global positioning system measurements of strain accumulation across the imperial valley california 1986 1989 global positioning system gps data collected in southern california from 1986 to 1989 indicate considerable strain accumulation across the imperial valley displacements are computed at 29 stations in and near the valley from 1986 to 1988 and at 11 sites from 1988 to 1989 the earlier measurements indicate 5 9 1 0 cm yr right lateral differential velocity across the valley although the data are heavily influenced by the 1987 superstition hills earthquake sequence the 1988 1989 gps displacements are best modeled by 5 0 0 9 cm yr of valley crossing deformation but rates calculated from 15 years of terrestrial geodetic measurements 3 7 0 1 cm yr fit the data nearly as weh1 the relatively high gps rates are suspect because some measurements especially the east trending displacements have large errors there is evidence from gps and very long baseline interferometry observations that the present strain rate along the southern san andreas fault is smaller than the long term geologic estimate suggesting a lower earthquake potential than is currently assumed correspondingly a higher earthquake potential is suggested for the san jacinto fault

antarctic climate at the eocene oligocene boundary climate model sensitivity to high latitude vegetation type and comparisons with the palaeobotanical record abstract simulated climate for the antarctic continent using the genesis version 2 1 global climate model with 34 ma boundary conditions is shown to be highly sensitive to polar vegetation type six experiments were run using different levels of atmospheric co2 orbital configurations ice sheet geometries and vegetation types to assess model sensitivity to antarctica being covered with either a needle leaf evergreen forest or tundra simulations using 2 pre industrial levels of co2 combined mean annual temperature mat 19 c are about 7 c cooler than minimum estimates from the antarctic cenozoic plant record mat 12 to 15 c however simulations using 3 co2 mat 7 c are in good agreement with our empirical estimates of mean annual temperature with ice sheets and orbits set up to represent early oligocene interglacial conditions the tundra climate is significantly cooler than the evergreen forest climate with local austral summer averages up to 6 c cooler in non glaciated areas and continental averages 2 5 c cooler in the model this is mainly due to higher albedo and decreases in net radiation and sensible and latent heat flux especially during spring and summer feedbacks between coastal and continental cooling marginal sea surface temperatures and sea ice also appear to be significant a review of the late palaeocene to earliest miocene plant fossil record in the antarctic peninsula and ross sea regions shows that the vegetation was in transition 34 ma from a relatively diverse mainly evergreen forest to a tundra vegetation the modelled sensitivity of continental temperatures to a change from forest to tundra suggests vegetation climate feedbacks during the eocene oligocene transition played a significant role in the initial rapid glaciation of the continent

organized structures in subsurface bubble clouds langmuir circulation in the open ocean observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface the measurements were obtained in the open ocean from a freely drifting self contained acoustical instrument equipped with side scan sonar dual frequency echo sounders and ambient sound recording systems the bubble clouds are organized into long narrow plumes aligned with the wind consistent with windrows caused by langmuir circulations they have widths of about 3 to 5 m and lengths up to 100 m with multiple scales coexisting their mean separation depends upon wind speed below 5 m s 1 mean spacings were about 5 m increasing to 10 m when winds exceeded 10 m s 1 the maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed with the greatest penetrations of 12 m occurring at the highest wind speeds 13 m s 1 however the time averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed whenever winds exceeded 5 m s 1 a mean penetration depth of 6 m was observed associated with these bubble plumes were maximum downward velocities of about 0 06 m s 1 these speeds were detected at 8 m depth in the center of the plumes magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes

the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

the effect of atmospheric carbon dioxide concentration on continental glaciation of the northern hemisphere using an ice sheet model that is asynchronously coupled to an atmospheric general circulation model gcm we have computed the volume and areal extent of northern hemisphere ice sheets as a function of atmospheric carbon dioxide concentration the ice sheet model includes an asthenosphere component for computing isostatic adjustment our basic strategy involves determining the net rate of snow accumulation in the gcm as a function of co2 by seeding all northern hemisphere land points initially with 10 m of water equivalent snow and determining the resultant net snow balance over the course of a year the net snow accumulations were then used as input to the ice sheet model which in turn was used to compute the resultant ice sheet height and extent after 25 kyr in order to judge model uncertainty two different but reasonable snowmelt formulations were used in the gcm the basic results of the simulations are 1 the ice sheet volume is strongly a function of co2 concentration with a more or less monotonic increase in volume as co2 is decreased 2 the computed ice sheet volumes are an order of magnitude less than those of reconstructed pleistocene ice sheets these results suggest that co2 changes are an important but not dominant factor in accounting for the pleistocene ice ages though considerable future work is required to clarify this more fully 3 while the choice of snowmelt formulation strongly affected the total volume of ice the rate of change dv d co2 appears to be basically unaffected 4 through use of a computed glaciation sensitivity index we show that greenland is by far the most likely to have an ice sheet tibet and siberia were more likely to have an ice sheet than canada or fenno scandinavia and alaska and north africa were least likely to have ice sheets though each did occur for at least one co2 value these results represent the first step in the process of asynchronous coupling of a gcm and ice sheet model these simulations do not account for such effects as the elevational feedback of the ice sheets on the overall climate or orbital insolation changes only albedo effects are accounted for in a future study we plan to continue the runs iteratively by using the gcm to determine new net snow accumulations consistent with the ice sheets computed in this present study and to investigate the possible effects of orbital insolation changes

holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

a low threshold for north atlantic ice rafting from low slung slippery late pliocene ice sheets 1 suborbital variability in late pleistocene records of ice rafted debris and sea surface temperature in the north atlantic ocean appears most extreme during times of enlarged ice sheets with a well constrained benthic oxygen isotope defined ice volume threshold 18ot for the 100 ka inter glacial world information on climate instability for the earlier pleistocene and late pliocene is more fragmentary and or of much lower temporal resolution but the data available suggest similar behavior with 18ot remaining more or less constant over the past 3000 ka this finding is puzzling because it implies that ice rafting is highly sensitive to ice volume on short suborbital glacial interglacial time scales but not to the long term changes in ice sheet composition associated with intensification of northern hemisphere glaciation nhg here we report new high resolution records of stable isotope change and ice rafting in the north atlantic at integrated ocean drilling program site u1308 reoccupation of deep sea drilling project site 609 during two glacials key to intensification of nhg marine isotope stages g4 2640 ka and 100 2520 ka we find a pattern of suborbital ice rafting events showing clear evidence of threshold behavior however contrary to previous reports we find that 18ot for the late pliocene is up to 0 45 vienna peedee belemnite vpdb lower than for the late pleistocene using published plio pleistocene global sea level records we evaluate different potential explanations for this finding we conclude that the observed pliocene pleistocene offset in 18ot is attributable to the existence of low slung pliocene ice sheets that flowed more readily than their late pleistocene counterparts associated with a smaller contemporaneous continental ice volume and less isotopically depleted ice

effects of spatial discretization in ice sheet modelling using the shallow ice approximation this paper assesses a two dimensional vertically integrated ice model for its numerical properties in the calculation of ice sheet evolution on a sloping bed using the shallow ice approxi mation we discuss the influence of initial conditions and individual model parameters on the model s numerical behaviour with emphasis on varying spatial discretizations the modelling results suffer badly from numerical problems they show a strong dependence on gridcell size and we conclude that the widely used gridcell spacing of 20 km is too coarse the numerical errors are small in each single time step but increase non linearly over time and with volume change as a result of feedback of the mass balance with height we propose a new method for the calculation of the surface gradient near the margin which improves the results significantly furthermore we show that we may use dimension analysis as a tool to explain in which situations numerical problems are to be expected

simulations of permian climate and comparisons with climate sensitive sediments abstract we use a climate model to simulate two intervals of permian climate the sakmarian ca 280 ma at the end of the major permo carboniferous glaciation and the wordian ca 265 ma we explore the climate sensitivity to various levels of atmospheric co2 concentration and to changes in geography and topography between the two periods the model simulates large seasonality and high aridity in the continental interiors of both hemispheres for both periods the northern summer monsoon weakens and the southern monsoon strengthens between the sakmarian and the wordian owing to changes in geography and topography the northern middle and high latitudes cool in winter between the sakmarian and wordian associated with northward shift of the continents this high latitude cooling strengthens the winter westerlies and shifts the maximum storm track precipitation south in the southern hemisphere the winter westerlies weaken from the sakmarian to the wordian starting the simulations with no permanent i

horizontal deformation in the imperial valley california between 1934 and 1980 a triangulation trilateration survey was performed by the national ocean survey national geodetic survey four months prior to the october 15 1979 imperial valley m 6 6 earthquake the survey was repeated in february march 1980 to gauge the coseismic displacement these data combined with triangulation surveys performed in 1934 1935 1939 1941 1954 1955 and 1967 provide a historical record of crustal deformation over a time period that also includes the 1940 el centro m 7 1 earthquake the total data set was analyzed in terms of a dislocation model that relates the observed surface deformation to slip on prescribed geologic faults according to the model the 1941 1980 fault slip at depths 10 km and greater is 1 9 0 3 m this value is approximately twice the 1 0 0 1 m estimated for the 1941 1980 slip on the imperial fault in the 0 to 10 km depth range this discrepancy can be explained by a range of possibilities including the following 1 some of the deep fault slip is due to aftereffects of the 1940 earthquake in which case the seismic cycle over which deep slip and shallow slip are equal is not the interval between large earthquakes but is delayed a few years 2 the deep slip should be equated to the shallow slip over several parallel faults 3 the crust experienced permanent inelastic deformation and 4 deep deformation is accommodated by a mechanism other than slip

coseismic fault slip associated with the 1992 m w 6 1 joshua tree california earthquake implications for the joshua tree landers earthquake sequence coseismic surface deformation associated with the mw 6 1 april 23 1992 joshua tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from global positioning system and trilateration measurements the rms signal to noise ratio for these inferred displacements is 1 8 with near fault displacement estimates exceeding 40 mm in order to determine the long wavelength distribution of slip over the plane of rupture a tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right lateral slip and zero surface slip constraints the resulting slip distribution yields a geodetic moment estimate of 1 7 1018 n m with corresponding maximum slip around 0 8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations from empirical green s function analyses a rupture duration of 5 s is obtained which implies a rupture radius of 6 8 km most of the inferred slip lies to the north of the hypocenter consistent with northward rupture propagation stress drop estimates are in the range of 2 4 mpa in addition predicted coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0 1 mpa in contrast predicted stress changes are near zero at the hypocenter of the mw 7 3 june 28 1992 landers earthquake which nucleated about 20 km beyond the northernmost edge of the joshua tree rupture based on aftershock migrations and the predicted static stress field we speculate that redistribution of joshua tree induced stress perturbations played a role in the spatio temporal development of the earthquake sequence culminating in the landers event

time dependent deformation and stress relaxation after strike slip earthquakes the quasi static time dependent behavior after strike slip earthquakes is studied using three dimensional finite element models the effects of lateral heterogeneities finite fault and nonuniform fault slip are included models of a great earthquake and a medium size earthquake are constructed using a set of possible viscosity structures deformation change after an earthquake is found to be sensitive to the viscosity structure located beneath the fault zone the presence of low viscosity beneath the fault concentrates the horizontal displacement near the fault zone and completely changes the character of vertical deformation thus leveling in addition to horizontal geodetic measurements after a strike slip event is important in determining the viscosity structure near fault zones the shear stress in front of the fault tip increases significantly within several decades after the event for models with a shallow low viscosity zone beneath the fault this accelerated stress accumulation is consistent with the observed phenomenon of earthquake migration along the north anatolian fault zone

oceanic winds measured from the seafloor acoustic ambient noise in the ocean at frequencies of 4 3 8 0 and 14 5 khz has been shown to be highly correlated with surface wind speed measurement of noise at the sea floor yields estimates of wind speed to within 1 m s r2 0 845 when compared with coincident surface observations the frequency dependence of the wind speed noise relation reported by others 17 db decade has been confirmed except that separate calibrations are required above and below the speed characterizing whitecap formation there is also some evidence of local hydrography influencing the absolute calibration

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

global ice volume variations through the last glacial cycle simulated by a 3 d ice dynamical model abstract a coupled ice sheet ice shelf bedrock model was run at 20 km resolution to simulate the evolution of global ice cover during the last glacial cycle the mass balance model uses monthly mean temperature and precipitation as input and incorporates the albedo mass balance feedback the model is forced by the grip and vostok derived temperature records and by sea level and solar radiation variations which are distributed spatially and seasonally model simulations show that at the last glacial maximum lgm a volume of ice representing about 65 m of sea level equivalent is stored on the eurasian continent whereas north america stores 42 m the other regions greenland tibet south america and antarctica contribute about 17 m to global sea level lowering at the lgm compared to present day sensitivity experiments show that the ice volumes of the eurasian and north american ice sheets are particularly sensitive to summer temperature changes finally the timing and mechanism of grounded ice formation and expansion for the last glacial were studied for the eurasian and north american continents ice sheet formation begins at 118 kyr bp in the barents and kara sea region over baffin island and in the high cordilleran range following a 4 c cooling alternating regions of ice sheet expansion by direct snowfall ice flow and grounding of ice shelves can be identified these relate to specific geomorphological traces and may as such help in model validation

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

can milankovitch orbital variations initiate the growth of ice sheets in a general circulation model the goddard institute for space studies climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations three different orbital configurations are used corresponding to 116 and 106 kyr b p and a modified insolation field with greater reductions in summer insolation at high northern latitudes the time periods chosen are those in which geophysical evidence suggests ice sheets may have been growing rapidly the reduced summer insolation field characteristic of all the experiments is thought to be a necessary condition for allowing snow to last through the summer and ice sheets to build the results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets despite the reduced summer and fall insolation when 10 m thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum the model failed to maintain it as well producing energy and mass imbalances which would remove the ice within 5 years only when the ocean surface temperatures were adjusted to their peak ice age values was the model able to keep any of the additional ice and then only in a very restricted region of northern baffin island the experiments indicate there is a wide discrepancy between the model s response to milankovitch perturbations and the geophysical evidence of ice sheet initiation as the model failed to grow or sustain low altitude ice during the time of high latitude maximum solar radiation reduction 120 110 kyr b p it is unlikely it could have done so at any other time within the last several hundred thousand years if the model results are correct it indicates that the growth of ice occurred in an extremely ablative environment and thus demanded some complicated strategy or else some other climate forcing occurred in addition to the orbital variation influence and co2 reduction which would imply we do not really understand the cause of the ice ages and the milankovitch connection if the model is not nearly sensitive enough to climate forcing it could have implications for projections of future climate change

crustal deformation around the gulf of california a number of satellite laser ranging slr sites in the southwest united states and mexico in operation for over ten years have been supporting the global laser network measuring tectonic plate motion and providing information for studies of regional crustal deformation observations of the laser geodetic satellite lageos collected in 1994 by the transportable satellite laser ranging system tlrs 4 at two sites on the baja peninsula now provide the means to extend the network of fixed stations at monument peak and otay mountain in southern california and mazatlan on mainland mexico after the third slr occupation of ensenada it s estimated site motion exhibits nearly the full plate rate predicted by the nnr nuvel1a model for a location on the pacific plate at the southern tip of the baja the motion of cabo san lucas has an azimuth that is more westerly than that expected from pacific modeled motion this discrepancy in azimuth in conjunction with the slower slr recovered velocity for mazatlan results in an apparent 6 mm yr faster spreading rate across the mouth of the gulf of california than that predicted by the nuvel 1a model the velocities of the monument peak and otay mountain sites show the expected long term difference from pacific plate motion due to their proximity to the san andreas fault system

post eemian glacial inception part ii elements of a cryospheric moisture pump abstract this paper extends the analyses of the glacial inception process described in a previous paper part i the impact of summer seasonal temperature bias the analyses described therein were based upon the use of the canadian centre for climate modelling and analysis cccma gcmii three simulations of the modern climate system were described that were respectively warm biased unbiased and cold biased with respect to the set of atmospheric model intercomparison project 2 ssts and land surface temperatures in summer these three control models were perturbed by the modification of the orbital insolation regime appropriate to the time 116 000 years before present 116 kyr bp during which the most recent period of continental glaciation began two of the three simulations do deliver perennial snow cover in polar latitudes analyses of the land surface energy balance hydrological cycle and energetics of the atmosphere in the northern hemisphere polar region at 116 kyr bp discussed in greater d

statistics of breaking waves observed as whitecaps in the open sea abstract conventional observations of waves carried out with a buoy in open sea conditions were supplemented with simultaneous visual observations of whitecaps to identify breaking events in the buoy records a statistical wave by wave analysis of these records indicates that such seemingly obvious parameters as wave steepness or wave asymmetry cannot be used to separate breakers from nonbreakers and the breaking occurs at wave steepness values much less than the theoretically expected steepness of a limiting wave the observed fraction of breaking waves varied from about 0 10 to about 0 16 depending on wind speed two thirds of the breaking waves were breaking in one third of the wave groups for which a hrms threshold definition was used

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

transient simulation of the last glacial inception part ii sensitivity and feedback analysis the sensitivity of the last glacial inception around 115 kyr bp 115 000 years before present to different feedback mechanisms has been analysed by using the earth system model of intermediate complexity climber 2 climber 2 includes dynamic modules of the atmosphere ocean terrestrial biosphere and inland ice the last of which was added recently by utilising the three dimensonal polythermal icesheet model sicopolis we performed a set of transient experiments starting at the middle of the eemiam interglacial and ran the model for 26 000 years with timedependent orbital forcing and observed changes in atmospheric co 2 concentration co2 forcing the role of vegetation and ocean feedback co 2 forcing mineral dust thermohaline circulation and orbital insolation are closely investigated in our model glacial inception as a bifurcation in the climate system appears in nearly all sensitivity runs including a run with constant atmospheric co 2 concentration of 280 ppmv a typical interglacial value and simulations with prescribed present day seasurface temperatures or vegetation cover although the rate of the ice sheets growth is smaller than in the case of the fully in teractive model only if we run the fully interactive model with constant present day insolation and apply present day co 2 forcing does no glacial inception appear at all this implies that within our model the orbital forcing alone is sufficient to trigger the interglacial glacial transition while vegetation ocean and atmospheric co2 concentration only provide additional although important positive feedbacks in addition we found that possible reorganisations of the thermohaline circulation influence the distribution of inland ice

terminating the last interglacial the role of ice sheet climate feedbacks in a gcm asynchronously coupled to an ice sheet model climatic deterioration in northeastern canada following the last interglacial resulted in the formation and abrupt expansion of the laurentide ice sheet however the physical mechanisms leading to rapid ice sheet expansion are not well understood here the authors report on experiments using an ice sheet model asynchronously coupled to a gcm to investigate the role of ice sheet climate feedbacks in terminating the last interglacial period in agreement with simpler models the experiments indicate that a specific type of ice albedo feedback the small ice cap instability is the dominant process controlling rapid expansion of the laurentide ice sheet as ice elevations increase in northeastern canada a stationary wave forms and strengthens over the laurentide ice sheet which acts to hinder further expansion of the ice margin and reduce the effect of the small ice cap instability the sensitivity of these feedbacks to ice topography results in a reduction in simulated ice volume when the communication interval between the gcm and ice sheet model is lengthened since this permits larger gains in ice elevation between gcm updates and biases the simulationtowardastrongerstationarywavefeedback theshortestcommunicationinterval 500 yr leadsto a laurentide ice volume of 6 3 10 6 km 3 in 10 kyr which is less than ice volume estimates based on the geological record but is a substantial improvement over previous gcm studies the authors discuss potential improvements to the asynchronous coupling scheme that would more accurately resolve ice sheet climate feedbacks potentially leading to greater simulated ice volume

dislocation model for aseismic crustal deformation at hollister california geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes we model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust frictional drag is represented by uniform dislocation on a fault surface we assume that friction occurs only on the upper brittle zone of each block boundary and that frictionless sliding occurs below this zone using a bayesian inversion procedure we invert geodetic data to determine the parameters of the block and dislocation model parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate dip angle slip angle and width of each rectangular fault patch for hollister we assume five blocks and nine rectangular fault patches rates of length change on 92 lines observed by trilateration make up the data triangulation leveling very long baseline interferometry satellite ranging and global positioning system data can also be easily included in our model our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data the block parameters are estimated from geological observations of fault displacement rates while the widths of the fault patches are estimated from earthquake hypocentral depths we choose prior estimates of the dislocation rate to match observed creep rates the estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates this suggests that the block motion is steady on time scales from 10 years to 1 m y the net motion across the san andreas calaveras fault system is 38 3 mm yr oriented s38 e thus some additional displacement possibly offshore is needed to match the rate of 56 mm yr predicted by plate tectonic models the depth of the transition from frictional to free sliding varies considerably from 0 4 km on the southern calaveras to 11 km on the central san andreas fault two segments of the san andreas north of 36 50 and south of 36 40 have the greatest potential for moderate to large earthquakes

preliminary determination of pacfic north america relative motion in the southern gulf of calfornia using the global positioning system global positioning system gps data from experiments conducted in 1985 and 1989 in the southern gulf of california mexico allow a determination of relative motion between the pacific and north american plates the data indicate motion of cabo san lucas on the pacific plate relative to north america at a rate of 47 7 mrn yr and azimuth of 57 6 west of north 1 errors equivalent within uncertainties to the nuvel 1 global plate motion model

the impact of paleogeography pco2 poleward ocean heat transport and sea level change on global cooling during the late ordovician abstract we performed sensitivity experiments with the global climate model genesis on two stages of the upper ordovician caradocian 454 ma ashgillian 446 ma under a range of atmospheric p co 2 values 8 18 pal pre industrial atmospheric level high and low sea level and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling we then coupled a three dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation all simulations with a high sea level and normal heat transport remain free of ice sheets even with p co 2 levels as low as 8 pal in the caradocian simulations ice sheets form in three scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport 2 with p co 2 of 8 pal and a high sea level and reduced 50 of normal poleward ocean heat transport and 3 with p co 2 of 15 pal and a low sea level and reduced poleward ocean heat transport in the ashgillian simulations ice sheets form in only two scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport or 2 with p co 2 of 8 pal and a high sea level and reduced poleward ocean heat transport the ice sheets in the ashgillian experiments are larger and thicker than the ice sheets in the caradocian simulations because the southward movement of gondwana increased land area in the higher southern latitudes where ice sheets could grow the threshold for glaciation under ashgillian paleogeography is 8 pal and either a low sea level exposed shelves or a reduced poleward ocean heat transport while the paleogeographic evolution and a drop in p co 2 during the late ordovician cooled the global climate changes in additional factors were required to initiate ice sheet formation such as a drop in sea level a reduction in poleward ocean heat transport or a combination of both

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

application of the global positioning system to crustal deformation measurements 3 result from the southern california borderlands five years of measurements from the global positioning system gps satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern california gps provides the first practical technique to make precise geodetic measurements in the region the geodetic network is situated along the california coastline from vandenberg 120 6 w 34 6 n to san diego with additional sites on santa cruz san nicolas santa catalina santa rosa and san clemente islands the precision of horizontal interstation vectors is subcentimeter and the interstation vector rate between ovro and vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation no significant motion is observed in the western santa barbara channel between vandenberg and santa rosa island 0 5 1 6 mm yr where the quoted uncertainties are one standard deviation motions in the eastern santa barbara channel are consistent with compressional deformation of 6 1 mm yr at n16 3 e this motion is in agreement with seismicity and an independent geodetic analysis for the period 1971 1987 larsen 1991 san clemente island is moving relative to san diego at the rate of 5 9 1 8 mm yr at a direction of n38 20 w the motion between san nicolas island and san clemente island 0 8 1 5 mm yr is insignificant

crustal deformation across the imperial fault results from kinematic gps surveys and trilateration of a densely spaced small aperture network kinematic global positioning system gps surveys in 1991 and 1993 of a 66 station small aperture network across a 12 km long segment of the imperial fault near el centro california reveal a fairly uniform secular displacement field consistent with right lateral slip along a single vertical dislocation surface total relative motion across the array equals 20 3 mm yr at n49 7 w for sites 5 6 km away from the fault trace on opposite sides these rates agree with simple dislocation models that assume 10 mm yr of creep along the fault trace from the surface to a locking depth of 9 km and a plate boundary rate of 45 mm yr oriented n40 w at depth model fit does not significantly improve if creep rates are permitted to vary along strike by several millimeters per year a comparison of 1991 1993 gps with 1987 high precision electronic distance measurements validates the dislocation models but suggests a smoother deformation field than that defined by the 1991 1993 velocities the difference can be attributed to the poorer 1991 kinematic positioning accuracy rather than to temporal variations in strain there is evidence for 20 mm of sympathetic coseismic slip from the 1987 superstition hills earthquake although aseismic slip along both faults appears to be completely decoupled

strain accumulation along the laguna salada fault baja california mexico strain accumulation observed over the 1978 1991 interval in a 30 100 km aperture trilateration network spanning the laguna salada fault is described by the principal strain rates 0 101 0 012 strain yr n80 e 2 and 0 021 0 012 strain yr n10 w 2 extension reckoned positive these strain accumulation rates have been corrected to remove coseismic effects of the nearby 1979 imperial valley m 6 5 1980 victoria baja california m 6 4 1987 superstition hills m 6 5 and 1987 elmore ranch m 5 9 earthquakes the observed strain rates indicate extension at a rate of about 0 08 strain yr perpendicular to the trend n35 w of the salton trough as well as a right lateral tensor shear strain rate 0 05 strain yr across it the extension perpendicular to the trough is observed neither farther north near the salton sea nor farther south across the gulf of california however holocene slip on the laguna salada fault about equal parts right lateral and normal slip is consistent with the observed strain accumulation a simple dislocation model intended to explain the observed strain accumulation as a product of slip at depth on the laguna salada fault would require that the fault be listric

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

coupled energy balance ice sheet model simulations of the glacial cycle a possible connection between terminations and terrigenous dust we apply a coupled energy balance ice sheet climate model in an investigation of northern hemisphere ice sheet advance and retreat over the last glacial cycle when driven only by orbital insolation variations the model predicts ice sheet advances over the continents of north america and eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses the orbital forcing alone however is unable to induce the observed rapid ice sheet retreat and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting in the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination these include marine instability thermohaline circulation effects carbon dioxide variations and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration for the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records of these various influences our analyses suggest that the albedo variations in the ice sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism using our parameterization of dirty snow in the ablation zone we find glacial retreat to be strongly accelerated such that complete collapse of the otherwise stable laurentide ice sheet ensues the last glacial maximum configurations of the laurentide and fennoscandian complexes are also brought into much closer accord with the ice 3g reconstruction of tushingham and peltier 1991 1992 and the ice 4g reconstruction of peltier 1994 when this effect is reasonably introduced

age constraints for the present fault configuration in the imperial valley california evidence for northwestward propagation of the gulf of california rift system releveling and other geophysical data for the imperial valley of southern california suggest the northern section of the imperial brawley fault system which includes the mesquite basin and brawley seismic zone is much younger than the age of the valley itself a minimum age of 3000 years is calculated for the northern segment of the imperial fault from correlations between surface topography and geodetically observed seismic interseismic vertical movements calculation of a maximum age of 100 000 years is based upon displacements in the crystalline basement along the imperial fault inferred from seismic refraction surveys this young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems which also suggest that the current patterns of seismicity and faulting in the imperial valley are not long lived the current fault geometry and basement morphology suggest a northwestward growth of the imperial fault and a northwestward migration of the brawley seismic zone if this localized process is representative of more regional tectonic processes along the extent of the salton trough we suggest that this migration is a manifestation of the propagation of the gulf of california rift system into the north american continent

simulation of the last glacial inception and rapid ice sheet growth in the mcgill paleoclimate model 1 a two dimensional 2 d dynamic ice sheet model coupled to the mcgill paleoclimate model mpm under milankovitch forcing and vostok derived atmospheric co2 levels is used to investigate the last glacial inception and subsequent rapid ice sheet growth in the northern hemisphere nh the impacts on ice sheet growth of the elevation effect of orography and the freezing of rain refreezing of meltwater are evaluated the results show that while milankovitch forcing only is sufficient to initiate the formation of permanent north american and eurasian ice at around 120 kyr bp rapid ice sheet growth during the next 10 kyr only occurs when the above two processes and an active ocean component are included the modelled ice volume equivalent drop in sea level during this growth period is estimated to be about two thirds of that found from sea level reconstructions finally the ice sheet thermohaline circulation interactions and ice sheet thickness distribution are also investigated

geologically and geomorphologically constrained numerical model of laurentide ice sheet inception and build up abstract the locations shapes and masses of evolving ice sheets during interglacial to glacial transitions remain elusive but need to be clarified for identification of the climatic feedbacks that amplified low amplitude insolation changes to become the major global climatic shift of the last glacial maximum the best way to explore probable spatial patterns of ice sheet inception and build up is through ice sheet modelling and validating model output against geomorphological and geological data we have modelled the 120 55 kyr bp evolution of the laurentide ice sheet focusing on the locations outline flow patterns and basal thermal regimes of its precursor ice centres for this we employed a time dependent mass balance driven finite element model forced with grip ice core oxygen isotope values calibrated to temperature the model achieves a good fit to relict ice flow systems in keewatin the interior plains and the hudson bay lowland thus our model indicates that the first coherent ice centre in north america was situated over baffin island and the melville and boothia peninsulas and expanded onto mainland canada at 90 75 kyr bp a previously unknown 200 km long moraine zone near the thlewiaza river keewatin may demarcate the marginal position of this early central arctic ice sheet cordilleran and keewatin ice merged and the hudson bay lowland was inundated from the east from quebec by 65 kyr bp

modeling of the surface static displacements and fault plane slip for the 1979 imperial valley earthquake synthesis of geodetic and seismological results for the 1979 imperial valley earthquake is approached using three dimensional finite element modeling techniques the displacements and stresses are calculated elastically throughout the modeled region the vertical elastic structure in the model is derived from compressional and shear wave velocities as used in the seismic data analysis fuis et al 1981 combined with a sediment density profile two strategies for applying initial conditions are followed in this modeling in the first strategy a sample seismological estimate for fault plane slip is used to predict the resultant surface motions we show that the geodetic strain results over distances of tens of kilometer from the fault snay et al 1982 are basically consistent with the model seismic fault displacements geodetic results from within a few kilometers of the fault trace mason et al 1981 seem to require more slip at shallow depths than appears at seismic time scales this is consistent with the occurrence of aftercreep at shallow depths in less well consolidated material which would bring surface displacements into line with maximum slip at depth but not greatly affect the net moment in the second strategy we consider stresses on the fault plane rather than displacements as model variables to constrain this part of our numerical modeling we assume that the fault driving stress is governed by ambient tectonic stress and an opposing coulomb friction derived from experiment the coseismic stress drop from point to point on the failed fault is given by the difference between the tectonic shear stress and the frictional stress after arriving at such a uniform model which adequately represents the snay et al results we further modify a small region near the seismic asperity to make the fault plane motions qualitatively and quantitatively resemble the model of coseismic motions used in the first strategy the observed offset on the fault trace sharp et al 1982 is approximated in this final stress driven model by removing the driving stress on the southern third of the fault thus the principal features of the coseismic slip pattern are explained by a stress driven fault model in which a a spatially unresolved asperity is found equivalent to a stress drop of 18 mpa averaged over an area of 15 km 2 and b driving stress is essentially absent on the fault segment overlapping the 1940 earthquake rupture zone

seismic gap hypothesis ten years after the seismic gap hypothesis states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries one of the earliest and clearest applications of the seismic gap theory to earthquake forecasting was by mccann et al 1979 who postulated zones of high medium and low seismic potential around the pacific rim in the 10 years since there have been over 40 large m 7 0 earthquakes enough to test statistically the earlier forecast we also analyze another forecast of long term earthquake risk that by kelleher et al 1973 the hypothesis of increased earthquake potential after a long quiet period can be rejected with a large confidence the data suggest that contrary to these forecasts places of recent earthquake activity have larger than usual seismic hazard whereas the segments of the circum pacific belt with no large earthquakes in recent decades have remained relatively quiet the clustering of earthquake times does not contradict the plate tectonic model which constrains only the long term average slip rate not the regularity of earthquakes

vertical crustal deformation associated with the 1979 m 6 6 imperial valley california earthquake implications for fault behavior elevation changes in the imperial valley california derived from repeated leveling surveys for the time period including the 1979 m 6 6 earthquake provide some constraints on fault geometry and slip distribution associated with this event many of the first order features of the observed vertical movements are well matched by simple models consisting of variable slip on planar faults in an elastic half space using fault offsets inferred from strong ground motion observations archuleta 1984 and measured afterslip the geodetic and seismic observations suggest that significant slip is confined to depths above 13 km with maximum right lateral offset reaching about 1 5 m on the imperial fault dip slip occurs predominantly in the sediments on the upper 5 km of the imperial fault and on the brawley fault right lateral afterslip is confined to the upper 5 km of the imperial fault and reaches about 30 cm for the period 1979 1981 in contrast observed elevation changes near the 1979 hypocenter on southern end of imperial fault and in the brawley seismic zone show significant deviations from those predicted by models of fault slip inferred from strong ground motion measurements specifically the geodetic data suggest that slip on the imperial fault is significantly lower in the vicinity of the earthquake hypocenter than along the central and northern sections of the fault in addition there is marginal evidence that the dip of the imperial fault changes along strike from approximately vertical just north of the u s mexico border to between 70 and 80 near the northernmost extent of the 1979 surface break this change in dip may be related to a change in local strike along the fault large elevation changes 15 cm also occur within the brawley seismic zone well north of the primary surface faulting while these movements are consistent with a number of possible fault models our prefered interpretation based on geodetic and seismic observations aftershock locations and focal mechanisms involves right lateral aseismic slip on a northwest striking fault along the east side of the brawley seismic zone and conjugate left lateral faulting on a northeast striking fault possibly associated with an m 5 8 aftershock buried creep on this same right lateral fault in the brawley seismic zone can also account for vertical deformation during the postseismic period of the 1940 m 7 1 imperial valley earthquake as well as deformation during the interseismic period between the 1940 and 1979 events substantial right lateral slip in the brawley seismic zone suggests that a significant part of the shear strain released during and following the 1940 and 1979 earthquakes on the imperial fault is transferred through the brawley seismic zone to the southern end of the san andreas fault

on the determination of breaking surface wave distributions using ambient sound when surface gravity waves break they generate an acoustic signal which can be used to derive information about their properties and distribution observations of sound formed in this way have been obtained at 1 10 and 40 m below the surface in the fetch limited environment of georgia strait british columbia interpretation of the results is aided by a model of randomly distributed sound sources on the ocean surface it is shown that the signal variability is governed by the number density or mean spacing of the breakers by the tendency for breakers to repeat at half the wave frequency and by the group structure of the wave field calculated wave field properties reveal several features which can be related to changes in the wind including variations in wave period mean breaker spacing and the average acoustic strength of individual breakers bubble clouds caused by the breaking waves can also be detected and there is evidence that they contribute to the trapping of sound in a shallow layer close to the surface

climate and ice sheet mass balance at the last glacial maximum from the genesis version 2 global climate model abstract at the last glacial maximum lgm about 21 000 calendar years ago 21 ka bp the overall mass balance of the laurentide and eurasian ice sheets should have been close to zero since their rate of change of total ice volume was approximately zero at that time the surface mass balance should have been zero or positive to balance any iceberg iceshelf discharge and basal melting but could not have been strongly negative in principle this can be tested by global climate model cgm simulations with prescribed ice sheet extents and topography we describe results from a suite of 21 ka bp simulations using a new global climate model genesis version 2 0 a with ssts prescribed from climap 1981 and predicted by a mixed layer ocean model and with ice sheets prescribed from clih the ice 4g peltier 1994 and climap 1981 reconstructions in common with previous gcm simulations using mixed layer oceans substantial cooling over and around antarctica occurs due to normal gcm dynamics and polar sea ice feedback without recourse to changes in thermohaline circulation we find slightly enhanced cooling and cloudiness over low latitude land masses compared to over ocean especially over the andes which decreases the disagreement with land based data and suggests that tropical climap ssts may only be 1 2 c too warm the gcm used here is well suited for ice sheet mass balance studies because i the surface can be represented at a finer resolution than the atmospheric gcm ii an elevation correction accounts for spectral distortions of the atmospheric gcm topography iii a simple post processing correction for refreezing of meltwater is applied and iv the model s precipitation and mass balances for present day greenland and antarctica are realistic however for all reasonable combinations of ssts and ice sheet configurations the predicted annual surface mass balances of the lgm laurentide and eurasian ice sheets are implausibly negative possible reasons for this discrepancy are discussed including increased ice age aerosols higher climap like ice sheet profiles in the few thousand years preceding lgm and a surge of the southern laurentide just before lgm to fleetingly produce the ice 4g profile at 21 ka bp

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

post eemian glacial inception part i the impact of summer seasonal temperature bias abstract post eemian glacial inception the transition between marine oxygen isotopic stage mois 5e and mois 5d began approximately 117 000 years before present 117 kyr bp and led to significant northern hemisphere glaciation within the ensuing 5000 yr previous sensitivity studies with atmospheric general circulation models agcms have had difficulty producing glacial nucleation in high northern latitude regions of the globe a base simulation of this process has been conducted using the canadian centre for climate modelling and analysis cccma gcmii with mixed layer slab ocean model constrained so as to ensure that the model reproduces the set of atmospheric model intercomparison project 2 amip2 modern sea surface temperatures ssts under conditions of modern radiative forcing this simulation demonstrates that entry into glacial conditions at 116 kyr bp requires only the introduction of post eemian orbital insolation and standard preindustrial co2 concentrations two additional sensitivity ex

dynap software for estimating crustal deformation from geodetic data abstract the acronym dynap dynamic adjustment program identifies a software package for estimating crustal dynamic parameters from a collection of geodetic data dynap accepts various data types including horizontal directions and angles distances azimuths zenith distances and intersite vectors such as those obtainable with the global positioning system gps or with very long baseline interferometry vlbi dynap incorporates the simultaneous reduction technique whereby crustal dynamic parameters are estimated simultaneously along with station positional coordinates and various nuisance parameters all via the least squares process the estimated crustal dynamics parameters come in groups of six or fewer where each group corresponds to a prespecified geographic region and period of time the six parameters convert mathematically to yield the three components of the horizontal strain rate tensor the horizontal rotation rate and the two components of the vertical tilt rate vector dynap s applicability is illustrated with two examples the first involves over 60 years of triangulation trilateration data in the cascadia subduction zone of northwestern california here a maximum shearing rate was found of 0 161 0 074 radians yr with the corresponding axis of maximum contraction oriented n25 e 14 the second involves a 1980 triangulation trilateration survey and a 1986 gps survey both spanning the imperial fault of southern california here a maximum dextral shearing rate was found of 1 073 0 171 radians yr in the direction of n31 w 5

initiation and development of the laurentide and cordilleran ice sheets following the last interglaciation abstract fossil records from sites overridden by or adjacent to the laurentide ice sheet indicate that the climate of the last interglaciation oxygen isotope substage 5e ca 130 116 ka was warmer than today following the last interglaciation the laurentide ice sheet first developed during stage 5 over keewatin quebec and baffin island along its northern margin the ice sheet reached its maximum extent of the last glaciation during stage 5 the ice sheet advanced across baffin island onto the continental shelf early during stage 5 5d whereas the advance into the western canadian arctic occurred late during stage 5 5b the ice sheet also may have advanced into the st lawrence lowland during substage 5b although this event may be younger stage 4 the hudson bay lowland became ice free during substage 5a retreat of the ice sheet on baffin island occurred during late stage 5 probably substage 5a the exact timing of retreat from the western canadian arctic is unknown but it occurred before 48 ka the southern sector including the st lawrence lowland was ice free during late stage 5 the hudson bay lowland may have remained ice free through stage 4 and much of stage 3 because of conflicting chronologies however it is more likely that this area was glaciated throughout stage 3 and perhaps stage 4 nevertheless the data demonstrate that the lowland was ice free during part of the last glaciation an independent ice cap developed over the appalachian uplands and advanced across nova scotia during stage 4 perhaps as far as the edge of the continental shelf the ice cap remained active over nova scotia as a setellite to the laurentide ice sheet throughout the remainder of the last glaciation the ice sheet advanced into the st lawrence lowland during stage 4 and subsequently overwhelmed the local ice cap in the appalachian uplands advancing perhaps into northern new england but not farther south the lowland remained covered by the ice sheet until late stage 2 the ice sheet may also have advanced into the lake ontario basin during stage 4 the position of the northern margin of the laurentide ice sheet during stage 4 is not known but it remained an unknown distance behind its maximum position reached during stage 5 cores from baffin bay indicate a substantial decrease in high latitude glaciation during stage 4 following retreat the keewatin sector of the ice sheet may have remained over much of northwestern canada as a quasi stable ice mass until it readvanced during stage 2 similarly the baffin island sector of the ice sheet may have remained largely intact the southern margin of the ice sheet may have advanced into the lake ontario basin and upper mississippi valley during the middle of stage 3 ca 50 ka reaching its maximum extent of the last glaciation during stage 2 ca 18 21 ka advance of the northern margin was younger ca 8 13 ka than that of the southern margin this advance was less extensive than the penultimate advance stage 5 paleoenvironmental records indicate that the last interglaciation in areas covered by and near the cordilleran ice sheet was as warm as or warmer than present the cordilleran ice sheet appears to have developed during stage 5 or 4 at that time it advanced over southern british columbia and into the northern puget lowland there is no record of this event in northern areas that were later covered by the ice sheet the ice sheet disappeared before 59 ka at the beginning of a lengthy nonglacial interval paleoenvironmental records indicate that climate was similar to the present during part of this interval the ice sheet was absent and glaciers probably were confined to mountain areas throughout stage 3 climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka by 14 15 ka the ice sheet had achieved its maximum extent of the last glaciation because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted we consider this synthesis as tentative and subject to significant revision as dating methods improve nevertheless this perspective of the north american ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume

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

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

passive acoustic localization techniques of eastern pacific grey whales eastern pacific grey whales eschrichtius robustus apparently do not actively echolocate yet still they thrive in shallow water environments where visibility is much reduced along their migration route and in their feeding grounds these whales are exposed to high levels of ambient noise highlyturbid waters and many underwater obstacles to test possible passive acoustic localization mechanisms e g acoustic daylight imaging and passive synthetic aperture we made extensive acoustic measurements during comprehensive field studies of these whales on their summer feeding grounds in british columbia canada in combination with visual observations of the whales and their behaviours we investigated the acoustical sources available to the whales when navigating within a feeding bay first we measured ambient noise levels to construct the acoustic landscape around the whales second we investigated how sound is altered when objects such as kelp beds and rocks are present we also measured acoustic chan

a model study of the role of high latitude topography in the climatic response to orbital insolation anomalies abstract the response of a combined global zonal averaged energy balance model and a continental ice sheet model to insulation anomalies produced by orbital perturbations is examined although the role of ice sheet dynamics is included in the model this paper focuses mostly on the response of the atmosphere hydrosphere to the perturbations the model developed from that of suarez and held 1979 explicitly incorporates surface topography surface energy balances snow and sea ice budgets and variable static stability the ice sheet model is based on a power law rheology it is found that the presence of a high latitude plateau significantly increases the sensitivity of the climate to the insolation perturbation the sensitivity is maximized when the elevation of the summer snowline is near the elevation of the plateau as appears to be the case with baffin island today there appears to be a rapid response of the seasonal cycle to an insolation anomaly through albedo temperature feedback and with the

late pliocene to pleistocene sensitivity of the greenland ice sheet in response to external forcing and internal feedbacks the timing and nature of ice sheet variations on greenland over the last 5 million years remain largely uncertain here we use a coupled climate vegetation ice sheet model to determine the climatic sensitivity of greenland to combined sets of external forcings and internal feedbacks operating on glacial interglacial timescales in particular we assess the role of atmospheric pco2 orbital forcing and vegetation dynamics in modifying thresholds for the onset of glaciation in late pliocene and pleistocene the response of circum arctic vegetation to declining levels of pco2 from 400 to 200 ppmv and decreasing summer insolation includes a shift from boreal forest to tundra biomes with implications for the surface energy balance the expansion of tundra amplifies summer surface cooling and heat loss from the ground leading to an expanded summer snow cover over greenland atmospheric and land surface fields respond to forcing most prominently in late spring summer and are more sensitive at lower pleistocene like levels of pco2 we find cold boreal summer orbits produce favorable conditions for ice sheet growth however simulated ice sheet extents are highly dependent on both background pco2 levels and land surface characteristics as a result late pliocene ice sheet configurations on greenland differ considerably from late pleistocene with smaller ice caps on high elevations of southern and eastern greenland even when orbital forcing is favorable for ice sheet growth

the response of northern hemisphere extratropical climate and vegetation to orbitally induced changes in insolation during the last interglaciation abstract the last interglaciation substage 5e provides an opportunity to examine the effects of extreme orbital changes on regional climates we have made two atmospheric general circulation model experiments p t approximated the northern hemisphere seasonality maximum near the beginning of 5e p t approximated the minimum near the end of 5e simulated regional climate changes have been translated into biome changes using a physiologically based model of global vegetation types major climatic and vegetational changes were simulated for the northern hemisphere extratropics due to radiational effects that were both amplified and modified by atmospheric circulation changes and sea ice feedback p t showed mid continental summers up to 8 c warmer than present mid latitude winters were 2 4 c cooler than present but in the arctic summer warmth reduced sea ice extent and thickness producing winters 2 8 c warmer than present the tundra and taiga biomes were displaced poleward while warm summer steppes expanded in the mid latitudes due to drought p t showed summers up to 5 c cooler than present especially in mid latitudes sea ice and snowpack were thicker and lasted longer polar desert tundra and taiga biomes were displaced equatorward while cool summer steppes and semideserts expanded due to the cooling a slight winter warming in mid latitudes however caused warm temperate evergreen forests and scrub to expand poleward such qualitative contrasts in the direction of climate and vegetation change during 5e should be identifiable in the paleorecord

deformation from 1973 through 1991 in the epicentral area of the 1992 landers california earthquake ms 7 5 deformation of a 50 60 km trilateration network that spans the epicenter of the 1992 landers earthquake ms 7 5 was measured by seven surveys over the 19 years preceding the earthquake three moderate earthquakes 1979 homestead valley ms 5 6 1986 north palm springs ms 6 0 and 1992 joshua tree ms 6 1 occurred within the network during those 19 years here we use geodetic and seismic data to construct a dislocation model for each of the three moderate earthquakes coseismic changes due to these three moderate earthquakes as predicted by the dislocation models are then removed from the trilateration data the residual geodetic changes appear to be uniform in time we take those changes to represent secular strain accumulation this strain accumulation clearly shows right lateral shear across the san andreas fault but also suggests a northwest southeast extension northeast of the eastern end of the big bend of the san andreas fault the landers earthquake ruptured along a north northwest trend across this region of northwest southeast extension

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

behavior of the east antarctic ice sheet as deduced from a coupled gcm ice sheet model while the possible instability of the west antarctic ice sheet has been widely recognized for some time as a potential source of sea level rise in an enhanced greenhouse warming the stability of the east antarctic ice sheet has only recently become the subject of such a conjecture we approach an aspect of this issue using a numerical experiment with an atmospheric general circulation model gcm coupled to a 3 dimensional ice sheet model response of the ice sheet model to the external forcing generated by an atmospheric gcm due to doubling the co2 concentration does not show any appreciable changes in the horizontal extent of the ice sheet due to normal creep and topographic instabilities suggesting that an antarctic collapse resulting from these factors is unlikely the mechanics of basal sliding non isothermal effects and ice shelves are as yet too poorly understood to make quantitative estimates of possible instabilities due to these processes

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

breaking of wind generated waves abstract breaking of wind generated waves was studies in a laboratory tank the critical surface slope and global wave steepness for inception of breaking were evaluated besides the frequency of occurrence two other characteristic quantities height and duration of breaking were measured the frequency of breaking was found to increase rapidly with wind velocity following a power law u2 2 the period of breaking remained about 7 of the wave period at all wind velocities the height of breaking was about 30 of the wave height portions of these results compare favorably with other available measurements

present and past ice sheet mass balance simulations for greenland and the tibetan plateau net annual mass balance was evaluated for greenland and the tibetan plateau using the meteorological forcings from the ncep reanalysis and two gcms foam1 0 and csm1 4 for modern climate and for different time periods extending back to the beginning of the holocene 11 000 years ago for the climate models the ice sheet budget calculations using the degree day methodology were performed on a finer grid than the model output by interpolating monthly precipitation and surface temperature and correcting the latter to account for the gcm s smoothed topography the computed net mass balance for greenland in the present day is positive and it ranges between 290 300 mm water equivalent w e year for the two models values close to the ncep estimate of 250 mm year the past climate simulations show that the greenland mass balance has become slightly more positive since the beginning of the holocene the tibetan plateau s present day area average net mass balance is negative and ranges between 1200 and 2000 mm w e year for the two models values bracketing the ncep estimate of 1700 mm year although the balance is positive over small regions of the plateau consistent with the existence of small ice caps and glaciers the calculated past mass balance shows an increasingly less negative value for foam from 11 000 years ago towards the present and expansion of the positive mass balance areas mainly due to decreased snow ablation as the summertime insolation decreases with the changes in orbital forcing in csm the opposite trend occurs but changes are smaller and less systematic the result from foam shows that the likelihood of ice sheets developing on the tibetan plateau may have increased since 11000 years ago which is consistent with some glacial records

sea ice feedback and cenozoic evolution of antarctic climate and ice sheets 1 the extent and thickness of antarctic sea ice have important climatic effects on radiation balance energy transfer between the atmosphere and ocean and moisture availability this paper explores the role of sea ice and related feedbacks in the cenozoic evolution of antarctic climate and ice sheets using a numerical climate model with explicit dynamical representations of sea ice and continental ice sheets in a scenario of decreasing cenozoic greenhouse gas concentrations our model initiates continental glaciation before any significant sea ice forms around the continent once variable ice sheets are established seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet s control on regional temperature and low level winds although the expansion of sea ice has significant climatic effects near the coast it has only minimal effects in the continental interior and on the size of the ice sheet therefore the cenozoic appearance of antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of paleogene and neogene glaciation the influence of the east antarctic ice sheet on sea ice southern ocean surface temperatures and winds has important implications for ocean circulation the marine carbon cycle and the development of the west antarctic ice sheet the sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long term stability of antarctic ice sheets

holocene activity of the rose canyon fault zone in san diego california the rose canyon fault zone in san diego california has many well expressed geomorphic characteristics of an active strike slip fault including scarps offset and deflected drainages and channel walls pressure ridges a closed depression and vegetation lineaments geomorphic expression of the fault zone from mount soledad south to mission bay indicates that the mount soledad strand is the most active a network of trenches excavated across the mount soledad strand in rose creek demonstrate a minimum of 8 7 m of dextral slip in a distinctive early to middle holocene gravel filled channel that crosses the fault zone the gravel filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development consequently the actual displacement of the channel could be greater than 8 7 m radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8 1 0 2 kyr the minimum amount of slip along with the maximum age yield a minimum slip rate of 1 07 0 03 mm yr on this strand of the rose canyon fault zone for much of holocene time other strands of the rose canyon fault zone which are east and west of our site may also have holocene activity based on an analysis of the geomorphology of fault traces within the rose canyon fault zone along with the results of our trenching study we estimate the maximum likely slip rate at about 2 mm yr and a best estimate of about 1 5 mm yr stratigraphie evidence of at least three events is present during the past 8 1 kyr the most recent surface rupture displaces the modern a horizon topsoil suggesting that this event probably occurred within the past 500 years stratigraphie and structural relationships also indicate the occurrence of a scarp forming event at about 8 1 kyr prior to deposition of the gravel filled channel that was used as a piercing line a third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event other events may also have occurred but these data suggest that the return time for surface rupturing earthquakes is no more than about 4 kyr

microplate versus continuum descriptions of active tectonic deformation whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi continuous flow has important implications for lithospheric dynamics fault mechanics and earthquake hazard assessment actively deforming regions of the western united states central asia japan and new zealand show features that argue for both styles of movement but new observations are necessary to determine which is most appropriate and at what scale the description applies geologic geodetic seismic and paleomagnetic measurements tend to sample complementary aspects of the deformation field so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases provided the total relative motion across each region is known and the distribution of active faults is well mapped determination of fault slip rates can provide potentially decisive constraints reconnaissance geological studies supply useful slip rate estimates but precise values depend upon detailed intensive investigation of individual sites geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long term deformation pattern earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere especially the strength and thickness of the elastic crust relative to the ductile lithosphere and dynamical models can provide contrasting forecasts of observable features with testable consequences

co seismic displacements of the 1992 landers earthquake sequence we present co seismic displacement vectors derived from global positioning system gps measurements of 92 stations in southern california these gps results are combined with five well determined gps displacement vectors from continuously tracking stations of the permanent gps geodetic array as well as line length changes from usgs geodolite and two color laser trilateration observations to determine a self consistent set of geodetic data for the earthquake these combined displacements are modeled by an elastic dislocation representation of the primary fault rupture planes on average the model residuals are about twice the estimated measurement errors

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

obliquity forcing with 8 12 times preindustrial levels of atmospheric pco2 during the late ordovician glaciation results from coupled ice sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the late ordovician were very sensitive to changes in atmospheric p co 2 and orbital forcing at the obliquity time scale 30 40 k y without orbital forcing ice sheets can grow with p co 2 level as high as 10 times preindustrial atmospheric level pal however with orbital forcing ice sheets can grow only with p co 2 levels of 8 times pal or lower these results indicate that the threshold of p co 2 for the initiation of glaciation is on the lower end of previously published estimates of 8 20 times pal the ice sheet model results further indicate that during exceptionally long periods of low summer insolation and low p co 2 levels 8 10 times pal large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher p co 2 values this finding suggests that in order to end the late ordovician glaciation with a rise in p co 2 atmospheric p co 2 must have risen to at least 12 times pal ice sheets therefore introduce nonlinearities and hysteresis effects to the ordovician climate system these nonlinearities might have also played a role in the initiation and termination of other glaciations in earth history

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

formation of sedimentary bedding couplets in the western interior seaway of north america implications from climate system modeling we use global climate simulations across one precessional cycle to investigate the effect of orbitally induced climatic changes on sedimentation in the western interior seaway wis of north america at the cenomanian turonian boundary the simulations include a control run with no orbital eccentricity and hence no precession cycle and four runs with varying precession with an eccentricity of 0 05 having 1 northern spring equinox at perihelion 2 northern winter solstice at perihelion 3 northern fall equinox at perihelion and 4 northern summer solstice at perihelion these numeric climate simulations and field observations suggest that the wis at the cenomanian turonian boundary can be divided into three latitudinal units 1 a northern unit alberta montana between 51 n and 71 n paleolatitude where conditions remained constant under the influence of steady inflow of low salinity cool waters which were devoid of calcareous plankton flowed in from the arctic preventing the development of bedding couplets 2 a central unit wyoming colorado between 41 n and 51 n paleolatitude where runoff from western north america wna was reduced by more than half when the northern hemisphere winter solstice coincided with perihelion where bedding couplets are well developed the central part of the wis was characterized by warm saline waters with abundant calcareous plankton however it experienced high summer surface runoff from the sevier highlands to the west during all orbital configurations except when the winters were unusually warm with the northern hemisphere winter solstice occurring at perihelion seasonal dilution of the surface waters of the seaway may have resulted in formation of a fresh water lid with stratification of the water column throughout most of the precession cycle when the northern hemisphere winter solstice was at perihelion reduced runoff would promote vertical mixing concomitantly a steady detrital sediment supply would occur in summer throughout the precession cycle except when the nh winter solstice was at perihelion when it would be much reduced thus the marlstone of the limestone marlstone couplets would represent most of the time of the precession cycle and the limestone layers would represent the time when the nh winter solstice was near perihelion 3 a southern region new mexico northern mexico from 21 to 42 n paleolatitude where the detrital sediment supply was much reduced and couplets are thicker and less well developed

regional deformation of the earth model for the san diego region california a mathematical model for historical horizontal crustal deformation is presented for the southern california region that extends northward from mexico for approximately 100 km to 33 5 n latitude model parameters are estimated from triangulation trilateration and astronomic azimuth data observed during the past century also geologic and seismic data indirectly participate in specifying the model the model relates crustal deformation to the episodic movement experienced during large earthquakes m 6 secular slip rates on geologic faults and secular strain rates over large geographic areas the episodic movement is modeled in accordance with the theory of dislocation in an elastic halfspace for the secular motion the modeled region is partitioned into a mosaic of districts that are allowed to individually translate rotate and homogeneously deform as a linear function of time the derived strain pattern supports the hypothesis that the modeled region spans the pacific north american plate boundary secular shear strain is maximum in the nw direction parallel to the major faults and shear strain rates decrease with distance from a nw se trending strip to the region s north this strip is 40 km wide and is bounded on the west by the san jacinto fault zone and on the east by the southern extension of the san andreas fault to the south this strip converges in width onto the imperial fault the secular velocity for a point near the city of san diego relative to a point at the region s northeast corner is 4 3 3 1 cm yr in the direction n 17 w 5 this rate provides a rough estimate for the relative velocity between the pacific and north american plates

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

strain localization along the san andreas fault consequences for loading mechanisms the orientation of the near fault strain accumulation associated with the relative motion between the pacific and north american plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion this observation follows from consideration of shear strain results from geodetic studies ranging from the imperial valley to shelter cove along the san andreas fault system this set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other both the localization of the strain associated with pacific north american motion and the orientation of the associated strain can be simply caused by failure of the lithosphere as opposed to localization of plate driving forces two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature 1 viscoelastic models which are dominated by horizontal layering of the crust and 2 elastic half space models with geometry which is dominantly vertical horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes vertical models produce strains which are parallel to the local structure at all times if the horizontal detachments of the viscoelastic models exist they must be at depths 2 3 times the seismogenic thickness geologic studies also indicate that major transcurrent features extend as ductile shear zones to depths much larger than would be expected in horizontal models these considerations lead us to favor models with vertical geometries the vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain softening processes associated with flow of composite materials such as quartzo feldspathic rocks independent of the horizontal rheological layering in the starting low strain material progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material thus shear zones such as the san andreas system develop vertical tabular geometries which extend far below the brittle plastic transition and which are mechanically persistent

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation

tectonic geomorphology of the san andreas fault zone in the southern indio hills coachella valley california geomorphic investigation of the san andreas fault zone in the indio hills indicates many tectonically produced landforms including beheaded streams right lateral deflected and offset streams sags shutter ridges pressure ridges and fault scarps near biskra palms an alluvial fan pediment complex has an apparent cumulative offset of about 0 7 km along the mission creek fault zone north branch san andreas fault many of the tectonic landforms as well as the fracture pattern that has developed during the pleistocene are explainable by simple shear or uplift associated with a small left bend in the main trace of the mission creek fault the ratio of vertical to horizontal displacement in the vicinity of the bend is about 0 04 exposed in pushawalla canyon 5 km northwest of the alluvial fan pediment complex are 1 a sequence of stream terraces 2 folded plio pleistocene fanglomerates and 3 an example of stream capture following a right lateral deflection or offset of several hundred metres a left step of the mission creek fault in pushawalla canyon is a probably cause of folds and sporadic uplift that produced the stream terraces possible cause of recent stream capture are 1 juxtaposition of pushawalla canyon with a relict canyon moving northwestward along the mission creek fault or 2 right lateral deflection or offset of pushawalla canyon along the fault with simultaneous headward erosion of a shorter steep stream flowing toward the coachella valley estimation of a slip rate and identification of paleoseismicity for the san andreas fault in the indio hills is difficult however degree of topographic dissection formation and preservation of desert pavement and relative soil profile development suggest that the age of the offset fan may be as old as 70 000 yr but that most likely it is on the order of 20 000 to 30 000 yr these age estimates for the offset fan indicate a minimum slip rate for the san andreas fault of 10 to 35 mm yr with about 23 to 35 mm yr the most likely the pattern of observed offset drainages is complex but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right lateral displacement

rupture process of the june 28 1992 big bear earthquake the june 28 1992 big bear earthquake in southern california was assumed to have ruptured along a northeast trending plane as suggested by long term aftershock distribution no surface rupture was found however and mainshock locations determined from both strong motion and terrascope data are mutually consistent and do not lie on the assumed fault plane an integrated study involving waveform modeling directivity and seismicity analyses suggests a complex rupture pattern with significant short and long period energy propagating northwest along the presumed conjugate fault plane

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

surface faulting along the superstition hills fault zone and nearby faults associated with the earthquakes of 24 november 1987 the m 6 2 elmore desert ranch earthquake of 24 november 1987 was associated spatially and probably temporally with left lateral surface rupture on many northeast trending faults in and near the superstition hills in western imperial valley three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the superstition hills fault zone as far as 9 km the maximum observed surface slip 12 5 cm was on the northern of the three the elmore ranch fault at a point near the epicenter twelve hours after the elmore ranch earthquake the m 6 6 superstition hills earthquake occurred near the northwest end of the right lateral superstition hills fault zone surface rupture associated with the second event occurred along three strands of the zone here named north and south strands of the superstition hills fault and the wienert fault for 27 km southeastward from the epicenter in contrast to the left lateral faulting which remained unchanged throughout the period of investigation the right lateral movement on the superstition hills fault zone continued to increase with time a behavior that was similar to other recent historical surface ruptures on northwest trending faults in the imperial valley region we measured displacements over 339 days at as many as 296 sites along the superstition hills fault zone and repeated measurements at 49 sites provided sufficient data to fit with a simple power law data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes the maximum right lateral slip at 1 day was about 50 cm near the south central part of the north strand of superstition hills fault and the predicted maximum final displacement is probably about 112 cm at imler road near the center of the south strand of the superstition hills fault the overall distributions of right lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture the average estimated final right lateral slip for the superstition hills fault zone is about 54 cm the average left lateral slip for the conjugate faults trending northeastward is about 23 cm the southernmost ruptured member of the superstition hills fault zone newly named the wienert fault extends the known length of the zone by about 4 km the southern half of this fault south of new river expressed only vertical displacement on a sinuous trace the maximum vertical slip by the end of the observation period there was about 25 cm but its growth had not ceased photolineaments southeast of the end of new surface rupture suggest continuation of the superstition hills fault zone in farmland toward mexico

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

coseismic and postseismic vertical movements associated with the 1940 m7 1 imperial valley california earthquake elevation changes derived from precise leveling surveys conducted by the national geodetic survey in 1931 1941 1972 and 1974 along two routes crossing the imperial fault in southern california represent the first documented examples of spatially coherent vertical movements associtated with strike slip faulting in the united states the leveling routes cross the northern section of the fault that broke at the time of the 1940 m7 1 imperial valley earthquake the observed vertical movements which extend approximately 25 km east and west of the fault are among the largest observed anywhere in the united states these movements are consistent with simple elastic models of coseismic and postseismic secondary vertical deformation due to right lateral strike slip movement along a finite length fault the spatial and temporal patterns of deformation suggest a simple scenario consisting of large coseismic slip on the southern part of the imperial fault which transferred stresses to the northern part of the imperial fault and the brawley fault en echelon fault northeast of imperial fault these stresses were subsequently released primarily by aseismic creep the estimates of coseismic faulting determined from the leveling observations are roughly consistent with field observations of horizontal surface offset made after the earthquake as well as with some estimates derived from triangulation measurements the postseismic fault offsets reported here suggest slip to shallower levels than previous estimates of postseismic slip determined from repeated triangulation measurements the overall shallow slip coseismic plus postseismic required by the leveling observations is larger on the southern part of the imperial fault than on the northern part of this fault or on the brawley fault the 1979 m6 6 imperial valley earthquake apparently eliminated some of this difference in overall shallow slip by causing surface offsets on the northern part of the imperial fault and the southern brawley fault even though the epicenter for the 1979 event was located on the southern imperial fault other geophysical observations in this part of the imperial valley fault creep seismicity seismic faulting suggest that stick slip on the southern part of the imperial fault and creep on the northern part of this fault and on the brawley fault may characterize the long term fault behavior in the imperial valley

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