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

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

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

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

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

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

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

reflection imaging with earthquake sources and dense arrays the mw 5 8 earthquake that occurred in louisa county virginia on 23 august 2011 provided an opportunity to record with several high density seismic arrays in addition to traditional sparse temporary seismic networks traditional aftershock networks consist of a few dozen stations spread over tens of kilometers as a result the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable here we report the results of recording with a large number of oil industry type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments the objective was to image subsurface structure with array methods using the aftershocks as sources the dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200 400 m and a 60 km long three component regional profile with stations every 2 km we demonstrate how processing techniques from vertical seismic profiling can produce high resolution 3 d reflection images of structure beneath the array these images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the u s geological survey of particular interest is a strong reflector imaged across multiple profiles our analysis demonstrates how a surface array of seismometers can provide 3 d images of structure using microearthquake sources when wavefields are sampled sufficiently densely

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

wrinkle ridges stress domains and kinematics of venusian plains wrinkle ridges are nearly ubiquitous landforms on the plains of venus by analogy with similar structures on other planets venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust an inference that is verified by geometrical relationships with positive and negative relief features on venus because plains are the dominant terrain on venus wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet in most places there are two or more sets of wrinkle ridges and commonly one of these persists over a very large area defining a regional stress domain intersection relationships indicate that these domains differ in age

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

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

bell regio venus integration of remote sensing data and terrestrial analogs for geologic analysis the geology and surface morphology of bell regio 18 42 n 32 58 e are investigated using a combination of magellan venera and analogous terrestrial data the properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models five major volcanic sources are identified from geologic mapping tepev mons nefertiti corona a large shield volcano east of tepev and two small edifices southeast of tepev the volcano api mons lies northeast of the main bell uplift the oldest volcanic units are associated with an extensive low shield volcano east of tepev mons and a small edifice southeast of tepev the annular flow apron of tepev mons formed next with volcanism at a second small edifice on the southeast flank of tepev mons producing the youngest flow units comparisons between magellan data terrrestrial radar images and field topography profiles suggest that only three units resemble terrestrial a a flows the remainder are consistent with smoother pahoehoe type surfaces this suggests that most of the flow units were erupted at relatively low volume effusion rates 100 m3 s over long periods of time or had very low viscosities one flow unit has a knobby texture which resembles large tumuli or ridge structures much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain there are no rift zones such as those which occur at beta regio atla regio or western eistla regio tepev mons is characterized by very steep slopes up to 40 along the east flank a relatively flat summit and two large 11 and 31 km calderas though covered in places by high dielectric material the tepev mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas these calderas are quite shallow on the order of a few hundred meters at most with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature magellan and venera data suggest that the eastern caldera is covered by a layer of unconsolidated material ejecta from miriam crater has produced a halo with a dielectric constant of 6 7 surrounding nefertiti corona and potanina crater ejecta is the likely source material for radar dark wind streaks south of tepev mons a low dielectric e 2 3 triangular region on the south flank of tepev mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind this region has undergone a wide variety of volcanic processes with a major shift in eruptive style from the older low relief eastern volcanic center to the steep slopes and large summit calderas characteristic of tepev mons and the two smaller edifices

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

interpretation of planetary radar observations the relationship between actual and inferred slope distributions we examined the distribution of surface slopes of a variety of terrestrial surfaces by field measurement representing surfaces formed by a wide range of processes and compared the results to planetary radar data slope distributions of the measured surfaces differed considerably from the distributions assumed by accepted models of radar scattering the rms slope values for the terrestrial surfaces ranged from 0 to 16 5 compared to a range of rms slope values of 1 to about 10 for planetary surfaces as inferred from radar observations while the great majority of planetary surfaces observed by radar have rms slope estimates in the lower end of this range 5 nearly all of the terrestrial surfaces we measured have rms slope values greater than 5 we also used hagfors model of radar scattering to predict the return that would be expected from surfaces where two discrete surface types were present within the radar field of view and found that the shapes of the resulting slope distributions differed from those predicted by the hagfors model for homogeneous surfaces additionally the resulting best fit rms slope was a nonlinear combination of those of the pure surfaces emphasizing the smoother surface together these results suggest that current methods of determining surface roughness from radar may significantly underestimate the roughness of planetary surfaces and that the derived rms slope can best be used as a qualitative guide to the physical interpretation of actual surface properties

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

the global distribution of wrinkle ridges on venus abstract new digital mapping of over 65 000 wrinkle ridges of the plains of venus shows a strong correlation between the location and orientation of these long low amplitude compressive anticlines and major features of the geoid and long wavelength topography regions with wrinkle ridges occupy 43 of the plains and are strongly skewed toward low elevations and negative geoid anomalies about 93 of the wrinkle ridge plains lie below mean planetary radius and 72 have negative geoid anomalies in contrast the extensional rift zones of venus are strongly skewed toward high elevations and positive geoid anomalies thus compressive deformation dominates the topographic and geoid lows whereas extensional rifting is generally restricted to the highs these observations are consistent with geoid based stress models that predict compression in geoid lows and extension in geoid highs the orientations of wrinkle ridges are generally consistent over regions extending for 1000 10 000 km many regions have multiple sets of wrinkle ridges of different orientations reflecting multiple episodes and directions of compression however about 80 of the regions display a single dominant wrinkle ridge orientation with secondary orientations subordinate the dominant wrinkle ridge orientations in many regions follow the contours of long wavelength topography and geoid or lie along the axes of troughs in the geoid thus the maximum horizontal compression recorded by the folds is commonly parallel to the present day gradient in geoid and topography although some regions are strong exceptions perhaps reflecting changes in topography and geoid with time the dominant wrinkle ridge trends ring several major geoid and topographic swells especially western aphrodite terra and lada terra with ring diameters of 75 120 8000 13 000 km in addition there are smaller rings of wrinkle ridges around the swells in themis eistla and bell regiones especially gula mons with ring diameters of 25 45 2500 5000 km in contrast there are no rings of compressive deformation surrounding some other geoid topographic highs including beta and atla regiones which have been proposed to be the youngest swells and display the largest peak geoid topography ratios and largest geoid gradients these observations are consistent with the idea that not all the major topographic geoid swells existed on venus at the time of wrinkle ridge formation

resolving focal depth in sparse network with local depth phase spl a case study for the 2011 mineral virginia earthquake sequenceresolving focal depth in sparse network with local depth phase spl

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

archaean cambrian crustal development of east antarctica metamorphic characteristics and tectonic implications abstract the east antarctic shield consists of a variety of archaean and proterozoic cambrian high grade terranes that have distinct crustal histories and were amalgamated at various times in the precambrian cambrian high grade pan african tectonism at 600 500ma is recognized from four distinct belts the dronning maud land lutzow holm bay prydz bay and denman glacier belts these high grade belts juxtapose distinct mesoproterozoic and neoproterozoic crustal provinces maud rayner and wilkes the rauer terrane and have also marginally affected archaean cratonic remnants in the napier complex and southern prince charles mountains the wilkes province experienced its principal tectonothermal events prior to 1130ma and was not affected by the younger events that characterize the maud province 1150 and 1030 990ma the rayner province 990 920ma and the rauer terrane 1030 990ma these differences between the isotopic event records of the basement provinces now separated by the pan african belts require that the older provinces were not formerly parts of a continuous grenville belt as proposed in the sw us east antartic model east antarctica was not a single unified crustal block within either east gondwana or rodinia until the cambrian which is now demonstrated to be the key phase of high grade and ultrahigh temperature uht metamorphism associated with supercontinent assembly the high grade pan african tectonism is characterized by extensive infracrustal melting clock wise p t paths rapid post peak exhumation along isothermal decompression paths to shallow or mid crustal levels by 500 ma and the generation at least locally of uht conditions a significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust these one dimensional models are inconsistent with present crustal thicknesses of 25 35km in the pan african domains of the east antarctic shield

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

flow and fracture maps for basaltic rock deformation at high temperatures understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state pressure and temperature and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems in particular it is essential to understand how rock strength at high temperatures is limited by fracture we have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps we have fitted an empirical flow law norton s law and a theoretical fracture criterion to these data the principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate temperature and pressure we have used these maps to re interpret the ductile brittle transition in basaltic rocks at high temperatures and show conceptually how these failure maps can be applied to volcanic systems using lava flows as an example c 2002 elsevier science b v all rights reserved

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

mechanical energy balance and apparent fracture toughness for dykes in elastoplastic host rock with large scale yielding the dynamics of dyke emplacement are typically modelled by assuming an elastic rheology for the host rock however the resulting stress field predicts significant shear failure in the region surro

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

experimental and theoretical fracture mechanics applied to antarctic ice fracture and surface crevassing recent disintegration of ice shelves on the antarctic peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally in this paper we present a fracture criterion incorporating new experimental fracture data coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the filchner ronne ice shelf we have developed experiments that have enabled us to quantify for the first time quasi stable crack growth in antarctic ice core specimens using a fracture initiation toughness kinit for which crack growth commences the tests cover a full range of near surface densities 560 871 kg m 3 10 9 75 7 m depth results indicate an apparently linear dependence of fracture toughness on porosity such that kinit 0 257 80 7 predicting a zero porosity toughness of ko 155 kpa m1 2 we have used this data to test the applicability to crevassing of a two dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field the growth of an initial flaw into a larger crevasse which involves a purely tensile crack opening depends on the size of the flaw the magnitude of kinit and the nature of the applied stress field by incorporating the criterion into a stress map of the filchner ronne ice shelf derived from a depth integrated finite element model of the strain rate field we have been able to predict regions of potential crevassing these agree well with satellite imagery provided an initial flaw size is assumed in the range 5 50 cm

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

the evolution of tectonic features on ganymede the bands of bright resurfaced terrain on ganymede are probably broad grabens formed by global expansion and filled with deposits of ice grooves within the bands are thought to be extensional features formed during the same episode of expansion the crust of ganymede is modeled as a viscoelastic material subjected to extensional strain with sufficiently high strain rates and stresses deep normal faulting will occur creating broad grabens that may then be filled continuing deformation at high strain rates and stresses will cause propagation of deep faults up into the flood deposits and normal faulting at the surface while lower strain rates and stresses will cause formation of open extension fractures or if the crustal strength is very low grabens at the surface the spacing between adjacent fractures may reflect the geothermal gradient at the time of deformation surface topography resulting from fracturing and normal faulting will decay with time as a result of viscous relaxation and mass wasting

experimental fracture and mechanical properties of antarctic ice preliminary results an experimental study of the fracture mechanics and rheology of ice from the ronne ice shelf antarctica is currently being undertaken the apparent critical stress intensity factor or apparent fracture toughness k q for crack propagation has been measured using a three point bend method for inducing crack growth perpendicular to the axis of cylindrical ice core specimens tensile crack nucleation under applied uniaxial compressive stress has also been evaluated both methods have allowed a profile of ice elastic and fracture properties with depth through the ice shelf to be constructed preliminary results indicate that the measured elastic modulus increases with depth through the firn and upper meteoric ice before reaching a constant value in the deeper dense meteoric and basal marine ice the resistance to fracture as measured by changes in apparent fracture toughness and crack nucleation stress increases with depth right through the firn and meteoric ice layers a simple fracture mechanics model applied to the ronne ice shelf indicates that crevasses form from small surface cracks less than 40 cm deep which quickly grow to depths of 40 60m and then remain stable

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

evidence for active hotspots on venus from analysis of magellan gravity data abstract the 500 myr average crater retention age for venus has raised questions about the present day level of tectonic activity in this study we examine the relationship between the gravity and topography of four large volcanic swells beta atla bell and western eistla regiones for clues about their stage of evolution the magellan line of sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation local compensation a flexural model with local and regional compensation of surface and subsurface loads and a hotspot model of compensation that includes top loading by volcanoes and subsurface loading due to a deep low density mass anomaly the coherence is also calculated in each region but yields an elastic thickness estimate only at bell regio in all models the long wavelengths are compensated locally the long wavelength estimates of the deep compensation depth for bell atla western eistla and beta regiones are 125 175 200 and 225 km respectively with an error of approximately 35 km the flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at bell regio and from other studies that model the topographic expression of flexure at bell regio and in other tectonic settings globally assuming a crustal thickness of 30 km the fit to the hotspot model at atla regio gives an effective elastic thickness of 30 5 km atla regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature at bell regio effective elastic thickness is 30 5 km at short wavelengths and 50 5 km at long wavelengths using a crustal thickness of 10 km the 30 km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced the 50 km value is believed to reflect the present day effective elastic thickness bell regio has a relatively shallow compensation depth 125 km and a weak bottom to top loading ratio determined from the coherence spectra these results may indicate a relatively old possibly inactive plume the data at beta and western eistla regiones are of relatively poor quality and do not permit detailed interpretations these areas are interpreted as active hotspots because of their large compensation depths greater than typical of the rest of the planet

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

conductive cooling of lava columnar joint diameter and stria width as functions of cooling rate and thermal gradient abstract much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate t t and thermal gradient t x at the point of fracture as the flow cools some evidence suggests that lava flows cool conductively a conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies both for extrusive and intrusive settings thermal gradient and cooling rate are then linked to stria width and column diameter with faster cooling leading to narrower columns and steeper gradients to narrower striae this model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant and predicts that the ratio of stria width to column diameter is also nearly constant this prediction is consistent with field observations both in this and other works the model also suggests that if conductive cooling dominates then joint spacing i e column diameter should increase inward from the margins of the body as should stria width

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

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

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

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

small scale fracture patterns on the volcanic plains of venus some of the most striking features of the magellan radar images of venus are the intricate and varied deformation patterns on the volcanic plains particularly puzzling are remarkably linear features with a regular spacing of about 1 km first observed in the gridded plains of guinevere planitia the very close spacing of these features requires an unreasonably thin lithospheric layer in conventional geophysical models for regular spacing we have found many sets of parallel regularly spaced lineations similar to the closely spaced features of the gridded plains they are composed of parallel thin straight lineations whose microwave reflectivity does not appear to depend on radar illumination direction these sets of parallel lineations which we interpret to be fractures typically cover areas with dimensions of hundreds of kilometers several examples of these regular lineations are characterized in terms of their average fracture spacing which is between 1 and 2 5 km the scatter in individual spacings is about 1 3 the average based on observations we hypothesize that these features are extension fractures in the brittle upper layers of the volcanic plains material which formed well after emplacement and solidification of the flows for parallel extension cracks models based on the stress distribution around a crack and observations of regular jointing in sedimentary layers predict that the spacing of fractures should be roughly equal to the depth of fracturing or to the thickness of the jointed layer however we observe that fracture sets in diverse locations all have nearly the same spacing implying either that the fractures always form with a depth extent of about 1 km or that the spacing is independent of the thickness of the layer containing the fractures we propose a model based on a shear lag mechanism used by materials scientists to explain morphologically similar patterns of regularly spaced tensile cracks on the brittle surfaces of layered composites in this model lithospheric extension causes a fracture to form in a surface layer of basalt several hundred meters to a kilometer thick a horizontal detachment corresponding to an older plains surface below the fractured layer allows frictionally resisted lateral displacement of the surface layer in the region near the extension fracture the model gives a spacing that is independent of layer thickness and is able to reproduce the observed fracture spacing using reasonable values of material constants it provides a framework for using these sets of lineations to help determine mechanical properties of the volcanic plains and to place constraints on the sequence of their tectonic evolution another unresolved problem of the gridded plains has been the presence of an orthogonal set of brighter structures with a markedly different appearance and a somewhat larger scale of spacing we utilize the observed morphology and self similarity of feature lengths to determine the reasons for their contrasting morphology it is concluded that they are primarily extensional structures which penetrate the entire 2 4 km thick brittle crustal layer and which are likely to have accommodated some shear deformation

tectonic processes on europa tidal stresses mechanical response and visible features abstract europa s orbital eccentricity driven by the resonance with io and ganymede results in diurnal tides 3 5 day period and possibly in nonsynchronous rotation both diurnal variation and nonsynchronous rotation can create significant stress fields on europa s surface and both effects may produce cracking patterns and time sequences of apparent tectonic features on europa include lineaments that correlate with both sources of stress if we take into account nonsynchronous rotation after initial crack formation by amounts ranging up to several tens of degrees for example the crosscutting time sequence of features in the cadmus and minos linea region is consistent with a combined diurnal and nonsynchronous tensile stress field as it evolves during tens of degrees of nonsynchronous rotation constraints on the rotation rate from comparing voyager and galileo images show that significant rotation requires 104yr but could be fast enough to have allowed significant rotation since the last global resurfacing even if such resurfacing was as recent as a few million years ago once cracking is initiated diurnal tides work cracks so that they open and close daily although the daily effect is small over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures according to a model in which underlying liquid water fills the open cracks partially freezes and is extruded during the daily closing of the cracks thus several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer

climate and interior coupled evolution on venus climate interior coupled evolution is investigated for venus by merging a partial melting parameterized mantle convection model with a gray radiative convective atmospheric model a positive feedback process can operate by the release of water to the atmosphere via mantle melting leading to an increase in atmospheric opacity and the radiative temperature gradient the resulting amplification of the greenhouse surface temperature raises the mantle temperature leading to an increase in the partial melting rate using thin lid convection a coupled model for venus running over an interval of 3 to 1 ga shows a significant increase in surface temperature partial melting extent and extrusive magma flux compared to a model where there is no communication between greenhouse modulated surface temperature and partial melting in the interior coupled and uncoupled models that transition with variable timing to stagnant lid convection either shut down partial melting via lithospheric thickening or evolve to large amounts of melt due to increased interior temperatures

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

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

experimental and theoretical fracture mechanics applied to fracture of the crust of venus mapping of closely spaced parallel extensional fractures in the guinevere and sedna planitia regions of venus reveals a concentric pattern of fractures around the edge of the large topographic rise of western eistla regio we have constructed 13 transects through these closely spaced parallel fractures cspf and find a mean spacing of between 0 8 and 1 2 km a two dimensional nonlayered fracture mechanics computer model for the formation of cspf is described for cracks extended by a remote tensile stress the stress intensity factor controls the depth of penetration which in turn governs the spacing between adjacent cracks based upon the stress shadow principle the stress required to initiate cracks depends upon the strength of the material thus spacing of cspf depends only on the physical properties of the preexisting rock mass using a new fracture mechanics apparatus designed to simulate venusian conditions 90 bar co2 450c the fracture toughness of basalt was measured for confining pressures up to 20 mpa and for temperature up to 600c fracture toughness was found to increase from 2 4 mpam1 2 at ambient pressure to 3 0 mpam1 2 at 10 mpa confining pressure fracture toughness showed no clear trend with temperature the experimental results for fracture toughness suggest that the preexisting rock mass that fits best with observations contains an inverse square distribution of flaws with maximum sizes of only 20 80 cm and that a remote tensile stress of 3 6 mpa is required to form the observed cspf

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

sedimentary provenance and age of metamorphism of the vestfold hills east antarctica evidence for a piece of chinese antarctica abstract the vestfold hills terrane in east antarctica is a granulite facies terrane that preserves a record of sedimentation magmatism and metamorphism that differs significantly from the other archaean terranes exposed in east antarctica sensitive high resolution ion mass spectrometry shrimp u pb isotopic data from detrital zircon cores metamorphic rims and metamorphic zircon and monazite from the granulite facies chelnok and taynaya paragneisses that demonstrate archaean ages that are incompatible with the established ages of other east antarctic source regions the original sediments to the chelnok and taynaya paragneisses were deposited between c 2575 and c 2520 ma and underwent a protracted period of high grade granulite facies metamorphism from c 2520 to c 2450 ma that is synchronous with the emplacement of the tonalitic mossel gneiss and the granitic to gabbroic rocks of the crooked lake gneiss group the distinctive detrital zircon age populations and the period metamorphism recorded by the vestfold hills paragneisses lead us to speculate that 1 these rocks were sourced from and deposited on the margin of the crystalline basement to the north china craton at the end of the archaean and 2 they underwent a period of high grade metamorphism related to orogenesis driven by the collision of the north china craton with the archaean nucleus of south india during the late archaean to early paleoproterozoic

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

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

poludnista dorsa venus history and context of a deformation belt 1 rolling lowlands comprise 80 of venus s surface and thus developing a geological understating of resurfacing and tectonism is critical for our understanding of venus evolution in this paper global regional and local approaches to interpreting magellan synthetic aperture radar and altimetry radar data are used to constrain modes of lowland evolution convection processes and lithospheric structure detailed geologic mapping is combined with altimetry data test models of deformation belt evolution in venus s lowlands poludnista dorsa a complex 2000 km long deformation belt is highly segmented and broadly time transgressive long wavelength deformation is spatially independent of local short wavelength deformation within the deformation belt and temporally predates regional deformation marked by wrinkle ridges a mechanically layered lithosphere of regional extent is not required by the local geologic history local lithospheric thinning or progressive regional thickening of the mechanical layer better explains the observed sequence of events

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

experimental determination of the emplacement pressure of proterozoic mafic dikes within portions of the east antarctic shield abstract in order to further constrain previous determinations of the p t pressure time path of two portions of the east antarctic shield the depth pressure under which two proterozoic dike suites were emplaced in the vestfold hills block was determined by experimentally reproducing the equilibrium phenocryst assemblage found in their chilled margins the phenocryst assemblage oliv opx of a high mg tholeiite sample ca 2420 ma could only be experimentally reproduced at 7 5 kbar it was also found that at 7 5 kbar the ca mg fe components of the synthetic orthopyroxene very closely matched the natural phenocryst concentrations based on high pressure experiments the phenocryst assemblage in a sample from the ca 1360 ma fe rich tholeiite suite oliv opx plag opx is only consistent with dike emplacement at pressures 5 5 kbar while the mg values of the natural clinopyroxene phenocrysts match the experimental pyroxene compositions at pressures of 4 5 kbar these results are consistent with pressures of 8 0 and 5 3 kbar obtained for the mg and fe rich tholeiite samples respectively using geobarometry applied to the groundmass assemblages emplacement pressures of compositionally similar dikes from the napier complex were estimated by comparison with previous experimental studies the comparison shows that a napier complex high mg tholeiite suite ca 2350 ma and an fe rich tholeiite suite ca 1190 ma were both emplaced at a pressure 8 kbar

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

tectonic evolution of bell regio venus regional stress lithospheric flexure and edifice stresses in order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of venus we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in bell regio using magellan synthetic aperture radar sar images and altimeter measurements of topography the major volcanoes of bell regio tepev mons and nyx mons exhibit tectonic characteristics that are unique relative to other volcanic edifices on venus the most prominent distinction is the lack of large rift zones within the overall highland uplift which characterize many other highland rises on venus also previous studies have determined that many large venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on earth and mars tepev and nyx montes exhibit both the radial tectonic features associated with other venusian edifices and numerous concentric graben nyx mons implies a more distributed magmatic system by its broad shape radial chains of pit craters and expansive flow fields whereas tepev mons is a more centralized volcanic system with limited associated long flows we investigate the regional stresses associated with bell regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading modeling both nyx mons and tepev mons as axisymmetric loads with gaussian mass distributions on an elastic plate the relationship between the tectonic features surrounding tepev mons and stresses associated with magma chamber inflation are also examined through finite element analysis using topography data to model the shape of the volcano we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in magellan images these results imply a shift in volcanic style within bell regio from an early phase of broad low shield formation to later steep sided more centralized edifice development such changes are consistent with an increase in the thickness of the lithosphere over time

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