global positioning system measurements of strain accumulation across the imperial valley california 1986 1989 global positioning system gps data collected in southern california from 1986 to 1989 indicate considerable strain accumulation across the imperial valley displacements are computed at 29 stations in and near the valley from 1986 to 1988 and at 11 sites from 1988 to 1989 the earlier measurements indicate 5 9 1 0 cm yr right lateral differential velocity across the valley although the data are heavily influenced by the 1987 superstition hills earthquake sequence the 1988 1989 gps displacements are best modeled by 5 0 0 9 cm yr of valley crossing deformation but rates calculated from 15 years of terrestrial geodetic measurements 3 7 0 1 cm yr fit the data nearly as weh1 the relatively high gps rates are suspect because some measurements especially the east trending displacements have large errors there is evidence from gps and very long baseline interferometry observations that the present strain rate along the southern san andreas fault is smaller than the long term geologic estimate suggesting a lower earthquake potential than is currently assumed correspondingly a higher earthquake potential is suggested for the san jacinto fault

3d wave equation interferometric migration of vsp free surface multiples interferometric migration of free surface multiples in vertical seismic profile vsp data has two significant advantages over standard vsp imaging 1 a significantly larger imaging area compared to migrating vsp primaries and 2 less sensitivity to velocity estimation and static errors than other methods for migration of multiples in this paper we present a 3d wave equation interferometric migration method that efficiently images vsp free surface multiples synthetic and field data results confirm that a reflectivity image volume comparable in size to a 3d surface seismic survey around the well can be computed economically the reflectivity image volume has less fold density and lower signal to noise ratio than that obtained by a conventional 3d surface seismic survey because of the relatively weak energy of multiples and the limited number of geophones in the well however the efficiency of this method for migrating vsp multiples suggests that it might sometimes be a useful tool for 4d seismic monitoring where reflectivity images can be computed quickly for each time lapse survey

triggered slip along the san andreas fault after the 8 july 1986 north palm springs earthquake in addition to minor surface cracks in the region of the 8 july 1986 north palm springs earthquake minor aseismic surficial rupture occurred along three segments of the san andreas fault 44 to 86 km southeast of the epicenter data from a creepmeter and a tiltmeter at one locality suggest that triggered slip occurred coseismically beneath the instruments but took 33 hr to propagate to the surface that slippage occurred coseismically at depth favors mechanisms for triggered slip that involve dynamic or static strain changes rather than creep migrating from the source region the distribution of slip along the san andreas fault associated with the north palm springs earthquake differed significantly from that recorded after the moderate 1968 borrego mountain california and 1979 imperial valley california earthquakes during these earthquakes triggered slip occurred along the san andreas fault in the durmid hill area and in the mecca hills triggered slip associated with the north palm springs earthquake occurred in these two areas again but also extended farther northwest into the indio hills where as much as 9 mm of dextral slip occurred in the mecca hills surface cracks in 1986 appeared over a shorter fault length than in previous events and the dextral displacement was smaller with maximum values of only 2 to 3 mm on durmid hill surface cracks in 1986 were localized along a 200 m long stretch of the fault spanning the mecca beach creepmeter and extending about 150 m to the southeast right lateral displacements on surface cracks in this area were 1 4 to 2 0 mm smaller than those observed in previous events although the mechanism of triggered aseismic slip is poorly understood examination of displacement rates for the past several decades to centuries may indicate whether the aseismic slip rate is constant or represents accelerating premonitory failiure of the southernmost san andreas fault

triggered surface slips in the salton trough associated with the 1999 hector mine california earthquake surface fracturing occurred along the southern san andreas superstition hills and imperial faults in association with the 16 october 1999 mw 7 1 hector mine earthquake making this at least the eighth time in the past 31 years that a regional earthquake has triggered slip along faults in the salton trough fractures associated with the event formed discontinuous breaks over a 39 km long stretch of the san andreas fault from the mecca hills southeastward to salt creek and durmid hill a distance from the epicenter of 107 to 139 km sense of slip was right lateral only locally was there a minor 1 mm vertical component of slip dextral slip ranged from 1 to 13 mm maximum slip values in 1999 and earlier triggered slips are most common in the central mecca hills field evidence indicates a transient opening as the hector mine seismic waves passed the southern san andreas fault comparison of nearby strong motion records indicates several periods of relative opening with passage of the hector mine seismic wave a similar process may have contributed to the field evidence of a transient opening slip on the superstition hills fault extended at least 9 km at a distance from the hector mine epicenter of about 188 to 196 km this length of slip is a minimum value because we saw fresh surface breakage extending farther northwest than our measurement sites sense of slip was right lateral locally there was a minor 1 mm vertical component of slip dextral slip ranged from 1 to 18 mm with the largest amounts found distributed or skewed away from the hector mine earthquake source slip triggered on the superstition hills fault commonly is skewed away from the earthquake source most notably in 1968 1979 and 1999 surface slip on the imperial fault and within the imperial valley extended about 22 km representing a distance from the hector mine epicenter of about 204 to 226 km sense of slip dominantly was right lateral the right lateral component of slip ranged from 1 to 19 mm locally there was a minor 1 2 mm vertical component of slip larger proportions of vertical slip up to 10 mm occurred in mesquite basin where scarps indicate long term oblique slip motion for this part of the imperial fault slip triggered on the imperial fault appears randomly distributed relative to location along the fault and source direction multiple surface slips both primary and triggered slip indicate that slip repeatedly is small at locations of structural complexity

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

behavior of the southernmost san andreas fault during the past 300 years surficial creep occurs at low rates along the coachella valley segment of the san andreas fault which has not produced a large earthquake during the period of historical record geodetic data indicate however that the crust adjacent to this segment of the san andreas fault is accumulating strain at a high rate furthermore neotectonic and paleoseismic data indicate that the fault does produce very large earthquakes every two to three centuries in view of its long term behavior the occurrence of creep along the surficial trace of the fault in the coachella valley is of particular interest along two short reaches of the san andreas fault in the coachella valley measurements of offset geological deposits and man made structures and from alignment arrays and creep meters show that slip rates of 2 4 mm yr near indio and near the salton sea have persisted for the past three centuries this slow aseismic surficial creep is not a transient precursor to seismic failure of this segment of the fault we suggest that the coachella valley segment of the san andreas fault creeps in its upper few kilometers this behavior may be due to tectonically induced high pore pressures in the coarse sediments that abut the fault

tutorial on seismic interferometry part 1 basic principles and applications seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the green s function between these receivers for the simple situation of an impulsive plane wave propagating along the x axis the crosscorrelation of the responses at two receivers along the x axis gives the green s function of the direct wave between these receivers when the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology then the crosscorrelation gives the green s function convolved with the autocorrelation of the source function direct wave interferometry also holds for 2d and 3d situations assuming the receivers are surrounded by a uniform distribution of sources in this case the main contributions to the retrieved direct wave between the receivers come from sources in fresnel zones around stationary points the main application of direct wave interferometry is the retrieval of seismic surface wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface seismic interferometry is not restricted to retrieving direct waves between receivers in a classic paper claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane wave reflection response of that medium this is essentially 1d reflected wave interferometry similarly the crosscorrelation of the transmission responses observed at two receivers of an arbitrary inhomogeneous medium gives the 3d reflection response of that medium one of the main applications of reflected wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface a common aspect of direct and reflected wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources

late holocene lacustrine chronology and archaeology of ancient lake cahuilla california freshwater lakes existed intermittently in the salton trough of southern california during the late holocene the lakes formed north of the subaerial colorado river delta whenever the colorado river flowed west into the trough instead of south to the gulf of california water filled the trough to a maximum altitude of 12 m stratigraphy radiocarbon dates and supplementary evidence document four lacustral intervals of lake cahuilla between a d 700 and 1580 archaeological sites are associated with the 12 m shoreline and their occupation correlates with these lacustral intervals

triggered slip on the calaveras fault during the magnitude 7 1 loma prieta california earthquake after the magnitude m 7 1 loma prieta earthquake on the san andreas fault the authors inspected selected sites along the calaveras fault for evidence of recent surface displacement in two areas along the calaveras fault they documented recent right lateral offsets of cultural features by at least 5 mm within zones of recognized historical creep the areas are in the city of hollister and at highway 152 near san felipe lake located approximately 25 km southeast and 18 km northeast respectively of the nearest part of the san andreas rupture zone on the basis of geologic evidence the times of the displacement events are constrained to within days or hours of the loma prieta mainshock they conclude that this earthquake on the san andreas fault triggered surface rupture along at least a 17 km long segment of the calaveras fault these geologic observations extend evidence of triggered slip from instrument stations within this zone of calaveras fault rupture

a comparison of synthetic p tau sections for proposed models of two ophiolites analyses of slant stacks of synthetic record sections generated from velocity models for ophiolites by the reflectivity method show that even 90 km long refraction lines having a 10 hz source and a 1 km intertrace spacing barely distinguish between fine scale crustal structures which are of current interest such data are sufficient however to distinguish between young and old oceanic crust tripling the spatial sampling period degrades the quality of the synthetic slant stacks for this temporal frequency content a major implication of the study is that most currently available field data being sparser and poorer in quality due to both noise and instrument responses offer little hope of resolving thin low velocity layers or small changes in velocity gradients by using newly proposed inversion techniques based on slant stacking last a simple first order correction to a downward continuation algorithm namely the inclusion of the phase shift caused by postcritical reflection at each ray parameter is proposed

structure of the crust and upper mantle beneath the great valley and allegheny plateau of eastern pennsylvania 1 comparison of linear inversion methods for sparse wide angle reflection data we use p data extracted from a small number of wide angle recordings of quarry blasts to construct averaged one dimensional velocity models of the crust beneath portions of the great valley newark basin valley and ridge and allegheny plateau of the central appalachians of pennsylvania using the linear form of the equations for p in terms of slowness depth structure we compare inversion results for the extremal backus gilbert and generalized least squares methods although the uncertainties are large the models do show a well constrained increase in midcrustal velocities and crustal thickness beneath the allegheny plateau inversion of precritical reflections from the moho suggests bounds of 7 1 7 6 km s on p wave velocity and 3 9 4 2 km s on s wave velocity near the base of the crust p wave velocity models for the great valley show a crustal thickness between 40 and 45 km with an average velocity between 6 4 and 6 6 km s models for the allegheny plateau show a larger crustal thickness 47 52 km and a much higher average velocity 6 8 6 9 km s estimates of average shear wave velocities for the great valley range from 3 6 to 3 8 km s with crustal thickness estimates between 37 and 44 km for the relatively small number of singular values retained standard errors in depth for models derived by generalized least squares range from 1 to 2 km for the p wave slowness models and from 2 to 4 km for the s wave models extremal depth bounds in general are 2 to 3 times as wide corresponding uncertainties in interval velocity estimated from resolving kernels in slowness range from 0 15 to 0 45 km s for p wave models and from 0 15 to 0 40 km s for s wave models t2 x2 inversion of pmp data gives similar estimates for total crustal thickness and average velocity after correction for refraction effects t2 x2 inversion of pmp data for a fourth profile suggests the possibility of a slight thickening of the crust 47 48 km directly beneath the axis of the great valley gravity low estimates of average vp vs for the crust based on average velocities for models derived by generalized least squares inversion range from 1 73 to 1 77 estimates based on travel time ratios for events interpreted as p wave and s wave reflections from the moho lie between 1 75 and 1 79 for a crust in which the effects of velocity anisotropy can be neglected these estimates correspond to crustal averages between 0 25 and 0 27 for poisson s ratio the one dimensional velocity models derived here provide estimates of the long wavelength component of velocity structure besides demonstrating the maximum resolving power inherent in a limited p data set these averaged models can be used for migrating reflection data and as starting models for determining two dimensional velocity structure

deep water subsurface imaging using obs interferometry abstractseismic interferometry processing is applied to an active seismic survey collected on ocean bottom seismometers obs deployed at 900 m water depth over a carbonate hydrates mound in the gulf of mexico common midpoint processing and stacking of the extracted green s function gives the subsurface pp reflectivity with a horizontal resolution of half the receiver spacing the obtained seismic section is comparable to classical upgoing downgoing wavefield decomposition and deconvolution applied on a common receiver gather seismic interferometry does not require precise knowledge of source geometry or shooting times but more accurate results are obtained when including this information for segmenting the signals before the crosscorrelations especially when signals from distant surveys are present in the data reflectivity estimates can be obtained with the crosscorrelation of pressure or vertical particle velocity signals but the pressure data gives the best resolution due to the wider frequency

surface displacement on the imperial and superstition hills faults triggered by the westmorland california earthquake of 26 april 1981 parts of the imperial and the superstition hills faults moved right laterally at the ground surface at the time of or shortly following the ml 5 6 westmorland earthquake of 26 april 1981 the displacements probably occurred before any significant aftershocks on either fault and thus are classed as triggered slips although the main shock was located in an exceptionally seismogenic part of imperial valley about 20 km distant from either fault no clear evidence of past surface faulting is known in the epicentral area horizontal displacement on the imperial and superstition hills faults southeast and southwest of the epicenter respectively reached maxima of 8 and 14 mm and the discontinuous surface ruptures formed along approximately equal lengths of northern segments of the two structures 16 8 and 15 7 km respectively the maximum vertical component of slip measured on surface cracks on the imperial fault 6 mm was located on the west side of mesquite basin near harris road leveling data at harris road suggest slightly larger vertical movement at the surface there fault dislocation modeling of this leveling data further suggests that the maximum shallow subsurface dip slip component of the triggered movement was as much a factor of 4 larger than that measured at the fault trace the map pattern of the rupture may reveal the principal strand among the several near the north end of the imperial fault no surface displacement was found along traces of the brawley fault zone the san andreas fault or the part of the coyote creek fault that slipped during the 1968 borrego mountain earthquake ground search in the epicentral area of the main shock and in the widely dispersed aftershock region in northern imperial valley failed to locate any definite evidence of new surface faulting

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

triggered seismicity and deformation between the landers california and little skull mountain nevada earthquakes this article presents evidence for the channeling of strain energy released by the ms 7 4 landers california earthquake within the eastern california shear zone ecsz we document an increase in seismicity levels during the 22 hr period starting with the landers earthquake and culminating 22 hr later with the ms 5 4 little skull mountain lsm nevada earthquake we evaluate the completeness of regional seismicity catalogs during this period and find that the continuity of post landers strain release within the ecsz is even more pronounced than is evident from the catalog data we hypothesize that regional scale connectivity of faults within the ecsz and lsm region is a critical ingredient in the unprecedented scale and distribution of remotely triggered earthquakes and geodetically manifest strain changes that followed the landers earthquake the viability of static strain changes as triggering agents is tested using numerical models modeling results illustrate that regional scale fault connectivity can increase the static strain changes by approximately an order of magnitude at distances of at least 280 km the distance between the landers and lsm epicenters this is possible for models that include both a network of connected faults that slip sympathetically and realistic levels of tectonic prestrain alternatively if dynamic strains are a more significant triggering agent than static strains ecsz structure may still be important in determining the distribution of triggered seismic and aseismic deformation

assimilation of paleoseismic data for earthquake simulation simulation of the complete earthquake generation process requires assimilation of observational data on long term behavior of faults over multiple rupture cycles paleoseismology the study of pre instrumental earthquakes from geological field investigations provides the only observations of long term fault behavior paleoseismic data present challenges for assimilation into models because data sets are small sparse analog and contain qualitative uncertainties observations can be categorized as primary measurements from field data or non primary extrapolations or interpretations of primary data assimilation of non primary data could yield biased simulation results we present a primary data set for the san andreas and imperial faults for comparison with non primary data in existing databases and propose standard formats and structure for paleoseismic data assimilation into numerical simulation models

experimental evidence of contact loss during stick slip possible implications for seismic behaviour abstract we present results of an experimental study on the stick slip phenomenon focused on detailed measurements of the relative normal displacement this was done mainly to determine whether or not the sliding surfaces become separated during the slip phase as suggested in previous works experiments were performed on pmma as it has mechanical properties comparable to those of brittle rocks and has proved to be a good analogue for rocks in rupture mechanics experiments results show clearly that normal displacement occurs during and after the slip phase the characterisation of the sliding surface enables us to compare the maximum relative normal displacement during the slip phase with asperity height in most cases the maximum relative normal displacement was found to be higher than the average value of the peak to trough relief of the surface showing that sliding surfaces were at least partially separated the phenomenon generally consists of a monotonous slip opening phase followed by a monotonous slip closure phase in some cases the slip closure phase shows oscillations which have been interpreted as elastic bounces this last phenomenon was observed irrespective of the experimental conditions with a probability ranging from 0 1 to 0 3 but no clear variations of this probability have been found as a function of normal stress and roughness these observations may give insights for the explanations of earthquake related problems such as the heat flow paradox anomalous p wave radiation or the inferred low average friction coefficients in subduction zones

surface deformation associated with fractures near the 2019 ridgecrest earthquake sequence contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detect millimeter scale deformation over broad regions of active faulting satellite radar observations revealed hundreds of previously unmapped linear strain concentrations or fractures surrounding the 2019 ridgecrest earthquake sequence we documented and analyzed displacements and widths of 169 of these fractures although most fractures are displaced in the direction of the prevailing tectonic stress prograde a large number of them are displaced in the opposite retrograde direction we developed a model to explain the existence and behavior of these displacements a major implication is that much of the prograde tectonic strain is accommodated by frictional slip on many preexisting faults

triggered surface slips in the coachella valley area associated with the 1992 joshua tree and landers california earthquakes the coachella valley area was strongly shaken by the 1992 joshua tree 23 april and landers 28 june earthquakes and both events caused triggered slip on active faults within the area triggered slip associated with the joshua tree earth quake was on a newly recognized fault the east wide canyon fault near the south western edge of the little san bernardino mountains slip associated with the land ers earthquake formed along the san andreas fault in the southeastern coachella valley surface fractures formed along the east wide canyon fault in association with the joshua tree earthquake the fractures extended discontinuously over a 1 5 km stretch of the fault near its southern end sense of slip was consistently right oblique west side down similar to the long term style of faulting measured offset values were small with right lateral and vertical components of slip ranging from 1 to 6 mm and 1 to 4 mm respectively this is the first documented historic slip on the east wide canyon fault which was first mapped only months before the joshua tree earthquake surface slip associated with the joshua tree earthquake most likely de veloped as triggered slip given its 5 km distance from the joshua tree epicenter and aftershocks as revealed in a trench investigation slip formed in an area with only a thin 3 m thick veneer of alluvium in contrast to earlier documented triggered slip events in this region all in the deep basins of the salton trough a paleoseismic trench study in an area of 1992 surface slip revealed evidence of two and possibly three surface faulting events on the east wide canyon fault during the late quaternary probably latest pleistocene first event and mid to late holocene second two events about two months after the joshua tree earthquake the landers earthquake then triggered slip on many faults including the san andreas fault in the southeastern coachella valley surface fractures associated with this event formed discontinuous breaks over a 54 km long stretch of the fault from the indio hills southeastward to durmid hill sense of slip was right lateral only locally was there a minor 1 mm vertical component of slip measured dextral displacement values ranged from 1 to 20 mm with the largest amounts found in the mecca hills where large slip values have been measured following past triggered slip events

rayleigh pulse dynamic triggering of fault slip we present an experimental and numerical investigation of the interaction of a rayleigh r pulse with a partially contacting strike slip fault between similar and dissimilar materials this study is intended to offer an improved understanding of the earthquake rupture mechanisms the fault is subjected to static normal and shear prestresses utilizing two dimensional dynamic photoelasticity in conjunction with high speed cinematography the evolution of time dependent isochromatic fringe patterns contours of maximum in plane shear stress associated with rayleigh pulse fault interaction is experimentally recorded it is shown that fault slip instability can be triggered by a pulse that propagates along the fault interface at rayleigh wave speed about 90 of the relevant shear wave speed and that the direction of the static shear preloading has an influence on the initiation of fault slip for the numerical studies a finite difference wave propagation simulator swifd solids wave impact fracture damage is used for a quantitative analysis of the problem under different combinations of contacting materials dynamic rupture in laterally heterogeneous structures is discussed by considering the effect of the acoustic impedance ratio of the two contacting materials on the wave patterns the results indicate that upon fault rupture mach head waves which carry a relatively large amount of concentrated energy can be generated that propagate from the fault contact region into the acoustically softer material such mach waves can cause concentrated damage in a particular region located inside an adjacent acoustically softer area this type of damage concentration might be another possible reason for the generation of the damage belt in kobe japan on the occasion of the 1995 hyogo ken nanbu earthquake

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

crosscorrelogram migration of inverse vertical seismic profile data we present the theory of crosscorrelogram migration of ghost reflections also known as interferometric imaging to delineate reflector geometries from inverse vertical seismic profile data the theory includes the equations for forward modeling migration asymptotic inversion and model resolution of crosscorrelgrams rather than using primary reflections crosscorrelogram migration can use ghost reflections to reconstruct the reflector geometry other multiples can be used as well including pegleg multiples and higher order multiples its main advantages over conventional kirchhoff migration are 1 both source location e g drill bit position and source wavelet need not be known such as when using a drill bit as a source in a deviated well 2 it is insensitive to source related static errors and 3 it has wider subsurface illumination than conventional kirchhoff migration of primary reflections crosscorrelation migration can effectively widen the source receiver aperture by more than 50 co

multiple precursors to the landers earthquake we present evidence for several temporally related premonitory movements preceding the landers earthquake of 28 june 1992 premonitory movements were measured at alignment arrays spanning the northeast painted canyon and san andreas faults in mecca hills california a transitory fault normal extension of 30 3 mm to eight targets was measured twice on 8 and 9 march 1992 and 7 and 8 june 1992 across the northeast and painted canyon faults at the hidden spring wash site hsws four kilometers northwest of the hsws simultaneous fault normal extensions of 24 21 and 6 6 mm were measured across a previously locked 12 km segment of the san andreas fault on 6 june 1992 22 days prior to the landers earthquake a linear regression fit of horizontal displacement at hsws showed a distinct increase in horizontal angles for those targets spanning active faults on the same dates as the fault normal extension for the first time other than a few small events on the north shore creepmeter dextral slip was measured on a nontranspressive parallel slip segment of the san andreas on 6 june 1992 we measured offsets of 14 8 and 3 arc sec 3 sec at the indian stone ring site isrs on the san andreas fault which translates into movement of 20 18 and 9 mm triggered slip was not observed on the main trace of the san andreas fault after the joshua tree earthquake but 17 mm of dextral slip possibly coseismic with the landers earthquake was measured along cracks on 5 july 1992 this amount of slip supports the magnitude of premonitory movement taken together substantial simultaneous fault normal extension and lateral slip at three alignment arrays across three faults followed by a large triggered slip event provide strong evidence for the occurrence of precursors to the landers earthquake

a comparison of coherency measurement using semblance and multiple signal classification from a seismic while drilling perspective abstracta diamond drill bit is usually considered to be an inadequate seismic vibration source to detect and use weak drill bit generated seismic wavefields in hard rock drilling we compared different coherency measures between a conventional method of semblance and a generalized multiple signal classification music algorithm we tested the detectability and resolution differences between semblance and music with synthetic examples music coherency has the advantage of higher resolution over semblance measurement when the source wavefronts are accurately predicted in addition we applied both methods to detect the coherent moveout of a diamond drill bit signal from a hard rock seismic while drilling experiment at hillside south australia we used the coherent moveout to estimate the overburden velocity around the borehole we also performed interferometry migration using the coherency measurements to image the drill bit position our analysis determined that the direct waves generated from a diamond

insar imaging of displacement on flexural slip faults triggered by the 2013 mw 6 6 lake grassmere earthquake central new zealand interferometric synthetic aperture radar insar data reveal surface slip on multiple faults triggered by the 2013 mw 6 6 lake grassmere earthquake new zealand surface offsets of 1 2 cm occurred on previously inferred flexural slip faults located 4 km from the epicenter we document dip slip on at least three different northeast striking northwest dipping flexural slip faults located in the western limb of a syncline the along strike extent of the triggered slip is 1 1 5 km for each fault dislocation models suggest that triggered slip is confined to shallow depths 800 m coulomb stress analysis indicates that slip was not triggered by the static stress change of the main shock but was likely caused by dynamic shaking during the passage of seismic waves our finding also provides an important clue on how some slip on shallow flexural slip faults takes place

effect of the loma prieta earthquake on surface slip along the calaveras fault in the hollister area over the past ten years the author has made over 800 measurements of slip rates at 20 sites on various faults in the san francisco bay region this data set enables them to compare rates and amounts of slip on these various faults before and after the loma prieta earthquake lpeq on the san andreas fault no surface slip rate changes associated with the earthquake occurred at any of the sites on the san andreas hayward northern calaveras concord green valley seal cove san gregorio antioch rodgers creek or west napa faults the lpeq apparently triggered up to 12 14 mm of right slip on the southern calaveras fault at two sites in the hollister area less than 50 km from the epicenter most of this slip was probably coseismic or nearly so about the same amount of slip was triggered at these sites in 1984 by the morgan hill earthquake this slip in contrast occurred as afterslip within about a 2 5 month interval the calaveras fault in the hollister area moves episodically with shorter times of more rapid slip alternating with longer times of slower slip the alternation occurs whether or not the times of faster slip are triggered by any nearby seismicmore event s less

ophiolites synthetic seismograms and oceanic crustal structure 2 a comparison of synthetic seismograms of the samail ophiolite oman and the rose refraction data from the east pacific rise synthetic seismograms for the samail ophiolite computed using the full reflectivity technique are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust if so then this ophiolite complex can be compared with the older 45 m y bay of islands boi ophiolite to develop an aging model for oceanic crust synthetics for this young emplacement age 5 8 m y ophiolite are compared with ocean bottom hydrophone data from 0 5 and 4 5 m y age crust obtained during the rivera ocean seismic experiment second arrival phases from reflected refractions from one ophiolite velocity depth model and data from 4 5 m y age crust are in excellent agreement this agreement suggests that the samail ophiolite is a good working model for young oceanic crust earlier work has shown that the boi ophiolite is a good prototype for mature oceanic crust the boi ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited we use the petrologic and geochemical properties of the samail and boi ophiolites to develop a two stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

new evidence on the slip rate renewal time and late holocene surface displacement southernmost san andreas fault mecca hills california we estimate a renewal time for large earthquakes on the southernmost san andreas fault saf of 260 100 years based on new paleoseismological data and a 34 millennia slip rate of 5 8 mm yr our mean recurrence interval 260 100 is longer and its variance is smaller than the 160 240 93 estimate for a m 7 earthquake used by the working group on california earthquake probabilities wgcep 1995 a smaller creep rate than on contiguous 12 km fault segments is explained by global positioning system data that reveals 2 4 mm yr of creep on the recently discovered witbaard fault that bypasses the 12 km long north shore strand of the saf geomorphic features near an archaeological site that cross the fault allow us to measure displacements of 155 25 cm based on right lateral gully offsets by the most recent earthquake e g 1690 an indian stone ring straddling the fault trace is currently offset 142 12 cm the ring lies on the 12 km long north shore segment of the saf that has accumulated a minimum of 30 mm of triggered slip and aseismic creep since 1969 contrasting slip rates of 23 35 mm yr near indio and 5 14 mm yr on the southernmost saf suggest that the coachella valley strand may be comprised of two distinct segments smaller long term strain accumulation along the southernmost segment probably reduces earthquake potential for that area

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

autocorrelogram migration ivspwd test we present the results of applying autocorrelogram migration to inverse vertical seismic while drilling ivspwd profile data collected in the austin chalk formation the seismic reflections were excited by a drill bit moving along a horizontal well at a depth of 2800 m the data were recorded by a receiver array on the surface there were 609 twenty second traces recorded at each of 10 three component stations after preprocessing the traces were autocorrelated and migrated two imaging conditions were examined the ghost reflection imaging condition produced a reflectivity section that agreed with a nearby common depth point section the migration section obtained with a primary reflection imaging condition produced a reflectivity section inferior in quality to that of the ghost image a possible explanation is that the drill bit location was not precisely known which can be shown to induce weak errors in the ghost imaging condition but stronger errors in the primary reflection imaging condition ghos

fault slip in southern california measurements of slip on major faults in southern california have been performed over the past 18 yr using principally theodolite alignment arrays and tautwire extensometers they provide geodetic control within a few hundred meters of the fault traces which complements measurements made by other techniques at larger distances approximately constant slip rates of from 0 5 to 5 mm yr over periods of several years have been found for the southwestern portion of the garlock fault the banning and san andreas faults in the coachella valley the coyote creek fault the superstition hills fault and an unnamed fault 20 km west of el centro these slip rates are typically an order of magnitude below displacement rates that have been geodetically measured between points at greater distances from the fault traces exponentially decaying postseismic slip in the horizontal and vertical directions due to the 1979 imperial valley earthquake has been measured it is similar in magnitude to the coseismic displacements analysis of seismic activity adjacent to slipping faults has shown that accumulated seismic moment is insufficient to explain either the constant or the decaying postseismic slip thus the mechanism of motion may differ from that of slipping faults in central california which move at rates close to the plate motion and are accompanied by sufficient seismic moment seismic activity removed from the slipping faults in southern california may be driving their relatively aseismic motion

seismic interferometry using multidimensional deconvolution and crosscorrelation for crosswell seismic reflection data without borehole sources crosswell reflection method is a high resolution seismic imaging method that uses recordings between boreholes the need for downhole sources is a restrictive factor in its application for example to time lapse surveys an alternative is to use surface sources in combination with seismic interferometry seismic interferometry si could retrieve the reflection response at one of the boreholes as if from a source inside the other borehole we investigate the applicability of si for the retrieval of the reflection response between two boreholes using numerically modeled field data we compare two si approaches crosscorrelation cc and multidimensional deconvolution mdd si by mdd is less sensitive to underillumination from the source distribution but requires inversion of the recordings at one of the receiver arrays from all the available sources we find that the inversion problem is ill posed and propose to stabilize it using singular value decomposition the results show that the reflections from deep boundaries are retrieved very well using both the cc and mdd methods furthermore the mdd results exhibit more realistic amplitudes than those from the cc method for downgoing reflections from shallow boundaries we find that the results retrieved from the application of both methods to field data agree well with crosswell seismic reflection data using borehole sources and with the logged p wave velocity

propagating episodic creep and the aseismic slip behavior of the calaveras fault north of hollister california a detailed kinematic study of fault slip occurring from the surface to a depth of about 7 km on the calaveras fault north of hollister was conducted during the summer of 1977 the observations coincided with a period of propagating episodic fault creep activity sensed along the fault trace data used in the investigation consist of creepmeter records near field strainmeter observations and high resolution geodetic measurements all collected contemporaneously over a period of 4 months detailed descriptions and analyses of the creepmeter and geodetic data have been presented elsewhere the near field strain measurements are here reported in detail and their analysis draws upon the previous two data sets for support the strainmeter observations are most sensitive to slip occurring in the upper 2 km hence the emphasis of the paper is placed upon the role of propagating episodic creep in the broad scale behavior of the fault the results suggest that propagating episodic fault creep as sensed along the fault trace is confined to the upper kilometer or so of the crust and represents the response of the surface layers to a longer term form of episodic aseismic slip occurring below the mean form of the advancing rupture front within the upper kilometer is ostensibly the same as that indicated by records from the surface creepmeters evidence is presented however which suggests that propagating creep events may not always break the surface and may propagate at velocities much slower and at amplitudes significantly larger than those generally observed at the surface

southern san andreas san jacinto fault system slip rates estimated from earthquake cycle models constrained by gps and interferometric synthetic aperture radar observations we use ground geodetic and interferometric synthetic aperture radar satellite observations across the southern san andreas saf san jacinto sjf fault systems to constrain their slip rates and the viscosity structure of the lower crust and upper mantle on the basis of periodic earthquake cycle maxwell viscoelastic finite element models key questions for this system are the saf and sjf slip rates the slip partitioning between the two main branches of the sjf and the dip of the saf the best fitting models generally have a high viscosity lower crust 10 21 pa s overlying a lower viscosity upper mantle 10 19 pa s we find considerable trade offs between the relative time into the current earthquake cycle of the san jacinto fault and the upper mantle viscosity with reasonable assumptions for the relative time in the earthquake cycle the partition of slip is fairly robust at around 24 26 mm a for the san jacinto fault system and 16 18 mm a for the san andreas fault models for two subprofiles across the saf sjf systems suggest that slip may transfer from the western coyote creek branch to the eastern clark superstition hills branch of the sjf from nw to se across the entire system our best fitting model gives slip rates of 2 3 12 9 12 9 and 17 3 mm a for the elsinore coyote creek clark and san andreas faults respectively where the large uncertainties in the slip rates for the sjf branches reflect the large uncertainty in the slip rate partitioning within the sjf system

timing of large earthquakes since a d 800 on the mission creek strand of the san andreas fault zone at thousand palms oasis near palm springs california paleoseismic investigations across the mission creek strand of the san andreas fault at thousand palms oasis indicate that four and probably five surface rupturing earthquakes occurred during the past 1200 years calendar age estimates for these earthquakes are based on a chronological model that incorporates radiocarbon dates from 18 in situ burn layers and stratigraphic ordering constraints these five earthquakes occurred in about a d 825 770 890 mean 95 range a d 982 840 1150 a d 1231 1170 1290 a d 1502 1450 1555 and after a date in the range of a d 1520 1680 the most recent surface rupturing earthquake at thousand palms is likely the same as the a d 1676 35 event at indio reported by sieh and williams 1990 each of the past five earthquakes recorded on the san andreas fault in the coachella valley strongly overlaps in time with an event at the wrightwood paleoseismic site about 120 km northwest of thousand palms oasis correlation of events between these two sites suggests that at least the southernmost 200 km of the san andreas fault zone may have ruptured in each earthquake the average repeat time for surface rupturing earthquakes on the san andreas fault in the coachella valley is 215 25 years whereas the elapsed time since the most recent event is 326 35 years this suggests the southernmost san andreas fault zone likely is very near failure the thousand palms oasis site is underlain by a series of six channels cut and filled since about a d 800 that cross the fault at high angles a channel margin about 900 years old is offset right laterally 2 0 0 5 m indicating a slip rate of 4 2 mm yr this slip rate is low relative to geodetic and other geologic slip rate estimates 26 2 mm yr and about 23 35 mm yr respectively on the southernmost san andreas fault zone possibly because 1 the site is located in a small step over in the fault trace and so the rate is not be representative of the mission creek fault 2 slip is partitioned northward from the san andreas fault and into the eastern california shear zone and or 3 slip is partitioned onto the banning strand of the san andreas fault zone

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

modulation of the earthquake cycle at the southern san andreas fault by lake loading 1 changes in the level of ancient lake cahuilla over the last 1500 years in the salton trough alter the state of stress by bending the lithosphere in response to the applied lake load and by varying the pore pressure magnitude within the crust the recurrence interval of the lake is similar to the recurrence interval of rupture on the southern san andreas and san jacinto faults both of which are partially covered by the lake at its highstand furthermore four of the last five ruptures on the southern san andreas fault have occurred near a time of substantial lake level change we investigate the effect of coulomb stress perturbations on local faults due to changing level of lake cahuilla to determine a possible role for the lake in affecting the timing of fault rupture coulomb stress is calculated with a three dimensional model of an elastic plate overlying a viscoelastic half space plate thickness and half space relaxation time are adjusted to match observed vertical deformation since the last lake highstand the lake cycle causes positive and negative coulomb stress perturbations of 0 2 0 6 mpa on the southern san andreas within the lake and 0 1 0 2 mpa on the southern san andreas outside the lake these coulomb stress perturbations are comparable to stress magnitudes known to have triggered events at other faults along the north america pacific plate boundary

slip on faults in the imperial valley triggered by the 4 april 2010 mw 7 2 el mayor cucapah earthquake revealed by insar 1 radar interferometry insar field measurements and creepmeters reveal surface slip on multiple faults in the imperial valley triggered by the main shock of the 4 april 2010 el mayor cucapah mw 7 2 earthquake co seismic offsets occurred on the san andreas superstition hills imperial elmore ranch wienert coyote creek elsinore yuha and several minor faults near the town of ocotillo at the northern end of the mainshock rupture we documented right lateral slip 40 mm on northwest striking faults and left lateral slip 40 mm on southwest striking faults slip occurred on 15 km and 20 km long segments of the san andreas fault in the mecca hills 50 mm and durmid hill 10 mm respectively and on 25 km of the superstition hills fault 37 mm field measurements of slip on the superstition hills fault agree with insar and creepmeter measurements to within a few millimeters dislocation models of the insar data from the superstition hills fault confirm that creep in this sequence as in previous slip events is confined to shallow depths 3 km

the response of creeping parts of the san andreas fault to earthquakes on nearby faults two examples rates of shallow slip on creeping sections of the san andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults one example of such perturbations occurred during the 26 january 1986 magnitude 5 3 tres pinos earthquake located about 10 km southeast of hollister california seven creepmeters on the san andreas fault showed creep steps either during or soon after the shock both left lateral ll and right lateral rl steps were observed a rectangular dislocation in an elastic half space was used to model the coseismic fault offset at the hypocenter for a model based on the preliminary focal mechanism the predicted changes in static shear stress on the plane of the san andreas fault agreed in sense ll or rl with the observed slip directions at all seven meters for a model based on a refined focal mechanism six of the seven meters showed the correct sense of motion two possible explanations for such coseismic and postseismic steps are 1 that slip was triggered by the earthquake shaking or 2 that slip occurred in response to the changes in static stress fields accompanying the earthquake in the tres pinos example the observed steps may have been of both the triggered and responsive kinds a second example is provided by the 2 may 1983 magnitude 6 7 coalinga earthquake which profoundly altered slip rates at five creepmeters on the san andreas fault for a period of months to years the xmm1 meter 9 km northwest of parkfield california recorded ll creep for more than a year after the event to simulate the temporal behavior of the xmm1 meter and to view the stress perturbation provided by the coalinga earthquake in the context of steady state deformation on the san andreas fault a simple time evolving dislocation model was constructed the model was driven by a single long vertical dislocation below 15 km in depth that was forced to slip at 35 mm yr in a rl sense a dislocation element placed in the seismogenic layer under xmm1 was given a finite breaking strength of sufficient magnitude to produce a parkfield like earthquake every 22 years when stress changes equivalent to a coalinga earthquake were superposed on the model running in a steady state mode the effect was to make a segment under xmm1 that could slip in a linear viscous fashion creep ll and to delay the onset of the next parkfield like earthquake by a year or more if static stress changes imposed by earthquakes off the san andreas can indeed advance or delay earthquakes on the san andreas by months or years then such changes must be considered in intermediate term prediction efforts

sawtooth segmentation and deformation processes on the southern san andreas fault california five contiguous 12 13 km fault segments form a sawtooth geometry on the south ernmost san andreas fault the kinematic and morphologic properties of each segment depend on fault strike despite differences of strike between segments of as little as 3 degrees oblique slip transpression of fault segments within the indio hills mecca hills and durmid hill results from an inferred 8 1 ratio of dex tral slip to convergence across the fault zone triggered slip and creep are confined almost en tirely to transpressive segments of the fault durmid hill has been formed in the last 28 n 6 ka by uplift at an average rate of 3 1 mm a

stress evolution of the san andreas fault system recurrence interval versus locking depth 1 major ruptures along the san andreas fault system safs are driven by stress that has accumulated in the upper locked portion of the crust the present day stress accumulation rate on any given fault segment is fairly well resolved by current geodetic measurements model stress accumulation rates vary between 0 5 and 7 mpa per century and are inversely proportional to earthquake recurrence intervals in contrast the total accumulated stress on a given fault segment is poorly resolved since it depends on the uncertain rupture history of each fault over the past few thousand years we simulate accumulated stress at crustal depths for both past and present day conditions by assuming complete release of accumulated slip deficit during major ruptures these speculative results indicate that the southern san andreas which has not ruptured in a major earthquake in over 300 years is currently approaching a threshold stress level

aseismic growth of durmid hill southeasternmost san andreas fault california our 2 27 km leveling line across and normal to the san andreas fault at durmid hill domed upward by 9 mm between september 1985 and december 1991 the flanks of the line tilted more than 1 rad yr implying that the uplift extends beyond the ends of the line and that absolute uplift of the crest of the hill may greatly exceed the measured rate of 1 5 mm yr trilateration measurements sampling 5 km of the fault zone within the hill from 1978 to 1992 yield a mean dextral slip rate of 2 4 0 3 mm yr and fault normal extension of 1 0 3 mm yr the inferred uplift rate during the 5 5 year leveling period as well as from geomorphic samples at 330 years and 35 000 years and from erosion of sedimentary strata younger than 738 000 years in monoclines abutting the fault at durmid hill is remarkably consistent at 1 to 4 mm yr and implies that the current processes may have persisted for millennia we investigate two mechanisms for the deformation that are defined by the symmetry of the observed doming transpression on a vertical or near vertical subsurface san andreas fault or slip on a shallow flat fault or detachment that terminates beneath durmid hill the resultant models suggest that uplift rates of 1 mm yr will result from 6 mm yr dextral shear along a vertical fault together with 1 mm yr horizontal convergence across that fault and this strain field is consistent with the paleoseismic record and our leveling data we conclude that interseismic processes account for most of the growth of durmid hill because the interseismic vertical deformation rates equal or exceed the long term vertical slip rate measured across the fault because dextral shear of 6 mm yr at durmid hill represents a small fraction of the 25 mm yr measured across the entire san andreas fault zone in coachella valley and because the san andreas fault at durmid hill may represent an earthquake nucleation zone the observed deformation processes may be of significant importance for understanding future earthquake rupture initiation or termination

slip on the superstition hills fault and on nearby faults associated with the 24 november 1987 elmore ranch and superstition hills earthquakes southern california alignment arrays and creepmeters spanning several faults in southern california recorded slip associated with the 24 november 1987 elmore ranch and superstition hills earthquakes no precursory slip had occurred on the superstition hills fault up to 27 october 1987 when the last measurement before the earthquakes was made about 23 days before the earthquake dextral creep events of about 13 mm and 0 5 mm may have occurred simultaneously on the imperial and southern san andreas faults respectively but the tectonic origin of the smaller event is questionable within 12 hr after the superstition hills earthquake 20 9 cm of dextral slip occurred on the main fault trace at the superstition hills alignment array and 39 8 cm of dextral slip was recorded over the entire 110 m width of the array despite this initial wide distribution of slip nearly all of the postseismic slip is occurring on the main fault trace as of 3 august 1988 the alignment array had recorded a total of 80 2 cm of dextral slip as of 5 days after the earthquakes 65 to 80 per cent of the total slip measured by the alignment array had occurred on discrete mappable fractures in addition the two earthquakes triggered slip on the coyote creek fault the southern san andreas fault and on the imperial fault telemetered data from creepmeters on the southern san andreas and imperial faults indicate that triggered slip began there within 3 min or less of each of the two earthquakes additional triggered slip occurred on the imperial fault beginning 3 5 hr after the second earthquake

dynamic triggering of shallow slip on forearc faults constrained by monitoring surface displacement with the ipoc creepmeter array abstract the m w 8 8 maule earthquake of february 27 2010 remotely triggered minor surface displacement along the atacama fault system in northern chile located 1500 1800 km from the epicenter here we present evidence that surface displacement events recorded by the ipoc integrated plate boundary observatory in n chile creepmeter array are related to earthquakes on the underlying subduction zone interface or to large earthquakes around the world our observations document dynamic triggering of shallow slip on faults in a forearc setting continuous monitoring revealed that up to 30 displacement events per year occur on the monitored fault segments 90 of them triggered by earthquakes in the near or far field synchronous to the passage of the seismic wave we can clearly attribute far field triggering to the passage of the surface wave and near field triggering to the passage of the body wave the most distant triggered displacement event monitored occurred after the 2011 m w 9 0 tohoku oki earthquake located 16 468 km away following the passage of large amplitude ss waves comparing the triggered displacement events with other triggered phenomena like seismicity volcanic eruptions hydrological changes as well as with data from the california creepmeter array we find that the seismic energy density appropriately describes a magnitude distance scaling relationship of slip triggering finally the observed surface displacement is found to not occur by continuous creep but rather through nearly exclusively discrete displacement events

new slip along parts of the 1968 coyote creek fault rupture california on 28 and 30 november 1987 we found new surface ruptures along the central break of the coyote creek fault s 1968 rupture mapped slip that may have been triggered by the superstition hills earthquakes of 24 november 1987 was dominantly right lateral and ranged up to 15 mm total length of the new ruptures was about 3 km distributed along two quasi continuous segments of the main fault plus one segment of a secondary fault all of which ruptured in 1968 the longest segment of new rupture followed the northeast side of the ocotillo badlands for 2 3 km newly mapped slip in 1987 occurred along parts of the coyote creek fault that were characterized in 1968 by several features these were large total dextral slip a prominent vertical slip component rupture along single fractures instead of en echelon cracks and large afterslip in the few years following the 1968 earthquake past episodes of triggered slip on the san andreas imperial and superstition hills faults were hypothesized by others to be precursory phenomena the 1987 slip on the coyote creek fault however may be an episode of afterslip this suggestion is supported by the decrease in creep rates since 1968 shown by data from two alinement arrays on the coyote creek fault the correct interpretation of triggered slip events and their value as earthquake precursors therefore depends upon a good knowledge of the slip history of a fault

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

long term slip rate of the southern san andreas fault from 10be 26al surface exposure dating of an offset alluvial fan we determine the long term slip rate of the southern san andreas fault in the southeastern indio hills using 10 be and 26 al isotopes to date an offset alluvial fan surface field mapping complemented with topographic data air photos and satellite images allows precise determination of piercing points across the fault zone that are used to measure an offset of 565 80 m a total of 26 quartz rich cobbles from three different fan surfaces were collected and dated the tight cluster of nuclide concentrations from 19 samples out of 20 from the offset fan surface implies a simple exposure history negligible prior exposure and erosion and yields an age of 35 5 2 5 ka the long term slip rate of the san andreas fault south of biskra palms is thus 15 9 3 4 mm yr this rate is about 10 mm yr slower than geological 0 14 ka and short term geodetic estimates for this part of the san andreas fault implying changes in slip rate or in faulting behavior this result puts new constraints on the slip rate of the san jacinto and on the eastern california shear zone for the last 35 kyr our study shows that more sites along the major faults of southern california need to be targeted to better constrain the slip rates over different timescales

surficial slip distribution on the central emerson fault during the june 28 1992 landers earthquake california we present the results of our mapping of a 5 6 km length of the central emerson fault that ruptured during the 1992 landers earthquake in the southwestern mojave desert california the right lateral slip along this portion of the rupture varied from about 150 to 530 cm along the main rupture zone in some locations a total of up to 110 cm of additional right lateral slip occurred on secondary faults up to 1 7 km away from the main rupture zone other secondary faults carried up to several tens of centimeters of left lateral or thrust displacement the maximum net vertical displacement was 175 cm east side up the sense of vertical slip across the main fault zone varied along strike but in most cases it was consistent with the sense of vertical slip in previous earthquakes as indicated by the locations of areas of older uplifted and abandoned alluvial fan surfaces although variations in surficial slip have been reported along previous strike slip ruptures our closely spaced slip measurements allow a much more detailed study of slip variability than was possible previously we document variations in slip as large as 1 m or more over distances ranging from 1 2 km to a few tens of meters suggesting that strains of the order of 10 1 may have occurred locally within the surficial sediments the long wavelength kilometer scale variations in surficial slip may be influenced by fault geometry and perhaps by the thickness of unconsolidated sediments the slip variations over shorter length scales tens of meters may be caused by variations in the proportion of the total shear that occurs on visible brittle fractures versus that which occurs as distributed shear warping or rotation the variability of slip along the ruptures associated with the landers earthquake calls for caution in interpreting geomorphic offsets along prehistoric fault ruptures

ophiolites synthetic seismograms and ocean crustal structure 1 comparison of ocean bottom seismometer data and synthetic seismograms for the bay of islands ophiolite a series of synthetic seismograms based on the bay of islands ophiolite data are compared with ocean bottom seismometer seismic refraction observations which sample 60 m y crust in the northeastern pacific the general agreement between the synthetics for the ophiolite and the observations support the idea that some ophiolites are representative of normal ocean crust this support extends to similarities in complex second arrivals in the synthetics and observations which often are not used in the interpretation of marine seismic data the use of the ophiolite model also allows us to show how the interpretation of seismic data is affected by experimental design and interpretation procedures as the sampling density increases and as more phases in the observations are accounted for the velocity depth model more closely approaches that found for the ophiolite the results of this modeling support sharper p velocity transitions within the crust than are found in young 12 m y age oceanic crust the need for a thin approximately 1 km thick p low velocity zone at the base of the crust is interpreted as support for serpentinization at the crust mantle boundary serpentinization of the ultramafic component in the bay of islands ophiolite therefore may include a small preemplacement component

vertical deformation 1952 1987 in the salton trough california from water level recordings analysis of 36 years of water level data from three sites on the shore of the salton sea reveals a history of vertical deformation in the salton trough data from two sites allow estimation of tilt along a 36 km nw se baseline between 1952 and 1970 since 1970 the addition of a 13 km wnw ese baseline allows the full tilt vector to be estimated we find down to the southeast tilting at an average rate of about 0 01 0 01 rad yr from the early 1960s to 1987 an anomalous period of more rapid 0 2 0 04 rad yr tilting occurs between the mid 1950s and early 1960s but this coincides with a period from which we have been unable to examine all the original data a previous analysis of the 1952 1978 data suggested a precursory increase in tilt rate prior to the 1968 borrego mountain earthquake and a tilt reversal in 1972 we show that there was no significant tilt change prior to the 1968 earthquake and that the apparent 1972 reversal resulted from uncorrected offsets in the water level observations tilts derived from water level and from reliable first order leveling surveys show limited agreement but the leveling data contain evidence for nonuniform tilting recent analyses of leveling data by others using different techniques show 20 30 cm uplift and subsidence events in the salton trough between 1968 and 1978 our data constrain the relative elevations of the three water level sites to be stable within 1 2 cm during these proposed events

strain rate profile across the elsinore san jacinto and san andreas faults near palm springs california 1973 81 a profile of the strain accumulation rate along a line trending n50 e across the subparallel elsinore san jacinto and san andreas faults near palm springs california has been constructed from trilateration surveys in the 1973 81 interval the strain accumulation is principally right lateral shear across a vertical plane parallel to fault strike n40 w the strain rate profile for that component exhibits two clearly resolved maxima one centered on the san jacinto fault max 0 35 0 02 rad a and the other on the san andreas fault max 0 40 0 02 rad a no maximum is associated with the elsinore fault this result clearly implies that slip at depth on both the san andreas and san jacinto faults is loading the shallower sections of these faults and that eventually rupture can be expected on both

moho and lower crustal reflectivity beneath a young rift basin results from a two ship wide aperture seismic reflection experiment in the valencia trough western mediterranean summary we present new images of the lower crust and moho beneath the valencia trough a young rift basin in the western mediterranean these images were obtained from a two ship wide aperture reflection experiment and show several features not distinguishable on previously available conventional single ship reflection profiles the moho which was previously only seen intermittently can now be confidently traced throughout the basin we have constructed a present day depth to moho map and estimated the degree of crustal thinning for the whole basin crustal thinning is at a maximum in the centre of the basin where values reach 3 15 0 25 at the margins of the basin the value decreases to 1 5 0 1 the reflective character of the lower crust and moho is different beneath different parts of the basin we have been able to correlate these differences with the amount of stretching we therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent neogene stretching event that opened the valencia trough along the iberian margin there is well developed lower crustal reflectivity consisting of 1 2s two way time twt of 1 4 km long near horizontal reflectors underlain by a more continuous although not significantly stronger reflector interpreted to be the reflection moho offshore this lower crustal reflective unit thins rapidly such that it is undetectable 40 50 km from the coastline where the crust has been stretched by a factor of 1 7 0 1 as the lower crustal reflectivity becomes undetectable the reflection moho becomes a robust continuous event where exceeds 2 4 0 2 however the moho is a weak event and difficult to trace we infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the moho where extension was moderate the balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the valencia trough the lower crust shows evidence of weak but locally variable lower crustal reflectivity it is possible that the lower crustal reflectivity was preserved simply because the moho was flexed downward and so decompression and hence melting of the upper mantle was restricted this suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification

real time test of the long range aftershock algorithm as a tool for mid term earthquake prediction in southern california result of the algorithm of earthquake prediction published in 1982 is examined in this paper the algorithm is based on the hypothesis of long range interaction between strong and moderate earthquakes in a region it has been applied to the prediction of earthquakes withm 6 4 in southern california for the time interval 1932 1979 the retrospective results were as follows 9 out of 10 strong earthquakes were predicted with average spatial accuracy of 58 km and average delay time the time interval between a strong earthquake and its best precursor 9 4 years varying from 0 8 to 27 9 years during the time interval following the period studied in that publication namely in 1980 1988 four earthquakes occurred in the region which had a magnitude ofm 6 4 at least in one of the catalogs caltech or noaa three earthquakes coalinga of may 1983 chalfant valley of july 1985 and superstition hills of november 1987 were successfully predicted by the published algorithm

theoretical seismogram calculation for laterally varying crustal structures summary a modification of the reflectivity method for calculating theoretical seismograms is introduced this technique enables allowances to be made for differing crustal structures beneath the source and the receiver the method has been applied to the 1971 long range refraction profile across france and it is shown that the observed effects can only be accounted for by structure in the upper mantle in this paper we develop a computational method which enables us to make such an allowance for the propagation effects of the different crustal structures beneath the source and the receiver using a modification of the reflectivity method we then apply this technique to the results of the long seismic refraction profile in france in 1971 hirn et al 1973 and use the experimentally determined crustal structures sapin prodehl 1973 to show that the observed effects can only be accounted for by structure in the upper mantle varying crustal structure and the reflectivity method the reflectivity method in its usual form fuchs miiller 1971 provides an effective computational technique for approximating the response of a horizontally layered elastic half space to excitation by a near surface source since the com putation of the complete response is exceedingly time consuming a number of simplifications are made only in the lower part of the layered half space referred to as the reflection zone are all multiple reflections and interconversions of wave

observations and implications of water well and creepmeter anomalies in the mojave segment of the san andreas fault zone four years of continuous water level data from a 1 900 m deep well near the san andreas fault in southern california show a 21 month period of unusual rapid fluctuations the anomalous water level activity ceases with the occurrence of the 8 july 1986 ml 5 6 north palm springs earthquake on the san andreas fault approximately 120 km southeast of the well site the water level data correlate with signals from two san andreas fault zone strain sensors the long term water level variations are similar in shape to those recorded by a borehole dilatometer 30 km from the well site while the rapid water level fluctuations correlate in time with anomalies in the record of a fault zone creepmeter located 35 km from the water level monitor the observations are interpreted in terms of strain occurring in a horizontally and vertically heterogeneous crust tectonic deformation is concentrated along weak block bounding fault zones the blocks are sufficiently rigid to generate correlated displacement phenomena along their weak boundaries as a consequence fault zone strain events can be related over large distances as in the case of the north palm springs earthquake and the coincident creep and water level anomalies the observed creep and water level events occur in a weak aseismic shallow 3 km material overlying stronger asperities along which the fault is locked and regional strain loading occurs local failure of the fault within the weak shallow material has no far field effects and is not important in the crustal stress budget but may signal accelerated strain accumulation at depth in this context the reported observations demonstrate the possibility of joint detection of fault zone anomalies that relate to and possibly anticipate major tectonic strain events on large faults

slip triggered on southern california faults by the 1992 joshua tree landers and big bear earthquakes five out of six functioning creepmeters on southern california faults recorded slip triggered at the time of some or all of the three largest events of the 1992 landers earthquake sequence digital creep data indicate that dextral slip was triggered within 1 min of each mainshock and that maximum slip velocities occurred 2 to 3 min later the duration of triggered slip events ranged from a few hours to several weeks we note that triggered slip occurs commonly on faults that exhibit fault creep to account for the observation that slip can be triggered repeatedly on a fault we propose that the amplitude of triggered slip may be proportional to the depth of slip in the creep event and to the available near surface tectonic strain that would otherwise eventually be released as fault creep we advance the notion that seismic surface waves perhaps amplified by sediments generate transient local conditions that favor the release of tectonic strain to varying depths synthetic strain seismograms are presented that suggest increased pore pressure during periods of fault normal contraction may be responsible for triggered slip since maximum dextral shear strain transients correspond to times of maximum fault normal contraction

tectonic geomorphology of the san andreas fault zone in the southern indio hills coachella valley california geomorphic investigation of the san andreas fault zone in the indio hills indicates many tectonically produced landforms including beheaded streams right lateral deflected and offset streams sags shutter ridges pressure ridges and fault scarps near biskra palms an alluvial fan pediment complex has an apparent cumulative offset of about 0 7 km along the mission creek fault zone north branch san andreas fault many of the tectonic landforms as well as the fracture pattern that has developed during the pleistocene are explainable by simple shear or uplift associated with a small left bend in the main trace of the mission creek fault the ratio of vertical to horizontal displacement in the vicinity of the bend is about 0 04 exposed in pushawalla canyon 5 km northwest of the alluvial fan pediment complex are 1 a sequence of stream terraces 2 folded plio pleistocene fanglomerates and 3 an example of stream capture following a right lateral deflection or offset of several hundred metres a left step of the mission creek fault in pushawalla canyon is a probably cause of folds and sporadic uplift that produced the stream terraces possible cause of recent stream capture are 1 juxtaposition of pushawalla canyon with a relict canyon moving northwestward along the mission creek fault or 2 right lateral deflection or offset of pushawalla canyon along the fault with simultaneous headward erosion of a shorter steep stream flowing toward the coachella valley estimation of a slip rate and identification of paleoseismicity for the san andreas fault in the indio hills is difficult however degree of topographic dissection formation and preservation of desert pavement and relative soil profile development suggest that the age of the offset fan may be as old as 70 000 yr but that most likely it is on the order of 20 000 to 30 000 yr these age estimates for the offset fan indicate a minimum slip rate for the san andreas fault of 10 to 35 mm yr with about 23 to 35 mm yr the most likely the pattern of observed offset drainages is complex but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right lateral displacement

paleoseismic evidence of clustered earthquakes on the san andreas fault in the carrizo plain california exposures we have excavated across the san andreas fault contradict the hypothesis that part of the fault in the carrizo plain is unusually strong and experiences relatively infrequent rupture the exposures record evidence of at least seven surface rupturing earthquakes which have been approximately dated by accelerated mass spectrometry radiocarbon analysis of detrital charcoal and buried in situ plants five large earthquakes have occurred since 1218 a d the most recent earthquake event a was the 1857 fort tejon earthquake which we have associated with 6 6 10 m of dextral slip along the main fault trace the penultimate earthquake event b most likely occurred within the period a d 1405 1510 slip from either events b and c combined or from event b alone totals 7 11 m three earthquakes events c d and e occurred in a temporal cluster prior to event b and after approximately a d 1218 the average recurrence interval within this cluster is 73 116 years depending on assumptions events f and g occurred after 200 years b c a depositional hiatus between events e and f may hide evidence of additional earthquakes events b and d within the carrizo cluster of a d 1218 1510 may correlate with events t a d 1329 1363 and v a d 1465 1495 at pallett creek on the mojave segment of the fault this suggests two fault ruptures similar in length to that of 1857 events c and e apparently did not rupture the mojave section which suggests that the carrizo segment has ruptured independently or in combination with segments to the north irregular repeat times of large earthquakes suggest a pattern of clustered events at the end of seismic supercycles

refining the spatial and temporal signatures of creep and co seismic slip along the southern san andreas fault using very high resolution uas imagery and sfm derived topography coachella valley california abstract the current interseismic period for an event on the coachella valley segment of the san andreas fault saf is 300 years much greater than the best estimated average recurrence interval of 180 years prompting questions regarding earthquake recurrence and fault behavior digital surface models dsms generated using structure from motion multiview stereo sfm mvs techniques on unmanned aerial systems uas imagery and very high resolution uas orthomosaics were used to identify and measure fault related offsets of small geomorphic features and develop a model of late holocene slip per event for the southern saf ssaf in the coachella valley we aim to better constrain slip estimates from recent earthquakes along the ssaf and combine new slip data with ages of past earthquakes from paleoseismic studies to understand earthquake recurrence slip per event for paleoearthquakes which ruptured this segment of the ssaf and the relationship between co seismic slip events creep or afterslip and triggered slip were evaluated examination of offsets suggests that five 5 large surface rupturing events produced average displacements of 2 6 3 1 m per event gaussian distribution analysis revealed small scale offset clusters that we attribute to climate modulated channel incision across the fault and subsequent creep these offset clusters support a creep rate of 3 mm yr in the mecca hills averaged over the past 160 years

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

fault slip rates effects of elastic heterogeneity on geodetic data and the strength of the lower crust in the salton trough region southern california 1 in southernmost california the salton trough sedimentary basin lies between the subparallel san jacinto and san andreas faults that together with the elsinore fault accommodate 80 of the 50 mm yr of pacific north american relative motion the slip rates on the san andreas and san jacinto faults have been a matter of recent debate and one source of uncertainty has been the influence of crustal strength heterogeneity due to elastically weak basin sediments on geodetic data to evaluate these effects we have modeled regional kinematics with elastic and viscoelastic finite element models that incorporate laterally and vertically varying crustal material properties we find that in general the effects of the sedimentary basin on surface deformation are relatively small and that to explain the geodetic velocity data the slip rate on the san andreas must be higher than that on the san jacinto fault consistent with traditional geologic estimates we also conclude that a relatively strong lower crust of viscosity 5 x 10 19 pa s exists in this region

seismic interferometry of teleseicmic s wave coda for retrieval of body waves an application to the philippine sea slab underneath the japanese islands summary the reconstruction of surface waves from the cross correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches however the reconstruction of body waves has not been extensively studied in this study we present a method for extracting body waves that is direct p and s waves and reflected waves from the philippine sea slab we use the cross correlation of the wavefield generated by the teleseismic s waves observed by hi net tiltmeters with a passband of 0 07 0 5 hz to enhance the contribution of the s coda we reduced the source time function and the deterministic phases ss scs and ss and the surface wave in s coda the records of the processed s coda observed at every station pair are cross correlated for each teleseismic event the cross correlation functions ccfs of different earthquakes are stacked to retrieve wavefields propagating between the station pairs as a result direct p s and the reflected waves could be extracted when the stacked ccfs are aligned as a function of distance of separation between two stations the gradients of the traveltime curves for the direct p and s waves are approximately 5 7 and 3k m s 1 respectively these velocities correspond to the seismic velocities of the crust or the p wave velocity of the uppermost mantle in order to enhance the reflected waves we searched for the reflection points by assuming that the later phases in the ccfs are ss reflections and map the amplitudes onto the depth sections as a result the negative phases dipping to the north can be traced right below the region of hypocentre distribution these negative phases probably correspond to the oceanic moho within the philippine sea slab we also show that the oceanic moho can also be traced by assuming pp reflections these results indicate that the ccfs plausibly contain information regarding both p and s waves propagating between the two receivers and are capable of detecting reflected phases in addition to the direct waves

triggered aseismic fault slip from nearby earthquakes static or dynamic effect 1 observations show that an earthquake can affect aseismic slip behavior of nearby faults and produce triggered aseismic fault slip two types of stress changes are often examined by researchers as possible triggering sources one is the static stress change associated with the faulting process and the other is the dynamic stress change or transient deformation generated by the passage of seismic waves no consensus has been reached however regarding the mechanism s of triggered aseismic fault slip we evaluate the possible triggering role of static stress changes by examining observations made after 10 large earthquakes in california most of the nearby fault segments that slipped aseismically were encouraged to move by the imposed positive changes in static coulomb failure stress cfs nonetheless three discrepancies or failures with this model exist which implies that static stress triggering either is or is not the sole mechanism causing the observed triggered slip we then use a spring slider system as a simplified fault model to study its slip behavior and the impact of transient dynamic loading on it we show that a two state variable rate dependent and state dependent frictional law can generate creep events transient loads are then put into the system certain types of them can cause a large time advance of or trigger the next creep event while our work examines triggered creep events near the surface it may well have implications for the occurrence of similar events near the bottom of the seismogenic zone where a transition in frictional stability occurs index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8168 tectonophysics evolution of the earth stresses general

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

the morphology of strike slip faults examples from the san andreas fault california fault zones are seldom straight nor are fractures within them continuous permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip these strain fields may be manifest as vertical deformation near the fault strain fields that increase area result in subsidence and strain fields that reduce area result in uplift although these effects are modified by erosion and deposition the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike graphical views of strain fields associated with simple vertical strike slip faults are derived using boundary element methods if the fault is straight and parallel to the applied shear strain the hills and valleys resulting from finite fault rupture are separated by the fault trace this is rarely observed in nature instead strike slip faults cut through hills and valleys apparently of their own making we show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments we test our theory with examples from parts of the san andreas fault system in california the morphologies of the parkfield region and the southern san andreas fault in the coachella valley are shown to be consistent with plausible distributions of slip on mapped surface faults the location of the salton sea appears to be a consequence of a reduction of slip on the san andreas fault toward the brawley seismic zone

migration methods for imaging different order multiples multiples contain valuable information about the subsurface and if properly migrated can provide a wider illumination of the subsurface compared to imaging with vsp primary reflections in this paper we review three different methods for migrating multiples the first method is model based and it is more sensitive to velocity errors than primary migration the second method uses a semi natural green s function for migrating multiples where part of the traveltimes are computed from the velocity model and part of the traveltimes i e natural traveltimes are picked from the data to construct the imaging condition for multiples the third method uses cross correlation of traces the last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use natural data to construct part of the migration imaging conditions compared with the interferometric i e crosscorrelation imaging method the semi natural green s function method is more computationally efficient and is sometimes less prone to migration artifacts numerical tests with 2 d and 3 d vsp data show that a wider subsurface coverage higher fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only however there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated one possible solution is to filter primaries and different orders of multiples before migration and another possible solution is least squares migration of all events a limitation of multiple migration is encountered for subsalt imaging here the multiples must pass through the salt body more than twice which amplifies the distortion of the image