finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

explosion quakes at stromboli italy abstract this paper reports the results of two seismic experiments aimed at determining the wave field of explosion quakes at stromboli island mediterranean sea southern italy the typical strombolian activity mostly consists of explosive phenomena causing pyroclastic materials to be emitted together with jets of volcanic gases from one or more craters stromboli is an active volcano characterized by persistent seismic activity consisting of explosion quakes that are seismic events associated with the explosive volcanic phenomena explosion quakes are short lived seismic events occurring intermittently whose amplitude tends to decrease with distance from the vent a distinctive feature of explosion quakes is the presence on seismograms of two often clearly distinct seismic phases the first low frequency seismic phase 3 4 hz after one second or more the first seismic phase of explosion quakes has been shown to be characterized by a nearly radial linear polarization and by an apparent propagation velocity estimated at 600 800 m s the second phase is characterized by a more chaotic motion and a lower apparent propagation velocity of 150 450 m s the wavefield associated with the first low frequency seismic phase appears to be generated by a resonating p wave seismic source accompanying gas explosion and emission of pyroclastic materials the wavefield associated with the second high frequency seismic phase of explosion quakes appears to be mainly composed of scattered and converted waves due to the critical topography of the volcano

late cenozoic sedimentation and metal deposition in the north pacific similar major chemical variations in cores from three sites across a wide area of the north pacific are modeled by coupling the vertical and horizontal motions of the pacific plate with a constant authigenic flux of elements into the sediments superimposed over this flux is an input of detrital material of relatively constant chemical composition the major features of sediment chemistry of cores from the north pacific apparently vary in a fairly uniform and systematic manner and these variations are consistent with cenozoic plate movement elements with a significant authigenic component e g mn co cu ni display high concentrations at depth in cores that correspond to the period when the seafloor while under the low productivity subtropical gyre of the n pacific accumulated sediment at an extremely slow rate 1 mm kyr these concentrations were diluted when the sites experienced an increase in the eolian mass accumulation rate first brought about by migration into the influence of the westerlies and then dominantly by the huge input of wind blown detritus which corresponded to the onset of northern hemisphere glaciation a comparison of 10be profiles to the profiles of metal concentrations is used to reveal the occurrence of haituses in the sediment column over the past 8 m y

existence of the threshold pressure for seismic excitation by atmospheric disturbances excitation of seismic waves by atmospheric pressure changes is examined from data for two tropical cyclones tropical storm lee 2011 and hurricane isaac 2012 they moved through the earthscope transportable array ta and generated variations in pressure and ground motions that spanned 4 5 orders of magnitude in power spectral density psd for vertical seismic ground velocity psd sv for frequencies between 0 01 and 0 02 hz there is a threshold pressure at about pressure psd sp of 10 pa2s below which vertical motion is not affected by local atmospheric pressure above this threshold pressure vertical ground motion increases with surface pressure as sv sp1 5 in order to understand the land atmosphere interaction pressure above this threshold is the only useful range horizontal component psds are about two orders of magnitude larger than vertical component psds and change with pressure for its entire range this overall trend is most likely caused by ground tilt

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

a study of infrasonic anisotropy and multipathing in the atmosphere using seismic networks we discuss the use of reverse time migration rtm with dense seismic networks for the detection and location of sources of atmospheric infrasound seismometers measure the response of the earth s

direct observation of rupture propagation during the 1979 imperial valley earthquake using a short baseline accelerometer array abstract the 1979 imperial valley california earthquake m s 6 9 was recorded on the el centro differential array a 213 m long linear array of 5 three component digital accelerometers 5 6 km from the nearest tectonic surface rupture although absolute time was not recorded on the array elements a relative time base was established using the main shock hypocentral p wave and the p and s waves from a later aftershock a cross correlation technique was used to measure the difference in arrival times of individual seismic waves in a moving 0 6 to 1 2 sec window at each array element which would then be converted into the wave9s slowness 1 velocity along the array when applied to the main shock vertical and horizontal accelerograms results from both components of motion indicated that the early arriving energy came from a source to the south of the array and the source of the energy moved rapidly to the north of the array during the strong shaking the ground motions at the array elements were well correlated for about the first 11 sec of motion these observations suggest that we have observed the initiation of rupture south of the array and its subsequent propagation along the fault to a position north of the array in about 10 sec and that the energy was radiated from a fairly compact region around the rupture front if the observed vertical and horizontal ground motions are assumed to be caused by p and s waves respectively then the observed slownesses show irregularities which can be interpreted as implying that the observed high frequency ground motions originated at irregularly distributed regions on the fault surface or that the rupture velocity was variable or both one possible interpretation of the data suggests that the rupture proceeded at near p wave velocity over a 7 km long section of fault average rupture velocities of about 2 7 to 3 2 km sec at 8 km depth are consistent with the data and 2 8 km sec is weakly preferred under the assumption that rupture propagates at a fixed fraction of the shear velocity the large vertical pulse which had a peak acceleration of 1 7 g at e06 was emitted from the portion of the fault extending 25 to 30 km northwest of the hypocenter near meloland overpass and not from the point on the fault closest to the differential array nothing can be said about fault behavior southeast of the hypocenter

analysis of the el centro differential array for the 1979 imperial valley earthquake two methods of analysis have been applied to the el centro differential array data from the 1979 imperial valley earthquake spectral ratios of array averages were developed as one way of estimating the spatial averaging of ground motion this spatial averaging will result in a reduction in the high frequency motion experienced by a large rigid foundation that is assumed to move as a single unit as an alternative covariance measurements in several frequency bands were made to illustrate how the fraction of the power propagating as single traveling wave decreases with increasing distance between stations base averaging reduction factors determined from these analyses show that reductions of 10 to 30 per cent would be expected for this particular earthquake for foundations of about 50 m in dimension and for frequencies above 20 hz the effect is frequency dependent and does not show any significant reduction at frequencies below 5 hz for this particular situation at a distance from the rupture surface of about 5 km the reduction is more pronounced for horizontal components than for vertical components

coherence of mach fronts during heterogeneous supershear earthquake rupture propagation simulations and comparison with observations 1 we study how heterogeneous rupture propagation affects the coherence of shear and rayleigh mach wavefronts radiated by supershear earthquakes we address this question using numerical simulations of ruptures on a planar vertical strike slip fault embedded in a three dimensional homogeneous linear elastic half space ruptures propagate spontaneously in accordance with a linear slip weakening friction law through both homogeneous and heterogeneous initial shear stress fields in the 3 d homogeneous case rupture fronts are curved owing to interactions with the free surface and the finite fault width however this curvature does not greatly diminish the coherence of mach fronts relative to cases in which the rupture front is constrained to be straight as studied by dunham and bhat 2008a introducing heterogeneity in the initial shear stress distribution causes ruptures to propagate at speeds that locally fluctuate above and below the shear wave speed calculations of the fourier amplitude spectra fas of ground velocity time histories corroborate the kinematic results of bizzarri and spudich 2008a 1 the ground motion of a supershear rupture is richer in high frequency with respect to a subshear one 2 when a mach pulse is present its high frequency content overwhelms that arising from stress heterogeneity present numerical experiments indicate that a mach pulse causes approximately an 1 7 high frequency falloff in the fas of ground displacement moreover within the context of the employed representation of heterogeneities and over the range of parameter space that is accessible with current computational resources our simulations suggest that while heterogeneities reduce peak ground velocity and diminish the coherence of the mach fronts ground motion at stations experiencing mach pulses should be richer in high frequencies compared to stations without mach pulses in contrast to the foregoing theoretical results we find no average elevation of 5 damped absolute response spectral accelerations sa in the period band 0 05 0 4 s observed at stations that presumably experienced mach pulses during the 1979 imperial valley 1999 kocaeli and 2002 denali fault earthquakes compared to sa observed at non mach pulse stations in the same earthquakes a 20 amplification of short period sa is seen only at a few of the imperial valley stations closest to the fault this lack of elevated sa suggests that either mach pulses in real earthquakes are even more incoherent that in our simulations or that mach pulses are vulnerable to attenuation through nonlinear soil response in any case this result might imply that current engineering models of high frequency earthquake ground motions do not need to be modified by more than 20 close to the fault to account for mach pulses provided that the existing data are adequately representative of ground motions from supershear earthquakes

the usgs parkfield california dense seismograph array upsar abstract an array of 14 irregularly spaced seismograph stations with an aperture of approximately 1 km has been installed near the middle of the rupture zone of the 1966 parkfield california earthquake m 5 5 the primary purpose of the array is to image sources of high frequency energy radiated by the next characteristic parkfield earthquake using recordings of its aftershocks at the array as empirical green9s functions in addition it will be used to characterize coda q in the parkfield region using beam forming techniques to isolate arrivals from specific volumes of the earth each of the 14 stations consists of a three component velocity transducer and an accelerometer package to ensure on scale recordings of the full range of ground motion from microearthquakes to large damaging shocks transducer output is digitized at each field site and telemetered to a computer at a central recording facility for event detection and storage each component of ground motion is sampled 200 times per second by a 16 bit analog to digital converter time code received from the goes west satellite synchronizes sampling across the array data telemetry is by arcnet a token passing local area network developed for pcs using a single coaxial cable arcnet is two way meaning that data or commands can be sent from any field station to the microcomputer or any other field station over the same medium using the same hardware and programming techniques specific site locations were chosen by a simulated annealing algorithm that chose the final site locations from a set of about 100 possible sites to optimize the beam pattern of the array the algorithm minimized the power of peaks in the beam pattern with slownesses less than 1 sec km relative to the central lobe the array configuration was required to include several embedded tripartite arrays to allow the examination of coherence across the array seismograms are shown for a m 2 3 earthquake which was located at middle mountain 12 9 km from the array the azimuth to the hypocenter is computed several ways using the array data and compared to the backazimuth from the epicenter location a least squares best fit to the arrival times of the first peak yields an azimuth of 7 9 which is similar to the azimuth of the peak in slowness computed using the music algorithm of 10 3 at 9 hz these angles are similar but consistently greater than the backazimuth to the hypocenter of 3 2 the amplitude of the first arrival is approximately 6 4 times greater for a stacked trace or beam computed for a ray aimed along the ray path from the hypocenter compared to a beam 180 opposite to that the amplitude of the first arrival of stacked traces diminishes over a range in azimuth of 80 on either side of the azimuth for a beam aimed along the upsar hypocenter ray path coda q calculated for a range of 0 to 350 and slownesses of 0 1 to 0 24 sec km show no dominant peaks in the frequency bands of 4 to 8 and 8 to 16 hz a large late arrival in the seismogram in the 2 to 4 hz band yields high q s at azimuths of around 0 with large errors coda q increases with frequency from about 150 in the 2 to 4 hz band to about 500 in the 8 to 16 hz band

earthquake ruptures modulated by waves in damaged fault zones faults are usually surrounded by damaged zones of lower elastic moduli and seismic wave velocities than their host rocks if the interface between the damaged rocks and host rocks is sharp enough earthquakes happening inside the fault zone generate reflected waves and head waves which can interact with earthquake ruptures and modulate rupture properties such as rupture speed slip rate and rise time we find through 2 d dynamic rupture simulations the following 1 reflected waves can induce multiple slip pulses the rise time of the primary pulse is controlled by fault zone properties rather than by frictional properties 2 head waves can cause oscillations of rupture speed and in a certain range of fault zone widths a permanent transition to supershear rupture with speeds that would be unstable in homogeneous media 3 large attenuation smears the slip rate function and delays the initial acceleration of rupture speed but does not affect significantly the rise time or the period of rupture speed oscillations 4 fault zones cause a rotation of the background stress field and can induce plastic deformations on both extensional and compressional sides of the fault the plastic deformations are accumulated both inside and outside the fault zone which indicates a correlation between fault zone development and repeating ruptures spatially periodic patterns of plastic deformations are formed due to oscillating rupture speed which may leave a permanent signature in the geological record our results indicate that damaged fault zones with sharp boundaries promote multiple slip pulses and supershear ruptures

finite element simulations of dynamic shear rupture experiments and dynamic path selection along kinked and branched faults we analyze the nucleation and propagation of shear cracks along nonplanar kinked and branched fault paths corresponding to the configurations used in recent laboratory fracture studies by rousseau and rosakis 2003 2009 the aim is to reproduce numerically those shear rupture experiments and from that provide an insight into processes which are active when a crack initially propagating in mode ii along a straight path interacts with a bend in the fault or a branching junction the experiments involved impact loading of thin homalite 100 a photoelastic polymer plates which had been cut along bent or branched paths and weakly glued back together everywhere except along a starter notch near the impact site strain gage recordings and high speed photography of isochromatic lines provided characterization of the transient deformation fields associated with the impact and fracture propagation we found that dynamic explicit 2 d plane stress finite element analyses with a simple linear slip weakening description of cohesive and frictional strength of the bonded interfaces can reproduce the qualitative rupture behavior past the bend and branch junctions in most cases and reproduce the principal features revealed by the photographs of dynamic isochromatic line patterns the presence of a kink or branch can cause an abrupt change in rupture propagation velocity additionally the finite element results allow comparison between total slip accumulated along the main and inclined fault segments we found that slip along inclined faults can be substantially less than slip along the main fault and the amount depends on the branch angle and kink or branch configuration

on reduction of long period horizontal seismic noise using local barometric pressure summary tilt from atmospheric loading has long been known to be the major source of long period horizontal seismic noise we try to quantify these effects for seismic data from the black forest observatory bfo which is known to be a very quiet station experimental transfer functions between local barometric pressure and horizontal seismic noise are estimated for two long time series by standard methods two simple analytical physical models are developed the local deformation model ldm and the acoustic gravity wave model twm subsequently these models with only two free parameters are fit using least squares to the observed seismic noise for time series of widely differing lengths the results are variable sometimes rather dramatic variance reductions are obtained and sometimes the reduction is hardly significant the method produces the best results when barometrically induced noise is high the resulting admittances for the ldm are compared to finite element calculations since the methods are simple and can result in conspicuous reductions in noise we provide one more reason for installing barometers at even the best broad band seismic stations

rupture process of the ms 6 6 superstition hills california earthquake determined from strong motion recordings application of tomographic source inversion abstract we analyze strong motion recordings of the m s 6 6 superstition hills earthquake to determine the timing location spatial extent and rupture velocity of the subevents that produced the bulk of the high frequency 0 5 to 4 hz seismic energy radiated by this shock the earthquake can be characterized by three principal subevents the largest ones occurring about 3 and 10 sec after initiation of rupture timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the superstition hills fault the two largest subevents display different directivity effects we apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance based on a one dimensional fault model the azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the superstition hills fault at a velocity about equal to the p wave velocity an alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent but it is not supported by the aftershock distribution the third subevent ruptured to the southeast along an 8 km long portion of the superstition hills fault at about the shear wave velocity this rupture propagation caused the relatively large accelerations and velocities observed in strong motion records for stations southeast of the hypocenter the long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development the southern half of the superstition hills fault did not generate significant high frequency strong ground motion although it showed substantial co seismic surface displacement the subevents are situated along the same northern portion of the fault where most of the aftershocks are located the locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth

infrasonic component of volcano seismic eruption tremor air ground and ground air elastic wave coupling are key processes in the rapidly developing field of seismoacoustics and are particularly relevant for volcanoes during a sustained explosive volcanic eruption it is typical to record a sustained broadband signal on seismometers termed eruption tremor eruption tremor is usually attributed to a subsurface seismic source process such as the upward migration of magma and gases through the shallow conduit and vent however it is now known that sustained explosive volcanic eruptions also generate powerful tremor signals in the atmosphere termed infrasonic tremor we investigate infrasonic tremor coupling down into the ground and its contribution to the observed seismic tremor our methodology builds on that proposed by ichihara et al 2012 and involves cross correlation coherence and cross phase spectra between waveforms from nearly collocated seismic and infrasonic sensors we apply it to datasets from mount st helens tungurahua and redoubt volcanoes

the zh ratio method for long period seismic data inversion for s wave velocity structure summary the particle motion of surface waves in addition to phase and group velocities can provide useful information fors wave velocity structure in the crust and upper mantle in this study we applied a new method to retrieve velocity structure using the zh ratio the ratio between vertical and horizontal surface amplitudes of rayleigh waves analysing data from the geoscope network we measured the zh ratios for frequencies between 0 004 and 0 05 hz period between 20 and 250 s and inverted them for s wave velocity structure beneath each station our analysis showed that the resolving power of the zh ratio is limited and final solutions display dependence on starting models in particular the depth of the moho in the starting model is important in order to get reliable results thus initial models for the inversion need to be carefully constructed we chose prem and crust2 0 in this study as a starting model for all but one station ech the eigenvalue analysis of the least squares problem that arises for each step of the iterative process shows a few dominant eigenvalues which explains the cause of the inversion s initial model dependence however the zh ratio is unique in having high sensitivity to near surface structure and thus provides complementary information to phase and group velocities application of this method to geoscope data suggest that low velocity zones may exist beneath some stations near hotspots our tests with different starting models show that the models with low velocity anomalies fit better to the zh ratio data such low velocity zones are seen near hawaii station kip crozet island crzf and djibuti atd but not near reunion island rer it is also found near echery ech which is in a geothermal area however this method has a tendency to produce spurious low velocity zones and resolution of the low velocity zones requires further careful study we also performed simultaneous inversions for volumetric perturbation and discontinuitydepth perturbation while its formulation and inversion were straightforward there seemed to be a difficult trade off problem between volumetric perturbation and discontinuity depth perturbation

analysis of near source static and dynamic measurements from the 1979 imperial valley earthquake gross features of the rupture mechanism of the 1979 imperial valley earthquake ml 6 6 are inferred from qualitative analysis of near source ground motion data and observed surface rupture a lower bound on the event s seismic moment of 2 5 1025 dyne cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture 40 5 cm far field estimates of moment suggest an average slip over the fault plane of 105 cm from which a static stress drop of 11 bars is obtained an alternative slip model consistent with the far field moment has 40 5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km this model suggests a static stress drop of 39 bars from the lower estimate of 11 bars an average strain drop of 32 strain is derived this strain drop is four times greater than the strain that could have accumulated since the 1940 el centro earthquake based on measured strain rates for the region hence a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940 unusually large amplitude 500 to 600 cm sec2 vertical accelerations were recorded at stations e05 e06 e07 e08 eda of the ei centro array and the five stations of the differential array near eda although the peak acceleration of 1705 cm sec2 at e06 is probably amplified by a factor of 3 due to local site conditions these large amplitude vertical accelerations are unusual in that they are evident on only a few stations all of which are near the fault trace and at about the same epicentral range two possible explanations are considered first that they are due to a direct p wave generated from a region about 17 km north of the hypocenter or second that they are due to a pp phase that is unusually strong in the imperial valley due to the large p wave velocity gradient in the upper 5 km of the imperial valley based on the distribution of both the horizontal and vertical offsets it is likely that the rupture went beyond stations e06 and e07 during the main shock by exploiting the antisymmetry of the parallel components of particle velocity between e06 and e07 and by examining polarization diagrams of the particle velocity at e06 and e07 an average rupture velocity in the basement of 2 5 to 2 6 km sec between the hypocenter and station e06 is obtained in addition several lines of evidence suggest that the imperial fault dips about 75 to the ne

slip pulse rupture behavior on a 2 m granite fault we describe observations of dynamic rupture events that spontaneously arise on meter scale laboratory earthquake experiments while low frequency slip of the granite sample occurs in a relatively uniform and crack like manner instruments capable of detecting high frequency motions show that some parts of the fault slip abruptly velocity 100 mm s 1 acceleration 20 km s 2 while the majority of the fault slips more slowly abruptly slipping regions propagate along the fault at nearly the shear wave speed we propose that the dramatic reduction in frictional strength implied by this pulse like rupture behavior has a common mechanism to the weakening reported in high velocity friction experiments performed on rotary machines the slip pulses can also be identified as migrating sources of high frequency seismic waves as observations from large earthquakes show similar propagating high frequency sources the pulses described here may have relevance to the mechanics of larger earthquakes

ground motion modeling of hayward fault scenario earthquakes part ii simulation of long period and broadband ground motions abstract we simulate long period t 1 0 2 0 s and broadband t 0 1 s ground motions for 39 scenario earthquakes m w 6 7 7 2 involving the hayward calaveras and rodgers creek faults for rupture on the hayward fault we consider the effects of creep on coseismic slip using two different approaches both of which reduce the ground motions compared with neglecting the influence of creep nevertheless the scenario earthquakes generate strong shaking throughout the san francisco bay area with about 50 of the urban area experiencing modified mercalli intensity vii or greater for the magnitude 7 0 scenario events long period simulations of the 2007 m w 4 18 oakland earthquake and the 2007 m w 5 45 alum rock earthquake show that the u s geological survey s bay area velocity model version 08 3 0 permits simulation of the amplitude and duration of shaking throughout the san francisco bay area for hayward fault earthquakes with the greatest accuracy in the santa clara valley san jose area the ground motions for the suite of scenarios exhibit a strong sensitivity to the rupture length or magnitude hypocenter or rupture directivity and slip distribution the ground motions display a much weaker sensitivity to the rise time and rupture speed peak velocities peak accelerations and spectral accelerations from the synthetic broadband ground motions are on average slightly higher than the next generation attenuation nga ground motion prediction equations we attribute much of this difference to the seismic velocity structure in the san francisco bay area and how the nga models account for basin amplification the nga relations may underpredict amplification in shallow sedimentary basins the simulations also suggest that the spudich and chiou 2008 directivity corrections to the nga relations could be improved by increasing the areal extent of rupture directivity with period

infrasound connecting the solid earth oceans and atmosphere the recently reinvigorated field of infrasonics is poised to provide insight into atmospheric structure and the physics of large atmospheric phenomena just as seismology has shed considerable light on the workings and structure of earth s solid interior although a natural tool to monitor the atmosphere and shallow earth for nuclear explosions it is becoming increasingly apparent that infrasound also provides another means to monitor a suite of natural hazards the frequent observation of geophysical sources such as the unsteady sea surface volcanoes and earthquakes that radiate energy both up into the atmosphere and down into the liquid or solid earth and transmission of energy across earth s boundaries reminds us that earth is an interconnected system this review details the rich history of the unheard sound in the atmosphere and the role that infrasonics plays in helping us understand the earth system

effects of supershear rupture speed on the high frequency content of s waves investigated using spontaneous dynamic rupture models and isochrone theory 1 in this paper we achieve three goals 1 we demonstrate that crack tips governed by friction laws including slip weakening rate and state dependent laws and thermal pressurization of pore fluids propagating at supershear speed have slip velocity functions with reduced high frequency content compared to crack tips traveling at subshear speeds this is demonstrated using a fully dynamic spontaneous three dimensional earthquake model in which we calculate fault slip velocity at nine points locations distributed along a quarter circle on the fault where the rupture is traveling at supershear speed in the in plane direction and subshear speed in the antiplane direction this holds for a fault governed by the linear slip weakening constitutive equation by slip weakening with thermal pressurization of pore fluid and by rate and state dependent laws with thermal pressurization the same is also true even assuming a highly heterogeneous initial shear stress field on the fault 2 using isochrone theory we derive a general expression for the spectral characteristics and geometric spreading of two pulses arising from supershear rupture the well known mach wave and a second lesser known pulse caused by rupture acceleration 3 we demonstrate that the mach cone amplification of high frequencies overwhelms the de amplification of high frequency content in the slip velocity functions in supershear ruptures consequently when earthquake ruptures travel at supershear speed a net enhancement of high frequency radiation is expected and the alleged low peak accelerations observed for the 2002 denali and other large earthquakes are probably not caused by diminished high frequency content in the slip velocity function as has been speculated

seismic recordings of landslides caused by typhoon talas 2011 japan 1 large deep seated landslides occurred in nara wakayama and mie prefectures of western japan when typhoon talas passed through the region on september 3 4 2011 signals of large landslides have been recorded by seismic networks around the world and overall force estimates have been previously determined for large landslides using long period waves this study focuses on the high frequency waveforms and presents signals of 18 landslides caused by typhoon talas 2011 the location of the landslides can be determined by a seismological back projection technique and these locations correlate with the observed surface features we have found that the volumes of the landslide correlated with an energy parameter derived from the seismic records

rupture process of an earthquake with kilometric size fault inferred from the modeling of near source records abstract we use the exceptional set of strong ground motion records obtained during one of the largest aftershocks m l 5 0 of the 1979 imperial valley earthquake to study the time history of rupture on a kilometric size fault we apply single crack composite cracks and dislocation earthquake models to simulate the source and calculate the ground velocity and acceleration at the recording sites the best fit to the data is obtained for a crack like source where rupture is initiated at one edge and expands circularly over the fault the stress drop inferred is 720 bars and the fractured area has a diameter of about 1 km the fault is vertical and strikes in the n320 e direction this simple model explains surprisingly well the amplitudes and waveforms of the ground velocities and accelerations recorded in the near source region during the event

high broadband seismoacoustic signature of vulcanian explosions at popocatepetl volcano mexico author s matoza robin samuel arciniega ceballos a sanderson r w mendo perez g rosado fuentes a chouet b a

subsurface structure at the insight landing site from compliance measurements by seismic and meteorological experiments measurements of ground compliance at the insight landing site describing the surface response to pressure loading are obtained from seismic and meteorological data compliance observations show an increase with frequency indicating the presence of a stiffer rock layer beneath the exposed regolith we performed a markov chain monte carlo inversion to investigate the vertical profile of the elastic parameters down to 20 m below insight compliance was inverted both freely and assuming prior knowledge of compaction in the regolith and the limitations and strengths of the methods were assessed on the basis of theoretical considerations and synthetic tests the inverted young modulus exhibits an increase by a factor of 10 100 over the first 10 15 m compatible with a structural discontinuity between 0 7 and 7 m the proposed scheme can be used for joint inversion of other seismic geological or mechanical constraints to refine the resulting vertical section

faulting characteristics of supershear earthquakes article i nfo recent observations of large continental strike slip earthquakes show that different fault segments may rupture at different speeds and that rupture may propagate faster than the shear wave velocity of surrounding rocks we report that all the documented occurrences of supershear rupture are associated with faults which have simple geometry the surface trace of these faults is described in the field or seen on satellite images as remarkably linear continuous and narrow segmentation features along these segments are small or absent and the deformation is highly localized as theoretically predicted supershear is clearly associated with mode ii rupture

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

evidence for azimuthal and temporal variations of the rupture propagation of the 1999 chi chi taiwan earthquake from dense seismic array observations data recorded by the smart 2 array and its nearby accelerometers in eastern taiwan were used to make direct measurements of the rupture propagation of the 1999 mw 7 6 chi chi taiwan earthquake the n th root stack method was employed to window and stack seismic waveforms and it revealed that coherent seismic energy showed clear temporal changes with regard to azimuth the results of this study elicited unambiguous observations as to rupture propagation direction rupture duration time and spatial distribution of the major asperities on the fault plane the rupture propagation of the chi chi earthquake was reconstructed and demonstrated that the source of the early arrived coherent energy was close to the epicenter and that the subsequent major energy moved southward twelve seconds after the initial rupture clear arrival energy was resolved and then propagated northward the horizontal apparent rupture velocity is estimated at 2 1 km sec for its northward propagation the largest source of radiated energy was identified and its location was near the northern end of the earthquake fault the strong shaking was continuous for about 24 seconds the analysed results of this study provide direct observations of the rupture properties of this earthquake these properties may serve as valuable constraints for other chi chi earthquake rupture propagation studies based on other indirect measurement methods

high frequency observations and source parameters of microearthquakes recorded at hard rock sites we have estimated the source parameters of 53 microearthquakes recorded in july 1983 which were aftershocks of the miramichi new brunswick earthquake that occurred on 9 january 1982 these events were recorded by local three component digital seismographs at 400 sps component from 2 hz velocity transducers sited directly on glacially scoured crystalline basement outcrop hypocentral distances are typically less than 5 km and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks the p wave velocity is typically 5 km sec at the surface and at least 6 km sec at depths greater than about 1 km the events have s wave corner frequencies in the band 10 to 40 hz and the calculated brune model seismic moments range from 1015 to 1018 dyne cm the corresponding stress drops are generally less than 1 0 bars but there is considerable evidence that the seismic source signals have been modified by propagation and or site effects the data indicate a there is a velocity discontinuity at 0 5 km depth b the top layer has strong scattering attenuating properties c some source receiver paths differentiate the propagated signal d there is a hard rock site p wave f max between 50 and 100 hz and e some hard rock sites are characterized by p wave resonance frequencies in the range 50 to 100 hz comparison of this dataset with the january 1982 new brunswick digital seismograms which were recorded at sites underlain by several meters of low velocity surface sediments suggests that some of the hard rock site phenomena listed above can be explained in terms of a layer over a half space model for microearthquakes this result implies that spectrally determined source dimension scales with site dimension thickness of the layer more generally it emphasizes that it is very difficult to accurately observe the details of seismogenic processes from the earth s surface

shallow elasticity structure from colocated pressure and seismic stations in the pinon flat observatory and estimation of vs30 author s tanimoto toshiro wang jiong abstract summary an algorithm developed in tanimoto a wang for deriving shallow elasticity structure from colocated pressure and seismic instruments is applied to data at nine stations in the pinon flat observatory pfo in southern california depth resolution kernels indicate that this approach can recover near surface shear modulus structure in the upper 50 100 m of the earth our estimate for vs30 at the closest station bph01 agrees with an independent result by yong et al which was based on an active source survey with a geophone array vs30 derived from the borehole s wave speed model by fletcher et al also agrees with estimates at nearby locations considering the size of uncertainties derived structures are much smoother than their models however this is because our method analyses slow quasi static deformation of the solid earth in the frequency range 0 005 0 05 hz that inherently limits depth resolution lack of detailed variations in structures does not seem to be the problem for estimating vs30 as vs30 is an averaged quantity from the upper 30 m of the earth also our estimates are not affected by different choices of frequency range in the inversion however our method may have some difficulty at stiff hard rock sites because ground deformation caused by surface pressure can become small and the key observables in our method the ratios between seismic amplitude and surface pressure change become difficult to measure accurately

high resolution backprojection at regional distance application to the haiti m7 0 earthquake and comparisons with finite source studies a catastrophic m w7 earthquake ruptured on 12 january 2010 on a complex fault system near port au prince haiti offshore rupture is suggested by aftershock locations and marine geophysics studies but its extent remains difficult to define using geodetic and teleseismic observations here we perform the multitaper multiple signal classification music analysis a high resolution array technique at regional distance with recordings from the venezuela national seismic network to resolve high frequency about 0 4 hz aspects of the earthquake process our results indicate westward rupture with two subevents roughly 35 km apart in comparison a lower frequency finite source inversion with fault geometry based on new geologic and aftershock data shows two slip patches with centroids 21 km apart apparent source time functions from usarray further constrain the intersubevent time delay implying a rupture speed of 3 3 km s the tips of the slip zones coincide with subevents imaged by backprojections the different subevent locations found by backprojection and source inversion suggest spatial complementarity between high and low frequency source radiation consistent with high frequency radiation originating from rupture arrest phases at the edges of main slip areas the centroid moment tensor cmt solution and a geodetic only inversion have similar moment indicating most of the moment released is captured by geodetic observations and no additional rupture is required beyond where it is imaged in our preferred model our results demonstrate the contribution of backprojections of regional seismic array data for earthquakes down to m 7 especially when incomplete coverage of seismic and geodetic data implies large uncertainties in source inversions

a preliminary investigation into the relationship between long period seismic noise and local fluctuations in the atmospheric pressure field summary the displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long period seismic noise field the study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space the numerical studies indicate that pressure waves with amplitudes of 100 pbar or more can contribute significantly to the long period vertical background noise observed at the surface provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre these same waves can also create significant tilt noise on long period horizontal seismographs located at or near the surface regardless of the rock type the seismic disturbances created by pressure waves decay rapidly away from the surface therefore it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths

effects of seawater of tokyo bay on short period strong ground motion the effects of the seawater of tokyo bay on strong ground motions are investigated in a short period range of less than about 2 0 sec for engineering importance a recent study based on observed accelerograms revealed that considerable short period surface waves were included in strong ground motions in the reclaimed zone of tokyo and that those surface waves were mainly love waves however it was not clarified whether short period rayleigh waves were present the present study is aimed at assessing the excitation of short period rayleigh waves on land and at exploring the effects of the shallow approximately 25 m water layer of tokyo bay on rayleigh waves in the reclaimed zone of tokyo we use a theoretical approach in which a 2d p sv wave field is calculated for a multilayered structural model the model is composed of a sea zone and a land zone and includes a water layer and soft surface layers to examine the effects of seawater we use models without and with a water layer as a result it is concluded that in the reclaimed zone of tokyo stripping the water layer off of a typical model for shallow tokyo bay has little influence on the short period ground motions around the theoretical predominant period of the ground of about 1 0 sec it is not essential to incorporate the water layer in modeling shallow tokyo bay for short period ground motion simulations

broadband seismic noise attenuation versus depth at the albuquerque seismological laboratory abstract seismic noise induced by atmospheric processes such as wind and pressure changes can be a major contributor to the background noise observed in many seismograph stations especially those installed at or near the surface cultural noise such as vehicle traffic or nearby buildings with air handling equipment also contributes to seismic background noise such noise sources fundamentally limit our ability to resolve earthquake generated signals many previous seismic noise versus depth studies focused separately on either high frequency 1 hz or low frequency

the effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station seismic investigations are mainly limited by seismic noise two microbarometers have been installed in the seismic vault of two different geoscope stations one at ssb and the other at tam all vertical components and most of the horizontal components show a significant correlation with pressure in order to correct the seismic signals from the atmospheric pressure noise a transfer function between the pressure data and the seismic data is inverted results show that after correction the noise levels reached on the horizontal components are similar between the two stations and the vertical components display noise levels below the low noise model as defined by peterson 1993 this technique reduces part of the noise and allows detection of small earthquakes and a better extraction of normal modes the analysis of the lowest normal modes of the earth excited by the m s 8 2 macquarie island earthquake is given to illustrate the perspectives of the method

supershear transition due to a free surface in 3 d simulations of spontaneous dynamic rupture on vertical strike slip faults supershear rupture propagation has been inferred from seismic observations for natural faults and observed in laboratory experiments we study the effect of the free surface on the transition of earthquake rupture from subshear to supershear speeds using simulations of spontaneous dynamic rupture on vertical strike slip faults we find that locally supershear rupture near the free surface can occur due to i the generalized burridge andrews mechanism that is a supershear loading field between p and sv wave arrivals generated by the main rupture front at depths and ii the phase conversion of sv to p diffracted waves at the free surface weaker supershear slip due to the generalized burridge andrews mechanism is caused by the low strength at shallow portions of the fault relative to deeper ones dominant supershear rupture is supported by the additional supershear loading field produced by phase conversion locally supershear propagation at the free surface occurs regardless of the level of prestress and can cause transition to supershear propagation over the entire seismogenic depth such global supershear transition which depends on prestress can occur under prestress levels lower than the theoretical estimates for models with no free surface although the effectiveness of supershear transition due to the free surface can be diminished by several potentially important factors it may play an important role on natural faults at least in those strike slip earthquakes that accumulate significant surface slip

earth motion caused by local atmospheric pressure changes summary observations have been made of the local atmospheric pressure field and the long period seismic noise fields both on the surface of the earth and in a mine at a depth of 183 metres the observations show that during windy intervals and in the period range 20 100 s there is a strong correlation between local atmospheric pressure changes and the noise recorded by a vertical seismograph located on the surface in contrast over the same range of periods there is no correlation between the seismic noise recorded in the mine and local atmospheric pressure changes except during the passage of acoustic waves it is shown that the noise in this pass band is not due to the buoyant response of the seismograph but is caused by the motion of the earth responding to atmospheric pressure changes

spatiotemporal properties of sub rayleigh and supershear ruptures inferred from full field dynamic imaging of laboratory experiments many earthquakes propagate at sub rayleigh speeds earthquakes propagating at supershear speeds though less common are by far more destructive hence it is important to quantify the motion characteristics associated with both types of earthquake ruptures here we report on the spatiotemporal properties of dynamic ruptures measured in our laboratory experiments using the dynamic digital image correlation technique earthquakes are mimicked by the frictional rupture propagating along the interface of two homalite plates digital images of the propagating ruptures are captured by an ultrahigh speed camera and processed with digital image correlation in order to produce sequences of evolving displacement and velocity maps our measurements reveal the full field structure of the velocity components bridge the gap between previous spatially sparse velocimeter measurements available only at two to three locations and enable us to quantify the attenuation patterns away from the interface

source effects on strong motion records and resolving power of strong motion arrays for source inversion abstract source effects are dominant in near source strong motion seismograms even in the high frequency range and source inversion is the most powerful tool for investigating source effects first source effects that are closely related to damage and intensity patterns are briefly shown large inconsistencies have been seen among source inversion results the cause seems to come mainly from insufficient station distribution for earthquake source studies however the deployment strategy of strong motion seismographs has been only poorly explored secondly three kinds of simulations conducted for the last several years are shown in order to indicate the vulnerability of the resolving power of strong motion arrays to a source inversion the simulations are 1 1 to obtain a relationship between the accuracy of the source inversion and fault array parameters 2 2 to present the optimum array geometry for the source inversion and 3 3 to describe the relative significance of various seismic waves in the source inversion thirdly basic guidelines on strong motion array layout for source inversion are provided and general comments are given on actual source inversion methods we find that the physical wave is a key concept for understanding the optimum array configuration and for choosing the most pertinent inversion method frequency consideration which is a remaining problem is required to decide the best inversion method and the best array layout we can design strong motion array for source inversion on the condition that frequency contents fault mechanism the spatial resolution and the inversion accuracy are prescribed

illuminating the near sonic rupture velocities of the intracontinental kokoxili mw 7 8 and denali fault mw 7 9 strike slip earthquakes with global p wave back projection imaging 1 the denali and kokoxili strike slip earthquakes are two of the longest recent intracontinental ruptures previous studies report a range of rupture velocities here we image these earthquakes by reverse time migration of the intermediate frequency p wave train recorded by global broadband seismometers this technique permits a relatively direct measure of rupture velocity speed and direction as constrained by the radiated seismic energy free from restrictive assumptions or rupture speed bounds placed on the solution we compare our results with published seismic gps displacement and surface slip inversion results both ruptures were initially subshear and transitioned over a distance no longer than 40 km to supershear speeds close to the p wave speed of 5 6 km s we investigate the accuracy of our results with synthetic data and experiment with using different imaging parameters and seismic subnetworks these tests allow us to rule out the possibility of subshear speeds along the supershear segments although we cannot exclude supershear speeds of 4 5 6 5 km s our most reliable rupture velocities of 5 6 km s are close to the local p wave speeds we hypothesize that these intracontinental faults have weak shear strengths or high breakdown slips or crustal rigidities and experience at least moderate slip or slip rate weakening our observations and previous published results lead us to speculate that very long surface extending faults with general homogeneity in prestress and fault strength together with smaller adjacent fault segments to provide triggering may be necessary ingredients for the sub rayleigh to supershear rupture speed transition in strike slip earthquakes

on the use of high resolution topographic data as a proxy for seismic site conditions vs30 an alternative method has recently been proposed for evaluating global seismic site conditions or the average shear velocity to 30 m depth v s 30 from the shuttle radar topography mission srtm 30 arcsec digital elevation models dems the basic premise of the method is that the topographic slope can be used as a reliable proxy for v s 30 in the absence of geologically and geotechnically based site condition maps through correlations between v s 30 measurements and topographic gradient here we evaluate the use of higher resolution 3 and 9 arcsec dems to examine whether we are able to resolve v s 30 in more detail than can be achieved using the lower resolution srtm data high quality dems at resolutions greater than 30 arcsec are not uniformly available at the global scale however in many regions where such data exist they may be employed to resolve finer scale variations in topographic gradient and consequently v s 30 we use the u s geological survey earth resources observation and science eros data center s national elevation dataset ned to investigate the use of high resolution dems for estimating v s 30 in several regions across the united states including the san francisco bay area in california los angeles california and st louis missouri we compare these results with an example from taipei taiwan that uses 9 arcsec srtm data which are globally available the use of higher resolution ned data recovers finer scale variations in topographic gradient which better correlate to geological and geomorphic features in particular at the transition between hills and basins warranting their use over 30 arcsec srtm data where available however statistical analyses indicate little to no improvement over lower resolution topography when compared to v s 30 measurements suggesting that some topographic smoothing may provide more stable v s 30 estimates furthermore we find that elevation variability in canopy based srtm measurements at resolutions greater than 30 arcsec are too large to resolve reliable slopes particularly in low gradient sedimentary basins

testing the applicability of correlations between topographic slope and vs30 for europe in the past few years a series of articles have been published concerning the use of topographic slope from digital elevation models dems constructed through remote sensing satellite imaging to give first order estimates of national earthquake hazards reduction program nehrp site classes based on the average shear wave velocity in the top 30 m vs30 wald and allen 2007 we evaluate the potential applicability of these methods taking advantage of a large 706 sites new database of measured and estimated vs30 values and their topographic slopes for locations in europe and the middle east novel statistical tests are performed to evaluate the predictive power of the procedure in this region we evaluate the percentage of sites correctly classified misclassified for each site class for active and stable regimes we also analyze the marginal distributions of the input vs30 and slope values and their impact on the vs30 slope correlations and we evaluate whether the method performs better than does chance we also consider the surface geology of sites and investigate whether differences in geology can help explain why certain sites are poorly classified by the method finally we use the city of thessaloniki greece as a test case for comparison between the results of a recent microzonation and the site classes predicted by vs30 slope correlations our results show that the method does a better job than blind chance for all site classes in active regions but only for class b rock and to a lesser extent class c stiff soil sites located in stable areas although the conclusions for stable areas are based on limited data we recommend that site classifications based on the vs30 slope correlations proposed by wald and allen 2007 be used only for regional or national and not local or site specific first order studies in active parts of europe and only in the absence of other more detailed information excluding sites inside small basins or those with special geological conditions that may affect results e g flat lying volcanic plateaus carbonate rocks continental glaciated terrain or a coastal location if slope is not calculated using bathymetric data

on noise reduction in vertical seismic records below 2 mhz using local barometric pressure we propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mhz the method consists in subtracting the locally recorded scaled atmospheric pressure signal from the gravity record the scale factor is frequency independent and can be linearly estimated from the data while such a pressure correction was previously only used in the analysis of tidal records it is equally efficient at the low frequency end of the normal mode band and we use gravity recordings of the big 1994 shikotan and bolivian quakes to demonstrate this point our examples also show that this correction permits to achieve noise levels well below the new low noise model nlnm peterson 1993 the success of the correction also sheds some light on the physical processes responsible for the generation of low frequency seismic noise at quiet observatories

identifying and removing tilt noise from low frequency 0 1 hz seafloor vertical seismic data low frequency 0 1 hz vertical component seismic noise can be reduced by 25 db or more at seafloor seismic stations by subtracting the coherent signals derived from 1 horizontal seismic observations associated with tilt noise and 2 pressure measurements related to infragravity waves the reduction in effective noise levels is largest for the poorest stations sites with soft sediments high currents shallow water or a poorly leveled seismometer the importance of precise leveling is evident in our measurements low frequency background vertical seismic spectra measured on a seafloor seismometer leveled to within 1 10 4 radians 0 006 degrees are up to 20 db quieter than on a nearby seismometer leveled to within 3 10 3 radians 0 2 degrees the noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides indicating that it is caused by tilting due to seafloor currents flowing across the instrument at low frequencies this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect to the earth s gravitational field the effect is much stronger on the horizontal components than on the vertical allowing significant reduction in vertical component noise by subtracting the coherent horizontal component noise this technique reduces the low frequency vertical noise on the less precisely leveled seismometer to below the noise level on the precisely leveled seismometer the same technique can also be used to remove background noise due to the seafloor pressure field up to 25 db noise reduction near 0 02 hz and possibly due to other parameters such as temperature variations

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

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

dependency of supershear transition and ground motion on the autocorrelation of initial stress abstract theoretical and observational studies show that earthquakes on strike slip faults can have rupture speeds exceeding the shear wave speed due to the close relationship between the rupture velocity and the radiated wave field it is important to understand the conditions leading to supershear ruptures and their effect on the resulting ground motion we compute dynamic strike slip ruptures in a 3d elastic half space using heterogeneous frictional properties on faults that are 60 km long we use a grid spacing of 60 m allowing us to compute ground motion for frequencies from 0 to 5 hz we analyze the resulting ground motion using isochrones to explain phenomena we observe we model the amplitudes of the initial shear stress as a self similar random field with cauchy distributed amplitudes the wavenumber amplitude spectrum of initial stress decays as a power law with exponent that controls the decay and the spatial correlation of the initial stress the faster the decay corresponding to larger value of the more correlated is the stress on the fault i e the stress field appears spatially smoother the strength on the fault is computed under the assumption of a constant s factor where s is the ratio of strength excess over stress drop on a fault with uniform strength and stress drop the s factor has to be less than a critical value for the supershear transition to occur for models with heterogeneous initial stress we find that both the s factor and the value of the spectral decay constant affect the occurrence of supershear rupture we observe that for a given but small enough s factor a smooth model 2 can run at supershear speed while a rough model 1 will rupture at subshear speeds for the same s factor based on the theory of fracture a non dimensional number was introduced to quantify the condition when a transition to supershear rupture velocity can occur during an earthquake transition will occur when exceeds a critical value we introduce a modified dimensionless parameter ac that is based on the original parameter the parameter ac incorporates a length scale w ac that reflects the degree of the autocorrelation of the stress field we compute ac for a large number of available dynamic ruptures that propagate at subshear and supershear speeds and find i there is a critical value ac c below which all ruptures propagate subshear ii for values larger than ac c there is only a finite probability that the rupture goes supershear i e it is a necessary but not a sufficient condition for the occurrence of supershear rupture propagation

precise measurement of seismic travel times from an air gun source summary seismic travel times for extrema zero crossings or entire body waves need to be determined precisely to one part in lo3 or better in several varieties of seismic studies employing an impulsive artificial source examples are crosshole surveys which delineate rock crack distribution separating the holes and monitoring of crustal seismic travel times in earthquake precursor studies a timing resolution of one part in lo3 has been achieved previously using digitally recorded seismic data these methods however do not use interpolation between digitized data points as a method to increase the timing resolution we report travel time determinations based on interpolation between digitized points which achieve a precision of two parts in lo4 a five fold improvement over the existing methods in addition the effects of seismic noise on travel time measurement have been compared for the extremum location the unnormalized correlation and the normalized correlation method the following conclusions are drawn from this comparison 1 the normalized correlation method provides an 18 55 per cent improvement in the standard deviation of the mean over the extremum location method and 2 results as accurate as those by the normalized correlation can be obtained by the unnormalized correlation if a complete up and down swing of the waveform is used as the master trace and if the master trace is close to being sinusoidal the advantage of the unnormalized correlation over the normalized correlation is speed the unnormalized correlation is faster by a factor of 28 in computing time

source dynamics of the 1979 imperial valley earthquake from near source observations of ground acceleration and velocity two sets of observations obtained during the 15 october 1979 imperial valley earthquake ms 6 9 are presented the data suggest different dynamic characteristics of the source when viewed in different frequency bands the first data set consists of the observed residuals of the horizontal peak ground accelerations and particle velocity from predicted values within 50 km of the fault surface the residuals are calculated from a nonlinear regression analysis of the data campbell 1981 to the following empirical relationships pga a 1 r c 1 d 1 pgv a 2 r c 2 d 2 in which r is the closest distance to the plane of rupture the so calculated residuals are correlated with a positive scalar factor signifying the focusing potential at each observation point the focusing potential is determined on the basis of the geometrical relation of the station relative to the rupture front on the fault plane the second data set consists of the acceleration directions derived from the windowed time histories of the horizontal ground acceleration across the el centro differential array ecda the horizontal peak velocity residuals and the low pass particle acceleration directions across ecda require the fault rupture to propagate northwestward the horizontal peak ground acceleration residuals and the high frequency particle acceleration directions however are either inconclusive or suggest an opposite direction for rupture propagation the inconsistency can best be explained to have resulted from the incoherence of the high frequency radiation which contributes most effectively to the registration of pga a test for the sensitivity of the correlation procedure to the souce location is conducted by ascribing the observed strong ground shaking to a single asperity located 12 km northwest of the hypocenter the resulting inconsistency between the peak acceleration and velocity observations in relation to the focusing potential is accentuated the particle velocity of delta station mexico in either case appears abnormally high and disagrees with other observations near the southeastern end of the fault trace from the observation of a nearly continuous counterclockwise rotation of the plane of p wave particle motion at ecda the average rupture velocity during the first several seconds of source activation is estimated to be 2 0 to 3 0 km sec a 3 km upper bound estimate of barrier dimensions is tentatively made on the basis of the observed quasiperiodic variation of the polarization angles

watching the wind seismic data contamination at long periods due to atmospheric pressure field induced tilting abstract following the installation of a new broadband seismic station near the pacific coast of the olympic peninsula washington the pacific northwest seismic network recorded elevated background seismic noise at periods exceeding 30 s we observed power spectral density variations of about 15 20 db in the horizontal component seismograms between times of high and low noise and differences of up to about 25 db between the horizontal and vertical components during noisy intervals the amplitude of the signal at long periods exhibited clear correlations with cyclic daytime fluctuations of air temperature and wind speed our observations provided measurable evidence that the variations of the local atmospheric pressure field associated with local wind systems represents an effective source of ground displacements and rotations the rotational component of the pressure field induced motion is recorded by horizontal component seismometers that below their low frequency corner are sensitive to surface tilts the response of seismometers to ground rotations is characterized by a transfer function which we use to estimate the amplitude of tilt we measured a pressure field induced surface tilt of the order of several tenths of rad based on simple modeling tilt contamination is expected to attenuate rapidly with depth and to become negligible just a few hundreds of meters below the surface removal of this unwanted effect from seismograms remains a challenging task as the details of tilting depend on the nonstationary character of the pressure field the compliance of the near surface to pressure variations and the design and construction of the seismic station online material animated time sequenced plots of daily seismic noise pdfs

multiasperity fault model and the nature of short period subsources we suggest to consider the breaking of an asperity i e a small contact patch between fault walls as a typical subsource producing an elementary short period radiation pulse from a source of a large earthquake based on the results ofdas andkostrov we propose formulas to describe amplitudes and spectra of acceleration for a multiasperity fault source model the stress drop over an asperity is determined in several ways the estimates agree to give the average value of several hundred bar theoretical acceleration spectral shapes for the case of similar asperities agree with the observed ones they reproduce such features as lower frequencyf1 slope peak and high frequency cutoff the statistical stress drop distribution over the population of asperities and also the related distribution of peak accelerations are discussed there distributions are found to be the power law ones with exponent near to 2 this means that acceleration peaks are formed normally by breaking of individual asperities we consider small earthquakes as produced by breaking of single asperities this idea explains the observed correlation between the upper cutoff frequency of acceleration spectrum and the typical characteristic frequency of small earthquakes