strike slip earthquakes on quasi vertical transcurrent faults inferences for general scaling relations the recent occurrence of several large strike slip earthquakes provides the opportunity to review and complement available data on the scaling of seismic moment mo with length of rupture l for large earthquakes depending on their tectonic setting and mechanism for strike slip earthquakes on quasi vertical transcurrent faults the mo versus l relation has a significant change of slope around mo 0 6 0 8 1020 n m and for larger earthquakes mo scales linearly with l this is compatible with models where slip is controlled by the width of the fault also it appears to be easier to categorize large earthquakes by their mechanism strike slip on vertical transcurrent fault versus pure thrust normal than their tectonic setting interplate intraplate

relationship of the presence of a non calcareous clay loam horizon to dem attributes in a gently sloping area abstract the initial objective of this study was to identify relationships between terrain attributes and soil cover over a loessic flat area covering a limestone plateau the second objective was to determine those energy factors flow path solar radiation wind intensity which could improve the understanding of morphology and soil genesis we describe 341 field observations taken over 1600 ha of an experimental area used for monitoring the water and nitrate supply in the petite beauce region 100 km to the southwest of paris for each observation several soil variables were encoded one was the presence of a non calcareous clay loam nccl horizon relief is very smooth in this region mean slope around 0 5 the main terrain attributes were derived from a digital elevation model dem at 20 20 m and assigned to the pedological observations a multiple logistic regression was used to analyse the relationship between the nccl horizon and terrain attributes special statistics were used for aspect because of the circular nature of this variable the results show a strong relationship between the presence of nccl horizon and slope gradient and slope aspect while hydrological parameters are not correlated with this horizon the mean angle of the aspect frequency of the nccl horizon was calculated and compared to the mean angles of wind direction and solar radiation balance there is a small difference between wind direction and aspect frequency of the nccl horizon this result confirms the role of wind in the spatial pattern of soils further data are required to better understand the combination of several factors role of vegetation and the age of the reshaping

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

source parameters of small earthquakes recorded at 2 5 km depth cajon pass southern california implications for earthquake scaling a 2 5 km deep triaxial seismometer at cajon pass in southern california has recorded several hundred earthquakes ml4 0 occurring within the san andreas fault system at 2 5 km seismic background noise is below amplifier sensitivity and the 2 250 hz spectral range of recorded seismic motion is wider and higher than that of most natural event catalogs compared with downhole recorded motion seismic amplitudes at the surface are amplified below 10 hz and severely attenuated above 30 hz we estimate that qs is at least 1000 for wave motion at 2 5 km and below and qp is over 2000 the range of source dimensions in the downhole recorded catalog is 10 m to 70 m ml 2 0 mo 108 nm to ml 2 7 mo 1013 nm the plot of log source radius vs log moment has a straight line trend compatible with earthquake scaling at constant stress drop inferred stress drops are scattered between 1 and 500 bars there is no evidence in the catalog for the proposed minimum source dimension at 100 m when the cajon pass borehole catalog containing some of the smallest recorded natural earthquakes is combined with 800 larger events from previous studies the moment radius trend suggests that natural earthquakes are self similar over a magnitude range m 2 to 8 we suggest that inferences of minimum source dimension are more likely due to bias in bandlimited individual catalogs than to properties of the seismic crust

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

observation of hurricane generated ocean swell refraction at the gulf stream north wall with the radarsat 1 synthetic aperture radar we analyze the refraction of long oceanic waves at the gulf stream s north wall off the florida coast as observed in imagery obtained from the radarsat 1 synthetic aperture radar sar during the passage of hurricane bonnie on august 25 1998 the wave spectra are derived from radarsat 1 sar images from both inside and outside the gulf stream from the image spectra we can determine both the long wave s dominant wavelength and its propagation direction with 180 spl deg ambiguity we find that the wavelength of hurricane generated ocean waves can exceed 200 m the calculated dominant wavelength from the sar image spectra agree very well with in situ measurements made by national oceanic and atmospheric administration national data buoy center buoys since the waves mainly propagate toward the continental shelf from the open ocean we can eliminate the wave propagation ambiguity we also discuss the velocity bunching mechanism we find that in this very long wave case the radarsat 1 sar wave spectra should not be appreciably affected by the azimuth falloff and we find that the ocean swell measurements can be considered reliable we observe that the oceanic long waves change their propagation directions as they leave the gulf stream current a wave current interaction model is used to simulate the wave refraction at the gulf stream boundary in addition the wave shoaling effect is discussed we find that wave refraction is the dominant mechanism at the gulf stream boundary for these very long ocean swells while wave reflection is not a dominant factor we extract 256 by 256 pixel full resolution subimages from the sar image on both sides of the gulf stream boundary and then derive the wave spectra the sar observed swell refraction angles at the gulf stream north wall agree reasonably well with those calculated by the wave current interaction model

euclidean and fractal models for the description of rock surface roughness accurate description of the topography of rock surfaces is essential because surface roughness affects frictional strength the flow of fluids in joints and fractures the seismic behavior of faults and the formation of gouge and breccia in fault zones real rock surfaces can be described using self similar and self affine fractal models of surface roughness if a surface is self similar a small portion of the surface when magnified isotropically will appear statistically identical to the entire surface if a surface is self affine a magnified portion of the surface will only appear statistically identical to the entire surface if different magnifications are used for the directions parallel and perpendicular to the surface at least two parameters are required to describe a fractal model one parameter typically describes how roughness changes with scale while the other specifies the variance or surface slope at a reference scale the divider method and the spectral method are in common use to determine the best fit fractal model from surface profile data power spectra from self similar surfaces have slopes of 3 on log log plots of power spectral density versus spatial frequency while spectra from self affine surfaces have slopes other than 3 power spectra can be interpreted with greater facility if dimensionless amplitude to wavelength ratios are contoured on plots of power spectral density versus frequency the topography of many natural rock surfaces including both fractures and faults is approximately self similar within the 6 5 order of magnitude wide wavelength band of 10 m to 40 m within smaller wavelength bands natural rock surfaces may exhibit self affine behavior

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

the consequences of sampling variability on the estimation of wave number and propagation direction from spaceborne sar image spectra even if it were true that sar synthetic aperture radar ocean surface imagery provides a perfect noiseless representation of ocean surface wave height each spectrum computed from such imagery would be but a single realization of the ensemble mean ocean spectrum there would be sampling variability associated with the parameters of dominant wave number and propagation direction extracted from such a spectrum the present study addresses two questions i what statistical distribution is applicable to the spectra of sar ocean images and ii what are the consequences of such statistics on the precision with which wave number and propagation direction can be extracted an examination is made of spectra computed from ocean imagery acquired during the sir b shuttle imaging radar b mission

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

hierarchical model of defect development and seismicity abstract the multiscale model of defect development kinetics is studied some critical effects have been found simulation produces phenomena which allow one to predict the time of appearance of large defects

observations and modeling of typhoon waves in the south china sea abstractbuoy based observations of surface waves during three typhoons in the south china sea were used to obtain the wave characteristics with the local wind speeds kept below 35 m s 1 the surface waves over an area with a radius 5 times that of the area in which the maximum sustained wind was found were mainly dominated by wind wave components and the wave energy distribution was consistent with fetch limited waves swells dominated the surface waves at the front of and outside the central typhoon region next the dynamics of the typhoon waves were studied numerically using a state of the art third generation wave model wind forcing errors made a negligible contribution to the surface wave results obtained using hindcasting near realistic wind fields were constructed by correcting the idealized wind vortex using in situ observational data if the different sets of source terms were further considered for the forcing stage of the typhoon which was defined as the half inertial period before and aft

asperity models for earthquakes a quasi static numerical method was used to simulate the failure of a strong stuck asperity on an otherwise creeping fault plane the numerical model produced the same slip distribution as analytical asperity models which for constant loading rate produced repeating events having a period t that scales with moment m 0 as t propto m 0 1 6 the scaling relation observed by nadeau and johnson 1998 at parkfield when the asperity is a composite of smaller hard unit asperities we still find t propto m 0 1 6 but the constant of proportionality depends on the density of unit asperities within the cluster since only clusters with a fixed asperity density follow the observed t propto m 0 1 6 scaling this result rules out the fractal spatial distribution cantor dust of unit asperities at parkfield suggested by sammis et al 1999 for solid asperities the average stress drop on the asperity is on the order of 100 mpa but the average stress drop over the entire rupture area is significantly lower equivalent to that estimated from spectral analysis however the stress drop for asperity models is not independent of seismic moment but decreases with earthquake size the energy release rate for asperity events is estimated to be mathcal g gtrsim 10 7 j m 2 near the upper limit of estimates by li 1987 using parameters from large events on the san andreas fault

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

directional wave spectra measured with the surface contour radar abstract the surface contour radar is a 36 ghz computer controlled airborne radar which generates a false color coded elevation map of the sea surface below the aircraft in real time and can routinely produce ocean directional wave spectra with post flight data processing which has much higher angular resolution than pitch and roll buoys when compared with waveriders and the xerb and edeco pitch and roll buoys there is good agreement among the nondirectional spectra there is also good agreement among the angles associated with a1 b1 and a2 b2 fourier coefficients of the spreading function for xerb endeco and the surface contour radar there are indications that the pitch and roll buoys in this study may have calibration problems with the magnitudes of the fourier coefficients of the spreading function and that the radar system determines the fourier coefficients with significantly less noise and bias the high spatial resolution and rapid mapping capability over extensive areas make the surface

development of texture contrast soils by a combination of bioturbation and translocation abstract soils and weathering profiles in a wide variety of parent materials and environmental settings exhibit coarse over fine vertical textural contrasts where these cannot be attributed to inherited texture contrasts or erosion deposition the most common explanations are based on translocation eluviation illuviation which removes clays from surface layers and deposits them in the subsoil or bioturbation where preferentially fine material is delivered to the surface by organisms from whence erosional winnowing creates a coarse surface layer in some soils of the lower coastal plain of north carolina u s a neither explanation is sufficient to explain the observed texture contrasts a heuristic model based on a combination of translocation of fine material from surface to subsoil and bioturbation driven delivery and recycling of material to the surface can explain the observed vertical textural contrasts the key elements in the model are coastal plain sediments which include some fine material eluviation illuviation by percolating water delivery of additional fine and mixed grain size material to surface by bioturbation making it available for translocation concentration of fine material originally scattered throughout the parent material in a b horizon and maintenance of vertical moisture fluxes by bioturbation the model is supported by morphological evidence of the key mechanisms argillic horizons that are finer than both the surface layers and underlying parent material evidence that argillic horizon formation is not limited by the rate of clay synthesis and the absence of texture contrasts in nearby soils formed from dune sands which lack fines

a simulation for spaceborne sar imagery of a distributed moving scene a computer simulation that is designed to represent aspects of spaceborn synthetic aperture radar sar imagery of the ocean surface is presented the simulation is unique in that a scatterer density per resolution cell is explicitly included thus allowing the incorporation of various scattering natures from purely specular to purely diffuse the simulation may be applied to ocean surface wave imaging since velocity bunching and scene coherence times are also included certain assumptions inherent in the velocity bunching formulation limit the applicability of the simulation in its present form to spaceborn sar systems only two experiments based on this simulation are considered 1 the effect of varying the target density and 2 the effect of the mean scene coherence time in the imaging of a ocean swell system

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

1 fβ fluctuations and self similarity in earthquake dynamics observational evidences in southern italy abstract recently it has been determined that many statistical features of earthquakes are scale invariant with universal values for exponents the analysis of scaling laws has led to the development of a wide variety of physical models of seismogenesis involving nonlinear dynamics and it can be used to better characterize the seismicity pattern in this work the presence of scale invariance properties in the seismicity of a selected area of the southern appennine chain is revealed 1 f fluctuations are detected from the analysis of earthquake sequences using the counting statistics and at the spatial scale a self similar structure in the epicentre aggregate is determined by the estimation of its fractal dimension finally correlations between temporal exponent fractal dimension and b value variations are described

the spatial distribution of ocean waves in tropical cyclones the spatial structure of both the wind and wave fields within tropical cyclones is investigated using two large databases the first of these was compiled from global overpasses of tropical cyclones by satellite altimeters the second dataset consists of an extensive collection of north american buoy measurements during the passage of tropical cyclones hurricanes the combined datasets confirm the vortex structure of the tropical cyclone wind field with the strongest winds to the right northern hemisphere of the storm the wave field largely mirrors the wind field but with greater right left asymmetry that results from the extended fetch to the right of the translating tropical cyclone the extensive in situ buoy database confirms previous studies indicating that the one dimensional spectra are generally unimodal the directional spectra are however directionally skewed consisting of remotely generated waves radiating out from the center of the storm and locally generated wind sea the one dimensional wave spectra have many similarities to fetch limited cases although for a given peak frequency the spectra contain less energy than for a fetch limited case this result is consistent with the fact that much of the wave field is dominated by remotely generated waves

imaging of ocean waves by synthetic aperture radar a comparison of results from seasat and sir b experiments in the north atlantic new analyses of wave spectra extracted from seasat sar images of the jasin site in the northeast atlantic are presented as tests of wave imaging theory spectra extracted from orbit 757 during a high sea state are dominated by the effects of smearing but the rms image smear is less than half that expected from in situ measurements of surface motions in a low sea state on orbit 547 fair agreement can be achieved between sar and buoy spectra but this requires that the smearing length be greater than was observed on orbit 757 these findings are compared with results obtained from the shuttle imaging radar sir b mission in order to assess the quantitative understanding of wave imaging by sar

seismotectonics and the earthquake frequency magnitude distribution in the aegean area summary a study of aegean seismotectonics and the resulting frequency magnitude distribution on a broad scale is undertaken using the tectonic model of le pichon angelier this implies a tectonic moment release rate due to the spreading of the aegean of 17 8 times 1018 n m 1 yr 1 over the past 13 ma if stretching is due mainly to a series of normal faults dipping at about 45 in a seismogenic crust 10 20 km deep the moment magnitude relation from an instrumental magnitude catalogue is log m0 a bm with a 10 970 0 294 b 1 206 in si units this is established from 50 events of magnitude ms 5 3 whose scalar seismic moments m0 have been evaluated in a homogeneous manner for the region by spectral analysis of surface waves at 30 60 s period by contrast published body wave analyses from waveform modelling at 20 s period and moment tensor analyses seem to produce seismic moments which are lower by a factor of two or so on average this moment magnitude relation implies a seismogenic moment release rate due to stretching in the aegean of 14 times 1018 n m 1 yr 1 during the time period 1918 81 and 17 times 1018 n m 1 yr 1 for the period 1964 81 within a factor of two or so uncertainty this implies that the energy release in the brittle crust due to aegean spreading is substantially seismic within the stated accuracy and is also relatively stationary even when compared with the tectonic moment release rates a similar comparison using the same tectonic model shows that the subducting slab is sinking beneath the hellenic arc almost completely aseismically and that processes such as near vertical slab pull and confined thermal expansion could explain most of the observed seismic energy release by internal deformation of the slab the frequency magnitude statistics for the area of active extension has a characteristic earthquake of magnitude 7ms corresponding approximately to fault depths of 10 14 km for an aspect ratio of 2 and a strain drop of 10 4 this is close to the observed seismogenic depth 10 20 km for well determined earthquakes in the aegean area

overview of results of spaceborne imaging radar c x band synthetic aperture radar sir c x sar the spaceborne imaging radar c x band synthetic aperture radar sir c x sar was launched on the space shuttle endeavour for two ten day missions in the spring and fall of 1994 radar data from these missions are being used to better understand the dynamic global environment during each mission radar images of over 300 sites around the earth were obtained returning over a terabit of data sir c x sar science investigations were focused on quantifying radar s ability to estimate surface properties of importance to understanding global change and focused studies in geology ecology hydrology and oceanography as well as radar calibration and electromagnetic theory studies in addition the second flight featured an interferometry experiment where digital elevation maps were obtained by interfering data from the first and second shuttle flight and from successive days on the second flight sir c x sar data have been used to validate algorithms which produce maps of vegetation type and biomass snow soil and vegetation moisture and the distribution of wetlands developed with earlier aircraft data

a case study of wave current interaction near an oceanic front high resolution directional wave spectrum data were obtained from two nasa airborne radars during the frontal air sea interaction experiment fasinex in february 1986 the observations show a significant change in the wave number spectrum across the front on the basis of surveys from a towed sensor and on satellite imagery the front location and current field are estimated a numerical model is developed for the wave current interaction and is used to model the wave refraction across the frontal current a parametric study is performed to demonstrate the effects of current meandering the main consequence of meandering is the formation of caustics and shadow zone regions in which the wave energy is significantly enhanced or reduced spectral simulation along the aircraft track reveals a reduction of more than 60 in wave energy in the shadow zone this is consistent with the observations

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

the lithosphere of the earth as a nonlinear system with implications for earthquake prediction the lithosphere of the earth can be viewed as a hierarchy of volumes from tectonic plates to grains of rock their relative movement against the forces of friction and cohesion is realized to a large extent through earthquakes the movement is controlled by a wide variety of independent processes concentrated in the thin boundary zones between the volumes a boundary zone has a similar hierarchical structure consisting of volumes separated by boundary zones etc altogether these processes transform the lithosphere into a large nonlinear system featuring instability and deterministic chaos from this background some integral grossly averaged empirical regularities emerge indicating a wide range of similarity collective behavior and the possibility of intermediate term earthquake prediction

magnitude frequency relation for small earthquakes a clue to the origin of ƒmax of large earthquakes in order to test the hyothesis that the max of large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3 we constructed the frequency magnitude relation for earthquakes with magnitude range from 1 3 to 4 recorded at a borehole seismograph station operated by the university of southern california at baldwin hills in the middle of the newport inglewood fault a procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time the cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit for ts p for each magnitude we found a clear depature of the observed log frequency magnitude relation for m 3 the observed frequency of earthquakes with m 0 5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3 we conclude that the observed frequency magnitude relation departs from self similarity for earthquakes with magnitude smaller than about 3 this departure coincides with the widely observed departure of the moment corner frequency relation from self similarity these observations are consistent with the interpretation of max of large earthquakes in terms of the slip weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers

small earthquakes tectonic forces earthquake scaling and frequency of occurrence relations require that small earthquakes be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension including plate boundaries

interaction of the san andreas fault creeping segment with adjacent great rupture zones and earthquake recurrence at parkfield three dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the san andreas fault in central california and the adjacent currently locked zones of the 1857 and 1906 great earthquakes vertically the model consists of an elastic upper crust over a maxwell viscoelastic region representing the entire lower crust or a narrower horizontal detachment layer and a stiffer and more viscous upper mantle the crust has a single vertical fault extending to the top of the mantle at 25 km depth in zones along strike corresponding to the 1857 and 1906 events the top 12 5 km of the fault is locked against slip except in great earthquakes below the locked zones and everywhere along the creeping region between them the fault is freely slipping the model parameters are compatible with seismological and geological observations and with a ratio of maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr km as established by li and rice 1987 and fares and rice 1988 based on fits to geodetic data along the san andreas fault an imposed constant far field shear motion and periodic 1857 and 1906 type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle shortly after an adjacent great earthquake slip rates in the creeping zone are higher than the far field velocity while later in the cycle they are lower hence time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion if parkfield earthquakes are a response to a time dependent loading of the type simulated here their recurrence interval would tend to lengthen with time since the 1857 event thus the hypothesis of characteristic periodic earthquakes at parkfield may not provide the best estimate of the occurrence time of the next event using for example the statistics of past events and assuming that parkfield earthquakes are a response to a slip deficit near middle mountain and that the elastic crustal layer is 17 5 km thick we find that the next event is predicted for about 1992 9 years if the lower crust is a 7 5 km thick layer having a material relaxation time of 15 years and 1995 11 years if the 7 5 km thick lower crust is characterized by a relaxation time of 7 5 years these values may be compared to the 1988 7 years estimate based on periodicity in time the modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small

ocean wave directional spectra and wave current interaction in the agulhas from the shuttle imaging radar b synthetic aperture radar sir b synthetic aperture radar ocean wave spectra for a 200 km pass crossing the agulhas current off the coast of africa are analyzed a significant enhancement of one spectral peak along the northern edge of the current is attributed both to amplification of the waves by refraction dominated wave current interaction and to transient specular backscatter contributions spatially correlated to the waves

estimation of sea surface wave spectra using acoustic tomography ocean acoustic tomography signals that interact with the ocean surface suffer travel time perturbations which have information on the frequency direction spectrum of that surface in this paper a technique for tomographically estimating spatially varying frequency direction spectra is introduced to demonstrate this technique synthetic signal spectra data were generated using an empirical model for a fetch limited sea the synthetic data were then inverted using linear inverse techniques to produce an estimate of the original sea surface spectra inverse solutions for both noise free and noisy experiments are presented and discussed

deep borehole log evidence for fractal distribution of fractures in crystalline rock summary sonic velocity and electrical resistivity logs run to a depth of 3 5 km in crystalline rock near the san andreas fault at cajon pass in southern california correlate over scale lengths both small sub metre and large tens to hundreds of metres no such correlations are seen with the more lithologically sensitive natural gamma intensity log the correlation between the sonic velocity and electrical resistivity logs suggests that a non lithologic property of the crystalline rock controls fluctuations in situ fracture intensity is a logical candidate for the controlling rock property the fluctuations of the individual sonic velocity and electrical resistivity logs are examined with the hurst rescaled range parameter over borehole log intervals 1 5 m l 1500 m for log fluctuations arising from a scale invariant physical process the hurst rescaled range scales with data interval as lh 0 h 1 a purely random sequence of in situ fractures produces a scaling exponent h 0 50 fluctuations in the cajon pass sonic velocity and electrical resistivity logs yield h 0 70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals the tendency for fracture clustering over log intervals 1 5 m l 1500 m suggests that fracture formation is a fractal process independent of length scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals seismic reflectivity derived from the borehole sonic velocity log is also scale independent over the range of data intervals 1 5m l 1500 m with a hurst exponent h 0 21 if we associate fracture clustering with crustal fault formation the cajon pass borehole sonic velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension d 2 30 the equivalent b value for earthquake size distribution is b d 2 1 15 on this hypothesis observed b values 1 15 indicate a tendency for earthquakes to cluster on existing weak faults a scale independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting thus in crustal regions of active but low strain fracture anisotropy is observed to be aligned with the inferred maximum principal stress while in actively faulted crust fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process

spatial variability of soil hydraulic conductivity along a tropical rainforest catena abstract we investigate the spatial variability of saturated hydraulic conductivity ksat along a tropical rainforest catena which consists of a typic kandiudult latossolo vermelho distrofico argissolico and plinthic hapludox latossolo amarelo distrofico tipico the former soil type extends from midslope to interfluve and has a high clay content throughout the profile whereas the latter soil type extends from the midslope to valley bottom and only has a high clay content at depths 90 cm we measured ksat along a transect at depths of 20 30 50 and 90 cm with a compact constant head permeameter ksat is not significantly different 0 10 between these soil types at any depth except for 90 cm ordinary and robust linear regression models with distance from the interfluve as in the independent variable showed no significant change in ksat as a function of topography loess regression showed some dependency of ksat on topography only at the 90 cm depth semivariograms showed no apparent spatial structure in ksat at distances 25 m for all depths we conclude that the strong topography dependence of soil types along this catena and hence primary soil attributes is not reflected in a similar dependence of ksat and tentatively attribute this lack of dependence to the overriding influence of bioturbation controlled macroporosity we outline a framework for detecting possible ksat topography relationships based on the distinction between bioturbation controlled nonstructural macroporosity and mineralogy controlled structural macroporosity

comparison of shuttle imaging radar b ocean wave image spectra with linear model predictions based on aircraft measurements during october 1984 coincident shuttle imaging radar b synthetic aperture radar sar imagery and wave measurements from airborne instrumentation were acquired the two dimensional wave spectrum was measured by both a radar ocean wave spectrometer and a surface contour radar aboard the aircraft in this paper we compare two dimensional sar image intensity variance spectra with these independent measures of ocean wave spectra to verify previously proposed models of the relationship between such sar image spectra and ocean wave spectra the results illustrate both the functional relationship between sar image spectra and ocean wave spectra and the limitations imposed on the imaging of short wavelength azimuth traveling waves

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

development of pedotransfer functions for coastal plain soils using terrain attributes abstract this study investigated the influence of terrain attributes and soil water content on soil genesis and developed pedotransfer functions ptfs for the coastal plain soils a two stage sampling procedure was adopted in the study in the first stage a total of 408 samples were collected along twelve toposequences while in the second stage 64 representative soil samples were collected on typical upper middle and lower slope positions particle size fractions bulk density hydraulic conductivity organic carbon field capacity water content fwc permanent wilting point pwp and available water content awc were determined elevation slope aspect curvatures flow direction and hill shade compound topographic index cti and stream power index spi were derived from a digital elevation model the data were analyzed using correlation principal component and stepwise multiple regression there was significant correlation p

estimating aircraft sar response characteristics and ocean wave spectra in the labrador sea extreme waves experiment the labrador extreme waves experiment lewex was an international effort to assess methods of measuring and modeling the directional aspects of wind generated ocean waves especially their evolution in the presence of rapidly turning winds the author describes data processing methods that have been developed to derive estimates of two dimensional wave height variance spectra from the ocean imagery obtained in lewex by a c band synthetic aperture radar sar system aboard a canadian cv 580 aircraft sar spectra at altitudes of 20000 and 12000 ft are compared for both large approximately 50 degrees and small approximately 25 degrees radar look angles independence spectral estimates from both a surface contour radar and a radar ocean wave spectrometer aboard a nasa aircraft are used to verify the surface wave spectrum

earthquakes in a model of seismic zone with embedded pre existing faults we construct a simple quasi static model of seismic zone with embedded pre existing faults and present a self consistent method for incorporating both the stick slip behavior at the fault surfaces and the long ranged stress redistribution following a rupture or a slip event driven by a slow tectonic plate relative motion the model evolves to a critical state exhibiting gutenberg richter power law for the frequency size distributions results on spatial and statistical features of earthquakes in the model are also presented

measurement of ocean surface slopes and wave spectra using polarimetric sar image data abstract methods have been investigated which use fully polarimetric synthetic aperture radar sar image data to measure ocean slopes and wave spectra independent techniques have been developed to measure wave slopes in the sar azimuth and range directions the azimuth slope technique in particular is a more direct measurement than conventional intensity based backscatter cross section measurements in the azimuth direction wave induced perturbations of the polarimetric orientation angle are used to sense the wave slopes in the range direction a new technique involving the alpha parameter from the cloude pottier h a entropy anisotropy and averaged alpha polarimetric scattering decomposition theorem is used to measure slopes both measurement types are sensitive to ocean wave slopes and are directional taken together they form a means of using polarimetric sar polsar image data to make complete measurements of either ocean wave slopes or directional wave spectra these measurements must still contend with fundamental nonlinearities in the sar image processing i e azimuth direction velocity bunching that are due to wave velocity and acceleration effects nasa jpl airsar l and p band data from california coastal waters were used in the studies wave parameters measured using the new methods are compared with those developed using both conventional sar intensity based methods and with in situ noaa national data center buoy measurement products

estimation of the two dimensional ocean current shear field with a synthetic aperture radar a double inverse method to convert sar data to estimates of current shear was developed theoretically problems of applying the technique to real sar data were examined the first step of the inversion process involved elimination of noise in the x vector k vector wave spectra deduced from sar images the second step estimation of current shear required the existence of well defined swell and a favorable single of interaction between swell and ocean current application of the method in the antarctic circumpolar current encountered sea conditions that both satisfied and violated these requirements where the wave current geometry was favorable realistic surface current velocity fields were obtained suggesting the presence of a substantial mesoscale eddy another attempt to apply the method in the gulf stream produced wave spectral estimates that showed no clear peak and hence produced no current estimates

fractal analysis applied to characteristic segments of the san andreas fault fractal theory is applied in a quantitative analysis of the san andreas fault saf geometry the method which directly measures the increase in total fault length with a decrease in ruler size gives the fractal dimension d and scaling properties for the chosen length band 0 5 1000 km a physical interpretation of d is that it measures the irregularity of the fault trace in the selected band the fault is subdivided into six segments of distinctive seismic behavior a main fault trace which shows either maximum coseismic slip or creeping was selected for analysis with three alternative branches examined for the saf system south of san bernardino branches of the fault trace are not considered in this work fractal dimensions calculated for the different segments range from 1 0008 to 1 0191 and are different from 1 0 to the 95 confidence interval these small changes reflect the overall smoothness of the main fault trace significant variations in d among segments indicate heterogeneities in the fault smoothness along strike d also changes significantly from the short length band to the long length band where the demarcation point ranges from 1 to 2 km the short length band has larger d values a slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting this indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view

improvement in the estimation of dominate wavenumber and direction from spaceborne sar image spectra when corrected for ocean surface movement because a synthetic aperture radar sar is a doppler device ocean surface movement can degrade a sar ocean image ocean surface movement causes the redistribution of apparent radar cross section in a sar image to neighboring image elements it is demonstrated in this paper that this aspect of sar imaging can be treated as a simple resolution loss in a linear system such an approach explains the falloff in response at high azimuth wavenumbers experienced in sar image spectra more importantly this approach allows for a straightforward method to alleviate the problem of ocean surface motion correction of sar image spectra using a linear systems approach results in sar image spectra in which the locations in wavenumber space of surface wave spectral peaks are more consistent with the estimated location of storm sources than are the locations of peaks from uncorrected spectra

detection of a locked zone at depth on the parkfield california segment of the san andreas fault the parkfield california segment of the san andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked carrizo plain segment to the southeast the rate of shallow fault slip decreases from 25 30 mm yr northwest of the epicenter of the 1966 parkfield earthquake to zero at the southeastern end of the 1966 rupture zone data from a network of trilateration lines spanning the san andreas fault near parkfield and extending to the pacific coast near san luis obispo shed light on the rate of fault slip at depth since the 1966 earthquake in this study average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the parkfield fault segment the fault is taken to be a vertical surface with unknown distribution of strike slip displacement in an elastic half space a striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 parkfield earthquake the data require that the locked zone extend nearly as far north as the 1966 epicenter however the vertical extent of the locked zone is not well resolved over much of the parkfield segment the fault is slipping faster at the earth s surface than it is at seismogenic depths in order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the san andreas the inversion results suggest a spatially uniform normal strain of 0 06 strain yr the orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region the magnitude of the contraction is consistent with convergence rates inferred from global plate motion models

seasat altimeter observations of dynamic topography in the gulf stream region individual seasat altimeter profiles in the western north atlantic have been differenced with the best available geoid model to remove the gravitational component the resulting sea surface height profiles compare remarkably well with independent oceanographic observations the gulf stream is clearly apparent in each profile as a 1 to 2 m step and known positions of cyclonic and anticyclonic rings correspond with depressions and elevations respectively with amplitudes as large as 95 cm some of the most important altimeter data for analyzing the dynamic ocean signal were gathered during the last month of the mission when the same ground track was repeated every 3 days these data allow detailed examination of time varying ocean phenomena since the gravitational component is time invariant one set of collinear passes clearly shows a cyclonic ring as it moves out from underneath the spacecraft s track an event which was simultaneously observed with the aid of a satellite tracked surface buoy another striking feature seen in the altimeter data set is the apparent variability exhibited by the gulf stream on time scales of a few days surface transport indicated by the sea surface height difference across the stream varied by nearly 30 and over the entire 3 month period much larger fluctuations were observed suggesting significant changes in total mass transport altimetry may provide an effective means of determining the time and space scales associated with these variations

a comparison of sir b directional ocean wave spectra with aircraft scanning radar spectra directional ocean wave spectra derived from shuttle imaging radar b sir b l band imagery collected off the coast of southern chile on 11 and 12 october 1984 were compared with independent spectral estimates from two airborne scanning radars in sea states with significant wave heights ranging from 3 to 5 meters the sir b derived spectra at 18 and 25 off nadir yielded reasonable estimates of wavelengths directions and spectral shapes for all wave systems encountered including a purely azimuth traveling system a sir b image intensity variance spectrum containing predominantly range traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum the prediction of a u s navy global spectral ocean wave model on 11 october 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30 espect to the mean of the aircraft and spacecraft estimates

observations of wind waves in the gulf stream frontal zone experimental results of a study of wind waves in the gulf stream frontal zone are presented measurements of two dimensional radar spectra of surface waves and surface current were performed during ship crossings of the gulf stream under different wind conditions waves reflected from the gulf stream were observed on the windward side for an along current wind these effects resulted in a widening of the spectrum angular distribution and a deviation of the mean wave direction from that of the wind trapped waves were observed under against current winds depending on wave age the trapped waves were either locally generated developing waves or entered into the jet downstream nearly fully developed waves trapped waves significantly increased the wave energy locally the experimental data are interpreted in terms of wave kinematics and the equation of spectrum evolution in the relaxation approximation

variation of the gutenberg richter b values and nontrivial temporal correlations in a spring block model for earthquakes we show that a two dimensional spring block model for earthquakes is equivalent to a continuous nonconservative cellular automaton model the level of conservation is a function of the relevant elastic parameters describing the model the model exhibits power law distributions for the energy released during an earthquake the corresponding exponent is not universal it is a function of the level of conservation thus the observed variation in the b value in the gutenberg richter law could be explained by a variation in the elastic parameters we address the problem of the boundary conditions and display results for two extreme possibilities furthermore we discuss the correlation in the interoccurrence time of earthquakes the model exhibits the features of real earthquakes the occurrence of small earthquakes is random while the larger earthquakes seem to be bunched primarily the results of our work indicate that the dynamic of earthquakes is intimately related to the nonconservative nature of the model which gives birth to both the change in the exponent and the correlations in interoccurrence time

waves in frazil and pancake ice and their detection in seasat synthetic aperture radar imagery a theoretical model of waves propagating into an ice cover composed of frazil and pancake ice is developed and compared with measurements of wavelength and direction derived from synthetic aperture radar sar imagery obtained from seasat in october 1978 the theoretical model is based on the concept that ice of these types which consists of small crystals or cakes has only a mass loading effect on the water surface we derive the dispersion relation for phase and group velocities finding that there is an upper frequency limit for propagation into the ice from the reflection coefficient at the ice edge we derive the wave radiation pressure exerted on the ice showing that it will cause a slick of frazil ice backed by thicker floes to become more dense or thick with increasing penetration the implications for radar scattering enabling detection on sar are that the bragg resonant wavelength corresponds to waves above the frequency limit for propagation so that a frazil slick appears dark on an sar image when the frazil ice becomes transformed into pancake ice through slick compression or other means the raised edges of the pancakes cause the ice to appear bright despite the fact that there are no waves present at the bragg wavelength these results are applied to a seasat sar image obtained from the chukchi sea the appearance of the ice in the image corresponds to what we expect for frazil ice gradually transforming itself into pancake ice backed by thicker floes we derive directional wave number spectra outside and inside the ice cover by digital fourier analysis of image subscenes and we find that the change of wavelength and angle of refraction of the dominant wave entering the ice field are both characteristic of the dispersion relation derived theoretically mean ice thicknesses extracted from the theory correspond to thicknesses expected for such slicks the technique offers a possible means of extracting the thickness of fields of frazil and pancake ice from sar imagery this may be of considerable utility when ers 1 sar is used to study the advancing winter ice edge in the antarctic which consists of vast areas of these ice types

tracking storm generated waves in the northeast pacific ocean with ers 1 synthetic aperture radar imagery and buoys this paper examines the capability of synthetic aperture radar imagery from ers 1 and buoys to track the wave field emanating from an intense storm over a several day period the first part of the study is a validation component that compares sar derived wave length and direction with buoy data from two locations over 10 different dates in late 1991 and national oceanic and atmospheric administration noaa wave model wam wave direction results when the sar is linear 8 out of 10 cases mean wavelength is within 5 of the buoy measurements and mean wave direction is within 1 of direction derived from the wave model albeit with a large standard deviation of 27 indicating close agreement the wave field generated from the intense storm in late december 1991 was measured by three separate ers 1 sar passes over a 3 day period a simple kinematic model was used for waves propagating from a storm comparing the model results with both sar and buoy data indicates that sar derived peak wavelength and direction measurements can be reliably used to predict arrival times and propagation direction over a several day period and considerable distances the measurements can also be used to derive estimated wave generation source regions about the storm as well such measurements are useful for comparing with wave model results which perform less accurately for direction than wave height for example and for predicting hazardous conditions for ship navigation and coastal regions

structure of the surface wind field from the seasat sar an intensive analysis of the vector wind field of one seasat data set pass 1339 is described wind speed and direction signatures are found in seasat sar synthetic aperture radar images and the resulting estimates are compared with the seasat a scatterometer system sass and simultaneous noaa p 3 aircraft measurements a power law is presented to relate sar measured backscatter to sass estimated wind speed and the sar estimates are shown to agree with the sass estimates to within a standard error of 0 7 m s over a range of wind speeds from 3 m s to 13 m s the surface expressions of atmospheric roll vortices are apparent in several of the sar images and may be responsible for the wind direction signature in these cases wind field estimates averaged over regions of variable sizes are possible because of the high resolution of sar imagery a method for extracting low wave number directionality and its variability from sar spectra is described and sar direction estimates obtained from spectra of 6 4 km square images are shown to have a precision of approximately 10 although a comparison data set that could validate these higher resolution estimates is lacking averages over 40 km square regions are in good agreement with the other wind field information the sar wind direction estimates yield a more complete interpretation in a region with a turning wind field near a front that is ambiguous with only sass observations in this region the flow patterns of the high resolution estimates appear consistent with our knowledge of the overall circulation considering all the observations however in another region the small scale variability is too large and random to represent real wind variability although averages derived from these estimates are still reliable

long term fault creep observations in central california the u s geological survey usgs has been monitoring aseismic fault slip in central california for more than 10 years as part of an earthquake prediction experiment since 1968 the usgs creep network has grown from one creep meter at the cienega winery south of hollister to a 44 station network that stretches from hayward east of san francisco bay to palmdale in southern california in general the long term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period e g calaveras fault near hollister and san andreas fault between san juan bautista and bear valley these fault sections are the most difficult to characterize with a single long term slip rate in contrast sections of the faults that are seismically relatively quiet e g san andreas fault between bear valley and parkfield produce the steadiest creep records and are easiest to fit with a single long term slip rate appendix is available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j82 004 1 00 payment must accompany order

the response of two joined quarter spaces to sh line sources located at the material discontinuity interface summary exact analytical solutions to problems of sh line force and line dislocation operating at the interface between two quarter spaces are derived and discussed it is shown that the resulting fields differ from that of a homogeneous half space in the distribution of travel times wave amplitudes and polarity of first motions these effects are documented in the literature it is therefore suggested that when constructing theoretical seismic displacement fields from source path and site effects it is important to use response functions that account for material heterogeneity in the source region an extension of the formulation used in the case of sh line dislocation leads to a 3 d representation theorem which gives the displacement field in terms of the displacement discontinuities along the rupture area the resulting overall sue scaling parameter is the potency which does not suffer from an ambiguity associated with seismic moment in the case of heterogeneous source region

a simplified spring block model of earthquakes the time interval between earthquakes is much larger than the actual time involved during slip in an individual event the authors have used this fact to construct a cellular automaton model of earthquakes this model describes the time evolution of a 2 d system of coupled masses and springs sliding on a frictional surface the model exhibits power law frequency size relations and can exhibit large earthquakes with the same scatter in the recurrence time observed for actual earthquakes

earthquake failure sequences along a cellular fault zone in a three dimensional elastic solid containing asperity and nonasperity regions numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three dimensional 3 d model representing approximately the central san andreas fault the model consists of an upper crust overlying a lower crust and mantle region together defining an elastic half space with a vertical half plane fault the fault contains a region where slip is calculated on a uniform grid of cells governed by a static kinetic friction law and regions where slip is prescribed so as to represent tectonic loading aseismic fault creep and adjacent great earthquakes the computational region models a 70 km long and 17 5 km deep section of the san andreas fault to the nw of the great 1857 rupture zone different distributions of stress drops on failing computational cells are used to model asperity parkfield asperity and nonasperity fault regions the model is inherently discrete and corresponds to a situation in which a characteristic size of geometric disorder within the fault i e cell size here a few hundreds of meters is much larger than the nucleation size of the order of tens of centimeters to tens of meters based on slip weakening or state evolution slip distances the computational grid is loaded by a constant plate motion imposed at the lower crust upper mantle and creeping fault regions and by a staircase slip history imposed at the 1857 and 1906 rupture zones stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3 d elastic dislocation theory the resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops the frequency magnitude statistics of the simulated failure episodes are approximately self similar for small events with b 1 2 the b value of statistics based on rupture area ba is about 1 but are strongly enhanced with respect to self similarity for events larger than a critical size this is interpreted as a direct manifestation of our 3 d elastic stress transfer calculations beyond certain rupture area and potency seismic moment divided by rigidity release values the event is usually unstoppable and it continues to grow to a size limited by a characteristic model dimension this effect is not accounted for by cellular automata and block spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size the simulations suggest that local maxima in observed frequency magnitude statistics correspond to dimensions of coherent brittle zones such as the width of the seismogenic layer or the length of a fault segment bounded by barriers the analysis indicates that a single cell size representing approximately a single scale of geometric disorder cannot induce self similarity in a 3 d elastic model over a broad range of magnitudes a representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self similar gutenberg richter statistics our simulations show a great diversity in the mode of failure of the parkfield asperity the earthquakes themselves define an irregular sequence of events the modeling like many other discrete fault models suggests that expectations for periodic parkfield earthquakes and or simple precursory patterns repeating from one event to the other are unrealistic

introduction studies of ocean wave spectra from the shuttle imaging radar b experiment imagery of ocean surface waves obtained from synthetic aperture radar sar provides a unique two dimensional fine resolution view of the ocean wave field which has been long thought to be of considerable value for studying air sea inter action wave generation and propagation and wave current refraction wave spectra from sar have also been thought to be extremely useful for updating and verifying global wave forecast models such promise prompted a sar to be placed on seasat an oceanographic satellite which operated in a rather glorious mode for only a little over 3 months in 1978 this imagery has been well studied and indeed has demonstrated the feasibility of acquiring useful directional wave spectra from space e g vesecky and stewart 1982

soil landscape modelling using fuzzy c means clustering of attribute data derived from a digital elevation model dem this study explores the use of fuzzy c means clustering of attribute data derived from a digital elevation model to represent transition zones in the soil landscape the conventional geographic model used for soil landscape description is not able to properly deal with these fuzzy c means clustering was applied to a hillslope within a small drainage basin in southern spain cluster validity evaluation was based on the coefficient of determination of regressing topsoil clay data on membership grades the resulting clusters occupied spatially contiguous areas we found a high degree of association with measured topsoil clay data r a 2 0 68 for three clusters and a weighting exponent of 2 1 location of the clusters coincided with observable terrain characteristics therefore we concluded that the coefficient of determination of regressing soil sample data on membership grades efficiently supports deciding upon the optimum fuzzy c partition the study confirms that fuzzy c means clustering of terrain attribute data enhances conventional soil landscape modelling as it allows representation of fuzziness inherent to soil landscape units

frequency magnitude distribution of earthquakes in vrancea relevance for a discrete model a high resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes h 60 km extending over an interval of 9 3 years emphasizes a significant deviation at low magnitudes ml 4 in a linear frequency magnitude distribution it appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve this is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity like earthquakes and the transition zone ml 3 3 3 9 from crack like earthquakes background seismicity to asperity like events these features are better pointed out on well individualized active zones the background seismicity shows a tremendeous decrease in its b slope from 1 03 to 0 57 during a 6 year interval before the occurrence of a major earthquake in 1986 mw 7 3 followed by a fast recovery to 1 03 in 2 years after this event such a behavior could be correlated to the continuous growth of the shear stress free surface on the fault as in a percolation process followed by its sudden diminution due to the locking of the fault the above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths

energetic balance in scaling organization of fracture tectonics abstract this paper presents a development of the seismicity model s o f t scaling organization and fracture tectonics we remain in the frame of this simple model which is based on an energy splitting combined with a renormalization group approach redistribution of energy over the entire considered domain after strong events was introduced in the previous model the present version displays some general features of real seismicity such as gutenberg richter law omori law of temporal decrease of the aftershock activity seismic cycle quiet periods with a background seismic activity periods of foreshock and aftershock activity this is shown by numerical experiments in both the single domain case and in the case of exchange of energy between several domains

premonitory transformation of steel fracturing and seismicity abstract transformation of microfracturing preceding the break up of steel and rock samples is similar to transformation of earthquakes flow prior to strong earthquakes in southern california the break up in a sample is preceded by transition from formation of new microcracks to coalescence or expansion of existing cracks this is reflected in the relation between the length and the number of the cracks similar transformation of seismicity precedes southern california earthquakes with magnitude m 6 6 1935 1994 specifically magnitude frequency relation bends downward for magnitudes from 3 to 4 5 and upward for magnitudes from about 4 5 to 6 in the usual notations the b value becomes larger in the first interval and smaller in the second one this transformation is accompanied by the increased share of aftershocks in the earthquakes flow in such a way the approach of a strong earthquake is reflected in both major traits of seismicity magnitude frequency relation and earthquake clustering imprecision of the earthquake catalog and reasonable variations in its analysis do not change our conclusions this phenomenon explains so far qualitatively a wide set of premonitory seismicity patterns we give it a formal definition allowing to test whether it takes place in other seismic regions

physical links between crustal deformation seismic moment and seismic hazard for regions of varying seismicity summary seismic moment release rates m0 inferred from a weibull frequency magnitude distribution and its extreme value equivalent are compared with observation the seismotectonically diverse regions studied all exhibit the curvature of a log linear frequency magnitude plot associated with applying a maximum magnitude to earthquake recurrence statistics the inferred seismic moment release rates are consistent with available crustal deformation data within uncertainties resulting from the line fit and in magnitude determination the uncertainties for the regions studied southern california the new madrid seismic zone the central and eastern mediterranean and mainland uk vary from at worst an order of magnitude down to a factor of 2 or 3 this agreement can be used to justify the extrapolation of frequency magnitude statistics beyond the historical and instrumental era in seismic hazard studies as a test of the stationarity of short term statistics against long term effects a striking example of a bimodal seismicity distribution is observed in the new madrid zone this can be interpreted as being due to the superposition of two distinct seismogenic source types observed in the area a quantitative analysis of the separate orders of seismicity observed in the frequency magnitude statistics comparing the different maximum magnitudes and inferred seismic moment release rates with those observed supports this hypothesis superposition of many such seismogenic sources can explain the linearity observed in global frequency versus seismic moment magnitude statistics

application of pattern recognition techniques to earthquake catalogs generated by model of segmented fault systems in three dimensional elastic solids techniques and seismicity parameters described by eneva and ben zion 1997 are used to examine synthetic earthquake catalogs generated by ben zion 1996 for precursory patterns of large model events different model realizations represent various levels of fault zone disorder these include models with uniform properties u a parkfield type asperity a fractal brittle properties f and multi size scale heterogeneities m the seismicity parameters used are based on information contained in typical earthquake catalogs reflecting earthquake distribution in space time and size the analysis highlights the complexity of the information content of the synthetic earthquake catalogs simple repetitive precursory signals have not been found however local extrema in the examined parameters are found to have significant association in time with large events thus our techniques and parameters may be useful for intermediate term earthquake prediction especially when parameters are used in combinations some analysis results are the same for all model realizations and some depend on the model features characterizing all catalogs are as follows 1 large model events are statistically predictable on the basis of patterns in the distribution of smaller events 2 for a given parameter the type of precursory extrema maxima or minima is the same for all models 3 the interparameter correlation for any parameter pair has the same sign positive or negative in all models 4 the large events are neither slip nor time predictable based on previous large events results that differ from model to model include the following 1 the degree of predictability of large events correlates with the degree of regularity in the assumed fault properties following the order u f a and m 2 there is no one to one correlation between type of precursory extrema maxima or minima and type of precursory trends increase or decrease this produces great variations in observable trends for any given parameter both from model to model and for different events in the same model 3 the interparameter correlations vary among models with the highest correlations in model f most discussed patterns are in agreement with observations from seismically active zones laboratory models and mining induced seismicity

spatial variations of ocean wave directional spectra from the seasat synthetic aperture radar seasat synthetic aperture radar ocean wave spectra for a 900 km pass are analyzed and interpreted in the context of both their probable generation sources and their surface current and bathymetric modifiers systematic vector wavenumber variations of several times the standard error of determination 1 5 in magnitude and 0 9 in direction occur along the entire 900 km pass the large scale spatial variation of a 200 m swell system can be accurately accounted for as a result of dispersion from a distant storm the more local variations are qualitatively well correlated in position with known currents and bathymetry but show systematic biases that appear partly due to an environmentally dependent instrument transfer function in the regions of high current and highest sea state there is also substantial evidence that a large angular deviation in the center of the pass is the result of a mesoscale eddy just to the east

earthquakes as self organized critical phenomena a cellular automation model with threshold elements previously proposed by bak et al 1988 which generates 1 f noise and self organizes into a critical state is studied as a model for earthquake occurrence the model explains some of fractal features of earthquakes such as the power law distribution of the size of the earthquake the model is modified so that every earthquake may trigger aftershocks the modified trigger model explains other fractal features of earthquakes such as the power law decay of aftershock frequency and the power law decay of aftershock frequency and the power law correlation of the hypocenter distribution

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

wave current interaction near the gulf stream during the surface wave dynamics experiment this paper presents a case study on the wave current interaction near the local curvature of a gulf stream meander the wave data were obtained from in situ measurements by a pitch roll discus buoy during the surface wave dynamics experiment swade conducted off wallops island virginia from october 1990 to march 1991 owing to the advection of the gulf stream by the semidiurnal tide the discus buoy was alternately located outside and inside the gulf stream the directional wave measurements from the buoy show the changes in wave direction wave energy and directional spreading when waves encountered the current in the gulf stream meanders a wave refraction model using the ray tracing method with an estimated gulf stream velocity field and meandering condition was used to simulate wave refraction patterns and to estimate wave parameters at relative locations corresponding to buoy measurements the numerical simulation shows that a focusing zone of wave rays was formed near the boundary and behind the crest of a simulated gulf stream meander the focusing of wave rays causes changes in wave direction increases in wave energy and decreases in wave directional spreading which are in good agreement with the results from the buoy measurements

saturated hydraulic conductivity and soil water retention properties across a soil slope transition the hydraulic properties of soil and their spatial structures are important for understanding soil moisture dynamics land surface and subsurface hydrology and contaminant transport we investigated whether landscape features including relative position on a slope contribute to the variability of soil hydraulic properties in a complex terrain of a glacial till material using 396 undisturbed soil cores collected along two orthogonal transects we measured saturated hydraulic conductivity ksat and soil water retention functions at two 15 and 30 cm depths across a glacial till landscape in central iowa that encompassed two soil types nicollet loam with 1 3 slope on the hilltop position and clarion loam with 2 5 slope on the shoulder position the van genuchten mualem model was fitted to the experimental data using the retc optimization computer code at the 15 cm depth a statistical comparison indicated significant differences in ksat saturated water content us water content at permanent wilting point u15 000 and van genuchten fitting parameters a and n between soil types and landscape positions at the 30 cm depth us u15 000 and residual water content ur were found to be significantly different across the soil slope transition available water content u333 15 000 did not show any significant difference across the soil slope transition for either depth no clear directional trend was observed with some exceptions for k sat us and a on specific transect limbs and depths drifts in the soil hydraulic parameters due to soil slope transition were removed using a mean polishing approach geostatistical analyses of these parameters showed several important characteristics including the following 1 the spatial correlation lengths and semivariogram patterns of the independently measured or estimated loge ksat and us at 30 cm depth matched extremely well 2 better spatial structures with large correlation lengths were observed for macro and micro porosity related loge ksat us and loge a than for texture related loge u333 15 000 loge u15 000 ur and loge n at 30 cm depth and 3 a higher nugget effect at 15 cm depth was evident for most soil hydraulic parameters indicating tillage and other surface disturbances these novel findings may prove to be critical for modeling and interpreting field scale or larger scale soil moisture dynamics surface and subsurface flow and solute transport

premonitory raise of the earthquakes correlation range lesser antilles abstract we apply to the observed seismicity of lesser antilles a short term earthquake precursor which has been recently found by analysis of synthetic seismicity the latter was generated by a lattice type colliding cascades model of interacting elements precursor named roc depicted premonitory increase of the earthquakes correlation range here this precursor is used as a second approximation to the intermediate term prediction as a first approximation we use the alarms determined in the previous publication by the algorithm seismic reversal sr it depicts premonitory reversal of territorial distribution of seismicity we consider combined performance of both algorithms in prediction of earthquakes with magnitude 5 5 or above four such earthquakes occurred in the territory considered during 1984 1998 the alarms occupy 0 5 of the total time space three alarms happened to be correct two alarms were false and one earthquake was missed by prediction the alarms are very stable to variation of adjustable parameters of prediction method in view of this stability such a prediction is unusually good even for retrospective analysis we present this prediction method as a hypothesis to be tested on advance prediction

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

on the estimation of wave slope and height varnance spectra from sar imagery a procedure is described for using synthetic aperture radar sar imagery to estimate two dimensional ocean wave slope and height variance spectra the logic underpinning the procedure is based both on the results of the numerical simulation of sar wave imagery and analytic descriptions of the sar imaging process the procedure when applied to sar imagery of waves acquired during the recent shuttle imaging radar mission sir b is shown to produce spectra that agree with independent measures of both the two dimensional slope and height variance spectra the implications of these results for future sar missions aimed at measuring ocean waves are considered

colliding cascades model for earthquake prediction a wide set of premonitory seismicity patterns is reproduced on a numerical model of seismicity and their performance in the prediction of major model earthquakes is evaluated seismicity is generated by the colliding cascades model recently developed by the authors the model has a hierarchical structure it describes the interaction of two cascades a direct cascade of loading which is applied to the top largest element and transfers down the hierarchy and an inverse cascade of failures which goes up the hierarchy from the smaller to the larger elements these cascades collide and interact loading leads to failures while failures release and redistribute the loading three basic types of earthquake precursors are considered i the clustering of earthquakes in space and time ii the intensity of earthquake sequences and iii the correlation distance between earthquakes patterns of the first two types are used in intermediate term earthquake prediction algorithms patterns of the third type are found in the colliding cascades model although they were hypothesized previously they have not been validated by observations for each precursor we explore what is called an error diagram showing the total duration of alarms the rate of failures to predict and the rate of false alarms

a characteristic earthquake model of the seismicity preceding the eruption of mount st helens on 18 may 1980 abstract the two classes of earthquakes which preceded the explosive eruption of mount st helens washington on 18 may 1980 can be related to two different forms of energy release this is manifest both in the predominant periods of the two classes of seismic energy release at 1 0 and 0 55 s and in the ratio of their source dimensions in the range 2 1 to 6 1 inferred from characteristic magnitudes of the two possible classes of earthquakes apparent in the discrete frequency magnitude distribution the observed increase in the ratio of long to short period amplitudes of surface waves recorded on mso a local wwssn station can be numerically related to the growth of a magma chamber or conduit within which are generated the transient oscillations thought to be responsible for volcanic tremor these calculations predict an increase in the chamber s diameter at a rate of 2 7 m day 1 at depth in the 2 months preceding the eruption and this is consistent with an observed growth rate of 2 m day 1 of the precursory bulge which appeared at the surface on the mountain s north flank at the time the frequency magnitude distribution is analysed in two parts corresponding to a power law at low magnitudes and a gaussian distribution of typical fault lengths at high magnitudes an extension of the characteristic earthquake model first developed for individual fault zones the power law can in general have a non integer exponent which can be equated to a fractal self similarity dimension d temporal changes in d which is proportional to the seismic b value are proposed as an alternative and complementary description of the way seismic energy is released to the more commonly used viewpoints of changes in stress and heterogeneity despite a reassessment of the completeness of the magnitude catalogue no clear seismic precursor to the explosive eruption is observed in the b value although an earlier phase of phreatic eruptions was strongly associated with a b value anomaly

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

statistics of characteristic earthquakes statistical methods are used to test the characteristic earthquake hypothesis several distributions of earthquake size seismic moment frequency relations are described based on the results of other researchers as well as my own tests evidence of the characteristic earthquake hypothesis can be explained either by statistical bias or statistical artifact since other distributions of earthquake size provide a simpler explanation for available information the hypothesis cannot be regarded as proven

statistics of earthquake stress drops on a heterogeneous fault in an elastic half space we investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault the model consists of an inherently discrete strike slip fault surrounded by a 3d elastic half space we consider various spatial distributions of frictional properties and analyze results generated by 150 300 model years in all cases the self organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process in particular the large system sized events have stress drops that are consistently 25 of predictions based on the average fault strength the type and amount of assumed spatial heterogeneity on the fault affect the stress drop statistics of small earthquakes m l 5 more than those of the larger events this produces a decrease in the range of stress drops as the earthquake magnitudes increase the results can resolve the discrepancy between traditional estimates of stress drops and seismological observations the general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes

large and small scale spatial evolution of digitally processed ocean wave spectra from seasat synthetic aperture radar precision digitally processed and corrected synthetic aperture radar sar image spectra along a large portion of seasat orbit 1339 september 28 1978 have revealed a number of unique potentials but also some substantial limitations of spaceborne sar for global ocean wave monitoring sar appears to be capable of monitoring the spatially evolving ocean wave number spectrum with high statistical reliability the large scale few hundred kilometer spatial evolution of the spectrum is generally consistent with the location in time and space of wave generating sources even when multiple wave systems are present on a more local scale of just a few tens of kilometers the nearshore wave number spectrum responds to local depth changes but is too noisy to respond reliably to local current changes the seasat sar was severely restricted in its ability to sense azimuthtraveling along track waves in high sea states azimuth waves corresponding to ocean wavelengths shorter than about 200 m were either undetected or severely attenuated in the higher sea states hs 2 m azimuth waves shorter than 100 m are not detectable in even the lowest sea states moreover there is a tendency for attenuated wave systems to be shifted toward the range direction this effect will significantly impede studies of spatial evolution of seasat sar spectra without some additional correction strategy

source dimensions and stress drops of small earthquakes near parkfield california abstract source dimensions and stress drops of 30 small parkfield california earthquakes with coda duration magnitudes between 1 2 and 3 9 have been estimated from measurements on short period velocity transducer seismograms times from the initial onset to the first zero crossing corrected for attenuation and instrument response have been interpreted in terms of a circular source model in which rupture expands radially outward from a point until it stops abruptly at radius a for each earthquake duration magnitude m d gave an estimate of seismic moment m o and m o and a together gave an estimate of static stress drop all 30 earthquakes are located on a 6 km long segment of the san andreas fault at a depth range of about 8 to 13 km source radius systemically increases with magnitude from about 70 m for events near m d 1 4 to about 600 m for an event of m d 3 9 static stress drop ranges from about 2 to 30 bars and is not strongly correlated with magnitude static stress drop does appear to be spatially dependent the earthquakes with stress drops greater than 20 bars are concentrated in a small region close to the hypocenter of the magnitude 5 1 2 1966 parkfield earthquake

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer