mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

review of crustal seismicity in the aleutian arc and implications for arc deformation abstract central and eastern aleutian arc is characterized by oblique convergence between the subducting pacific and overriding bering plates this results in westward arc translation and formation of rotating crustal blocks in the forearc in 2006 2010 several moderate shallow crustal earthquakes up to magnitude 6 7 occurred in the region these events are located about 150 km away from the trench on the volcanic axis and have either strike slip west of 174 w or normal east of 174 w faulting mechanisms we improve aftershock locations by applying precise relocation methods to aid in identifying preferred fault planes we also review similar earthquakes that occurred prior to 2006 for the central aleutian arc we conclude that while some of these events occurred along the boundaries of the rotating blocks the majority are left lateral strike slip events on nw to n oriented fault planes in the unrotated bering massif these manifest riedel shearing in response slip partitioning due to the oblique convergence normal faulting events in eastern aleutian arc reflect along arc extension

near continuous tremor and low frequency earthquake activities in the alaska aleutian subduction zone revealed by a mini seismic array author s li b ghosh a abstract 2017 american geophysical union all rights reserved tectonic tremor and low frequency earthquakes lfes are relatively poorly studied in the alaska aleutian subduction zone due to the limited data availability difficult logistics and rugged terrain using 2 months of continuous data recorded by a mini seismic array in the akutan island we detect near continuous tremor activity with an average of 1 3 h of tectonic tremor per day using a beam backprojection method tremor sources are clustered in two patches with an 25 km gap in between them in addition we visually identify three low frequency earthquakes and using them as templates we detect 1300 additional lfes applying a matched filter method tremor and lfe activities agree well in space and time and lfes show a much smaller recurrence interval during tremor than during non tremor time periods tremor sources propagate both along the strike and dip directions of the subduction fault with velocities ranging between 13 and 110 km h prolific patchy tremor and lfe activities suggest lateral heterogeneity in the locked to freely slipping transition zone indicating that slow earthquakes may play an important role in the earthquake cycles in this subduction zone

evidence for a scale limited low frequency earthquake source process we calculate the seismic moments for 34 264 low frequency earthquakes lfes beneath the olympic peninsula washington lfe moments range from 1 4 1010 to 1 9 1012 n m mw 0 7 2 1 while regular earthquakes follow a power law moment frequency distribution with a b value near 1 the number of events increases by a factor of 10 for each unit increase in mw we find that while for large lfes the b value is 6 for small lfes it is 1 the magnitude frequency distribution for all lfes is best fit by an exponential distribution with a mean seismic moment characteristic moment of 2 0 1011 n m the moment frequency distributions for each of the 43 lfe families or spots on the plate interface where lfes repeat can also be fit by exponential distributions an exponential moment frequency distribution implies a scale limited source process we consider two end member models where lfe moment is limited by 1 the amount of slip or 2 slip area we favor the area limited model based on the observed exponential distribution of lfe moment and geodetically observed total slip we estimate that the total area that slips within an lfe family has a diameter of 300 m assuming an area limited model we estimate the slips subpatch diameters stress drops and slip rates for lfes during episodic tremor and slip events we allow for lfes to rupture smaller subpatches within the lfe family patch models with 1 10 subpatches produce slips of 0 1 1 mm subpatch diameters of 80 275 m and stress drops of 30 1000 kpa while one subpatch is often assumed we believe 3 10 subpatches are more likely

introduction to special section on phenomenology underlying processes and hazard implications of aseismic slip and nonvolcanic tremor 1 this paper introduces the special section on the phenomenology underlying processes and hazard implications of aseismic slip and nonvolcanic tremor by highlighting key results of the studies published in it many of the results indicate that seismic and aseismic manifestations of slow slip reflect transient shear displacements on the plate interface with the outstanding exception of northern cascadia where tremor sources have been located on and above the plate interface differing models of the plate interface there also need to be reconciled slow slip phenomena appear to result from propagating deformation that may develop with persistent gaps and segment boundaries results add to evidence that when tectonic deformation is relaxed via slow slip most relaxation occurs aseismically but with seismic signals providing higher resolution proxies for the aseismic slip instead of two distinct slip modes as suggested previously lines between fast and slow slip more appropriately may be described as blurry zones results reported also show that slow slip sources do not coincide with a specific temperature or metamorphic reaction their associations with zones of high conductivity and low shear to compressional wave velocity ratios corroborate source models involving pore fluid pressure buildup and release these models and spatial anticorrelations between earthquake and tremor activity also corroborate a linkage between slow slip and frictional properties transitional between steady state and stick slip finally this special section highlights the benefits of global and multidisciplinary studies which demonstrate that slow phenomena are not confined to beneath the locked zone but exist in many settings

seismicity patterns 1963 1983 as stress indicators in the shumagin seismic gap alaska earthquakes 1963 to 1983 of magnitude 5 0 to 6 5 form a rim of high seismic activity around a central region of relative seismic quiescence that coincides with the shumagin seismic gap this pattern which can be inferred both from local network and teleseismic earthquake locations shows high activity near the eastern and western ends of the seismic gap and along the down dip end of the main thrust zone the rim of seismicity surrounding an area of seismic quiescence may reflect strong couplign along the elastic brittle part of the plate boundary where the great earthquakes occur the temporal behavior of the microseismicity recorded by the shumagin network is characterized by a burst of activity in 1978 and 1979 and a low or average level of activity from 1980 to january 1983 the increased microearthquake activity during 1978 1979 is located mainly near the down dip end of the main thrust zone and along the benioff zone down to 100 to 200 km depth composite fault plane solutions of earthquakes occurring in 1978 1979 show down dip tension between 50 to 120 km depth in the upper plane of the benioff zone composite fault plane solutions of 1981 earthquakes however indicate in plate compression in the same region hence the rate of occurrence and focal mechanisms of microearthquakes located in the benioff zone below the main thrust both show coincident temporal and spatial variations that may reflect fluctuations in local stresses

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

transition from locked to creeping subduction in the shumagin region alaska 1 gps velocities from the alaska peninsula are modeled to determine the extent of locking on the alaska aleutian subduction interface the observations which span from the semidi islands to sanak island encompass the 1938 mw 8 3 rupture zone and the transition into the shumagin gap model parameters are optimized using a simulated annealing method coupling variation along strike of the plate interface show a nearly fully locked 90 subduction zone at the semidi islands decreasing to about 30 locked at the shumagin islands and freely slipping to the west of the shumagins independent rupture of the shumagin segment could produce repeated mw 7 6 earthquakes unless a significant fraction of the slip on the interface occurs as afterslip following large earthquakes southwest directed velocities at most of the sites may be attributed to clockwise rotation of a bering block

automatic detection and characterization of seismic tremors in northern cascadia 1 we develop an algorithm that automatically detects and characterizes seismic waveforms associated with episodic tremor and slip ets events in northern cascadia for each hour long seismogram the process begins with the calculation of moving average and scintillation index functions a logic chart is designed to sort the observed waveform patterns into a set of classes based on these functions an ets event is always manifest by many station hours with coherent tremor ct patterns automatic analysis of seismic waveforms recorded in the past three years gives results consistent with the work done by tedious visual inspection of seismograms our algorithm which is free from human oversight is ideal for an automatic tremor monitoring system to give immediate notice of an ets event

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

evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska 1 global positioning system gps measurements are used to examine the hypothesis of a clockwise rotating bering plate originally proposed on the basis of seismicity the bering plate encompasses the bering sea western alaska and the aleutian islands gps measurements from the bering plate s interior show south to southwest motions of 3 5 mm a where a is years we construct elastic dislocation models to determine the spatial distribution and intensity of locked patches on the aleutian subduction interface we use these to remove interseismic strain from the gps observation and determine an arc translation velocity for each region of the aleutians revealing south and southwest motions of 4 8 mm a we combine the arc translation rates with measurements from the bering plate s interior sites and estimate the euler pole for the bering plate relative to north america to be located at 42 5 n 121 3 e with an angular speed of 6 0 ma the clockwise rotation of the bering plate may cause left lateral faulting in interior alaska the bering plate s interaction with south central alaska may be responsible for the decreased slip rate on the western denali fault and for contraction across the central alaska range we analyze slip partitioning along the aleutian arc on the basis of both gps measurements and slip azimuths of thrust earthquakes and find a systematic discrepancy between plate convergence direction and slip azimuths we conclude that slip partitioning in the back arc only develops west of amchitka pass whereas slip partitioning in the fore arc is present throughout the arc

future cascadia megathrust rupture delineated by episodic tremor and slip 1 a suite of 15 episodic tremor and slip events imaged between 1997 and 2008 along the northern cascadia subduction zone suggests future coseismic rupture will extend to 25 km depth or 60 km inland of the pacific coast rather than stopping offshore at 15 km depth an ets derived coupling profile accurately predicts gps measured interseismic deformation of the overlying north american plate as measured by 50 continuous gps stations across western washington state when extrapolated over the 550 year average recurrence interval of cascadia megathrust events the coupling model also replicates the pattern and amplitude of coseismic coastal subsidence inferred from previous megathrust earthquakes here for only the washington state segment of the cascadia margin this translates into an mw 8 9 earthquake with significant moment release close to its metropolitan centers

the rupture process and tectonic implications of the great 1964 prince william sound earthquake we have determined the rupture history of the march 28 1964 prince william sound earthquake m w 9 2 from long period wwssn p wave seismograms source time functions determined from the long period p waves indicate two major pulses of moment release the first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture the second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds because of the large size of this event and thus a deficiency of on scale digitizable p wave seismograms it is impossible to uniquely invert for the location of moment release however if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault the first moment pulse most likely initiated near the epicenter at the northeastern down dip edge of the aftershock area and then spread over the fault surface in a semi circular fashion until the full width of the fault was activated the rupture then extended toward the southwest approximately 300 km ruff and kanamori 1983 the second moment pulse was located in the vicinity of kodiak island starting at 500 km southwest of the epicenter and extending to about 600 km although the aftershock area extends southwest past the second moment pulse by at least 100 km the moment release remained low we interpret the 1964 prince william sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major but smaller asperity in the kodiak island region

triggered tremor sweet spots in alaska 1 to better understand what controls fault slip along plate boundaries we have exploited the abundance of seismic and geodetic data available from the richly varied tectonic environments composing alaska a search for tremor triggered by 11 large earthquakes throughout all of seismically monitored alaska reveals two tremor sweet spots regions where large amplitude seismic waves repeatedly triggered tremor between 2006 and 2012 the two sweet spots locate in very different tectonic environments one just trenchward and between the aleutian islands of unalaska and akutan and the other in central mainland alaska the unalaska akutan spot corroborates previous evidence that the region is ripe for tremor perhaps because it is located where plate interface frictional properties transition between stick slip and stably sliding in both the dip direction and laterally the mainland sweet spot coincides with a region of complex and uncertain plate interactions and where no slow slip events or major crustal faults have been noted previously analyses showed that larger triggering wave amplitudes and perhaps lower frequencies 0 03 hz may enhance the probability of triggering tremor however neither the maximum amplitude in the time domain or in a particular frequency band nor the geometric relationship of the wavefield to the tremor source faults alone ensures a high probability of triggering triggered tremor at the two sweet spots also does not occur during slow slip events visually detectable in gps data although slow slip below the detection threshold may have facilitated tremor triggering

deep low frequency earthquakes in tectonic tremor along the alaska aleutian subduction zone 1 we characterize and locate tremor not associated with volcanoes along the alaska aleutian subduction zone using continuous seismic data recorded by the alaska volcano observatory and the alaska earthquake information center from 2005 to present visual inspection of waveform spectra and time series reveal dozens of 10 to 20 min bursts of tremor along the length of the alaska aleutian subduction zone we use autocorrelation to demonstrate that these tremor signals are composed of hundreds of repeating low frequency earthquakes lfes the tremor activity we characterize is localized in four segments from east to west kodiak island shumagin gap unalaska and andreanof islands although the geometry age thermal structure frictional and other relevant properties of the alaska aleutian subduction zone are poorly known these characteristics are likely to differ systematically from east to west locations near kodiak island are the most reliable because station coverage is more complete lfe hypocenters in this region are located on the plate interface near the down dip limit of the 1964 mw 9 2 alaska earthquake rupture area lfe hypocenters in the remaining areas along the arc are also located down dip of the most recent mw 8 megathrust earthquakes although these locations are less well constrained our results support the hypothesis that tremor activity marks the down dip rupture limit for great megathrust earthquakes in this subduction zone lastly there is no correlation between the presence of tremor and particular aspects of over riding or subducting plate geology or coupling it appears that lfes are a fundamental characteristic of the alaska aleutian subduction zone

a 15 year catalog of more than 1 million low frequency earthquakes tracking tremor and slip along the deep san andreas fault low frequency earthquakes lfes are small rapidly recurring slip events that occur on the deep extensions of some major faults their collective activation is often observed as a semicontinuous signal known as tectonic or nonvolcanic tremor this manuscript presents a catalog of more than 1 million lfes detected along the central san andreas fault from 2001 to 2016 these events have been detected via a multichannel matched filter search cross correlating waveform templates representing 88 different lfe families with continuous seismic data together these source locations span nearly 150 km along the central san andreas fault ranging in depth from 16 to 30 km this accumulating catalog has been the source for numerous studies examining the behavior of these lfe sources and the inferred slip behavior of the deep fault the relatively high temporal and spatial resolutions of the catalog have provided new insights into properties such as tremor migration recurrence and triggering by static and dynamic stress perturbations collectively these characteristics are inferred to reflect a very weak fault likely under near lithostatic fluid pressure yet the physical processes controlling the stuttering rupture observed as tremor and lfe signals remain poorly understood this paper aims to document the lfe catalog assembly process and associated caveats while also updating earlier observations and inferred physical constraints the catalog itself accompanies this manuscript as part of the electronic supplement with the goal of providing a useful resource for continued future investigations

large accelerations and tectonic setting of the may 1993 shumagin islands earthquake sequence variability of earthquake rupture can lead to large disparities between expected and observed ground motion to better understand the range of ground motions generated by subduction zone earthquakes we examine teleseismic and nearsource strong motion records from an interplate thrust earthquake sequence that occurred near the shumagin islands alaska peninsula in may 1993 the two largest events show source depths of 36 km may 13 and 42 km may 25 from inversion of teleseismic waveforms which place them in the deepest part of the interplate thrust zone this inference is verified by single station epicenter estimates derived from p wave polarization and s p times and by the focal mechanisms the 13 may 1993 mainshock ms 6 9 shows peak accelerations of 0 7 to 1 4 m sec 2 within 75 km of the epicenter consistent with scaling relations observed elsewhere the largest aftershock 25 may mw 6 1 shows peak acceleration near 8 m sec 2 at a station 25 km away the 8 m sec 2 acceleration is far larger than expected for an earthquake of this size and depth and is perhaps the largest recorded in alaska simple explanations such as site effects and rupture directivity cannot fully account for the anomalous accelerations the short rupture time of the 25 may event 2 8 sec contributes to high accelerations but cannot explain the factor of 12 variability observed in peak acceleration near the epicenter unusual focusing effects and rupture patterns are suspected but cannot be confirmed whatever the cause the unusually high accelerations demonstrate that deep and moderate sized earthquakes can occasionally produce significant ground motions in subduction zones

low frequency earthquakes reveal punctuated slow slip on the deep extent of the alpine fault new zealand we present the first evidence of low frequency earthquakes lfes associated with the deep extension of the transpressional alpine fault beneath the central southern alps of new zealand our database comprises a temporally continuous 36 month long catalog of 8760 lfes within 14 families to generate this catalog we first identify 14 primary template lfes within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross correlation routine the hypocentres of 12 of the 14 lfe families lie within 10 km of the inferred location of the alpine fault at depths of approximately 20 30 km in a zone of high p wave attenuation low p wave speeds and high seismic reflectivity the lfe catalog consists of persistent discrete events punctuated by swarm like bursts of activity associated with previously and newly identified tremor periods the magnitudes of the lfes range between ml 0 8 and ml 1 8 with an average of ml 0 5 we find that the frequency magnitude distribution of the lfe catalog both as a whole and within individual families is not consistent with a power law but that individual families frequency amplitude distributions approximate an exponential relationship suggestive of a characteristic length scale of failure we interpret this lfe activity to represent quasi continuous slip on the deep extent of the alpine fault with lfes highlighting asperities within an otherwise steadily creeping region of the fault

nonvolcanic tremor in the aleutian arc nonvolcanic tremors nvt have been observed across the aleutian arc the tremorlike events are observed over a large region spanning seismic networks on several volcanoes and are therefore unlikely to be related to any one volcanic complex although locating the events is not possible at the present time we provide general locations based on move out times and amplitudes across multiple networks the majority of nvt events are recorded in regions where the subducting pacific plate is inferred to be locked or transitioning from creeping to locked one nvt event was recorded during a time of increased seismic and volcanic activity in the area near the rat and andreanof islands similar simultaneous increases have been observed in the aleutians in the past and are hypothesized to be caused by large regional stress changes possibly caused by slow slip events a prominent recent nvt event was recorded on the korovin and great sitkin seismic networks approximately one hour after the onset of the 12 july 2008 eruption of okmok volcano nvt does not appear sporadically across the aleutian arc but seems to concentrate in a few regions with 2500 km of subduction zone we have an opportunity to examine these regions and compare them to regions not producing nvt in an attempt to identify factors that may play a role in its creation

a large slow slip event and the depth of the seismogenic zone in the south central alaska subduction zone abstract we modeled crustal deformation before during and after a slow slip event sse on the plate interface in southern alaska we divided the gps time series into three time periods before 1998 1998 to 2001 and 2001 2002 the first and last periods represent the steady deformation while the middle period represents the sse we found a stationary zone of 100 slip deficit in the shallow part of the seismogenic zone to depths of 25 30 km which we identify with the main asperity that ruptured in 1964 earthquake the sse occurs downdip of the asperity on a section that accumulated some slip deficit during the steady time periods during the sse 20 000 sq km of the plate interface slipped 10 cm for a cumulative moment magnitude of m w 7 2 the area of the sse did not have a significant slip in the 1964 earthquake compared to the slip of the main asperity we propose that between earthquakes this zone fails repeatedly in sses and remains at relatively low shear stress compared to the main asperity up dip of it

an autocorrelation method to detect low frequency earthquakes within tremor 1 recent studies have shown that deep tremor in the nankai trough under western shikoku consists of a swarm of low frequency earthquakes lfes that occur as slow shear slip on the down dip extension of the primary seismogenic zone of the plate interface the similarity of tremor in other locations suggests a similar mechanism but the absence of cataloged low frequency earthquakes prevents a similar analysis in this study we develop a method for identifying lfes within tremor the method employs a matched filter algorithm similar to the technique used to infer that tremor in parts of shikoku is comprised of lfes however in this case we do not assume the origin times or locations of any lfes a priori we search for lfes using the running autocorrelation of tremor waveforms for 6 hi net stations in the vicinity of the tremor source time lags showing strong similarity in the autocorrelation represent either repeats or near repeats of lfes within the tremor we test the method on an hour of hi net recordings of tremor and demonstrates that it extracts both known and previously unidentified lfes once identified we cross correlate waveforms to measure relative arrival times and locate the lfes the results are able to explain most of the tremor as a swarm of lfes and the locations of newly identified events appear to fill a gap in the spatial distribution of known lfes this method should allow us to extend the analysis of shelly et al 2007a to parts of the nankai trough in shikoku that have sparse lfe coverage and may also allow us to extend our analysis to other regions that experience deep tremor but where lfes have not yet been identified

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

triggered seismic events along the eastern denali fault in northwest canada following the 2012 mw 7 8 haida gwaii 2013 mw 7 5 craig and two mw 8 5 teleseismic earthquakes we conduct a systematic search for remotely triggered seismic activity along the eastern denali fault edf in northwest canada an intraplate strike slip region we examine 19 distant earthquakes recorded by nine broadband stations in the canadian national seismograph network and find that the 2012 m w 7 8 haida gwaii and 2013 m w 7 5 craig alaska earthquakes triggered long duration 10 s emergent tremor like signals near the southeastern portion of the edf in both cases tremor coincides with the peak transverse velocities consistent with love wave triggering on right lateral strike slip faults the 2011 m w 9 0 tohoku oki and 2012 m w 8 6 indian ocean earthquakes possibly triggered tremor signals although we were unable to locate those sources in addition we also identify many short duration 5 s bursts that were repeatedly triggered by the rayleigh waves of the 2012 m w 7 8 haida gwaii earthquake although we were unable to precisely locate the short duration 5 s events they appear to be radiating from the direction of the klutlan glacier and from a belt of shallow historical seismicity at the eastern flank of the wrangell st elias mountain range the fact that these events were triggered solely by the rayleigh waves suggests a different source mechanism as compared with triggered tremor observed along the edf and other plate boundary regions online material description of rayleigh wave polarization detection figures of spectograms s wave amplitudes polarization analysis and static stress changes tables of earthquake catalogs and velocity model and animations of tremors and bursts

excitation of a buried magmatic pipe a seismic source model for volcanic tremor recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so called long period or low frequency events accordingly we can view the long period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor generating system we present a seismic model in which the source of tremor is the acoustic resonance of a fluid filled volcanic pipe triggered by excess gas pressure the model consists of three elements namely a triggering mechanism a resonator and a radiator for simplicity we assume a hemispherical trigger cylindrial resonator and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk shaped radiator shutting off its bottom considering the simple case of a source buried in a homogeneous half space we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances the results demonstrate that the displacement attributed to the pipe dominates the near field motion while that due to the disk is representative of the intermediate and far fields the trigger itself has a smaller contribution mainly limited to the field in the proximity of the source the characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well developed harmonic wave train dominated by rayleigh waves at larger distances no clear shear arrival can be detected in the synthetic seismograms the displacement spectrum reflects the organ pipe modes of the conduit and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid in the case of the cylindrical magma column considered the radiation loss is proportional to the square of the pipe radius while the loss related to viscous damping is inversely proportional to the same factor indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir the relative importance of the pipe and disk elements likewise is a function of the conduit cross section this suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near and far field observations

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

using systematically characterized low frequency earthquakes as a fault probe in guerrero mexico studies of low frequency earthquakes lfes have focused on detecting events within previously identified tectonic tremor however the principal lfe detection tools of matched filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event in this study we therefore focus on generating the largest number possible of lfe templates by uniformly applying a recently developed lfe template detection method to a 2 5 yearlong data set in guerrero mexico using each of the detected templates in a matched filter search we then form event families that each represents a single source we finally develop simple empirical statistics to select the event families that represent lfes our resulting catalog contains 1120 unique lfe sources and a total of 1 849 486 detected lfes over the 2 5 yearlong data set the locations of the lfe sources are then divided into subcatalogs based on their distance from the subduction trench considering each lfe as a small unit of slip along the subduction interface we observe discrete episodes of lfe activity in the region associated with large slow slip events this is in direct contrast to the near continuous activity observed 35 km farther downdip within the previously identified lfe tremor sweet spot

determination of slow slip episodes and strain accumulation along the cascadia margin 1 continuous gps stations in the pacific northwest geodetic array network clearly record subduction related strain accumulation and slow slip episodes along the cascadia convergent margin many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor leading to the previous discovery of episodic tremor and slip ets in this study we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the gps time series independent of seismic tremor data we then apply this technique to study the patterns of strain accumulation and release associated with ets events and characterize patterns of coupling associated with the locked and transition zones of the plate interface we demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements recurrence patterns in the distribution of gps observations demonstrate coherence among neighboring stations over time and apparent along strike segmentation of the subduction interface when slow slip events are removed from the time series we can estimate the total site velocities between slow slip events these velocities decay as depth to the subduction interface increases but they diverge from the long term trends expected from the interseismic cycle at 30 60 km above the interface consistent with the location where slow slip displacements occur forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30 coupling from 20 to 35 km depth which is needed to produce the observed back slip displacements intriguingly our best fitting models have a decrease in coupling down to 30 at 20 km depth followed by a peak of greater than 80 coupling at 30 35 km depth suggesting the source zone for ets events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone although a zone of constant 30 coupling cannot be ruled out with this data set such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition we propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle

automated detection and location of cascadia tremor 1 a new autonomous seismic location and detection methodology enables real time opportunities for high resolution spatio temporal monitoring of non volcanic tremor combining a unique cross correlation technique with epicenter clustering analysis in northern cascadia automatically yields thousands of tremor epicenters from the may 2008 episodic tremor and slip ets event while routinely detecting and locating inconspicuous inter ets tremor bursts although ets events in this area produce about two weeks of continuous tremor we find a nearly equal amount of tremor during the last 15 month inter ets period the resulting ets and inter ets epicenters occur in the slow slip region where the plate interface is 30 45 km deep and have a sharp well resolved updip boundary about 75 km east of the downdip edge of the seismogenic megathrust zone this ability to track tremor with high spatio temporal resolution facilitates automatic tremor monitoring and the mapping of the transition zone and regions of locked zone stress accumulation

correction to evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska 1 in the paper evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska by ryan s cross and jeffrey t freymueller journal of geophysical research 113 b07405 doi 10 1029 2007jb005136 2008 the magnitudes of the estimated angular velocities for the bering plate were off by two orders of magnitude or by a factor of 100 because of a unit conversion error the same error affected all forward and inverse calculations so only the bering plate rotation rate was incorrect the pole location and all other calculations in the paper are correct the correct angular speed for the best model is 0 064 0 033 degrees ma an updated table 3 is given below with the correct rates for all models tested

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

three dimensional elastic dislocation modeling of the postseismic response to the 1964 alaska earthquake 1 gps velocity observations from the kenai peninsula and kodiak island alaska display a pattern of spatial variability suggesting the presence of multiple active processes on the underlying pacific north american plate interface velocities of sites on the eastern kenai peninsula and kodiak island are consistent with elastic strain accumulation at the plate interface whereas velocities of sites on the western kenai peninsula and cook inlet are oriented in the opposite direction and suggest an ongoing postseismic response to the 1964 alaska earthquake we can reproduce the observed velocities using a three dimensional elastic dislocation model to estimate the variation in coupling between the pacific and north american plates two different inversion schemes are examined to check the robustness of the results we find that the gps data can be satisfied by the presence of a locked area near southwest prince william sound a locked area near southwest kodiak island and an area of postseismic reverse slip beneath and north of the western kenai peninsula the shallow plate interface trenchward of the western kenai peninsula does not appear to be locked the locked areas correspond to the prince william sound and kodiak island asperities that ruptured in 1964 and the area of postseismic slip lies downdip of the 1964 rupture area the correspondence between the present shallow coupling distribution and the 1964 slip distribution suggests that the locked regions repeat from one earthquake cycle to another