upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

the 1981 microearthquake swarm near moodus connecticut from august through october 1981 an intense microearthquake swarm of over 500 events was detected near moodus connecticut the site of persistent small earthquake activity throughout the historical record the swarm consisted of 39 foreshocks which preceeded the magnitude 2 1 mainshock a normal pattern of aftershocks which immediately followed the main shock and later smaller bursts of microearthquakes associated with migrations of the event epicenters the shocks were all within 1 km of the earth s surface and located along a 5 km long segment of the salmon river earthquakes of magnitude less than 1 were felt and events as small as magnitude 0 were heard by people living near the epicenters

mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

evidence for ancient lithospheric deformation in the east european craton based on mantle seismic anisotropy and crustal magnetics abstract we present new shear wave splitting measurements performed at 16 stations on the east european craton and discuss their implications in terms of upper mantle anisotropy for this geophysically poorly known region previous investigations of mantle anisotropy in central europe have shown fast directions aligning smoothly with the craton s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen in deformation here we aim at investigating the continuation of this shear wave splitting pattern further to the east into the east european craton for the craton the interpretation appears to be less ambiguous than for central europe since several arguments support lithospheric anisotropy in this region 1 the large scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small scale 2 the weak correlation with absolute plate motion vectors and 3 the good correlation between anisotropy and crustal features for which we use magnetic field alignments as a proxy rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust the observed splitting orientations thus reflect the last tectonic events of each block frozen in into the lithosphere for hundreds of millions of years

the 23 april 1984 martic earthquake and the lancaster seismic zone in eastern pennsylvania a seismic zone that has been a persistent source of seismicity for at least two centuries contains most of the known earthquakes in southeastern pennsylvania and is named the lancaster seismic zone lsz from the county it approximately occupies the lsz is identified by new epicenters for known and previously unknown earthquakes obtained from a systematic search of newspapers covering one and a half centuries of the preinstrumental period 1750 to 1900 the new preinstrumental epicenters and reliable instrumental epicenters indicate that the lsz is about 50 km long and spans the belt of allochthonous appalachian crystallines between the great valley and the martic line an early paleozoic suture the lsz also bridges across the newark gettysburg triassic basin the overall level and character of seismicity on the lsz has been uniform at least since 1800 nevertheless three bursts can be identified the latest burst started in the middle 1960 s the 23 april 1984 martic earthquake mblg 4 1 is the largest in this burst and resembles in size the largest events in the other bursts the 1984 event is located in the southernmost portion of the lsz hypocenter distribution and first motion data from several locally recorded events in the 1984 sequence suggest a seismogenic fault in a narrow depth range centered at 4 5 km the inferred rupture dips steeply east with reverse and right lateral movement consistent with ene horizontal maximum compression the rockhill jurassic dike and associated faults strike nne and may control the geometry of the 1984 rupture and the overall distribution of seismicity in the lsz

localized imaging of the uppermost mantle with usarray pn data 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states this allows increased resolution for regional studies of the lithosphere and deeper structure of the north american continent in this study we use pn phases in the usarray data set to solve for velocity structure in the uppermost mantle in the western united states this article focuses on localized imaging techniques that complement traditional pn tomography analysis we apply waveform cross correlation to obtain inter station travel times between the closely and uniformly spaced usarray stations this allows us to use traces without phase picks and reduces errors associated with the picking we obtain differential times that can directly be used to fit locally for slowness and depending on the approach for the direction and curvature of the incoming wavefront the various measurements of incoming wavefronts at different sub arrays provide constraints on azimuthal variations in velocity the traditional tomography approach and the local fitting method reveal similar large scale features no regularization is applied with the local method and the resulting velocity maps reveal smaller scale structures than the tomographic images

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

statistical aspects of new england seismicity from 1975 to 1982 and implications for past and future earthquake activity abstract the seismicity of new england from 1 october 1975 to 30 november 1982 has been studied to determine how well it represents past seismicity and what it might imply about future earthquakes just over 100 earthquakes with magnitudes between 2 0 and 4 7 were recorded during this modern time period while the historic earthquake catalog of chiburis 1981 lists 760 earthquakes in new england from 1568 to 1975 the largest having maximum modified mercalli intensity vii shaking the spatial distribution of the recent seismicity reproduces that for the historical seismicity patterns quite well with the larger earthquakes in the modern record having preferentially occurred along coastal and lowland new england or in central new hampshire space time plots show that the seismicity displays considerable spatial variation and that there are only vague hints of migrations of epicenters the number of earthquakes as well as the amount of seismic energy release appear essentially random in time and the recurrence curve for the modern time period log number of events versus magnitude is a straight line for events between magnitudes 2 and 4 5 with a b value of 0 84 0 03 the sizes of the largest earthquakes in the historic record are those which would be expected by extrapolating the present seismicity rate backward in time using a gumbel type i distribution one major discrepancy between the statistics of the recent and historic catalogs is that the latter does not have as many moderate magnitudes 5 to 5 8 earthquakes as would be expected from the recent data this discrepancy could be due to inaccuracies in the historic catalog to unusually high recent seismicity or to a real historical shortage of such moderate magnitude earthquakes large earthquakes may be more likely to happen in areas of past and present high seismicity especially in central and southern new hampshire along coastal maine and along the south coast to new england

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

seismic anisotropy beneath the mississippi embayment and the new madrid seismic zone a study of shear wave splitting shear wave splitting associated with the mississippi embayment me is determined using teleseismic sks phases recorded by the northern embayment lithosphere experiment nele the usarray transportable array ta and the new madrid seismic network for the period 2005 2016 our data set consists of 5900 individual splitting measurements from 257 earthquakes recorded at 151 stations within and outside the me stations outside of the me exhibit significant shear wave splitting with average delay times between 0 4 s and 1 8 s to the northeast and east of the me nearly all observed fast orientations are approximately oriented northeast southwest in agreement with absolute plate motion apm predicted by hs3 nuvel 1a the homogeneity of the fast orientations in this region suggests that the splitting is due to active flow in the asthenosphere a counterclockwise rotation in the splitting orientation is observed moving northeast to northwest across the study area inside the me some stations show large and systematic deviations of the measured fast orientations from the apm the delay times within the entire me range from 0 9 s to 2 1 s splitting complexity is attributed to relic lithospheric fabrics formed during past tectonic events including passage of a hot spot in mid cretaceous time the anisotropy may also be linked to the presence of a southwest dipping region of low p and s wave velocities below the me or to deeper flow in the asthenosphere

shear wave splitting and the pattern of mantle flow beneath eastern oregon a r t i c l e i n f o article history the tectonic and geologic setting of eastern oregon includes the volcanically active high lava plains hlp province and the accreted terrains of the blue and wallowa mountains and is bounded by the columbia river ood basalts to the north basin and range extension to the south the cascade arc to the west and stable north america to the east several models have been proposed to explain the tectonic evolution of eastern oregon and in particular the voluminous volcanic activity in the hlp but a consensus on which model fully describes the complex range of processes remains elusive measurements of the seismic anisotropy that results from active mantle ow beneath the region can provide a crucial test of such models to constrain this anisotropy here we present new sks splitting results obtained at approximately 200 broadband seismic stations in eastern oregon and the surrounding region data come from the usarray transportable array ta and two temporary experiments carried out in the hlp and in the wallowa mountains our splitting data set includes 2900 individual splitting measurements from sks phases recorded between 2006 and 2008 stations in eastern oregon exhibit signi cant shear wave splitting with average delay times at individual stations between 0 8 s and 2 7 s in the hlp nearly all observed fast directions are approximately e w while to the north in the blue and wallowa mountains there is more variability in the splitting patterns the average delay time observed at stations located in the heart of the hlp province is 2 s well above the global average of 1 s for continental regions we infer from the large split times and homogeneous fast directions that there must be signi cant active ow in a roughly e w direction in the asthenosphere beneath the hlp this inferred ow eld places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region in the wallowa region the anisotropic signature is more complicated and there may be a signi cant contribution from fossil fabrics in the crust or mantle lithosphere

sks splitting beneath southern california measurements of sks phase splitting were obtained from nineteen seismic stations in southern california the fast polarization directions are 53 at the southern end of the great valley 82 8 in the western transverse ranges and northern peninsular ranges 95 4 in mojave desert and 70 on san clemente island the splitting time ranges from 0 8 to 1 8 seconds which is consistent with an anisotropic layer of 100 to 200 km thick for 4 anisotropy

seismically imaged relict slab from the 55 ma siletzia accretion to the northwest united states teleseismic tomography images a large high velocity curtain extending vertically beneath the ca 50 ma challis magmatic trend to maximum depths of 230 600 km we interpret this structure as subducted farallon ocean lithosphere that stalled with the ca 55 ma accretion of the siletzia microplate to north america within the columbia embayment and we consider the regional tectonic implications the abrupt switch ca 53 ma from laramide thrusting and magmatic quiescence to extension and vigorous magmatism in the northwestern united states is evidence for foundering of the fl at subducting farallon slab to account for the imaged curtain foundering apparently occurred by rollback after siletzia accretion terminated subduction within the columbia embayment the magnitude of the high seismic velocity curtain is approximately that expected for 20 50 m y old ocean lithosphere that stalled in the upper mantle ca 50 ma we suggest that the stalled slab became nearly neutrally buoyant as a result of removal of its basaltic crust the melting of which likely contributed to the short lived vigor of challis magmatism after siletzia accretion normal dip cascadia subduction initiated west of siletzia evidenced by arc volcanism in oregon and washington beginning ca 45 40 ma while continued quiescence of the sierra nevada arc suggests persistence of flsubduction to the south kinematically this requires a tear in the subducted farallon slab near the latitude of southern oregon we propose that north to south propagation of this torn slab edge propagated the ignimbrite fl are up across what now is the northern basin and range and ended the laramide orogeny

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

seismic structure of the central us crust and shallow upper mantle uniqueness of the reelfoot rift abstract using seismic surface waves recorded with earthscope s transportable array we apply surface wave imaging to determine 3d seismic velocity in the crust and uppermost mantle our images span several proterozoic and early cambrian rift zones mid continent rift rough creek graben rome trough birmingham trough southern oklahoma aulacogen and reelfoot rift while ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences the reelfoot rift is unique in its association with low mantle seismic velocity its mantle low velocity zone lvz is exceptionally pronounced and extends down to at least 200 km depth this lvz is of variable width being relatively narrow 50 km wide within the northern reelfoot rift which hosts the new madrid seismic zone nmsz we hypothesize that this mantle volume is weaker than its surroundings and that the reelfoot rift consequently has relatively low elastic plate thickness which would tend to concentrate tectonic stress within this zone no other intraplate ancient rift zone is known to be associated with such a deep mantle low velocity anomaly which suggests that the nmsz is more susceptible to external stress perturbations than other ancient rift zones

origin of water salinity in a lake and coastal aquifer system the studied area is in the algerian northeast a number of interacting factors at the site are able to modify salinity the main influences are 1 the lithology 2 fedzara lake 3 marine waters and 4 a prolonged drought 1 the lithology is very clearly delineated by the outcrop of metamorphic formations gneiss marbles micaschists and quartzites and sedimentary formations limestones clay sandstones and sands all these formations can influence the chemical composition of waters 2 fedzara lake evacuates its salted waters via the wadi meboudja which connects with the wadi seybouse and the phreatic surface these lake waters might induce changes in salinity 3 the marine waters represent the third source of salinity change over pumping of water from wells and drillings can cause an imbalance to the fresh salted water interface 4 the effects of a local prolonged drought of more than 10 years may increase water salinity over pumping to augment water supply generates a fall in water levels and the drier atmosphere increases evapotranspiration both may contribute to increased salinity results of water analysis were used to observe the evolution of these various factors

the fault pattern in the northern negev and southern coastal plain of israel and its hydrogeological implications for groundwater flow in the judea group aquifer abstract on the basis of a broadly expanding data base the hydrogeological properties of the judea group sequence in the northern negev and southern coastal plain of israel have been reassessed the updated subsurface model is based on data derived from water and oil wells and on recent large scale geophysical investigations a new regional pattern of the reassessed geological through the subsurface of the study area has been revealed in view of the reassessed geological and hydrological subsurface setting it appears that the judea group aquifer should not be regarded as one continuous and undisturbed hydrological unit owing to the occurrence of regional faults its subaquifers are locally interconnected these subaquifers which contain mainly high quality water are juxtaposed as a result of faulting against kurnub group sandstones containing brackish paleowater the latter group is faulted against late jurassic formations containing highly saline groundwater in the beer sheva area the judea group aquifer is vertically displaced against the senonian and eocene mt scopus and avdat groups which also contain brackish and saline water in the southern coastal plain major faults locally dissect also the pleistocene kurkar group facilitating inflow of mg rich groundwater deriving from judea group dolomites the new geological evidence and its hydrogeological implications provide new solutions for previously unexplained salinization phenomena

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

a uniformly processed data set of sks shear wave splitting measurements a global investigation of upper mantle anisotropy beneath seismic stations anisotropy in the earth s upper mantle is a signature of past and present deformation here we present a new data set of 50 000 uniformly processed sks shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0 02 0 1 hz the data set consists of measurements obtained at 2000 seismic stations from 2000 events we identify several stations characterized by an apparent absence of shear wave splitting so called null stations station averaged measurements are obtained by stacking shear wave splitting error surfaces the stacked data set shows excellent agreement with a compilation of previous sks measurements the average amount of splitting beneath seismic stations after error surface stacking is 0 8 s slightly lower than that found previously by vectorial averaging of non null measurement splitting parameters the data set disagrees however with an azimuthally anisotropic surface wave tomography model dkp2005 suggesting that caution should be exercised when using such models for geodynamic interpretation especially in continental regions studying our data set in detail we find evidence that flow in the asthenosphere exerts partial control over sks splitting in orogenic regions globally in the active orogenic environment of the western usa where we have the densest coverage our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east

hotspot origin of the mississippi embayment and its possible impact on contemporary seismicity abstract previous authors have related the late cretaceous early tertiary subsidence of the mississippi embayment to the opening of the gulf of mexico but the gulf opened earlier in triassic jurassic time we offer an alternative hypothesis that development of the embayment was coeval with the passage of the mississippi valley graben system over the bermuda hotspot about 90 ma several lines of evidence of significant uplift of the embayment axis accompanying mid cretaceous magmatism and prior to late cretaceous subsidence support this proposal first reactivation of the pascola arch in the northern embayment is recorded by flanking deposits of basal upper cretaceous gravel second beneath a regional mid cretaceous unconformity subcrops of jurassic and early cretaceous strata define a pronounced southwest plunging arch in the southern embayment this arch is collinear with an arch revealed in paleozoic rocks after restoration to mid cretaceous structural geometries third a deep weathering profile on mid cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by paleocene sediments and rapid mid cretaceous cooling of these intrusions has been interpreted from apatite fission tracks moreover exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal upper cretaceous strata fourth there was an anomalous influx of clastic sediment into the northern gulf of mexico during mid cretaceous time and subsequent clastic facies patterns suggest the mississippi river drainage began to enter the gulf in the late cretaceous passage of the mississippi valley graben over the bermuda hotspot during elevated hotspot activity of cretaceous time may have significantly weakened the previously rifted lithosphere rifted continental margin at charleston south carolina also passed over this hotspot in latest cretaceous time similarly the st lawrence rift system passed over the great meteor hotspot during the cretaceous it is important to note that these rift systems are the principal loci of strong seismicity in eastern north america and thus weakening by increased cretaceous hotspot activity may be an important common factor for these seismic rift zones

shear wave splitting and implications for mantle flow beneath the melt region of the east pacific rise shear wave splitting across the fast spreading east pacific rise has been measured from records of sks and skks phases on the ocean bottom seismometers of the mantle electromagnetic and tomography melt experiment the direction of fast shear wave polarization is aligned parallel to the spreading direction delay times between fast and slow shear waves are asymmetric across the rise and off axis values on the pacific plate are twice those on the nazca plate splitting on the pacific plate may reflect anisotropy associated with spreading induced flow above a depth of about 100 km as well as a deeper contribution from warm asthenospheric return flow from the pacific superswell region

timing of post depositional events in the burano formation of the secchia valley upper triassic northern apennines clues from gypsum anhydrite transitions and carbonate metasomatism the burano evaporite formation from the secchia river valley is an up to 2200 m thick sequence composed of meter to decameter scale interbeds of gypsum anhydrite and dolostones with minor halite the deposit has been affected by a complex array of post depositional modifications thermal events and large scale evaporite dissolution preventing a satisfactory reconstruction of the environment of deposition the modifications are intense because these rocks were the main decollement horizon during the formation of the northern apennines chain the carbonate rocks are massive and or laminated dolomitic mudstone wackestone oolitic packstones and oolitic peloidal bioclastic grainstones which commonly appear as mega boudins within a sulfate groundmass the dolostones to 3 7 to 3 0 pdb have been affected by mg metasomatic replacement by magnesite to 2 6 to 1 4 pdb induced by hydrothermal circulation total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310 c the anhydrite rocks are characterized by flow structures such as centimeter scale pseudo lamination composed of aligned prismatic crystals with transposed isoclinal folds outlined by dolostones fragments homogenization temperatures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305 c emilia and from 230 to 315 c tuscany the gypsum rocks are composed of xenotopic irregular cloudy crystals and more rarely by centimeter scale idiotopic crystals showing the same structures as the anhydrite rocks the origin of the gypsum rocks is due to late alteration of anhydrite by migration of sharp hydration fronts the hydration is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro relics and authigenic quartz crystals which include anhydrite the role of the burano evaporites during the apennines tectogenesis can be depicted as follows a prevalent deposition of gypsum in the upper triassic b gypsum dehydration at burial conditions to form anhydrite cretaceous c syn tectonic flow of anhydrite rocks brecciation of dolostones syn tectonic growth stage of quartz euhedra at deep burial conditions possibly related to the development of the oligocene miocene greenschist facies apuane metamorphic complex d hydrothermal deposition of sparry magnesite and partial mg metasomatic replacement of dolostones by magnesite e sub surface dissolution of halite to form thick matrix supported residual caprock like anhydrite mega breccias f complete gypsification of anhydrite at sub surface conditions and g evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of most clasts calcare cavernoso

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

an alternative scenario for earthquake prediction experiments b current earthquake prediction experiments investigate behaviour before potentially large earthquakes there is some evidence that comparatively small typical events in isolated swarms of earthquakes may have precursory behaviour similar to that before much larger earthquakes in more complicated areas of seismicity such typical events in isolated swarms frequently recur with similar magnitudes in similar locations with repetition times sometimes as little as a few days it is argued that monitoring such typical events in isolated swarms could be an effective way to gain experience of precursory activity and might well be a good guide for investigations of precursors to large earthquakes

lehmann discontinuity as the base of an anisotropic layer beneath continents long period surface wave r 1 g 1 body wave s ss sss and scs reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample australia and the western pacific these models support the proposal that the lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

on the interpretation of sks splitting measurements in the presence of several layers of anisotropy summary concerns over the validity of expressions derived by montagner et al that link sks splitting measurements to the variation with depth of anisotropic parameters in the upper mantle have been recently expressed pointing out that the long period approximations applied by these authors may not be valid for the frequency range commonly used in sks studies and in particular that the anisotropy splitting parameters should depend on the order in which different anisotropy layers are arranged with depth we show here that indeed measurements of splitting time and fast axis direction performed at individual azimuths do depend on the order of layering however the expressions of montagner et al concern station averaged quantities that do not depend on the order of layers it is therefore correct to use these expressions in joint inversions of surface waveforms and sks station averaged splitting measurements on the other hand the depth dependent sensitivity of surface waveforms naturally provides constraints on the order of layering having clarified this confusion we extend the expressions of montagner et al to the case of a tilted axis of symmetry and non vertical incident waves and show that station averaged estimates of effective splitting parameters splitting time fast axis direction and tilt of the fast axis can be related to the integral with depth of quantities which now depend not only on the local splitting time and fast axis direction but also on the local tilt of the fast axis thus providing constraints also on the variation of the tilt with depth we show that the effective parameters used as constraints in the inversion can be obtained either from the measurement of splitting intensity or through a parameter search and cross convolution method in particular in the case when the effective tilt is significant the splitting intensity no longer presents 180 periodicity with azimuth providing a diagnostic tool for the presence of such tilts in the upper mantle thus combining body wave and surface wave observations also has the potential of constraining the variation with depth of the tilt of the fast axis of anisotropy a geodynamically important parameter

possible neoproterozoic early paleozoic grabens in mississippi alabama and tennessee low intensity magnetic and negative gravity anomalies in northern mississippi north western alabama and western tennessee are viewed as being similar to the magnetic and gravity low associated with the mississippi valley graben the anomalies are interpreted as shallow grabens and it is proposed that the grabens like the mississippi valley graben resulted from northwest southeast extension during neoproterozoic early paleozoic fragmentation of laurentia if real the grabens indicate that extension affected a larger part of laurentia than has been previously documented they imply hitherto unsuspected oil and gas potential and if the area is coupled to the stress field affecting the mississippi valley graben represent structures that warrant evaluation for seismic risk

mantle flow and lithosphere asthenosphere coupling beneath the southwestern edge of the north american craton constraints from shear wave splitting measurements abstract high quality broadband seismic data recorded by the usarray and other stations in the southwestern united states provide a unique opportunity to test different models of anisotropy forming mechanisms in the vicinity of a cratonic margin systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations the western and southern edges of the north american craton show edge parallel fast directions with larger than normal splitting times and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of north america at the majority of the stations no significant systematic azimuthal variations of the splitting parameters are observed suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations the spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere based on previous results of seismic tomography and geodynamic modeling we propose a model involving deflecting of asthenospheric flow by the cratonic root as the cause of the observed edge parallel fast directions and large splitting times along the western and southern edges of the north american craton

neotectonism along the atlantic passive continental margin a review abstract an extensive body of geologic data including the modern state of stress historical seismicity surface and subsurface stratigraphy numerical models of crustal deformation surficial geomorphic systems and historical precise leveling and tidal gauge records constrain the style and rate of neotectonic deformation for the appalachians and atlantic passive continental margin there are two major styles of neotectonism in the eastern united states the northeastern united states is dominated by isostatic uplift and northward migration of peripheral bulge collapse in response to deglaciation this locally rapid but decreasing rate of deformation is superimposed upon slower long term deformation along the atlantic margin most of the long term continental margin deformation is attributed to lithospheric flexuring in response to sediment loading in sedimentary basins especially the baltimore canyon trough and carolina trough isostatic deformation in response to continental denudation and water loading of the shelf and stress from far field plate tectonic sources significant deformational features include an uplift anomaly near cape fear n c northward and southward tilting of the coastal plain into the salisbury and southeast georgia embayments respectively seaward tilting of the coastal plain piedmont and a complex pattern of postglacial uplift and later subsidence in the northeast estimates of vertical crustal velocities for similar locations vary over several orders of magnitude measurement interval bias and systematic leveling errors may account for some of the discrepancies evidence for periodic deformation in the eastern united states in substantial and it is possible that historic data indicate a period of accelerated deformation along the atlantic continental margin

a uniform database of teleseismic shear wave splitting measurements for the western and central united states we present a shear wave splitting sws database for the western and central united states as part of a lasting effort to build a uniform sws database for the entire north america the sws measurements were obtained by minimizing the energy on the transverse component of the pks skks and sks phases each of the individual measurements was visually checked to ensure quality this version of the database contains 16 105 pairs of splitting parameters the data used to generate the parameters were recorded by 1774 digital broadband seismic stations over the period of 1989 2012 and represented all the available data from both permanent and portable seismic networks archived at the incorporated research institutions for seismology data management center in the area of 26 00 n to 50 00 n and 125 00 w to 90 00 w about 10 000 pairs of the measurements were from the 1092 usarray transportable array stations the results show that approximately 2 3 of the fast orientations are within 30 from the absolute plate motion apm direction of the north american plate and most of the largest departures with the apm are located along the eastern boundary of the western us orogenic zone and in the central great basins the splitting times observed in the western us are larger than and those in the central us are comparable with the global average of 1 0 s the uniform database has an unprecedented spatial coverage and can be used for various investigations of the structure and dynamics of the earth

the correlation between low tectonic stress and the appalachian basin quiet zone abstract in order to assess the nature of the appalachian basin quiet zone abqz a region of low seismic activity a minimum horizontal stress profile from sonic logs was calibrated using hydraulic fracture stress measurements in a well from mckean county pa the calibrated minimum horizontal stress profile that best fits with the hydraulic fracture stress data was established using an extended eaton s model that treats rock as a transverse isotropic material with a vertical symmetry axis tiv to achieve a best fit several assumptions are introduced 1 a constant tectonically induced elastic strain of ehmax 3 3 10 4 2 an anisotropic velocity model annie approximation for shales and 3 a bimodal pore pressure distribution with abnormally high pore pressure in the marcellus gas shale other calibration methods assume an isotropic medium a constant tectonic strain gradient corresponding to zero stress at ground surface or constraints from sidewall core data to estimate stiffness constants by testing four calibration methods we find that stress profiles are affected by multiple factors including the ratio of maximum tectonically induced elastic strain over minimum tectonic strain pore pressure in the marcellus biot s coefficients and elastic properties sensitivity analysis was conducted to analyze the influence of these parameters our stress profile is most influenced by pore pressure and tectonically induced elastic strain while other factors had much smaller effects tectonically included strain in the abqz is insufficient to achieve the critical stress necessary to induce frictional slip on favorably oriented faults in the sedimentary cover which presumably speaks to the lack of earthquakes in the basement of the abqz however induced earthquakes from deep disposal wells suggest that basement at the edge of the abqz may be decoupled from the sedimentary cover in which case the cover has relaxed through a time dependent deformation mechanism such as pressure solution creep

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

depth dependent azimuthal anisotropy in the western us upper mantle abstract we present the results of a joint inversion of long period seismic waveforms and sks splitting measurements for 3d lateral variations of anisotropy in the upper mantle beneath the western us incorporating recent datasets generated by the usarray deployment as well as other permanent and temporary stations in the region we find that shallow azimuthal anisotropy closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle 70 150 km depth whereas at depths greater than 150 km it is dominated by northward and upward flow associated with the extension of the east pacific rise under the continent constrained to the east by the western edge of the north american craton and to the north by the presence of the east west trending subduction zone in particular the depth integrated effects of this anisotropy explain the apparent circular pattern of sks splitting measurements observed in nevada without the need to invoke any local anomalous structures

the information content of high frequency seismograms and the near surface geologic structure of hard rock recording sites due to hardware developments in the last decade the high frequency end of the frequency band of seismic waves analyzed for source mechanisms has been extended into the audio frequency range 20 hz in principle the short wavelengths corresponding to these frequencies can provide information about the details of seismic sources but in fact much of the signal is the site response of the nearsurface several examples of waveform data recorded at hard rock sites which are generally assumed to have a flat transfer function are presented to demonstrate the severe signal distortions includingfmax produced by near surface structures analysis of the geology of a number of sites indicates that the overall attenuation of high frequency 1 hz seismic waves is controlled by the whole path q between source and receiver but the presence of distinctfmax site resonance peaks is controlled by the nature of the surface layer and the underlying near surface structure models of vertical decoupling of the surface and nearsurface and horizontal decoupling of adjacent sites on hard rock outcrops are proposed and their behaviour is compared to the observations of hard rock site response the upper bound to the frequency band of the seismic waves that contain significant source information which can be deconvolved from a site response or an array response is discussed in terms offmax and the correlation of waveform distortion with the outcrop scale geologic structure of hard rock sites it is concluded that although the velocity structures of hard rock sites unlike those of alluvium sites allow some audio frequency seismic energy to propagate to the surface the resulting signals are a highly distorted limited subset of the source spectra

shear wave splitting along a nascent plate boundary the north anatolian fault zone summary the north anatolian fault zone nafz is a transform structure that constitutes the boundary between the anatolian plate to the south and the eurasia plate to the north we analysed the properties of the upper mantle strain field and mantle anisotropy in the vicinity of nafz via splitting of sks and skks phases we used data from the north anatolian fault naf passive seismic experiment this is the first study that analyses the upper mantle anisotropy in this region and our results indicate that the observed upper mantle strain field is uniform underneath the array with consistent ne sw polarization directions for fast split waves the measured lag times between the arrivals of the fast and slow split waves varies from 0 5 to 1 6 s for the study area smaller lag times are obtained consistently in the eastern part of the array however we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary nafz the uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy the regional tectonic framework favours a sw direction of asthenospheric flow due to the forces acting on the upper mantle exerted by the slab roll back taking place along the aegean and the cyprean subduction zones

the study of the hydrogeological setting of the chamshir dam site with special emphasis on the cause of water salinity in the zohreh river downstream from the chamshir dam southwest of iran dissolution of evaporite formations emergence of salty water springs and intrusion of deep saline waters are important causes in changing the quality of surface water the study area is part of the reservoir and downstream of chamshir dam which is located in watershed of the zohreh river 20 km southeast of gachsaran city southwest iran to construct powerhouse and related structures for supplying water to agricultural lands located in downstream of dam water quality of zohreh river was studied by eight sampling stations in the study area early studies showed that water quality of the zohreh river decreases severely downstream of the chamshir dam spatial variations diagram of major ions piper and composition diagrams of water samples in selected stations mark the presence of two slight and major contaminating zones at sampling station r4 and r5 in these zones concentration of ca so4 and na cl ions increase suddenly results of hydrogeological hydrochemical lithological and tectonics studies showed that even though there are several low discharges springs in the contaminated zone they cannot be related to surface dissolution of evaporate layers by zohreh river there is an important fault zone including chamshir faults i and ii in the contamination zones through which intrusion of sulfate brackish and chloride brine waters occur along the fault zone and then enter zohreh river below its base level in the absence of any surface evidence the fault zone is the main cause of salinity evaluation of water balance salinity in contaminated zones shows that the discharge rate of saline waters to the river is not low and cannot be separated these findings show that there are serious restrictions upon the purposes of the project

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

seismic anisotropy and mantle flow beneath the baikal rift zone seismic studies have shown that continental rifts such as lake baikal and the great rift valley of east africa are like mid ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1 4 the direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow induced orientation of mantle olivine crystals5 8 seismic studies of the mid atlantic ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8 10 here we present results from an array of seismic stations across the baikal rift zone in southern siberia the splitting in arrival times of sks seismic waves indicates that the upper mantle beneath the rift zone is anisotropic with the fast direction which reflects the direction of mantle flow being horizontal and normal to the rift axis this suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid ocean rifts this may help to elucidate the processes involved in continental rifting

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

anisotropic convection with implications for the upper mantle on the basis of polycrystalline theory describing the plasticity in olivine and enstatite the flow in a convection cell has been simulated using a finite element formulation the spatial variations in anisotropic properties are computed from the textures that evolve with the flow a kinematically constrained equilibrium based assumption is used to partition the macroscopic deformation among crystals within an aggregate we model the convection for one specific cell geometry and two sets of boundary conditions a complete map of textures throughout the cell is obtained the textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain the results of the simulations indicate that during up welling a strong texture develops rapidly it convects during spreading and is attenuated during subduction results are compared with features of the upper mantle in our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear this is contrary to hess 1964 intuition that 001 slip planes of olivine orient themselves parallel to the flow planes yet the pattern is consistent with natural fabric data significant differences are observed as a function of depth within the cell the variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

seismic anisotropy and mantle flow beneath the northern great plains of north america a diverse set of tectonic features and the recent availability of high quality broadband seismic data from the usarray and other stations on the northern great plains of north america provide a distinct opportunity to test different anisotropy forming mechanisms a total of 4138 pairs of well defined splitting parameters observed at 445 stations show systematic spatial variations of anisotropic characteristics azimuthally invariant fast orientations subparallel to the absolute plate motion apm direction are observed at most of the stations on the superior craton and the southern yavapai province indicating that a single layer of anisotropy with a horizontal axis of symmetry is sufficient to explain the anisotropic structure for areas with simple anisotropy the application of a procedure for estimating the depth of anisotropy using spatial coherency of splitting parameters results in a depth of 200 250 km suggesting that the observed anisotropy mostly resides in the upper asthenosphere in the vicinity of the northern boundary of the yavapai province and the wyoming craton the splitting parameters can be adequately explained by a two horizontal layer model the lower layer has an apm parallel fast orientation and the upper layer has a fast orientation that is mostly consistent with the regional strike of the boundary based on the splitting measurements and previous results from seismic tomography and geodynamic modeling we propose a model involving deflecting of asthenosphere flow by the bottom of the lithosphere and channeling of flow by a zone of thinned lithosphere approximately along the northern boundary of the yavapai province

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

crust and upper mantle structure of the new madrid seismic zone insight into intraplate earthquakes we determine a 3 d p wave velocity model of the crust and upper mantle down to 400 km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the new madrid seismic zone we used 4871 high quality arrival times from 187 local earthquakes and 30 846 precise travel time residuals from 1041 teleseismic events recorded by the earthscope usarray transportable array our results show that beneath the reelfoot rift a significant low velocity low v zone exists in the upper mantle down to 200 km depth with a large volume of 200 x 200 x 150 km 3 the origin of the low v zone may be related to the passage of the bermuda hotspot and the stalled ancient farallon slab materials foundering in the mantle transition zone this low v zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust leading to the large intraplate earthquakes in the new madrid seismic zone c 2014 elsevier b v all rights reserved

shear wave splitting measurements problems and solutions abstract we compare sensitivity and reliability of three basic techniques of shear wave splitting analysis cross correlation of wave components minimization of covariance matrix eigenvalues and minimizing energy on the transverse component of both synthetic and real data point out some problems and propose solutions we discuss major difficulties of evaluating shear wave splitting parameters which are not always obvious in routine processing and can sometimes lead to false results and wrong interpretations we describe effects of pre processing of digital records of shear waves e g filtering and setting lengths of analyzed signals and discuss reliability of results to minimize outliers in evaluated splitting parameters for reliable evaluation of splitting of medium quality sks wave forms which are valuable mainly in temporary deployments of stations we recommend a careful band pass filtering of the signal using a window length not shorter than the dominant period inspecting the signal noise ratio on t component and the particle motion the transverse minimization method provides usually more robust and stable solutions than the other two methods in the conclusion we suggest how to proceed in evaluation of splitting parameters to ensure reliable solutions even for medium or low quality shear waveforms recorded frequently during temporary field measurements

detailed three dimensional shear wave velocity structure of the northwestern united states from rayleigh wave tomography abstract since the mid miocene the northwestern united states has experienced extensive flood basalt volcanism followed by the formation of two time progressive tracks of silicic volcanism the yellowstone snake river plains ysrp and the high lava plains hlp the ysrp track progresses towards the northeast parallel to north american plate motion and has therefore often been attributed to a deep mantle plume source however the hlp track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion the causes of the mid miocene flood basalts and the tracks of the ysrp and hlp are a matter of ongoing debate we present results of rayleigh wave phase velocity inversions and inversions for 3 d shear wave velocity structure of the northwestern united states using data collected from the high lava plains seismic experiment and the earthscope usarray transportable array ta the large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area our velocity images indicate that low s wave velocities in the uppermost mantle do not well match the track of hlp volcanism while at the surface the newberry caldera appears to anchor the nw end of the hlp hotspot track the seismic results show that it lies in a separate north south trending low velocity band just east of the cascades that is distinct from the main hlp trace the ultra low s wave velocities beneath the ysrp track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region along axis the ysrp hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows narrows and intensifies to the northeast but then deepens rapidly to the north beneath yellowstone the shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate coupled with basin and range extension and heating

imprints of diverse mantle deformational episodes in the cauvery suture zone south india abstract this study attempts to understand the upper mantle deformation pattern beneath the northern part of southern granulite terrain sgt located to the south of archean dharwar craton in south india the study region comprises the tectonic blocks nilgiri biligiri rangan shevaroy and madras blocks separated from the northern part of madurai block by the cauvery suture zone csz associated with subduction accretion collision tectonics shear wave splitting observed on high quality xk k s sks skks pks waveforms from 119 teleseismic events recorded by 19 broadband seismic stations mostly within the csz and the northern part of madurai block is utilized in this study the 122 splitting measurements obtained show an average delay time of 0 8 s between the fast and slow waves strong lateral variations in the fast polarization azimuths fpa are observed with the splitting parameters at the majority of stations showing a dependence on back azimuth importantly excluding the madras block a poor correlation between the fpa and the present day absolute plate motion apm direction is observed this dominance of non apm strain reflects complex anisotropy preserved in the lithosphere by plotting the fast polarization trends based on the ray piercing points at a depth of 150 km three distinct splitting patterns associated with the i biligiri rangan block and the eastern part of the nilgiri block ii eastern part of the csz and the northern madurai block nmb and iii madras block and its vicinity within the csz are observed the strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions two episodes of ocean closure and the collision that followed plate motion related strain or a combination of these similar deformation patterns in the northern part of madurai block might be related to remnants of subducted oceanic lithosphere

shear wave splitting beneath the central tien shan and tectonic implications 1 shear wave splitting analyses were performed at 30 seismic stations in the central tien shan and its vicinity fast orientation at most of the stations on the tien shan range is ene wsw parallel to the strike of the tien shan it largely reflects a coherent deformation of the lithosphere in response to the ns shortening anisotropy with nne ssw fast axes is observed within adjacent basins around the issyk kul and near the northwestern tarim where the lithosphere is presumably strong less deformed the nne ssw orientation which coincides with the surface velocity of the central tien shan can be best explained by shear between the lithosphere and asthenosphere on a regional scale rather than small scale convection as proposed in previous studies the dextral talasso fergana fault seems to have little effect on anisotropy at its nearby stations suggesting this fault is probably confined to the shallow part of the lithosphere

seismic anisotropy in the eastern united states deep structure of a complex continental plate we have analyzed shear wave splitting recorded by portable and permanent broadband and long period stations located in the eastern united states teleseismic shear waves sks skks and pks were used to retrieve the splitting parameters the orientation of the fast wave polarization plane and the delay time t in total 120 seismic events were processed allowing for more than 600 splitting measurements within the appalachians stations located in the western external part are characterized by t 1s and trending n50 70 e in the south and central regions and n30 40 e in the north closely following the trend of the orogenic belt in these areas the transition region between north and central is characterized by t 1 1 3 s and by e w trending that are at a high angle to the regional geologic trend measurements at two stations located in the eastern internal part of the belt indicate very weak anisotropy the large scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate splitting along the southern and eastern margins of the continent is consistent with that expected for grenvillian deformation an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out within the cratonic core the correlation between t and lithospheric thickness suggests a lithospheric anisotropy smaller length scale variations also argue for a significant contribution of lithospheric structures the fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern united states i e the grenville and appalachian orogenies and the subsequent rifting of the north atlantic ocean our observations in the western appalachians suggest that the anisotropy may be preserved since the grenvillian orogeny the absence of detectable splitting in the two stations in the eastern appalachians is attributed to the igneous intrusions related to the atlantic rifting the measurements in the transition between the northern and central southern appalachians constitute an intriguing anomaly whose e w have little obvious relation to the regional surface geology we suggest two possible causes 1 the local dominance of asthenospheric flow motivated by the proximity of a pervasive low velocity anomaly and 2 lithospheric deformation in a transcontinental strike slip fault zone active during the appalachian collision

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

saline groundwater in israel its bearing on the water crisis in the country abstract one of the major causes for the deterioration of water quality bearing heavily on the water crisis in israel is the ongoing contamination of its water resources by saline water bodies the present paper reviews the geochemical processes forming saline water lists and explains certain chemical and isotopic parameters which enable understanding these processes and describes the saline groundwater bodies and various salinization phenomena occurring in the country s various aquifers deterioration of groundwater in israel is caused by numerous natural processes such as encroachment of sea water migration of connate highly pressurized brines penetrating into fresh groundwater by subsurface dissolution of soluble salts originating in surrounding country rocks and by water rock interaction in addition to sea water two saline water bodies were identified as the main factors causing salinization of fresh groundwater a ca chloride brines encountered in the jordan dead sea rift valley in various parts of the negev and of the coastal plain and b na chloride saline water identified in the subsurface of the negev and in the southern part of the coastal plain intensive exploitation of groundwater in israel has disturbed the natural equilibrium which prevailed between fresh and saline water the newly established groundwater flow regimes have facilitated the migration of saline water bodies their participation in the active hydrological cycle and the progressive contamination of fresh groundwater these processes which were not anticipated by planners and water resources managers emphasize that large scale groundwater exploitation was undertaken without giving sufficient consideration to the occurrence and subsurface migration of saline water and brines

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

shear wave splitting in the rio grande rift fast directions and lag times associated with sks shear wave splitting are determined for six digital stations in the rio grande rift rgr the mean fast direction for three stations in the central rgr is about 45 oblique to the axis of the rgr in contrast fast directions approximately parallel to the axis of the rgr are found for two stations in the southern rgr 10 and one station in the northern rgr 26 the fast directions for southern stations are not aligned with the ne sw extension direction of the oligocene to early miocene stage of extension the significant n s lateral variation in the sks shear wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the rgr is underlain by hot material with low seismic wave velocities hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle because none of the fast directions are parallel to the present e w extension direction of the rgr the dominant upper mantle flow direction is not perpendicular to the axis of the rgr the pattern of the fast directions along the rgr provides evidence for a rising asthenosphere beneath the central to southern rgr and suggests that thermally driven small scale convection may be faster for flow parallel to the rift than perpendicular to it

forward modeling of the development of seismic anisotropy in the upper mantle development of seismic anisotropy in response to upper mantle flow is approached through an integrated numerical model this model allows to predict the splitting parameters for a shear wave propagating across an upper mantle which deformed in response to a given geodynamic process it consists of 1 thermo mechanical modeling of the finite strain field 2 modeling olivine lattice preferred orientation lpo generated by this strain field 3 calculation of the 3 d elastic properties associated with this lpo and 4 estimation of the shear wave splitting parameters the time lag between the fast and slow split shear wave arrivals dt and the polarization azimuth of the fast wave modeled olivine lpo are constrained relative to lpo measured in naturally and experimentally deformed peridotites comparison of predicted shear wave splitting parameters with seismological data allows us to quantify the possible contribution of the modeled upper mantle flow to the measured splitting values and hence to constrain the interpretation of shear wave splitting data in terms of upper mantle flow we use this forward model to investigate the seismic anisotropy generated in ocean basins by a velocity gradient between the plate and the deep mantle fast shear wave polarizations calculated assuming a constant plate motion are in good agreement with both the sks polarization and the fast propagation direction for p and rayleigh waves observed in the pacific and indian oceans suggesting that away from mid ocean ridges seismic anisotropy in oceanic basins primarily results from asthenospheric deformation by resistive drag beneath the plate delay times are however overestimated this may be ascribed to a stronger strain localization in nature or to partial erosion of the anisotropic layer by hotspots indeed hotspot activity may explain the short length scale variations of dt in the southern pacific finally twolayer models that simulate a change in pacific plate motion as suggested by the bend in the hawaii emperor chain fail to reproduce the observed shear wave splitting this is consistent with previous suggestions that the emperor chain track may not faithfully record the pacific plate absolute motion before 43 ma 1998 elsevier science b v all rights reserved

lithospheric and asthenospheric contributions to shear wave splitting observations in the southeastern united states we present observations of both null and non null sks splitting from temporary deployments across the southeastern united states in order to evaluate the relative contributions of lithospheric deformation and asthenospheric flow to regional anisotropy data for this study come from four temporary broadband seismic deployments the appalachian seismic transect ast the test experiment for eastern north america teena the south carolina earth physics project scepp and the florida to edmonton array fled in general we find fast directions aligned roughly parallel to absolute plate motion of the north american plate apm within and west of the southern appalachians whereas to the southeast we find a broad area dominated by complex splitting patterns consisting of well constrained null splitting measurements over a range of backazimuths along with a very small number of resolved non null measurements this change in splitting patterns is consistent with a transition from drag induced asthenospheric flow beneath the older sections of the north american continent to vertical or incoherent mantle flow likely in combination with complex lithospheric anisotropy beneath the younger accreted terranes to the southeast in addition to these general patterns we find a number of non null splitting measurements that are not aligned with apm but are instead aligned with prominent magnetic anomalies that may correspond to ancient continental suture zones or faults this would suggest that in these areas a strongly anisotropic but localized lithospheric fabric dominates over any ambient asthenospheric anisotropic signature in areas with generally strong apm parallel splitting this would imply a thick sheared mantle lithosphere whose deformation induced anisotropy is strong enough to overprint the anisotropy induced by apm and is aligned with the shallower crustal structures responsible for generating the observed magnetic anomalies in the southeastern areas dominated by null splitting measurements there may be no strong signature from asthenospheric anisotropy to override but a substantial lithospheric thickness is still required to generate the magnitude of the observed sks splitting 1 s more data are required to verify these results but future datasets including data from usarray may be able to exploit the correlations between null and non null sks splitting measurements and magnetic lineaments to better constrain the provenance of the regional anisotropic signature

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

time corrections to teleseismic p delays derived from sks splitting parameters and implications for western u s p wave tomography 1 upper mantle anisotropy will affect teleseismic p wave arrival times and create artifacts in isotropic travel time tomography compiled sks splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western u s we use sks splitting parameters and a hexagonally anisotropic elastic tensor fast symmetry axis to estimate azimuthal anisotropy contributions to p wave delay times and evaluate the effects on isotropic p wave tomography estimated anisotropy correction times have a root mean square rms value of 0 16 s which is 37 of p wave delay time rms the magnitude of azimuthal anisotropy rather than its azimuth has the strongest effect on p delay times however if the anisotropy symmetry axis has a plunge p wave travel times may be strongly affected by both magnitude and azimuth applying azimuthal anisotropy corrections significantly increases tomographic p wave velocity beneath the high lava plains and central california coast and decreases velocity beneath the great basin

shear wave anisotropy in the crust around the san andreas fault near parkfield spatial and temporal analysis summary we systematically analysed shear wave splitting sws for seismic data observed at a temporary array and two permanent networks around the san andreas fault saf observatory at depth the purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the saf in this segment and its temporal behaviour in relation to the occurrence of the 2004 parkfield m 6 0 earthquake the dense coverage of the networks the accurate locations of earthquakes and the high resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the saf zone the results show that the primary fast polarization directions pds in the region including the saf zone and the northeast side of the fault are nw se nearly parallel or subparallel to the saf strike some measurements on the southwest side of the fault are oriented to the nne ssw direction approximately parallel to the direction of local maximum horizontal compressive stress there are also a few areas in which the observed fast pds do not fit into this general pattern the strong spatial variations in both the measured fast pds and time delays reveal the extreme complexity of shear wave anisotropy in the area the top 2 3 km of the crust appears to contribute the most to the observed time delays however substantial anisotropy could extend to as deep as 7 8 km in the region the average time delay in the region is about 0 06 s we also analysed temporal patterns of sws parameters in a nearly 4 yr period around the 2004 parkfield main shock based on similar events the results show that there are no appreciable precursory coseismic or post seismic temporal changes of sws in a region near the rupture of an m 6 0 earthquake about 15 km away from its epicentre

significant seismic anisotropy beneath the southern lhasa terrane tibetan plateau 1 shear wave splitting measurements using teleseismic pks skks and sks phases recorded by station lsa on the southern part of the lhasa terrane of the tibetan plateau reveal significant azimuthal anisotropy with a splitting time of up to 1 5 s a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy in addition systematic variations of the splitting parameters fast polarization direction and splitting time with the arriving azimuth of the seismic ray path are observed suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry the measurements are consistent with a model with two layers of anisotropy the top layer has a ne sw fast direction which is consistent with gps revealed direction of surface movement and can be associated with lattice preferred orientation of middle lower crustal minerals such as amphibole the lower layer has a nearly e w fast direction and can be the consequence of either the n s directed compressional stress originated from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia this study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

shear wave splitting and mantle flow associated with the deflected pacific slab beneath northeast asia 1 a total of 361 sks and five local s wave splitting measurements obtained at global and regional seismic network stations in ne china and mongolia are used to infer the characteristics of mantle fabrics beneath northeast asia fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features the dominant fast directions at the eastern part beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone mtz are about 100 from north which are almost exactly the same as the motion direction of the eurasian plate relative to the pacific plate and are independent of the direction of local geological features the splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3 anisotropy the shear wave splitting observations complemented by the well established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the paleozoic era can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected pacific slab in the mtz or by back arc asthenospheric flow in the mantle wedge above the slab

descent of the ancient farallon slab drives localized mantle flow below the new madrid seismic zone 1 the great earthquake sequence that occurred in the central mississippi river valley during the winter of 1811 1812 is unprecedented in the historical record of seismicity within stable continental plate interiors we show using viscous flow models based on high resolution seismic tomography that the descent of the ancient farallon slab into the deep mantle beneath central north america induces a highly localized flow directly below the new madrid seismic zone nmsz this localization arises because of structural variability in the farallon slab and the low viscosity of the sub lithospheric upper mantle and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity

na sws 1 1 a uniform database of teleseismic shear wave splitting measurements for north america 1 this version of shear wave splitting sws database db for north america na contains 6224 pairs of splitting parameters fast polarization directions and splitting times measured using all the available data recorded by digital broadband seismic stations over the period of 1980 2007 and archived at the incorporated research institutions for seismology data management center the measurements were produced using a set of robust procedures that involve automated batch processing and manual screening and were ranked quantitatively on the basis of a uniform criterion the result is a homogeneous database of individual rather than station averaged splitting parameters that can be used by a variety of geoscientists for the understanding of the structure and dynamics of the earth s deep interior beneath na this data brief describes the seismic data and techniques used to generate the sws results the spatial and azimuthal coverage and the limitations of the db the db will be periodically updated and enhanced by adding newly recorded data and by measuring splitting parameters using the pks and skks phases in addition to sks the database can be accessed at http www mst edu liukh sws

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

mapping p wave azimuthal anisotropy of the new madrid seismic zone abstract we determine a detailed tomographic model of 3 d p wave azimuthal anisotropy in the crust and upper mantle beneath the new madrid seismic zone nmsz using a large number of high quality arrival time data of 877 local earthquakes and relative travel time data of 1511 teleseismic events recorded at 62 usarray stations the tomographic inversion is conducted by using a limited memory broyden fletcher goldfarb shanno algorithm for bound constrained optimization because damping regularization is not required in this inversion scheme the obtained results are less affected by human subjectivity a prominent low velocity zone with ne sw fast velocity directions of azimuthal anisotropy down to 160 km depth beneath the nmsz is imaged which may reflect upwelling mantle materials probably triggered by passage of the bermuda hotspot from the southwest of the mississippi embayment our results also reveal a subhorizontal high velocity anomaly around the moho discontinuity with a width of 150 200 km confirming the existence and defining the geometry of a lens shaped high density rift pillow near the moho which may result from the mantle upwelling nw se fast velocity directions of azimuthal anisotropy appear in the lower crust beneath the mississippi embayment this result may reflect recent weakening of a detachment fault above the rift pillow which drives recurrently sinking of the rift pillow and as a consequence causing large repeating intraplate earthquakes in the nmsz

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

quaternary tectonic faulting in the eastern united states abstract paleoseismological study of geologic features thought to result from quaternary tectonic faulting can characterize the frequencies and sizes of large prehistoric and historical earthquakes thereby improving the accuracy and precision of seismic hazard assessments greater accuracy and precision can reduce the likelihood of both underprotection and unnecessary design and construction costs published studies proposed quaternary tectonic faulting at 31 faults folds seismic zones and fields of earthquake induced liquefaction phenomena in the appalachian mountains and coastal plain of the 31 features seven are of known origin four of the seven have nontectonic origins and the other three features are liquefaction fields caused by moderate to large historical and holocene earthquakes in coastal south carolina including charleston the central virginia seismic zone and the newbury massachusetts area however the causal faults of the three liquefaction fields remain unclear charleston has the highest hazard because of large holocene earthquakes in that area but the hazard is highly uncertain because the earthquakes are uncertainly located of the 31 features the remaining 24 are of uncertain origin they require additional work before they can be clearly attributed either to quaternary tectonic faulting or to nontectonic causes of these 24 14 features most of them faults have little or no published geologic evidence of quaternary tectonic faulting that could indicate the likely occurrence of earthquakes larger than those observed historically three more features of the 24 were suggested to have had quaternary tectonic faulting but paleoseismological and other studies of them found no evidence of large prehistoric earthquakes the final seven features of uncertain origin require further examination because all seven are in or near urban areas they are the moodus seismic zone hartford connecticut dobbs ferry fault zone and mosholu fault new york city lancaster seismic zone and the epicenter of the shallow cacoosing valley earthquake lancaster and reading pennsylvania kingston fault central new jersey between new york and philadelphia and everona fault mountain run fault zone washington d c and arlington and alexandria virginia