accurate tremor locations from coherent s and p waves nonvolcanic tremor is an important component of the slow slip processes which load faults from below but accurately locating tremor has proven difficult because tremor rarely contains clear p or s wave arrivals here we report the observation of coherence in the shear and compressional waves of tremor at widely separated stations which allows us to detect and accurately locate tremor events an event detector using data from two stations sees the onset of tremor activity in the cascadia tremor episodes of february 2003 july 2004 and september 2005 and confirms the previously reported south to north migration of the tremor event detectors using data from three and four stations give sand p arrival times of high accuracy the hypocenters of the tremor events fall at depths of 30 to 40 km and define a narrow plane dipping at a shallow angle to the northeast consistent with the subducting plate interface the s wave polarizations and p wave first motions define a source mechanism in agreement with the northeast convergence seen in geodetic observations of slow slip tens of thousands of locations determined by constraining the events to the plate interface show tremor sources highly clustered in space with a strongly similar pattern of sources in the three episodes examined the deeper sources generate tremor in minor episodes as well the extent to which the narrow bands of tremor sources overlap between the three major episodes suggests relative epicentral location errors as small as 1 2 km

a slower fault may produce a smaller preseismic moment rate non 1 tf acceleration of moment rate during nucleation and dependency on the background slip rate 1 a recent global study has revealed that seismicity near the hypocenter prior to large earthquakes which could be a proxy for preseismic moment rate accelerates before interplate earthquakes while it rarely does before intraplate earthquakes understanding the amplitude of preseismic deformation is important in assessing the possibility of its detection for a class of rate state friction laws without a characteristic speed related parameter e g aging law and slip law a dimensional analysis has shown that if the moment rate increases more mildly than 1 tf where tf is the time to failure then the amplitude of preseismic moment rate is smaller for a smaller quasistatic slip rate three dimensional numerical simulations have revealed that the aging law yields 1 tf acceleration while the slip law causes milder acceleration if the latter is the case faults with very low long term slip rates e g intraplate faults may have very small preseismic moment rates

seismic hazard assessment for mymensingh bangladesh mymensingh municipality lies in one of the most earthquake prone areas of bangladesh the town was completely destroyed during the great indian earthquake of 12 june 1897 for which the surface wave magnitude was 8 1 in this study the 1897 great indian earthquake was used as a scenario event for developing seismic microzonation maps for mymensingh for microzonation purposes spt data from 87 boreholes were collected from different relevant organizations to verify those data ten boreholes of depth up to 30 m were drilled intensity values obtained for different events were calibrated against attenuation laws to check applicability to the study area vibration characteristics at diverse points of the study area were estimated by employing the one dimensional wave propagation software shake shake discretizes the soil profile into several layers and uses an iterative technique to represent the non linear behavior of the soil by adjusting the material properties at each iteration step the required input information includes depth shear wave velocity damping factor and unit weight of each soil layer the liquefaction resistance factor and the resulting liquefaction potential were estimated to quantify the severity of liquefaction quantification of secondary site effects and the weighting scheme for combining the various seismic hazards were heuristic based on judgment and expert opinion

earthquake nucleation and triggering on an optimally oriented fault seismic surface waves from large distant earthquakes commonly trigger smaller earthquakes however delay times of hours to days between the surface waves and the triggered earthquakes weaken the causal connection furthermore when there is no delay the triggered earthquakes are typically too small or too obscured to obtain reliable source mechanisms we present observations of instantaneous triggering of a strike slip earthquake in central alaska shear strain from the optimally aligned teleseismic love wave induced a 24 s exponential foreshock signal leading to the triggered earthquake this nucleation phase and the alignment of the triggered earthquake source mechanism with the teleseismic stress field reveal the behavior of an existing fault under well calibrated strain conditions the alaska earthquake provides the first observation of combined nucleation and triggering and it suggests that transient stresses during nucleation may influence the subsequent earthquake rupture laboratory and theoretical studies of nucleation and triggering may help discriminate between different interpretations for the alaska earthquake

characterization of nucleation during laboratory earthquakes 1 we observe the nucleation phase of in plane ruptures in the laboratory we show that the nucleation is composed of two distinct phases a quasi static and an acceleration stage followed by dynamic propagation we propose an empirical model which describes the rupture length evolution the quasi static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time the transition from quasi static to accelerating rupture is related to the critical nucleation length which scales inversely with normal stress in accordance with theoretical predictions and to a critical surfacic power which may be an intrinsic property of the interface finally we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions

a phase coherence approach to identifying co located earthquakes and tremor we present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions co located earthquakes and tremor the method used is a phase coherence based implementation of empirical matched field processing modified to suit tremor analysis it works by comparing the frequency domain phases of waveforms generated by two sources recorded at multiple stations we first cross correlate the records of the two sources at a single station if the sources are co located this cross correlation eliminates the phases of the green s function it leaves the relative phases of the source time functions which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength we therefore search for cross correlation phases that are consistent across stations as an indication of co located sources we also introduce a method to obtain relative locations between the two sources based on back projection of interstation phase coherence we apply this technique to analyse two tremor like signals that are thought to be composed of a number of earthquakes first we analyse a 20 s long seismic precursor to a m 3 9 earthquake in central alaska the analysis locates the precursor to within 2 km of the mainshock and it identifies several bursts of energy potentially foreshocks or groups of foreshocks within the precursor second we examine several minutes of volcanic tremor prior to an eruption at redoubt volcano we confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence the amplitude of the tremor diminishes about 30 s before the eruption but the phase coherence results suggest that the tremor may persist at some level through this final interval

preslip and cascade processes initiating laboratory stick slip recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a cascade up process to explore such a case in the laboratory we study the initiation of dynamic rupture stick slip of a smooth saw cut fault in a 76 mm diameter cylindrical granite laboratory sample at 40 120 mpa confining pressure we use a high dynamic range recording system to directly compare the seismic waves radiated during the stick slip event to those radiated from tiny m 6 discrete seismic events commonly known as acoustic emissions aes that occur in the seconds prior to each large stick slip the seismic moments focal mechanisms locations and timing of the aes all contribute to our understanding of their mechanics and provide us with information about the stick slip nucleation process in a sequence of 10 stick slips the first few microseconds of the signals recorded from stick slip instabilities are nearly indistinguishable from those of premonitory aes in this sense it appears that each stick slip begins as an ae event that rapidly 20 s grows about 2 orders of magnitude in linear dimension and ruptures the entire 150 mm length of the simulated fault we also measure accelerating fault slip in the final seconds before stick slip we estimate that this slip is at least 98 aseismic and that it both weakens the fault and produces aes that will eventually cascade up to initiate the larger dynamic rupture

superresolution with seismic arrays using empirical matched field processing scattering and refraction of seismic waves can be exploited with empirical matched field processing of array observations to distinguish sources separated by much less than the classical resolution limit to describe this effect we use the term superresolution a term widely used in the optics and signal processing literature to denote systems that break the diffraction limit we illustrate superresolution with pn signals recorded by the arces array in northern norway using them to identify the origins with 98 2 accuracy of 549 explosions conducted by closely spaced mines in northwest russia the mines are observed at 340 410 kilometers range and are separated by as little as 3 kilometers when viewed from arces many are separated by just tenths of a degree in azimuth this classification performance results from an adaptation to transient seismic signals of techniques developed in underwater acoustics for localization of continuous sound sources matched field processing is a potential competitor to frequency wavenumber and waveform correlation methods currently used for event detection classification and location it operates by capturing the spatial structure of wavefields incident from a particular source in a series of narrow frequency bands in the rich seismic scattering environment closely spaced sources far from the observing array nonetheless produce distinct wavefield amplitude and phase patterns across the small array aperture with observations of repeating events these patterns can be calibrated over a wide band of frequencies e g 2 5 12 5 hertz for use in a power estimation technique similar to frequency wavenumber analysis the calibrations enable coherent processing at high frequencies at which wavefields normally are considered incoherent under a plane wave model

detection and analysis of near surface explosions on the kola peninsula seismic and infrasonic observations of signals from a sequence of near surface explosions at a site on the kola peninsula have been analyzed norsar s automatic network processing of these events shows a significant scatter in the location estimates and to improve the automatic classification of the events we have performed full waveform cross correlation on the data set although the signals from the different events share many characteristics the waveforms do not exhibit a ripple for ripple correspondence and cross correlation does not result in the classic delta function indicative of repeating signals using recordings from the arces seismic array 250 km w of the events we find that a correlation detector on a single channel or three component station would not be able to detect subsequent events from this source without an unacceptable false alarm rate however performing the correlation on each channel of the full arces array and stacking the resulting traces generates a correlation detection statistic with a suppressed background level which is exceeded by many times its standard deviation on only very few occasions performing f k analysis on the individual correlation coefficient traces and rejecting detections indicating a non zero slowness vector results in a detection list with essentially no false alarms applying the algorithm to 8 years of continuous arces data identified over 350 events which we confidently assign to this sequence the large event population provides additional confidence in relative travel time estimates and this together with the occurrence of many events between 2002 and 2004 when a temporary network was deployed in the region reduces the variability in location estimates the best seismic location estimate incorporating phase information for many hundreds of events is consistent with backazimuth measurements for infrasound arrivals at several stations at regional distances at lycksele 800 km sw of the events as well as at arces infrasound is detected for most of the events in the summer and for few in the winter at apatity some 230 km s of the estimated source location infrasound is detected for most events as a first step to providing a ground truth database for this useful source of infrasound we provide the times of explosions for over 50 events spanning 1 year

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

a family of repeating low frequency earthquakes at the downdip edge of tremor and slip we analyze an isolated low frequency earthquake lfe family located at the downdip edge of the main episodic tremor and slip ets zone beneath western washington state the 9000 individual lfes from this repeating family cluster into 198 swarms that recur roughly every week cumulative lfe seismic moment for each swarm correlates strongly with the time until the next swarm suggesting that these lfe swarms are time predictable precise double difference relative locations for 700 individual lfes within this family show a distribution that is approximately 2 km long and 500 m wide elongated parallel to the relative plate convergence direction the distribution of locations 300 m vertical spread lies within a few hundred meters of two different plate interface models and has a similar dip peak to peak lfe s wave amplitudes range from 0 2 to 18 nm individual lfes exhibit a trend of increasing magnitude during swarms with smaller events at the beginning and the largest events toward the end the largest lfes cluster in a small area 300 m radius coincident with maximum lfe density we propose that the less concentrated smaller lfes act to unlock this patch core allowing it to fully rupture in the largest lfes usually toward the end of a swarm we interpret the patch responsible for producing these lfes as a subducted seamount on the downgoing juan de fuca jdf plate lfe locking efficiency slip estimated during 5 years from summing lfe seismic moment divided by plate rate determined slip is at most 20 and is highly concentrated in two 50 m radius locations in the larger patch core estimated individual lfe stress drops range from 1 to 20 kpa but could also be significantly larger

overview of holocene tsunami deposits along the nankai suruga and sagami troughs southwest japan tsunami deposits provide a basis for reconstructing holocene histories of great earthquakes and tsunamis on the pacific coast of southwest japan the deposits have been found in the past 15 years at lakes lagoons outcrops and archaeological excavations the inferred tsunami histories span 3000 years for the nankai and suruga troughs and nearly 10 000 years for the sagami trough the inferred histories contain recurrence intervals of variable length the shortest of these 100 200 years for the nankai trough 150 300 years for the sagami trough resemble those known from written history of the past 1000 1500 years longer intervals inferred from the tsunami deposits probably reflect variability in rupture mode incompleteness of geologic records and insufficient research the region s tsunami history could be clarified by improving the geologic distinction between tsunami and storm dating the inferred tsunamis more accurately and precisely and using the deposits to help quantify the source areas and sizes of the parent earthquakes

imaging slow slip fronts in cascadia with high precision cross station tremor locations 1 we apply a new method to obtain accurate locations of tremor sources beneath southern vancouver island unlike more standard cross time methods which compare waveforms from different time windows at the same station this cross station method compares waveforms from the same time window at widely separated stations it performs well relative to cross time methods when the response to an impulsive tremor source is dominated by the main arrival rather than coda and when multiple colocated sources are active within the time window examined we focus on a region roughly 10 km across that was monitored by the polaris deployment from 2003 to 2006 relative location errors appear to be 1 km allowing us to image in great detail rapid and small scale tremor migrations that arise behind the main slow slip front the secondary fronts tend to a start at or within about 1 km of the main tremor front and propagate back along strike at rates of 10 20 km h b less commonly do the reverse ending at the main front or c propagate up or downdip at or within 1 2 km of the main front estimated stress drops in the secondary events are comparable to that in the main event implying that their 25 50 times greater propagation speed results from a similarly greater slip speed

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

nucleation and arrest of dynamic slip on a pressurized fault 1 elevated pore pressure can lead to reactivation of slip on pre existing fractures and faults when the static coulomb failure is reached locally as the pressurized region spreads diffusively slip can accumulate quasi statically paced by the pore fluid diffusion or dynamically in this work we consider a prestressed fault with a locally peaked diffusively spreading pore pressure field to study 1 conditions leading to the escalation of slip and nucleation of dynamic rupture and 2 rupture run out distance before it is arrested nucleation appears in this model when the fault friction decreases from its peak value with slip while arrest of dynamic propagation is imminent on aseismic faults i e such that prestress b is less than the residual fault strength rat ambient conditions when fluid overpressure is a small to moderate fraction of the ambient value of normal effective stress and prestress is large enough for fault slip to be activated by overpressure dynamic rupture always nucleates and the nucleation length increases with decreasing prestress practically independently of the overpressure value transition from the ultimately unstable b r to the ultimately stable b r fault loading is marked by a strong increase of the nucleation length 1 b r 2 as b approaches r from above for aseismic faults b r no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress the largest run out distances of dynamic slip on aseismic faults correspond to overpressure prestress just sufficient for slip activation in such cases the dynamically accumulated slip can lead to enhanced dynamic fault weakening resulting in a sustained dynamic rupture and generating a large earthquake this is consistent with field observations when the largest injection induced seismicity occurred after fluid injection ended

high resolution imaging of rapid tremor migrations beneath southern vancouver island using cross station cross correlations we develop a cross station method to detect and locate tremor and low frequency earthquakes lfes based on the original work of armbruster et al 2014 that compares waveforms from the same time window at stations separated by roughly 10 km to improve the signal to noise ratio we first rotate the horizontal components into the empirical shear wave particle motion direction the large scale rapid tremor reversals beneath southern vancouver island are best recorded by stations that exhibit pronounced shear wave splitting which obscures this optimal direction we correct for splitting using the stacked templates of 11 low frequency earthquake families obtained in this region by bostock et al 2012 we find that the style of rapid tremor migrations rtms evolves as the main front passes over a region very close to the main front numerous small scale migrations occur with recurrence intervals far shorter than tidal periods these usually propagate along the main front even when that is not parallel to dip several larger rtms propagating along the main front have prominent elongation orthogonal to the propagation direction inconsistent with the interpretation that their large propagation speed is an apparent velocity caused by the slow main front intersecting a preexisting linear structure on the plate interface farther behind the main front rtms gradually progress to being tidally modulated and have generally slower propagation speeds many reversal like rtms are observed some of which evolve from fronts initially propagating along the main front these reversals are sometimes coherent across regions of low tremor density

foreshocks during the nucleation of stick slip instability 1 we report on laboratory experiments which investigate interactions between aseismic slip stress changes and seismicity on a critically stressed fault during the nucleation of stick slip instability we monitor quasi static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike slip fault 2 m long and 0 4 m deep in a saw cut sample of sierra white granite with 14 piezoelectric sensors we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture we observe localized aseismic fault slip in an approximately meter sized zone in the center of the fault while the ends of the fault remain locked clusters of high frequency foreshocks mw 6 5 to 5 0 can occur in this slowly slipping zone 5 50 ms prior to the initiation of dynamic rupture their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault the meter sized nucleation zone is generally consistent with theoretical estimates but source radii of the foreshocks 2 to 70 mm are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur we propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections which would typically slip aseismically may radiate seismic waves if they are rapidly stressed fault behavior of this type may provide physical insight into the mechanics of foreshocks tremor repeating earthquake sequences and a minimum earthquake source dimension

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

decay of aftershock density with distance does not indicate triggering by dynamic stress the most predictable earthquakes are aftershocks which invariably follow mainshocks so what triggers aftershocks it was recently argued controversially that the decay of aftershocks with distance from the main earthquake could only be explained by dynamic triggering keith richards dinger and colleagues have tested this hypothesis and conclude that the observed decay can be better explained by static triggering resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard it has recently been argued that the decay of aftershocks with distance from the main earthquake could be explained only by dynamic triggering this hypothesis has now been tested the conclusion being that the observed decay can be better explained by static triggering resolving whether static1 2 3 or dynamic4 5 6 7 8 stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard9 felzer and brodsky10 examined the distance distribution of earthquakes occurring in the first five minutes after 2 m 3 and 3 m 4 mainshocks and found that their magnitude m 2 aftershocks showed a uniform power law decay with slope 1 35 out to 50 km from the mainshocks from this they argued that the distance decay could be explained only by dynamic triggering here we propose an alternative explanation for the decay and subject their hypothesis to a series of tests none of which it passes at distances more than 300 m from the 2 m 3 mainshocks the seismicity decay 5 min before the mainshocks is indistinguishable from the decay five minutes afterwards indicating that the mainshocks have no effect at distances outside their static triggering range omori temporal decay the fundamental signature of aftershocks is absent at distances exceeding 10 km from the mainshocks finally the distance decay is found among aftershocks that occur before the arrival of the seismic wave front from the mainshock which violates causality we argue that felzer and brodsky10 implicitly assume that the first of two independent aftershocks along a fault rupture triggers the second and that the first of two shocks in a creep or intrusion driven swarm triggers the second when this need not be the case

large nucleation before large earthquakes is sometimes skipped due to cascade up implications from a rate and state simulation of faults with hierarchical asperities 1 does a large earthquake have a large quasi static preparation a hierarchical asperity model in which a large tough patch patch l contains smaller fragile patches patch s enables a large earthquake to start with only small preparation because of cascade up rupture growth we realized such a model in a rate and state framework by heterogeneous distributions of the state evolution distance and its consequences are investigated by earthquake sequence simulations focus is put on elementary processes the interaction between two scales one patch l containing one patch s is simulated with their size ratio as a parameter if patch s is larger than the nucleation size of patch l the system falls into a limit cycle consisting of only one earthquake that starts with patch s nucleation and grows dynamically to rupture entire patch l if patch s is considerably smaller than the nucleation size of patch l small earthquakes never dynamically cascade up and the large earthquakes are initiated by large quasi static nucleation in between large earthquakes start in various ways by large nucleation dynamic cascade up or delayed cascade up in the final stage of quasi static nucleation the preseismic moment release rate increases roughly inversely proportional to the time to failure tf with its amplitude depending on the nucleation size for a patch s rupture to cascade up strength in the adjacent region must have been reduced manifested by a higher creep velocity before the patch s nucleation starts following 1 tf acceleration large nucleation sometimes has a precursory small earthquake characterized by larger afterslip than nonprecursory ones