earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

occurrence patterns of foreshocks to large earthquakes in the western united states observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake unfortunately the apparent randomness of foreshock occurrence they precede some events and not others has severely hampered their use in reliable earthquake prediction understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short term prediction will ever be possible1 here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western united states the incidence of foreshocks decreases with increasing depth of the mainshock and also depends on the mainshock slip orientation this pattern of occurrence may be explained by a decrease in small scale crustal heterogeneity with increasing depth and suggests that increasing normal stress both regional tectonic stress and lithostatic load inhibits the occurrence of foreshocks no relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process or that foreshocks are not part of this process

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

evidence for a buried fault system in the coastal plain of the carolinas and virginia implications for neotectonics in the southeastern united states geomorphic geologic and geophysical data suggest the presence of an 600 km long north northeast trending buried fault system in the coastal plain of the carolinas and virginia herein named the east coast fault system the east coast fault system is expressed by anomalous changes in fluvial geomorphology that locally coincide with one or more of the following features linear aeromagnetic anomalies buried faults interpreted from seismic reflection data surface faults offsetting pliocene pleistocene surficial units locally brecciated phyllites and argillites gently upwarped sediments topographic highs and seismicity near summerville south carolina river anomalies where the rivers traverse unconsolidated upper pleistocene holocene flood plain sediments are evidence of deformation along the east coast fault system during the past 130 to 10 k y which may be ongoing the fault system traverses the epicentral area of the 1886 charleston south carolina earthquake and lies west of paleoliquefaction sites along the outer south carolina coastal plain thus the fault system could be the source of the charleston earthquake and other large prehistoric earthquakes therefore confirmation and demarcation of the east coast fault system and elucidation of its history will help better assess seismic hazards in the southeastern united states

the 1930 santa monica and the 1979 malibu california earthquakes the malibu earthquake m l 5 0 that occurred 1 january 1979 in santa monica bay was located at 33 56 9 n and 118 41 3 w and at a depth of 10 km the focal mechanism of the main shock as determined from first motions recorded by short period high gain seismic stations in southern california shows reverse faulting with one plane dipping 55 to 60 to the north striking 275 to 290 and the second plane dipping 30 to 38 to the south and striking 100 to 120 more than 400 aftershocks were recorded during 1979 focal mechanisms for 20 aftershocks of m 3 0 show similar reverse mechanisms as the main shock the main shock and aftershocks m 2 5 were relocated using a local velocity model and a set of corresponding station delays the aftershocks define a volumetric hypocentral zone 4 km 6 km west trending with depths ranging from 6 to 11 km this 1979 malibu main shock aftershock sequence probably occurred on the eastern end of the anacapa dume fault the santa monica earthquake m l 5 2 that occurred 31 august 1930 was relocated using station delays calculated from the 1979 date set the relocated 1930 epicenter is located near the western end of the santa monica fault the 1979 and 1930 main shocks can be interpreted to define a 8 to 10 km north south sidestep between the anacapa dume and santa monica faults possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake

recurrence rates of large earthquakes in the south carolina coastal plain based on paleoliquefaction data we present a reanalysis of results of 15 years of paleoliquefaction investigations in the south carolina coastal plain all earlier radiocarbon age data and locations of organic material collected by various investigators were reviewed and recalibrated to obtain a uniform data set the calibrated dates and the spatial extent of the sandblows having similar dates were used to estimate ages and magnitudes of prehistoric earthquake episodes the results of this analysis suggest seven episodes episodes a g of prehistoric liquefaction in the past 6000 years and two possible scenarios for their occurrence in the first scenario three seismic sources exist within the coastal plain of south carolina at charleston a b e and g with magnitudes m 71 georgetown c and f and bluffton d with magnitudes m 6 in the second scenario episodes c and d are combined into one episode episode c9 in this scenario all earthquakes occurred at charleston and with m 71 episodes a and b seem to be more representative of the earthquake cycle and suggest a recurrence time of 500 600 years for m 71 earthquakes at charleston the recurrence times and magnitudes for episodes c and d are estimated at 2000 years and 6 0 respectively the older episodes are less frequent a fact that may be attributable to times of low ground water table before 6000 years b p the ground water table was too low to permit observable liquefaction features to develop at the surface

quantification of the effects of eustasy subsidence and sediment supply on miocene sequences mid atlantic margin of the united states we use backstripping to quantify the roles of variations in global sea level eustasy subsidence and sediment supply on the development of the miocene stratigraphic record of the mid atlantic continental margin of the united states new jersey delaware and maryland eustasy is a primary infl uence on sequence patterns determining the global template of sequences i e times when sequences can be preserved and explaining similarities in miocene sequence architecture on margins throughout the world sequences can be correlated throughout the mid atlantic region with sr isotopic chronology 0 6 m y to 1 2 m y eight miocene sequences correlate regionally and can be correlated to global 18 o increases indicating glacioeustatic control this margin is dominated by passive subsidence with little evidence for active tectonic overprints except possibly in maryland during the early miocene however early miocene sequences in new jersey and delaware display a patchwork distribution that is attributable to minor tens of meters intervals of excess subsidence backstripping quantifi es that excess subsidence began in delaware at ca 21 ma and continued until 12 ma with maximum rates from ca 21 16 ma we attribute this enhanced subsidence to local fl exural response to the progradation of thick sequences offshore and adjacent to this area removing this excess subsidence in delaware yields a record that is remarkably similar to new jersey eustatic estimates we conclude that sea level rise and fall is a fi rstorder control on accommodation providing similar timing on all margins to the sequence record tectonic changes due to movement of the crust can overprint the record resulting in large gaps in the stratigraphic record smaller differences in sequences can be attributed to local fl exural loading effects particularly in regions experiencing largescale progradation

upper cretaceous sequences and sea level history new jersey coastal plain we developed a late cretaceous sea level estimate from upper cretaceous sequences at bass river and ancora new jersey odp ocean drilling program leg 174ax we dated 11 14 sequences by integrating sr isotope and biostratigraphy age resolution 0 5 m y and then estimated paleoenvironmental changes within the sequences from lithofacies and biofacies analyses sequences generally shallow up section from middle neritic to inner neritic paleodepths as shown by the transition from thin basal glauconite shelf sands transgressive systems tracts tst to medial prodelta silty clays highstand systems tracts hst and finally to upper delta front quartz sands hst sea level estimates obtained by backstripping accounting for paleodepth variations sediment loading compaction and basin subsidence indicate that large 25 m and rapid 1 m y sea level variations occurred during the late cretaceous greenhouse world the fact that the timing of upper cretaceous sequence boundaries in new jersey is similar to the sea level lowering records of exxon production research company epr northwest european sections and russian platform outcrops points to a global cause because backstripping seismicity seismic stratigraphic data and sediment distribution patterns all indicate minimal tectonic effects on the new jersey coastal plain we interpret that we have isolated a eustatic signature the only known mechanism that can explain such global changes glacio eustasy is consistent with foraminiferal 18 o data either continental ice sheets paced sea level changes during the late cretaceous or our understanding of causal mechanisms for global sea level change is fundamentally flawed comparison of our eustatic history with published ice sheet models and milankovitch predictions suggests that small 5 10 10 6 km 3 ephemeral and areally restricted antarctic ice sheets paced the late cretaceous global sea level change new jersey and russian eustatic estimates are typically one half of the epr amplitudes though this difference varies through time yielding markedly different eustatic curves we conclude that new jersey provides the best available estimate for late cretaceous sea level variations

role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges california evidence for intracontinental subduction bouguer gravity anomalies in the transverse ranges of southern california show a pronounced asymmetry in north south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike while individual gravity topography profile pairs could be consistent with a number of isostatic compensation mechanisms the unusual relationship between gravity and topography in the san bernardino mountains severely limits the class of acceptable models if the bouguer gravity field is caused by flexure of the crust mantle boundary the simplest solution consistent with the data in the western transverse ranges the san gabriel mountains and the san bernardino mountains models the great valley and mojave provinces to the north as a 50 km thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south the best fit to the gravity data in the san bernardino mountains requires a terminal bending moment on the southern plate beneath the mountains the magnitude of the terminal bending moment decreases to the west in the san gabriel and western transverse ranges we equate the load responsible for this terminal moment with the high velocity slablike feature mapped in the upper mantle beneath the transverse ranges using seismic tomography the coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small scale convective feature

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

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

the search for evidence of large prehistoric earthquakes along the atlantic seaboard the spacial distribution of seismically induced liquefaction features discovered along the atlantic seaboard suggest that during the last 2000 to 5000 years large earthquakes body wave magnitude m b 5 8 0 4 in this region may have been restricted exclusively to south carolina paleoliquefaction evidence for six large prehistoric earthquakes was discovered there at least five of these past events originated in the charleston south carolina area the locale of a magnitude 7 event in 1886 during the past two millennia large events may have occurred about every 500 to 600 years

investigation of coupling between surface processes and induced flow in the lower continental crust as a cause of intraplate seismicity many studies have highlighted the role of coupling between surface processes and flow in the lower continental crust in deforming the crust and creating topographic relief over quaternary timescales on the basis of the rheological knowledge gained it is suggested that intraplate seismicity can also be caused by coupling between surface processes and flow in the lower continental crust this view is shown to be a natural consequence of the modern idea that isostatic equilibrium is maintained by flow in the weak lower crust in response to erosion and sedimentation it is supported by a general correlation between the vigour of surface processes and rates of intraplate seismicity and by instances of seasonal seismicity that correlates with seasonal climate human interference in the environment can affect surface loading for instance deforestation for agriculture or urban development can cause increased erosion rates global warming is expected to cause increased storminess and thus increased erosion rates and or global sea level rise the possibility of increased rates of seismicity resulting from these processes should thus be considered in future hazard assessment copyright 2006 john wiley sons ltd

mining induced microseismicity monitoring and applications of imaging and source mechanism techniques the study of microseismicity in mines provides an ideal method for remote volumetric sampling of rock masses the nature and uniqueness of microseismic monitoring is outlined in the context of acquisition hardware and software requirements several topics are used to highlight the potential for novel applications of microseismicity and to outline areas where further study is required these topics reflect some of the current interest areas in seismology namelyb values and source parameters fault plane solutions modes of failure and moment tensor inversion imaging and seismicityvelocity correlations these studies suggest potential correlations between zones of high seismic velocity high microseismic activity and maximal stress drops which can be interpreted spatially to be the locations of highly stressed ground with a potential for rock bursting fault plane solutions are shown to be useful in determining the slip potential of various joint sets in a rock mass source parameter studies and moment tensor analysis clearly show the importance of non shear components of failure andb values for microseismicity appear to be magnitude limited and related to spatial rather than temporal variations in effective stress levels

fault plane solutions and relocations of recent earthquakes in middleton place summerville seismic zone near charleston south carolina the middleton place summerville seismic zone mpssz located about 20 km northwest of charleston is the most active seismic zone in south carolina between 1980 and 1991 58 events with md 0 8 to 3 3 were recorded in mpssz they lie in a diffuse area of 11 km by 14 km of which over two thirds are located in a narrow 5 km by 6 km zone the hypocentral depths range from 2 to 11 km with over 90 deeper than 4 km single fault plane solutions were obtained for 35 events based on the focal mechanisms the earthquakes were grouped into five subsets the mean p axis of all fault plane solutions is oriented n63 e in general agreement with the direction of shmax obtained from in situ stress measurements of the 35 events 18 are associated with reverse faulting on nw se striking and sw dipping fault planes these events were inferred to be associated with the ashley river fault zone which is not a planar feature but is composed of short segments of varying strikes n20 w to n70 w and dips 40 to 70 sw eleven events were associated with strike slip motion on nne ssw striking vertical faults and with thrust faulting on n s oriented faults dipping to the west respectively these two sets are identified as being parts of the woodstock fault zone the concentrated zone of seismicity included events associated with both the arf and wf zones suggesting that it is at the intersection of these two fault zones

geology of the continental margin beneath santa monica bay southern california from seismic reflection data we interpret seismic reflection data which were collected in santa monica bay using a 70 in 3 generator injector air gun to show the geologic structure of the continental shelf and slope and of the deep water santa monica and san pedro basins the goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults these data reveal that northwest of the palos verdes peninsula the palos verdes fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface however other major faults in the study area such as the dume and san pedro basin faults were active recently as indicated by an arched seafloor and offset shallow sediment rocks under the lower continental slope are deformed to differing degrees on opposite sides of santa monica canyon northwest of this canyon the continental slope is underlain by a little deformed sediment apron the main structures that deform this apron are two lower slope anticlines that extend toward point dume and are cored by faults showing reverse or thrust separation southeast of santa monica canyon lower slope rocks are deformed by a complex arrangement of strike slip normal and reverse faults the san pedro escarpment rises abruptly along the southeast side of santa monica canyon reverse faults and folds underpinning this escarpment steepen progressively southeastward locally they form flower structures and cut downward into basement rocks these faults merge downward with the san pedro basin fault zone which is nearly vertical and strike slip the escarpment and its attendant structures diverge from this strike slip fault zone and extend for 60 km along the margin separating the continental shelf from the deep water basins the deep water santa monica basin has large extent but is filled with only a thin less than 1 5 km section of what are probably post miocene rocks and sediment extrapolating ages obtained from ocean drilling program site 1015 indicates that this sedimentary cover is quaternary possibly no older than 600 ka folds and faults along the base of the san pedro escarpment began to form during 8 13 ka ago refraction velocity data show that high velocity rocks probably the catalina schist or miocene volcanic rocks underlie the sedimentary section the san pedro basin developed along a strike slip fault widens to the southeast and is deformed by faults having apparent reverse separation and by folds near redondo canyon and the palos verdes peninsula

holocene to pliocene tectonic evolution of the region offshore of the los angeles urban corridor southern california 1 quaternary tectonism in the coastal belt of the los angeles urban corridor is diverse in this paper we report the results of studies of multibeam bathymetry and a network of seismic reflection profiles that have been aimed at deciphering the diverse tectonism and at evaluating the relevance of published explanations of the region s tectonic history rapid uplift subsidence in basins folds and thrusts extensional faulting and strike slip faulting have all been active at one place or another throughout the quaternary period the tectonic strain is reflected in the modern physiography at all scales los angeles la basin has filled from a deep submarine basin to its present condition with sediment impounded behind a large sill formed behind uplifts near the present shoreline newport trough to the south southeast of la basin also accumulated a large volume of sediment but remained at midbathyal depths throughout the period there is little or no evidence of quaternary extensional tectonism in either basin although as much as 6 km of subsidence which mainly occurred by sagging has been recorded in places since the middle miocene the uplifts include folded and thrust faulted terranes in the palos verdes hills and the shelves of santa monica and san pedro bays the uplifted areas have been shortened in a southwest northeast direction by 10 or slightly more and some folds are reflected in the bathymetry two large adjacent midbathyal basins santa monica and san pedro show strong evidence of subsidence and slight west northwest extension 10 during the same time folding was taking place in the uplifts the tectonic boundaries between uplifts and basins are folded normal faulted reverse faulted and strike slip faulted depending on location the rapid quaternary uplift and subsidence along with the filling of la basin have produced a reversal in the regional physiography in the early pliocene la basin was a submarine deep palos verdes and the shelves comprised a northeast basin slope and the present offshore basins and catalina island formed an emergent or shallowly submerged shelf since extensional compressional and lateral strains are all locally in evidence simple notions that this part of southern california underwent a change from miocene transtension to quaternary transpression fail to explain our observations

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

evidence of uplift near charleston south carolina in spite of extensive research the causal structure of the 1886 magnitude 7 earthquake near charleston south carolina has not been identified in this study the authors analyzed digital surface topography and river morphology in light of earlier studies using seismic reflection seismic refraction earthquake seismology and gravity and magnetic surveys this analysis revealed an area approximately 400 km sup 2 northwest of charleston that may have been repeatedly uplifted by earthquakes geologic and seismic reflection data confirm alteration of formations at depth deformation of the surface is supported by observations on aerial and landsat photographs therefore the structure on which the 1886 earthquake occurred may be within the uplifted area defined in this report

three dimensional seismo tectonic imaging an example from the southern california transverse ranges abstract we present a new technique to image deformation 3d seismo tectonic imaging which juxtaposes instantaneous and accumulated deformation by combining earthquake and structural geology data we test the technique using three independent data sets we find concordance between the predicted finite deformation field and related instantaneous deformation surfaces inferred from aftershock and background seismicity data as well as structure imaged by seismic reflection data these results indicate that 1 the 1994 northridge event ruptured a transverse high angle reverse fault linking two ew striking listric thrust systems the pico and oak ridge the rupture terminated on the thrusts and was capped by a north dipping forward breaking imbricate of the santa susana fault 2 the los angeles seismic zone is a region of thrusting associated with the development of a new mountain front forming south of the present one represented by the santa monica anticline the thrust is rooted on a mid crustal ramp forming beneath the los angeles basin we conclude that seismo tectonic imaging by linking earthquake data to an empirically based quantitative description of the finite deformation field provides a new and fruitful method for integrating geological and seismological data

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

coseismic folding earthquake recurrence and the 1987 source mechanism at whittier narrows los angeles basin california static deformation associated with the october 1 1987 whittier narrows california m 6 0 earthquake was detected by geodetic elevation changes the earthquake uplifted a 1 5 km high quaternary fold the santa monica mountains anticlinorium by 50 mm but caused no fault rupture at the ground surface this suggests that folding and faulting of the los angeles basin sediments are coincident and continuing after correction for surveying errors and nontectonic subsidence we model the 214 geodetic observations with a simple dislocation in an elastic half space a thrust fault with reverse slip of 1 1 0 3 m and dipping 30 4 n with an upper edge at a depth of 12 1 km and a lower edge at 17 1 km fits the geodetic data best and is consistent with the main shock hypocenter fault plane solution and initial aftershock distribution the upper limit on the static stress drop is 17 5 5 0 mpa 175 50 bar the lower limit on the geodetic moment is 1 0 0 2 1025 dyne cm in accord with seismic estimates indicating that most of the slip took place during seismic rupture if earthquakes of m 6 characterize the blind thrust fault on which the earthquake occurred we estimate a 200 year repeat time at whittier narrows and a 5 18 year rate of earthquake occurrence within a 150 km long band in the northern los angeles basin and santa monica bay to the west the 25 year rate of historical occurrence since 1860 is less than this prediction the deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here

seismotectonics of the newport inglewood fault zone in the los angeles basin southern california the newport inglewood fault zone nif strikes northwest along the western margin of the los angeles basin in southern california the seismicity 1973 to 1985 of m l 2 5 that occurred within a 20 km wide rectangle centered on the nif extending from the santa monica fault in the north to newport beach in the south is analyzed a simultaneous full inversion scheme velest is used to invert for hypocentral parameters two velocity models and a set of station delays arrival time data from three quarry blasts are included to stabilize the inversion the first velocity model applies to stations located along the rim and outside the los angeles basin and is well resolved it is almost identical to the starting model which is the model routinely used by the cit usgs southern california seismic network for locating local earthquakes the second velocity model applies to stations located within the los angeles basin it shows significantly lower velocities down to depths of 12 to 16 km which is consistent with basement of catalina schist below the sediments in the western los angeles basin the distribution of relocated hypocenters shows an improved correspondence to mapped surface traces of late quaternary fault segments of the nif a diffuse trend of seismicity is observed along the inglewood fault from the dominguez hills across the baldwin hills to the santa monica fault in the north the seismicity adjacent to long beach however is offset 4 to 5 km to the east near the trace of the subsurface los alamitos fault the depth distribution of earthquakes along the nif shows clustering from 6 to 11 km depth which is similar to average seismogenic depths in southern california thirty nine single event focal mechanisms of small earthquakes 1977 to 1985 show mostly strike slip faulting with some reverse faulting along the north segment north of dominguez hills and some normal faulting along the south segment south of dominguez hills to newport beach the results of an inversion of the focal mechanism data for orientations of the principal stress axes and their relative magnitudes indicate that the minimum principal stress is vertical along the north segment while the intermediate stress is vertical along the south segment the maximum principal stress axis is oriented 10 to 25 east of north reverse faulting along the north segment indicates that a transition zone of mostly compressive deformation exists between the los angeles block and the central transverse ranges

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

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

earthquake locations in the inner continental borderland offshore southern california the inner continental borderland region offshore southern california is tectonically active and contains several faults that are potential seismic hazards to nearby cities however fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms to improve the accuracy of event locations in this area we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the borderland the regions are defined by either temporal or spatial clustering of seismic activity in the southern california seismic network scsn catalog obtaining accurate locations for these events is difficult due to the lack of nearby stations the limited azimuthal coverage and uncertainties in the velocity structure for this area our location procedure is based on the l 1 norm grid search waveform cross correlation method of shearer 1997 except that we use a nearest neighbor approach astiz et al 2000 to identify suitable event pairs for waveform cross correlation and we explore the effect of different velocity models on the locations and associated station terms in general our relocated events have small estimated relative location errors and the events are more clustered than the scsn catalog locations a quarry on the south tip of catalina island provides a test of our location accuracy and suggests that under ideal conditions offshore events can be located to within 1 to 2 km of their true locations our final locations for most clusters are well correlated with known local tectonic features we relate the 1981 santa barbara island m l 5 3 earthquake with the santa cruz fault the 13 july 1986 oceanside m l 5 3 sequence with the san diego trough fault zone and events near san clemente island with the known trace of the san clemente fault zone over 3000 of the offshore events during this time period are associated with the 1986 oceanside earthquake and its extended aftershock sequence our locations define a northeast dipping fault plane for the oceanside sequence but in cross section the events are scattered over a broad zone about 4 km thick this could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure events that occur near coronado bank in the scsn catalog are relocated closer to the san diego coast and suggest a shallow angle northeast dipping fault plane at 10 to 15 km depth manuscript received 12 february 1999

liquefaction potential assessment of pleistocene beach sands near charleston south carolina abstract liquefaction potential of four pleistocene beach sand deposits in the greater charleston area south carolina is assessed the assessment is based on a review of 51 sites of conspicuous craterlets and horizontal ground displacement that occurred in beach sand deposits during the 1886 charleston earthquake and an analysis of 82 seismic cone penetration tests with pore pressure measurements of the 51 ground failure sites 23 are associated with the ten mile hill beds 13 with the wando formation 13 with the silver bluff terrace and younger deposits that lie adjacent to the harbor rivers and creeks and two with the ladson formation liquefaction potential is analyzed using the seismic cone data with and without correction for age related processes diagenesis and then expressed in terms of the liquefaction potential index lpi probability curves are developed from the lpi calculations for different earthquake ground shaking parameters the probability curves for the wando formation overpred

an analysis and comparison of observed pleistocene south carolina usa shoreline elevations with predicted elevations derived from marine oxygen isotope stages geological maps of south carolina covering 6800 km2 confirm the existence of eight preserved pleistocene shorelines above current sea level marietta 42 6 m wicomico 27 4 m penholoway 21 3 m ladson 17 4 m ten mile hill 10 7 m pamlico 6 7 m princess anne 5 2 m and silver bluff 3 m current geochronologic data suggest that these eight shorelines correlate with marine oxygen isotope stages mis as follows marietta older than mis 77 wicomico mis 55 45 penholoway mis 19 or 17 ladson mis 11 ten mile hill mis 7 pamlico mis 5 princess anne mis 5 and silver bluff mis 5 or 3 except for the mis 5e pamlico and possibly the mis 11 ladson the south carolina elevations are higher than predicted by isotope proxy based reconstructions the 4 m of total relief from the pamlico to the silver bluff shoreline in south carolina for which other reconstructions suggest an expected relief of 80 m illustrates the lack of match our results suggest that processes affecting either post depositional changes in shoreline elevations or the creation of proxy sea level estimates must be considered before using paleo sea level position on continental margins

the 1987 whittier narrows earthquake in the los angeles metropolitan area california the whittier narrows earthquake sequence local magnitude m l 5 9 which caused over 358 million damage indicates that assessments of earthquake hazards in the los angeles metropolitan area may be underestimated the sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east west length of the northern margin of the los angeles basin peak horizontal accelerations from the main shock which were measured at ground level and in structures were as high as 0 6g where g is the acceleration of gravity at sea level within 50 kilometers of the epicenter the distribution of the modified mercalli intensity vii reflects a broad north south elongated zone of damage that is approximately centered on the main shock epicenter

qβ estimates from spectral ratios and multiple lapse time window analysis results from an underground research laboratory in granite an experiment to measure quality factor q in situ was carried out at the underground research laboratory in manitoba canada using the spectral ratio technique and a form of the mulitple lapse time window analysis technique the volume of rock examined measures approximately 30 30 30 m at a depth of 420 m in lac du bonnet granite the site encompases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls the source for the seismic pulses was a mechanical hammer placed at the bottom of specially located boreholes results from the spectral ratio experiment gave s wave quality factor q of 114 18 based upon the short duration of the first arrival pulses and low scattered wave energy in the signal coda we conclude that the spectral ratio technique measured the intrinsic attenuation of the rock mass further analysis using the more recently developed multiple lapse time window analysis gave results for intrinsic q qi of between 85 and 100 conversely values for scattering q qs were greater than 800 the high correlation for qi values between the two methods help to confirm the nature and absolute values of the attenuation and also support the use of the later technique where spectral ratios cannot be applied

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

estimated ground motions in los angeles due to mw 7 earthquake on the elysian thrust fault abstract the elysian thrust fault has been identified as a blind thrust fault davis et al 1989 hauksson and jones 1989 representing a potentially serious seismic hazard to the metropolitan los angeles and its neighboring areas we have simulated time histories peak ground accelerations and their uncertainties using a semi empirical method for a m w 7 0 earthquake on the elysian thrust fault using a flat layered crustal structure the accelerograms from the 4 october 1987 whittier narrows aftershock 10h 50m m l 5 3 are used to represent the source functions of each subfault on the fault surface to account for the velocity variation in the surface sediments we compared simulated ground motions using two separate shearwave velocities 0 6 km sec and 0 9 km sec respectively in the top layer of the crustal model the use of such a simple crustal model is validated by modeling the accelerograms recorded during the 1 october 1987 whittier narrows mainshock m l 6 0 the duration and the relative frequency content observed on these accelerograms are successfully modeled at some stations the simulated peak accelerations agree well with the observed values however at some sites the simulated and observed values differ by a factor of 2 to 3 the variation is attributed to a laterally varying crustal structure that is significant for small earthquakes additional site specific study is needed for an improved prediction of the observed peak accelerations for large magnitude earthquakes the near source peak ground acceleration appears to be controlled by the source receiver geometry relative to the fault as well as the location of asperities on the fault surface this is validated by simulating the peak acceleration data of the 1989 loma prieta earthquake recorded within 30 km of the source for our simulations on the elysian thrust fault an additional component is added to the uncertainty by analyzing several asperity models finally an analytical representation is given to the simulated average peak horizontal ground accelerations y r which is expressed by ln y r 5 38 0 085 2 09 0 0268 ln r 8 0 0 343 in the range of 7 5 r 35 km r is the closest distance to the seismogenic rupture zone this functional form predicts a rapid fall off rate for the simulated ground motions compared with the fall off rate predicted by the other published empirical attenuation relations we use the finite difference method to investigate the effects of irregular structure on ground motions resulting from point sources on the elysian park fault the response computed at several depths beneath the basin suggests that the response is dominated by the initial direct arrivals for sources located interior to the basin when the receivers are located at one end of the basin and the seismic sources are located at the opposite edge the characteristic features on the seismograms are the long durations caused by the trapped energy within the basin the level of trapped energy decreases as the depth of the source increases thus the fault model becomes important in determining the level of peak ground motions and shaking duration

geologic evidence for recurrent moderate to large earthquakes near charleston south carolina multiple generations of earthquake induced sand blows in quaternary sediments and soils near charleston south carolina are evidence of recurrent moderate to large earthquakes in that area the large 1886 earthquake the only historic earthquake known to have produced sand blows at charleston probably caused the youngest observed blows older late quaternary sand blows in the charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event

evidence of possible tectonic upwarping along the south carolina coastal plain from an examination of river morphology and elevation data although the fault s responsible for the 1886 charleston south carolina earthquake have not yet been identified primarily because of the lack of surface rupture evaluation of landsat imagery aerial photography and topographic maps have revealed an 200 km long 15 km wide north northeast trending zone composed of subtle topographic highs and morphologic changes in rivers that may be associated with tectonic activity river anomalies observed within this zone include river bends that are convex toward the north northeast incised channels changes in river patterns and convex upward longitudinal profiles analyses of geologic and geophysical data further indicate that these surface features may be the result of ongoing tectonic uplift along a north northeast trending fault zone possibly associated with recent seismicity near charleston

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

a new seismic site coefficient model based on conditions in the south carolina coastal plain abstract a new seismic site coefficient model is developed in this article from the results of over 48 000 total stress 1d equivalent linear and nonlinear ground response simulations assuming conditions at seven locations in the south carolina coastal plain site coefficients f computed from the simulations are plotted versus average shear wave velocity in the top 30 m v s 30 and grouped by location spectral acceleration and spectral period in all plots it can be seen that f increases from zero to a zone of peak values as v s 30 increases from zero and then f decreases to a value of 1 as v s 30 approaches the reference soft rock value variables found to be most influential on f in addition to v s 30 and spectral acceleration are stiffness of material in the top 100 m depth to top of rock and frequency content of the rock outcrop motion a mathematical model for median values of f defined by these five variables is proposed significant differences exist between the new seismic site coefficient model and the site coefficients commonly assumed in current seismic design codes particularly where v s 30 is

a laboratory acoustic emission experiment under in situ conditions in this paper we revisit acoustic emission ae data from an in situ rock fracture experiment conducted at the underground research laboratory url in manitoba canada the mine by experiment a large scale excavation response test was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel during the experiment a small array of 16 panametrics v103 ae sensors enclosed a 0 7 m 0 7 m 1 1 m rectangular prism of lac du bonnet granite located in the tunnel wall the v103 sensors were later calibrated in the laboratory and a source parameter analysis was undertaken using a spectral fitting method corner frequency and moment magnitude were found to be inside the ranges 250 khz fc 490 khz and 7 5 mw 6 8 respectively static stress drops ranged from 0 3 to 4 mpa which is consistent with large seismicity recorded at the url

long period ground motions and dynamic strain field of los angeles basin during large earthquakes abstract during a big earthquake along the san andreas fault in southern california high excitation and low attenuation of long period 3 to 10 sec strong ground motions will cause wave motions to propagate efficiently far from the epicentral area these ground motions could potentially be destructive to large dimension structures in the los angeles basin we performed calculations using the surface wave gaussian beam method for a 3d southern california crustal structure displacement field as well as the associated dynamic strain field produced by large propagating ruptures along the san andreas fault are evaluated results indicate that in the presence of lateral heterogeneity focusing and multipathing interference contribute significantly to a complex pattern of the displacement field and the associated dynamic strain field for a big event on the san andreas fault with a seismic moment of 1 8 10 28 dyne cm long period displacement in the los angeles basin could reach a maximum amplitude of meters in places since this calculation is fast we have evaluated the displacement field for a dense grid of points a differentiation gives the corresponding effective horizontal dynamic strain field at times the maximum effective dynamic strains may reach mid 10 3 to even 10 3 high enough to be of engineering concern this computational result probably gives the upper bound values due to the large source assumed for events of smaller seismic moment release along less extensive ruptures these results can easily be scaled down proportionally different scenarios are considered in this study with different slip distributions it is found that with a given seismic moment a more evenly distributed fault slip over the rupture surface will result in lower peak values on both displacements and dynamic strains our displacement results give similar values to those obtained by kanamori using empirical green9s functions but substantially higher than bouchon and aki9s results

direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern california in inverting populations of focal mechanisms to estimate stress misfits may arise owing to 1 errors in determining the fault plane solutions and 2 real variations in the state of stress among a number of techniques that invert for stress most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on p wave first motions in a highly nonlinear manner which is difficult to propagate into stress inversions to address these issues we present a new method for constraining stress tensors in which first motion observations are directly inverted for stress without assuming that focal mechanisms are known the technique produces estimates of four stress parameters a suite of focal mechanisms consistent with both the stress tensor and the first motions and estimates of the uncertainty in stress it also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor when tested against artificial data the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks as expected utilizing first motions from the southern california catalog the method is applied to several clusters and aftershock sets from 1981 to 1992 typical uncertainties in stress orientation exceed 20 30 at the 95 confidence level resulting from 5 mispick rates typical of even the cleanest data these uncertainties exceed some previous estimates for similar populations presumably because they account for focal mechanism un certainties and perhaps indicating that some previously reported stress variations are not statistically significant most populations are found to be internally consistent with a homogeneous stress field the most significant exceptions being aftershock sequences of some major earthquakes in at least one case the 1992 landers sequence aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses overall the method provides a useful and robust approach for evaluating stress constraints provided by p wave first motions

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

fault geometry and earthquakes in continental interiors abstract although large earthquakes in continental interiors are much less frequent than those along plate boundaries they have been responsible for a disproportionate amount of destruction i present a testable model for strain accumulation and seismicity in the upper crust of continental interiors intersecting faults provide a focus for strain build up resulting in local pockets of high strain accumulation rigid plates typically deform at strain rates 10 10 to 10 9 strain yr 1 but at intersecting fault zones strain accumulates at anomalously high rates of 10 8 to 10 7 strain yr 1 i e the strain rates are comparable with those at plate boundaries the resulting large earthquakes are associated with relatively short return times hundreds of years typically one of the intersecting faults appears to split and to offset the other main fault defined as a fault on which the larger offset occurs forming two parallel strands separated by a jog i also observe a rotation of one leg of the main fault with respect to the other the region of intersection is usually associated with intense fracturing increased microseismicity and uplift

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

seismicity and active compressional tectonics in santa monica bay southern california during 1973 1986 the seismicity in santa monica bay consisted of an average of four events per year of m l 2 5 in addition to the 1979 malibu m l 5 0 mainshock aftershock sequence the 1979 sequence ruptured a north dipping thrust fault at 10 15 km depth beneath the bay a diffuse distribution of seismicity extends across the bay from the terminus of the palos verdes fault to point dume two thirds of the single event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west northwest striking planes a third of the focal mechanisms exhibit strike slip faulting on north to north northwest striking planes a stress inversion of the focal mechanism data shows that the maximum principal stress is oriented n13 e in a horizontal plane and the minimum principal stress is oriented n102 e and plunging 47 hence the tectonics in santa monica bay are dominated by compression with thrust reverse and strike slip faulting the coexistence of thrust and strike slip faults suggests that the bay is not a part of the peninsular ranges terrane but forms a transition zone that accommodates the change from strike slip in the peninsular ranges to the south to reverse faulting in the transverse ranges to the north mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to los angeles and the southern california coastal zone through seismic shaking and possible generation of small tsunamis

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

modern seismicity and the fault responsible for the 1886 charleston south carolina earthquake abstract an eight station seismic network was installed in august 2011 in the epicentral area of the 1886 charleston south carolina earthquake within and near the town of summerville the network operated for one year and located 134 earthquakes with duration magnitudes ranging from 1 8 to 2 6 the earthquakes occurred in a tabular zone striking n186 e with dip of about 43 to the west the focal depths range from 13 km to the top of the early mesozoic section at a depth 1 km the dimensions of the seismic zone are 23 km along strike and 15 km down dip many of the earthquakes are concentrated in the 2 6 5 km depth range near shallow faults imaged on seismic reflection profiles adjacent to the ashley river our hypocenter relocations of 228 earthquakes occurring in the period 1977 2005 are consistent with the results obtained from the recent one year station deployment the 48 well constrained focal mechanisms derived from the one year deployment are predominantly reverse motion and 30 exhibit north south trending nodal planes consistent with the orientation of the seismic zone the seismicity near summerville has characteristics in common with the aftershock sequence of the 23 august 2011 mineral virginia earthquake we interpret these results to indicate that the modern seismicity is the lingering aftershock sequence of the 1886 shock and that the mainshock occurred on a south striking west dipping fault plane with predominantly reverse motion possibly with a component of right lateral strike slip most moment release probably occurred at depths greater than 6 km online material tables listing hypocenters and focal mechanisms and an animation showing hypocenter profiles oriented along different azimuths

internally consistent pattern of seismicity near charleston south carolina an improved velocity model for the meizoseismal area of the 1886 charleston earthquake was used to relocate current seismicity which showed marked separation into clusters the relocated hypocenters and composite focal plane solutions were compared with available geophysical data to interpret their tectonic significance and possible association with the 1886 earthquakes there is a distinct velocity discontinuity at a depth of about 10 km where vp increases from 5 9 to 6 45 km s the relocated hypocenters and composite focal plane solutions delineate two main source zones lying at different depths the shallower zone at 4 to 8 km depth and collinear with the ashley river is herein named the ashley river seismogenic zone the composite focal plane solution suggests reverse faulting on a steeply dipping northwest striking fault with the southwest side upthrown this zone is also associated with aeromagnetic and gravity anomalies the deeper zone at 9 to 13 km suggests a right slip on a fault extending n26 e from east of ravanel to jedburg a distance of more than 25 km and dipping steeply to the west northwest its location and extent are similar to the so called woodstock fault examination of geomorphic data suggests that there may be some ongoing tectonic uplift and subsidence in the area the inferred p axes from fault plane solutions are oriented s60 w firsthand accounts of the 1886 earthquakes suggest that two source areas were active in 1886 and the months that followed i postulate that the two zones of current seismicity are coincident with the 1886 source areas

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

paleoseismic evidence for recurrence of earthquakes near charleston south carolina a destructive earthquake that occurred in 1886 near charleston south carolina was associated with widespread liquefaction of shallow sand structures and their extravasation to the surface several seismically induced paleoliquefaction structures preserved within the shallow sediments in the meizoseismal area of the 1886 event were identified field evidence and radiocarbon dates suggest that at least two earthquakes of magnitudes greater than 6 2 preceded the 1886 event in the past 3000 to 3700 years the evidence yielded an initial estimate of about 1500 to 1800 years for the maximum recurrence of destructive intraplate earthquakes in the charleston region

the 1987 whittier narrows california earthquake a metropolitan shock just 3 hours after the whittier narrows earthquake struck it became clear that a heretofore unseen geological structure was seismically active beneath metropolitan los angeles contrary to initial expectations of strike slip or oblique slip motion on the whittier fault whose north end abuts the aftershock zone the focal mechanism of the mainshock showed pure thrust faulting on a deep gently inclined surface hauksson et al 1988 this collection of nine research reports spans the spectrum of seismological geodetic and geological investigations carried out as a result of the whittier narrows earthquake although unseen the structure was not unforeseen namson 1987 had published a retrodeformable geologic cross section meaning that the sedimentary strata could be restored to their original depositional position 100 km to the west of the future earthquake epicenter in which blind or subsurface thrust faults were interpreted to be active beneath the folded southern transverse ranges working 25 km to the west hauksson 1987 had also found a surprising number of microearthquakes with thrust focal mechanisms south of the santa monica mountains another clue to a subsurface system of thrust faults finally davis 1987 had presented a preliminary cross section only 18 km to the west of whittier narrows that identified as fault b the thrust that would rupture later that year not only was the earthquake focus and its orientation compatible with the 10 15 km depth and north dipping orientation of davis proposed thrust but fault b appears to continue beneath the northern flank of the los angeles basin skirting within 5 km of downtown los angeles an area of dense commercial high rise building development these results are refined and extended by davis et al this issue

structural and tectonic setting of the charleston south carolina region evidence from the tertiary stratigraphic record eleven upper eocene through pliocene stratigraphic units occur in the subsurface of the region surrounding charleston south carolina these units contain a wealth of information concerning the long term tectonic and structural setting of that area these stratigraphic units have a mosaic pattern of distribution rather than a simple layered pattern because deposition erosion and tectonic warping have interacted in a complex manner through time by generating separate structure contour maps for the base of each stratigraphic unit an estimate of the original basal surface of each unit can be reconstructed over wide areas changes in sea level over geologic time generate patterns of deposition and erosion that are geographically unique for the time of each transgression such patterns fail to persist when compared sequentially over time in some areas however there has been persistent repetitive net downward or upward movement over the past 34 m y these repetitive patterns of persistent motion are most readily attributable to tectonism the spatial pattern of these high and low areas is complex but it appears to correlate well with known tectonic features of the region this correlation suggests that the tectonic setting of the charleston region is controlled by scissors like compression on a crustal block located between the north trending adams run fault and the northwest trending charleston fault tectonism is localized in the charleston region because it lies within a discrete hinge zone that accommodates structural movement between the cape fear arch and the southeast georgia embayment