lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

ultrashallow depth imaging of a channel stratigraphy with first arrival traveltime inversion and prestack depth migration a case history from bull creek oklahoma depth imaging in ultrashallow 50 m environments presents twofold challenge 1 coda available for depth migration is very limited and 2 conventional time processing with limited coda generally fails to estimate reliable velocity models for depth migration we studied the combining of first arrival traveltime inversion and prestack depth migration psdm for depth imaging of ultrashallow paleochannel stratigraphy associated with the bull creek drainage system oklahoma restricted by a limited number of geophones 24 we acquired data for inversion and migration through two coincident profiles the first profile for inversion has a wider survey aperture 115 m maximum shot receiver spacing and consequently sparse cmp spacing 2 5 m whereas the second profile for psdm has denser cmp spacing 1 m and consequently a narrower survey aperture 46 m maximum shot receiver spacing we also found that the velocity model from traveltime inversion of the wider aperture data set is more preferable for dep

anatomy of the chesapeake bay impact structure revealed by seismic imaging delmarva peninsula virginia usa 1 a 30 km long radial seismic reflection and refraction survey completed across the northern part of the late eocene chesapeake bay impact structure cbis on the delmarva peninsula virginia usa confirms that the cbis is a complex central peak crater we used a tomographic p wave velocity model and low fold reflection images constrained by data from two deep boreholes located on the profile to interpret the structure and composition of the upper 5 km of crust the seismic images exhibit well defined structural features including with increasing radial distance a collapsed central uplift a breccia filled moat and a collapsed transient crater margin which collectively constitute a 40 km wide collapsed transient crater and a shallowly deformed annular trough these seismic images are the first to resolve the deep structure of the crater 1 km and the boundaries between the central uplift moat and annular trough several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift moat and annular trough within the moat breccia extends to a minimum depth of 1 5 km or a maximum of 3 5 km depending upon the interpretation of the deepest layered materials the images show 350 to 500 m of postimpact sediments above the impactites the imaged structure of the cbis indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling

sensitivity of prestack depth migration to the velocity model to get a correct earth image from seismic data acquired over complex structures it is essential to use prestack depth migration a necessary condition for obtaining a correct image is that the prestack depth migration is done with an accurate velocity model in cases where we need to use prestack depth migration determination of such a model using conventional methods does not give satisfactory results thus new iterative methods for velocity model determination have been developed the convergence of these methods can be accelerated by defining constraints on the model in such a way that the method only looks for those components of the true earth velocity field that influence the migrated image in order to determine these components the sensitivity of the prestack depth migration result to the velocity model is examined using a complex synthetic data set the marmousi data set for which the exact model is known the images obtained with increasingly smoothed versions of the true model are compared

3 d tomographic seismic inversion of a paleochannel system in central new south wales australia buried paleochannels are of significant interest for understanding hydrological mechanisms and their potential as alluvial gold deposits seismic tomographic methods are a suitable solution for resolving the vertical and horizontal structure of such features we assess a method for seismic 3 d tomographic inversion from refraction arrivals with reflection control over a suspected paleochannel adjacent to the wyalong gold fields in the lachlan fold belt of central new south wales australia a standard multichannel engineering seismic recording and cable receiver system was used on a 3 d field geometry of multiple linear arrays more than 3000 p wave first arrival traveltime values were inverted using a regularized inversion scheme for which simplified 2 d models served as initial velocity depth models for the complete 3 d inversion seismic reflection arrivals provided additional depth estimates to the bedrock and compensated for a lack of refraction phases at that depth correlating the 3 d seismic veloc

seismotectonics of the loma prieta california region determined from three dimensional v p v p v s and seismicity three dimensional vp and vp vs velocity models for the loma prieta region were developed from the inversion of local travel time data 21 925 p arrivals and 1 116 s arrivals from earthquakes refraction shots and blasts recorded on 1700 stations from the northern california seismic network and numerous portable seismograph deployments the velocity and density models and microearthquake hypocenters reveal a complex structure that includes a san andreas fault extending to the base of the seismogenic layer a body with high vp extends the length of the rupture and fills the 5 km wide volume between the loma prieta mainshock rupture and the san andreas and sargent faults we suggest that this body controls both the pattern of background seismicity on the san andreas and sargent faults and the extent of rupture during the mainshock thus explaining how the background seismicity outlined the along strike and depth extent of the mainshock rupture on a different fault plane 5 km away new aftershock focal mechanisms based on three dimensional ray tracing through the velocity model support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression the subvertical or steeply dipping san andreas fault and the fault surfaces that ruptured in the 1989 loma prieta earthquake are both parts of the san andreas fault zone and this section of the fault zone does not have a single type of characteristic event

fracture surface energy of the punchbowl fault san andreas system the energetics of earthquake rupture slip weakening and rupture surface formation are poorly understood by measuring the energy associated with fracture damage along the punchbowl fault in the devil s punchbowl los angeles county park part of the san andreas system chester et al show that the fracture surface energy for rupture along a mature fault of this type accounts for only a small fraction of the energy released during large earthquakes as determined by seismologists this suggests that mechanisms such as melt lubrication and thermal pressurization are responsible for slip weakening fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening1 2 3 fracture energy has been estimated from seismological and experimental rock deformation data4 5 6 7 8 yet its magnitude mechanisms of rupture surface formation and processes leading to slip weakening are not well defined8 9 10 here we quantify structural observations of the punchbowl fault a large displacement exhumed fault11 12 in the san andreas fault system and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake if fracture energy is attributed entirely to the production of fracture surfaces then all of the fracture surface area in the punchbowl fault could have been produced by earthquake displacements totalling 1 km but this would only account for a small fraction of the total energy budget and therefore additional processes probably contributed to slip weakening during earthquake rupture

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

faulting apparently related to the 1994 northridge california earthquake and possible co seismic origin of surface cracks in potrero canyon los angeles county california apparent southward dipping reverse fault zones are imaged to depths of about 1 5 km beneath potrero canyon los angeles county california based on their orientation and projection to the surface we suggest that the imaged fault zones are extensions of the oak ridge fault geologic mapping by others and correlations with seismicity studies suggest that the oak ridge fault is the causative fault of the 17 january 1994 northridge earthquake northridge fault our seismically imaged faults may be among several faults that collectively comprise the northridge thrust fault system unusually strong shaking in potrero canyon during the northridge earthquake may have resulted from focusing of seismic energy or co seismic move ment along existing related shallow depth faults the strong shaking produced ground surface cracks and sand blows distributed along the length of the canyon seismic reflection and refraction images show that shallow depth faults may underlie some of the observed surface cracks the relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have devel oped from nontectonic alluvial movement but others may be fault related imme diately beneath the surface cracks p wave velocities are unusually low 400 m sec and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m on the basis of velocity data we suggest that unconsolidated soils 800 rrdsec extend to depths of about 15 to 20 m beneath our datum 25 m below ground surface the underlying rocks range in velocity from about 1000 to 5000 m sec in the upper 100 m this study illustrates the utility of high resolution seismic imaging in assessing local and regional seismic hazards

a method and example of seismically imaging near surface fault zones in geologically complex areas using vp vs and their ratios abstract the determination of near surface vadose zone and slightly below fault locations and geometries is important because assessment of ground rupture strong shaking geologic slip rates and rupture histories occurs at shallow depths however seismic imaging of fault zones at shallow depths can be difficult due to near surface complexities such as weathering groundwater saturation massive nonlayered rocks and vertically layered strata combined p and s wave seismic refraction tomography data can overcome many of the near surface fault zone seismic imaging problems because of differences in the responses of elastic bulk and shear moduli of p and s waves to shallow depth fault zone properties we show that high resolution refraction tomography images of p to s wave velocity ratios v p v s can reliably identify near surface faults we demonstrate this method using tomography images of the san andreas fault saf surface rupture zone associated with the 18 april 1906 m 7 9 san francisco earthquake on the san francisco peninsula in california there the saf cuts through franciscan melange which consists of an incoherent assemblage of greywacke chert greenstone and serpentinite a near vertical zone 75 northeast dip of high p wave velocities up to 3000 m s low s wave velocities 150 600 m s high v p v s ratios 4 8 8 and high poisson s ratios 0 44 0 49 characterizes the main surface rupture zone to a depth of about 20 m and is consistent with nearby trench observations we suggest that the combined v p v s imaging approach can reliably identify most near surface fault zones in locations where many other seismic methods cannot be applied

near surface location geometry and velocities of the santa monica fault zone los angeles california high resolution seismic reflection and seismic refraction imaging combined with existing borehole earthquake and paleoseismic trenching data suggest that the santa monica fault zone in los angeles consists of multiple strands from several kilometers depth to the near surface we interpret our seismic data as showing two shallow depth low angle fault strands and multiple near vertical 85 faults in the upper 100 m one of the low angle faults dips northward at about 28 and approaches the surface at the base of a topographic scarp on the grounds of the wadsworth va hospital wvah the other principal low angle fault dips northward at about 20 and projects toward the surface about 200 m south of the topographic scarp near the northernmost areas of the los angeles basin that experienced strong shaking during the 1994 northridge earthquake the 20 north dipping low angle fault is also apparent on a previously published seismic reflection image by pratt et al 1998 and appears to extend northward to at least wilshire boulevard where the fault may be about 450 m below the surface slip rates determined at the wvah site could be significantly underestimated if it is assumed that slip occurs only on a single strand of the santa monica fault or if it is assumed that the near surface faults dip at angles greater than 20 28 at the wvah tomographic velocity modeling shows a significant decrease in velocity across near surface strands of the santa monica fault p wave velocities range from about 500 m sec at the surface to about 4500 m sec within the upper 50 m on the north side of the fault zone at wvah but maximum measured velocities on the south side of the low angle fault zone at wvah are about 3500 m sec these refraction velocities compare favorably with velocities measured in nearby boreholes by gibbs et al 2000 this study illustrates the utility of combined seismic reflection and seismic refraction methods which allow more accurate reflection imaging and compositional estimations across areas with highly variable velocities a property that is characteristic of most fault zones

reply to comment on near surface location geometry and velocities of the santa monica fault zone los angeles california by r d catchings g gandhok m r goldman d okaya m j rymer and g w bawden by t l pratt and j f dolan in a comment on our 2008 paper catchings gandhok et al 2008 on the santa monica fault in los angeles california pratt and dolan 2010 herein referred to as p d cite numerous objections to our work inferring that our study is flawed however as shown in our reply their objections contradict their own published works published works of others and proven seismic methodologies rather than responding to each repeated invalid objection we address their objections by topic in the subsequent sections in catchings gandhok et al 2008 we presented high resolution seismic reflection images that showed two near surface faults in the upper 50 m beneath the grounds of the wadsworth veterans administration hospital wvah although p d suggest we effectively duplicated their seismic acquisition our survey was not a duplication of their efforts rather we conducted a seismic imaging survey over a similar profile as pratt et al 1998 but used a different data acquisition system and different data processing methods to evaluate methods of seismically imaging blind faults in the wake of the 17 january 1994 m 6 7 northridge earthquake we used an acquisition method that provides both tomographic seismic velocities and reflection images our combined data approach allowed for shallower imaging 2 5 m minimum than the 20 m minimum of pratt et al 1998 clearer images of the fault zone and more accurate depth determinations rather than time images in processing the reflection images we used prestack depth migration which is generally accepted as the only proper imaging method for imaging subsurface structures with strong lateral velocity variations versteeg 1993 a condition shown to exist at the wvah site we correlated our reflection images with refraction tomography images borehole lithology and velocity data interferometric synthetic aperture radar images and changes in groundwater depths except for some minor differences our seismic reflection images coincide with previously published seismic reflection

near surface geophysical techniques for geohazards investigations some canadian examples over the last 40 years there has been an expansion of activity in applications of near surface geophysical techniques for various types of geohazards investigations in canada numerous national and international research groups have been working with the near surface geophysics section of the geological survey of canada to develop techniques for specific canadian engineering and environmental geohazards problems a few of the more interesting examples from widespread parts of the country are discussed in this paper

three dimensional vp and vp vs structure at loma prieta california from local earthquake tomography p and s p times from local earthquakes and p times from explosions at seismic stations in the vicinity of the 1989 loma prieta earthquake are inverted simultaneously to produce three dimensional images of vp and vp vs structure along the rupture zone the results confirm the presence of the major heterogeneities identified in previous studies using p waves only including high vp along the rupture in areas of significant slip and a low velocity wedge along the san andreas at the se edge of the rupture a change in rock properties evidenced by a significant vp vs increase equal to a 30 poisson s ratio increase from the middle crust to the upper crust may have been responsible for the upward termination of the main shock rupture we speculate that the deep high vp body that lies along the se portion of the rupture zone consists of two bodies based on model results indicating that the sw part of the body has relatively high vp vs while the ne part has relatively low vp vs

tectonic geomorphology of the san andreas fault zone from high resolution topography an example from the cholame segment abstract high resolution topographic data along fault zones are important aids in the delineation of recently active breaks a 15 km long portion of the south central san andreas fault saf along the southern cholame segment contains well preserved tectonic landforms such as benches troughs scarps and aligned ridges that indicate recurring earthquake slip recently acquired lidar topographic data along the entire southern saf b4 project have shot densities of 3 4 m 2 computed from the lidar returns digital elevation models dems of 0 25 to 0 5 m resolution using local binning with inverse distance weighting and 0 8 m or larger search radii depict the tectonic landforms at paleoseismic sites well enough to assess them confidently mapping of recently active breaks using a lidar only based approach compares well with aerial photographic and field based methods the fault zone varies in width from meters to nearly 1 km and is comprised of numerous en echelon meter to kilometer length overlapping sub parallel fault surfaces bounding differentially moving blocks that elongate parallel to the saf the semantic variations of what constitutes active and the importance of secondary traces influence the breadth and complexity of the resulting fault trace maps

near surface mapping using sh wave and p wave seismic land streamer data acquisition in illinois u s human activities have considerable impact on the upper 300 m of sediments of the earth conflicting activities in highly populated areas involving groundwater exploitation aggregate extraction and hazardous chemical related production and storage induce stress on the environment nearly half the population living around the great lakes depends on shallow groundwater aquifers a coalition of four great lakes states illinois indiana michigan and ohio in conjunction with the u s geological survey has initiated a program to map this sensitive upper 300 m layer as part of this effort the illinois state geological survey isgs has undertaken the task of preparing 3d quadrangle maps of the upper 100 m of the entire state at a scale of 1 24 000 although large scale maps indicating potential aquifer contamination already exist figure 1 very little is known about the groundwater pathways through the upper sedimentary column which mainly consists of deposits from the quaternary continental glacier

developing a map of geologically defined site condition categories for california consideration of site conditions is a vital step in analyzing and predicting earthquake ground motion the importance of amplification by soil conditions has long been recognized but though many seismic instrument sites have been characterized by their geologic conditions there has been no consistent simple classification applied to all sites as classification of sites by shear wave velocity has become more common the need to go back and provide a simple uniform classification for all stations has become apparent within the pacific earthquake engineering research center s next generation attenuation equation project developers of attenuation equations recognized the need to consider site conditions and asked that the california geological survey provide site conditions information for all stations that have recorded earthquake ground motion in california to provide these estimates we sorted the available shear wave velocity data by geologic unit generalized the geologic units and prepared a map so that we could use the extent of the map units to transfer the velocity characteristics from the sites where they were measured to sites on the same or similar materials this new map is different from the california geological survey preliminary site conditions map of california in that 19 geologically defined categories are used rather than national earthquake hazards reduction program categories although this map does not yet cover all of california when completed it may provide a basis for more precise consideration of site conditions in ground motion calculations

multicomponent high resolution seismic reflection profiling multicomponent seismic reflection methods are a new tool for oil and gas exploration and reservoir monitoring miles 1988 but such technologies have not yet been extensively exploited for near surface exploration related to hydrogeological and or geotechnical investigations with the advantage of relatively inexpensive recording systems for near surface applications we show that the use of multicomponent high resolution seismic reflection methods has great potential as a new means of observing and characterizing the physical parameters of the shallow subsurface and in particular of groundwater reservoirs

microfracture analysis of fault growth and wear processes punchbowl fault san andreas system california abstract fabric and timing relations of mode i microfractures are used to test current hypotheses for the origin of damage along large displacement faults by the processes of fault growth and wear oriented samples 0 075 m to 1 km from the punchbowl fault surface i e ultracataclasite layer document an increase in development of preferred orientation and increase in density of microfractures towards the ultracataclasite layer defining a zone of fault related microfracture damage about 100 m thick a distinct microfracture set that is perpendicular to the slip direction of the fault is present throughout the damage zone this implies that the average orientation of the maximum principal compressive stress within the damage zone was nearly normal to the fault surface two additional microfracture sets are present one oriented at low angles to the fault within meters of the ultracataclasite layer and another low angle set that occurs in the outermost damage zone the preferred orientations and timing relations are most consistent with local damage accumulation from stress cycling associated with slip on a geometrically irregular relatively weak fault surface low angle microfractures nearest the ultracataclasite layer also may record wear associated with the passage of earthquake ruptures and those in the outermost damage zone may be consistent with andersonian fault formation and subsequent fault weakening

three dimensional vp vs variations for the coso region california recent seismological studies of the coso region of southeastern california document both low p wave velocities and abnormal sv attenuation in indian wells valley south of the pleistocene volcanics of the coso range in order to learn more about the physical nature of these colocated anomalies a tomographic inversion for the three dimensional variations of vp vs the ratio of compressional to shear velocity was performed iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that vp vs is generally high at the surface and decreases systematically to 10 km depth superimposed on this trend are several large anomalies near devil s kitchen in the coso geothermal area vp vs values are very low near the surface consistent with measured values for steam dominated geothermal systems abnormally high values of vp vs are observed in portions of indian wells valley and also below the cactus peak rhyolite dome from 2 to 5 km in depth in indian wells valley the co occurrence of low p velocities low s velocities high vp vs and anomalous sv attenuation are indicative of subsurface partial melt because the inversion is based on vertical data only however the results cannot be considered conclusive only suggestive

delineating a shallow fault zone and dipping bedrock strata using multichannal analysis of surface waves with a land streamer the multichannel analysis of surface waves masw seismic method was used to delineate a fault zone and gently dipping sedimentary bedrock at a site overlain by several meters of regolith seismic data were collected rapidly and inexpensively using a towed 30 channel land streamer and a rubberband accelerated weight drop seismic source data processed using the masw method imaged the subsurface to a depth of about 20 m and allowed detection of the overburden gross bedding features and fault zone the fault zone was characterized by a lower shear wave velocity vs than the competent bedrock consistent with a large scale fault secondary fractures and in situ weathering the masw 2d vs section was further interpreted to identify dipping beds consistent with local geologic mapping mapping of shallow fault zones and dipping sedimentary rock substantially extends the applications of the masw method

large explosive source wide recording aperture seismic profiling on the columbia plateau washington clear subsurface seismic images have been obtained at low cost on the columbia plateau washington the columbia plateau is perhaps the most notorious of all bad data areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield this degradation was mitigated in this study via a large explosive source wide recording aperture shooting method the shooting method emphasizes the wide angle portion of the wavefield where fermat s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces the basalt diving wave normally discarded in standard common midpoint cmp seismic profiling can be used to image basalt velocity structure via traveltime inversion maximum depth penetration of the diving wave tightly constrains basalt sediment interface depth an arrival observed only at shot receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

application of shallow shear wave seismic reflection methods in earthquake hazards studies in engineering geophysics the use of shear wave seismic methods for earthquake hazards analysis has primarily been limited to site specific downhole crosshole and refraction studies for velocities to model site response to earthquake shaking and or liquefaction potential this paper demonstrates the utility of s wave reflection methods in earthquake hazards studies focusing on areas where collection of high quality compressional wave reflection data is problematic specific examples will show how s wave reflection data help interpret the age style and extent of near surface deformation in the new madrid seismic zone of the central united states and in mapping the holocene pleistocene surface beneath the fraser river delta in southwestern british columbia canada

abstract distinguishing water saturation changes from porosity or clay content changes using multicomponent seismic data abstract it is difficult to predict whether gas saturation is low or high in reservoir pore spaces prior to drilling when reservoirs include lateral porosity or clay content changes this task is even more difficult however the problem is easier to address with high quality multicomponent seismic data this paper proposes to use rps rpp as a partial gas indicator pgi where rps and rpp are defined as the change in the p sv and p p reflection coefficients respectively the target portion of the reservoir is compared to an inferred background portion of the reservoir which is assumed to be 100 water saturated the rps rpp ratio behaves quite differently for high and low gas saturations as shown by theoretical reflection coefficient computation for a range of examples using the zoeppritz and gassmann s equations the responses of rps rpp to porosity and clay content changes are computed using empirical relationships among velocities porosity and clay content wang and nur 1992 han et al 1986 eberhart phillips et al 1989 castagna et al 1985 the ratio is insensitive to the magnitude of porosity or clay content changes and this behavior is very different from the variations in the ratio associated with changes in gas saturation theoretical reflection coefficient computation modeling and synthetic seismograms show that rps rpp is an effective direct hydrocarbon indicator and pgi for all three classes of gas reservoirs rutherford and willams 1989 at both shale sand and sand shale interfaces the three classes of reservoirs are classified based on their acoustic impedance contrasts with their overlaying shales the rps rpp ratio can distinguish water saturation changes from porosity or clay content changes and separate regions of high gas saturation from low saturation areas

multichannel analysis of surface waves to map bedrock in many geologic settings topographic variations and discontinuities in the surface of bedrock can influence the transport and eventual fate of contaminants introduced at or near the ground surface determining the nature and location of anomalous bedrock can be an essential component of hydrologic characterization preliminary analysis of the hydrologic characteristics of a site in olathe kansas u s based primarily on borehole data alone suggested that a cluster of fractures and or an unmapped buried stream channel may influence fluid movement along the drill defined bedrock surface

waveform tomography at a groundwater contamination site surface reflection data wehaveappliedacoustic waveformtomographyto452d seismic profiles to image the 3d geometry of a buried paleochannel at a groundwater contamination site at hill air force base in utah the paleochannel which is incised into an alluvium covered clay aquitard acts as a trap for dense nonaqueous phase liquids dnapls that contaminate the shallowest groundwater system in the study area the 2d profiles were extracted from a 3d surface reflection data set first arrival traveltime tomography provided initial velocity models for the waveform tomography we inverted for six frequency components in the band 30 90 hz of the direct and refracted waves to produce 45 2d velocity models the flanks and bottom of a channel with a maximum depth of about 15 m were well modeled in most of the 45 parallel 2d slices whichallowedustoconstructa3dimageofthechannel by combining and interpolating between the 45 image slices the 3d model of the channel will be useful for siting extraction wells within the site remediation program the alluvium that fills the channel showed marked vertical and lateral velocity heterogeneity traveltime tomography and waveform tomography can be complementary approaches used together they can provide high resolution images of complicatedshallowstructures

mixing zone dolomites in the gully oolite lower carboniferous south wales the gully oolite at st lythams glamorgan is extensively dolomitized towards its base the dolomite is non ferroan and is both replacive and void filling in nature 18 o varies from 6 06 0 to 2 96 0 pdb and 13 c from 2 32 0 to 3 55 0 pdb with a marked covariation of 18 o and 13 c dolomitization is believed to have occurred within a mixing zone environment at salinities up to 50 of that of seawater the variation of dolomite isotopic composition versus depth in the sediment is explained in terms of successive upwards migration of the mixing zone and associated multiple phases of dolomitization the tops of these successive mixing zones coincide with evidence of sediment emergence and the presence of dolomite intraclasts implies that dolomitization occurred at shallow depths within the sediment this suggests dolomitization occurred in the absence of an overlying meteoric phreatic lens the high degree of dolomitization at st lythams as opposed to that occurring in modern mixing zones is related to 1 longer duration of the mixing zone environment in the sediment 2 the proximity of the mixing zone to the sediment surface 3 possible chemical differences between carboniferous and modern seawater particularly with respect to pco 2

detailed p and s wave velocity models along the larse ii transect southern california abstract structural details of the crust determined from p wave velocity models can be improved with s wave velocity models and s wave velocities are needed for model based predictions of strong ground motion in southern california we picked p and s wave travel times for refracted phases from explosive source shots of the los angeles region seismic experiment phase ii larse ii we developed refraction velocity models from these picks using two different inversion algorithms for each inversion technique we calculated ratios of p to s wave velocities v p v s where there is coincident p and s wave ray coverage we compare the two v p inverse velocity models to each other and to results from forward modeling and we compare the v s inverse models the v s and v p v s models differ in structural details from the v p models in particular dipping tabular zones of low v s or high v p v s appear to define two fault zones in the central transverse ranges that could be parts of a positive flower structure to the san andreas fault these two zones are marginally resolved but their presence in two independent models lends them some credibility a plot of v s versus v p differs from recently published plots that are based on direct laboratory or down hole sonic measurements the difference in plots is most prominent in the range of v p 3 to 5 km s or v s 1 25 to 2 9 km s where our refraction v s is lower by a few tenths of a kilometer per second from v s based on direct measurements our new v s v p curve may be useful for modeling the lower limit of v s from a v p model in calculating strong motions from scenario earthquakes

compressive failure of rocks by shear faulting a model for the compressive failure of rocks via the process of shear faulting is presented the model addresses the progressive growth of damage that leads to the formation of a critical fault nucleus which grows unstably in its own plane by fracturing the grain boundaries in an increasingly rapid succession the model uses a two parameter weibull type shear strength distribution for defining the nucleation of initial damage followed by the use of stress enhancement factors for addressing the increased probability of failure in the vicinity of already cracked grain boundaries these factors essentially involve surface averaging of enhanced stresses in the neighboring grains with the appropriate strength distribution as the weighting function as the stress is further increased similar correlated fracturing events get preferentially aligned to the crack cluster resulting in en echelon of cracks this crack cluster is modeled as an elliptical inhomogeneity within which the cracks interact and lead to material pulverization the effect of which mechanistically is to lower the shear modulus compared with the uncracked material on the outside the shear stress concentration resulting from this moduli mismatch is calculated and used to compute the stress enhancement factors for defining the nucleation of additional cracking events near the crack cluster eventually the size of the crack cluster becomes sufficiently large that it carries a stress concentration high enough to fracture all grain boundary elements in from of it in an increasingly rapid succession the stress associated with this event is taken as the failure stress since the model allows other cracking events to occur within the material volume in accordance with the assumed strength distribution function formation of other competing but subcritical shear faults naturally occurs besides the faulting stress for a prescribed confinement the model is able to predict the angle of the shear fault fairly well the model is applied to a variety of rock types including granite eclogite gabbro aplite rock salt sandstone dunite limestone and marble and the predicted failure envelopes compare rather well with the failure data available in the literature

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

application of seismic refraction methods in groundwater modeling studies in new england seismic refraction methods have been extensively used in petroleum mineral and engineering investigations and to some extent for hydrologic applications during the past 30 years recent advances in equipment sound sources and computer interpretation techniques make seismic refraction methods highly effective and economical for obtaining data for groundwater modeling studies aquifers that can be defined by one or more seismic velocity surfaces such as alluvial or glacial deposits in consolidated rock valleys limestone or sandstone underlain by metamorphic or igneous rock or saturated unconsolidated deposits overlain by unsaturated unconsolidated deposits are ideally suited for seismic refraction methods seismic refraction allows economical collection of data for one or more model input parameters and provides the basis for efficient collection of subsurface data by test drilling or aquifer tests seismic refraction studies were conducted over unconfined glacial aquifers in new england to deter

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

steep dip seismic imaging of the shallow san andreas fault near parkfield seismic reflection and refraction images illuminate the san andreas fault to a depth of 1 kilometer the prestack depth migrated reflection image contains near vertical reflections aligned with the active fault trace the fault is vertical in the upper 0 5 kilometer then dips about 70 to the southwest to at least 1 kilometer subsurface this dip reconciles the difference between the computed locations of earthquakes and the surface fault trace the seismic velocity cross section shows strong lateral variations relatively low velocity 10 to 30 high electrical conductivity and low density indicate a 1 kilometer wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace

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

san andreas fault geometry at desert hot springs california and its effects on earthquake hazards and groundwater abstract the mission creek and banning faults are two of the principal strands of the san andreas fault zone in the northern coachella valley of southern california structural characteristics of the faults affect both regional earthquake hazards and local groundwater resources we use seismic gravity and geological data to characterize the san andreas fault zone in the vicinity of desert hot springs seismic images of the upper 500 m of the mission creek fault at desert hot springs show multiple fault strands distributed over a 500 m wide zone with concentrated faulting within a central 200 m wide area of the fault zone high velocity up to 5000 m sec rocks on the northeast side of the fault are juxtaposed against a low velocity m 6 0 earthquakes in the area in 1948 and 1986 occurred at or near the depths 10 to 12 km of the merged san andreas fault large magnitude earthquakes that nucleate at or below the merged fault will likely generate strong shaking from guided waves along both fault zones and from amplified seismic waves in the low velocity basin between the two fault zones the mission creek fault zone is a groundwater barrier with the top of the water table varying by 60 m in depth and the aquifer varying by about 50 m in thickness across a 200 m wide zone of concentrated faulting