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

floor subsidence and rebound of large venus craters the topography and geology of large craters on venus reveal no evidence for floor rebound or relaxation rather the floors have subsided depressions of the floor centers relative to the margins are evident in topography and floor faulting is interpreted as contractional failure of the likely processes responsible only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas assuming volcanism occurred within a few tens of millions of years after the impact thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere further the predicted stresses are generally consistent with observed floor fracturing we constrain the impact heat deposited in the lithosphere to be less than roughly 5 1023 j for diameters of 100 km the absence of perceptible floor subsidence at craters this size on the moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter but also on gravity and target density the unfractured melt sheets of three large young bright floored craters imply sufficient lithospheric rigidity to support the crater cavities an elastic flexural rebound model restricts the elastic plate thickness to at least 10 15 km for the three craters corresponding to maximum geotherms of 20 30 k km 1 structural evidence for early rebound in older dark floored craters may have been buried by subsequent volcanism

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

migration based microseismic event location in the schlema alberoda mining area abstract mining operations in the schlema alberoda uranium mine located in south west saxony germany stopped in 1990 and the mine was then allowed to flood the region is seismically active and has been monitored for a long time we have located some of the microseismic events which occurred between 2005 and 2012 in the mining area to investigate the nature of the seismicity as well as the long term role of mining induced and triggered seismicity in this study we used pseudo synthetic data obtained from the vertical component records of a local seismic network to accurately locate the hypocenters the location procedure is based on a kirchhoff prestack depth migration approach in order to assess the accuracy of the migration results a homogeneous and a 3 d velocity model are tested in the migration algorithm both of these velocity models were previously used to migrate data acquired by a 3 d reflection seismic survey in this area the resulting images were comparable with minor differences to the reflectors in our area of interest in contrast we found that the errors in the velocity model can strongly affect the migration based earthquake localization the located hypocenters are compared to the 3 d reflection seismic image of the area to determine possible correlations between the seismicity and reflectivity this comparison shows that some of reflectors can be directly correlated with the induced and triggered seismicity of the area the results also indicate that the main causes of the post mining seismicity are changes in the mine flooding level and the consequent changes in the pore pressure of the subsurface structures finally we confirm that the hypocenters are often bound to preexisting subsurface structures like faults and fractures in the area in particular the spatial pattern of the hypocenters proves the existence of a previously unmapped extension of the ruhmvoll fault this pattern also suggests a connection between this structure and the roter kamm fault a major component of the larger gera jachymov fault zone

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

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

automated seismic event location by waveform coherence analysis summary automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring since microseismic records are generally characterized bylowsignal to noiseratio automated locationmethods arerequested to be noise robust and sufficiently accurate most of the standard automated location routines arebasedontheautomatedpicking identificationandassociationofthefirstarrivalsofpands waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases although current methods can accurately pick p onsets the automatic picking of the s onset is still problematic especially when the p coda overlaps the s wave onset in this paper we propose a picking free earthquake location method based on the use of the short term average long term average sta lta traces at different stations as observed data for the p phases we use the sta lta traces of the vertical energy function whereas for the s phases we use the sta lta traces of a second characteristic function which is obtained using the principal component analysis technique in order to locate the seismic event we scan the space of possible hypocentral locations and origin times and stack the sta lta traces along the theoretical arrival time surface for both p and s phases iterating this procedure on a 3 d grid we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event a pilot application was performed in the campania lucania region southern italy using a seismic network irpinia seismic network with an aperture of about 150km we located 196 crustal earthquakes depth 20km with magnitude range 1 1 ml 2 7 a subset of these locations were compared with accurate manual locations refined by using a double difference technique our results indicate a good agreement with manual locations moreover our method is noise robust and performs better than classical location methods based on the automatic picking of the p and s waves first arrivals

combining seismic arrays to image the high frequency characteristics of large earthquakes summary this study investigates the source properties of the 2007 8 0 pisco peru 2007 8 4 and 7 9 mentawai islands indonesia 2009 8 1 samoa islands and 2010 8 8 maule chile earthquakes using a backprojection technique that utilizes data from multiple seismic arrays combining seismic arrays increases azimuth and distance coverage and improves lateral resolution the four subduction interface events show rupture propagation towards the bottom of the seismogenic zone in addition all of the earthquakes show evidence of multiple rupture segments with varying rupture speeds and directions relating these segments to the specific features of the subduction zones e g interseismic coupling suggests that asperity sizes may be controlled by subtle features of the subducted slab

imaging supershear rupture for the 2014 mw 6 9 northern aegean earthquake by backprojection of strong motion waveforms the seismic source spatiotemporal evolution of the mw 6 9 event on 24 may 2014 in northern aegean is imaged by backprojection of strong motion envelopes the results indicate that the event ruptured on two different fault segments in the first one rupture propagated from the hypocenter westward for 20 km in the second delayed segment to the east rupture propagated eastward for 65 km with a supershear velocity 5 5 km s at the end of this rupture the largest stacking amplitudes are imaged associated with possible stopping phases from the abrupt cessation of a fast slip low aftershock seismicity on the supershear segment implies a simple and linear fault geometry there this is the third large event in the northern aegean trough northern anatolian fault zones that has ruptured with supershear speed this characteristic should be taken into account in studying past events and estimating seismic hazard in this area

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

comparison of migration based location and detection methods for microseismic events microseismic monitoring in the oil and gas industry commonly uses migration based methods to locate very weak microseismic events the objective of this study is to compare the most popular migration based methods on a synthetic dataset that simulates a strike slip source mechanism event with a low signal to noise ratio recorded by surface receivers vertical components the results show the significance of accounting for the known source mechanism in the event detection and location procedures for detection and location without such a correction the ability to detect weak events is reduced we show both numerically and theoretically that neglecting the source mechanism by using only absolute values of the amplitudes reduces noise suppression during stacking and consequently limits the possibility to retrieve weak microseismic events on the other hand even a simple correction to the data polarization used with otherwise ineffective methods can significantly improve detections and locations a simple stacking of the data with a polarization correction provided clear event detection and location but even better results were obtained for those data combined with methods that are based on semblance and cross correlation

probabilistic earthquake location in complex three dimensional velocity models application to switzerland 1 we present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of switzerland a three dimensional 3 d p wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3 d p wave velocity structure a 3 d p wave velocity model derived from controlled source seismology css is used as an initial reference model the final 3 d model thus combines all available information from both css and local earthquake data the probabilistic nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model in particular the combination of nonlinear global search algorithms such as the oct tree importance sampling with probabilistic earthquake location provides a fast and reliable tool for earthquake location the comparison of hypocenter locations obtained routinely by the swiss seismological service sed to those relocated in the new 3 d velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths we can attribute these large shifts in part to large uncertainties in the hypocenter location events with a low number of observations 8 and no observation within the critical focal depth distance typically show large location uncertainties improved hypocentral locations particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for switzerland

earthquake locations in southern california obtained using source specific station terms we relocate 297 400 events recorded by the southern california seismic network scsn between 1975 and 1998 using spatially varying station terms to improve relative location accuracy our method uses existing scsn p and s picks a smooth one dimensional velocity model and an iterative grid search approach based on the l1 norm we apply empirical corrections for three dimensional structure by computing station timing corrections that continuously vary as a function of source position station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor delaunay tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth our locations exhibit much less scatter particularly in depth than those of the scsn catalog and a greater tendency to align into linear and planar features suggestive of fault structures our results appear comparable to and in some cases better than previous scsn relocation studies using joint hypocenter velocity inversion techniques plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity we observe no simple relationship between the maximum depth of seismicity and surface geology

fast location of seismicity a migration type approach with application to hydraulic fracturing data we propose a new approach for the location of seismic sources using a technique inspired by gaussian beam migration of three component data this approach requires only the preliminary picking of time intervals around a detected event and is much less sensitive to the picking precision than standard location procedures furthermore this approach is characterized by a high degree of automation the polarization information of three component data is estimated and used to perform initial value ray tracing by weighting the energy of the signal using gaussian beams around these rays the stacking is restricted to physically relevant regions only event locations correspond to regions of maximum energy in the resulting image we have successfully applied the method to synthetic data examples with 20 30 white noise and to real data of a hydraulic fracturing experiment where events with comparatively small magnitudes 0 were recorded

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

the internal generation of the atlantic ocean interdecadal variability abstractnumerical simulations of a realistic ocean general circulation model forced by prescribed surface fluxes are used to study the origin and structure of intrinsic interdecadal variability of

multiband array detection and location of seismic sources recorded by dense seismic networks we present a new methodology for detection and space time location of seismic sources based on multiscale frequency selective coherence of the wave field recorded by dense large scale seismic networks and local antennas the method is designed to enhance coherence of the signal statistical features across the array of sensors and consists of three steps signal processing space time imaging and detection and location the first step provides for each station a simplified representation of seismic signal by extracting multiscale non stationary statistical characteristics through multiband higher order statistics or envelopes this signal processing scheme is designed to account for a priori unknown transients potentially associated with a variety of sources e g earthquakes tremors and to prepare data for a better performance in posterior steps following space time imaging is carried through 3 d spatial mapping and summation of station pair time delay estimate functions this step produces time series of 3 d spatial images representing the likelihood that each pixel makes part of a source detection and location is performed in the final step by extracting the local maxima from the 3 d spatial images we demonstrate the efficiency of the method in detecting and locating seismic sources associated with low signal to noise ratio on an example of the aftershock earthquake records from local stations of international maule aftershock deployment in central chile the performance and potential of the method to detect locate and characterize the energy release associated with possibly mixed seismic radiation from earthquakes and low frequency tectonic tremors is further tested on continuous data from southwestern japan

rapid identification of earthquake rupture plane using source scanning algorithm summary accurate identification of an earthquake s rupture plane is critical in many aspects ranging from scientific such as seismology and plate tectonics to societal such as emergency rescue planning we modify the recently developed source scanning algorithm ssa to rapidly image the rupture pattern of an earthquake using waveform data recorded at local and regional distances the method calculates the brightness function of a grid point by summing the observed amplitudes of the p wave envelopes at the corresponding predicted arrival times at all stations the composite image consisting of brightness functions of all grid points within a prescribed source volume and time interval illuminates the locations on the rupture plane where significant seismic energy is emitted synthetic tests indicate that the proposed method is robust even under the presence of hypocentral mislocation and origin time errors application of this method to the 2003 san simeon and 2004 parkfield earthquakes both occurred in central california clearly identifies the nw se striking planes as the actual rupture planes a conclusion consistent with all available evidences limited azimuthal distribution of seismic stations will not severely affect the identification result making this method ideal for offshore earthquake studies because the proposed method is able to identify the rupture plane using seismic waveform data only it is perfectly suited for near real time operation and may enable routine report of earthquake rupture planes by local seismic networks in the future

delineating complex spatiotemporal distribution of earthquake aftershocks an improved source scanning algorithm summary conventional earthquake location methods depend critically on the correct identification of seismic phases and their arrival times from seismograms accurate phase picking is particularly difficult for aftershocks that occur closely in time and space mostly because of the ambiguity of correlating the same phase at different stations in this study we introduce an improved source scanning algorithm issa for the purpose of delineating the complex distribution of aftershocks without time consuming and labour intensive phase picking procedures the improvements include the application of a ground motion analyser to separate p and s waves the automatic adjustment of time windows for brightness calculation based on the scanning resolution and a modified brightness function to combine constraints from multiple phases synthetic experiments simulating a challenging scenario are conducted to demonstrate the robustness of the issa the method is applied to a field data set selected from the ocean bottom seismograph records of an offshore aftershock sequence southwest of taiwan although visual inspection of the seismograms is ambiguous our issa analysis clearly delineates two events that can best explain the observed waveform pattern

image domain velocity inversion and event location for microseismic monitoring abstractmicroseismic event locations obtained from seismic monitoring data sets are often a primary means of determining the success of fluid injection programs such as hydraulic fracturing for oil and gas extraction geothermal projects and wastewater injection event locations help the decision makers to evaluate whether operations conform to expectations or parameters need to be changed and may be used to help assess and reduce the risk of induced seismicity however obtaining accurate event location estimates requires an accurate velocity model which is not available at most injection sites common velocity updating techniques require picking arrivals on individual seismograms this can be problematic in microseismic monitoring particularly for surface acquisition due to the low signal to noise ratio of the arrivals we have developed a full wavefield adjoint state method for locating seismic events while inverting for p and s wave velocity models that optimally focus multiple complementary ima

picking versus stacking in a modern microearthquake location comparison of results from a surface passive seismic monitoring array in oklahoma abstractwe present location results for a group of 200 microearthquakes that occurred in 2012 in a region of oklahoma hosting ongoing exploration activities using a local passive surface seismic monitoring network of 15 broadband stations we applied two modern location techniques that use fundamentally different approaches the first is a pick based double difference relocation method with waveform crosscorrelation multiple event location techniques such as these are generally regarded as the best approach for obtaining high precision locations from pick data the second approach is an automated waveform migration stacking method these types of methods are becoming increasingly common due to increasing network station density and computer power the results from the two methods show excellent agreement and provide similar results for the interpreter both methods reveal spatial and temporal patterns in the locations that are not visible in results obtained using a more traditional pick based approach

impact cratering on venus physical and mechanical models results are presented on laboratory theoretical and numerical simulations of various phenomena related to impact cratering processes specific to the venusian environment such processes are influenced mainly by two factors the high atmospheric density and the high surface temperature model results are presented for the influence of the atmosphere on high velocity meteoroids the influence of high velocity meteoroids on the atmosphere the influence of atmospheric shock waves on the venusian surface and the influence of high crustal temperature on impact melting and melt cooling new modeling techniques based on the two dimensional free lagrangian method have been employed and show that an icy body of 1 km radius may reach the bottom of the venusian atmosphere in a partially dispersed state and form an impact crater approximately 20 km in diameter a significant amount of energy is transferred to the venusian atmosphere during meteoroid transit and the gas dynamic flow will be similar to a strong explosion modeling shows that a significant wake of low density will form behind the meteoroid and have an important influence on crater formation and ejecta distribution according to our estimates the boundary of the flow of dense gas may correspond to the region of hummocky ejecta deposition observed for venusian impact craters the collapse of the gas rock debris column on the boundary between the low and high density regions may result in a turbiditelike flow outside the hummocky ejecta area we carried out preliminary laboratory simulations of the interaction of meteoroid shock waves with the surface considerable surface disruption is possible even under the trajectory of an oblique impact the possibility of back venting caused by shock wave pressuring and depressuring in ground pores may provide an explanation for the origin of dark margins the differences between venus and earth for impact melting and melt cooling were examined melt generation on venus should be 20 to 40 greater than on earth for the same crater diameter melt volumes should stay molten about an order of magnitude longer on venus leading to substantial melt flows and smooth crater floors

coalescence microseismic mapping earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations if mis picks are made these will contribute to the location therefore causing potential bias for data recorded by a dense seismic array direct imaging methods can be applied instead we describe the coalescence microseismic mapping method which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array by continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio temporal coordinates of the origins of seismic events noisy data are migrated away from the correct origin so do not contribute to errors in location the method is rooted in a bayesian formulation of event location traveltime inversion allows imaging of source locations and has the capacity to handle errors in modelled traveltimes it has the advantage of working with any 3 d velocity model which therefore may include anisotropy it also automatically incorporates both p and s wave data a multiresolution grid search leads to an efficient implementation with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality we discuss the theory and implementation of this method and illustrate it with real data from microseismic events in iceland caused by melt intrusion in the crust

simultaneous microseismic event localization and source mechanism determination abstractmicroseismic monitoring has become a tool of choice for the development and optimization of oil and gas production from unconventional reservoirs the primary objective of micro seismic monitoring includes localization of micro seismic events and characterization of their source mechanisms most seismic events are of a nonexplosive nature and thus there are waveform polarity differences among receivers specifically double couple sources represented a challenge for migration based localization techniques we developed and applied a new migration type location technique combined with source mechanism inversion that allowed for constructive interference of signal in seismic waveforms the procedure included constructing image functions by stacking the amplitudes with compensated polarity changes the compensation weights were calculated by using moment tensor inversion this method did not require any picking of arrivals at individual receivers but it required receivers to be distributed i

real time event location using model based estimation of arrival times and back azimuths of seismic phases abstracti have devised a real time procedure for locating events using an estimation method that analyzes arrival times and back azimuths of phases the new procedure is applicable to data acquired by local array receivers such as those used in single well monitoring as well as by dense receiver networks and also to noisy waveforms such as those observed in hydraulic fracturing monitoring if the signal to noise ratio is greater than approximately 6 db the new procedure uses coalescence microseismic mapping to obtain predictions of arrival times based on these predictions arrival times were estimated by picking the maximum of the ratio of the short term average to the long term average of a characteristic function computed for waveforms in an appropriate time window the estimated arrival times were used in a probabilistic location method and the probability density function pdf of the event location was generated to locate events for a local array of receivers the pdfs of event back azimuths ob

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

recent deformation rates on venus constraints on the recent geological evolution of venus may be provided by quantitative estimates of the rates of the principal resurfacing processes volcanism and tectonism this paper focuses on the latter using impact craters as strain indicators the total postimpact tectonic strain lies in the range 0 5 6 5 which defines a recent mean strain rate of 10 18 10 17 s 1 when divided by the mean surface age interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 ma catastrophic resurfacing model if distributed tectonic resurfacing contributed strongly before that time as suggested by the widespread occurrence of tessera as inliers the mean global strain rate must have been at least 10 155 s 1 which is also typical of terrestrial active margins numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years parameterized convection models predict that the mean global thermal gradient decreases by only about 5 k km over this time even with the exponential dependence of viscosity upon temperature the surface strain rate drops by little more than one order of magnitude strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model an alternative uniformitarian resurfacing hypothesis holds that venus is resurfaced in quasi random patches several hundred kilometers in size that occur in response to changing mantle convection patterns under such a model the observed crater strain distribution indicates that about 1 of the planet s surface is tectonically active at any time however this model requires a very weak crustal rheology to achieve surface velocities 100 mm yr appropriate to the required patch size without well developed lateral weak zones venus is essentially a one plate planet but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement the net rate of tectonic activity is logarithmically intermediate between earth and mars about 100 times slower than plate tectonics but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

wavefield extrapolation of body waves for 3 d imaging of earthquake sources summary reverse time imaging of earthquake source parameters is extended from two to three spatial dimensions and from two to three component recordings provided that the recording aperture is sufficiently large and the data are not spatially aliased source radiation patterns in both time and space can be reconstructed by elastic reverse time propagation of body waves such reconstructions are necessarily partial as only the energy that was recorded is available for reconstruction for a point source the origin time and the 3 d spatial location of an event can be reconstructed by extracting the time and position of the best focused energy from the backward propagating wavefield for a spatially and temporally extended source biased estimates of fault position and rupture slip time history can be estimated if the recording aperture is sufficient for smaller recording apertures the time history can still be estimated if the fault location and geometry are known a priori the latter is viable for a reasonable number 200 of three component recordings and so is potentially applicable to real data all these processes assume that a smoothed representation of the 3 d velocity distribution in the volume containing the source and receivers is available

moment tensor migration imaging we develop and apply an imaging procedure for simultaneous location and characterization of seismic source properties called moment tensor migration imaging the procedure constructs images for moment tensor components using a weighted diffraction stack migration and combines ray theoretical green s functions with a reverse time moment tensor imaging methodology by applying an approximation we term the ray angles only approximation we form an expression for moment tensor migration imaging where the migration weights depend only on the take off and arrival angles for rays leaving receiver positions and incident upon the image points moment tensor migration imaging retains the benefits of diffraction stack procedures for source location and characterization namely speed flexibility and the potential for incorporating non linear stacking procedures whilst also providing the benefits of moment tensor imaging such as the inclusion of multiple phase and multiple component data the collapsing of the source radiation pattern estimation of the moment tensor we examine variations of the imaging procedure through a synthetic test we show that although the assumptions required for the imaging and ray angles only approximation may not be strictly valid for realistic survey geometries a simple weight adjustment can be used to obtain more accurate and stable results in these situations in our synthetic example we find that the use of a p wave only migration without this reweighting structure produces poor results whereby the resulting images show activity upon incorrect moment tensor components however many of these effects are mitigated by use of the reweighting scheme and the results are further improved through the introduction of non linear stacking operators such as semblance weighted stacks the highest quality moment tensor images for the synthetic test examined here are obtained through the use of both p wave and s wave wave fields this highlights the importance of multicomponent data and multiphase modelling when characterizing seismic sources we also find that the imaged moment tensor components vary proportionately when the input velocities are perturbed by a scale factor this suggests for the geometry investigated here derived source properties such as fault plane solutions and shear tensile components will not be influenced by bulk changes in seismic velocities finally we show the application to a real microseismic event observed using a surface array during hydraulic fracturing we find that the procedure collapses the seismic radiation pattern into an anomaly with a maximum at the hypocentre and our derived mechanism is consistent with the observed radiation pattern from the source

real time earthquake location using kirchhoff reconstruction real time location of earthquakes can be achieved by using direct imaging of the recorded wave field based on a kirchhoff reconstruction method similar to that used in the migration of seismic reflection data the standard method of event location requires the wave arrival at each sensor to be picked and associated with an event by using direct imaging the event is identified once in the imaged wave field the computation is independent of the level of seismic activity and can be carried out on a typical desktop computer the procedure has been successfully demonstrated in two and three dimensions using data from the southern california seismic network trinet at higher resolutions the reconstruction method can identify finite source effects further work considers extending the method by implementing full elastic theory and solving for moment tensors at all locations in the mesh

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

microseismic image domain velocity inversion marcellus shale case study abstractseismic monitoring at injection wells relies on generating accurate location estimates of detected micro seismicity event location estimates assist in optimizing well and stage spacings assessing potential hazards and establishing causation of larger events the largest impediment to generating accurate location estimates is an accurate velocity model for surface based monitoring the model should capture 3d velocity variation yet rarely is the laterally heterogeneous nature of the velocity field captured another complication for surface monitoring is that the data often suffer from low signal to noise levels making velocity updating with established techniques difficult due to uncertainties in the arrival picks we use surface monitored field data to demonstrate that a new method requiring no arrival picking can improve microseismic locations by jointly locating events and updating 3d p and s wave velocity models through image domain adjoint state tomography this approach creates a co

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels