seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

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

long term acceleration of aseismic slip preceding the mw 9 tohoku oki earthquake constraints from repeating earthquakes a decadal scale deformation transient preceding the 2011 mw 9 tohoku oki japan earthquake was reported from continuous gps data and interpreted as accelerating aseismic slip on the japan trench megathrust given the unprecedented nature of this transient independent confirmation of accelerating slip is required here we show that changes in the recurrence intervals of repeating earthquakes on the japan trench megathrust in the period 1996 to 2011 are consistent with accelerating slip preceding the tohoku oki earthquake all sequences of repeating earthquakes with statistically significant trends in recurrence interval at 95 confidence offshore south central tohoku occurred at an accelerating rate furthermore estimates of the magnitude of slip acceleration from repeating earthquakes are consistent with the completely independent geodetic estimates from a joint inversion of the gps and seismicity data we infer that a substantial portion of the megathrust experienced accelerating slip partly surrounding the eventual rupture zone of the mw 9 earthquake

systematic search for repeating earthquakes along the haiyuan fault system in northeastern tibet repeating earthquakes have been found at many faults around the world and they provide valuable information on diverse faulting behavior at seismogenic depth the haiyuan fault is a major left lat

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

characteristic repeating earthquakes in an arc continent collision boundary zone the chihshang fault of eastern taiwan abstract creeping crustal faults often generate a large number of microearthquakes and less commonly they may also produce major earthquakes that rupture the brittle crust the chihshang fault in eastern taiwan is characterized by such behavior and has been known to undergo 2 3 cm yr surface creep making it one of the most active creeping thrust faults known in the world it gives an excellent opportunity to study how a creeping fault can generate large earthquakes however the understanding of fault behavior at depth in this area has been limited due to sparse sampling from seismic and geodetic stations in this study we determine and evaluate a population of repeating earthquakes to improve the understanding of deep fault deformation we propose a repeating sequence identification scheme in the region where the station coverage is sparse and one sided using this identification method we found 30 m 2 3 characteristic repeating sequences under the northern half of the chihshang fault at 7 23 km depth slip estimates from these sequences indicate an average slip rate of 3 cm yr which is consistent with the rate inferred from surface geodetic measurements we infer that the 30 km long chihshang fault is creeping along its northern half and locked in the south consistent with the occurrence of the 2003 ml 6 4 earthquake on the southern fault section

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

seismicity rate immediately before and after main shock rupture from high frequency waveforms in japan 1 we analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the hi net borehole array in japan by scrutinizing high frequency signals we detect 5 times as many aftershocks in the first 200 s as in the japan meteorological agency catalogue after correcting for the changing completeness level immediately after the main shock the aftershock rate shows a crossover from a slower decay with an omori s law exponent p 0 58 0 08 between 20 and 900 s after the main shock to a faster decay with p 0 92 0 04 after 900 s the foreshock seismicity rate follows an inverse omori s law with p 0 73 0 08 from several tens of days up to several hundred seconds before the main shock the seismicity rate in the 200 s immediately before the main shock appears steady with p 0 35 0 50 these observations can be explained by the epidemic type aftershock sequence etas model and the rate and state model for a heterogeneous stress field on the main shock rupture plane alternatively nonseismic stress changes near the source region such as episodic aseismic slip or pore fluid pressure fluctuations may be invoked to explain the observation of small p values immediately before and after the main shock

seismic clustering in the sea of marmara implications for monitoring earthquake processes abstract quantifying regional earthquake cluster style is essential for providing a context for studies of seismicity patterns and earthquake interactions here we identify clusters of seismicity in the sea of marmara region of the north anatolian fault nw turkey using a recently derived high resolution seismicity catalog and the nearest neighbor earthquake cluster approach the detected earthquake clusters are utilized for 1 determining spatial distribution of mainshock and aftershock rates and estimating the proximity to failure on different fault segments 2 identifying fault sections having earthquake repeaters and 3 finding areas with enhanced foreshock activity about 6 70 and 24 of the events are identified as foreshocks mainshocks and aftershocks respectively with the largest concentration of aftershocks and foreshocks located along the western high and the cinarcik fault respectively the method successfully identifies regions where previous studies reported earthquake repeaters as indicator for fault creep and suggests additional repeater areas in the gulf of gemlik the largest proportion of mainshocks with associated foreshocks and aftershocks are along the western high and cinarcik fault segments potentially indicating that these segments are closer to failure and have increased susceptibility to seismic triggering continuing studies can contribute to monitoring possible preparation phase of a large m 7 earthquake in the marmara region near the istanbul metropolitan region

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

variability of fault slip behavior along the san andreas fault in the san juan bautista region an improved understanding of the time history of fault slip at depth is an essential step toward understanding the underlying mechanics of the faulting process using a waveform cross correlation approach we document spatially and temporally varying fault slip along the northernmost creeping section of the san andreas fault near san juan bautista sjb california by systematically examining spatiotemporal behaviors of characteristically repeating earthquakes cres the spatial distribution of pre 1998 sjb earthquake 1984 1998 fault slip rate inferred from the cres reveals a 15 km long low creep or partially locked section located near the 1998 mw 5 1 sjb earthquake rupture a finite fault slip inversion reveals that the rupture of the 1998 sjb earthquake is characterized by the failure of a compact 4 km2 asperity with a maximum slip of about 90 cm and corresponding peak stress drop of up to 50 mpa whereas the mean stress drop is about 15 mpa following the 1998 earthquake the cre activity was significantly increased in a 5 10 km deep zone extending 2 7 km northwest of the main shock which indicates triggering of substantial aseismic slip the postseismic slip inferred from the cre activity primarily propagated to the northwest and released a maximum slip of 9 cm in this 5 10 km depth range the estimated postseismic moment release is 8 6 1016 n m which is equivalent to mw 5 22 the aseismic slip distribution following the 1998 earthquake is not consistent with coseismic stress driven afterslip but represents a triggered long lasting slow earthquake

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

towards a universal rule on the recurrence interval scaling of repeating earthquakes 1 a recent analysis of recurrence properties of small repeating earthquakes on a creeping oblique thrust fault in eastern taiwan reveals a weak variation in recurrence interval tr with seismic moment mo compared to the scaling of tr with mo from repeating earthquake data near parkfield in california the repeating data from eastern taiwan has recurrence intervals that are 2 times shorter also in northeastern japan tr of repeating quakes are 4 times shorter than those expected from the parkfield scaling law when adjusted to account for differences in the geodetically derived slip rates for the three fault zones however the tr mo scaling is remarkably consistent among the three regions it suggests that the tectonic loading rate is likely the most important factor that controls the repeat time it also suggests that there seems to exist a universal rule on recurrence interval scaling of repeating earthquakes in diverse tectonic settings

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

measurement of interseismic strain across the haiyuan fault gansu china by insar the haiyuan fault is part of a major left lateral fault system at the northeastern edge of the tibet qinghai plateau two m8 earthquakes 1920 and 1927 occurred along the fault bracketing an unbroken section of the fault identified as the tianzhu seismic gap we use interferometric synthetic aperture radar data from descending orbits of the ers satellites acquired between 1993 and 1998 along two adjacent tracks covering the gap to measure the current surface movements and better understand the present day mechanical behavior of this fault section the analysis of the radar data involves first the combined correction of orbital errors and errors associated with the phase delay through the troposphere a subset of the data is then selected based on the analysis of the residual noise spectra for each pair of data the selected interferograms are stacked and the average phase change rate is converted in fault parallel velocity assuming that the ground movement is horizontal and parallel to the fault velocity maps from both tracks show a zone of high velocity gradient across the fault a few kilometers wide consistent with left lateral slip on the haiyuan fault the average velocity field from the two tracks in their overlapping area is well fit with a single screw dislocation model in an elastic half space the derived fault slip rate at depth 4 2 8 mm yr is consistent with recent gps results the corresponding shallow apparent locking depth 0 4 2 km can be explained by a current low stress accumulation on the fault due to creep almost on the entire fault plane however unless it is transient this creep would be paradoxical with the occurrence of past large earthquakes along this fault section as revealed by paleoseismology an alternative model implying both shallow creep in the brittle upper crust and deep aseismic slip beneath the seismogenic layer separated by a locked section would be consistent with insar observations and with the potential for large earthquakes on the fault as well a two dislocation model with slip at 5 mm yr beneath 15 km and a transient creep rate of 11 mm yr between 2 and 7 km fits the insar data however the width and creep rate of the shallow creeping zone and their possible along strike variations are still poorly resolved with the present data set

slip rate increase at parkfield in 1993 detected by high precision edm and borehole tensor strainmeters on two of the instrument networks at parkfield california the two color electronic distance meter edm network and borehole tensor strainmeter btsm network we have detected a rate change starting in 1993 that has persisted at least 5 years these and other instruments capable of measuring crustal deformation were installed at parkfield in anticipation of a moderate m6 earthquake on the san andreas fault many of these instruments have been in operation since the mid 1980s and have established an excellent baseline to judge changes in rate of deformation and the coherence of such changes between instruments the onset of the observed rate change corresponds in time to two other changes at parkfield from late 1992 through late 1994 the parkfield region had an increase in number of m4 to m5 earthquakes relative to the preceding 6 years the deformation rate change also coincides with the end of a 7 year period of sub normal rainfall both the spatial coherence of the rate change and hydrological modeling suggest a tectonic explanation for the rate change from these observations we infer that the rate of slip increased over the period 1993 1998

mapping of crustal scale tectonic boundaries in the ossa morena zone using reprocessed iberseis reflection seismic data the iberseis deep seismic reflection profile imaged crustal scale structures in the sw iberian variscan belt crossing the south portuguese zone the ossa morena zone and the central iberian zone in spain two subsets of the profile corresponding to the south portuguese zone ossa morena zone and the ossa morena zone central iberian zone tectonic contacts have been reprocessed with the aim of investigating the influence of cross dip and to better image steeply dipping features alternative strategies for binning midpoints into common depth point cdp bins using different azimuths were examined for synthetic data we show that the choice of the cdp processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile multi azimuth binning normal moveout dip moveout and migration velocity analysis on synthetic and real data show the presence of clear sub vertical upper crustal structures near the south portuguese zone ossa morena zone suture the aroche fault this sub vertical reflectivity that was not imaged earlier projects into a location in the lower crust with low reflectivity

repeating earthquakes in western corinth gulf greece implications for aseismic slip near locked faults an extensive search for repeating earthquakes was performed in the western corinth gulf by applying waveform cross correlation on 22 000 earthquakes that occurred in 2008 2014 event pairs with high correlation coefficient cc 0 95 recorded by two or more stations are classified as multiplets of repeating events the highly similar event pairs have typically interevent distances less than a quarter wavelength 150 m for a dominant frequency of 10 hz and are used to estimate the accuracy of the relocated catalog a detailed analysis of the spatiotemporal properties of the repeating sequences revealed two types of repeating events namely burst like and continuous type repeaters burst like repeaters are widespread in the entire study area mostly associated with seismic excitations located at depths between 5 and 9 km triggered either by fluid intrusion or stress transfer their duration is short with high values of coefficient of variation in recurrence intervals cov 1 and high slip rates the continuous type repeaters which last 1 to 7 years with cov 1 and slip rates almost 0 26 cm yr form a very narrow shallow north dipping seismic zone at 10 km depth along the psathopyrgos and aigion faults that kind of activity provides strong evidence for aseismic slip in the western corinth gulf and defines the boundaries between brittle ductile transition in the area

seismicity and fault geometry of the san andreas fault around parkfield california and their implications abstract fault geometry is a consequence of tectonic evolution and it provides important information on potential seismic hazards we investigated fault geometry and its properties in parkfield california on the basis of local seismicity and seismic velocity residuals refined by an adaptive velocity hypocentral parameter inversion method the station correction terms from the hypocentral parameter inversion present characteristic seismic velocity changes around the fault suggesting low seismic velocities in the region east of the fault and high seismic velocities in the region to the west large seismic velocity anomalies are observed at shallow depths along the whole fault zone at depths of 3 8 km seismic velocity anomalies are small in the central fault zone but are large in the northern and southern fault zones at depths 8 km low seismic velocities are observed in the northern fault zone high seismicity is observed in the southwest fracture zone which has developed beside the creeping segment of the san andreas fault the vertical distribution of seismicity suggests that the fault has spiral geometry dipping ne in the northern region nearly vertical in the central region and sw in the southern region the rapid twisting of the fault plane occurs in a short distance of approximately 50 km the seismic velocity anomalies and fault geometry suggest location dependent piecewise faulting which may cause the periodic m6 events in the parkfield region

rescaled earthquake recurrence time statistics application to microrepeaters slip on major faults primarily occurs during characteristic earthquakes the recurrence statistics of characteristic earthquakes play an important role in seismic hazard assessment a major problem in determining applicable statistics is the short sequences of characteristic earthquakes that are available worldwide in this paper we introduce a rescaling technique in which sequences can be superimposed to establish larger numbers of data points we consider the weibull and log normal distributions in both cases we rescale the data using means and standard deviations we test our approach utilizing sequences of microrepeaters microearthquakes which recur in the same location on a fault it seems plausible to regard these earthquakes as a miniature version of the classic characteristic earthquakes microrepeaters are much more frequent than major earthquakes leading to longer sequences for analysis in this paper we present results for the analysis of recurrence times for several microrepeater sequences from parkfield ca as well as ne japan we find that once the respective sequence can be considered to be of sufficient stationarity the statistics can be well fitted by either a weibull or a log normal distribution we clearly demonstrate this fact by our technique of rescaled combination we conclude that the recurrence statistics of the microrepeater sequences we consider are similar to the recurrence statistics of characteristic earthquakes on major faults

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

periodic pulsing of characteristic microearthquakes on the san andreas fault deep fault slip information from characteristically repeating microearthquakes reveals previously unrecognized patterns of extensive large amplitude long duration quasiperiodic repetition of aseismic events along much of a 175 kilometer segment of the central san andreas fault pulsing occurs both in conjunction with and independent of transient slip from larger earthquakes it extends to depths of 10 to 11 kilometers but may be deeper and it may be related to similar phenomena occurring in subduction zones over much of the study area pulse onset periods also show a higher probability of larger earthquakes which may provide useful information for earthquake forecasting

paleogene laterites bearing the highest insect ichnodiversity in paleosols research on laterites deals mostly with geomorphological and geochemical characterization of extant tropical systems with few examples of sedimentologic and paleosol analyses of mid latitude detrital records extra tropical lower eocene laterites from uruguay include fe rich indurated horizons duricrusts that preserve paleosol features and nodular beds discrete trace fossils are generally absent in laterites despite that they are formed in tropical ecosystems with high biodiversity uruguayan laterites bear abundant highly diverse and superbly preserved insect trace fossils association distribution orientation and abundance of trace fossils preserved in strongly developed paleosols ultisols are key for reconstructing cyclic depositional and weathering processes governed by hydrology and tectonics each cycle lasted 420 ka and included iterative stages of fluvial sedimentation in a pericratonic scenario pedogenesis and insect nesting and pupation in a seasonal tropical savanna duricrust formation by desiccation and interruption of insect activities and duricrust disaggregation nodular beds when rainfall increased the proposed cycle can be used as a heuristic tool in the study of laterites

episodic creep events on the san andreas fault caused by pore pressure variations recent seismic and geodetic observations indicate that interseismic creep rate varies in both time and space the spatial extent of creep pinpoints locked asperities while its temporary accelerations known as slow slip events may trigger earthquakes although the conditions promoting fault creep are well studied the mechanisms for initiating episodic slow slip events are enigmatic here we investigate surface deformation measured by radar interferometry along the central san andreas fault between 2003 and 2010 to constrain the temporal evolution of creep we show that slow slip events are ensembles of localized creep bursts that aseismically rupture isolated fault compartments using a rate and state friction model we show that effective normal stress is temporally variable on the fault and support this using seismic observations we propose that compaction driven elevated pore fluid pressure in hydraulically isolated fault zone and subsequent frictional dilation cause the observed slow slip episodes we further suggest that the 2004 mw6 parkfield earthquake might have been triggered by a slow slip event which increased the coulomb failure stress by up to 0 45 bar per year this implies that while creeping segments are suggested to act as seismic rupture barriers slow slip events on these zones might promote seismicity on adjacent locked segments

emergence and disappearance of interplate repeating earthquakes following the 2011 m9 0 tohoku oki earthquake slip behavior transition between seismic and aseismic depending on the loading rate we investigated spatiotemporal change in the interplate seismic activity following the 2011 tohoku oki earthquake m9 0 in the region where interseismic interplate coupling was relatively weak and large postseismic slip was observed we classified earthquakes by their focal mechanisms to identify the interplate events and conducted hypocenter relocation to examine the detailed spatiotemporal distribution of interplate earthquakes in the mostly creeping area the results show that many interplate earthquakes including m 6 events emerged immediately after the tohoku oki earthquake in areas where very few interplate earthquakes had been observed in the 88 previous years the emergent earthquakes include repeating sequences and the extremely long quiescence of small to moderate earthquakes before the tohoku oki earthquake suggests that the source areas for the post m9 events slipped aseismically during the quiescence the repeaters magnitudes decayed over time following the tohoku oki earthquake and some sequences disappeared within a year the emergence of interplate earthquakes suggests that areas where aseismic slip had been dominant before the tohoku oki earthquake started to cause seismic slip after the earthquake probably due to the increased loading rate from the afterslip the magnitude decrease and disappearance of repeaters can be interpreted as shrinkage in seismic areas around the repeaters sources as the loading rate decreased due to the afterslip decay over time these observations suggest that changes in the loading rate can cause slip behavior transition between seismic and aseismic this indicates that such loading rate dependent slip behavior plays an important role in the spatiotemporal distribution of earthquakes in interplate seismogenic zones

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

analysis of deformation data at parkfield california detection of a long term strain transient analysis of more than a decade of high quality data particularly those from the two color electronic distance meter edm in the parkfield california area reveals a significant transient in slip rate along the san andreas fault this transient consists of an increase in fault slip rate of 3 3 0 9 mm yr during 1993 0 to 1998 0 the most reliable fault creep instruments show a comparable increase in slip rate suggesting that the deformation is localized to the fault which breaks the surface there was also an increase in precipitation around 1993 it is unlikely however that this anomaly is due directly to hydrology as its spatial distribution is what would be expected for increased slip on the san andreas fault the increase in slip rate corresponds temporally to a dramatic increase in seismicity including the four largest earthquakes in the period 1984 1999 that occurred along a 6 km segment of the fault just to the north of the edm network there was also a previously reported anomaly in borehole shear strain gwyther et al 1996 that closely corresponds temporally to the transient in edm data solely on the basis of edm data the transient can be modeled as a slip event on a 10 km long segment of the fault the calculated shear strains from this model however are not consistent with the observed ones a compatible model can be found if there is increased aseismic slip to the northwest in conjunction with the four earthquakes support for this northwestern slip is provided by a recent study of slip rate based on microearthquake activity we speculate that this northwestern event served to load the fault to the southeast with the stress being partially released by the observed slip

time dependent model of creep on the hayward fault from joint inversion of 18 years of insar and surface creep data 1 spatial and temporal variations of aseismic fault creep influence the size and timing of large earthquakes along partially coupled faults to solve for a time dependent model of creep on the hayward fault we invert 18 years of surface deformation data 1992 2010 obtained by interferometric processing of 52 and 50 sar images acquired by the ers1 2 and envisat satellites respectively and surface creep data obtained at 19 alinement and 4 creepmeter stations for multi temporal analysis of the sar data we developed a method for identifying stable pixels using wavelet multi resolution analysis we also implement a variety of wavelet based filters for reducing the effects of environmental artifacts using a reweighted least squares approach we inverted the interferometric data to generate a time series of surface deformation over the san francisco bay area with a precision of better than a few millimeters to jointly invert the insar displacement time series and the surface creep data for a time dependent model of fault creep we use a robust inversion approach combined with a kalman filter the time dependent model constrains a zone of high slip deficit that may represent the locked rupture asperity of past and future m 7 earthquakes we identify several additional temporal variations in creep rate along the hayward fault the most important one being a zone of accelerating slip just northwest of the major locked zone we estimate that a slip rate deficit equivalent to mw 6 3 6 8 has accumulated on the fault since the last event in 1868

aseismic slip and fault normal strain along the central creeping section of the san andreas fault 1 we use gps data to measure the aseismic slip along the central san andreas fault csaf and the deformation across adjacent faults comparison of edm and gps data sets implies that except for small scale transients the fault motion has been steady over the last 40 years we add 42 new gps velocities along the csaf to constrain the regional strain distribution shear strain rates are less than 0 083 0 010 strain yr adjacent to the creeping saf with 1 4 5 mm yr of contraction across the coast ranges dislocation modeling of the data gives a deep long term slip rate of 31 35 mm yr and a shallow 0 12 km creep rate of 28 mm yr along the central portion of the csaf consistent with surface creep measurements the lower shallow slip rate may be due to the effect of partial locking along the csaf or reflect reduced creep rates late in the earthquake cycle of the adjoining saf rupture zones

inversion of gps data for spatially variable slip rate on the san andreas fault near parkfield ca we analyze gps data collected from 1991 1998 at 35 sites near the parkeld segment of the san andreas fault inverting the resultant site velocities for the distri bution of interseismic slip rate on the san andreas reveals an area of low slip rate on thefault extendingfrom between middle mountain and carr hill to southeast of gold hill this slip rate patternis similar to thatfoundbyharris and segall 1987 using trilateration data collected between 1966 and 1984 we infer a deep slip rate 33 mm yr and depth of the transition between seismogenic and non seismogenic slip 14km thatagree betterwithindependentgeologic ev idence than those found in the 1987 study in contrast to harris and segall 1987 wendnoevidenceoffault normal contraction

reflection signature of seismic and aseismic slip on the northern cascadia subduction interface at the northern cascadia margin the juan de fuca plate is underthrusting north america at about 45 mm yr 1 ref 1 resulting in the potential for destructive great earthquakes2 3 the downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake thought to have been in 1700 4 occurred before instrumental recording thermal and deformation studies5 indicate that off southern vancouver island the interplate interface is presently fully locked for a distance of 60 km downdip from the deformation front great thrust earthquakes on this section of the interface with magnitudes of up to 9 4 5 have been estimated to occur at an average interval of about 590 yr ref 3 further downdip there is a transition from fully locked behaviour to aseismic sliding where high temperatures allow ductile deformation with the deep aseismic zone exhibiting slow slip thrust events6 here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked to a broad reflection band at greater depth where aseismic slip is thought to be occurring this change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones

spatial temporal distribution of early aftershocks following the 2016 ms 6 4 menyuan qinghai china earthquake abstract the lenglongling fault located in the north qilian fold zone along the northeastern margin of the tibetan plateau plays an important role in accommodating the tectonic deformation between the tibetan plateau and the alxa block on 21 january 2016 the ms 6 4 menyuan earthquake occurred near the lenglongling fault in this study we utilize the waveform matched filter technique and double difference earthquake location algorithm to examine the spatial temporal evolution of aftershocks following the menyuan mainshock events from both china earthquake networks center cenc routine catalog and relocated catalog are used as templates to scan through the continuous waveform data 21 january 2016 to 31 december 2016 we identify three and eleven times more events than listed in cenc routine catalog and relocated catalog respectively we find that the earliest aftershocks e g in the first 10 min or so mostly illuminated a steeply dipping plane while aftershocks at later times outlined a near vertical dipping plane with mostly strike slip focal mechanisms the relocated aftershocks also suggest that the menyuan mainshock occurred in a secondary fault rather than the major lenglongling fault which is consistent with geodetic and geological observations moreover we find that aftershocks systematically expand with time along the fault strike and 16 repeating earthquake clusters occur in the rupture zone which are likely driven by afterslip of the mainshock

aseismic slip and fault interaction from repeating earthquakes in the loma prieta aftershock zone 1 along creeping sections of the san andreas and other faults small asperities in the fault zone load and fail in characteristic repeating earthquake sequences which can be used as subsurface creepmeters here we use these virtual creepmeters to examine and compare deep slip rates on both the northwestern end of the creeping section of the san andreas fault near san juan bautista and on the nearby subparallel sargent fault while creep on the san andreas increased dramatically due to static stress changes in response to the 1989 loma prieta earthquake the sargent showed very little immediate response consistent with loma prieta finite slip models that put this section of the fault in a region of less than 3 bar coulomb stress increase after about 10 years the san andreas creep rate fell back close to the interseismic rate and variations in creep became coherent in time with the sargent indicating a mutual driving force in the system

fault slip rates at depth from recurrence intervals of repeating microearthquakes unique attributes in sequences of recurring similar microearthquakes at parkfield california provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events application of the method using an 11 year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault zone slip if this phenomenon is found to be generally common behavior in active faults it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone

repeating earthquakes and interplate coupling along the western part of the north anatolian fault abstract the sea of marmara accommodates segments of the north anatolian fault naf in turkey and remains the only part of the western naf that has not ruptured during the last century at its nearest the segment is 20 km from istanbul the largest city in turkey thus it is important to understand the locking state of the fault since it illuminates the strain accumulation rate along the fault segments which in turn is an important input parameter in seismic hazard studies to infer the interplate locking state we used repeating earthquakes that indicate fault creep in the surrounding area using long term april 2005 to may 2013 seismic observations at 12 broadband seismic stations operated by bogazici university kandilli observatory and earthquake research institute we defined repeating earthquakes from waveform coherences that were 0 95 for 40 second long waveforms of the vertical component using the selection procedure we found 21 repeating earthquakes with magnitude 2 3 to 3 2 that are grouped into 9 sequences they are distributed along the main naf comprising three groups of activity one group in the sea of marmara and a group either side to the east and west the three groups are located near the boundary of previous large earthquake ruptures suggesting relatively weak coupling there we also estimated the fault creep rate from the cumulative slip of the repeating earthquakes using a scaling relationship between repeating earthquakes moment and slip the slip rate for these three groups are similar 3 4 cm yr and comparable to albeit slightly higher than those expected from global plate models 2 4 cm yr this suggests relatively weak locking around the groups the relocation results of the repeating earthquake hypocenters in the sea of marmara suggest the creep is occurring at 10 to 20 km depth these results suggest heterogeneous coupling in the segment

time dependent inversion study of the slow thrust event in the nankai trough subduction zone southwestern japan we estimate time variation of interplate coupling between the subducting philippine sea and the overriding continental plates along the western part of the nankai trough japan during the period between 1996 and 1999 after subtracting annual components and average annual velocities deviation from steady crustal deformation in the shikoku and kyushu islands is examined around 15 gps stations facing the bungo channel detected the following three stage ground displacements of up to 3 cm in 1997 with a duration of 1 year 1 increased displacement rate associated with the 1996 hyuga nada earthquakes 2 an increasingly rapid anomalous period of deformation from june 1997 to the end of 1997 and 3 a slowdown period and return to normal rates of deformation after the end of 1997 linearized least squares locate the slip area of this slow event near the plate boundary beneath the bungo channel the estimated slip is opposite the motion of the philippine sea plate these results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the philippine sea plate time dependent inversion analysis shows the following slip history during the initial period of increased displacement rate a visible slip area is resolved beneath the coastal area of southwestern shikoku in the next stage of accelerated displacements the slip area expanded southwestward to the bungo channel beginning around june 1997 when postseismic slip of the 1996 hyuga nada earthquakes almost ended after this accelerated period the estimated thrust slip ends with time

seismic and hydroacoustic tremor generated by colliding icebergs 1 iceberg harmonic tremor iht emanating from tabular icebergs in the southern ocean and along calving margins of the antarctic ice sheet is a complex evolving signal at frequencies above approximately 0 5 hz iht has been observed as t phases on islands in the equatorial pacific as hydroacoustic signals in the indian ocean and by local and regional antarctic seismic networks to identify the iht source mechanism and to understand its relevance to iceberg calving evolution and breakup we deployed seismometers on a giant 25 by 50 km tabular iceberg called c16 in the ross sea antarctica during a uniquely accessible period austral summer 2003 2004 when it was aground against the northern shore of ross island during the deployment period c16 was in sporadic contact with another giant tabular iceberg b15a that was moving under the influence of local ocean currents this study reveals that the c16 associated iht was a manifestation of extended episodes of discrete repeating stick slip icequakes typically thousands of individual subevents per hour produced when the cliff like edges of the tabular icebergs underwent glancing strike slip collisions the iht signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes but is instead the consequence of long sequences of very regularly spaced and similar pulses of seismic radiation from these constituent stick slip subevents iht represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice shelf disintegration processes

fault coupling and potential for earthquakes on the creeping section of the central san andreas fault the 150 km long central section of the san andreas fault csaf in central california creeps at the surface and has not produced a large earthquake historically however sections of the san andreas fault to the north and south are known to have ruptured repeatedly in m 7 8 earthquakes it is currently unclear whether the creeping csaf could rupture in large earthquakes either individually or along with earthquakes on the locked sections to the north and south we invert global positioning system and interferometric synthetic aperture radar data with elastic block models to estimate the degree of locking on the csaf and place bounds on the moment accumulation rate on the fault we find that the inferred moment accumulation rate is highly dependent on the long term fault slip rate which is poorly constrained along the csaf the inferred moment accumulation rate normalized by shear modulus ranges from 3 28 104 to 5 85 107 m3 yr which is equivalent to a mw 5 5 7 2 earthquake every 150 years for a long term slip rate of 26 mm yr and mw 7 3 7 65 for a long term slip rate of 34 mm yr the comparisons of slip distributions with microseismicity and repeating earthquakes indicate a possible locked patch between 10 and 20 km depth on the csaf that could potentially rupture with mw 6 5

behavior of repeating earthquake sequences in central california and the implications for subsurface fault creep repeating earthquakes res are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth we investigate the occurrence of these res along faults in central california to determine which faults exhibit creep and the spatiotemporal distribution of this creep at the juncture of the san andreas and southern calaveras paicines faults both faults as well as a smaller secondary fault the quien sabe fault are observed to produce res over the observation period of march 1984 through may 2005 res in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time cumulative slip over the observation period at individual sequence locations is determined to range from 5 5 58 2 cm on the san andreas fault from 4 8 14 1 cm on the southern calaveras paicines fault and from 4 9 24 8 cm on the quien sabe fault creep at depth appears to mimic the behaviors seen for creep on the surface in that evidence of steady slip triggered slip and episodic slip phenomena are also observed in the re sequences for comparison we investigate the occurrence of res west of the san andreas fault within the southern coast range events within these re sequences occurred only minutes to weeks apart from each other and then did not repeat again over the observation period suggesting that res in this area are not produced by steady aseismic creep of the surrounding fault surface

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses