correlation of changes in gravity elevation and strain in southern california measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity elevation and strain in several southern california networks precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long laser ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern california during 1978 and 1979 although the precision of these measuring systems has been questioned the rather good agreement among them suggests that the observed changes reflect true crustal deformation

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

southern california permanent gps geodetic array continuous measurements of regional crustal deformation between the 1992 landers and 1994 northridge earthquakes the southern california permanent gps geodetic array pgga was established in 1990 across the pacific north america plate boundary to continuously monitor crustal deformation we describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes we compare displacement rates at four site locations with those reported by feigl et al 1993 which were derived from an independent set of global positioning system gps and very long baseline interferometry vlbi measurements collected over nearly a decade prior to the landers earthquake the velocity differences for three sites 65 100 km from the earthquake s epicenter are of order of 3 5 mm yr and are systematically coupled with the corresponding directions of coseismic displacement the fourth site 300 km from the epicenter shows no significant velocity difference these observations suggest large scale postseismic deformation with a relaxation time of at least 800 days the statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets two possible noise models fit the pgga data equally well as described in the companion paper by zhang et al this issue the pre landers data are too sparse and heterogeneous to derive a reliable noise model under a fractal white noise model for the pgga data we find that the velocity differences for all three sites are statistically different at the 99 significance level a white noise plus flicker noise model results in significance levels of only 94 43 and 88 additional investigations of the pre landers data and analysis of longer spans of pgga data could have an important effect on the significance of these results and will be addressed in future work

nature of the recent vertical ground movements inferred from high precision leveling data in an intraplate setting ne ardenne belgium yearly high precision leveling surveys have been performed from 1993 to 1998 along a 48 km long local network centered on gileppe lake at the foot of the ardenne massif belgium institut geographique militaire and institut geographique national igm ign levelings of 1948 and 1974 yielded additional information the data were corrected for collimation and thermal errors other sources of error earth tides atmospheric refraction rod miscalibration were estimated and corrections were made when necessary random errors of individual surveys range from 0 15 to 0 74 mm km the recorded elevation changes indicate the existence of tilted segments some 5 to 10 km in length with yearly tilt values amounting up to 2 rads fault movements of a few mm yr are also demonstrated the segment limits are spatially stable from one year to the next but very frequent sense reversals of the vertical motions are observed this explains why the total movement over a 20 to 30 year period is barely higher than the yearly displacements various nontectonic causes of vertical ground movement e g monument instability varying water loading of the lake sediment compaction etc have been considered yielding little satisfactory explanation of the patterns of observed elevation changes we propose that the recorded motions which we call reversible fault creep and reversible tilt respectively are a true tectonic signal reflecting the tectonic noise which accompanies the long term deformation of uplifting areas in intraplate settings such movements are characteristic of fault segments which may occasionally generate earthquakes

the heterogeneous upper mantle low velocity zone abstract the upper mantle low velocity zone lvz is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals the zone is present below a relatively constant depth of 100 km in most continental parts of the world both in cratonic areas with high average velocity and tectonically active areas with low average velocity evidence for the low velocity zone arises from controlled and natural source seismology including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces calculations of receiver functions and absolute velocity tomography the available data indicates a more pronounced reduction in seismic velocity and q value for s waves than p waves as well as high electrical conductivity in the lvz seismic waves are strongly scattered by the zone which demonstrates the existence of small scale heterogeneity the depth to the base of the lvz is systematically shallower in cold stable cratonic areas than in hot active regions of the world because of its global occurrence below a relative constant depth of 100 km the lvz cannot be explained by metamorphic or compositional variation and rheological changes calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth provided that the rocks contain water and carbon dioxide the presence of even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80 100 km depth the seismic velocities and q values are significantly reduced of rocks which are close to the solidus or contain small amounts of partial melt hence the lvz may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km assuming that the rocks contain only limited amounts of fluids this mechanism may explain the low velocities q values and resistivity as well as the intrinsic scattering and the characteristic variation in thickness of the low velocity zone

three dimensional prestack plane wave migration of teleseismic p to s converted phases 1 theory 1 we present the theoretical foundations for a prestack migration technique to image teleseismic p to s converted phases the method builds on teleseismic p wave deconvolution pseudostation stacking neal and pavlis 1999 and on the idea of using a plane wave decomposition for imaging as introduced by treitel et al 1982 deconvolution operators are constructed by pseudostation stacking of the array aligned to the incident p wave arrival times to produce a space variable deconvolution operator the resulting data are then muted to remove the deconvolved direct p wave pulse and pseudostation stacked over a grid of feasible slowness vectors the pseudostation stack interpolates the wave field onto a regular grid along earth s surface producing a series one per slowness vector of uniformly sampled three dimensional data cubes two space variables and time the plane wave components can be propagated downward using a form of approximate ray tracing with a three dimensional earth model this yields a series of distorted cubes topologically equivalent to the original uniformly sampled data cubes these data volumes are summed as a weighted stack with the weights derived from an integration formula for inverse scattering based on the generalized radon transform this allows an image of the subsurface to be constructed on an event by event basis beneath the array we apply this technique to data from the lodore array that was deployed in northwestern colorado the results suggest the presence of a major lithospheric scale discontinuity defined by a south dipping boundary

imaging crustal discontinuities and the downgoing slab beneath western crete the migration of teleseismic receiver functions yields high resolution images of the crustal structure of western crete data were collected during two field campaigns in 1996 and 1997 by networks of six and 47 short period three component seismic stations respectively a total of 1288 seismograms from 97 teleseismic events were restituted to true ground displacement within a period range from 0 5 to 7 s the application of a noise adaptive deconvolution filter and a new polarization analysis technique helped to overcome problems with local coda and noise conditions the computation and migration of receiver functions results in images of local crustal structures with unprecedented spatial resolution for this region the crust under crete consists of a continental top layer of 15 20 km thickness above a 20 30 km thick subducted fossil accretionary wedge with a characteristic en echelon fault sequence the downgoing oceanic moho lies at a depth of 40 60 km and shows a topography or undulation with an amplitude of several kilometres as a consequence of slab depth and distribution of local seismicity the mediterranean ridge is interpreted as the recent accretionary wedge

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

a seismic based cross section of the grenville orogen in southern ontario and western quebec a schematic crustal cross section is presented for the southwestern grenville province based on reprocessed lithoprobe near vertical incidence seismic reflection data and compiled seismic refraction wide angle velocity models interpreted with geological constraints the schematic crustal architecture of the southwest grenville province from southeast to northwest comprises allochthonous crustal elements frontenac adirondack belt and composite arc belt that were assembled prior to ca 1160 ma and then deformed and transported northwest over reworked rocks of pre grenvillian laurentia and the laurentian margin primarily between 1120 and 980 ma reworked pre grenvillian laurentia and laurentian margin rocks are interpreted to extend at least 350 km southeast of the grenville front beneath all of the composite arc belt three major structural boundary zones the grenville front and adjacent grenville front tectonic zone the central metasedimentary belt boundary thrust zone and the elzevir frontenac bou

crustal thickness and vp vs variations in the grenville orogen ontario canada from analysis of teleseismic receiver functions abstract we have developed a simple semblance weighted stacking technique to estimate crustal thickness and average vp vs ratio using teleseismic receiver functions we have applied our method to data from 32 broadband seismograph stations that cover a 700 400 km2 region of the grenville orogen a 1 2 0 98 ga himalayan scale collisional belt in eastern north america our seismograph network partly overlaps with l ithoprobe and other crustal refraction surveys in 8 out of 9 cases where a crustal refraction profile passes within 30 km of a seismograph station the two independent crustal thickness estimates agree to within 7 our regional crustal thickness model constructed using both teleseismic and refraction observations ranges between 34 0 and 52 4 km crustal thickness trends show a strong correlation with geological belts but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium the thickest crust 52 4 1 7 km was found at a station located within the 1 1 ga mid continent failed rift the central gneiss belt which contains rocks exhumed from deep levels of the crust is characterized by vp vs ranging from 1 78 to 1 85 in other parts of the grenville orogen vp vs is found to be generally less than 1 80 the thinnest crust 34 5 37 0 km occurs northeast of the 0 7 ga ottawa bonnechere graben and correlates with areas of high intraplate seismicity

the 1979 palmdale california strain event in retrospect although the long term 1971 1984 strain accumulation across the san andreas fault near palmdale california is simply right lateral shear at the rate of 0 20 0 01 strain a tensor shear annual measurements indicate that an anomalous contraction perpendicular to the fault accumulated in the interval 1973 1978 and was abruptly released in mid 1979 this anomaly is referred to as the 1979 palmdale strain event more frequent measurements of strain since 1979 suggest a more irregular accumulation of strain than prior to 1979 this irregularity may be due to measurement errors larger than previously estimated as a consequence of this post 1979 irregularity the 1979 strain event no longer appears as anomalous as it had in the shorter 1971 1980 record to provide independent support for existence of the 1979 palmdale strain event we review gravity measurements at palmdale by jachens 1983 and water level measurements in wells near palmdale by merifield and lamar 1981 that have previously been shown to provide a marginal correlation with the 1979 strain recovery these data taken together with the original strain observations suggest a real strain anomaly did occur near palmdale in 1979 but the evidence is not overwhelming

shear wave structure in the grenville province beneath the lower great lakes region from rayleigh wave tomography 1 we have constructed a high resolution 3 d shear wave velocity model of the upper mantle beneath the grenville province in the lower great lakes region a two plane wave tomography method was applied to fundamental mode rayleigh wave data recorded at 35 broadband seismic stations rayleigh wave phase velocities were obtained at 15 periods from 20 s to 143 s and then inverted for shear wave velocity variation a high velocity lid to 150 km depth is imaged beneath most of the studied grenville province one low velocity anomaly of 4 appears at a failed rift in the northeast above 100 km depth coinciding with the great meteor hot spot track and the high seismicity band which can be interpreted as high temperature and high iron and or volatile content related to the hot spot activity low velocity anomaly of 2 at the depths of 100 200 km are found beneath the east of lake ontario and lake huron where the lithosphere is relatively thin these anomalies follow the nw se trend of the slow indentation imaged on the continental scale models we conclude that the location of low velocity anomalies is not only related to the hot spot track but also controlled by the pre existing lithosphere morphology and strength a high velocity column appears between southern lake ontario and the appalachian front centered at 100 km depth it is likely caused by high mg content at the formation of the lithosphere or the presence of ecologite or garnet phases generated during the opening and closure of the iapetus ocean

viscoelastic coupling model of the san andreas fault along the big bend southern california the big bend segment of the san andreas fault is the 300 km long segment in southern california that strikes about n65 w roughly 25 counterclockwise from the local tangent to the small circle about the pacific north america pole of rotation the broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation continuous slip on the fault below the locking depth at the rate of relative plate motion whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km the discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust thus the broad distribution of deformation observed across the big bend segment can be largely associated with the san andreas fault itself not subsidiary faults distributed throughout the region the working group on california earthquake probabilities 1995 in using geodetic data to estimate the seismic risk in southern california has assumed that strain accumulated off the san andreas fault is released by earthquakes located off the san andreas fault thus they count the san andreas contribution to total seismic moment accumulation more than once leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their type c zones

displacement of surface monuments vertical motion several measurements of vertical ground motion at pinon flat observatory california indicate the overall weakness and instability of the earth s weathered surface with respect to the underlying rock cumulative long period motions of order 0 5 mm per year dominate these records though smaller elastic deformations caused by precipitation loading atmospheric loading and tidal strains are evident at higher frequencies all of these help to characterize the near surface material the long period records suggest that near surface weathering is the dominant influence on monument motion at least at this site on crystalline rock in a semi arid environment rainfall loading gives an average vertical modulus of 2 6 gpa for the material in the uppermost 26 m of the ground compared with 88 gpa for granite under moderate confining stress atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours earth tide records show poisson s ratio to be 0 09 in contrast to the normative range of 0 2 0 25 establishing that horizontal strains couple only weakly into vertical ones so that vertical strains near the surface are a poor measure of areal strain the form of the ground surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument positioning error budget that increases with time because of ground instability and the generally small rate of crustal tectonic motions deeply emplaced monuments will be needed for observational programs designed to detect short term changes in crustal deformation over baselengths of order 10 km and less

salton trough regional deformation estimated from combined trilateration and survey mode gps data the salton trough in southeastern california united states has one of the highest seismicity and deformation rates in southern california including 20 earthquakes m 6 or larger since 1892 from 1972 through 1987 the u s geological survey usgs measured a 41 station trilateration network in this region we re measured 37 of the usgs baselines using survey mode global positioning system methods from 1995 through 1999 we estimate the salton trough deformation field over a nearly 30 year period through combined analysis of baseline length time series from these two datasets our primary result is that strain accumulation has been steady over our observation span at a resolution of about 0 05 strain yr at 95 confidence with no evidence for significant long term strain transients despite the occurrence of seven large regional earthquakes during our observation period similar to earlier studies we find that the regional strain field is consistent with 0 5 0 03 strain yr total engineering shear strain along an axis oriented 311 6 23 east of north approximately parallel to the strike of the major regional faults the san andreas and san jacinto all uncertainties in the text and tables are standard deviations unless otherwise noted we also find that 1 the shear strain rate near the san jacinto fault is at least as high as it is near the san andreas fault 2 the areal dilatation near the southeastern salton sea is significant and 3 one station near the southeastern salton sea moved anomalously during the period 1987 95 1995 11 online material tables of stations used in this study and of estimated strain fields manuscript received 8 january 2003

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

interseismic deformation along the san andreas fault in southern california eight trilateration networks located along the san andreas fault in southern california have been surveyed 8 to 19 times within a 14 to 17 year interval between 1971 and 1992 the data measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models the corrected data a total of 2027 measurements are displayed in the form of plots of measured distance versus time for each of the 167 lines measured the hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error a significant deviation from steady deformation is found only for the network located neax palmdale california in that network many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys the deviations are not of the form fixed proportional error one would generally expect from systematic survey error if the survey of early 1982 is excluded the remaining data at palmdale are consistent with steady deformation thus the apparent strain event observed near palmdale in 1982 was transient

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

a source extent analysis of the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 a new method for estimating source extent parameters has been applied to the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 from the stations of the world wide standard seismograph network and canadian seismic network estimates of the duration of far field long period sh waves were made by computing the variance or second central moment 2 of instrument corrected attenuation corrected pulses seventeen measurements for the imperial valley event and 25 for the victoria event were made we inverted for the source duration t the fault length l and the directivity parameter d by least squares we find that for the imperial valley t 9 0 0 5 s d 130 43 km s and l 46 13 km and for the victoria event t 10 9 0 6 s and d 251 71 km s l is not well constrained for this second event the inversion gives 23 km the lower bound obtained by constraining 2 to be positive everywhere on the focal sphere although a value as high as 40 km would still be within the uncertainties the positive values of d for both events indicate that rupture propagation was predominantly to the northwest in order to estimate the degree of unilateralness we have computed the rupture mode index defined by r d lt which is 1 for a unilateral rupture and zero for a bilateral rupture we find a value of r 0 31 0 14 for the imperial valley event significantly less than unity which indicates that a southern component of rupture is also present assuming uniform slip we estimate the northern and southern fault segments to be 26 and 20 km respectively the presence of a bilateral component is supported by near field studies which have made use of stations along the southern half of the imperial fault for the victoria event due primarily to the uncertainty in l the value of r can vary from 0 6 to 1 with the value of 1 being most probable assuming that the rupture is unilateral we can obtain an estimate of fault length from the relation l d t which yields l 23 6 km geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5 8 0 4 for imperial valley and 4 8 0 4 for victoria in units of 1018 n m the presence of a southern component of rupture for the imperial valley event would suggest a closer and perhaps causal relationship to the victoria event which occurred 8 months later and about 60 km to the south in a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of cerro prieto

intermediate term prediction of occurrence times of strong earthquakes pattern recognition procedures for infrequent events are adapted to the problem of identifying patterns of clustering of small and intermediate scale seismicity before large earthquakes identification procedures derived from analysis of large california and nevada earthquakes yield a high success rate when applied to other parts of the world

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

effects of karst and geological structure on groundwater flow the case of yarqon taninim aquifer israel summary this study demonstrates the significant influences of the geological structure especially folding and lithology and the karst system on groundwater flow regime folds divert groundwater flow from the general hydraulic gradient marly layers sustain several perched sub aquifers above the regional aquifer and karstification increases the hydraulic conductivity by several orders of magnitude these phenomena are quantitatively demonstrated within the yarqon taninim yt basin israel which is a complex groundwater system combining several extremely opposite characteristics humid and arid recharge zones phreatic and confined parts shallow and deep sub aquifers stratified and relatively homogeneous sub basins saline and fresh water bodies as well as stagnant and fast flowing groundwater regions we have introduced a 3d geological based grid for the basin for the first time it was implemented into a numerical code feflow which was used thereafter to analyze quantitatively the flow regime the groundwater mass balance and the aquifer hydraulic properties we present up to date conceptual understanding and numerical modeling of the yt flow field especially at its mountainous parts based on the calibration procedure and the sensitivity analyses we obtained the best fitted hydraulic conductivity values for the aquifer mesh the general phenomenon observed is that as groundwater flow quantity increases the hydraulic conductivity also increases we interpret this result by the karstification mechanism including paleo karst thus where groundwater flow lines converge and where groundwater discharge amount increases the karstification process intensifies and permeability increases consequently at the mountainous region along the syncline axes where groundwater flow lines converge higher conductivities are found modeling results also exhibit that at the lowland confined area the geological structure does not play a major role in directing groundwater flow rather the flow field is controlled by the well developed karst system and the relatively homogenous carbonate section it is hypothesizes that the extensive karstification took place at the messinian salinity crises 5 5 ma during which groundwater heads as well as sea level were lowered by several 100 m

mantle fabric at multiple scales across an archean proterozoic boundary grenville front canada abstract in eastern ontario and southwest quebec canada the proterozoic grenville province abuts against the archean superior province the complex tectonic history of the region is reflected in the pattern of electrical and seismic anisotropy within the lithosphere while asthenospheric anisotropy is expected to reflect current patterns of mantle flow magnetotelluric and teleseismic data from the polaris and fednor experiments and the lithoprobe abitibi grenville transect are examined for sks splitting and geoelectric strike and receiver functions are generated at selected stations in order to characterise both vertical and horizontal variations in anisotropy in the eastern ontario upper mantle the average shear wave split direction coincides with the direction of plate motion split times are found to be strongest in the southern part of the grenville province where asthenospheric flow is enhanced by the presence of a lithospheric divot the ottawa bonnechere graben and the immediate vicinity of the grenville front are regions of altered split direction indicative of a lithospheric component of anisotropy north of the grenville front there is a gradual reorientation of the split direction from ese to ene which is not easily attributable to crustal tectonics and may represent the northern limit of lithospheric deformation produced by the grenville orogen electrical anisotropy is pervasive in the study area the pattern of magnetotelluric strikes is more complex than the sks pattern though after correction for local distortion the geoelectric strikes correlate fairly well with sks measurements at nearby stations obliquity between sks and magnetotelluric results shows no consistent orientation across the study area receiver function analysis at three selected stations is indicative of a subcrustal anisotropic layer with a consistent sse fast direction underlain at station gac by a sequence of anisotropic layers with varying directions this sequence is not observed at station sado combining these results we interpret the strength and direction of anisotropic fabric in the grenville to vary strongly with depth the upper part of the lithosphere contains thin anisotropic layers perhaps related to eclogitization and relict slabs but which are insufficient to explain the observed sks splits the lower lithosphere is likely to be more ductile and uniformly anisotropic and may be an important control on magnetotelluric strike directions as well as a significant contributor to sks splitting the largest contributor to sks splitting in this region is interpreted to be asthenospheric anisotropy related to absolute plate motion

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

the seismic 8 discontinuity and partial melting in continental mantle strong scattered reflections beyond 8 degrees 8 offset are characteristic features of all high resolution seismic sections from the continents the reflections identify a low velocity zone below approximately 100 kilometers depth beneath generally stratified mantle this zone may be caused by partial melting globally initiated at equal depth in the continental mantle solid state is again attained at the lehmann discontinuity in cold stable areas whereas the zone extends to near the 400 kilometer discontinuity in hot tectonically active areas thus the depth to the lehmann discontinuity may be an indicator of the thermal state of the continental mantle

the earthquake deformation cycle recurrence and the time predictable model geodetic and geologic observations from a handful of well studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation in ideal circumstances the time predictable model requires knowledge of only the seismic slip and time of the last earthquake and the rate of relative plate motion or fault slip practically indirect measurements must usually suffice local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate permanent nonrecoverable deformation often roughly half the coseismic strain offset near great thrust earthquakes complicates the inference of interplate seismic slip owing to short and long term postseismic transients commonly 20 40 of the coseismic strain drop deformation rates are variable and not simply related to the plate motion rate at strike slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly when these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are most severe

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

fault slip and earthquake recurrence along strike slip faults contributions of high resolution geomorphic data article i nfo understanding earthquake eq recurrence relies on information about the timing and size of past eq ruptures along a given fault knowledge of a fault s rupture history provides valuable information on its potential future behavior enabling seismic hazard estimates and loss mitigation stratigraphic and geomorphic evidence of faulting is used to constrain the recurrence of surface rupturing eqs analysis of the latter data sets culminated during the mid 1980s in the formulation of now classical eq recurrence models now routinely used to assess seismic hazard within the last decade light detection and ranging lidar surveying technology and other high resolution data sets became increasingly available to tectono geomorphic studies promising to contribute to better informed models of eq recurrence and slip accumulation patterns after reviewing motivation and background we outline requirements to successfully reconstruct a fault s offset accumulation pattern from geomorphic evidence we address sources of uncertainty affecting offset measure ment and advocate approaches to minimize them a number of recent studies focus on single eq slip distribu tions and along fault slip accumulation patterns we put them in context with paleoseismic studies along the respective faults by comparing coefficients of variation cv for eq inter event time and slip per event and find that a single event offsets vary over a wide range of length scales and the sources for offset variability differ with length scale b at fault segment length scales single event offsets are essentially constant c along fault offset accumulation as resolved in the geomorphic record is dominated by essentially same size large offset in crements and d there is generally no one to one correlation between the offset accumulation pattern constrained in the geomorphic record and eq occurrence as identified in the stratigraphic record revealing the higher resolution and preservation potential of the latter while slip accumulation along a fault segment may be dominated by repetition of large nearly constant offset increments timing of surface rupture is less regular

slip rate increase at parkfield in 1993 detected by high precision edm and borehole tensor strainmeters on two of the instrument networks at parkfield california the two color electronic distance meter edm network and borehole tensor strainmeter btsm network we have detected a rate change starting in 1993 that has persisted at least 5 years these and other instruments capable of measuring crustal deformation were installed at parkfield in anticipation of a moderate m6 earthquake on the san andreas fault many of these instruments have been in operation since the mid 1980s and have established an excellent baseline to judge changes in rate of deformation and the coherence of such changes between instruments the onset of the observed rate change corresponds in time to two other changes at parkfield from late 1992 through late 1994 the parkfield region had an increase in number of m4 to m5 earthquakes relative to the preceding 6 years the deformation rate change also coincides with the end of a 7 year period of sub normal rainfall both the spatial coherence of the rate change and hydrological modeling suggest a tectonic explanation for the rate change from these observations we infer that the rate of slip increased over the period 1993 1998

correlated errors in geodetic time series implications for time dependent deformation analysis of frequent trilateration observations from the two color electronic distance measuring networks in california demonstrate that the noise power spectra are dominated by white noise at higher frequencies and power law behavior at lower frequencies in contrast earth scientists typically have assumed that only white noise is present in a geodetic time series since a combination of infrequent measurements and low precision usually preclude identifying the time correlated signature in such data after removing a linear trend from the two color data it becomes evident that there are primarily two recognizable types of time correlated noise present in the residuals the first type is a seasonal variation in displacement which is probably a result of measuring to shallow surface monuments installed in clayey soil which responds to seasonally occurring rainfall this noise is significant only for a small fraction of the sites analyzed the second type of correlated noise becomes evident only after spectral analysis of line length changes and shows a functional relation at long periods between power and frequency of 1 where is frequency and 2 with 2 this type of correlated noise is termed random walk noise and its source is mainly thought to be small random motions of geodetic monuments with respect to the earth s crust though other sources are possible because the line length changes in the two color networks are measured at irregular intervals power spectral techniques cannot reliably estimate the level of 1 noise rather we also use here a maximum likelihood estimation technique which assumes that there are only two sources of noise in the residual time series white noise and random walk noise and estimates the amount of each from this analysis we find that the random walk noise level averages about 1 3 mm yr and that our estimates of the white noise component confirm theoretical limitations of the measurement technique in addition the seasonal noise can be as large as 3 mm in amplitude but typically is less than 0 5 mm because of the presence of random walk noise in these time series modeling and interpretation of the geodetic data must account for this source of error by way of example we show that estimating the time varying strain tensor a form of spatial averaging from geodetic data having both random walk and white noise error components results in seemingly significant variations in the rate of strain accumulation spatial averaging does reduce the size of both noise components but not their relative influence on the resulting strain accumulation model

interseismic strain accumulation and anthropogenic motion in metropolitan los angeles 1 we use global positioning system gps geodesy and synthetic aperture radar sar interferometry to distinguish between interseismic strain accumulation and anthropogenic motion in metropolitan los angeles we establish a relationship between horizontal and vertical seasonal oscillations of the santa ana aquifer use this relationship to infer cumulative horizontal anthropogenic motions from cumulative vertical motions caused by water and oil resource management and estimate horizontal interseismic velocities corrected for anthropogenic effects vertical anthropogenic rates from 1992 to 1999 are slower than 3 mm yr 1 in the santa ana and san gabriel aquifers and faster than 5 mm yr 1 in the chino aquifer and in many oil fields inferred horizontal anthropogenic velocities are faster than 1 mm yr 1 at 18 of 46 gps sites northern metropolitan los angeles is contracting with the 25 km south of the san gabriel mountains shortening at 4 5 1 mm yr 1 95 confidence limits the thrust fault in an elastic edge dislocation model of the observed strain is creeping at 9 2 mm yr 1 beneath and north of a position 6 2 km deep and 8 8 km north of downtown los angeles the model fault is near the los angeles segment of the puente hills thrust but south of the sante fe springs segment of the thrust disagreement between the 6 km locking depth in the model and the 15 km seismogenic depth inferred from earthquakes suggests that the elastic continuum model may be unsatisfactory models with different stiffnesses of sedimentary basin and crystalline basement must be investigated

strain accumulation near yucca mountain nevada 1983 1993 over the decade 1983 1993 the u s geological survey has measured the deformation of a 50 km aperture trilateration network centered on yucca mountain the proposed disposal site for high level nuclear waste in the united states the network was surveyed in 1983 1984 and 1993 the average annual principal strain rates are 0 010 0 020 strain yr n90 w 24 and 0 009 0 021 strain yr n00 e 24 indicating no significant strain accumulation the southeast corner of the network was disturbed on june 29 1992 by the little skull mountain earthquake ms 5 4 the epicenter of which is about 20 km southeast of the yucca mountain site using the seismically determined fault plane dip 54 s55 e we find that 0 580 0 075 m of normal slip on a 5 km square rupture surface at a depth of about 8 km provides a good fit to the observed deformation in the southeast corner of the network the inferred seismic moment is 4 4 0 6 1017 n m which compares well with the observed seismic moment of 4 1 1017 n m

structural fabric of the central metasedimentary belt of southern ontario canada from deep seismic profiling abstract the central metasedimentary belt boundary tectonic zone cmbbtz is a 10 20 km wide zone of intense structural deformation within the 1 3 1 0 ga grenville orogen of southeastern canada the crustal structure of the exposed cmbbtz has been well studied but its sub phanerozoic location and geometry beneath the urban development and nuclear stations of the toronto region are not well known a new 75 km lithoprobe reflection profile acquired close to toronto provides a clear image of the cmbbtz as a panel of southeast dipping reflections that extends with moderate dip

a scattered wave image of subduction beneath the transverse ranges over 5600 short period recordings of teleseismic events were used to create deterministic maps of p wave scatterers in the upper mantle beneath southern california between depths of 50 and 200 kilometers the southern flank of the slab subducting beneath the transverse ranges is marked by strong scattering the marked scattering indicates that the edge of the slab is a sharp thermal boundary such a boundary could be produced by slab shearing or small scale convection in the surrounding mantle the northern limb of the slab is not a strong scatterer consistent with thicker lithosphere north of the transverse ranges

stress redistribution and slow earthquakes abstract earthquakes are usually assumed to result when the gradual stress buildup in a region eventually exceeds some initial local strength there are observations which suggest that a number of mechanisms must have a significant role in stress redistribution in active earthquake regions the stress build up could be non linear in time there is evidence that this is so in the case of some major earthquakes one mechanism that might explain non linear strain buildup is a slow earthquake this is similar to a normal earthquake except that the rupture and slip velocities are very low so that negligible energy is radiated in the normally observed seismic frequency band the slow earthquakes studied here have been recorded by a network of sacks evertson borehole strainmeters installed along the seismically active pacific coast of honshu japan larger slow events may have slow foreshocks and aftershocks in a manner similar to that for normal earthquakes the aftershocks appear to have lower stress drops than the foreshocks the static field changes associated with the izu oshima earthquake m 7 of january 1978 were sufficiently large that they were clearly recorded by the nearest three strainmeters interpretation of the records indicate that this earthquake was in essence a normal quake followed by a slow event of similar moment this interpretation is corroborated by reports of ground vibration and faulting on the izu peninsula

a unified analysis of crustal motion in southern california 1970 2004 the scec crustal motion map 1 to determine crustal motions in and around southern california we have processed and combined trilateration data collected from 1970 to 1992 vlbi data from 1979 to 1992 and gps data from 1986 to 2004 a long temporal coverage required in part by the occurrence of several large earthquakes in this region from a series of solutions for station positions we have estimated interseismic velocities coseismic displacements and postseismic motions within the region from 31 n to 38 n and east to 114 w the final product includes estimated horizontal velocities for 1009 gps 190 trilateration and 16 vlbi points with ties between some of these used to stabilize the solution all motions are relative to the stable north american reference frame snarf as realized through the velocities of 20 gps stations this provides a relatively dense set of horizontal velocity estimates with well tested errors for the past quarter century over the plate boundary from 31 n to 36 5 n these velocities agree well with those from the plate boundary observatory which apply to a later time period we also estimated vertical velocities 533 of which have errors below 2 mm yr most of these velocities are less than 1 mm yr but they show 2 4 mm yr subsidence in the ventura and los angeles basins and in the salton trough our analysis also included estimates of coseismic and postseismic motions related to the 1992 landers 1994 northridge 1999 hector mine and 2003 san simeon earthquakes postseismic motions increase logarithmically over time with a time constant of about 10 days and generally mimic the direction and relative amplitude of the coseismic offsets

high resolution topography derived offsets along the 1857 fort tejon earthquake rupture trace san andreas fault the great fort tejon earthquake of 1857 with an 350 km long surface rupture was the most recent major earthquake along the south central san andreas fault saf prior reconstruction of its surface slip distribution and reconstruction of preceding earthquakes along the 1857 rupture trace have contributed to formulation of the characteristic earthquake cem and uniform slip models usm for earthquake recurrence that find wide application in seismic hazard assessment and earthquake forecasting we used the high resolution b4 light detection and ranging lidar topographic data set sufficient for depiction of meter scale tectonic landforms to reevaluate the distribution of surface displacement along the 1857 rupture trace we present 450 offset measurements with displacements below 60 m increasing observation density relative to previous studies by a factor of 2 our results show that the 1857 earthquake had overall an average displacement below 3 5 m with 4 6 m released along the northwestern half of the rupture its along fault slip distribution is smooth at 10 km length scales at 28 less than prior estimates assuming a 10 km rupture depth and a 30 gpa shear modulus reconstruction of pre 1857 earthquakes is impeded by an exponential decrease in observation density with increasing displacement amount offset observation density soon reaches a level where only tentative and nonunique reconstructions are permitted the results of this study question prior surface slipreconstructions of pre 1857 rupture s that were based on a distinctly lower number of offset observations challenging the applicability of the aforementioned earthquake recurrence models to explain saf rupture behavior online material catalog of offset measurements along the 1857 surface rupture trace as well as an annotated map 1 embed inline graphic 1 gif

seismicity of the southern great lakes revised earthquake hypocenters and possible tectonic controls using data from 27 seismograph stations for the period 1990 2001 we have relocated 106 hypocenters of earthquakes with magnitudes from 0 9 to 5 4 in the region of the southern great lakes two complementary methods were used for relocation a conventional least squares approach lienert and havskov 1995 and joint hypocentral determination pujol 2000 these two methods yielded mutually consistent spatial patterns of seismicity with an average difference of 3 7 km in epicentral locations and 1 1 km in focal depths we show that the hypocenter locations are not very sensitive to realistic uncertainties in 1d crustal velocity our sharpened definition of zones of seismicity delineates several clusters beneath lake ontario around niagara falls and near the south shore of lake erie these seismicity zones appear to correlate with areas where the regional magnetic data exhibit prominent short wavelength 5 km linear anomalies the magnetic anomalies are associated with basement structures that formed during the precambrian mesoproterozoic grenville orogen both the seismicity and magnetic anomalies exhibit statistically significant preferred orientations at n40 e n45 e but the correlation of the earthquake clusters with specific aeromagnetic lineaments remains uncertain three preliminary focal mechanisms of earthquakes with magnitudes m n 3 1 to 3 8 show unusual normal faulting with nodal planes in almost the same direction as the magnetic trends n42 e n52 e proximity of the earthquake clusters to large bodies of water coupled with colinearity with magnetic anomaly trends suggests that both surface water and pre existing basement structures may play significant roles in controlling intraplate seismicity in the southern great lakes region manuscript received 3 january 2003

images of crustal variations in the intermountain west 1 we develop a map of crustal thickness variations across the great basin colorado plateau rocky mountain and great plains provinces of the western united states using common conversion point stacking of teleseismic receiver functions below the rocky mountains and high plains in colorado we find the thickest crust in the region at 45 50 km thick beneath the basin and range thinner between 30 and 40 km crust is found thin 30 km thick crust is present in the northern portion of nevada and utah despite elevations similar to those farther south crustal thickness across the colorado plateau can be characterized as a broad transitional region between the thin crust of basin and range to the thicker crust of the rocky mountains the impedance contrast across the mohorovicic discontinuity decreases below the colorado plateau as converted arrivals recorded in this region appear weak compared to surrounding areas variations in vp vs across the region indicate higher values along the western boundary of the basin and range in the rocky mountains and in the western great plains we are not able to characterize vp vs in the colorado plateau we find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western united states

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

lithospheric imaging via teleseismic scattering tomography summary the coda of the teleseismic p phase consists largely of energy scattered by small inhomogeneities in the receiver side lithosphere given large collections of teleseismic data from dense permanent networks previous workers have successfully back propagated coda energy back to scattering source points using various kinematic migration schemes as well as by inverting using an inverse scattering radon transform approach under the born approximation seismic scattering is a linear process therefore it is possible to approach coda scattering as a linear waveform inversion problem mathematically similar to transmission based tomography assuming ray theoretical propagation and rayleigh scattering we pose the inverse scattering problem in tomographic form and recover perturbations in density and p and s velocities from pp and ps scattered data the method is applied to data from the southern california seismic network scsn covering the san jacinto anza region the results show a considerable correlation between seismicity and velocity perturbation structure particularly in the region between the mission creek and banning fault branches features connecting the coyote creek and elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression

receiver functions in the western united states with implications for upper mantle structure and dynamics 1 investigations into mechanisms driving surface tectonics commonly search for mantle sources but few observations constrain flow in the upper mantle and transition zone here variations in the upper mantle discontinuities at 410 km and 660 km below the western united states are revealed through mapping depths of compressional to shear wave conversions recorded by broadband seismometers the resulting image exhibits 20 and 30 km of topography on the 410 and 660 km discontinuities similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone the pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the rocky mountains colorado plateau or basin and range province providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region at least at scale lengths of a few hundred kilometers furthermore we find no clear correlation between the depths of the 410 and 660 km discontinuities undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km if the topography were due only to lateral temperature differences such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone alternatively the topography may reflect lateral variations in composition variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth

strain accumulation north of los angeles california as a function of time 1977 1992 no significant change in the rate of strain accumulation in a 40 120 km trilateration network spanning the san gabriel mountains was observed from 1977 5 to 1991 8 despite an apparent increase in seismicity ml 4 5 beginning in late 1987 in the northern los angeles basin immediately to the south the observed deformation 0 13 0 01 strain yr right lateral shear across a vertical plane striking n63 w 1 can be attributed to strain accumulation on the san andreas fault local strike n65 w no significant accumulation of compression normal to the strike of the san andreas fault was observed in the network as a whole

the buried afiq canyon eastern mediterranean israel a case study of a tertiary submarine canyon exposed in late messinian times abstract the afiq submarine canyon was one of a series of canyons initially incised in a drowned shelf edge and slope of the eastern mediterranean margins in early oligocene times p19 zone during most of the early miocene submarine erosion or non deposition prevailed this was followed by deposition of pelagic marls and debris flows in early middle miocene n8 times large scale sliding in late middle miocene times n14 resulted in the collapse of the slope the removal of most of the middle miocene sequence and the formation of a box shaped scar back cutting incision ultimately resulted in the incision of the shelf in late miocene times a sea level fall ranging between 50 and 800 m below the canyon s rim resulted in the deposition of the messinian lower evaporites mavqiim formation within the canyon a subsequent rise of a similar extent led to the deposition of the upper evaporites be eri gypsum which are found on the canyon s southern shoulder the final messinian sea level drop below the canyon s floor resulted in a subaerial environment in the canyon the erosion of the upper evaporites and the subsequent deposition of fluvial and brackish sediments afiq formation the latter correspond to the lago mare sediments known throughout the mediterranean the rapidly rising sea level during the pliocene along with the high rate of nilotic clastic sedimentation resulted in the final burial of the afiq canyon and a pronounced seaward progradation 20 30 km of the shelf edge

strong motion and broadband teleseismic analysis of the 1991 sierra madre california earthquake short period and broadband teleseismic waveform data and three component strong motion records were analyzed to obtain the source parameters of the 1991 sierra madre earthquake close in strong motion velocity records analyzed from 5 s to 5 hz show two distinct pulses about 0 35 s apart requiring some rupture complexity the near field shear wave displacement pulse from this event has a relatively short duration about 1 s for the magnitude of the event requiring a particularly high average stress drop to further constrain the rupture process the data were used in a finite fault source inversion to determine the temporal and spatial distribution of slip we chose a fault plane orientation striking s 62 w and dipping 50 toward the northwest as required by the distribution of aftershocks the first motion mechanism and the teleseismic bodywave point source inversion in addition to the aftershock locations depth constraints are provided by teleseismic short period and broadband recordings which require a centroid depth of 10 11 km our inverse modeling results indicate that both the teleseismic and strong motion data sets can be fit with a compact rupture area about 12 km2 southwest and up dip from the hypocenter the average slip is approximately 50 cm and the maximum slip is 120 cm the seismic moment obtained from either of the separate data sets or both sets combined is about 2 8 0 3 1024 dyn cm and the potency is 0 01 km3 using the area of significant slip estimated from the finite fault inversion the resulting stress drop is on the order of 150 200 bars

evolution of the stress field in southern california and triggering of moderate size earthquakes a 200 year perspective changes in stress in southern california are modeled from 1812 to 2025 using as input 1 stress drops associated with six large 7 0 7 5 earthquakes through 1995 and 2 stress buildup associated with major faults with slip rates 3 mm yr as constrained by geodetic paleoseismic and seismic measurements evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner we present snapshots of the cumulative coulomb failure function acff as a function of time for faults of various strike dip and rake throughout southern california we take acff to be zero everywhere just prior to the great shock of 1812 we find that about 95 of those well located m 6 earthquakes whose mechanisms involve either strike slip or reverse faulting are consistent with the coulomb stress evolutionary model that is they occurred in areas of positive acff the interaction between slow moving faults and stresses generated by faster moving faults significantly advanced the occurrence of the 1933 long beach and 1992 landers events in their earthquake cycles coulomb stresses near major thrust faults of the western and central transverse ranges have been accumulating for a long time future great earthquakes along the san andreas fault especially if the san bernardino and coachella valley segments rupture together can trigger moderate to large earthquakes in the transverse ranges as appears to have happened in the santa barbara earthquake that occurred 13 days after the great san andreas shock of 1812 maps of current acff provide additional guides to long term earthquake prediction

3 d crust and mantle structure in southern ontario canada via receiver function imaging abstract a teleseismic data set from the polaris project is used to obtain 3 d images of southern ontario using two imaging techniques scattering tomography and common conversion point stacking the resulting images reveal a layered crust the layering being interrupted by discontinuities associated with major crustal scale faulting breaks in crustal continuity and moho deflections associated with the ottawa bonnechere graben indicate that the graben is associated with faulting on a whole crust scale we also detect similar discontinuities across the mississauga domain supporting the previous interpretation that the domain is bounded by crustal scale faults we locate discontinuous sub lithospheric negative polarity arrivals which indicate complex three dimensional structures within the lithosphere and may be associated with subduction remnants or a mid lithosphere discontinuity

upper mantle velocity structure of the lower great lakes region abstract the lithospheric root beneath north america contains a prominent indentation beneath the lower great lakes region that is approximately aligned with the track of the new england seamounts by combining data from the recently installed polaris network in southern ontario canada with data acquired in 1996 during the abitibi grenville teleseismic experiment we have performed a tomographic inversion using 4543 p wave traveltimes from 213 events 5 0 m b 6 6 and 1860 s wave traveltimes from 98 events 5 0 m b 6 6 to obtain high resolution images of the upper mantle beneath the lower great lakes two salient features of the 3 d models are 1 a patchy nnw trending low velocity region and 2 a linear ne striking high velocity anomaly s wave images show that the low velocity anomaly changes from an arcuate feature at 400 km depth to a nw striking linear feature at 100 km depth beneath the neoproterozoic ottawa bonnechere graben the linear high velocity anomaly extends to at least 300 km depth and strikes parallel to surface geological belts and the laurentian continental margin we interpret the high velocity anomaly as a possible relict slab associated with ca 1 35 1 3 ga subduction beneath the composite arc belt whereas the low velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the late cretaceous monteregian plutons our data support an interpretation of these plutons as melts generated by the passage of north america across a mantle plume rather than a far field response to opening of the north atlantic

computing a large refined catalog of focal mechanisms for southern california 1981 2010 temporal stability of the style of faulting using the method developed by hardebeck and shearer 2002 2003 termed the hash method we calculate focal mechanisms for earthquakes that occurred in the southern california region from 1981 to 2010 when available we use hypocenters refined with differential travel times from waveform cross correlation using both the p wave first motion polarities and the s p amplitude ratios computed from three component seismograms we determine mechanisms for more than 480 000 earthquakes and analyze the statistical features of the whole catalog we filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high quality catalog with approximately 179 000 focal mechanisms as more s p amplitude ratios become available after 2000 the average nodal plane uncertainty decreases significantly compared with mechanisms that include only p wave polarities in general the parameters of the focal mechanisms have been stable during the three decades the dominant style of faulting is high angle strike slip faulting with the most likely p axis centered at n5 e for earthquakes of m 2 5 events using the 210 moment tensor solutions in tape et al 2010 as benchmarks we compare the focal plane rotation angles of common events in the catalog seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty the common events with relatively large rotation angles are either located around the edge of the scsn network or poorly recorded

shortening and thickening of metropolitan los angeles measured and inferred by using geodesy geodetic observations using the global positioning system gps and other techniques record a high rate of north south shortening in an east southeast trending 5 40 km wide belt in northern metropolitan los angeles california downtown los angeles is observed to be converging upon the southern san gabriel mountains at 6 mm yr aside from the elastic strain that will be released during earthquakes rupturing the san andreas and san jacinto faults east west lengthening across northern metropolitan los angeles is minor 2 5 mm yr therefore north south shortening is accommodated mainly by vertical crustal thickening

the 1982 cocrust seismic experiment across the ottawa bonnechere graben and grenville front in ontario and quebec summary during the summer of 1982 the canadian consortium for crustal reconnaissance using seismic techniques cocrust conducted a major long range seismic refraction and wide angle reflection experiment across the grenville province of the canadian shield three seismic lines each approximately 300 km in length were located i along the ottawa bonnechere graben ii perpendicular to the graben and iii perpendicular to the crenville front geological evidence indicates that the graben may have originated from a failed arm of the st lawrence rift system and the grenville front marks the boundary between the grenville province and the much older superior province other geological features of the grenville province that were traversed by the profiles were the central metasedimentary belt and the central gneiss belt analysis of the data involved conventional traveltime procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures results from the survey indicate a variation in near surface seismic velocity

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

geodetic measurements of horizontal strain near the white wolf fault kern county california 1926 1993 the white wolf fault located north of the big bend segment of the san andreas fault is the ne sw trending left lateral oblique reverse fault responsible for the ms 7 8 1952 kern county earthquake we combined global positioning system gps measurements with historical triangulation and trilateration data to determine changes in the strain rate over 7 decades 1926 1993 we reanalyzed the historical geodetic data and calculated an elevated preseismic 1926 1952 maximum shear strain rate of 0 62 0 16 strain yr across the white wolf fault the maximum shear strain rate decreased with distance toward the garlock fault to 0 09 0 08 strain yr in the decade following the earthquake 1952 1963 the near fault was high 0 85 0 23 strain yr and decreased to 0 23 0 13 strain yr across the garlock fault in 1993 we resurveyed many of the same monuments with gps receivers to estimate fault crossing and off fault strain rates for the preceding 30 years across the white wolf fault the maximum shear strain rate dropped to 0 19 0 07 strain yr the azimuths of the maximum principal strain rates for the 1963 1993 epoch rotate from a fault normal orientation 57 15 across the white wolf fault to 11 3 e across the garlock fault

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

constraining the kinematics of metropolitan los angeles faults with a slip partitioning model due to the limited resolution at depth of geodetic and other geophysical data the geometry and the loading rate of the ramp decollement faults below the metropolitan los angeles are poorly understood here we complement these data by assuming conservation of motion across the big bend of the san andreas fault using a bayesian approach we constrain the geometry of the ramp decollement system from the mojave block to los angeles and propose a partitioning of the convergence with 25 5 0 5 mm yr and 3 1 0 6 mm yr of strike slip motion along the san andreas fault and the whittier fault with 2 7 0 9 mm yr and 2 5 1 0 mm yr of updip movement along the sierra madre and the puente hills thrusts incorporating conservation of motion in geodetic models of strain accumulation reduces the number of free parameters and constitutes a useful methodology to estimate the tectonic loading and seismic potential of buried fault networks