seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

accelerated stress buildup on the southern san andreas fault and surrounding regions caused by mojave desert earthquakes a sequence of four mw 6 earthquakes including the 1992 mw 5 7 3 landers and mw 5 7 1 hector mine earthquakes occurred in the mojave desert in the 1990s in close proximity to the southern san andreas fault inducing stress changes on several of its segments we calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and or upper mantle has led to a coulomb stress increase of 2 3 3 5 bar on the san bernardino mountain segment of the southern san andreas fault between 1992 and 2001 with a projected increase of 3 6 4 9 bar by the year 2020 in comparison the calculated coseismic stress increase is 1 8 bar for this segment this accelerated buildup of stresses is predicted to bring the san bernardino mountain segment which last ruptured more than 190 yr ago closer to a potentially major rupture meanwhile we project a net stress decrease of as much as 23 5 bar between 1992 and 2020 for the western coachella valley segment if the fault is governed by low effective friction or an increase of 1 5 bar if the fault is governed by high effective friction coulomb stresses are calculated to decrease on the mojave segment by as much as 21 bar between 1992 and 2020 accelerated stress buildup is also predicted to occur on parts of the san jacinto elsinore and calico faults the pattern of the observed post landers aftershock clustering and the calculated coulomb stress buildup on the calico fault is similar to that noted in the hector mine region prior to the 1999 mw 5 7 1 earthquake these results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

small scale stress heterogeneity in the anza seismic gap southern california focal mechanism inversions reveal significant lateral variations in stress orientations along the anza segment of the san jacinto fault zone the most notable stress anomaly is within the 20 km aseismic seismic gap portion of the fault zone where 1 the maximum compressive stress is nearly horizontal and is oriented at 74 13 relative to the fault strike this contrasts with orientations ranging from 62 11 to 49 7 along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap regional stress results found by inverting all focal mechanisms simultaneously indicate that 1 is horizontal and trends north south while 3 is horizontal and trends east west approximately 15 km west of the seismic gap in the off fault cahuilla swarm area 1 and 3 solutions are rotated clockwise by about 25 relative to the regional model roughly 10 km southeast of the seismic gap near the buck ridge fault 1 and 3 are rotated counterclockwise by about 10 relative to the regional solution northwest of the seismic gap along the fault zone 3 plunges about 30 from the horizontal correlating with a local increase in reverse faulting between the hot springs and san jacinto faults southeast of the seismic gap 1 plunges about 45 from the horizontal correlating with a local increase in normal faulting in the trifurcation region of the buck ridge clark and coyote creek faults we propose a simple mechanical model in which a block rotation superimposed on the dominant right lateral strikeslip motion of the fault zone satisfies the first order observations of stress orientation faulting and horizontal surface strain under this model the anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap

postseismic variations in seismic moment and recurrence interval of repeating earthquakes in laboratory experiments longer stationary contact time leads to larger seismic moment during repeated ruptures however not all observations in natural fault systems agree with the prediction we analyze a subset of 34 m 0 4 2 1 repeating earthquake sequences res from 1987 to 2009 at parkfield to examine the variation of their recurrence properties in space and time following a 2004 m6 earthquake many of the repeating events have greatly reduced recurrence intervals tr that systematically increase with time in addition to this change in timing we also find systematic changes in seismic moment mo where most sequences experienced an immediate increase in mo and subsequent decay as tr approached pre quake durations the res at shallower depth tend to have a larger range in both tr and mo whereas deeper res show smaller variation the shallowest res with the greatest magnitude m1 8 2 1 among the events we studied reveal a large variation in tr but small variation in mo these observations are qualitatively consistent with earthquake simulations in 3d continuum fault models with rate and state dependent friction in the models res are produced on velocity weakening patches surrounded by velocity strengthening fault areas the models show that the degree of postseismic variation in mo and tr is a function of radius r and nucleation zone size h of the velocity weakening patch a ratio of r h 1 produces negative mo tr slopes whereas larger ratios of r h yield weak positive slopes given the same nucleation size h i e the same frictional properties and effective normal stress smaller radii and hence smaller seismic moments result in negative mo tr slopes whereas larger radii and hence larger moments lead to weak positive mo tr slopes which are consistent with observations conversely with only a small percentage of its slip accumulated seismically a small asperity appears to grow in mo under high loading rate which is contrary to the view that mo should decrease due to a reduced strength recovery time

preliminary velocity and resistivity models of the loma prieta earthquake region a preliminary three dimensional velocity model of the loma prieta epicentral region in the santa cruz mountains of california has been derived using raypaths from aftershocks recorded by the u s g s seismic network in addition a magnetotelluric sounding profile was completed prior to the earthquake and a two dimensional resistivity model computed the velocity and resistivity models include a low resistivity 3 10 ohm m low velocity 3 3 5 8 km s wedge of material between the zayante and san andreas faults that extends to depths of over 8 km and is interpreted to be largely tertiary marine sedimentary rocks a high resistivity 1000 ohm m high velocity 6 4 6 7 km s zone between the san andreas and sargent faults may represent largely mafic intrusive rocks

an independent assessment of the load unload response ratio lurr proposed method of earthquake prediction the load unload response ratio lurr method is a proposed technique to predict earthquakes that was first put forward by yin 1987 lurr is based on the idea that when an area enters the damage regime the rate of seismic activity during loading of the tidal cycle increases relative to the rate of seismic activity during unloading in the months to one year preceding a large earthquake since earth tides generally contribute the largest temporal variations in crustal stress it seems plausible that earth tides would trigger earthquakes in areas that are close to failure e g vidale et al 1998 however the vast majority of studies have shown that earth tides do not trigger earthquakes e g vidale et al 1998 heaton 1982 rydelek et al 1992 in this study we conduct an independent test of the lurr method since there would be important scientific and social implications if it were proven to be a robust method of earthquake prediction smith and sammis 2004 undertook a similar study and found no evidence that there was predictive significance to the lurr method we have repeated calculations of lurr for the northridge earthquake in california following both the parameters of x c yin personal communication and the somewhat different ones of smith and sammis 2004 though we have followed both sets of parameters closely we have been unable to reproduce either set of results our examinations have shown that the lurr method is very sensitive to certain parameters thus it seems likely that the discrepancies between our results and those of previous studies are due to unaccounted for differences in the calculation parameters a general agreement was made at the 2004 aces workshop in china between research groups studying lurr to work cooperatively to resolve the differences in methods and results and thus permit more definitive conclusions on the potential usefulness of the lurr method in earthquake prediction

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

earthquake depths and the relation to strain accumulation and stress near strike slip faults in southern california earthquakes in the major fault zones are predominantly deep earthquakes in the crustal blocks bounding the fault zones are predominantly shallow in the san jacinto fault zone maximum earthquake depths correlate with surface heat flow these relations together with focal mechanisms geodetic strain measurements and fault zone models are consistent with the following ideas 1 interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes 2 the aseismic slip in a narrow fault zone in the brittle plastic transition region concentrates strain at the base of the brittle fault zone 3 deep earthquakes occur in thelower part of the brittle fault zone due to stick slip failure of highly stressed patches 4 background earhtquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding deep and stick slip shallow regions of the fault zone furthermore the difference in seismicity between the san jacinto and southern san andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases 5 shear stress may be lessmore in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks 6 in the stress field produced by plate motion and slip in the deep fault zone the upper brittle fault zone is not oriented favorably for shear failure lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation less

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

effects of three dimensional velocity structure on the seismicity of the 1984 morgan hill california aftershock sequence abstract a three dimensional velocity model for the area surrounding the 24 april 1984 morgan hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival time data this velocity model corresponds well to the surface geology of the region predominantly showing a low velocity region associated with the sedimentary sequence to the southwest of the madrone springs fault this low velocity sequence extends to a depth of approximately 5 km previously constructed geologic cross sections for this region had suggested that these sediments extended to a depth of less than 2 km relocation of 2 179 hypocenters with the three dimensional velocity model did not show great differences when compared to locations done with a one dimensional velocity model with station corrections in both cases the calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the calaveras fault in the southern half of the rupture zone the greatest differences between the two sets of locations was that the locations using the three dimensional velocity model were deeper and fewer earthquakes were located within the low velocity region the focal mechanisms were also determined for 946 earthquakes using both the one dimensional and three dimensional earth models both earth models yield similar focal mechanisms for these earthquakes

triggering effect of m 4 5 earthquakes on the earthquake cycle of repeating events at parkfield california stress perturbations influence earthquake recurrence and are of funda mental importance to understanding the earthquake cycle and determining earthquake hazard the large population of repeating earthquakes on the san andreas fault at parkfield california provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes using 187 m 0 4 to 1 7 repeating earthquake sequences from the high resolution seismic network cat alog we find that the time to recurrence of repeating events subsequent to nearby m 4 5 earthquakes is shortened suggesting triggering by major events the triggering effect is found to be most evident within a distance of 5 km corresponding to static coseismic stress changes of 0 6 26 6 kpa and decays with distance we also find coherently reduced recurrence intervals from 1993 to 1998 this enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s online material figures of event chronologies of res estimates of dt and coseismic static stress changes

three dimensional tomography of the 1992 southern california earthquake sequence constraints on dynamic earthquake rupture tomographic inversion of p wave arrival times from aftershocks of 1992 southern california earthquakes is used to produce three dimensional images of subsurface velocity the preliminary 1992 data set augmented by the 1986 m 5 9 north palm springs sequence consists of 6458 high quality events recorded by the permanent regional network providing 76306 raypaths for inversion the target area consisted of a 104 x 104 x 32 km 3 volume divided into 52 x 52 x 10 rectilinear blocks significant velocity perturbations appear to correlate with rupture properties of recent major earthquakes preliminary results indicate that a low velocity anomaly separates the dynamic rupture of the m 6 5 big bear event from the m 7 4 landers main shock a similar low velocity region separates the m 6 1 joshua tree sequence from the landers rupture high velocity anomalies occur at or near nucleation sites of all four recent main shocks north palm springs joshua tree landersbig bear a high velocity anomaly is present along the san andreas fault between 5 and 12 km depth through san gorgonio pass this high velocity area may define an asperity where stress is concentrated and where future large earthquakes may begin

upper mantle structure from long period differential traveltimes and free oscillation data summary we have performed an inversion to model upper mantle structure using ss s differential traveltime residuals in conjunction with previously reported structure coefficient estimates obtained from free oscillation frequency and attenuation measurements the model we obtain is consistent with both the differential traveltime and structure coefficient data sets and provides nearly the same level of fit as independent inversions of these data sets we find that a relatively simple model parametrization appears to be adequate for the data used upper mantle velocity perturbations and lateral variations in the radii of boundaries at 400 and 670 km depth are parametrized with spherical harmonic expansions up to degree and order 8 the expansions are fixed at specific depth knots separated by roughly 200 km spanning the range 20 670 km radial variations in velocity are obtained by linear interpolation between these depth knots smoothness constraints were applied as part of the inversion procedure as well as corrections for the thickness and velocity variations of the crust the smoothness constraints were adjusted to require a smoother structure in the bottom portion 400 to 670 km depth of the model where the structure coefficients have the greatest sensitivity the upper part of the model is primarily constrained by the differential traveltime data the resulting misfit of the final model shows that on average the individual structure coefficients and ss s measurements are fit to within approximately 1 5 standard deviations we suggest that some of the misfit is due to lower mantle structure which is not included in the modelling the power as a function of harmonic degree for the top portion of the model 20 to 220 km depth does not fall off significantly at the higher harmonic degrees suggesting that the differential traveltime data may require models which include structure for degrees greater than 8 boundary perturbations at depths of 400 and 670 km trade off strongly with the velocity variations allowing variations of the order of f10 km in the radii of the discontinuities at 400 and 670 km provides a slight improvement to the data fit comparisons of several depths of our upper mantle velocity model to the geoid show the strongest correlation at degree 2 for structure near 670 km depth the distribution of slab structures is highly correlated to the degree 2 components of the model for depths of 200 to 670 km examination of the model in the vicinity of hotspots shows no consistent hotspot related signal

tectonic and seismic implications of an intersegment rupture the damaging may 11th 2011 mw 5 2 lorca spain earthquake on may 11th 2011 a mw 5 2 earthquake stroke the city of lorca in the se spain this event caused 9 fatalities 300 injuries and serious damage on the city and the surrounding areas the lorca earthquake occurred in the vicinity of a region bounding two well known segments of a large active fault the alhama de murcia fault amf the lorca earthquake offers a unique opportunity to study how strain is accommodated in an intersegment region of a large strike slip fault we map recent tectonic structures in the epicentral region and we use radar interferometry to analyze the coseismic deformation combining these data with seismological observations of lorca seismic sequence we first model the source of the earthquake then we analyze the influence of our preferred model in the adjacent segments by coulomb failure stress modeling the proposed earthquake source model suggests that this event ruptured an area of 4 3 km within the complex structure that limits the gonar lorca and lorca totana segments of the amf the induced static stress change on the adjacent segments of the fault represents a seismic cycle advance equivalent to 200 to 1000 years of tectonic loading

do earthquakes talk to each other triggering and interaction of repeating sequences at parkfield 1 knowledge of what governs the timing of earthquakes is essential to understanding the nature of the earthquake cycle and to determining earthquake hazard yet the variability and controls of earthquake recurrences are not well established the large population of small characteristically repeating earthquakes at parkfield provides a unique opportunity to study how the interaction of earthquakes affects their recurrence properties we analyze 112 m 0 4 3 0 repeating earthquake sequences ress to examine the triggering effect from nearby microseismicity we find that the repeating events with a smaller number of neighboring earthquakes in their immediate vicinity tend to recur in a more periodic manner i e the coefficient of variation in recurrence intervals is less than 0 3 the total static stress perturbation from close by earthquakes however does not seem to strongly influence res regularity the uneven distribution of stress changes in time has a modest but significant impact on recurrence intervals a significant reduction of recurrence intervals occurs in the case of very high stress changes from neighboring events close by events influence res timing in a matter of several days or less by short term triggering events that occurred within less than 1 day of an res often imposed or experienced high stress changes a static stress increment of 30 kpa can be enough to produce such short term triggering we find that the triggered repeating events are often near the end of their average earthquake cycle but some events occur following a substantially shortened interval when comparing the accelerated occurrence at the time of res events following neighboring events with varying magnitudes we find that the distance of short term triggering increases from 1 km to 4 km for m1 to m4 events

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

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

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

repeating volcano tectonic earthquakes at mt etna volcano sicily italy during 1999 2009 abstract repeating volcano tectonic vt earthquakes taking place at mt etna during 1999 2009 were detected and analyzed to investigate their behavior we found 735 families amounting to 2479 vt earthquakes representing 38 of all the analyzed vt earthquakes the number of vt earthquakes making up the families ranges from 2 to 23 over 70 of the families comprise 2 or 3 vt earthquakes and only 20 families by more than 10 events the occurrence lifetime is also highly variable ranging from some minutes to ten years in particular more than half of the families have a lifetime shorter than 0 5 day and only 10 longer than 1 year on the basis of these results most of the detected families were considered burst type i e show swarm like occurrence and hence their origin cannot be explained by a temporally constant tectonic loading indeed since the analyzed earthquakes take place in a volcanic area the rocks are affected not only by tectonic stresses related to the fairly steady regional stress field but also by local stresses caused by the volcano such as magma batch intrusions movements and gravitational loading we focused on the five groups of families characterized by the longest repeatability over time namely high number of events and long lifetime located in the north eastern eastern and southern flanks of the volcano unlike the first four groups which similarly to most of the detected families show swarm like vt occurrences group v located in the north eastern sector exhibits a more tectonic behavior with the events making up such a group spread over almost the entire analyzed period it is clear how both occurrence and slip rates do not remain constant but vary over time and such changes are time related to the occurrence of the 2002 2003 eruption finally by fpfit algorithm a good agreement between directions identified by nodal planes and the earthquake epicentral distribution was generally found

detailed kinematics structure and recurrence of micro seismicity in the safod target region 1 large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics structure and recurrence interval scaling properties of micro seismicity in a 4 4 km lateral and 6 km deep crustal volume encompassing the region of the safod deep drilling experiment the characterization reveals that the seismically active san andreas fault in the vicinity of safod s repeating magnitude 2 target earthquakes is composed of two sub parallel fault strands that are creeping at comparable rates and that one of the strands lies between the safod drilling platform and safod s target events in the region 55 of the earthquakes are members of 52 characteristically repeating earthquake sequences the recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences however this scaling is contrary to that expected from standard constant stress drop theory

the 1992 landers earthquake sequence earthquake occurrence and structural heterogeneities the june 28 1992 mw 7 3 landers earthquake occurred in the southeastern mojave desert california over 10 000 aftershocks of the earthquake were recorded by the caltech usgs southern california seismic network scsn in 1992 to investigate the relationship between complexities in the crustal structure and variations in seismicity we have used a tomographic method to invert 145 098 p wave arrival times from 3740 landers earthquake aftershocks and 1148 other events recorded by 60 permanent and temporary scsn stations we determined a detailed p wave tomographic image with a spatial resolution of about 5 km and relocated the hypocenters with the obtained 3 d velocity model the results show a correlation between seismicity patterns and velocity patterns and a tendency for regions rich in seismicity to be associated with higher velocities the higher velocity areas are considered to be strong and brittle parts of the fault zone which are apt to generate earthquakes in contrast low velocity areas are probably more ductile and weaker allowing aseismic slippage

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

assessing interaction of active extensional faults from structural and paleoseismological analysis the teruel and concud faults eastern spain abstract the relationship of independence interaction or linkage between two neighbouring intraplate active extensional faults the teruel and concud faults are investigated from structural and paleoseismological data and the results are discussed to improve seismic hazard assessment for the region this paper provides the structural and paleoseismological characterization of the almost unknown teruel fault from detailed mapping and trench analysis and discusses its kinematic and dynamic relationships with the concud fault four individual events occurred between 76 0 ka and 9 2 ka bp have been recorded at two branches of the teruel fault unfortunately these only represent a small fraction of its overall activity during such time lapse and their time constraints do not allow correlating them with those at the concud fault the teruel and concud faults are independent structures from the geometric and kinematic point of view as evinced by their distinct i transport directions n275 e and n220 e respectively and ii average coseismic displacements 0 5 m and 1 9 m respectively these displacements are consistent with their respective lengths 9 0 km and 14 2 km and significantly smaller than those expected for a hypothetically joint concud teruel 23 km long fault however their displacement gradients close to the relay zone indicate that both faults undergo dynamic interaction thus suggesting a transient stage from independence to linkage we hypothesize that slip on both structures occurred at the scale of the seismic cycle in a broadly alternating manner which induced strain partitioning between them and allowed accommodating bulk biaxial extension in the region such deformation pattern would have increased the earthquake frequency with respect to the scenario of a hypothetically linked concud teruel fault but diminished the potential seismic magnitude

near surface velocities and attenuation at two boreholes near anza california from logging data to investigate near surface site effects in granite rock we drilled 300 m deep boreholes at two sites which are collocated with stations from the digital array at anza california the first borehole was sited at station knw keenwild fire station which is located along a ridge line about 8 7 km east of the san jacinto fault zone station pfo pinon flat observatory chosen for the second site is another 6 km further to the east of station knw and is located on a gently sloping hillside we logged each borehole for p and s wave velocities as well as for crack density and orientation p waves were generated by striking a plate with a hammer at the surface a tool consisting of weighted anvils driven by compressed air against end plates along a 3 5 m beam was used to generate shear waves signals were recorded downhole with a three component sensor package at 2 5 m intervals from the surface to 50 m depth and at 5 m intervals from 50 m depth to the bottom of the hole velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth travel times were computed for the first breaks at shallow depths however below about 100 m depth times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable the knw site yielded a shear velocity of 1 9 km sec by only 30 m in depth and reached close to 2 6 km sec at the bottom of the hole p wave velocities at knw were also high at 5 4 km sec starting at 120 m depth the pfo site had similar but slightly higher shear wave velocities the bottom hole shear wave velocity reached 3 0 km sec and the p wave velocity was 5 4 km sec shear wave attenuation was computed using both the pulse rise time and spectral ratio methods at station knw attenuation was significant only in an interval between 17 5 and approximately 40 m in depth over the top 50 m attenuation corresponding to a q of about 8 was obtained a total t of 0 004 sec was measured for this interval pulse rise times also increased rapidly in this zone the spectral ratio data for station pfo yields two peaks in attenuation above 50 m similar to the attenuation found for station knw the peak in attenuation corresponds to a q of about 11 averaged over the top 50 m spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 hz significant motion perpendicular to the polarizations of the first shear wave arrival was recorded within a few meters of the surface apparently the rock structure is sufficiently complicated that body waves are being converted sh to sv at oblique incidence very close to the surface the presence of these elliptical particle motions within a mere few m of the pure shear wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not by itself a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth s crust crack densities and orientations were determined from televiewer records these records showed cracks with a preferred direction at station knw and of a greater density than at station pfo at station pfo crack densities were smaller and more diffuse in orientation

crustal and uppermost mantle structure under southern california the travel times of pg and pn arrivals are used together as a new approach to estimate the station delays lateral velocity variations in the upper crust and upper mantle crustal thickness variations and pn anisotropy in the upper mantle under southern california the main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from pn arrivals significant negative station delays are found in the colorado desert san jacinto mountains san gabriel mountains central mojave desert and from the southern ventura basin to santa cruz island in addition to the positive delays associated with well known sedimentary basins lateral velocity variations in the upper crust exceed a few percent only in the coso ranges these variations show no clear association with surface faulting they also show little resemblance to the upper mantle velocity variations which have fast velocity under the mojave and near the offshore area and slow velocity in most other areas the southern california moho shows large variations large crustal thinning occurs near the salton sea northwest of the white wolf fault in the coso range near san diego and offshore while the san bernardino mountains western granite mountains and eastern mojave south of the ludlow fault are underlain by roots a significant amount 3 of pn anisotropy is present in the uppermost mantle with a fast direction n 75 4 w this could be explained very well by averaging the pn anisotropy estimates east and west of the san andreas

inferring fault mechanical conditions from the source parameters of a complex microseismic multiplet in the corinth rift greece we develop a mechanical model of tight clusters of coplanar seismic asperities to investigate a particular microearthquake swarm located at 8 km depth in the corinth rift in greece which was active between 2001 and 2007 although it is classified as a multiplet based on waveform similarity this seismic sequence is much more complex than a repeating earthquake sequence and cannot be interpreted as the regular failure of a single asperity forced by surrounding aseismic creep here we suggest that such complex sequences could be generated by the failure of a set of coplanar asperities interacting in a small region of a fault segment we show that in order to reproduce the dynamics of the observed sequence and the characteristics of the events the cluster of asperities has to be located very close to an aseismically slipping fault segment which could be an updip extension of the deep detachment zone in the rift creeping at 1 5 cm yr for more general cases of coplanar clustered asperities we show that the shape of the cumulative coseismic displacement pattern associated with the repeated failures of the asperities is strongly controlled by the behavior of the fault area surrounding the asperity cluster in particular if the cluster is part of a locked fault area the resulting long term cumulative displacement is maximum at the center of the cluster in contrast an asperity cluster surrounded by aseismic creep leads to a uniform cumulative coseismic slip pattern the ratio between cumulative slip at the center of the seismogenic patch and cumulative slip at its periphery could therefore be an indicator of the mechanical conditions prevailing on the fault a systematic study of the source parameters of complex microseismic sequences could therefore provide insights into the mechanical state of active faults continuously generating microseismicity

importance of the reference model in linearized tomography and images of subduction below the caribbean plate in linearized delay time tomography the choice of a one dimensional reference model is very important for correct mapping of seismic anomalies we demonstrate that the use of an inadequate background model results in artifacts in tomographic images which may give rise to erroneous interpretations particularly the absence or presence of upper mantle discontinuities in the reference model influences the mapping of structures e g subducting slabs at the transition between upper and lower mantle new tomographic images of the mantle below the caribbean region are presented these show the extent of the lesser antilles subduction zone down to 600 km which is well below the seismic zone a high velocity anomaly is imaged from the mid american trench to lower mantle depths 1400 km

geological and geomorphologic asymmetry across the rupture zones of the 1943 and 1944 earthquakes on the north anatolian fault possible signals for preferred earthquake propagation direction summary the east and west rupture directions of the 1943 and 1944 earthquakes on the north anatolian fault naf are hypothesized to represent respectively long term preferred propagation directions on the corresponding sections of the naf fault sections with preferred rupture direction are expected to have an asymmetric damage structure with respect to the slipping zone to test the above hypothesis we study geological and geomorphologic manifestations of structural asymmetry with respect to the active trace of the naf along the 1943 and 1944 sections the following fault zone elements are mapped gouge fabric in the cm scale fault core structure in a metre scale and secondary faults and fault rocks in tens of metres scale mapping results at three sites on the 1943 rupture and one site on the 1944 rupture are consistent with accumulation of more rock damage on the south side of the 1943 section and on the north side of the 1944 section erosion patterns adjacent to the fault that are not correlated with the distribution of intrinsic and extrinsic erosion controlling variables e g rock type are interpreted as morphologic responses to the damage content of rocks and its impact on rock erodibility the valleys of 11 rivers are parallel to the studied fault sections about 75 per cent of the total river valleys length along the 1943 rupture is on the south side of the fault and about 89 per cent of the total river valleys length along the 1944 rupture is on the north side of the fault morphometric analysis of watersheds in two correlative terrains displaced along the 1944 rupture section shows that stream erosion is considerably more intense in the terrain north of the fault with drainage density values almost double in the north compare to the south badland topography at two sites along the 1943 rupture section is substantially more developed at the 100 m scale on the south side of fault our observations along the 1943 1944 rupture sections including various types of signals that span a large range of scales are systematically compatible with an opposite sense of damage asymmetry between the two fault sections these observations are consistent with opposite preferred direction of ruptures for the two sections similar to the propagation directions of the two recent earthquakes if those rupture directions are dictated by the velocity structure at depth we infer that the south side of the 1943 rupture has faster seismic velocity at seismogenic depth than the north side and that the sense of velocity contrast is reversed along the 1944 rupture zone

triggering of repeating earthquakes in central california dynamic stresses carried by transient seismic waves have been found capable of triggering earthquakes instantly in various tectonic settings delayed triggering may be even more common but the mechanisms are not well understood catalogs of repeating earthquakes earthquakes that recur repeatedly at the same location provide ideal data sets to test the effects of transient dynamic perturbations on the timing of earthquake occurrence here we employ a catalog of 165 families containing 2500 total repeating earthquakes to test whether dynamic perturbations from local regional and teleseismic earthquakes change recurrence intervals the distance to the earthquake generating the perturbing waves is a proxy for the relative potential contributions of static and dynamic deformations because static deformations decay more rapidly with distance clear changes followed the nearby 2004 mw6 parkfield earthquake so we study only repeaters prior to its origin time we apply a monte carlo approach to compare the observed number of shortened recurrence intervals following dynamic perturbations with the distribution of this number estimated for randomized perturbation times we examine the comparison for a series of dynamic stress peak amplitude and distance thresholds the results suggest a weak correlation between dynamic perturbations in excess of 20 kpa and shortened recurrence intervals for both nearby and remote perturbations

the anza array a high dynamic range broadband digitally radiotelemetered seismic array a small aperture seismic array has been deployed along a 30 km stretch of the san jacinto fault in the vicinity of the town of anza california the array was installed to study specifically the scaling laws of body wave spectra the character of high frequency ground motion the physical interpretation of seismic stress drops and the interaction of earthquakes this region was chosen for these studies due to its high rate of seismic activity in the 2 m 4 5 range the likelihood of a m 6 event in the next decade and the existence of the southern california batholith on either side of the fault reducing problems associated with attenuation and large scale anisotropy these studies employ an instrument package with a frequency response of 100 hz a dynamic range of 138 db a sampling rate of 250 times per second per component and a 16 bit a d converter the array consists of 10 three component stations telemetered via digital vhf radio to a nearby mountain peak and thence via a microwave link to la jolla california a minicomputer system monitors the array s performance detects events and records data upon demand initial results demonstrate the feasibility of digital transmission with an inherent increase in data quality over analog systems

systematic variations in recurrence interval and moment of repeating aftershocks 1 the recurrence intervals for 194 repeating clusters on the calaveras fault follow a power law decay relation with elapsed time after the 1984 m6 2 morgan hill california mainshock the decay rates of repeating aftershocks in the immediate vicinity of a high slip patch that failed during the mainshock systematically exceed those that are farther away the trend between relative moment and recurrence interval which is a measure of the fault healing rate varies systematically with depth and changes from negative to positive value as the distance between the repeating aftershock and the mainshock slip patch increases we speculate that high strain rate in the early postseismic period may cause transient embrittlement and strengthening of the deep repeating clusters in the areas adjacent to the mainshock slip patch resulting in large moments that decrease with time as the strain rate diminishes our observations suggest that systematic behavior of repeating aftershocks reflect variations in the fault zone rheology

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

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

relations among fault behavior subsurface geology and three dimensional velocity models the development of three dimensional p wave velocity models for the regions surrounding five large earthquakes in california has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities these relations point toward a physical understanding of why faults are divided into segments that can fail independently an understanding that could improve our ability to predict earthquakes and strong ground motion

dynamic stress variations due to shear faults in a plane layered medium summary a complete set of expressions is presented for the computation of elastic dynamic stress in plane layered media we use a discrete wavenumber reflectivity method to compute the stress field radiated by arbitrary moment tensor sources the expressions derived here represent an interesting tool for both the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena dynamic stress changes associated with a strike slip fault having unilateral rupture are shown this modelling which is similar to the 1992 landers california earthquake illustrates the effects of distance directivity and depth on transient stress changes

p wave image of the crust and uppermost mantle in southern california we have determined a p wave tomographic image of the crust and uppermost mantle in southern california by using 131 372 p wave arrival times from 6 437 local earthquakes recorded by the caltech usgs southern california seismic network in the past twelve years the obtained image has a spatial resolution of 25 km in the horizontal direction and 8 11 km in depth the tomographic image is found to correlate well with major surface geological features in the shallow crust sedimentary basins such as the los angeles basin ventura basin and santa maria basin are well imaged as low velocities while batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities in the deeper crust the velocity is low beneath the mojave desert coso volcanic area and salton trough while it is high beneath the great valley continental borderland and the major basins a high velocity layer exists in the mid crust beneath the salton trough in good agreement with a previous study using seismic explosions and gravity data in the uppermost mantle the velocity is low beneath southeastern sierra nevada and the volcanic areas while it is high beneath the mojave desert and along the pacific coast

fault healing inferred from time dependent variations in source properties of repeating earthquakes we analyze two sets of repeating earthquakes on the calaveras fault to estimate in situ rates of fault strengthening healing earthquake recurrence intervals tr range from 3 to 803 days variations in relative moment and duration are combined to study changes in stress drop rupture dimension rupture veloc ity and particle velocity as a function of t healing rates and source variations are compared with predictions of laboratory derived friction laws two interpretations of event duration z are used one in which is given by the ratio of slip to particle ve locity and one in which it scales as rupture dimension divided by rupture velocity our data indicate that faults strengthen during the interseismic period we infer that source dimension de creases with t r due to aseismic creep within the region surround ing the repeating events stress drop increases 1 3mpa per dec ade increase in twhich represents an increase of a factor of 2 3 relative to events with tbetween 10 and 100 days this rate of fault healing is consistent with extrapolations of laboratory meas urements of healing rates if fault strength is high on order of 60mpa and stress drop is roughly 10 of this value

geological controls on soil parent material geochemistry along a northern manitoba north dakota transect abstract as a pilot study for mapping the geochemistry of north american soils samples were collected along two continental transects extending east west from virginia to california and north south from northern manitoba to the us mexican border and subjected to geochemical and mineralogical analyses for the northern manitoba north dakota segment of the north south transect x ray diffraction analysis and bivariate relations indicate that geochemical properties of soil parent materials may be interpreted in terms of minerals derived from shield and clastic sedimentary bedrock and carbonate sedimentary bedrock terranes the elements cu zn ni cr and ti occur primarily in silicate minerals decomposed by aqua regia likely phyllosilicates that preferentially concentrate in clay sized fractions cr and ti also occur in minerals decomposed only by stronger acid physical glacial processes affecting the distribution and concentration of carbonate minerals are significant controls on the variation of trace metal background concentrations

repeat times of large earthquakes implications for earthquake mechanics and long term prediction information on the time intervals between large earthquakes is now available for several fault segments along plate boundaries in japan alaska california cascadia and turkey when dates in a sequence are known historically as along much of the nankai trough they provide information on the natural intrinsic variability of the rupture process most sets of repeat times however are dominated by paleoseismic determinations of dates of older large earthquakes which contain measurement uncertainties in addition to intrinsic variability a bayesian technique along with prior information on measurement uncertainties is used to make maximum likelihood estimates of intrinsic repeat time and its normalized standard deviation the coefficient of variation cv it is these intrinsic parameters and their uncertainties that are most useful for understanding the mechanics of earthquakes and for prediction for timescales of a few decades our estimates of intrinsic cv are small 0 to 0 25 for several very active fault segments where deformation is relatively simple large events do not appear to be missing in historic and paleoseismic records and data are available at or near major asperities and away from the ends of rupture zones cv is larger for regions of multibranched faulting overlapping slip near the ends of rupture zones and for data from uplifted terraces at subduction zones a poisson process is an inferior characterization of all of the 11 segments we examined scenarios used by recent working groups that assume either poissonian behavior or renewal processes with cv of 0 5 0 2 for the most active fault segments in the san francisco bay area likely lead to incorrect 30 year probability estimates the hayward fault and perhaps the peninsular segment of the san andreas fault in the san francisco bay area appear to be advanced in their buildup of stress that will be released in future large earthquakes multibranched faulting may account for why the predicted tokai earthquake in japan has not occurred as of 2006 parkfield earthquakes from 1857 to 2004 were characterized by the largest uncertainty of the sequences we studied cv 0 37 which may account for the failure of past predictions the large cv for parkfield fits our hypothesis that relatively weak fault segments are characterized by more irregular earthquake recurrence paleoseismic data from coastal sites along the cascadia subduction zone are characterized by cvs of about 0 3

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

stress interaction effect on the occurrence probability of characteristic earthquakes in central apennines 1 in this study we compute the effect of stress change due to previous historical earthquakes on the probability of occurrence of future earthquakes on neighboring faults following a methodology developed in the last decade we start from the estimate of the probability of occurrence in the next 50 years for a characteristic earthquake on known seismogenic structures based on a time dependent renewal model then a physical model for the coulomb stress change caused by previous earthquakes on these structures is applied the influence of this stress change on the occurrence rate of characteristic earthquakes is computed taking into account both permanent clock advance and temporary rate and state perturbations we apply this method to the computation of earthquake hazard of the main seismogenic structures recognized in the central and southern apennines region for which both historical and paleoseismological data are available this study provides the opportunity of reviewing the problems connected with the estimate of the parameters of a renewal model in case of characteristic earthquakes characterized by return times longer than the time spanned by the available catalogues and the applicability of the concept of characteristic earthquake itself the results show that the estimated effect of earthquake interaction in this region is small compared with the uncertainties affecting the statistical model used for the basic time dependent hazard assessment

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

predictability of repeating earthquakes near parkfield california summary we analyse sequences of repeating microearthquakes that were identified by applying waveform coherency methods to data from the parkfield high resolution seismic network because by definition all events in a sequence have similar magnitudes and locations the temporal behaviour of these sequences is naturally isolated which coupled with the high occurrence rates of small events makes these data ideal for studying interevent time distributions to characterize the temporal predictability of these sequences we perform retrospective forecast experiments using hundreds of earthquakes we apply three variants of a simple algorithm that produces sequence specific time varying hazard functions and we find that the sequences are predictable we discuss limitations of these data and more generally challenges in identifying repeating events and we outline the potential implications of our results for understanding the occurrence of large earthquakes

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

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

modification of secondary craters on the martian south polar layered deposits secondary crater fields are important stratigraphic markers that can shed light on resurfacing processes that have occurred since their formation we examine the morphologies of secondary craters formed from the ejection of material from two large impacts on the martian south polar layered deposits spld mcmurdo crater at 84 5 s 0 w and an unnamed impact at 80 8 s 284 w the morphologies of these secondary craters allow us to impose constraints on the modification history of the spld we have quantified crater morphologies using data sets from the mars global surveyor and mars odyssey missions we find a complete lack of secondary craters smaller than 300 m in diameter in both crater fields which implies that at least the upper 30 m of the deposits have been resurfaced since the time of these impacts secondary crater depth to diameter ratios are low average of 0 016 indicating that significant degradation has occurred since their emplacement we find that vertical resurfacing alone is not enough to explain the observed depth to diameter distribution and suggest that viscous relaxation of craters coupled with a small amount of vertical resurfacing best fits the data in the mcmurdo field high depth to diameter craters are found preferentially on steeper terrain associated with scarps cutting through the secondary field this observation suggests that crater modification exhibits a dependence on slope we comment on possible mechanisms that may explain this observation the morphologies of secondary craters on the spld point to modification processes without lunar parallel and not yet fully modeled for mars

fixed recurrence and slip models better predict earthquake behavior than the time and slip predictable models 1 repeating earthquakes 1 the behavior of individual events in repeating earthquake sequences in california taiwan and japan is better predicted by a model with fixed inter event time or fixed slip than it is by the time and slip predictable models for earthquake occurrence given that repeating earthquakes are highly regular in both inter event time and seismic moment the time and slip predictable models seem ideally suited to explain their behavior taken together with evidence from the companion manuscript that shows similar results for laboratory experiments we conclude that the short term predictions of the time and slip predictable models should be rejected in favor of earthquake models that assume either fixed slip or fixed recurrence interval this implies that the elastic rebound model underlying the time and slip predictable models offers no additional value in describing earthquake behavior in an event to event sense but its value in a long term sense cannot be determined these models likely fail because they rely on assumptions that oversimplify the earthquake cycle we note that the time and slip of these events is predicted quite well by fixed slip and fixed recurrence models so in some sense they are time and slip predictable while fixed recurrence and slip models better predict repeating earthquake behavior than the time and slip predictable models we observe a correlation between slip and the preceding recurrence time for many repeating earthquake sequences in parkfield california this correlation is not found in other regions and the sequences with the correlative slip predictable behavior are not distinguishable from nearby earthquake sequences that do not exhibit this behavior

seismic characterization of a highly stressed rock mass using tomographic imaging and induced seismicity a seismic tomography study was carried out at falconbridge s lockerby mine sudbury canada in april 1989 the objective was to determine the integrity of a rock mass at 1 km depth and to investigate the correlation between induced seismicity and p wave velocity structure the tomographic image showed two distinct and significant p wave velocity anomalies a low velocity zone at the source region of a mn 2 3 mining induced tremor recorded in 1985 and a high velocity zone at the source region of a mn 2 6 mining induced tremor recorded in 1988 analysis of mining induced microseismicity showed that the high velocity zone was the focus for seismic activity prior to and after the 1988 tremor in contrast to the low velocity zone which remained aseismic for the same period the seismic anomalies were diamond drilled following the tomographic imaging to verify the physical characteristics of the zones the seismically active high velocity zone showed severe core disking indicative of a highly stressed strong rock while the low velocity zone was characterized by extensive fracturing and failed rock the results showed that the 1985 tremor and rock burst sequence resulted in a major failure of the rock mass in the source region of the event and stress was redistributed and concentrated in the source region of the 1988 tremor the 1988 event and rock burst sequence did not result in failure of the source region which therefore remains a seismically active high velocity zone with a potential for significant seismic energy release and further rock bursting the study has highlighted a correlation between p wave velocity structure and induced seismicity which can be used to characterize highly stressed rock masses in underground mines

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

3 d velocity structure of the san jacinto fault zone near anza california i p waves summary seismic arrival times from microearthquakes ml 4 on the san jacinto fault near anza california are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone preliminary modelling of the 1 d p wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent the traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections however the variance of the traveltime residuals is still 2 25 times larger than the variance of the picking error the spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3 d velocity structure because the 3 d inverse problem is non unique smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error where a smooth model is defined such that the gradient of the velocity perturbation from the original 1 d model is small with small non zero station corrections a 3 d velocity model can be found that fits the data well the structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade off between station corrections and poorly resolved near surface structure the model shows a horizontal gradient with overall faster velocities in the north east side of the fault zone at 3 6 km depth the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault however at 9 km depth higher seismic velocities are found extending into the fault zone from the north east block this higher velocity region occurs where there is a distinct lack of seismicity on the fault there is also a localized feature in the south west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average

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

spatiotemporal evolution of a transient slip event on the san andreas fault near parkfield california 1 in 1993 several baselines of the two color electronic distance meter edm network at parkfield california deviated from their long term rates coincident with anomalous observations from nearby strain meters and a creep meter as well as an increase in microseismicity between october 1992 and december 1994 three m 4 5 earthquakes occurred beneath middle mountain near the hypocenter of the 1934 and 1966 parkfield m6 events we analyzed the two color edm data using a kalman filtering based technique to image the spatiotemporal evolution of slip on the fault at parkfield between the mid 1980s and 2003 this method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines we find that a slip rate increase occurred between january 1993 and july 1996 on the upper 8 km of the fault near middle mountain the peak estimated slip rate during this time was 49 mm yr which exceeds the long term geologic rate of 35 mm yr the slip rate evolution appears episodic with an initial modest increase after the m4 3 earthquake and a much larger jump following the shallower m4 7 event in december 1994 this temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip the edm data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 m6 parkfield earthquakes however transient slip and its associated stress release in the hypocentral area of previous parkfield events is consistent with the nucleation of the 2004 m6 parkfield earthquake elsewhere on the fault

variations in rupture process with recurrence interval in a repeated small earthquake in theory and in laboratory experiments friction on sliding surfaces such as rock glass and metal increases with time since the previous episode of slip1 this time dependence is a central pillar of the friction laws widely used to model earthquake phenomena2 3 on natural faults other properties such as rupture velocity4 5 porosity and fluid pressure6 11 may also vary with the recurrence interval eighteen repetitions of the same small earthquake separated by intervals ranging from a few days to several years allow us to test these laboratory predictions in situ the events with the longest time since the previous earthquake tend to have about 15 larger seismic moment than those with the shortest intervals although this trend is weak in addition the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact

earthquake travel time tomography of the southern santa cruz mountains control of fault rupture by lithological heterogeneity of the san andreas fault zone the 1989 loma prieta earthquake occurred along the stretch of the san andreas fault zone within the southern santa cruz mountains that last failed as a major earthquake in 1906 the southeastern end of the 1989 rupture marks the transition from stable aseismic slip on the central creeping section of the san andreas fault to unstable failure on the locked 1906 segment we investigate this transition and the rupture characteristics of the 1989 earthquake using a three dimensional p wave velocity model of the southern santa cruz mountains section of the fault zone the velocity model was determined by joint progressive inversion of 5422 p wave arrival times from 173 earthquakes which included aftershocks of the 1989 earthquake and premainshock background events recorded by the u s geological survey central california network this velocity model was used to relocate 424 background earthquakes the loma prieta mainshock and 292 aftershocks the velocity model is calibrated in terms of lithology using laboratory data for california coast ranges rock types and available seismic refraction profiles and is shown to be in good agreement with the surface geology the model images a large anomalous high velocity body at midcrustal depths within the rupture zone of the 1989 earthquake that the available evidence suggests might have gabbroic or other mafic composition on the basis of the relationship of the lithological features interpreted from the velocity model to the seismicity and surface creep we propose a model in which the high velocity body is primarily responsible for the transition from stable to unstable fault slip at pajaro gap the active plane of the san andreas fault cuts throughout the body the fault system attempts to circumvent this barrier by transferring slip to secondary faults including splay faults that have propagated along the frictionally favorable contact between the high velocity rock mass and franciscan country rocks however the near arrest of stable sliding causes stress to concentrate within the body and the high strength unstable contact within it evolves from a barrier to the asperity that failed in the 1989 earthquake the general features of the 1989 rupture predicted by this asperity model agree with several rupture histories computed for the earthquake the model implies that as proposed by other workers the loma prieta earthquake did not involve a repeat of the 1906 slip which has an important bearing on earthquake recurrence estimates for the santa cruz mountains segment of the fault