pulse like ruptures induced by low velocity fault zones low velocity fault zones lvfzs are found in most mature faults they are usually 100 400 m wide and have 20 60 wave velocity reductions relative to the country rock to study the effect of lvfzs on earthquake rupture and the radiated wavefield we conducted two dimensional 2 d simulations of dynamic rupture on faults that bisect a lvfz considering a range of velocity reductions and widths most earthquakes apparently have slip rise times much shorter than their overall rupture duration a number of dynamic mechanisms for such pulse like ruptures have been proposed including frictional self healing fault strength heterogeneities and bimaterial effects we find that ruptures in lvfzs with strong enough wave velocity contrast behave as pulses these pulses are generated by fault locking induced by waves reflected from the boundaries of the lvfz this mechanism of pulse generation is robust to variations of initial stress smoothness of the lvfz structure rupture mode and exclusion of frictional healing moreover we find that lvfzs can generate complex rupture patterns lvfzs with low velocity reduction induce multiple rupture fronts involving coexisting pulses and cracks lvfzs with certain widths can accelerate the transition to supershear rupture speed these additional effects of lvfzs on dynamic rupture can contribute to the complexity of high frequency ground motions

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

amplification of ground motion in fault and fracture zones observations from the tremestieri fault mt etna italy abstract a series of ambient noise measurements have been performed to evaluate their use in detecting the possible influence of tectonic structures on local amplification of seismic waves the area studied is located on the southeastern flank of mt etna a few kilometres north of the town of catania using a mobile station data were collected along three profiles which cross both a fault and an eruptive fracture in addition ambient noise was sampled at two permanent stations installed in close proximity to the fault and at another station located within the city limits of catania and considered to be a reference site analysis using spectral ratio techniques shows there is a tendency towards amplification at the sites located near the fault in h v spectral ratios such amplification is observed in the frequency range 4 0 8 0 hz and the amplitudes of the dominant spectral peaks decrease rapidly over the first few tens of meters from the discontinuity a similar tendency was observed for the sites located on the eruptive fracture zone dominant spectral peaks at 1 8 hz are also common in the study area the theoretical transfer function obtained through 1 d modelling indicates that such peaks can be related to the local stratigraphy the mean h h standard spectral ratio of microtremor evaluated at the permanent station located on the fault also shows an amplification peak at 4 0 hz azimuthal analysis shows that the resonance at 4 0 hz is directional with the maximum in the azimuth range n30 e n80 e

modeling the effect of san andreas fault structure on receiver functions using elastic 3d finite difference abstract teleseismic p wave data collected at the san andreas observatory sao seismic station show strong frequency dependent nonrectilinear particle motions consisting of off azimuth horizontal displacements that are sometimes larger than the vertical displacements the azimuthal dependence of the horizontal components amplitude is directly related to the symmetry of the fault zone a 3d finite difference program frankel and vidale 1992 is used to examine the response of the san andreas fault zone velocity structure to incident teleseismic p waves synthetic seismograms are formed using a 3d representation of an assumed 2d fault zone structure the fault zone velocity model produces large amplitude phases that contain combinations of refracted and internally reflected p to s converted phases diffractions and possible p to rayleigh converted phases the largest amplitude phases observed in the data are nondispersive and have complicated particle motions confined to the vertical plane perpendicular to the fault zone the data are fit best by synthetic receiver functions for field points similar to the location of sao relative to the san andreas fault zone since the velocity model used in the computations is nonunique sensitivity studies were performed by modifying the structure synthetics match the timing and nature of the observed phases at sao showing that assumption of plane layered structure is invalid in the area of this station

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

semiannual patterns of erosion and deposition in upper monterey canyon from serial multibeam bathymetry recently acquired 3 m resolution 244 khz multibeam seafloor bathymetry 0 5 m depth precision reveals geomorphology at sufficient detail to interpret small scale features and short term processes in the upper 4 km of monterey canyon california the study area includes the continental shelf and canyon features from 10 m to 250 m depth the canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor the sand waves are strongly asymmetrical indicating net down canyon sediment transport in this reach terraces including a broad 25 m tall terrace complex near the head of the canyon bear evidence of recent degradation of the canyon floor slump scars and gullies having a variety of sizes and relative ages shape the canyon walls serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period september 2002 to march 2003 and a 24 h period 24 march to 25 march significant changes over the six month period include 1 complete reorganization of the sand waves on the channel floor 2 local channel degradation creating new 2 m tall erosional terraces on the channel margins 3 local channel widening that laterally eroded older channel margin terraces and 4 60 m extension of one minor gully head on a steep canyon wall there were no discernable changes in morphology during the 24 h study period raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study erosion of 320 000 m 3 80 000 m 3 of sediment occurred mainly in the tributaries along the margins of the axial channel and in the lowest 700 m of the analyzed reach this eroded volume was approximately balanced by 260 000 m 3 70 000 m 3 of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head there was no measurable sediment gain or loss during the 24 h study period

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

large amplitude variations recorded by an on fault seismological station during the l aquila earthquakes evidence for a complex fault induced site effect 1 a station fagn installed on a segment of the fault system that generated the april 2009 l aquila earthquakes shows larger ground motions compared to nearby stations spectral ratios using 304 earthquakes result in a station amplification significantly varying event by event in the frequency band 1 8 hz the resulting pattern of amplitude dependence on causative earthquake location reveals that the strongest up to a factor of 10 amplifications occur for tightly clustered aftershocks aligned with the fault dip beneath fagn thus indicating a fault guided effect fault models are investigated in a grid search approach by varying velocity q width and depth of the fault zone although the problem solution is not unique and there are strong trade offs among the model parameters constraints from observations yield a deep trapping structure model where the most likely values of velocity reduction q and damage zone width are 25 20 and 280 m respectively

depth dependent structure of the landers fault zone from trapped waves generated by aftershocks we delineate the internal structure of the johnson valley and kickapoo faults landers southern rupture at seismogenic depth using fault zone trapped waves generated by aftershocks trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 m7 5 landers earthquake show large amplitudes and dispersive wave trains following the s waves group velocities of trapped waves measured from multiple band pass filtered seismograms for aftershocks occurring at different depths between 1 8 km and 8 2 km show an increase in velocity with depth velocities range from 1 9 km s at 4 hz to 2 6 km s at 1 hz for shallow events while for deep events velocities range from 2 3 km s at 4 hz to 3 1 km s at 1 hz coda normalized amplitude spectra of trapped waves peak in amplitudes at 3 4 hz for stations located close to the fault trace the amplitude decays rapidly with the station offset from the fault zone normalized amplitudes also decrease with distance along the fault giving an apparent q of 30 for shallow events and 50 for deep events we evaluated depth dependent fault zone structure and its uncertainty from these measurements plus our previous results from near surface explosion excited trapped waves li et al 1999 in a systematic model parameter searching procedure using a three dimensional 3 d finite difference computer code graves 1996 our best model of the landers fault zone is 250 m wide at the surface tapering to 100 150 m at 8 2 km depth the shear velocity within the fault zone increases from 1 0 to 2 5 km s and q increases from 20 to 60 in this depth range fault zone shear velocities are reduced by 35 to 45 from those of the surrounding rock and also vary along the fault zone with an increase of 10 near ends of the southern rupture zone

seismic radiation from an sh line source in a laterally heterogeneous planar fault zone a solution to the class of problems in which an sh line source is operating in a medium consisting of two quarter spaces with an arbitrary number of semi infinite vertical layers in between is given in a frequency wavenumber domain the general result is applied to a model with a single vertical layer and synthetic seismograms are calculated for a source specified as an sh line dislocation at the interface between the vertical layer and the faster quarter space the numerical examples demonstrate that moderate fault normal material heterogeneity can have significant effects on the seismic response of a medium the overall distribution of travel times amplitudes and motion polarities in the presence of fault zone material heterogeneity is different from a corresponding distribution in a homogeneous half space the effects are most prominent near the fault zone where head waves surface waves and trapped modes result in waveform complexities large amplifications over an order of magnitude in the fault zone itself and first motion polarities which are reversed from homogeneous medium predictions the results imply that material heterogeneity should be included in fault zone models which are used for the interpretation of observed seismic data and theoretical calculations of near fault responses such models can be utilized to obtain important fault zone and earthquake source parameters

shallow seismic trapping structure in the san jacinto fault zone near anza california summary we analyse fault zone trapped waves generated by 500 small earthquakes for high resolution imaging of the subsurface structure of the coyote creek clark valley and buck ridge branches of the san jacinto fault zone near anza california based on a small number of selected trapped waves within this data set a previous study concluded on the existence of a lowvelocity waveguide that is continuous to a depth of 15 20 km in contrast our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3 5 km this is based on the following lines of evidence 1 earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones 2 a traveltime analysis of the difference between the direct s arrivals and trapped wave groups shows no systematic increase moveout with increasing hypocentral distance or event depth estimates based on the observed average moveout values indicate that the propagation distances within the low velocity fault zone layers are 3 5 km 3 quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low velocity fault zone layers the results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones the waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch this may result from a preferred propagation direction of large earthquake ruptures on the san jacinto fault

fault zone trapped seismic waves two instances of fault zone trapped seismic waves have been observed at an active normal fault in crystalline rock near oroville in northern california trapped waves were excited with a surface source and recorded at five near fault borehole depths with an oriented three component borehole seismic sonde at parkfield california a borehole seismometer at middle mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the san andreas fault zone at oroville recorded particle motions indicate the presence of both love and rayleigh normal modes the love wave dispersion relation derived for an idealized wave guide with velocity structure determined by body wave travel time inversion yields seismograms of the fundamental mode that are consistent with the observed love wave amplitude and frequency applying a similar velocity model to the parkfield observations we obtain a good fit to the trapped wavefield amplitude frequency dispersion and mode time separation for an asymmetric san andreas fault zone structure a high velocity half space to the southwest a low velocity fault zone a transition zone containing the borehole seismometer and an intermediate velocity half space to the northeast in the parkfield borehole seismic data set the locations depth and offset normal to fault plane of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone we conclude that it is feasible to obtain near surface borehole records of fault zone trapped waves because trapped modes are excited only by events close to the fault zone proper thereby fixing these events in space relative to the fault a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body wave tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane it is an open question whether near surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device

a seismic refraction study of the santa clara valley and southern santa cruz mountains west central california abstract two seismic refraction profiles were recorded in the santa clara valley region of central california to determine the upper crustal seismic velocity structure a reversed 8 km long profile was recorded across the valley near gilroy and an unreversed 38 km long profile was recorded in the southern santa cruz mountains the data indicate that the valley is wedge shaped in cross section with the basement dipping 10 to the east the alluvial fill of the valley has a maximum thickness of 1 5 km and has an average compressional wave velocity of 2 3 km sec the top of the basement has a velocity of 4 3 km sec the velocity reaches 6 0 km sec at a depth of 2 km the calaveras fault zone located due east of the valley was crossed by a short unreversed profile it appears to be characterized by a low velocity v p 2 7 km sec to a depth of 2 5 km the southern santa cruz mountain profile indicates a 4 6 km sec near surface refractor and a layer with a velocity of 6 1 km sec at 2 km depth thus considered together the similarity in velocity structure determined from the two profiles indicates that the franciscan rocks of the santa cruz mountains continue beneath the valley the presence of high compressional wave velocities 6 0 km sec at 2 km depth in this franciscan terrain can best be explained by an abundance of metabasalts greenstones at that depth these seismic observations indicate that an area of at least 120 km 2 is underlain by this high velocity basaltic material therefore the upper crustal structure in the study area differs dramatically from the franciscan terrain of the diablo range where greywackies and metagreywackies v p 5 4 km sec dominate in the uppermost crust

san jacinto fault zone guided waves a discrimination for recently active fault strands near anza california we deployed three 350 m long eight element linear seismic arrays in the san jacinto fault zone sjfz near anza california to record microearthquakes starting in august through december 1995 two arrays were deployed 18 km northwest of anza across the casa loma fault clf and the hot springs fault hsf strands of the sjfz the third array was deployed across the san jacinto fault sjf in the anza slip gap we observed fault zone guided waves characterized by low frequency large amplitudes following s waves at the clf array and the sjf array for earthquakes occurring within the fault zone however we did not observe guided waves at the hsf array for any events the amplitude spectra of these guided waves showed peaks at 4 hz at the clf and 6 hz at the sjf which decreased sharply with the distance from the fault trace in contrast no spectral peaks at frequency lower than 6 hz were registered at the hsf array we used a finite difference method to simulate these guided modes as 5 waves trapped in a low velocity waveguide sandwiched between high velocity wall rocks the guided mode data are adequately fit by a waveguide on the clf with the average width of 120 m and s velocity of 2 5 km s about 25 reduced from the s velocity of the surrounding rock this waveguide becomes 40 to 60 m wide with the 5 velocity of 2 8 km s in the anza slip gap on the other hand there is not a continuous waveguide on the hsf at depth locations of the events with guided modes suggest that the fault plane waveguide extends along the clf between the towns of san jacinto and anza dipping northeastward at 75 80 to a depth of about 18 km it becomes nearly vertical in the anza gap we speculate that the existence of a continuous low velocity waveguide on the clf can be caused by the rupture of the magnitude 6 9 earthquake on april 21 1918 occurring near the towns of san jacinto and hemet further the lack of a clear waveguide on the hsf suggests that it was not ruptured in this event

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

fault interaction at the junction of the transverse ranges and eastern california shear zone a case study of intersecting faults abstract a case study from southern california illustrates the value of detailed geologic data for understanding the kinematics of complex fault systems the neotectonic behavior of faults at the junction of the eastern california shear zone ecsz and transverse ranges has implications for the interaction of intersecting segmented fault systems and regional plate boundary evolution paleoseismic observations indicate that the north frontal thrust system nfts has ruptured once in the holocene with 1 7 m displacement despite previous speculations of inactivity based on its dissection by younger strike slip faults simple polyphase deformation in which dextral shear has replaced and overprinted thrusting is thus not a valid explanation for this system of intersecting faults this illustrates the limitations of inferring rupture behavior from mapped fault patterns alone neotectonic and geomorphic observations along the thrust system also suggest that the thrust segment west of the intersection with the dextral helendale fault is significantly more active than the segment to the east this is consistent with a simple block velocity model in which dextral slip on the helendale fault is balanced by convergence on the western thrust segment dextral motion on the poorly studied pipes creek fault to the southeast and inactivity on the eastern thrust segment this divides the san bernardino mountains into domains dominated by thrusting west and strike slip east the union of which is a quasi stable triple junction we speculate that this union has migrated to the west as the mojave desert has been translated southwards along the san andreas fault

focusing and defocusing of teleseismic p waves by known three dimensional structure beneath pahute mesa nevada test site the focusing and defocusing of teleseismic p waves predicted by a known three dimensional structure beneath the nevada test site is calculated by dynamic raytracing and superposition of gaussian beams the 20 to 100 km scale lengths of this model having velocity fluctuations of several per cent account for a factor of 3 fluctuation in the azimuthal pattern of p amplitudes since similar sized scale lengths and intensities of velocity fluctuation are commonly resolved in three dimensional inversions of mantle structure in other regions focusing and defocusing of teleseismic p waves is likely to be a ubiquitous feature of every test site hence network averages of mb that weight azimuthal windows in which p energy is either focused or defocused will tend to either over or underestimate the yields of underground tests the results obtained with a known nevada test site structure suggest that structural inversions having similar structural resolution may be capable of reducing 25 per cent of the variance in amplitudes due to the focusing and defocusing of upper mantle structure near the source assuming that scattering processes are concentrated in the crust and upper most mantle beneath the source and receiver the integrated energy in the p coda should be more stable than direct p because it tends to homogenize the effect of focusing defocusing structure near the source for scattering into direct p near the source for direct p scattering near the receiver however even the late coda is not capable of completely removing the effects of focusing defocusing near the source

continuity of the west napa franklin fault zone inferred from guided waves generated by earthquakes following the 24 august 2014 mw 6 0 south napa earthquake we measure peak ground velocities from fault zone guided waves fzgws generated by on fault earthquakes associated with the 24 august 2014 m w 6 0 south napa earthquake the data were recorded on three arrays deployed across north and south of the 2014 surface rupture the observed fzgws indicate that the west napa fault zone wnfz and the franklin fault ff are continuous in the subsurface for at least 75 km previously published potential field data indicate that the wnfz extends northward to the maacama fault mf and previous geologic mapping indicates that the ff extends southward to the calaveras fault cf this suggests a total length of at least 110 km for the wnfz ff because the wnfz ff appears contiguous with the mf and cf these faults apparently form a continuous calaveras franklin wnfz maacama cfwm fault that is second only in length 300 km to the san andreas fault in the san francisco bay area the long distances over which we observe fzgws coupled with their high amplitudes 2 10 times the s waves suggest that strong shaking from large earthquakes on any part of the cfwm fault may cause far field amplified fault zone shaking we interpret guided waves and seismicity cross sections to indicate multiple upper crustal splays of the wnfz ff including a northward extension of the southhampton fault which may cause strong shaking in the napa valley and the vallejo area based on travel times from each earthquake to each recording array we estimate average p s and guided wave velocities within the wnfz ff 4 8 5 7 2 2 3 2 and 1 1 2 8 km s respectively with fzgw velocities ranging from 58 to 93 of the average s wave velocities

short baseline variations in site response and wave propagation effects and their structural causes four examples in and around the santa clara valley california abstract ground motion records of local and regional events from a portable array are used to investigate the structural causes of variations in ground motion over distances of a few hundred meters to a few kilometers in the sedimentary basin environment of the santa clara valley california and its margins arrays of portable seismic stations are used to target four study areas with different ground motion patterns 1 an edge of the alluvial basin extending up onto a marginal ridge blossom hill 2 a cenozoic basin with a nearly flat bottom cupertino basin 3 a long narrow cenozoic basin with a steep v profile evergreen basin and 4 a line perpendicular to the trace of the hayward fault average peak velocities on blossom hill from local earthquakes are a factor of 2 5 times higher than nearby valley sites three dimensional 3d modeling is used to conclude that the majority of the amplification is due to lower shear wave velocities along a local fault zone shannon berrocal site amplification over the cupertino basin in the frequency band 0 5 4 hz is generally low less than 2 0 relative to a mesozoic rock site and spatially uniform this response is attributed to the shallow flat bottomed shape of the basin and the uniform flat laying sedimentary fill in contrast site amplification in the evergreen basin generally exceeds 3 0 and is attributed to the deep v shaped geometry of the basin and younger sedimentary fill 3d waveform modeling shows the elongated shape of the evergreen basin causes more efficient trapping of long period waves for sources along the long axis of the basin a low velocity zone is postulated along the hayward fault with a width between 100 and 200 m based on elevated site response along the fault trace and 4 5 hz fault zone guided waves on the horizontal components of stations near the fault

seismic site characterization of an urban sedimentary basin livermore valley california site response basin edge induced surface waves and 3d simulations thirty two accelerometers were deployed in the livermore valley california for approximately one year to study sedimentary basin effects many local and near regional earthquakes were recorded including the 24 august 2014 m w 6 0 napa california earthquake the resulting ground motion data set is used to quantify the seismic response of the livermore basin a major structural depression in the california coast range province bounded by active faults site response is calculated by two methods the reference site spectral ratio method and a source site spectral inversion method longer period 1 s amplification factors follow the same general pattern as bouguer gravity anomaly contours site response spectra are inverted for shallow shear wave velocity profiles which are consistent with independent information frequency wavenumber analysis is used to analyze plane wave propagation across the livermore valley and to identify basin edge induced surface waves with back azimuths different from the source back azimuth finite element simulations in a 3d velocity model of the region illustrate the generation of basin edge induced surface waves and point out strips of elevated ground velocities along the margins of the basin online material figures comparing 5 damped acceleration response spectra for data and synthetics

observations of guided waves from the pamir seismic zone provide additional evidence for the existence of subducted continental lower crust abstract as part of the tipage tien shan pamir geodynamic program project passive seismological observations were made along an approximately n s profile crossing the pamir seismic zone for about one year from these observations guided waves were recognized these guided waves occur as a single continuous secondary compressional p wave phase behind the first p wave arrivals an equivalent phase in the shear s wavefield is hardly recognizable modelling of the phase shows that an approximately 10 km thick low velocity zone lvz between the moho and about 160 km depth reproduces the guided waves as a single continuous phase much better than a 15 20 km thick lvz modelling of the arrival times of the guided waves reveals that a model with a p wave velocity of 6 3 km s above about 100 km depth and a velocity of 7 6 km s between this depth and the deep cluster of earthquakes at about 150 km depth provides the best fit to the observed travel time data one plausible way to explain the low velocity of 6 3 km s is to invoke the presence of melts in the lvz then taking a velocity of 6 9 km s for the lower crust being subducted about 10 13 melt is required to obtain a velocity of about 6 3 km s in the lvz between the moho and about 100 km depth this would be in keeping with the estimated burial depths from xenoliths of gondwana terrane affinity brought to the surface in the southeastern pamir around 11 million yr ago the present day lvz is interpreted to comprise continental lower crust although guided waves are known to exist associated with subducted oceanic crust or fault zones this is the first time to the knowledge of the authors that guided waves have been observed resulting from a lvz associated with subducted continental lower crust

earthquake ground motions in this review we discuss some of the many phenomena that can determine the nature of strong ground motion some of these phenomena have been inferred from the study of existing ground motion records and others are inferred from theoretical models of earthquake sources and wave propagation

fault zone guided waves from explosions in the san andreas fault at parkfield and cienega valley california fault zone guided waves were successfully excited by near surface explosions in the san andreas fault zone both at parkfield and cienega valley central california the guided waves were observed on linear three component seismic arrays deployed across the fault trace these waves were not excited by explosions located outside the fault zone the amplitude spectra of guided waves show a maximum peak at 2 hz at parkfield and 3 hz at cienega valley the guided wave amplitude decays sharply with observation distance from the fault trace the explosion excited fault zone guided waves are similar to those generated by earthquakes at parkfield but have lower frequencies and travel more slowly these observations suggest that the fault zone wave guide has lower seismic velocities as it approaches the surface at parkfield we have modeled the waveforms as s waves trapped in a low velocity wave guide sandwiched between high velocity wall rocks resulting in love type fault zone guided waves while the results are nonunique the parkfield data are adequately fit by a shallow wave guide 170 m wide with an s velocity 0 85 km sec and an apparent q 30 to 40 at cienega valley the fault zone wave guide appears to be about 120 m wide with an s velocity 0 7 km sec and a q 30

surface seismic and electrical methods to detect fluids related to faulting in the absence of drilling surface based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths these in situ physical properties can then be used to infer the presence and distribution of fluids along faults although such observations are by nature indirect and become less exact with greater depth multiple observations of a range of such geophysical properties as compressional and shear seismic velocity vp and vs vp v5 ratio related to poisson s ratio resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry fluids in the fault zone and fluid reservoirs in the surrounding crust for simple geometries fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide to address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid rock ratios we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry large changes in material properties and numerous earthquakes within the fault zone a synthetic 0 5 km wide fault zone with 20 velocity reduction is well imaged using local earthquake tomography when sequential velocity inversions are done the true fault width is found even to 9 km depth although the calculated amplitude of the velocity reduction is lower than the actual amplitude vp vs is as well determined as vp magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km further a narrow 1 km very low resistivity 3 ohm m fault core can be imaged within a broad 5 km low resistivity 10 ohm m fault zone illustrating that regions of a fault containing large quantities of interconnected fluids within a broader conductive fault zone should be detectable thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade off with other factors such as intrinsic variations in porosity mineralogy and pore geometry the best approach is combined modeling of varied seismic and electrical data

amplification of ground motion and waveform complexity in fault zones examples from the san andreas and calaveras faults summary p wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of gaussian beams for point sources located in a low velocity wedge surrounding a fault the computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the san andreas and haywardcalaveras faults calculations in these models show that the 10 20hz vertical displacements of earthquakes located at 3 10 km depth are amplified by up to an order of magnitude in a 1 2 km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust this amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike slip earthquake depending on the source depth of the earthquake and the structure and velocity contrast of the wedge multiple triplications in the travel time curve of direct p and s waves will occur at stations in the fault zone a wedge model successfully predicts the triplications observed in the p waveforms of aftershocks of the coyote lake earthquake recorded in the fault zone showing that body waves from microearthquakes can be used to determine the three dimensional velocity structure of the fault zone the amplification waveform complexity and distortion of ray paths introduced by the lowvelocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone for realistic models of the wedge asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20hz numerical methods may be necessary to calculate accurately the wavefield at lower frequencies

directional topographic site response at tarzana observed in aftershocks of the 1994 northridge california earthquake implications for mainshock motions abstract the northridge earthquake caused 1 78 g acceleration in the east west direction at a site in tarzana california located about 6 km south of the mainshock epicenter the accelerograph was located atop a hill about 15 m high 500 m long and 130 m wide striking about n78 e during the aftershock sequence a temporary array of 21 three component geophones was deployed in six radial lines centered on the accelerograph with an average sensor spacing of 35 m station c00 was located about 2 m from the accelerograph we inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion we identified a 3 2 hz resonance that is a transverse oscillation of the hill a directional topographic effect the top base amplification ratio at 3 2 hz is about 4 5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill this resonance is seen most strongly within 50 m of c00 other resonant frequencies were also observed a strong lateral variation in attenuation probably associated with a fault caused substantially lower motion at frequencies above 6 hz at the east end of the hill there may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill causing particle motions not spectral ratios at the top of the hill to be rotated about 20 away from the direction transverse to the hill the resonant frequency but not the amplitude of our observed topographic resonance agrees well with theory even for such a low hill comparisons of our observations with theoretical results indicate that the 3d shape of the hill and its internal structure are important factors affecting its response the strong transverse resonance of the hill does not account for the large east west mainshock motions assuming linear soil response mainshock east west motions at the tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill probable variations in surficial shear wave velocity do not account for the observed differences among mainshock acceleration observed at tarzana and at two different sites within 2 km of tarzana

effects of three dimensional crustal structure on the estimated slip history and ground motion of the loma prieta earthquake abstract the strong ground motion of the loma prieta earthquake is synthesized using a known three dimensional 3d crustal model of the region a rupture model determined under the assumption of laterally homogeneous structure and green9s functions computed by superposition of gaussian beams compared to results obtained assuming a laterally homogeneous crust stations within 30 km of the hypocenter and 5 km of the fault trace are predicted to be strongly focused by factors up to 10 or more while stations lying at the northeastern edge of the santa clara valley are predicted to be weakly defocused by factors of 0 5 the focusing is caused by low velocities lying between the zayante and san andreas faults and the defocusing is caused by a wedge of high velocity material between the san andreas and sargent faults the magnitude of amplitude variations resulting from known 3d structure is similar to that incorporated in site effects resulting from variations in surface impedance and shallow structure beneath receivers the existence of these effects add to the uncertainty in estimates of future strong ground motion the strong path dependence of the effects makes it possible to include them in estimates of future ground motions only by knowing both the 3d structure and the sites of localized high slip in future earthquakes to match observed waveforms with the structure model of eberhart phillips et al 1990 the slip model of beroza 1991 is modified by reducing the magnitude of the northwest maximum in slip by a factor of 0 7 and increasing the magnitude of the southeast maximum slip by a factor of 2 5 these modifications are in the range of variations in the estimated slip history published by investigators using different data sets and inversion procedures

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

seismic channel waves in the accretionary prism of the middle america trench a visual examination of seismograms of earthquakes located near the middle america trench and recorded on vbb seismographs along the pacific coast of mexico when bandpass filtered between 6 and 30 s shows the presence of two types of waves the first is a short pulse and the second is a dispersed wavetrain the first type of wave is quickly attenuated while the second type propagates without significant attenuation we applied a frequency mime analysis to identify the nature of these waves a comparison of period group velocity diagrams for signals recorded on different stations shows that the first arrival is a rayleigh wave propagating in the continental plate while the second one propagates along the trench in the thickened sediment wedge of the accretionary prism we have also measured group velocities of rayleigh wave propagating in the cocos plate far from the coast at periods between 15 and 20 s the group velocities of the wave propagating along the trench are lower than the velocity of rayleigh waves propagating in adjacent continental and oceanic plates we conclude that the dispersed wavetrain observed near the coast is a wave trapped in a channel formed by low velocity sediments of the accretionary prism

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites

can seismic waves be trapped inside an inactive fault zone the case study of nocera umbra central italy in march 1998 a small aperture 350 m linear array of five broadband stations was installed across a pliocene strike slip fault crossing the town of nocera umbra central italy three component seismograms of local earthquakes recorded during the umbria marche seismic sequence have shown that the degradation of mechanical properties in damaged rock causes a significant amplification of ground motion within the fault zone marra et al 2000 during the operation of the array an m w 5 3 subcrustal earthquake occurred at a depth of 48 km and an epicentral distance of 10 km from nocera umbra the near vertical incidence of this event offers a unique opportunity for constraining the 2d geometry of the laterally heterogeneous low velocity structure responsible for the large amplification across the fault based on the observed symmetry of the amplification pattern a 2d wedge shaped model of the fault zone is used for the computation of antiplane sh synthetic waveforms to be compared with observations along the array two parameters of the wedge model can be constrained by independent geophysical and geological data 1 shear wave velocity in pristine rock outside the fault zone v s 2200 m sec and 2 the lateral extent of the damaged rock zone l 160 m the other two model parameters namely the wedge depth h and shear wave velocity of the fault zone v f s are varied computer simulations show that trapped waves which have been observed so far for earthquakes occurring inside the fault zone can develop inside a 2d wedge model when struck by external vertically incident seismic radiation results of numerical modeling indicate that in spite of its extreme simplicity such an idealized model is successful in reproducing many of the essential characters of ground motion within the fault zone of nocera umbra even though the best fit solution is not unique and the model parameters trade off each other a study of parameter sensitivity comparing synthetics with recorded data indicates that the variability of h and v f s can be restricted to a limited range of values the quality of fit is optimal when the shear velocity reduction lies in the range of 40 50 and the depth of the trapping zone is comprised between 1 and 2 km

ground motion modeling of the 1906 san francisco earthquake part i validation using the 1989 loma prieta earthquake we compute ground motions for the beroza 1991 and wald et al 1991 source models of the 1989 magnitude 6 9 loma prieta earthquake using four different wave propagation codes and recently developed 3d geologic and seismic velocity models in preparation for modeling the 1906 san francisco earthquake we use this well recorded earthquake to characterize how well our ground motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the san francisco bay area all of the simulations generate ground motions consistent with the large scale spatial variations in shaking associated with rupture directivity and the geologic structure we attribute the small variations among the synthetics to the minimum shear wave speed permitted in the simulations and how they accommodate topography our long period simulations on average under predict shaking intensities by about one half modified mercalli inten sity mmi units 25 35 in peak velocity while our broadband simulations on average under predict the shaking intensities by one fourth mmi units 16 in peak velocity discrepancies with observations arise due to errors in the source models and geologic structure the consistency in the synthetic waveforms across the wave propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure in agree ment with earlier studies we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the beroza and wald source models although the new 3d seismic velocity model improves upon previous velocity models we identify two areas needing improvement nevertheless we find that the seismic velocity model and the wave propagation codes are suitable for modeling the 1906 earthquake and scenario events in the san francisco bay area online material modified mercalli intensities and velocity waveforms and a movie of simulated wave propagation