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

hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

automated seismic event location by waveform coherence analysis summary automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring since microseismic records are generally characterized bylowsignal to noiseratio automated locationmethods arerequested to be noise robust and sufficiently accurate most of the standard automated location routines arebasedontheautomatedpicking identificationandassociationofthefirstarrivalsofpands waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases although current methods can accurately pick p onsets the automatic picking of the s onset is still problematic especially when the p coda overlaps the s wave onset in this paper we propose a picking free earthquake location method based on the use of the short term average long term average sta lta traces at different stations as observed data for the p phases we use the sta lta traces of the vertical energy function whereas for the s phases we use the sta lta traces of a second characteristic function which is obtained using the principal component analysis technique in order to locate the seismic event we scan the space of possible hypocentral locations and origin times and stack the sta lta traces along the theoretical arrival time surface for both p and s phases iterating this procedure on a 3 d grid we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event a pilot application was performed in the campania lucania region southern italy using a seismic network irpinia seismic network with an aperture of about 150km we located 196 crustal earthquakes depth 20km with magnitude range 1 1 ml 2 7 a subset of these locations were compared with accurate manual locations refined by using a double difference technique our results indicate a good agreement with manual locations moreover our method is noise robust and performs better than classical location methods based on the automatic picking of the p and s waves first arrivals

continuous kurtosis based migration for seismic event detection and location with application to piton de la fournaise volcano la reunion abstract we present an automatic earthquake detection and location technique based on migration of continuous waveform data data are preprocessed using a kurtosis estimator in order to enhance the first arrival information then migrated onto a predefined search grid using precalculated p wave travel times and finally stacked local maxima in the resulting 4d space time grid indicate the locations and origin times of seismic events we applied our technique to earthquake swarms occurring on piton de la fournaise volcano la reunion france we located 5000 events from 12 different swarms that occurred between 2009 and 2011 our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level multiplet analysis of the detected events and subsequent double difference relocation produce sharper images of the earthquake swarms our code waveloc is released in open source online material figures of seismicity distributions from waveloc synthetic test and stack amplitude values versus magnitudes

hypocenter determination in strongly heterogeneous earth models using the shortest path method the shortest path method moser 1991 for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed by tarantola and valette 1982 it uses the analogy between seismic rays in the earth and shortest paths in networks to construct first arrival times from one point to all other points of a three dimensional grid simultaneously in a fast robust way in earth models of arbitrary complexity doing this for all stations of a seismic array one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three dimensional grid the concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location due to uncertainties in the travel time data numerical errors in the calculated travel times and to a limited extent incomplete knowledge about the earth model the result is a three dimensional contour map of regions of equal confidence for the earthquake location the method becomes especially attractive when more than one event recorded by the same array is studied because the calculation of the travel times which is relatively the most time consuming operation has to be done only once the method is applied on the location of an event that occurred on january 18 1989 in israel

stress release in exhumed intermediate and deep earthquakes determined from ultramafic pseudotachylyte stresses released by coseismic faults during subduction toward lawsonite eclogite facies conditions in the alpine subduction complex of corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured congruent peridotite melting at ambient conditions of 1 5 gpa and 470 c requires a temperature increase of 1280 c to 1750 c we assume that more than 95 of the work is converted to heat during faulting hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain minimum estimates of released stress are typically greater than 220 mpa and as high as 580 mpa the abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth

seismicity and one dimensional velocity structure of the himalayan collision zone earthquakes in the crust and upper mantle 1 earthquakes beneath the himalayan collision zone occur at depths between near surface and around 100 km below sea level after relocating earthquakes with two one dimensional 1 d velocity models we found a clear bimodal depth distribution for earthquakes in the himalayas of eastern nepal and the southern tibetan plateau and evidence that some earthquakes originate at upper mantle depths seismicity in nepal shows an accumulation of earthquakes along the front of the himalayan arc with a seismic gap between longitudes 87 3 e and 87 7 e although upper crustal seismicity along the topographic front of the high himalaya is consistent with a region of high strain accumulation associated with convergence on the main himalayan thrust fault microearthquakes do not necessarily occur on this fault instead they concentrate in the hanging wall seismic activity in the sub himalaya and the terai plains is almost exclusively limited to the vicinity of the location of the magnitude 6 5 20 august 1988 udayapur earthquake with most of the earthquakes in the lower crust and the upper mantle clusters of earthquakes in the lesser and high himalayas and south tibet tethyan himalayas mark very well defined zones of seismicity at depths between 50 and 100 km confirming the presence of earthquakes in the upper mantle in the region of continental collision the occurrence of earthquakes at sub moho depths favors the idea that the continental upper mantle deforms by brittle processes

time reversal imaging of seismic sources and application to the great sumatra earthquake the increasing power of computers and numerical methods like spectral element methods allows continuously improving modelization of the propagation of seismic waves in heterogeneous media and the development of new applications in particular time reversal in the three dimensional earth the concept of time reversal hereafter referred to as tr was previously successfully applied for acoustic waves in many fields like medical imaging underwater acoustics and non destructive testing we present here the first application at the global scale of tr with associated reverse movies of seismic waves propagation by sending back long period time reversed seismograms we show that seismic wave energy is refocused at the right location and the right time of the earthquake when tr is applied to the sumatra andaman earthquake 26 dec 2004 the migration of the rupture from the south towards the north is retrieved therefore tr is potentially interesting for constraining the spatio temporal history of complex earthquakes

intermediate depth earthquakes beneath the india tibet collision zone we report on three intermediate depth earthquakes in the india tibet collision zone two under the himalayan thrust belt htb and one beneath the indus zangbo suture the mb magnitudes of these three events are from 4 3 to 4 9 and are too small to be well located by conventional means however from modeling their broadband waveforms recorded at near regional distances on a temporary passcal array we can confidently confine the sources to be below the crust between 70 and 80 km deep the existence of these intermediate depth earthquakes in this area suggests relatively low temperatures in the mantle lithosphere the two events under the htb display strike slip mechanisms with some normal faulting component this is quite different from the shallow thrust events typical of the same area the principal p and t axes of all 3 fault plane solutions show roughly ns compression and ew extension consistent with a regional stress field produced by the indenting of the india continent

long period long duration seismic events during hydraulic stimulation of shale and tight gas reservoirs part 2 location and mechanisms abstractlong period long duration lpld seismic events that have been observed during hydraulic stimulation of shale gas and tight gas reservoirs appear to represent slow shear slip on relatively large faults within the limitations of the recording geometry we determine the areas in the reservoirs where the events are located in two case studies in the barnett shale in one data set lpld events appear to occur in the region where the density of natural fractures as well as the fluid pressure during pumping were highest in the other data set the lpld events are observed to occur between two wells and seem to establish a hydraulic connection between them in both data sets the lpld events occur in areas with very few located microearthquakes a combination of factors such as high fluid pressure and or high clay content is potentially responsible for the slowly slipping faults the lpld events appear to be occurring only on faults large enough to produce a sequence of slow slip events we suggest that

reverse modelling for seismic event characterization summary the localization of seismic events is of utmost importance in seismology and exploration current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network weak events not visible in the individual seismogram of the network are missed out we present an approach where no picking of events in the seismograms of the recording network is required the observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling assuming the correct velocity model the reversely modelled wavefield focuses on the hypocentre of the seismic event the origin time of the event is given by the time where maximum focussing is observed the spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition this automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data the method is particularly useful for the upcoming large passive networks since no picking is required it has great potential for localizing very weak events not detectable in the individual seismogram since the reverse modelling sums the energy of all recorded traces and therefore enhances the signal to noise ratio similar to stacking in seismic exploration the method is demonstrated by 2 d and 3 d numerical case studies which show the potential of the technique events with a s n ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

location of the focus and tectonics of the focal region of the california earthquake of 18 april 1906 modern relocation constrains the hypocenter of the 18 april 1906 california earthquake within an 8 24 km southwest northeast trending zone to the north of san francisco that intersects the offshore san andreas fault system west of san francisco here we further constrain the location of the 1906 focus and examine the tectonics of the focal region using recent microseismicity relocated in a 3d model along with other seismological geological and marine evidence in agreement with previous work we associate the 1906 focus with an extensional right stepover in the san andreas fault system a zone of sea floor subsidence and complex normal and strike slip faulting we define a likely focal volume between lake merced and offshore of the golden gate centered near 37 77 n 122 55 w and 10 km depth we hypothesize possible 1906 rupture in this region by normal deformation around a 10 km long steeply west dipping structure trending about 20 clockwise to the san andreas fault zone and bilateral strike slip fault rupture along the san andreas fault system away from this structure rupture to the northwest on a vertical currently aseismic faulting structure under the golden gate fault and rupture to the southeast under the san francisco peninsula either along a steeply southwest dipping structure showing present day extensional tectonism or along a near vertical currently aseismic fault zone all three of these structures extend from the near surface to about 10 km depth we also identify a deeper 15 20 km long linear trend of clustered seismicity at 10 13 km depth rotated about 6 clockwise relative to the strike of the san andreas fault which connects the north end of the focal volume with the south end of the seismicity defining the southwest dipping structure under the san francisco peninsula we propose that this trend shows faulting at the base of a brittle upper crust in response to underlying shear in a ductile lower crust these interpretations on the focal region of the 1906 earthquake have implications for seismotectonic understanding earthquake monitoring and seismic hazard assessment in the san francisco bay area

advancement in source estimation techniques using broadband regional seismograms one important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases such as pnl surface wave sv sh because the misfit errors used in some waveform inversions are normalized by the data and synthetics the amplitude information in the data has not been fully utilized in this article we modify the cut and paste source estimation technique zhao and helmberger 1994 by removing this type of normalization it is shown that the modified method increases the stability and resolution of inversion when multiple stations at different distance ranges are used a distance scaling factor is introduced to compensate for the amplitude decay with distance by applying the technique to the terrascope data we have determined source mechanisms and depths of 335 southern californian events with m l 3 5 the amplitude decays with distance are r 1 13 for pnl r 0 55 for love waves and r 0 74 for rayleigh waves in contrast to generally shallow source depths reported by the southern california short period network the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km

seismoionospheric gps total electron content anomalies observed before the 12 may 2008 mw7 9 wenchuan earthquake 1 the global ionospheric map gim is used to observe variations in the total electron content tec of the global positioning system gps associated with 35 m 6 0 earthquakes that occurred in china during the 10 year period of 1 may 1998 to 30 april 2008 the statistical result indicates that the gps tec above the epicenter often pronouncedly decreases on day 3 5 before 17 m 6 3 earthquakes the gps tec of the gim and electron density profiles probed by six microsatellites of formosat3 cosmic f3 c are further employed to simultaneously observe seismoionospheric anomalies during an mw7 9 earthquake near wenchuan china on 12 may 2008 it is found that gps tec above the forthcoming epicenter anomalously decreases in the afternoon period of day 6 4 and in the late evening period of day 3 before the earthquake but enhances in the afternoon of day 3 before the earthquake the spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions respectively the f3 c results further show that the ionospheric f2 peak electron density nmf2 and height hmf2 significantly decreases approximately 40 and descends about 50 80 km respectively when the gps tec anomalously reduces

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 statistical study of ionospheric earthquake precursors monitored by using equatorial ionization anomaly of gps tec in taiwan during 2001 2007 abstract in this paper we examine pre earthquake ionospheric anomalies by the total electron content tec derived from a ground based receiver of the global positioning system gps a network of eight gps receivers is used to construct daily latitude time tec ltt plots to monitor the crest of equatorial ionization anomaly eia in the taiwan area three parameters of the strength location and formation time of the eia crest are extracted a 15 day running medians of the three parameters and the associated upper and lower quartiles are utilized as the references for identifying abnormal signals for all of the 150 m 5 0 earthquakes in the taiwan area during 2001 2007 results show that the eia crest significantly moves equatorward poleward and appears in an earlier later time of the afternoon period a few days before after the earthquakes along the taiwan longitude the two parameters of the eia crest location and occurrence time can be employed to detect ionospheric earthquake precursors the results further imply that atmospheric electric fields generated around the epicenter of a forthcoming earthquake during the preparation period are essential

depth dependent activity of non volcanic tremor in southwest japan 1 with a new location method we obtained a high resolution map of non volcanic tremor along the subducting philippine sea plate in southwest japan and clarified the depth dependent behavior of the tremor activity a bimodal distribution of tremor along dip is apparent in regions where short term slow slip events sses and very low frequency earthquakes are frequently detected the separation of bimodal peaks is 5 to 10 km in depth the updip tremor activity occurs episodically coincident with major bursts that often accompany geodetically detectable sses the downdip tremor activity is however rather stable with frequent minor bursts this depth dependent tremor activity likely reflects variation of interplate slip properties specifically weakening with increasing depth and temperature in westernmost shikoku anomalous tremor activity was detected in only the updip section in late 2003 this is interpreted as being triggered by a long term sse in the shallower edge of the tremor zone

seismic velocities in southern tibet lower crust a receiver function approach for eclogite detection summary beneath the tibet plateau the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock mass transfer to nourish the unsettled debate of the relative importance of these two processes we propose a new approach based on the s to p and the p to s wave conversions enabling the precise determination of the seismic velocities the weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and vp vs ratio separately for two sets of p and s receiver functions for each set of receiver functions coherent stack gives the vp vs ratio and thickness for the considered layer the grid search stacking method the values of vp vs ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions but using the p and s receiver functions allows us to solve this indetermination and to find the effective parameters of the layer velocity vs vp vs ratio and thickness we use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties for the southern lhasa block the migrated sections of both p and s receiver functions hi climb experiment data show a layer in the lower crust that may be related to the lower indian crust underplated beneath tibet with the grid search stacking method high shear wave velocities vs 4 73 km s 1 and low vp vs ratios 1 69 are detected in this layer such values are typical for high grade eclogites and the low vp vs ratio precludes the confusion with mafic granulites there is no evidence for partial eclogitization near and south of the yarlung tsangpo suture and the about 19 km thick eclogitic layer extends northwards only to about the middle of the lhasa terrane

m w dependence of the preseismic ionospheric electron enhancements ionospheric electron enhancement was reported to have occurred 40 min before the 2011 tohoku oki mw9 0 earthquake japan by observing total electron content tec with global navigation satellite systems receivers their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference tec curves from which anomalies are defined here we propose a numerical approach based on akaike s information criterion to detect positive breaks sudden increase of tec rate in the vertical tec time series without using reference curves we demonstrate that such breaks are detected 25 80 min before the eight recent large earthquakes with moment magnitudes mw of 8 2 9 2 the amounts of precursory rate changes were found to depend upon background tec as well as mw the precursor times also showed mw dependence and the precursors of intraplate earthquakes tend to start earlier than interplate earthquakes we also performed the same analyses during periods without earthquakes to evaluate the usefulness of tec observations for short term earthquake prediction

earthquake processes of the himalayan collision zone in eastern nepal and the southern tibetan plateau summary focal mechanisms determined from moment tensor inversion and first motion polarities of the himalayan nepal tibet seismic experiment himnt coupled with previously published solutions show the himalayan continental collision zone near eastern nepal is deforming by a variety of styles of deformation these styles include strike slip thrust and normal faulting in the upper and lower crust but mostly strike slip faulting near or below the crust mantle boundary moho one normal faulting earthquake from this experiment accommodates east west extension beneath the main himalayan thrust of the lesser himalaya while three upper crustal normal events on the southern tibetan plateau are consistent with east west extension of the tibetan crust strike slip earthquakes near the himalayan moho at depths 60 km also absorb this continental collision shallow plunging p axes and shallow plunging ew trending t axes proxies for the predominant strain orientations show active shearing at focal depths 60 90 km beneath the high himalaya and southern tibetan plateau beneath the southern tibetan plateau the plunge of the p axes shift from vertical in the upper crust to mostly horizontal near the crust mantle boundary indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate

aftershock seismicity of the 2010 maule mw 8 8 chile earthquake correlation between co seismic slip models and aftershock distribution 1 the 27 february 2010 maule chile mw 8 8 earthquake is one of the best instrumentally observed subduction zone megathrust events here we present locations magnitudes and cumulative equivalent moment of the first 2 months of aftershocks recorded on a temporary network deployed within 2 weeks of the occurrence of the mainshock using automatically determined onset times and a back projection approach for event association we are able to detect over 30 000 events in the time period analyzed to further increase the location accuracy we systematically searched for potential s wave arrivals and events were located in a regional 2d velocity model additionally we calculated regional moment tensors to gain insight into the deformation history of the aftershock sequence we find that the aftershock seismicity is concentrated between 40 and 140 km distance from the trench over a depth range of 10 to 35 km focal mechanisms indicate a predominance of thrust faulting with occasional normal faulting events increased activity is seen in the outer rise region of the nazca plate predominantly in the northern part of the rupture area further down dip a second band of clustered seismicity showing mainly thrust motion is located at depths of 40 45 km by comparing recent published mainshock source inversions with our aftershock distribution we discriminate slip models based on the assumption that aftershocks occur in areas of rapid transition between high and low slip surrounding high slip regions of the mainshock

striations duration migration and tidal response in deep tremor deep tremor in subduction zones is thought to be caused by small repeating shear slip events on the plate interface with significant slow components it occurs at a depth of about 30 kilometres and provides valuable information on deep plate motion and shallow stress accumulation on the fault plane of megathrust earthquakes tremor has been suggested to repeat at a regular interval migrate at various velocities and be modulated by tidal stress here i show that some time invariant interface property controls tremor behaviour using precise location of tremor sources with event duration in western shikoku in the nankai subduction zone japan in areas where tremor duration is short tremor is more strongly affected by tidal stress and migration is inhibited where tremor lasts longer diffusive migration occurs with a constant diffusivity of 10 4 m 2 s 1 the control property may be the ratio of brittle to ductile areas perhaps determined by the influence of mantle wedge serpentinization on the plate interface the spatial variation of the controlling property seems to be characterized by striations in tremor source distribution which follows either the current or previous plate subduction directions this suggests that the striations and corresponding interface properties are formed through the subduction of inhomogeneous structure such as seamounts for periods as long as ten million years

observations and simulations of seismoionospheric gps total electron content anomalies before the 12 january 2010 m7 haiti earthquake 1 in this paper the total electron content tec of the global ionosphere map gim is used to detect seismoionospheric anomalies associated with the 12 january 2010 m7 haiti earthquake and an ionospheric model is applied to simulate the detected anomalies the gim temporal variation shows that the tec over the epicenter significantly enhances on 11 january 2010 1 day before the earthquake the latitude time tec ltt plots reveal three anomalies 1 the northern crest of equatorial ionization anomaly eia moves poleward 2 the tecs at the epicenter and its conjugate increase and 3 the tecs at two dense bands in the midlatitude ionosphere of 35 n and 60 s further enhance the spatial analysis demonstrates that the tec enhancement anomaly appears specifically and persistently in a small region of the northern epicenter area the simulation well reproduces the three gim tec anomalies which indicate that the dynamoelectric field of the ionospheric plasma fountain might have been perturbed by seismoelectric signals generated around the epicenter during the earthquake preparation period

a study of the ionospheric disturbances associated with strong earthquakes using the empirical orthogonal function analysis abstract in this study an ionospheric logtec model has been constructed using an empirical orthogonal function eof decomposition method the main orders 1 9 orders of this eof model represent the ionospheric background while the rest high orders represent ionospheric tec perturbations based on this model logtec data during 16 ms 8 0 earthquakes from 2000 to 2012 have been analyzed we found that ionospheric disturbances occurred in 10 earthquakes ionospheric abnormities were clearly observed 5 days before the earthquakes but ionospheric disturbances can also be seen 1 day before and 1 day and 2 days after the earthquakes for the tec data the possibility of positive enhanced tec and negative decreased tec abnormities is almost the same furthermore by using the simulation results of ionospheric electric field and sami2 model we show that an abnormal westward electric field can lead to electron density enhancements equatorward and decrease polarward of the epicenters and vice versa for an eastward abnormal electric field

source mechanism of a lower crust earthquake beneath the himalayas and its possible relation to metamorphism abstract the nature of deep crustal intermediate and deep focus earthquakes and their relation to metamorphic reactions is a topic of debate here we seek to better understand a possible link between the earthquake process and metamorphism by analyzing the mechanism of ongoing deep crustal earthquakes we focus on a region in the himalayas with observed seismicity at depths expected to experience active eclogite facies metamorphism and dehydration reactions there are few permanent seismic stations in the region therefore we use waveform data from a temporary seismic array deployment we find two earthquakes with magnitude and station coverage adequate for moment tensor inversion for a given earthquake we estimate its seismic full moment tensors and magnitude together with uncertainties using all available waveforms for the largest earthquake mw 3 7 we obtain a best fitting moment tensor and uncertainties that show a double couple with a tensional crack component in the context of geological records that document similar processes and of laboratory experiments conducted at spatial scales that are 5 6 orders of magnitude smaller this mechanism may be related to dehydration driven stress changes triggering slight crack opening and ambient stresses favoring slip along a fault

recent progress in ionospheric earthquake precursor study in china a brief review abstract the study of ionospheric anomaly variation before large earthquakes has attracted the attention of geophysicists for many years this is primarily because ionospheric anomaly variation has been considered to constitute one possible method to identify earthquake precursors since china covers several major earthquake zones it is critical for chinese scientists to study possible ionospheric earthquake precursors in this report we provide a brief summary of recent progress in the study of ionospheric earthquake precursors mainly from chinese mainland researchers as well as some other scientists this report focuses on 1 some case studies of ionospheric abnormal behaviors before some great earthquakes around the world e g m7 9 wenchuan earthquake m9 0 tohoku oki earthquake m7 0 haiti earthquake and 2 some statistical characteristics of ionospheric anomalies before earthquakes including their temporal and spatial distributions and the relationship with the forthcoming earthquake the above mentioned studies may provide some new and beneficial insights for the future study of ionospheric earthquake precursors

temporal and spatial precursors in the ionospheric global positioning system gps total electron content observed before the 26 december 2004 m9 3 sumatra andaman earthquake 1 we report seismo ionospheric precursors of anomalous decreases in the total electron content tec appearing day 5 prior to an m9 3 earthquake the largest one in the last five decades which occurred in sumatra andaman indonesia on 26 december 2004 sequences of global ionosphere maps of the tec derived from worldwide ground based receivers of the global positioning system gps are used to statistically study the temporal and spatial precursors of the earthquake it was found that the temporal precursor of the gps tec around the epicenter was significantly reduced during the afternoon period on d 5 before the earthquake the spatial precursors prominently persistently and simultaneously appear around the epicenter and its conjugate areas of the sumatra andaman earthquake

variations of ionospheric total electron content during the chi chi earthquake on 20 september1999 ut 21 septemberin local time lt a large earthquake mw 7 7 struck central taiwan nearthe small town of chi chi the greatest plasma frequency in the ionosphere fof2 observed by the chung li ionosonde 25 0 n 121 2 e reveals three clear precur sors at 1 3 and 4 days prior to the earthquake this paper examines the ionospheric total electron content tec ob served by a network of the global positioning system gps receivers in taiwan area it is found that variations in fof2 and overhead tec recorded at chung li have a similar ten dency combining the data of the network of 13 gps re ceivers time and spatial variations of tec prior to the chi chi earthquake are examined results show that the equa torial anomaly crest moves equatorward and its tec value significantly decreases 1 3 and 4 days before the earth quake a comparison between the disturbed and reference previous 15 day median days confirms that tec decreases significantly around the epicenter in the afternoons of these days finally possible mechanisms are proposed and dis cussed

ionospheric and geomagnetic disturbances caused by the 2008 wenchuan earthquake a revisit previous works have shown that coseismic ionospheric disturbances cids after the tsunamigenic 2011 tohoku earthquake tohoku eq mw9 1 covered a vast area and were observed thousands of kilometers away from the epicenter for the purpose of making a comprehensive comparison between powerful oceanic and inland eqs we conduct a retrospective investigation of cids and geomagnetic responses to the 2008 wenchuan eq mw7 9 using a combination of techniques total electron content hf doppler and ground magnetometer it is the very first study to present cids recorded by different techniques at co located sites and profiled with regard to changes of both ionospheric plasma and current geomagnetic field simultaneously the integrated observation also shows that 1 in the wenchuan case most of the ionospheric and geomagnetic disturbances were observed within 1000 km distance which is far less than the tohoku case 2 two groups of cids were found with maximum amplitudes in the direction of azimuth 150 and 135 respectively and 3 the geomagnetic changes were only registered by three magnetometers located to the east and southeast of the epicenter all the facts indicate that the main directional lobe of wenchuan eq energy propagation is to southeast and perpendicular to the direction of the fault rupture but this kind of directivity is not that distinct in the tohoku case we suggest that the different fault slip inland or submarine affecting the way of couplings of lithosphere with atmosphere may contribute to the discrepancies between the two events

spatiotemporal distribution of seismic energy radiation from low frequency tremor in western shikoku japan 1 the spatiotemporal patterns of the radiated seismic energy from low frequency tremor associated with slow slip events in western shikoku japan have been estimated a spatially decaying mean square amplitude and differential traveltime measurement from envelope correlations observed at hi net stations were used to locate tremors and to estimate their seismic energies tremor amplitude was corrected for the site amplification factors estimated using the coda normalization method the location of the tremor is estimated as the position where both the decay of the observed energy and the differential travel times are well explained the temporal pattern of the total energy released agrees very well with the observed tilt records although the spatial patterns of the energy radiation vary strongly among the six slow slip events sses in western shikoku the region where they overlap always radiates relatively high levels of energy the estimated band limited energy rate of the tremor for a 2 to 10 hz bandwidth was typically between 105 and 106 j min though there is temporal variation of the radiation energy the released total energy associated with the sses whose magnitude is 5 9 6 1 is on the order of 108 j within the duration of sses of 8 13 days this is quite small energy radiation compared to the moment release of accompanying sse the scaled energy which is the ratio of the seismic moment and the seismic energy for the slow slip and a very low frequency earthquake was estimated to be 5 orders of magnitude smaller than that of a regular earthquake

multiband array detection and location of seismic sources recorded by dense seismic networks we present a new methodology for detection and space time location of seismic sources based on multiscale frequency selective coherence of the wave field recorded by dense large scale seismic networks and local antennas the method is designed to enhance coherence of the signal statistical features across the array of sensors and consists of three steps signal processing space time imaging and detection and location the first step provides for each station a simplified representation of seismic signal by extracting multiscale non stationary statistical characteristics through multiband higher order statistics or envelopes this signal processing scheme is designed to account for a priori unknown transients potentially associated with a variety of sources e g earthquakes tremors and to prepare data for a better performance in posterior steps following space time imaging is carried through 3 d spatial mapping and summation of station pair time delay estimate functions this step produces time series of 3 d spatial images representing the likelihood that each pixel makes part of a source detection and location is performed in the final step by extracting the local maxima from the 3 d spatial images we demonstrate the efficiency of the method in detecting and locating seismic sources associated with low signal to noise ratio on an example of the aftershock earthquake records from local stations of international maule aftershock deployment in central chile the performance and potential of the method to detect locate and characterize the energy release associated with possibly mixed seismic radiation from earthquakes and low frequency tectonic tremors is further tested on continuous data from southwestern japan

seismotectonics of bhutan evidence for segmentation of the eastern himalayas and link to foreland deformation abstract the instrumental record of bhutan is characterized by a lower seismicity compared to other parts of the himalayan arc to understand this low activity and its impact on the seismic hazard a seismic network was installed in bhutan for 22 months between 2013 and 2014 recorded seismicity earthquake moment tensors and local earthquake tomography reveal along strike variations in structure and crustal deformation regime a thickened crust imaged in western bhutan suggests lateral differences in stresses on the main himalayan thrust mht potentially affecting the interseismic coupling and deformation regime sikkim western bhutan and its foreland are characterized by strike slip faulting in the indian basement strain is particularly localized along a nw se striking mid crustal fault zone reaching from chungthang in northeast sikkim to dhubri at the northwestern edge of the shillong plateau in the foreland the dextral dhubri chungthang fault zone dcf causes segmentation of the indian basement and the mht between eastern nepal and western bhutan and connects the deformation front of the himalaya with the shillong plateau by forming the western boundary of the shillong block the kopili fault the proposed eastern boundary of this block appears to be a diffuse zone of mid crustal seismicity in the foreland in eastern bhutan we image a seismogenic flat portion of the mht which might be either related to a partially creeping segment or to increased background seismicity originating from the 2009 m w 6 1 earthquake in western central bhutan clusters of micro earthquakes at the front of the high himalayas indicate the presence of a mid crustal ramp and stress buildup on a fully coupled mht the area bounded by the dcf in the west and the seismogenic mht in the east has the potential for m7 8 earthquakes in bhutan similarly the dcf has the potential to host m7 earthquakes as documented by the 2011 sikkim and the 1930 dhubri earthquakes which were potentially associated with this structure

long period long duration seismic events during hydraulic stimulation of shale and tight gas reservoirs part 1 waveform characteristics abstractlong period long duration lpld seismic events are relatively low amplitude signals that have been observed during hydraulic fracturing in several shale gas and tight gas reservoirs these events are similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries lpld events are predominantly composed of s waves but weaker p waves have also been identified in some cases microearthquakes are observed during the events based on the similarity with tectonic tremors and our observations of several impulsive s wave arrivals within the lpld events we interpret the lpld events as resulting from the superposition of slow shear slip events on relatively large faults most large lpld waveforms appear to start as a relatively slower low amplitude precursor lacking clear impulsive arrivals we estimate the energy carried by the larger lpld events to be 1000 times greater than a mw 2 microseismic event that is typical of the events that occur during hyd

nonvolcanic tremors deep beneath the san andreas fault we have discovered nonvolcanic tremor activity i e long duration seismic signals with no clear p or s waves within a transform plate boundary zone along the san andreas fault near cholame california the inferred epicentral region of the 1857 fort tejon earthquake moment magnitude 7 8 the tremors occur between 20 to 40 kilometers depth below the seismogenic zone the upper 15 kilometers of earth s crust where earthquakes occur and their activity rates may correlate with variations in local earthquake activity

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

rapid identification of earthquake rupture plane using source scanning algorithm summary accurate identification of an earthquake s rupture plane is critical in many aspects ranging from scientific such as seismology and plate tectonics to societal such as emergency rescue planning we modify the recently developed source scanning algorithm ssa to rapidly image the rupture pattern of an earthquake using waveform data recorded at local and regional distances the method calculates the brightness function of a grid point by summing the observed amplitudes of the p wave envelopes at the corresponding predicted arrival times at all stations the composite image consisting of brightness functions of all grid points within a prescribed source volume and time interval illuminates the locations on the rupture plane where significant seismic energy is emitted synthetic tests indicate that the proposed method is robust even under the presence of hypocentral mislocation and origin time errors application of this method to the 2003 san simeon and 2004 parkfield earthquakes both occurred in central california clearly identifies the nw se striking planes as the actual rupture planes a conclusion consistent with all available evidences limited azimuthal distribution of seismic stations will not severely affect the identification result making this method ideal for offshore earthquake studies because the proposed method is able to identify the rupture plane using seismic waveform data only it is perfectly suited for near real time operation and may enable routine report of earthquake rupture planes by local seismic networks in the future

source estimation from broadband regional seismograms recently developed source inversion techniques do not take full advantage of the broadband nature of regional seismograms the reason is that green s functions generated from flat layered models are not correct for the complicated propagational phenomena associated with realistic crustal structure the inversion is usually performed by removing short periods and inverting only selected portions of the records in this article we introduce a source estimation technique that allows for better use of the entire broadband record when only imperfect green s function are available the procedure desensitizes the timing between the principal crustal arrivals by fitting portions of the green s functions independently the preferred source is the orientation that minimizes the average l1 and l2 norms in term of the four parameters m 0 strike dip and rake the l2 error emphasizes the longer period surface waves while the l1 error weights the shorter period body waves we obtain estimates of duration by modeling long period and broadband signals the depth determination can be obtained directly from the surface reflected phases sp mp or ss ms or by cycling through the depth dependent green s functions in addition to the source parameters we obtain t phase alignments shifts that can be used as green s function corrections for relocating other events or as a guide to deriving new crustal models

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

is an unusual large enhancement of ionospheric electron density linked with the 2008 great wenchuan earthquake 1 on 12 may 2008 at 0628 ut a major earthquake ms 8 0 struck wenchuan county 31 0 n 103 4 e in southwest china the maximum ionospheric electron density at f2 peak nmf2 recorded an unusual large enhancement during the afternoon sunset sector by the chinese ionosondes over wuhan 30 5 n 114 4 e and xiamen 24 4 n 123 9 e which are close to the earthquake epicenter an averaged increase at these two stations is about 2 times on a geomagnetic quiet day 9 may kp 2 3 days prior to the earthquake relative to the median value of 1 12 may whereas the increase was much less significant over yamagawa 31 2 n 130 6 e and okinawa 26 7 n 128 2 e in japan combining the data from the network of 58 global positioning system receivers around china and the global ionospheric map the variations of the total electron content reveal the region where enhancement persisted for a long period to be within longitudes 90 130 e our results suggest that this abnormal enhancement is most possibly a seismo ionospheric signature

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

laboratory earthquakes triggered during eclogitization of lawsonite bearing blueschist abstract the origin of intermediate depth seismicity has been debated for decades a substantial fraction of these events occurs within the upper plane of wadati benioff double seismic zones believed to represent subducting oceanic crust we deformed natural lawsonite rich blueschist samples under eclogite facies conditions 1 5 p 3 5 gpa 583 k t 1121 k using a d dia apparatus installed at a synchrotron beamline continuously monitoring stress strain phase changes and acoustic emissions aes two distinct paths were investigated i heating during deformation at pressures 2 5 gpa to maximum temperatures ranging from 762 to 1073 k during which lawsonite and glaucophane became gradually unstable while entering the stability field of lawsonite eclogite and the breakdown reaction of lawsonite was only crossed in case of the highest final temperature ii heating while deforming at a pressure

delineating complex spatiotemporal distribution of earthquake aftershocks an improved source scanning algorithm summary conventional earthquake location methods depend critically on the correct identification of seismic phases and their arrival times from seismograms accurate phase picking is particularly difficult for aftershocks that occur closely in time and space mostly because of the ambiguity of correlating the same phase at different stations in this study we introduce an improved source scanning algorithm issa for the purpose of delineating the complex distribution of aftershocks without time consuming and labour intensive phase picking procedures the improvements include the application of a ground motion analyser to separate p and s waves the automatic adjustment of time windows for brightness calculation based on the scanning resolution and a modified brightness function to combine constraints from multiple phases synthetic experiments simulating a challenging scenario are conducted to demonstrate the robustness of the issa the method is applied to a field data set selected from the ocean bottom seismograph records of an offshore aftershock sequence southwest of taiwan although visual inspection of the seismograms is ambiguous our issa analysis clearly delineates two events that can best explain the observed waveform pattern

coalescence microseismic mapping earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations if mis picks are made these will contribute to the location therefore causing potential bias for data recorded by a dense seismic array direct imaging methods can be applied instead we describe the coalescence microseismic mapping method which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array by continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio temporal coordinates of the origins of seismic events noisy data are migrated away from the correct origin so do not contribute to errors in location the method is rooted in a bayesian formulation of event location traveltime inversion allows imaging of source locations and has the capacity to handle errors in modelled traveltimes it has the advantage of working with any 3 d velocity model which therefore may include anisotropy it also automatically incorporates both p and s wave data a multiresolution grid search leads to an efficient implementation with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality we discuss the theory and implementation of this method and illustrate it with real data from microseismic events in iceland caused by melt intrusion in the crust

the characteristics of quasistatic electric field perturbations observed by demeter satellite before large earthquakes abstract this paper presents new results after processing the ulf electric field dc 15 hz observed by demeter satellite h 660 710 km typical perturbations were picked up in quasistatic electric field around some large earthquakes in 2010 at first and then 27 earthquakes were selected to be analyzed on quasistatic electric field in two seismic regions of indonesia and chile at equatorial and middle latitude area respectively three component electric field data related to earthquakes were collected along all the up orbits in local nighttime in a limited distance of 2000 km to the epicenters during 9 days with 7 days before and 1 day after those cases and totally 57 perturbations were found around them all the results show that the amplitude of quasistatic electric field perturbations varies from 1 5 to 16 mv m in the upper ionosphere mostly smaller than 10 mv m and the perturbations were mainly located just over the epicentral area or at the end of seismic faults constructed by a series of earthquakes where electromagnetic emissions may be easily formed during preparation and development processes of seismic sequences among all 27 cases there are 10 earthquakes with perturbations occurring just one day before which demonstrates the close correlation in time domain between quasistatic electric field in ionosphere and large earthquakes finally combined with in situ observation of plasma parameters the coupling mechanism of quasistatic electric field in different earth spheres was discussed

detection and characterization of seismic phases using continuous spectral estimation on incoherent and partially coherent arrays summary seismic arrays are employed in the global monitoring of earthquakes and explosions because of their superior ability to detect and estimate the direction of incident seismic arrivals traditional beamforming and f k analysis require waveform semblance over the full array aperture and cannot be applied in many situations where signals are incoherent between sensors the norsar and mjar arrays are two primary ims stations where this is the case for high frequency regional phases large intersite distances and significant geological heterogeneity at these arrays result in waveform dissimilarity which precludes coherent array processing in the frequency bands with optimal snr multitaper methods provide low variance spectral estimates over short time windows and seismic arrivals can be detected on single channels using a non linear spectrogram transformation which attains local maxima at times and frequencies characterized by an energy increase this detection procedure requires very little a priori knowledge of the spectral content of the signal the transformed spectrograms can be beamformed over large aperture arrays or networks according to theoretical time delays resulting in an incoherent detection system which does not require waveform semblance at any frequencies we outline a real time automatic detection system for regional phase arrivals on the norsar array and demonstrate how stable and accurate slowness and azimuth estimates can be obtained for quite marginal signals in the case of partially coherent arrays the procedure described may provide stable if low resolution estimates which can subsequently be refined using coherent processing over subsets of sensors in particular we illustrate how the spectrogram beamforming method facilitates a stable and accurate slowness estimate for the incoherent high frequency pn arrival at the mjar array in japan from the 2006 october 9 underground nuclear test in north korea

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

seismic evidence for thermal runaway during intermediate depth earthquake rupture 1 intermediate depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected there are two leading candidates for the physical mechanism behind these earthquakes dehydration embrittlement and self localizing thermal shear runaway a complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate depth earthquake rupture the combination of high stress drop and low radiation efficiency that we observe for mw 4 5 earthquakes in the bucaramanga nest implies a temperature increase of 600 1000 c for a centimeter scale layer during earthquake failure this suggests that substantial shear heating and possibly partial melting occurs during intermediate depth earthquake failure our observations support thermal shear runaway as the mechanism for intermediate depth earthquakes which would help explain differences in their behavior compared to shallow earthquakes

earthquake induced transformation of the lower crust the structural and metamorphic evolution of the lower crust has direct effects on the lithospheric response to plate tectonic processes involved in orogeny including subsidence of sedimentary basins stability of deep mountain roots and extension of high topography regions recent research shows that before orogeny most of the lower crust is dry impermeable and mechanically strong 1 during an orogenic event the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones in the bergen arcs of western norway shear zones initiate as faults generated by lower crustal earthquakes seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms however normal earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust 2 here we show that the volume of lower crust affected by such aftershocks is substantial and that fluid driven associated metamorphic and structural transformations of the lower crust follow these earthquakes this provides a top down effect on crustal geodynamics and connects processes operating at very different timescales during continent collision and associated mountain building a surprisingly large volume of the lower crust is shown to be affected by earthquake aftershocks producing a top down effect on crustal geodynamics

temporal and spatial occurrence of deep non volcanic tremor from washington to northern california 1 regional network data reveal deep non volcanic tremor occurs in five independent sections along the u s section of the cascadia subduction zone with different recurrence intervals sizeable tremor episodes occur during periods with no measurable geodetic deformation as well as coincident with episodic slow slip events and the displacement amplitudes of the tremor with and without geodetic deformation are similar the tremor depths cover a wide range that does not coincide directly with the subducting slab interface the tremor can migrate bi directionally from the point of initiation and more than one spatially distinct tremor source can be active at a given time these results indicate that the tremor is not directly related to the slow slip event instead we suggest that the tremor is related to the presence and movement of fluids at depth which can explain the locations and episodic occurrence as well as the seismic signal characteristics themselves namely the limited frequency content small size emergent onsets and prolonged durations with and without geodetic deformation

non double couple earthquakes 1 theory historically most quantitative seismological analyses have been based on the assumption that earth quakes are caused by shear faulting for which the equiv alent force system in an isotropic medium is a pair of force couples with no net torque a double couple or dc observations of increasing quality and coverage however now resolve departures from the dc model for many earthquakes and find some earthquakes especially in volcanic and geothermal areas that have strongly non dc mechanisms understanding non dc earth quakes is important both for studying the process of faulting in detail and for identifying nonshear faulting processes that apparently occur in some earthquakes this paper summarizes the theory of moment tensor expansions of equivalent force systems and analyzes many possible physical non dc earthquake processes contrary to long standing assumption sources within the earth can sometimes have net force and torque components described by first rank and asymmetric sec ond rank moment tensors which must be included in analyses of landslides and some volcanic phenomena non dc processes that lead to conventional symmetric second rank moment tensors include geometrically complex shear faulting tensile faulting shear faulting in an anisotropic medium shear faulting in a heteroge neous region e g near an interface and polymorphic phase transformations undoubtedly many non dc earthquake processes remain to be discovered progress will be facilitated by experimental studies that use wave amplitudes amplitude ratios and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor com ponents

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

improving the detection of low magnitude seismicity preceding the mw 6 3 l aquila earthquake development of a scalable code based on the cross correlation of template earthquakes

fault slip rates at depth from recurrence intervals of repeating microearthquakes unique attributes in sequences of recurring similar microearthquakes at parkfield california provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events application of the method using an 11 year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault zone slip if this phenomenon is found to be generally common behavior in active faults it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone

dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust intermediate depth earthquakes in cold subduction zones are observed within the subducting oceanic crust as well as the mantle in contrast intermediate depth earthquakes in hot subduction zones predominantly occur just below the mohorovicic discontinuity these observations have stimulated interest in relationships between blueschist facies metamorphism and seismicity particularly through dehydration reactions involving the mineral lawsonite here we conducted deformation experiments on lawsonite while monitoring acoustic emissions in a griggs type deformation apparatus the temperature was increased above the thermal stability of lawsonite while the sample was deforming to test whether the lawsonite dehydration reaction induces unstable fault slip in contrast to similar tests on antigorite unstable fault slip that is stick slip occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed microstructural observations indicate that strain is highly localized along the fault r1 and b shears and that the fault surface develops slickensides very smooth fault surfaces polished by frictional sliding the unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate a thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones indicating the potential for unstable frictional sliding within natural lawsonite layers

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

real time earthquake location using kirchhoff reconstruction real time location of earthquakes can be achieved by using direct imaging of the recorded wave field based on a kirchhoff reconstruction method similar to that used in the migration of seismic reflection data the standard method of event location requires the wave arrival at each sensor to be picked and associated with an event by using direct imaging the event is identified once in the imaged wave field the computation is independent of the level of seismic activity and can be carried out on a typical desktop computer the procedure has been successfully demonstrated in two and three dimensions using data from the southern california seismic network trinet at higher resolutions the reconstruction method can identify finite source effects further work considers extending the method by implementing full elastic theory and solving for moment tensors at all locations in the mesh

an automatic monitoring system for nonvolcanic tremors in southwest japan 1 the recent development of a system for exchanging and distributing seismic waveform data over high speed networks enables seismic events to be monitored in real time throughout japan in the present study we have developed an automatic real time monitoring system for deep nonvolcanic tremors in southwest japan the system automatically detects the occurrence of nonvolcanic tremors and determines their hypocenters in real time in addition the system creates image files for the detected tremor activities and makes them accessible via the world wide web to detect tremors we carry out a two step numerical hypothesis test in which the first step is to test whether two given envelope seismograms are correlated and the second step is to test whether an event occurs using the results of the first test this two step test is applied to real time data every 2 min once an event is detected we can regard time lags that generate maximum cross correlations as the travel time differences which we use for hypocenter determination although this procedure detects nontremor signals most of them can be rejected using several criteria since the start of monitoring in 2006 the system has worked well for detecting a wide variety of tremor activities results from the present system will contribute to understanding the stress relaxation process in the transition zone between the locked and stable slip zones of the subducting philippine sea plate interface

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

a reanalysis of the hypocentral location and related observations for the great 1906 california earthquake we determine probabilistic hypocentral locations for the foreshock and mainshock of the great 1906 california earthquake through reanalysis of arrival time observations in conjunction with modern velocity models and advanced event location techniques we obtain two additional observations for the mainshock and one for the foreshock that were not used in previous location studies using a robust likelihood function for event location we generate a usable subset of the predominantly unreliable teleseismic readings and determine new wave type identifications for some of the local and teleseismic readings our locations are much better constrained than those of earlier studies even though we do not assume that the epicenter lies on the san andreas fault as did previous authors we confirm the conclusions of earlier studies that the local and teleseismic arrival time observations can be explained by a single foreshock focus and a single mainshock focus on the san andreas fault and that there is no single unique hypocenter that satisfies all available local observations the maximum likelihood point latitude 37 78 n longitude 122 51 w for our preferred mainshock location indicates a hypocenter to the west of san francisco close to the san andreas fault zone this hypocenter has a 68 confidence error of about 8 km parallel to the san andreas fault and about 24 km perpendicular to the fault and a depth in the midcrust of about 12 7 km the closest point on the san andreas fault to this hypocenter lies about 10 km to the northwest of the widely accepted mainshock epicenter of bolt 1968 our mainshock location is consistent with the association of initial rupture of the 1906 mainshock with a dilatational right bend or step over in the submerged san andreas fault system offshore of the golden gate our foreshock location is less well constrained than our mainshock location but is consistent with the foreshock hypocenter being at the same location as the mainshock hypocenter online material visualization of 3d probabilistic hypocentral locations associated with the 1906 earthquake

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process