litho1 0 an updated crust and lithospheric model of the earth we present the litho1 0 model which is a 1 tessellated model of the crust and uppermost mantle of the earth extending into the upper mantle to include the lithospheric lid and underlying asthenosphere the model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers such as water ice sediments crystalline crust lithospheric lid and asthenosphere litho1 0 is created by constructing an appropriate starting model and perturbing it to fit high resolution surface wave dispersion maps love and rayleigh group and phase over a wide frequency band 5 40 mhz we examine and discuss the model with respect to key lithospheric parameters such as average crustal velocity crustal thickness upper mantle velocity and lithospheric thickness we then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency it appears that litho1 0 represents a reasonable starting model of the earth s shallow structure crust and uppermost mantle for the purposes in which these models are used such as traveltime tomography or in efforts to create a 3 d reference earth model the model matches surface wave dispersion over a frequency band wider than the band used in the inversion there are several avenues for improving the model in the future by including attenuation and anisotropy as well as making use of surface waves at higher frequency

long range triggered earthquakes that continue after the wave train passes 1 large earthquakes can trigger distant earthquakes in geothermal areas some triggered earthquakes happen while the surface waves pass through a site but others occur hours or even days later does this prolonged seismicity require a special mechanism to store the stress from the seismic waves that differs from ordinary aftershock mechanisms these questions have driven studies of long range triggering since the phenomenon s discovery here i attempt to answer the questions by examining the statistics of triggered sequences two separate observations are consistent with the prolonged sequences being simply local aftershocks of earthquakes triggered early in the wave train first the sequences obey omori s law over both short 1 hour and longer 5 day time intervals secondly the number of observed triggered earthquakes in the first hour after the wave train can be predicted from the number of earthquakes triggered during the wave train even the very vigorous 10 day triggering at long valley from the 1992 landers mw 7 3 earthquakes can be interpreted as the aftershocks of either a local m 4 1 earthquake or an equivalent combination of several smaller mainshocks therefore long range triggering does not need to include a mechanism to produce sustained stresses other than the process that generates aftershocks of the earthquakes that occur while the wave train is passing

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

earthquakes a hand on the aftershock trigger received wisdom relates that static stress change associated with an earthquake mainshock is the prime mover of its aftershocks but a fresh look at the data points the finger at wave surfing dynamic stress

moho geometry and upper mantle images of northeast india 1 images of the crust and mantle beneath northeast india obtained by 2d migration of 1000 broadband p receiver functions clearly trace a northward dipping moho from the himalayan foredeep reaching depths up to 50 km further north beneath the himalayan convergence zone also these images reveal presence of largely coherent 410 km and 660 km discontinuities that conform to the iasp91 model marginal variations in the depth of the 410 km interface are observed that appear region specific the thickness of the mantle transition zone does not deviate significantly from a global average of 250 km interestingly our results reveal consistent presence of a signal from an interface around 300 km origin of such a boundary known as x discontinuity and unrelated to the lehmann discontinuity is discussed possible presence of the x discontinuity from the indian region is reported here for the first time

heterogeneous lithosphere and the underlying mantle of the indian subcontinent abstract p and s receiver functions prfs and srfs respectively for 21 broad band seismograph stations of the india meteorological department imd illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the indian sub continent our analysis demonstrates that the archean and early proterozoic lithospheric keel of the indian shield has been reworked by younger processes we find very low s wave velocities in the uppermost mantle from 4 0 to 4 3 km s to the north of the deccan volcanic province kutch region and aravalli craton 1 in the south southern granulite terrain and sri lanka 2 and in the north east gangetic plane bengal basin and singhbuhm craton 3 the anomalies 1 and 2 may extend into the transition zone early arrivals of the s410p seismic phase are indicative of anomalously high vp vs ratio 1 9 in the upper mantle of the low velocity regions whereas late arrivals in the western himalaya ladakh and western tibet are consistent with the previously found indications of anomalously low vp vs ratio a transition from the high s velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere asthenosphere boundary is problematic a mafic s velocity in the upper crust and a pronounced low s velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the deccan traps a separation in depths between the 410 km and 660 km discontinuities varies laterally in a range from 240 to 270 km the largest uplift of the 410 km discontinuity up to 390 km is observed beneath the foothills of the himalaya where it is caused by cooling of the transition zone by the ongoing continental collision

direct test of static stress versus dynamic stress triggering of aftershocks 1 aftershocks observed over time scales of minutes to months following a main shock are plausibly triggered by the static stress change imparted by the main shock dynamic shaking effects associated with passage of seismic waves from the main shock or a combination of the two we design a direct test of static versus dynamic triggering of aftershocks by comparing the near field temporal aftershock patterns generated by aseismic and impulsive events occurring in the same source area the san juan bautista california area is ideally suited for this purpose because several events of both types of m 5 have occurred since 1974 we find that aftershock rates observed after impulsive events are much higher than those observed after aseismic events and this pattern persists for several weeks after the event this suggests that at least in the near field dynamic triggering is the dominant cause of aftershocks and that it generates both immediate and delayed aftershock activity

3 d seismic tomography of the lithosphere and its geodynamic implications beneath the northeast india region we have evolved 3 d seismic velocity structures in northeast india region and its adjoining areas to understand the geodynamic processes of indian lithosphere that gently underthrusts under the himalayas and steeply subducts below the indo burma ranges the region is tectonically buttressed between the himalayan arc to the north and the indo burmese arc to the east the tomographic image shows heterogeneous structure of lithosphere depicting different tectonic blocks though our results are limited to shallower depth 0 90 km it matches well with the deeper continuation of lithospheric structure obtained in an earlier study koulakov 2011 we observe low velocity structure all along the eastern himalayas down to 70 km depth which may be attributed to deeper roots thicker crust developed by underthrusting of indian plate parallel to this low velocity zone lies a high velocity zone in foredeep region represents the indian lithosphere the underthrusting indian lithosphere under the himalayas as well as below the indo burma ranges is well reflected as a high velocity dipping structure the buckled up part of bending indian plate in study region the shillong plateau mikir hills tectonic block is marked as a high velocity structure at shallower depth the eastern himalayan syntaxis tectonic block where the two arcs meet is identified as a high velocity structure the bengal basin tectonic block to the south of shillong plateau shows low velocity due to its thicker sediments based on the tomographic image a schematic model is presented to elucidate the structure and geodynamics of indian lithosphere in study region

depth variant azimuthal anisotropy in tibet revealed by surface wave tomography azimuthal anisotropy derived from multimode rayleigh wave tomography in china exhibits depth dependent variations in tibet which can be explained as induced by the cenozoic india eurasian collision in west tibet the e w fast polarization direction at depths 100 km is consistent with the accumulated shear strain in the tibetan lithosphere whereas the n s fast direction at greater depths is aligned with indian plate motion in northeast tibet depth consistent nw se directions imply coupled deformation throughout the whole lithosphere possibly also involving the underlying asthenosphere significant anisotropy at depths of 225 km in southeast tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the himalaya and burma respectively the multilayer anisotropic surface wave model can explain some features of sks splitting measurements in tibet with differences probably attributable to the limited back azimuthal coverage of most sks studies in tibet and the limited horizontal resolution of the surface wave results

earthquake triggering at alaskan volcanoes following the 3 november 2002 denali fault earthquake the 3 november 2002 m w 7 9 denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at alaskan volcanoes the alaska volcano observatory operates short period seismic networks on 24 historically active volcanoes in alaska 247 2159 km distant from the mainshock epicenter we searched for evidence of triggered seismicity by examining the unfiltered waveforms for all stations in each volcano network for 1 hr after the m w 7 9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock we found compelling evidence for triggering only at the katmai volcanic cluster kvc 720 755 km southwest of the epicenter where small earthquakes with distinct p and s arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock peak dynamic stresses of 0 1 mpa at augustine volcano 560 km southwest of the epicenter are significantly lower than those recorded in yellowstone and utah 3000 km southeast of the epicenter suggesting that strong directivity effects were at least partly responsible for the lack of triggering at alaskan volcanoes we describe other incidents of earthquake induced triggering in the kvc and outline a qualitative magnitude distance dependent triggering threshold we argue that triggering results from the perturbation of magmatic hydrothermal systems in the kvc and suggest that the comparative lack of triggering at other alaskan volcanoes could be a result of differences in the nature of magmatic hydrothermal systems

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

application of multivariate statistical analyses to itraxtm core scanner data for the identification of deep marine sedimentary facies a case study in the galician continental margin abstract the validity and usefulness of multivariate statistical tools for the facies characterization in deep marine environments have been applied on the geochemical sedimentological and magnetic data from a piston core extracted from the transitional zone in the galician continental margin the combination of geochemical profiles of fe mn ti ba and ca and magnetic susceptibility ms obtained using the itraxtm core scanner at the university of vigo together with the grain size grey level and r red g green b blue colour analyses have allowed characterizing and classifying the sediments of the core pc13 3 using spss package v 23 cluster analysis ca displays in the first level of the hierarchy two major groups that correspond with clay silt and sand facies in a second level it is possible to observe six subfacies that match de visu preliminary classification and allowed us to complete and improve the characterization and the facies limits in the whole core discriminant analysis da confirmed the validity of the cluster analyses and enhanced the results of the classification the principal component analysis pca shows four principal components coarse lithogenic fraction pc1 fine lithogenic fraction pc2 high density fraction pc3 and biogenic fraction pc4 these results are in concordance with the pearson correlation coefficient and the sem observations in general terms the results confirm the utility of the multivariate statistical methods applied on high resolution geochemical and magnetic data acquired with itraxtm corer scanner as a quick and complementary tool in sedimentary facies analysis and description in deep marine environments

the tectonic origin of the bay of bengal and bangladesh we are able to decipher the tectonic evolution of the bay of bengal a puzzle which has not been satisfactorily solved in the past and we are also able to shed new light on origin of the buried 85 e ridge we do so by incorporating a number of disparate items into a unified solution these items are the marine magnetic anomalies in the western basin of the bay of bengal the rajmahal and sylhet traps and deep seismic sounding lines in india a prominent magnetic anomaly doublet and seismic seaward dipping reflectors in bangladesh and a new precise gravity map of the bay of bengal we identify seafloor spreading magnetic anomalies ranging in age from 132 ma m12n to 120 ma m0 in the western basin these anomalies are one sided the conjugate anomalies lie in the western enderby basin off east antarctica the direction of spreading was approximately nw se and the half spreading rates varied from 2 5 to 4 0 cm yr with the arrival of the kerguelen plume around m0 time seafloor spreading was reorganized and a new spreading axis opened at or close to the line joining the rajmahal and sylhet traps the prominent magnetic anomaly doublet connecting the rajmahal and sylhet traps indicates that these traps are not individual eruptions at about 118 ma but rather together define the new line of opening spreading started at this line and subsequently india changed direction from west to north the new oceanic crust thus generated underlies bangladesh and the eastern basin of the bay of bengal and is younger than 118 ma the western boundary of the new ocean floor is a transform fault which was generated by the spreading axis jump this transform fault appears as the 85 e ridge and further north on land as a negative free air gravity anomaly strip a unique feature of the northern boundary of the new oceanic crust is that due to the later deposition of enormous sediments derived from the himalayan orogeny it lies onshore bangladesh in contrast to most continent ocean boundaries in the world which lie offshore

morphological feature analyses of the prestige half graben on the sw galicia bank abstract morphological analysis was carried out on the sw flank of the galicia bank as part of detailed geological studies of the half graben where the prestige oil tanker wreckage is located broad multibeam echosounder simrad em 12s 120 and seismic reflection airgun data were collected on board rv hesperides three groups of morphological features were identified as tectonic erosive and depositional the main tectonic morphological feature is a n s oriented fault scarp in the eastern part of the study area main erosive features are gullies slide scars e w oriented inter lobe channels and a nne ssw oriented main channel dominant depositional morphologies comprise thirteen e w oriented sedimentary wedges seven large e w to ne sw oriented sedimentary lobes and the relatively smooth sedimentary layer covering the eastern and western ridges that comprise the galicia bank s sw flank characterization classification and distribution of these features indicate that the study area constitutes a half graben structure comprising a n s oriented normal fault scarp abruptly connecting steep gradients in the eastern area with the gentle slope of the central area a thick but narrow wedge of sediment has accumulated at the foot of the scarp as a result of mass movement processes that govern the formation of a series of short lived wedges overlapping lobes and also channels this particular kind of slope apron system corresponds to a tectonic controlled drainage pattern from a sharply tilted hanging wall block supplied by sediment from the ridge finally we can conclude that the prestige stern and bow are located at the steep fault scarp close to a channel head and at the axis of inter lobe channels developed by mass movement processes thus morphologic tectonic and sedimentary factors can be considered as potential hazards

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

the failure of earthquake failure models in this study i show that simple heuristic models and numerical calculations suggest that an entire class of commonly invoked models of earthquake failure processes cannot explain triggering of seismicity by transient or dynamic stress changes such as stress changes associated with passing seismic waves the models of this class have the common feature that the physical property characterizing failure increases at an accelerating rate when a fault is loaded stressed at a constant rate examples include models that invoke rate state friction or subcritical crack growth in which the properties characterizing failure are slip or crack length respectively failure occurs when the rate at which these grow accelerates to values exceeding some critical threshold these accelerating failure models do not predict the finite durations of dynamically triggered earthquake sequences e g at aftershock or remote distances some of the failure models belonging to this class have been used to explain static stress triggering of aftershocks this may imply that the physical processes underlying dynamic triggering differs or that currently applied models of static triggering require modification if the former is the case we might appeal to physical mechanisms relying on oscillatory deformations such as compaction of saturated fault gouge leading to pore pressure increase or cyclic fatigue however if dynamic and static triggering mechanisms differ one still needs to ask why static triggering models that neglect these dynamic mechanisms appear to explain many observations if the static and dynamic triggering mechanisms are the same perhaps assumptions about accelerating failure and or that triggering advances the failure times of a population of inevitable earthquakes are incorrect

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

triggering of volcanic eruptions although earthquakes and volcanic eruptions are each manifestations of large scale tectonic plate and mantle motions it is usually thought that the occurrences of these events are not directly related there have been some studies however in which triggering of volcanic eruptions by earthquakes remote from the volcano has been proposed1 2 the 1992 landers southern california earthquake caused triggered seismicity at very large distances3 including the magmatically active4 long valley caldera region which also experienced a significant coincident deformation transient5 motivated by this demonstration of the ability of a distant earthquake to disturb a volcanic system and the earlier studies of specific cases of eruption triggering we examine here the historical record of eruptions and earthquakes to see if there are indeed significantly more eruptions immediately following large earthquakes we find that within a day or two of large earthquakes there are many more eruptions within a range of 750 km than would otherwise be expected additionally it is well known6 that volcanoes separated by hundreds of kilometres frequently erupt in unison the characteristics of such eruption pairs are also consistent with the hypothesis that the second eruption is triggered by earthquakes associated with the first

high resolution seismic stratigraphy of the galicia bank region and neighbouring abyssal plains nw iberian continental margin abstract the high resolution seismic stratigraphy of the galicia bank region and adjacent deposits of the neighbouring iberian and biscay abyssal plains was included as part of the geological studies conducted in the area where the oil tanker prestige wreck is located this seismic stratigraphy is characterized by five seismic units 5 to 1 from oldest to youngest lying above an irregular acoustic basement defined by a highly fractured system of horsts and grabens these faulted systems have controlled the local depositional architecture deforming fracturing relocating and distributing sediments since the valanginian three depositional models of facies can be recognized on the galicia bank region and surrounding abyssal plains bank escarpment and abyssal plain facies the bank facies reflects the depositional evolution from initial filling to obliterating stages of the horsts and grabens the escarpment facies has a local distribution and represents deposition associated with active slopes of the faulted escarpments the abyssal plain facies represents gravity flow deposits coming mostly from the bounded fault flanks of the galicia bank region the valanginian to quaternary sedimentary evolution of the galicia bank region reflects depositional responses to tectonic condition the occurrence of mass movements confirms that the syn rift period valanginian to aptian age along the galicia margin was diachronous beginning in the interior basin and shifting later toward the west of the galicia bank region deep galicia margin the albian to quaternary post rift evolution is characterized by a decrease in tectonic activity and sedimentary variations in the style of deposition valley and related features mass movement deposits contourites and drape deposits conditioned by changes in the sedimentary processes paleoenvironments and tectonic reactivation of the regional slopes the effects of tectonic movements persist as evidenced in areas where faulted escarpments and highs outcrop and in areas of near surface faulting the syn rift and post rift stages have also conditioned the depositional architecture of the prestige sinking area the evolution of the surrounding abyssal plains does not reflect the same pattern to tectonic condition

seismic response of the katmai volcanoes to the 6 december 1999 magnitude 7 0 karluk lake earthquake alaska a sudden increase in earthquake activity was observed beneath volcanoes in the katmai area on the alaska peninsula immediately following the 6 december 1999 magnitude m w 7 0 karluk lake earthquake beneath southern kodiak island alaska the observed increase in earthquake activity consisted of small m l 1 3 shallow z 5 0 km events these earthquakes were located beneath mount martin mount mageik trident volcano and the katmai caldera and began within the coda of the karluk lake mainshock all of these earthquakes occurred in areas and magnitude ranges that are typical for the background seismicity observed in the katmai area seismicity rates returned to background levels 8 to 13 hours after the karluk lake mainshock the close temporal relationship with the karluk lake mainshock the onset of activity within the mainshock coda and the simultaneous increase beneath four separate volcanic centers all suggest these earthquakes were remotely triggered modeling of the coulomb stress changes from the mainshock for optimally oriented faults suggests negligible change in static stress beneath the katmai volcanoes this result favors models that involve dynamic stresses as the mechanism for triggered seismicity at katmai

triggered seismicity in utah from the 3 november 2002 denali fault earthquake immediately after the arrival of the surface waves from the m w 7 9 denali fault earthquake on 3 november 2002 the university of utah regional seismic network recorded an abrupt increase in local microseismicity throughout most of utah s main seismic belt we examined this increase in the context of the regional background seismicity using a catalog of 2651 earthquakes from 1 january 2000 to 30 june 2003 statistical analyses of this catalog above spatially varying magnitudes of completeness ranging from 1 2 to 1 7 allow us to reject with 95 confidence the null hypothesis that the observed increases were due to random occurrence the elevated seismicity was most intense during the first 24 hr 10 times the average prior rate but continued above background level for 25 days at the 95 confidence level in most areas we conclude that the increased seismicity was triggered by the denali fault earthquake which occurred more than 3000 km from the study region high peak dynamic stresses of 0 12 to 0 35 mpa that occurred during the passage of the love waves are consistent with the interpretation of triggering the peak dynamic stresses were estimated by measuring peak vector velocities at 43 recording sites 37 of which were relatively new strong motion stations of the advanced national seismic system the triggered events ranged in magnitude m c and or m l from less than 0 to 3 2 and were widely distributed across the state primarily in seismically active regions in contrast to many previously published observations of remotely triggered seismicity the majority of the triggered earthquakes did not occur near quaternary volcanic vents or in areas of magma related geothermal activity in several areas the triggered seismicity was spatially clustered more than five earthquakes each separated by 5 km double difference relative relocations for the earthquakes in three of these clusters indicate that most but not all of the triggered events were spatially separated from source zones of prior seismicity during 2000 2002 focal mechanisms for the two largest triggered events have northeast to northwest trending tension axes which are unusual for the region where they occurred the temporal decay of the triggered activity was similar to that of utah aftershock sequences and can be described by the modified omori s law with a p value of 0 6 to 0 7 the frequency magnitude distribution of the triggered earthquakes is also similar to that of utah aftershocks and for the study area as a whole can be described by the gutenberg richter relation with a b value of 0 81 0 16 these similarities between the triggered seismicity and utah aftershock sequences suggest the possibility that the initiation and development of both could result from the same causative mechanisms online material catalog of earthquakes used in this study

dynamic versus static stress triggering and friction parameters inferences from the november 23 1980 irpinia earthquake this paper concentrates on the problem of fault interaction and earthquake triggering through the 1980 irpinia italy sequence more specifically this paper deals with the problem of the triggering of the second subevent by the mainshock the interaction between the two segments is modeled through a dynamic coulomb failure function the aims of this paper are first to discriminate between the dynamic and the static stress effects on the triggering if these effects exist second to estimate the fault strength relative to the initial state of stress third to determine the parameters of a slip dependent friction law that lead to the observed delay of 20 s numerical simulations show that the critical slip dc may range from 0 03 m up to 1 7 m and that the initial slope of the friction law 0 must be lower than 0 04 m 1 we show that the relative magnitude of the fault strength and the initial state of stress govern the existence and value of a dc lower threshold under which the fault always ruptures before 13 s a close to failure fault is not consistent with a critical slip dc less than 0 8 m whereas small values of dc typically 0 03 m imply a far from failure fault general results concern the effect of a dynamic stress pulse we show that an event can be triggered by a transient stress pulse and that in this case the event can have an initiation duration much longer than the pulse duration we show that it is possible to explain both the triggering and the time delay only with the effect of the transient stress pulse this may explain aftershock triggering even in regions of negative coulomb failure function or long distance triggering of earthquakes by propagating waves

detrital and early diagenetic processes in late pleistocene and holocene sediments from the sw galicia bank inferred from high resolution enviromagnetic and geochemical records abstract u channel based high resolution magnetic and geochemical measurements supported by scanning electron microscopy x ray diffraction and fluorescence and grain size analyses of discrete samples were carried out on six sediment cores recovered from the sw flank of the galicia bank at the prestige s wreck area five magnetochemical facies reflecting the different sedimentary environments i e hemipelagic contouritic turbiditic sediment provenances and early diagenetic redoxomorphic processes i e mn peaks are defined on the basis of a cluster analysis the records showed climatically forced detrital signatures of regional importance associated with continental slope sedimentary processes the chronostratigraphy and correlation of the cores is based on 20 14 c ams dating recognition of heinrich like events h1 to h3 and well documented holocene climatic features from a geochemical and enviromagnetic perspective the late pleistocene to holocene evolution of the area shows two different sedimentation contexts mis 2 is dominated by down slope depositional processes the highest sedimentation rates 9 1 cm ky were achieved during this interval the major turbidite activity is recorded between 31 000 yr bp and 13 500 yr bp successive arrivals of ird h1 to h3 are also identified during this period in contrast holocene sedimentation was dominated by a mixture of pelagic hemipelagic and along slope depositional processes and presented considerable lower sedimentation rates 3 cm ky bottom currents intensification at 7000 yr bp and 4500 yr bp coinciding with late holocene warm events are also recognised during this stage

seismic signature of a hydrous mantle transition zone abstract although water has a major influence on tectonic and other geodynamic processes little is known about its quantity and distribution within the deep earth in the last few decades laboratory experiments on nominally anhydrous minerals nams of the transition zone have shown that these minerals can contain significant amounts of water up to 3 3 wt in this study we investigate if it is possible to use seismic observations to distinguish between a hydrous and anhydrous transition zone we perform an extensive literature search of mineral experimental data to generate a compilation of the water storage capacities elastic parameters and phase boundary data for potentially hydrous minerals in the transition zone and use thermodynamic modelling to compute synthetic seismic profiles of density v p and v s at transition zone temperatures and pressures we find that large uncertainties on the mineral phase equilibria ca 2 gpa and elastic properties produce a wide range of seismic profiles in particular there is a lack of data at temperatures corresponding to those along a 1300 c adiabat or hotter which may be expected at transition zone pressures comparing our hydrous transition zone models with equivalent profiles at anhydrous conditions we see that the depths of the 410 and 660 discontinuities cannot at present be used to map the water content of the transition zone due to these uncertainties further while average velocities and densities inside the transition zone clearly decrease with increasing water content there is a near perfect trade off with increases in temperature it is therefore difficult to distinguish thermal from water effects and the conventional view of a slow and thick transition zone for water and slow and thin transition zone for high temperature should be regarded with caution a better diagnostic for water may be given by the average velocity gradients of the transition zone which increase with increasing water content but decrease for increasing temperature however the significance of this effect depends on the degree of water saturation and partitioning between the nams since seismology is better able to constrain the thickness of the transition zone than velocity gradients our study indicates that the most useful input from future mineral physics experiments would be to better constrain the phase relations between hydrous olivine and its high pressure polymorphs especially at high temperatures additionally the uncertainties on the mineral seismic properties could be reduced significantly if the experimentally observable correlations between bulk and shear moduli and their corresponding pressure derivatives would be published

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

galicia bank sediment transport activity in response to continuous sedimentary instability dynamics a geotechnical perspective the sedimentary instability dynamics occurring over time throughout the isolated galicia bank atlantic ocean nw iberian peninsula have been studied using a sedimentological and geotechnical approach featuring a quantitative assessment of slope stability under different scenarios including earthquake activity the erosion of the scarps in the galicia bank includes the continuous development of small instabilities evacuated along gullies and channels or deposited on slopes these deposits may be subsequently mobilized evolving into new sedimentary gravity flows e g turbidity flow the studied sediments consist mostly of poorly to very poorly sorted sands and silts transported by turbidity currents and occasionally by debris flow processes the sediments in the study area identified as normally consolidated and located on gently sloping areas gradients less than 5 may become unstable if low magnitude seismic events occur pga 10 without any trigger other than oversteepening in contrast overconsolidated sediments may remain stable under static conditions and may become unstable on slope gradients 10 when earthquakes occur with the maximum peak ground acceleration pga 0 34 the sedimentological and geotechnical models presented herein are complementary approaches that can be utilized to understand the long term sedimentary instability dynamics observed within the study area such results are critical for better understanding sedimentary models of the dismantling processes of deep seamounts located far away from continental sedimentary inputs

seismicity analysis at the prestige oil tanker wreck area galicia margin nw of iberia an evaluation of the geological hazard related to the sinking of the prestige oil tanker in november 2002 in the galicia margin nw of iberia was supported by the spanish research agency it included a specific seismic study of the local activity and of the regional seismicity in this margin results of which are presented here published global catalogues show that diffuse seismicity of moderate magnitude events up to 5 6 mb can be associated with the northern flank of the galicia margin with a roughly e w distribution however event locations in this area are poorly constrained due to the inappropriate geometry of permanent seismic networks we collected and analyzed new data from a dense deployment of 10 obs around the wreck site and from a network of 12 land stations at the nw corner of iberia during a 7 months operation period the land network permitted location of 120 events with magnitudes ranging from 1 6 to 4 8 more than 30 of which were not reported by the spanish permanent array confirming the existence of a moderate seismicity in this area five new focal solutions have been obtained for events on land providing additional information about the active faults in the region the obs network ran for 1 month and allowed the location of three earthquakes of magnitude 2 5 near the sinking site four more events have been identified with just a rough determination of their epicenters based on polarization of the p wave arrivals besides these earthquakes the obs data recorded numerous seismic events of different types they include i groups of very short and regularly spaced pulses that can be explained as acoustic calls generated by finback whales ii a large number of short duration seismic events 2 3 s long identified only at individual obs iii local intense monochromatic signals lasting for up to 4 h that can be described as harmonic tremors these last two types of seismic events might be related to hydrothermal activity within the uppermost sediments and deserve additional studies to appropriately establish the seismic hazard of this area 2007 elsevier b v all rights reserved

the absence of remotely triggered seismicity in japan remote dynamic triggering occurred in the western united states and greece following the following the denali hector mine landers and izmit main shocks in seismically active largely geothermal areas that experienced shaking from the mainshock typically exceeding 0 2 cm sec because triggered earthquakes are small often below mw 2 investigations of whether remote triggering is a generally occurring phenomena must be concentrated in well instrumented locations as a seismically active geothermal well instrumented location japan constitutes an ideal location for such study we systematically examine whether remote dynamic trig gering occurs following mainshocks in japan producing amplitudes in the study region above 0 2 cm sec at distances greater than two estimated fault lengths from 1998 to 2004 we use both catalog data and filtered waveforms to search for seis micity increases one significant regional increase follows a mw 7 6 mainshock in march 2000 largely due to a volcanic swarm that began the day before and increased in intensity in the following days two localized increases in cataloged seismicity occur in kyushu which is the only area in the study region with onshore tectonic extension two factors possibly inhibit regional triggering in japan 1 compres sional tectonics and 2 the frequent occurrence of large mainshocks the ambient seismicity in japan combined with the lack of observable widespread triggering ren ders rate state friction and stress corrosion to be improbable triggering mechanisms fracture unclogging as a triggering mechanism is consistent with the observations

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

transient stresses at parkfield california produced by the m 7 4 landers earthquake of june 28 1992 observations from the upsar dense seismograph array the m 7 4 landers earthquake triggered widespread seismicity in the western united states because the transient dynamic stresses induced at regional distances by the landers surface waves are much larger than the expected static stresses the magnitude and the characteristics of the dynamic stresses may bear upon the earthquake triggering mechanism the landers earthquake was recorded on the upsar u s geological survey parkfield small aperture array array a group of 14 triaxial accelerometers located within a 1 square km region 10 km southwest of the town of parkfield california 412 km northwest of the landers epicenter no triggered earthquakes were observed at parkfield multiple filter analysis shows that the displacements obtained by double integration are dominated by the fundamental mode love and rayleigh modes with some higher mode contributions for periods shorter than 10 s most of the surface waves propagated along the great circle path from landers but a late arriving surface wave appears to have been scattered from the sierra nevada mountains we used a standard geodetic inversion procedure to determine the surface strain and stress tensors as functions of time from the observed displacements peak dynamic strains and stresses at earth s surface are about 7 strain and 0 035 mpa respectively and they have a flat amplitude spectrum between 2 s and 15 s period these stresses agree well with stresses predicted from a simple equation using the ground velocity spectrum observed at a single station peak stresses ranged from about 0 035 mpa at the surface to about 0 12 mpa between 2 and 14 km depth with the sharp increase of stress away from the surface resulting from the rapid increase of rigidity with depth and from the influence of mode shapes because of the free surface boundary conditions the horizontal components of the stress tensor tend to dominate in the top 5 6 km of the crust which might cause triggered seismicity to have strike slip or normal mechanisms comparison of dynamic stresses induced by the landers loma prieta and petrolia earthquakes at a variety of sites indicates that the landers stresses were not spectacularly larger than those induced by the other sources landers dynamic stresses were comparable to coalinga static stresses at parkfield the effective strain caused by landers at parkfield where no earthquakes were triggered are the same amplitude as those at some sites in nevada where earthquakes were triggered comparing various authors observations of dynamic stresses there is no obvious characteristic of these stresses that correlates with the triggered seismicity

multibeam backscatter as a tool for sea floor characterization and identification of oil spills in the galicia bank this study is focused on the potential of using the multibeam acoustic backscatter both for sea floor characterization of the galicia bank and for identification of sea bottom oil spills an extended multibeam data set was acquired during the rg 1 and rg 2 surveys aboard rv hesperides in 2003 for the geological and geophysical characterization of the galicia bank where the oil tanker prestige wreck is located 3 d images have been made by integrating bathymetry and backscatter data multibeam data have been additionally supported by gravity core information in order to correlate sea floor acoustic backscatter with the surficial sediments and their physical properties the major factors that seem to control the zonation of the backscatter intensity images are i the type of the sedimentary processes gravitational turbiditic hemipelagic which control at the same time the grain size distribution and ii the sea bottom morphologies such as basement outcrops however there are some backscatter halos that cannot be explained by these processes in the backscatter maps the bow and stern of the prestige wreck are clearly marked by a high backscatter intensity anomaly whose existence is discussed

small 660 km seismic discontinuity beneath tibet implies resting ground for detached lithosphere 1 using seismic profiles comprising of high resolution triplicate waveforms across apertures of over 1 000 km we show that because of high p wave speed vp near the bottom of the mantle transition zone the contrast in vp across the 660 km discontinuity beneath central tibet is small only about 70 of that beneath the northern indian shield this subhorizontal anomaly of high vp is most likely a remnant of detached mantle lithosphere that recently sank to depth thus providing key evidence for a direct connection between continental collision near the surface and deep seated dynamics in the mantle

crustal structure and tectonics of bangladesh new constraints from inversion of receiver functions abstract an understanding of the sedimentary and crustal structure of the bengal basin and of the tectonics deforming it remains elusive due to lack of seismic data from bangladesh taking advantage of recently available data from 11 seismic stations deployed over bangladesh we determine the crustal structure beneath each station using 2768 high quality receiver functions rfs inversion of the rfs reveals a highly variable thickness of the overlying sediments beneath the bengal basin the thickness of the sediments increases dramatically across the hinge zone of the early cretaceous passive margin from 3 to 17 km the thick sediments partly represent progradation of the continental margin due to the influx of clastic sediments from the himalayas the moho shallows across the region this reflects thinning of the crystalline crust from 38 km in the indian craton to 34 km at the hinge zone to 4 0 km s at lower crustal levels supports an influence of the kerguelen plume igneous activity during rifting we invert data for a station near the dauki fault which marks the southern boundary of the uplifted shillong plateau for dip and anisotropic effects our results show the dauki as a north dipping thrust fault at jaflong with a dip angle of 32 and strike 110 close to its surficial expression a strong anisotropy 15 and the sense of shear plunge 24 trend 79 compliment the dipping geometry and deformation seems to be related to the initiation of the dauki fault

structure and evolution of the galicia interior basin atlantic western iberian continental margin abstract an industrial multichannel seismic reflection data set is presented on the galicia interior basin g i b a sedimentary basin that has been poorly studied until now seismic stratigraphic units defined in other areas of the western iberian continental margin as well as some deeper reflections can also be identified within this basin although they show some particular characteristics the g i b is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs western banks the structural framework of the area consists of nnw sse trending normal faults cross cut by ne sw transfer faults the inception of the g i b probably ocurred during triassic times within the late jurassic early cretaceous rifting phase it probably experienced several extensional episodes the most intense one being during valanginian times evidence for compressional tectonic activity in this basin during cenozoic times has also been found when comparing the g i b with the adjacent deep galicia margin a gap in the timing of the main extensional events as well as some differences in the structural styles have been found in a wider regional context an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western iberian margin is consistent with a northward propagating rift for the late jurassic early cretaceous phase

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

continental scale body wave tomography of india evidence for attrition and preservation of lithospheric roots we assemble p and s waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the indian plate from the southern tip of india to the himalayan collision belt and generate an accurate data set of 52 050 p and 30 423 s arrival times through the multichannel cross correlation approach these traveltimes are then inverted to obtain 3 d p and s velocity structures of the subcontinent at a 2 2 lateral resolution the heterogeneous nature of the indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale the key cratonic segments of the indian shield are characterized by pockets of high velocity anomalies 3 at shallow depths 300 km with the diamondiferous regions like wajrakarur revealing high shear wave anomalies down to 300 km in contrast to the southern deccan volcanic province dvp the northwestern dvp is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of deccan volcanism which was facilitated by the reactivation of the rift systems

post rift compression on the sw iberian margin eastern north atlantic a case for prolonged inversion in the ocean continent transition zone abstract an extensive grid of 2d seismic reflection data imaging the non volcanic continental margin of sw iberia is used to investigate the post rift compressional evolution in the transition zone between continental and oceanic crust tectonic compression affected the margin almost continuously since the latest cretaceous but was predominantly focused during the mid eocene and the late oligocene mid miocene the detailed interpretation of post rift structures and their adjacent strata shows that crustal shortening in the various sectors of the margin is neither synchronous nor similar in style main post rift structures include 1 thick skinned transpressional deformation on the distal margin 2 localized basin shortening on the outer proximal margin 3 limited reverse faulting and inversion on the inner proximal margin the location and magnitude of crustal shortening are dependent on the inherited syn rift geometry the existence of evaporite or clay rich detachments at depth the rheological behaviour of previously extended continental crust and the position of the ocean continent transition zone the mechanisms of compression are mainly dominated by plate collision associated with the southeastern migration of the iberian microplate during the alpine orogeny and with recent westward convergence with the oceanic domain

the anza array a high dynamic range broadband digitally radiotelemetered seismic array a small aperture seismic array has been deployed along a 30 km stretch of the san jacinto fault in the vicinity of the town of anza california the array was installed to study specifically the scaling laws of body wave spectra the character of high frequency ground motion the physical interpretation of seismic stress drops and the interaction of earthquakes this region was chosen for these studies due to its high rate of seismic activity in the 2 m 4 5 range the likelihood of a m 6 event in the next decade and the existence of the southern california batholith on either side of the fault reducing problems associated with attenuation and large scale anisotropy these studies employ an instrument package with a frequency response of 100 hz a dynamic range of 138 db a sampling rate of 250 times per second per component and a 16 bit a d converter the array consists of 10 three component stations telemetered via digital vhf radio to a nearby mountain peak and thence via a microwave link to la jolla california a minicomputer system monitors the array s performance detects events and records data upon demand initial results demonstrate the feasibility of digital transmission with an inherent increase in data quality over analog systems

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

lehmann discontinuity beneath north america no role for seismic anisotropy 1 the lehmann discontinuity in the upper mantle is often interpreted as a boundary separating anisotropic and isotropic media we demonstrate however that seismic anisotropy plays if any a minor role in the origin of the lehmann discontinuity our data are obtained with s receiver function technique from recordings of the moma seismograph array in the eastern usa part of the data is processed with a new modification of this technique we observe sp phase converted from the lehmann discontinuity at a depth around 200 km with comparable amplitudes and similar polarities in two azimuths differing by about 90 contrary to the observations the polarity in the synthetic seismograms for models with azimuthal anisotropy is opposite for the azimuths differing by 90 radial anisotropy with a slow vertical direction results in the sp phase with the polarity opposite to the observed one

distinct lateral contrast of the crustal and upper mantle structure beneath northeast tibetan plateau from receiver function analysis abstract we investigate the crustal thickness poisson s ratio and the thickness variation of the mantle transition zone using the p receiver function technique on data recorded by the 68 seismic station of the ascent experiment beneath the northeast tibetan plateau our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast tibetan plateau along two migrated profiles in the eastern region the moho is relatively smooth and continuous and varies in depth between 80 and 51 km whereas the depth of the moho beneath the western region varies between 77 and 52 km and is offset 20 km beneath the boundary between the kunlun fault and qaidam basin the delay times for the ps phases from both the 410 km and 660 km discontinuities in the western region are 1 2 s later than that in the eastern region indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region crustal rocks with low to moderate poisson s ratios are interpreted to be dominated by felsic to intermediate compositions implying that large scale middle lower crustal flow does not occur easily the nearly constant thickness of the mantle transition zone with an average value of 255 km 24 4 s from south to north implies that no lithospheric fragments correlating to the india asia collision have entered into the mantle transition zone

recent sedimentary processes in the prestige site area galicia bank nw iberian margin evidenced by high resolution marine geophysical methods an echo character analysis of the oil tanker prestige wreck area was undertaken using high resolution marine geophysical methods topas and airgun seismic reflection profiles multibeam echosounder and tobi sidescan sonar integration and comparison of the results using all these methods is presented given some practical applications for indirect near surface and seafloor interpretations ten different echo types were identified and grouped into four main classes i distinct ii indistinct iii irregular and iv hyperbolic echo character distribution enabled us to determine recent sedimentary processes in the area two major depositional systems can be found through the identification of these sedimentary processes a a slope depositional system sds located in the eastern and central area where mass movement processes are dominant and b a turbiditic main channel depositional system tds located in the western area where channel related processes are dominant both of these interact over the half graben structure of the southwestern sector of the galicia bank where the prestige wreck is located within the sds erosive and depositional mass movement processes characterised a complex depositional system erosive processes occur on the fault scarp channels head inter lobe channels and distal part of the sedimentary lobes moreover depositional processes take place on the top of the fault scarp sedimentary wedges sedimentary lobes and on the west flank of the main channel both depositional systems interact but the sds should be more active during fault scarp reactivation periods through relief rejuvenation and new exposed deposits microearthquake activity would favour the available materials being weathered eroded and transported by mass movements in such a situation the tds acts as the main collector of eroded sediment derived from the fault scarp throughout the sds being responsible for its evacuation into the iberian abyssal plain however outside of fault reactivation periods the sds is less active such as during the present situation

the accuracy of seismic estimates of dynamic strains an evaluation using strainmeter and seismometer data from pinon flat observatory california abstract the dynamic strains associated with seismic waves may play a significant role in earthquake triggering hydrological and magmatic changes earthquake damage and ground failure we determine how accurately dynamic strains may be estimated from seismometer data and elastic wave theory by comparing such estimated strains with strains measured on a three component long base strainmeter system at pinon flat california we quantify the uncertainties and errors through cross spectral analysis of data from three regional earthquakes the m 0 4 10 17 n m st george utah m 0 4 10 17 n m little skull mountain nevada and m 0 1 10 19 n m northridge california events at distances of 470 345 and 206 km respectively our analysis indicates that in most cases the phase of the estimated strain matches that of the observed strain quite well to within the uncertainties which are about 0 1 to 0 2 cycles however the amplitudes are often systematically off at levels exceeding the uncertainties about 20 in one case the predicted strain amplitudes are nearly twice those observed we also observe significant strains tangential direction which should be zero theoretically in the worst case the rms strain exceeds the other nonzero components these nonzero strains cannot be caused by deviations of the surface wave propagation paths from the expected azimuth or by departures from the plane wave approximation we believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

seismicity in idaho and montana triggered by the denali fault earthquake a window into the geologic context for seismic triggering we present a case study of dynamically triggered seismicity in idaho and western montana from the 2002 mw 7 9 denali fault earthquake to investigate the relationship between measured geological discriminants and propensity for trig gering we first establish triggering we find events that are not reported in the advanced national seismic system catalog in idaho and yellowstone following the denali fault earthquake by filtering broadband waveforms an ml 4 6 earthquake is discovered near new meadows idaho during the passage of the rayleigh waves and another earthquake probably located near yellowstone we find that central western idaho and yellowstone have statistically significant seismicity increases by applying a b test to the cataloged events in the 48 hr after the denali fault earthquake in idaho and surrounding regions we also find that pine idaho may have triggered events but the measurement is not robust because of uncertainty in the background seismicity rate both central western idaho and yellowstone triggered previously during the 1992 landers earthquake we then try to determine the local geological conditions necessary for triggered seismicity to occur geothermal regions with high total dissolved solids and background seismicity appear to be favorable conditions to trigger earthquakes in the study

thermal evolution of a hyperextended rift basin mauleon basin western pyrenees onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma poor rifted margins however critical questions remain concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation in the western pyrenees the mauleon basin preserves the structural and stratigraphic record of cretaceous extension exhumation and sedimentation of the proximal to distal margin development pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating making the mauleon basin an ideal locality to study the temporal and thermal evolution of magma poor hyperextended rift systems through coupling bedrock and detrital zircon u th he thermochronometric data from transects characterizing different structural rifting domains these new data indicate the basin was heated during early rifting to 180 c with geothermal gradients of 80 100 c km the proximal margin recorded rift related exhumation cooling at ca 98 ma whereas the distal margin remained 180 c until the onset of paleocene pyrenean shortening lithospheric scale numerical modeling shows high geothermal gradients 80 c km and synrift sediments 180 c can be reached early in rift evolution via heat advection by lithospheric depth dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments mauleon basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma poor continental rifted margins

quaternary chronostratigraphic framework and sedimentary processes for the gulf of cadiz and portuguese contourite depositional systems derived from natural gamma ray records abstract the contourite depositional systems cds in the gulf of cadiz and on the west iberian margin preserve a unique archive of mediterranean outflow water mow variability over the past 5 3 ma these cds have been recently drilled in several places during the iodp expedition 339 these drill sites now offer a new window to the internal pliocene and quaternary architecture of the cds in this study we use downhole and core gamma ray gr data acquired from 5 sites drilled in the cds along the middle slope and 1 site drilled in the deeper setting of the lower slope out of the mow path the gr data primarily tracks the clay content in the sediment and is the expression of sediment supply and for sites drilled in the cds of the bottom current processes both appear astronomically controlled as shown by spectral analysis performed on the gr data results also reveal that the gr log patterns correlate well across the sites over the last 1 4 my several gr horizons corresponding to drops in gr values were identified most of which fit with coarse grained deposits observed in cores and interpreted as contourite beds the gr horizons are interpreted as isochronous horizons providing a regional scale chronostratigraphic framework for the cds depositional records with an accuracy of 20 ky we further assess the spatial and temporal variability of the cds hiatuses at the regional scale

reinterpretation of the quaternary sedimentary infill of the ria de vigo nw iberian peninsula as a compound incised valley abstract seismic data have been used to investigate the stratigraphy of the galician rias for more than two decades here we present a new interpretation of the sedimentary infill of an incised valley ria de vigo nw iberian peninsula based on high resolution seismic profiles core sediment analysis and radiocarbon 14 c data the new data indicate that the stratigraphic architecture of the galician rias result from multiple incision infill phases and therefore they are reclassified as compound rather than simple incised valleys seven seismic units were identified one of tertiary age u 1 four of pleistocene age u 2 u 5 which are interpreted as 4 th order sequences deposited between mis 11 and mis 2 and late pleistocene u 6 and holocene u 7 units corresponding with post glacial sedimentation the sedimentary infill overlies a highly faulted irregular granitic and metamorphic basement the inherited morphology is shown to be important for controlling the pathway and evolution of the fluvial network as well as preservation of the sedimentary deposits during several glacial interglacial cycles the presence of a rocky barrier at the mouth of the ria is a distinctive feature that conditions sedimentation and exchange of sediment between the ria and the adjacent shelf for the first time faults and tilted blocks affecting late pleistocene mis 3 deposits have been identified the new data presented here provide the opportunity to reconstruct the evolution of the sedimentary infill of a ria with especially high resolution during the last post glacial transgression

quantifying the remote triggering capabilities of large earthquakes using data from the anza seismic network catalog southern california received 24 august 2006 revised 22 june 2007 accepted 13 august 2007 published 3 november 2007 1 various studies have examined remote earthquake triggering in geothermal areas but few studies have investigated triggering in nongeothermal areas we search the anza southern california network catalog for evidence of remote triggering using three statistical tests binomial kolmogorov smirnov and wilcoxon rank sum we determine the significance of the rates and timing of earthquakes in southern california following large teleseismic events to validate our statistical tests we identify 20 local main shocks ml 3 1 with obvious aftershock sequences and 22 local main shocks ml 3 0 that lack obvious aftershock sequences our statistical tests quantify the ability of these local main shocks to trigger aftershocks assuming that the same triggering characteristic i e a particular seismic wave amplitude perhaps in a specific frequency band is evident for both local and remote main shocks we apply the same tests to 60 remote main shocks mb 7 0 and assess the ability of these events to trigger seismicity in southern california we find no obvious signature of remote triggering we find minimal differences between the spectral amplitudes and maximum ground velocities of the local triggering and nontriggering earthquakes similar analysis of a select few of our remote earthquakes shows that the related ground motion regularly exceeds that of local earthquakes both at low frequencies and in maximum velocity this evidence weakly suggests that triggering requires larger amplitudes at high frequencies and that a maximum ground velocity alone is not the primary factor in remote triggering our results are complex suggesting that a triggering threshold if it exists may depend on several factors

influence of dominant wind patterns in a distal region of the nw iberian margin during the last glaciation this work combines planktonic foraminiferal assemblages stable isotope transfer function estimated sea surface paleotemperatures sst and export productivity pexp with sedimentological records from the pc7 core to document paleoclimatic and paleoceanographic changes on a distal region of the nw iberian margin since the last glaciation 87 sr 86 sr indicate that the studied locality mostly received sediments from european ice sheets eis during h5 during the hs and d o stadial events ssts were in general higher than in other cores collected in inner areas of the nw iberian margin the presence and intensification of portugal current probably induced heat transfer to this distal region of the iberian margin our results also suggest the occurrence of high variability in the intensity and wind patterns during the last glaciation in some periods the intensity of northerly winds could have been very strong and contributed to a significant stimulation of ocean productivity in this distal area through the intensification of upwelling in general oceanic productivity decreased during the hs and d o as the presence of cold and less saline water prevented nutrient rich waters from deep levels reaching the surface the high productivity events were bracketed by periods characterized by the highest influence of the azores current eastern branch during which northerly winds and the oceanic productivity should have been weakened supplementary material geochemical and foraminiferal data analysed in this work as well as pearson correlations between the analysed data are available at https doi org 10 6084 m9 figshare c 3931813

geomorphology of the iberian continental margin abstract the submarine features and processes around the iberian peninsula are the result of a complex and diverse geological and oceanographical setting this paper presents an overview of the seafloor geomorphology of the iberian continental margin and the adjacent abyssal plains the study covers an area of approximately 2 3 million km 2 including a 50 to 400 km wide band adjacent to the coastline the main morphological characteristics of the seafloor features on the iberian continental shelf continental slope continental rise and the surrounding abyssal plains are described individual seafloor features existing on the iberian margin have been classified into three main groups according to their origin tectonic and or volcanic depositional and erosional major depositional and erosional features around the iberian margin developed in late pleistocene holocene times and have been controlled by tectonic movements and eustatic fluctuations the distribution of the geomorphological features is discussed in relation to their genetic processes and the evolution of the margin the prevalence of one or several specific processes in certain areas reflects the dominant morphotectonic and oceanographic controlling factors sedimentary processes and the resulting depositional products are dominant on the valencia catalan margin and in the northern part of the balearic promontory strong tectonic control is observed in the geomorphology of the betic and the gulf of cadiz margins the role of bottom currents is especially evident throughout the iberian margin the galicia portuguese and cantabrian margins show a predominance of erosional features and tectonically controlled linear features related to faults

dynamic stress variations due to shear faults in a plane layered medium summary a complete set of expressions is presented for the computation of elastic dynamic stress in plane layered media we use a discrete wavenumber reflectivity method to compute the stress field radiated by arbitrary moment tensor sources the expressions derived here represent an interesting tool for both the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena dynamic stress changes associated with a strike slip fault having unilateral rupture are shown this modelling which is similar to the 1992 landers california earthquake illustrates the effects of distance directivity and depth on transient stress changes

stress strain changes and triggered seismicity following the mw 7 3 landers california earthquake calculations of dynamic stresses and strains constrained by broadband seismograms are used to investigate their role in generating the remotely triggered seismicity that followed the june 28 1992 mw7 3 landers california earthquake i compare straingrams and dynamic coulomb failure functions calculated for the landers earthquake at sites that did experience triggered seismicity with those at sites that did not bounds on triggering thresholds are obtained from analysis of dynamic strain spectra calculated for the landers and mw6 1 joshua tree california earthquakes at various sites combined with results of static strain investigations by others i interpret three principal results of this study with those of a companion study by gomberg and davis this issue first the dynamic elastic stress changes themselves cannot explain the spatial distribution of triggered seismicity particularly the lack of triggered activity along the san andreas fault system in addition to the requirement to exceed a coulomb failure stress level this result implies the need to invoke and satisfy the requirements of appropriate slip instability theory second results of this study are consistent with the existence of frequency or rate dependent stress strain triggering thresholds inferred from the companion study and interpreted in terms of earthquake initiation involving a competition of processes one promoting failure and the other inhibiting it such competition is also part of relevant instability theories third the triggering threshold must vary from site to site suggesting that the potential for triggering strongly depends on site characteristics and response the lack of triggering along the san andreas fault system may be correlated with the advanced maturity of its fault gouge zone the strains from the landers earthquake were either insufficient to exceed its larger critical slip distance or some other critical failure parameter or the faults failed stably as aseismic creep events variations in the triggering threshold at sites of triggered seismicity may be attributed to variations in gouge zone development and properties finally these interpretations provide ready explanations for the time delays between the landers earthquake and the triggered events

upper mantle discontinuity structure beneath eastern and southeastern tibet new constraints on the tengchong intraplate volcano and signatures of detached lithosphere under the western yangtze craton we present new constraints on the upper mantle transition zone structure beneath eastern and southeastern tibet based on p wave receiver functions for a large broadband data set from two very dense seismic arrays a clear depression of both the 410 km and 660 km discontinuities is detected west of the red river fault relative to the east the correlated topographic variations across the red river fault are indicative of temperature changes in the upper mantle above the transition zone which suggests that the fault is a deep rooted structure that penetrates into the upper mantle and separates indochina from south china west of the red river fault the transition zone thickness under the tengchong volcano is found to be normal compared to the global average this strongly suggests that the intraplate volcano may originate from slab tearing of the eastward subducting indian plate at shallow depths in the upper mantle rather than from dehydration of a flattened plate within the transition zone our results further show that the 660 km discontinuity is significantly depressed under the western yangtze craton and that the transition zone therefore thickens by up to 20 km this thickening is suggestive of lowered temperatures associated with a remnant of detached lithosphere in response to overlying asthenospheric escape flow in and around the western yangtze craton in addition we find that the transition zone thickness beneath much of the sichuan basin is similar to the global average

low shear velocities in the sub lithospheric mantle beneath the indian shield 1 ever since its breakup from the gondwanaland 140 myr ago the indian plate was ravaged by four hot spots although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the deccan and the fast drift of the indian plate the modifications to the deep structure remain to be grasped in this study we investigate the mantle transition zone tz structure beneath the indian shield region using 14 000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains the arrival times of the p to s ps conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by 2 s in comparison with the times observed for other precambrian shield regions like africa australia and canada such delays in the conversions from the 410 km discontinuity below the indian shield suggest low shear wave speeds in the lithospheric and sub lithospheric mantles due to higher temperatures together with a thinner high velocity lid that contrasts with a thicker one found beneath most archean cratons a thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the tz itself also a further delay of the tz discontinuities is observed for stations on the southern granulite terrain which was under the influence of the marion plume that is responsible for the separation of madagascar from india although the data do not conclusively show evidence for a 520 km discontinuity an lvl atop the 410 cannot be ruled out beneath certain geological provinces of the indian shield

seismic signatures of detached lithospheric fragments in the mantle beneath eastern himalaya and southern tibet abstract in this study we investigate the mantle transition zone mtz structure beneath eastern himalaya and southern tibet using 9000 high quality receiver functions from 96 broadband stations spanning the entire region the pervasive early arrival times 1 7 s compared to iasp91 of the p to s conversions from the 410 km discontinuity are attributed to the high shear wave velocities associated with the subducted indian and asian lithosphere s global and regional shear wave velocity models obtained from seismic tomography evince such high velocity anomalies both in the uppermost mantle and within the mtz in this complex collision environment in contrast the conversions from the 660 km discontinuity are either normal or delayed up to 1 s providing evidences for a thickened mtz beneath most of the study region although presence of water in the mtz can result in such a thickening issues like 1 low amplitudes of the p410s and p660s conversions 2 small dependence of their amplitudes with frequency 3 absence of a detectable low velocity layer atop 410 and 4 lack of evidence for a 520 km discontinuity preclude such an interpretation instead we attribute this thickening to lowered temperatures affected by the possible presence of detached cold and dense lithospheric slabs within the mtz such a detachment might have been facilitated either by convection or gravity removal of the lithosphere thickened due to continued subduction of the indian plate since the mesozoic

the le danois contourite depositional system interactions between the mediterranean outflow water and the upper cantabrian slope north iberian margin abstract the le danois contourite depositional system cds located in an intraslope basin along the cantabrian margin is unique with respect to the known sedimentary systems along the upper slope of the biscay margin whereas the steep biscay slopes are dominated by downslope processes the le danois cds has been generated by alongslope processes and has a strong potential to contain a record of the neogene palaeoceanography this paper will focus on the onset development and present day functioning of this system with respect to its unique morphological control and the responsible local oceanographic processes new bathymetric and seismic reflection data show that the past and present le danois cds is shaped by the mediterranean outflow water conditioned by seafloor irregularities and two topographic highs the large le danois bank and the smaller vizco high the seismic stratigraphic analysis carried out on the contourite deposits has allowed to identify 3 seismic sequences separated by 3 major regional discontinuities changes in depositional styles the vertical stacking of seismic units and the nature of the discontinuities suggest a correlation with the development of the cadiz cds and well known palaeoceanographic events along the ne atlantic margin the first clues for bottom current deposits are identified in the lower sequence which is developed after tentatively the lower pliocene the drift deposits of both the lower and middle sequences were confined into two palaeobasins within the intraslope basin however from the middle pleistocene revolution 0 9 ma onwards the contouritic deposition is intensified due to the switch to a full glacial mode with 100 ka cyclicity this has allowed the development of the present day depositional and erosive features such as respectively elongated mounded and separated drifts plastered drifts moats and slide scars

sedimentary processes and cold water coral mini mounds at the ferrol canyon head nw iberian margin abstract the western iberian margin has a complex morphology controlled by both geological and oceanographic processes compared to the submarine canyons in the southern part of this margin e g nazare and setubal the canyons in the northern part e g ferrol and a coruna have received little attention this study maps the geomorphological features around the ferrol canyon head and combines them with oceanographic observations to infer the sedimentary and oceanographic processes active in this area furthermore the occurrence of a cold water coral cwc mini mound province near the canyon head and the ortegal spur pockmark field is investigated with regard to seepage processes oceanographic conditions and anthropogenic impact the ferrol canyon head and outer ortegal spur are characterized by erosional erosional and abraded surfaces contourite channels and furrows depositional contourite drifts and sediment waves and mixed contourite terrace features indicating a dominant control of bottom currents on the sedimentary processes bottom currents are related to the interaction of the canyon head topography with both the mediterranean outflow water mow slope current and with m2 internal tides associated to the interface pycnocline between eastern north atlantic central water enacw and mow due to this interaction the ferrol canyon head is subject to active sediment resuspension and forms a potential source area for the intermediate nepheloid layers along the nw iberian upper slope the cwc mini mounds 1 9 2 7 m high and 75 141 m in diameter occur between 400 and 560 m depth seismic facies signatures indicative of fluid flow and morphometric analysis implies that larger cwc mini mounds adjacent to the pockmark field initiated by colonization of pre existing pockmarks while smaller more clustered mounds developed independently the observed mini mounds are relict features likely related to their position outside of the contemporary enacw mow interface which favours cwc growth through 1 increased bottom currents enhancing food supply and 2 the presence of a potential density envelope 27 35 27 65 kg m3 proposed to control coral larvae transport along the ne atlantic margin alternatively the lack of contemporary coral growth might be related to habitat destruction by bottom trawling however a preliminary age constraint reveals cwc growth occurred during the early holocene coeval to relict cwc mini mounds near canyon heads on the armorican margin their existence suggests a regional shift in the ne atlantic density profile where the enacw mow interface occurred up to 200 m above its contemporary position

seismology dynamic triggering of earthquakes the landers earthquake of 1992 triggered a series of earthquakes in other parts of california focusing attention on the mechanism of remote triggering laboratory experiments on granular material under pressure suggest that triggering is a result of seismic waves impinging on a fault and inducing elastic nonlinear behaviour of the fault core accompanied by instantaneous weakening and failure the resulting softening to weakening model fits in well with field observations of real earthquakes after an earthquake numerous smaller shocks are triggered over distances comparable to the dimensions of the mainshock fault rupture although they are rare at larger distances here we analyse the scaling of dynamic deformations the stresses and strains associated with seismic waves with distance from and magnitude of their triggering earthquake and show that they can cause further earthquakes at any distance if their amplitude exceeds several microstrain regardless of their frequency content these triggering requirements are remarkably similar to those measured in the laboratory for inducing dynamic elastic nonlinear behaviour which suggests that the underlying physics is similar1 2

crustal structure and continent ocean boundary along the galicia continental margin nw iberia insights from combined gravity and seismic interpretation the magma poor rifted continental margin of galicia has an extremely complex structure its formation involved several rifting episodes that occurred ultimately during the early cretaceous near a ridge triple junction which produced a change in the orientation of the main structures in its transition to the north iberia margin in addition there is a superimposed partial tectonic inversion along its northwest and northern border which developed from the late cretaceous to at least oligocene times the present study integrates a large volume of new geophysical information mainly marine gravity data and 2d seismic reflection profiles to provide insights on the formation of this rift system and on the development of its later inversion the combined interpretation and modeling of this data enable the presentation of a new crustal and structural domains map for the whole galicia margin this includes the rift domains related to the extreme thinning of the crust and the lithospheric mantle stretched necking and hyperextension and mantle exhumation hme domains as well as a domain of intense compressional deformation new constraints arise on the origin the deep structure and the characterization of the along and across strike variation of the continent ocean transition of the margin where a progressive change from hyperextension to partial inversion is observed the development of both rifting and later partial tectonic inversion is influenced by the existence of former first order tectonic features most of the tectonic inversion is focused on the hme domain which in some areas of the northwestern margin is completely overprinted by compressional deformation

a unified model for dynamic and static stress triggering of aftershocks antishocks remote seismicity creep events and multisegmented rupture aftershocks have been thought to be triggered by static stress transfer the 1992 landers and 1999 hector mine california earthquakes have demonstrated the possibility of remote triggering of seismicity highlighting the role of dynamic seismic waves static stress changes and transient deformations have different timescales the mechanisms by which they trigger earthquakes are thought to be different leading to an unwieldy vision of aftershock triggering we propose a model that encompasses both static and dynamic triggering of aftershocks and antishocks as well as the occurrence of stable slip creep events or multisegmented rupture the framework of this model is based on the stability instability transition of faults we propose that dynamic and static stress triggering pertain to the same physical process we study the effect of the complete coulomb failure function we demonstrate the possibility of the triggering of seismicity by a dynamic stress pulse in a stress shadow zone characterized by a negative static stress field the limited time efficiency of dynamic triggering shown by the data is explained by the model we suggest that dynamic waves trigger only the most unstable faults associated with a relatively short characteristic time while the static stress field triggers also faults that are stable associated with a larger characteristic time

tidal triggering caught in the act over the past three decades many researchers have searched for the tidal triggering of earthquakes in the hope that small triggered shocks would illuminate faults critically stressed for failure in future large earthquakes the results have been equivocal but not anymore according to the perspective by stein he highlights a recent paper by tanaka ohtake and sato in earth planets and space where they report the detection of detected tidal triggering in 10 of the japanese archipelago and show that earthquakes are only triggered when the tidal stress acts in the same direction as the regional tectonic stress if the results can be reproduced in places such as california and taiwan a long sought goal of seismology would be achieved

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

insight into the origin of the tengchong intraplate volcano and seismotectonics in southwest china from local and teleseismic data received 18 june 2008 revised 22 february 2009 accepted 10 march 2009 published 13 may 2009 1 a high resolution tomographic image of the crust and upper mantle under yunnan province in southwest china was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local earthquake arrival times recorded by the dense yunnan seismic network the grid spacing of the tomographic model is 70 km 78 km in the horizontal directions and 4 70 km in depth our resulting model shows a clear low velocity low v 0 3 with respect to the average one dimensional velocity model column extending from the surface down to about 400 km depth under the active tengchong volcano and some high velocity high v anomalies existing in the mantle transition zone furthermore the low vanomaly extends horizontally toward the northeast at 250 to 400 km depth combining our tomographic results with geochemical and geological investigations we infer that the tengchong volcano is a rift related volcano caused by the subduction and dehydration of the indian plate as well as corner flow in the mantle wedge though the subducted slab is a continental plate our results show that the upwelling flow under tengchong originates at 400 km depth a prominent low v anomaly along the red river fault zone extends down to the upper mantle 100 km depth reflecting that the red river fault zone may have cut through the crust to the upper mantle the two dayao earthquakes m 6 1 and m 6 2 in 2003 occurred on the margin of high v anomalies and are underlain by a prominent low v anomaly in the lower crust and upper mantle suggesting that the dayao earthquakes may be associated with fluids released from the dehydration of the subducted indian slab these results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the india asia collision

late pleistocene and holocene sedimentary facies on the sw galicia bank atlantic nw iberian peninsula abstract five main pleistocene holocene lithofacies are defined in three different sedimentary environments fault scarp sedimentary lobe and inter lobe channel of the sw galicia bank 1 turbidites biogenous and terrigenous 2 hemipelagites 3 pelagites 4 debrites and 5 heinrich sediments in the sedimentary lobe and inter lobe channel the stratigraphic record consists mainly of turbidites interbedded with debrites and heinrich sediments and hemipelagites that are covered by hemipelagites or pelagites in the fault scarp the stratigraphy comprises hemipelagites and turbidites covered by pelagites frequency of turbidite events has varied between 1 1 2 ka and 1 3 1 ka since 31 3 ka bp at least four turbidite events 1 to 4 between 9 1 and 31 3 ka bp have been correlated between the different sedimentary environments the downslope of turbidity flows prevailed until 9 1 ka after which vertical settling and slow lateral advection have controlled sedimentation the source area of turbidites and debrites is the fault scarp erosion of the slope near surface pelagites hemipelagites and ancient outcropping deposits could explain the presence of biogenous turbidites and terrigenous turbidites respectively the rhythmic development of turbidites interrupted by hemipelagites could represent the manifestation of different pulses of sedimentary instability induced by a combination of oversteepening up to 29 and occurrence of earthquakes

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust

insight into the subducted indian slab and origin of the tengchong volcano in se tibet from receiver function analysis abstract the subduction of the indian plate beneath se tibet and its related volcanism in tengchong are important geologic processes that accompany the evolution of the tibetan plateau however it is still not clear whether the subduction and volcanism are confined to the upper mantle or if they extend deep into the mantle transition zone mtz here we imaged mtz structures by using receiver function methods with the waveforms recorded by more than 300 temporary stations in se tibet the results show significant depressions of both the 410 km and 660 km discontinuities and a thickened mtz 260 280 km beneath se tibet the depression of the 660 km discontinuity by 10 30 km and the thickened mtz correlate well with high p wave velocity anomalies in the mtz indicating the presence of a subducted indian slab within the mtz significant depression of the 410 km discontinuity by 10 20 km beneath the tengchong volcano indicates that the volcano originates from the mtz and is closely related to the subducted indian slab our results confirm the deep subduction of the indian plate and the deep origin of the tengchong volcano however it remains unknown whether a slab gap exists and contributes to the tengchong volcano