tectonic motion in the western united states inferred from very long baseline interferometry measurements 1980 1986 we use over 6 years of mobile very long baseline interferometry vlbi baseline measurements between 12 sites in the western united states to infer their velocities relative to the north american plate we find these velocities to be generally consistent with those determined from geologic data and contemporaneous satellite laser ranging measurements in the same region the discrepancy between the largest velocities determined from the vlbi measurements of 40 48 mm yr 1 and the relative plate velocity of 50 56 mm yr 1 predicted from plate motion models is found to be consistent with a broadened distribution of interseismic strain from cyclic activity on the san andreas and subsidiary faults the vlbi data are best explained by a cumulative rate of strike slip motion near the plate boundary of approximately 48 mm yr 1 although exclusion of competing values of 56 and 41 mm yr 1 is based upon very few data the rates of offshore fault slip inferred from this study range from 15 mm yr 1 in central california to negligible amounts in the san francisco region finite element calculations of multiple fault strain distributions show good agreement with systematic variations in the distribution of shear strain along the san andreas system as revealed by previous geodetic measurements

a multilayer model of time dependent deformation following an earthquake on a strike slip fault a multilayer half space model of the earth is used in finite element calculations of time dependent deformation and stress following an earthquake on an infinitely long strike slip fault the model involves shear properties of an elastic upper lithosphere a standard viscoelastic linear solid lower lithosphere a maxwell viscoelastic asthenosphere and an elastic mesosphere time dependent displacements strains and stresses are computed both at the surface of the earth and at depth the analysis includes both systematic variations of fault and layer depths and comparisons with simpler elastic lithosphere over viscoelastic asthenosphere calculations for conditions which may be appropriate for the earth the creep of both the lower lithosphere and the asthenosphere can contribute to the postseismic deformation the magnitude of the deformation is enhanced by a short distance between the bottom of the fault slip zone and the top of the creep layer but is less sensitive to the thickness of the creeping layer furthermore postseismic restressing is increased as the lower lithosphere becomes more viscoelastic but the tendency for the width of the restressed zone to grow with time is retarded

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

pacific north america plate motions new results from very long baseline interferometry expansion and densification of the very long baseline interferometry vlbi network by the nasa crustal dynamics project continues to clarify details of pacific north america motions in this paper i construct for the first time rigid plate tectonic models based purely on space geodetic data construction of vlbi plate models has the advantages of averaging the bias and errors of individual site velocities making failures in the rigid plate assumption easy to document and allowing the consistency of specified plate models to be quantified against the vlbi data set as a whole vlbi based rigid plate models vm2 and vm3 obtained by fitting vector velocities of 10 sites in the stable interiors of the north america and pacific plates reveal a misfit of vlbi observations and rm2 minster and jordan 1978 predictions significant at the 99 confidence level nuvel l demets et al 1990 reproduces the velocities of these 10 sites far better than rm2 and offers a fit to the data which is statistically indistinguishable from vm2 and vm3 by including velocities of 16 additional stations in the deforming plate margin vm2 and vms can resolve all four terms vpn total pacific north america motion vsa san andreas slip ve w path integrated deformation east and west of the san andreas in the vector equation characterizing pacific north america plate boundary deformation vpn vsa ve vw for several points along the san andreas without reference to a global plate model geological or local geodetic measurements or restrictions on distributed shear in most cases instantaneous vlbi determined rates agree with geologically based estimates mapped to point c 36 0 n 120 6 w in central california vm2 gives vpn c 46 8 1 4 mm yr n36 2 w vsa c 38 6 1 3 mm yr n36 2 w and ve c 8 2 1 3 mm yr n34 7 w vlbi substantially eliminates vw c the component of the san andreas discrepancy west of c although a few millimeters per year of right lateral strike slip on offshore faults can not be rejected at point d 34 1 n 117 0 w in southern california vm2 returns vsa d 25 0 1 1 mm yr n52 3 w ve d 8 8 1 1 mm yr n25 8 w and vw d 13 3 1 6 mm yr n21 5 w vw d is accommodated west of the san andreas to the channel islands the azimuth of san andreas slip suggests that the transverse ranges north of los angeles are absorbing 8 1 mm yr of compression perpendicular to the trend of the fault

identifying changing fluvial conditions in low gradient alluvial archaeological landscapes can coleoptera provide insights into changing discharge rates and floodplain evolution using examples from dated alluvial deposits in the trent basin uk this paper suggests that the waterlogged remains of insects mainly beetles can be used to detect the occurrence of varying discharge and changing patterns of flood plain evolution in low gradient alluvial landscapes of holocene age this approach may provide invaluable data where the finer grained nature of sedimentation means that other types of information such as coarse boulder berms are lacking comparison of coleoptera assemblages from three high energy and two lower energy deposits indicates that a number of distinctive characteristics can be identified and criteria can be set for the distinction of these two contrasting depositional environments this approach is tested using coleoptera remains recovered from a palaeochannel fill at spalford meadows in the lower trent valley the application of this methodology could contribute to the reconstruction of river histories a key factor in understanding the distribution of valley floor archaeological settlement patterns

vertical saltmarsh accretion and its relationship to sea level in the severn estuary u k an investigation using foraminifera as tidal indicators saltmarsh vertical accretion has been used as a proxy for sea level rise in previous studies on the assumption that accretion is in quasi equilibrium with sea level rise this assumption requires verification in each case as accretion may lag or exceed sea level rise leading to under or over estimates of sea level rise respectively verification may be provided by biostratigraphic analysis where biological remains in saltmarsh sediments may be related to former tide levels foraminifera are well suited as studies of modern saltmarshes have demonstrated distinct tidal relationships which may be used to calibrate downcore temporal foraminifera sequences it is hypothesized that depending on the relationship between sea level rise and accretion quasi equilibrium submergence and emergence foraminifera sequences may be recognized these are explored here in a severn estuary u k saltmarsh where an emergence foraminifera sequence is identified suggesting that previous sea level rise rates for the severn estuary based on saltmarsh accretion rates may be over estimates wider consequences of this may include over estimation of regional crustal subsidence rates required to accommodate artificially high sea level rise rates saltmarsh accretion rates are clearly not a suitable proxy for sea level rise in all cases and as such each saltmarsh requires biostratigraphic evaluation to establish sea level rise accretion relationships preferably independent sea level index points slips should be used over saltmarsh accretion rates in constructing sea level histories

mechanical characterization of limestone from sound velocity measurement the aim of this research was to establish correlations between different physical properties sound velocity vp bulk density and open porosity p0 and mechanical properties compressive strength rc tensile strength rt static elasticity modulus e0 and poisson s ratio i i of limestones the first step of the work consisted in an experimental campaign on limestone cores drilled from a gothic building in paris eight cores were extracted from the monument and tested in the laboratory secondly the experimental results were added to those proposed by 14 authors in the international scientific literature and all of these data were analyzed using the generalized least squares method so that correlation curves could be suggested seven correlations are proposed rc vp rc vp p0 e0 rc i i vp rt rc finally the dispersion of the results led to an estimated confidence level of 90 according to statistical considerations highlights we carried out an experimental physical and mechanical campaign on limestones cores correlations between different physical and mechanical properties are established dispersion of results led to an estimated confidence level according to statistical considerations

global positioning system reoccupation of early triangulation sites tectonic deformation of the southern coast ranges we study tectonic deformation in the southern coast ranges california we use triangulation and astronomic azimuth data collected since 1875 trilateration since 1970 and global positioning system data collected from 1986 to 1987 two modeling techniques have been used an elastic block fault model is applied to study the tectonic motion of the san andreas fault and the san gregorio hosgri fault station velocities are modeled to study regional deformations results show that the regional deformation is predominantly controlled by deep strike slip motion along the san andreas fault at a rate of 33 2 mm yr deep slip along the san gregorio hosgri fault is about 0 4 mm yr assuming a locked fault to a depth of 20 km convergence normal to the san andreas fault in the southern coast ranges is not greater than 0 02 rad yr

locating where archaeological sites occur in intertidal sequences the use of archaeoentomological data as a proxy for tidal regime intertidal archaeological deposits occur worldwide particularly in the temperate latitudes these deposits can contain archaeological sites that were constructed at the time these were terrestrial landscapes but subsequently were inundated as a result of rising sea levels part of this process can include the development of salt marshes there is a need therefore to identify where archaeological sites lie within the cline of past tidal regimes this paper presents the results of a survey of uk archaeoentomological data recovered from intertidal deposits which was undertaken in order to identify patterns in archaeoentomological data that might indicate a deposit s position within a saltmarsh such an approach has potential to establish indicator groups for saltmarsh zones thereby facilitating archaeological interpretation of intertidal deposits a statistical ordination of the archaeoentomological dataset has been undertaken to explore the security and strength of proposed archaeoentomological indicator groups for various ecological zones within saltmarsh intertidal environments and the results are presented here these indicator groups also are crossed checked against the known modern ecology of the various beetles included within each grouping to determine if they make good ecological sense the dataset discussed here is specific to northern europe but the approach is applicable worldwide

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

reconstructing relative sea level change using uk salt marsh foraminifera abstract the position of past sea levels can be reconstructed using a series of sea level index points that possess information on age location altitude and a quantified vertical relationship with a former tidal frame the indicative meaning whilst these points fix the altitude of relative sea level at one instant in time they provide no information on its variation between them here a foraminiferal transfer function is used to reconstruct changes in water depth from fossil assemblages preserved within salt marsh sediments the transfer function performs reliably in the high marsh zone where agglutinated foraminiferal species are dominant in lower marsh environments where calcareous species are prevalent postmortem test dissolution alters the fossil assemblages and leaves them without modern analogues to circumvent this problem a new transfer function based on agglutinated foraminifera and utilising preserved test linings is developed this new transfer function significantly reduces the number of samples without modern analogues but at the expense of diminished sensitivity in the low marsh environment additionally the transfer function is used to establish a series of sea level index points that place observed lateral shifts in depositional environment into a vertical and temporal context the combination of age altitude and high resolution water level reconstructions offers the potential to obtain more detailed and reliable records of relative sea level change from salt marsh sediments

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

a theory of intraplate tectonics the theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation a new theory of intraplate tectonics is developed to calculate the first order intraplate deformation induced by horizontal displacement of deformable plate boundaries it is based on simple assumptions that link the well established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries inward outward and tangential displaced boundaries the theory predicts the direction of intraplate displacement displacement rate strain and stress fields in terms of small circles great circles and 45 loxodromes around the pole of rotation of two adjacent plates the 45 loxodromes are two orthogonal sets of directions clockwise and counterclockwise that intersect both small and great circles at 45 the principal axis of the maximum horizontal stress follows small circles for inward displaced boundaries great circles for out ward displaced boundaries and loxodromes for tangential displaced boundaries the theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project zoback 1992 the theory indicates that the first order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid ocean ridges and plate interiors ridge push the simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system

diatom based tidal level transfer functions as an aid in reconstructing quaternary history of sea level movements in the uk this research analyses the diatom asssemblages recorded from six uk coastal sites and relates these diatom assemblages to tidal levels the relationship between diatom assemblages and tidal levels is examined statistically in order to develop a diatom based tidal level transfer function the results suggest that there is highly significant correlation between the diatom assemblages and water levels from mean high water of neap tides to highest astronomical tide p 0 01 99 random permutations a weighted average wa transfer function is thus established and the predictive ability of this transfer function is highly satisfactory finally this transfer function is applied successfully to estimate palaeotidal levels from fossil diatom data recorded in late holocene coastal sequences copyright 1999 john wiley sons ltd

can we characterise openness in the holocene palaeoenvironmental record modern analogue studies of insect faunas and pollen spectra from dunham massey deer park and epping forest england this paper examines the degree to which tree associated coleoptera beetles and pollen could be used to predict the degree of openness in woodland the results from two modern insect and pollen analogue studies from ponds at dunham massey cheshire and epping forest greater london are presented we explore the reliability of modern pollen rain and sub fossil beetle assemblages to represent varying degrees of canopy cover for up to 1000 m from a sampling site modern woodland canopy structure around the study sites has been assessed using gis based mapping at increasing radial distances as an independent check on the modern insect and pollen data sets these preliminary results suggest that it is possible to use tree associated coleoptera to assess the degree of local vegetation openness in addition it appears that insect remains may indicate the relative intensity of land use by grazing animals our results also suggest most insects are collected from within a 100 200 m radius of the sampling site

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

the application of local and regional transfer functions to the reconstruction of holocene sea levels north norfolk england foraminiferal assemblages from thornham and brancaster marshes norfolk uk illustrate statistically significant relationship with elevation with respect to the tidal frame we develop local data from thornham and brancaster marshes and regional data from thornham and brancaster marshes combined with those from 11 other sites around the uk predictive foraminifera based transfer functions to reconstruct former sea levels from a holocene sediment sequence from holkham north norfolk uk the two transfer functions produce similar patterns of tidal elevation change during the holocene the vertical error ranges of the local transfer function are smaller than those of the regional transfer function although the difference 0 09 m is not significant when compared to other factors affecting the reconstructed elevation the value of the reconstructed elevations also differ between the two transfer functions by up to 0 43 m and this is primarily due to the lack of modern analogues in the local transfer function we conclude that the reconstructions derived from the regional transfer function are more reliable than those of the local transfer function since the latter achieves its slight increase in precision at the expense of a significant decrease in predictive power the regional transfer function is used to construct a relative sea level curve from fossil assemblages within a sediment core from north norfolk uk these results are consistent with existing sea level data and geophysical model predictions and illustrate the utility of the foraminifera based transfer function approach

a late bronze age insect fauna from the river avon warwickshire england its implications for the terrestrial and fluvial environment and for climate abstract insect faunas from organic deposits laid down by the warwickshire avon dating from the late bronze age were examined and their environmental significance assessed it was concluded that the local countryside was open grassland with evidence of large grazing mammals the river avon at this time seems to have had a clean stony bed without the blanket of mud and silt which forms the bottom today a climate slightly warmer than that of the present time is suggested comparison is made with faunas of sites of the same or similar ages

geodetic measurement of deformation in the ventura basin region southern california we have measured the deformation in the ventura basin region southern california with global positioning system gps measurements carried out over 4 6 years between 1987 and 1992 the deformation within our network is spatically variable on scales of tens of kilometers with strain rates reaching 0 6 1 micro rad yr in the east central basin blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower 0 06 1 micro rad yr to the south we also observed clockwise rotations of 1 deg 7 deg m y shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and gps measurements give consistent though less precise results the geodetic rates of shortening across the basin and western transverse ranges are lower than those estimated from geological observations but the patterns of deformation from the two methods agree qualitatively

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

north america pacific plate boundary an elastic plastic megashear evidence from very long baseline interferometry i investigate a number of lithospheric deformation models based upon the concept that the motions of points near the north america pacific plate boundary are a linear combination of north america and pacific velocities this concept entails a shear strain transition zone in the vicinity of the boundary the best of these models fit 95 of the variance in 139 very long baseline interferometry vlbi length and transverse velocity observations taken over 5 5 years and require only 90 of the relative plate motion predicted by rm2 residual misfits in the megashear models are largely explained by a 6 1 mm yr component of basin and range spreading perpendicular to the plate boundary instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the san andreas fault some of this deformation will be recovered through elastic rebound the rest will be permanently set through plastic processes thus depending on the elastic plastic composition of the borderlands the width and pattern of strain accumulation in the megashear over geological time may or may not be significantly different than what is currently observed because vlbi data have been assembled for only a small fraction of the earthquake cycle plastic and elastic strains are indistinguishable other information will have to be brought to bear to discriminate elastic and plastic behaviors and reconcile instantaneous and geologic deformation near the north america pacific margin

reconstructing holocene sea level using salt marsh foraminifera and transfer functions lessons from new jersey usa we present an expanded training set of salt marsh foraminifera for reconstructing holocene relative sea level change from 12 sites in new jersey that represent varied physiographic environments seven groups of foraminifera are recognized including four high or transitional marsh assemblages and a low salinity assemblage a weighted averaging transfer function trained on this dataset was applied to a dated core from barnegat bay to reconstruct sea level with uncertainties of 14 of tidal range we evaluate the transfer function using seven tests 1 leave one site out cross validation suggests that training sets of salt marsh foraminifera are robust to spatial autocorrelation caused by sampling along transects 2 segment wise analysis shows that the transfer function performs best at densely sampled elevations and overall estimates of model performance are over optimistic 3 dissimilarity and 4 non metric multi dimensional scaling evaluated the analogy between modern and core samples the closest modern analogues for core samples were drawn from six sites demonstrating the necessity of a multi site training set 5 goodness of fit statistics assessed the validity of reconstructions 6 the transfer function failed a test of significance because of the unusual properties of some cores selected for sea level reconstruction 7 agreement between reconstructed sea level and tide gauge measurements demonstrates the transfer function s utility copyright 2013 john wiley sons ltd

determination of relative site motions in the western united states using mark iii very long baseline interferometry four years of very long baseline interferometry vlbi measurements involving fixed and mobile stations in the western united states are analyzed using weighted least squares baseline solution time series are used to estimate rates of change of the vector baselines between pairs of vlbi stations horizontal vector motions are determined for 18 sites with respect to a site in the mojave desert six sites west of and two sites just to the east of the san andreas fault show northwestward motion closely parallel to the san andreas fault their motions range from 18 to 43 mm yr and are significant at confidence levels greater than 99 motions in california relative to interior north american plate sites are used to estimate an integrated contribution from basin and range spreading to the total relative pacific plate motion as 9 10 mm yr a maximum relative velocity of 50 51 mm yr is found between the interior north american sites and the western california sites in close agreement with the nuvel i plate motion model

measurement of horizontal motions in alaska using very long baseline interferometry we have analyzed 6 years of very long baseline interferometry vlbi data beginning in 1984 from a network of sites in alaska the yukon and the conterminous united states the analysis of these 53 data sets includes constrained continuous piecewise linear estimation of the site atmospheres and clocks application of values and correlations from a self consistent earth orientation series derived from other vlbi data and simultaneous estimation of site positions and velocities from a solution that includes 21 007 observations with a fit of 40 ps in delay and 64 fs s in delay rate we have determined the horizontal velocities of seven sites with precisions of 0 5 2 mm yr 1 standard deviation the three vlbi sites within the alaska aleutian subduction boundary zone yakataga kodiak and sand point have higher instantaneous velocities relative to eastern north america than the interior sites of alaska fairbanks and sourdough the estimated velocity of yakataga relative to fairbanks is 37 8 1 3 mm yr at n27 w 2 modeling of the yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip the estimated velocity of kodiak relative to fairbanks is 13 6 0 8 mm yr at n41 w 3 this result is lower than the rate predicted on the basis of simple elastic dislocation models the estimated velocity of sand point relative to fairbanks is 6 4 0 9 mm yr at n65 w 8 the component of this velocity in the plate direction 3 5 1 1 mm yr is not consistent with either an aseismic slip model i e no strain accumulation in the overriding plate or a model in which the site is undergoing compressional strain 10 9 mm yr at n28 w due to simple subduction at the predicted orientation of pacific plate convergence in a north america fixed reference frame the estimated velocities of fairbanks 1 5 0 5 mm yr at s12 e 20 and sourdough 3 9 1 0 mm yr at s13 w 13 are both southward these results may imply motion of this region relative to stable north america but more observations are required to reduce the uncertainties in the azimuths the time evolution of the estimated site positions from vlbi in alaska are in general consistent with constant velocity however in 1988 the yakataga site showed a significant departure from the position predicted from a constant velocity model obtained from the previously estimated position this departure may be static displacement associated with two major strike slip earthquakes that occurred in the gulf of alaska between the 1987 and 1988 measurement sessions

quasi static propagator matrices creep on strike slip faults abstract this paper presents a method for computing viscoelastic flow in a layered earth by means of quasi static propagator matrices the method has advantages over approximate or purely numerical attacks in that exact semi analytical solutions are obtained the procedure enables a more rapid calculation than is possible with finite elements yet it does not sacrifice exactness as do analytical approximations to illustrate the technique i constructed a plausible model of the san andreas fault and investigated the time and space behavior of displacement displacement rate shear stress and shear strain rate at depth as well as at the surface viscoelastic relaxation speeds the restressing of the fault for events of magnitude 6 2 and 6 9 viscoelasticity reduces recharge time relative to the base strain rate by 15 and 50 respectively for events of magnitude 6 2 and less viscoelasticity has only a small influence and a linear extrapolation of stress accumulation will predict the time of recharge reasonably well relative plate velocities measured within 400 km of the fault are highly variable in space and time direct plate velocity measurements made as far as 100 km from a major fault could differ by a factor of two from the average rate features of the fault model at depth include stresses and strain rates which exceed surface values by a factor of three sign reversals in strain rates and positive coseismic stress drops induced for limited periods in narrow thin zones the latter feature could initiate and terminate aftershock sequences stress recharge does not occur simultaneously at all depths on the fault for all magnitude events recurrence times of earthquakes estimated from surface observations may thus be biased

a kinematic model of southern california we propose a kinematic model for southern california based on late quaternary slip rates and orientations of major faults in the region internally consistent motions are determined assuming that these faults bound rigid blocks relative to north america most of california west of the san andreas fault is moving parallel to the san andreas fault through the transverse ranges and not parallel to the motion of the pacific plate this is accomplished by counterclockwise rotation of california south of the san andreas fault and by the westward movement of central california north of the gar lock fault the velocities of the blocks are calculated along several paths in southern california that begin in the mojave desert and end off the california coast a path that crosses the western transverse ranges accumulates the accepted relative north america pacific plate velocity whereas paths to the north and south result in a significant missing component of motion this implies the existence of a zone of active deformation in southern california that is interpreted to include the western transverse ranges and northwest trending predominately strike slip faults close to the coast both north and south of the transverse ranges strain on this system accounts for about a third of the total north america pacific plate motion

the use of ostracods from marginal marine brackish waters as bioindicators of modern and quaternary environmental change abstract ostracoda microscopic aquatic crustacea from brackish waters have a great potential for ecological monitoring and palaeoenvironmental analyses in highly variable environments this has been proven in many articles during recent decades but their potential has yet to be fully developed or utilised the analysis of ostracod assemblage composition species distributions eco phenotypic variability and the analysis of stable isotopes and trace elements in ostracod shells provide valuable information on present and past water salinity temperature and chemistry hydrodynamic conditions substrate characteristics climate sea level variations oxygen and nutrient availability this article provides an overview on the application of ostracods from brackish waters for palaeoenvironmental reconstruction in the quaternary and for environmental studies in present day environments

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

precision geodesy using the mark iii very long baseline interferometer system very long baseline interferometry vlbi has been used to make precise measurements of the vector separation between widely separated antennas the system for acquiring and processing vlbi data known as mark iii is described tests of the system show it to have millimeter level accuracy on short baselines measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters vlbi experiments conducted between 1976 and 1983 have demonstrated the stability of the north american plate by showing that there is no change in the distance between easternl california and massachusetts at the level of a few millimeters per year or greater experiments made from 1980 to 1984 indicate that the distance from massachusetts to sweden is increasing by 1 7 1 cm year where the quoted standard deviation includes the estimated effects of systematic atic errors

present tectonic motion across the coast ranges and san andreas fault system in central california geodetic results from very long baseline interferometry vlbi satellite laser ranging slr and the global positioning system gps are used to estimate angular velocities between the sierran microplate pacific plate and north american plate the sierra pacific pole of rotation lies nearer to the san andreas fault than does the pacific north america pole of rotation and leads to different tectonic implications than if the latter is used the angular velocities show that the san andreas fault system and central california coast ranges accommodate motion of 39 2 mm yr mainly by strike slip faulting all confidence limits following signs in this paper are 95 confidence limits fault normal motion is small is mainly convergent at rates up to 3 3 1 0 mm yr and varies along the coast but is divergent at 2 6 1 2 mm yr across san pablo bay and associated topographic lows across which the sierran and central valley watershed drains to the pacific ocean the mountain ranges tend to be larger where the fault normal convergence rates are larger the low convergence rate 0 5 1 8 mm yr normal to the san andreas fault in the carrizo plain differs sharply from that previously inferred 8 2 1 2 mm yr and 4 9 1 6 mm yr by feigl et al 1993 the difference is due to differences between their and our elastic strain accumulation models and between how their and our pacific plate reference frames are defined the ranges in most places require a minimum of 4 2 1 m y of fault normal convergence at the present rate to attain their present cross sectional area if erosion is neglected more if it is not the amount of convergence previously estimated from a balanced cross section across the diablo range in central california requires 10 8 3 m y of convergence at the present rate the former is consistent with widely held views about the onset of the coast range orogeny but the latter is not both are consistent however with the recent plate reconstructions by s cande j stock and colleagues which indicate that pacific plate motion relative to north america changed to a more convergent direction 20 25 clockwise of its prior direction at ca 8 to 6 ma and not at 3 5 ma as had been previously inferred the inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the san andreas fault of 0 7 4 7 from which we infer that the sierran microplate changed motion relative to north america at the same time ca 8 to 6 ma as did the pacific plate we further infer that the motion accommodated across the great basin must also have changed at the same time we also examine the hypothesis that stable sliding occurs along the san andreas fault and other northwest striking strike slip faults in central california where the fault normal convergence rate is low or negative and that these faults are unstable where the fault normal convergence rate is high such a relationship appears to hold in general but fails in detail in particular there are substantial sections of fault with small inferred rates of fault normal convergence across which the san andreas fault is locked moreover the creeping section of the san andreas fault i e the section between parkfield and the calaveras junction is the locus of greater fault normal convergence 3 2 1 4 mm yr than is the locked part of the fault 0 5 1 8 mm yr south of parkfield thus this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault

san luis obispo transform fault and middle miocene rotation of the western transverse ranges california a fault disrupted linear belt of lower middle miocene pyroclastic and volcanic rocks was erupted 15 17 m y b p along a trend 140 to 200 km long in the southern coast ranges and along the northern flank of the western transverse ranges of california offset segments of the belt are coincident with the oceanic west huasna santa maria river little pine and lompoc solvang fault zones the volcanic rocks thin eastward and also vary in thickness along strike the latter reflecting the undulation of porpoising highs and lows of basins to account for the linear distribution of over 3000 km2 of bimodal volcanic rock types it is proposed that they were intruded extruded and ejected into a continental submarine margin environment along a feature here named the san luis obispo transform slot the fault was near the mid miocene continental margin and evidently joined the early mid miocene mendocino triple junction with the crest of the east pacific rise subduction of the farallon plate had stopped along the adjacent continental margin before miocene magmatism in this region the inferred position of the east pacific rise crest was too far to the south and the inferred position of the mendocino triple junction was too far to the north between 15 and 20 m y b p to have been directly related to the miocene volcanic event near 35 n latitude at that time a part of the western transverse ranges was transported northwestward immediately following miocene volcanism earlier movement is possible but evidence is inconclusive microplate transport was concurrent with the development of pull apart structures and the 12 to 16 m y old period of volcanism along the southern margin of the microplate within the los angeles basin variable paleomagnetic directions that display clockwise deflections in the oligocene e g morro rock islay hill complex and miocene igneous rocks e g western transverse ranges in the vicinity of slot can be accounted for by extension and rotational tectonics during pull apart development rotation of blocks between faults during the northwestward transport of the western transverse ranges along slot and by rotation and possible northward translation of a part of the western transverse ranges