determination of focal mechanisms of nonvolcanic tremor using s wave polarization data corrected for the effects of anisotropy we propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy corrected s polarizations measured directly from noisy tremor signals the successful retrieval of polarization angles in noisy signals owes much to the observation that in general tremors propagate slowly and therefore do not change their location immediately this feature of tremors enables us to use longer time windows to compute polarization angles e g 1 min or longer resulting in a stack of particle motions we applied the method to a tremor episode that occurred beneath the kii peninsula in april 2013 although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface some events with strike slip components were also found to have occurred the proposed method provides us with a tool for investigating smaller scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones

a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

sandwave migration in monterey submarine canyon central california abstract repeated high resolution multibeam bathymetric surveys from 2002 through 2006 at the head of the monterey submarine canyon reveal a sandwave field along the canyon axis between 20 and 250 m water depth these sandwaves range in wavelength from 20 to 70 m and 1 to 3 m in height a quantitative measure was devised to determine the direction of sandwave migration based on the asymmetry of their profiles despite appreciable spatial variation the sandwaves were found to migrate in a predominantly down canyon direction regardless of season and tidal phases a yearlong adcp measurement at 250 m water depth showed that intermittent internal tidal oscillations dominated the high speed canyon currents 50 80 cm s which are not correlated with the spring neap tidal cycle observed currents of 50 cm s or higher were predominantly down canyon applying a simple empirical model flows of such magnitudes were shown to be able to generate sandwaves of a size similar to the observed ones

crustal anisotropy from tectonic tremor under washington state in the cascadia we present new observations of crustal shear wave anisotropy extracted from nonvolcanic tremor in cascadia under washington state measurements of crustal anisotropy are extremely sparse and limited in this area mainly due to low level of seismicity abundance of tremor activity during slow earthquakes offers a unique opportunity to measure anisotropy parameters of the continental crust using tremor signal to accomplish this polarization and splitting analyses of nonvolcanic tremor are performed using three component broadband seismic stations splitting times measurements range between 0 08 and 0 17 s and similar to the splitting magnitude typically observed in the continental crust fast direction of shear wave anisotropy generally trends ese wnw fast polarization directions are in general perpendicular to the prevailing maximum compressive stress field but tend to be parallel to several mapped ew and ese wnw trending faults in this area the observed spatial pattern of anisotropy is likely controlled by faulting that accommodates ns compression resulting from the tectonic movement of the oregon block toward north existence of several ew trending crustal faults and source parameters of crustal earthquakes at depth consistent with the regional stress regime indicate that these faults may be the dominant factor causing the observed pattern of shear wave anisotropy

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

u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

very low frequency earthquakes related to small asperities on the plate boundary interface at the locked to aseismic transition 1 very low frequency vlf seismic signals observed in southwestern japan are evidently radiated from shear slips on the upper surface of the subducting philippine sea plate we used grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes between 3 1 and 3 8 from continuous seismograms recorded over a 5 year period by using a very dense broadband seismic network at least 5 10 sequences of repetitive activity were observed during the 5 years the vlf events formed clustered distributions along the 35 km isodepth contour of the subducting plate surface the nodal planes which dip landward of moment tensor solutions of the vlf events reflected the configuration of the subducting plate interface the slip vectors were consistent with the direction of movement of the subducting plate the dip and strike of the slip vectors clearly reflected the configuration of the upper surface of the subducting plate we found that the rates of seismic moment release per unit area associated with five major vlf clusters were very similar although both the seismic magnitudes and sizes of the clusters varied considerably the rate of seismic moment released from detectable vlf sources was 0 1 of the rates of short term slow slip events suggesting that the source areas occupied only 0 1 of the fault segment on which the short term slow slip events occurred

characteristics of slow earthquakes in the very low frequency band application to the cascadia subduction zone deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency vlf signals detectable by broadband seismometers but usually hidden in large ambient noise by stacking broadband seismograms relative to tremor hypocentral times we can recover the vlf signals the stacked signals are then inverted to determine a moment tensor solution using a procedure previously applied to vlf signals in japan taiwan and mexico here we apply this method to the cascadia subduction zone where tremors and slow slip events are clearly observed as expected we successfully recover vlf signals for almost the entire tremor region beneath southern vancouver island and northern washington state the moment tensors are mostly well determined as low angle thrust type but source depths are poorly constrained the slip direction is slightly rotated counterclockwise with respect to the local plate motion direction probably due to bending of the subducting plate the seismic moment measured in vlf band is proportional to the seismic energy of tremors with a scaled energy of about 3 10 9 the widespread observability of vlf signals suggests that the deformation associated with tremors and sses is actually a very broadband phenomenon as suggested by stochastic models

deposits related to degradation processes on piton des neiges volcano reunion island overview and geological hazard abstract piton des neiges pn volcano on reunion island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material including extensive debris avalanche deposits pn litho structural units first studied by upton and wadsworth 1965 philos trans r soc lond a 271 pp 105 130 are re examined this review highlights the importance of long volcanic repose periods and erosion processes during pn history volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of pn degradation processes the deposits of these processes have been classified into 1 talus 2 mudflow and debris flow and 3 debris avalanche lithology frequency and estimated volumes of each deposit type imply that the structural evolution of pn can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes the occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of pn which implies a very low risk at present on the contrary mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets moreover if smaller debris avalanches can occur in the cirques of pn another major risk must be evaluated

universality of slow earthquakes in the very low frequency band deep tectonic tremors have been observed together with signals in the very low frequency vlf band from 0 02 to 0 05 hz which have been identified as vlf events in limited regions of subduction zones by stacking broadband seismograms relative to the timing of tremors we have detected similar signals in all regions where tremors occur in western japan these signals are inverted to obtain the moment tensor and the fault normal and slip vectors are generally consistent with the geometry of the plate interface and the direction of plate motion therefore these signals are probably radiated by shear slip on the plate interface the ratio between the seismic energy rate estimated from the tremors and seismic moment rate in the vlf band is almost proportional with a proportionality constant i e scaled energy of around 5 10 10 the spatial distribution of scaled energy may reflect spatial variations in the frequency dependent characteristics of slow deformation

very low frequency earthquakes along the ryukyu subduction zone 1 a total of 1314 very low frequency earthquakes vlfes were identified along the ryukyu trench from seismograms recorded at broadband networks in japan f net and taiwan bats in 2007 the spectra of typical vlfes have peak frequencies between 0 02 to 0 1 hz among those waveforms from 120 vlfes were inverted to obtain their centoroid moment tensor cmt solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11 14 in latitude and longitude and at depths of 0 to 60 km most of the vlfes occur on shallow thrust faults that are distributed along the ryukyu trench which are similar to those earthquakes found in honshu and hokkaido japan the locations and mechanisms of vlfes may be indicative of coupled regions within the accretionary prism or at the plate interface this study highlights the need for further investigation of the ryukyu trench to identify coupled regions within it

crustal anisotropy in a subduction zone forearc northern cascadia s wave splitting analyses using low frequency earthquake templates at three component stations across southern vancouver island and northern washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the north american plate for southern vancouver island we investigate contributions to anisotropy from the leech river complex a terrane composed of strongly foliated phyllites and schists with steeply dipping foliations striking east west fast directions across mainland southern vancouver island are subparallel to the dominant leech river complex foliation direction east to west increases in delay times and small scale azimuthal variations in fast directions indicate heterogeneous anisotropy we test azimuthally anisotropic leech river complex models constrained by previous geological and seismic reflection studies through forward modeling using 3 d spectral element method simulations the preferred model of a north northeast shallowly dipping wedge of leech river complex material with varying orientation of anisotropy terminating at midcrustal levels explains the splitting observations at a majority of southern vancouver island stations for stations where anisotropic leech river complex models do not recreate observations fast directions are subparallel to local estimates of maximum compressive horizontal stress suggesting that fluid filled cracks could be a source of anisotropy we assert that the leech river complex is the primary source of crustal anisotropy beneath southern vancouver island not cracks as suggested by prior studies fast directions at stations on northern washington exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the olympic core rocks these slates may also underlay stations on southern vancouver island and represent another source of anisotropy

crustal anisotropy structure related to lateral and down dip variations in interplate coupling beneath the kii peninsula sw japan 1 anisotropy is an important feature of elastic wave propagation in the earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or lattice preferred orientations we studied regional variations in shear wave anisotropy beneath southern kii peninsula which is located above the subducting philippine sea plate using waveforms of local earthquakes observed with a dense seismic network the fast directions of polarization are normal to the strike of the plate margin in the proximity of the locked portion of the underlying mega thrust fault but they are subparallel to the strike of the margin along the fault s down dip extension the distribution of the fast directions roughly coincides with that of the maximum horizontal compressive stress axis directions as inferred from earthquake focal mechanisms and borehole stress measurements this indicates that the observed anisotropy is likely controlled by crustal stresses furthermore the directions of the stress induced anisotropy correlate well with variations of the plate coupling ratio along the plate boundary as inferred from geodetic measurements these results indicate the possibility that crustal anisotropy within an overlying plate can be used as an effective measure to monitor temporal variations of the plate coupling strength during the interseismic period

deep structure and metallogeny of early proterozoic mobile belts in the light of superdeep drilling in russia abstract superdeep drilling in russia and ukraine provides new direct information about the internal structure of the continental crust its evolution and associated ore forming processes deep levels of early proterozoic mobile belts have been investigated with two superdeep boreholes in russia and ukraine 1 in the kola peninsula in the baltic shield and 2 in the krivoy rog basin in the ukrainian shield the investigations of the kola superdeep borehole invalidate previous hypotheses on the conrad discontinuity they prove intensive proterozoic tectonic and metamorphic reworking of archaean rocks and reveal mineralizations of sulphide copper nickel and gold at depth in the light of the new evidence two geotectonic models of the pechenga imandra varzuga belt are reviewed 1 an intracontinental rift subject to the later compression and 2 an ocean opening followed by development of a collisional suture zone the krivoy rog belt has traditionally been described as a deep synclinorium filled with banded iron formations and cut in the west by a steep regional fault the krivoy rog superdeep hole was predicted to intersect all members of the early proterozoic krivoy rog group and to meet tonalites of the archaean basement at 9 10 km depth contrary to expectations however the borehole reached the basement complex at a depth of 2350 m the drilling stimulated geological and geophysical studies of adjacent territories and the results are reviewed in the light of two geotectonic models for the central part of the ukranian shield 1 a plate tectonic model involving westward subduction of oceanic crust and 2 a model implying a rise of asthenolite during early proterozoic times west of the krivoy rog basin the superdeep borehole data lead to better understanding of the structures of the continental crust and its metallogeny but at the same time pose new questions to which answers have yet to be found

rifting evolution in geological history abstract rift and rift like zones global linear extensional structures of the crust have appeared during most periods of the earth s history rifting as a geological process has undergone a complex evolution with its historical roots in the early precambrian the development of the oldest linear mobile tectonic zones archean greenstone belts and granulite belts combined the features of both rift and geosynclinal processes in the following epochs early proterozoic mobile zones protogeosynclinal regions and protorift zones evolved under similar tectonic regimes of crustal destruction in the early stages and compression in the late stages of their development but different in size scale and intensity of the deformations and thermal processes experienced typical rift structures have existed since the late proterozoic in the late proterozoic and paleozoic continental intracratonic rifting in the form of aulacogen and aulacogeosynclinal zone development was a process related and subordinated to a geosynclinal process in the adjacent mobile belts in the paleozoic the first epiorogenic postorogenic rift zones appeared within the epigeosynclinal folded belts during the meso cenozoic epoch of the earth s history rifting increased considerably and became independent of geosynclinal processes it is believed to be one of the main forms of continental crust destruction and formation of the oceanic basins and probably occurred under conditions of some general expansion of the earth

major stages of rifting evolution in the earth s history abstract linear extensional rift and rift like structures have been generated many times during various periods of the earth s history since early times these structures have varied with time however in scale morphology formations paragenesis of tectonic features regularity of distribution and evolution i distinguish and consider five major stages of rifting they are katarchean archean early proterozoic late proterozoic paleozoic and mesozoic cenozoic linear structures of the oldest stage combine features of development and structure similar to those typical of the rift and geosynclinal zones in the later periods of the earth s history at the second stage proto rift and proto geosynclinal structures can be distinguished for the first time though differences are not distinct paleo rift zones of the third and fourth stages differ from geosynclinal belts and zones their structural and genetic evolution however is related to the development of the latter in contrast the distribution and evolution of the rift zones and belts during the fifth stage are connected mainly with the destruction of the continents and formation of the secondary oceanic basins they are complementary to the mesozoic cenozoic geosynclinal orogenic belts periods of the greatest development of rift and rift like structures in the archean riphean and mesozoic cenozoic probably correspond to the major phases of the earth s expansion which can be distinguished in its general pulsating evolution

complex evolution of transient slip derived from precise tremor locations in western shikoku japan transient slip events which occur more slowly than traditional earthquakes are increasingly being recognized as important components of strain release on faults and may substantially impact the earthquake cycle surface based geodetic instruments provide estimates of the overall slip distribution in larger transients but are unable to capture the detailed evolution of such slip either in time or in space accompanying some of these slip transients is a relatively weak extended duration seismic signal known as nonvolcanic tremor which has recently been shown to be generated by a sequence of shear failures occurring as part of the slip event by precisely locating the tremor we can track some features of slip evolution with unprecedented resolution here we analyze two weeklong episodes of tremor and slow slip in western shikoku japan we find that these slip transients do not evolve in a smooth and steady fashion but contain numerous subevents of smaller size and shorter duration in addition to along strike migration rates of 10 km d observed previously much faster migration also occurs usually in the slab dip direction at rates of 25 150 km h over distances of up to 20 km we observe such migration episodes in both the updip and downdip directions these episodes may be most common on certain portions of the plate boundary that generate strong tremor in intermittent bursts the surrounding regions of the fault may slip more continuously driving these stronger patches to repeated failures tremor activity has a strong tidal periodicity possibly reflecting the modulation of slow slip velocity by tidal stresses

a review of retrospective stress forecasts of earthquakes and eruptions changes in shear wave splitting sws monitor stress induced changes to the geometry of the stress aligned fluid saturated microcracks pervading almost all sedimentary igneous and metamorphic rocks in the earth s crust and upper mantle changes in sws implying stress accumulation and stress relaxation suggesting crack coalescence before large earthquakes have been observed retrospectively in the rock mass surrounding large or larger earthquakes in one case the time magnitude and fault plane of a m 5 earthquake in sw iceland was successfully stress forecast 3 days before it occurred similar characteristic behaviour of shear wave splitting has been observed retrospectively before 17 other earthquakes and before three volcanic eruptions these retrospective stress forecasts have been published in different formats in different journals for clarification this paper redraws all observations of stress accumulation and stress relaxation in a consistent normalised format that allows the overall similarities in behaviour to be recognised before earthquakes and volcanic eruptions such behaviour inconsistent with conventional sub critical geophysics confirms the compliance of the new geophysics of a critically microcracked earth where the microcracks are so closely spaced that they verge on failure and hence are critical systems that impose a range of fundamentally new properties on conventional sub critical geophysics one of the implications of new geophysics is that there are similarities in the behaviour of stress before earthquakes and volcanic eruptions the normalised formats show such similarities and include the opportunity to stress forecast both earthquakes and eruptions

low frequency earthquakes in the mexican sweet spot 1 we use data from the meso america subduction experiment to detect and locate low frequency earthquakes lfes in the mexican subduction zone we use visually identified templates to perform a network waveform correlation search that produced 17 000 robustly detected lfes that form 15 distinct families stacking an lfe family s corresponding detections results in seismograms with high signal to noise ratios and clear p and s wave arrivals we use these travel times to locate the sources the resulting locations superpose a previously identified region of permanent non volcanic tremor nvt activity husker et al 2012 called this region a sweet spot suggesting that the local conditions are adequate to continuously generate nvt the lfe hypocenters have been located at a depth of 40 45 km in an area that is surrounding the upper slab plate interface we characterize their focal mechanisms by comparing their stacked seismograms to synthetic seismograms this analysis reveals a common low dipping focal mechanism

phenomenology of deep slow earthquake family in southwest japan spatiotemporal characteristics and segmentation 1 the phenomenology of deep slow earthquakes including low frequency tremors very low frequency earthquakes and short term slow slip events on the subducting plate interface in southwest japan is investigated on the basis of their spatiotemporal characteristics the belt like distribution of tremors is divided into segments bounded by gaps the repetition of tremor episode depends on the magnitude of the episode which is defined by the number of detected tremors within each episode major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months accompanying the short term slow slip events in large segments in small segments tremor episodes with small numbers of tremors have recurrence intervals of a few months some of them occasionally accompany the slight ground tilting caused by a small size slow slip event even in the same segment there exist different recurrence intervals according to the magnitude of the tremor episode the migration pattern varies greatly in initiation termination direction and speed however the migration direction has a general tendency for each segment on both sides of the gap the tremor starts next to the gap and propagates to the other side assuming all tremor episodes are caused by slow slip events the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode the equivalent slip history estimated from all tremor episodes is almost constant in each segment this indicates that tremor activity is a good proxy for slow slips

meltwater pulse 1a from antarctica as a trigger of the bolling allerod warm interval meltwater pulse 1a mwp 1a was a prominent feature of the last deglaciation which led to a sea level rise of 20 meters in less than 500 years concurrent with mwp 1a was the onset of the bolling allerod interstadial event 14 600 years before the present which marked the termination of the last glacial period previous studies have been unable to reconcile a warm northern hemisphere with mwp 1a originating from the laurentide or fennoscandian ice sheets with the use of a climate model of intermediate complexity we demonstrate that with mwp 1a originating from the antarctic ice sheet consistent with recent sea level fingerprinting inferences the strength of north atlantic deep water nadw formation increases thereby warming the north atlantic region and providing an explanation for the onset of the bolling allerod warm interval the established mode of active nadw formation is then able to respond to subsequent freshwater forcing from the laurentide and fennoscandian ice sheets setting the stage for the younger dryas cold period the melting of continental ice sheets during the last deglaciation provided a freshwater source to the ocean that affected global sea level and the strength of the thermohaline circulation an exceptionally large melting event inferred from far field relative sea level records occurred

slow earthquakes and nonvolcanic tremor nonvolcanic tremor is observed in close association with geodetically observed slow slip events in subduction zones accumulating evidence points to these events as members of a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below by virtue of their locations and their properties slow earthquakes are certain to provide new insights into the behavior of earthquakes and faulting and into the hazard they embody

a monochromatic earthquake suggesting deep seated magmatic activity beneath the izu ooshima volcano japan after a quiescence of 12 years the izu ooshima volcano located about 30 km east of the izu peninsula in central japan erupted on november 15 1986 on august 27 1985 about 1 year before the eruption a peculiar earthquake of magnitude 2 7 took place near the moho boundary just beneath the izu ooshima volcano where seismic activity is otherwise absent analysis of digital data obtained by the telemetered seismic network of the national research center for disaster prevention reveals the following distinctive features of the seismic waves 1 the main part of the wave train is composed of s waves and a small p phase precedes it 2 the spectral density has a sharp peak at 1 0 hz which is independent of the epicentral distance azimuth and time indicating a monochromatic spectrum at the source 3 ground motion for the initial part of the s wave is polarized in the n s direction at almost all the stations the amplitude ratios of s to p waves and the polarization pattern of s waves suggest that a traction force caused by magma flow is a more probable source than a double couple mechanism free oscillation of the magma reservoir or opening of a tensile crack we propose a traction force model for this earthquake in which magma flow through a conduit produces unidirectional viscous force on the conduit walls the observations imply a n s force direction and a maximum force of about 2 8 1010 n the pressure difference between the ends of the conduit is estimated as 6 1 107 pa 610 bar under the assumption of conduit dimensions of 2 km 1 km the monochromatic nature of the source spectrum is difficult to account for by the free oscillation of a magma reservoir we tentatively suggest the intermittent opening of a barrier in the conduit as a plausible mechanism of the periodic oscillation

anatomy of piton de la fournaise volcano la reunion indian ocean the aim of this work is to propose a general model of piton de la fournaise volcano using information from geological and geophysical studies firstly we make a graphical compilation of all available geophysical information along a w e profile secondly we construct a geological section that integrates both the geophysical information and the geological information the lithosphere beneath piton de la fournaise is not significantly flexed and the crust is underlain by an underplating body which might represent the deep magma reservoir for la reunion volcanism piton de la fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to les alizes volcano indeed if the differentiated rocks observed at the bottom of the riviere des remparts are the top of les alizes volcano the interface with piton de la fournaise may be located at about sea level beneath the summit area the endogenous constructions intrusive complexes related to les alizes and piton de la fournaise volcanoes represent a large volume the huge intrusive complex of les alizes volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano tectonic activity of piton de la fournaise the early piton de la fournaise edifice was built around a focus located beneath the plaine des sables area the center subsequently moved 5 6 km eastward to its current location the dense high velocity body beneath the plaines des sables and the western part of the enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present day position magma reservoirs may have existed and may still exist as illustrated by the march 1998 crisis at the mechanical and density interface between the oceanic crust and the les alizes edifice strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit the march 1998 pre eruptive seismic pattern location and upward migration seems to be evidence for a transfer of magma between the two reservoirs the dominant structural feature of the central zone is a collapse structure beneath the summit craters above the inferred magma reservoir near sea level the collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic intrusive and hydrothermal activity because of its higher permeability and weaker mechanical strength

a new model for the evolution of la reunion volcanic complex from complete marine geophysical surveys results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands in 2006 cruises forever and eroder 1 investigated the submarine flanks and the surrounding abyssal plain of la reunion indian ocean to obtain for the first time a complete geophysical survey of the area combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes piton des neiges and piton de la fournaise we show that debris avalanche deposits extend on the abyssal plain only offshore the active piton de la fournaise volcano attesting the occurrence of large flank collapse events the absence of such deposits offshore piton des neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano such as sliding or spreading the slow deformation of piton des neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island this study proposes a new model using the most complete marine data set available slow deformation controls the evolution of piton des neiges whereas piton de la fournaise formed on the flanks of a pre existing edifice experienced catastrophic large flank collapse events

very low frequency earthquakes in cascadia migrate with tremor we find very low frequency earthquakes vlfes in cascadia under northern washington during 2011 episodic tremor and slip event vlfes are rich in low frequency energy 20 50 s and depleted in higher frequencies higher than 1 hz compared to local earthquakes based on a grid search centroid moment tensor inversion we find that vlfes are located near the plate interface in the zone where tremor and slow slip are observed in addition they migrate along strike with tremor activity their moment tensor solutions show double couple sources with shallow thrust mechanisms consistent with shear slip at the plate interface their magnitude ranges between mw 3 3 and 3 7 seismic moment released by a single vlfe is comparable to the total cumulative moment released by tremor activity during an entire episodic tremor and slip event the vlfes contribute more seismic moment to this episodic tremor and slip event than cumulative tremor activity and indicate a higher seismic efficiency of slow earthquakes in cascadia than previously thought spatiotemporal correlation of vlfe and tremor activity suggests that they are the results of the same physical processes governing slow earthquakes

comparative study of tectonic tremor locations characterization of slow earthquakes in guerrero mexico deep tectonic tremor in guerrero mexico has been observed using dense temporal seismic networks i e the meso american subduction experiment and guerrero gap experiment g gap arrays during two different time periods we apply a set of seismic waveform analysis methods to these data sets to constrain the locations of tremors and determine the associated moment tensors first we detect and locate the tremors next very low frequency vlf signals are identified by stacking waveform data during tremor bursts and their moment tensors are determined finally to better investigate the link between tremors and vlf earthquakes we detect vlf events using a matched filtering algorithm to search continuous seismic records none of the 11 vlf events detected by this method occurred in the absence of tremor bursts suggesting they are indeed part of the same phenomena unlike previous investigations our results for the g gap period reveal that downdip tremor activity i e in the so called sweet spot is segmented into two patches separated by 40 km in the along trench direction indicating possible variations in the geometry of the plate interface and or slab effective pressure moment tensors of vlf signals are consistent with shear slip on the near horizontal plate interface but source depths are about 5 km deeper than the established plate interface the slip directions of the vlf events are slightly 10 counterclockwise of the plate convergence direction indicating that strain energy promoting left lateral strike slip motion may accumulate in the continental crust during the interseismic period

reunion hotspot activitity through tertiary time initial results from the ocean drilling program leg 115 abstract leg 115 of the ocean drilling program recovered basaltic rocks from four sites along the ancient trail of the reunion hotspot the age of volcanism determined from biostratigraphic data at the four sites increases to the north and records the motion of india away from the reunion hotspot through tertiary time hotspot activity began with the eruption of the deccan flood basalt flows at the time of the cretaceous tertiary boundary the reunion hotspot has been stationary with respect to other hotspots in the atlantic and indian ocean basins through tertiary time the geochemical compositions of the drilled basalts differ according to the relative contributions of magmas from hotspot and morb mantle sources

low frequency earthquakes below southern vancouver island 1 the nature and distribution of low frequency earthquakes lfes in subduction zones provide insight into plate boundary deformation downdip of the locked seismogenic zone we employ network autocorrelation detection to identify lfe families beneath southern vancouver island and environs an initial suite of 5775 lfes detected in 2004 and 2005 at a select set of 7 stations is grouped into 140 families using waveform cluster analysis these families are used as templates within an iterative network cross correlation scheme to detect lfes across different tremor episodes incorporate new stations and improve lfe template signal to noise ratio as in southwest japan representative lfe locations define a relatively tight dipping surface several km above the locus of intraslab seismicity within a prominent dipping low velocity zone lvz lfe polarizations for near vertical source receiver geometries possess a remarkably uniform dipolar signature indicative of point source double couple excitation focal mechanisms determined fromp wave first motions are characterized by a combination of strike slip and thrust faulting we suggest that lfes and regular intraslab seismicity occur in distinct structural and stress regimes the lvz inferred to represent weak overpressured porous and mylonitized metabasalts of oceanic crustal layer 2 separates lfes manifesting deformation within a plate boundary shear zone from intraslab earthquakes generated by tensional stresses and dehydration embrittlement within a more competent lower oceanic crustal layer 3 and underlying mantle

understanding sediment dynamics of two large submarine valleys from seafloor data blanes and la fonera canyons northwestern mediterranean sea abstract the studied area appears as a major place for dense shelf water dsw formation and export towards the deep basin with blanes and la fonera canyons as most efficient pathways these two canyons are so deeply indented on the continental shelf and slope of the northern catalan margin western mediterranean sea that their heads significantly reduce the width of the shelf and interfere with coast and contour parallel sediment transport patterns this study presents the results of the analysis of swath bathymetry data and parametric seismic reflection profiles showing the detailed morphology and uppermost sedimentary structure of blanes and la fonera canyons with the focus on their heads and adjacent upper courses and shelf the main aim is to understand their functioning and assess how they influence and respond to the hydrosedimentary processes active in the study area the n s oriented blanes canyon head whose shortest distance to shore is only 4 km extends along 21 7 km from 70 to 1300 m water depth with an average rim to rim width of 8 km and a canyon floor width up to 750 m la fonera canyon head extends along 28 km from 60 to 1700 m water depth rim to rim and canyon floor widths are up to 7 km and up to 700 m respectively the canyon head follows a general wnw ese trending course in its deeper part but trends n s in its shallower course formed by cap begur branch south of it the illa negra branch trends nw se and its tip located at 60 m water depth and 800 m from the coastline intersects the littoral sedimentary prism backscatter data show high reflectivity into both blanes and la fonera canyon floors as well as on tributary gullies which at these locations is indicative of coarse sediment the seafloor and subseafloor observations here presented are explained and best understood from the sedimentological viewpoint by the interplay of event driven dsw flows permanent mesoscale circulation and storm action while dsw flows from the gulf of lion and the shelf area around the studied canyons are the most dynamic agent in terms of sediment transport and seafloor shaping the northern current ensures background sedimentation of fines and coastal storms promote episodic entries through canyon heads and upper course rims canyon wall morphology and sediment draping respond to the currentward or leeward position of each wall with respect to the main southward moving water flows in the area either episodic or permanent that are dsw and the northern current other relevant morphosedimentary features such as a 40 km long southbound subdued channel on the roses outer shelf that originates off cap de creus promontory to finally feed cap begur branch of la fonera canyon and a contouritic ridge and moat attached to the northern wall of the canyon are interpreted in terms of dsw flows a comprehensive model of the sedimentary functioning of the canyoned north catalan margin that integrates seafloor information and background data on dynamic processes is presented which could be of application to other continental margins worldwide such an integrative view is eased by the enormous unprecedented multidisciplinary research effort carried out in the study area over the last three decades

very broadband analysis of a swarm of very low frequency earthquakes and tremors beneath kii peninsula sw japan 1 we have conducted a temporal broadband seismic observation in kii peninsula southwest japan and detected a swarm of 110 very low frequency earthquakes vlfs and deep low frequency tremors during three days of the swarm activity vlfs and tremors occur concurrently in two localized regions separated by 10 km stacking analyses are also employed to detect vlf signals of a period longer than 50 s whose focal mechanisms are determined for the first time from data and shown to be consistent with the subducting plate motion evaluation of the vlf seismic moment implies that a substantial portion of the sse seismic moment is released as a vlf swarm

late glacial to preboreal sea level rise recorded by the rhone deltaic system nw mediterranean abstract a unique late glacial preboreal record of changes in sea level and sediment fluxes originating from the alps is recorded in the rhone subaqueous delta in the western mediterranean sea the compilation of detailed bathymetric charts together with high resolution seismic profiles and long cores reveals the detailed architecture of several sediment lobes related to periods of decreased sea level rise and or increased sediment flux they are situated along the retreat path of the rhone distributaries from the shelf edge and canyon heads up to the modern coastline they form transgressive backstepping parasequences across the shelf the late holocene highstand deltas being confined to the inner shelf the most prominent feature is an elongated paleo shoreface deltaic system with an uppermost sandy fraction remolded into subaqueous dunes a long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system in seismic data it displays aggradation starting at 15 cal kyr bp followed by progradation initiated during the first phase of the younger dryas a period of reduced sea level rise or stillstand the delta kept pace with resumed sea level rise during the preboreal which is estimated at about 1 cm yr as a result of increased sediment supply from the alps melting of glaciers and more humid climate flushing the sediment down to the sea abandonment of the delta occurred around 10 500 cal yr bp that is to say about 1000 yr after the end of the younger dryas probably because of decreased sediment flux

in situ measurements of velocity structure within turbidity currents 1 turbidity currents are thought to be the main mechanism to move 500 000 m3 of sediments annually from the head of the monterey submarine canyon to the deep sea fan indirect evidence has shown frequent occurrences of such turbidity currents in the canyon but the dynamic properties of the turbidity currents such as maximum speed duration and dimensions are still unknown here we present the first ever in situ measurements of velocity profiles of four turbidity currents whose maximum along canyon velocity reached 190 cm s two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year long deployment

simple relationship between seismic activity along philippine sea slab and geometry of oceanic moho beneath southwest japan summary many earthquakes with various types of focal mechanisms occur along the subducting philippine sea slab phs beneath southwest japan and the hypocentre distribution is quite inhomogeneous in this region distinct non volcanic deep low frequency tremors are often observed to clarify the cause of these apparently complicated and peculiar seismic activities we first construct a detailed configuration of the oceanic moho slab moho within the phs based on a receiver function rf analysis to convert the time domain rfs into their depth domain equivalents we adopt a fine scaled velocity model based on the traveltime tomography the depth contours of the oceanic moho exhibit complexities but the active region of the intraslab earthquakes corresponds well to the oceanic moho geometry in the western part of shikoku the intraslab seismicity is very high and the focal mechanisms mainly exhibit normal faults with east west extension axes since the phs is subducting to the west and the dip angle of the slab becomes steep in the western part of this region the stress field in this region can be attributed to the slab pull induced by the westwards slab subduction similar types of focal mechanisms are observed in the tokai district with a focal depth greater than 35 km since the oceanic moho exhibits a ridge shape in this region its stress field can be attributed to the slab pull and the effect of slab bending shallow earthquakes in the tokai and eastern shikoku regions have strike slip focal mechanisms and their extension axes are parallel to the strike of the oceanic moho these focal mechanisms may be influenced by both the oblique subduction of the slab and low slab dip angle earthquakes beneath the kii peninsula exhibit strike slip or downdip compression focal mechanisms with east west compression axes since the moho beneath this peninsula shows a valley shape compressional stress occurs around the moho moreover the slab convergence compresses this valley this also induces the east west compression field in this region we have newly confirmed the existence of an aseismic slab beneath the northern part of the kinki district central part of japan in this aseismic region the dip direction of the slab is parallel to that of the slab convergence direction this tendency is observed in the central part of shikoku where the slab seismicity is relatively low these features imply that the intraslab seismicity is constrained by the interaction between the slab geometry and slab motion we also revealed the possibility that the oceanic moho beneath not only ise bay but also the kii channel has a ridge shape in both these regions the non volcanic low frequency tremor activity becomes low more than 80 per cent of the tremor epicentres are distributed within contour depths of 38 3 km of the oceanic moho we find that the interaction between the moho geometry and the plate motion of the subducting phs plays an important role in determining the local stress field and the location of both seismicity and low frequency tremors

anisotropic structures of oceanic slab and mantle wedge in a deep low frequency tremor zone beneath the kii peninsula sw japan 1 anisotropy is an important feature of elastic wave propagation in the earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or preferred crystal orientations we studied the regional variations in shear wave anisotropy around a deep low frequency earthquake lfe zone beneath the kii peninsula sw japan using waveforms of local earthquakes observed by a dense linear array along the lfe zone the fast directions of polarization are subparallel to the strike of the margin for both crustal and intraslab earthquakes the delay time of the split shear waves in intraslab earthquakes is larger than that in crustal earthquakes and shows a down dip variation across the lfe zone this indicates that anisotropy exists in the mantle wedge and in the lower crust and or oceanic slab we explain the observed delay time of 0 015 0 045 s by suggesting that the mantle wedge consists of a deformed 1 15 km thick serpentine layer if the mantle wedge is completely serpentinized in addition to high fluid pressures within the oceanic crust the sheared serpentine layer may be a key factor driving lfes in subduction zones

generation and migration of coarse grained sediment waves in turbidity current channels and channel lobe transition zones large scale sediment waves composed of gravels and sands have been studied using deep water sidescan systems new data are presented from submarine channels off the canary islands and from canyon mouths off portugal data from other areas are briefly reviewed including a re interpretation of data from laurentian fan in order to summarise the varied morphology and setting of these bedforms coarse grained sediment waves are found in the proximal dominantly bypassing areas of deep water turbidite systems within canyons channels and channel lobe transition zones wave heights are in the region of 1 10 m and wavelengths are up to several hundred metres the distribution of waves and sparse sedimentological evidence from modern and ancient sediment wave fields suggests that initial transport and deposition of coarse sediment occurs within a high density turbidity current and not as a non newtonian debris flow in some cases the development of pronounced wave asymmetry and evidence of wave disruption and reworking suggests that the wave morphology is at least partially controlled by a later phase of low density turbidity flow grain size also appears to exert some control on wave morphology for example gravel rich waves have a greater height for the same wavelength than sand rich waves coarse grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents and are equally difficult to recognise in outcrop because of their large size

a comparative study of low frequency earthquake templates in northern cascadia abstract using network correlation methods we generate low frequency earthquake templates for a set of 4 composite arrays on vancouver island and washington state that employ data from earthscope transportable and flexible arrays plate boundary observatory polaris and permanent network canadian national seismograph network pacific northwest seismic network sources on the basis of empirical and semi analytical arguments the templates can be viewed as green s function sections corresponding to moment tensor point sources with step function time dependence in displacement low frequency earthquake hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth with washington locations averaging slightly deeper than those on vancouver island we invert template waveforms for moment tensor mechanisms and find that data are well modelled by double couple sources for southern vancouver island with the highest quality templates the majority of mechanisms are consistent with shallow thrusting in the direction of plate motion the three other data sets with lower signal to noise levels show predominantly thrust mechanisms with more variable orientations taken together with other constraints our observations support the hypothesis that low frequency earthquakes manifest shear slip on a relatively thin plate boundary

geological contributions extinct rift valleyson the nature of greenstone belts in the precambrian at present there is considerable speculation concerning the origin of greenstone belts they are compared with phanerozoic geosynclines fold belts modern island arcs or with the rift structures formed above subduction zones in marginal seas four stages in greenstone belt development can be determined 3 8 3 0 3 0 2 6 2 6 1 9 and 1 9 b y and younger sections of early greenstone belts contain all the essential features of the modern oceanic sequences their peculiarities are great thickness presence of jaspilites and scarce occurrence of very coarse clastics on the contrary the latter type of sediment is widespread in younger greenstone belts and in those of the last stage which are completely formed in continental environment whole rock chemistry of precambrian volcanics more than 1800 analyses all over the world confirms that alkali olivine basalts are absent till 1 9 b y and andesites are generally rare widespread type of early precambrian basalts are ol hy and q normative ones showing close similarity to typical abyssal tholeiites the presence of basaltic komatiites is not a unique petrochemical feature of volcanism in the early precambrian as one thinks now similar rocks have been found in phanerozoic rift zones there is no evidence suggesting significant shortening of initial crustal complexes and greenstone belts these data along with slight metamorphism and above features can only be explained by the formation of greenstone belts in rifting environment during the early stages 3 8 1 9 b y the thickness of the protocrust was not more than 10 20 km it allows us to understand the rare occurrence of intermediate volcanics at early stages of the earth s evolution in summary it is argued that in the early precambrian the spread of primitive oceanic lithosphere was not compensated by the consumption in subduction zones consequently plate tectonics cannot explain the formation of the crust in the early history of the earth

seismic anisotropy in the shallow crust of the loma prieta segment of the san andreas fault system we study seismic anisotropy beneath the santa cruz mountains using digital recordings of s waves from 60 aftershocks of the 1989 loma prieta earthquake m 7 0 recorded at 18 local stations three component particle motion covariance matrix decomposition combined with a cross correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves results from this procedure are compared with those obtained through visual inspection of horizontal particle motions inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of s to p converted phases from our study uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20 and 10 ms respectively about half of the s wave records examined show clear evidence of shear wave splitting two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events 52 of the polarizations are parallel to the strike of the san andreas fault nw se and 24 are parallel to the probable direction of local maximum horizontal compressive stress ne sw the fault parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary the fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the san andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy eda where parallel alignment of fluid filled fractures produces the anisotropy the predominance of fault parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results the splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth this suggests that the seismic anisotropy is no deeper than 2 km the depth of the shallowest earthquakes analyzed for an anisotropic zone 2 km thick the average velocity anisotropy is about 5 in our study region

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

bridging the gap between seismically and geodetically detected slow earthquakes 1 recently observed unusual seismic events deep low frequency earthquakes and tremor very low frequency earthquakes and slow slip events form a family of slow earthquakes that are governed by a scaling law different from ordinary earthquakes guided by this scaling law we have observed previously unknown events with source duration of 20 200 s and moment magnitude of 3 4 under the kii peninsula in western japan these events radiate seismic energy in the 2 8 hz band in direct proportion to their seismic moment rate though the constant of proportionality is 4 5 orders of magnitude smaller than for ordinary earthquakes slow earthquakes in this region may be comprised of numerous shear slip events which in aggregate are manifest seismically as tremor or isolated longer events up to 200 s and geodetically as slow slip events

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

s wave polarization inversion of the 15 october 1979 23 19 imperial valley aftershock evidence for anisotropy and a simple source mechanism s polarizations from the 16 near source strong motion stations of the 15 october 1979 23 19 imperial valley aftershock have been analyzed for constraining the source mechanism they show a significant delay of the east component relative to the north component at most stations assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments where the rays are dominantly vertical the measured fast s polarizations are mainly oriented between n0 e and n25 e and show a mean delay of 0 25 s between slow and fast s this is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it with a mean fluid filled crack density of 0 1 to 0 2 the records once corrected for the splitting effect show s polarizations stable in time which have been used to estimate the fault mechanism parameters this is done by applying a new inversion method which is based on a model probability estimate the error function is related to the polarization misfits and the associated joint probability function of the data and parameter is computed in the whole model space the inversion of the corrected s polarizations provides a unique and well constrained solution for the imperial valley aftershock mechanism strike 145 5 dip 85 10 slip 155 15 which is compatible with the available p polarities and is quite similar to the main shock mechanism

a probability method for local earthquake focal mechanisms we propose a method for obtaining focal mechanisms of earthquakes recorded on a local network it is based on the non linear estimation of the probability function for strike dip and slip of the focal mechanism from p polarities s polarizations and s p amplitude ratios the method is shown to be effective for the analysis of small earthquakes recorded by local networks in particular the s p amplitude ratios bring strong constraints on fault parameters even if large errors are present an application to a small earthquake recorded at the volcanic zone of campi flegrei naples italy during a seismic outburst is presented

source duration of deep very low frequency earthquakes in western shikoku japan 1 the source durations of very low frequency earthquakes in western shikoku japan were estimated nineteen very low frequency earthquakes were located in this region between 14 and 17 march 2007 that migrated southward with tremor activity the source durations of four mw 3 5 3 8 very low frequency earthquakes were estimated by comparing simulated and observed seismograms to be 12 18 s the ratio of seismic moment and source duration is 1 5 1013 nm s this value is several to ten times larger than those in southern kii japan such a ratio difference may reflect the nature of very low frequency earthquake between these regions if a stress drop of 0 003 3 mpa is assumed then the slip propagation velocity of very low frequency earthquakes ranges from several tens to several hundreds of m s

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

direct sediment transfer from land to deep sea insights into shallow multibeam bathymetry at la reunion island abstract submarine canyon heads are key areas for understanding the triggering factors of gravity currents responsible for the transfer of detrital sediment to the deep basins this contribution offers a detailed picture of canyon heads off la reunion island with high resolution multibeam bathymetry in the water depth range of 4 220 m the present feeding of the cilaos turbidite system one of the largest modern volcaniclastic systems in the world is deduced from morphological and sedimentological interpretations of newly acquired data the study highlights small scale sedimentary features indicating hydrodynamic and sedimentary processes a direct connexion between the saint etienne river mouth and submarine canyons is evidenced by the complete incision of the shelf and the presence of canyon heads connected to the modern deltaic bar this direct connection supplied by river torrential floods cyclonic floods every two or three years suggests the continuity of high density fluvial flows to submarine gravity flows forming hyperpycnal flows in the canyon the initiation of secondary submarine gravity flows by storm waves large austral waves and cyclonic waves is also proposed for submarine canyons with large canyon heads developed in the surf zone from a sandy coastal bar bedforms in active canyon axis are considered as an indicator of the frequent activity of high density turbidity currents moreover a morphological record of last glacial and deglacial sea level variations is preserved and particularly the last glacial maximum sea level with the presence of small vertical cliffs observed in this bathymetric data which likely corresponds to a paleo shoreline or paleo reefs

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

cascadia tremor polarization evidence for plate interface slip 1 new seismic measurements of the repeated phenomenon of episodic tremor and slip in northern cascadia indicate identical source processes of tremor and slow slip predicted polarization directions of upgoing s waves radiated from shear slip on the plate interface align with the relative motion between the juan de fuca and north american plates seismic observations from small aperture array data on the olympic peninsula of the cascadia subduction zone show uncharacteristically stable linear particle motion coincident with the passage of tremor sources beneath the array the azimuth of this horizontal ground motion matches expected polarizations from slip on the plate interface this finding suggests that cascadia tremor is shear slip on the plate interface implying that as in japan geodetically observed slow slip and seismically observed tremor are manifestations of the same phenomenon

split from slip and schist crustal anisotropy beneath northern cascadia from non volcanic tremor 1 polarization and splitting analyses of minute long windows of non volcanic tremor at 3 component stations in the vicinity of southern vancouver island provide new evidence for the presence of anisotropy within the crustal portion of the north american plate beneath cascadia as reported previously tremor particle motions are predominantly horizontal and inferred to manifest an upgoing wavefield composed primarily of s waves under this assumption estimates of incidence angle and back azimuth can be deduced from orientation of the smallest principle direction of motion when subjected to standard s wave splitting analysis most stations yield systematic and reproducible fast polarization directions estimates of original polarization direction are more scattered and splitting times often exhibit multimodal distributions numerical simulations wherein band limited synthetic seismograms comprising contributions from multiple sources are analyzed for splitting neatly reproduce these characteristics and thereby provide a framework for interpretation a number of stations in our study sit astride the san juan fault which based on reflection studies dips approximately 60 to 70 to the north underlying these stations is the leech river complex consisting of strongly foliated greenschist facies phyllites with steeply dipping foliations striking parallel to fast polarization directions of split s waves detailed laboratory velocity measurements at elevated pressures show that as little as 2 to 3 km of vertically dipping leech river phyllite are required to produce the observed splitting these phyllites have some of the highest s wave anisotropies measured to date and are part of a once continuous belt of anisotropic crust extending over 2000 km along the western north american coast from southern vancouver island to southwest alaska

three dimensional seismic velocity structure and configuration of the philippine sea slab in southwestern japan estimated by double difference tomography 1 three dimensional seismic velocity structure in and around the philippine sea plate subducting beneath southwestern sw japan is determined by applying double difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in japan a region of low s wave velocity and high vp vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from tokai to kyushu this characteristic layer dips shallowly in the direction of slab subduction compared with the upper surface of the philippine sea slab based on seismic reflection and refraction surveys on seven survey lines we interpret that the low vs and high vp vs layer corresponds to the oceanic crust of the philippine sea slab on the basis of the position of the low vs and high vp vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes the upper surface of the philippine sea slab is reliably determined for the entire area of sw japan nonvolcanic deep low frequency earthquakes that occurred associated with the subduction of the philippine sea slab are distributed along the isodepth contour of 30 km in sw japan except for the tokai district where the depth of deep low frequency earthquakes becomes gradually deeper toward northeast

spatial distribution of seismic energy rate of tectonic tremors in subduction zones the sizes of deep tectonic tremors have never been accurately evaluated as a physical quantity here we estimate tremor size as the band limited seismic energy rate at 2 8 hz with accurate evaluation of the path attenuation and site amplification of seismic waves in four subduction zones nankai cascadia jalisco and south chile the size frequency statistics of seismic energy rate which are characterized by the median measure for each subregion are spatially variable the spatial variations are categorized into three types with each type corresponding to a different tremor migration behavior in type a regions where tremor zone is wider seismic energy rates are highly variable in the dip direction and tremor activities are usually initiated in the less energetic tremor zone some of them further penetrate into the energetic tremor zone and subsequently migrate for long distances in the strike direction type b regions are characterized by relatively narrow tremor zones minor variations in energy rates in the dip direction and long distance migration in the strike direction type c regions are characterized by isolated clusters of tremor activities without migration and by independent failure of each small tremor cluster given that the spatial distributions of tremor energy rates reflect heterogeneities in the strength of the plate interface such distributions which would be controlled by the width of tremor zone may determine the regional style of slow earthquake behavior some energetic tremor regions act as switches that trigger large slow slip events especially in type a regions

enhanced detection capability of non volcanic tremor using a 3 level vertical seismic array network va net in southwest japan 1 this study evaluates a method that enhances detection of non volcanic low frequency tremor nvt using va net a 3 level vertical seismic array network that was recently constructed in southwest japan the method employs semblance analysis for continuous vertical array records and calculates semblance coefficients as a function of time and apparent vertical velocity the sign of the best apparent velocity provides information about propagation direction either upward or downward allowing easy discrimination between seismic signals of natural origin and unwanted noise such as man made vibrations we applied the method to a nvt episode that occurred beneath the kii peninsula during november 2008 to demonstrate that nvt signals can be detected using only the 3 level array furthermore we demonstrate that the method enables us to detect minor nvt activity which is not detected by a conventional envelope cross correlation method resulting in a dramatic improvement in detection capability

evidence for the splitting of shear waves from waveform and focal mechanism analyses abstract a temporary seismograph station was sited above the aftershock area of an intermediate crustal earthquake m 4 9 in the kinki district japan the results from analysing digital three component seismograms and numerically determining focal mechanisms of aftershocks have provided clear evidence for the splitting of shear waves attributed to crustal anisotropy faster shear waves from most of the aftershocks are polarized wnw ese regardless of their azimuths and incident angles such shear wave motions are often significantly inconsistent with the focal mechanisms of the aftershocks determined using p wave first motions for many of the events slower shear waves polarized orthogonally to the faster shear wave direction nne ssw are observed to arrive about 0 1 s later two vectors showing particle motions of faster and slower shear waves in the horizontal plane are summed to reconstruct polarizations of shear waves before splitting for events with sufficiently high signal to noise ratios of shear waves and well determined focal mechanisms the reconstructed polarizations are consistent with the focal mechanisms within the range of uncertainty such consistency is not clearly detected for the other events possibly because of large noise effects and poorly constrained fault plane solutions the direction of faster shear wave polarization wnw ese generally coincides with that observed at three other stations located within 15 km of the temporary station in the study area the fabric of crustal anisotropy is almost uniform over the scale of 15 km but appears to exhibit small local fluctuations

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses