automated detection and location of microseismicity at mount st helens with a large n geophone array in the summer of 2014 a dense array of 904 geophones was deployed at mount st helens along the road and trail system within 15 km distance of the summit crater the array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano a reverse time imaging method is applied to short term average over long term average time series data to automatically detect and locate microseismicity these efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the pacific northwest seismic network earthquake locations resolve a narrow 1 km wide vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater this feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface

the current state of knowledge about shatter cones introduction to the special issue shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments the occurrence of shatter cones is the only accepted meso to macroscopic recognition criterion for impact structures shatter cones exhibit a number of geometric characteristics orientation apical angles striation angles sizes that can be best described as varied from case to case possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them a database of shatter cone occurrences has been produced for this introduction paper to the special issue of meteoritics and planetary science on shatter cones distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter with a few currently questionable exceptions all pertinent hypotheses of formation are presented and discussed several may be discarded in light of the most recent observations the branching fracture mechanism and the interference models proposed respectively by sagy et al 2002 and baratoux and melosh 2003 require further evaluation new observations experiments or theoretical considerations presented in this special issue promise an important step forward based on a renewed effort to resolve the enigmatic origin of these important features

sandstone sample analysis and additional structural data from jabal rayah a possible impact structure in saudi arabia the 5 5 km sized jabal rayah ring structure located at 28 390 n 37 120 e in saudi arabia has been classified as a possible complex impact structure located in flat lying paleozoic clastic sediments previous detailed mapping showed that erosional processes led to a relief inversion with displaced folded and faulted blocks of silurian to early devonian strata interpreted to form a ring syncline now forming a topographically outstanding 150 m high ring crest the drainage toward the center of the structure seems controlled by a set of radial faults this central part is eroded to the level of the surrounding plateau and partially covered with gravel analysis of 28 qusaiba formation sandstones showed that at the present outcrop level the sediments seem devoid of shock features measurement of fold axes in the central part of the structure shows radially outward plunging fold axes becoming steeper toward the center and also fold axes of other orientation and folded folds this fold axis pattern is interpreted as an upward pointing kilometer sized sheath fold assuming an impact scenario and using the present size of the structure the minimum central structural uplift is estimated at 500 m which is consistent with qusaiba formation occupying the center of the ring structure

determination of fault planes at coalinga california by analysis of patterns in aftershock locations the may 2 1983 coalinga california earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline in a region that had little historic seismicity the main shock hypocenter was at 10 km depth and faulting did not break the surface there has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane conflicting results from geodetic geologic and aftershock data imply that slip occurred along one or the other or both nodal planes the aftershock zone is large several planar features within the aftershock zone indicate that faulting was complex to delineate the planes of slip we applied the three point method a statistical technique by which event planes can be determined from aftershock locations we obtained several important results from our study first we identified a number of planes using locations of aftershocks some of which have not been previously identified from visual inspection of the aftershock locations second the method allows us to choose slip planes by associating three point planes with fault plane solutions based on proximity and similar orientation finally three of the planes identified by the method intersect near themore main shock hypocenter making it difficult to determine which plane is the main shock slip plane nevertheless aftershocks during the first 24 hours after the main shock clearly define the high angle ne dipping nodal plane whereas few aftershocks were located along the conjugate plane assuming that the aftershocks took place along the rupture zone we conclude that slip during the main shock occurred predominantly along the high angle ne dipping plane from the fault plane solution for this event copyright american geophysical union 1989 less

observing earthquakes triggered in the near field by dynamic deformations we examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments our analysis focuses on seismicity at close range within the aftershock zone complementing published studies of long range triggering our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction therefore we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically with a majority of triggered earthquakes in the direction of rupture propagation bilaterally propagating ruptures also may be directive and we propose simple criteria for assessing their directivity we compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes m 5 7 to m 8 1 that occurred in california and idaho in the united states the gulf of aqaba syria guatemala china new guinea turkey japan mexico and antarctica nine of these mainshocks had clearly directive unilateral ruptures of these nine seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction the two exceptions include an earthquake preceded by a comparable magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results similar but weaker correlations were found for the bilaterally rupturing earthquakes we studied although the static stress change also may trigger seismicity it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus for several of the directive earthquakes we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change manuscript received 1 march 2002

chemical modification of projectile residues and target material in a memin cratering experiment in the context of the memin project a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite campo del cielo as projectile accelerated to 4 56 km s 1 and a block of seeberger sandstone as target material the ejecta collected in a newly designed catcher are represented by 1 weakly deformed 2 highly deformed and 3 highly shocked material the latter shows shock metamorphic features such as planar deformation features pdf in quartz formation of diaplectic quartz glass partial melting of the sandstone and partially molten projectile mixed mechanically and chemically with target melt during mixing of projectile and target melts the fe of the projectile is preferentially partitioned into target melt to a greater degree than ni and co yielding a fe ni that is generally higher than fe ni in the projectile this fractionation results from the differing siderophile properties specifically from differences in reactivity of fe ni and co with oxygen during projectile target interaction projectile matter was also detected in shocked quartz grains the average fe ni of quartz with pdf about 20 and of silica glasses about 24 are in contrast to the average sandstone ratio about 422 but resembles the fe ni ratio of the projectile about 14 we briefly discuss possible reasons of projectile melting and vaporization in the experiment in which the calculated maximum shock pressure does not exceed 55 gpa

testing the ability of surface arrays to monitor microseismic activity recently there has been much interest in the use of data from surface arrays in conjunction with migration based processing methods for passive seismic monitoring in this study we use an example of this kind of data recorded whilst 18 perforation shots with a variety of positions and propellant amounts were detonated in the subsurface as the perforation shots provide signals with known source positions and origin times the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface in all but one case the signals from the perforation shots are not visible in the raw or preprocessed data however clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events even when the signals are not visible in individual traces we find that point source locations are within typically 45 m laterally of the true shot location however the depths are less well constrained 150 m we test the sensitivity of our imaging method to the signal to noise ratio in the data using signals embedded in realistic noise we find that the position of the imaged shot location is quite insensitive to the level of added noise the primary effect of increased noise being to defocus the source image given the migration approach the array geometry and the nature of coherent noise during the experiment signals embedded in noise with ratios 0 1 can be used to successfully image events furthermore comparison of results from data and synthetic signals embedded in noise shows that in this case prestack corrections of traveltimes to account for near surface structure will not enhance event detectability although the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods

saqqar a 34 km diameter impact structure in saudi arabia here we present the first proof of an impact origin for the saqqar circular structure in northwestern saudi arabia neville et al 2014 with an apparent diameter of 34 km centered at 29 35 n 38 42 e the structure is formed in cambrian devonian siliciclastics and is unconformably overlain by undeformed cretaceous and paleogene sediments the age of impact is not well constrained and lies somewhere between 410 and 70 ma the subsurface structure is constrained by 2 d reflection seismic profiles and six drilled wells first order structural features are a central uplift that rises approximately 2 km above regional datums surrounded by a ring syncline the crater rim is defined by circumferential normal faults the central uplift and ring syncline correspond to a bouguer gravity high and an annular ring like low respectively the wells were drilled within the central uplift the deepest among them exceed 2 km depth sandstone core samples from these wells show abundant indicators of a shock metamorphic overprint planar deformation features pdfs were measured with orientations along 0001 103 and less frequently along 101 and 104 planar fractures pfs predominantly occur along 0001 and 101 and are locally associated with feather features ffs in addition some shocked feldspar grains and strongly deformed mica flakes were found the recorded shock pressure ranges between 5 and 15 gpa the preserved level of shock and the absence of an allochthonous crater fill suggest that saqqar was eroded by 1 2 km between the devonian and maastrichtian the documentation of unequivocal shock features proves the formation of the saqqar structure by a hypervelocity impact event

earthquake scaling laws for creeping and non creeping faults the dependence of the summed moment release on the fault dimension differs between non creeping faults and creeping faults or volcanic regions this can be attributed to different earthquake scaling laws the number n of events with fault length l scales for non creeping faults as n l l 2 and for creeping faults as n l l 3 this difference offers a means for mapping the distribution of creep in the seismogenic crust

the mw 5 8 mineral virginia earthquake of august 2011 and aftershock sequence constraints on earthquake source parameters and fault geometry the mw 5 8 earthquake of 23 august 2011 17 51 04 utc moment m0 5 7 10 17 n m occurred near mineral virginia within the central virginia seis mic zone and was felt by more people than any other earthquake in united states history the u s geological survey usgs received 148 638 felt reports from 31 states and 4 canadian provinces the usgs pager system estimates as many as 120 000 people were exposed to shaking intensity levels of iv and greater with approximately 10 000 exposed to shaking as high as intensity viii both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast striking reverse fault that nucleated at a depth of approximately 7 2 km the distri bution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western united states horton and williams 2012 near source and far field damage reports which extend as far away as washington d c 135 km away and baltimore maryland 200 km away are consistent with an earthquake of this size and depth in the eastern united states eus within the first few days following the earthquake several government and aca demic institutions installed 36 portable seismograph stations in the epicentral region making this among the best recorded aftershock sequences in the eus based on modeling of these data we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry area of rupture and observations of the aftershock sequence mag nitude frequency and temporal distribution the observed slope of the magnitude frequency curve or b value for the aftershock sequence is consistent with previous eus studies b 0 75 suggesting that most of the accumulated strain was released by the mainshock the aftershocks define a rupture that extends between approxi mately 2 8 km in depth and 8 10 km along the strike of the fault plane best fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes n36 e and dips to the east southeast at 49 5 moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations both indicating reverse slip along a northeast southwest striking southeast dipping fault plane

focal mechanism analyses for virginia earthquakes 1978 1984 abstract focal mechanisms are presented for 11 earthquakes from the giles county virginia seismic zone and for 12 earthquakes from the central virginia seismic zone these earthquakes 0 m 4 were monitored by local networks between january 1978 and october 1984 in giles county six single event focal mechanisms and five composite event focal mechanisms were determined in central virginia eleven single event focal mechanisms and four composite event focal mechanisms were obtained a computer program focmec that systematically searches the focal sphere for valid mechanism solutions based on both p wave polarities and sv p z amplitude ratios was used to determine the focal mechanism solutions the results for the giles county seismic zone show predominately strike slip mechanisms on steeply dipping 73 16 nne right lateral motion and ese left lateral motion trending nodal planes however some 4 11 of the solutions show similar movement on nodal planes rotated 45 counterclockwise the means and standard deviations for the trends and plunges of the p axes are n46 e 24 and 13 5 20 respectively as computed from the six single event focal mechanisms and two composite event focal mechanisms focal mechanisms from central virginia exhibit much more scatter in mechanism types and nodal plane orientations than observed in giles county the p axes in central virginia are generally northeast trending for shallow earthquakes 8 km the focal mechanisms exhibit a mixture of reverse and strike slip faulting on planes that dip 62 16 the two virginia seismic zones are separated by only 200 km yet their seismogenic characteristics are very different in giles county the earthquakes occur beneath the appalachian decollement and the faulting and inferred stress orientations are uniform in central virginia however the seismicity is occurring at and above the decollement and the associated fault planes and stress orientations are quite variable

the 9 december 2003 central virginia earthquake sequence a compound earthquake in the central virginia seismic zone the 9 december 2003 central virginia earthquake sequence was a compound earthquake consisting of two nearly identical events occurring about 12 sec apart the second event is separated by about 300 m from the first event along the azimuth of approximately 195 10 the source mechanism determined from regional waveform inversion indicates predominantly thrust faulting at a depth of approximately 10 2 km the sequence with two events attained the combined seismic moment of m 0 2 64 1 01 10 15 n m m w 4 3 the focal mechanism indicates a subhorizontal p axis trending 301 and plunging 19 a regional stress model for the central virginia seismic zone cvsz derived from the 9 december 2003 events and 11 previous earthquakes indicates a thrust faulting stress regime with 1 trending 133 12 and plunging 14 3 the least principal stress axis 3 trends 25 10 and plunges 52 3 the 1 axis is rotated approximately 68 clockwise relative to the average maximum horizontal compressional stress s hmax direction for eastern north america approximately 65 the 9 december 2003 earthquake sequence occurred among the systems of paleozoic and mesozoic faults above the southern appalachian decollement which is at depths from 12 to 19 km in the piedmont geologic province of central virginia

continuous kurtosis based migration for seismic event detection and location with application to piton de la fournaise volcano la reunion abstract we present an automatic earthquake detection and location technique based on migration of continuous waveform data data are preprocessed using a kurtosis estimator in order to enhance the first arrival information then migrated onto a predefined search grid using precalculated p wave travel times and finally stacked local maxima in the resulting 4d space time grid indicate the locations and origin times of seismic events we applied our technique to earthquake swarms occurring on piton de la fournaise volcano la reunion france we located 5000 events from 12 different swarms that occurred between 2009 and 2011 our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level multiplet analysis of the detected events and subsequent double difference relocation produce sharper images of the earthquake swarms our code waveloc is released in open source online material figures of seismicity distributions from waveloc synthetic test and stack amplitude values versus magnitudes

a revised picture of the structure of the monsoon and land itcz over west africa this article presents an overview of the land itcz intertropical convergence zone over west africa based on analysis of ncar ncep reanalysis data the picture that emerges is much different than the classic one the most important feature is that the itcz is effectively independent of the system that produces most of the rainfall rainfall linked directly to this zone of surface convergence generally affects only the southern sahara and the northern most sahel and only in abnormally wet years in the region a second feature is that the rainbelt normally assumed to represent the itcz is instead produced by a large core of ascent lying between the african easterly jet and the tropical easterly jet this region corresponds to the southern track of african easterly waves which distribute the rainfall this finding underscores the need to distinguish between the itcz and the feature better termed the tropical rainbelt the latter is conventionally but improperly used in remote sensing studies to denote the surface itcz over west africa the new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift which in turn is controlled by the characteristics of the african easterly jet and tropical easterly jet rather than by moisture convergence this new picture also includes a circulation feature not generally considered in most analyses of the region this feature a low level westerly jet termed the african westerly jet plays a significant role in interannual and multidecadal variability in the sahel region of west africa included are discussions of the how this new view relates to other aspects of west africa meteorology such as moisture sources rainfall production and forecasting desertification climate monitoring hurricanes and interannual variability the west african monsoon is also related to a new paradigm for examining the interannual variability of rainfall over west africa one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north south displacements of this feature the new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the african easterly waves the disturbances that are linked to rainfall over west africa and spawn hurricanes over the atlantic and with our knowledge of the prevailing synoptic and dynamic features this article demonstrate a new aspect of the west africa monsoon a bimodal state with one mode linked to dry conditions in the sahel and the other linked to wet conditions the switch between modes appears to be linked to an inertial instability mechanism with the cross equatorial pressure gradient being a critical factor the biomodal state has been shown for the month of august only but this month contributes most of the interannual variability this new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability but also to the multidecadal variability evidenced in the region between the 1950s and 1980s

oxygen isotopic compositions of quartz in the sand seas and sandy lands of northern china and their implications for understanding the provenances of aeolian sands abstract a better understanding of aeolian sand provenance in deserts would be useful for studying interactions between various earth surface processes occurring in arid and semiarid regions in this study we examined oxygen isotopic compositions of quartz in sand samples taken from taklamakan and badain jaran deserts of northwestern china and from the hunshandake and hulunbeier sandy lands in northeastern china typical fractions of grain sizes were chosen for examination sample preparation followed the chemical principals described in geochemistry and final measurements were performed with mass spectrometers in the sands from the badain jaran desert the 18o value is generally low with a mean of 12 1 in the coarse fraction 0 200 0 250 mm and a mean of 13 2 in the fine fraction 0 125 0 154 mm the sands of the taklamakan desert show a mean 18o value of 15 4 in the main grain size fraction 0 064 0 150 mm in contrast the mean 18o values in all grain size fractions of sand samples from the hunshandake sandy land and from the hulunbeier sandy land are much lower varying between 6 8 and 9 9 the outcome established a preliminary database about 18o distributions in quartz sands throughout china s large deserts and sandy lands the results show regional differences between each of the sand seas and sandy lands indicating different provenances of the aeolian sands further detailed comparison of the 18o values between the deserts and loess stratigraphy in the loess plateau would provide information about changes in areas of loess sources and evidence of palaeocirculation

high resolution relocation of aftershocks of the mw 7 1 darfield new zealand earthquake and implications for fault activity 1 low slip rate regions often represent under recognized hazards and understanding the progression of seismicity when faults in such areas rupture will help us to better understand earthquake rupture patterns the 3 september 2010 utc mw 7 1 darfield earthquake revealed a formerly unrecognized set of faults in the canterbury region of new zealand an area that had previously been mapped as one of the lower hazard areas in the country in this study we analyze the first four months of its aftershock sequence to identify active faults and temporal changes in seismicity along them we jointly invert for three dimensional p wave and s wave velocities and hypocentral locations using data for 2840 aftershocks recorded at 36 temporary and permanent seismic stations within 70 km of the main shock epicenter these relocations delineate eight individual faults active prior to the 22 february 2011 mw 6 3 christchurch earthquake the largest aftershock of the darfield earthquake two of these faults are in the christchurch region one of which corresponds to geodetically determined rupture planes of the christchurch earthquake using focal mechanisms calculated from first motion polarities we find mainly strike slip faulting events with some reverse and normal faulting events as well we compare the orientations of these faults to the prevailing regional stress directions to identify which faults may have been active prior to the darfield earthquake and which may be newly developed

imaging widespread seismicity at midlower crustal depths beneath long beach ca with a dense seismic array evidence for a depth dependent earthquake size distribution we use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in long beach california poor signal to noise ratio due to anthropogenic activity is mitigated via downward continuation of the recorded wavefield the downward continued data are continuously back projected to search for coherent arrivals from sources beneath the array which reveals numerous previously undetected events the spatial distribution of seismicity is uncorrelated with the mapped fault traces or with activity in the nearby oil fields many events are located at depths larger than 20 km well below the commonly accepted seismogenic depth for that area the seismicity exhibits temporal clustering consistent with omori s law and its size distribution obeys the gutenberg richter relation above 20 km but falls off exponentially at larger depths the dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area because the event size distribution above 20 km depth obeys a power law whose exponent is near one this improvement yields a hundred fold decrease in the time needed for effective characterization of seismicity in long beach

reverse modelling for seismic event characterization summary the localization of seismic events is of utmost importance in seismology and exploration current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network weak events not visible in the individual seismogram of the network are missed out we present an approach where no picking of events in the seismograms of the recording network is required the observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling assuming the correct velocity model the reversely modelled wavefield focuses on the hypocentre of the seismic event the origin time of the event is given by the time where maximum focussing is observed the spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition this automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data the method is particularly useful for the upcoming large passive networks since no picking is required it has great potential for localizing very weak events not detectable in the individual seismogram since the reverse modelling sums the energy of all recorded traces and therefore enhances the signal to noise ratio similar to stacking in seismic exploration the method is demonstrated by 2 d and 3 d numerical case studies which show the potential of the technique events with a s n ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

luminescence dating of the wabar meteorite craters saudi arabia 1 luminescence dating has been used to find the age of meteorite impact craters at wabar al hadida in saudi arabia the luminescence characteristics of the shocked material were determined using a variety of luminescence dating techniques applied to impactite formed by the meteorite and to the underlying sand the age is found to be 290 38 years a comparison is made with two possible historically recorded ages an impact as young as this has implications for the assessment of hazards from the impact on earth of small meteorites

cathodolumenescence spectra of quartz as provenance indicators revisited abstract we present a discrimination scheme for cathodoluminescence cl spectra of quartz as a tool in provenance studies we analyzed 1000 quartz crystals from 58 samples of different plutonic volcanic metamorphic and pegmatitic rocks the technique is based on the measurement of the relative intensity of the two main emission centers in visible light at 470 490 nm and 600 640 nm the results confirm a red violet or bright to medium blue luminescence for volcanic phenocrysts mainly bright blue colors for felsic plutonic and high temperature metamorphic quartz as well as brown to dark blue cl for quartz of low temperature metamorphic origin these and additional results for mafic plutonic dark blue and pegmatitic quartz bright blue lead to a possible discrimination with the following rock grouping 1 volcanic quartz 2 low temperature metamorphic and mafic plutonic quartz and 3 felsic plutonic high temperature metamorphic and pegmatitic quartz in the proposed scheme 87 of the spectra are classified correctly the three quartz groups can be taken as an estimate for the amount of volcanic metamorphic and plutonic quartz in sediments unless major input from plutonic high temperature metamorphic or pegmatitic quartz has taken place hence despite criticism within the literature during the last decades the cl colors of detrital quartz still can be used as provenance indicators we finally conclude that the measurement of wavelength spectra is a fast and straightforward method to determine quartz bearing source rocks of siliciclastic sediments

back projection stacking of p and s waves to determine location and focal mechanism of microseismic events recorded by a surface array we present an automatic method of processing microseismic data acquired at the surface by a star like array the back projection approach allows successive determination of the hypocenter position of each event and of its focal mechanisms one component vertical geophone groups and three component accelerometers are employed to monitor both p and s waves hypocenter coordinates are determined in a grid by back projection stacking of the short time average to long time average ratio of absolute amplitudes at vertical components and polarization norm derived from horizontal components of the p and s waves respectively to make the location process more efficient calculation is started with a coarse grid and zoomed to the optimum hypocenter using an oct tree algorithm the focal mechanism is then determined by stacking the vertical component seismograms corrected for the theoretical p wave polarity of the focal mechanism the mechanism is resolved in the coordinate space of strike dip and rake angles the method is tested on 34 selected events of a dataset of hydraulic fracture monitoring of a shale gas play in north america it was found that by including s waves the vertical accuracy of locations improved by a factor of two and is equal to approximately the horizontal location error a twofold enhancement of horizontal location accuracy is achieved if a denser array of geophone groups is used instead of the sparse array of three component seismometers the determined focal mechanisms are similar to those obtained by other methods applied to the same dataset

gebel kamil the iron meteorite that formed the kamil crater egypt abstract the 45 m in diameter kamil impact crater was formed 10 g inferred from the density map compiled in this study is 3400 kg field data indicate that the iron bolide approached the earth s crust from the northwest 305 340 n travelling along a moderately oblique trajectory upon hypervelocity impact the projectile was disrupted into thousands of fragments shattering was accompanied by some melting of the projectile and of the quartz arenite target rocks which also suffered shock metamorphism

location of aftershocks of the 4 april 2010 mw 7 2 el mayor cucapah earthquake of baja california mexico a magnitude m w 7 2 earthquake occurred on 4 april 2010 at approximately 50 km southwest of the city of mexicali baja california mexico the regional networks red sismica del noroeste de mexico resnom that the centro de investigacion cientifica y de educacion superior de ensenada cicese operates and the southern california seismic network scsn located the main event between the sierras el mayor and cucapah at the southeastern end of the pescadores fault twenty four hours after the origin time of the main event a temporal seismic network was installed around the faults that ruptured during this event we used body wave arrival times recorded by the first 14 stations installed to obtain precise hypocentral locations of this important sequence of earthquakes most of the aftershocks located on 6 and 7 april are distributed near the traces of the pescadores and cucapah faults particularly at the southern end of these faults the spatial distribution of the epicenters indicates that these faults and possibly others may extend southeast of sierra cucapah where the faults were buried by sediments and were activated during the seismic sequence in this area north of the sierra el mayor where no faults were previously mapped the epicenters align in the northwest southeast direction most hypocenters have depths less than 15 km suggesting that an important portion of the seismic slip was shallow based on the distribution of focal depths of the aftershocks we infer that the seismogenic zone must be between 5 and 10 km

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

the aftershock sequence of the 2011 mineral virginia earthquake temporal and spatial distribution focal mechanisms regional stress and the role of coulomb stress transfer abstract the aftershocks of the 23 august 2011 m w 5 7 mineral virginia earthquake were recorded by 36 temporary stations installed by several institutions we located 3960 aftershocks from 25 august 2011 through 31 december 2011 a subset of 1666 aftershocks resolves details of the hypocenter distribution we determined 393 focal mechanism solutions aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane other aftershocks occurred 10 20 km to the northeast a large percentage of the aftershocks occurred in regions of positive coulomb static stress change and 80 of the focal mechanism nodal planes were brought closer to failure however the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity aftershocks at depths less than 4 km exhibit reverse mechanisms with north northwest trending nodal planes most focal mechanisms at depths greater than 6 km are similar to the mainshock with north northeast trending nodal planes a concentration of aftershocks in the 4 6 km depth range near the mainshock are mostly of reverse type but display a 90 range of nodal plane trend those events appear to outline the periphery of mainshock rupture where positive coulomb stress transfer is largest the focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km along with the mainshock point to the possibility of a depth dependent stress field prior to the occurrence of the mainshock analysis of earthquake occurrence using a new magnitude scale indicates a gutenberg richer law b value of 0 864 and an omori law p value of 1 085 indicative of a typical aftershock sequence online material catalogs of aftershock location magnitude and focal mechanisms

joint location and source mechanism inversion of microseismic events benchmarking on seismicity induced by hydraulic fracturing summary seismic monitoring can greatly benefit from imaging events with a low signal to noise ratio snr as the number of the events with a low signal grows exponentially one way to detect weaker events is improvement of a snr by migration type stacking of waveforms from multiple stations we have developed a new method of location of seismic events that involves stackingofseismicphasesandamplitudesalongdiffractiontraveltimecurvestosuppressnoise and detect seismic events with a snr lower than that on individual receivers the stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function we applied this method to locate microseismicity induced by hydraulic fracturing first we calibrated the velocity model by locating perforation shots at known locations then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration type locations of the largest events that were manually located the detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals the proposed technique is fully automated and feasible for real time seismic monitoring

remote sensing of terrestrial impact craters the tandem x digital elevation model with the tandem x digital elevation model dem the terrestrial solid surface is globally mapped with unprecedented accuracy tandem x is a german x band radar mission whose two identical satellites have been operated in single pass interferometer configuration over several years the acquired data are processed to yield a global dem with 12 m independent posting and relative vertical accuracies of better than 2 m and 4 m in moderate and mountainous terrain respectively this dem provides new opportunities for space borne remote sensing studies of the entire sample of terrestrial impact craters in addition it represents an interesting repository to aid in the search for new impact crater candidates we have used the tandem x dem to investigate the current set of confirmed impact structures for a subsample of the craters including small midsized and large structures we compared the results with those from other dems this quantitative analysis demonstrates the excellent quality of the tandem x elevation data our findings help to estimate what can be gained by using the tandem x dem in impact crater studies they may also be beneficial in identifying the regions and morphologies where the search for currently unknown impact structures might be most promising

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

tectonics of the southern and central appalachian internides the appalachian orogen was constructed along the ancient precambrian continental margin of eastern north america by a series of compressional events that began in the ordovician and episodically spanned much of the paleozoic era the processes of accretion of suspect and exotic terranes together with terrane collision and ultimately completion of the wilson cycle by continent continent collision resulted in construction of the appalachian orogen figure 1 these paleogeographic elements are rep resented by the various lithotectonic units visible today in the southern and central appalachians figure 2 these subdivisions represent the late precambrian rifted margin and early paleozoic platform that were formed directly upon north american grenvillian basement cumberland allegheny plateau valley and ridge and western blue ridge the offshore north american slope rise assemblage hamburg klippe southern appalachian eastern blue ridge and inner piedmont and higher thrust sheets of the maryland northern virginia piedmont and the exotic carolina or avalon and wilmington terranes the carolina ter rane contains the alleghanian high grade metamorphic core these elements form the basis for this analysis and review of the present state of knowledge of the tectonic history of the southern and central appalachian internides the appalachian orogen has been the subject of study by geologists at least since the early nineteenth century early fundamental concepts such as the geosynclinal theory of mountain building dana 1873 hall 1883

coseismic reverse and oblique slip surface faulting generated by the 2008 mw 7 9 wenchuan earthquake china the mw 7 9 wenchuan china earthquake ruptured two large thrust faults along the long menshan thrust belt at the eastern margin of the tibetan plateau this earthquake generated a 240 km long surface rupture zone along the beichuan fault and an additional 72 km long surface rupture zone along the pengguan fault maximum vertical and horizontal offsets of 6 5 m and 4 9 m respectively were measured along the beichuan fault a maximum vertical offset of 3 5 m was measured along the pengguan fault coseismic surface ruptures integrated with aftershocks and industry seismic profiles show that two imbricate structures have ruptured simultaneously resulting in the largest continental thrust event ever documented large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region as everywhere along the edge of tibetan plateau

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

ash shutbah a possible impact structure in saudi arabia we have investigated the ash shutbah circular structure in central saudi arabia 21 degrees 37n 45 degrees 39e using satellite imagery field mapping thin section petrography and x ray diffraction of collected samples the approximately 2 1km sized structure located in flat lying jurassic tuwaiq mountain limestone has been nearly peneplained by erosional processes satellite and structural data show a central area consisting of dhruma formation sandstones with steep bedding and tight folds plunging radially outward open folding occurs in displaced younger tuwaiq mountain limestone formation blocks surrounding the central area but is absent outside the circular structure an approximately 60cm thick unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure with exception of a possible concave shatter cone found in the orthoquartzite of the central area other diagnostic shock features are lacking some quartz rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures this deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea or groundwater heating the combination of central stratigraphic uplift of 140m concussion features in discolored sandstone outward dipping concentric folds in the central area deformation restricted to the rocks of the ring structure a complex circular structure of 2 1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets and a possible shatter cone all point to an impact origin of the ash shutbah structure in fact the ash shutbah structure appears to be a textbook example of an eroded complex impact crater located in flat lying sedimentary rocks where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure less

hidden aftershocks of the 2011 mw 9 0 tohoku japan earthquake imaged with the backprojection method 1 the first 25 h of the aftershock sequence following the 11 march 2011 mw 9 0 tohoku japan earthquake is investigated using a backprojection method in total 600 aftershocks are imaged during this time period these aftershocks are distributed over a 500 by 300 km area and include many events in the outer rise the backprojection events are compared with the japan meteorological agency jma catalogue which is composed of earthquakes recorded by local seismic networks in japan surprisingly half of the backprojection events are not found in the jma catalogue these events cluster near the japan trench and in the outer rise and fill in gaps in the spatial distribution of the early aftershock sequence where large main shock slip is thought to have occurred these results show that the jma magnitude of completeness is very high near the trench following the 2011 tohoku main shock and earthquakes as large as magnitude 6 8 went undetected by local seismic networks

optimized automatic pickers application to the ancorp data set summary it has become increasingly important to develop fast and accurate automatic procedures to process and fully exploit increasing large seismic data sets traditionally these data sets are processed manually which requires significant amounts of both manpower and time with sometimes variable results we have developed a cost minimization approach to train three automatic pickers an sta lta tpd and the pai k picker at each station within a dense temporary network located in northern chile and southern bolivia the optimum picking parameters for each station show regional variability and need to be adjusted individually to achieve the best performance we developed a weighting scheme that uses four independent predictors of weight calibrated using a handpicked data subset which mimics the picking by an expert seismologist we use the fact that each of the three pickers highlights different properties of the observed seismic trace to combine two pickers that work in tandem the first makes an initial pick before the second picker refines and improves the accuracy of the automatic pick we find the tandem pickers improve the accuracy of the automatic picks when compared to the single automatic pickers we demonstrate that following the cost minimization procedure described here the automatic picks have sufficient accuracy that they would be suitable for high precision earthquake location focal mechanism determination or high resolution seismic tomography

interaction of aluminum projectiles with quartz sand in impact experiments formation of khatyrkite cual 2 and reduction of sio 2 to si abstract we analyzed the interaction of spherical 6 36 mm diameter cu bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at nasa ames vertical gun range impact velocities and inferred peak shock pressures varied between 5 9 and 6 5 km s and 41 and 48 gpa respectively shocked particles impact melt particles coated with thin crusts of molten projectile material were recovered from the floors of the ca 33 cm diameter craters and the respective ejecta blankets through petrographic and chemical optical microscopy fe empa sem edx and xrf analysis we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism these can be characterized by the following physical and chemical reactions and alteration products i complete melting and subsequent recrystallization of the projectile forming a distinct crystallization texture in the fused metal crust ii projectile target mixing involving a redox reaction between cu bearing al alloy und sio 2 leading to formation of khatyrkite cual 2 al 2 o 3 melt euhedral silicon crystals and spherical droplets of silicon iii melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains and iv shock lithification of quartz grains with fracturing of grains grain boundary melting planar deformation features and complete loss of porosity to our knowledge this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments these results have implications for the understanding of a similar redox reaction between al cu metal and siliceous impact melt recently postulated for the khatyrka cv3 carbonaceous chondrite moreover these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres such as asteroids in the main belt e g 4 vesta and on the moon specifically impacts of mm sized projectiles at velocities between 4 and 6 km s into regolith covered asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects i e partially to completely molten projectile remnants adhering to impact melted regolith agglomerates as well as projectile contaminated impact melts and local shock melting along grain boundaries

backprojection of volcanic tremor backprojection has become a powerful tool for imaging the rupture process of global earthquakes we demonstrate the ability of backprojection to illuminate and track volcanic sources as well we apply the method to the seismic network from okmok volcano alaska at the time of an escalation in tremor during the 2008 eruption although we are able to focus the wavefield close to the location of the active cone the network array response lacks sufficient resolution to reveal kilometer scale changes in tremor location by deconvolving the response in successive backprojection images we enhance resolution and find that the tremor source moved toward an intracaldera lake prior to its escalation the increased tremor therefore resulted from magma water interaction in agreement with the overall phreatomagmatic character of the eruption imaging of eruption tremor shows that time reversal methods such as backprojection can provide new insights into the temporal evolution of volcanic sources

seismicity seismic reflection studies gravity and geology of the central virginia seismic zone part i seismicity the central virginia seismic zone is an area of persistent low level seismicity in the piedmont province of that state its north south dimension is 120 km and it extends some 150 km in an east west direction from richmond to lynchburg the results of instrumental monitoring there by a seismic network during the past 5 yr are reported herein to provide a useful constraint on geologic structural models considerable care must be exercised in the location of earthquakes and in the specification of the accuracy of those locations our procedures for accomplishing those tasks were as follows first a velocity model specific to the network locale was developed from seismic refraction and reflection data second evaluation of the detection capability of the network indicated that the detection of seismic events should be virtually complete down to a magnitude of 1 5 third the locational capability of the network was assessed by locating blasts originating from three different quarries operating within the network area as if they were earthquakes of unknown location the average calculated locations when compared to the known blast sites indicated average differences of 2 km horizontally and 4 km vertically except for the northeastern portion there the location capability degrades to fair for epicenters 4 km and to poor for estimation of focal depths 8 km the resultant decrease in locational capability is probably due to the presence of low velocity coastal plain sediments fourth the location of all of the earthquakes relative to each other was tested by use of special joint hypocenter calculations the recalculated locations differed from the original locations in a random manner on the average that difference was 3 km and it indicates the absence of serious bias introduced by combining earthquake locations derived from different subnetworks of seismographs ninety six pre network pre 1978 earthquakes for the most part felt events with noninstrumental locations and 34 network determined hypocenters 1978 1982 for mostly smaller not felt shocks form the basic data set of this study the spatial distribution exhibited by the 130 hypocenters is diffuse both vertically and horizontally such a pattern favors multiple rather than singular seismogenic structures seventy five percent of the focal depths are in the upper one third of the crust at depths of 11 km or less these multiple shallow sources will be shown in part 4 of this sequence of papers to be along and above a master detachment fault

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

the wabar impact craters saudi arabia revisited the very young wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933 we describe field observations made during a visit to the wabar impact site provide analytical data on the material collected and combine these data with poorly known information discovered during the recovery of the largest meteorites during our visit in march 2008 only two craters philby b and 11m were visible philby a was completely covered by sand mapping of the ejecta field showed that the outcrops are strongly changing over time combining information from different visitors with our own and satellite images we estimate that the large seif dunes over the impact site migrate by approximately 1 0 2 0myr 1 southward shock lithification took place even at the smallest 11m crater but planar fractures pfs and undecorated planar deformation features pdfs as well as coesite and stishovite have only been found in shock lithified material from the two larger craters shock lithified dune sand material shows perfectly preserved sedimentary structures including cross bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding slickensides and radiating striation resembling shatter cones the composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite we observed a partial decoupling of fe and ni in the black impact glass probably due to partitioning of ni into unoxidized metal droplets the absence of a ca enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet which has an estimated thickness of 20 30m less

teleseismic p wave imaging of the 26 december 2004 sumatra andaman and 28 march 2005 sumatra earthquake ruptures using the hi net array 1 seismograms from a dense high quality seismic network in japan are used to investigate the characteristics of the 26 december 2004 sumatra andaman and the 28 march 2005 sumatran earthquakes the onset of the p waveforms are aligned through cross correlation and a simple concept of back projecting seismic energy to a grid of potential source locations is applied the waveform alignment removes the effects due to lateral variations in wave speed between the hypocenter and each station to better approximate the effects of three dimensional heterogeneity for paths originating from grid points away from the hypocenter cross correlation results of the p waveforms from aftershocks are introduced this additional information leads to improved resolution of smaller scale features near many of the aftershocks by reducing wavefront distortion the back projection analysis provides a quick assessment of the spatiotemporal extent and variability of relative high frequency energy release which can be translated into an estimate of the moment magnitude as well as an unparalleled view of high frequency rupture propagation the results are in general consistent with those obtained from more involved source inversion methods the 2004 sumatra andaman earthquake released most energy in a region northwest of the sumatra island and the rupture extended to the northern andaman islands about 1300 km from the epicenter this northern portion of the rupture radiated a considerable amount of energy but there is little evidence of slow slip the 2005 event is imaged to have bilateral rupture with northwestern slip occurring for about 50 s before it moved to the southeast of the epicenter

insights into the mechanism of intermediate depth earthquakes from source properties as imaged by back projection of multiple seismic phases 1 this study investigates the spatial and temporal distribution of energy release of large intermediate depth earthquakes using a modified back projection technique first used to study the 2004 sumatra andaman megathrust event multiple seismic phases are included in the back projection analysis which provides the capability to determine the energy distribution with respect to depth and time a total of 22 intermediate depth earthquakes with moment magnitudes greater than or equal to 6 5 are investigated with hypocentral depths between 100 and 300 km for most of these events the vertical extent of energy release is either below the resolution of this study 5 km or slightly above 15 km this observation agrees with previous studies that find large intermediate depth earthquakes have subhorizontal rupture planes the results also show a significant portion of the events have multiple rupture planes that are well separated in depth the closeness in time of the ruptures on separate planes and the distance between the planes suggest dynamic triggering where the p waves from the first rupture initiate rupture on the second plane we propose that a dehydration embrittlement mechanism combined with preferentially hydrated subhorizontal faults can explain the observations of dominant subhorizontal rupture planes and the frequent occurrence of rupture complexity involving multiple subevents

illuminating the near sonic rupture velocities of the intracontinental kokoxili mw 7 8 and denali fault mw 7 9 strike slip earthquakes with global p wave back projection imaging 1 the denali and kokoxili strike slip earthquakes are two of the longest recent intracontinental ruptures previous studies report a range of rupture velocities here we image these earthquakes by reverse time migration of the intermediate frequency p wave train recorded by global broadband seismometers this technique permits a relatively direct measure of rupture velocity speed and direction as constrained by the radiated seismic energy free from restrictive assumptions or rupture speed bounds placed on the solution we compare our results with published seismic gps displacement and surface slip inversion results both ruptures were initially subshear and transitioned over a distance no longer than 40 km to supershear speeds close to the p wave speed of 5 6 km s we investigate the accuracy of our results with synthetic data and experiment with using different imaging parameters and seismic subnetworks these tests allow us to rule out the possibility of subshear speeds along the supershear segments although we cannot exclude supershear speeds of 4 5 6 5 km s our most reliable rupture velocities of 5 6 km s are close to the local p wave speeds we hypothesize that these intracontinental faults have weak shear strengths or high breakdown slips or crustal rigidities and experience at least moderate slip or slip rate weakening our observations and previous published results lead us to speculate that very long surface extending faults with general homogeneity in prestress and fault strength together with smaller adjacent fault segments to provide triggering may be necessary ingredients for the sub rayleigh to supershear rupture speed transition in strike slip earthquakes

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

the march 11 2011 tohoku oki earthquake and cascading failure of the plate interface 1 a continuous back projection analysis using data recorded in north america between march 9 2011 and april 7 2011 is applied to the mw 9 0 2011 tohoku oki earthquake and the foreshocks and aftershocks of this event as with the mw 8 8 2010 maule chile earthquake back projection results of the mainshock show variations in rupture behavior when data filtered to different frequency ranges are used in particular there is a relationship between the frequency of data used and the position along the dip direction of the energy release with the highest frequency result imaging energy at the down dip edge of the rupture and progressively lower frequencies showing a continuous shift updip in addition these results show that late in the rupture 200 seconds after the hypocentral time energy at all frequencies is imaged very close to the trench at about 37 n which with the energy imaged near and updip of the epicenter may have acted as a tsunami source comparing the mainshock rupture area to the area that ruptured during m 6 foreshocks and aftershocks between march 9th and april 7th shows that total failure of the plate interface nearly doubled compared to the mainshock alone building upon the idea that the mainshock occurred through interactions between multiple segments the results imply that the total failure area of the interface may represent the area that can rupture during a single event as a series of in phase interface segments

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

the villalbeto de la pena meteorite fall i fireball energy meteorite recovery strewn field and petrography an impressive daylight fireball was observed from spain portugal and the south of france at 16h46m45s utc on january 4 2004 the meteoroid penetrated into the atmosphere generating shock waves that reached the ground and produced audible booms the associated airwave was recorded at a seismic station located 90 km north of the fireball trajectory in spain and at an infrasound station in france located 750 km north east of the fireball the absolute magnitude of the bolide has been determined to be 18 1 from a casual video record the energy released in the atmosphere determined from photometric seismic and infrasound data was about 0 02 kilotons kt a massive fragmentation occurred at a height of 28 0 2 km resulting in a meteorite strewn field of 20 6 km the first meteorite specimen was found on january 11 2004 near the village of villalbeto de la pena in northern palencia spain to date about 4 6 kg of meteorite mass have been recovered during several recovery campaigns the meteorite is a moderately shocked s4 l6 ordinary chondrite with a cosmic ray exposure age of 48 5 ma radioisotope analysis shows that the original body had a mass of 760 150 kg which is in agreement with the estimated mass obtained from photometric and seismic measurements

depth dependence of earthquake frequency magnitude distributions in california implications for rupture initiation statistics of earthquakes in california show linear frequency magnitude relationships in the range of m2 0 to m5 5 for various data sets assuming gutenberg richter distributions there is a systematic decrease in b value with increasing depth of earthquakes we find consistent results for various data sets from northern and southern california that both include and exclude the larger aftershock sequences we suggest that at shallow depth 0 to 6 km conditions with more heterogeneous material properties and lower lithospheric stress prevail rupture initiations are more likely to stop before growing into large earthquakes producing relatively more smaller earthquakes and consequently higher b values these ideas help to explain the depth dependent observations of foreshocks in the western united states the higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock at greater depth 9 15 km any rupture initiation is more likely to continue growing into a larger event so there are fewer foreshocks if one assumes that frequency magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake then a small m2 rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a m5 5 or larger event compared to the same small rupture initiation at 0 to 3 km

imaging short period seismic radiation from the 27 february 2010 chile mw 8 8 earthquake by back projection of p pp and pkikp waves 1 teleseismic short period 0 5 5 s p waves from the 27 february 2010 chile earthquake mw 8 8 are back projected to the source region to image locations of coherent short period seismic wave radiation several receiver array configurations are analyzed using different p wave arrivals including networks of stations in north america p japan pkikp and europe pp as well as a global configuration of stations with a broad azimuthal distribution and longer period p waves 5 20 s coherent bursts of short period radiation from the source are concentrated below the chilean coastline along the downdip portion of the megathrust the short period source region expands bilaterally with significant irregularity in the radiation comparison with finite fault slip models inverted from longer period seismic waves indicates that the regions of large slip on the megathrust are located updip of the regions of short period radiation a manifestation of frequency dependent seismic radiation similar to observations for the great 2011 tohoku earthquake mw 9 0 back projection of synthetic p waves generated from the finite fault models demonstrates that if the short period energy had radiated with the same space time distribution as the long period energy back projection analysis would image it in the correct location updip we conclude that back projection imaging of short period signals provides a distinct view of the seismic source that is missed by studies based only on long period seismic waves geodetic data and or tsunami observations

estimating a new date for the wabar meteorite impact the wabar meteorite craters were discovered in the desert of al rub al khali saudi arabia by philby 1933 who was the first to identify them as created in a meteorite impact the age of these ctarers is still a subject of debate the first approximate date was 1863 based on a reported meteorite fall at an unidentified site in the al rub al khali recently conducted luminescence dating suggests an age of 289 46 years prescott et al 2002 it is the aim of this article to present evidence contained in historical poems which substantiates this dating the texts quoted indicate that nearly 30 ago i e in 1704 a great fireball in the skies was seen from tarim yemen southwest of the wabar site this dating seems to roughly coincide with the results of recent scientific investigations

earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod 1 arrival time data from 453 local earthquakes and 6 explosions at parkfield ca are inverted for earthquake locations and three dimensional vp and vp vs structure the structure is dominated by the velocity contrast across the saf with the southwest side about 20 25 faster consistent with previous studies nearly all the earthquakes occur almost directly beneath the fault trace we find high vp vs anomalies that correlate with low resistivity features in a magnetotelluric model that are interpreted to represent fluids we locate a magnitude 2 earthquake that is a potential target event for the final stage of safod drilling yielding a depth of 3 1 km below surface and an epicenter 100 m southwest of the fault trace nonlinear analyses indicate 95 confidence relative and absolute uncertainties on the order of 500 700 m vertically and 200 300 m horizontally for this target earthquake

complex faulting associated with the 22 december 2003 mw 6 5 san simeon california earthquake aftershocks and postseismic surface deformation we use data from two seismic networks and satellite interferometric synthetic aperture radar insar imagery to characterize the 22 december 2003 m w 6 5 san simeon earthquake sequence absolute locations for the mainshock and nearly 10 000 aftershocks were determined using a new three dimensional 3d seismic velocity model relative locations were obtained using double difference the mainshock location found using the 3d velocity model is 35 704 n 121 096 w at a depth of 9 7 0 7 km the aftershocks concentrate at the northwest and southeast parts of the aftershock zone between the mapped traces of the oceanic and nacimiento fault zones the northwest end of the mainshock rupture as defined by the aftershocks projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the oceanic fault near the santa lucia range front and the 5 mm postseismic insar imagery contour the oceanic fault in this area as mapped by hall 1991 is therefore probably a second order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth the southeast end of the rupture projects closer to the mapped oceanic fault trace suggesting much of the slip was along this fault or at a minimum is accommodating much of the postseismic deformation insar imagery shows 72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture and 48 and 45 mm at the northwest and southeast end of the aftershock zone respectively from these observations we model a 30 km long northwest trending northeast dipping mainshock rupture surface called the mainthrust which is likely the oceanic fault at depth a 10 km long southwest dipping backthrust parallel to the mainthrust near the hypocenter several smaller southwest dipping structures in the southeast and perhaps additional northeast dipping or subvertical structures southeast of the mainshock plane discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic nacimiento fault zone at depth the insar data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone the well defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the santa lucia range as a popup block aftershocks in the southeast part of the zone also indicate a popup block but it is less well defined the absence of backthrust features in the central part of the zone suggests range front uplift by fault propagation folding or backthrusts in the central part were not activated during the mainshock

aftershock triggering by complete coulomb stress changes 1 we examine the correlation between seismicity rate change following the 1992 m7 3 landers california earthquake and characteristics of the complete coulomb failure stress cfs changes cfs t that this earthquake generated at close distances the time varying dynamic portion of the stress change depends on how the rupture develops temporally and spatially and arises from radiated seismic waves and from permanent coseismic fault displacement the permanent static portion cfs depends only on the final coseismic displacement cfs diminishes much more rapidly with distance than the transient dynamic stress changes a common interpretation of the strong correlation between cfs and aftershocks is that load changes can advance or delay failure stress changes may also promote failure by physically altering properties of the fault or its environs because it is transient cfs t can alter the failure rate only by the latter means we calculate both cfs and the maximum positive value of cfs t peak cfs t using a reflectivity program input parameters are constrained by modeling landers displacement seismograms we quantify the correlation between maps of seismicity rate changes and maps of modeled cfs and peak cfs t and find agreement for both models however rupture directivity which does not affect cfs creates larger peak cfs t values northwest of the main shock this asymmetry is also observed in seismicity rate changes but not in cfs this result implies that dynamic stress changes are as effective as static stress changes in triggering aftershocks and may trigger earthquakes long after the waves have passed

the jiddat al harasis 073 strewn field sultanate of oman the recently discovered jiddat al harasis jah 073 strewn field is the largest found so far in the sultanate of oman covering an area of 19 6 km the 3463 single stones collected range in weight from 52 2 kg down to 1 g total weight 600 8 kg and show a pronounced mass sorting the strewn field shape can be approximated by a nw se oriented ellipsoid indicating an atmospheric entry from se at a low angle relative to the surface the meteorite belongs to the l6 ordinary chondrite group and shows s4 average shock grade smaller stones generally show a higher weathering grade resulting in a spread from w2 and w4 enhanced weathering of the stones causing fragmentation after the fall is observed in sandy depressions five 14c measurements on stones of variable size and weathering grade yielded 14c from 3 8 to 49 9 dpm kg three samples give a 14c 10be age consistent with about 14 4 ka for two samples the cosmogenic trapped and radiogenic noble gases were measured the ratio of the 4he and 40ar gas retention ages of 0 29 0 10 and that of the 3he and 21ne cosmic ray exposure ages of 0 36 0 08 ma indicate that jah 073 experienced a complex exposure history and lost 4he and 40ar due to a major collision fragmentation statistics indicate a single major atmospheric disruption and an originally relatively spherical shape of the object assuming the material collected represents the majority of fallen mass and 90 99 of the original weight was lost by ablation the pre atmospheric minimum radius of the meteoroid with density 3 4 g cm 3 would have been at least 75 cm