the 1994 northridge earthquake 3 d crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms a detailed 3 d p wave velocity structure of the crust in the epicentral area of the 17 january 1994 northridge earthquake is determined by using 104 709 arrival times from 1673 northridge aftershocks and 2948 other local earthquakes a test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3 d model are accurate to about 2 km we found that regions with high aftershock activity are generally associated with faster p wave velocities the velocity is high around the main south dipping fault of the 1994 northridge earthquake and the north dipping fault of the 1971 san fernando earthquake a linear distribution of strike slip aftershocks was found along a ne sw boundary between high velocity and low velocity structures to the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low velocity area while to the east aftershocks with thrust mechanisms occurred in a high velocity area these observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the transverse ranges a better understanding of these features is important for long term seismic hazard assessment in the los angeles area

three dimensional attenuation tomography at loma prieta inversion of t for q three dimensional q 1 variations in the aftershock region of loma prieta are derived by tomographic inversion the data set consists of over 4000 aftershock recordings at 22 passcal program for array seismic studies of the continental lithosphere stations deployed after the loma prieta mainshock of 1989 estimates of attenuation are determined from nonlinear least squares best fits to the fourier amplitude spectrum of p and s wave arrivals the linear attenuation inversion is accomplished by using three dimensional velocity variations derived previously in nonlinear velocity inversions low q is observed near the surface and q generally increases with depth the southwest side of the san andreas fault exhibits lower q than does the northeast side and this feature apparently extends to approximately 7 km depth the fault zone as determined by the dipping plane of aftershock activity is characterized by slightly higher qp and lower qs compared to regions immediately adjacent to the fault these correlate with high velocity anomalies associated with seismicity at depth the results are in agreement with earlier observations regarding the association of high velocity anomalies seismicity and fault zone asperities

associations between temporal velocity changes and induced seismicity a new technique to image temporal velocity changes is proposed and applied to mining induced microseismic data recorded at falconbridge s strathcona mine sudbury canada the motivation for this work is to back analyze temporal changes in the velocity structure which may shed light upon observed correlations between seismicity and velocity structure the sequential imaging method involves cross correlating waveforms of event doublets and computing the relative event location from the time delays time delays which are corrected for the travel time delay due to the differences in locations are then used to compute images of temporal velocity differences between the recording periods of the doublets controlled blast data was used to examine the accuracy of relative locations which was found to be 2 8 m compared to 13 5 m for the absolute locations corrected blast doublet time delays were inverted to test the sensitivity of the method which confirmed the occurrence of no significant velocity changes a case study of imaging velocity differences with event doublets recorded at different times demonstrated a correlation between velocity decreases and both the location of induced microseismic events and a mn 2 6 tremor the velocity decreases were attributed to fracture unclamping due to stress field rotations associated either with mining or the tremor the events were found to locate in zones of anomalously high velocity in the static velocity image corroborating similar observations made in both mining induced and natural seismicity studies

on the stability of p wave tomography at loma prieta a comparison of parameterizations linear and nonlinear inversions we investigate the stability of tomographic analysis by comparing the results of two different methods of parameterizing the three dimensional p wave velocity variations in the vicinity of the 1989 loma prieta earthquake a block inversion is implemented using 55 by 45 by 10 blocks of 1 km width and varying thickness to a depth of 25 km below the surface linear and nonlinear analysis are presented the nonlinear analysis is achieved by iterating over three dimensional raytracing and earthquake relocation relative to current three dimensional models until solutions show only small improvements a second parameterization is achieved by using cubic b spline functions to span the space of the model which is rotated by 46 5 nonlinear results are presented with several different starting models illustrating the robustness of the technique to the initial conditions all the nonlinear results produced essentially the same final model which was structurally the same as the model obtained by linear analysis using a reasonable starting model

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

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

the relation of crustal scattering to seismicity in southern california dense regional array recordings of 377 teleseismic events are used to map geographic variability in upper crustal p to s scattering in southern california scattered wave energy is sensitive to short wavelength heterogeneity and maps of scattering potential offer a powerful complement to travel time tomography in characterizing seismic crustal heterogeneity the scale length of resolved scattering variability is commensurate with the scale lengths of surface fault trace length and microseismicity variations and both mapped faults and seismicity are concentrated in regions of strong scattering fault proximal scattering averaged across strike of predominately strike slip fault zones is highly correlated with microseismicity levels and is predictive of the pattern of microseismicity variation but not of absolute scale north south profiles through the transverse ranges reveal coincident strong gradients in microseismicity and scattering whereas mapped segment bounds along the san andreas fault zone from san francisco to the salton sea coincide with strong along strike gradients in the level of scattering the overall correlation of microseismicity and scattering potential is consistent with structural control of aftershock and background microseismicity production strain energy control of scattering or both prior evidence for fault offset in the pattern of scattering intensity revenaugh and reasoner 1997 favors the former

variable rates of late quaternary strike slip on the san jacinto fault zone southern california three strike slip displacements of strata with known approximate ages have been measured at two locations on the san jacinto fault zone minimum horizontal offset between 5 7 and 8 6 km in no more than 0 73 m y northeast of anza indicates 8 12 mm yr average slip rate since late pleistocene time two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the coyote creek fault in western imperial valley horizontal slip of 1 7 m has been calculated for the youngest sediment of lake cahuilla since its deposition 275 510 yr b p the corresponding slip rate is 2 8 5 0 mm yr right lateral offset of 10 9 m measured on a buried stream channel older than 5060 yr b p but younger than 6820 yr b p yields average slip rates for the intermediate time periods 400 to 6000 yr b p of 1 2 mm yr the average rates of slip for these three time intervals suggest a relatively quiescent period for the san jacinto fault zone from about 4000 b c to about 1600 a d to the extent that long term variations in seismic activity of major strike slip faults elsewhere are known fluctuations in slip rate for the san jacinto fault would not appear to be abnormal if the san jacinto and adjacent segments of the san andreas fault alternately assume dominant roles in absorbing motion between the pacific and american plates perhaps even more recently than 400 yr ago the san andreas fault south of the transverse ranges expressed most of the motion but has since become relatively inactive

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

tectonic evolution of the meatiq infrastructure central eastern desert egypt the pan african basement exposed in the meatiq area central eastern desert egypt was found to possess tectonic styles belonging to infrastructure transition and super structure levels tectonic style of the infrastructure was developed during an old orogeny in the lower pan african and overprinted by those of the transition and superstructure levels of a younger orogeny in the middle and upper pan african respectively in the lower pan african old sequences of sediments and volcanics together with ophiolitic rocks were intensely deformed and regionally highly metamorphosed into gneisses and amphibolites in the infrastructure level such rocks were migmatized plastically deformed and intruded by synkinematic granitoid diorite to alkali granite lenses sheets and plutons which were probably crystallized as amphibolite facies gneisses later on these infrastructural rocks were cut by mafic to intermediate dykes and sills isostically uplifted and cratonized parts were exposed to erosion in the middle pan african the cratonized rocks sank into a transition tectonic level where they underwent large scale thrusting varying degrees of cataclasis and mylonitization and low to medium grade regional metamorphism an episode of plutonism guided by thrusts followed mainly in the form of synkinematic intrusion of tonalite granodiorite and quartz monzodiorite in the upper pan african isostatic uplift occurred and the mylonitic rocks attained the superstructure level there they were subjected to two main phases of folding resulting in the major meatiq domal structure subsequently the folded rocks were intruded by a postkinematic quartz monzonite pluton

comparison of seismic waveform inversion results for the rupture history of a finite fault application to the 1986 north palm springs california earthquake the july 8 1986 north palm springs earthquake is used as a basis for comparison of several different approaches to the solution for the rupture history of a finite fault the inversion of different waveform data is considered both teleseismic p waveforms and local strong ground motion records linear parametrizations for slip amplitude are compared with nonlinear parametrizations for both slip amplitude and rupture time inversions using both synthetic and empirical green s functions are considered in general accurate green s functions are more readily calculable for the teleseismic problem where simple ray theory and flat layered velocity structures are usually sufficient however uncertainties in the variation in t with frequency most limit the resolution of teleseismic inversions a set of empirical green s functions that are well recorded at teleseismic distances could avoid the uncertainties in attenuation in the inversion of strong motion data the accurate calculation of propagation path effects other than attenuation effects is the limiting factor in the resolution of source parameters the assumption of a laterally homogeneous velocity structure is usually not a good one and the use of empirical green s functions is desirable considering the parametrization of the problem any degree of fault rupture complexity can be described in terms of a linear parametrization for slip amplitudes however a nonlinear parametrization for rupture times and slip amplitudes can have a distinct advantage over a simple linear one by limiting the number of unknown parameters regardless of the choice of data or the type of parametrization the model or solution will be affected by the choice of minimization norm and the type of stabilization used

corals move boulders an unusual mechanism of sediment transport tidal currents over the sill of knight inlet a fjord on the western canadian coast are frequently in excess of 1 m s 2 this shallow sill 65 m is composed of cobble to boulder sized sediments that support extensive growths of large gorgonian corals although current generated shear stresses are normally far from sufficient to initiate boulder movement drag forces due to the attached corals do cause a significant rearrangement of the boulders on the sill

two orogenies in the meatiq area of the central eastern desert egypt abstract the pan african basement exposed in the meatiq area west of quseir egypt consists of an infracrustal basement overthrusted by a supracrustal cover the infracrustal rocks were developed as a result of an old orogeny referred to as the meatiqian orogeny where granite gneiss migmatitic gneisses and migmatized amphibolites were formed the granite gneiss represents a deformed granite pluton emplaced at 626 2 ma whereas the migmatitic gneisses and amphibolites are of mixed igneous and sedimentary parentage in view of the data so far available the nature of the meatiqian orogeny could not be deciphered in spite of the young isotopic ages it is suggested that at least the metasedimentary gneisses represent older rocks in the stratigraphic sequence of the infracrustal basement the supracrustal cover represents a part of an extensive ophiolitic melange obducted onto the infracrustal basement during the next orogeny abu ziran orogeny which culminated at 613 2 ma an active continental margin type regime can adequately explain the evolution of such a supracrustal cover during obduction the ophiolitic melange and the upper 2 km thick part of the infracrustal basement were intensely deformed and metamorphosed under pt conditions of the greenschist epidote amphibolite facies the deformed infracrustal basement was converted into mylonitic blastomylonitic rocks and schists composing five thrust sheets and subsequently intruded by synkinematic granitoid sheets later both the infracrustal basement and the overlying supracrustal cover were isostatically uplifted subjected to complex shallow folding giving rise to the major meatiq domal structure and were intruded by a postkinematic adamellite pluton at 579 6 ma

biogeochemical cycling in an organic rich coastal marine basin 7 sulfur mass balance oxygen uptake and sulfide retention sulfur and oxygen fluxes were quantified in the seasonally varying anoxic marine sedimentary system of cape lookout bight n c u s a over the three year study period 1981 1983 the mean annual sulfate reduction rate was determined to be 18 2 1 6 moles m 2 y 1 this value added to the estimate of the detrital sulfur input of 1 2 4 4 gave a total sulfur input of 19 4 4 7 moles m 2 y 1 the sulfide flux to the sediment water interface measured in anaerobic benthic chambers was 4 6 0 5 moles m 2 y 1 and represented 37 of the annual oxygen uptake rate of 25 2 2 8 moles m 2 y 1 the sulfide burial rate determined to be 15 5 3 1 moles m 2 y 1 was within 5 of the value predicted by summing the fluxes at the sediment water interface the cs weight ratio of sediment below the depth of diagenetic reaction was determined to be 2 75 the sulfide retention rate in these rapidly accumulating sediments 10 cm yr was 77 19 comparison of this result with previous studies shows that rapid sediment accumulation and the lack of bioturbation control this unusually high degree of sulfide retention

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

relation of the 1992 landers california earthquake sequence to seismic scattering measurements of crustal scattering for the area surrounding the 1992 landers earthquake sequence obtained from regional array recordings of teleseismic events for the 10 year period before the sequence showed that the slip distribution on faults could be deducible from the preshock elastic structure scattering intensity correlated strongly with the distribution of aftershocks and slip of the moment magnitude mw 7 3 landers main shock mw 6 1 joshua tree and mw 6 2 big bear events which implies that aftershocks and slip are structurally controlled and broadly predictable scattering within the fault zones was directional and consistent with variable along strike alignment of stress induced cracks

vertical gps ground motion rates in the euro mediterranean region new evidence of velocity gradients at different spatial scales along the nubia eurasia plate boundary we use 2 5 to 14 years long position time series from 800 continuous global positioning system gps stations to study vertical deformation rates in the euro mediterranean region we estimate and remove common mode errors in position time series using a principal component analysis obtaining a significant gain in the signal to noise ratio of the displacements data following the results of a maximum likelihood estimation analysis which gives a mean spectral index 0 7 we adopt a power law white noise stochastic model in estimating the final vertical rates and find 95 of the velocities within 2 mm yr with uncertainties from filtered time series 40 smaller than from the unfiltered ones we highlight the presence of statistically significant velocity gradients where the stations density is higher we find undulations of the vertical velocity field at different spatial scales both in tectonically active regions like eastern alps apennines and eastern mediterranean and in regions characterized by a low or negligible tectonic activity like central iberia and western alps a correlation between smooth vertical velocities and topographic features is apparent in many sectors of the study area glacial isostatic adjustment and weathering processes do not completely explain the measured rates and a combination of active tectonics and deep seated geodynamic processes must be invoked excluding areas where localized processes are likely or where subduction processes may be active mantle dynamics is the most likely process but regional mantle modeling is required for a better understanding

coseismic displacement field and slip distribution of the 2005 kashmir earthquake from sar amplitude image correlation and differential interferometry the coseismic surface displacement field and slip distribution at depth due to the kashmir earthquake mw 7 6 2005 have been analysed by different authors using subpixel correlation of synthetic aperture radar sar images and optical images teleseismic analysis gps measurements as well as in situ field measurements in this paper first we use 23 sets of measurement from subpixel correlation of sar images and differential interferometry to retrieve the 3 d coseismic surface displacement field the obtained horizontal and vertical components along the fault trace are then compared respectively to equivalent measurements obtained from subpixel correlation of two optical aster images and in situ field measurements second the coseismic fault geometry parameters and slip distribution at depth are estimated in addition to the one segment slip model as reported in previous work a two segments slip model that better fits the surface fault break is proposed the improvement of the two segments slip model in interpreting the measured displacement field is highlighted through comparison of residuals of both slip models taking advantage of differential interferometry measurements that provide precise and continuous information in the far field of the fault firstly a wedge thrust according to bendick et al to the northwest of the main rupture built on our two segments model is tested according to the obtained results the residual of the two segments main rupture plus wedge thrust model is slightly smaller than the residual of the two segments model to the northwest of the balakot bagh fault secondly we test the sensitivity of our slip model to the presence of slip along a decollement as evidenced by jouanne et al through post seismic analysis the results indicate that the estimations of the coseismic displacement field and slip distribution in this paper are not significantly biased by such post seismic displacement and that most coseismic displacement is located on a 40 ne dipping fault as previously reported

numerical simulations of large earthquakes dynamic rupture propagation on heterogeneous faults our current conceptions of earthquake rupture dynamics especially for large earthquakes require knowledge of the geometry of the faults involved in the rupture the material properties of the rocks surrounding the faults the initial state of stress on the faults and a constitutive formulation that determines when the faults can slip in numerical simulations each of these factors appears to play a significant role in rupture propagation at the kilometer length scale observational evidence of the earth indicates that at least the first three of the elements geometry material and stress can vary over many scale dimensions future research on earthquake rupture dynamics needs to consider at which length scales these features are significant in affecting rupture propagation

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

kinematic inversion of the 2004 m 6 0 parkfield earthquake including an approximation to site effects the 2004 m 6 0 parkfield earthquake yielded one of the largest amounts of near source strong ground motion seismic data ever we invert strong motion seismograms to obtain a model for the space time distribution of coseismic slip on the fault to reduce noise in the inversion we take into account local amplifications that affect each station by using records of the 1983 m 6 5 coalinga earthquake site amplification correlates well with large peak ground velocities registered during the 2004 parkfield mainshock the inversion for a kinematic rupture model yields a nonunique solution we therefore analyze various rupture models that explain the data equally well our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter where the maximum slip is 57 cm a secondary slip area over which contours are not well resolved is located northwest of the hypo center the rupture speed is highly heterogeneous we infer an average rupture ve locity of 2 8 km sec close to the hypocenter and of 3 3 km sec in the secondary region of large slip to the northwest of the hypocenter by correlation of our rupture model with both microseismicity and velocity structure we identify six patches on the fault plane that behave in seismically distinct ways online material kinematic rupture model parameters

sequential velocity imaging and microseismic monitoring of mining induced stress change sequential imaging of the temporal changes inp wave velocity offers a practical tool to monitor a rock mass using established correlations between the location of seismic events and velocity structure the temporal seismic potential characteristics of the rock may be monitored furthermore the temporal velocity differences isolate the time dependent factors effecting velocity such as stress while cancelling static factors such as lithology various sequential imaging techniques were compared with respect to accuracy differences between successive velocity images were found to have relatively high associated error estimates however images of velocity differences calculated from measured travel time delays between successive velocity surveys were found to have lower error estimates in particular travel time delays measured using cross correlation techniques resulted in the most accurate sequential image two 3d sequential imaging studies were carried out at strathcona mine in sudbury ontario canada results of the average static images indicated an association between the location of induced microseismicity and a zone of both high velocity and high gradient additional examples are described from the global seismology literature which also show a similar correlation between seismicity and velocity structure we attribute this association to an interrelated stress and strength effect the strathcona mine sequential images show zones of significantly decreased velocity in regions of concentrated microseismic activity which are postulated to be indications of localized destressing and relaxation of the clamping forces resulting in the microseismicity the zones of decreased velocity corresponded to an increase in the velocity gradient one of the case studies also shows an increase in velocity in a zone of high static velocity which is later the site of amn 2 5 mining induced seismic tremor the increase in velocity is believed to correspond to a region of stress concentration resulting in the subsequent seismic tremor

tomographic p wave velocity images of the loma prieta earthquake asperity tomographic inversion is applied to delay times from local earthquakes to image 3 d velocity variations surrounding the main rupture of the 1989 loma prieta earthquake the 55 times 45 square km region is represented by blocks of 1 km per side laterally and by 8 layers of varying thickness to 18 km depth high quality p wave arrival times recorded on the usgs calnet array from 549 crustal earthquakes with depths of 0 to 25 km were used as sources preliminary results note several velocity variations 5 12 that correlate with specific characteristics of the 1989 rupture these include prominent high velocity anomalies near the mainshock hypocenter and prominent low velocity anomalies where the dip of the san andreas fault appears to change significantly the termination of prominent low velocity features existing primarily in the hanging wall to depths of 7 9 km correlates with the top of the rupture zone high velocity variations along the fault dominate where aftershock activity is high the high velocity anomaly located at depth along the fault is interpreted as imaging the asperity on which the loma prieta earthquake occurred

seismic behavior of the southern san andreas fault zone in the northern coachella valley california comparison of the 1948 and 1986 earthquake sequences moderate earthquakes in 1948 and 1986 that occurred along the southern san andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern coachella valley the 1986 north palm springs earthquake occurred between the banning and mission creek traces of the san andreas fault zone based on data from both portable and permanent stations aftershocks of the 1986 event define a nearly planar surface that strikes about n60 to 70 w is about 15 km long and dips northeast from near the banning surface trace at about 45 to 50 relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34 n00 26 116 w36 34 and a focal depth of 10 4 km the first motion focal mechanism for the mainshock indicates pure right slip on a plane dipping northeast at 40 to 45 with a strike of n60 w previous moment tensor solutions e g hartzell 1989 exhibit significant oblique reverse slip on a plane with a more westerly strike and a seismic moment of 1 4 to 1 7 1018 n m mw 6 1 actual and synthetic wood anderson recordings indicate a local magnitude of ml 6 0 the 1948 desert hot springs earthquake produced waveforms similar to 1986 although amplitudes for 1948 are typically 20 to 30 larger the 1948 mainshock is relocated at 33 n55 2 116 w28 9 with a focal depth of 12 km a revised magnitude of ml 6 3 and an estimated seismic moment of 2 to 3 1018 n m based on arrival time information from close 20 km portable stations deployed in 1948 to 1949 the 1948 aftershock distribution is about 15 km long 8 to 10 km wide and abuts the 1986 aftershock sequence to the southeast along the banning fault the 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right slip motion rake 169 on a plane that strikes n55 w and dips more steeply northeast at 60 to 70 in 1986 the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry while the down dip extent may have been controlled by the presence at depth of a regional detachment in 1948 the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern indio hills where the surface trace of the banning fault undergoes an approximate 7 change in strike these results indicate that the banning fault is nonvertical is likely segmented according to fault dip as well as fault strike and has been the primary locus of recent moderate sized earthquake activity in the northern coachella valley

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

the 1992 landers earthquake sequence earthquake occurrence and structural heterogeneities the june 28 1992 mw 7 3 landers earthquake occurred in the southeastern mojave desert california over 10 000 aftershocks of the earthquake were recorded by the caltech usgs southern california seismic network scsn in 1992 to investigate the relationship between complexities in the crustal structure and variations in seismicity we have used a tomographic method to invert 145 098 p wave arrival times from 3740 landers earthquake aftershocks and 1148 other events recorded by 60 permanent and temporary scsn stations we determined a detailed p wave tomographic image with a spatial resolution of about 5 km and relocated the hypocenters with the obtained 3 d velocity model the results show a correlation between seismicity patterns and velocity patterns and a tendency for regions rich in seismicity to be associated with higher velocities the higher velocity areas are considered to be strong and brittle parts of the fault zone which are apt to generate earthquakes in contrast low velocity areas are probably more ductile and weaker allowing aseismic slippage

travel time tomography in the northern coachella valley using aftershocks of the 1986 ml 5 9 north palm springs earthquake tomographic inversion is applied to delay times from aftershocks of the 1986 ml 5 9 north palm springs nps earthquake to image 3 d velocity variations within the northern coachella valley p wave arrival times from 1074 earthquakes with depths ranging from 3 to 20 km were used as sources recorded by 12 portable and 4 permanent stations preliminary results show well defined high and low velocity anomalies 2 7 that correlate with the rupture distribution of the 1986 mainshock at depths less than 8 km a low velocity anomaly predominates between the two ne dipping banning and mission creek faults from 8 to 12 km in depth where the nps mainshock and most of the aftershocks occur a high velocity anomaly is observed this high velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture and suggests that velocity information may help to define important elements such as asperities that control fault rupture and thus may help to predict the location and size of future events

integrated assessment of the carbon budget in the southeastern bering sea abstract during the primary field program for the bering ecosystem study 2008 2010 independent seasonal estimates of net primary production npp net community production ncp vertical export production cexp and benthic carbon consumption bcc were used to construct a shelf wide carbon budget for the southeastern bering sea here we quantify the annual production utilization and transport of npp for the southeastern shelf region of the bering sea spatially partitioned into outer middle and coastal domains we observed that approximately 25 and 30 of npp on the shelf is exported horizontally from the middle and outer domains respectively this horizontal transport was the dominant mode of carbon export in the outer domain exceeding cexp by more than 30 g c m 2 yr 1 99 g c m 2 yr 1 compared to 67 g c m 2 yr 1 respectively in the middle domain cexp was more prominent than lateral transport 65 g c m 2 yr 1 and 46 g c m 2 yr 1 respectively and vertically exported carbon was more efficiently recycled in this domain than in the outer domain 53 and 32 of cexp respectively in the coastal domain lateral transport was a source of carbon to the bottom layer with estimated input of carbon exceeding npp by as much as 54 g c m 2 yr 1 while the source of this additional carbon is unknown one possible source is transport from the middle domain during wind events that induce coastal convergence overall the combined carbon reservoir attributed to burial and transport in the middle and outer domains is similar to a previous budget for this region 47 walsh and mcroy 1986 although some qualitative differences are apparent the data presented here indicate a more pelagic character in the outer domain relative to the middle domain and that the middle and coastal domain carbon budgets are balanced only when combined

the dynamic controls on carbonate mineral saturation states and ocean acidification in a glacially dominated estuary abstract recently a number of studies have shown that the intrusion of anthropogenic carbon dioxide co2 into the ocean has created an acidification effect leading to the reduction in carbonate mineral saturation states however the uptake of atmospheric co2 is not the only climate induced phenomenon that leads to a reduction of in marine environments over the past 250 years glacier bay ak glba has experienced rapid deglaciation leading to an increase in the amount of freshwater entering the marine ecosystem this excess freshwater discharge is low in total alkalinity and reduces the buffering capacity of surface waters and enhances the vulnerability of the estuary to further reductions in ph the corresponding reduction in may cause these waters to become corrosive to shell building organisms to better understand these processes we collected monthly samples within glba that show the variability in throughout the water column low values were well correlated with the timing of maximum glacial discharge events and most prominent within the two regions where tidewater glacial discharge was highest the saturation state with respect to aragonite reached a minimum of 0 40 at the surface during the summer of 2011 before rebounding to a maximum value of 3 26 in the spring of 2012 aragonite was undersaturated at the surface throughout the entire bay during fall months sept and oct here we present results from a year long study designed to constrain the effects of glacial freshwater discharge on the marine carbonate system and discern the primary controls on in this pristine estuarine environment 1

three dimensional basin and fault structure from a detailed seismic velocity model of coachella valley southern california the coachella valley in the northern salton trough is known to produce destructive earthquakes making it a high seismic hazard area knowledge of the seismic velocity structure and geometry of the sedimentary basins and fault zones is required to improve earthquake hazard estimates in this region we simultaneously inverted first p wave travel times from the southern california seismic network 39 998 local earthquakes and explosions 251 land sea shots from the 2011 salton seismic imaging project to obtain a 3 d seismic velocity model earthquakes with focal depths 10 km were selected to focus on the upper crustal structure strong lateral velocity contrasts in the top 3 km correlate well with the surface geology including the low velocity 5 km s sedimentary basin and the high velocity crystalline basement rocks outside the valley sediment thickness is 4 km in the southeastern valley near the salton sea and decreases to 2 km at the northwestern end of the valley eastward thickening of sediments toward the san andreas fault within the valley defines coachella valley basin asymmetry in the peninsular ranges zones of relatively high seismic velocities 6 4 km s between 2 and 4 km depth may be related to late cretaceous mylonite rocks or older inherited basement structures other high velocity domains exist in the model down to 9 km depth and help define crustal heterogeneity we identify a potential fault zone in lost horse valley unassociated with mapped faults in southern california from the combined interpretation of surface geology seismicity and lateral velocity changes in the model

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

tomographic images of p wave velocity variation at parkfield california tomographie inversion is applied io delay times from local earthquakes to image three dimensional velocity variations near parkfield california the 25 20 square km region is represented by nearly cubic blocks of 0 5 km per side arrival times of p waves from 551 local earthquakes with depths of 0 to 15 km were used as sources producing 3135 rays covering the target region the data were recorded on low noise downhole seismographs a conjugate gradient method is used to invert the resulting sparse system of simultaneous equations to diminish the effects of noisy data the laplacian of the model parameters is constrained to be small within horizontal layers providing smoothing of the model the resolution of the model is estimated by calculating point spread functions at blocks of interest estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure the results of the inversion show correlation with some of the local geological and geophysical features station corrections removed the long wavelength anomaly associated with the contrast of the salinian block southwest of the san andreas fault versus the franciscan to the northeast a velocity low located a few kilometers northwest of parkfield depth 2 5 3 5 km appears to lie along the gradient of the large bouguer gravity anomaly associated with the parkfield syncline the south southeastward extension of the low velocities may relate to reflections observed on the parkfield consortium for continental reflection profiling cocorp lines we speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment the correlation of higher velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower velocity less competent rocks

3 d velocity structure in southern haiti from local earthquake tomography we investigate 3 d local earthquake tomography for high quality travel time arrivals from aftershocks following the 2010 m7 0 haiti earthquake on the leog ne fault the data were recorded by 35 stations including 19 ocean bottom seismometers from which we selected 595 events to simultaneously invert for hypocenter location and 3 d vp and vs velocity structures in southern haiti we performed several resolution tests and concluded that clear features can be recovered to a depth of 15 km at 5 km depth we distinguish a broad low velocity zone in the vp and vs structure offshore near gonave island which correlate with layers of marine sediments results show a pronounced low velocity zone in the upper 5 km across the city of leog ne which is consistent with the sedimentary basin location from geologic map at 10 km depth we detect a low velocity anomaly offshore near the trois baies fault and a nw se directed low velocity zone onshore across petit go ve and jacmel which is consistent with a suspected fault from a previous study and that we refer to it in our study as the petit go ve jacmel fault these observations suggest that low velocity structures delineate fault structures and the sedimentary basins across the southern peninsula which is extremely useful for seismic hazard assessment in haiti

cubic b splines tomography at loma prieta abstract a high resolution tomographic study using cubic b splines parameterization and employing a systematic approach to the choosing of appropriate damping and smoothing parameters provided a three dimensional p wave velocity map of the loma prieta area used in the inversion were 11 977 high quality raypaths from 844 aftershocks of the 1989 loma prieta earthquake the velocity model exhibits a low velocity feature between the san andreas and zayante vergeles faults in the top 10 km of the crust this low velocity feature is interpreted as a sedimentary unit exposed to the northwest and separated from the salinian block by the zayante vergeles fault below 10 km no consistent change is observed between the salinian and the franciscan blocks there appears to be a high correlation of aftershock activity and localized high velocity anomalies southeast of the loma prieta mainshock whereas this anomaly may represent brittle rocks associated with a fault zone asperity that failed after the mainshock there is evidence to suggest it may be a body of serpentinite the serpentinite exhibits high velocities and is potentially less competent than surrounding country rock thus providing a sector along the fault more likely to be associated with many smaller earthquakes or creep behavior

why is the challenger deep so deep abstract recent sidescan surveys of the deepest segment of the southern mariana trench in the western pacific ocean provide the first detailed images of this plate boundary which includes the world s greatest ocean depth the challenger deep the surveys reveal details of the southern mariana plate margin identify another deep rivaling the challenger and document widespread deformation of the overriding plate our data show a subduction generated deep ocean trench not the transform fault boundary suggested by other work d e karig geol soc am bull 82 1971 323 344 d e karig et al j geophys res 83 1978 1213 1226 d e karig b ranken in the tectonic and geologic evolution of southeast asian seas and islands part 2 american geophysical union washington dc 1983 pp 266 280 k fujioka et al geophys res lett 19 2002 1 4 we present the geological characteristics of the region including seismic evidence for a tear in the subducting plate that has influenced the deformation of the overriding plate the rollback caused by this tear creates greater depths along the southern part of the trench than elsewhere along its length

the 27 may 1995 ms 7 6 northern sakhalin earthquake an earthquake on an uncertain plate boundary on 27 may 1995 a large earthquake ms 7 6 occurred in the northern part of sakhalin island near an uncertain plate boundary between the okhotsk or the north american plate and the eurasian or the amurian plate the source process of the mainshock the detailed aftershock activity the b value of the fre quency magnitude relationship for aftershocks and the 3d p wave velocity distri bution have been investigated thirty one body wave records were inverted to de termine the sequence of subevents of the mainshock using an iterative deconvolution method the source parameters obtained are the location of the initial epicenter 52 64 n 142 83 e the centroid depth 9 km strike dip and slip equal to 191 87 172 for the first subevent 198 7 6 175 for a second subevent and 196 7 9 174 for the combined source and the total seismic moment m0 4 2 10 19 n m and mw 7 0 we also deployed 12 temporary seismographic stations in the focal area from 7 june to 8 july 1995 more than 2000 events were located in an aftershock area of 60 km 10 km that trends in a n15e direction which is consistent with the location of the surface rupture tracing the gyrgylan i ossoy fault the p wave velocity patterns were compared with the aftershock seismicity and the spatial distribution of the b values with results suggesting that high velocity corre lates with high seismicity and small b value our research strongly suggests that the 1995 sakhalin earthquake was one of the outstanding events that define the uncertain plate boundary

effects of a low velocity zone on a dynamic rupture dynamic crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous tomographic and near fault seismic studies indicate that the crust near faults is instead comprised of rocks of varying material velocities we have tested the effects of adding material velocity variation to simulations of spontaneously propagating earthquakes we used two dimensional plane strain conditions coupled with a slip weakening fracture criterion and examined earthquakes on faults that bisect finite width low velocity zones embedded in country rock and earthquakes on faults that bound two different velocity materials when a fault bisects a low velocity zone the normal stress remains unchanged but both the rupture velocity and slip velocity pulse shape are perturbed the presence of the low velocity zone induces high frequency oscillations in the slip function near the rupture front when the fault is on the edge of the low velocity zone the oscillations are more pronounced and repeated sticking and slipping can occur near the rupture front for the slip weakening velocity independent friction model however the temporary sticking does not lead to permanent arrest of slip and slip duration is still controlled by the overall rupture dimension when an earthquake ruptures a fault juxtaposing a lower velocity material against a higher velocity material the normal stress across the fault near the crack tip is perturbed the sign of the normal stress perturbation depends on the direction of rupture leading in some cases to a directional dependence of rupture velocity when slip is accompanied by stress reduction a positive feedback develops between the normal and shear stress changes as previously noted by andrews and ben zion 1997 resulting in an apparently unavoidable grid size dependence in computation of stress change near the rupture front numerical experiments indicate however that the rupture velocity is insensitive to this zone size dependence which is highly localized immediately behind the crack tip the factors controlling the rupture velocity in the simulations including directional dependence are further elucidated by a new analytical solution for rupture of an asperity on a frictionless interface

seismic velocity constraints in the thessaloniki and chalkidiki areas northern greece from a 3 d tomographic study abstract three dimensional tomographic inversion of p wave travel time data is used to investigate the seismic velocity structure of the crust in thessaloniki and chalkidiki northern greece local earthquakes recorded by two networks operating in the area are used as natural seismic sources two different target volumes defined on the surface by 39 50 41 50 n and 21 25 24 20 e and 40 10 41 00 n and 22 45 23 50 e are investigated the first dataset is recorded by 12 stations and the second by 29 the size of the blocks used to parameterize the areas is 10 10 km and 3 3 km in the horizontal respectively with varying depth layering the major seismic velocity anomalies within the crust obtained by the tomographic inversion are resolved with a horizontal spatial resolution of about 20 and 7 km for the first and second target volume respectively our particular interest is to illuminate velocity anomalies and more detailed characteristics of the two main neogene quaternary basins in the area vardar axios and struma strymon these basins are identified as low velocity features overlying relatively higher p wave velocity structures in the lower crust the complex mygdonian area reveals a similar pattern of low velocity basin overlying higher velocity basement overall the velocity patterns correlate well with the location and strike of the main geological and tectonic units in the area as well as the basic assumptions on basin development this highlights the utility of local tomography to illuminate structural tectonic and rheological properties within the crust

shallow structure of the landers fault zone from explosion generated trapped waves with fault zone trapped waves generated by near surface explosions within the fault zone of the 1992 landers california earthquake and recorded by linear seismic arrays deployed along and across the fault we have delineated the shallow seismic structure near the 30 km long southern rupture on the johnson valley fault the explosion excited trapped waves with relatively large amplitude and long duration wave train after the s waves are similar to those generated by aftershocks li et al 1994a b but have lower frequencies and travel more slowly coda normalized amplitude spectra of explosion excited trapped waves show a maximum at 2 hz which decreases rapidly with the station offset from the fault trace on the cross fault profiles normalized amplitudes of trapped waves on the along fault profile also decreased with distance between the explosion and station giving an apparent q of 18 at 1 2 hz in the fault zone for the shot near the profile the dispersion of trapped waves from 0 6 to 2 5 hz recorded on this along fault profile implies a shear velocity of 1 0 km s for the fault zone and 1 8 km s for the wall rock while the data from the farther shot show an increase in velocity and q with depth measured group velocities and q values were used as constraints in the numerical modeling of trapped waves on cross fault and along fault profiles results reveal that the shallow johnson valley fault is marked by a zone 250 m wide where the shear velocity is 1 km s and q is 20 calculation of finite difference synthetics for a depth varying fault structure show that these model parameters apply to the depth of 1 km below which the fault zone shear velocity increases to 1 9 km s and q increases to 30

geophysical evidence for wedging in the san gorgonio pass structural knot southern san andreas fault zone southern california geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the san gorgonio pass region this picture serves as the basis for inferring past fault movements within the san andreas system which are fundamental to understanding the tectonic evolution of the san gorgonio pass region interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of san gorgonio pass beneath basement rocks of the southeast san bernardino mountains subtle long wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of san gorgonio pass and south under peninsular ranges basement and has a southern edge that is roughly parallel to but 5 6 km south of the surface trace of the banning fault this deep magnetic body is composed either of upper plate rocks of san gabriel mountains basement or rocks of san bernardino mountains basement or both we suggest that transpression across the san gorgonio pass region drove a wedge of peninsular ranges basement and its overlying sedimentary cover northward into the san bernardino mountains during the neogene offsetting the banning fault at shallow depth average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data seismicity suggests a deeper detachment surface beneath the deep magnetic body this interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross sectional view given the multilayered nature of deformation it is unlikely that the san andreas fault will rupture cleanly through the complex structures in san gorgonio pass

mapping coral and sponge habitats on a shelf depth environment using multibeam sonar and rov video observations learmonth bank northern british columbia canada abstract efforts to locate and map deep water coral and sponge habitats are essential for the effective management and conservation of these vulnerable marine ecosystems here we test the applicability of a simple multibeam sonar classification method developed for fjord environments to map the distribution of shelf depth substrates and gorgonian coral and sponge dominated biotopes the studied area is a shelf depth feature learmonth bank northern british columbia canada and the method was applied aiming to map primarily non reef forming coral and sponge biotopes aside from producing high resolution maps 5 m2 raster grid biotope substrate associations were also investigated a multibeam sonar survey yielded bathymetry acoustic backscatter strength and slope from benthic video transects recorded by remotely operated vehicles rovs six primary substrate types and twelve biotope categories were identified defined by the primary sediment and dominant biological structure respectively substrate and biotope maps were produced using a supervised classification mostly based on the inter quartile range of the acoustic variables for each substrate type and biotope twenty five percent of the video observations were randomly reserved for testing the classification accuracy the dominant biotope defining corals were red tree coral primnoa pacifica and small styasterids of which stylaster parageus was common demosponges and hexactinellid sponges were frequently observed but no sponge reefs were observed the substrate classification readily distinguished fine sediment sand and bedrock from the other substrate types but had greater difficulty distinguishing bedrock from boulders and cobble the biotope classification accurately identified gardens dense aggregations of sponges and corals and primnoa dominated biotopes 67 accuracy but most other biotopes had lower accuracies there was a significant correspondence between learmonth s biotopes and substrate types with many biotopes strongly associated with only a single substrate type this strong correspondence allowed substrate types to function as a surrogate for helping to map biotope distributions our results add new information on the distribution of corals and sponges at learmonth bank which can be used to guide management at this location

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

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

san andreas fault geometry at desert hot springs california and its effects on earthquake hazards and groundwater abstract the mission creek and banning faults are two of the principal strands of the san andreas fault zone in the northern coachella valley of southern california structural characteristics of the faults affect both regional earthquake hazards and local groundwater resources we use seismic gravity and geological data to characterize the san andreas fault zone in the vicinity of desert hot springs seismic images of the upper 500 m of the mission creek fault at desert hot springs show multiple fault strands distributed over a 500 m wide zone with concentrated faulting within a central 200 m wide area of the fault zone high velocity up to 5000 m sec rocks on the northeast side of the fault are juxtaposed against a low velocity m 6 0 earthquakes in the area in 1948 and 1986 occurred at or near the depths 10 to 12 km of the merged san andreas fault large magnitude earthquakes that nucleate at or below the merged fault will likely generate strong shaking from guided waves along both fault zones and from amplified seismic waves in the low velocity basin between the two fault zones the mission creek fault zone is a groundwater barrier with the top of the water table varying by 60 m in depth and the aquifer varying by about 50 m in thickness across a 200 m wide zone of concentrated faulting

contemporary block tectonics california and nevada well determined fault plane solution and the gross pattern of late cenozoic faulting in california and nevada show a systematic relation between the orientation of fault planes and slip directions in general normal faults have northerly strikes reverse faults have easterly strikes and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes respectively kinematically this relation is consistent with the response of clusters of fault bounded crustal blocks to a regional stress field generated by the relative motion between the pacific and north american plates in this stress field the greatest and least principal compressive stresses are restricted to northerly and easterly striking vertical planes respectively clusters of crustal blocks bound by faults having the above attitudes form three basic regimes 1 a spreading regime with a combination of normal and conjugate strike slip faults 2 a converging regime with a combination of reverse and conjugate strike slip faults and 3 a wrench regime with sets of subparallel throughgoing northwest striking dextral faults or northeast striking sinstral faults these three regimes are typified by faulting patterns in the basin and range province spreading the transverse ranges converging and the san andreas garlock fault systems wrench respectively the gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north south shortening and east west extension with the ratio of extensional to shortening strains and the areal dilitation decreasing systematically from spreading to wrench to compressional regimes the wrench regime involves a component of net rotational deformation clockwise for dextral slip and counter clockwise for sinstral slip while deformation of the spreading and converging regimes is irrotational local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength byerlee s law along preexisting fractures and that pore pressure in the brittle crust is laterally uniform a minimum strength difference between active block bounding faults and block interiors is 15 30 simple arrangements of such block clusters mimic the gross kinematic pattern of quaternary faulting in california and nevada some implications for contemporary tectonics emphasized by this model involve the westward displacement of the sierra nevada block with respect to the stable interior of the north american plates oblique thrusting of the salinian block over the pacific plate and a progressive increase in the offset of the san andreas fault represented by the big bend through the transverse ranges