sequence stratigraphy depositional history and middle to late quaternary sea levels of the ventura shelf california abstract high resolution shallow seismic profiles from the ventura shelf reveal two contrasting structurally controlled styles of deposition on the northern shelf folded plio pleistocene strata comprising the south limb of the ventura avenue anticline are unconformably overlain by undeformed late quaternary strata in contrast a thicker section of undeformed prograding deltaic sequences preserved on the subsiding southern shelf contains a detailed record of at least five middle and late quaternary glacioeustatic sea level oscillations a transgressive surface at 140 m in upper pleistocene saugus deposits is overlain by transgressive and highstand systems tracts a shelf margin systems tract subsequently developed as the sea level lowered marking the end of saugus deposition thereafter over 50 m of post saugus upper pleistocene sediment accumulated during a period of smaller scale sea level oscillations followed by the development of a pair of marine terraces presently at 50 and 33 m and a large fluvial channel now buried at 113 m correlations of saugus and terrace marker horizons with the middle to late quaternary paleosea level curve suggest development of the saugus marker horizons during oxygen isotopic stages 7 5e and formation of the terraces during wisconsinan high sea levels maximum rates of vertical separation on the oak ridge and pitas point faults are estimated to be 0 13 and 0 30 m 1000 yr respectively

seismicity triggered by fluid injection induced aseismic slip anthropogenic fluid injections are known to induce earthquakes the mechanisms involved are poorly understood and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited we directly measure fault slip and seismicity induced by fluid injection into a natural fault we observe highly dilatant and slow 4 micrometers per second m s aseismic slip associated with a 20 fold increase of permeability which transitions to faster slip 10 m s associated with reduced dilatancy and micro earthquakes most aseismic slip occurs within the fluid pressurized zone and obeys a rate strengthening friction law 0 67 0 045ln v v with v 0 1 m s fluid injection primarily triggers aseismic slip in this experiment with micro earthquakes being an indirect effect mediated by aseismic creep

high rate injection is associated with the increase in u s mid continent seismicity an unprecedented increase in earthquakes in the u s mid continent began in 2009 many of these earthquakes have been documented as induced by wastewater injection we examine the relationship between wastewater injection and u s mid continent seismicity using a newly assembled injection well database for the central and eastern united states we find that the entire increase in earthquake rate is associated with fluid injection wells high rate injection wells 300 000 barrels per month are much more likely to be associated with earthquakes than lower rate wells at the scale of our study a well s cumulative injected volume monthly wellhead pressure depth and proximity to crystalline basement do not strongly correlate with earthquake association managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes

reach scale channel geometry of mountain streams abstract the basic patterns and processes of steep channels remain poorly known relative to lower gradient channels in this analysis characteristics of step pool plane bed and pool riffle channels are examined using a data set of 335 channel reaches from the western united states nepal new zealand and panama we analyzed differences among the three channel types with respect to hydraulics channel geometry boundary roughness and bedforms step pool channels have significantly steeper gradients coarser substrate higher values of shear stress and stream power for a given discharge and larger ratios of bedform amplitude wavelength h l pool riffle channels have greater width depth ratios and relative grain submergence r d84 than the other channel types plane bed channels tend to have intermediate values for most variables examined relative form submergence r h is statistically similar for step pool and pool riffle channels despite the lesser relative grain submergence and greater bedform amplitude of step pool channels mean values of darcy weisbach friction factor do not change in response to changes in relative grain submergence these patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types

bankfull discharge recurrence interval in gravel bed rivers bankfull discharge was identified in some 30 gravel bed rivers representing in total c 40 gauging stations the catchment sizes vary from 4 km 2 to nearly 2700 km 2 bankfull discharge value increases with basin size in the case of gravel bed rivers developed on an impermeable substratum the following equation emerges q b 0 087 a 1 044 bankfull discharge recurrence interval was determined by fitting maximum annual floods t a into gumbel s distribution and then using the partial duration series t p in this same distribution recurrence interval is below 0 7 years t p for small pebble bed rivers developed on an impermeable substratum it reaches 1 1 to 1 5 years when the catchment size of these rivers exceeds 250 km 2 rivers incised in the soft schists of the famenne show larger channel capacity at bankfull stage a small width depth ratio and thus higher recurrence intervals 1 4 5 3 years with t a and 1 4 4 years with t p baseflow dominated gravel bed streams and sandy or silty rivers experience less frequent bankfull discharges with a recurrence interval higher than 2 or even 3 years t p 1997 by john wiley sons ltd

regional bankfull geometry relationships for southern california mountain streams and hydrologic applications abstract this study develops and intercompares regional relationships for bankfull channel width hydraulic depth and cross sectional area for southern california mountain streams based on several data sources surveyed streams us geological survey stream survey reports and existing literature although considerable uncertainty exists in estimating bankfull conditions the relationships developed from the varying data sources show significant agreement for small watersheds with drainage area ranging from 15 to 2000 km 2 the estimates of bankfull top width ranged from 7 2 to 44 5 m and hydraulic depth estimates ranged from 0 35 to 1 15 m the utility of the developed bankfull geometry regional curves is demonstrated for southern california catchments through a the computation of the bankfull discharge and b the estimation of the surface runoff response necessary to produce bankfull conditions in the streams at the outlet of these catchments for selected locations with instantaneous flow records the occurrence frequency of events exceeding bankfull flow was examined for the available 10 15 year span of observational records bankfull discharge estimates for all small watersheds in the region ranged from 1 3 to 74 m 3 s while the range at the selected gauged stream locations was from 2 6 to 16 4 m 3 s stream locations along the transverse mountains of southern california showed an average occurrence frequency of less than 1 year whereas along the peninsular mountains the average return period tended to be greater than 1 year the application of the regional curves to the estimation of the surface runoff response necessary to produce bankfull conditions at the channel outlets of small catchments may be used as an index for conditions of minor flooding with saturated soils this surface runoff response index ranges from 2 0 to 5 5 mm for a 3 hour rainfall duration for southern california watersheds greater than 15 km 2 in area differences between the values for the peninsular and transverse mountain ranges are linked to geological climatic and geomorphologic differences the developed regional geometry relationships are suitable for use in various hydrologic modeling applications including distributed modeling with high resolution pertinent to flash flood forecasting

finite fault slip model of the 2011 mw 5 6 prague oklahoma earthquake from regional waveforms the slip model for the 2011 mw 5 6 prague oklahoma earthquake is inferred using a linear least squares methodology waveforms of six aftershocks recorded at 21 regional stations are used as empirical green s functions egfs the solution indicates two large slip patches one located around the hypocenter with a depth range of 3 5 5 km the other located to the southwest of the epicenter with a depth range from 7 5 to 9 5 km the total moment of the solution is estimated at 3 37 1024 dyne cm mw 5 65 the peak slip and average stress drop for the source at the hypocenter are 70 cm and 90 bars respectively approximately one half the values for the mw 5 8 2011 mineral virginia earthquake the stress drop averaged over all areas of slip is 16 bars the relatively low peak slip and stress drop may indicate an induced component in the origin of the prague earthquake from deep fluid injection

reach scale channel geometry of a mountain river mountain rivers can be subject to strong constraints imposed by changes in gradient and grain size supplied by processes such as glaciation and rockfall nonetheless adjustments in the channel geometry and hydraulics of mountain rivers at the reach scale can produce discernible patterns analogous to those in fully alluvial rivers mountain rivers can differ in that imposed reach scale gradient is an especially important control on reach scale channel characteristics as indicated by examination of north st vrain creek in colorado north st vrain creek drains 250 km2 of the rocky mountains we used 25 study reaches within the basin to examine controls on reach scale channel geometry variables measured included channel geometry large woody debris grain size and mean velocity drainage area at the study reaches ranged from 2 2 to 245 km2 and gradient from 0 013 to 0 147 m m 1 we examined correlations among 1 potential reach scale response variables describing channel bankfull dimension and shape hydraulics bedform wavelength and amplitude grain size ow resistance standard deviation of hydraulic radius and volume of large woody debris and 2 potential control variables that change progressively downstream drainage area discharge or that are likely to re ect a reach speci c control bed gradient we tested the hypothesis that response variables correlate most strongly with local bed gradient because of the segmented nature of mountain channels results from simple linear regression analyses indicate that most response variables correlate best with gradient although channel width and width depth ratio correlate best with discharge multiple regression analyses using mallow s cp selection criterion and log transformation of all variables produced similar results in that most response variables correlate strongly with gradient these results suggest that the hypothesis is partially supported channel bed gradient is likely to be a good predictor for many reach scale response variables along mountain rivers but discharge is also an important predictor for some response variables copyright 2004 john wiley sons ltd

bankfull discharge in mountain streams theory and practice the results are presented of an investigation of bankfull discharge in two polish carpathian streams skawica and krzyworzeka existing definitions of river bankfull were reviewed and applied in tests carried out on selected cross sections of the streams the woodyer method was given special attention with a correspondingly detailed survey of plants characterizing river benches riley s bench index method and the methods of williams wolman schumm and brown and woloszyn were tested the report concludes that bankfull discharge value for a mountain stream should not be reported as a single number but rather as a range of discharges within which one could expect the bankfull value to lie copyright 2002 john wiley sons ltd

locations of injection induced earthquakes in oklahoma controlled by crustal structures in recent years many small to moderate size earthquakes occurred in central northern oklahoma likely associated with wastewater injection the most recent one is the m5 8 pawnee earthquake occurred on 3 september 2016 it is still not clear what controls the locations of these injection induced earthquakes here we conduct 2 d pg wave tomography with anisotropy to image seismogenic structures in this region using more than 10 years of pg arrivals recorded by 81 seismic stations our high resolution pg wave tomography shows two high velocity zones with northwest fast direction alternated by low velocity zones with northeast fast direction the two dominant anisotropy directions are consistent with conjugated microfractures from surface faults and two nodal planes from focal mechanisms of moderate size earthquakes most moderate size m 4 earthquakes occurred either close to the boundaries between high and low seismic velocity zones or within the high velocity zones suggesting that they are associated with geological boundaries of different basement rock properties or with strong material properties in the upper crust we suggest that although these moderate size earthquakes were induced by fluid injection their spatial locations were likely controlled by local geologic structures

downstream variation in bankfull width of wadeable streams across the conterminous united states abstract bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology fluvial geomorphology and stream ecology we developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area a w a dhg wa for nine aggregate ecoregions comprising the conterminous united states using 1588 sites from the u s environmental protection agency s national wadeable streams assessment wsa including 1152 sites from a randomized probability survey sample sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10 000 km 2 in drainage area the dhg wa exponent which expresses the rate at which bankfull stream width scales with drainage area fell into three distinct clusters ranging from 0 22 to 0 38 width increases more rapidly with basin area in the humid eastern highlands encompassing the northern and southern appalachians and the ozark mountains and the upper midwest great lakes region than for the west both mountainous and xeric areas the southeastern coastal plain and the northern plains the dakotas and montana stream width increases least rapidly with basin area in the temperate plains cornbelt and southern plains great prairies in the heartland the coefficient of determination r 2 was least in the noncoastal plains 0 36 0 41 and greatest in the appalachians and upper midwest 0 68 0 77 dhg wa equations differed between streams with dominantly fine bed material silt sand and those with dominantly coarse bed material gravel cobble boulder in six of the nine analysis regions where dhg wa equations varied by sediment size fine bedded streams were consistently narrower than coarse bedded streams within the western mountains ecoregion where there were sufficient sites to develop dhg wa relationships at a finer spatial scale and ranged from 1 23 to 3 79 and 0 23 to 0 40 respectively with r 2 0 50 for 10 of 13 subregions range 0 36 to 0 92 enhanced dhg equations incorporating additional data for three landscape variables that can be derived from gis mean annual precipitation elevation and mean reach slope significantly improved equation fit and predictive value in several regions most notably the western mountains and the temperate plains channel width was also related to human disturbance we examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance contrary to our expectations the data suggest that the dominant response of channel width to human disturbance in the united states is a reduction in bankfull width in streams with greater disturbance particularly in the western mountains where population density road density agricultural land use and local riparian disturbance were all negatively related to channel width and in the appalachians and new england where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width

limits of downstream hydraulic geometry adjustments to flow width depth and velocity in response to changes in discharge are commonly characterized by using downstream hydraulic geometry relationships the spatial limits of these relationships within a drainage basin have not been systematically quantified where the erosional resistance of the channel substrate is sufficiently large hydraulic driving forces presumably will be unable to adjust channel form data sets from 10 mountain rivers in the united states panama nepal and new zealand are used in this study to explore the limits of downstream hydraulic geometry relationships where the ratio of stream power to sediment size d 84 exceeds 10 000 kg s3 downstream hydraulic geometry is well developed where the ratio falls below 10 000 kg s3 downstream hydraulic geometry relationships are poorly developed these limitations on downstream hydraulic geometry have important implications for channel engineering and simulations of landscape change

monitoring the earthquake source process in north america with the implementation of the usgs national earthquake information center prompt assessment of global earthquakes for response system pager rapid determination of earthquake moment magnitude is essential especially for earthquakes that are felt within the contiguous united states we report an implementation of moment tensor processing for application to broad seismically active areas of north america this effort focuses on the selection of regional crustal velocity models codification of data quality tests and the development of procedures for rapid computation of the seismic moment tensor we systematically apply these techniques to earthquakes with reported magnitude greater than 3 5 in continental north america that are not associated with a tectonic plate boundary using the 0 02 0 10 hz passband we can usually determine with few exceptions moment tensor solutions for earthquakes with m w as small as 3 7 the threshold is significantly influenced by the density of stations the location of the earthquake relative to the seismic stations and of course the signal to noise ratio with the existing permanent broadband stations in north america operated for rapid earthquake response the seismic moment tensor of most earthquakes that are m w 4 or larger can be routinely computed as expected the nonuniform spatial pattern of these solutions reflects the seismicity pattern however the orientation of the direction of maximum compressive stress and the predominant style of faulting is spatially coherent across large regions of the continent

coseismic slip and early afterslip of the 2015 illapel chile earthquake implications for frictional heterogeneity and coastal uplift great subduction earthquakes are thought to rupture portions of the megathrust where interseismic coupling is high and velocity weakening frictional behavior is dominant releasing elastic deformation accrued over a seismic cycle conversely postseismic afterslip is assumed to occur primarily in regions of velocity strengthening frictional characteristics that may correlate with lower interseismic coupling however it remains unclear if fixed frictional properties of the subduction interface coseismic or aftershock induced stress redistribution or other factors control the spatial distribution of afterslip here we use interferometric synthetic aperture radar and global position system observations to map the distribution of coseismic slip of the 2015 mw 8 3 illapel chile earthquake and afterslip within the first 38 days following the earthquake we find that afterslip overlaps the coseismic slip area and propagates along strike into regions of both high and moderate interseismic coupling the significance of these observations however is tempered by the limited resolution of geodetic inversions for both slip and coupling additional afterslip imaged deeper on the fault surface bounds a discrete region of deep coseismic slip and both contribute to net uplift of the chilean coastal cordillera a simple partitioning of the subduction interface into regions of fixed frictional properties cannot reconcile our geodetic observations instead stress heterogeneities either preexisting or induced by the earthquake likely provide the primary control on the afterslip distribution for this subduction zone earthquake we also explore the occurrence of coseismic and postseismic coastal uplift in this sequence and its implications for recent hypotheses concerning the source of permanent coastal uplift along subduction zones

in situ stress and active faulting in oklahoma abstract appreciable injection induced seismicity has been occurring in north central oklahoma since 2009 to better understand these earthquakes we have compiled new information on the state of stress in the state to compare it with both mapped faults and faults inferred from earthquake epicenters and focal plane mechanisms seventy five new in situ stress orientations are available from wellbores throughout the state in the north central part of the state where the induced seismicity is occurring stress orientation and relative magnitude from focal mechanism inversions show excellent agreement with the wellbore stress orientations all of the data show remarkably uniform stress directions the azimuth of s hmax the maximum horizontal stress is about n85 5 e strike slip faulting is occurring in central oklahoma with strike slip normal and normal faulting observed in northern oklahoma and southern kansas as very few of the thousands of m 2 5 earthquakes that have recently occurred are located on or near already mapped faults we utilize the stress information to interpret the likely fault planes associated with over 300 well constrained focal plane mechanisms in the vicinity of the january 2016 sequence of magnitude 4 and 5 earthquakes in the fairview oklahoma region we illustrate how knowledge of the stress field can be used to identify the faults responsible for the seismicity and better evaluate the potential hazard associated with possible future earthquakes in the area the september 2016 m w 5 8 pawnee event occurred on an unmapped n70 w trending strike slip fault expected to be active in the local stress field

induced earthquake magnitudes are as large as statistically expected a major question for the hazard posed by injection induced seismicity is how large induced earthquakes can be are their maximum magnitudes determined by injection parameters or by tectonics deterministic limits on induced earthquake magnitudes have been proposed based on the size of the reservoir or the volume of fluid injected however if induced earthquakes occur on tectonic faults oriented favorably with respect to the tectonic stress field then they may be limited only by the regional tectonics and connectivity of the fault network in this study we show that the largest magnitudes observed at fluid injection sites are consistent with the sampling statistics of the gutenberg richter distribution for tectonic earthquakes assuming no upper magnitude bound the data pass three specific tests 1 the largest observed earthquake at each site scales with the log of the total number of induced earthquakes 2 the order of occurrence of the largest event is random within the induced sequence and 3 the injected volume controls the total number of earthquakes rather than the total seismic moment all three tests point to an injection control on earthquake nucleation but a tectonic control on earthquake magnitude given that the largest observed earthquakes are exactly as large as expected from the sampling statistics we should not conclude that these are the largest earthquakes possible instead the results imply that induced earthquake magnitudes should be treated with the same maximum magnitude bound that is currently used to treat seismic hazard from tectonic earthquakes

hydraulic geometry of stream reaches abstract variation in river hydraulic geometry throughout stream networks is central to the problems of catchment management because of its influence on flow and sediment routing physical habitats and channel floodplain interactions in its simplest form at a station hydraulic geometry is described by power functions relating the surface width water depth and mean velocity at a channel cross section to discharge this paper extends hydraulic geometry analysis to represent the mean and variation of hydraulic characteristics along a river reach as power functions of discharge using 54 surveys of 17 river reaches mostly in south east australia it is shown that five reach hydraulic variables are sufficient to characterise the gross hydraulic conditions along a river reach the means and coefficients of variation of surface width and hydraulic depth plus the coefficient of variation of cross sectional velocity exponents for the mean width and mean depth hydraulic geometry relations are sensitive to measurement and cross section location and may best be treated as constant for regional studies coefficients for the mean width and mean depth hydraulic geometry relations are correlated positively with mean discharge and negatively with reach gradient respectively further analysis of variations in reach hydraulic geometry parameters particularly the power function coefficients in relation to river type flow parameters sediment load bank material and bank vegetation is warranted reach hydraulic geometry relations show considerable potential for improving analyses of fluvial processes and physical habitats across catchments

injection induced seismicity poroelastic and earthquake nucleation effects the standard model of injection induced seismicity considers changes in coulomb strength due solely to changes in pore pressure we consider two additional effects full poroelastic coupling of stress and pore pressure and time dependent earthquake nucleation we model stress and pore pressure due to specified injection rate in a homogeneous poroelastic medium stress and pore pressure are used to compute seismicity rate through the dieterich 1994 model for constant injection rate the time to reach a critical seismicity rate scales with t r2 cfc where r is distance from the injector c is hydraulic diffusivity and fc is a factor that depends on mechanical properties and weakly on r the seismicity rate decays following a peak consistent with some observations during injection poroelastic coupling may increase or decrease the seismicity rate depending on the orientation of the faults relative to the injector if injection induced stresses inhibit slip abrupt shut in can lead to locally sharp increases in seismicity rate tapering the flux mitigates this effect the maximum magnitude event has been observed to occur postinjection we suggest the seismicity rate at a given magnitude depends on the nucleation rate the size distribution of fault segments and if the background shear stress is low the time varying volume of perturbed crust this leads to a rollover in frequency magnitude distribution for larger events with a corner that increases with time larger events are absent at short times but approach the background frequency with time larger events occurring post shut in are thus not unexpected

rupture model of the 2011 mineral virginia earthquake from teleseismic and regional waveforms 1 we independently invert teleseismic p waveforms and regional crustal phases to examine the finite fault slip model for the 2011 mw 5 8 mineral virginia earthquake theoretical and empirical green s functions are used for the teleseismic and regional models respectively both solutions show two distinct sources each about 2 km across and separated by 2 5 km the source at the hypocenter is more localized in the regional model leading to a higher peak slip of 130 cm and higher average stress drop of 250 bars compared with 86 cm and 150 bars for the same source in the teleseismic model both sources are centered at approximately 8 km depth in the regional model largely below the aftershock distribution in the teleseismic model the sources extend updip to approximately 6 km depth into the depth range of the aftershocks the rupture velocity is not well resolved but appears to be near 2 7 km s

finite fault source inversion using teleseismic p waves simple parameterization and rapid analysis we examine the ability of teleseismic p waves to provide a timely image of the rupture history for large earthquakes using a simple 2d finite fault source parameterization we analyze the broadband displacement waveforms recorded for the 2010 m w 7 darfield new zealand and el mayor cucapah baja california earthquakes using a single planar fault with a fixed rake both of these earthquakes were observed to have complicated fault geometries following detailed source studies conducted by other investigators using various data types our kinematic finite fault analysis of the events yields rupture models that similarly identify the principal areas of large coseismic slip along the fault the results also indicate that the amount of stabilization required to spatially smooth the slip across the fault and minimize the seismic moment is related to the amplitudes of the observed p waveforms and can be estimated from the absolute values of the elements of the coefficient matrix this empirical relationship persists for earthquakes of different magnitudes and is consistent with the stabilization constraint obtained from the l curve in tikhonov regularization we use the relation to estimate the smoothing parameters for the 2011 m w 7 1 east turkey 2012 m w 8 6 northern sumatra and 2011 m w 9 0 tohoku japan earthquakes and invert the teleseismic p waves in a single step to recover timely preliminary slip models that identify the principal source features observed in finite fault solutions obtained by the u s geological survey national earthquake information center usgs neic from the analysis of body and surface wave data these results indicate that smoothing constraints can be estimated a priori to derive a preliminary first order image of the coseismic slip using teleseismic records

likelihood testing of seismicity based rate forecasts of induced earthquakes in oklahoma and kansas likelihood testing of induced earthquakes in oklahoma and kansas has identified the parameters that optimize the forecasting ability of smoothed seismicity models and quantified the recent temporal stability of the spatial seismicity patterns use of the most recent 1 year period of earthquake data and use of 10 20 km smoothing distances produced the greatest likelihood the likelihood that the locations of january june 2015 earthquakes were consistent with optimized forecasts decayed with increasing elapsed time between the catalogs used for model development and testing likelihood tests with two additional sets of earthquakes from 2014 exhibit a strong sensitivity of the rate of decay to the smoothing distance marked reductions in likelihood are caused by the nonstationarity of the induced earthquake locations our results indicate a multiple fold benefit from smoothed seismicity models in developing short term earthquake rate forecasts for induced earthquakes in oklahoma and kansas relative to the use of seismic source zones

oklahoma experiences largest earthquake during ongoing regional wastewater injection hazard mitigation efforts the 3 september 2016 mw 5 8 pawnee earthquake was the largest recorded earthquake in the state of oklahoma seismic and geodetic observations of the pawnee sequence including precise hypocenter locations and moment tensor modeling shows that the pawnee earthquake occurred on a previously unknown left lateral strike slip basement fault that intersects the mapped right lateral labette fault zone the pawnee earthquake is part of an unprecedented increase in the earthquake rate in oklahoma that is largely considered the result of the deep injection of waste fluids from oil and gas production if this is indeed the case for the m5 8 pawnee earthquake then this would be the largest event to have been induced by fluid injection since 2015 oklahoma has undergone wide scale mitigation efforts primarily aimed at reducing injection volumes thus far in 2016 the rate of m3 and greater earthquakes has decreased as compared to 2015 while the cumulative moment or energy released from earthquakes has increased this highlights the difficulty in earthquake hazard mitigation efforts given the poorly understood long term diffusive effects of wastewater injection and their connection to seismicity

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

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture