stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

mechanics of low angle normal faulting an example from roosevelt hot springs geothermal area utah abstract roosevelt hot springs geothermal area is located in the mineral mountains of southern utah the geothermal reservoir is formed by systems of faults and joints in cenozoic plutonic and precambrian metamorphic rocks low angle denudation faults dipping between 5 and 35 to the west form an important component of the reservoir s structure these faults developed simultaneously with steeply dipping faults that dissect the low angle fault plates and merge into denudation faults at depth gently westward dipping joints provided planes of weakness along which the denudation faults nucleated the average coefficient of sliding friction along the faults was less than 0 5 and probably ranged between 0 15 and 0 4 the maximum depth for formation of the denudation faults was estimated as 5 km hydrothermally altered cataclasite preserved in the fault zones indicates that faulting occurred under brittle conditions in the presence of chemically reactive fluids the hydrothermal alteration may have significantly reduced friction in the fault zones hydrothermal alteration along fractures in the present geothermal reservoir is similar to that observed in the exhumed denudation faults indicating that the frictional resistance along faults and joints in the reservoir could be significantly lower than along similar structures in unaltered granitic rock studies of the structural stability of the reservoir as a consequence of fluid withdrawal and reinjection should consider possible mechanical effects of this hydrothermal alteration

contemporary tectonics of the wasatch front region utah from earthquake focal mechanisms we have completed a comprehensive study of focal mechanisms of digitally recorded earthquakes ml 4 4 that occurred in the wasatch front region in utah during 1980 to 1986 single event solutions for 24 events were determined using recently revised crustal models and a computerized grid search technique overall the mechanisms show predominantly normal faulting on n s striking nodal planes of moderate to steep dip 30 tension axis azimuths average 96 12 thus in general the mechanisms indicate e w to ese wnw crustal extension and vertical crustal shortening oblique slip when observed is characterized by left lateral motion on planes striking n to ne or right lateral motion on planes striking n to nw most of the mechanisms with significant amounts of oblique slip motion occur in the southern part of the study area where compression axis orientations range from near vertical to near horizontal thus the mechanisms suggest a possible change in stress regime from north to south along the wasatch front despite geologic evidence for low angle faults in the study area shallowly dipping nodal planes are relatively uncommon

mining related and tectonic seismicity in the east mountain area wasatch plateau utah u s a as part of a larger multi institutional seismic monitoring experiment during june august 1984 in the eastern wasatch plateau utah data from a subarray of 20 portable seismographs were used to investigate seismicity in the east mountain area an area of active underground coal mining and intense microseismicity eight stations of the subarray were concentrated on top of east mountain about 600 m above mine level at an average spacing of 2 to 3 km the primary objective was the accurate resolution of hypocenters and focal mechanisms for seismic events originating at submine levels data from high resolution seismic reflection profiles and drill hole sonic logs yielded a detailed velocity model this model features a strong velocity gradient in the uppermost 1 km which has a significant effect on takeoff angles for first arriving p waves from shallow seismic events two hundred epicenters located with a precision of 500 m cluster within an area about 5 km in diameter and show an evident spatial association with four sites of longwall mining during the study period a special set of foci rigorously tested for focal depth reliability indicates submine seismicity predominating within 500 m of mine level and extending at least to 1 km and perhaps to 2 km below mine level continuous monitoring for a 61 day period june 15 august 15 bracketed a 16 day mining shutdown july 7 22 during which significant seismicity comparable to that observed before the shutdown was observed ten focal mechanisms for seismic events originating at or down to 2 km below mine level nearly all imply reverse faulting consistent with previous results and the inferred tectonic stress field enigmatic events recorded with all dilatational first motions can be fit with double couple normal faulting solutions if they in fact occur above mine level perhaps reflecting overburden subsidence if these events are constrained to occur at mine level their first motion distributions are incompatible with a double couple source mechanism

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

dependence of sediment filled valley response on input amplitude and valley properties abstract for three sediment filled valleys average site responses were experimentally determined and found to apply to within 0 25 log units to earthquakes at any azimuth and to s waves and coda waves for one of these valleys ground accelerations were measured over the range 10 5 to 0 2 g and no significant differences in site responses were apparent in two valleys with sediment basement impedance contrasts of 5 1 and 6 1 average site amplification is as much as a factor of five in certain frequency bands in another valley with a sediment basement impedance contrast of 3 1 sediment amplification is about a factor of two the duration of motion on the valley sediments in narrow frequency bands is equal to that on the bordering rock implying that the observed amplification is not due to a high q resonance of the sediments

modelling generation and dissipation of overpressure in sedimentary basins an example from the halten terrace offshore norway a pressure simulator modelling the dynamic overpressure history in sedimentary basins is developed using a pressure compartment methodology simple models for porosity reduction are used for controlling pressure generation and different empirical models are used to estimate the lateral and vertical sealing dissipation mechanisms the equations describing the flow of formation water between the pressure compartments are used to calculate the resulting pressure dissipation on a geological time scale the methodology is tested and applied to the jurassic fangst group in the halten terrace area offshore norway dynamic allocation and an explicit numerical solution technique ensure a fast and accurate solution of the flow equations the results suggest a the probable mechanisms and processes that have generated the overpressure b the timing and location of hydraulic fracturing and c the nature and behaviour of the overpressure in addition to the overpressure history

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

effects of local geologic structure on yucca flats nevada test site explosion waveforms two dimensional linear finite difference simulations abstract two dimensional linear elastic finite difference calculations were performed for a two dimensional geologic model of yucca flats nevada test site nevada the calculations were used to produce synthetic teleseismic p wave seismograms for explosive line sources in yucca flats p wave coda in the first 5 sec is observed to be highly dependent on take off angle for the teleseismic synthetics p wave coda also varies with the position of the source in the valley structure and may produce variations in the individual station teleseismic p wave m b magnitude of up to 0 3 magnitude units however these magnitude variations should be substantially reduced by averaging over stations at multiple azimuths the reverberant coda appears to rise from scattered modal waves that are initially excited in the low velocity near surface structures of the yucca flats deposits of alluvium and tuff scattering of the waves occurs at offsets in the basement structure and at the sides of the valley the combined effects of scattering source function intrinsic attenuation and instrument response serve to obscure the p pp spectral scalloping that is expected from a linear model this loss of spectral resolution is the product of p coda filling in the p pp interference notches and the lengthening of the initial p wave source time function by a convolution of the source time function intrinsic attenuation operator and instrument response therefore short time windows 1 to 2 sec that do not include p coda energy do not have sufficient resolution to reliably detect the p pp interference notches

horizontal strain across the wasatch front near salt lake city utah two adjacent geodetic networks span a 105 km extent of the wasatch fault zone in north central utah one of the networks located in the vicinity of salt lake city was observed by the national ocean service national geodetic survey nos ngs in 1962 1963 triangulation mode and again in 1973 1974 combined triangulation trilateration mode to the north in the vicinity of ogden the u s geological survey usgs performed five separate trilateration surveys of the second network during the 1972 1981 interval when fit to a model assuming both temporal and spatial homogeneity for the strain rate the two independent data sets yield mutually consistent strain rate tensors subsets of the nos ngs data however suggest spatial variation in the strain field and a subset of the usgs data indicates a possible temporal variation in the strain rate the quality of the data is such that we are uncertain whether or not these apparent variations represent actual tectonic signals the strain rate tensors for the two complete data sets suggest an underlying deformational pattern characterized by east west e w maximum principal strain extension positive this pattern may also be inferred from regional seismicity and quaternary geology the suspected variations would imply that the area to the north of salt lake city experienced significant e w compression preceding 1978 in contradiction to the underlying strain pattern this compression may be caused by the mountainous block eastward adjacent to the fault zone displacing westward along a shallow angle west dipping detachment surface we speculate on the existence of an e w trending zone of weakness for example secondary faults located north of salt lake city to explain how the stress field to the north may be decoupled from that to the south where e w compression has not been detected while seismic and geologic information contribute some plausibility to our speculations the evidence for the existence of the suspected variations is inconclusive

smectite dehydration its relation to structural development and hydrocarbon accumulation in northern gulf of mexico basin a comparison of clay diagenesis data obtained from a study of tertiary shales from the brazos colorado river system of texas the mississippi river system of louisiana and the niger river system of nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs the threshold temperature required to initiate diagenesis ranges from about 160 f 71 c in mississippi river sediments to more than 300 f 150 c in the niger delta water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time in either situation the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported observations from the northern gulf of mexico basin indicate a close relation between buildup of high fluid pressure and the smectite illite transformation process abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed the depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting dips of some faults in texas have been observed to change abruptly within the interval of smectite diagenesis and some faults formed in the overpressured miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum although these faults may be important for fluid redistribution in shallow sandstone shale sections they are a minor factor in moving hydrocarbons out of shale below the faults in much of the texas offshore area fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern gulf of mexico basin this flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present shale tectonism is also the primary mechanism for structural development in the tertiary section of the niger delta however seismic data indicate that the rate of dip change of seaward dipping listric growth faults is commonly less than that observed in texas where dips as low as 10 15 can occur at depths as shallow as 10 000 15 000 ft 3 048 4 572 m syndepositional faults in nigeria are formed in sandstone shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section subtle differences in structural styles in the tertiary sections of texas and nigeria are probably the result of differences in clay composition of the shaly sections being deformed

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

inversion of field data in fault tectonics to obtain the regional stress i single phase fault populations a new method of computing the stress tensor summary we attempt a general definition of the inverse problem of computing the components of the regional stress tensor from a set of field data including the measurements of the strike and dip of several faults and the directions and senses of relative motion along these faults as indicated by slickenslides in previous treatments of this problem no experimental errors could be taken into account except those in measuring the pitch of slickenslides thus errors in the orientation of the fault strike and dip which have considerable practical importance were neglected in our work all experimental errors in the field measurements are taken into account so that the agreement between the computed stress tensor and the set of field measurements can be rigorously checked

stresses and strains in a twisted subduction zone fiordland new zealand summary in the fiordland region of the south island of new zealand the subducted australian plate steepens from south to north becoming near vertical below 75 km depth between doubtful and milford sounds this steepening is accompanied by a sharp change in strike of the subduction zone of about 17 here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three month deployment of 24 portable digital seismographs we determine focal mechanisms of 115 earthquakes of ml 2 5 4 5 using both first motion polarity data and amplitudes of seismogram envelopes we also invert for the stress tensor orientation for regions within both the subducted and overlying plates using a method which invokes the coulomb failure criterion and considers all p wave first motions together regardless of whether or not they are sufficient to define single event focal mechanisms focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths this stress and strain regime is related to the 17 change in strike of the subduction zone which leads to an arching up of the subducted plate at the bend such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend earthquakes in the subducted plate show predominantly normal faulting relative to the plate arrival time inversions for 3 d seismic velocity structure indicate that a zone of high vp 8 5 km s 1 in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone the steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike this mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction

seismicity and shear strain in the southern great basin of nevada and california this study examines the relationship between the distribution of small earthquakes ml 4 3 and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events the southern great basin seismic network sgbsn region the great basin is a unique continental extensional province characterized by normal and strike slip faulting high heat flow crust of thin to normal thickness and high elevations the sgbsn is operated to provide data to address suitability issues pertaining to yucca mountain nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository suitability issues include estimation of the probability of occurrence of future damaging earthquakes the characterization of the mechanisms that drive hydrologic flow and the identification of fractures faults that might act as flow conduits or barriers this study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field where strain concentrates there should be a greater number of small earthquakes strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field these strain field models are then interpreted with regard to the regional tectonics and site suitability issues modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults clusters in a wide band along and extending northeast of the northern half of the furnace creek fault may correspond to elevated shear strains along the fault and a broad cluster in the pahranagat shear zone may be associated with shear strain arising from a distribution of smaller localized faults the relatively large number of small earthquakes in the southern and eastern portions of the nevada test site is consistent with the strain field models a minimum in shear strain at yucca mountain is predicted by all models consistent with an almost total lack of earthquakes observed there the region to the west of the death valley furnace creek fault system the portion of the study area with the most active deformation but few small earthquakes is an area of low shear strain a possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep while there is also a relative lack of earthquakes at yucca mountain this may be indicative of a lack of accumulating strain energy and thus a lower potential for a large earthquake

inferences about the local stress field from focal mechanisms applications to earthquakes in the southern great basin of nevada focal mechanisms determined from regional network earthquake data or aftershock field investigation often contain members ranging from strike slip to normal slip in extensional tectonic environments or from strike slip to thrust slip in compressional environments although the coexistence of normal and strike slip faulting has suggested to some investigators that the maximum and intermediate principal stresses are of approximately equal magnitude several have asserted that the directions of principle stresses can or must interchange to accommodate both types of mechanisms zoback and zoback 1980b vetter and ryall 1983 a coulomb navier criterion of slip is invoked to demonstrate that both types of mechanisms as well as oblique members having preferred nodal plane dips intermediate between those of the strike slip and normal mechanisms may be observed in a region where the stress field resolved into principal components is axially symmetric the proximate coexistence of earthquakes having diverse focal mechanisms could be interpreted as evidence for an approximately axially symmetric stress field in a region where optimally oriented planes of weakness are known to exist in the host rock 10 refs 6 figs

state of stress and modern deformation of the northern basin and range province constraints on the current stress regime of the actively extending northern basin and range province are provided by deformation data focal mechanisms and fault slip studies hydraulic fracturing in situ stress measurements borehole elongation breakouts analyses and alignment of young volcanic vents the integrated data indicate significant variations both in principal stress orientations and magnitudes an approximately e w least principal stress direction appears to characterize both the eastern and western margins of the basin and range province whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented nw to n60 w the contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins active deformation along the southeastern and western province margins is characterized by a combination of strike slip and normal faulting focal mechanisms along northeastern province margin wasatch front and in central nevada indicate a combination of normal and oblique normal faulting temporal regional and depth dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles however some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated evidence for significant temporal variation or multiple cycles of variation in relative stress magnitude comes from the sierran front basin and range boundary region where recent earthquakes are predominantly strike slip whereas the profound relative vertical relief across the sierra frontal fault zone in the last 9 10 m y implies a normal faulting stress regime using the best data on stress orientation relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements analysis of well constrained slip vectors in the owens valley california area indicate that large temporal variations in the magnitude of the approximately n s oriented maximum horizontal stress are required to explain dominantly dip slip and strike slip offsets on subparallel faults similar faulting relations are observed throughout much of the boundary zone between the basin and range sierra nevada including the walker lane belt along the eastern province margin in the wasatch front area in utah available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of 4 5 km earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth furthermore superimposed sets of young fault striae along a segment of the wasatch fault also indicate temporal variations of relative stress magnitudes sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion thermally induced flow and the influence of the san andreas plate boundary although difficult to date accurately the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle 1000 5000 years and multiple earthquake cycles 10 000 years

local magnitude determined from a simulated wood anderson seismograph abstract the design of a seismograph to simulate the output of a wood anderson seismograph is reported results demonstrate that the seismograph can satisfactorily simulate the wood anderson seismograph at least in the frequency range from 1 to 10 hz the relationships between local magnitude m l body wave magnitude m b seismic moment m o and duration magnitude m d for taiwan earthquakes are m l 0 604 0 485 1 268 0 094 m b log m o 14 571 1 683 1 598 0 236 m l and m d 0 187 0 373 0 862 0 066 m l the first two relationships are different from those for southern california earthquakes for the same m b or m o the local magnitude value of a taiwan earthquake is larger than that of a southern californian one thick sediments just below the simulated wood anderson seismograph site probably amplify the short period seismic waves thus yielding larger local magnitude values for taiwanese earthquakes the different source properties in the two regions are also considered to contribute to the differences

geologic setting of the 1884 bear lake idaho earthquake rupture in the hanging wall of a basin and range normal fault revealed by historical and geological analyses analysis of historical accounts of the m 6 3 1884 earthquake in northern utah reveals that the earthquake had an epicenter near 42 3 n 111 4 w approximately 30 km northwest of the event s original location we use detailed reports of damage to structures and the consequences of ground shaking to define a felt area of approximately 70 000 km2 and estimate the peak ground accelerations as 100 300 cm sec2 analysis of the geologic structure of the area indicates that the epicentral area is a half graben bounded on the east by the listric bear lake fault and on the west by the steeply dipping west bear lake fault the earthquake epicenter was on the west side of the basin and we interpret the event to have been the result of slip on the west bear lake fault zone at a depth of 4 5 km this zone consists of steeply dipping antithetic faults in the hanging wall of the east bear lake fault these data suggest that moderate magnitude earthquakes on antithetic or small displacement faults pose a significant if local seismic hazard in the northeastern basin and range province we also demonstrate the utility of combining geological and historigraphic analyses to examine pre instrument era earthquakes online material felt reports and town summaries manuscript received 16 july 2002

contributions of compaction and aquathermal pressuring to geopressure and the influence of environmental conditions reply luo and vasseur 1992 presented a useful methodology of simplifying the complex equations that describe basin compaction and thereby directly assessing the contribution of aquathermal effects in the development of abnormal pressures their methodology leads to the conclusion that aquathermal pressuring plays only a small role in the development of overpressures in sedimentary basins although this conclusion follows from the cases studied it is possible to conclude otherwise using 1 a more general model of sediment compaction and 2 material properties that are more representative of conditions at depth the primary factor behind objection to the conclusion in luo and vasseur 1992 is that is fundamental equations which formed the basis for the simplifying methodology do not account fully for conditions in which a sediment s effective stresses are reduced we emphasize here that we do not claim that thermal effects are the dominant cause of overpressures rather our calculations suggest that they are not negligible in all cases there is a complex interplay of the various mechanisms that cause excess pore water pressures no one mechanism of overpressuring dominates all others for all situations because thermal effects not only cause higher excess pore pressures but also reduce effective stresses thermal effectsmore will be minimized by default in calculations that do not account for effective stress reduction the resulting reduced effective stresses also have important implications for geological questions such as acoustic velocities and fracture potential less

spatial heterogeneity of tectonic stress and friction in the crust the complex geometry of faults seismicity and diversity of earthquake mechanisms suggest that the stress and strength in earth s crust are spatially heterogeneous we investigated the degree of heterogeneity using the following two end member models in one end member model we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies in this model the variability of earthquake mechanisms means that friction during faulting must vary for each event we computed friction from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure the values of vary over a large range from 0 to 1 5 in the other extreme model we assumed optimally oriented slip and a constant 0 6 as is suggested by byerlee s law for all the earthquakes and determined the local stress orientation for each earthquake the orientation of the stress changes drastically from one earthquake to another and the assumption of uniform stress field commonly used in stress inversion is not warranted an important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive the actual situation in the crust is most likely to be intermediate between these two end member models from the existing data alone we cannot determine the degree of heterogeneity uniquely but both and the local stress field near earthquake faults are likely to vary substantially and studies on earthquake rupture dynamics must take these heterogeneities into consideration

triggered seismicity in utah from the 3 november 2002 denali fault earthquake immediately after the arrival of the surface waves from the m w 7 9 denali fault earthquake on 3 november 2002 the university of utah regional seismic network recorded an abrupt increase in local microseismicity throughout most of utah s main seismic belt we examined this increase in the context of the regional background seismicity using a catalog of 2651 earthquakes from 1 january 2000 to 30 june 2003 statistical analyses of this catalog above spatially varying magnitudes of completeness ranging from 1 2 to 1 7 allow us to reject with 95 confidence the null hypothesis that the observed increases were due to random occurrence the elevated seismicity was most intense during the first 24 hr 10 times the average prior rate but continued above background level for 25 days at the 95 confidence level in most areas we conclude that the increased seismicity was triggered by the denali fault earthquake which occurred more than 3000 km from the study region high peak dynamic stresses of 0 12 to 0 35 mpa that occurred during the passage of the love waves are consistent with the interpretation of triggering the peak dynamic stresses were estimated by measuring peak vector velocities at 43 recording sites 37 of which were relatively new strong motion stations of the advanced national seismic system the triggered events ranged in magnitude m c and or m l from less than 0 to 3 2 and were widely distributed across the state primarily in seismically active regions in contrast to many previously published observations of remotely triggered seismicity the majority of the triggered earthquakes did not occur near quaternary volcanic vents or in areas of magma related geothermal activity in several areas the triggered seismicity was spatially clustered more than five earthquakes each separated by 5 km double difference relative relocations for the earthquakes in three of these clusters indicate that most but not all of the triggered events were spatially separated from source zones of prior seismicity during 2000 2002 focal mechanisms for the two largest triggered events have northeast to northwest trending tension axes which are unusual for the region where they occurred the temporal decay of the triggered activity was similar to that of utah aftershock sequences and can be described by the modified omori s law with a p value of 0 6 to 0 7 the frequency magnitude distribution of the triggered earthquakes is also similar to that of utah aftershocks and for the study area as a whole can be described by the gutenberg richter relation with a b value of 0 81 0 16 these similarities between the triggered seismicity and utah aftershock sequences suggest the possibility that the initiation and development of both could result from the same causative mechanisms online material catalog of earthquakes used in this study

large mid holocene and late pleistocene earthquakes on the oquirrh fault zone utah abstract the oquirrh fault zone is a range front normal fault that bounds the east side of tooele valley and it has long been recognized as a potential source for large earthquakes that pose a significant hazard to population centers along the wasatch front in central utah scarps of the oquirrh fault zone offset the provo shoreline of lake bonneville and previous studies of scarp morphology suggested that the most recent surface faulting earthquake occurred between 9000 and 13 500 years ago based on a potential rupture length of 12 to 21 km from previous mapping moment magnitude mw estimates for this event range from 6 3 to 6 6 in contrast our results from detailed mapping and trench excavations at two sites indicate that the most recent event actually occurred between 4300 and 6900 yr b p 4800 and 7900 cal b p and net vertical displacements were 2 2 to 2 7 m much larger than expected considering estimated rupture lengths for this event empirical relations between magnitude and displacement yield mw 7 0 to 7 2 a few short discontinuous fault scarps as far south as stockton utah have been identified in a recent mapping investigation and our results suggest that they may be part of the oquirrh fault zone increasing the total fault length to 32 km these results emphasize the importance of integrating stratigraphic and geomorphic information in fault investigations for earthquake hazard evaluations at both the big canyon and pole canyon sites trenches exposed faulted lake bonneville sediments and thick wedges of fault scarp derived colluvium associated with the most recent event bulk sediment samples from a faulted debris flow deposit at the big canyon site yield radiocarbon ages of 7650 90 yr b p and 6840 100 yr b p all lab errors are 1 a bulk sediment sample from unfaulted fluvial deposits that bury the fault scarp yield a radiocarbon age estimate of 4340 60 yr b p stratigraphic evidence for a pre bonneville lake cycle penultimate earthquake was exposed at the pole canyon site and although displacement is not well constrained the penultimate event colluvial wedge is comparable in size to the most recent event wedges charcoal from a marsh deposit which overlies the penultimate event colluvium and was deposited during the bonneville lake cycle transgression yields an ams radiocarbon age of 20 370 120 yr b p multiple charcoal fragments from fluvial deposits faulted during the penultimate event yield an ams radiocarbon age of 26 200 200 yr b p indirect stratigraphic evidence for an antepenultimate event was also exposed at pole canyon charcoal from fluvial sediments overlying the eroded free face for this event yields an ams age of 33 950 1160 yr b p providing a minimum limiting age on the antepenultimate event ages for the past two events on the oquirrh fault zone yield a recurrence interval of 13 300 to 22 100 radiocarbon years and estimated slip rates of 0 1 to 0 2 mm yr temporal clustering of earthquakes on the nearby wasatch fault zone in the late holocene does not appear to have influenced activity on the oquirrh fault zone however consistent with findings on the wasatch fault zone and with some other quaternary faults within the bonneville basin we found evidence for higher rates of activity during interpluvial periods than during the bonneville lake cycle if a causal relation between rates of strain release along faults and changes in loads imposed by the lake does exist it may have implications for fault dips and mechanics however our data are only complete for one deep lake cycle the past 32 000 radiocarbon years and whether this pattern persisted during the previous cutler dam and little valley deep lake cycles is unknown

the little skull mountain nevada earthquake of 29 june 1992 aftershock focal mechanisms and tectonic stress field implications the ml 5 6 29 june 1992 little skull mountain earthquake occurred in the southern nevada test site nye county nevada within 20 km of a potential national nuclear waste repository at yucca mountain nevada while microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock foreshock activity ml 2 were also sufficiently well recorded to provide local network p wave focal mechanism solutions the mainshock and the majority of the aftershocks through september 1992 display predominantly normal slip focal mechanisms but unambiguous strike slip and oblique slip focal mechanism solutions are also present in the aftershock series the mainshock and immediate foreshock depths were 10 1 km below sea level and depths of focus of aftershocks range from about 6 to 12 km a large subset of the aftershock hypocenters define a southeast dipping structure probably the mainshock fault plane no variation of mechanism style with depth is apparent two mainshock and 84 aftershock focal mechanism solutions were input into gephart s 1990 stressfield inversion program and principal compressional stress directions j j 1 2 3 were determined as well as the stress difference ratio r 1 2 1 3 the results indicate a uniform stress field in which 3 minimum stress is oriented 116 5 azi plunge and 1 maximum stress is oriented 219 69 with r 0 35 the inversion s selection of preferred nodal planes invokes bott s 1959 criterion which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces for the predominantly normal slip little skull mountain earthquake mechanisms having northeast striking planes the southeast moderately to steeply dipping dip 55 nodal planes are selected however for the dip slip earthquake mechanisms exhibiting more northerly trending strikes shallowly to moderately west dipping 25 dip 65 nodal planes are selected when a coulomb mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio 1 3 the ratio of resolved shear stress to normal stress n on the southeast dipping nodal plane of the mainshock exhibited a near maximum level suggesting a high fault strength in contrast a wide range of fault strengths or apparent coefficients of friction was suggested by the n ratios for little skull mountain aftershock preferred nodal planes seismicity on misoriented faults i e those on which shear stress was relatively low may have been the result of a temporarily elevated fluid pore pressure following the mainshock

strain accumulation in the rocky mountain states strain accumulation in trilateration networks in montana utah and new mexico has been measured for over a decade the network in new mexico spans the rio grande rift near socorro the absence of any observed strain accumulation in that network places an upper limit 2 standard deviations of 3 mm a on east west spreading across the 100 km wide rift the network in utah spans the wasatch fault near ogden although the 1972 1984 average rate of strain accumulation in that network is not significant at a 2 standard deviation detection threshold of 0 03 strain a short term strain fluctuations of marginal significance have been detected in the network the network in montana spans the epicentral area of the 1959 hebgen lake earthquake magnitude 7 1 the principal strain rates observed in that network for the 1973 1984 interval were e 1 0 28 0 02 strain an14 e 1 and e 2 0 05 0 02 strain an76 w 1 extension reckoned positive the principal extension is perpendicular to the 1959 rupture trace the deformation of this network suggests rather rapid reloading of the 1959 rupture zone but whether this is steady strain accumulation or still postseismic relaxation is not known the strain rate does appear to be constant over the 1973 1984 interval

strain accumulation across the central nevada seismic zone 1973 1994 five trilateration networks extending for 280 km along the central nevada seismic zone 1915 pleasant valley m 7 3 1954 dixie valley m 6 8 1954 stillwater m 6 8 1954 rainbow mountain m 6 6 1954 fairview peak m 7 1 and 1932 cedar mountain m 7 2 have been surveyed 6 times since 1973 to determine deformation along the zone within the precision of measurement the deformation appears uniform along the zone and is described by the principal strain rates 0 036 0 008 strain yr n60 w 3 and 0 031 0 008 strain yr n30 e 3 extension reckoned positive the observed strain rates are consistent with simple right lateral tensor shear at the rate of 0 033 strain yr across a shear zone striking n15 w this central nevada shear zone appears to be the northward continuation of the eastern california shear zone the orientation of the strike slip and normal slip ruptures within the central nevada seismic zone are consistent with principal stress axes parallel to the measured principal strain rate axes space based geodetic measurements very long baseline interferometry indicate that the relative motion accommodated across the basin and range province west of ely nevada is about 9 1 1 5 mm yr n16 w 8 dixon et al 1995 notice that the right lateral shear zone postulated to explain deformation in the central nevada seismic zone is properly oriented to accommodate that relative motion however a 135 km effective width of the shear zone would be required to accommodate all of the 9 1 mm yr relative motion at the strain rates observed in the nevada seismic zone only about 3 mm yr of that relative motion is accommodated within the span of the trilateration networks

characterization of regional stress patterns in the western great basin using grouped earthquake focal mechanisms abstract focal mechanisms from the mammoth round valley chalfant area the mono excelsior seismic zone and western central nevada are compared with results from southern nevada a small area in the southern sierra nevada and the truckee region the most important conclusions are the following the regional extension direction changes from ne sw in the mammoth lakes area to nw se in western and southern nevada the walker lane a clear boundary between differing styles of tertiary and quaternary faulting as deduced from geology shows no such relationship to the focal mechanisms earthquakes with common focal characteristics straddle this structure strike slip faulting dominates everywhere with the exception of west central nevada and the southern sierra nevada but this result is based mainly on small to moderate shocks e g none greater than m l 6 3 and appears to be inconsistent with the record of holocene normal faulting in these regions

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

physical properties of sediments from the ulleung basin east sea results from second ulleung basin gas hydrate drilling expedition east sea korea abstract physical properties including basic index properties undrained shear strength and thermal conductivities were measured on conventional cores retrieved from the ulleung basin during the second ulleung basin gas hydrate expedition ubgh2 the ubgh2 logged 13 sites and cored 10 sites to locate potential sites for an offshore test production a total of 211 conventional cores were recovered undrained shear strength thermal conductivity and index properties including porosity and grain density were measured onboard mineral composition analyses and grain size analyses were completed onshore after the expedition the averaged porosity at each site ranges from 65 to 71 the averaged grain density at each site ranges from 2 57 to 2 66 g cm3 and the mean grain size mostly lies between 4 m and 25 m with sparsely scattered coarse grained intervals the relatively high porosity and low grain density are due to the large portion of diatomaceous sediments the thermal conductivities average around 0 8 w mk and the low porosity and the abundance of clay mineral and oapl a may have caused the relatively low thermal conductivity in the ulleung basin the geomechanical analyses revealed a few relevant findings important for sediment physical behaviors during gas hydrate production the particle migration in coarse grained layers during production necessitates proper measure for sand productions while fine grained layers are mostly self filtering the vertical deformation estimated from the compression index suggested that the subsidence induced by the pore pressure change from the depressurization is much higher than those induced by the lost of gas hydrate particles from dissociation massive vertical deformation in fine grained layers has been predicted to occur due to the pore pressure change from depressurization induced gas hydrate production the drilling mud weight induced pressure window is mostly determined by the water depth in deep sea drilling but caution should be taken in determination of fracture pressures in hydrate bearing sediments because gas hydrate saturations alter poisson s ratio

the taiwan telemetered seismographic network abstract in 1972 the taiwan telemetered seismographic network ttsn was installed to monitor earthquakes occurring in an area from 21 to 26 n latitude and from 119 to 123 e longitude the number of stations increased from six in 1972 to 24 in 1987 the stations are equipped with velocity type seismometers with a natural frequency of 1 hz since 1983 horizontal component sensors have been added at several stations signals recorded by the field stations are transmitted to taipei recording center through telephone lines or radio and then recorded on fm magnetic tapes paper and nine track magnetic tapes through a d converters hypocenters of earthquakes are routinely determined by a hypoc2 computer program the size of an earthquake is estimated from the signal duration moreover a simulated wood anderson seismographic station is in operation at taipei for the determination of local magnitude a seismic work system sws is being established for automatic data acquisition and data processing to avoid clipping of signal amplitude an optimum telemetry system is being linked to the ttsn

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

waveform analysis of two preshock main shock aftershock sequences in utah we use waveform cross correlation to study clustering of small earthquakes that occurred from 1981 to 1983 within 20 km of 1 an ml 4 3 main shock near salt lake city utah on 8 october 1983 and 2 an ml 4 0 main shock near richfield utah on 24 may 1982 no clusters of earthquakes were observed prior to the salt lake city event during the time period studied however a cluster of four nearly identical preshocks within 80 m of each other possibly representing the failure of a critical asperity occurred 5 km northwest of the richfield event during a 4 hour period 10 months before the main shock we found no similar clusters among the other preshocks within the richfield study area all of which were located more than 10 km from the main shock the aftershocks in both study regions during the first 17 days after the respective main shocks occupied areas smaller than the rupture areas estimated for the main shocks from initial pulse width measurements both aftershock sequences showed a spatial migration of the initial events that could only be discerned from the waveform data one possible explanation for this migration could be propagating stress changes caused by the occurrence of the aftershocks themselves or by postseismic creep

high resolution seismic profiling reveals faulting associated with the 1934 ms 6 6 hansel valley earthquake utah usa the 1934 ms 6 6 hansel valley utah earthquake produced an 8 km long by 3 km wide zone of north south trending surface deformation in an extensional basin within the easternmost basin and range province less than 0 5 m of purely vertical displacement was measured at the surface although seismologic data suggest mostly strike slip faulting at depth characterization of the origin and kinematics of faulting in the hansel valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension here we address three questions 1 how does the 1934 surface rupture compare with faults in the subsurface 2 are the 1934 fault scarps tectonic or secondary features 3 did the 1934 earthquake have components of both strike slip and dip slip motion to address these questions we acquired a 6 6 km long high resolution seismic profile across hansel valley including the 1934 ruptures we observed numerous east and west dipping normal faults that dip 40 70 and offset late quaternary strata from within a few tens of meters of the surface down to a depth of 1 km spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin our data clearly show complex basin faulting that is most consistent with transtensional tectonics although the kinematics of the 1934 earthquake remain underconstrained we interpret the disagreement between surface normal and subsurface strike slip kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities we infer that the 1934 earthquake occurred along an 3 km wide off fault damage zone characterized by distributed deformation along small displacement faults that may be alternatively activated during different earthquake episodes

changes in strain trajectories in three different types of plate tectonic boundary deduced from earthquake focal mechanisms abstract principal strain orientations minimum horizontal compression ex and maximum horizontal compression ey were established at three different types of plate tectonic boundary two transform faults an oceanic ridge located on the southeast indian ridge and a trench located close to the south sandwich archipelago to establish the strain patterns in each zone 104 earthquake focal mechanisms centroid moment tensor solutions for earthquakes with mb 4 harvard seismology data cmt were examined by fault population analysis despite the existence of only one tectonic process that controlled deformation in these zones divergence convergence or passive displacement and only one main strain tensor several coeval strain ellipsoids were found these differed from the main strain tensor in the location of the principal strains in general permutations were observed between the principal strains i e interchanges between the location of the principal strain axes maintaining the strain ellipsoids in the same 3d orientation only in some cases were changes in the ellipsoid orientation associated with major structures

characteristics of seismic energy release of subduction zones examples from taiwan abstract the characteristics of seismic energy releasing for oceanic subduction and continental collision or subduction zones were explored by a proposed observational approach based on contour maps of seismic energy release on both horizontal and vertical planes magnitude energy curve and magnitude energy time relationship it was found that distinct features of energy releasing behavior exist for the two types of subduction the oceanic subduction tends to have a more active background noise by continuously emitting small magnitude earthquakes and to release energy from a wider range of depth owing to the existence of subducted plate on the other hand surficial strata of a continental subduction tend to release almost all of the seismic energy through a major earthquake with relatively much less long term noise meanwhile several seismic gaps have been found based on the proposed release energy contour together with the tectonic framework and the regional geologic setting these gaps potentially may involve aseismic zones e g accretionary deposit or zones awaiting to release its accumulated energy e g the upper crust the quite zones prone to yielding major earthquakes are accordingly highlighted for a further study

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

delineation and interpretation of seismotectonic domains in western north america indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western north america the san andreas east central california ecc mendocino triple junction mtj and pacific northwest pnw domains are found to have similar stress fields although the style and rate of deformation in each is different we propose that all of these domains are generated by shear between the pacific and north american plates in the mtj and pnw domains this shear may be evidence for the initiation of a transform fault connection between the san andreas and the queen charlotte island fault systems the ecc a region including much of the sierra nevada appears to result from the relatively stiff nature of the san andreas fault system the big bend and region of relatively new faulting in northern california act to make the san andreas fault system appear stiff on a regional scale two other domains the northern basin and range nbr and the intermountain seismic belt isb show evidence of being controlled by more local forces the nbr has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the juan de fuca north america plate boundary the isb can be explained by readjustments between the colorado plateau and the basin and range province which may be caused by the lowering of the basin and range due to extension

shallow subsurface structure of the wasatch fault provo segment utah from integrated compressional and shear wave seismic reflection profiles with implications for fault structure and development integrated vibroseis compressional and experimental hammer source shear wave seismic reflection profiles across the provo segment of the wasatch fault zone in utah reveal near surface and shallow bedrock structures caused by geologically recent deformation combining information from the seismic surveys geologic mapping terrain analysis and previous seismic first arrival modeling provides a well constrained cross section of the upper 500 m of the subsurface faults are mapped from the surface through shallow poorly consolidated deltaic sediments and cutting through a rigid bedrock surface the new seismic data are used to test hypotheses on changing fault orientation with depth the number of subsidiary faults within the fault zone and the width of the fault zone and the utility of integrating separate elastic methods to provide information on a complex structural zone although previous surface mapping has indicated only a few faults the seismic section shows a wider and more complex deformation zone with both synthetic and antithetic normal faults our study demonstrates the usefulness of a combined shallow and deeper penetrating geophysical survey integrated with detailed geologic mapping to constrain subsurface fault structure due to the complexity of the fault zone accurate seismic velocity information is essential and was obtained from a first break tomography model the new constraints on fault geometry can be used to refine estimates of vertical versus lateral tectonic movements and to improve seismic hazard assessment along the wasatch fault through an urban area we suggest that earthquake hazard assessments made without seismic reflection imaging may be biased by the previous mapping of too few faults

on the inter relationship between grain size sensitive creep and dynamic recrystallization of olivine abstract the recent theoretical results of twiss 1976 and goetze 1978 suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain boundary sliding or nonlinear coble creep mechanisms this would result following the production of a fine grain size by dynamic recrystallization for the high temperature experimental work performed by karato et al 1982 dry single crystals of olivine were almost totally recrystallized during creep and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates however the activation energy for creep determined by karato et al 1982 was far higher than that predicted by the models of twiss 1976 or goetze 1978 although the conditions required for operation of at least the model of twiss 1976 apparently were satisfied the data for the highly recrystallized specimens from the higher stress lower temperature experiments of zeuch and green 1979 and zeuch 1980 are in good agreement with the results of karato et al 1982 these latter experiments were conducted under conditions where either the model of twiss 1976 or goetze 1978 should have been applicable i tentatively conclude that although fine grain sizes were produced in the experiments of karato et al 1982 zeuch and green 1979 and zeuch 1980 by dynamic recrystallization there is no evidence for a transition to grain boundary diffusional creep mechanisms at high or low stresses despite the predictions of twiss 1976 or goetze 1978 instead deformation is dominated by dislocation movement with recovery by dynamic recrystallization

a microearthquake study of southwest utah northwest arizona transition between the basin and range province and intermountain seismic belt abstract we conducted a microearthquake study of southwest utah northwest arizona during june and july of 1982 in order to better determine the variation of the stress field in what appears to be a transition between stress domains eleven single event and composite fault plane solutions were determined including two from a microearthquake swarm which occurred near enterprise utah the region can be divided into two domains defined by fault plane solutions the western and southern parts contain no intermountain seismic belt type fault plane solutions while the remaining area contains both the intermountain seismic belt and northern basin and range type fault plane solutions the mixed fault plane solutions may be caused by a low horizontal deviatoric stress which corresponds to the stress transition between the northern basin and range colorado plateau and structural complexities which may locally affect the stress field the average t axis direction determined from the fault plane solutions for the entire study area is approximately n65 w as compared to the minimum horizontal principal stress direction of n65 w inferred from holocene geologic indicators and n85 e calculated from seismic moment tensors found by other workers in addition we observed as have other workers in the intermountain seismic belt no relationship between seismicity and quaternary faulting finally the microearthquake swarm which is characteristic of southwest utah northwest arizona displayed two distinctly different fault plane solutions from events that migrated in time and space the two fault plane solutions are inferred to represent stress release by the earlier microearthquakes which thereby altered the local stress field

an integrated model for seismogenesis in the intermountain seismic belt abstract the idea that all late quaternary surface faults in the intermountain seismic belt isb are steeply dipping and penetrate from the surface to depths of 12 to 15 km may be an over simplification of the actual tectonic seismogenic process in the isb eastern basin and range transition zone studies of late quaternary faulting in southwestern wyoming and northcentral utah 125 km east of the wasatch fault zone indicate evolving tectonic structural relationships exist with time and location the hebgen lake and borah peak earthquakes represent a geologically mature stage of tectonic seismogenic development manifested by fault bounded mountain blocks and recurrent late quaternary fault movements normally reactivated thrust faults and listric normal faults represent an early to intermediate stage of deformation in preexisting thrust faulted terrains the degree to which preexisting structures unrelated to the modern stress field affect tectonic development is complex and a variety of structural relations may exist with time and location an incipient seismogenic fault in the early stages of extension may exist only as a subdecollement blind structure as extension continues blind seismogenic faults propagate upward eventually rupturing the surface as 45 to 60 planar structures normally reactivated thrust faults and listric normal faults characteristic of early to intermediate extension are tectonically beheaded and become inactive with propagation of planar seismogenic faults to the surface the seismic potential of normally reactivated thrust faults and listric normal faults is open to debate whether the seismic potential of low angle faults can be assessed from analysis of surface rupture parameters alone is a vitally important problem in seismic hazard assessment

false alarms and mine seismicity an example from the gentry mountain mining region utah as part of a large discrimination study in the western united states taylor et al 1989 one earthquake was consistently classified as an explosion the magnitude m 3 5 disturbance occurred on 14 may 1981 and was conspicuous in its lack of love waves relative lack of high frequency energy low lg pg ratio and high moment tensor additionally a moment tensor solution by patton and zandt 1991 indicated that the event had a large implosional component the event occurred in the gentry mountain coal mining region in the eastern wasatch plateau utah previous microearthquake studies in the region have demonstrated the existence of numerous small implosional events associated with the mining activities the event is more comparable in appearance at the broadband station at kanab utah knb to the atrisco collapse at the nevada test site nts than to another double couple event near the mining region using a simple source representation we modeled the event as a tabular excavation collapse that occurred as a result of normal mining activities this study raises the importance of having a good catalog of seismic data and information about mining activities from potential proliferant nations this information will be critical in resolving potential false alarms that may be generated from natural mine seismicity

extensional tectonics in northern utah southern idaho u s a and the 1934 hansel valley sequence abstract the hansel valley pocatello valley region of northern utah and southern idaho is one of the most seismically active areas of the eastern basin and range study of the seismicity of this region provides insight into the crustal processes taking place near the physiographic boundary between the basin and range and the middle rocky mountains this study presents source parameters determined for the march 12 1934 hansel valley utah mainshock and largest aftershock these results are then integrated with studies of regional and local seismicity to determine better the tectonic processes currently active in the region

regional relationships among earthquake magnitude scales various magnitude scales commonly used and the interrelationships among them are reviewed it is shown that problems exist with all of the magnitude scales being used in the united states when using regional catalogs for example it is often necessary to determine how the reported magnitudes were determined often such information is not available although the potential errors are quite large both the ms and the mb scales were designed to be universal scales however both ms and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales furthermore the ms magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs the mb magnitudes are more generally available than ms values however there is also much greater variation in the way mb is determined in particular a significant change in the mb scale occurred in the early 1960 s when the world wide standard seismograph network wwssn was established this change in instrumentation used to determine mb values had a significant effect on estimated magnitudes post 1960 values are lower and the saturation level of the mb scale the older longer period instruments recorded larger mb magnitudes than can be recorded with the wwssn instruments in addition great care must be taken when selecting the mb magnitudes of western u s earthquakes because the values are often in considerable error owing to the fact that they were determined at distances less than 25 and were not properly corrected for attenuation in the upper mantle or asthenosphere the seismic body wave magnitude mb of an earthquake is strongly affected by regional variations in the q structure composition and physical state within the earth therefore because of differences in attenuation of p waves between the western and eastern united states a problem arises when comparing mb values for the two regions a regional mb magnitude bias exists which depending on where the earthquake occurs and where the p waves are recorded can lead to magnitude errors as large as unit there is also a significant difference between mb and ml values for earthquakes in the western united states an empirical link between the mb of an eastern u s earthquake and the ml of an equivalent western earthquake is given by ml 0 57 0 92 mb east this result is important when comparing ground motion between the two regions and for choosing a set of real western u s earthquake records to represent eastern earthquakes

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

the pocatello valley idaho utah border earthquake sequence of march to april 1975 abstract an earthquake sequence during march and april 1975 which included a main shock of magnitude m l 6 0 on 28 march was centered in pocatello valley on the idaho utah border a foreshock of magnitude m l 4 2 preceded the main shock by 22 hr and was followed by at least 156 microearthquakes that behaved as aftershocks of the first event accurate locations of 587 aftershocks that occurred during the first 20 days after the main shock define irregular zones of concentrated activity beneath the south central western and northern parts of pocatello valley the main shock is interpreted to have involved nonuniform dipslip rupture without surface faulting on a previously unrecognized normal fault that dips nw obliquely transecting the north trending pocatello valley graben the speace time pattern of aftershocks and composite focal mechanisms indicate subsidiary faulting and the interaction of diverse fracture trends resulting in a mixture of normal strike slip and oblique faulting details of the earthquake sequence imply a complex episode of basin and range graben subsidence that may typify similar source regions elsewhere in utah9s wasatch front area

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

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

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

contemporary seismicity faulting and the state of stress in the colorado plateau the contemporary seismicity of the colorado plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude and contrary to earlier views of a low to moderate rate of occurrence with earthquakes widely distributed concentrations of earthquakes have been observed in a few areas of the plateau the most seismically active area of the colorado plateau is the eastern wasatch plateau book cliffs where abundant small magnitude seismicity is induced by coal mining the largest earthquakes observed to date of estimated richter magnitude m l 5 6 have generally occurred in northern arizona although very few earthquakes can be associated with known geologic structures or tectonic features in the colorado plateau seismicity appears to be the result of the reactivation of pre existing faults lacking surficial expression but favorably oriented to the tectonic stress field the small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted precambrian basement and upper crust and a moderate level of differential tectonic stress earthquakes in the plateau generally occur in the upper crust from the near surface to a depth of 15 20 km although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow the latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible the predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest to north northwest striking faults with some localized occurrences of strike slip faulting on north west or northeast striking planes at shallow depths the contemporary state of stress within the plateau is characterized by approximate northeast trending extension in contrast to the previous belief that the plateau was being subjected to east west tectonic compression one area of the plateau the eastern wasatch plateau and book cliffs may still be characterized by compressive stresses however the nature of these stresses is not well understood

characterization of intrabasin faulting and deformation for earthquake hazards in southern utah valley utah from high resolution seismic imaging we conducted active and passive seismic imaging investigations along a 5 6 km long east west transect ending at the mapped trace of the wasatch fault in southern utah valley using two dimensional 2d p wave seismic reflection data we imaged basin deformation and faulting to a depth of 1 4 km and developed a detailed interval velocity model for prestack depth migration and 2d ground motion simulations passive source microtremor data acquired at two sites along the seismic reflection transect resolve s wave velocities of approximately 200 m s at the surface to about 900 m s at 160 m depth and confirm a substantial thickening of low velocity material westward into the valley from the p wave reflection profile we interpret shallow 100 600 m bedrock deformation extending from the surface trace of the wasatch fault to roughly 1 5 km west into the valley the bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front further west in the valley the p wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the wasatch fault another inferred fault about 4 km west of the mapped wasatch fault displaces horizons within the valley to as shallow as 100 m depth the overall deformational pattern imaged in our data is consistent with the wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults as part of the evolution of an inferred 20 km wide half graben structure within utah valley finite difference 2d modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

observed variations of earthquake motion across a sediment filled valley the response to earthquake motion of a site in one sediment filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal s azimuth and incidence angles the observed input motion was weak varying between 10 5 and 10 3 g measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites the valley was 400 m wide and 700 m long with a maximum sediment thickness of 60 m the average seismic impedance contrast between the sediments and underlying basement rock was about 6 1 ratios of fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten depending strongly on frequency and the distance of the site from the valley edge the scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes ground motion at valley edge and mid valley sites separated by less than 100 m differed by as much as a factor of five this difference could cause large differential motion in structures spanning these sites a theoretical flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge the lowest frequency responance apparently involves the sediments across the width of the valley not just the vertical column of soil beneath each site

the enigma of the arthur s pass new zealand earthquake 2 the aftershock distribution and its relation to regional and induced stress fields the aftershock distribution of the 1994 arthur s pass earthquake mw6 7 is unusual for a reverse faulting event in that it extends 12 km nnw and 30 km sse of the actual fault plane which strikes ne sw we have used several methods to infer the regional stress field in the region including geodetic results earthquake mechanisms and inversion of p wave polarity data for the stress tensor orientation the inversion method is new and does not require the focal mechanisms of the events used it also incorporates the coulomb failure criterion all results point to a stress field favoring strike slip faulting not thrusting with near horizontal 1 and 3 principal axes striking at 298 and 28 using dislocation theory we calculate the stress induced by the arthur s pass earthquake and its largest aftershock a strike slip event and add this to the regional field there is a fair correspondence between the hypocenters of aftershocks away from the mainshock fault plane and regions of high induced coulomb failure stress cfs on optimally oriented fault planes however there are regions of high induced cfs that are devoid of aftershocks it appears that earthquake slip in this region of oblique 19 plate convergence is as observed elsewhere partitioned into components parallel and perpendicular to the plate margin most of the slip is parallel as occurs on the nearby dextral alpine fault the boundary between the pacific and australian plates however occasional reverse events such as the arthur s pass earthquake account for at least some of the perpendicular component of slip and the uplift that produced the southern alps

the sea99 ground motion predictive relations for extensional tectonic regimes revisions and a new peak ground velocity relation the values of 3 listed in table 1 of pankow and pechmann 2004 are too large by a factor of 2 because they were

local magnitude and sediment amplification observations from earthquakes in the northern baja california southern california region abstract the data set that is analyzed in this study consists of strong motion and digital velocity records obtained during the last 6 yr for earthquakes along faults in baja california north and southern california from this set local magnitudes for 60 earthquakes recorded at stations on hard rock and sediments are calculated following the technique proposed by kanamori and jennings 1978 the calculated near source motion magnitudes m lsm are then compared with reported local magnitude values m l which are in the range from 3 to 6 6 from these comparisons it is found that for earthquakes with magnitude m l between 3 and 5 5 almost all the m lsm values estimated from data registered on sediments exceed the m l values by amounts which at the shorter distances less than about 10 km may be as high as an order of magnitude or even higher for a few cases the strong motion records associated with these high m lsm values have highly coherent pulse like s wave signals of very short period and large accelerations similar estimations of local magnitude from more limited data obtained on hard rock were comparable or lower than the standard m l values we conclude that the high m lsm values are the result of a combination of high frequency source spectra and a strong shear wave energy amplification due to the presence of sediments a sediment amplification factor of 3 2 is inferred from the data and is in good agreement with a theoretical factor of 3 4 estimated on the basis of the mcmechan and mooney 1980 velocity model for the imperial valley it is also found from the analyzed data that near source local magnitude decreases between 0 and 10 km indicating that for smaller earthquakes the attenuation function log a 0 richter 1958 used in local magnitude determinations needs to be modified to take into account the characteristics of the seismic energy radiated at short distances near source strong motion data for the 1979 imperial valley m l 6 6 and 1980 victoria m l 6 1 events do not show the strong discrepancy between m lsm and m l observed for the smaller magnitude events factors that might be responsible for this include nonlinear sediment attenuation for larger amplitude waves as well as complex near source spreading out of the waveform resulting from larger source size near source magnitude saturation we give a numerical example using small earthquakes as green9s functions to show that for a complex source function represented as a superposition of smaller earthquakes m lsm and m l for the 1980 victoria earthquake can be explained by the magnitude saturation phenomenon

variation of the regional stress tensor at the western great basin boundary from the inversion of earthquake focal mechanisms the orientation of the principal axes of the regional stress tensor in and near the western great basin boundary is calculated from the inversion of earthquake focal mechanism data data were taken from three regions along the western great basin sierra nevada boundary mammoth lakes round valley and chalfant valley and at two regions within the great basin interior mono walker lake and west central nevada the depth range is similar for all data earthquakes are essentially restricted to the uppermost 16 km of the crust the stress tensor changes from the great basin boundary to the basin interior for example the extension direction changes by about 50 from roughly e w at the boundary in the west to roughly nw se inside of the basin in the boundary region the mean t axis inferred from focal mechanisms rotates clockwise by 27 from n64 e in the mammoth lakes area to n91 e in the chalfant valley about 50 km southeast of mammoth lakes however the components of the best fitting stress tensors in the two regions vary only by 12 so that a common stress tensor with a greatest principal stress azimuth of n10 w and a least principal stress azimuth of n80 e both components nearly horizontal favoring strike slip describes the seismic deformation of the boundary region quite well the stress tensors for the mono region and west central nevada have similar greatest and least principal stress azimuths northeasterly and northwesterly respectively however the plunges of the greatest principal stress are different nearly horizontal for the mono area and about 45 for central nevada reflecting the strong contribution of normal faulting in the central nevada earthquake data the resulting values which describe the relative sizes of the three stress components vary from 0 5 to 0 65 which indicates that the magnitudes of the three principal stresses are of similar size