noise based monitoring and imaging of aseismic transient deformation induced by the 2006 basel reservoir stimulation abstractwe have analyzed the time dependent properties of the ambient seismic wavefield between 0 1 and 8 hz to detect resolve monitor and image the deformation induced by the water injection associated with the stimulation of the 2006 deep heat mining project in the city of basel switzerland the application of passive methods allowed the detection of an aseismic transient of approximately 35 days duration that began with the onset of the reservoir stimulation peak deformation was reached some 15 days after the bleed off and after the induced seismicity ceased we resolved a significant increase in seismic velocities and a simultaneous decorrelation of the noise correlation coda waveforms the wavefield properties implied that the material response was monitored mainly in the sedimentary layer 2 5 km above the stimulated volume that was approximately 4 5 km deep we inverted the velocity change and decorrelation data to estimate the spatial distribution of the medium changes the resulting imag

evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

changes in attenuation related to eruptions of mt ruapehu volcano new zealand abstract temporal variations of seismic attenuation have been observed at different volcanoes around the world and are often claimed to correlate with volcanic activity seismic attenuation and its variation in time at mt ruapehu volcano new zealand was measured for the period october 1990 to may 2005 using local earthquakes originating from a nearby cluster of earthquakes called the waiouru swarm as recorded on 5 permanent seismometer stations cross correlation of p and s waves reveals a similarity of events from this ongoing seismic source which suggests that alterations due to source and path effects are minimized therefore the swarm is suitable for the study of temporal changes in seismic attenuation the attenuation q c 1 of coda waves following the direct p and s waves are calculated for five frequency bands centred at 1 5 3 0 6 0 9 0 and 12 0 hz using the single scattering model a best fit equation of q c q 0 f with q 0 54 7 and 1 02 0 06 shows an overall high frequency dependence of coda attenuation which is characteristic for tectonically active regions small fluctuations of q c 1 are observed at all stations over the entire time period and do not act as an apparent indicator of volcanic activity relative changes in integrated direct wave attenuation t along the whole path are obtained using a spectral ratio method in which spectra of individual 1 second p wave windows are compared to a reference spectrum during the time of volcanic activity the following changes were significant at the 95 confidence level in the low frequency band 1 5 6 hz a high attenuation anomaly with an abrupt increase of t by 0 23 s was detected in waveforms that have travelled through the volcano and a shift from negative to positive t by 0 25 s was observed at a station next to the volcano we suggest that these observations are related to variations detected in seismic anisotropy studies which propose a model of a change in stress caused by pressurisation of magma dykes under the volcano

evidence for temporal variation of seismic velocity within the upper continental crust observations of systematic temporal variations of seismic anisotropy are presented for an induced seismicity experiment at 9 km depth these observations were made under particularly well controlled conditions in the german continental deep drilling program ktb borehole using shear wave splitting from similar events recorded at a three component instrument located at 4 km depth from a hydraulic fracturing experiment at 9 km depth in a large set of seismic events recorded during the experiment many can be associated with multiplets exhibiting essentially identical waveforms since they must have approximately the same source location and source radiation pattern these events are particularly useful for testing the hypothesis of time dependent anisotropy anisotropy itself is clearly a very prominent feature in the data a simple approach for waveform matching of split shear waves allows unprecedented resolution of variations in shear wave splitting importantly the variation of shear wave splitting with time is a relative measurement which can be performed with higher accuracy than the associated absolute measurement in particular the relative measurement is not affected by timing errors nor by event distance variations during the experiment the difference between shear wave velocities decreases by 2 within 12 hours after that the medium apparently approaches a state which is stable for at least 5 hours we suggest that the temporal variation is due to the tectonic stress release from seismic events caused by the fluid injection this model requires the presence of fluid filled cracks at depths larger than 4 kin

postseismic fault healing on the rupture zone of the 1999 m 7 1 hector mine california earthquake we probed the rupture zone of the october 1999 m 7 1 hector mine earthquake using repeated near surface explosions in october 2000 and november 2001 three dense linear seismic arrays were deployed across the north and south lavic lake faults llfs that broke to the surface in the mainshock and across the bullion fault bf that experienced minor slip in that event two explosions each year were detonated in the rupture zone one on the middle and one on the south llf we found that p and s velocities of fault zone rocks increased by 0 7 1 4 and 0 5 1 0 between 2000 and 2001 respectively in contrast the velocities for p and s waves in surrounding rocks increased much less this trend indicates that the hector mine rupture zone has been healing by strengthening after the main shock most likely due to the closure of cracks that opened during the 1999 earth quake the observed fault zone strength recovery is consistent with an apparent crack density decrease of 1 5 within the rupture zone the ratio of travel time decrease for p to s waves was 0 72 consistent with partially fluid filled cracks near the fault zone this restrengthening is similar to that observed after the 1992 m 7 4 landers earthquake which occurred 25 km to the west li and vidale 2001 we also find that the velocity increase with time varies from one fault segment to another at the hector mine rupture zone we see greater changes on the llfs than on the bf and the greatest change is on the middle llf at shallow depth we tentatively conclude that greater damage was inflicted and thus greater healing is observed in regions with larger slip in the mainshock

faulting style controls for the space time aftershock patterns abstract to understand the complexity and thus to predict earthquake occurrence in size time and space seismicity patterns are characterized by two robust empirical laws the exponential distribution of magnitude and the power law decrease of aftershock rate over time these laws are known as the gutenberg richter law and the omori law respectively using global earthquake catalogs we resolve that on average the k aftershock productivity and the p value exponent of the power law decrease of aftershock rate over time are dependent on the mainshock faulting style strike slip events have a lower aftershock rate n and k values and a larger p value than thrust and normal events respectively within the epidemic type aftershock sequence model strong k n values are driven by a high branching ratio value n within the same framework a relatively higher n value for the thrust events also predicts the lower p value we observe for thrust events as compared to strike slip and normal faulting events respectively furthermore we observed that earthquake interactions through time and space are a function of the faulting style when measured by t the exponent of the power law decrease of earthquake density over space the t values of thrust events for different time windows always remain smaller than those of the strike slip events when changes in faulting styles are driven by stress pattern the anderson faulting theory predicts thrust faulting that requires somewhat larger stresses in absolute magnitude than do normal and strike slip faulting within the framework of rate and state friction law changes in the stress heterogeneity patterns reproduce the p value changes we observe our results suggest that only stress perturbations associated with mainshock rupture affect the productivity and decay rate over time of aftershocks

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

time frequency coherency analysis of three component crosshole seismic data for arrival detection a processing method for three component wide band crosshole seismic data is presented it is based on the observed phenomena of a decrease in the coherency between the coda waves recorded by closely spaced sensors or excited by closely spaced seismic sources we transfer signals to the time frequency domain and compute the instantaneous coherency at each moment of time between signals propagated on adjacent raypaths as a result we obtain the distribution of the high coherency time frequency areas connected with coherent components of the signal we can then see different modes of the recorded seismic signal and identify them as direct p and s waves reflected waves and converted tube waves a reliable method of evaluation of the onset s time for different modes of the seismic signal is also proposed the technique has good resolution and allows separation of even very closely spaced wavefield components

stability of coda wave attenuation during the loma prieta california earthquake sequence the loma prieta california earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the october 1989 mainshock this affords an unprecedented opportunity to detect changes in seismic wave propagation in the earth s crust associated with a major earthquake in this study we use pairs of nearly identical earthquakes doublets to search for temporal changes of coda attenuation in the vicinity of the loma prieta earthquake we analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic coseismic and postseismic intervals and measure the change in coda q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 hz this method provides an estimate of changes in coda q that is insensitive to other factors that influence coda amplitudes our observations place an upper bound of about 5 on preseismic coseismic and postseismic changes of coda q in the epicentral region of the loma prieta earthquake even at this low level the changes are neither spatially coherent nor correlated between adjacent frequency bands the only hint of a signal is in the preseismic data where there is a possible precursory increase in coda q of approximately 5 in the two years before the mainshock the stability of coda q throughout the loma prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda q for other earthquakes

differential analysis of coda q using similar microearthquakes in seismic gaps part 1 techniques and application to seismograms recorded in the anza seismic gap abstract a large but conflicting literature exists that suggests that the scattering properties of the earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes to search for temporal variations in coda q in the vicinity of an expected moderate earthquake nucleation region along the san jacinto fault zone pairs and clusters of earthquakes recorded by the anza seismic network between 1982 and 1992 are examined to minimize differences arising from path and source variability only the most similar examples of microearthquake wave fields are intercompared to assess coda q differences between similar microearthquake signals we search for systematic temporal changes in moving window spectral amplitude ratios for a single scattering coda model with an envelope decay function of the form t m e ft qc the relationship between the natural log amplitude spectral ratio and q 1 q 1 c 2 q 1 c 1 is linear i e ln r f t ft q 1 f w f where w f is a time independent relative source term to obtain robust uncertainty estimates of q 1 f and w f we utilize multi taper spectral analysis coupled with a nonparametric monte carlo confidence interval estimation procedure for the most similar events in the anza region coda cross correlation values of approximately 0 7 for 16 sec of coda we find that coda q as a function of time is stable at a typical 1 resolution of only between 25 and 50 of the reference coda q value for the area even in the best constrained frequency bands comparison of similar microearthquake coda signals with both short and long temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda q to obtain estimates of relative coda q that are more precise than this level the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the anza region such earthquakes have been observed to occur in other regions of the san andreas fault system and are analyzed in antolik et al 1996

a search for temporal variations in station terms in southern california from 1984 to 2002 we use relative arrival times and locations for similar earthquake pairs that are found using a cross correlation method to analyze the time dependence of p and s station terms in southern california from 1984 to 2002 we examine 494 similar event clusters recorded by southern california seismic network scsn stations and compute absolute arrival time variations from the differential arrival time resi duals obtained following event relocation we compute station terms from the robust means of the absolute arrival time residuals from all events recorded by each station at 3 month intervals we observe nine stations with abrupt offsets in timing of 20 70 msec which are likely caused by equipment changes during our study period taking these changes into account could improve the relative location accuracy for some of the event clusters for other stations we generally do not see systematic temporal variations greater than about 10 msec analysis of residuals along individual ray paths does not reveal any clear localized regions of apparent velocity changes at depth these results limit large scale long lasting temporal variations in p and s ve locities across southern california during this time period to less than about 0 2 however there is an increased fraction of individual travel time residuals exceeding 20 msec immediately following major earthquakes from source regions near the main shock rupture

coseismic velocity change associated with the 2011 van earthquake m7 1 crustal response to a major event monitoring coseismic velocity changes is a major challenge since the earth crust has to be uniformly sampled at preseismic coseismic and postseismic stages using repeating active or natural sources here we investigate the crustal response to the 2011 van turkey earthquake using ambient noise which provides the best possible temporal resolution combined recordings from the nearest five broadband stations are analyzed for a time period of 6 months framing the main shock we observe a coseismic velocity decrease of up to 0 76 in the vicinity of the main shock in the frequency range of 0 05 0 3 hz the velocity drop is largest at close proximity to the earthquake hypocenter and decreases systematically with distance we also find a correlation between coseismic velocity decrease and the amount of coseismic slip on the rupture plane the observed velocity drop shows the drastic response of the brittle crust in response to a major earthquake

seismic waveform attributes before and after the loma prieta earthquake scattering change near the earthquake and temporal recovery during 1987 1995 several clusters of nearly identical seismic events multiplets occurred near the loma prieta source region these multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 loma prieta main shock for seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency post loma prieta events have reduced coherencies with pre loma prieta events in a spatially limited region close to the loma prieta hypocenter while other paths remain nearly unaffected these changes gradually recover within a time interval of 5 years after the loma prieta earthquake a possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the loma prieta event postseismic relaxation processes such as crack healing fluid diffusion or after deformations lead to progressive closure of these cracks with time after the main shock thus the scattering properties of the local crust approach the pre main shock state

changes in source and site effects compared to coda q 1 temporal variations using microearthquakes doublets in california we analyse spectral ratio of the coda of doublets of microearthquakes our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake a moving window cross spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram for several frequency bands sr t f plot from a certain pattern in thesr t f plot we should infer a temporal change in coda attenuation several doublets recorded in central california by the usgs network are analysed

dating cleavage formation in slates and phyllites with the 40ar 39ar laser microprobe an example from the western new england appalachians usa determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts however when applied to cleavages in slates and phyllites conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations in situ high spatial resolution 40ar 39ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains three regionally developed cleavages s2 s3 and s4 are present in the northern taconic allochthon of vermont and new york representative samples were studied from a variety of localities where these cleavages which are defined by white micas are well developed in the suite of samples only s3 and s4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40ar 39ar geochronology mean 40ar 39ar laser microprobe ages for these domains are 370 7 1 0 myr for s3 and 345 5 1 7 myr for s4 because estimates of the ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining s3 and s4 these values are interpreted as periods since cleavage formation this interpretation is consistent with independent geochronological constraints on the age of the acadian orogeny in the region

relation of the 1992 landers california earthquake sequence to seismic scattering measurements of crustal scattering for the area surrounding the 1992 landers earthquake sequence obtained from regional array recordings of teleseismic events for the 10 year period before the sequence showed that the slip distribution on faults could be deducible from the preshock elastic structure scattering intensity correlated strongly with the distribution of aftershocks and slip of the moment magnitude mw 7 3 landers main shock mw 6 1 joshua tree and mw 6 2 big bear events which implies that aftershocks and slip are structurally controlled and broadly predictable scattering within the fault zones was directional and consistent with variable along strike alignment of stress induced cracks

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

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

migration of seismic scatterers associated with the 1993 parkfield aseismic transient event the time varying deformation field within a fault zone particularly at depths where earthquakes occur is important for understanding fault behaviour and its relation to earthquake occurrence1 2 3 but detection of this temporal variation has been extremely difficult although laboratory studies have long suggested that certain structural changes such as the properties of crustal fractures should be seismically detectable4 here we present evidence that such structural changes are indeed observable in particular we find a systematic temporal variation in the seismograms of repeat microearthquakes that occurred on the parkfield segment of the san andreas fault over the decade 1987 97 our analysis reveals a change of the order of 10 m in the location of scatterers which plausibly lie within the fault zone at a depth of 3 km the motion of the scatterers is coincident in space and time with the onset of a well documented aseismic transient deformation event we speculate that this structural change is the result of a stress induced redistribution of fluids in fluid filled fractures caused by the transient event

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

source array analysis of coda waves near the 1989 loma prieta california mainshock implications for the mechanism of coseismic velocity changes coda waves are often considered to be generated by backscattering of primary waves from randomly distributed heterogeneities in the crust however modeling and experimental work indicates that much of the coda may be due to scattering near the receiver knowing where the coda is generated is important for several reasons coda waves have been used to characterize site amplification scattering and attenuation throughout the crust and velocity changes associated with large earthquakes however a lack of knowledge of where coda waves are generated can lead to ambiguous interpretations in this study we analyze 26 s of coda waves recorded at up to 78 stations using slowness stacking on two source arrays and find that at nearly all stations regardless of distance from the source nearly all the arrivals are within one hemisphere most are strongly clustered near the direct arrival and the clusters are in general symmetrically disposed about the array axis fault plane this finding suggests that the coda consists primarily of waves scattered near the stations rather than waves scattered throughout the crust this result holds even at stations very close to the sources where the amount of coda examined is about 6 times the direct s travel time and it suggests that much of the coseismic velocity decrease associated with the 1989 loma prieta earthquake ellsworth et al 1992 occurred in the shallow crust near the stations one possible mechanism for inducing such a change is a decrease in the mean stress from the mainshock slip however on the basis of static stress modeling we find that this mechanism would produce a velocity increase in the region where velocity was observed to decrease we conclude that the velocity decrease was more likely caused by crack opening or crack coalescence due to strong shaking from the mainshock

seismic cycles recorded in late quaternary calcite veins geochronological geochemical and microstructural evidence southwest turkey is seismically active as a result of the hellenic subduction process in the eastern mediterranean we conducted high resolution micro sampling high precision u series dating and microchemical analysis on an extensional vein system in a tectonically active but non hydrothermal area u th age data and microscopic observations provide evidence of repeated fracturing of a previously sealed crack system followed by a new increment of veining repeated injection of veinlets suggests that the vein system was formed by the crack seal mechanism four major u th age groups for the emplacement of the vein system fall between 23 9 0 2 ka and 23 2 0 4 ka 21 7 0 4 ka and 19 2 0 2 ka 17 3 0 1 ka and 16 2 0 3 ka and at 11 8 0 2 ka stable and sr isotope geochemistry of the calcite vein samples indicates that surface water interacting with the soil cover was the major component of the groundwater system from which the extensional veins precipitated trace element and 0 isotope data of the vein system are interpreted to reflect carbonate precipitation associated with seismic cycles involving fluids with different trace element compositions and co2 contents initial carbonate precipitation during a single seismic cycle occurred from co2 dominated fluids that were degassed from the original co2 water mixture this was followed consecutively by carbonate precipitation from the remaining water which was relatively impure with higher trace element contents millimetre to submillimetre scale u series dating in conjunction with geochemistry of carbonate veins related to active tectonism offers an innovative means of constraining the absolute timing of late quaternary seismic and inter seismic events c 2010 elsevier b v all rights reserved

origins of amplitude variations in seismic doublets source or attenuation process summary accurate estimates of spectral ratios of two doublets i e similar seismic events occurring along the san andreas fault in the hollister region have been computed using a cross spectral method both events of each doublet occurred within a 24 hr interval although both waveforms of a doublet are remarkably similar computed spectral ratios exhibit strong but steady variations with frequency in the first arrivals the attenuation process can explain neither the sign nor the order of magnitude of these variations on the contrary they are closely related to the source mechanism we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events one doublet exhibits an azimuthal distribution of the spectral ratio variations that is interpreted as the result of a change in the position of the hypocentres relative to the rupture area for this doublet location of hypocentres varies only from 10 to 30 per cent of the source length this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets i e in the amplitude of each event for the other doublet the variations are related to a change in source length we also have computed spectral ratios all along the seismograms using a moving window technique when an azimuthal distribution of the spectral ratio is observed in the first arrivals we notice a decrease of the spectral ratio all along the seismograms the doublet with strong but non azimuthal variations of the spectral ratio in the first arrivals does not indicate such a decrease of the spectral ratio along the seismogram this implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters these variations have the same order of magnitude as those provoked by a temporal change in coda q of 10 per cent they can occult time changes of spectral ratio for instance in case the attenuation in the crust would vary before a strong earthquake

time lapse monitoring of rock properties with coda wave interferometry 1 the coda of seismic waves consists of that part of the signal after the directly arriving phases in a finite medium or in one that is strongly heterogeneous the coda is dominated by waves which have repeatedly sampled the medium small changes in a medium which may have no detectable influence on the first arrivals are amplified by this repeated sampling and may thus be detectable in the coda we refer to this use of multiple sampling coda waveforms as coda wave interferometry we have exploited ultrasonic coda waves to monitor time varying rock properties in a laboratory environment we have studied the dependence of velocity on uniaxial stress in berea sandstone the temperature dependence of velocity in granite and in aluminum and the change in velocity due to an increase of water saturation in sandstone there are many other possible applications of coda wave interferometry in geophysics including dam and volcano monitoring time lapse reservoir characterization earthquake relocation and stress monitoring in mining and rock physics

coseismic and postseismic velocity changes measured by repeating earthquakes 1 repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the earth s crust since source and path effects are common to all earthquakes in a repeating earthquake sequence multiplet differences in their waveforms can be attributed to changes in the characteristics of the medium we have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 loma prieta and 1984 morgan hill california earthquakes postmain shock events reveal delays of phases in the early s wave coda of as much as 0 2 s relative to premain shock events the delay amounts to a path averaged coseismic velocity decrease of about 1 5 for p waves and 3 5 for s waves since most of the multiplets are aftershocks and follow omori s law we have excellent temporal sampling in the immediate postmain shock period we find that the amplitude of the velocity decrease decays logarithmically in time following the main shock in some cases it returns to the premain shock values while in others it does not similar results are obtained for the morgan hill main shock because the fractional change in s wave velocity is greater than the fractional change in p wave velocity it suggests that the opening or connection of fluid filled fractures is the underlying cause the magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change our results suggest that the changes are predominantly near the stations and shallow but we cannot exclude the possibility that changes occur at greater depth as well if the variations are shallow we may be detecting the lingering effects of nonlinearity during main shock strong ground motion if the variations are deep it suggests that pore pressures at seismogenic depths are high which would likely play a key role in the earthquake process

seismic evidence for rock damage and healing on the san andreas fault associated with the 2004 m 6 0 parkfield earthquake we deployed a dense linear array of 45 seismometers across and along the san andreas fault near parkfield a week after the m 6 0 parkfield earthquake on 28 september 2004 to record fault zone seismic waves generated by aftershocks and explosions seismic stations and explosions were co sited with our previous exper iment conducted in 2002 the data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 m 6 0 mainshock show 1 0 1 5 decreases in seismic wave velocity within an 200 m wide zone along the fault strike and smaller changes 0 2 0 5 beyond this zone most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 m 6 0 earthquake the width of the damage zone characterized by larger velocity changes is consistent with the low velocity waveguide model on the san andreas fault near parkfield that we derived from fault zone trapped waves li et al 2004 the damage zone is not symmetric but extends farther on the southwest side of the main fault trace waveform cross correlations for repeated aftershocks in 21 clusters with a total of 130 events located at different depths and distances from the array site show 0 7 1 1 increases in s wave velocity within the fault zone in 3 months starting a week after the earthquake the velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time most likely due to the closure of cracks opened during the mainshock we estimate that the net decrease in seismic velocities within the fault zone was at least 2 5 caused by the 2004 m 6 0 parkfield earthquake the healing rate was largest in the earlier stage of the postmainshock healing process the magnitude of fault healing varies along the rupture zone being slightly larger for the healing beneath middle mountain correlating well with an area of large mapped slip the fault healing is most promi nent at depths above 7 km

seismic velocity reductions caused by the 2003 tokachi oki earthquake 1 we use four repeating earthquake sequences located near hokkaido to identify velocity changes caused by the mw 8 0 2003 tokachi oki earthquake using a moving window cross correlation technique we identify delays in the arrival time of seismic waves that accumulate linearly with time into the seismogram this behavior is indicative of multiple scattering within a medium where the seismic velocity has been reduced for all of our earthquake receiver geometries we find evidence of significant velocity reductions close to the receiver the correlation of the size of the velocity reductions with both strong shaking and site characteristics suggests that these velocity reductions are caused by damage to near surface materials created by nonlinear strong ground motion for earthquake receiver geometries where the seismic waves cross the tokachi oki rupture zone we identify particularly large reductions in velocity as a result of the earthquake for these geometries we believe that the rupture zone of the tokachi oki earthquake or the shallow crust above it represents a second region where seismic velocities are reduced as a result of the main shock

temporal changes in p wave attenuation in the loma prieta rupture zone 1 clusters of recurring nearly identical microearthquakes have been recorded near loma prieta california such events make possible direct measurement of temporal changes in the attenuation operator t along fixed paths under what are tantamount to laboratory conditions analysis of almost 10 years of p wave data following the 18 october 1989 loma prieta california main shock m 7 0 reveals a large amplitude t increase within the first 3 weeks of the main shock a gradual t recovery toward its level prior to the main shock is seen in the years ensuing the results point to changes in porosity and fluid saturation taking place in parts of the main shock source volume stemming from coseismic crack opening and postseismic relaxation processes such as postseismic fluid flow into the cracks crack healing and rock compaction

observations of changes in waveform character induced by the 1999 m w7 6 chi chi earthquake 1 we observe changes in the waveforms of repeating earthquakes in eastern taiwan following the 1999 mw7 6 chi chi earthquake while their recurrence intervals appear to be unaffected there is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the chi chi event these changes are limited to stations in and paths that cross the 70 100 km region surrounding the chi chi source area the area where seismic intensity and co seismic surface displacements were largest this suggests that damage at the near surface is responsible for the observed waveform changes delays are largest in the late s wave coda reaching approximately 120 ms this corresponds to a path averagedswave velocity reduction of approximately 1 there is also evidence that damage in the fault zone caused changes in waveform character at sites in the footwall where source receiver paths propagate either along or across the rupture the reduction in waveform similarity persists through the most recent repeating event in our study november 15 2007 indicating that the subsurface damage induced by the chi chi earthquake did not fully heal within the first 8 years following the chi chi earthquake

depth extent of damage zones around the central calaveras fault from waveform analysis of repeating earthquakes summary we systematically investigate spatial variations of temporal changes and depth extent of damage zones along the calaveras fault that ruptured during the 1984 morgan hill earthquake by the waveform analysis of 333 sets of repeating earthquakes we use a sliding window waveform cross correlation technique to measure traveltime changes in waveforms generated by each repeating cluster we find clear traveltime delays in the s and early s coda waves for events immediately after the morgan hill main shock the amplitudes of the time delays decrease logarithmically with time since the main shock indicating a time dependent recovery healing process following the abrupt coseismic temporal changes the largest temporal changes are observed at station cco that is the closest to the rupture zone of the morgan hill main shock the time delays at this station are larger for clusters in the top 6 km and decrease systematically at larger depth in comparison the time delays observed at other five stations are much smaller and do not show clear relationship with hypocentral depth we suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near surface layers while the temporal changes at station cco are likely associated with the damage zone around the calaveras fault that is well developed in the top few kilometres of the upper crust our results are consistent with the inference of a widespread damage and non linearity in the near surface layers associated with strong ground motions of nearby large earthquakes and localized damages and flower type structures around active faults based on previous studies of fault zone structures and recent 3 d numerical simulations

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

seismic imaging of scatterer migration associated with the 2004 parkfield earthquake using waveform data of repeating earthquakes and active sources we used borehole seismic records of four repeating earthquake clusters and two explosions to investigate the coseismic changes of the scattering wave field of the 28 september 2004 m 6 parkfield earthquake we found systematic changes in p wave and s wave coda recorded from repeating events that occurred before and immediately after the earthquake applying the coda wave interferometry technique allowed us to determine that the observed changes are caused by a localized change in the scattering field we further developed a technique based on decorrelation indexes calculated from running time windows to locate migrating scatterers synthetic tests showed that the technique is relatively insensitive to changes in background velocity of the medium and source location and thus can be applied to records of loosely colocated clusters we found a localized change of material property within the fault zone at 3 km depth beneath the middle mountain area the change is shown most clearly in the p to s scattering mode of the active source data and both the p to s and s to s scattering modes of the repeating earthquakes data suggesting that the observed scattering property change is a result of charge or discharge of fluids in fractures caused by the 2004 parkfield earthquake the same scatterer s was found to respond to the 1993 aseismic stress strain transient event acting like an in situ stress strain meter at seismogenic depth four dimensional time lapse imaging of the scattering wave field thus provides an effective way to monitor the subsurface stress strain field

stability of coda q in the region of parkfield california view from the u s geological survey parkfield dense seismograph array many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate to large earthquakes parkfield california on the san andreas fault is a promising location for studying premonitory changes in coda q qc because a large earthquake is likely to occur there we have investigated qc using recordings from the u s geological survey parkfield dense seismograph array which is a digital array with 14 triaxial sensors and an aperture of about 1 km for each earthquake we can measure qc from up to 42 recordings their average is more stable than the measurement from a single station using clustered seismicity we have developed criteria for selecting events and reducing scatter in the measurement the qc value determined from a seismogram depends on the position and length of the analysis window thus qc should always be measured from the same length window starting at the same lapse time regardless of the source location in addition the band limited signal to noise ratio at the end of the analysis window is important qc determined in two frequency bands 4 8 hz and 8 16 hz from a tight cluster of 26 events which occurred between december 1989 and january 1994 has not changed despite m 4 7 and m 4 6 events in october 1992 and november 1993 qc measured from local events 60 km in three frequency bands shows larger scatter but has also not changed during this period for monitoring qc observations should include array averaged measurements from a single lapse time because qc measurements made using an analysis window that starts at a constant multiple of the s wave lapse time depend on epicentral distance a procedure combining the evaluation of the time and distance dependences of qc also gives stable observations

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

postseismic deformation following the 1989 m 7 1 loma prieta california earthquake postseismic deformation along a 90 km profile bisecting the projected surface trace of the coseismic rupture of the 1989 loma prieta earthquake has been monitored by frequent gps surveys for 3 3 years following the earthquake in addition to the expected deformation associated with secular strain accumulation on the san andreas and calaveras faults deformation associated with postseismic readjustment has been detected most of that deformation can be attributed to 1 5 m right lateral and 0 9 m reverse postseismic slip on a 5 km wide downdip extension of the loma prieta rupture in addition there seems to be a 0 1 m postseismic collapse of the loma prieta rupture zone in the direction perpendicular to the plane of the rupture the fault normal n48 e surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1 4 years similar plots of the fault parallel n42 w displacement components do not exhibit significant curvature presumably the deformation shown in those plots is dominated by secular strain accumulation along the san andreas and calaveras faults rather than postseismic relaxation

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

azimuthal seismic anisotropy in the upper crust of the japanese islands induced by the 2011 tohoku oki megathrust earthquake

time dependent triggered afterslip following the 1989 loma prieta earthquake we employ a modified version of the network inversion filter to investigate time dependent slip following the 1989 loma prieta earthquake previous analysis of global positioning system gps and leveling data suggests afterslip on the loma prieta rupture as well as aseismic slip on a thrust fault northeast of the san andreas fault which we identify with the foothills thrust belt we analyzed 173 daily gps solution files at 62 stations collected from 1989 8 to 1998 3 a total of 1 134 three dimensional relative baseline determinations the observed position changes are assumed to result from secular deformation random benchmark motions and temporally varying fault slip the data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip the amount of temporal smoothing is estimated by maximum likelihood conditional on this estimate reverse slip on the foothills thrust decays from 45 12 mm yr immediately after the earthquake to zero by 1992 reverse slip on the loma prieta rupture surface decays from 57 11 mm yr to zero by 1994 right lateral slip on the loma prieta rupture surface decays monotonically from 30 10 mm yr to zero by 1994 these results suggest that 1 triggered afterslip can occur off the main rupture zone on adjacent faults 2 shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake and 3 postseismic slip on the foothills thrust may account for a significant portion of its total slip budget

monitoring change in volcanic interiors using coda wave interferometry application to arenal volcano costa rica 1 multiply scattered waves are a new tool for monitoring changes in the earth s subsurface in this study we use the air pressure and displacement recorded at the arenal volcano to detect temporal changes the pressure pulses emitted by the volcano are associated with a complex displacement signal of overlapping coda waves by deconvolving the ground motion with the air pressure for different time intervals we obtain a highly repeatable displacement signal that has the character of diffusive wave propagation this deconvolved displacement signal changes with the time interval analyzed in the employed data this change can be explained by a shift in the point of excitation of the elastic waves over a distance of about 15 meters

differential analysis of coda q using similar microearthquakes in seismic gaps part 2 application to seismograms recorded by the parkfield high resolution seismic network abstract nearly two thirds of the microseismicity occurring at parkfield california consists of repeating clusters of closely located events exhibiting waveform cross correlation values exceeding 0 95 this observation coupled with a high probability for an m 6 earthquake within the next decade working group on california earthquake probabilities 1988 make this segment of the san andreas fault system a prime location to search for temporal variations in coda q associated with crustal changes that may occur during the earthquake cycle the coda comparison analysis method described in part 1 of this study aster et al 1996 is applied to data from four borehole stations of the parkfield high resolution seismic network hrsn for the period january 1987 through february 1994 we examine 21 of the nearly 300 repeating clusters observed to date the restriction of the data set to only the most highly similar sources is observed to be critical small differences in the source process even for event pairs with centroid locations within approximately 20 m significantly increase the spurious noise level in the parameter estimates absolute 68 confidence bound estimates obtained on the repeatability of coda q for all microearthquake pairs are 10 for frequencies below 20 hz tighter constraints are obtainable by reconstructing the temporal history of coda q from the first difference measurements between microearthquake pairs these functional reconstructions show that the mean value of coda q has not varied by more than approximately 5 from 3 to 30 hz at the 1 level during the study period however other observables such as an increased level of microseismicity suggest that a preparatory stage for the next parkfield earthquake has begun indicating that coda q is so far showing no sign of sensitivity to preparatory processes at parkfield

postseismic response of repeating aftershocks the recurrence intervals of repeating earthquakes on the san andreas fault in the loma prieta aftershock zone follow the characteristic 1 t decay of omori s law a model in which these earthquakes occur on isolated patches of the fault that fail in stick slip with creep around them can explain this observation in this model the recurrence interval is inversely proportional to the loading rate due to creep logarithmic velocity strengthening friction predicts 1 t decay in creep rate following the mainshock the time dependence of recurrence is inconsistent with a simple viscous constitutive relationship which predicts an exponential decay of loading rate thus our observations imply postseismic slip at seismogenic depth under a power law rheology the time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths

compaction of porous rock by dissolution on discrete stylolites a one dimensional model 1 compaction of sedimentary porous rock by dissolution and precipitation is a complex deformation mechanism that is often localized on stylolites and pressure solution seams we consider a one dimensional model of compaction near a thin clay rich stylolite embedded in a porous rock under the assumption that the clay enhances solubility the model predicts a reactive transport away from the clay layer followed by pore cementation the evolution of the porosity reactant transport and compaction rate are studied as functions of model parameters and shown to reach a stationary state we find good agreement between the porosity distribution predicted by the model and previously reported field measurements the model provides quantitative estimates for compaction rates on stylolitic surfaces