metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

real time reliable magnitudes for large earthquakes from 1 hz gps precise point positioning the 2011 tohoku oki japan earthquake 1 the early warning issued after the onset of the mw9 0 tohoku oki earthquake significantly underestimated its magnitude saturating 120 seconds after the earthquake began at mw8 1 here we investigate whether real time deformation data from japan s dense network of continuously recording global positioning system gps stations could have been used to provide a more reliable rapid estimate of the earthquake s magnitude and ultimately a more robust tsunami forecast we use precise point positioning in real time mode with broadcast clock and orbital corrections to give station positions every 1 s we then carry out a simple static inversion on a subset of stations to determine the portion of the fault that slipped and the earthquake magnitude unlike most previous methods our method produces estimates of seismic moment before the earthquake rupture has completed we find that the deformation data allow a robust magnitude estimate just 100 s after the earthquake onset we also investigated the density of stations required for a robust moment magnitude estimate fewer than 1 station every 100 km are needed we recommend that gps data be incorporated into earthquake early warning systems for regions at threat from large magnitude earthquakes and tsunamis

imaging a crustal low velocity layer using reflected seismic waves from the 2014 earthquake swarm at long valley caldera california the magmatic system roof

very high rate 10 hz gps seismology for moderate magnitude earthquakes the case of themw6 3 l aquila central italy event accepted for publication in journal of geophysical research copyright 2010 american geophysical union

basement diapirism associated with the emplacement of major ophiolite nappes some constraints abstract the association of basement uplifts with major ophiolite nappes in some phanerozoic orogenic belts suggests that gravitational instability results in the local diapiric uplift of the basement following ophiolite emplacement in previous analyses of diapirism in crustal silicate rocks viscous behaviour of rocks has been assumed it is argued that this assumption is not valid an alternative analysis is offered to determine whether or not the stress would be sufficient for diapirism to occur the negative buoyancy stress resulting from the emplacement of an ophiolite nappe 5 15 km thick onto continental basement may be in the range 10 50 mpa if the horizontal deviatoric stress is zero this will be the maximum principal compressive stress after ophiolite emplacement the thermal profile through the ophiolite and the basement will relax from a saw tooth form to an equilibrium profile if the ophiolite is young and thick there will be a zone of ductile strain in the lower part of the ophiolite and in the upper part of the continental basement results from steady state creep experiments suggest that temperatures in this zone may be high enough for a short time after ophiolite emplacement 3 ma or more for the rocks in this zone to deform at geologically significant strain rates 10 14 or greater in response to the negative buoyancy stress a thin ophiolite or rapid erosion will result in this ductile zone being absent or too short lived for significant strain aquaeous fluids may reduce the strength of brittle rocks by decreasing the effective normal stress or by encouraging pressure solution creep evidence suggests that the deviatoric stress across presently active faults may be as low at 10 mpa thus diapirism in response to ophiolite emplacement may occur through brittle strain gravity spreading within the ophiolite is an alternative mechanism for accommodating the gravitational instability the critical evidence lies in the field

slip segmentation and slow rupture to the trench during the 2015 mw8 3 illapel chile earthquake the 2015 mw8 3 illapel chile earthquake is the latest megathrust event on the central segment of that subduction zone it generated strong ground motions and a large up to 11 m runup tsunami which prompted the evacuation of more than 1 million people in the first hours following the event observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust the deep portion generates strong shaking while slow large slip on the shallow fault is responsible for the tsunami it is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this here we show that the illapel event ruptured both deep and shallow segments with substantial slip we resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection we find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior we forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements independently validating the slip model these results show that the central portion of the chilean subduction zone has accumulated a significant shallow slip deficit and indicates that given enough time shallow slip might be possible everywhere along the subduction zone

comments on tectonic setting of ophiolite obduction in oman by robert coleman the oman ophiolite project coleman 1981a contributed a considerable amount of valuable detailed geologic petrologic and geophysical data which added to our understanding of the origin and emplacement of the ophiolites however as in any study involving a large number of researchers some inconsistencies appear hop son et al 1981 are aware of this problem in the structural and tectonic interpretations particularly there is a certain tendency to present in text and illustrations one particular line of arguments in support of one preconceived idea

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

the measurement of velocity dispersion and frequency dependent intrinsic attenuation in sedimentary rocks a series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the imperial college borehole test site these experiments fall into four categories vertical seismic profiles vsps within the frequency range 30 280 hz crosshole surveys 0 2 2 3 khz sonic logging 8 24 khz and laboratory measurements 300 900 khz the intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible velocity dispersion is observed for both compressional and shear waves the intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band the data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio the modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 super 3 to 10 super 4

a new seismogeodetic approach applied to gps and accelerometer observations of the 2012 brawley seismic swarm implications for earthquake early warning 1 the 26 august 2012 brawley seismic swarm of hundreds of events ranging from m1 4 to m5 5 in the salton trough california provides a unique data set to investigate a new seismogeodetic approach that combines global positioning system gps and accelerometer observations to estimate displacement and velocity waveforms first in simulated real time mode we analyzed 1 5 hz gps data collected by 17 stations fully encircling the swarm zone at near source distances up to about 40 km using precise point positioning with ambiguity resolution ppp ar we used a reference network of north american gps stations well outside the region of deformation to estimate fractional cycle biases and satellite clock parameters which were then combined with ultrarapid orbits from the international gnss service to estimate positions during the brawley seismic swarm next we estimated seismogeodetic displacements and velocities from gps phase and pseudorange observations and 100 200 hz accelerations collected at three pairs of gps and seismic stations in close proximity using a new tightly coupled kalman filter approach as an extension of the ppp ar process we can clearly discern body waves in the velocity waveforms including p wave arrivals not detectable with the gps only approach for earthquake magnitudes as low as mw 4 6 and significant static offsets for magnitudes as low as mw 5 4 our study shows that gps networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance

high resolution seismic attenuation imaging from wide aperture onshore data by visco acoustic frequency domain full waveform inversion summary here we assess the potential of the visco acoustic frequency domain full waveform inversion fwi to reconstruct p wave velocity vp and p wave attenuation factor q from surface onshore seismic data first we perform a sensitivity analysis of the fwi based upon a grid search analysis of the misfit function and several synthetic fwi examples using velocity and q models of increasing complexity subsequently we applied both the acoustic and viscoacousticfwitorealsurfacewide apertureonshoreseismicdatafromthepolishbasin wherea strongattenuationoftheseismicdataisobserved thesensitivityanalysisofthevisco acoustic fwi suggests that the fwi can jointly reconstruct the velocity and the attenuation factor if the signatureoftheattenuationissufficientlystronginthedata asyntheticexamplecorresponding to a homogeneous background model with an inclusion shows a reliable reconstruction of vp and q in the inclusion when q is as small as 90 and 50 in the background model and in the inclusion respectively a first application of acoustic fwi to real data shows that a heuristic normalization of the data with offset allows us to compensate for the effect of the attenuation in the data and reconstruct a reliable velocity model alternatively we show that visco acoustic fwi allows us to reconstruct jointly both a reliable velocity model and a q model from the true amplitude data we propose a pragmatical approach based upon seismic modelling and source wavelet estimation to infer the best starting homogeneous q model for visco acoustic fwi we find the source wavelet estimation quite sensitive to the quality of the velocity and attenuation models used for the estimation for example source to source wavelets are significantly more consistent when computed in the final fwi model than in the initial one a good kinematic and amplitude match between the early arriving phases of the real and time domain synthetic seismograms computed in the final fwi model provides an additional evidence of the reliability of the final fwi model we find the recovered velocity and attenuation models consistent with the expected lithology and stratigraphy in the study area we link high attenuation zones with the increased clay content and the presence of the mineralized fluids

retrieving real time co seismic displacements using gps glonass a preliminary report from the september 2015 mw 8 3 illapel earthquake in chile

pressure solution lithification as a mechanism for the stick slip behavior of faults many major faults including a large fraction of the san andreas appear to be virtually quiescent between great earthquakes the locked sections of the san andreas near san francisco and los angeles have little or no seismic activity on the primary fault trace although secondary faults may be active stick slip behavior on a fault can be explained in terms of a static coefficient of friction which is larger than the dynamic or sliding coefficient in this paper we propose a modification of the friction hypothesis in which chemical lithification cementation occurs on the fault between earthquakes as the stress on the fault increases it becomes large enough to break the cemented bonds between particles causing slip on the fault we treat this problem quantitatively assuming that pressure solution is responsible for the cementation an advantage of this model is that the hydrostatic pressure approaches the lithostatic pressure during cementation so that low fault strengths are predicted

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

instantaneous geodetic positioning with 10 50 hz gps measurements noise characteristics and implications for monitoring networks 1 recent earthquake recordings from high rate 1 hz continuous gps stations in california indicate that even higher temporal resolution provided by modern gps receivers is desirable we measured seven baselines during time intervals devoid of detectable transient signals at sampling rates of 1 50 hz with geodetic receivers from four manufacturers to investigate the noise characteristics of these data our tests over short distances meters to typical station spacing tens of kilometers of regional gps networks show no loss of spatial resolution compared to 1 hz samples measurement noise is red with the typical ramp profile of log log spectra below about 0 5 hz above this frequency noise is essentially white low pass filtering of high rate positions achieves improved spatial resolution compared to decimated raw samples averaging 20 hz measurements to 2 hz samples on a 40 km baseline for example yields about 0 5 mm horizontal and about 3 4 mm vertical accuracy at high frequencies these estimates are a factor of 2 2 5 better than for 2 hz raw samples the improvements in spatial resolution due to averaging at high frequencies are substantial and approach the theoretical square root of n expectation for independent samples however since noise spectral densities rise rapidly below about 0 5 hz low pass filtering is only effective above this frequency these results have important implications for the design of continuous gps networks for crustal deformation and structural monitoring and for positioning and attitude determination of dynamic platforms

seismic attenuation tomography using the frequency shift method we present a method for estimating seismic attenuation based on frequency shift data in most natural materials seismic attenuation increases with frequency the high frequency components of the seismic signal are attenuated more rapidly than the low frequency components as waves propagate as a result the centroid of the signal s spectrum experiences a downshift during propagation under the assumption of a frequency independent q model this downshift is proportional to a path integral through the attenuation distribution and can be used as observed data to reconstruct the attenuation distribution tomographically the frequency shift method is applicable in any seismic survey geometry where the signal bandwidth is broad enough and the attenuation is high enough to cause noticeable losses of high frequencies during propagation in comparison to some other methods of estimating attenuation our frequency shift method is relatively insensitive to geometric spreading reflection and transmission effects source and receiver coupling and radiation patterns and instrument responses tests of crosswell attenuation tomography on 1 d and 2 d geological structures are presented

towards the application of seismogeodesy in central italy a case study for the 2016 august 24 mw 6 1 italy earthquake modelling

application of real time gps to earthquake early warning 1 we explore the use of real time high rate gps displacement data for earthquake early warning using 1 hz displacement waveforms from the april 4 2010 mw 7 2 el mayor cucapah earthquake we compare these data to those provided by the broadband velocity and accelerometer instrumentation of the southern california seismic network the unique information provided by the gps based displacement timeseries is the permanent static displacement using a simple algorithm that can be applied in real time we extract the static offset shortly after the s wave arrival around the time of the observed peak shaking at the same site and before shaking at more distant locations these data can be used as they become available to provide a robust estimate of the earthquake magnitude which ranges from 6 8 to 7 0 in this case we therefore conclude that real time high rate gps can provide a useful and independent assessment of earthquake magnitude for the purpose of earthquake early warning and real time earthquake information systems in general including tsunami warning systems

q estimation from vertical seismic profile data and anomalous variations in the central north sea four different methods of estimating q from vertical seismic profile vsp data based on measurements of spectral ratios pulse amplitude pulse width and zeroth lag autocorrelation of the attenuated impulse are described the last procedure is referred to as the pulse power method practical problems concerning nonlinearity in the estimating procedures uncertainties in the gain setting of the recording equipment and the influence of structure are considered in detail vsp data recorded in a well in the central north sea were processed to obtain estimates of seismic attenuation these data revealed a zone of high attenuation from approximately 4 900 ft to 5600 ft with a value of 25 3 results of the spectral ratio analysis show that the data conform to a linear constant q model in addition since the pulse width measurement is dependent upon the dispersive model adopted it is shown that a nondispersive model cannot possibly provide a match to the real data no unambiguous evidence is presented that e

gravity profiles across the samail ophiolite oman four hundred and sixty gravity stations have been established over the samail ophiolite and adjacent units exposed in the oman mountains the bouguer anomaly pattern follows more or less parallel to ophiolite exposures along the coastal areas the bouguer anomaly values range from 20 to 140 mgal and increase northward four gravity profiles across the ophiolite are correlated with known geology and structure the main gravity profile extending 325 km northward from ibra to muscat and into the gulf of oman shows two positive bouguer anomalies 85 over the southern folded ophiolite nappe and 110 mgal near the shoreline at muscat that are associated with two nappes of tabular form the nappes are nearly continuous and there is no evidence that they are rooted in the mantle the gravity high across the southern nappe is centered over the axis of the synform maximum nappe thickness its position over the gabbro peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite at the southern end of the profile in the ibra dome area the negative bouguer anomaly 10 to 15 mgal is interpreted as an infracambrian salt piercement similar to those in the zagros fold belt geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units the nature of the gravity anomaly across the continental margin indicates a transition from 42 km thick continental crust to a thinner 16 18 km oceanic crust the two gravity profiles in the north oman region show a doublet of positive residual bouguer anomalies 50 to 90 mgal the anomalies correlate well with imbricated ophiolite exposures a result of low angle thrusting caused by strong compressive forces in the absence of imbricate thrusting the gravity profile in the central region has only one large positive residual bouguer anomaly 130 mgal the anomalies of all the four profiles are further analyzed to elucidate the nappe transport distances

real time inversion of gps data for finite fault modeling and rapid hazard assessment 1 responses to recent great earthquakes and ensuing tsunamis in sumatra chile and japan with the resulting loss of life and damage to infrastructure demonstrate that our ability to ascertain the full extent of slip of catastrophic earthquakes and their tsunamigenic potential in the first minutes after the initiation of rupture is problematic regional gps networks such as those in western north america and japan are complementary to seismic networks by being able to directly measure displacements close to the source during large earthquakes in real time we report on rapid modeling of two large earthquakes the 2003 mw 8 3 tokachi oki earthquake 100 km offshore hokkaido island using 356 geonet stations and the 2010 mw 7 2 el mayor cucapah earthquake in northern baja california using 95 crtn stations in southern california about 75 km northwest of the epicenter working in a simulated real time mode we invert for finite fault slip in a homogeneous elastic half space using green s functions obtained from okada s formulation we compare two approaches the first starts with a catalog of pre defined faults while the second uses a rapid centroid moment tensor solution to provide an initial estimate of the ruptured fault plane in either case we are able to characterize both earthquakes in less than two minutes reducing the time necessary to obtain finite fault slip and moment magnitude for medium and greater earthquakes compared to traditional methods by an order of magnitude

comparison of seismic attenuation models using zero offset vertical seismic profiling vsp data for seismic frequencies it is common to use an empirical equation to model attenuation usually the attenuation coefficient is modeled with linear frequency dependence a model referred to as the kolsky futterman model other models have been suggested in the geophysical literature we compare eight of these models on a zero offset vertical seismic profiling vsp data set the kolsky futterman the power law the kjartansson the muller the azimi second the azimi third the cole cole and the standard linear solid sls models for three separate depth zones we estimate velocities and q values for all eight models a least squares model fitting algorithm gives almost the same normalized misfit for all models thus none of the models can be preferred or rejected based on the given data set slightly better overall results are obtained for the kolsky futterman model for one depth zone the sls model gave the best result

the determination of the seismic quality factor q from vsp data a comparison of different computational methods1 ten methods for the computation of attenuation have been investigated namely amplitude decay analytical signal wavelet modelling phase modelling frequency modelling rise time pulse amplitude matching technique spectral modelling and spectral ratio in particular we have studied the reliability of each of these methods in estimating correct values of q using three synthetic vsp seismograms for plane p waves with different noise contents the investigations proved that no single method is generally superior rather some methods are more suitable than others in specific situations depending on recording noise or geology the analytical signal method has been demonstrated to be superior if true amplitude recordings are available otherwise spectral modelling or in the noise free case the spectral ratio method is optimal finally two field vsps in sediments are investigated only in the case of the highest quality vsp can significant information be deduced from the computed attenuation

middle paleozoic to cenozoic geology and tectonic evolution of the northwestern himalaya abstract previously published data and data collected by the authors from the northwestern himalaya are summarized and assessed the account focuses on kohistan and the karakorum range in pakistan and on ladakh and the zanskar range in india five palaeotectonic units are recognized from south to north these are 1 1 palaeozoic eocene continental shelf sequence on the northern margin of the indian continent 2 2 a triassic jurassic continental slope sequence the lamayuru unit of the indus zone and of the spongtang klippe 3 3 the cretaceous tertiary kohistan ladakh magmatic arc which includes the kokistan sequence the ladakh batholith the dras unit the spongtang klippe the indus molasse unit and the shyok zone 4 4 the karakorum batholith a late tertiary zone of granitoid magmatism and metamorphism 5 5 a palaeozoicmesozoic continental shelf and slope sequence deposited on the southern margin of an asian continent the following sequence of events is tentatively proposed 1 1 permian carboniferous continental rifting 2 2 triassic jurassic collapse of continental shelf and formation of deep oceanic basin 3 3 cretaceous formation of kohistan ladakh magmatic arc in response to subduction from the south as ocean closed 4 4 late cretaceous eocene emplacement of kohistan arc and of thrust sheets of indus zone magmatic arc units and lamayuru unit southwards onto indian continental margin as india approached and collided with magmatic arc 5 5 silicic phases of ladakh batholith palaeocene eocene and karakorum batholith miocene may be related to subduction of oceanic basin between magmatic arc and asian margin alternatively they are late and post collisional products 6 6 the shyok zone between the ladakh and karakorum batholiths was emplaced to the south in oligocene or miocene time 7 7 final collision between india the magmatic arc and asia in oligocene miocene time was followed by continued crustal shortening which disrupted thrust units

southern california permanent gps geodetic array spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 landers earthquake the june 28 1992 m w 7 3 landers california earthquake was the first earthquake to be surveyed by a continuously operating global positioning system gps array the coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake we employ a new spatial filtering technique that removes common mode errors from the coordinate time series this approach provides precise estimates of site specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations all sites indicate significant coseismic horizontal displacements of 5 65 mm with uncertainties of 1 2 mm horizontal displacements are in general agreement with elastic dislocation models in particular for sites closer to the epicenter vertical displacements range from 13 to 7 mm with uncertainties of 2 4 mm the observed vertical displacements in all cases show 5 10 mm more subsidence than expected from geodetic and seismic geologic models significant postseismic horizontal displacements totaling 6 2 mm 10 20 of the coseismic displacement are detected at the three sites closest to the epicenter these displacements are modeled as a short term exponential relaxation with a decay time of 22 10 days superimposed on a longer term linear interseismic trend scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain which agrees well with the direction and magnitude determined from more precise laser strain meter data the time series do not show any detectable preseismic displacements

accuracy of high rate gps for seismology 1 we built a device for translating a gps antenna on a positioning table to simulate the ground motions caused by an earthquake the earthquake simulator is accurate to better than 0 1 mm in position and provides the ground truth displacements for assessing the technique of high rate gps we found that the root mean square error of the 1 hz gps position estimates over the 15 min duration of the simulated seismic event was 2 5 mm with approximately 96 of the observations in error by less than 5 mm and is independent of gps antenna motion the error spectrum of the gps estimates is approximately flicker noise with a 50 decorrelation time for the position error of 1 6 s we find that for the particular event simulated the spectrum of surface deformations exceeds the gps error spectrum within a finite band more studies are required to determine whether a generally optimal bandwidth exists for a target group of seismic events

rapid resolution of crustal motion at short ranges with the global positioning system on the basis of the analysis of global positioning system gps data collected at the u s geological survey kennedy ranch house alinement array near parkfield california in november 1990 and february 1991 we assess the temporal resolution of crustal deformation for gps receiver systems operating as strain meters and provide guidelines for achieving millimeter level precision with short range kinematic type gps measurements site occupation of about one half hour is sufficient for day to day repeatability of submillimeter precision in relative position during good satellite geometry signal multipath effects which dominate dispersive baseline errors are essentially averaged out and need not be modeled however as occupation times decrease baseline solutions are increasingly sensitive to multipath effects which together with nondispersive receiver measurement noise are of millimeter to centimeter magnitude in epoch by epoch solutions on repeatedly monitored baselines we can account for both of these noise sources separately the mostly random receiver noise is effectively suppressed by low pass filters with corner frequencies of about 20 mhz the multipath noise is filtered using the fact that it essentially repeats from day to day residual errors in filtered 1 s epoch by epoch solutions are below 2 mm during periods of good satellite geometry hence for a continuously monitored baseline actual deformation in excess of this threshold can be detected with a time lag comparable to the group delay of the low pass filter i e typically within a minute in terms of strain along a typical short baseline of 1 km sampled once per second we can detect 1 2 s in l min with multipath modeling without multipath modeling it takes 10 min to resolve the same magnitude of deformation in 30 min we can detect 0 1 s independent of multipath modeling these results have important implications for kinematic type gps surveys for millimeter level precision without impractical multipath modeling it is necessary to occupy a site during good satellite geometry for a period longer than that of the multipath signature our experience in both benign and difficult multipath environments indicates that approximately 10 min are sufficient the measurements at the kennedy ranch house alinement array suggest that this segment of the san andreas fault appears to be virtually locked at the surface during the 3 month period between the two surveys

cretaceous high pressure metamorphism in ne oman and its relationship to subduction and ophiolite nappe emplacement an occurrence of glaucophane bearing schists is reported from near muscat in ne oman in rocks which underlie the upper cretaceous semail ophiolite nappe these blueschists formed by the metamorphism of quartz sandstones shales and basic igneous rocks belong to a pre permian basement in the area metamorphic micas phengites from these rocks have been dated at between 101 and 80 ma k ar ages indicating a mid to late cretaceous age for the high pressure event a syn to late f 1 assemblage of almandine glaucophane phengite in the pelites shows that the peak of metamorphism reached about 400 450 c and 4 8 kbars pressure this was followed by a greenschist facies overprint with the formation of epidote and albite porphyroblasts in the meta igneous rocks and a widespread retrogression to chlorite the high pressure metamorphism is ascribed to the effects of partial subduction of the ne oman continental margin during the initial stages of ophiolite obduction and the subsequent greenschist facies event to uplift of the area during the later stages of emplacement of the nappes

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

time lapse full waveform inversion of vertical seismic profile data workflow and application to the co2crc otway project vertical seismic profile vsp is one of the technologies for monitoring hydrocarbon production and co2 geosequestration however quantitative interpretation of time lapse vsp is challenging due to its irregular distribution of source receiver offsets one way to overcome this challenge is to use full waveform inversion fwi which does not require regular offsets we present a workflow of elastic fwi applied to offset vertical seismic profile data for the purpose of identification and estimation of time lapse changes introduced by injection of 15 000 tonnes of co2 rich gas mixture at 1 5 km depth application of this workflow to both synthetic and field data shows that elastic fwi is able to detect and quantify the time lapse anomaly in p wave velocity with the magnitude of 100 150 m s

measurements of attenuation from vertical seismic profiles values of attenuation have been extracted from five detailed velocity surveys significant amounts of attenuation are observed in all five wells measured values of attenuation vary by a factor of 10 ranging from less than 0 1 to 0 9 db wavelength correlation of attenuation with lithology is good considering the resolution limits of the surveys one of the surveys was performed in west texas and the other four were carried out in the gulf coast area the total number of recording depths in each survey varied from 31 to 208 with downhole recordings taken every 10 or 20 ft over key stratigraphic zones in all five surveys a monitor geophone was placed near the well to check for source consistency our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse a value of cumulative attenuation ca was then extracted from each spectral ratio and plotted as a function of geophone depth this method allowed us to use many m