stratigraphy and radiocarbon chronology of a subaerially exposed holocene coral reef dominican republic a subaerially exposed coral reef records a period of marine conditions following earliest holocene transgression into the technically active subsea level enriquillo valley dominican republic field mapping detailed leveling of reef elevations and i4c dating of fossil corals and mollusks reveal the following chronology of reef development in an unusual geologic and tectonic environment 1 earliest holocene 9760 pm 100 yr b p marine transgression and growth of oysters on an alluvial fan surface approximately 33 m below sea level bsl 2 colonization of reef corals at approximately 34 m bsl on the oyster shell deposits 8990 pm 60 yr b p 3 middle holocene development of a typical caribbean fringing reef in a protected environment 6200 pm 80 to 4760 pm 90 yr b p 4 late holocene sedimentation over the reef with fossil bivalves 5710 pm 90 to 2820 pm 40 yr b p that record a gradual change from marine to brackish water that accompanied growth of shoreline th

from transtension to transpression along the northern caribbean plate boundary off cuba implications for the recent motion of the caribbean plate abstract marine geophysical surveys using seabeam single channel seismic reflection gravimetric and magnetic measurements have been conducted along a segment of the northern caribbean transcurrent plate boundary seacarib ii cruise the data allow a better definition of the geometry and the tectonic regime of this major strike slip area they support the following results 1 1 along the southern cuban margin the oriente fault displays a discontinuous trace mainly composed of dextral offset en echelon segments some pull apart basins are located between fault segments cabo cruz basin chivirico and baitiquiri basins in the windward passage area the plate boundary enters into the tortue channel and is not connected with the subduction front off northern hispaniola 2 2 the eastern part of the oriente deep and the santiago promontory are characterised by active compressional tectonics they form the santiago deformed belt described here for the first time this deformed belt can be divided longitudinally into three main segments each one characterised by a particular tectonic style its development is related to a transpressional mechanism along the left lateral oriente strike slip fault our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area probably in the late pliocene which may be compared with the recent geological events recorded on land in the northern caribbean domain the precise knowledge of both geometry and structures along the oriente strike slip fault south of cuba provides new constraints for the recent kinematic evolution along the northern caribbean transcurrent plate boundary it leads us to infer the existence of a convergence component associated with the slip component along the oriente transform fault

the 1946 hispaniola earthquakes and the tectonics of the north america caribbean plate boundary zone northeastern hispaniola we have determined focal mechanisms for the largest earthquake ms 7 8 recorded instrumentally in the caribbean basin the august 4 1946 hispaniola earthquake and three of its large magnitude ms 6 1 aftershocks we also relocated 63 aftershocks and one foreshock of the event series the aftershock series is elongate trends wnw and is centered on the samana peninsula of northeast hispaniola shallow aftershocks are in a 75 km wide linear zone and intermediate depth 70 to 130 km aftershocks apparently delineate a moderately south or ssw dipping slab it is not clear however whether these events indicate active subduction of north american atlantic ocean lithosphere or are strike slip events on the interface between subducted but no longer sinking slab and caribbean mantle we constrained focal mechanisms of the main shock and three aftershocks by combining observed p and s polarities and amplitude ratios and also by waveform modeling the two methods yield consistent results the mechanisms include strike slip and thrust displacements on nw striking nodal planes fault dip is variable ne or sw the nw striking fault planes parallel mapped terrane boundaries and faults in the north america na caribbean ca plate boundary zone and are also parallel to the aftershock series trend we interpret the events to be motions on a wnw trending restraining bend segment of the na ca plate boundary in eastern hispaniola we have calculated magnitudes for eight of the earthquakes in the series for the three events including the main shock for which data are available our magnitudes are systematically less than the previously published magnitude estimates given the high magnitude and large aftershock area of the august 4 1946 event these earthquakes probably represent the true long term interplate motions between north america and the terranes in this portion of the plate boundary zone

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

behavior of heavy metals in sulfide mine tailings and bottom sediment salair kemerovo region russia the given work focused on solving the problem of environmental geochemistry related to investigation of element speciation their mobility and migration in polluted areas the purpose was to describe quantitatively migration distribution and redistribution of heavy metals by the example of the old tailings talmovaya sands of the lead zinc concentration plant salair kemerovo region russia and technogenic bottom sediments of the malaya talmovaya river contents of elements in the sulfide tailings range in the following limits zn 1 100 27 000 ppm cd 1 3 240 ppm pb 0 01 0 81 ppm cu 220 960 ppm as 15 970 ppm fe 19 000 76 000 ppm and ba 80 000 1 00 000 ppm element concentrations in the river sediment are proportional to the element contents in the sulfide tailings element speciations in the sulfide tailings and technogenic bottom sediments were investigated by the modified sequential extraction procedure chemical forms of heavy metals in pore water and surface water were calculated by wateq4f software principles of heavy metal migration in the sulfide tailings and technogenic bottom deposits were established the obtained results about element species in the sulfide tailings and sediment explain the main principles of element migration and redeposition in the mine waste and technogenic bottom deposits there is vertical substance transformation with formation of geochemical barriers

global geodesy using gps without fiducial sites baseline lengths and geocentric radii have been determined from gps data without the use of fiducial sites data from the first gps experiment for the iers and geodynamics gig 91 have been analyzed with a no fiducial strategy a baseline length daily repeatability of 2 mm 4 parts per billion was obtained for baselines in the northern hemisphere comparison of baseline lengths from gps and the global vlbi solution glb659 caprette et al 1990 show rms agreement of 2 1 parts per billion the geocentric radius mean daily repeatability for all sites was 15 cm comparison of geocentric radii from gps and sv5 murray et al 1990 show rms agreement of 3 8 cm given n globally distributed stations the n n 1 2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well defined center of mass geodetic information can be obtained by examining the structure of the polyhedron and its change with time

monument motion and measurements of crustal velocities it is usually assumed in geodetic studies that measurement errors are independent from one measurement to the next and that the rate of deformation velocity is constant over the duration of the experiment any temporal correlation between measurements can substantially affect the uncertainty in this velocity estimate when it is determined from the time series of measurements one source of possible long term correlation is motion of the geodetic monument with respect to the deep crust available measurements suggest that this motion introduces errors that have the form of a random walk process we show how such errors affect the uncertainty of velocity estimates for a geodetic experiment of set duration we calculate the velocity uncertainty as a function of the number of observations and of the relative amount of correlated and uncorrelated noise we find that 1 neglecting long term temporal correlations makes the uncertainty in the estimated velocities much too small and that 2 when the correlated and independent noise sources are of similar magnitude the expected improvement in uncertainty from having more measurement is not realized there is almost no improvement in some cases we have also examined the effect of outliers blunders on the velocity uncertainty for a frequency of outliers typical of geodetic field campaigns the previous two conclusions remain unchanged these results suggest that long term correlations have a large effect on estimating deformation rates unless these correlations are small frequent observations give little advantage if frequent observations are planned the amount of correlated noise due to monument instability must be kept small if the full capabilities of the measurement technique are to be realized

tectonic motion and deformation from satellite laser ranging to lageos satellite laser ranging slr to lageos acquired during the period 1978 1988 has been analyzed to yield estimates of tectonic motion for 22 laser tracking stations situated on seven major plates the analysis is based on the precise modeling of the orbit dynamics of lageos and includes the determination of other geodynamic and nonconservative force model parameters involved in the orbit determination problem at the centimeter level site velocities were recovered from station positions determined each calendar quarter using a network adjustment procedure which maintains the reference frame within the stable interior of tectonic plates the recovered motions indicate good agreement with existing estimates of stress fields particularly across northern europe the eastern pacific and in the basin and range province of north america a comparison of the intersite motions for stations centrally located on separate plates between slr and those implied by the nuvel 1 geologic motion model yield a strong positive correlation of 0 989 and a 3 7 scale difference which may be attributable to recent changes in relative velocities or geologic time scale uncertainties sites located at or within 250 km of convergent plate boundaries have additional components of motion indicating a need for further tectonic modeling beyond that provided by simple rigid plate motions for lines crossing the north atlantic the san andreas fault and within the basin and range province the geodesic rates determined by slr are in good agreement with those determined by very long baseline interferometry an alternative space geodetic technique

reply to comment on the 1946 hispaniola earthquake and the tectonics of the north america caribbean plate boundary zone northeastern hispaniola by r m russo and a villasenor

seamarc ii mapping of transform faults in the cayman trough caribbean sea seamarc ii maps of the southern wall of the cayman trough between honduras and jamaica show zones of continuous well defined fault lineaments adjacent and parallel to the wall both to the east and west of the cayman spreading axis these lineaments mark the present active traces of transform faults which intersect the southern end of the spreading axis at a triple junction the swan islands transform fault to the west is dominated by two major lineaments that overlap with right stepping sense across a large push up ridge beneath the swan islands the fault zone to the east of the axis named the walton fault is more complex containing multiple fault strands and a large pull apart structure the walton fault links the spreading axis to jamaican and hispaniolan strike slip faults and it defines the southern boundary of a microplate composed of the eastern cayman trough and western hispaniola the presence of this microplate raises questions about the veracity of caribbean plate velocities based primarily on cayman trough opening rates

muertos trough subduction microplate tectonics in the northern caribbean the relative plate motion between the north american and caribbean plates has been commonly assumed to take place in the form of oblique subduction along the puerto rico trench1 west of hispaniola this plate boundary becomes pure strike slip nevertheless it has been suggested that this boundary is more diffuse with part of the plate motion taken up by subduction along the muertos trough mt south of hispaniola2 5 fig 1 but there has been no direct seismological evidence of subduction to confirm this here we present the results of a study of the 24 june 1984 earthquake beneath mt the focal mechanism shows thrusting at a depth of 32 km which together with reports of a great earthquake in 1751 confirms that mt is an active plate boundary this implies that plate motion is accommodated by several tectonic boundaries delineating microplates caught between the caribbean and north american plates fig 4

relative motion between the caribbean and north american plates and related boundary zone deformation from a decade of gps observations global positioning system gps measurements in 1986 1994 and 1995 at sites in dominican republic puerto rico cuba and grand turk define the velocity of the caribbean plate relative to north america the data show eastward motion of the caribbean plate at a rate of 21 1 mm yr 1 standard error in the vicinity of southern dominican republic a factor of 2 higher than the nuvel 1a plate motion model prediction of 11 3 mm yr independent measurements on san andres island and an euler vector derived from these data also suggest a rate that is much higher than the nuvel 1a model available data combined with simple elastic strain models give the following slip rate estimates for major left lateral faults in hispaniola 1 the north hispaniola fault offshore the north coast of hispaniola 4 3 mm yr 2 the septentrional fault in northern dominican republic 8 3 mm yr and 3 the enriquillo fault in southern dominican republic and haiti 8 4 mm yr the relatively high plate motion rate and fault slip rates suggested by our study combined with evidence for strain accumulation and historical seismicity imply that seismic risk in the region may be higher than previous estimates based on low plate rate low fault slip rate models and the relatively low rate of seismicity over the last century

effects of atmospheric azimuthal asymmetry on the analysis of space geodetic data we develop a formalism for parameterizing and evaluating the effects of lateral variations in the properties of earth s atmosphere on the propagation of microwave signals a parametric form is incorporated into our analysis of very long baseline interferometry vlbi data and the estimated atmospheric delay gradients are compared with those calculated from three dimensional weather analysis fields from the national center for environmental prediction ncep for a 12 day series of experiments in january 1994 the vlbi and the ncep analyses show common atmospheric gradient delays with amplitudes of up to 30 mm at 10 elevation angle comparison of the characteristics over a longer period of time reveals common mean north south gradients with amplitudes up to 10 mm at 10 elevation at midlatitudes no discernible mean east west gradients were found in either data set the root mean square rms variations of the gradient effects determined from the ncep analysis are similar in the north south and east west directions with a typical rms scatter of 6 10 mm at 10 elevation after accounting for gradients detailed analysis of the january 1994 vlbi data shows clearly that the residual station height variations of 10 mm at westford massachusetts are almost totally explained by the effects of atmospheric pressure loading

influence of cohesion on scour under submerged circular vertical jets the results of an experimental study on scour under submerged circular vertical jets of water in cohesionless and cohesive sediments are presented the difference between scour patterns in cohesionless and cohesive sediments is identified in cohesive sediments the variations of maximum depth and volume of scour have been studied with respect to the percentage of clay content dry density antecedent moisture content etc empirical relationships have been proposed for the maximum depth and volume of scour for both nonplastic and plastic cohesive sediments the range of data for the applicability of the proposed relationships is specified

estimating culvert scour in cohesive material an investigation of scour at culvert outlets in a cohesive bed material is presented twelve experiments of 1000 minutes duration were conducted using a cohesive bed material culvert 27 3 cm 10 in 35 6 cm 14 in and 45 7 cm 18 in diameters were tested with discharges ranging from 0 054 m sec 0 824 m sec 1 91 cfs to 29 13 cfs the tailwater elevation was maintained at 0 45 d n 0 05 d above the culvert invert where d is the diameter of the culvert the results yielded a series of empirical relationships expressing the depth width length and volume of scour as a function of the culvert diameter culvert outlet velocity soil plasticity index soil shear strength and fluid density general observations concerning scour hole formation growth and stabilization are reported

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

a gps estimate of relative motion between north and south america gps velocity data are used to estimate the euler vector describing rigid body motion of north america relative to south america assuming the boundary between the north and south american plates is located near the fifteen twenty fracture zone in the equatorial atlantic the euler vector predicts extension across the royal trough up to 1 mm yr and convergence across the barracuda ridge at about 2 mm yr in agreement with geological estimates averaged over tens of millions of years further west convergence between north and south america at rates up to 8 mm yr may contribute to deformation of the caribbean plate along its southwest boundary with south america

plate motion and crustal deformation estimated with geodetic data from the global positioning system we use geodetic data taken over four years with the global positioning system gps to estimate 1 motion between six major plates and 2 motion relative to these plates of ten sites in plate boundary zones the degree of consistency between geodetic velocities and rigid plates requires the one dimensional standard errors in horizontal velocities to be 2 mm yr each of the 15 angular velocities describing motion between plate pairs that we estimate with gps differs insignificantly from the corresponding angular velocity in global plate motion model nuvel 1a which averages motion over the past 3 m y the motion of the pacific plate relative to both the eurasian and north american plates is observed to be faster than predicted by nuvel 1a supporting the inference from very long baseline interferometry vlbi that motion of the pacific plate has sped up over the past few m y the eurasia north america pole of rotation is estimated to be north of nuvel 1a consistent with the independent hypothesis that the pole has recently migrated northward across northeast asia to near the lena river delta victoria which lies above the main thrust at the cascadia subduction zone moves relative to the interior of the overriding plate at 30 of the velocity of the subducting plate reinforcing the conclusion that the thrust there is locked beneath the continental shelf and slope

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

cohesion influences on erosion and bed load transport 1 studies on erosion and transport rates of cohesive sediment mixtures containing gravel are not reported so far in the literature to the best of our knowledge results from laboratory experiments on the process of erosion of cohesive sediment mixtures by open channel flow are presented the sediment used for experimentation consisted of fine gravel mixed with clay in proportions varying from 10 to 50 and fine gravel fine sand in equal proportion by weight mixed with clay proportions again varying from 10 to 50 by weight experimental observations have revealed that the process of detachment of cohesive sediment mixtures containing gravel is significantly different from that of cohesionless sediments also it is different from the detachment process of cohesive sediment mixtures not containing coarser size material experimental data on transport rates of the bed load of the cohesive sediment mixtures are presented on the basis of dimensional considerations a computational procedure is proposed to determine the bed load transport rates generated from erosion of the different size fractions of sediment mixtures clay percentage and unconfined compressive strength of the sediment bed were the main parameters controlling bed load transport rate of such sediments in the absence of cohesion the proposed method reduces to the patel and ranga raju 1996 method for bed load transport rate computation of nonuniform cohesionless sediments

earthquake slip vectors and estimates of present day plate motions although horizontal slip directions determined for earthquakes along transform faults and shallow subduction thrust faults are commonly assumed to parallel the direction of motion between two plates systematic biases of earthquake slip vectors relative to the local long term average plate direction can be introduced by a variety of causes including deformation of forearcs above subduction zones unmodeled lateral velocity heterogeneities near earthquake sources and unrecognized changes in plate velocities within the averaging interval of the kinematic model given that earthquake slip vectors comprise nearly two thirds of the data used to derive the nuvel 1 model for present day global plate velocities it is important to determine the extent to which potential systematic biases of earthquake slip vectors could degrade estimates of plate velocities given by nuvel 1 here two alternative models for global plate velocities are derived and compared to nuvel 1 the first model nuvel sz is derived from a data set that omits the 240 subduction zone sup vectors in the nuvel 1 data set but includes all remaining 882 kinematic data the velocities predicted by nuvel sz and nuvel 1 differ little with maximum differences of only 1 mm yr 1 and 2 along all plate boundaries except those surrounding the caribbean plate the second model nuvel g is derived from a data set that omits all 724 earthquake sup vectors but includes the remaining 277 spreading rates and 121 transform fault azimuths velocities predicted by nuvel g and nuvel 1 also differ little with maximum differences of 2 mm yr 1 and 4 except for the caribbean plate boundaries these results indicate that even in the unlikely event that all earthquake slip vectors in the nuvel 1 data set are unreliable recorders of long term plate directions the slip vectors do not significantly degrade the model it thus appears that the decision to use or not to use earthquake slip vectors to derive a global plate motion model has little practical effect on the ability to derive an accurate description of present day plate velocities little emphasis is placed on the changes in estimates of caribbean plate velocities that occur upon exclusion of earthquake slip vectors nearly all of the data from caribbean plate boundaries are suspect and it is unlikely that the nuvel sz or nuvel g models represent a significantly improved description of caribbean plate velocities comparison of 677 slip vectors from transform faults to directions predicted by nuvel g shows statistically insignificant differences along 12 of 15 spreading centers the good agreement between the nearly instantaneous estimates of slip directions provided by slip vectors and the longer term average directions given by nuvel g suggest that in general transform fault slip vectors parallel the longer term average directions along transform faults interestingly a statistically significant difference between slip vectors from right slipping and left slipping transform faults is noted for nearly all spreading centers with slip vectors along right slipping and left slipping faults rotated clockwise and counterclockwise respectively from the predicted direction the cause of this bias is unknown but may be related to biases introduced by unmodeled lateral heterogeneities in the mantle near transform faults

paleomagnetic evidence for late miocene counterclockwise rotation of north coast carbonate sequence puerto rico a paleomagnetic study of the essentially undeformed middle tertiary carbonate sequence along the north coast of puerto rico reveals statistically significant pre pliocene discordance of characteristic component directions against those expected from cratonic north america for much of the section despite generally weak to moderately weak magnetic intensities confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization intrasite bipolarity and or the reproducibility of measurements the mean geographic direction for the upper oligocene to middle miocene strata is 335 2 degree 32 9 degree and the corrected mean paleomagnetic pole is 207 6 degree 66 5 degree n 3 alpha 95 4 3 degree this suggests a counter clockwise ccw block rotation of puerto rico and its microplate of 24 5 degrees plus minus 5 8 degrees during the late miocene using a width of 250 km for the northern caribbean plate boundary zone ncpbz between the north american plate and caribbean plate the mean left lateral displacement implied is 1 8 to 2 4 cm yr which agrees fairly well with published relative motion rates for the two plates average rotation rate for 50 ma to 20 ma was 0 7 degree my but perhaps as great as 4 degree my in the miocene resolution of mean paleolatitude indicates northward motion of a degree or lessmore during the period of rotation causes of this short lived rotation may include 1 tectonic escape from the inhibiting presence of the bahama banks and beata ridge during eastward motion of puerto rico along the sinistral transpressive puerto rico trench and muertos trough fault systems or 2 changes in relative plate motions of the caribbean and north american plate during the late miocene less

extension and transtension in the plate boundary zone of the northeastern caribbean the authors propose that the caribbean ca north american na plate boundary zone pbz from the puerto rico trench to the venezuelan basin from mona canyon east has been in left transtension over the last 15 20 ma a boundary normal component of extension occurs throughout the pbz and is a principal cause of the puerto rico trench such extension is due to wnw velocity of na ca and the northward pullaway of na from its s dipping slab which is below puerto rico strike slip motion may be taken up among terranes in the pbz by rigid ccw rotation and by oblique slip at their boundaries rotation of the largest terrane puerto rico virgin islands prvi has caused such major structures as the muertos thrust and anegada passage the model implies na ca velocity estimated from cayman transforms is more accurate than that from slip vectors from seisms in the na slab

continuous monitoring of surface deformation at long valley caldera california with gps continuous global positioning system gps measurements at long valley caldera an active volcanic region in east central california have been made on the south side of the resurgent dome since early 1993 a site on the north side of the dome was added in late 1994 special adaptations for autonomous operation in remote regions and enhanced vertical precision were made the data record ongoing volcanic deformation consistent with uplift and expansion of the surface above a shallow magma chamber measurement precisions 1 standard error for absolute position coordinates i e relative to a global reference frame are 3 4 mm north 5 6 mm east and 10 12 mm vertical using 24 hour solutions corresponding velocity uncertainties for a 12 month period are about 2 mm yr in the horizontal components and 3 4 mm yr in the vertical component high precision can also be achieved for relative position coordinates on short 10 km baselines using broadcast ephemerides and observing times as short as 3 hours even when data are processed rapidly on site comparison of baseline length changes across the resurgent dome between the two gps sites and corresponding two color electronic distance measurements indicates similar extension rates within error 2 mm yr once we account for a random walk noise component in both systems that may reflect spurious monument motion both data sets suggest a pause in deformation for a 3 5 month period in mid 1995 when the extension rate across the dome decreased essentially to zero three dimensional positioning data from the two gps stations suggest a depth 5 8 1 6 km and location west side of the resurgent dome of a major inflation center in agreement with other geodetic techniques near the top of a magma chamber inferred from seismic data gps systems similar to those installed at long valley can provide a practical method for near real time monitoring and hazard assessment on many active volcanoes

a composite focal mechanism for microearthquakes along the northeastern border of the caribbean plate seismograms from a local seismic network in the virgin islands portion of the northeastern caribbean are used to determine a composite focal mechanism for 26 microearthquakes along the north america caribbean plate boundary only one nodal plane of the focal mechanism could be constrained from p wave first motion data alone p sv amplitude ratios observed for these events were compared to theoretical amplitude ratios calculated for different focal mechanisms this procedure constrained the dip of the second nodal plane to be shallower than about 50 ruling out the possibility of a transform fault in this portion of the plate boundary the resulting focal mechanism indicates that oblique underthrusting of the north american plate beneath the caribbean plate occurs in the area this oblique motion is accommodated along a thrust plane that dips at a relatively shallow angle beneath the virgin islands platform

postseismic relaxation theory on the spherical earth abstract postseismic displacements and strains observed at the earth9s surface can be explained through the relaxation of a viscoelastic asthenosphere underlying a purely elastic crust a theory is presented for modeling the effects of postseismic relaxation on a spherically symmetric earth for an earthquake point source located in the upper elastic layer the displacement field is decomposed into its toroidal and spheroidal components a linear maxwell rheology in the viscoelastic layers is assumed enabling the use of the correspondence principle for the solutions of the equations of static equilibrium displacement and strain fields are then calculated using normal mode summation computational results are presented for two simple earthquake sources a strike slip fault and a uniaxial thrust fault the patterns of postseismic displacements and strains are found to depend strongly on both the earth model and earthquake source geometry in particular the elastic plate thickness asthenosphere thickness fault type and fault length each play a major role in determining the spatial pattern of postseismic relaxation effects asthenospheric viscosity controls the temporal pattern of relaxation

southern california permanent gps geodetic array spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 landers earthquake the june 28 1992 m w 7 3 landers california earthquake was the first earthquake to be surveyed by a continuously operating global positioning system gps array the coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake we employ a new spatial filtering technique that removes common mode errors from the coordinate time series this approach provides precise estimates of site specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations all sites indicate significant coseismic horizontal displacements of 5 65 mm with uncertainties of 1 2 mm horizontal displacements are in general agreement with elastic dislocation models in particular for sites closer to the epicenter vertical displacements range from 13 to 7 mm with uncertainties of 2 4 mm the observed vertical displacements in all cases show 5 10 mm more subsidence than expected from geodetic and seismic geologic models significant postseismic horizontal displacements totaling 6 2 mm 10 20 of the coseismic displacement are detected at the three sites closest to the epicenter these displacements are modeled as a short term exponential relaxation with a decay time of 22 10 days superimposed on a longer term linear interseismic trend scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain which agrees well with the direction and magnitude determined from more precise laser strain meter data the time series do not show any detectable preseismic displacements

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr