coseismic and postseismic slip from the 2003 san simeon earthquake and their effects on backthrust slip and the 2004 parkfield earthquake 1 the 2003 m6 5 san simeon earthquake occurred less than a year before and 50 km west of the long awaited parkfield earthquake of 2004 aftershocks of the san simeon event occurred not only on the presumptive rupture plane but also delineated subconjugate structures to the east backthrusts insar data show a complex pattern of deformation during the postseismic time interval and suggest that shallow postseismic slip may have occurred on the backthrust structures in this paper we present a model inversion for slip during and after the earthquake from four interferograms and gps data we find that the data are well fit when coseismic slip is allowed on two planes the main rupture plane and a backthrust structure to the south the backthrust structure accommodates slip amounting to about one third of the coseismic moment release shallow postseismic slip also occurred on the main rupture plane updip from the coseismic slip and on the southern backthrust postseismic slip on the more northern backthrust structure is not well constrained and the data is fit nearly as well without including it the areas of postseismic slip on the backthrusts correspond well with areas that experienced unclamping and little slip is found in regions of clamping no value for the coefficient of friction below 1 0 results in a coulomb failure stress field that matches the aseismic slip distributions better than the normal stress alone we also investigate whether static stress changes from the san simeon earthquake acted to encourage right lateral strike slip on the parkfield segment of the san andreas fault we find that the san simeon earthquake s rupture planes were oriented in such a way that the maximum increase in coulomb failure stress on the san andreas fault occurred on the parkfield segment the 2004 parkfield earthquake differed from its expected behavior by nucleating in the southern portion of the parkfield segment an area that experienced increased shear stress of 0 1 0 14 bars from coseismic slip in the san simeon earthquake postseismic slip from san simeon further loaded the southern portion of the parkfield segment preferentially over the northern portion though the size of the stress changes was small 0 03 0 04 bars

modeling low frequency earthquake recurrence patterns summary tectonic tremor and low frequency earthquakes lfe have been identified globally but the physical mechanisms responsible for different types of lfe recurrence patterns are still elusive here we use a brittle ductile friction model to study the frictional conditions leading to different lfe recurrence patterns observed in central california we do a comprehensive search of the friction parameters including the brittle contact failure length and ductile damping strength to generate synthetic lfe bursts that match three different occurrence patterns of lfe bursts we find that the output synthetic lfe burst recurrence type varies under different frictional conditions the chaotic recurrence lfe sources require a higher brittle to ductile friction ratio consistent with the observation that they are mainly at shallower part of the brittle ductile transition zone the tri modal recurrence lfe sources require a wide distribution of asperity sizes indicating that the segment of deep saf between monarch peak and parkfield is likely to be more fragmented and weaker than the surrounding segments

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

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

temporal changes in attenuation associated with the 2004 m6 0 parkfield earthquake 1 elevated seismic attenuation is often observed in fault zones due to the high degree of fracturing and fluid content however temporal changes in attenuation at the time of an earthquake are poorly constrained but can give indications of fracture damage and healing in this study spectral ratios between earthquakes within repeating clusters are calculated in an attempt to resolve temporal variations in attenuation at the time of the 2004 m6 0 parkfield earthquake a sharp increase in attenuation is observed immediately after the earthquake which then decays over the next 2 years influences of intercluster magnitude variations time window length and previously reported postseismic velocity changes are investigated the postseismic decay is fit by a logarithmic function the timescale of the decay is found to be similar to that in gps data and ambient seismic noise velocities following the 2004 m6 0 parkfield earthquake the amplitude of the attenuation change corresponds to a decrease of approximately 10 in qp at the time of the earthquake the greatest changes are recorded on the northeast of the fault trace consistent with preferential damage in the extensional quadrant behind a north westerly propagating rupture tip our analysis suggests that significant changes in seismic attenuation and hence fracture dilatancy during coseismic rupture are limited to depths of less than about 5 km

deep low frequency tremors as a proxy for slip monitoring at plate interface 1 we propose a new method to monitor slip at the plate interface using non volcanic deep low frequency dlf tremors we assume that a dlf tremor is the superposition of frequently excited intermittent events meaning that the envelope of the reduced displacement of the dlf tremor provides an apparent moment rate function we estimate a conversion factor from the apparent moment to the seismic moment with an assumption that a total size of dlf tremors of an episode is proportional to the size of corresponding slow slip event sse the cumulative seismic moment estimated by dlf tremors is consistent with that estimated from geodetic methods and provides appropriate slip and slip rate at the plate interface this proves our assumptions and demonstrates that dlf tremors are useful tool for real time monitoring of the slip at the plate interface

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

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

the frequency size scaling of non volcanic tremors beneath the san andreas fault at parkfield possible implications for seismic energy release abstract we analyse the frequency size distribution of non volcanic tremors observed along the parkfield section of the san andreas fault we suggest that these non volcanic tremors follow a power law scaling typical of scale invariant stick slip tectonic earthquakes but with an unusually high scaling exponent of more than 2 0 and a systematic depth dependency while each individual non volcanic tremor releases only a minuscule amount of energy and slip this is more than compensated by their sheer numbers consequently the integrated contribution of this largely invisible seismicity non volcanic tremors and nano earthquakes is non negligible and could potentially account in selected patches along the san andreas fault for up to 100 of the plate motion

short term slow slip and correlated tremor episodes in the tokai region central japan 1 we observe the related phenomenon of short term slow slip event sse and low frequency nonvolcanic tremor on the deeper extension of the 1944 tonankai and the anticipated tokai earthquake rupture areas central japan japan nation wide seismometer and tiltmeter array nied hi net has recorded these correlated episodes of sse and tremor known as episodic tremor and slip ets in 2004 and 2005 the events are located on two areas in the tokai region one is near the shima peninsula and the other is around the aichi area both episodes have a typical duration of 2 3 days and released seismic moment corresponding to mw 6 0 moreover these episodes repeatedly occur with a recurrence interval of about six months in both areas this evidence suggests that the coincidence between the sse and the tremor is a characteristic behavior in the whole tremor belt like zone in the nankai trough subduction zone

tectonic tremor and deep slow slip on the alpine fault 1 tectonic tremor is characterized by persistent low frequency seismic energy seen at major plate boundaries although predominantly associated with subduction zones tremor also occurs along the deep extension of the strike slip san andreas fault here we present the first observations of tectonic tremor along new zealand s alpine fault a major transform boundary that is late in its earthquake cycle we report tectonic tremor that occurred on the central section of the alpine fault on 12 days between march 2009 and october 2011 tremor hypocenters concentrate in the lower crust at the downdip projection of the alpine fault coincide with a zone of high p wave attenuation low qp and bright seismic reflections occur in the 25 45 km depth range below the seismogenic zone and may define the deep plate boundary structure extending through the lower crust and into the upper mantle we infer this tremor to represent slow slip on the deep extent of the alpine fault in a fluid rich region marked by high attenuation and reflectivity these observations provide the first indication of present day displacement on the lower crustal portion of the australia pacific transform plate boundary

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

triggering of tremor and inferred slow slip by small earthquakes at the nankai subduction zone in southwest japan the correlation of earthquakes with tremor and slow slip has not been clearly quantified we investigate 12 year earthquake and tremor catalogs for southwest japan and find that nearby small intraslab earthquakes are weakly correlated with tremor in particular the intraslab earthquakes with magnitudes 2 7 tend to be followed by tremor more often than expected at random by a factor of 2 to 6 the excess number of tremor before earthquakes is not as significant although marginally more than expected the underlying physical mechanism of the observed triggering of tremor and inferred slow slip by earthquakes is most likely to be the dynamic stress changes several to several tens of kilopascals rather than the much smaller static stress changes the rate of triggering of tremor by earthquakes is similar to although somewhat lower than rates observed for similar amplitude stress changes due to the lower frequency teleseismic surface waves and tidal stressing

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

structural explanation for low creep rates on the san andreas fault near monarch peak central california comparison of fault slip throughout most of the central creeping section of the san andreas fault zone indicates that near fault about 100 m aperture and intermediate scale about 1 to 2 km aperture geodetic measurements of right lateral slip are in good agreement an exception is present near monarch peak near the center of the creeping section here near fault measuring techniques yield no more than two thirds of the slip rate determined by intermediate scale measurements anomalously low near fault creep at this site has continued during 10 yr of measurement detailed geologic mapping shows the presence of geologically youthful subsidiary faults along an area about 7 km long and parallel with the main trace centered on monarch peak we infer that the subsidiary faults are active taking up the difference in horizontal offset if the subsidiary faults are in fact active it makes the structural setting near monarch peak anomalous in the creeping section of the fault zone

synchronous low frequency earthquakes and implications for deep san andreas fault slip a b s t r a c t low frequency earthquakes lfes are slip events that occur repeatedly at source locations within the lower crust lfes and the associated seismic broadcast known as tremor have been observed in a diverse array of tectonic environments here we develop a suite of statistical tools to conduct a systematic study of the spatial and temporal correlations of the event occurrence patterns of the 88 lfe sources beneath the greater parkfield section of the san andreas fault we first examine correlations in the occurrence patterns on long time scales to show that the regions to the north and south of parkfield behave independently we next use the cumulative event signatures of each source to characterize the individual occurrence patterns on shorter time scales through application of a statistical clustering algorithm we demonstrate that individual lfe sources form spatially coherent clusters that may represent localized elastic structures or asperities on the deep fault interface we conclude by examining the fine scale features of the event rates within the lfe occurrence patterns through quantitative comparison to analogous laboratory shear experiments on granular fault gouge like materials we infer that the distinctive features of lfe occurrence patterns reflect variations in the in situ stress and frictional conditions at the individual lfe source locations these observations provide a framework to understand the spatial and temporal diversity of fault slip that occurs within the lower crust beneath parkfield and that may influence seismic hazard in the region 2015 elsevier b v all rights reserved

heat capacity and thermodynamic functions for gehlenite and staurolite with comments on the schottky anomaly in the heat capacity of staurolite

postearthquake relaxation after the 2004 m6 parkfield california earthquake and rate and state friction 1 an unusually complete set of measurements including rapid rate gps over the first 10 days of postseismic deformation is available at 12 continuous gps stations located close to the epicenter of the 2004 m6 0 parkfield earthquake the principal component modes for the relaxation of the ensemble of those 12 gps stations were determined the first mode alone furnishes an adequate approximation to the data thus the relaxation at all stations can be represented by the product of a common temporal function and distinct amplitudes for each component north or east of relaxation at each station the distribution in space of the amplitudes indicates that the relaxation is dominantly strike slip the temporal function which spans times from about 5 min to 900 days postearthquake can be fit by a superposition of three creep terms each of the form l loge 1 t l with characteristic times l 4 06 0 11 and 0 0001 days it seems likely that what is actually involved is a broad spectrum of characteristic times the individual components of which arise from afterslip on different fault patches perfettini and avouac 2004 have shown that an individual creep term can be explained by the spring slider model with rate dependent no state variable friction the observed temporal function can also be explained using a single spring slider model i e single fault patch that includes rate and state dependent friction a single state variable and either of the two commonly used aging and slip state evolution laws in the latter fits the rate and state friction parameter b is negative

a comparison of mechanisms generating vertical transport in the brazilian coastal upwelling regions 1 several mechanisms can drive vertical velocities in the coastal ocean including wind forcing and through gradients in the vorticity field generated by flow topography interactions a two layer steady wind driven analytical model is applied to the major upwelling systems of brazil cabo frio cf and cabo de santa marta csm regions comparisons are made between the relative roles of wind and flow topography interaction in inducing upwelling over these regions ekman pumping is the weakest mechanism over the shelf but does influence the along shelf temperature in the cf area away from coastline irregularities wind driven upwelling ekman transport dominates over all mechanisms however in the vicinity of capes and coastal features topographically driven upwelling plays a significant role and its transports may vary from 43 to 94 of wind driven upwelling upstream of capes topographically driven vertical motions are downwelling favorable and act against the wind driven coastal upwelling while downstream of capes they are upwelling favorable where all mechanisms add up to create strong upwelling peaks in total upwelling in the cf region are about twice as large as those in the csm area because the cf region has stronger winds and larger coastline perturbations than in the csm region observed sea surface temperatures sst agree well with variability in the vertical transports where upwelling peaks are in phase with low temperature peaks along the coast results suggest that on larger scales the sst variability along the coast is mainly controlled by wind driven upwelling while upwelling due to flow topography interaction is responsible for the smaller scale sst variability

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

a mixed seismic aseismic stress release episode in the andean subduction zone in subduction zones stress is released by earthquakes and transient aseismic slip the latter falls into two categories slow slip and afterslip slow slip events emerge spontaneously during the interseismic phase and show a progressive acceleration of slip with a negligible contribution of synchronous tremors or microseismicity to the energy or moment release1 2 3 4 5 6 7 8 9 10 11 12 in contrast afterslip occurs immediately after large and moderate earthquakes decelerates over time and releases between 20 and 400 of the moment released by the preceding earthquake13 14 15 16 17 18 here we use seismic and gps data to identify transient aseismic slip that does not fit into either of these categories we document a seismic aseismic slip sequence which occurred at shallow depths along a weakly coupled part of the andean subduction zone19 in northern peru and lasted seven months the sequence generated several moderate earthquakes that together account for about 25 of the total moment released during the full sequence equivalent to magnitude 6 7 transient slip immediately followed two of the earthquakes with slip slowing at a logarithmic rate considered separately the moment released by transient slip following the second earthquake was more than 1 000 of the moment released during the earthquake itself a value incompatible with classical models of afterslip synchronous seismic swarms and aseismic slip may therefore define a stress release process that is distinct from slow slip events and afterslip

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

continuous borehole strain and pore pressure in the near field of the 28 september 2004 m 6 0 parkfield california earthquake implications for nucleation fault response earthquake prediction and tremor near field observations of high precision borehole strain and pore pres sure show no indication of coherent accelerating strain or pore pressure during the weeks to seconds before the 28 september 2004 m 6 0 parkfield earthquake minor changes in strain rate did occur at a few sites during the last 24 hr before the earth quake but these changes are neither significant nor have the form expected for strain during slip coalescence initiating fault failure seconds before the event strain is stable at the 10 11 level final prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 10 12 nm m 2 2 and a source size less than 30 m ground displacement data indicate similar constraints localized rupture nucleation and runaway precludes useful prediction of damaging earthquakes coseismic dynamic strains of about 10 microstrain peak to peak were superimposed on volumetric strain offsets of about 0 5 microstrain to the northwest of the epicenter and about 0 2 microstrain to the southeast of the epicenter consistent with right lateral slip observed strain and global positioning system gps offsets can be simply fit with 20 cm of slip between 4 and 10 km on a 20 km segment of the fault north of gold hill m 0 7 10 17 n m variable slip inversion models using gps data and seismic data indicate similar moments observed postseismic strain is 60 to 300 of the coseismic strain indicating incomplete release of accumulated strain no measurable change in fault zone compliance preceding or following the earthquake is indicated by stable earth tidal response no indications of strain change accompany nonvolcanic tremor events reported prior to and following the earthquake

temporal variation of tectonic tremor activity in southern taiwan around the 2010 ml6 4 jiashian earthquake deep tectonic tremor which is extremely sensitive to small stress variations could be used to monitor fault zone processes during large earthquake cycles and aseismic processes before large earthquakes in this study we develop an algorithm for the automatic detection and location of tectonic tremor beneath the southern central range of taiwan and examine the spatio temporal relationship between tremor and the 4 march 2010 ml6 4 jiashian earthquake located about 20 km from active tremor sources we find that tremor in this region has a relatively short duration short recurrence time and no consistent correlation with surface gps data we find a short term increase in the tremor rate 19 days before the jiashian mainshock and around the time when the tremor rate began to rise one gps station recorded a flip in its direction of motion we hypothesize that tremor is driven by a slow slip event that preceded the occurrence of the shallower jiashian mainshock even though the inferred slip is too small to be observed by all gps stations our study shows that tectonic tremor may reflect stress variation during the pre nucleation process of a nearby earthquake

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

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

locating nonvolcanic tremors beneath the san andreas fault using a station pair double difference location method 1 it has been a challenging task to locate nonvolcanic tremors because of the lack of impulsive wave arrivals a station pair double difference dd location method is developed to determine absolute tremor locations by directly using the station pair travel time differences measured from cross correlating tremor waveform envelopes multiple tremors are located together for inverting for station corrections to take into account velocity model inaccuracy the new method is applied to tremors in the parkfield region of central california between 27 july 2001 and 21 february 2009 compared to the tremor catalog locations determined from a grid search location method most of the newly located tremors are located at depths between 20 and 35 km well below the seismogenic zone in the area the tremors beneath cholame ca are more clearly separated into two zones laterally distributed across the san andreas fault with most tremors occurring to the southwest and exhibiting a periodic pattern of occurrence the new tremor locations help better delineate the spatial and temporal distribution of tremor activity and therefore are helpful for better understanding tremor origin and process

temporal variations in crustal scattering structure near parkfield california using receiver functions abstract we investigate temporal variations in teleseismic receiver functions using 11 yr of data at station pkd near parkfield california by stacking power spectral density psd functions within 12 month windows we find that psd levels for both radial and transverse components drop by 5 db following the 2003 san simeon m 6 5 earthquake with a persistent reduction in background levels of 2 db relative to the pre 2003 levels after the 2004 parkfield m 6 earthquake corresponding to an estimated decrease in shear wave velocity of 0 12 and respectively or equivalent negative changes in poisson s ratio of 0 02 and 0 01 our results suggest that the perturbation originates at middle to lower crustal levels possibly caused by the redistribution of crustal pore fluids consistent with increased and sustained tremor activity near parkfield following both earthquakes this study shows that we can resolve temporal variations in crustal scattering structure near a major seismogenic fault using the receiver function method

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

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

nonvolcanic tremor evolution and the san simeon and parkfield california earthquakes nonvolcanic tremors occur adjacent to locked faults and may be closely related to the generation of earthquakes monitoring of the san andreas fault in the parkfield california region revealed that after two strong earthquakes tremor activity increased in a nearly dormant tremor zone increased and became periodic in a previously active zone and has remained elevated and periodic for over 4 years static shear and coulomb stress increases of 6 to 14 kilopascals from these two earthquakes are coincident with sudden increases in tremor rates the persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the san andreas fault which ruptured in the moment magnitude 7 8 ft tejon earthquake of 1857

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

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

episodic tremors and slip in cascadia in the framework of the frenkel kontorova model received 27 august 2010 revised 20 october 2010 accepted 28 october 2010 published 13 january 2011 1 the seismic moment for regular earthquakes is proportional to the cube of rupture time a second class of phenomena collectively called slow earthquakes has very different scaling we propose a model inspired from the phenomenology of dislocation dynamics in crystals that is consistent with the scaling relations observed in the cascadia episodic tremor and slip ets events two fundamental features of ets are periodicity and migration in the northern cascadia subduction zone ets events appear every 14 5 months or so during these events tremors migrate along strike with a velocity of 10 km day and simultaneously zip back and forth in the relative plate motion direction with at ypical velocity of 50 km h our model predicts the formation of a sequence of slip pulses on the boundary of the plates which describes the major features of fault dynamics including periodicity and the migration pattern of tremors citation gershenzon n i g bambakidis e hauser a ghosh and k c creager 2011 episodic tremors and slip in cascadia in the framework of the frenkel kontorova model geophys res lett 38 l01309

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

foreshocks and aftershocks of the great 1857 california earthquake the san andreas fault is the longest fault in california and one of the longest strike slip faults anywhere in the world yet we know little about many aspects of its behavior before during and after large earthquakes we conducted a study to locate and to estimate magnitudes for the largest foreshocks and aftershocks of the 1857 m 7 9 fort tejon earthquake on the central and southern segments of the fault we began by searching archived first hand accounts from 1857 through 1862 by grouping felt reports temporally and by assigning modified mercalli intensities to each site we then used a modified form of the grid search algorithm of bakun and wentworth derived from empirical analysis of modern earthquakes to find the location and magnitude most consistent with the assigned intensities for each of the largest events the result confirms a conclusion of sieh that at least two foreshocks dawn and sunrise located on or near the parkfield segment of the san andreas fault preceded the mainshock we estimate their magnitudes to be m 6 1 and m 5 6 respectively the aftershock rate was below average but within one standard deviation of the number of aftershocks expected based on statistics of modern southern california mainshock aftershock sequences the aftershocks included two significant events during the first eight days of the sequence with magnitudes m 6 25 and m 6 7 near the southern half of the rupture later aftershocks included a m 6 event near san bernardino in december 1858 and a m 6 3 event near the parkfield segment in april 1860 from earthquake logs at fort tejon we conclude that the aftershock sequence lasted a minimum of 3 75 years

tremor reveals stress shadowing deep postseismic creep and depth dependent slip recurrence on the lower crustal san andreas fault near parkfield 1 the 2003 magnitude 6 5 san simeon and the 2004 magnitude 6 0 parkfield earthquakes induced small but significant static stress changes in the lower crust on the central san andreas fault where recently detected tectonic tremor sources provide new constraints on deep fault creep processes we find that these earthquakes affect tremor rates very differently consistent with their differing transferred static shear stresses the san simeon event appears to have cast a stress shadow north of parkfield where tremor activity was stifled for 3 6 weeks in contrast the 2004 parkfield earthquake dramatically increased tremor activity rates both north and south of parkfield allowing us to track deep postseismic slip following this event rates initially increased by up to two orders of magnitude for the relatively shallow tremor sources closest to the rupture with activity in some sources persisting above background rates for more than a year we also observe strong depth dependence in tremor recurrence patterns with shallower sources generally exhibiting larger less frequent bursts possibly signaling a transition toward steady creep with increasing temperature and depth

delayed dynamic triggering of deep tremor along the parkfield cholame section of the san andreas fault following the 2014 m6 0 south napa earthquake large distant earthquakes are known to trigger deep tectonic tremor along the san andreas fault and in subduction zones however there are relatively few observations of triggering from regional distance earthquakes here we show that a small tremor episode about 12 18 km nw of parkfield was triggered during and immediately following the passage of surface waves from the 2014 mw 6 0 south napa main shock more notably a major tremor episode followed beginning about 12 h later and centered se of parkfield near cholame this major episode is one of the largest seen over the past several years containing intense activity for 3 days and taking more than 3 weeks to return to background levels this episode showed systematic along strike migration at 5 km d suggesting that it was driven by a slow slip event our results suggest that moderate size earthquakes are capable of triggering major tremor and deep slow slip at regional distances

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

regional seismicity a potential pitfall for identification of long period long duration events long period long duration lpld events are tremorlike signals that have been observed during monitoring of hydraulic fracture treatment programs lpld events have been interpreted to reflect slow deformation processes on fractures or faults that are misoriented for reactivation with respect to the present day stress field regional earthquakes could easily be mistaken for lpld events because both are characterized by similar frequency content 100 hz and duration 1 min using data from a 10 5 month continuous downhole deployment of a 15 hz geophone array in a tight sand gas field in western canada we compared recordings of small earthquakes with previously published lpld events we determined that regional earthquakes can show similar waveform characteristics to lpld events underscoring the importance of distinguishing regional earthquake signals from lpld events to ensure robust interpretation of reservoir deformation processes

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