factors controlling riffle scale hyporheic exchange flows and their seasonal changes in a gaining stream a three dimensional groundwater flow model 1 subsurface flow within a single riffle of a low gradient gravel bed stream was modeled in three dimensions using modflow a finite difference groundwater flow model model simulations showed that exchange flows can only occur in this low gradient gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment k 10 4 m s 1 compared with k 10 6 to 10 8 m s 1 the key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium the hydraulic gradient between upstream and downstream ends of the riffle and the flux of groundwater entering the alluvium from the sides and beneath in the study riffle each of these factors changes with season causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0 2 0 5 m 3 d per meter stream length in summer to about 0 008 0 04 m 3 d 1 per meter stream length during fall to spring the model also revealed that exchange flows are up to twice as strong but more variable at the sides of the stream than near the center and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

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

imaging the nonvolcanic tremor zone beneath the san andreas fault at cholame california using station pair double difference tomography abstract nonvolcanic tremors have been detected deep beneath the san andreas fault saf near cholame california the tremors are modulated by small stress changes induced by earth tides and regional and teleseismic earthquakes implying elevated pore fluid pressure in the tremor zone in this area tremors lie beneath the seismogenic zone limiting the usefulness of local earthquakes for resolving seismic velocity properties in the tremor zone station pair double difference tomography that uses differential tremor arrival times is used for the first time to provide new detailed information about the complexity of the lower crust and upper mantle transition and the occurrence of deep tremors in zones of localized low shear wave velocity anomalies the anomalies may represent zones containing high pore pressure fluids that migrate into the tremor zone episodically from the southwest and that are ultimately derived from dehydration of deep serpentinized mantle wedge material these results improve our understanding of the tremor process why tremors are concentrated in specific zones along the saf and possible relationships between tremors and large historic earthquakes

dynamic triggering by strong motion p and s waves evidence from the 1999 chi chi taiwan earthquake high frequency band pass filtering of acceleration records from the 1999 chi chi taiwan earthquake m w 7 6 resolves the continuous signal into a series of relatively short duration discrete energy bursts we hypothesize that these bursts originate near the individual stations as small shallow events that have been dynamically triggered by the p and s waves generated by the chi chi mainshock however we cannot rule out the possibility that they originate from some nonseismic source associated with the surface such as buildings or trees bursts are observed only during the seismic signal and not in the pre or postsignal noise the bursts are not likely to be generated by the instruments because groupings of three colocated individual instruments record identical bursts also we show that the bursts are not due to the band pass filtering of instrumentally generated step functions the hypothesis that they are local events is supported by the observation of bursts in the 40 hz frequency band at distances up to 170 km from the epicenter if the bursts originated on the chi chi fault plane as hypothesized by chen et al 2006 based on their analysis of recordings within 20 km from the chelungpu fault then they should not be observable at this distance assuming any reasonable value of crustal attenuation assuming a local origin we estimate an average local event magnitude of m w 0 2 and a source receiver distance of approximately 1 km we extended our analysis to lower stress levels by analyzing records from a smaller m w 5 3 event that was recorded by many of the same instruments used in the chi chi analysis for this event bursts are observed only on the accelerograms from stations relatively close to the mainshock hypocenter analysis of the combined data set from both mainshocks suggests a stress threshold for triggering in the range 0 03 0 05 mpa for s wave triggering and 0 0013 0 0033 mpa for p wave triggering consistent with prior observations of surface wave triggering

nonvolcanic tremor observed in the mexican subduction zone nonvolcanic tremor nvt activity is revealed as episodes of higher spectral amplitude at 1 8 hz in daily spectrograms from the continuous seismological records in guerrero mexico the analyzed data cover a period of 2001 2007 when in 2001 2002 a large slow slip event sse had occurred in the guerrero oaxaca region and then a new large sse occurred in 2006 the tremor burst is dominated by s waves more than 100 strong nvt bursts were recorded in the narrow band of 40 150 km 2 to the south of iguala city and parallel to the coastline depths of nvt hypocenters are mostly scattered in the continental crust between 5 and 40 km depth tremor activity is higher during the 2001 2002 and 2006 sse compared with that for the quiet period of 2003 2005 while resistivity pattern in guerrero does not correlate directly with the nvt distribution gravity and magnetic anomaly modeling favors a hypothesis that the nvt is apparently related to the dehydration and serpentinization processes

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

modeling dynamic triggering of tectonic tremor using a brittle ductile friction model 1 we study the physics of dynamically triggered tectonic tremor by applying a brittle ductile friction model in which we conceptualize the tremor source as a rigid block subject to driving and frictional forces to simulate dynamic triggering of tremor we apply a stress perturbation that mimics the surface waves of remote earthquakes the tectonic and wave perturbation stresses define a phase space that demonstrates that both the timing and amplitude of the dynamic perturbations control the fundamental characteristics of triggered tremor tremor can be triggered instantaneously or with a delayed onset if the dynamic perturbation significantly alters the frictional state of the tremor source

effective stress friction and deep crustal faulting author s beeler nm hirth g thomas a burgmann r abstract 2015 american geophysical union all rights reserved studies of crustal faulting and rock friction invariably assume the effective normal stress that determines fault shear resistance during frictional sliding is the applied normal stress minus the pore pressure here we propose an expression for the effective stress coefficient f at temperatures and stresses near the brittle ductile transition bdt that depends on the percentage of solid solid contact area across the fault f varies with depth and is only near 1 when the yield strength of asperity contacts greatly exceeds the applied normal stress for a vertical strike slip quartz fault zone at hydrostatic pore pressure and assuming 1 mm and 1 km shear zone widths for friction and ductile shear respectively the bdt is at 13 km f near 1 is restricted to depths where the shear zone is narrow below the bdt f 0 is due to a dramatically decreased strain rate under these circumstances friction cannot be reactivated below the bdt by increasing the pore pressure alone and requires localization if pore pressure increases and the fault localizes back to 1 mm then brittle behavior can occur to a depth of around 35 km the interdependencies among effective stress contact scale strain rate and pore pressure allow estimates of the conditions necessary for deep low frequency seismicity seen on the san andreas near parkfield and in some subduction zones among the implications are that shear in the region separating shallow earthquakes and deep low frequency seismicity is distributed and that the deeper zone involves both elevated pore fluid pressure and localization

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

non volcanic tremor resulting from the combined effect of earth tides and slow slip events swarms of non volcanic tremor in southeastern japan are associated with slow slip events and tend to occur with a periodicity of 12 or 24 h this periodicity can be reproduced by a combination of stresses due to earth tides and transient stress changes caused by slow slip events non volcanic tremors may therefore be useful for understanding stress relaxation at the subduction zone interface slow slip events1 are accompanied by swarms of non volcanic tremor2 3 along the subduction zone of the philippine sea plate in southwest japan the swarms often occur with a periodicity of about 12 or 24 h refs 4 5 these episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface1 6 7 here we analyse seismic data to record the locations and durations of non volcanic tremor swarms that occurred in may 2005 and february 2006 we evaluate the magnitude of stress changes produced by slow slip events as well as the effective normal stress on the plate interface where slow slip events occur we find that the observed periodicity in tremor occurrence originates from a combined effect of the periodic stress due to earth tides and the transient stress due to slow slip events our calculations show that non volcanic tremor is sensitive to stress change this phenomenon can therefore be effective in monitoring the process of stress relaxation at subducting plate interfaces

precise location of san andreas fault tremors near cholame california using seismometer clusters slip on the deep extension of the fault 1 we examine a 24 hour period of active san andreas fault saf tremor and show that this tremor is largely composed of repeated similar events utilizing this similarity we locate the subset of the tremor with waveforms similar to an identified low frequency earthquake lfe master template located using p and s wave arrivals to be 26 km deep to compensate for low signal to noise we estimate event pair differential times at clusters of nearby stations rather than at single stations we find that the locations form a near linear structure in map view striking parallel to the saf and near the surface trace therefore we suggest that at least a portion of the tremor occurs on the deep extension of the fault likely reflecting shear slip similar to subduction zone tremor if so the saf may extend to the base of the crust 10 km below the deepest regular earthquakes on the fault

dynamic triggering of microearthquakes in three geothermal volcanic regions of california geothermal volcanic regions are most susceptible to local earthquake triggering by regional and remote earthquakes transient stresses caused by surface waves of these earthquakes can activate critically stressed faults though earthquakes can be triggered in geothermal volcanic regions it is less understood how these regions differ in their triggering responses to distant earthquakes we conduct a systematic survey of local earthquakes triggered by distant earthquakes in three geothermal volcanic regions of california long valley caldera coso geothermal field and geysers geothermal field we examine waveforms of distant earthquakes with magnitudes 5 5 occurring between 2000 and 2012 and compute statistics to confirm the significance of our findings we find that long valley coso and geysers vary in triggering frequency 2 0 3 8 and 6 8 in the 12 year period respectively and when compared to the triggering of deep tectonic tremors along the parkfield cholame section of san andreas fault 9 2 in the 12 year period stress triggering thresholds vary among the regions with long valley having the highest of 5 kpa and 1 kpa for the other regions because dynamic stresses from distant earthquakes are similar in these three regions the varying triggering behavior likely reflects faults having a tendency to be at or near failure this is compatible with geysers having a higher a value in the gutenberg richter relationship and higher geothermal productivity than the other two regions the observation of more frequent triggering of tremor than microearthquakes is consistent with recent laboratory studies on increasing triggerability with lower effective stress

comparisons of triggered tremor in california abstract we conduct a visual inspection of deep nonvolcanic tremor triggered by large teleseismic earthquakes around the calaveras fault in northern california nc and the san jacinto fault in southern california sc out of the 42 large m w 7 5 earthquakes between 2001 and 2010 only the 2002 m w 7 9 denali fault earthquake triggered clear tremor in these two regions this is in marked contrast with the parkfield cholame section of the san andreas fault in central california cc where 12 earthquakes have triggered tremor in that region the amplitude of the triggered tremor correlates with that of the triggering surface waves in cc and is consistent with the clock advance model the lack of widespread triggered tremor in nc and sc is not simply a consequence of their different background noise levels from cc but rather reflects different background tremor rates in these regions online material tables of parameters for triggered nontriggered teleseismic earthquakes and triggered tremors and figures of band pass filtered seismograms maximum vertical pgvs versus median amplitude of band pass filtered envelope functions tremor amplitude versus pgv and s wave spectra and corresponding noise of local selected earthquakes

hyporheic exchange in gravel bed rivers with pool riffle morphology laboratory experiments and three dimensional modeling received 7 june 2005 revised 30 june 2006 accepted 25 august 2006 published 31 january 2007 1 we report the first laboratory simulations of hyporheic exchange in gravel pool riffle channels which are characterized by coarse sediment steep slopes and three dimensional bed forms that strongly influence surface flow these channels are particularly important habitat for salmonids many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange with surface subsurface mixing measured directly from the concentration decay of a conservative tracer fluorescein injected into the surface flow near bed pressure measurements were also used to predict hyporheic exchange from a three dimensional pumping transport model comparison of the predicted and observed hyporheic exchange shows good agreement indicating that the major mechanism for exchange is bed form induced advection however the effect of bed forms is modulated by discharge and the degree of topographic submergence we also tested the performance of the hydrostatic pressure as a proxy for the observed near bed pressure in driving hyporheic exchange which would facilitate field measurement and analysis of hyporheic flow in natural rivers we found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows

dynamic response of frequent tremors at aso volcano to teleseismic waves from the 1999 chi chi taiwan earthquake abstract teleseismic waves from the 1999 chi chi taiwan earthquake dynamically triggered isolated tremor and micro earthquakes at aso volcano japan which is at a distance of 1400 km immediately after the arrival of the p waves short period tremor spt began to occur more frequently for about 2000 s and during and after the arrival of surface waves 2 micro earthquakes were observed around the crater observations of frequent spt are an indication of triggered isolated tremor which consists of both spt and long period tremor lpt components for the cases of arrivals of seismic waves from other large teleseisms from march 1995 to june 2002 triggering of isolated tremor was observed during 1998 1999 during this period increased thermal supply to the crater was observed while the background activity was relatively low and the volcanic system beneath the crater was more responsive to external stress perturbations

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

seismic wave triggering of nonvolcanic tremor episodic tremor and slip and earthquakes on vancouver island 1 we explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on vancouver island british columbia during and immediately after the arrival of large amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes we identify tremor triggered by four of the teleseismic earthquakes the close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure it is particularly susceptible to triggering of further events the amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor tremor and energy radiated from regional local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller more frequent events we confidently identify triggered local earthquakes following only one teleseism that with the largest amplitude and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates we also note an interesting possible correlation between large teleseismic events and episodic tremor and slip ets episodes whereby ets events that are late and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large distant event while ets events that are early or on time are not

seismological studies at parkfield ii search for temporal variations in wave propagation using vibroseis abstract for more than 3 years the propagation characteristics of shear waves have been monitored for paths near the 1966 hypocenter at parkfield the presumed nucleation site for the expected next m 6 earthquake there data have been collected repeatedly 33 sets as of april 1991 from eight s wave vibroseis source positions into the 10 borehole installed three component seismometers of the local high sensitivity digital network twenty second correlated records from a 6 to 24 hz sweep are acquired and the entire seismogram is viewed for analysis as the elastic response of the local crustal structure which includes the san andreas fault zone amplitudes travel times spectra and particle motions of the p and s waves are monitored for indications of any changes in these properties that may be attributed to processes associated with nucleation the horizontal vibrator at each source point is positioned at three surface orientations to study anisotropy unorthodox methods have been developed to display the waveform properties in time in order to visualize the resulting massive data sets the first order variations seen in some of the parameters are attributed to changes from dry to wet conditions in the shallow subsurface due to the seasonal rainfall which affects the source function of the vibrator corrections have been devised for these source specific variations secular variations not obviously coupled to seasonal near surface changes are also seen in some localized time intervals within the 20 sec records the most striking of such changes is a progressive travel time decrease at rates of 3 to 7 msec year seen for late arrivals 7 to 11 sec travel time on at least five paths into station vca which sample the region southeast of the anticipated epicenter at middle mountain this anomaly appears to be genuine and is now the subject of intensified study in the same general area along the fault in the southwestern block the direct s wave is clearly split with the faster of the split phases polarized parallel to the fault zone a result in agreement with that from the vsp survey in the varian well on the northeast side of the fault

possible relationship between nonvolcanic tremor and the 1998 2001 slow slip event south central alaska 1 we document the occurrence of nonvolcanic tremor nvt in south central alaska for 3 months of each year between 1999 and 2001 to provide a first order attempt to understand the relationship between nvt and a large slow slip event that occurred over a 2 to 3 year period between 1998 and 2001 the majority of the nvt signals observed after the onset of the slow slip event are bursts lasting between 10 and 15 min with frequencies ranging from 1 to 6 hz by the summer of 2001 the quantity of nvt has decreased dramatically from the previous summers and episodes rarely last up to 10 min the decrease in nvt activity in the summer of 2001 is coincident with the end of the slow slip event there is a clear spatial correlation between the observed nvt and the slow slip event with most of the nvt events located on the downdip edge of the region of highest slip depth estimates range widely over the region with approximately half concentrated near the interpreted north america yakutat pacific plate interface and the remainder located above the plate interface however owing to inadequate station coverage and poor knowledge of the local structure determining accurate depths is difficult and thus the relationship between the nvt and the plate interface is problematic this is the first study giving evidence that the relationship between slow slip events and nvt seen in cascadia and southwest japan may also exist in alaska

rapid continuous streaking of tremor in cascadia nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move its spatiotemporal distribution however is complex and varies over different time scales and the causal physical mechanisms remain unclear here we use a beam backprojection method to show rapid continuous slip parallel streaking of tremor over time scales of several minutes to an hour during the may 2008 episodic tremor and slip event in the cascadia subduction zone the streaks propagate across distances up to 65 km primarily parallel to the slip direction of the subduction zone both updip and downdip at velocities ranging from 30 to 200 km h we explore mainly two models that may explain such continuous tremor streaking the first involves interaction of slowly migrating creep front with slip parallel linear structures on the fault the second is pressure driven fluid flow through structurally controlled conduits on the fault both can be consistent with the observed propagation velocities and geometries although the second one requires unlikely condition in addition we put this new observation in the context of the overall variability of tremor behavior observed over different time scales

remotely triggered microearthquakes and tremor in central california following the 2010 mw 8 8 chile earthquake 1 we examine remotely triggered microearthquakes and tectonic tremor in central california following the 2010 mw 8 8 chile earthquake several microearthquakes near the coso geothermal field were apparently triggered with the largest earthquake ml 3 5 occurring during the large amplitude love surface waves the chile mainshock also triggered numerous tremor bursts near the parkfield cholame section of the san andreas fault saf the locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from coso but can be identified by applying high pass or matched filters both triggered tremor along the saf and the ml 3 5 earthquake in coso are consistent with frictional failure at different depths on critically stressed faults under the coulomb failure criteria the triggered tremor however appears to be more phase correlated with the surface waves than the triggered earthquakes likely reflecting differences in constitutive properties between the brittle seismogenic crust and the underlying lower crust

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

simulations of tremor related creep reveal a weak crustal root of the san andreas fault 1 deep aseismic roots of faults play a critical role in transferring tectonic loads to shallower brittle crustal faults that rupture in large earthquakes yet until the recent discovery of deep tremor and creep direct inference of the physical properties of lower crustal fault roots has remained elusive observations of tremor near parkfield ca provide the first evidence for present day localized slip on the deep extension of the san andreas fault and triggered transient creep events we develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near parkfield is consistent with triggered fault creep governed by laboratory derived friction laws between depths of 20 35 km on the fault simulated creep and observed tremor northwest of parkfield nearly ceased for 20 30 days in response to small coseismic stress changes of order 104 pa from the 2003 m6 5 san simeon earthquake simulated afterslip and observed tremor following the 2004 m6 0 parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust these creep responses require very low effective normal stress of 1 mpa on the deep san andreas fault and near neutral stability frictional properties expected for gabbroic lower crustal rock

triggering of tremors and slow slip event in guerrero mexico by the 2010 mw 8 8 maule chile earthquake 1 we investigate the triggering of seismic tremor and slow slip event in guerrero mexico by the february 27 2010 maule earthquake mw 8 8 triggered tremors start with the arrival of s wave generated by the maule earthquake and keep occurring during the passing of scs ss love and rayleigh waves the rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor indicating a strong modulation of the source of tremors by the passing surface wave this correlation and modulation by the passing waves is progressively lost with time over a few hours the tremor activity continues during the weeks months after the earthquake gps time series suggest that the second sub event of the 2009 2010 sse in guerrero is actually triggered by the maule earthquake the southward displacement of the gps stations starts coincidently with the earthquake and tremors the long duration of tremors indicate a continuing deformation process at depth which we propose to be the second sub event of the 2009 2010 sse we show a quasi systematic correlation between surface displacement rate measured by gps and tremor activity suggesting that the nvt are controlled by the variations in the slip history of the sse this study shows that two types of tremors emerge 1 those directly triggered by the passing waves and 2 those triggered by the stress variations associated with slow slip this indicates the prominent role of aseismic creep in the mexican subduction zone response to a large teleseismic earthquake possibly leading to large scale stress redistribution

statistical properties of low frequency earthquakes triggered by large earthquakes in southern taiwan abstract the recent discovery of triggered tremors tts and low frequency earthquakes lfes in various tectonic environments provides an opportunity for studying the fundamental properties and physical mechanisms of deep tectonic tremor here we quantify the relationship between tts and lfes beneath the central range in southern taiwan and their statistical properties during the teleseismic waves of six large distant earthquakes using waveforms of 11 lfes triggered by the 2005 m w 8 6 nias earthquake as templates we scan through 12 hours of waveform data around six mainshocks and identify a total of 783 lfes the lfes were mainly located in a compact region between 12 and 36 km in depth near the chaochou lishan fault most of lfes occurred within tt during the passage of large amplitude surface waves and the increase of the lfe rate during the surface waves is statistically significant the lfe rates do not follow an omori s type decay but rather abruptly return to the background rate immediately after the surface wave passage these findings suggest that lfes do not trigger any additional lfes at later times and are primarily driven by an external forcing our observations are consistent with the inference that tts consist of many reoccurring lfes

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

surface wave potential for triggering tectonic nonvolcanic tremor corrected abstract source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle are anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction is 0 2 documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan however are associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

high resolution deep tectonic tremor locations beneath the san andreas fault near cholame california using the double pair double difference location method we determine high precision deep tectonic tremor locations beneath the san andreas fault saf near cholame california region using a newly developed double pair double difference location method and a new 3 d vs model compared to the previous results newly determined tremor locations are more consistent with known low frequency earthquake lfe locations and are more concentrated along the saf new tremor locations near cholame show clear contrast in the predominant depth of the tremor activity across the saf and also show contrasts in the degree of periodic behavior both along and across the saf they collectively define a very special tremor generation region vstr near the intersection corner between the saf and the adjacent san juan fault sjf in the vstr new locations reveal some isolated tremor clusters which may occur on multiple deep fault patches for these isolated clusters some are closer to the saf and some are closer to the sjf implying that both the saf and sjf may extend to the ductile lower crust possible interaction between the saf and sjf in the lower crust beneath the vstr combined with the trapped high pressure fluids may explain the strong tremor activity the isolated tremor clusters and the characteristic tremor occurrence patterns and depth distributions in this region

remotely triggered nonvolcanic tremor in sumbawa indonesia we present for the first time evidence for triggered tremor beneath the island of sumbawa indonesia we show triggered tremor in response to three teleseismic earthquakes the mw9 0 2011 tohoku earthquake and two oceanic strike slip earthquakes mw 8 6 and mw8 2 offshore of sumatra in 2012 we constrain an apparent triggering threshold of 1 mm s ground velocity that corresponds to about 8 kpa dynamic stress peak tremor amplitudes of about 180 nm s are observed and scale with the ground velocity induced by the remote earthquakes triggered tremor responds to 45 65 s period surface waves and predominantly correlates with rayleigh waves even though the 2012 oceanic events have stronger love wave amplitudes we could not locate the tremor because of minimal station coverage but data indicate several potential source volumes including the flores thrust the java subduction zone or tambora volcano

upper ocean thermal structure and heat content off the us west coast during the 1997 1998 el nino event based on axbt and satellite altimetry data abstract during the 1997 1998 el nino event extensive oceanic temperature profiles were taken off the coast of california in january and february 1998 using airborne expendable bathythermographs axbts these axbt measurements are compared with altimetry based upper ocean temperature estimates using topex and ers satellite altimetry data the altimetry based temperature estimates are well correlated with the axbt data in particular when combining the two satellite data sets together to form a blended altimeter temperature estimate both the axbt and altimetry data show that the nearshore coastal el nino signal differed from that further offshore the axbt data show that near shore the warm anomalies extended to much greater depths and had greater amplitude a time series of the satellite derived layer averaged temperatures averaged separately over the nearshore and offshore halves of the axbt analysis domain also shows a larger el nino signal in the nearshore half the role of local atmospheric forcing of the coastal oceanic temperature anomalies is analyzed using ncep reanalysis and coastal upwelling data sets the forcing terms include ekman pumping radiation surface heat fluxes precipitation and alongshore wind stresses that drive coastal upwelling expressed as a coastal downwelling index cdi the temperature forcing from all of the terms except the cdi anomalies are small the cdi anomalies can explain most of the slowly varying temperature changes that occur near the coast during a two year period spanning the el nino event as well as some of the larger amplitude rapid monthly warming episodes that appear to be part of the el nino signal several distinct rapid warming episodes however are not correlated with the cdi anomalies and therefore we conclude that the nearshore el nino signal originates from a combination of both a remote oceanic pathway and local atmospheric forcing

slow slip and tremor search at kilauea volcano hawaii 1 kilauea volcano hawaii has hosted a long series of slow slip events observed since the installation of the continuous gps network in 1996 kilauea s slow slip events are inferred to occur on the decollement fault at 8 km depth beneath its south flank with a location updip of the epicenters of large regular earthquakes fault slip typically lasts about two days and the events have magnitudes equivalent to mw 5 3 6 0 while slow slip events in subduction zones are commonly accompanied by tectonic tremor also called nonvolcanic tremor no tremor has yet been reported in association with kilauea s slow slip events instead there are swarms of small triggered earthquakes which is a characteristic only seen at select subduction zones e g boso and hikurangi a temporary array of seismometers was installed at kilauea in 2007 in anticipation of a slow slip event here we use several established methods to perform a systematic search for tectonic tremor during geodetically defined slow slip events as well as searching for tremor triggered by teleseismic surface waves we do not detect tectonic tremor using any of these methods although we are able to detect episodes of previously identified deep offshore volcanic tremor at 15 20 km depth and volcanic tremor from kilauea although kilauea s seismic network may not be adequate to observe tectonic tremor because hawaii is seismically noisy and its crust is highly attenuating it is also possible that the specific fault conditions on kilauea s decollement are not conducive to such tremor generation

dynamic stresses coulomb failure and remote triggering dynamic stresses associated with crustal surface waves with 15 30 sec periods and peak amplitudes 5 km the latter is consistent with the observation that extensional or transtensional tectonic regimes are more susceptible to remote triggering by rayleigh wave dynamic stresses than compressional or transpressional regimes locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal volcanic systems

dynamic strains at regional and teleseismic distances abstract we develop formulas for the size of dynamic strains caused by seismic waves from an earthquake of given magnitude and distance these formulas include peak strain peak dissipated power and total dissipated energy and they are applicable at regional and teleseismic distances the formulas are fits to data from 89 large 6 5 m w 9 0 shallow earthquakes with source distances between 500 and 16 000 km recorded between 1977 and 2013 by three long base laser strainmeters at pinon flat observatory in southern california these strainmeters provide uniquely well calibrated measurements of tensor strain the residuals to the fits suggest that strain values can usually be predicted to within a factor of 2 these data also show that the strain tensor can be substantially different from that expected for plane waves in particular the extension perpendicular to the back azimuth which should be zero is always 20 or more of the extension along that azimuth how much the strains resemble those for plane waves depends on their path perhaps because inhomogeneities along different paths produce different amounts of multipathing the observed strains are systematically 10 30 larger at nearby laser strainmeter sites in the salton trough suggesting local amplification from inhomogeneous crustal structure online material tables of earthquake catalogs and dynamic strain analysis and figures of earthquake magnitude distance distributions and time delays of maximum strain

model of deep nonvolcanic tremor part i ambient and triggered tremor there is evidence of triggering of tremor by seismic waves emanating from distant large earthquakes the frequency content of triggered and ambient tremor are largely identical suggesting that tremor does not depend directly on the nature of the source we show here that the model of plate dynamics developed earlier by us is an appropriate tool for describing the onset of tremor in the framework of this model tremor is an internal response of a fault to failure triggered by external disturbances the model predicts generation of radiation in a frequency range defined by the fault parameters other specific features predicted are 1 the upper limit of the size of the emitting area is a few dozen kilometers 2 tremor accompanies earthquakes and aseismic slip 3 the frequency content of tremor depends on the type of failure the model also explains why a tremor has no clear impulsive phase in contrast to earthquakes a comparatively small effective normal stress hence a high fluid pressure is required to make the model consistent with observed tremor parameters our model indicates that tremor is not necessarily a superposition of low frequency earthquakes as commonly assumed although the latter may trigger tremor the approach developed complements the conventional viewpoint which assumes that tremor reflects a frictional process with low rupture speed essentially our model adds the hypothesis that resonant type oscillations exist inside a fault this addition may change our understanding of the nature of tremor in general and the methods of its identification and location in particular

remote triggering of non volcanic tremor around taiwan summary we perform a systematic survey of triggered deep non volcanic tremor beneath the central range cr in taiwan for 45 teleseismic earthquakes from 1998 to 2009 with mw 7 5 and epicentral distance 1000 km to the broad band station tpub triggered tremors are visually identified as bursts of high frequency 2 8 hz non impulsive and long duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves out of the 45 earthquakes we identified nine teleseismic events associated with nine tremor sources in the southern and five in the northern cr most of the tremor sources are located within the depth range of 15 25 km in the lower crust above the moho we find that the amplitudes of the surface waves play an important role in determining the triggering potential and the apparent triggering threshold is 0 1 cm s 1 or 7 8 kpa however such threshold is partially controlled by the background noise level which could prevent weaker tremor triggered by surface waves with smaller amplitudes from being identified the amplitudes of the triggered tremor show a positive correlation with the amplitudes of the triggering surface waves consistent with the predictions by the clock advance model in addition to amplitudes other factors such as frequency contents and incidence angles also affect the triggering potential we find that intermediate period 30 10 s surface waves could trigger modulate tremors suggesting that long period 30 s surface waves are not always required in long range triggering tremors appear to be triggered by both love and rayleigh waves when the incidence angles are parallel to the strike of the cr all six events triggered tremor primarily during the rayleigh waves for strike normal incidence only the 2001 mw7 8 kunlun earthquake showed predominant love wave triggering this observation can be qualitatively explained by a simple coulomb failure for a left lateral shear on the low angle detachment fault beneath the southern cr

spatiotemporal variations in the b value of earthquake magnitude frequency distributions classification and causes abstract interpretation of the b value of earthquake frequency magnitude distributions has received considerable attention in recent decades this paper provides a comprehensive review of previous investigations of spatial and temporal variations in b value including their classification and possible causes based on least squares regression of seismicity data compiled from the neic iris and isc catalogs we find an average value of 1 02 0 03 for the whole earth and its two hemispheres consistent with the general view that in seismically active regions the long term average value is close to unity nevertheless wide ranging b variations 0 3 b 2 5 have been reported in the literature this variability has been interpreted to arise from one or more of the following factors prevailing stress state crustal heterogeneity focal depth pore pressure geothermal gradient tectonic setting petrological environmental geophysical characteristics clustering of events incomplete catalog data and or method of calculation excluding the latter all of these factors appear to be linked directly or indirectly with the effective state of stress although time dependent changes in b value are well documented conflicting observations reveal either a precursory increase or decrease in b value before major earthquakes our compilation of published analyses suggests that statistically significant b variations occur globally on various timescales including annual monthly and perhaps diurnal taken together our review suggests that b variations are most plausibly linked with changes in effective stress

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

s wave triggering of tremor beneath the parkfield california section of the san andreas fault by the 2011 tohoku japan earthquake observations and theory the dynamic stresses that are associated with the energetic seismic waves generated by the m w 9 0 tohoku earthquake off the northeast coast of japan triggered bursts of tectonic tremor beneath the parkfield section of the san andreas fault saf at an epicentral distance of 8200 km the onset of tremor begins midway through the 100 s period s wave arrival with a minor burst coinciding with the shsh arrival as recorded on the nearby broadband seismic station pkd a more pronounced burst coincides with the love arrival followed by a series of impulsive tremor bursts apparently modulated by the 20 to 30 s period rayleigh wave the triggered tremor was located at depths between 20 and 30 km beneath the surface trace of the fault with the burst coincident with the s wave centered beneath the fault 30 km northwest of parkfield most of the subsequent activity including the tremor coincident with the shsh arrival was concentrated beneath a stretch of the fault extending from 10 to 40 km southeast of parkfield the seismic waves from the tohoku epicenter form a horizontal incidence angle of 14 with respect to the local strike of the saf computed peak dynamic coulomb stresses on the fault at tremor depths are in the 0 7 10 kpa range the apparent modulation of tremor bursts by the small strike parallel rayleigh wave stresses 0 7 kpa is likely enabled by pore pressure variations driven by the rayleigh wave dilatational stress these results are consistent with the strike parallel dynamic stresses s associated with the s shsh and surface wave phases triggering small increments of dextral slip on the fault with a low friction 0 2 the vertical dynamic stresses d do not trigger tremor with vertical or oblique slip under this simple coulomb failure model

slip acceleration generates seismic tremor like signals in friction experiments 1 since their discovery nearly a decade ago the origin of seismic tremor remains unclear recent studies indicate that various driving phenomena such as earth and ocean tides regional and teleseismic earthquakes enhance tremor activity observations of the coincidence with slow slip events and of fast migrations of tremors have led frictional slip to be considered as the possible source of tremors indeed laboratory friction experiments succeeded in generating and recording tremor like signals tls here we show a systematic correlation between the onset of slip acceleration and the emission of tls in a laboratory friction experiment tls are generated when the shear stress reaches the peak static resistance and the dilatancy meets its maximum that is when the mature interface is close to failure this robust result provides a comprehensive image of how natural seismic tremors might be generated and or triggered by passing seismic waves tides or even slow slip events

non volcanic tremor beneath the central range in taiwan triggered by the 2001 mw 7 8 kunlun earthquake summary non volcanic tremor nvt is an extended duration and non impulsive seismic signal observed away from volcanic regions although nvt has been found along the circum pacific subduction zones and the transform plate boundary in california it is not clear whether nvt occurs in other tectonic environments nvt is often associated with slow slip events and can be triggered instantaneously during the surface waves of teleseismic events however the underlying mechanisms of tremor generation remain mysterious here we show clear evidence of nvt beneath the central range cr in taiwan triggered by the 2001 m w 7 8 kunlun earthquake in northern tibet tremor occurs when the love wave displacement is to the southwest the lateral motion direction for the cr suggesting a simple frictional response to the driving force we hypothesize that tremor occurred on the weak basal detachment fault beneath the cr our observations indicate that tremor may exist at a wide range of active tectonic environments

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

remote triggering of tremor along the san andreas fault in central california received 30 august 2008 accepted 16 april 2009 published 18 july 2009 1 we perform a systematic survey of triggered tremor along the san andreas fault in central california for the 31 teleseismic earthquakes with mw 7 5 since 2001 we identify 10 teleseismic events associated with clear triggered tremor about 52 of the tremor is concentrated south of parkfield near cholame where ambient tremor has been identified previously and the rest is widely distributed in the creeping section of the san andreas fault north of parkfield tremor is generally initiated and is in phase with the love wave particle velocity however the pattern becomes complicated with the arrival of the rayleigh waves and sometimes tremor continues after the passage of the surface waves we identify two cases in which tremor is triggered during the teleseismic pkp phase these results suggest that while shear stress from the passage of the love waves plays the most important role in triggering tremor in central california other factors such as dilatational stresses from the rayleigh and p waves also contribute we also examine the ambient tremor occurrence rate before and after the teleseismic events and find a transient increase of stacked tremor rate during the passage of the teleseismic surface waves this observation implies that the occurrence time of tremor is temporally advanced by the dynamic stresses of the teleseismic waves the amplitude of the teleseismic waves correlates with the occurrence of triggered tremor and the inferred tremor triggering threshold is 2 3 kpa the relatively low triggering threshold indicates that the effective stress at the tremor source region is very low most likely due to near lithostatic fluid pressure

dynamic triggering of shallow earthquakes near beijing china summary we perform a comprehensive analysis of dynamic triggering around the babaoshan and huangzhuang gaoliying faults near beijing china the triggered earthquakes are identified as impulsive seismic arrivals with clear p and s waves in 5 hz high pass filtered three component velocity seismograms during the passage of large amplitude body and surface waves of large teleseismic earthquakes we find that this region was repeatedly triggered by four earthquakes in east asia including the 2001 mw 7 8 kunlun 2003 mw 8 3 tokachi oki 2004 mw 9 2 sumatra and 2008 mw 7 9 wenchuan earthquakes in most instances the microearthquakes coincide with the first few cycles of the love waves and more are triggered during the large amplitude rayleigh waves such an instantaneous triggering by both the love and rayleigh waves is similar to recent observations of remotely triggered non volcanic tremor along major plate boundary faults and can be explained by a simple coulomb failure criterion we are able to locate five of the earthquakes triggered by the kunlun and tokachi oki earthquakes these events occurred at shallow depth 5 km above the background seismicity near the boundary between nw striking babaoshan and huangzhuang gaoliying faults and the fangshan pluton we suggest that these triggered earthquakes occur near the transition between the velocity strengthening and weakening zones in the top few kilometres of the crust and are likely driven by relatively large dynamic stresses on the order of few tens of kpa

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

delayed and sustained remote triggering of small earthquakes in the san jacinto fault region by the 2014 mw 7 2 papanoa mexico earthquake we apply a move max matched filter method to detect heightened seismicity triggered in the san jacinto fault sjf zone by the 2014 mw 7 2 papanoa mexico earthquake the move max matched filter detects 5 4 and 1 7 times the number of earthquakes in the southern california seismic network catalog and those detected by the matched filter method respectively the seismicity rate increases significantly 3 5 hr after the passage of the teleseismic waves and persists above the background level for about 1 week this observation of delayed triggering may imply that dynamic stresses had initiated a time dependent advance to failure or a secondary process a highly active triggered patch is located 10 km west of the sjf near anza on a previously unmapped fault focal mechanisms and a best fit plane suggest a normal fault perpendicular to the sjf the unmapped fault may indicate higher seismic hazard in the surrounding areas if a large earthquake nucleated around the anza seismic gap

remote triggering of fault strength changes on the san andreas fault at parkfield a 20 year seismological record of from the parkfield area of the san andreas fault has been used to identify two occasions when long term changes in fault strength seem to have been induced remotely by large seismic events the 2004 sumatra andaman and the 1992 landers earthquakes in both cases two changes occurred that are consistent with variations in fault strength variation in the properties of seismic scatterers with time reflecting stress induced migration of fluids and systematic variation in the characteristics of repeating earthquake sequences these findings suggest that the largest earthquakes have a global influence on the strength of the earth s fault systems in support of that hypothesis an unusually large number of m 8 earthquakes occurred in the three years following the 2004 sumatra andaman earthquake changes in fault strength over time can increase or decrease the likelihood of failure and the ultimate triggering of seismic events observation of the parkfield area at the san andreas fault over the years 1987 2008 now reveals two occasions the 2004 sumatra andaman earthquake and the 1992 landers earthquake where long term changes in fault strength have probably been induced remotely by large seismic events fault strength is a fundamental property of seismogenic zones and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events although changes in fault strength have been suggested to explain various phenomena such as the remote triggering of seismicity1 there has been no means of actually monitoring this important property in situ here we argue that 20 years of observation 1987 2008 of the parkfield area at the san andreas fault have revealed a means of monitoring fault strength we have identified two occasions where long term changes in fault strength have been most probably induced remotely by large seismic events namely the 2004 magnitude m 9 1 sumatra andaman earthquake and the earlier 1992 m 7 3 landers earthquake in both cases the change possessed two manifestations temporal variations in the properties of seismic scatterers probably reflecting the stress induced migration of fluids and systematic temporal variations in the characteristics of repeating earthquake sequences that are most consistent with changes in fault strength in the case of the 1992 landers earthquake a period of reduced strength probably triggered the 1993 parkfield aseismic transient2 3 4 5 as well as the accompanying cluster of four m 4 earthquakes at parkfield the fault strength changes produced by the distant 2004 sumatra andaman earthquake are especially important as they suggest that the very largest earthquakes may have a global influence on the strength of the earth s fault systems as such a perturbation would bring many fault zones closer to failure it should lead to temporal clustering of global seismicity this hypothesis seems to be supported by the unusually high number of m 8 earthquakes occurring in the few years following the 2004 sumatra andaman earthquake

tremor bands sweep cascadia 1 during the slow slip events in cascadia and japan a well documented feature of nonvolcanic tremor nvt is its puzzling slow along strike migration but the cause possible implications and underlying physics of this long term tremor migration and its relationship with slow slip remain elusive here we use tremor data recorded by a dense seismic array in cascadia and apply a beam backprojection technique to map the spatiotemporal evolution of tremor with high resolution it reveals that elongated slip parallel bands of tremor activity illuminate the slipping part of the plate interface over the time scale of several hours and sweep cascadia along strike from south to north we propose that small changes in static stress due to slow slip in a section of the fault cause slip and associated nvt activity in the adjacent section and the resulting progressive along strike transfer of stress is responsible for the long term tremor migration during a slow slip event

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

dynamic triggering of high frequency bursts by strong motions during the 2004 parkfield earthquake sequence high pass filtering 20 hz of acceleration records from the usgs parkfield dense seismograph array upsar reveals a series of bursts that occur only during strong shaking from the 2004 mw6 parkfield california earthquake and its immediate aftershocks because there is no correlation between these high frequency bursts observed at closely spaced stations we hypothesize that they are associated with dynamically triggered events occurring within 20 meters of the stations in the highly fractured shallow crust the triggering threshold was found to be 0 02 mpa consistent with a previous estimate based on a similar analysis of high frequency bursts observed in strong motion data from the 1999 chi chi earthquake in taiwan fischer et al 2008 the consistent observation of high frequency bursts at both parkfield and taiwan suggest that they may be a common phenomenon associated with strong motion in the very shallow crust

complex nonvolcanic tremor near parkfield california triggered by the great 2004 sumatra earthquake 1 in several instances the passing surface waves from large earthquakes have ignited nonvolcanic tremor nvt on major faults still the mechanism of tremor and its reaction to the dynamic stressing from various body and surface waves is poorly understood we examine tremor near parkfield california beneath the san andreas fault triggered by the mw 9 2 2004 sumatra earthquake the prolonged shaking produces the richest and the most varied observations of dynamically triggered tremor to date the tremor appears in at least three distinct locations and shows activity pulsing with encouraging stress as has been observed in other cases the greatest amount of triggering and tremor modulation accompanies the long period love waves rayleigh waves on the other hand appear to be less effective in exciting tremor sources also at times the tremor stops before the surface waves are complete at other times it continues quivering after the waves have passed while tremor is found to be sensitive to small stress changes there are times when stresses of comparable magnitudes do not trigger noticeable tremor some tremors in this nvt sequence appear to be associated with the passage of p waves which is unusual and surprising given the small stresses they impart

tectonic tremor on vancouver island cascadia modulated by the body and surface waves of the mw 8 6 and 8 2 2012 east indian ocean earthquakes author s kundu b ghosh a mendoza m burgmann r gahalaut vk saikia d abstract 2016 american geophysical union all rights reserved the 2012 east indian ocean earthquake mw 8 6 so far the largest intraoceanic plate strike slip event ever recorded modulated tectonic tremors in the cascadia subduction zone the rate of tremor activity near vancouver island increased by about 1 5 times from its background level during the passage of seismic waves of this earthquake in most cases of dynamic modulation large amplitude and long period surface waves stimulate tremors however in this case even the small stress change caused by body waves generated by the 2012 earthquake modulated tremor activity the tremor modulation continued during the passage of the surface waves subsequent to which the tremor activity returned to background rates similar tremor modulation is observed during the passage of the teleseismic waves from the mw 8 2 event which occurs about 2 h later near the mw 8 6 event we show that dynamic stresses from back to back large teleseismic events can strongly influence tremor sources

strong tremor near parkfield ca excited by the 2002 denali fault earthquake 1 we show clear evidence of non volcanic tremor triggered by 2002 mw7 8 denali fault earthquake near parkfield triggered tremor is identified as bursts of high frequency 2 8 hz non impulsive seismic energy whose envelope is coherent among many stations and has the same periodicity as the passing surface waves the tremor originates from at least three hypocenters near the san andreas fault with differing frictional regimes two in the creeping section and the other where the san andreas is transitional between creeping and locked all the sources originate below the seismogenic zone suggesting that transitional frictional properties are necessary conditions for tremor generation tremor is excited by the love waves when the san andreas is sheared in a right lateral sense encouraging slip and is absent when the san andreas is sheared in a left lateral sense consistent with a simple frictional response to the driving stress

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

lack of additional triggered tectonic tremor around the simi valley and the san gabriel mountain in southern california abstract we conduct a systematic search of tectonic tremor triggered by distant large earthquakes around the simi valley sv and the san gabriel mountain sgm in southern california out of 59 large earthquakes between 2000 and 2013 only the 2002 m w 7 9 denali fault earthquake triggered clear tremor in the region the observed travel times of the triggered tremors are consistent with theoretical predictions from tremor sources that are spatially clustered in the sv close to the rupture zone of the 1994 m w 6 7 northridge earthquake we also estimate the triggering stress threshold as 12 kpa from measuring the peak ground velocities near the tremor source the lack of clear tremor beneath the sgm provides a negative example for a region where tremor is expected to occur because of clear evidence of fluid rich zones at the middle crust the results imply that the necessary conditions for tremor to occur are more than fluid induced low effective normal stress online material figures of additional tremor inspection tables of earthquake catalogs and tremor locations

the 2004 sumatra earthquake and tsunami lessons learned in subduction zone science and emergency management for the cascadia subduction zone the 26 december 2004 mw 9 3 sumatra earthquake and tsunami was a pivotal turning point in our awareness of the dangers posed by subduction zone earthquakes and tsunamis this earthquake was the world s largest in 40 years and it produced the world s deadliest tsunami this earthquake ruptured a subduction zone that has many similarities to the cascadia subduction zone in this article i summarize lessons learned from this tragedy and make comparisons with potential rupture characteristics slip distribution deformation patterns and aftershock patterns for cascadia using theoretical modeling and interseismic observations both subduction zones are approximately 1 100 1 300 km in length both have similar convergence rates and represent oblique subduction slip along the subduction fault during the 26 december earthquake is estimated at 15 25 m similar to values estimated for cascadia the width of the rupture 80 150 km estimated from modeling seismic and geodetic data is similar to the width of the locked and transition zone estimated for cascadia coseismic subsidence of up to 2 m along the sumatra coast is also similar to that predicted for parts of northern cascadia based on paleoseismic evidence in addition to scientific lessons learned the 2004 tsunami provided many critical lessons for emergency management and preparedness as a result of that tragedy a number of preparedness initiatives are now underway to promote awareness of earthquake and tsunami hazards along the west coast of north america and plans are underway to develop prototype tsunami and earthquake warning systems along cascadia lessons learned from the great sumatra earthquake and tsunami tragedy both through scientific studies and through public education initiatives will help to reduce losses during future earthquakes in cascadia and other subduction zones of the world

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

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

evidence suggesting fluid flow beneath japan due to periodic seismic triggering from the 2004 sumatra andaman earthquake 1 we show that very large surface waves from the 2004 sumatra andaman earthquake m9 2 triggered deep low frequency dlf seismic tremors beneath japan over a 500 km wide region the triggered seismicity was periodically excited synchronized with the amplitudes and phases of the oscillatory surface waves incident on the region periodic excitation implies that the specific phase of the surface waves plays an important role for the triggering estimation of stress changes in the source regions showed that the volumetric dilatation larger than 10 8 was the predominant factor for triggering thereby strongly supporting the hypothesis that dlf events are caused by processes related to fluid flow

tidal triggering of low frequency earthquakes near parkfield california implications for fault mechanics within the brittle ductile transition 1 studies of nonvolcanic tremor nvt have established the significant impact of small stress perturbations on nvt generation here we analyze the influence of the solid earth and ocean tides on a catalog of 550 000 low frequency earthquakes lfes distributed along a 150 km section of the san andreas fault centered at parkfield lfe families are identified in the nvt data on the basis of waveform similarity and are thought to represent small effectively co located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km we calculate the sensitivity of each of these 88 lfe families to the tidally induced right lateral shear stress rlss fault normal stress fns and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity lfe occurrence is most strongly modulated by fluctuations in shear stress with the majority of families demonstrating a correlation with rlss at the 99 confidence level or above producing the observed lfe rate modulation in response to shear stress perturbations requires low effective stress in the lfe source region there are substantial lateral and vertical variations in tidal shear stress sensitivity which we interpret to reflect spatial variation in source region properties such as friction and pore fluid pressure additionally we find that highly episodic shallow lfe families are generally less correlated with tidal stresses than their deeper continuously active counterparts the majority of families have weaker or insignificant correlation with positive tensile fns two groups of families demonstrate a stronger correlation with fault normal tension to the north and with compression to the south of parkfield the families that correlate with fault normal clamping coincide with a releasing right bend in the surface fault trace and the lfe locations suggesting that the san andreas remains localized and contiguous down to near the base of the crust the deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth synthesizing our observations with those of other lfe hosting localities will help to develop a comprehensive understanding of transient fault slip below the seismogenic zone by providing constraints on parameters in physical models of slow slip and lfes

detection of triggered deep low frequency events from the 2003 tokachi oki earthquake 1 we show the level of increased decreased seismicity mapped across japan immediately following the 2003 tokachi oki earthquake mw 8 1 of september 25 2003 ut by use of the statistic significant dynamic triggering of events is found near the nankai subduction zone 900 1400 km from the earthquake the events are inferred to be dynamically triggered deep low frequency earthquakes of a type that often occur in this region the triggering appears to be caused by the long period oscillations of the surface waves which may be effective in triggering fluid related events the region that showed the highest level of triggered events was an area of elevated activity in the bungo strait where a slow slip event was in progress

widespread triggering of nonvolcanic tremor in california we identified seven locations on or near the transform plate boundary in california where nonvolcanic tremor was triggered by the 2002 denali earthquake this result implies that the conditions essential for nonvolcanic tremor exist in a range of tectonic environments models explaining tremor typically require conditions endemic to subduction zones that is high temperatures and fluid pressures because previously tremor was nearly exclusively documented in subduction zones the absence of tremor in geothermal areas is inconsistent with such models additionally we found no correlation between creeping or locked faults and tremor contrary to predictions of frictional models of tremor

surface wave potential for triggering tectonic nonvolcanic tremor source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle is anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction 0 2 however documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan is associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

mysterious tremor like signals seen on the reelfoot fault northern tennessee abstract a phased array of 19 broadband seismometers was deployed from november 2009 to september 2011 in an effort to detect nonvolcanic tremor or tectonic tremor associated with the reelfoot fault northern tennessee an autodetection algorithm using broadband frequency wavenumber analysis was used to search for the recurrence of signals first reported during an active source experiment in 2006 the original signals appeared as short duration impulsive arrivals with a high phase velocity ranging from 3 to 25 km s we have identified thousands of similar signals on the 2 year long array data two distinct detection peaks are observed with event azimuths from the west and northeast the detections are most similar to the events seen in 2006 and are inferred to come from very small m l 1 microearthquakes that occur in the shallow basement on faults adjacent to the reelfoot fault these include detections with coherent s wave energy that reinforce the interpretation of very small local and regional events other signals detected show distinct changes in slowness and azimuth as a function of time these events were interpreted as atmospheric acoustic sources the high frequency content and impulsive arrivals of the nonacoustic arrivals are not consistent with tectonic tremor as seen in other parts of the world but do indicate seismic activity in the crust near the reelfoot thrust fault that was previously unknown

remotely triggered earthquakes in south central tibet following the 2004 mw 9 1 sumatra and 2005 mw 8 6 nias earthquakes