magma chamber and crustal channel flow structures in the tengchong volcano area from 3 d mt inversion at the intracontinental block boundary southeast of the tibetan plateau

high resolution seismic velocities and shallow structure of the san andreas fault zone at middle mountain parkfield california a 5 km long high resolution seismic imaging survey across the san andreas fault saf zone and the proposed san andreas fault observatory at depth safod drill site near parkfield california shows that velocities vary both laterally and vertically velocities range from 4 0 km sec probably correspond to granitic rock of the salinian block which is exposed a few kilometers southwest of the saf the depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed safod site we observe a prominent lateral low velocity zone lvz beneath and southwest of the surface trace of the saf the lvz is about 1 5 km wide at 300 m depth but tapers to about 600 m wide at 750 m depth at the maximum depth of the velocity model 750 m the lvz is centered approximately 400 m southwest of the surface trace of the saf similar velocities and velocity gradients are observed at comparable depths on both sides of the lvz suggesting that the lvz is anomalous relative to rocks on either side of it velocities within the lvz are lower than those of san andreas fault gouge and the lvz is also anomalous with respect to gravity magnetic and resistivity measurements because of its proximity to the surface trace of the saf it is tempting to suggest that the lvz represents a zone of fractured crystalline rocks at depth however the lvz instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the saf such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver suggesting a zone of fault strands at least 1 5 km wide at a depth of 300 m tapering to about 600 m wide at 750 m depth fluids within the sedimentary sliver are probably responsible for observed low resistivity values

magnetotelluric imaging of the median tectonic line in western shikoku southwest japan implications of the fault related low resistivity zone abstract we carried out a magnetotelluric mt survey along a profile crossing the median tectonic line mtl in western shikoku japan the mtl is a terrane boundary that formed during the cretaceous between the sanbagawa belt consisting of high pressure temperature p t metamorphic rocks and the ryoke belt of granites and low p t metamorphic rocks the mt image shows a zone of remarkably low resistivity dipping northward at 40 at the surface coincides with the surface trace of the mtl the low resistivity zone probably corresponds to a fluid filled damaged zone of porous media composed of clay minerals and cracked rocks formed by repeated faulting of the mtl since late cretaceous time the calculated maximum porosity of the damaged zone is 7 1 which is clearly higher than that of the non damaged crystalline rocks

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

joint inversion of body wave arrival times and surface wave dispersion for three dimensional seismic structure around safod we incorporate body wave arrival time and surface wave dispersion data into a joint inversion for three dimensional p wave and s wave velocity structure of the crust surrounding the site of the san andreas fault observatory at depth the contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations we find that the trade off between fitting the two data types controlled by the weighting defines a clear optimal solution varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type all the acceptable solutions yield structures with similar primary features but the smaller scale features change substantially when there is a lower relative weight on the surface wave data it appears that the solution over fits the body wave data leading to a relatively rough vs model whereas for the optimal weighting we obtain a relatively smooth model that is able to fit both the body wave and surface wave observations adequately

magnetotelluric evidence for thick skinned tectonics in central taiwan taiwan is the type example of an arc continent collision numerous tectonic models have been proposed for this orogen and include both thin skinned and thick skinned lithospheric deformation these models predict very different structures at middle and lower crustal depths but insuffi cient geophysical data exist to unequivocally distinguish between them long period magnetotelluric mt data were collected in central taiwan in 2006 2007 to constrain the crustal resistivity structure a two dimensional inversion of these mt data revealed a prominent electrical conductor that extends across the decollement predicted by the thin skinned model this feature is interpreted to be due to 1 2 saline fl uids and is inconsistent with the thin skinned model in contrast the thick skinned model predicts this feature since fl uids are generated in the crustal root through metamorphism quantitative correlation of the resistivity and seismic velocity models supports small volume high salinity fl uids in a thickened crust as the cause of this conductor

internal structure of the precordilleran fault system chile insights from structural and geophysical observations abstract geological field work and audiomagnetotelluric data from two profiles crossing the west fissure zone in northern chile were used to describe the geometry and structure of the fracture damaged zone surrounding this strike slip fault system the w e profiles show that the width of the west fissure zone based on the fracture density distribution derived from aerial photographs is 4000 m on profile a and 7000 m on profile b the estimated widths correspond roughly to the region where the fault is kinematically uniform the ratio of the fault width to the fault length ranges from 0 024 to 0 041 this ratio compares favourably with the measured ratio of small metre scale natural faults audio frequency magnetotelluric imaging shows low electrical resistivity zones resistivity of 5 30 m coincident with the mapped surface traces of the fault however these zones are very narrow width about 100 m and only 50 m profile a and 200 m profile b deep respectively on profile b the shallow high conductive zone is underlain by a resistive zone 1000 m the conductivity enhancement in these shallow and narrow zones contrasts with the broad process zone revealed by structural analysis we assume that the conductivity enhancement is due to meteoric water entering a zone of ruptured rocks along the fault trace fault core at present we have no indication for seismic slip along the investigated segments of the west fissure zone fault models have shown that during aseismic periods of fault evolution fault healing i e strength recovery due to compaction and cementation is active fluids and fluid transport which are probably responsible for enhanced conductivity are confined to the remaining fractures of the fault core at shallow depth

the role of fluids in faulting deformation a case study from the dead sea transform jordan the geochemistry of carbonate fault rocks has been examined in two areas of the arava fault segment which forms the major branch of the dead sea transform between the dead sea and the gulf of aquaba the role of fluids in faulting deformation in the selected fault segment is remarkably different from observations at other major fault zones our data suggest reduced fluid rock interactions in both areas and limited fluid flow the fault did not act as an important fluid conduit there are no indications that hydrothermal reactions cementation dissolution did change the strength and behavior of the fault zone although the two areas show considerable differences with respect to fluid sources and fluid flow in one area the investigated calcite mineralization reveals an open fluid system with fluids originating from a variety of sources stable isotopes 13c 18o strontium isotopes and trace elements indicate both infiltration of descending meteoric and or sea water and ascending hydrothermal fluids in the other area all geochemical data indicate only local small scale fluid redistribution these fluids were derived from the adjacent limestones under nearly closed system conditions

joint inversion of gravity and arrival time data from parkfield new constraints on structure and hypocenter locations near the safod drill site 1 taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the parkfield california region we generated an image consistent with both types of data among a variety of strategies the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data the preferred model looks essentially the same as the best fit arrival time model in areas where ray coverage is dense with differences being greatest at shallow depths and near the edges of the model where ray paths are few earthquake locations change by no more than about 100 m the general effect being migration of the seismic zone to the northeast closer to the surface trace of the san andreas fault

shallow architecture of the wadi araba fault dead sea transform from high resolution seismic investigations in a high resolution small scale seismic experiment we investigated the shallow structure of the wadi araba fault waf the principal fault strand of the dead sea transform system between the gulf of aqaba eilat and the dead sea the experiment consisted of 8 sub parallel 1 km long seismic lines crossing the waf the recording station spacing was 5 m and the source point distance was 20 m the first break tomography yields insight into the fault structure down to a depth of about 200 m the velocity structure varies from one section to the other which were 1 to 2 km apart but destinct velocity variations along the fault are visible between several profiles the reflection seismic images show positive flower structures and indications for different sedimentary layers at the two sides of the main fault often the superficial sedimentary layers are bent upward close to the waf our results indicate that this section of the fault at shallow depths is characterized by a transpressional regime we detected a 100 to 300 m wide heterogeneous zone of deformed and displaced material which however is not characterized by low seismic velocities at a larger scale at greater depth the geophysical images indicate a blocked cross fault structure the structure revealed fault cores not wider than 10 m are consistent with scaling from wear mechanics and with the low loading to healing ratio anticipated for the fault

electrical resistivity changes not observed with the 28 september 2004 m6 0 parkfield earthquake on the san andreas fault california 1 an 18 a long experiment to capture changes in electrical resistivity prior to during and after a m6 earthquake on the san andreas fault at parkfield california culminated with an event of that magnitude on 28 september 2004 resistivity was monitored with a telluric array giving precisions in measured parameters of 0 2 or less no coseismic resistivity changes greater than 5 could have occurred or they would have been detected by the array and any precursory changes would have been much smaller because maximum possible precursory strains were 4000 times smaller than the coseismic strain given that the earthquake ruptured directly beneath the telluric array this experiment places a bound on the amount resistivity might change at the hypocentral location changes measured with instruments farther from earthquakes can only be smaller it is still possible that a measurable change with a larger earthquake or other fault geometries are capable of generating larger signals but our results show clearly that future experiments should be designed with precisions of much better than 0 1 if they are to be successful

resistivity structure of a seismic gap along the atotsugawa fault japan abstract seismicity along the atotsugawa fault located in central japan shows a clear heterogeneity the central segment of the fault with low seismicity is recognized as a seismic gap although a lot of micro earthquakes occur along this fault in order to elucidate the cause of the heterogeneity in seismicity the electrical resistivity structure was investigated around the atotsugawa fault by using the magnetotelluric mt method the regional geoelectrical strikes are approximately parallel to the fault in a low frequency range we constructed two dimensional resistivity models across the fault using tm mode mt responses to minimize three dimensional effects on the modeling process a smooth inversion algorithm was used and the static shifts on the apparent resistivity were corrected in the inversion process a shallow low resistivity zone along the fault is found from the surface to a depth of 1 2 km in the best fit model across the high seismicity segment of the fault on the other hand the corresponding low resistivity zone along the low seismicity segment is limited to a shallower depth less than 1 km the low resistivity zone along the atotsugawa fault is possibly due to fluid in the fracture zone the segment with higher levels of seismicity may have higher fluid content in the fault zone compared with the lower seismicity segment on a view of the crustal structure a lateral resistivity variation in a depth range of 3 12 km is found below the fault trace in the high seismicity segment while a resistive layer of wide extent is found at a depth of about 5 km below the fault trace in the low seismicity segment the resistive layer is explained by less fluid condition and possibly characterized as high rigidity differences in the resistivity structures between low and high seismicity segments of the fault suggest that the seismic gap in the central part of the atotsugawa fault may be interpreted as a locked segment thus mt is an effective method in evaluating a cause and future activity of seismic gaps along active faults the lower crust appears as a conductive zone beneath the low seismicity segment less conductive beneath the high seismicity segment fluid is inferred as a preferable cause of the conductive zone in this study it is suggested that the conductive lower crust beneath the low seismicity segment is recognized where fluid is trapped by an impermeable layer in the upper crust on the other hand fluid in the lower crust may upwell to the surface along the high seismicity segment of the fault

a magnetotelluric study of the damara belt in namibia 1 regional scale conductivity anomalies abstract the namibian margin is dominated by the late proterozoic to early cambrian fold belts of the damara orogen which wrap around and separate the congo and kalahari cratons this mosaic of relatively soft fold belts and hard cratons apparently controlled the path for the opening of the south atlantic in the early cretaceous the continents split along the coast parallel fold belts of the damara orogen while the inland fold belt damara belt was effected by extension and widespread igneous intrusion but never developed to the rift stage this paper is concerned with the interpretation of magnetotelluric mt data along a 200 km nw se profile across the damara belt in nw namibia the regional two dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones i a generally very resistive upper crust which is typical for the granites and metasediments of the damara belt ii two subvertical conductors in upper to mid crustal levels which correlate with major tectonic zone boundaries and iii a highly conductive middle to lower crust in the southern part of the profile the geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust we interpret the high electrical conductivity in terms of graphite or other forms of mineralization enrichment along the shear planes this zone of crustal weakness may have originated in pre damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the cretaceous rifting and magmatism associated with the opening of the south atlantic

geophysical data reveal the crustal structure of the alaska range orogen within the aftershock zone of the mw 7 9 denali fault earthquake geophysical information including deep crustal seismic reflection magnetotelluric mt gravity and magnetic data cross the aftershock zone of the 3 november 2002 mw 7 9 denali fault earthquake these data and aftershock seis micity jointly interpreted reveal the crustal structure of the right lateral slip denali fault and the eastern alaska range orogen as well as the relationship between this structure and seismicity north of the denali fault strong seismic reflections from within the alaska range orogen show features that dip as steeply as 25 north and extend downward to depths between 20 and 25 km these reflections reveal crustal structures probably ductile shear zones that most likely formed during the late cretaceous but these structures appear to be inactive having produced little seis micity during the past 20 years furthermore seismic reflections mainly dip north whereas alignments in aftershock hypocenters dip south the denali fault is nonre flective but modeling of mt gravity and magnetic data suggests that the denali fault dips steeply to vertically however in an alternative structural model the denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the alaska range orogen modeling of mt data indicates a rock body having low electrical resistivity 10 xm that lies mainly at depths greater than 10 km directly beneath aftershocks of the denali fault earthquake the maxi mum depth of aftershocks along the denali fault is 10 km this shallow depth may arise from a higher than normal geothermal gradient alternatively the low electrical resistivity of deep rocks along the denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after shock zone

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

new interpretation of the reduction phenomenon in the electrical resistivity of rock masses before local earthquakes abstract based on long term observational data from the area of peter the great ridge pamir a decrease in the electrical resistivity of the rock massif before local earthquakes has been observed to explain the observed phenomenon a hypothesis has been proposed the structure of the pore space of a rock in the focal zone of an earthquake changes during the compression tension associated with earthquake preparation however the models proposed have only qualitatively interpreted the experimental data this article attempts to quantify this hypothesis based on 3d modeling of the observed electromagnetic effects it is shown that variations in the apparent resistivity are due to changes in the electrical resistivity in a narrow fault zone located at the focus of the dipole dipole sounding system the changes occur at depths of

electrical resistivity structure of the xiaojiang strike slip fault system sw china and its tectonic implications abstract it is widely accepted that large scale strike slip faults play an important role in accommodating the india eurasia convergence but the relationships between surface displacements and deep deformation along the faults remain largely uncertain to address this issue new magnetotelluric mt images were obtained along two profiles across the xiaojiang strike slip fault system xjfs one of the most tectonically active areas in southwest china although different in details these two profiles show similar overall crustal structures the upper crust is generally resistive but separated by several narrow subvertical conductors that broadly coincide with the surface traces of the xjfs these fault zone conductors fzcs are interpreted to represent fault damage zones that formed by strike slip faulting shearing along the faults and filled with deeply sourced magmatic and or metamorphic fluids the lower crust is broadly characterized by enhanced conductivity and likely associated with partial melting at high temperatures these observations are consistent with other geophysical measurements in this area and in favor of the development of ductile flow along the xjfs although the pattern of flow is more complex than previously suggested based on these observations we propose that active deformation along the xjfs can be better described in the brittle upper crust by strike slip motion along the fluid weakened faults and by shear enhanced ductile flow in the partially molten lower crust these two mechanisms are not mutually exclusive but rather can be complementary to each other

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

electromagnetic heterogeneity of the seismogenic region of 1962 m6 5 northern miyagi earthquake northeastern japan magnetotelluric surveys were carried out to in vestigate the electrical resistivity structure of the seismo genic region of the 1962 northern miyagi earthquake m6 5 in miyagi prefecture northeastern japan a 2 d inversion reveals the existence of a deep conductive zone and overly ing re sistivezonein theuppe r crust a positiveae romag netic anomaly is also recognized above the resistive zone which is interpreted as a buried kitakami granitoid pluton by comparison with these ismicity wefound that themi croearthquakes occur just above the deep conductor and in the resistive zone and that several s wave reflectors are just above the deep conductor since the s wave reflectors sug gest the existence of fluid beneath them the deep conduc tor can be interpreted as a fluid filled zone moreover we suggest that the seepage of the fluid from the conductive fluid filled zone to the resistive granitoid pluton can become a trigger of the earthquakes

large scale intrusion of shallow water into a vertical fracture zone in crystalline bedrock initial hydrochemical perturbation during tunnel construction at the aspo hard rock laboratory southeastern sweden on march 13 1991 construction of the entrance tunnel to the aspo hard rock laboratory opened a vertical fracture zone at a depth of 70 m this provides an opportunity to study geochemical changes resulting from shallow water inflow into a crystalline bedrock aquifer as anticipated during construction and operation of a deep repository for spent nuclear fuel chloride ion is a natural conservative tracer for mixing between the dilute cl 10 mg l 1 shallow groundwater and the saline cl 5000 mg l 1 native groundwater of the fracture zone a sharp dilution front corresponding to 80 dilution of the native groundwater indicated arrival of shallow groundwater in the entrance tunnel after 3 weeks in spite of this large input of shallow water the fracture zone remains predominantly anoxic major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of alkalinity and biogenic co2 g input of organic carbon with shallow groundwater provides a possible energy and carbon source for anaerobic respiration there is no evidence for sulfate reduction and fe iii oxyhydroxide fracture minerals are the only other dominant electron acceptor observed

high resolution electromagnetic imaging of the san andreas fault in central california although there is increasing evidence that fluids may play a significant role in the earthquake rupture process direct observation of fluids in active fault zones remains difficult since the presence of an electrically conducting fluid such as saline pore water strongly influences the overall conductivity of crustal rocks electrical and electromagnetic methods offer great potential for overcoming this difficulty here we present and compare results from high resolution magnetotelluric mt profiles across two segments of the san andreas fault saf which exhibit very different patterns of seismicity parkfield which has regular small earthquakes and creep events and in the carrizo plain where the fault is seismically quiescent and apparently locked in both surveys electric fields were sampled continuously with 100 m long dipoles laid end to end across the fault from 100 to 0 1 hz the data from both profiles are consistent with a two dimensional 2 d fault parallel resistivity model when both transverse electric and magnetic te and tm mode data are included in the interpretation narrow approximately 300 endash 600 m wide zones of low resistivity extending to depths of 2 endash 4 km in the core of the fault are required at both locations however at parkfield the conductance conductivity thickness product of the anomalous region ismore an order of magnitude larger than at carrizo plain suggesting much higher concentrations of fluids for the more seismically active parkfield segment we also image structural differences between the two segments at carrizo plain resistive presumably crystalline rocks are present on both sides of the fault at depths below 3 endash 4 km in particular we clearly image resistive basement extending approximately 10 km or more east of the saf beneath the elkhorn hills and temblor range at parkfield the situation is quite different with a resistive block of salinian granite west of the fault and an electrically conductive presumably fluid rich franciscan complex to the east it is possible that these structural differences control the difference in mechanical behavior of the fault either directly by affecting fault strength or indirectly by controlling fluid supply copyright 1999 american geophysical union less

electrical structure of the tectonically active kalabsha fault aswan egypt in this work we use the magnetotelluric mt method to detect geoelectrical conductivity anomalies in the earth s crust and link them to local seismic activity this application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity mt measurements were carried out in the period range 0 0046 420 s at nine sites along a 15 km long north south profile crossing the kalabsha fault on the western bank of lake aswan data were analysed by 2d simultaneous inversion of both polarisations the resulting model is compared with the local seismicity map and reveals the conductive signature of the fault as well as geological and tectonic stresses prevailing in the aswan area our mt investigations show the following features the measured mt strike aligns with the seismic epicentre axis corresponding to the kalabsha fault while crossing the fault enhanced conductivity is found down to depths of 5 km on a 1 2 km profile segment at mid crustal depths 20 km a very high conductive body is found to coincide with the main seismic cluster in the aswan area these observations indicate that seismic activity and high electrical conductivity are related the link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed their presence is also required to trigger the observed seismicity in addition we explain the lower conductivity of the local upper crust in terms of stress modulated rock porosity we believe that these results are of general significance as they could explain the mid crustal seismicity of tectonically active zones d 2005 elsevier b v all rights reserved

refining the image of the san andreas fault near parkfield california using a finite difference travel time computation technique abstract the parkfield area seismic observatory paso was a dense telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the san andreas fault observatory at depth safod drill site the main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for safod drilling and in the process develop a high for passive seismological techniques resolution image of the fault zone structure a challenging aspect of the analysis of this data set was the known existence of large 20 25 contrasts in seismic wavespeed across the san andreas fault the resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques in order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer thurber c roecker s roberts k gold m powell m l and rittger k 2003 earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod geophys res lett 30 3 10 1029 2002gl016004 with one derived from a relatively fine grid and an application of a finite difference algorithm hole j a and zelt b c 1995 3 d finite difference reflection traveltimes geophys j int 121 2 427 434 in both cases we inverted arrival time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three dimensional vp and vp vs structure included are data from an active source seismic experiment around the safod site as well as from a vertical array of geophones installed in the 2 km deep safod pilot hole drilled in summer 2002 our results show that the main features of the original analysis are robust hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same at the same time we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated and our estimate of the depth of the main part of the seismogenic zone decreases in places by 100 m tests using virtual earthquakes borehole receiver gathers of picks for surface shots indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically

eastern termination of the altyn tagh fault western china constraints from a magnetotelluric survey the left lateral altyn tagh fault forms the northern boundary of the tibetan plateau the strike slip rate of the active altyn tagh fault decreases northeastward and reduces close to zero as it passes north of the qilian shan this geometry raises controversies on whether and how the fault terminates or extends further east to address these controversies wide band magnetotelluric data were collected along four profiles across the altyn tagh fault ranging from 135 to 261 km in length all four profiles are located in the foreland of the qilian shan ranges and are oriented perpendicular to the inferred fault zone that could be the continuation of altyn tagh fault both the two dimensional and three dimensional electrical resistivity models derived from our magnetotelluric data show that the hexi corridor crust is generally of low resistivity whereas the crust of the huahai jinta basin is in general of high resistivity with a local and isolated low resistivity anomaly within the mid lower crust the generally high resistivity crust of the huahai jinta basin may be rheologically unfavorable for the altyn tagh fault passing through the basin toward the northeast the entirely different electrical structure between the hexi corridor and its northern neighbors indicates the existence of a tectonic boundary that coincides with the altyn tagh fault in the west and reverse faults in the east the two dimensional electrical conductivity models suggest that the altyn tagh fault transfers from a single fault in the west to a branching set of mainly dip slip faults in the east

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

geophysical images of the dead sea transform in jordan reveal an impermeable barrier for fluid flow 1 high resolutionseismictomography and magneto telluric mt soundings of the shallow crust show strong changes in material properties across the dead sea transform fault dst in the arava valley in jordan 2d inversion results of the mt data indicate that the dst is associated with a strong lateral conductivity contrast of a highly conductive layer at a depth of approximately 1 5 km cut off at a position coinciding with the surface trace of the dst at the same location we observe a sharp increase of p wave velocities from 5 km s to the east the high velocities in the east probably reflect precambrian rocks while the high electrical conductivity west of the dst is attributed to saline fluids within the sedimentary filling in this sense the dst appears to act as an impermeable barrier between two different rock formations such a localized fluid barrier is consistent with models of fault zone evolution but has so far not been imaged by geophysical methods the situation at the dst is remarkably different from active segments of the san andreas fault which typically show a conductive fault core acting as a fluid conduit

fault zone guided waves from explosions in the san andreas fault at parkfield and cienega valley california fault zone guided waves were successfully excited by near surface explosions in the san andreas fault zone both at parkfield and cienega valley central california the guided waves were observed on linear three component seismic arrays deployed across the fault trace these waves were not excited by explosions located outside the fault zone the amplitude spectra of guided waves show a maximum peak at 2 hz at parkfield and 3 hz at cienega valley the guided wave amplitude decays sharply with observation distance from the fault trace the explosion excited fault zone guided waves are similar to those generated by earthquakes at parkfield but have lower frequencies and travel more slowly these observations suggest that the fault zone wave guide has lower seismic velocities as it approaches the surface at parkfield we have modeled the waveforms as s waves trapped in a low velocity wave guide sandwiched between high velocity wall rocks resulting in love type fault zone guided waves while the results are nonunique the parkfield data are adequately fit by a shallow wave guide 170 m wide with an s velocity 0 85 km sec and an apparent q 30 to 40 at cienega valley the fault zone wave guide appears to be about 120 m wide with an s velocity 0 7 km sec and a q 30

electromagnetic emissions from dry and wet granite associated with acoustic emissions 1 triaxial deformation experiments were performed to compare electromagnetic emissions emes from dry and wet granite specimens associated with acoustic emissions aes cylindrical specimens of initially intact granite were tested at room temperature we measured both high frequency 10 khz to 1 mhz and low frequency dc to 500 hz components of the electric potential variations in general it is difficult to detect emes in wet conditions because wet rock has very short electric relaxation time which is proportional to its electric resistivity however in the wet experiment using a granite specimen saturated with distilled water under a constant pore pressure emes in the high frequency range were detected and accompanied with aes although emes in the low frequency range were not observed in contrast in the dry granite experiment emes were detected in both the high frequency and low frequency ranges in the high frequency range the dry specimen generated eme more frequently than the wet specimen the generation mechanism can be explained based on piezoelectricity and compensation charges the difference in the eme generation from wet and dry granite is attributed to the difference of electric relaxation time also electromagnetic fields due to an electric polarization in a dielectric and conductive medium are discussed including the frequency dependence of the skin depth

a magnetotelluric study from over dharwar cratonic nucleus into billigiri rangan charnockitic massif india abstract the electrical resistivity structure of the crust beneath the dharwar craton in southern india was investigated by magnetotelluric method in the present study a northwest southeast oriented 220 km long profile of 18 stations with a station spacing of 10 15 km is used the profile extends from dharwar cratonic nucleus in the north to billigiri rangan charnockitic massifs in the south time series data are processed to get the apparent resistivity and phase the dominant geoelectric strike direction 75 was calculated in a period range of 0 01 1000 s the data are rotated to 75 strike direction two dimensional inversion is carried out by using the non linear conjugate gradient scheme for both apparent resistivity and phase our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile the mid lower crust in southern part of the profile shows less resistive 2 flushed terranes regional scale carbonation occurred in late archaean is associated with chitradurga shear zone and in late proterozoic is associated with salem attur shear zone the co 2 rich fluids derived during that time might have exhausted in dehydration reactions later events such as the indian plate passing over several hotspots and the metasomatized fluids associated with the cretaceous tertiary magmatism in the region is the reason for observed low resistivity near billigiri rangan massif and surrounding regions in the south

magnetic character of a large continental transform an aeromagnetic survey of the dead sea fault new high resolution airborne magnetic hram data along a 120 km long section of the dead sea transform in southern jordan and israel shed light on the shallow structure of the fault zone and on the kinematics of the plate boundary despite infrequent seismic activity and only intermittent surface exposure the fault is delineated clearly on a map of the first vertical derivative of the magnetic intensity indicating that the source of the magnetic anomaly is shallow the fault is manifested by a 10 20 nt negative anomaly in areas where the fault cuts through magnetic basement and by a 5 nt positive anomaly in other areas modeling suggests that the shallow fault is several hundred meters wide in agreement with other geophysical and geological observations a magnetic expression is observed only along the active trace of the fault and may reflect alteration of magnetic minerals due to fault zone processes or groundwater flow the general lack of surface expression of the fault may reflect the absence of surface rupture during earthquakes the magnetic data also indicate that unlike the san andreas fault the location of this part of the plate boundary was stable throughout its history magnetic anomalies also support a total left lateral offset of 105 110 km along the plate boundary as suggested by others finally despite previous suggestions of transtensional motion along the dead sea transform we did not identify any igneous intrusions related to the activity of this fault segment

microearthquake s wave observations from 0 to 1 km in the varian well at parkfield california abstract high gain three component seismometers from 0 to 1 km deep along the varian a 1 well at parkfield california were used to record the waveforms of nearby microearthquakes despite being in the thick tertiary sediments of the parkfield syncline the s wave amplification at this site is only about a factor of 3 the spectral content and spectral ratios of s waves along the well show that the average qs in the top 1 km at this site is 37 with the qs in different subintervals varying between 8 and 65 based on initial s wave polarizations a complex s wave velocity structure must exist at and below the varian site this structure appears to include position dependent anisotropy as well as steep lateral velocity gradients at a depth of 1 km s wave splitting parallel and normal to the san andreas fault zone is consistently observed this splitting scales at roughly 0 01 sec km subsequent to the split s waves the particle motion seems to be controlled by event focal mechanism above 1 km the upgoing s waves attenuate and change directions of polarization with a new splitting rate of 0 1 sec km uniquely for some events on the san andreas fault immediately below the varian site large post s wave signals with normal dispersion are present we propose that these phases are fault zone guided waves channeled from the san andreas fault to the varian site along the gold hill fault

conductivity structure of the san andreas fault parkfield revisited 1 a previous interpretation of a high resolution magnetotelluric profile across the san andreas fault at parkfield attributed an anomalously conductive region just west of the fault and extending to depths of 3 km to fractured rock saturated with brines laboratory measured electrical resistivities of samples of sedimentary rocks known to border the fault to the east are comparable to resistivities of the anomalous zone we suggest that the anomalous region results from conductive sedimentary rocks in a plunging syncline that is adjacent to the fault rather than caused by fractured rock if correct the current surface trace of the san andreas fault at middle mountain does not form the boundary between the salinian block and the north american plate instead that boundary is 1 km west and collocated with current seismicity

correlation of electrical conductivity and structural damage at a major strike slip fault in northern chile 1 large scale strike slip fault zones are often imaged as electrically conductive structures in the brittle crust however the relationship of conductivity and internal architecture of the fault zone remains largely unclear this paper presents results of a study designed to compare the record of structural deformation across a fault zone with its electrical conductivity image two high resolution magnetotelluric profiles trend perpendicularly across the west fault a branch of the precordilleran fault system in northern chile the magnetotelluric and geomagnetic response functions in the frequency range from 1000 hz to 0 1 hz clearly image a fault zone conductor fzc about 350 m wide and 1500 m deep trending along the surface trace of the fault the position of the fzc and its geometric properties width dip correlate with a region of intense fluid alteration and the orientation of fault related damage elements minor faults fractures according to estimates of fluid salinity and rock porosity the conductivity anomaly results from meteoric water penetrating into a permeable zone this suggests that the increase in electrical conductivity is causally related to a mesh of minor faults and fractures acting as a pathway for fluids the fzc reflects the central and most fractured part of the damage zone in view of similar published magnetotelluric studies we infer a dependency of a fault s state of activity and the characteristics of the fzc such as its conductance width and depth extent ongoing deformation is the controlling factor for maintaining a permeable and electrically conductive fracture network

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

earthquake doublet in an active shear zone southwest japan constraints from geophysical and geochemical findings abstract in 1997 the kagoshima earthquake doublet consisting of two closely associated mw 6 strike slip events five km and 48 days apart has occurred in southwest japan the location is where an e w trending discontinuity along 32 n latitude on southern kyushu island is clearly defined in gps velocities indicating the presence of a highly active left lateral shear zone however there have not been any obvious indications of active faulting at the surface prior to the earthquake doublet which could be associated with this shear zone three dimensional inversion of magnetotelluric sounding data obtained in the source region of the earthquake doublet reveals a near vertical conductive zone with a width of 20 km extending down to the base of the crust and perhaps into the upper mantle toward the okinawa trough the prominent conductor corresponds to the western part of the active shear zone elevated 3 he 4 he ratios in groundwaters sampled from hot spring and drinking water wells suggest the emission of mantle derived helium from the seismic source region the geophysical and geochemical observations are significant indications that the invasion of mantle fluids into the crust driven by upwelling asthenosphere from the okinawa trough triggers the notable left lateral shearing in the zone in the present day subduction system in addition the existence of aqueous fluids in and below the seismogenic layer could change the strength of the zones and alter the local stress regime resulting in the occurrence of the 1997 earthquake doublet

reply to comment on seismomagnetic effects from the long awaited 28 september 2004 m 6 0 parkfield earthquake by m j s johnston y sasai g d egbert and r j mueller by p varotsos and s uyeda the comment by varotsos and uyeda 2008 vu hereafter does not have much to do with our article which reports electromagnetic data and their implications prior to during and following the 2004 m 6 parkfield earthquake eq in fact our article did not include any extensive discussion of the possible flaws in the seismic electric signal ses approach to eq prediction the four main points vu discuss in their comment are from a summary sentence in the introduction that is preceded by the phrase controversy about these ses results exists because 1 2 4 where 1 4 are the specific points that vu list we respond to each of these points in the following discussion a final comment made by vu concerns our observation that ses type signals are not seen in the data that we have we could have provided more detailed evidence about the lack of ses in the months preceding the earthquake but see little point in including long data plots that show nothing note that all of these data are freely available from the northern california earthquake data center ncedc www ncedc org last accessed october 2007 in our article we presented null results from sites above the parkfield eq rupture without extrapolating to the conclusion that sess do not exist anywhere in fact park et al 2007 report additional data from 10 20 km lines over the entire region that also show no evidence for ses type signals preceding the parkfield eq such signals were also not seen in 16 yr of previous monitoring with these lines park 1991 1997 furthermore no signals were seen in 2 yr of data on 10 km lines both along and within the fault and orthogonal to the fault a little farther to the north johnston 1989 to suggest that all of these observations resulted from an

two dimensional seismic image of the san andreas fault in the northern gabilan range central california evidence for fluids in the fault zone a joint inversion for two dimensional p wave velocity vp p to s velocity ratio vp vs and earthquake locations along the san andreas fault saf in central california reveals a complex relationship among seismicity fault zone structure and the surface fault trace a zone of low vp and high vp vs lies beneath the saf surface trace safst extending to a depth of about 6 km most of the seismic activity along the saf occurs at depths of 3 to 7 km in a southwest dipping zone that roughly intersects the safst and lies near the southwest edge of the low vp and high vp vs zones tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected a second high vp vs zone extends to the northeast apparently dipping beneath the diablo range another zone of seismicity underlies the northeast portion of this vp vs high the high vp vs zones cut across areas of very different vp values indicating that the high vp vs values are due to the presence of fluids not just lithology the close association between the zones of high vp vs and seismicity suggests a direct involvement of fluids in the faulting process

magnetotelluric imaging of the fault rupture area of the 1999 izmit turkey earthquake abstract wide band 320 0 001 hz and long period 0 01 0 0001 hz magnetotelluric mt data were acquired along two profiles crossing the western part of the north anatolian fault zone nafz turkey which consists of two main fault branches the first profile izmit profile crosses the epicentral area of the 17 august 1999 izmit earthquake in fact the mt measurements along this profile started just a few weeks before the occurrence of the mainshock and four instruments happened to be in operation in the vicinity of the fault when the earthquake took place the second profile adapazari profile is located about 30 km east of the first profile two dimensional modeling shows the following results first the hypocenters of mainshock and aftershocks are located on the highly resistive side near the edge of a conductive zone second the long period mt data show a low resistivity zone extending down to 50 km between the two fault branches such a deep conductive zone is interpreted as representing partial melting resulted from the past complex tectonics in this region and it is related to the non seismic after slip in the layer below the seismogenic zone characterized by the highly resistive layer

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

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones

a large intraplate earthquake triggered by latent magmatism 1 a magnitude mj 7 3 intraplate earthquake occurred in the western tottori area southwest japan on 6 october 2000 where there was no apparent prefaulting subsurface indication of the source fault of the 2000 earthquake magnetotelluric soundings were taken in and around where the aftershocks occurred in order to image three dimensional electrical resistivity structure at depths of up to 40 km which could indicate an anomalously conductive body in the middle crust to the upper mantle on the southwestern side of the source fault free gas and dissolved gases collected from groundwater wells around the seismic source region are characterized by 3he 4he ratios several times higher than the atmospheric value the highest value of 5 1ra is similar to those of typical arc related volcanic gases although alkali basalts of early pleistocene age are sparsely distributed in the western tottori area the observed 3he 4he ratios are higher than the calculated 3he 4he ratio derived from the ancient magmatism considering postextrusive radiogenic ingrowth of 4he by decay of u and th included in the magma therefore it is concluded that the geophysical anomaly imaged to the southwest of the source fault is attributed to latent magmatism in the present day subduction system aqueous fluids separated from the cooling crustal magma could cause deep low frequency earthquakes around the moho discontinuity and migrate into the brittle upper crust in addition the presence of aqueous fluid is expected to weaken the crustal materials locally anelastic deformation implying notable compressive deformation in the e w direction was observed in the region where aftershocks were distributed under overpressure conditions the existing fault could serve as a pathway for aqueous fluids expelled from magma with high 3he 4he ratios so the upwelling of overpressurized fluids toward the earth s surface results in the emanation of groundwaters with high 3he 4he ratios along the trace of the source fault segments

adaptive mesh seismic tomography based on tetrahedral and voronoi diagrams application to parkfield california 1 we have developed an adaptive mesh seismic tomography method based on tetrahedral and voronoi diagrams to automatically match the inversion mesh to the data distribution two different irregular mesh interpolation methods linear and natural neighbor are evaluated a synthetic test shows that the adaptive mesh seismic tomography method recovers the true synthetic model well in both the linear and natural neighbor interpolation cases use of more accurate differential data is helpful in removing artifacts in the model that result from the noisier absolute data application of the adaptive mesh scheme to parkfield california results in much more uniform sampling of the model as measured by the derivative weight sum on the optimized irregular inversion mesh compared to a regular grid the across strike cross sections resulting from using either just the absolute data or both the absolute and differential data show a clear velocity contrast at the san andreas fault in the linear and natural neighbor interpolation cases the velocity model obtained using natural neighbor interpolation is smoother than that from linear interpolation including the differential data in the inversion yields more clustered event locations and removes a velocity reversal to the southwest of the fault which is likely caused by noise in the absolute data