s velocity variations beneath north america 1 we investigate s velocity variation in the upper mantle beneath north american to better understand the effects of data heterogeneity model parameterization and regularization to this end we analyzed and fit regional s and rayleigh wave trains generated by earthquakes around north america that occurred between the years 2000 through 2006 including waveforms from the transportable array stations of earthscope s usarray these new data were combined with constraints used for the 3 d s velocity model na04 in order to create a new model na07 another model was created from a suite of good fit models to provide a useful guide for model velocities and uncertainties by estimating ranges of probable velocity variations throughout the upper mantle we find that the southern and eastern edges of the north american craton appear to be defined by paleozoic orogens rather than proterozoic ones on average the archean portion of the craton is 200 km thick while the paleozoic part averages 175 km thick with an 80 m s lower s velocity the horizontal gradients in velocity are over 1 0 100 km at the western margin of the craton 0 5 100 km in the south and 1 100 km at the eastern margin

long period perturbations in starlette orbit and tide solution continuous and dynamically consistent orbits have been computed using satellite laser ranging slr data to starlette for a one year period during 1976 and 1977 and the three year period from 1983 through 1985 a low degree and order ocean tide solution consisting of a total of 66 tidal parameters from 14 tidal constituents was obtained in a combined solution using both the one and three year starlette orbits an enhanced model of the schwiderski 1980 tide solution was used both as the a priori and the background tide model the model contains 368 tidal coefficients from 21 tidal constituents yearly values for the second degree annual tides sa which produce seasonal variations on the starlette node were obtained also these variations were found to have year to year fluctuations of more than 25 about the mean value and produced significant long period perturbations in the starlette orbit the starlette tide solution is in good agreement with the schwiderski tide solutions and with other satellite solutions including multi satellite tide solutions the average uncertainties are approximately 0 1 cm for the estimated semidiurnal and diurnal tides and 0 4 cm for the long period tides the discrepancies of effective k2 inferred from the starlette tide solution and other tide solutions are less than 0 01 the tide solutions were evaluated using orbit fits of a three year lageos and a one year starlette long arc the results indicate that the starlette tide solution produces significantly improved orbit fits for both lageos and starlette the uncertainties for the estimated tide model coefficients were used to predict tidal errors for the lageos orbit and were found to be consistent with the apparent tidal errors present in this orbit the computed secular change in the moon s mean motion due to tidal dissipation is 24 8 0 8 arc second century2 which is in good agreement with the results obtained from other investigations

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

neogene tephra correlations in eastern idaho and wyoming implications for yellowstone hotspot related volcanism and tectonic activity the explosive rhyolitic eruptions that define the track of the snake river plain yellowstone volcanism have produced a large volume of tephra found in late miocene and younger basin fill sediments throughout the western united states here we use 40ar 39ar isotopic dating paleomagnetic analysis major and trace element geochemistry and standard optical techniques to establish regional tephra correlations we focus on tephra deposits in three neogene basins in spatially separated areas grand valley in eastern idaho jackson hole in northwestern wyoming and the granite mountains area in central wyoming these basins have experienced relatively continuous deposition from the late miocene to the holocene we found tephra layers that directly tie the stratigraphy between all three basins using these correlations we found that basins experienced discrete pulses of extension separated by long periods of relative quiescence the dates of which are staggered between basins an early pulse of extension occurred in grand valley and jackson hole between 10 3 ma and 16 33 ma with a second pulse initiating between 4 54 ma and 2 09 ma the granite mountains basin experienced a single pulse of extension sometime between 11 14 ma and 6 75 ma these pulses of accelerated extension along with evidence of similar pulses in other basins present a pattern of west to east migration that we suggest is related to the yellowstone hotspot the later pulse of activity in grand valley and jackson hole corresponds to the migration of the north american plate over the tail of the yellowstone hotspot we speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location where the columbia river plateau flood basalt province initiated in southeastern oregon to its outermost edge under central wyoming our results are consistent with this model of a plume head though not unique to it the results of our study also indicate that previous suggestions that the rate of snake river plain explosive volcanism has decreased by a factor of 2 or 3 since emplacement of the middle miocene trapper creek tuffs likely underestimate post trapper creek eruption rates we have discovered a large number of previously unidentified post middle miocene major eruptive events both as ash flow tuffs and as vitric air fall tuffs recalculation to include these newly discovered events results in a rate of major eruptions that is fairly uniform until ca 4 54 ma however there is a substantial gap in major silicic eruptions in the interval between 4 54 ma and 2 09 ma which we call the post heise eruptive gap with the exception of this gap the rate of major eruptions on the snake river plain has been roughly constant since inception of the eastern snake river plain yellowstone volcanic track between 16 ma and 17 ma

source parameters of the 22 december 1964 mb 5 4 ms 6 2 offshore ensenada earthquake abstract the largest offshore earthquake that has occurred west of northern baja california has been studied recalculated magnitudes are m b 5 4 and m s 6 2 p wave first motions polarization of the s waves and wave form modeling were used to calculate the fault plane solution and source parameters the source mechanism indicates a right lateral strike slip fault with a plane that strikes 309 and dips 76 northwest and a compressional axis that trends approximately north south it was not possible to relocate this event therefore it cannot be determined whether it occurred in the maximinos or the san isidro fault these faults have a different trend 330 from the strike of the inferred fault plane body wave modeling gave a seismic moment of 1 8 0 5 10 25 dyn cm and constrained the source depth to 8 2 km the dislocation time function indicates a simple rupture pattern with a total duration of 8 1 2 s apparent stress calculated from the relation of kanamari and anderson was 20 bars

an analysis of tidal variations in the length of day observations of the length of day lod corrected for the effects of variations in the angular momentum due to changes in wind velocity and atmospheric pressure ocean tide heights and currents and solid earth zonal tides were analyzed the apparent effect of atmospheric angular momentum aam on the astronomically determined lod is found to be much less at the highest frequencies than at lower frequencies for periods shorter than ten days there appears to be essentially no correlation between aam and lod the international earth rotation service iers standards 1992 model for the effects of zonal tides on the earth s rotation which includes ocean tidal effects adequately accounts for the observations of the variations in the length of day with periods between five and 180 days at the present level of accuracy a currently unexplained semi annual variation in the length of day remains but this may be due to the unmodeled effects of stratospheric winds the power spectrum of the remaining variations with periods less than twenty days is essentially that of a white noise process the amplitudes of the remaining unexplained variations in length of day are less than thirty microseconds this paper has been accepted by geophysical journal international for publication

some thoughts on the spring neap cycle of tidal dissipation abstract the spring neap cycle of global energy dissipation by ocean tides is calculated with a view to trying to explain an observed anomaly in the tidal fluctuations in the length of day calculations are performed in three ways dissipation by friction linearly proportional to semidiurnal tide velocity by quadratic friction and by the torques of the lunar and solar tidal forces on the solar and lunar ocean tides respectively all methods give comparable results equivalent to an amplitude of about 0 1 s change in the length of day with a small phase lag these are inadequate to explain the observed anomaly of about 3 s and 0 1 rad phase lag further investigations to determine the generation of a non equilibrium global msf wave of equatorial amplitude 0 9 mm by nonlinear interactions in shallow seas using global tide models and observations are suggested

phase relations for high frequency core mantle coupling and the earth s axial angular momentum budget abstract although it has long been thought that dynamical core mantle coupling is necessary for understanding decadal variations in the length of day lod the physics of the coupling mechanism are very poorly constrained there several hypotheses electromagnetic topographic gravitational viscous but it is notoriously difficult to distinguish between them on observational grounds there is both an expectation geophys j int 138 1999 679 and some evidence j geophys res 100 1995 8233 geophys res lett 24 1997a 1799 that there is significant core mantle coupling on much faster timescales down to subannual periods at such frequencies the core would no longer be the dominant driving force for lod but would instead interact with both the mantle and external reservoirs of angular momentum such as the atmosphere and ocean on these much faster timescales it is possible that some information that is obscured on decadal timescales becomes available in particular any characteristic timescales at very short periods may become observable here i examine the angular momentum budget of the earth on subannual to annual timescales for any additional implied constraints on the physics of angular momentum exchange between the core and the mantle i find that the discrepancy between atmospheric angular momentum and mantle angular momentum on timescales where the ocean is thought to be unimportant has a distinctive signature which is difficult to reproduce using simple models of core mantle coupling although an ad hoc frictional model does moderately well rather than indicating simplicity this may instead be symptomatic of great complexity in the physics of the core mantle boundary region

oceanic tidal angular momentum and earth s rotation variations abstract luni solar tides affect earth s rotation in a variety of ways we give an overview of the physics and focus on the excitation of earth rotational variations by ocean tides under the conservation of angular momentum various models for diurnal and semidiurnal tidal height and tidal current fields have been derived following a legacy of a number of theoretical tide models from the topex poseidon t p ocean altimetry data we review the oceanic tidal angular momenta otam predicted by these t p models for the eight major tides q1 o1 p1 k1 n2 m2 s2 k2 and their excitations on both earth s rotational speed variation in terms of length of day or ut1 and polar motion prograde diurnal semidiurnal components and retrograde semidiurnal components these small high frequency effects have been unambiguously observed in recent years by precise earth rotation measurements via space geodetic techniques here we review the comparison of the very long baseline interferometry vlbi data with the t p otam predictions the agreement is good with discrepancies typically within 1 2 microseconds for ut1 and 10 30 microarcseconds for polar motion the eight tides collectively explain the majority of subdaily earth rotation variance during the intensive vlbi campaign cont94 this establishes the dominant role of otam in exciting the diurnal semidiurnal polar motion and paves the way for detailed studies of short period non otam excitations such as atmospheric and oceanic angular momentum variations earthquakes the atmospheric thermal tides earth librations and the response of the mantle lateral inhomogeneities to tidal forcing these studies await further improvements in tide models and earth rotation measurements

crustal structure and thickness along the yellowstone hot spot track evidence for lower crustal outflow from beneath the eastern snake river plain 1 receiver functions from seismic stations about the yellowstone hot spot track are migrated to depth using a vp vs map constructed from stacking of the direct and free surface moho reverberations i e h k analysis and a shear velocity tomogram constructed from surface wave measurements the thickest crust 48 54 km resides in the wyoming province beneath the sampled laramide age blocks and the thinnest crust 32 37 km resides in the montana basin and range province the eastern snake river plain esrp crust is thickest 47 km at its ne end beneath the young calderas and thinnest 40 km at its sw end beneath the older twin falls caldera two esrp crustal thickness domains are found 1 at the older twin falls and picabo calderas the mean esrp crust is 4 km thicker with respect to its margins and 2 adjacent to the heise caldera field the mean esrp crust is 4 km thicker with respect to its se margin crust but no thicker with respect to its nw margin crust this lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the heise caldera field confirmation of these crustal thickness variations is provided by inspection of common conversion point ccp stacks that delineate several secondary features the top of a thick high velocity 3 9 km s lower crust layer within the wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the yellowstone caldera this paired arrival would be consistent with a low velocity zone perhaps associated with magma staging beneath the caldera our most important finding is that the magmatic loads injected into the esrp crust over the last 4 12 myr in tandem with the esrp crustal viscosity structure have been sufficient to drive significant outflow of the esrp lower crust

density and lithospheric strength models of the yellowstone snake river plain volcanic system from gravity and heat flow data abstract the structure and composition of the yellowstone snake river plain ysrp volcanic system were analyzed using gravity data taken at over 30 000 stations in the ysrp and surrounding region additional constraints were provided by tomographic seismic velocity models new heat flow and temperature information gps derived strain rates earthquake locations and chemical analyses of volcanic rocks p wave velocity models and velocity density data based on petrologic information were used to constrain three dimensional density models rheology and strength properties were calculated at selected locations and compared to earthquake focal depths results of this study suggest that the lower crust of the snake river plain srp has been thickened by the addition of an underplated layer composed primarily of clinopyroxene having a density of 3200 kg m3 a mid crustal high density sill is composed of a series of gabbroic lenses inter fingering with the granitic upper crust this geometry yields a bulk composition comparable to diorite and a density of 2900 kg m3 the mid crustal sill varies from 4 to 11 km in thickness resulting in a series of se nw trending gravity highs observed along the axis of the srp the mid crustal sill extends up to 20 km southeast of the srp volcanic field and causes asymmetry of the gravity field the yellowstone plateau volcanic field density model reveals low density partial melt 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by 20 the caldera melt body has a density of 2520 kg m3 and a significantly lower value of 2470 kg m3 for the northeastern caldera melt body southwest of yellowstone the crustal section occupied by the mid crustal sill in the srp and the partial melt in yellowstone constitute a transition area between the active yellowstone magma system and the now volcanically quiescent srp with a density of 2820 kg m3 strength models show that the crust of the ysrp becomes progressively stronger and cooler with increasing distance from yellowstone and tectonic earthquakes within the yellowstone caldera are unlikely to nucleate below 4 6 km depth thus limiting the maximum magnitude of earthquakes to m 6 5

earth s variable rotation recent improvements in geodetic data and practical meteorology have advanced research on fluctuations in the earth9s rotation theinterpretation of these fluctuations is inextricably linked with studies of the dynamics of the earth moon system and dynamical processes in the liquid metallic core of the earth where the geomagnetic field originates other parts of the earth9s interior and the hydrosphere and atmosphere fluctuations in the length of the day occurring on decadal time scales have implications for the topography of the core mantle boundary and the electrical magnetic and other properties of the core and lower mantle investigations of more rapid fluctuations bear on meteorological studies of interannual seasonal and intraseasonal variations in the general circulation of the atmosphere and the response of the oceans to such variations

mass transfer along the yellowstone hotspot track ii kinematic constraints on the volume of mantle derived magma abstract this paper evaluates the neogene kinematic evolution of the eastern snake river plain esrp to estimate the volume of mafic magma transferred from mantle to crust during passage over the yellowstone hotspot four major kinematic processes affected the esrp including densification due to emplacement of gabbro into the middle crust subsidence and consequent filling of a 6 km deep basin with volcanic and plutonic rocks 20 extension via dike injection or faulting ductile attenuation and lower crustal flow from esrp to the adjacent basin and range two kinematic models involving these processes are proposed overall the two models are viable only if the initial crustal thickness were 43 47 km thick otherwise the lost mass of the esrp is not balanced by the excess mass in the basin and range in the first model densification eruption and extension by dike injection the esrp crust receives mantle derived gabbro equivalent to a 16 km thick layer but would need to lose 10 km of crust via lower crustal flow in the second model densification eruption and extension by faulting and ductile attenuation the esrp crust receives a 9 4 km equivalent thickness of mantle derived gabbro and would need to lose 6 km of crust via lower crustal flow the cumulative amount of mantle derived gabbro in the esrp crust derived by this kinematic analysis equivalent to a 9 4 16 km thick layer is nearly identical to the amount estimated independently using a mass balance petrologic model suggesting it is a robust result

deterministic signals in precipitation in arkansas louisiana and mississippi evidence for a signal near 19 years is found in 51 out of 65 rain gauge records in the tri state region of arkansas louisiana and mississippi while another term with period 10 to 11 years is found in 39 instances these are identified as the 18 6 year luni solarm n and solar cycles c signals in climate variability in terms of both amplitude and phase the luni solar wave trains are highly nonstationary in particular the waves commonly change phase by 180 examples of them n ands c terms in corn yield for arkansas and missouri are presented and the implications for economic science briefly discussed

subdaily earth rotation during the epoch 92 campaign global positioning system gps data were used to estimate earth rotation variations over an 11 day period during the epoch 92 campaign in the summer of 1992 earth orientation was measured simultaneously by several very long baseline interferometry vlbi networks gps and vlbi estimates of ut1 with 3 hour time resolution were then compared and analyzed the high frequency behavior of both data sets is similar although drifts between the two series of 0 1 ms over 2 5 days are evident the geodetic results were also compared with models for ut1 fluctuations at tidal periods and with estimates of atmospheric angular momentum made at 6 hour intervals most of the geodetic signal in the diurnal and semidiurnal frequency bands can be attributed to tidal processes whereas ut1 variations over a few days are mostly atmospheric in origin

the moon and el nino regional climates around the world display cycles corresponding to the 18 61 year maximum lunar declination mld periodicity we suggest that these cycles are created by a relationship between mld and el nino southern oscillation enso both equatorial pacific sea surface temperature and south pacific atmospheric pressure significantly correlate with maximum lunar declination low mlds are associated with warmer equatorial pacific sea surface temperatures and negative values of the southern oscillation index a lunar influenced change in the pacific gyre circulation presents a viable physical mechanism for explaining these relationships we suggest that the gyre is enhanced by tidal forces under high mlds inducing cold water advection into the equatorial region but is restricted by the weak tidal forcing of low mlds thereby favoring el nino episodes an astronomical model utilizing this relationship produces a forecast of increased non el nino either la nina or neutral activity for the early part of this decade

geophysical analysis of zonal tidal signals in length of day summary the earth s zonal response coefficient k is estimated from the tidal signals in the observed length of day lod data its magnitude and phase are functionals of the earth s internal structure and dynamics in this paper an analysis of 13 years of precise lod data 1980 1992 reveals strong signals for nine zonal tidal groups ranging from 5 to 35 days in period numerical estimates of k for 27 major tides are thus obtained 11 among which are considered sufficiently high in signal to noise ratio to provide meaningful geophysical constraints on the earth s rotational dynamics the results favour a k magnitude close to but somewhat smaller than 0 315 which is the theoretical value for an elastic mantle completely decoupled from the fluid core plus equilibrium oceans a small amount of dispersion is also detectable where shorter periods tend to have lower k magnitude and larger phase lag our k magnitude estimates are consistent with two recently published non equilibrium ocean tide models and an anelastic response in the mantle although an equilibrium response in the ocean and a purely elastic response in the mantle is not disallowed phase lags of a few degrees are required by both ocean tide models and by our data

testing five of the simplest upper mantle anisotropic velocity parameterizations using teleseismic s and sks data from the billings montana passcal array 1 five of the simplest parameterizations of upper mantle anisotropy are tested and ranked for a data set collected on a dense temporary passcal seismic array located 100 km ne of yellowstone these hexagonal symmetry anisotropy models possess either one or two layers with either flat or dipping fast velocity axis fva recordings from fifteen high quality direct s and sks arrivals are stacked to provide accurate waveform and error estimates source normalization is accomplished using the cross convolution technique a direct monte carlo neighborhood algorithm is used to map the posteriori model probability density ppd volume using the f test we find that models with purely flat fva can be rejected at 97 probability our best model p5 is a two layer dipping fva parameterization albeit the two layer model with one flat and one dipping fva can only be rejected at 80 probability the best model has a lower layer with a n65 e fva strike and a 12 dip down to the southwest and an upper layer with a n20 w fva strike and a 47 fva dip down to the southeast the bottom asthenospheric layer fva strikes parallel to north america s absolute plate motion direction and dips opposite to what passive plate shear of the asthenosphere would predict the upper lithospheric layer is consistent with lpo accretion associated with north directed drift of the north american plate during the mesozoic comparison between the sks and direct s wave data sets shows that the direct s waves improve resolution of the double layer anisotropic model parameters

source mechanism and surface wave excitation for two earthquakes in northern baja california mexico summary two earthquakes of comparable local magnitude from different regions of northern baja california the 1975 july 17 pino solo earthquake ml 5 1 from the peninsular ranges region and the 1976 december 7 mesa de andrade earthquake ml 5 3 from the colorado river delta region are studied in detail to determine possible causes for the observed stronger excitation of surface waves by earthquakes from the delta region relative to earthquakes of comparable local magnitude in the peninsular ranges region data from distant stations are complemented with data from aftershock studies using local arrays and for the mesa de andrade earthquake with data from two strong motion stations the strong motion records for ihe mesa de andrade earthquake show that it consisted of two events 45 s apart which produced maximum recorded horizontal accelerations of 0 21 and 0 24g respectively at a distance of 18 km from the epicentre synthetic seismograms are used to help determine the depth of these events and their source time functions both events are relatively simple the second event is sharp and impulsive and could be thought of as an aftershock since it has a considerably smaller moment than the first event even though its recorded acceleration was higher comparison of measured parameters shows that while the local magnitude of the mesa de andrade earthquake is only 0 2 units larger than the pino solo earthquake and its source dimension is 2 2 times larger source areas 4 8 times larger its moment is larger by a factor of 6 2 8 5 this is approximately explained by the cy2 scaling law however the near source spectra and accelerations recorded on the strong motion accelerograph at riito at a distance of about 18 km are considerably larger than predicted by the scaling law this is probably a results of high stress drop asperities on the fault surface although near field focusing directivity may also have contributed

a combined length of day series spanning 1832 1997 lunar97 abstract universal time ut measurements taken by the techniques of lunar occultation optical astrometry lunar laser ranging and very long baseline interferometry have been combined using a kalman filter to produce a smoothed length of day lod series lunar97 spanning 1832 5 1997 5 at yearly intervals decadal lod variations having amplitudes as large as a few milliseconds ms are clearly evident in this newly determined series as they have been in previously determined series comparing the lunar97 lod series with other available series shows differences that are generally less than the uncertainty in the determination of the lunar97 lod values which is about 0 6 ms during the early 19th century improving by more than an order of magnitude to about 0 03 ms during the late 20th century decadal lod variations are therefore a robust feature of the lunar occultation optical astrometric and space geodetic measurements of the earth s rotation that have been taken since 1832

experiments in tidal mass conservation summary a common feature of theoretical ocean tide models is their failure to conserve mass over the oceans this failure distorts the tidal solution and imparts ambiguity to the calculation of tidal effects on gravity crustal loading and earth rotation for example predicted effects of long period tides on rotation could be uncertain by up to 50 per cent in magnitude and 100 per cent in phase using the spherical harmonic tide theory developed by dickman 1989 the influence of various global conservation constraints on the tidal solution is explored an ocean wide mass conservation constraint implemented using lagrange multipliers produces a realistic and significantly more accurate and unambiguous tidal solution

earth orientation from lunar laser ranging and an error analysis of polar motion services the rotational orientation of the earth has been determined from lunar laser ranging llr measurements acquired at mcdonald observatory in texas the values and uncertainties of ut0 are calculated from 1971 through may 1982 in four forms raw results on 799 separate days kalman filtered values gaussian smoothed values and fourier smoothed values the effects of the different smoothings are assessed llr being dominated by a single station operation is sensitive to one projection of polar motion the component along the mcdonald meridian variation of latitude results are presented for the polar motion component along the mcdonald meridian analyses of the llr data assess the errors of polar motion sources bureau internationale de l heure bih defense mapping agency hydrographic topographic center dmahtc doppler the international polar motion service ipms lageos goddard space flight center gsfc lageos university of texas texas and the lerch analysis of artificial satellite data the rms errors of both the llr data and the determination of polar motion by the various services decrease in recent times for data taken from august 1976 through may 1982 doppler and bih values are of comparable quality estimated error doppler 20 5 cm bih 22 4 cm with ipms results being somewhat larger 38 5 cm lageos polar motion error drops in early 1979 in both analyses gsfc 14 7 cm texas 18 6 cm for data from february 1979 through may 1981 this drop parallels instrumental improvements and the increased number of stations in the laser system the doppler polar motion error 17 5 cm is especially low for the period may 1977 through february 1981 the analysis of lerch february 1979 through may 1981 gives an error estimate of 14 7 cm all analyses are consistent with 11 cm as an estimate for the combined llr modeling fitting and instrumental error noise for august 1976 to may 1982

geodetic studies in baja california mexico and the evaluation of short range data from 1974 to 1982 the short range geodetic data from northern baja california mexico for the period 1974 1982 are carefully analyzed these data contribute to an understanding of the complex pattern of faulting associated with the pacific north american plate boundary in this region survey precisions are evaluated and significant systematic errors are found to exist a technique of studying a scale free displacement solution is developed as an aid to interpreting the data we conclude that 1 the motion on the san miguel vallecitos fault system is presently in a right lateral sense and at a level that warrants trilateration surveys at least annually 2 present geodetic data permit no statement about movement on the agua bianca fault and 3 the mesa whose flank delineates the cerro prieto fault in the valle de mexicali lies in a zone which has undergone significant horizontal areal compression at a rate of 14 5 ppm yr in addition to a right lateral tensor shear at a rate of 4 1 ppm yr oriented n 33 9 w between 1980 and 1982

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

teleseismic p wave tomogram of the yellowstone plume 1 inversion of a new data set of teleseismic p wave travel times from three passcal seismic deployments around the yellowstone hotspot reveals a 100 km diameter upper mantle plume that extends from the yellowstone volcanic caldera to 500 km depth and dips 20 to the northwest a monotonic decrease in the velocity perturbation of the plume from 3 2 at 100 km to 0 9 at 450 km is consistent with a uniform thermal anomaly of 180 c where the plume crosses the 410 km discontinuity previous research shows a depression in the 410 km discontinuity consistent with a warm plume fee and dueker 2004 additionally a region of high velocities extends to 250 km beneath the wind river basin in nw wyoming that may represent a convective downwelling of the lithosphere

evaluating processing choices for the geodetic estimation of earth orientation parameters with numerical models of global geophysical fluids different earth orientation parameter eop time series are publicly available that typically arise from the combination of individual space geodetic technique solutions the applied processing str

shear wave splitting beneath the snake river plain suggests a mantle upwelling beneath eastern nevada usa the snake river plain srp a 90 km wide topographic depression in southern idaho is a topographically anomalous feature in the western u s previous seismic studies focused on the northeastern srp to study its relationship with the yellowstone hotspot we present new teleseismic shear wave splitting data from six broadband seismic stations deployed along the axis of the srp from june 2000 to september 2001 we also analyze splitting at hlid a permanent station of the national seismic network located f100 km north of the plain splitting of individual teleseismic phases is consistent at all stations within 2r errors and we favor the interpretation of anisotropy with a single horizontal fast axis although a dipping axis interpretation is statistically permitted at two of the stations our station fast directions as well as shear wave splitting data from numerous other stations throughout the basin and range are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume like upwelling material beneath eastern nevada into a southwestward flowing asthenosphere with respect to a fixed hotspot reference frame this parabolic asthenospheric flow paf model for the great basin is attractive because it explains the observed high elevations high mantle buoyancy low velocity anomaly beneath eastern nevada high heat flow and depleted geochemistry of some erupted basalts the lack of pliocene recent major volcanism in eastern nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy our splitting station delay times vary in a way not predicted by the paf model and can be explained by a zone of aligned magma filled lenses and or partially molten dikes beneath the srp lithosphere a depleted olivine rich residuum underneath the sides of the eastern srp and or the effect of lateral lower crustal flow from beneath the esrp toward its adjacent flanks d 2004 elsevier b v all rights reserved

polar motion and earth rotation during the period from 1979 through 1982 polar motion and earth rotation activity centered on the development of new observation techniques and the investigations of the relation between the observed motions and geophysical phenomena determinations of polar motion through classical techniques continued but the number of united states observing sites was reduced however significant advances in alternate methods for polar motion and earth rotation determination were achieved regular polar motion and length of day determinations are being reported now by groups analyzing satellite doppler laser ranging and very long baseline interferometry vlbi data and preliminary determinations of ut1 by these methods are now being reported

the effect of ocean tides on the earth s rotation as predicted by the results of an ocean tide model the published ocean tidal angular momentum results of seiler 1991 are used to predict the effects of the most important semidiurnal m2 s2 n2 diurnal k1 o1 p1 and long period mf mf mm and ssa ocean tides on the earth s rotation the separate as well as combined effects of ocean tidal currents and sea level height changes on the length of day ut1 and polar motion are computed the predicted polar motion results reported here account for the presence of the free core nutation and are given in terms of the motion within the rotating body fixed terrestrial reference frame of the celestial ephemeris pole so that they can be compared directly to the results of observations outside the retrograde diurnal tidal band the summed effect of the semidiurnal and diurnal ocean tides studied here predict peak to peak polar motion amplitudes as large as 2 mas within the retrograde diurnal tidal band the resonant enhancement caused by the free core nutation leads to predicted polar motion or equivalently nutation amplitudes as large as 9 mas at the unobservable retrograde k1 tidal frequency

daily earth rotation determinations from iris very long baseline interferometry classical optical determinations of earth rotation ut1 require several weeks of observations to achieve an accuracy of 1 5 ms of time whereas modern techniques have demonstrated an accuracy of 0 7 0 9 ms on timescales of 1 5 days l 2 here we have probed the limits of the accuracy and time resolution of very long baseline inteferometry vlbi determinations of ut1 by conducting a series of 1 h observing sessions using only one baseline comparison of the results from the 1 h sessions with corresponding determinations from 24 h multi baseline sessions establishes that the accuracy of the 1 h determinations is close to 0 1 ms of time these observations were scheduled as part of the project merit monitor earth rotation and intercompare techniques of observation and analysis3 intensive observing campaign

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

degree 2 harmonics of the earth s mass load estimated from gps and earth rotation data a fluid mobile atmosphere and oceans surrounds the solid earth and upon its land surface lays a continually changing distribution of ice snow and ground water the changing distribution of mass associated with the motion of these surficial fluids changes the earth s rotation by changing its inertia tensor and changes the earth s shape by changing the load on the solid earth it has recently been demonstrated that large scale changes of the earth s shape and hence of the mass load causing the earth s shape to change can be measured using the global network of gps receivers here the degree 2 mass load coefficients determined from gps data are compared with those obtained from earth orientation observations from which the effects of tides winds and currents have been removed good agreement is found between these two estimates of the degree 2 mass load particularly at seasonal frequencies

magmatism and extension the thermal and mechanical effects of the yellowstone hotspot currently there is a parabolic shaped pattern of seismicity and latest quaternary faulting which centers on the axis of the eastern snake river plain srp and has its apex at the yellowstone plateau the pattern of activity surrounds a region of aseismicity that includes the eastern srp geologic evidence suggests that this pattern has migrated in tandem with rhyolitic volcanism induced by the yellowstone hotspot during its 16 m y trek across western north america similarly there is a migration in the onset of basaltic lava flows and the emplacement of up to 12 km of midcrustal mafic intrusions that follows the track of rhyolitic volcanism these observations suggest that the intrusion of magmas has a significant influence on the pattern of seismicity and faulting we present a one dimensional finite difference thermomechanical model that accounts for the observed pattern of increased faulting followed by fault quiescence within the circum eastern srp in this model mafic magmas are intruded into a lithosphere that is already extending the intrusions heat the surrounding rocks resulting in locally increased strain rates as the intruded magmas solidify the length of time required to return strain rates to their pre intrusion level is then determined the model assumes constant horizontal tectonic forces and maps strain rate as a function of yield strength and time since intrusion we vary model parameters such as crustal thickness initial geothermal gradient and amount of magma intruded in order to assess how they affect turnaround time for strain rates the amount and timing of intruded material allowed by the model are constrained by seismic refraction data as well as by regional geochronologic and petrologic studies we assume intruded rock rheology to be a maryland diabase mantle rheology we base on dry olivine we represent the crust by both dry granite and dry anorthosite rheologies our results suggest that 2 to 3 m y appears to be a reasonable length of time for strain rates to return to levels present before a midcrustal mafic intrusion equivalent in magnitude to the yellowstone intrusion this corresponds closely to the length of time between the onset of along axis accelerated faulting and subsequent fault quiescence assuming a hotspot velocity of 3 5 to 4 0 cm yr relative to the north american plate

long period tidal variations in the length of day a new model of long period tidal variations in length of day is developed the model comprises 80 spectral lines with periods between 18 6 years and 4 7 days and it consistently includes effects of mantle anelasticity and dynamic ocean tides for all lines the anelastic properties follow wahr and bergen experimental confirmation for their results now exists at the fortnightly period but there remains uncertainty when extrapolating to the longest periods the ocean modeling builds on recent work with the fortnightly constituent which suggests that oceanic tidal angular momentum can be reliably predicted at these periods without data assimilation this is a critical property when modeling most long period tides for which little observational data exist dynamic ocean effects are quite pronounced at shortest periods as out of phase rotation components become nearly as large as in phase components the model is tested against a 20 year time series of space geodetic measurements of length of day the current international standard model is shown to leave significant residual tidal energy and the new model is found to mostly eliminate that energy with especially large variance reduction for constituents sa ssa mf and mt

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

fortnightly earth rotation ocean tides and mantle anelasticity summary this study of the fortnightly mf tide comprises three main topics 1 a new determination of the fortnightly component of polar motion and length of day lod from a multidecade time series of observed space geodetic data 2 the use of the polar motion determination as one constraint in the development of a hydrodynamic ocean model of the mf tide and 3 the use of these results to place new constraints on mantle anelasticity at the mf tidal period our model of the mf ocean tide assimilates more than 14 years of altimeter data from the topex poseidon and jason 1 satellites because the mf altimetric signal to noise ratio is very small it is critical that altimeter data not be overweighted the polar motion data plus tidegauge data and independent altimeter data give useful additional information with only the polar motion putting constraints on tidal current velocities the resulting ocean tide model plus the dominant elastic body tide leaves a small residual in observed lod caused by mantle anelasticity the inferred effective tidal q of the anelastic body tide is 90 and is in line with a frequency dependence with in the range 0 2 0 3

subsidence of a volcanic basin by flexure and lower crustal flow the eastern snake river plain idaho the eastern snake river plain esrp is a linear volcanic basin interpreted by many workers to reflect late cenozoic migration of north america over the yellowstone hotspot thermal subsidence of this volcanic province with respect to yellowstone has been documented by several workers but no one has characterized subsidence with respect to the adjacent basin and range province this paper documents crustal flexure along the northwest edge of the esrp uses flexure to model the dimensions of a dense load beneath the basin and presents evidence in support of density driven subsidence and lower crustal flow away from the basin crustal flexure adjacent to the esrp is reflected by the attitudes of mesozoic fold hinges and neogene volcanic rocks fold hinges formed with a subhorizontal plunge and a trend perpendicular to the esrp but now show a southward plunge near the esrp of as much as 20 25 we present a contour map of equal fold plunges proximal to the esrp that shows flexure is roughly parallel to and extends 10 20 km north of the average edge of the esrp flexural profiles indicate the minimum amount of esrp subsidence with respect to the basin and range subsidence ranges from 4 5 to 8 5 km the structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models these models indicate the flexed crust is very weak flexural parameter of 4 10 km interpreted to be a result of the high heat flow of the esrp assuming subsidence was induced by emplacement of a dense crustal layer beneath the esrp a midcrustal sill identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure new dimensions include a thickness of 17 25 km and a half width of 40 50 km which place the edge of the sill beneath the edge of the esrp the dimensions of the esrp sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30 km and because esrp seismic gravity and heat flow data support lower crustal compensation density driven lower crustal flow away from the esrp is proposed to accommodate subsidence and maintain isostatic equilibrium timing of subsidence is constrained by esrp exploratory wells where 6 6 ma rhyolites at a depth of 1 5 km indicate most subsidence occurred prior to their emplacement and by strong spatial correlations between plunge contours and quaternary volcanic rift zones two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at 10 ma which provided the heat source for the initial rhyolitic volcanism on the esrp and continuing localized loads from dikes and sills associated with quaternary basalts widespread 10 ma magmatism and subsidence conflicts with simple time transgressive migration of the yellowstone hotspot indicating a need for revision of the hotspot paradigm

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

crustal scale recycling in caldera complexes and rift zones along the yellowstone hotspot track o and hf isotopic evidence in diverse zircons from voluminous rhyolites of the picabo volcanic field idaho abstract rhyolites of the picabo volcanic field 10 4 6 6 ma in eastern idaho are preserved as thick ignimbrites and lavas along the margins of the snake river plain srp and within a deep 3 km borehole near the central axis of the yellowstone hotspot track in this study we present new o and hf isotope data and u pb geochronology for individual zircons o isotope data for major phenocrysts quartz plagioclase and pyroxene whole rock sr and nd isotope ratios and whole rock geochemistry for a suite of picabo rhyolites we synthesize our new datasets with published ar ar geochronology to establish the eruptive framework of the picabo volcanic field and interpret its petrogenetic history in the context of other well studied caldera complexes in the srp caldera complex evolution at picabo began with eruption of the 10 44 0 27 ma u pb tuff of arbon valley tav a chemically zoned and normal 18o 18o magma 7 9 unit with high zoned 87sr 86sri 0 71488 0 72520 and low e nd 0 18 and e hf 0 28 the tav and an associated post caldera lava flow possess the lowest end 0 23 indicating 40 60 derivation from the archean upper crust normal 18o rhyolites were followed by a series of lower 18o eruptions with more typical lower crustal sr nd hf isotope ratios and whole rock chemistry the voluminous 8 25 0 26 ma west pocatello rhyolite has the lowest 18o value 18omelt 3 3 and we correlate it to a 1 000 m thick intracaldera tuff present in the inel 1 borehole with published zircon ages 8 04 8 35 ma and similarly low 18o zircon values the significant 4 5 decrease in magmatic 18o values in picabo rhyolites is accompanied by an increase in zircon 18o heterogeneity from 1 variation in the tav to 5 variation in the late stage low 18o rhyolites a trend similar to what is characteristic of heise and yellowstone and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption however due to the greater abundance of low 18o rhyolites at picabo the eruptive framework may reflect an intertwined history of caldera collapse and coeval basin and range rifting and hydrothermal alteration we speculate that the source rocks with pre existing low 18o alteration may be related to 1 deeply buried and unexposed older deposits of picabo age or twin falls age low 18o volcanics and or 2 regionally abundant late eocene challis volcanics which were hydrothermally altered near the surface prior to or during peak picabo magmatism basin and range extension specifically the formation of metamorphic core complexes exposed in the region could have facilitated the generation of low 18o magmas by exhuming heated rocks and creating the large water rock ratios necessary for shallow hydrothermal alteration of tectonically rift zones and volcanically calderas buried volcanic rocks these interpretations highlight the major processes by which supereruptive volumes of magma are generated in the srp mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics

a fixed sublithospheric source for the late neogene track of the yellowstone hotspot implications of the heise and picabo volcanic fields the heise and picabo volcanic fields of eastern idaho are part of the more extensive time transgressive yellowstone snake river plain hotspot track calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern snake river plain along with some limited geophysical data a 1 5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields using these new caldera locations we calculate an extension adjusted rate of 2 35 cm yr for the north american plate over the last 6 66 m y and a velocity of 2 30 cm yr over the 10 27 m y recalculation of a previously determined plate velocity based migration of the deformation field surrounding the eastern snake river plain yields an extension adjusted rate of 2 38 0 21 cm yr these migration rates all fall within the previously published range of north american plate velocities of 2 2 0 8 cm yr 2 4 cm yr and 2 68 0 78 cm yr based on a global hot spot reference frame the consistency of these rates suggest that over the last 10 m y the yellowstone hot spot is fixed with respect to the motion of the north american plate and therefore consistent with a classical deep sourced hotspot model

geological and seismic evidence of a new branch of the agua blanca fault the agua blanca fault is a major trans peninsular right lateral fault located in northern baja california its wnw ese orientation is markedly different from the general trend of the san andreas gulf of california fault system from a geological point of view the agua blanca fault is considered active but there has been little significant seismic activity directly associated with it on october 9 1981 the onset of an earthquake swarm was detected at the ensenada seismic station enx with a s p time average of 2 sec shortly after this an array of up to seven portable seismic stations was installed by cicese around todos santos bay more than 180 events were recorded during the following 10 days after which the local seismicity decreased a second swarm consisting of 100 microearthquakes was detected between november 28 and december 5 1981 the epicentral locations of those events recorded at four or more local stations all lie inside todos santos bay the composite fault plane solution for both swarms indicates a right lateral strike slip vertical fault with a strike of n52 w geological observations allow us to conclude that the seismic activity reported here is associated with the agua blanca fault zone

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

evidence for excitation of polar motion by fortnightly ocean tides the second degree zonal tide raising potential which is responsible for tidal changes in the earth s rotation rate and length of day is symmetric about the polar axis and hence can excite the earth s polar motion only through its action upon nonaxisymmetric features of the earth such as the oceans ocean tidal excitation of polar motion in the diurnal and semidiurnal tidal bands has been previously detected and examined here the detection of ocean tidal excitation of polar motion in the long period tidal band specifically at the mf 13 63 day and mf 13 66 day tidal frequencies is reported spectra of the space94 polar motion excitation series exhibit peaks at the prograde and retrograde fortnightly tidal periods after removing effects of atmospheric wind and pressure changes an empirical model for the effect of the fortnightly ocean tides upon polar motion excitation is obtained by least squares fitting periodic terms at the mf and mf tidal frequencies to the residual polar motion excitation series the resulting empirical model is then compared with the predictions of two hydrodynamic ocean tide models

deep probe imaging the roots of western north america analysis of the lithoprobe deep probe and southern alberta refraction experiment data sets focusing on the region between deep probe shots 43 and 55 has resulted in a continental scale velocity structural model of the lithosphere of platformal western laurentia reaching depths of 150 km three major lithospheric blocks were investigated i the hearne province a typical continental archean cratonic province lying beneath the western canada sedimentary basin ii the wyoming province an even older block of phanerozoic modified archean crust with an enigmatic lower lithosphere and iii the yavapai mazatzal province proterozoic terranes underlying the colorado plateau and southern rocky mountains in this study the northern two of these regions are investigated with a modified ray theoretical traveltime inversion routine that respects the spherical geometry of the earth the resulting crustal velocity structure combined with supporting geological and geophysical data reveals that the medicine hat block mhb lying between the hearne and wyoming provinces is a third independent archean crustal block the subcrustal lithosphere along the profile is homogeneous in velocity structure but two significant northward dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three archean blocks the hearne crust is typical of an archean shield or platform both in its thickness of 34 50 km and its seismic velocity structure the crust of the archean mhb and wyoming province which ranges in thickness from 49 to 60 km includes a 10 30 km thick high velocity layer interpreted to be proterozoic in age such a feature is unexpected beneath archean crustal provinces but if the region is considered to be the remanent marginal portion of a larger archean continent then the interpreted proterozoic underplating and lack of an archean lithospheric root can be explained the variable topography along the reflective upper and lower boundaries of this layer especially within the mhb suggests considerable variability in its emplacement and subsequent tectonic history

evidence for 18 6 year mn signal in temperature and drought conditions in north america since ad 1800 evidence for the 18 6 year lunar nodal mn term in air temperature is found at 30 stations in eastern north america since ad 1800 the mean discrepancy in epoch between maxima in temperature and maxima in the drought area index dai for the western united states vis a vis the maxima in the mn tide is 0 9 and 0 1 yr respectively also a cluster of 9 stations in western canada yields the 18 6 yr term which is out of phase with stations in eastern north america and is indicative of a standing tidal wave in the atmosphere this suggests that enhanced drought conditions in the western united states are neither recurrent nor rhythmic on a time scale of 20 yr but rather periodic with a period of 18 6 yr finally the rocky mountain system is seen as an influence for atmospheric tidal phenomena at epochs of maximum in the mn tide as well as for maxima in the temperature records and the dai

the 1978 yellowstone eastern snake river plain seismic profiling experiment crustal structure of the yellowstone region and experiment design in 1978 a major seismic profiling experiment was conducted in the yellowstone eastern snake river plain region of idaho and wyoming fifteen shots were recorded that provided coverage to distances of 300 km in this paper travel time and synthetic seismogram modeling was used to evaluate an average p wave velocity and apparent q structure of the crust from two seismic profiles reversed across the yellowstone national park region this area includes the well known hydrothermal features of yellowstone national park geysers fumeroles etc a large collapse caldera and extensive silicic volcanism of quaternary age features attributed to shallow crustal sources of magma the averaged crustal structure for this region as interpreted from the seismic data consists of 1 a highly variable near surface layer approximately 2 km thick with variable velocities of 3 0 to 4 8 km s and a low apparent q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill 2 an upper crustal layer 3 to 4 km thick with variable velocities of 4 9 to 5 5 km s and apparent q of 50 to 200 that is thought to represent the accumulation of the pleistocene quaternary rhyolite flows ash flow tuffs and possible paleozoic and precambrian metamorphic equivalents 3 the crystalline upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4 0 to 6 1 km s averaging 5 km thick with a q of 300 this layer appears to be a cooling but still hot body of granitic composition beneath the yellowstone caldera it is thought to be a remnant of the magma chambers that produced the quaternary silicic volcanic rocks of the yellowstone plateau and may still be a major contributor to the high heat flow 4 a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6 5 km s and apparent q of 100 to 300 5 a homogeneous 25 km thick lower crust with a velocity of 6 7 to 6 8 km s and an apparent q of 300 and 6 a total crustal thickness of 43 km the upper crustal layer 5 5 to 6 0 km s is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed archean crust an interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of yellowstone the high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation partial melting and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the yellowstone plateau this seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern snake river plain to their present location beneath the yellowstone hydrothermal system while these findings do not bear directly on the origin and source of the heat i e mantle lumes lithospheric fractures mantle radiogenetic heat basal lithospheric shearing etc they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries

the relative movement of the north american and pacific plates in 1984 1985 detected by the pacific vlbi network heki k takahashi y kondo t kawaguchi n takahashi f and kawano n 1987 the relative movement of the north american and pacific plates in 1984 1985 detected by the pacific vlbi network in r l wesson mechanics of earthquake faulting tectonophysics 144 151 158 very long basline interferomet vlbi experiments from stations on the north american and pacific plates in the period 1984 1985 revealed that the interplate baselines have undergone changes by several centimeters which are consistent with the values expected from the model of present plate kinematics it was also clarified that the intraplate baseline lengths are almost stationary in this period these results demonstrate that the relative movement of these two plates in the last two years is similar to the plate motion observed on a geological time scale say a few millions of years

crustal structure of southwestern montana and east central idaho results of a reversed seismic refraction line on october 20 1982 we collected seismic refraction data on a 250 km line segment between butte montana and challis idaho these data are from two open pit mine blasts one at anaconda minerals companys southeast berkely pit and the other at cyprus mines corporations thompson creek mine eight of our ten recorders wrote clear records of both blasts with measured travel times accurate to 0 2 seconds we supplemented these data with data from permanent stations in the butte area the pg crustal velocity is 5 8 km s to the southwest and 6 0 km s to the northeast the pn mantle velocity is 7 9 km s to the northeast and 8 1 km s to the southwest total travel times are equal the s wave velocity in the crust is about 3 5 km s the symmetry of the travel time results with their intersection within one kilometer of the midpoint between the pit blasts strongly suggests the refraction line overlies a horizontal moho or parallels the strike line of a dipping moho surface these data indicate the crust in southwestern montana and east central idaho is approximately 33 km thick contrasting tectonic processes compression and thickening during the laramide orogeny with extension and thinning since the paleocene resulted in a crust with slightly less than normal thickness

revised velocity structure of western montana using a one dimensional 1d layered earth approach we determined a new crust and upper mantle velocity model for western montana to improve earthquake hypocenter locations p wave arrival times recorded on 280 stations from 1432 well recorded earthquakes provided input for a sparse damped least squares sensitivity matrix we solved for 1d velocity structure refractor depths station corrections and hypocentral positions by minimizing travel time residuals the new model has three layers with p wave velocities of 5 70 6 12 and 6 53 km sec with corresponding interface depths of 7 0 19 8 and 39 7 km below the surface the upper mantle velocity is 8 00 km sec we determined station corrections and show that the yellowstone caldera is the only regional geologic feature not adequately modeled by our new 1d velocity model use of the new model and station corrections reduced hypocenter location uncertainties and travel time residuals implying improved hypocenter locations for western montana earthquakes online material table of seismographic stations

a comparison of geodetic and seismic estimates of inner core rotation seismological observations suggest the inner core has been rotating faster than the mantle during the past 30 50 years differential rotation can produce large gravitational torques on the mantle which if not balanced by other torques would be detectable as changes in the rotation rate of the mantle length of day or lod we compare seismological estimates of inner core rotation with estimates derived by inverting lod observations using models of gravitational coupling model predictions depend on several parameters but for the expected range of parameter values the seismological estimates of relative rotation are at least an order of magnitude larger than those predicted by the lod data furthermore the lod data predict oscillation in the angular alignment of the inner core and mantle during the past 30 50 years while the seismological data seem to require a component of steady rotation we show that steady rotation of the inner core implies a steady torque on the mantle this torque is not accelerating the rotation rate of the mantle so it must be opposed by another torque the required torque is consistent with a frictional electromagnetic stress on the base of the mantle due to a westward flow at the top of the core

tidal variations of earth rotation we have rederived the periodic variations of the earth s rotation due to the tidal deformation of the earth by the sun and moon and included all terms with amplitudes 0 002 milliseconds 1 mm this series applies to the mantle plus crust and oceans which rotate together for characteristic tidal periods the parameter which scales the rotational series is k c where k is that fraction of the love number which causes the tidal variation in the moment of inertia of the coupled mantle and oceans while c is the dimensionless polar moment of inertia of the coupled units if the whole earth minus oceans were coupled rotationally then k 0 30 and c 0 33 ocean tides increase k by 0 04 decoupling of the fluid core from the mantle decreases k by 0 06 while c cmantle 0 29 since neither pressure viscous nor hydromagnetic coupling locks the fluid to the mantle for periods less than 5 to 25 years from the analysis of lunar laser ranging data we find that k c at monthly and fortnightly frequencies equals 0 99 0 15 and 0 99 0 20 as compared with a theoretical value of 0 94 0 04 in addition we have estimated theoretically the effects of ocean tides on earth rotation nutation and polar motion using models based on the laplace tidal equations

martian precession and rotation from viking lander range data the spatial orientation of the martian pole of rotation and axial rotation parameters have been determined at a midpoint in the viking epoch january 1 1980 the mean obliquity e and node angles are e 25 1894 0 0001 35 4796 0 0002 2 the mean precession rate is o 7 83 0 30 arc sec yr 1 the corresponding moment estimate is c mre2 0 355 0 015 and covers two extreme theoretical estimates seasonal annual and semiannual variations in mars rotation angle have also been observed and are 279 100 and 311 100 milliarc second of angle mas respectively after correcting for a general relativity effect a simple model has been constructed for the main contribution involving seasonal mass exchange between the ice caps and atmosphere where the air pressure at the viking 1 lander site is a proxy for the global ice cap air pressure histories the annual term is about 190 mas smaller and differs in phase by 30 compared to the model prediction the semiannual term is 100 mas larger and also differs in phase by 30 compared to the model a combination of tidally driven rotation changes zonal winds polar motion and ice cap model deficiencies may account for any discrepancy the upcoming pathfinder lander and mars global surveyor missions carry ranging transponders which can dramatically improve our knowledge of mars interior through precise determination of the precession rate and detection of core and tidal effects we advocate that plans be developed for a coordinated campaign of simultaneous ranging and doppler tracking to each transponder

upper mantle p wave velocity structure from passcal teleseismic transects across idaho wyoming and colorado 1 inversion of teleseismic p time residuals along two linear seismic arrays provides well resolved inverse images of the upper mantle low velocity bodies beneath the yellowstone hotspot track and the grand mesa volcanic field in western colorado are similar in shape and magnitude in the two places where our transects cross the 1 78 ga archean proterozoic cheyenne suture high velocities are imaged to 150 km beneath the leucite hills volcanic field in wyoming a small upper mantle low velocity body is imaged whether these anomalies are lithospheric or asthenospheric in origin is poorly constrained but the consistent high velocities beneath the cheyenne suture suggest a lithospheric origin likewise the lack of extension beneath the western colorado volcanic fields suggests that a low solidus lithosphere is being disturbed by a regional heating event

a parametrized solid earth tide model and ocean tide loading effects for global geodetic baseline measurements summary an observation model for earth tide displacements in application to very long baseline interferometry and similar precise geodetic techniques is developed it incorporates effects from anelasticity ellipsoidal figure and fluid core resonance based on a harmonic development of the external potential the model follows the familiar love number concept the transfer function of the earth to each harmonic is formulated in terms of coupled harmonics in the space domain and fulfils the causality condition in the time domain solve for parameters can be chosen flexibly the guideline however has been to provide a minimum set of well defined and well resolvable tide response parameters for analysis of observations the aim of accuracy for tide displacements prediction is below 1 mm being the major perturbation of the solid earth tide ocean tide loading effects are computed and the accuracy of the models involved is discussed it appears that the major error source relates to those ocean tide frequencies for which global models are not available these frequencies form a continuum with a power spectrum being largely a result of non linear tide interaction the associated loading effects cannot be reliably interpolated from global tide models which are available only for a few distinct frequencies and which disregard tidal intermodulation thus an accuracy of 1 mm for computed loading tide displacements cannot always be achieved

can earth s rotation and tidal despinning drive plate tectonics abstract we re evaluate the possibility that earth s rotation contributes to plate tectonics on the basis of the following observations 1 plates move along a westerly polarized flow that forms an angle relative to the equator close to the revolution plane of the moon 2 plate boundaries are asymmetric being their geographic polarity the first order controlling parameter unlike recent analysis the slab dip is confirmed to be steeper along w directed subduction zones 3 the global seismicity depends on latitude and correlates with the decadal oscillations of the excess length of day lod 4 the earth s deceleration supplies energy to plate tectonics comparable to the computed budget dissipated by the deformation processes 5 the gutenberg richter law supports that the whole lithosphere is a self organized system in critical state i e a force is acting contemporaneously on all the plates and distributes the energy over the whole lithospheric shell a condition that can be satisfied by a force acting at the astronomical scale assuming an ultra low viscosity layer in the upper asthenosphere the horizontal component of the tidal oscillation and torque would be able to slowly shift the lithosphere relative to the mantle

stratigraphic and structural evolution of the middle miocene synvolcanic oregon idaho graben the oregon idaho graben is a newly identified north south trending synvolcanic graben in southeastern oregon and southwestern idaho within the middle miocene backarc rift system that extends 1100 km from southern nevada to southeastern washington the graben formed along the western margin of the north american craton shortly after the largest volumes of tholeiitic flood basalt erupted columbia river basalt group steens pueblo basalt basalt of malheur gorge basalt and latite unit of ekren et al 1981 1 rhyolite flows and ash flow tuffs 16 1 14 0 ma erupted from northeastern oregon dooley volcanics to northern nevada mcdermitt volcanic field shortly after the flood basalt was emplaced subsidence of the oregon idaho graben 15 5 15 3 ma coincides with eruption of rhyolite flows and caldera related ash flow tuffs from vents along the margins and within the graben mafic and silicic intragraben volcanism accompanied sedimentation from about 15 3 to 10 5 ma sedimentary and volcanic rocks from extrabasinal sources especially southwestern idaho were introduced periodically after initial subsidence the evolution of the oregon idaho graben is divided into three stages stage 1 15 3 14 3 ma followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben stage 2 14 3 12 6 ma movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc alkalic volcanism fine grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence synsedimentary hot spring alteration and precious metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits during stage 3 12 6 10 5 ma the subbasins were filled and graben wide fluviatile and lacustrine sedimentation resumed at about the same time renewed rhyolitic volcanism occurred on both flanks and tholeiitic volcanism resumed within the oregon idaho graben subsidence in the oregon idaho graben ceased as west northwest striking faults related to the formation of the western snake river plain became active 1 ref 28

a global geodetic reference frame from lageos ranging sl5 1ap a summary of the results obtained for a new comprehensive geodetic parameter solution from the analysis of lageos laser ranging data for the period may 1976 to the end of 1982 is presented estimates of each component of the polar motion and earth rotation the station coordinates the value of the earth s gravitational constant gm and the elements of the lageos orbit comprise this sl5 1ap solution the results differ from previously published values primarily through incorporation of more rigorous dynamic models for the ocean and solid earth tides the precision of the geodetic parameters are on average 5 marc sec polar motion 0 2 ms length of day better than 5 cm center of mass geodetic positioning 3 cm global baselines and 2 cm regional baselines an assessment of the contribution of systematic errors in the interstation distance determination is presented

influence of viscoelastic coupling on the axial rotation of the earth and its fluid core summary linear equations governing the axial rotation of the earth are dcveloped for a model with a maxwell homogeneous mantle and a homogeneous inviscid fluid core having a differential rotation relative to the mantle we find three eigenfrequencies for the axial perturbations in rotation the first one is relative to friction acting at the core mantle boundary and the others are relaxation modes associated with viscoelastic love numbers we point out the differential rotation of the core when the earth is submitted to zonal geophysical excitations like glaciation and deglaciation tidal torque or other processes occurring at the core mantle boundary cmb we show that the tidal deceleration involves an eastward drift of the core with respect to the mantle however for precise values of the mantle viscosity and of the frictional constant at the cmb we find that the last deglaciation involves a westward drift of the core with respect to the mantle which may be correlated with the observed westward drift of the geomagnetic field of the earth

the variation with frequency of the long period tides the oceanic response to long period tidal forcing in closed basins approaches equilibrium for periods much longer than a month for moderate values of dissipation at a given frequency the response is closest to equilibrium near the eastern wall of the basin global calculations of the mf and mm tides made using a 2 2 grid are presented changes in the length of day have also been computed for these tides

analysis of the seismicity detected in 1982 1984 in the northern peninsular ranges of baja california the san miguel fault system strikes in the northwest direction within the peninsular ranges of baja california the study concentrates primarily on the southern sector of the san miguel fault ssm which is about 20 km long and had a large rupture in 1956 there is a high rate of seismicity along the ssm in a time period of 23 days 109 events were located with duration magnitudes between 1 4 and 2 8 focal depths are between 5 and 15 km epicentral locations show three clusters of seismicity two clusters are on the ssm another cluster is closer to a normal fault that strikes parallel 8 km to the southwest of ssm the seismicity is stationary with an average of 20 events per day a b value of 0 92 0 02 is characteristic for the background seismicity a larger b value of 1 4 0 07 is associated with the cluster at the northwest end of ssm those inconsistent b values suggest a heterogeneous state of stress most fault plane solutions in the northern peninsular ranges of baja california indicate strike slip motions one normal mechanism is also observed the greatest principal stress is oriented approximately north south