late cenozoic tectonics of the east ventura basin transverse ranges california the east ventura basin originated in the middle miocene as a rift system bounded on one side by the oak ridge simi hills structural shelf and on the other side by a granitic ridge parallel to the san gabriel fault this fault began accumulating right slip 10 12 m y ago at a rate of 4 5 9 mm yr depending on whether total slip is 45 or 60 km slowing to about 1 mm yr in the quaternary north of the santa clara river rifting ended prior to deposition of the uppermost miocene lower pliocene towsley formation south of the santa clara river the rift axis shifted southwest toward the oak ridge simi hills shelf as the towsley formation accumulated against a normal fault ancestor of the santa susana fault a change to contractile tectonics occurred in the pliocene with depos tion of the fernando formation when the newhall potrero anticline developed as a monocline above a blind reverse fault the pico anticline to the southeast and the temescal and hopper ranch modelo anticlines to the northwest may have a similar origin tectonic inversion and displacement on the southwest verging santa susana fault began about 0 5 ma based on appearance of locally derived clasts in the upper saugus formation and its equivalents and continues today along with the southwest verging san cayetano fault farther west also active are northeast verging backthrusts occurring in the east ventura basin fold belt and a segment of the san gabriel fault which now acts as a northeast dipping oblique slip reverse fault northeast trending discontinuities and structures divide the present deformation zone into four segments in the hopper canyon segment at the west end of the area in the west ventura basin the san cayetano fault places miocene modelo formation over pliocene pleistocene strata more than 5 km thick largely at maximum burial to the southeast the newhall potrero segment is characterized by north vergent backthrusts within the east ventura fold belt and by southward thrusting of the basin sequence tectonic inversion over the structural shelf on the santa susana fault farther southeast the placerita segment is marked by reverse faulting on both the santa susana fault and the san gabriel fault southeast of the basin in the san fernando valley the sylmar segment contains a thick pli cene pleistocene sequence overridden by basement rocks of the san gabriel mountains as well as the south verging mission hills granada hills and northridge hills fault zones

seismically active fold and thrust belt in the san joaquin valley central california structural analysis of late cenozoic folds along the western and southern margins of the san joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics the folds have formed by the processes of fault bend folding and fault propagation folding which commonly occur in fold and thrust belts our structural interpretation attributes the seismically active coalinga and kettleman hills north dome anticlines to fault bend folding above a thrust fault s that steps up from a detachment within the franciscan assemblage to a detachment at the base of the great valley group this thrust does not reach the surface blind thrust and its slip is consumed in backthrusting and formation of subsurface folds under the san joaquin valley movement on the postulated thrust s would explain the cause of the may 2 1983 coalinga earthquake and the august 15 1985 avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes well documented examples of fault bend and fault propagation folding also occur at wheeler ridge in the san emigdio mountains and at kettleman hills south dome deformation associated with fold and thrust belts can be extremely complicated considerable fault movement can occur without accompanying surface rupture or shallow level faulting conversely surface rupture can occur that has no direct relationship to fault slip at depth an example would be flexural slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth consequently traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts our interpretation of the structural style that developed in central california during the late cenozoic requires that the strain associated with the transpressive motion between the pacific and north american plates be resolved into normal and tangential components thrust faulting and folding normal to the plate boundary and strike slip faulting parallel to the plate boundary san andreas fault the thrust faults root in a deecollement at the brittle ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction

estimation and applicability of attenuation characteristics for source parameters and scaling relations in the garhwal kumaun himalaya region india abstract source parameters of the small to moderate earthquakes are significant for understanding the dynamic rupture process the scaling relations of the earthquakes and for assessment of seismic hazard potential of a region in this study the source parameters were determined for 58 small to moderate size earthquakes 3 0 m w 5 0 occurred during 2007 2015 in the garhwal kumaun region the estimated shear wave quality factor q f values for each station at different frequencies have been applied to eliminate any bias in the determination of source parameters the q f values have been estimated by using coda wave normalization method in the frequency range 1 5 16 hz a frequency dependent s wave quality factor relation is obtained as q f 152 9 7 f 0 82 0 005 by fitting a power law frequency dependence model for the estimated values over the whole study region the spectral low frequency spectral level and corner frequency and source static stress drop seismic moment apparent stress and radiated energy parameters are obtained assuming 2 source model the displacement spectra are corrected for estimated frequency dependent attenuation site effect using spectral decay parameter kappa the frequency resolution limit was resolved by quantifying the bias in corner frequencies stress drop and radiated energy estimates due to finite bandwidth effect the data of the region shows shallow focused earthquakes with low stress drop the estimation of zuniga parameter e suggests the partial stress drop mechanism in the region the observed low stress drop and apparent stress can be explained by partial stress drop and low effective stress model presence of subsurface fluid at seismogenic depth certainly manipulates the dynamics of the region however the limited event selection may strongly bias the scaling relation even after taking as much as possible precaution in considering effects of finite bandwidth attenuation and site corrections although the scaling can be improved further with the integration of large dataset of microearthquakes and use of a stable and robust approach

slip distribution of the 1997 umbria marche earthquake sequence joint inversion of gps and synthetic aperture radar interferometry data 1 in september october 1997 a seismic sequence struck the southern part of the umbria marche region in central italy the three largest earthquakes took place on 26 september 0033 ut mw 5 7 09 40 gu mw 6 0 near colfiorito and on 14 october mw 5 6 near sellano we present a slip model for the main sequence over two separate fault planes we use previously published global positioning system gps and differential interferometric synthetic aperture radar dinsar data as well as new dinsar data covering the sellano event as input to a simulated annealing solution for the distribution of fault slip the combination of both types of geodetic data as well as the combination of sar interferograms from both ascending and descending satellite tracks allows us to make a more complete geodetically based assessment of the fault slip for these earthquakes for the solutions presented here we required the slip distribution to match the observed surface displacements while maintaining a total scalar seismic moment approximately equal to the seismologically inferred moments for this sequence we find the slip was concentrated in a zone at 4 5 5 km depth with a shallower tail extending to near the surface at the northern end of the colfiorito sequence for both the second colfiorito event and the sellano event individual patches were calculated with maximum slip of 2 m implying stress drops as high as 20 mpa coulomb stress calculations show that the coseismic slip distribution for the colfiorito events facilitated slip of the adjacent sellano earthquake

source parameters for small events associated with the 1986 north palm springs california earthquake determined using empirical green functions using small events as empirical green functions which include path site and instrument effects on the p waveforms source parameters were estimated for 25 ml 3 4 to 4 4 events associated with the 1986 north palm springs earthquake the static stress drops ranged from 3 to 80 bars for moments of 0 7 to 11 1021 dyne cm there was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock the highest values were outside the main area of slip and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock the relatively high stress drop of a preshock in 1983 located close to the hypocenter of the main shock is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

overlapping fault planes of the 1971 san fernando and 1994 northridge california earthquakes aftershocks of the 1971 san fernando and 1994 northridge earthquakes were relocated using a three dimensional velocity model that was derived from inverting p wave travel time data the hypocenters show clear orientations of the dipping fault planes the san fernando aftershocks form a plane extending from a depth of 15 km to the surface dipping toward the northeast at about 40 the northridge aftershocks delineate a fault extending from a depth of 18 km up to about 5 km dipping toward the southwest at about 40 in the region the aftershocks overlap in map view the san fernando plane cuts off the northridge plane at a depth of 5 to 8 km preventing it from reaching the surface the similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast southwest compression

the santa barbara california earthquake of 13 august 1978 the 5 1 m l santa barbara earthquake of 13 august 1978 occurred at 22h54m 52 8s utc the epicenter was located 3 km southeast of santa barbara at 34 23 9 n latitude and 119 40 9 w longitude with a focal depth of 12 7 km the main shock was followed between 13 august and 30 september by 373 aftershocks that were located with the caltech usgs array the aftershock zone extended 12 km wnw from the epicenter and was 6 km wide in the n s direction and it had a very clear temporal development during the first 20 min of activity all the aftershocks were located in a cluster 7 km wnw of the main shock epicenter during the next 24 hr the aftershock zone grew to 11 km in the wnw direction and 4 km in the n s direction during succeeding weeks the zone extended to 12 by 6 km this temporal spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the wnw and the initial rupture plane may have been considerably smaller than the eventual aftershock zone this smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone in cross section the aftershock hypocenters outline a nearly horizontal plane dipping 15 or less at 13 km depth the main shock focal mechanism indicates nne ssw compression and vertical extension the preferred fault plane strikes n80 w and dips 26 nne indicating north over south thrusting with a component of left lateral movement focal mechanisms for 40 aftershocks also indicate compression in the general n s direction for most of these events the north dipping nodal plane dips between 7 and 45 with most dipping 25 or more which is significantly steeper than the plane delineated by the hypocenters themselves these observations are consistent with a tectonic model in which much of the slip during the santa barbara earthquake occurred on a nearly horizontal plane the after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip hence the santa barbara earthquake may be taken as evidence for mid crustal horizontal shearing in the western transverse ranges

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

advancement in source estimation techniques using broadband regional seismograms one important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases such as pnl surface wave sv sh because the misfit errors used in some waveform inversions are normalized by the data and synthetics the amplitude information in the data has not been fully utilized in this article we modify the cut and paste source estimation technique zhao and helmberger 1994 by removing this type of normalization it is shown that the modified method increases the stability and resolution of inversion when multiple stations at different distance ranges are used a distance scaling factor is introduced to compensate for the amplitude decay with distance by applying the technique to the terrascope data we have determined source mechanisms and depths of 335 southern californian events with m l 3 5 the amplitude decays with distance are r 1 13 for pnl r 0 55 for love waves and r 0 74 for rayleigh waves in contrast to generally shallow source depths reported by the southern california short period network the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km

using palaeoenvironmental dna to reconstruct past environments progress and prospects palaeoenvironmental dna palendna is defined as ancient dna adna originating from disseminated genetic material within palaeoenvironmental samples sources of palendna include marine and lake sediments peat loess till ice permafrost palaeosols coprolites preserved gut contents dental calculus tephras and soils as well as deposits in caves rockshelters and at archaeological sites palendna analysis provides a relatively new tool for quaternary and archaeological sciences and its applications have included palaeoenvironmental and palaeodietary reconstructions testing hypotheses regarding megafaunal extinctions human environment interactions taxonomic studies and studies of dna damage because palendna samples comprise markedly different materials and represent wide ranging depositional and taphonomic contexts various issues must be addressed to achieve robust reproducible findings such issues include climatic and temporal limitations the biological origin and state free versus bound of palendna stratigraphic reliability sterile sampling ability to distinguish modern from adna signals dna damage and pcr amplification dna extraction methods and taxonomic resolution in this review we provide a non specialist introduction to the use of palendna for quaternary and archaeological researchers assess attributes and limitations of this palaeoenvir onmental tool and discuss future prospects of using palendna to reconstruct past environments copyright 2014 the authors journal of quaternary science published by john wiley sons ltd on behalf of quaternary research association

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

discrepancy between geological and geodetic deformation rates in the ventura basin space geodesy provides a new tool for measuring neotectonic deformation which when combined with geological and geophysical observations and models can be used to infer rates and styles of deformation in the ventnra basin region of southern california the geodetically measured convergence rates of 7 10 mm yr 1 ref 1 are significantly lower than the 20 25 mm yr 1 estimated from geological data2 3 here we investigate the possible implications of this discrepancy it might be for example that temporal variations cause the short and long term convergence rates to differ but if this were the case we would expect to observe the long term rate by extending the geodetic measurements over a broader region yet only 11 3 mm yr 1 of regional convergence is observed4 our preferred explanation of the discrepancy is that the geological models require revision and to this end we present two deformation models that satisfy both the geological and geodetic constraints our modelling suggests that the faults bounding the basin are locked at the surface but are slipping at depths below 2 5 km

a tomographic image of mantle structure beneath southern california we determined the variations in seismic structure beneath southern california by using a tomographic method of inversion on teleseismic p delays recorded with the southern california array the algorithm employed was a modified form of an algebraic reconstruction technique art used in medical x ray imaging deconvolution with an empirically estimated point spread function was also used to help in focusing the image the inversion reveals two prominent features beneath the region the first is a thin vertical wedge directly beneath the transverse ranges that is 2 3 faster than the surrounding region this feature deepens to the east attaining a maximum depth of about 250 km beneath the san bernardino mountains the second feature is a major zone of low velocity material that is 2 4 slow under the salton trough rift valley extending to a depth of about 125 km two possible explanations for the spatial association of the transverse ranges with the velocity anomaly below are lithospheric subduction or small scale sublithospheric convection in the region of the big bend of the san andreas fault the low velocity anomaly beneath the salton trough is consistent with convective upwelling there

estimating finite source properties of small earthquake ruptures many of the most fundamental questions in earthquake science are currently limited by a lack of knowledge about small earthquake ruptures small earthquakes are difficult to study owing to the poor constraints placed on many of the interesting physical parameters by bandlimited far field seismic data traditionally dynamic models such as an expanding circular crack have been utilized to bridge the gap between the easily measurable quantities for small earthquakes and more interesting physical parameters such as stress drop and rupture velocity here i present a method for estimating the basic finite source properties of a rupture that is independent of any a priori model and utilizes the description of a finite source the second moments that far field waves are inherently sensitive to application to two magnitude 5 events in southern california demonstrates the ability of an empirical green9s function approach to estimating the second moments to resolve the fault plane ambiguity rupture length and overall directivity additional results are presented for two example m 2 7 events from the creeping section of the san andreas fault to examine the likely lower bound on event size that can be studied with surface seismometers the creeping section earthquakes have very similar rupture areas but would be improperly interpreted as significantly different using the traditional methodology one of these events presents a relatively clear interpretation of the velocity of rupture front propagation which is about 0 8 of the rayleigh speed suggesting little difference in rupture velocity between it and typical large earthquakes

fault plane determinations for three small earthquakes along the san jacinto fault california search for cross faults using rupture directivity and aftershock locations three earthquakes ml 4 2 4 3 were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane all three events occurred near the san jacinto fault zone during december 1989 and february 1990 and had a large component of strike slip motion with nodal planes oriented in the northeast and northwest directions smaller earthquakes were used as empirical green functions to deconvolve the path and site complications from the short period p waveforms the resultant far field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data the results of the waveform inversion showed that the earthquake near the anza gap december 2 1989 and the earthquake near san bernardino december 28 1989 ruptured northwest trending planes the earthquake near buck ridge february 18 1990 appeared to rupture a northeast trending plane but the waveform results for this event were not considered as reliable available aftershock data for the three events were also examined using master event relocations the aftershock sequence following the buck ridge event showed a clear lineation trending toward the northeast the results of this study indicate that the anza and san bernardino earthquakes ruptured areas on or parallel to the san jacinto fault while the buck ridge event ruptured a northeast trending cross fault in addition to determining the fault plane the waveform analysis resolved details of the slip distribution for these relatively small earthquakes

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

apparent stress scaling for tectonic and induced seismicity model and observations abstract a macroscopic model of seismic sources provides a scaling relationship for the apparent stress treated as a function of three independent parameters seismic moment rupture area size and average slip acceleration these parameters represent three different factors kinematic geometric and material this relationship allows us to distinguish and explain the following statistical characteristics of the log apparent stress versus log seismic moment plot the regional trends represented by a series of 1 2 slope lines are related to the averaged shape of slip velocity pulses so they reflect kinematic characteristics of the rupture process the global trend represented by the 1 6 slope line is expected to characterize sets of events of wide range of rupture area sizes and assumes dependence of rupture area size on total slip so it is related to the rupture initiation propagation and arrest conditions therefore it reflects earthquake rupture dynamics additional shiftings among the trend lines obtained for the smallest induced tremors larger tectonic earthquakes and slow tsunami earthquakes reflect differences between the intact rock failure and the frictional slip failure that is between fracture energies of these different earthquake classes

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

source parameters and rupture velocity of small m 2 1 reservoir induced earthquakes summary we calculate stress drop and rupture speed for ml 2 1 shallow reservoir induced earthquakes and find them to be similar to those of large natural earthquakes previous studies have suggested that hydrofractures mining and reservoir induced earthquakes have lower average stress drop than natural tectonic earthquakes this difference might result from the different tectonic setting or the shallower hypocentral depths of induced earthquakes alternatively difficulties in correcting for attenuation and site effects in earlier studies may lead to underestimation of stress drop in addition most studies assume the rupture velocity of small reservoir induced earthquakes to be the same as for the large earthquakes we analyse a set of 101 ml 2 1 earthquakes induced by changing water level in the acu reservoir ne brazil the earthquakes are shallow depth 5 km and the region has negligible natural seismicity we use three different approaches to calculate the source parameters of the six largest 1 9 ml 2 1 earthquakes we model the individual spectra to find corner frequency frequency independent q and long period amplitude we use collocated small earthquakes as empirical green s functions to calculate the spectral ratios and determine the relative source time functions estimates of the source duration and corner frequency imply stress drops in the range of 26 179 mpa these are similar to or higher than tectonic earthquakes suggesting that the shallow hypocentral depth and the presence of water do not affect stress drop we observe clear directivity for one of the earthquakes and use the azimuthal variation in pulse width to estimate a rupture velocity of 0 6

fine structure of the rupture zone of the april 26 and 27 1997 northridge aftershocks we investigated the rupture geometry of two m w 4 5 earthquakes april 26 1997 and april 27 1997 that occurred on the western edge of the aftershock zone of the 1994 m w 6 7 northridge earthquake both events have thrust mechanisms with a steep plane dipping 75 se and a shallow plane dipping 45 ne an empirical green s function deconvolution followed by a waveform inversion was used to determine the slip distribution of the two events the inversion results show that the steep plane fits the data slightly better than the shallow plane the background seismicity aftershocks of the 1994 northridge event in the epicentral region shows the existence of a north dipping fault plane similar to that of the 1971 san fernando earthquake however the spatial trend of the two april 1997 events and their aftershocks reveals tightly clustered seismicity on a steep plane dipping south relative relocation of the april 26 and 27 events shows that the april 27 event ruptured 1 4 km n70 e of the april 26 event and at a slightly shallower depth i e almost along strike and on a steep plane dipping south these observations suggest that the steep plane is the fault plane of both events the two events ruptured on a plane which is almost perpendicular to the trend of the regional background seismicity thus the seismogenic structure beneath the transverse ranges exhibits complexity on scales of a few kilometers the rupture area for both the april 26 and 27 events is 1 km 2 with a stress drop of at least 20 to 30 bars

stress drops and radiated seismic energies of microearthquakes in a south african gold mine 1 we estimate stress drops and radiated seismic energies of 20 microearthquakes 0 0 mw 1 3 in a south african gold mine to investigate their rupture characteristics and scaling relationships to large earthquakes we analyze seismograms of borehole accelerometers recorded with high sampling rate 15 khz within 200 m of the hypocenters at the depth of 2650 m the waveform data have very high signal to noise ratio and no significant later phases are observed at all stations corner frequencies and quality factors of the anelastic attenuation q are estimated from spectra of velocity seismograms by assuming the omega squared model of boatwright 1978 we also investigate moment tensors for double couple solutions and volumetric components from the waveform inversion static stress drops of the 20 earthquakes calculated from the model of madariaga 1976 are from 3 2 to 88 mpa and scaled energies er mo the ratio of the radiated energy er to the seismic moment mo are from 4 2 10 6 to 1 1 10 4 we find that both the static stress drops and the scaled energies of the analyzed earthquakes are comparable to those values of larger earthquakes our results indicate that the dynamic rupture processes of these microearthquakes are similar to those of larger earthquakes

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

source inversion of the 1988 upland california earthquake determination of a fault plane for a small event abstract we examined short period p waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small m l 4 6 earthquake near upland california we removed path and site complications by choosing a small aftershock m l 2 7 as an empirical green function the main shock p waves were deconvolved by using the empirical green function to produce simple far field displacement pulses we used a least squares method to invert these pulses for the slip distribution on a finite fault both nodal planes strike 125 dip 85 and strike 221 dip 40 of the first motion focal mechanism were tested at various rupture velocities the southwest trending fault plane consistently gave better fitting solutions than the southeast trending plane we determined a moment of 4 2 10 22 dyne cm the rupture velocity and thus the source area could not be well resolved but if we assume a reasonable rupture velocity of 0 87 times the shear wave velocity we obtain a source area of 0 97 km 2 and a stress drop of 38 bars choice of a southwest trending fault plane is consistent with the trend of the nearby portion of the transverse ranges frontal fault zone and indicates left lateral motion this method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations

the front speed of intrusive gravity currents this paper describes an experimental and numerical study of an intrusion propagating along the interface of a two layer fluid in a channel we find that when the density of the intrusion is the depth weighted mean of the layer densities the interface ahead of the intrusion is undisturbed but for other densities the interface ahead of the intrusion is displaced vertically we find that this vertical displacement which takes the form of an upstream propagating long wave depends on the properties of the intrusion and not only on the relative depths of the two layers for the case when the interface is undisturbed the intrusion propagation speed is a minimum we develop an energy argument that describes the observed variation of the intrusion speed from this minimum speed as a function of the intrusion and layer densities and the ratio of the layer depths we also show that if and only if the layer depths are equal the speed of the intrusion is independent of the density of the intrusion

evidence for possible horizontal faulting in southern california from earthquake mechanisms we find that 36 of the 505 fault plane solutions m 3 0 1981 1990 in southern california have a nodal plane dipping no more than 30 with the assumption of the low angle nodal planes being the fault planes four cross sections are constructed to show the possible horizontal faults in the middle and upper crust more than half of these low angle faults are located within or adjacent to the transverse ranges the focal depths vary from 1 km in the southern end of the sierra nevada and the southwestern mojave desert to 20 km in the transverse ranges the slip directions are also diverse in general east west extensional movements are dominant in the boundary between the southern sierra nevada extending to the san emigdio mountains and the western mojave desert whereas north south compressional movements are dominant in the transverse ranges in the peninsular ranges and the salton trough both the slip directions and focal depths vary these features suggest that seismically active low angle faults in southern california may exist at different depths and slip in various directions our data do not support the existence of a regional scale seismically active detachment in southern california only in the western transverse ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km

the san gabriel mountains bright reflective zone possible evidence of young mid crustal thrust faulting in southern california abstract during the los angeles region seismic experiment larse a reflection refraction survey was conducted along a line extending northeastward from seal beach california to the mojave desert crossing the los angeles basin and san gabriel mountains shots and receivers were spaced most densely through the san gabriel mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area a stack of common midpoint cmp data reveals a bright reflective zone 1 s thick that dominates the stack and extends throughout most of the mid crust of the san gabriel mountains the top of this zone ranges in depth from 6 s 18 km depth in the southern san gabriel mountains to 7 5 s 23 km depth in the northern san gabriel mountains the zone bends downward beneath the surface traces of the san gabriel and san andreas faults it is brightest between these two faults where it is given the name san gabriel mountains bright spot sgmbs and becomes more poorly defined south of the san gabriel fault and north of the san andreas fault the polarity of the seismic signal at the top of this zone is clearly negative and our analysis suggests it represents a negative velocity step the magnitude of the velocity step is approximately 1 7 km s in at least one location an event with positive polarity can be observed 0 2 s beneath the top of this zone indicating a thickness of the order of 500 m for the low velocity zone at this location several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone possibly a master decollement 1 it represents a significant velocity reduction if the rocks in this zone contain fluids such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the sgmbs near lithostatic fluid pressure is required in the sgmbs such differential changes are believed to occur in the neighborhood of active fault zones where fault value action has been postulated less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke 2 it occurs at or near the brittle ductile transition at least in the southern san gabriel mountains a possible zone of concentrated shear 3 a thin reflection rising from its top in the southern san gabriel mountains projects to the hypocenter of the 1987 m 5 9 whittier narrows earthquake a blind thrust fault earthquake with one focal plane subparallel to the reflection alternatively one could argue that the bends or disruptions in the reflective zone seen at the san gabriel and san andreas faults are actually offsets and that the reflective zone is therefore an older feature possibly an older fault zone

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

quaternary flake tectonics of the california transverse ranges p residual studies suggest that the pacific american plate boundary in the upper mantle lies in the central mojave desert northeast of its crustal position at the san andreas fault focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north dipping thrust faults resulting in a mid crustal detachment in which the central transverse ranges move south relative to the mantle this makes sense geologically if the central transverse ranges and western mojave desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile pelona schist or catalina schist which makes up the zone of decollement such ductile schists would occur today at depths comparable to those at which they were metamorphosed the detachment breaks through to the surface at the point dume malibu coast fault zone and the red mountain san cayetano santa susana sierra madre cucamonga fault zone the latter coinciding with the southern edge of the palmdale bulge the transverse ranges flake now moves west northwest relative to the western mojave flake south southwest relative to the pacific plate and northwest relative to the great valley sierra nevada the mantle boundary may rejoin the san andreas fault north of a southwest trending zone of high seismicity in the tehachapi mountains which may be produced by convergence the age of the present tectonic system may be the same as the age of the break through thrust faults these are probably no older than 2 m y and may be even younger

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

stress drops and radiated energies of aftershocks of the 1994 northridge california earthquake we study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern california earthquakes we use empirical green functions to obtain source time functions for 47 of the larger m 4 0 aftershocks of the 1994 northridge california earthquake m6 7 we estimate static and dynamic stress drops from the source time functions and compare them to well calibrated estimates of the radiated energy our measurements of radiated energy are relatively low compared to the static stress drops indicating that the static and dynamic stress drops are of similar magnitude this is confirmed by our direct estimates of the dynamic stress drops combining our results for the northridge aftershocks with data from other southern california earthquakes appears to show an increase in the ratio of radiated energy to moment with increasing moment there is no corresponding increase in the static stress drop this systematic change in earthquake scaling from smaller to larger m3 to m7 earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults

determination of rupture duration and stress drop for earthquakes in southern california a simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3 5 and 4 0 using the time between the p wave onset and the first zero crossing 1 2 on seismograms from local seismic networks this method is applied to 10 main shocks in southern california to investigate regional variations in stress drop the initial pulse widths of 65 foreshocks or aftershocks of these events were measured values of 1 2 for small earthquakes below about magnitude 2 2 are generally observed to remain constant with decreasing magnitude in four sequences studied the relative pulse width of a particular main shock m 3 5 at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station these observations are interpreted to signify that the waveforms of these small events m 2 2 are essentially the impulse response of the path between the source and receiver values of 1 2 determined from small foreshocks and aftershocks are therefore subtracted in effect deconvolved from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects significant variations in rupture duration and stress drop are observed for the main shocks studied aftershock locations and azimuthal variations in 1 2 both indicate that the rupture zone of one earthquake expanded unilaterally a factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the san jacinto fault system the first event in this pair had the highest stress drop of the events studied 860 bars and was followed within 8 months by a magnitude 5 5 earthquake 2 km away

investigating microearthquake finite source attributes with iris community wavefield demonstration experiment in oklahoma author posting the authors 2018 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 215 2018 1072 1087 doi 10 1093 gji ggy203