evolution of shallow crustal thermal structure from subduction to collision an example from taiwan we study crustal thermal evolution by examining heat flow patterns along a convergent boundary from a young subduction zone to a more structurally mature collision zone more than 8000 km of seismic profiles covering an offshore region of 45 000 km 2 in southern taiwan show widespread bottom simulating reflectors bsrs we derived 1107 bsr based heat flows before combining 42 additional published offshore thermal probe data and 86 on land heat flow data to document the shallow forearc thermal structures from the subduction zone to the collision zone in the subduction zone the geothermal gradient ranges mostly within 30 80 c km and decreases toward the arc due to slab cooling intensive dewatering at the toe sediment blanketing topographic effects and other processes the geothermal gradient ranges mostly from 30 to 90 c km in the collision zone and increases instead of decreases toward the arc possibly caused by exhumation erosion topographically induced ground water circulation and some upper mantle processes related to collision heat flow in the collision zone ranges from 80 to 250 mw m 2 the high heat flow in the collision zone correlates with a shallower seismicity zone and high seismic attenuation while the lower heat flow in the subduction zone might allow the earthquakes to rupture to greater depth the heat flow increases along the topographic high from subduction to collision zone due to increasing geothermal gradients and higher thermal conductivities of the exhumed basement rocks this heat flow variation may generate an artificial exhumation rate pattern if a conventional 30 c km geothermal gradient was used in fission track studies

a viscoelastic earthquake cycle model for taiwan 1 while recent geophysical measurements including global positioning system gps surveys have clarified the mechanisms of active mountain building in the taiwan arc continent collision the geometry of faults beneath the island remains unclear in this paper we use gps data collected before the 1999 chi chi earthquake to image the geometry of active faults under taiwan and estimate slip rates on these faults we invert the data using kinematic and mechanical earthquake cycle models consisting of faulting in an elastic lithosphere overlying a viscoelastic asthenosphere in the kinematic model uniform slip is prescribed on the faults at a constant rate while in the mechanical model the faults slip in response to shear stresses in the lithosphere these models explain the essential features of both the horizontal and vertical gps velocity fields we find a 27 39 km thick elastic lithosphere overlying a viscoelastic asthenosphere with maxwell relaxation time of 15 80 years or viscosity of 0 5 4 1019 pa s the models produce a combined slip rate of 30 50 mm yr on the frontal thrusts in western taiwan and 60 80 mm yr of slip on the longitudinal valley fault inversion of the data using both models indicates a decollement dipping 10 under western taiwan and increasing in dip by no more than 10 under the central ranges and eastern taiwan

subtidal variability of exchange flows produced by river pulses wind stress and fortnightly tides in a subtropical stratified estuary abstract time series of current velocity profiles were obtained at the mouth and head of a brazilian subtropical estuary the ararangua to study its dry period response to fortnightly tidal forcing river pulses and wind forcing the ararangua exemplifies a series of river dominated estuaries along southern brazil with widths 30 m3 s but 200 m3 s appeared in two other occasions late in the observation period and caused seaward flushing of the entire system overall the estuarine circulation resembled that of a fjord when river discharge was moderate and weaker an active upper layer driven by river discharge moving over a semi stagnant lower layer strongest subtidal exchange developed 2 3 days after neap tides in contrast to other studies where fortnightly modulation is determined by vertical mixing from tides in the lower estuary subtidal flows seemed to result from a combination of vertical mixing from tides and wind and from tidal advection moreover the lower estuary was much more sensitive to wind forcing than the upper estuary the upper estuary responded preferentially to river discharge and tidal forcing

large scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge seismic waves can trigger further fault slip analysis of seismic and geodetic data shows that seismic waves from the 2016 kaik ura new zealand earthquake were amplified by subduction zone sediments triggering slow fault slip up to 600 km away

horizontal coseismic deformation of the 1999 chi chi earthquake measured from spot satellite images implications for the seismic cycle along the western foothills of central taiwan 1 the 1999 chi chi earthquake mw 7 6 broke a major thrust fault along the western foothills of the central range of taiwan we have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake these data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall at the surface coseismic slip increases from 5 6 m near the epicenter to 10 11 m to the north in the epicentral area we observe a left lateral strike slip zone trending n125 e and farther north a fault zone trending n s with a right lateral component the data were modeled using elastic dislocations the fault geometry consists of a shallow

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 seismicity in continental china following the 2008 mw 7 9 wenchuan earthquake we perform a systematic search of remotely triggered seismicity in con tinental china following the 2008 mw 7 9 wenchuan earthquake we visually identify earthquakes as impulsive seismic energy with clear p and s arrivals on 5 hz high pass filtered three component velocity seismograms 1 hr before and after the wenchuan earthquake out of the 271 stations in the updated chinese digital seismic network csn 17 stations show statistically significant seismicity increase with statistic values larger than 2 following the wenchuan earthquake these include 11 stations in the north china block which is seismically active and is in the rupture propagation direction of the wenchuan earthquake three stations along the coastal lines in the relatively stable south china block one station near the haiyuan fault zone in north west china and two stations near the tanlu fault zone and the longgang volcano in northeast china these observations suggest that dynamic triggering in intraplate regions tends to occur near active faults that have ruptured in historic times and in the rupture propagation directions of the mainshock however it is worth noting that many sites that satisfy the criteria are not triggered suggesting that these conditions would help but are not sufficient enough to guarantee remote triggering in intraplate regions the tectonic environments near the sites with clear triggered activity range from transpressional to tranextentional and most regions are not associated with active geothermal or volcanic activity indicating that dynamic triggering could occur in a wide range of tectonic environments online material tables of station information and measured parameters and two supplementary figures showing locally triggered earthquakes

slow slip predictions based on granite and gabbro friction data compared to gps measurements in northern cascadia received 7 october 2008 revised 17 april 2009 accepted 12 june 2009 published 29 september 2009 1 for episodic slow slip transients in subduction zones a large uncertainty in comparing surface deformations predicted by forward modeling based on rate and state friction to gps measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along shallow subduction faults in this study we apply the laboratory rate and state friction data of granite and gabbro gouges under hydrothermal conditions to a cascadia like 2 d model to produce spontaneous aseismic transients and we compare the resulting intertransient and transient surface deformations to gps observations along the northern cascadia margin an inferred region along dip of elevated fluid pressure is constrained by seismological observations where available and by thermal and petrological models for the cascadia and sw japan subduction zones for the assumed

infrequent triggering of tremor along the san jacinto fault near anza california we examine the conditions necessary to trigger tremor along the san jacinto fault sjf near anza california where previous studies suggest triggered tremor occurs but observations are sparse we investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near anza california we examine 44 mw 7 4 teleseismic events between 2001 and 2011 these events occur at a wide range of back azimuths and hypocentral distances in addition we included one smaller magnitude regional event the 2009 mw 6 5 gulf of california earthquake because it induced extremely high strains at anza we find the only episode of triggered tremor occurred during the 3 november 2002 mw 7 8 denali earthquake the tremor episode lasted 300 s was composed of 12 tremor bursts and was located along sjf at the northwestern edge of the anza gap at approximately 13 km depth the tremor episode started at the love wave arrival when surface wave particle motions are primarily in the transverse direction we find that the denali earthquake induced the second highest stress 35 kpa among the 44 teleseismic events and 1 regional event the dominant period of the denali surface wave was 22 8 s at the lower end of the range observed for all events 20 40 s similar to periods shown to trigger tremor in other locations the surface waves from the 2009 mw 6 5 gulf of california earthquake had the highest observed strain yet a much shorter dominant period of 10 s and did not trigger tremor this result suggests that not only the amplitude of the induced strain but also the period of the incoming surface wave may control triggering of tremors near anza in addition we find that the transient shear stress 17 35 kpa required to trigger tremor along the sjf at anza is distinctly higher than what has been reported for the well studied san andreas fault

tidal modulation of continuous nonvolcanic seismic tremor in the chile triple junction region 1 we located continuous seismic tremor with coherent amplitude wave trains in the chile ridge subduction region 46 5 s in two clusters north and south of the chonos archipelago between the chile trench and the north patagonian fore arc tremor persisted from december 2004 to february 2007 the entire period of the chile ridge subduction project temporary seismic deployment and lasted 17 h on six occasions tremor in the more active southern cluster reached a maximum duration of 48 h and we observed no more than 3 continuous days without tremor activity the cluster locations coincide with the surface projections of subducted transform faults formed at the chile ridge we also detected simultaneous colocated low frequency microearthquakes with well defined impulsive waves within the tremor signals distributed from the surface to 40 km depth suggesting tremors and earthquakes are part of the same process the periodicity of tremor duration is strongly correlated with semidiurnal diurnal and long period tides m2 n2 k1 o1 p1 and mm 12 421 h 12 000 h 23 934 h 25 819 h 24 066 h and 27 555 days respectively we found a significant correlation between tremor occurrence and earth tides when tidal stress is calculated for the slip plane of a right lateral strike slip fault with strike n95 e which is near parallel to subducted transform faults n78 e of the chile ridge indicating that the very small stresses resulting from the combination of ocean loading and solid earth tides 1 kpa are sufficient to facilitate or suppress tremor production tremors occur when shear stresses are maximum and wane or are low when shear stresses are minimum

endmember unmixing of compositional data abstract manipulation of the n endmember mixing equations appropriate to compositional data leads to a general method 1 to test hypotheses about endmember compositions and 2 to recover the concentrations of all compositional components in an endmember when the concentrations of n 1 components are known or measured in that endmember additionally pearce element ratio analysis is shown to be a special and not efficiently posed two endmember case of n endmember unmixing information available on pearce element ratio diagrams as values of the slopes of lines is contained on harker diagrams as values of points

slow earthquakes and nonvolcanic tremor nonvolcanic tremor is observed in close association with geodetically observed slow slip events in subduction zones accumulating evidence points to these events as members of a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below by virtue of their locations and their properties slow earthquakes are certain to provide new insights into the behavior of earthquakes and faulting and into the hazard they embody

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

triggered tectonic tremor in various types of fault systems of japan following the 2012 mw8 6 sumatra earthquake tectonic tremor an extremely stress sensitive seismic phenomenon occurring in the brittle ductile transition section of a fault is associated with the shearing mechanism of slow slip observations of triggered tremor can facilitate the evaluation of the existence of background ambient tremor and slow slip events this paper presents widespread triggered tremor sources in japan initiated by the surface waves of the 2012 mw8 6 sumatra earthquake on strike slip and thrust faults in the deep volcanic low frequency earthquake active area in the shallow tectonic tremor and very low frequency earthquake active regions and in the subduction zone in most regions the amplitudes of triggered tremor are generally logarithmically proportional to the dynamic stress caused by various triggering earthquakes our observations suggest that triggered tremor in the newly discovered sources is the result of a more rapid rate of background ambient tremor and evidence has suggested the existence of ambient tremor in some regions

tectonic tremor along the northern hikurangi margin new zealand between 2010 and 2015 we present the first comprehensive tectonic tremor catalog for the northern hikurangi margin obtained from the systematic analysis of data from the new zealand seismic network between 2010 and 2015 the recent densification of this array along the northern hikurangi margin has allowed for the detection and location of tectonic tremor associated with 20 out of 26 identified transients in the continuous gps record along the east coast of the raukumara peninsula eighteen of these transients have never been reported in the literature and are likely to be slow slip events with magnitudes smaller than those previously studied we also investigate the spatiotemporal relationships between shallow offshore slow slip tectonic tremor and earthquakes for the duration of the study period

detecting low frequency earthquakes within non volcanic tremor in southern taiwan triggered by the 2005 mw8 6 nias earthquake 1 we use a matched filter technique to detect 41 low frequency earthquakes lfes within 700 s of triggered tremor signals in the southern central range in taiwan during the surface waves of the 2005 mw8 6 nias earthquake off the coast of northern sumatra the depth distributions of lfes after double difference relocations concentrate at the depth range of 12 38 km below the background seismicity and above the moho depth inferred from receiver function studies the locations of lfes are close to the downward extension of the steep dipping chaochou lishan fault with only modestly high vp vs ratios 1 75 1 85 our observation indicates that at least portions of triggered tremor consists of many lfes similar to ambient tremor observed at other major plate boundary faults

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

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

inferred fault geometry and slip distribution of the 2010 jiashian taiwan earthquake is consistent with a thick skinned deformation model abstract we invert measurements of coseismic displacements from 139 continuously recorded gps sites from the 2010 jiashian taiwan earthquake to solve for fault geometry and slip distribution using an elastic uniform stress drop inversion the earthquake occurred at a depth of 23 km in an area between the western foothills fold and thrust belt and the crystalline high mountains of the central range providing an opportunity to examine the deep fault structure under taiwan the inferred rupture plane is oblique to the prominent orientation of thrust faults and parallel to several previously recognized nw striking transfer zones that appear to connect stepping thrusts we find that a fault striking 318 344 with dip of 26 41 fits the observations well with oblique reverse sinistral slip under a low stress drop of about 0 5 mpa the derived geodetic moment of 2 92 10 18 n m is equivalent to a m w 6 24 earthquake coseismic slip is largely concentrated within a circular patch with a 10 km radius at the depth between 10 and 24 km and maximum slip of 190 mm we suggest this earthquake ruptured the nw striking chishan transfer fault zone which we interpret as a listric ne dipping lateral ramp with oblique slip connecting stepping thrust faults ramps the inferred slip on the lateral ramp is considerably deeper than the 7 15 km deep detachment identified in previous studies of western taiwan we infer an active basal detachment under western taiwan at a depth of at least 20 23 km based on these inversion results the earthquake may have nucleated at the base of the lateral ramp near the intersection with the basal detachment coulomb stress change calculations suggest that this earthquake moved several ne striking active thrust faults in western taiwan nearer to failure

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

comparative study of tectonic tremor locations characterization of slow earthquakes in guerrero mexico deep tectonic tremor in guerrero mexico has been observed using dense temporal seismic networks i e the meso american subduction experiment and guerrero gap experiment g gap arrays during two different time periods we apply a set of seismic waveform analysis methods to these data sets to constrain the locations of tremors and determine the associated moment tensors first we detect and locate the tremors next very low frequency vlf signals are identified by stacking waveform data during tremor bursts and their moment tensors are determined finally to better investigate the link between tremors and vlf earthquakes we detect vlf events using a matched filtering algorithm to search continuous seismic records none of the 11 vlf events detected by this method occurred in the absence of tremor bursts suggesting they are indeed part of the same phenomena unlike previous investigations our results for the g gap period reveal that downdip tremor activity i e in the so called sweet spot is segmented into two patches separated by 40 km in the along trench direction indicating possible variations in the geometry of the plate interface and or slab effective pressure moment tensors of vlf signals are consistent with shear slip on the near horizontal plate interface but source depths are about 5 km deeper than the established plate interface the slip directions of the vlf events are slightly 10 counterclockwise of the plate convergence direction indicating that strain energy promoting left lateral strike slip motion may accumulate in the continental crust during the interseismic period

high frequency identification of non volcanic tremor triggered by regional earthquakes 1 subsequent to the discovery of ambient non volcanic tremor activity along the parkfield cholame section of the san andreas fault in central california triggered tremors associated with the surface waves of large teleseismic earthquakes have been recognized however no evidence of triggered tremors from regional earthquakes has previously been found either here or in other tremor regions by systematically filtering seismograms to higher frequencies i e above 20 hz associated with 99 regional m5 earthquakes since 2001 we identify four regional earthquakes that have triggered tremors in central california significant high frequency energy is also observed in previously identified teleseismically triggered and ambient tremors suggesting a common mechanism we find that long period and large amplitude surface waves from both regional and teleseismic events have a greater potential of triggering tremor in the same region and that the inferred minimum triggering dynamic stress is 1 kpa

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

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

new detection of tremor triggered in hokkaido northern japan by the 2004 sumatra andaman earthquake 1 the transient stress change caused by the passage of surface waves resulting from the 26 december 2004 sumatra andaman earthquake triggered seismic tremor in two regions in hokkaido in the northernmost part of the japanese islands where tectonic tremor associated with the subducting plate has not previously been detected the amplitude pattern of the tremor envelope in both regions is characterized by a periodic enhancement at an interval of about 20 s which correlates with the surface wave one tremor that was triggered in central hokkaido at a depth of around 10 20 km coincides with active seismicity linked to previously known deep low frequency microearthquakes related to volcanic activity another tremor occurred in northernmost hokkaido where there are no known active faults volcanoes or microearthquake seismicity if the source of the northern tremor is located near the ground surface it would be possible that the tremor is related to fluid pressure change because the periodic enhancement of tremor amplitude is in phase with the largest compressional strain caused by surface waves

investigating the lithospheric velocity structures beneath the taiwan region by nonlinear joint inversion of local and teleseismic p wave data slab continuity and deflection the interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the taiwan region whether and in what form the eurasian plate subducts deforms under taiwan island is critical to the debate of tectonic models although an east dipping high velocity anomaly down to a depth below 200 km has been reported previously its detailed morphology remains uncertain and leads to different interpretations with a two step strategy of nonlinear joint inversion the slab images of the eurasian plate were retrieved in a geometry that is hyperthin in the south becoming massive and steeper in the central and severely deformed in the north the possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn moreover the slab deflection found at 23 2 n latitude seems to correspond to where the nonvolcanic tremors and recent nw se striking structures have occurred in southern taiwan

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

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

detection and location of low frequency earthquakes using cross station correlation source geometry and simple propagation of s waves generated by low frequency earthquakes lfes in northern cascadia results in strong waveform coherence on horizontal channels for stations at smaller epicentral distances as recognized by previous workers this cross station similarity can be exploited for detection of lfes in tremor we develop a cross station correlation approach based on waveform coherence and travel time consistency that exploits a full complement of network stations and we demonstrate its application to separate arrays on vancouver island and washington state our approach yields thousands of impulsive lfe detections per slow slip episode and hundreds of events between episodes lfe epicentral distributions reveal the presence of well defined regions of high asperity density with those regions farther downdip that are active during interepisodic tremor and slow slip periods the lfe epicenters also display pronounced spatiotemporal clustering that compares favorably with independent tremor and lfe template catalogs and that defines rapid tremor reversals with complex propagation patterns the lfe source depths can be estimated for some detections in which p waves are identified on the vertical channel through correlation with the horizontal component s waveforms lfe hypocenters cluster between two recently developed plate interface models below southern vancouver island our approach enables the identification of high signal to noise ratio impulsive lfe detections from targeted regions on the plate boundary for generation of lfe templates to be used in studies of structure and seismogenesis in cascadia online material additional maps of station density for southern vancouver island epicenter locations for northern washington and additional time distance plots for southern vancouver island

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

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

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

tectonic tremor and lfes on a reverse fault in taiwan we compare low frequency earthquakes lfes from triggered and ambient tremor under the southern central range taiwan we apply the pagerank algorithm used by aguiar and beroza 2014 that exploits the repetitive nature of the lfes to find repeating lfes in both ambient and triggered tremor we use these repeaters to create lfe templates and find that the templates created from both tremor types are very similar to test their similarity we use both interchangeably and find that most of both the ambient and triggered tremor match the lfe templates created from either data set suggesting that lfes for both events have a common origin we locate the lfes by using local earthquake p wave and s wave information and find that lfes from triggered and ambient tremor locate to between 20 and 35 km on what we interpret as the deep extension of the chaochou lishan fault

ambient tremors in a collisional orogenic belt deep seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt in this study we explore the general features of collisional tremors in taiwan and discuss the possible generation mechanism in the 4 year data we find 231 ambient tremor episodes with durations ranging from 5 to 30 min in addition to a coseismic slip induced stress change from nearby major earthquake increased tremor rate is also highly correlated with the active normal faulting earthquake swarms at the shallower depth both the tremor and earthquake swarm activities are confined in a small area where the high attenuation high thermal anomaly the boundary between high and low resistivity and localized veins on the surfaces distributed suggesting the involvement of fluids from metamorphic dehydration within the orogen

tidal modulation and triggering of low frequency earthquakes in northern cascadia we analyze the influence of earth and ocean tides on the triggering of low frequency earthquakes lfes in northern cascadia using three lfe catalogs for southern vancouver island and washington state from episodic tremor and slip events between 2003 and 2013 sensitivities of lfe families to tidally induced fault normal stress updip shear stress udss and corresponding time derivatives are computed and their geographic variability is mapped we find localized areas showing higher sensitivity to udss than their surroundings suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface we observe that sensitivity of lfes to udss rises dramatically from near zero on the first day of strong activity to a maximum 4 days later in addition the peak lfe rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events we identify 64 rapid tremor reversals rtrs that start a few days after the main slip front the rtrs have an average stress drop of 0 8 kpa and a majority 72 occurs during periods of large positive udss the combined observations imply that rtrs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity

remote triggered seismicity caused by the 2011 m9 0 tohoku oki japan earthquake 1 seismic waves from large earthquakes have been shown to trigger seismicity large distances from a mainshock and this is termed remotely or dynamically triggered seismicity we performed a global search for seismicity potentially triggered by the seismic waves from the 2011 m9 0 tohoku oki japan earthquake using seismograms from global seismic networks and an event catalog we search for earthquakes and tremors instantaneously triggered during the passing of the seismic waves as well as for statistically significant changes in local and global seismic rates after the passing of the waves for earthquakes we find potential cases of instantaneous triggering in the united states russia china ecuador and mexico while for tremors we find evidence for triggering in taiwan armenia cuba and the united states in addition we observed a potential case of delayed triggering of larger magnitude earthquakes including a m5 2 in baja california mexico

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

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

a large mw 6 0 aftershock of the 2008 mw 7 9 wenchuan earthquake triggered by shear waves reflected from the earth s core because the earth s outer core is liquid shear waves generated by a large earthquake that travel to the core scs are totally reflected with strong seismic energy it is interesting to note that one large aftershock m w 6 0 of the 2008 wenchuan earthquake m w 7 9 in china occurred when the scs phase returned around 15 5 m after the mainshock estimated peak dynamic stress 3 3 kpa of the scs amplitude reaches the critical level of the lower boundary capable of triggering earthquakes both the temporal correlation between aftershock occurrence and scs arrival as well as the dynamic stress capable of triggering an earthquake suggest that the large aftershock might well have been triggered dynamically by scs waves a similar feature of another large aftershock m b 5 3 occurring around the time of scs arrival has been also found from the 2003 hokkaido earthquake m w 8 3 further statistical examinations of 25 major earthquakes m w 8 0 between 1994 and 2008 shows some large aftershocks m w 5 0 also occurring around the time of scs phase arrival it is reasonable to conclude that these observations suggest aftershocks being triggered dynamically by scs phase arrivals and thus a temporal warning for potentially destructive aftershocks might be considered for future great earthquakes

dynamic triggering of creep events in the salton trough southern california by regional m 5 4 earthquakes constrained by geodetic observations and numerical simulations abstract since a regional earthquake in 1951 shallow creep events on strike slip faults within the salton trough southern california have been triggered at least 10 times by m 5 4 earthquakes within 200 km the high earthquake and creep activity and the long history of digital recording within the salton trough region provide a unique opportunity to study the mechanism of creep event triggering by nearby earthquakes here we document the history of fault creep events on the superstition hills fault based on data from creepmeters insar and field surveys since 1988 we focus on a subset of these creep events that were triggered by significant nearby earthquakes we model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate and state dependent friction we find that the static stress changes from the causal earthquakes are less than 0 1 mpa and too small to instantaneously trigger creep events in contrast we can reproduce the characteristics of triggered slip with dynamic perturbations alone the instantaneous triggering of creep events depends on the peak and the time integrated amplitudes of the dynamic coulomb stress change based on observations and simulations the stress change amplitude required to trigger a creep event of a 0 01 mm surface slip is about 0 6 mpa this threshold is at least an order of magnitude larger than the reported triggering threshold of non volcanic tremors 2 60 kpa and earthquakes in geothermal fields 5 kpa and near shale gas production sites 0 2 0 4 kpa which may result from differences in effective normal stress fault friction the density of nucleation sites in these systems or triggering mechanisms we conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the superstition hills fault

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

co seismic strike slip and rupture length produced by the 2001 ms 8 1 central kunlun earthquake field investigations show that the surface wave magnitude ms 8 1 central kunlun earthquake tibetan plateau of 14 november 2001 produced a nearly 400 kilometer long surface rupture zone with as much as 16 3 meters of left lateral strike slip along the active kunlun fault in northern tibet the rupture length and maximum displacement are the largest among the co seismic surface rupture zones reported on so far the strike slip motion and the large rupture length generated by the earthquake indicate that the kunlun fault partitions its deformation into an eastward extrusion of tibet to accommodate the continuing penetration of the indian plate into the eurasian plate

a global search for triggered tremor following the 2011 mw 9 0 tohoku earthquake abstract the 2011 m w 9 0 tohoku japan earthquake triggered deep tectonic tremor and shallow microearthquakes in numerous places worldwide here we conduct a systematic survey of triggered tremor in regions where ambient or triggered tremor has been previously identified tremor was triggered in the following regions south central alaska the aleutian arc shikoku in southwest japan the north island of new zealand southern oregon the parkfield cholame section of the san andreas fault in central california the san jacinto fault in southern california taiwan and vancouver island we find no evidence of triggered tremor in the calaveras fault in northern california one of the most important factors in controlling the triggering potential is the amplitude of the surface waves data examined in this study suggest that the threshold amplitude for triggering tremor is 0 1 cm s which is equivalent to a dynamic stress threshold of 10 kilopascals the incidence angles of the teleseismic surface waves also affect the triggering potentials of love and rayleigh waves the results of this study confirm that both love and rayleigh waves contribute to triggering tremor in many regions in regions where both ambient and triggered tremor are known to occur tremor triggered by the tohoku event generally occurred at similar locations with previously identified ambient and or triggered tremor further supporting the notion that although the driving forces of triggered and ambient tremor differ they share similar mechanisms we find a positive relationship between the amplitudes of the triggering waves and those of the triggered tremor which is consistent with the prediction of the clock advance model online material table of measured parameters and other information related to triggering nontriggering information and figures of observed seismograms

shallow microearthquakes near chongqing china triggered by the rayleigh waves of the 2015 m7 8 gorkha nepal earthquake abstract we present a case of remotely triggered seismicity in southwest china by the 2015 04 25 m7 8 gorkha nepal earthquake a local magnitude m l 3 8 event occurred near the qijiang district south of chongqing city approximately 12 min after the gorkha mainshock within 30 km of this m l 3 8 event there are 62 earthquakes since 2009 and only 7 m l 3 events which corresponds to a likelihood of 0 3 for a m l 3 on any given day by a random chance this observation motivates us to investigate the relationship between the m l 3 8 event and the gorkha mainshock the m l 3 8 event was listed in the china earthquake national center cenc catalog and occurred at shallow depth 3 km by examining high frequency waveforms we identify a smaller local event m l 2 5 15 s before the m l 3 8 event both events occurred during the first two cycles of the rayleigh waves from the gorkha mainshock we perform seismic event detection based on envelope function and waveform matching by using the two events as templates both analyses found a statistically significant rate change during the mainshock suggesting that they were indeed dynamically triggered by the rayleigh waves both events occurred during the peak normal and dilatational stress changes 10 30 kpa consistent with observations of dynamic triggering in other geothermal volcanic regions although other recent events i e the 2011 m9 1 tohoku oki earthquake produced similar peak ground velocities the 2015 gorkha mainshock was the only event that produced clear dynamic triggering in this region the triggering site is close to hydraulic fracturing wells that began production in 2013 2014 hence we suspect that fluid injections may increase the region s susceptibility to remote dynamic triggering

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

deep tremor in new zealand triggered by the 2010 mw8 8 chile earthquake 1 deep non volcanic tremor nvt is usually associated with episodic slow slip events new zealand is one notable exception where numerous slow slip events have been identified yet nvt has remained undetected here we present the first known case of triggered nvt at new zealand s hikurangi subduction margin following the mw8 8 chilean earthquake of february 27 2010 we identify coherent high frequency tremor signals that are in phase with and modulated by the passing rayleigh waves this is consistent with the surface wave triggering potential for strike normal incidence on a low angle thrust fault after constraining the tremor depth on the plate interface we locate the tremor source within 20 km of the source area of episodic slow slip the tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated underplated sediment we speculate that reduced interface friction and high fluid pressures provided by fluid rich underplated sediment facilitates the tremor generation

simultaneous long term and short term slow slip events at the hikurangi subduction margin new zealand implications for processes that control slow slip event occurrence duration and migration 1 we document a sequence of simultaneous short term and long term slow slip events sses at the hikurangi subduction zone during the 2010 2011 period the sequence of short term events each 2 3 weeks in duration ruptured much of the shallow plate interface 15 km at central and northern hikurangi over a 6 month period was accompanied by microseismicity and involved patchy irregular migration of sse slip we suggest that the patchy migration of the short term sse is due to large scale 100 3500 km heterogeneities on the plate interface related to seamount subduction and sediment subduction and or underplating this is in contrast to a 2010 2011 long term sse at the central hikurangi margin which evolved steadily over 1 5 years and ruptured much of the plate interface between 20 and 70 km depth we suggest that the occurrence of long term versus short term sses at hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface the long term sse sequence began 1 year before the short term sequence coulomb stress change models suggest that the long term sse may have triggered initiation of the subsequent short term sse sequence initiation of the short term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior alternatively it could be characteristic of a region undergoing partial interseismic coupling this is in contrast to sses observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

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

detecting deep tectonic tremor in taiwan with a dense array abstract we perform a systematic detection of deep tectonic tremor beneath the southern central range in taiwan using a dense small aperture seismic array deployed in february 2011 the array has been recording tectonic tremor nearly continuously for 134 days including tremor triggered by the 2011 m w 9 0 tohoku oki earthquake we use broadband frequency wavenumber beamforming and moving window grid search methods to compute array parameters for all continuous data and identify tremor as coherent nonimpulsive seismic signals with deep incidence angles the obtained array parameters closely match with those of relocated local earthquakes and triggered tremor bursts located by the waveform envelope correlation and clustering wecc method indicating the robustness of our array technique during the 134 day study period we detect tremor for 44 days with a total duration of 1481 min three to six times as long as the detection of tremor by the wecc method we find a relative quiescence in ambient tremor activity for about 20 days following the 2011 tohoku oki earthquake suggesting that dynamic stresses from the distant mainshock triggered most tremor close to failure resulting in a temporary lack of tremor activity in some cases we observe rapid tremor migration with a speed at the order of 40 50 km hr that is similar to the speed of fast tremor migration along dip on narrow streaks in japan and cascadia our results suggest that dense array techniques are capable of capturing detailed spatiotemporal evolutions of tremor behaviors in southern taiwan online material tables of station coordinates and tremor event parameters and figures of the moving window grid search test with local earthquakes tremor triggered by the 2011 tohoku oki earthquake and additional examples of detected ambient tremors

an automatic monitoring system for nonvolcanic tremors in southwest japan 1 the recent development of a system for exchanging and distributing seismic waveform data over high speed networks enables seismic events to be monitored in real time throughout japan in the present study we have developed an automatic real time monitoring system for deep nonvolcanic tremors in southwest japan the system automatically detects the occurrence of nonvolcanic tremors and determines their hypocenters in real time in addition the system creates image files for the detected tremor activities and makes them accessible via the world wide web to detect tremors we carry out a two step numerical hypothesis test in which the first step is to test whether two given envelope seismograms are correlated and the second step is to test whether an event occurs using the results of the first test this two step test is applied to real time data every 2 min once an event is detected we can regard time lags that generate maximum cross correlations as the travel time differences which we use for hypocenter determination although this procedure detects nontremor signals most of them can be rejected using several criteria since the start of monitoring in 2006 the system has worked well for detecting a wide variety of tremor activities results from the present system will contribute to understanding the stress relaxation process in the transition zone between the locked and stable slip zones of the subducting philippine sea plate interface

3d velocity structure around the source area of the 1999 chi chi taiwan earthquake before and after the mainshock 3d v p and v p v s models for the source area before and after the chi chi earthquake are derived from a tomographic inversion of seismic travel time data from the taiwan central weather bureau seismic network cwbsn resulting 3d velocity models reveal a strong lateral variation in the crust particularly at a depth range of 5 to 25 km along the trend of the dipping chelungpu thrust in general low v p and high v p v s anomalies are found in and around the source volume of the chi chi earthquake a volume centered around the entire 100 km 40 km chelungpu rupture surface we pay special attention to the crustal properties and their changes due to the chi chi event and the association of the crustal anomalies with seismicity combining the results with those of other geological and geophysical surveys the crustal anomalies are interpreted as images of a highly fractured and fluid filled chi chi source region fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the chi chi earthquake the v p models obtained before and after the chi chi event show some interesting heterogeneities regions of low v p anomaly appear to have further expanded with the occurrence of the chi chi earthquake the v p v s model shows high anomalies near the source rupture especially near where the rupture is believed to have nucleated after the chi chi event the v p v s model shows a substantial change that may reflect an outward shift of the fluid filled fractured source region this shift is shown by a general expansion in v p v s anomalies to a broader region e g where two intensive aftershock clusters occurred and a significant v p v s reduction to more normal values near the chelungpu rupture we attribute these changes in v p and v p v s anomalies as well as the expansion and shift of the anomalous regions to be a response to the localized stress change caused by the slip of the chi chi rupture the location of the seismic events is correlated with that of the crustal v p and v p v s anomalies large events appear to occur in the transition zones of both v p and v p v s anomalies this finding may be of use in the identification of weakened crustal regions in a seismic zone thus suggesting where impending large earthquakes are likely to occur all input data and initial and final models of this article are given as electronic files in the cd attached to the end of this issue

nonvolcanic tremor along the oaxaca segment of the middle america subduction zone 1 the oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates shallow subduction and short trench to coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland previous analysis of slow slip events in southern mexico suggests that they may represent motion in the transition zone defining the downdip edge of future megathrust earthquakes a new deployment consisting of broadband seismometers distributed inland along the oaxaca segment provide the means to examine whether nonvolcanic tremor nvt signals can also be used to characterize the boundary between the seismogenic and transition zones in this study we established that nvt exists in the oaxaca region based on waxing and waning of seismic energy on filtered day long seismograms that were correlated across neighboring stations and were further supported by appropriate relative time moveouts in record sections and spectrograms with narrow frequency bands eighteen prominent nvt episodes that lasted upwards of a week were identified during the 15 months analyzed june 2006 to september 2007 recurring as frequently as every 2 3 months in a given region we analyze nvt envelope waveforms with a semiautomated process for identifying prominent energy bursts and analyst refined relative arrival times are inverted for source locations nvt burst epicenters primarily occur between the 40 50 km contours for depth of the plate interface except in eastern oaxaca where they shift toward the 30 km contour as the slab steepens nvt hypocenters correlate well with a high conductivity zone that is interpreted to be due to slab fluids nvt is more frequent shorter in duration and located further inland than gps detected slow slip while the latter is associated with a zone of ultra slow velocity interpreted to represent high pore fluid pressure this zone of slow slip corresponds to approximately 350 c 450 c with megathrust earthquakes microseismicity and strong long term coupling occurring immediately updip from it this leaves nvt primarily in a region further inland from the thermally defined transition zone suggesting that transition from locking to free slip may occur in more than one phase

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

resolving the detailed spatiotemporal slip evolution of deep tremor in western japan we study the detailed spatiotemporal behavior of deep tremor in western japan through the development and application of a new slip inversion method although many studies now recognize tremor as shear slip along the plate interface manifested in low frequency earthquake lfe swarms a conventional slip inversion analysis is not available for tremor due to insufficient knowledge of source locations and green s functions here we introduce synthetic template waveforms which are typical tremor waveforms obtained by stacking lfe seismograms at arranged points along the plate interface using these synthetic template waveforms as substitutes for green s functions we invert the continuous tremor waveforms using an iterative deconvolution approach with bayesian constraints we apply this method to two tremor burst episodes in western and central shikoku japan the estimated slip distribution from a 12 day tremor burst episode in western shikoku is heterogeneous with several patchy areas of slip along the plate interface where rapid moment releases with durations of 100 s regularly occur we attribute these heterogeneous spatiotemporal slip patterns to heterogeneous material properties along the plate interface for central shikoku where we focus on a tremor burst episode that occurred coincidentally with a very low frequency earthquake vlf we observe that the source size of the vlf is much larger than that estimated from tremor activity in western shikoku these differences in the size of the slip region may dictate the visibility of vlf signals in observed seismograms which has implications for the mechanics of slow earthquakes and subduction zone processes

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event