geochemistry zircon u pb and hf isotope for granitoids nw sanandaj sirjan zone iran implications for mesozoic cenozoic episodic magmatism during neo tethyan lithospheric subduction abstract the sanandaj sirjan zone ssz of iran comprises sedimentary and metamorphic basement rocks which are generally regarded as having been derived from the southern active margin of the eurasian plate within the ssz a number of mesozoic to cenozoic granitoid intrusions of various size elongated in a nw se direction are exposed with the benefit of precise age dating geochemical and isotopic data over the past decade the magmatic history of these intrusions has become clearer this study presents further geochronological and geochemical data for previously dated and undated granitoids together with considerable zircon lu hf isotopic data which were limited in the past combined with previous work these new data including the finding of ca 170 ma adakite lead to improved constraints on the meso cenozoic tectonic evolution of neo tethyan lithospheric subduction the dominant early middle jurassic magmatism is proposed to have occurred with a major contribution of crustal components during the initiation of neo tethyan subduction subsequent late jurassic magmatism suggests the involvement of lower crust partial melting in an extensional tectonic setting cretaceous magmatism almost ceased after formation of a flat slab caused by a trench retreat rate exceeding that of slab roll back late paleocene eocene magma sources in the ssz are dominated by mantle derived input through an asthenospheric window with subordinate crustal contamination during the subduction of the neo tethyan ridge spreading center this interpretation differs from paleocene eocene magmatic flare up proposed for the northern iranian interior which may have been driven by an episode of slab retreat or slab roll back following cretaceous flat slab subduction

transient luminous events above two mesoscale convective systems storm structure and evolution 1 two warm season mesoscale convective systems mcss were analyzed with respect to their production of transient luminous events tles mainly sprites the 20 june 2007 symmetric mcs produced 282 observed tles over a 4 h period during which the storm s intense convection weakened and its stratiform region strengthened tle production corresponded well to convective intensity the convective elements of the mcs contained normal polarity tripole charge structures with upper level positive charge 40 c midlevel negative charge 20 c and low level positive charge near the melting level in contrast to previous sprite studies the stratiform charge layer involved in tle production by parent positive cloud to ground cg lightning resided at upper levels this layer was physically connected to upper level convective positive charge via a downward sloping pathway the average altitude discharged by tle parent flashes during tle activity was 8 2 km above mean sea level msl 25 c the 9 may 2007 asymmetric mcs produced 25 observed tles over a 2 h period during which the storm s convection rapidly weakened before recovering later unlike 20 june tle production was approximately anticorrelated with convective intensity the 9 may storm which also had a normal tripole in its convection best fit the conventional model of low altitude positive charge playing the dominant role in sprite production however the average altitude discharged during the tle phase of flashes still was higher than the melting level 6 1 km msl 15 c based on these results it is inferred that sprite production and sprite parent positive charge altitude depend on mcs morphology

petrogenesis of andalusite kyanite sillimanite veins and host rocks sanandaj sirjan metamorphic belt hamadan iran quartz rich veins in metapelitic schists of the sanandaj sirjan belt hamadan region iran commonly contain two al2sio5 polymorphs and more rarely three coexisting al2sio5 polymorphs in most andalusite and sillimanite schists the types of polymorphs in veins correlate with al2sio5 polymorph s in the host rocks although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks e g vein andalusite is enriched in fe2o3 relative to host rock andalusite low grade rocks contain andalusite quartz veins medium grade rocks contain andalusite sillim anite quartz plagioclase veins and high grade rocks contain sillimanite quartz plagioclase veins leucosomes although most andalusite and sillimanite bearing veins occur in host rocks that also contain al2sio5 kyanite quartz veins crosscut rocks that lack al2sio5 e g staurolite schist granite a quartz vein containing andalusite kyanite sillimanite staurolite muscovite occurs in andalusite sillimanite host rocks textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite this crystallization sequence conflicts with the observation that kyanite quartz veins post date andalusite sillimanite veins and at least one intrusive phase of a granite that produced a low pressure high temperature contact aureole these relationships imply a sequence of andalusite to sillimanite to kyanite varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by p t paths of different rocks passing near slightly above slightly below the al2sio5 triple point and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

shear wave splitting lithospheric anisotropy and mantle deformation beneath the arabia eurasia collision zone in iran article i nfo records of core refracted phases from permanent and temporary seismological stations are analysed to provide a map of seismic anisotropy covering most tectonic provinces of iran anisotropy is coherent at the scale of a single province but it differs from one province to another no significant shear wave splitting is observed in the mountain ranges of zagros alborz and kopeh dagh anisotropy with average time lags of 1 1 0 3 s between fast and slow shear waves with axes of the faster polarization oriented mostly nw se is observed in central iran but the faster orientation is ne sw in ne iran the orientations of fast axes do not coincide with the directions of absolute plate motion in the no net rotation frame or in the hot spot frame suggesting that the observed anisotropy cannot be explained by a simple model of asthenospheric flow the heterogeneous orientation of fast axes with differences in length scales of a few hundreds km suggests that the anisotropy originates in the lithospheric mantle in the area where strike slip shear dominates the fast axes lie approximately parallel to the orientations of maximum shear estimated from gps measurements either with right lateral shear associated with the ns trending right lateral faults in central iran or left lateral shear associated with ew trending left lateral faults in northeast iran in the mountain belts the null measurements may be related to horizontal shortening and thickening by pure shear as proposed from synthetic models of olivine fabric which both imply a large amount of vertical strain not seen by sks splitting our data therefore do not agree with previous explanations of shear wave splitting in the neighbouring regions of eastern anatolia and saudi arabia in terms of simple asthenospheric flow they rather support the view that iran deforms over a broad area with vertically coherent deformation at a lithospheric scale that includes both shear between blocks in central iran and shortening at the mountain belts

kinematic microphysical and electrical aspects of an asymmetric bow echo mesoscale convective system observed during steps 2000 1 five hours of detailed observations of the asymmetric bow echo mesoscale convective system on 11 12 june 2000 during the steps project are presented data sources included polarimetric and doppler radars a vhf lightning mapper and the national lightning detection network nearly every 99 vhf lightning source occurred within 10 km of the convective line charge identification within the much of the convective line revealed evolution during the first 3 h from a normal polarity dipole upper positive charge near 9 km msl 30 c over mid level negative charge near 7 km 16 c to a normal polarity tripole with the addition of lower positive charge near 4 km 1 c during the final 2 h the charge structure resembled an inverted dipole with negative charge overlaying positive when the bow echo occurred during hour 2 part of the cells associated with severe winds developed an inverted tripolar charge structure and produced predominantly positive cloud to ground lightning stratiform lightning was infrequent normally initiating in the convective line and propagating rearward along two pathways a downward sloping upper pathway near 9 km and a more common constant altitude lower pathway near 6 km 10 c some flashes initiated within the stratiform region during this time but only after stratiform reflectivities increased significantly in the mixed phase region these flashes initiated near the bright band 4 km and tapped negative charge above and positive charge below possible mechanisms to explain the observations are discussed

mica deformation fabrics and the seismic properties of the continental crust abstract seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica however naturally deformed micaceous rocks commonly show composite e g s c fabrics it is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields an outcrop analogue for granitic mid crustal deformed zones is used here to calibrate the seismic response against both parameters seismic responses are modelled using crystallographic preferred orientations for polymineralic micaceous granitic gneisses measured using electron back scatter diffraction the sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation so called rock and fabric recipes the maximum p and s anisotropy are calculated at 16 6 and 23 9 for single foliation gneisses but for mixed i e s c foliation gneisses these values reduce to 5 8 and 7 5 respectively furthermore mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy this effect coupled with the geographical orientation of fabrics in nature can generate substantial variations in the orientation and magnitude of seismic anisotropy especially for shear waves as measured for the continental crust using existing receiver function and teleseismic near vertical incidence methods thus maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth varying deformation kinematics and tectonic decoupling

evidence for near frictionless faulting in the 1989 m 6 9 loma prieta california earthquake and its aftershocks comment and reply

evolution of the total lightning structure in a leading line trailing stratiform mesoscale convective system over houston texas 1 line normal vertical cross sections of houston lightning detection and ranging ldar vhf radiation sources and radar reflectivity provide new insights into the three dimensional total lightning structure and evolution of a leading line trailing stratiform llts mesoscale convective system mcs over houston texas on 31 october 2005 previous research examining only the mature stage of an mcs showed that the overwhelming majority of vhf lightning sources occurred in the convective region with a lightning pathway extending rearward and descending in altitude into the stratiform region this descending pathway was most likely associated with small charged ice particles advected from the convective line unlike previous research the lightning pathway observed during the evolution of the mcs on 31 october 2005 initially extended rearward 40 km at a nearly constant height of 9 10 km in less than an hour the lightning pathway evolved into a sloped pathway similar to that found by previous research with a horizontal extent between 50 to 60 km and downward descent of 4 to 5 km during the lightning pathway evolution radar analysis showed an increase of reflectivity in the midlevels of the stratiform region consistent with increased depositional and aggregational growth above the melting layer broadening increased range of values of radar reflectivity above the melting layer was most likely an indication of a strengthening mesoscale updraft in the stratiform region this strengthening updraft may have contributed to an increase in the growth of the small charge carrying ice crystals giving them a greater fall speed in addition the mesoscale updraft may have promoted an environment conducive to local stratiform region charge generation in the mixed phase region just above the melting layer

application of the global positioning system to crustal deformation measurement 1 precision and accuracy in this paper we assess the precision and accuracy of interstation vectors determined using the global positioning system gps satellites these vectors were between stations in california separated by 50 450 km using data from tracking the seven block i satellites in campaigns from 1986 through 1989 we examine the precision of gps measurements over time scales of a several days and a few years we characterize gps precision by constant and length dependent terms the north south component of the interstation vectors has a short term precision of 1 9 mm 0 6 parts in 108 the east west component shows a similar precision at the shortest distances 2 1 mm with a larger length dependence 1 3 parts in 108 the vertical precision has a mean value of 17 mm with no clear length dependence for long term precision we examine interstation vectors measured over a period of 2 2 to 2 7 years when we include the recent results of davis et al 1989 for distances less than 50 km we can describe long term gps precision for baselines less than 450 km in length as 3 4 mm 1 2 parts in 108 5 2mm 2 8 parts in 108 11 7mm 13 parts in 108 in the north south east west and vertical components accuracy has been determined by comparing gps baseline estimates with those derived from very long baseline interferometry vlbi a comparison of eight interstation vectors shows differences ranging from 5 to 30 mm between the mean gps and mean vlbi estimates in the horizontal components and less than 80 mm in the vertical a large portion of the horizontal differences can be explained by local survey errors at two sites in california a comparison which suffers less from such errors is between the rates of change of the baselines the horizontal rates estimated from over 4 years of vlbi data agree with those determined with 1 2 years of gps data to within one standard deviation in the vertical both gps and vlbi find insignificant vertical motion

a reappraisal of earthquake focal mechanisms and active shortening in the zagros mountains of iran summary the zagros mountains of sw iran are one of the most seismically active intra continental fold and thrust belts on earth and an important element in the active tectonics of the middle east surface faulting associated with earthquakes is extremely rare and so most information about the active faulting comes from earthquakes we use long period teleseismic p and sh body waves to determine the orientation and depth of faulting in 16 new earthquakes and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the zagros we use this information to investigate the style and distribution of active faulting in the zagros and how it contributes to the n s shortening of the arabia eurasia collision when the data are ranked in quality and carefully evaluated simple patterns are seen that are not apparent when routine catalogue data are taken at face value an important change in the fault configuration occurs along strike of the belt in the nw overall convergence is oblique to the trend of the belt and the surface anticlines and is achieved by a spatial separation partitioning of the orthogonal strike slip and shortening components on separate parallel fault systems by contrast in the se overall convergence is orthogonal to the regional strike and achieved purely by thrusting in the central zagros between these two structural regimes deformation involves parallel strike slip faults that rotate about vertical axes allowing extension along the strike of the belt the overall configuration is similar to that seen in other curved shortening belts such as the himalaya and the java sumatra trench all the zagros earthquakes we have been able to check have centroids shallower than 20 km and are confined to the upper crust many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover which is active even beneath areas of known shallow structural decollement such as the dezful embayment the dominant style of shortening is high angle reverse faulting with dips 30 though some lower angle thrusting occurs in places active thrust and reverse faulting is relatively confined to the lower topography on the sw edge of the belt today and only strike slip faulting affects the higher topography profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the simple folded belt but has progressively migrated sw over the last 5 ma there is no evidence for a seismically active structural decollement such as a low angle thrust beneath the zagros nor is there any seismic evidence for active subduction either beneath the zagros or beneath central iran instead the arabian margin seems to have shortened by distributed thickening of the basement only in the syntaxis of the oman line at the se end of the zagros is there any evidence for a low angle thrust of regional extent here earthquakes continue 50 km north of the zagros thrust line the geological suture between the arabian margin and central iran reaching depths of 30 km and may represent thrusting of arabian basement beneath central iran to this extent

detecting strain in the yucca mountain area nevada from repeated surveys of the relative motions between geodetic stations claim black mile 67tjs and wahomie fig 1 from 1991 through 1997 wernicke et al 1 deduce a n65 w strain accumulation rate of 50 9 nanostrains per year 2 across the proposed high level radioactive waste disposal repository at yucca mountain nevada that strain rate is sufficient to indicate a higher than expected earthquake hazard at the repository an earlier 1983 1993 u s geological survey usgs measurement table 1 of the strain accumulation in the network black triangles in fig 1 found a n65 w strain rate of 8 20 nanostrains per year a rate not significantly different from zero 3 although the two measurements are marginally consistent at the 95 confidence level that is the difference 42 22 nanostrains per year is less than 2 standard deviations sd the earthquake hazard implications are somewhat different the usgs strain determination is the better of the two measurements wernicke et al did not include the effects of monument instability in their error budget and as a consequence have significantly underestimated the uncertainties in their measurements moreover in their interpretation of the data they did not give proper weight to the coseismic and postseismic effects of the little skull mountain earthquake june 29 1992 m 5 4

cloud to ground lightning in linear mesoscale convective systems recently three distinct archetypes for midlatitude linear mesoscale convective systems mcss have been identified those comprising convective lines with trailing stratiform precipitation ts leading stratiform precipitation ls and parallel stratiform precipitation ps while cloud to ground cg lightning in ts mcss has received a great deal of study in recent years linear mcss exhibiting leading or parallel stratiform precipitation have received comparatively little attention this paper documents the arrangement and frequency of cg lightning for four ls and four ps mcss that occurred during may 1996 and may 1997 on average the ls cases bore some similarity to previously studied ts mcss they produced predominantly positive cg lightning 1cg early in their lifetimes concomitant with convective precipitation and late in their lifetimes concomitant with stratiform precipitation in contrast with previously published results for mcss in general the ls mcss in the present study overall had relatively low negative cg 2cg flash densities yielding higher percentages of 1cgs than other mcs archetypes while the ps systems had higher average flash densities than the ls systems they did not exhibit prolonged 1cg modes early in their lifetimes and they produced a lower overall fraction of 1cgs notably the presence of line parallel flow may render a somewhat unique electrical character in ps systems in particular the ps mcss in this study exhibited enhanced cg lightning in the vicinity of decaying convective cells that were embedded within their stratiform precipitation regions

convective structure of hurricanes as revealed by lightning locations cloud to ground lightning flash locations were examined for nine atlantic basin hurricanes using data from the national lightning detection network a common radial distribution in ground flash density was evident a weak maximum in the eyewall region a clear minimum 80 100 km outside the eyewall and a strong maximum in the vicinity of outer rainbands 210 290 km radius these results are consistent with the authors previous study of hurricane andrew none of the storms showed this characteristic radial structure during prehurricane stages the results support the division of precipitation in the hurricane into three distinct regimes the eyewall is a unique phenomenon but shares some attributes with deep weakly electrified oceanic monsoonal convection the region outside the eyewall and under the central dense overcast has characteristics of the trailing stratiform region of mesoscale convective systems including a relatively high fraction of positive polarity flashes the outer bands with mean maximum flash density at the 250 km radius contain the vast majority of ground flashes in the storms eyewall lightning defined as that within 40 km of the center was examined for four moderate to strong hurricanes such lightning occurred episodically during hurricane stage with 93 of hourly intervals containing no detected flashes eyewall lightning outbreaks over water always occurred at the beginning of or during times of intensification but often were indicative of the imminent end of deepening it is proposed that the existence of such inner core lightning might reveal the presence of an eyewall cycle for the one storm with available aircraft reconnaissance data eyewall cycles were reliably identified by the occurrence of inner core lightning and inner core lightning appeared only during such cycles suggestions are made as to how eyewall flashes in existing hurricanes might be used to help predict hurricane intensity change

p sh conversions in layered media with hexagonally symmetric anisotropy a cookbook reflectivity synthetic seismograms demonstrate that the type layering and orientation of 1 d anisotropy influences strongly the coda of teleseismic p waves at periods t 1 sec particularly p sh converted waves we assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis of arbitrary orientation the resulting phase velocities vary as cos 2 with respect to that axis using three families of simple crustal models we compare the effects of an anisotropic surface layer with reverberations caused by both thick and thin layers of anisotropy at depth if anisotropy in the surface layer is significant the polarization of direct p can be distorted to generate a transverse component followed by ps and a prominent shear reverberation converted from direct p at the free surface if the anisotropic layer is buried the first and often the most prominent arrival on the transverse component is the p to sh conversion at its upper surface if the anisotropic layer is sufficiently thin p to sh conversions from its boundaries interfere to form a derivative pulse shape on the transverse component which could be mistaken as the signature of shear wave splitting if is horizontal compressional p and shear s anisotropy both produce similar waveform perturbations with four lobed azimuthal patterns suggesting that a weighted stack of p coda from different back azimuths would improve signal to noise for tilted between the horizontal and vertical however the effects of p and s anisotropy differ greatly the influence of p anisotropy on p to s conversion is greatest for a symmetry axis tilted at 45 to the vertical where its azimuthal pattern has two lobes rather than four combinations of p and s anisotropy typically lead to a composite azimuthal dependence in the p coda reverberations

cloud to ground lightning observations from toga coare selected results and lightning location algorithms abstract recently observations of electrified oceanic convection and associated cloud to ground cg lightning were obtained over the tropical western pacific ocean during toga coare tropical ocean global atmosphere coupled ocean atmosphere response experiment during coare observations of convection were made using a variety of instrument platforms including ship and airborne doppler radars an advanced lightning direction finder aldf network and a shipborne inverted electric field mill this study focuses on data collected by the coare aldf network fusion of those data with observations and the methods used to calculate accurate cg return stroke locations analysis of cg lightning data and doppler radar data indicates that lightning producing oceanic convection is characterized by deep vertically developed convective cells with radar reflectivities exceeding 30 dbz above the height of the 10 c level in several cases a peak in cg frequency occurred coincident with the descent of precipitation

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

layered crustal anisotropy around the san andreas fault near parkfield california the rheology of the earth s crust controls the long term and short term strength and stability of plate boundary faults and depends on the architecture and physical properties of crustal materials in this paper we examine the seismic structure and anisotropy of the crust around the san andreas fault saf near parkfield california using teleseismic receiver functions these data indicate that the crust is characterized by spatially variable and strongly anisotropic upper and middle crustal layers with a moho at 35 km depth the upper layer is 5 10 km thick and is characterized by strong 30 anisotropy with a slow axis of hexagonal symmetry where the plane of fast velocity has a strike parallel to that of the saf and a dip of 40 we interpret this layer as pervasive fluid filled microcracks within the brittle deformation regime the 10 15 km thick midcrustal layer is also characterized by a weak axis of hexagonal symmetry with 20 anisotropy but the dip direction of the plane of fast velocity is reversed the midcrustal anisotropic layer is more prominent to the northeast of the san andreas fault we interpret the mid crustal anisotropic layer as fossilized fabric within fluid rich foliated mica schists when combined with various other geophysical observations our results suggest that fault creep behavior around parkfield is favored by intrinsically weak and overpressured crustal fabric

pliocene growth of the dowlatabad syncline in frontal fars arc folding propagation across the zagros fold belt iran this research was funded by the darius programme as well as the group of dynamics of the lithosphere geosciences barcelona geo3bcnconsejo superior de investigaciones cientificas csic spain we thank the paleomagnetic laboratory universitat de barcelona scientific and technological centers geo3bcn csic where the paleomagnetic analyses were conducted and also the laboratorio de geocronologia y geoquimica isotopica of the universidad complutense de madrid spain where the sr samples were processed in addition the study of calcareous nannofossils was supported by the research group of the basque university system it 930 16 we are also grateful to the national iranian oil company for their support during fieldwork we thank darius programme and its sponsors and projects alpimed pie csic 201530e082 and subtetis pie csic 201830e039 for additional support

healing of the shallow fault zone from 1994 1998 after the 1992 m7 5 landers california earthquake we conducted seismic surveys at the johnson valley fault in 1994 1996 and 1998 we found that the shear velocity of the fault zone rock increased by 1 2 between 1994 and 1996 and increased further by 0 7 between 1996 and 1998 this trend indicates the landers rupture zone has been healing by strengthening after the mainshock most likely due to the closure of cracks that opened during the 1992 earthquake the observed fault zone strength recovery is consistent with a decrease of 0 03 in the apparent crack density within the fault zone the ratio of decrease in travel time for p to s waves changed from 0 75 in the earlier two years to 0 65 in the later two years between 1994 and 1998 suggesting that cracks near the fault zone are partially fluid filled and have became more fluid saturated with time

site velocities before and after the loma prieta and gulf of alaska earthquakes determined from vlbi we use geodetic data from vlbi to determine the pre and postseismic velocities of two sites we then place limits on variations in interseismic strain buildup the 1987 and 1988 gulf of alaska earthquakes each ms 7 6 broke the pacific plate interior during the earthquakes the cape yakataga site moved 78 mm toward southwest during the 1989 loma prieta earthquake ms 7 1 the fort ord site moved 48 mm toward north baselines a from fairbanks to cape yakataga and b from mojave to fort ord change at nearly the same rate before and after the earthquakes postseismic transients which we determine from differences between post and preseismic rates are minor at cape yakataga the transient is 3 4 mm in a postseismic interval of 23 months and at fort ord the transient is 6 5 mm in 21 months the slip beneath the loma prieta rupture needed to generate the fort ord transient is 0 22 0 19 m one tenth the coseismic slip 2 m we analyze elastic lithosphere viscous asthenosphere models to determine that the characteristic time describing exponential decay in deep fault slip is longer than 6 years the vlbi measurements are consistent with uniform interseismic strain buildup they disagree with fast postseismic rates caused by an asthenosphere with very low viscosity

velocity shift in heterogeneous media with anisotropic spatial correlation summary converted waves analysis is used to map the anisotropy as a function of depth in the upper mantle stacking of sv a ndt component records of several seismic events well distributed in azimuth helps to detect these converted waves weighting coe cients are used to extract the anisotropic signal in the records synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy eiects from the eiects of lateral heterogeneity such as those resultingfrom dipping structures a good azimuthal coverage of the data set is important in order to separate the diierent eiects in the stack sections obtained for diierent azimuthal orders the procedure is applied to data observed at geoscope station can and gives a model with two anisotropic layers in the upper mantle the upper layer located between the crust and a depth of140 km has its fast axis in an east west direction the lower layer 40 km thick has its fast axis in a north south direction the fast axis directions are in agreement with results obtained from surface wave analysis and tectonic information

geodetic estimate of coseismic slip during the 1989 loma prieta california earthquake offsets in the relative positions of geodetic stations resulting from the loma prieta earthquake can be explained with a dislocation model that includes buried oblique slip on a rupture surface extending 37 km along the strike of the san andreas fault dipping 70 degree to the sw and extending from a depth of about 5 to 17 5 km assuming uniform slip on a rectangular surface the mean values for a range of reasonable fault geometries are 1 6 plus minus 0 3 m right lateral strike slip and 1 2 plus minus 0 3 m reverse slip slip on an adjacent extension of the rupture to the southeast recorded in the aftershock sequence is not well constrained by the geodetic data the geodetic data clearly preclude rupture extending near the surface

horizontal structure of the electric field in the stratiform region of an oklahoma mesoscale convective system 1 this analysis combines vertical electric field components ez observed by two research aircraft flying horizontally at two levels with vertical soundings of thermodynamic parameters and ez made by five balloons to produce a quasi three dimensional view of the space charge distribution in the trailing stratiform cloud region behind a mesoscale convective system mcs that developed in central oklahoma late in the afternoon of 2 june 1991 the balloons were launched serially at one hour intervals from two sites separated by 80 km along a north south line as the mcs moved eastward yielding two east west time height cross sections of the ez structure within the quasi steady state trailing stratiform region behind the mcs the balloon measurements are consistent with a vertical stack of five rearward and downward sloping horizontal sheets of charge of alternating polarity beginning at the bottom with a negative charge layer below the 0 c level and a positive layer near the 0 c level this structure persisted for more than 2 hours the two aircraft flew back and forth along a north south line through the balloon launch sites during the balloon launch period aircraft measurements demonstrated that the vertical electric field ez at constant altitude varied in the north south direction the peak magnitudes of ez deduced from the airborne instrument systems agreed with the magnitudes deduced from the balloon measurements at the aircraft altitudes of 4 5 km and 5 8 km agl rapid reversals in polarity of ez with peak magnitude 50 kv m 1 observed by the aircraft at 4 5 km just above the 0 c level confirms the thin concentrated positive charge layer observed there by balloons and suggests that this charge layer is undulating above and below 4 5 km altitude at least in the north south direction microphysically this layer contained large aggregates and pockets of low cloud liquid water concentration at the 5 8 km level the polarity of ez was always positive but the magnitude varied from zero to 25 kv m 1 aircraft observed ez at both altitudes varied on horizontal scales of 10 km or greater at both levels suggesting that the charge density derived using the one dimensional infinite layer gauss s law approximation applied to the balloon soundings of ez is valid in this study these observations show that layers of charge can persist for hours as they advect rearward in a storm relative sense possibly due to continuing in situ charge separation and or due to weak dispersion slow recombination and slow settling of charge attached to low mobility low terminal velocity ice hydrometeors

vertical radar reflectivity structure and cloud to ground lightning in the stratiform region of mcss further evidence for in situ charging in the stratiform region abstract this study presents further evidence in support of an in situ noninductive charging mechanism as the process likely responsible for significant electrification of the trailing stratiform regions of mesoscale convective systems mcss in contrast to previous studies of mcs electrification that have investigated observations of radar reflectivity and cloud to ground lightning in the horizontal e g orville et al rutledge et al here the relationship between the location and occurrence of cloud to ground lightning in the stratiform regions of midlatitude and tropical mcss and the vertical profile of radar reflectivity are examined the vertical profile of radar reflectivity at elevations above the 0 c level is used as a proxy for the amount of mass present in the mixed phase region of the stratiform clouds which in turn is related to the generation of charge through a noninductive charging mechanism to further explore the relationship between radar reflectivity mixed phase microphysics a

complexity of the coseismic rupture for 1999 chi chi earthquake taiwan from inversion of gps observations abstract based on the elastic half space model and the layered earth model we use global position system observations to invert the distribution of coseismic slip along the chi chi earthquake rupture bending fault planes are introduced in our models the results indicate that the shear slip model can not fit horizontal and vertical coseismic displacements simultaneously unless the fault normal motion tensile component is considered in the inversion we interpret that the fault normal motion reflects the roughness of fault planes demonstrating the complexity of the rupture in property and geometry the distribution of the fault normal motion appropriately fits with the complexity of surface coseismic rupture from the field investigation and the distribution of the strike dip slip of coseismic rupture is similar to the results from inversion of the teleseismic data

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

cloud to ground lightning associated with the evolution of a multicell storm 1 the relationship of cloud to ground cg lightning strikes to the stages of updraft downdraft evolution is investigated in a multicell thunderstorm that occurred on 11 july 1989 during the north dakota thunderstorm project with only single doppler radar data available for this storm reflectivity and single doppler velocity signatures are used to identify individual cells within the storm and to deduce the stage of evolution within each cell cell evolution appeared to follow the thunderstorm project model of the late 1940s updraft growth and vertical development during the growing cumulus stage updraft peaking in the upper portion of the cell and rainy downdraft descending from the middle portion during the mature stage and lighter precipitation settling out during the dissipating stage on 11 july cells tended to occur in clusters each cluster consisting of growing mature and dissipating cells close together in both time and space no cg strikes were detected when the convective region of the storm contained only one cluster of cells all strikes occurred when there were two or more clusters of cells with only two exceptions strikes occurred in the convective region of the storm typically closer to growing mature cells than to dissipating cells these observations support the hypothesis that descending precipitation creates electrical conditions conducive to flashes reaching ground however these observations also suggest that the greater complexity of electrical structure due to the juxtaposition of several cells is especially conducive to cg flashes and can cause cg flashes to occur earlier in a new cell s life cycle than would otherwise be the case

crustal structure and deformation across a mature slab tear zone the case of southern tyrrhenian subduction italy we compute s velocity profiles of the crust across the messina strait italy the tear zone at the southern end of the ionian subduction zone separating sicily from calabria the messina strait hosted some of the strongest earthquakes to ever occur in italy here the motion of the ionian slab with respect to sicily creates a complex tectonic setting characterized by lithospheric tearing we show velocity models of the crust computed from teleseismic receiver function inversion outlining the differences between sicily and calabria strong deformation across the messina strait between 10 15 and 30 km depth is expressed by strong anisotropy up to 10 developed in a ductile shear zone of the crust the top of these ductile weaker layers could limit the depth extent of future ruptures

the neotethyan sanandaj sirjan zone of iran as an archetype for passive margin arc transitions the sanandaj sirjan zone of iran is a northwest trending orogenic belt immediately north of the zagros suture which represents the former position of the neotethys ocean the zone contains the most extensive best preserved record of key events in the formation and evolution of the neotethys from its birth in late paleozoic time through its demise during the mid tertiary collision of arabia with eurasia the record includes rifting of continental fragments off of the northern margin of gondwanaland formation of facing passive continental margins initiation of subduction along the northern margin and progressive development of a continental magmatic arc the latter two of these events are critical phases of the wilson cycle that elsewhere in the world are poorly preserved in the geologic record because of superimposed events our new synthesis reaffirms the similarity between this zone and various terranes to the north in central iran late paleozoic rifting preserved as a type granites and accelerated subsidence was followed by a phase of pronounced subsidence and shallow marine sedimentation in permian through triassic time marking the formation and evolution of passive margins on both sides of the suture subduction and arc magmatism began in latest triassic early jurassic time culminating at 170 ma the extinction of arc magmatism in this zone and its shift northeastward to form the subparallel urumieh dokhtar arc occurred diachronously along strike in late cretaceous or paleogene time post cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid tertiary time

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

principal component analysis of geodetically measured deformation in long valley caldera eastern california 1983 1987 typical geodetic measurements of deformation consist of repeated surveys of a particular geodetic network such deformation data can be interpreted as a consequence of one or more self coherent sources by means of principal component analysis a self coherent source is defined as any source that produces deformation that is time and space separable principal component analysis then gives the time and space factors that characterize the deformation attributed to each self coherent source geodetic measurements of deformation at long valley caldera provide two examples of the application of principal component analysis a 40 line trilateration network surrounding the caldera was surveyed in midsummer 1983 1984 1985 1986 and 1987 principal component analysis indicates that the observed deformation can be represented by a single coherent source the time dependence for that source displays a rapid rate of deformation in 1983 1984 followed by less rapid but uniform rate in the 1984 1987 interval the spatial factor seems consistent with expansion of a magma chamber beneath the caldera plus some shallow right lateral slip on a vertical fault in the south moat of the caldera an independent principal component analysis of the 1982 1983 1984 1985 1986 and 1987 leveling across the caldera requires two self coherent sources to explain the deformation the deformation pattern produced by the larger of these two sources appears to be roughly consistent with that found from the trilateration data the deformation due to the second source is a nearly uniform tilt in the uplift profile presumably that tilt is simply an artifact of systematic error in the leveling

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

seismic imaging of the lithospheric structure of the zagros mountain belt iran we present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across central and northern zagros the receiver function analysis of teleseismic earthquake records gives a high resolution image of the moho beneath the seismic transects on both cross sections the crust has an average thickness of 43 2 km beneath the zagros fold and thrust belt and the central domain the crust is thicker beneath the hanging wall of the main zagros reverse fault mzrf with a larger maximum moho depth in central 69 2 km than in northern zagros 56 2 km the thickening affects a narrower region 170 km beneath the sanandaj sirjan zone of central zagros and a wider region 320 km in northern zagros we propose that this thickening is related to overthrusting of the crust of the arabian margin by the crust of central iran along the mzrf considered as a major thrust fault cross cutting the whole crust the fault is imaged as a lowvelocity layer in the receiver function data of the northern zagros profile moreover the crustal scale thrust model reconciles the imaged seismic moho with the bouguer anomaly data measured on the central zagros transect at upper mantle depth p wave tomography confirms the previously observed strong contrast between the faster velocities of the arabian margin and the lower velocities of the iranian micro blocks our higher resolution tomography combined with surface wave analysis locates the suture in the shallow mantle of the sanandaj sirjan zone beneath central zagros the arabian upper mantle has shield like shear wave velocities while the lower velocities of the iranian upper mantle are likely due to higher temperature but these velocities are not low enough and the low velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath iran the lack of a high velocity anomaly in the mantle beneath central iran suggests that the neotethyan oceanic lithosphere is probably detached from the arabian margin

analysis of a small vigorous mesoscale convective system in a low shear environment part i formation radar echo structure and lightning behavior the evolution of a small vigorous mesoscale convective system mcs over northern alabama is described using doppler radar goes satellite surface mesonet lightning and sounding data the mcs formed near noon in a relatively unstable environment having weak synoptic forcing and weak shear the initiation of separate lines and clusters of deep convection occurred in regions exhibiting cumulus cloud streets horizontal variations in stratocumulus cloud cover and variations in inferred soil moisture mcs growth via merger of storms within clusters and lines and among the clusters was accomplished largely through intersection of storm scale and mesoscale outflow boundaries the mcs maximum anvil area 60 000 km2 at 220 k and lifetime 8 h were about 50 that of the typical great plains mesoscale convective complex mcc despite its smaller size this mcs displayed many aspects that typify the mostly nocturnal great plains mcs the precipitation output was highly variable due to the transient nature of the intense convective elements many of which produced microbursts the radar measurements documented the formation of a stratiform region along the trailing side of an intense convective line this stratiform region formed as decaying convective cores coalesced rather than through advection of precipitation particles directly from the convective region combined goes ir imagery and radar reflectivity analyses within the stratiform region show a sinking anvil cloud top in the presence of increases in the vertical radar reflectivity gradient within the cloud during the maturation of the stratiform region during its intense developing stages the mcs generated a peak cloud to ground cg flash rate of 2400 h 21 comparable to rates produced by larger mccs early on positive cg flashes were most prevalent around intense convective core regions exhibiting strong divergence at anvil level during the latter stages the emergence of positive cg was coincident with the formation of a prominent radar bright band within the stratiform region thus a bipole was established but its length was quite short at approximately 50 km 25 50 of the distance documented in other mcss

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

postseismic deformation following the 1906 san francisco earthquake we have reevaluated triangulation data from northern california following the 1906 san francisco earthquake thereby increasing the temporal and spatial resolution of postseismic deformation following that event we have calculated uniform shear strain rates and average station velocities at point arena using data from 1906 1907 1929 1930 and 1973 1975 and for the point reyes petaluma arc using data from 1929 1930 1938 1939 and 1960 1961 with the addition of recent geodetic data we infer an effective relaxation time for long term postseismic deformation following the 1906 earthquake of 36 16 years the point arena data are satisfactorily fit with accelerated afterslip at depth along the san andreas fault plane for a 10 km deep 25 km wide afterslip zone the average slip rate between 1906 and 1930 is 11 2 cm yr between 1929 and 1975 it is 4 6 cm yr deformation in the point reyes petaluma arc is clearly asymmetric with respect to the trace of the san andreas fault especially between 1929 and 1939 after inverting for the range of acceptable accelerated deep afterslip and horizontal detachment models a detailed analysis using geologically reasonable geometries revealed that both model types have trouble explaining the spatial variations in the deformation field through time in particular accelerated deep afterslip models cannot reproduce the breadth of the observed deformation field to the northeast of the san andreas fault as a result more complex time varying deformation mechanisms are required to explain the point reyes petaluma arc observations

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

contractional salt tectonic system in the south dezful embayment zagros abstract the dezful embayment in zagros has traditionally been interpreted as a typical fold belt with multiple detachment levels including a lower detachment at the basement cover interface and an upper detachment along the miocene gachsaran halite bearing evaporites this paper constrains diapiric processes in the gachsaran detachment coeval with regional shortening that resulted in complex and hitherto unstudied salt structures and flanking synclinal minibasins in this paper well tied seismic interpretation is integrated with field and remote sensing mapping cross section construction and stepwise tectono sedimentary restoration the results indicate that after the deposition of mishan lower aghajari pre kinematic overburden sediments detachment folding initiated above the gachsaran evaporites during late tortonian times as a result of regional shortening the salt cored anticlines later actively pierced their roof to subsequently rise as passive diapirs and evolve into salt walls synclinal minibasins developed due to salt evacuation into flanking walls during the deposition of the lahbari mb in messinian pliocene remarkable sedimentary thickening and lateral shifts in minibasin depocenter towards the salt walls accompanied by overturned collars and hook halokinetic sequences are diagnostics of near surface syndepositional passive growth of diapirs during this stage from pliocene to recent the major episode of regional shortening has squeezed these precursor salt walls to form thrust welds in conclusion the well exposed structures of south dezful show an early shortening stage then a transition to a combination of shortening and passive diapirism and final thrusting along diapirs this evolution model may be more common in other salt influenced fold thrust belts than is currently recognized

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

postseismic deformation following the 1989 m 7 1 loma prieta california earthquake postseismic deformation along a 90 km profile bisecting the projected surface trace of the coseismic rupture of the 1989 loma prieta earthquake has been monitored by frequent gps surveys for 3 3 years following the earthquake in addition to the expected deformation associated with secular strain accumulation on the san andreas and calaveras faults deformation associated with postseismic readjustment has been detected most of that deformation can be attributed to 1 5 m right lateral and 0 9 m reverse postseismic slip on a 5 km wide downdip extension of the loma prieta rupture in addition there seems to be a 0 1 m postseismic collapse of the loma prieta rupture zone in the direction perpendicular to the plane of the rupture the fault normal n48 e surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1 4 years similar plots of the fault parallel n42 w displacement components do not exhibit significant curvature presumably the deformation shown in those plots is dominated by secular strain accumulation along the san andreas and calaveras faults rather than postseismic relaxation

the south zagros suture zone in teleseismic images abstract the geometry of intra continental lithosphere boundaries along the zagros orogenic belt in the arabia eurasia collision is investigated by means of teleseismic data the data are gathered over a seismic linear profile extending across south zagros the sanandaj sirjan metamorphic zone the urumieh dokhtar magmatic arc central iran and the kopeh dagh binalud mountains we exploit the p and s receiver functions leading to map the geometry of the crustal and subcrustal interfaces the migrated depth sections reveal an abrupt crustal thickening and a gentle crustal thinning 60 km north and 30 km south of the zagros suture respectively associated to the buckled antiformal moho south of the suture a deeper synform in the lithospheric lid of the lower arabia plate is shown by migrated depth sections affecting the lithospheric mantle of the arabia plate beneath the suture zone this geometry implies an unexpected intra lid decoupling these features imply that the central iran lithosphere acts as a relatively strong backstop producing significant internal deformation expressed by shortening and thickening at the edge of the arabian lithosphere the 410 km and 660 km transition zones are imaged by p to s converted phases and showed lateral continuity implying an originally low dip angle subduction of the oceanic arabian plate beneath central iran

postseismic deformation associated with the 1992 m ω 7 3 landers earthquake southern california following the 1992 m 7 3 landers earthquake a linear array of 10 geodetic monuments at roughly 5 km spacing was established across the emerson fault segment of the landers rupture the array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it the array was surveyed by global positioning system 0 034 0 048 0 381 1 27 1 88 2 60 and 3 42 years after the landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation the temporal behavior is described roughly by a short term decay time 84 23 days exponential relaxation superimposed upon an apparently linear trend because the linear trend represents motions much more rapid than the observed preseismic motions we attribute that trend to a slower decay time greater than 5 years postseismic relaxation the curvature of which cannot be resolved in the short run 3 4 years of postseismic data about 100 mm of right lateral displacement and 50 mm of fault normal displacement accumulated across the geodetic array in the 3 4 year interval covered by the postseismic surveys those displacements are attributed to postseismic right lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace the right lateral slip amounted to about 1 m directly beneath the geodetic array and the fault normal displacement is apparently primarily a consequence of the curvature of the rupture these conclusions are based upon dislocation models fit to the observed deformation however no dislocation model was found with rms residuals as small as the expected observational error

electrification of stratiform regions in mesoscale convective systems part ii two dimensional numerical model simulations of a symmetric mcs abstract model simulations of a symmetric mesoscale convective system mcs observations discussed in part i were conducted using a 2d time dependent numerical model with bulk microphysics a number of charging mechanisms were considered based on various laboratory studies the simulations suggest that noninductive ice ice charge transfer in the low liquid water content regime characteristic of mcs stratiform regions is sufficient to account for observed charge density magnitudes and as much as 70 of the total charge in the stratiform region the remaining 30 is contributed by charge advection from the convective region the strong role of in situ charging is consistent with previous water budget studies which indicate that roughly 70 of the stratiform precipitation results from condensation in the mesoscale updraft thus both in situ charging and charge advection the two previously identified hypotheses appear to be important contributors to the electrical budget of the stratiform region the

time dependent triggered afterslip following the 1989 loma prieta earthquake we employ a modified version of the network inversion filter to investigate time dependent slip following the 1989 loma prieta earthquake previous analysis of global positioning system gps and leveling data suggests afterslip on the loma prieta rupture as well as aseismic slip on a thrust fault northeast of the san andreas fault which we identify with the foothills thrust belt we analyzed 173 daily gps solution files at 62 stations collected from 1989 8 to 1998 3 a total of 1 134 three dimensional relative baseline determinations the observed position changes are assumed to result from secular deformation random benchmark motions and temporally varying fault slip the data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip the amount of temporal smoothing is estimated by maximum likelihood conditional on this estimate reverse slip on the foothills thrust decays from 45 12 mm yr immediately after the earthquake to zero by 1992 reverse slip on the loma prieta rupture surface decays from 57 11 mm yr to zero by 1994 right lateral slip on the loma prieta rupture surface decays monotonically from 30 10 mm yr to zero by 1994 these results suggest that 1 triggered afterslip can occur off the main rupture zone on adjacent faults 2 shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake and 3 postseismic slip on the foothills thrust may account for a significant portion of its total slip budget

seismic anisotropy in central north anatolian fault zone and its implications on crustal deformation abstract we investigate the crustal seismic structure and anisotropy around the central portion of the north anatolian fault zone a major plate boundary using receiver function analysis the characterization of crustal seismic anisotropy plays a key role in our understanding of present and past deformation processes at plate boundaries the development of seismic anisotropy in the crust arises from the response of the rocks to complicated deformation regimes induced by plate interaction through the analysis of azimuthally varying signals of teleseismic receiver functions we map the anisotropic properties of the crust as a function of depth by employing the harmonic decomposition technique although the moho is located at a depth of about 40 km with no major offset across the area our results show a clear asymmetric distribution of crustal properties between the northern and southern blocks divided by the north anatolian fault zone heterogeneous and strongly anisotropic crust is present in the southern block where complex intra crustal signals are the results of strong deformation in the north a simpler and weakly anisotropic crust is typically observed the strongest anisotropic signal is located in the first 15 km of the crust and is widespread in the southern block stations located on top of the main active faults in the area indicate the highest amplitudes together with fault parallel strikes of the fast plane of anisotropy we interpret the origin of this signal as due to structure induced anisotropy and roughly determine its depth extent up to 15 20 km for these stations away from the faults we suggest the contribution of previously documented uplifted basement blocks to explain the observed anisotropy at upper and middle crustal depths finally we interpret coherent ne sw orientations below the moho as a result of frozen in anisotropy in the upper mantle as suggested by previous studies

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

lightning location relative to storm structure in a leading line trailing stratiform mesoscale convective system 1 horizontal and line normal vertical cross sections and composite images of dallas fort worth lightning detection and ranging ldar ii vhf radiation sources and radar reflectivity over a 30 min period provide a unique perspective on lightning pathways within a leading line trailing stratiform llts mesoscale convective system mcs on 16 june 2002 the overwhelming majority of vhf lightning sources occurred within the leading convective line in a bimodal pattern in the vertical assuming that vhf source density maxima were most likely associated with positive charge then the ldar ii observations suggest that the gross charge structure of the convective region of the mcs was characterized by a tripole with net positive charge centered at 4 5 km agl 3 c and 9 5 km agl 35 c and net negative charge centered roughly in the relative minimum of the vhf source density maximum at 7 km agl 17 c a persistent lightning pathway and inferred positive charge zone sloped rearward by 40 50 km and downward by 4 5 km from the upper vhf source maximum in the convective line through the transition zone and into the radar bright band of the stratiform region in the stratiform region vhf lightning sources and inferred positive charge were concentrated in three layers centered at 4 5 6 and 9 km agl 2 c 11 c and 31 c respectively consistent with past electric field studies of symmetric llts mcss positive cloud to ground lightning flashes in the stratiform region were initiated in the convective line and followed the slanting pathway from the top of convective cores to the stratiform precipitation where they were horizontally extensive layered and highly branched the sloping lightning pathway was identical to hypothetical trajectories taken by snow particles these observations provide further support for the advection of charge on snow along the sloping pathway and the in situ generation of charge in the horizontal lightning layers as primary contributors to electrification and positive lightning production rearward of the convective line

mapping seismic anisotropy using harmonic decomposition of receiver functions an application to northern apennines italy 1 isotropic and anisotropic seismic structures across the northern apennines italy subduction zone are imaged using a new method for the analysis of teleseismic receiver functions rfs more than 13 000 p wave coda of teleseismic records from the 2003 2007 retreating trench extension and accretion tectonics retreat experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of mediterranean evolution a new methodology for the analysis of receiver functions is developed which combines both migration and harmonic decomposition of the receiver function data set the migration technique follows a classical common conversion point scheme and helps to focus on a crucial depth range 20 70 km where the mantle wedge develops harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of p to s converted phases the separation of the back azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures the application of our method to the retreat data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the northern apennines chain where previous studies failed to clearly retrieve the geometry of the subducted interfaces we present our results in closely spaced profiles across and along the northern apennines chain to highlight the convergence of the tyrrhenian and the adriatic microplates which differ in their crustal structure where the adriatic microplate subducts beneath tuscany and the tyrrhenian sea a signature of the dipping adriatic moho is clearly observed beneath the tyrrhenian moho in a large portion of the forearc region in the area where the two mohos overlap our new analysis reveals the presence of an anisotropic body above the subducted moho there is a strong ps converted phase with anisotropic characteristics from the top of the adriatic plate to a depth of at least 80 km because the ps conversion occurs much deeper than similar ps phases in cascadia and japan dehydration of oceanic crust seems unlikely as a causative factor rather the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting

on the origin of diverse aftershock mechanisms following the 1989 loma prieta earthquake summary we test the hypothesis that the origin at the diverse suite of aftershock mechanisms following the 1989 m 7 1 loma prieta california earthquake is related to the post main shock static stress field we use a 3 d boundary element algorithm to calculate static stresses combined with a coulomb failure criterion to calculate conjugate failure planes at aftershock locations the post main shock static stress field is taken as the of a sum of a pre existing stress field and changes in stress due to the heterogeneous slip across the loma prieta rupture plane the background stress field is assumed to be either a simple shear parallel to the regional trend of the san andreas fault or approximately fault normal compression a suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared allowing for uncertainties in both mechanism parameters and earthquake locations to well constrained mechanisms reported in the us geological survey northern california seismic network catalogue we also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes assuming either plane to be a likely rupture plane various permutations of the assumed background stress field fractional coefficients of aftershock fault planer methods of comparisons etc explain between 52 and 92 per cent of the aftershock mechanisms we can explain a similar proportion of mechanisms however by comparing a randomly reordered catalogue with the various suites of synthetic aftershocks the inability to duplicate aftershock mechanisms reliably on a one to one basis is probably a function of the combined uncertainties in models of main shock slip distribution the background stress field and aftershock locations in particular we show theoretically that any specific main shock slip distribution and a reasonable background stress field are able lo generate a highly variable suite of failure planes such that quite different aftershock mechanisms may be expected to occur within a kilometre of less of each other this scale of variability is less than the probable location error of aftershock earthquakes in the loma prieta region we successfully duplicate a measure of the variability in the mechanisms of the entire suite of aftershocks if static stress changes are responsible for the generation of aftershock mechanisms we are able to place quantitative constraints on the level of stress that must have existed in the upper crust prior to the loma prieta rupture this stress level appears to be too low to generate the average dip across the main shock rupture plane possible reasons for this result range from incorrect initial assumptions of homogeneity in the background stress field friction and fault geometry to driving stresses that arise from deeper in the crust or upper mantle alternatively aftershock focal mechanisms may be determined by processes other than or in addition to static stress changes such as pore pressure changes or dynamic stresses

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow

upper mantle stratification by p and s receiver functions summary seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the graefenberg array in southern germany the anisotropic p receiver function technique kosarev et al 1984 vinnik montagner 1996 is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the sv and t components of the pds phases the summary events provide better separation of the second azimuthal harmonic than the individual events the parameters of the second harmonics of sv and t thus evaluated should be similar if they reflect the effects of azimuthal anisotropy this can be used as a criterion to identify the anisotropy to detect the sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique which can be termed the s receiver function technique it includes axis rotation to separate interfering p and s arrivals determination of the principal m component of the s wave motion deconvolution of the p components of many recordings by their respective m components and stacking of the deconvolved p components with weights depending on the level of noise and the angle between the m direction and the backazimuth of the event both techniques yield consistent results for the graefenberg array as indicated by the p receiver functions the upper layer of the mantle between the moho and 80 km depth is anisotropic with dvs vs around 0 03 and the fast direction close to 20 clockwise from north the fast direction of anisotropy below this layer is around 110 the boundary between the upper and the lower anisotropic layers is manifested by the detectable pds and sdp converted phases shear wave splitting in sks is strongly dominated by azimuthal anisotropy in the lower layer asthenosphere