a continuum of stress strength and slip in the cascadia subduction zone movement of the down going oceanic plate in subduction zones is accommodated by earthquakes slow slip and free slip with increasing depth analysis of accompanying tremor reveals a continuum of slow slip events in the cascadia subduction zone which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone

ground motion prediction from tremor 1 the widespread occurrence of tremor coupled with its frequency content and location provides an exceptional opportunity to test and improve strong ground motion attenuation relations for subduction zones we characterize the amplitude of thousands of individual 5 min tremor events in cascadia during three episodic tremor and slip events to constrain the distance decay of peak ground acceleration pga and peak ground velocity pgv we determine the anelastic attenuation parameter for ground motion prediction equations gmpes to a distance of 150 km which is sufficient to place important constraints on ground motion decay tremor pga and pgv show a distance decay that is similar to subduction zone specific gmpes developed from both data and simulations however the massive amount of data present in the tremor observations should allow us to refine distance amplitude attenuation relationships for use in hazard maps and to search for regional variations and intrasubduction zone differences in ground motion attenuation

source parameters and time dependent slip distributions of slow slip events on the cascadia subduction zone from 1998 to 2008 1 we invert for the time dependent slip history of slow slip events on the cascadia subduction zone using gps data from 1998 to 2008 the 16 slip transients have sufficient station coverage to solve for the slip distribution on the plate interface gps time series are inverted for fault slip using the extended network inversion filter limited station coverage south of portland 45 5 n latitude restricts our analysis to events on the northern half of the subduction zone slip is resolved at the base of the seismogenic zone and the slip distributions suggest a potential segment boundary near seattle 47 6 n that correlates roughly with geologic and tectonic boundaries events that initiate to the north and south tend to overlap at about this latitude we compile statistics on source parameters such as propagation rate recurrence interval and stress drop which can be used to constrain proposed models of the source mechanics over a 10 year period total strain release from slow slip events is nonuniform along strike with the greatest cumulative slip 27 cm centered beneath port angeles 48 1 n this slip patch also exhibits the most regular recurrence of mw 6 events relative to other locations along strike the spatial extent of the slip patch beneath port angeles correlates with the along strike bend of the cascadia subduction zone in northwestern washington suggesting that plate geometry plays an important role in controlling the along strike characteristics of slow slip

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

settling velocity diameter and density for flocs of illite kaolinite and montmorillonite abstract the relation between floc settling velocity and diameter was measured for illite kaolinite and montmorillonite the curves between floc diameter and settling velocity show a non stokes relation that indicated the floc density decreased or drag properties increased as the floes increased in size

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

earthquake spectra and near source attenuation in the cascadia subduction zone f 1 for slow events and as f 2 for fast earthquakes we show that at least in cascadia wave propagation effects likely control spectral decay rates above 2 hz we use seismograms from multiple small aperture arrays to estimate the spectral decay rates of near source spectra of 37 small events and find strong correlation between source location and decay rate the decay rates 1 vary overall by an amount in excess of that inferred to distinguish slow sources from fast earthquakes 2 are indistinguishable for sources separated by a few tens of km or less and 3 separate into two populations that correlate with propagation through and outside a low velocity zone imaged tomographically we find that some events repeat as is characteristic of low frequency earthquakes lfes but have spectra similar to those of non repeating earthquakes we also find no correlation between spectral decay rates and rates of ambient tremor activity these results suggest that earthquakes near the plate boundary at least in cascadia do not distinctly separate into slow and fast classes and correctly accounting for propagation effects is necessary to characterize sources

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

magnitudes and moment duration scaling of low frequency earthquakes beneath southern vancouver island we employ 130 low frequency earthquake lfe templates representing tremor sources on the plate boundary below southern vancouver island to examine lfe magnitudes each template is assembled from hundreds to thousands of individual lfes representing over 269 000 independent detections from major episodic tremor and slip ets events between 2003 and 2013 template displacement waveforms for direct p and s waves at near epicentral distances are remarkably simple at many stations approaching the zero phase single pulse expected for a point dislocation source in a homogeneous medium high spatiotemporal precision of template match filtered detections facilitates precise alignment of individual lfe detections and analysis of waveforms upon correction for 1 d geometrical spreading attenuation free surface magnification and radiation pattern we solve a large sparse linear system for 3 d path corrections and lfe magnitudes for all detections corresponding to a single ets template the spatiotemporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12 24 h of lfe activity during an ets episode when tidal sensitivity is low the remainder is released in bursts over several days particularly as spatially extensive rapid tremor reversals rtrs during which tidal sensitivity is high rtrs are characterized by large magnitude lfes and are most strongly expressed in the updip portions of the ets transition zone and less organized at downdip levels lfe magnitude frequency relations are better described by power law than exponential distributions although they exhibit very high b values 5 we examine lfe moment duration scaling by generating templates using detections for limiting magnitude ranges mw 1 5 mw 2 0 lfe duration displays a weaker dependence upon moment than expected for self similarity suggesting that lfe asperities are limited in fault dimension and that moment variation is dominated by slip this behavior implies that lfes exhibit a scaling distinct from both large scale slow earthquakes and regular seismicity

intraslab earthquakes dehydration of the cascadia slab we simultaneously invert travel times of refracted and wide angle reflected waves for three dimensional compressional wave velocity structure earthquake locations and reflector geometry in northwest washington state the reflector interpreted to be the crust mantle boundary moho of the subducting juan de fuca plate separates intraslab earthquakes into two groups permitting a new understanding of the origins of intraslab earthquakes in cascadia earthquakes up dip of the moho9s 45 kilometer depth contour occur below the reflector in the subducted oceanic mantle consistent with serpentinite dehydration earthquakes located down dip occur primarily within the subducted crust consistent with the basalt to eclogite transformation

generation mechanism of slow earthquakes numerical analysis based on a dynamic model with brittle ductile mixed fault heterogeneity 1 various characteristics have been discovered for small slow earthquakes occurring along subduction zones which are deep nonvolcanic tremor low frequency earthquakes lfes and very low frequency earthquakes vlfs in this study we model these slow earthquakes using a dynamic model consisting of a cluster of frictionally unstable patches on a stable background the controlling parameters in our model are related to the patch distribution and the viscosity of both the patches and the background by decreasing patch density or increasing viscosity we observed the transition in rupture propagation mechanism that is from fast elastodynamic interactions characterized by an elastic wave propagation to slow diffusion limited by viscous relaxation times of traction on fault patches and or background some sets of these geometrical and frictional parameters collectively explain the moment rate functions source spectra and scaled energy of observed slow earthquakes in addition we successfully explain both parabolic and constant velocity migrations in the case of the diffusion limited rupture therefore the observed various characteristics of tremor lfes vlfs and potentially slow slip events may be essentially explained by our simple model with a few parameters describing source structures and frictional properties of brittle ductile transition zones along plate boundaries

low frequency earthquakes at the southern cascadia margin we use seismic waveform data from the mendocino experiment to detect low frequency earthquakes lfes beneath northern california during the april 2008 tremor and slip episode in southern cascadia 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct lfe families the main front of tremor epicenters migrates along strike at 9 km d 1 we also find one instance of rapid tremor reversal observed to propagate in the opposite direction at 10 20 km h 1 as in other regions of cascadia lfe hypocenters from this study lie several kilometers above a recent plate interface model south of cascadia lfes were discovered on the maacama and bucknell creek faults the bucknell creek fault may be the youngest fault yet observed to host lfes these fault zones also host shallow earthquake swarms with repeating events that are distinct from lfes in their spectral and recurrence characteristics

periodic chaotic and doubled earthquake recurrence intervals on the deep san andreas fault earthquake recurrence histories may provide clues to the timing of future events but long intervals between large events obscure full recurrence variability in contrast small earthquakes occur frequently and recurrence intervals are quantifiable on a much shorter time scale in this work i examine an 8 5 year sequence of more than 900 recurring low frequency earthquake bursts composing tremor beneath the san andreas fault near parkfield california these events exhibit tightly clustered recurrence intervals that at times oscillate between 3 and 6 days but the patterns sometimes change abruptly although the environments of large and low frequency earthquakes are different these observations suggest that similar complexity might underlie sequences of large earthquakes

complex characteristics of slow slip events in subduction zones reproduced in multi cycle simulations 1 since the discovery of slow slip events along subduction zone interfaces worldwide dense geodetic and seismic networks have illuminated detailed characteristics of these events and associated tremor high resolution observations of tremor where the spatial temporal evolution is presumed to reflect that of the underlying slow slip events show highly complex patterns in which the origins remain poorly understood we present a new computationally efficient modeling technique that reproduces many features of observed slow slip events including slow initiation coalescence of separate events and rapid back propagation of renewed slip over previously slipped regions rapid back propagation speeds are explained as a consequence of rate and state dependent frictional healing consistent with analytical solutions developed in support of the simulations

identifying low frequency earthquakes in central cascadia using cross station correlation abstract we use stations in the willamette valley and oregon coast ranges to identify low frequency earthquakes that occurred during the august september 2009 episodic tremor and slip event while autocorrelation techniques are often successful at identifying lfe templates in central cascadia this technique systematically fails due to the lack of densely spaced high quality stations instead we use cross station methods that have been successful at identifying lfes in northern cascadia to register initial candidate templates network cross correlation to register additional lfe detections and stacking to refine the identified templates using this procedure we detect nearly 16 thousand events comprising a total of 18 lfe families located in central oregon between 30 and 40 km depth either at or near the plate boundary the time history of detections between families is consistent with the slip front in the 2009 sse migrating from north to south at a velocity of 5 km day the templates we identify have more complicated waveforms than those previously identified in northern cascadia these differences in waveform character likely a consequence of the small number of contributing stations

estimating seismic moment magnitude mw of tremor bursts in northern cascadia implications for the seismic efficiency of episodic tremor and slip 1 we develop a method to estimate the seismic moments of deep non volcanic tremor bursts observed in northern cascadia for each tremor burst the maximum amplitudes at individual stations within a time window 5 s around the predicted arrivals of the s phase are measured and compared to the maximum s amplitudes measured from synthetic seismograms the proposed method is thoroughly calibrated using 464 local earthquakes and the results show excellent consistency between the reported ml and the estimated mw we apply the method to northern cascadia tremors and infer that most bursts have mw 1 0 1 7 the corresponding b value appears to be 1 consistent with that of ordinary earthquakes but over a narrower mw range comparison of cumulative tremor mw and the mw estimated from the accompanying slow slip suggests that the seismic efficiency of the episodic tremor and slip ets is of the order of 0 1 or less

laterally propagating slow slip events in a rate and state friction model with a velocity weakening to velocity strengthening transition we investigate the behavior of simulated slow slip events using a rate and state friction model that is steady state velocity weakening at low slip speeds but velocity strengthening at high slip speeds our simulations are on a one dimensional line fault but we modify the elastic interactions to mimic the elongate geometry frequently observed in slow slip events simulations exhibit a number of small events as well as periodic large events the large events propagate approximately steadily along strike and stress and slip rate decay gradually behind the propagating front their recurrence intervals can be determined by considering what is essentially an energy balance requirement for long distance propagation it is possible to choose the model parameters such that the simulated events have the stress drops slip velocities and propagation rates observed in cascadia

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

multi event simulations of slow slip events for a cascadia like subduction zone 1 we employ the earthquake simulator rsqsim which incorporates rate and state dependent friction to investigate characteristics of slow slip events sses along a cascadia like mega thrust the simulations consist of 100 000 sses with equivalent moment magnitudes mw4 0 7 0 the largest simulated sses mw6 4 7 0 have inter event times of 19 months durations of 10 40 days mean slips of 2 2 4 1 cm and along strike propagation speeds of 7 20 km day which are comparable to observations from cascadia the simulations show quiescence after mw 6 4 sses followed by a progressive increase in both the magnitude and frequency of sses prior to the next large event small sses below mw5 6 develop in an incoherent manner and have irregular geometries while larger sses show highly coherent growth of the slip region and spontaneous but transient event to event segmentation the change in event characteristics at mw5 6 corresponds to a break in the scaling of seismic moment with slip fault area and duration

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

high resolution images of tremor migrations beneath the olympic peninsula from stacked array of arrays seismic data episodic tremor and slip ets in subduction zones is generally interpreted as the manifestation of shear slip near the base of earthquake generating portion of the plate interface here we devise a new method of cross correlating stacked array of arrays seismic data that provides greatly improved tremor locations a proxy for the underlying slow slip beneath the olympic peninsula this increased resolution allows us to image many features of tremor that were not visible previously we resolve the spatial transition between the rupture zones of the inter ets and major ets episodes in 2010 suggesting stress redistribution by the former most tremor migrations propagated along the slowly advancing main tremor front during both the inter ets and the major ets episodes even though the main front of the former deviated strongly from its usual along dip orientation we find a distinct contrast between along dip rupture extent of large scale rapid tremor reversals rtrs to the south and that to the north in our study region that anticorrelates with the locations of inter ets events these rtrs originate from the main front similar to smaller scale rtrs previously observed at high resolution and many start by propagating along the main front this could be consistent with rtrs being triggered by a cascading failure of brittle asperities after initiation the rtrs repeatedly occupy the same source region and the early repetitions appear not to be tidally driven their stress drop may come from continuing fault weakening processes within the tremor zone or loading by aseismic slip in surrounding regions

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

tremor asperities in the transition zone control evolution of slow earthquakes 1 slow earthquakes characterized by slow slip and associated seismic radiation called non volcanic tremor have been observed in major subduction zones worldwide they constitute an important mode of stress release for the fault s transition zone which lies directly downdip of the locked segment the nucleation zone of large damaging earthquakes however the depth of tremor in cascadia and the factors governing tremor generation and rupture propagation during slow quakes remain enigmatic here we develop a novel multibeam backprojection mbbp method to detect and locate tremor using multiple mini seismic arrays we apply this technique to image tremor activity during an entire ets cycle including a large episodic tremor and slip ets event in cascadia with unprecedented resolution our results suggest that the majority of the tremor is occurring near the plate interface we observe strongly heterogeneous tremor distribution with patches in the transition zone that experience repeated tremor episodes and produce most of the tremor the patches tens of kilometers in dimension behave like asperities on the fault plane during the large ets event rupture propagation velocity varies at least by a factor of five and seems to be modulated by these tremor asperities these observations support a model in which the transition zone is heterogeneous and consists of patches of asperities with surrounding regions slipping aseismically the asperities fail quasi periodically releasing stress and appear to regulate rupture propagation and tremor generation during slow earthquakes this study presents new observations revealing the tectonic characteristics of the transition zone controlling the generation and evolution of slow 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

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

seismic and geodetic constraints on cascadia slow slip 1 automatically detected and located tremor epicenters from episodic tremor and slip ets episodes in northern cascadia provide a high resolution map of washington s slow slip region thousands of epicenters from the past four ets events from 2004 to 2008 provide detailed map view constraints that correlate with geodetic estimates of the simultaneous slow slip each of these ets events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip analysis of the latest 15 month inter ets period also reveals ageodetic tremor activity similar both in duration and extent to ets tremor epicenters from both ets and inter ets tremor are bounded between the 30 and 45 km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting juan de fuca plate inter ets tremor overlaps but is generally downdip of ets tremor and does not yet correlate with geodetically observed slip but this is likely because the slip is below current gps detection levels based on the tremor and slip correlation and the tremor duration and slip magnitude relationship we suggest that the well resolved sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties the region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake alternatively plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of gps observations

precise tremor source locations and amplitude variations along the lower crustal central san andreas fault 1 we precisely locate 88 tremor families along the central san andreas fault using a 3d velocity model and numerous p and s wave arrival times estimated from seismogram stacks of up to 400 events per tremor family maximum tremor amplitudes vary along the fault by at least a factor of 7 with by far the strongest sources along a 25 km section of the fault southeast of parkfield we also identify many weaker tremor families which have largely escaped prior detection together these sources extend 150 km along the fault beneath creeping transitional and locked sections of the upper crustal fault depths are mostly between 18 and 28 km in the lower crust epicenters are concentrated within 3 km of the surface trace implying a nearly vertical fault a prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6 0 parkfield earthquake

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

scaling relationship between the duration and the amplitude of non volcanic deep low frequency tremors 1 we investigate a duration amplitude relation of non volcanic deep low frequency dlf tremors in the tokai region southwest japan to constrain the source process of the tremors we apply two models to the distribution one is an exponential model as a scale bound distribution and the other a power law model as a scale invariant distribution the exponential model shows a better fit to the duration amplitude distribution of the tremors than a power law model implying that the dlf tremors are caused by a scale bound source process the source process of the dlf tremors therefore differs from those for earthquakes we suggest that the non volcanic dlf tremor is possibly caused by a fixed source dimension with variable excess pressure of fluid or variable stress drop

sonic to ultrasonic q of sandstones and limestones laboratory measurements at in situ pressures laboratory measurements of the attenuation and velocity dispersion of compressional and shear waves at appropriate frequencies pressures and temperatures can aid interpretation of seismic and well log surveys as well as indicate absorption mechanisms in rocks construction and calibration of resonant bar equipment was used to measure velocities and attenuations of standing shear and extensional waves in copper jacketed right cylinders of rocks formula in length formula in diameter in the sonic frequency range and at differential pressures up to formula we also measured ultrasonic velocities and attenuations of compressional and shear waves in formula diameter samples of the rocks at identical pressures extensional mode velocities determined from the resonant bar are systematically too low yielding unreliable poisson s ratios poisson s ratios determined from the ultrasonic data are frequency corrected and used to calculate thesonic frequency compressional wave velocities and attenuations from the shear and extensional mode data we calculate the bulk modulus loss the accuracies of attenuation data expressed as formula where q is the quality factor are formula for compressional and shear waves at ultrasonic frequency formula for shear waves and formula for compressional waves at sonic frequency example sonic frequency data show that the energy absorption in a limestone is small formula greater than 200 and stress independent and is primarily due to poroelasticity whereas that in the two sandstones is variable in magnitude formula ranges from less than 50 to greater than 300 at reservoir pressures and arises from a combination of poroelasticity and viscoelasticity a graph of compressional wave attenuation versus compressional wave velocity at reservoir pressures differentiates high permeability formula formula brine saturated sandstones from low permeability formula formula sandstones and shales

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets