearly reversing the goldstrike blasts a small earthquake near bridgeport ca also produced pickable p wave arrivals across the transect providing fan shot data arrivals from m5 events in the mariana and kuril islands also appear in the records this refraction survey observes an unexpectedly deep crustal root under the northern sierra nevada range over 50 km in thickness and possibly centered west of the topographic crest pn delays of 4 6 s support this interpretation at battle mountain nevada we observe anomalously thin crust over a limited region perhaps only 150 km wide with a moho depth of 19 23 km pn crossover distances of less than 80 km support this anomaly which is surrounded by observations of more normal 30 km thick crust a 10 km thick and high velocity lower crustal pillow is an alternative hypothesis but unlikely due to the lack of volcanics west of battle mountain large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array even over distances exceeding 600 km new elastic synthetic seismogram modeling suggests that pn can be strong as a first arrival easing the modeling and interpretation of crustal refraction data fast eikonal computations of first arrival time can match pickable pn arrival times