earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

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

late cretaceous seasonal ocean variability from the arctic palaeoclimate records for the arctic ocean and its environs in the late cretaceous greenhouse era 65 99 million years ago are sparse using electron microscope techniques applied to cores of shallow buried cretaceous laminated sediments from the alpha ridge of the arctic ocean davies et al have obtained a seasonally resolved sediment record from the cretaceous that permits a reconstruction of the annual cycle of marine productivity and flux to the sediment they find that the period was characterized by seasonal diatom productivity in a stratified ocean with possible winter sea ice cover although the modern arctic ocean is regarded as a barometer of global change and an amplifier of global warming little is known about its state in the greenhouse period of the late cretaceous epoch 65 99 million years ago here a seasonally resolved cretaceous sedimentary record from the alpha ridge of the arctic ocean is presented evidence suggests seasonal diatom productivity in a stratified ocean with possible winter sea ice cover the modern arctic ocean is regarded as a barometer of global change and amplifier of global warming1 and therefore records of past arctic change are critical for palaeoclimate reconstruction little is known of the state of the arctic ocean in the greenhouse period of the late cretaceous epoch 65 99 million years ago yet records from such times may yield important clues to arctic ocean behaviour in near future warmer climates here we present a seasonally resolved cretaceous sedimentary record from the alpha ridge of the arctic ocean this palaeo sediment trap provides new insight into the workings of the cretaceous marine biological carbon pump seasonal primary production was dominated by diatom algae but was not related to upwelling as was previously hypothesized2 rather production occurred within a stratified water column involving specially adapted species in blooms resembling those of the modern north pacific subtropical gyre3 or those indicated for the mediterranean sapropels4 with increased co2 levels and warming currently driving increased stratification in the global ocean5 this style of production that is adapted to stratification may become more widespread our evidence for seasonal diatom production and flux testify to an ice free summer but thin accumulations of terrigenous sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter supporting a wide body of evidence for low temperatures in the late cretaceous arctic ocean6 7 8 rather than recent suggestions of a 15 c mean annual temperature at this time9

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

systematic stress variations in the southern san joaquin valley and along the white wolf fault implications for the rupture mechanics of the 1952 ms 7 8 kern county earthquake and contemporary seismicity the southern san joaquin valley ssjv stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes as well as local and possibly temporal variations that appear to be the result of the 1952 ms 7 8 kern county earthquake the regional maximum horizontal principal stress shmax orientation in the san joaquin valley systematically rotates from ne sw compression along the western margins of the valley to n s compression in the ssjv this n s shmax stress direction in the ssjv is consistent with active development of a e w trending structural fabric of fold axes and shallow thrust and the reverse dip slip motion observed during the kern county earthquake on the south southeast dipping white wolf fault contemporary seismicity 30 40 years after the mainshock clusters in two separate areas along the white wolf fault the southwest region where the 1952 earthquake nucleated and the northeast region near where the earthquake rupture terminated earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike slip reverse oblique and to a lesser extent normal slip inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike slip reverse stress regime with s1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake inversion of focal plane mechanisms in the northeast region indicates a strike slip stress regime with s2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength these results indicate near complete stress drops for fault planes associated with the 1952 earthquake and some of the contemporary earthquakes implying fault surfaces which are frictionally weak i e slip planes subparallel to principal stress planes based on the observed stress state in the southern san joaquin valley much of the seismicity may be the result of elevated fluid pressures within these active fault zones

comparison of geophysical investigations for detection of massive ground ice pingo ice 1 six different geophysical investigations 1 ground penetrating radar 2 dc resistivity sounding 3 seismic refraction 4 very low frequency vhf electromagnetic 5 helicopter borne electromagnetic hem and 6 transient electromagnetic tem techniques were employed to obtain information on the ice body properties of pingos near fairbanks alaska the surface nuclear magnetic resonance nmr data were also compared from similar sites near one of the study pingos the geophysical investigations were undertaken along with core sampling and permafrost drilling to enable measurement of the ground temperature regime drilling ground truthing results support field geophysical investigations and have led to the development of a technique for distinguishing massive ice and overburden material of the permafrost the two dimensional dc resistivity sounding tomography and ground penetrating radar profiling are useful for ice detection under heterogeneous conditions however the dc resistivity sounding investigation required high quality ground contact and less area coverage the active layer thickness and the homogeneous horizontal structure of the overburden material are important parameters influencing detection of massive ice in permafrost for most methods such as seismic tem or surface nmr

electromagnetic propagation of gpr signals in martian subsurface scenarios including material losses and scattering a study on the electromagnetic propagation in various models of the martian subsurface is performed with a relevance to ground penetrating radar gpr operating onboard rover missions measurements of the electromagnetic properties of mars soil simulants are obtained on this basis the attenuation features of the gpr signals are estimated including both electric and magnetic losses the effect on propagation of inhomogeneities inside the soil is also taken into account by means of a specific model with randomly distributed scatterers the gpr performance in terms of resolution and maximum penetration depth is evaluated in the considered scenarios for different operating frequencies thus providing a basic information for the design of systems for future subsurface sounding investigations on mars

rheology of a long lava flow at pavonis mons mars 1 dimensions of lava flows can be used to unravel the relative roles of viscosity changes and concurrent formation of levees stationary margins and stagnant zones this approach is applied to data derived from the mars orbiter laser altimeter mola experiment for a long lava flow on the plains north of pavonis mons mars we obtain a formula for the relative change in viscosity on the basis of a steady state newtonian flow rate our approach features a new length scale that describes the transfer of lava from the active advancing component to passive components this length scale can be determined from planetary image and topographic data by estimating the volume fraction of lava contained in flow margins relative to the total flow volume we find only modest changes in viscosity over the distal 175 km of the pavonis flow allowing the flow to also lose volume through degassing resulting in a density increase does little to affect the overall viscosity change thickening and widening of the flow with distance are as expected for a single coherent isothermal viscous flow this dynamic regime features a balance between the formation of an outer skin and shedding of lava into stationary zones requirements for attaining such a regime include a thick flow shallow slopes over extended distances and preexisting surface roughness that is small compared to flow thickness this style of emplacement may explain why many of the long thick sheet like flows on the plains of mars often exhibit an unexpected lack of thickening with distance

radar sounding of temperate permafrost in alaska analogy to the martian midlatitude to high latitude ice rich terrains 1 radar detection of subsurface ice on mars has been widely debated in part because the dielectric signature of ice as deduced from the dielectric constant can be confused with dry silicate rich materials to identify the ice dielectric signature it is crucial to estimate the imaginary part of the dielectric permittivity inferred from the dielectric attenuation after removing the scattering loss unfortunately the latter remains poorly quantified at both mars advanced radar for subsurface and ionospheric sounding marsis and shallow subsurface radar sharad frequencies to address this ambiguity we conducted multiple frequency ground penetrating radar and resistivity investigations in well characterized temperate permafrost in fairbanks alaska the area shows several geomorphologic similarities to midlatitude and high latitude terrains on mars this approach allowed us to quantify the dielectric and scattering losses in temperate permafrost over the 10 to 1000 mhz frequency band at 20 mhz our results suggest an average dielectric loss rate of 0 25 0 03 db m whereas the corresponding average scattering loss rate is 0 94 0 37 db m the scattering loss was found to represent 69 of the total signal attenuation considering this result and the study by heggy et al 2006a in volcanic environments we revised the interpretation of the attenuation coefficient calculated from sharad data over the deuteronilus mensae region and amazonis planitia we then used the reevaluated dielectric loss to estimate the imaginary part of the dielectric permittivity our results suggest that even if deuteronilus mensae deposits and the vastitas borealis formation may have similar dielectric constants their imaginary parts are different this implies that the two regions have different bulk compositions with the former being ice rich sediments and the latter being nonconsolidated volcanic deposits

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

vertical resolution of thick beds thin beds and thin bed cyclothems resolution has two common definitions the first of these relates to determination of the position of a reflector in time or space resolution by this definition is inversely related to pulse width or sharpness of the wavelet and potentially directly related to bandwidth of the frequency spectrum the second definition of resolution relates to determination of the spacing of close features such as thin beds this type of resolution is determined by the rayleigh criterion which inversely relates resolution to wavelength resolution of thin beds improves with shorter wavelength or higher frequency in comparison to the rayleigh criterion the ricker and widess criteria improve the resolution potential of thin beds but they rely on models of the reflectors and require that the beds be isolated it is a long established and widely accepted axiom that the best wavelets contain bandwidths of at least a couple of octaves this results in wavelets with reduced side lobes and ringiness a more easily interpret

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

state of stress before and after the 1994 northridge earthquake the state of tectonic stress in the epicentral area of the 17 january 1994 northridge earthquake mw 6 7 is investigated by applying a stress inversion method to p wave polarity data from earthquakes in northridge from july 1981 to january 1994 and from the northridge aftershocks during january 1994 to december 1995 a 3 d crustal model is used to trace the rays taking off from the hypocenter which reduced the effects of large structural heterogeneities on the determination of the stress tensor we found significant temporal changes of stress orientations induced by the northridge earthquake the principal pressure p axis is oriented n32 e from 1981 to june 1992 and n30 e from 28 june 1992 to 16 january 1994 suggesting that the stress field in northridge was not affected by the 1992 landers earthquake during two weeks following the northridge mainshock the p axis is oriented n13 e which is a significant 17 change from that before the earthquake n30 e between february 1994 and august 1995 the p axis orientation changes from n18 e to n26 e and finally ends up at n34 by the end of 1995 which is close to that before the northridge earthquake these results suggest that the stresses rotated coseismically then rotated more slowly back to their original orientation the aftershocks caused by the mainshock changed the stress distribution in the crust which showed up as a regional stress change the stress recovery appears to have completed within two years after the mainshock which is very short compared to the time scale of the earthquake cycle

the dielectric permittivity of terrestrial ground ice formations considerations for planetary exploration using ground penetrating radar 1 exploration of the polar ice caps and apparent glacial and periglacial landforms on mars will aid our understanding of its ancient climate conditions and the history of water on the planet given that ground penetrating radar gpr is likely to be used to understand these features we investigated the real component of the complex dielectric permittivity of stratified segregation ice non stratified segregation ice and polygonal ice wedge deposits in the canadian arctic we acquired moveout profiles with a 450 mhz gpr on ground ice formations that had active layer sediments excavated prior to surveying using ice core data collected from these sites we found that the volumetric fraction of ice plays the greatest role in defining the dielectric permittivity of the deposit and that it can be described using a modified complex refractive index method crim dielectric mixing model using the modified crim model we estimate the dielectric permittivity of several ground ice deposits on earth and present further estimates for similar features on mars using permittivity values for martian sediments derived from both theory and laboratory methods

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

effects of magnetite on high frequency ground penetrating radar large concentrations of magnetite in sedimentary deposits and soils with igneous parent material have been reported to affect geophysical sensor performance we have undertaken the first systematic experimental effort to understand the effects of magnetite for ground penetrating radar gpr characterization of the shallow subsurface laboratory experiments were conducted to study how homogeneous magnetite sand mixtures and magnetite concentrated in layers affect the propagation behavior velocity attenuation of high frequency gpr waves and the reflection characteristics of a buried target important observations were that magnetite had a strong effect on signal velocity and reflection at magnitudes comparable to what has been observed in small scale laboratory experiments that measured electromagnetic properties of magnetite silica mixtures magnetite also altered signal attenuation and affected the reflection characteristics of buried targets our results indicated important implications for several fields including land mine detection martian exploration engineering and moisture mapping using satellite remote sensing and radiometers

discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions it is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area considering earthquake generating stress to be uniform in a studied area we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of p waves first the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution with intervals of 10 this calculation is made for all the events used then four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum a numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 25 readings it is found that a stable solution of the stress tensor is determined by the inversion and about 60 of fault planes are distinguished from auxiliary planes there are no events whose auxiliary planes are incorrectly determined as fault planes this method is used to estimate the earthquake generating stress in the aftershock area of the 1984 western nagano prefecture earthquake central japan the results obtained show that the maximum principal stress is horizontal in the direction of n80 w s80 e there are strike slip events having fault planes almost perpendicular to the fault plane of the main shock

direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern california in inverting populations of focal mechanisms to estimate stress misfits may arise owing to 1 errors in determining the fault plane solutions and 2 real variations in the state of stress among a number of techniques that invert for stress most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on p wave first motions in a highly nonlinear manner which is difficult to propagate into stress inversions to address these issues we present a new method for constraining stress tensors in which first motion observations are directly inverted for stress without assuming that focal mechanisms are known the technique produces estimates of four stress parameters a suite of focal mechanisms consistent with both the stress tensor and the first motions and estimates of the uncertainty in stress it also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor when tested against artificial data the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks as expected utilizing first motions from the southern california catalog the method is applied to several clusters and aftershock sets from 1981 to 1992 typical uncertainties in stress orientation exceed 20 30 at the 95 confidence level resulting from 5 mispick rates typical of even the cleanest data these uncertainties exceed some previous estimates for similar populations presumably because they account for focal mechanism un certainties and perhaps indicating that some previously reported stress variations are not statistically significant most populations are found to be internally consistent with a homogeneous stress field the most significant exceptions being aftershock sequences of some major earthquakes in at least one case the 1992 landers sequence aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses overall the method provides a useful and robust approach for evaluating stress constraints provided by p wave first motions

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

measurements of dielectric loss factors due to a martian dust analog 1 radar signals can penetrate loose sediments such as dust sand and alluvium to reveal buried geologic structures obscured in visible and infrared images in anticipation of data from instruments such as the marsis and sharad radar sounders and potential future imaging sar and rover mounted gpr instruments measurements have been made to characterize electrical loss factors of a martian dust analog and an iron rich soil this paper presents results from dielectric measurements of carbondale red clay crc and the palagonitic mars jsc 1 soil simulant from 0 2 to 1 3 ghz and from transmission measurements of radar penetration of crc up to 12 ghz attenuations predicted from dielectric measurements are compared to values measured in the experiment and are discussed relative to the frequencies at which imaging radars operate over the frequencies considered loss tangent tan decreases with increasing frequency but attenuation increases due to the influence of wavelength attenuation in crc ranges from 8 to 22 db m over p and l bands and jumps to 36 and 67 db m at c and x bands respectively mars jsc 1 has lower attenuation of 5 db m at p band and 12 db m at l band compared to attenuation measurements of sand both crc and mars jsc 1 cause significantly greater attenuation likely due to their compositions containing iron bearing minerals because martian fine sediments also contain ferric mineral components constraining higher losses of analogs for martian dust and fine sediments is important for predicting the performance of radar instruments operating in orbit or on the surface of mars

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

design and field experiments of a ground penetrating radar for mars exploration 1 we developed a wideband low power lightweight prototype ground penetrating radar gpr for subsurface exploration for mars the transmitter and receiver subsystems were constructed using a commercially available radio frequency rf and digital integrated circuits connectorized components and evaluation boards the transmit receive system antennas laptop controller and batteries are all accommodated on a 2 5 m sled field experiments were conducted in kansas and alaska the experiments in kansas tested the operation of the system the primary objective of the alaska experiments was to investigate the ability of a gpr to detect and distinguish between subsurface deposits of ice and ice cemented soils the investigation depths of these experiments ranged from 1 m to 30 m and the subsurface geology included near surface thaw discontinuous permafrost water saturated soils and lenses of pure ice dielectric contrasts within the ground were detected with near meter resolution however identifying the geologic context of an interface was difficult due to ambiguities associated with reflection data these investigations demonstrate some of the difficulties associated with inverting reflection data to obtain dielectric properties of the subsurface fortunately previous geophysical investigations such as drilled cores and seismic surveys helped to constrain the geology and numerical simulations provided an additional resource for the interpretations on mars additional geological context will be limited and numerical simulations will become extremely important for data interpretation due to these difficulties we demonstrate in one experiment how dielectric information can be obtained directly from bistatic measurements with a fixed transmitter and mobile receiver finally a comparison with a commercially available geophysical survey systems inc gssi radar system is presented and we discuss how the system can be modified and improved for future exploration on mars

seismicity and active compressional tectonics in santa monica bay southern california during 1973 1986 the seismicity in santa monica bay consisted of an average of four events per year of m l 2 5 in addition to the 1979 malibu m l 5 0 mainshock aftershock sequence the 1979 sequence ruptured a north dipping thrust fault at 10 15 km depth beneath the bay a diffuse distribution of seismicity extends across the bay from the terminus of the palos verdes fault to point dume two thirds of the single event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west northwest striking planes a third of the focal mechanisms exhibit strike slip faulting on north to north northwest striking planes a stress inversion of the focal mechanism data shows that the maximum principal stress is oriented n13 e in a horizontal plane and the minimum principal stress is oriented n102 e and plunging 47 hence the tectonics in santa monica bay are dominated by compression with thrust reverse and strike slip faulting the coexistence of thrust and strike slip faults suggests that the bay is not a part of the peninsular ranges terrane but forms a transition zone that accommodates the change from strike slip in the peninsular ranges to the south to reverse faulting in the transverse ranges to the north mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to los angeles and the southern california coastal zone through seismic shaking and possible generation of small tsunamis

ground penetrating radar geologic field studies of the ejecta of barringer meteorite crater arizona as a planetary analog 1 ground penetrating radar gpr has been a useful geophysical tool in investigating a variety of shallow subsurface geological environments on earth here we investigate the capabilities of gpr to provide useful geologic information in one of the most common geologic settings of planetary surfaces impact crater ejecta three types of ejecta are surveyed with gpr at two wavelengths 400 mhz 200 mhz at meteor crater arizona with the goal of capturing the gpr signature of the subsurface rock population in order to ground truth the gpr characterization subsurface rocks are visually counted and measured in preexisting subsurface exposures immediately adjacent to and below the gpr transect the rock size frequency distribution from 10 to 50 cm based on visual counts is well described by both power law and exponential functions the former slightly better reflecting the control of fragmentation processes during the impact ejection event gpr counts are found to overestimate the number of subsurface rocks in the upper meter by a factor of 2 3x and underestimate in the second meter of depth 0 6 1 0x results attributable to the highly scattering nature of blocky ejecta overturned ejecta that is fractured yet in which fragments are minimally displaced from their complement fragments produces fewer gpr returns than well mixed ejecta the use of two wavelengths and division of results into multiple depth zones provides multiple aspects by which to characterize the ejecta block population remote gpr measurement of subsurface ejecta in future planetary situations with no subsurface exposure can be used to characterize those rock populations relative to that of meteor crater