dynamics of wind forced coherent anticyclones in the open ocean we numerically study the dynamics of coherent anticyclonic eddies in the ocean interior for the hydrostatic rotating stably stratified turbulence we use a high resolution primitive equation model forced by small scale winds in an idealized configuration many properties of the horizontal motions are found to be similar to those of two dimensional and quasi geostrophic turbulence major differences are a strong cyclone anticyclone asymmetry linked to the straining field exerted by vortex rossby waves which is also found in shallow water flows and the complex structure of the vertical velocity field which we analyze in detail locally the motion can become strongly ageostrophic and vertical velocities associated with vortices can reach magnitudes and levels of spatial complexity akin to those reported for frontal regions transport and mixing properties of the flow field are further investigated by analyzing lagrangian trajectories particles released in the pycnocline undergo large vertical excursions because of the vertical velocities associated to the vortices with potentially important consequences for marine ecosystem dynamics

stalling of near inertial waves in a cyclone 1 observations at the edge of the loop current after hurricane katrina show inertial energy amplified at a depth of approximately 600 700 m ray analysis using the eddy field obtained from a numerical simulation with data assimilation suggests that the amplification is due to inertial motions stalled in a deep cyclone

enhanced turbulence and energy dissipation at ocean fronts the ocean surface boundary layer mediates air sea exchange in the classical paradigm and in current climate models its turbulence is driven by atmospheric forcing observations at a 1 kilometer wide front within the kuroshio current indicate that the rate of energy dissipation within the boundary layer is enhanced by one to two orders of magnitude suggesting that the front rather than the atmospheric forcing supplied the energy for the turbulence the data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence the resulting boundary layer is stratified in contrast to the classically well mixed layer these effects will be strongest at the intense fronts found in the kuroshio current the gulf stream and the antarctic circumpolar current all of which are key players in the climate system

cold filamentary intensification and oceanic surface convergence lines 1 the reflectance images of the oceanic surface are full of surfactant lines created by surface velocity convergences with typical line spacings of 0 1 10 km i e in the submesoscale range here we propose a dynamical explanation by the process of filamentary intensification that shrinks the transverse scale of rectilinear surface temperature extrema at a super exponential rate by the action of horizontal deformation flows associated with mesoscale eddies this process is analogous to deformation induced frontogenesis with sharpening temperature gradients accelerated by its ageostrophic secondary circulation in the particular case of cold filaments the shrinking rate surface convergence and downwelling are especially strong because of the configuration of ageostrophic acceleration this behavior is demonstrated theoretically with idealized two dimensional solutions and illustrated in a realistic regional circulation simulation near peru

constraints on the timing and duration of orogenic events by combined lu hf and sm nd geochronology an example from the grenville orogeny abstract grenville age basement rocks of the virginia blue ridge province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent rodinia although the igneous origins of these rocks have been the subject of much previous work few studies have examined the complex metamorphic history of these rocks during the grenville orogeny to better constrain the timing and duration of metamorphism we determined lu hf and sm nd garnet and u pb zircon rim dates of garnet bearing rocks exposed in the virginia blue ridge multiple periods of mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before 1065 ma and subsequent local episodes occurring at least until 1000 ma zircon overgrowths and monazite dates suggest metamorphic events between 1050 and 960 ma our new lu hf garnet dates range from 1032 3 to 1019 3 ma new sm nd dates determined on the same garnet dissolutions define a younger range from 965 6 to 949 6 ma our zircon dates are consistent with previously measured ages 1090 940 ma and often overlap with the lu hf garnet dates from the same samples there is a large and systematic difference 60 80 m y between the garnet dates from the two isotopic systems although sm nd dates slightly younger than those recorded by lu hf are not uncommon the large discrepancies between the two systems that we see here are unusual the dated garnets are unzoned and yield lu hf dates that coincide with the zircon overgrowth dates sm nd ages overlap with the younger monazite dates we interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems further these data require slow cooling following large scale crustal thickening after the assembly of rodinia during grenville orogenesis these data illustrate that the use of the coupled lu hf and sm nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events

lagrangian assessment of simulated surface current dispersion in the coastal ocean 1 a purely lagrangian assessment of dispersion from modeled surface current trajectories in the coastal ocean is presented modeled trajectories come from roms simulations for the southern california bight during the 1996 through 1999 period data are from surface current trajectories collected primarily in the santa barbara channel with code style drifters distributions of particle positions from trajectories emanating from launch locations within 10 kilometers of the coast throughout the santa barbara channel that advect for one through four days lagrangian pdfs are evaluated descriptively and quantitatively the two dimensional kolmogorov smirnov k s statistical test for comparing discrete sampled data with a known probability distribution is the quantitative basis in general dispersion distributions from observations are similar to lagrangian pdfs computed from modeled trajectories and the k s statistic quantifies this accordingly a few specific regions of poor model data agreement are indicated and discussed the purely lagrangian assessment elucidates an improved understanding of model performance and ocean circulation beyond that offered in a eulerian sense and is necessary when modeled trajectories are utilized for applied oceanographic and marine ecology problems

material transport in oceanic gyres part i phenomenology abstract material spreading and mixing by oceanic mesoscale eddies are analyzed in an idealized numerical model of the wind driven midlatitude oceanic circulation the main focus is on the regime with large reynolds number re and vigorous mesoscale eddies although brief comparisons are made with less turbulent solutions at smaller re the analyses are based on ensembles of lagrangian particle trajectories the authors find that tracer transport by mesoscale eddies differs in many ways from the commonly used model of homogeneous isotropic eddy diffusion the single particle dispersion which describes the spreading process is generally anisotropic and inhomogeneous and in most places it is not diffusive i e not linear in time during intermediate time intervals after tracer release in most of the basin and especially in the deep layers subdiffusive single particle dispersion occurs due to long time trapping of material by coherent structures such as vortices near the strong currents and planeta

friction frontogenesis and the stratification of the surface mixed layer the generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air sea fluxes and stresses in this paper it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum a classic example is the strong restratification resulting from frontogenesis in regions of confluent flow frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving ekman flows that differentially advect density ekman flow induced during spindown always tends to restratify the fluid while wind driven ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front i e directed against or with the geostrophic shear respectively scalings are constructed for the relative importance of friction versus frontogenesis in the restratification of the mixed layer and are tested using numerical experiments of mixed layer fronts forced by both winds and a strain field the scalings suggest and the numerical experiments confirm that for wind stress magnitudes mixed layer depths and cross front density gradients typical of the ocean wind induced friction often dominates frontogenesis in the modification of the stratification of the upper ocean the experiments reveal that wind induced destratification is weaker in magnitude than restratification because the stratification generated by up front winds confines the turbulent stress to a depth shallower than the ekman layer which enhances the frictional force ekman flow and differential advection of density frictional destratification is further reduced over restratification because the stress associated with the geostrophic shear at the surface tends to compensate a down front wind stress

regional change of coda q in the oceanic lithosphere coda q of 22 local earthquakes recorded on islands located in young and old oceanic lithosphere areas has been measured we found that the q values at periods of from 1 to 2 seconds increase from young to old ocean lithosphere in the same way as for the continent where the intensity of current tectonic activity shows a strong negative correlation with the coda q the q values increase with frequency proportional to fn where n was found to be 0 46 0 61 with the higher values for younger oceanic lithosphere which is again similar to the continents where the active regions have higher n values

new constraints on relative motion between the pacific plate and baja california microplate mexico from gps measurements summary we present a new surface velocity field for baja california using gps data to test the rigidity of this microplate calculate its motion in a global reference frame determine its relative motion with respect to the north american and the pacific plates and compare those results to our estimate for pacific north america motion determination of pacific plate motion is improved by the inclusion of four sites from the south pacific sea level and climate monitoring project these analyses reveal that baja california moves as a quasi rigid block but at a slower rate in the same direction as the pacific plate relative to north america this is consistent with seismic activity along the western edge of baja california the baja california shear zone and may reflect resistance to motion of the eastern edge of the pacific plate caused by the big bend of the san andreas fault and the transverse ranges in southern california

diagnosing lateral mixing in the upper ocean with virtual tracers spatial and temporal resolution dependence several recent studies diagnose lateral stirring and mixing in the upper ocean using altimetry derived velocity fields to advect virtual particles and fields offline however the limited spatiotemporal resolution of altimetric maps leads to errors in the inferred diagnostics because unresolved scales are necessarily imperfectly modeled the authors examine a range of tracer diagnostics in two models of baroclinic turbulence the standard phillips model in which dispersion is controlled by large scale eddies and the eady model where dispersion is determined by local scales of motion these models serve as a useful best and worst case comparison and a valuable test of the resolution sensitivity of tracer diagnostics the effect of unresolved scales is studied by advecting tracers using model velocity fields subsampled in space and time and comparing the derived tracer diagnostics with their true value obtained from the fully resolved flow the authors find that eddy diffusivity and absolute dispersion which are governed by largescaledynamics areinsensitivetospatialsamplingerrorin eitherflow measuresthatdependstronglyonsmall scales such as relative dispersion andfinite timelyapunov exponents are highly sensitive to spatial sampling in the eady model temporal sampling error is found to have a more complicated behavior because of the onset of particle overshoot leading to scrambling of lagrangian diagnostics this leads to a potential restrictionontheutilityofrawaltimetrymapsforstudyingmixingintheupperocean theauthorsconcludethat offline diagnostics of mixing in ocean flows with an energized submesoscale should be viewed with some caution

bath s law and its relation to the tectonic environment a case study for earthquakes in mexico abstract we studied 66 mainshocks and their largest aftershocks in the mexican subduction zone and in the gulf of california with magnitudes in the range of 5 2 m w

pattern formation at the ocean surface sargassum distribution and the role of the eddy field positively buoyant organisms such as the macroalga sargassum and the cyanobacterium trichodesmium often form surface accumulations visible in satellite imagery that have lateral scale separation of tens of kilometers and cannot be explained by langmuir circulation here we discuss the accumulation of floating materials in the ocean in presence of meso and submesoscale activity using high resolution simulations of the ocean mesoscale in both idealized a 3 dimensional box where coherent eddies are forced by small scale winds and realistic domains western gulf of mexico where extensive concentrations of floating sargassum have been recorded in satellite images we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few tens of meters below it such distribution does not resemble that observed for passive tracers in quasi geostrophic turbulence the strong divergence and convergence zones generated at the surface by ageostrophic processes in the submesoscale range are responsible for the creation of areas where the floating material accumulates floating particles are expelled from the core of mesoscale eddies and concentrate in convergence regions in patterns comparable to the ones observed through the satellite images in light of those results sargassum and or trichodesmium may provide a useful proxy to track convergence divergence processes resulting from ageostrophic processes at the ocean surface

a bottom driven mechanism for distributed faulting in the gulf of california rift abstract observations of active faulting in the continent ocean transition of the northern gulf of california show multiple oblique slip faults distributed in a 200 70 km 2 area developed some time after a westward relocation of the plate boundary at 2 ma in contrast north and south of this broad pull apart structure major transform faults accommodate pacific north america plate motion here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the northern gulf where basal shear is distributed between the cerro prieto strike slip fault southernmost fault of the san andreas fault system and the ballenas transform fault we hypothesize that in oblique extensional settings whether deformation is partitioned in a few dip slip and strike slip faults or in numerous oblique slip faults may depend on 1 bottom driven distributed extension and shear deformation of the lower crust or upper mantle and 2 the rift obliquity to test this idea we explore the effects of bottom driven shear on the deformation of a brittle elastic plastic layer with the help of pseudo three dimensional numerical models that include side forces strain localization results when the basal shear abruptly increases in a step function manner while oblique slip on numerous faults dominates when basal shear is distributed we further explore how the style of faulting varies with obliquity and demonstrate that the style of delocalized faulting observed in the northern gulf of california is reproduced in models with an obliquity of 0 7 and distributed basal shear boundary conditions consistent with the interpreted obliquity and boundary conditions of the study area

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

wind ringing of the ocean in presence of mesoscale eddies 1 recent findings have highlighted the impact of high frequency hf winds on the inertial motions and the consequences on the large scale oceanic circulation wunsch and ferrari 2004 within this context the present study focuses on the role of intermediate scales related to oceanic mesoscale eddies results show that a turbulent eddy field does not affect the impact of hf winds on the inertial motions however the eddies efficiently and permanently concentrate the wind forced inertial motions in small scale anticyclonic structures this leads to a spatially heterogeneous vertical mixing strongly related to the eddy field properties

new kinematic models for pacific north america motion from 3 ma to present i evidence for steady motion and biases in the nuvel 1a model we use velocities derived from 2 4 5 years of continuous gps observations at 21 sites on the pacific and north american plates along with a subset of the nuvel 1a data to examine the steadiness of pacific north america motion since 3 16 ma the transfer of baja california to the pacific plate and the magnitude of biases in the nuvel 1a estimate of pacific north america motion we find that pacific north america motion has remained steady since 3 16 ma but at rates significantly faster than predicted by nuvel 1a in the vicinity of baja california our gps derived model and recent seafloor spreading rates in the southern gulf of california both indicate that the nuvel 1a model underestimates pacific north america rates by 4 2 mm yr 1 steady pacific north america motion since 3 16 myr and increasing seafloor spreading rates since 3 58 myr in the gulf of california imply that pacific north america motion was partitioned between seafloor spreading in the gulf of california and decelerating slip along faults in or offshore from the baja peninsula

eddy mean flow decomposition and eddy diffusivity estimates in the tropical pacific ocean 1 methodology 1 eddy diffusivity of the surface velocity field in the tropical pacific ocean was estimated using satellite tracked drifting buoys 1979 through mid 1996 the tropical pacific surface current system is characterized by nonstationarity strong meridional shear and an energetic mesoscale velocity field eddy diffusivity may be defined as the integral of the autocovariance of lagrangian eddy velocities requiring both stationary and homogeneous statistics of the eddy field eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned 1 spatially to group data that have similar dispersion characteristics and 2 temporally to create stationary eddy statistics zonal diffusivity estimates are up to 7 times larger than meridional diffusivity estimates in the high eddy energy regions this anisotropy is associated with the meridional mesoscale wave motion i e by equatorial and tropical instability waves that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion at least for initial time lags simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics an independent confirmation of the diffusivity estimate in the central eastern pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy reynolds stress flux divergence our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field

westward migration of extension in the northern gulf of california mexico interpretation of industry seismic lines indicates that the eastern margin of the northern gulf of california contains the inactive adair tepoca and upper tiburon basins the western margin is active and includes the wagner consag upper delfin and lower delfin basins these basin systems are separated by a wide basement high across which the upper strata in the inactive basins correspond to the middle and lower strata in the active basins recording the westward migration of strain and subsidence during late pliocene time our results illustrate the formation of an abandoned rift margin along the eastern gulf of california

mean flow in the gulf of mexico abstract several independent data sources suggest that there is a net upper layer mass flux o 3 sv sv 106 m3 s 1 to the west in the central gulf of mexico even though the western gulf is a closed basin a plausible explanation is that this net flux is pumped downward by the convergent wind driven ekman pumping as is typical of all midlatitude anticlyclonic gyres the downward flux can follow isopycnals to depths o 500 600 m and deeper by eddy mixing a mechanism for forcing deep water to the south through the yucatan channel is provided by the intrusion and ring shedding cycle of the loop current potential vorticity maps show that a deep flow from the western gulf back to the yucatan channel is likely

two dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large scale deformation 1 ageostrophic secondary circulation asc in frontal data sets have been previously diagnosed using only adiabatic versions of omega equations this study includes the effect of vertical mixing on such diagnostics modified quasi geostrophic and semigeostrophic balance equations are examined for the existing azores front data sets and idealized fronts in the presence of diabatic vertical mixing as well as a large scale deformation field the relevant dimensionless parameter representing the relative importance of large scale deformation and vertical mixing is the inverse of the ekman number based on the deformation rate h2 ao where is large scale deformation rate h is vertical scale of the front and ao is the maximum vertical eddy viscosity when this parameter is smaller than o 102 the influence of vertical mixing may be important other relevant parameters include burger number bu and prandtl number pr the vertical velocity in ascs caused by diabatic mixing in all diagnostics solutions except for large bu cases shows good agreement with the garrett loder scaling the structure of ascs is found to be sensitive to pr the diagnostics of ascs for the azores front data set indicates that the vertical mixing intensifies the asc in the upper ocean 100 m and should not be neglected when the large scale deformation rate is o 10 6 s 1 or smaller the magnitude of vertical velocity in ascs due to mixing contributions reaches o 10 m d 1 with downwelling on the dense side and upwelling on the less dense side of azores front

rayleigh wave dispersion measurements reveal low velocity zones beneath the new crust in the gulf of california rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the gulf of california low velocity zones lvzs are found on axis between 26 and 50 km depth beneath the guaymas basin but mostly off axis under the other rift basins with the largest feature underlying the ballenas transform fault we interpret the broadly distributed lvzs as regions of partial melting in a solid mantle matrix the pathway for melt migration and focusing is more complex than an axis centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments we also consider the existence of solid lower continental crust in the gulf north of the guaymas basin where the association of the lvzs with asthenospheric upwelling suggests lateral flow assisted by a heat source these results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

evolution and kinematics of a loop current eddy in the gulf of mexico during 1985 the large eddy that broke off from the loop current in july 1985 was the most extensively studied eddy ever to appear in the gulf of mexico other investigators have described its early evolution based on lagrangian drifters and its later evolution using moored current meters in the western gulf this paper provides additional insight on the early evolution of the eddy using results from air dropped expendable bathythermographs and air dropped expendable current profilers in early may a hydrographic ship survey in mid july and a detailed ship survey in august using expendable bathythermographs and a current profiler the may survey established a center of circulation at about 26 n but showed that the eddy had not separated from the loop current a maximum velocity of 171 cm s was observed near the northern edge of the feature the evidence suggests that a large elongated eddy then separated from the loop current and later split into two smaller eddies the july hydrographic cruise showed a clear separation of the large eddy from the loop current to the southeast two weeks later the august survey showed an asymmetric eddy with the maximum surface current 178 cm s south of the center of circulation and 132 cm s to the north a western eddy named ghost eddy then separated from an eastern eddy named fast eddy using the current profiles and data from the drifters we constructed a simple kinematic feature model for eddies in the gulf of mexico

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

local and regional p wave spectral attenuation model for iran a robust frequency dependent local and regional p wave attenuation model is estimated for continental paths in the iranian plateau in order to calculate the average attenuation parameters 46 337 vertical component waveforms related to 9267 earthquakes which are recorded at the iranian seismological center irsc stations have been selected in the distance range 10 1000 km the majority of the event s magnitudes are less than 4 5 this collection of records provides high spatial ray path coverage results indicate that the shape of attenuation p wave curve versus distance is not uniform and has three distinct sections with hinges at 90 and 175 km a trilinear model for attenuation of p wave amplitude in the frequency range 1 10 hz is proposed in this study fourier spectral amplitudes are found to decay as r 1 2 where r is hypocentral distance corresponding to geometric spreading within 90 km from the source there is a section from 90 to 175 km where the attenuation is described as r 0 8 and the attenuation is described well beyond 175 km by r 1 3 moreover the average quality factor for pg and pn waves qpg and qpn related to anelastic attenuation is obtained as qpg 54 2 2 6 f 1 0096 0 07 and qpn 306 8 7 4 f 0 51 0 05 there is a good agreement between the results of the model and observations also the attenuation model shows compatibility with the recent regional studies from the results it turns out that the amplitude of p waves attenuates more rapidly in comparison with the global models in local distances

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

mesoscale to submesoscale transition in the california current system part ii frontal processes this is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized equilibrium subtropical eastern boundary upwelling current system similar to the california current the emergent upper ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives using a combination of composite averaging and separation of distinctive subregions of the flow the initiating dynamical process for the transition is near surface frontogenesis the frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics i e uniform interior potential vorticity q and diagnostic force balance in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field however there are significant behavioral differences compared to the surface dynamics model wind stress acts on fronts through nonlinear ekman transport and creation and destruction of potential vorticity the strain induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events submesoscale vortices and excitation of even smaller scale flows intermittent submesoscale breakdown of geostrophic and gradient wind force balance occurs during the intense frontogenesis and frontal instability events

submesoscale impacts on horizontal and vertical transport in the gulf of mexico 1 impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions 1 and 5 km the focus is the northwestern gulf of mexico where the loop current eddies are surrounded by smaller vortices by increasing the horizontal resolution the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities inside the coherent eddies and at their peripheries increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer but are found at all depths horizontal velocities on the contrary are similar lagrangian isobaric tracers are deployed close to the surface and at 100 m and three dimensional neutrally buoyant particles are released close to the surface at the base of the mixed layer and at 100 m the modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution close to the ocean surface however convergence zones generated by submesoscale ageostrophic motions and resolved at 1 km resolution influence the details of the tracer distributions vertical dispersion increases by several folds for increasing resolution at all depths explored with the largest differences found close to the surface therefore submesoscales processes play a fundamental role in driving vertical transport in eddy dominated flows both within and below the mixed layer for times comparable to the eulerian time scale

mixed layer instabilities and restratification abstract the restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied the lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical these instabilities which are referred to as mixed layer instabilities mlis differ from instabilities in the ocean interior because of the weak surface stratification spatial scales are o 1 10 km and growth time scales are on the order of a day linear stability analysis and fully nonlinear simulations are used to study mlis and their impact on mixed layer restratification the main result is that mlis are a leading order process in the ml heat budget acting to constantly restratify the surface ocean climate and regional ocean models do not resolve the scales associated with mlis and are likely to underestimate the rate of ml restratification and consequently suffer from a bias in sea surface temperatures and ml depths in a

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

an attenuation study of body waves in the south central region of the gulf of california mexico abstract we studied the seismic attenuation of body waves in the south central region of the gulf of california goc with records from the network of autonomously recording seismographs of baja california nars baja from the centro de investigacion cientifica y de educacion superior de ensenada s broadband seismological network of the goc resban and from the ocean bottom seismographs obs deployed as part of the sea of cortez ocean bottom array experiment scooba we examine 27 well located earthquakes reported in sumy et al 2013 that occurred from october 2005 to october 2006 with magnitudes m w between 3 5 and 4 8 we estimated s wave site effects by calculating horizontal to vertical spectral ratios and determined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes of 21 frequencies between 0 13 and 12 59 hz for the scooba obs stations and 19 frequencies between 0 16 and 7 94 hz for nars baja and resban stations we calculated the geometrical spreading and the attenuation 1 q factors for two distance intervals 10 120 and 120 220 km respectively for each frequency considered the estimates of q obtained with the scooba obs records for the interval 10 120 km indicate that the p waves attenuate more than s waves q p 34 1 2 f 0 82 0 10 q s 59 1 1 f 0 90 0 03 for frequencies between 0 6 and 12 6 hz whereas for the 120 220 km interval where ray paths travel deeper s waves attenuate more than p waves q p 117 1 3 f 0 44 0 19 q s 51 1 2 f 1 12 0 11 the estimates of q obtained using nars baja and resban records within 10 120 km indicate that p waves attenuate more than s waves q p 69 1 2 f 0 87 0 16 q s 176 1 4 f 0 61 0 26 at frequencies between 0 3 and 6 3 hz whereas at the 120 220 km distance interval s waves attenuate slightly more than p waves q p 39 1 1 f 0 64 0 06 q s 48 1 1 f 0 37 0 07 at high frequencies f 3 hz these results based on a unique obs dataset provide an indirect mean to constrain future models of the thermal structure beneath the goc

rapid changes in mixed layer stratification driven by submesoscale instabilities and winds 1 submesoscale eddies generated by baroclinic instability of upper ocean fronts lead to rapid restratification of the mixed layer on a time scale of days this restratification can be opposed by a down front wind stress acting in the direction of the geostrophic velocity that drives a surface ekman flow from the dense side to the light side of the front to arrest the slumping of isopycnals a scaling diagnostic is suggested to determine whether the effect of eddies or wind dominates under different conditions using a numerical model we investigate the juxtaposition of submesoscale eddies and down front winds acting on the mixed layer by estimating the eddy induced overturning stream function in the mixed layer we separate the along and cross isopycnal fluxes of buoyancy associated with submesoscale mixed layer eddies and demonstrate the need for parameterization of the advective along isopycnal flux though the cross front transport of buoyancy induced by the down front component of the wind opposes restratification by mixed layer eddies it becomes diminished as the eddies and growth of the frontal instability disrupt alignment between the wind and frontal axis

coda q in different tectonic areas influence of processing parameters abstract published results of coda q show a large variation in values these variations are often claimed to be related to different tectonics whereas they might just be related to using different assumptions in the processing leading to different input parameters for the analysis in this study the effect of using different processing parameters is investigated and significant differences particularly at low frequencies are observed we find a new set of optimal parameters which we recommend using in future studies using a short lapse time of 30 s and optimal parameters data from both similar and very different tectonic regions are used to calculate coda q using the same program and the same parameters the regions considered are eastern anatolia the azores jan mayen northwestern and central argentina the shanxi rift system in china and southwestern norway we obtain the following relations eastern anatolia q 88 f 0 66 azores q 86 f 0 70 jan mayen q 90 f 0 72 northwestern and central argentina q 89 f 0 94 shanxi rift system q 99 f 0 89 and southwestern norway q 124 f 0 91 the results show that coda q is very similar for regions of similar tectonics and significantly different for regions with varying tectonics using alternative more common parameters gives different q but the regional differences remain so which parameters to use to get correct coda q values is still uncertain however coda q can clearly distinguish different tectonic areas provided identical processing parameters are used even if they are not optimal

lagrangian drifter dispersion in the southwestern atlantic ocean abstractin the framework of monitoring by ocean drifters mondo project a set of lagrangian drifters were released in proximity of the brazil current the western branch of the subtropical gyre in the south atlantic ocean the experimental strategy of deploying part of the buoys in clusters offers the opportunity to examine relative dispersion on a wide range of scales adopting a dynamical systems approach the authors focus their attention on scale dependent indicators like the finite scale lyapunov exponent fsle and the finite scale mean square relative velocity fsrv between two drifters as a function of their separation and compare them with classic time dependent statistical quantities like the mean square relative displacement between two drifters and the effective diffusivity as functions of the time lag from the release the authors find that dependently on the given observable the quasigeostrophic turbulence scenario is overall compatible with their data analysis with discrepancies fr

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

mesoscale to submesoscale transition in the california current system part i flow structure eddy flux and observational tests in computational simulations of an idealized subtropical eastern boundary upwelling current system similar to the california current a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to o 1 km this first paper in a series of three describes the transition in terms of the emergent flow structure and the associated time averaged eddy fluxes in addition to the mesoscale eddies that arise from a primary instability of the alongshore wind driven currents significant energy is transferred into submesoscale fronts and vortices in the upper ocean the submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers in turn some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll up vortices associated with this phenomenon are a large vertical vorticity and rossby number a large vertical velocity relatively flat horizontal spectra contrary to the prevailing view of mesoscale dynamics a large vertical buoyancy flux acting to restratify the upper ocean a submesoscale energy conversion from potential to kinetic a significant spatial and temporal intermittency in the upper ocean and material exchanges between the surface boundary layer and pycnocline comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean with characteristics similar to the simulations

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

site response attenuation and source spectra of s waves along the guerrero mexico subduction zone acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra the site response and the quality factor q of s waves this method is applied to moderate earthquakes 4 0 0 8 hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions when the source spectra are interpreted in terms of brune s model the stress drops obtained are less than 20 bars for the four larger events analyzed the site functions obtained indicate that although all the stations used are on rock the effect of the near surface geology caused important amplifications at some stations

do altimeter wavenumber spectra agree with the interior or surface quasigeostrophic theory in high eddy energy regions it is generally assumed that sea level wavenumber spectra compare well with quasigeostrophic qg turbulence models and that spectral slopes are close to the expected k 5 law this issue is revisited here sea level wavenumber spectra in the gulf stream kuroshio and agulhas regions are estimated using the most recent altimeter datasets the ocean topography experiment topex poseidon jason 1 the environmental satellite envisat and the geosat follow on the authors show that spectral slopes in the mesoscale band are significantly different from a k 5 law in disagreement with the qg turbulence theory however they very closely follow a k 11 3 slope which indicates that the surface quasigeostrophic theory sqg is a much better dynamical framework than the qg turbulence theory to describe the ocean surface dynamics because of the specific properties of the sqg theory these results offer new perspectives for the analysis and interpretation of satellite data

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

fast deep sinking in mediterranean eddies i large up to 0 03 m s 1 downward vertical water velocities w are observed using yearlong moored adcp at 2400 m in the deep mediterranean sea s algerian basin once every 2 3 months w rapidly increases to o 0 01 0 002 m s 1 before slowly decreasing during a few weeks in association with the passage of mesoscale eddies formed nearby these amplitudes of negative w are o 100 times larger than those commonly linked to mesoscale eddies near the surface subduction and o 10 times larger than settling velocities for marine snow our observations suggest that mesoscale eddies which are important for biological productivity near the surface can also convect nearly fresh material down to the bottom 3000 m there within a few days

zircon u pb geochronology and nd pb isotope geochemistry of blue ridge basement in the eastern great smoky mountains u s a implications for the proterozoic tectonic evolution of the southeastern laurentian margin the mesoproterozoic to paleozoic history of the southeastern laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events the great smoky mountains basement complex gsmbc within the southern appalachian blue ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of laurentian margin evolution from ca 1 9 ga to 450 ma the gsmbc consists primarily of 1 1 34 to 1 31 ga pre elzevirian granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in appalachian grenville massifs correlated with pre elzevirian crustal components in the adirondack green mountains new jersey highlands and french broad massifs 2 ca l 15 to 1 05 ga augen and granitic orthogneiss produced during shawinigan and ottawan phases of grenville age magmatism and metamorphism respectively and 3 paragneiss derived from protoliths with either grenville age 1 1 1 0 ga or post grenville neoproterozoic depositional ages based on presence absence of ca 1 0 ga metamorphic zircon and 1 9 to 1 1 ga detrital zircon all lithologies experienced taconian metamorphism and variable migmatization pre ottawan paragneiss exhibits major detrital zircon ages modes at 2 0 to 1 6 and 1 4 to 1 3 ga that require a component of older proterozoic crust in the sediment source region detrital zircon grains in post ottawan paragneiss exhibits the full spectrum of grenville age modes that correlate with the five phases of grenville magmatic metamorphic events in eastern laurentia these paragneiss samples also contain scattered 750 to 600 ma detrital zircon grains that constrain their maximum depositional age to late neoproterozoic the sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all grenville crustal age components during post orogenic exhumation and crustal extension leading up to late neoproterozoic breakup of rodinia most zircon u pb age systematics exhibit variable discordance that can be attributed to disturbance of the u pb system and or new zircon growth during either high grade ottawan ca 1 04 ga or taconian ca 0 46 ga regional metamorphism and migmatization neodymium tdm model ages for granodioritic orthogneiss and paragneiss range from 1 8 to 1 6 ga indicating that the rocks were derived from recycling of proterozoic crust that is they are not juvenile consistent with the 1 9 to 1 6 ga detrital zircon grains in pre ottawan paragneiss and with 1 8 to 1 7 ga inherited zircon in some 1 33 ga orthogneisses and a 1 35 ga xenolith whole rock pb isotope compositions of gsmbc rocks overlap the field of compositions characteristic of amazonian crust and of other basement rocks from the south central appalachians the pb isotopes and geochronology in orthogneiss mafic xenoliths and pre ottawan paragneiss are consistent with a correlation of the gsmbc with the mars hill terrane within the french broad massif and with the greater grenvillian south central appalachian basement scab that is considered exotic to laurentia and transferred during rodinian collision prior to ca 1 2 ga similarities in protolith ages and pb isotopes point to the paragua terrane of amazonia southwestern brazil and eastern bolivia as a likely match for scab initial amazonia laurentia collision occurred at ca 1 35 to 1 32 ga with final transfer of scab to laurentia occurring after 1 20 ga within the sinistral oblique collision zone between laurentia and amazonia defined by previous workers

source functions and path effects from earthquakes in the farallon transform fault region gulf of california mexico that occurred on october 2013 we determined source spectral functions q and site effects using regional records of body waves from the october 19 2013 m w 6 6 earthquake and eight aftershocks located 90 km east of loreto baja california sur mexico we also analyzed records from a foreshock with magnitude 3 3 that occurred 47 days before the mainshock the epicenters of this sequence are located in the south central region of the gulf of california goc near and on the farallon transform fault this is one of the most active regions of the goc where most of the large earthquakes have strike slip mechanisms based on the distribution of the aftershocks the rupture propagated northwest with a rupture length of approximately 27 km we calculated 3 component p and s wave spectra from ten events recorded by eleven stations of the broadband seismological network of the goc resban these stations are located around the goc and provide good azimuthal coverage the average station gap is 39 the spectral records were corrected for site effects which were estimated calculating average spectral ratios between horizontal and vertical components hvsr method the site corrected spectra were then inverted to determine the source functions and to estimate the attenuation quality factor q the values of q resulting from the spectral inversion can be approximated by the relations q text p 48 1 pm 1 1 f 0 88 pm 0 04 and q text s 135 4 pm 1 1 f 0 58 pm 0 03 and are consistent with previous estimates reported by vidales basurto et al bull seism soc am 104 2027 2042 2014 for the south central goc the stress drop estimates obtained using the 2 model are below 1 7 mpa with the highest stress drops determined for the mainshock and the aftershocks located in the ridge zone we used the values of q obtained to recalculate source and site effects with a different spectral inversion scheme we found that sites with low s wave amplification also tend to have low p wave amplification except for stations bahb guyb and sfqb located on igneous rocks where the p wave site amplification is higher

the 4 january 2006 mw 6 6 san pedro martir earthquake example of an earthquake for calibrating excitation and attenuation studiesthe mw 6 6 san pedro martir earthquake for calibrating excitation and attenuation studies

diagnosis of the three dimensional circulation associated with mesoscale motion in the california current a high resolution upper ocean survey of a cyclonic jet meander and an adjacent cyclonic eddy in the california current region near 388n 1268w was conducted as part of the summer of 1993 eastern boundary currents program temperature and salinity were measured from a seasoar vehicle and velocity was measured by shipboard acoustic doppler current profiler adcp seasoar data show a density front at a depth of 70 100 m with strong cyclonic curvature the geostrophic velocity fields referenced to the adcp data at 200 m show a strong surface intensified jet maximum speed of 0 9 m s 21 that follows the density front along a cyclonic meander relative vorticities within the jet are large ranging from 20 8 f to 11 2 f where f is the local coriolis parameter the seasoar density and adcp velocity data are used to diagnose the vertical velocity via the q vector form of the quasigeostrophic omega equation the diagnosed vertical velocity field shows a maximum speed of 40 45 m d21 the lateral distribution of vertical velocity is characterized by two length scales a large 75 km pattern where there is downwelling upstream and upwelling downstream of the cyclonic bend and smaller patches arrayed along the jet core with diameters of 20 30 km geostrophic streamline analysis of vertical velocity indicates that water parcels make net vertical excursions of 20 30 m over 2 3 days resulting in net vertical velocities of 7 15 m d21 water parcels moving along geostrophic streamlines experience maximum vertical velocities in the regions of maximum alongstream change in relative vorticity an indication of potential vorticity conservation

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

mechanisms for vertical nutrient transport within a north atlantic mesoscale eddy abstract prompted by observational evidence for an enhanced source of surface nutrients within an anticyclonic eddy in the ne atlantic we investigate vertical transport processes that may produce such a phenomenon for the eddy investigated the dominant mechanism is found to be ageostrophic circulation resulting from a perturbation of the circular flow of the eddy this can produce upwelling velocities of order 10 m d 1 ekman pumping due to wind stress on the eddy also produces upwelling within but on a smaller scale of 0 5 m d 1 there is no evidence that self propagation of the eddy leads to an enhanced nutrient flux by displacement upwards of nutrient laden deeper isopycnals over the core of the eddy deeper winter mixing within the eddy relative to the surroundings however may contribute to the elevated nutrient levels

velocity and hydrographic structure of two gulf of mexico warm core rings results from an extensive survey of two gulf of mexico warm core rings are presented the data were taken during a 1 week period in december 1983 using a shipboard acoustic doppler current profiler adcp expendable current probes xcp expendable bathythermographs xbt and a ctd two rings were observed an older one in the western gulf adjacent to the west texas shelf and one just separated from the loop current the western ring was of order 200 km in diameter with peak currents of 1 m s 1 at the 100 m level water properties were uniform and characteristic of the high salinity caribbean subtropical underwater the eastern ring was of order 300 km in diameter with peak currents of near 2 m s 1 at the 100 m level water below 200 m is of caribbean origin while the surface waters show more variability suggesting some inflow of gulf of mexico common water the evolution of the rings is also described based on the cruise measurements satellite avhrr and other observations of opportunity

the production and dissipation of compensated thermohaline variance by mesoscale stirring abstract temperature salinity profiles from the region studied in the north atlantic tracer release experiment natre show large isopycnal excursions at depths just below the thermocline it is proposed here that these thermohaline filaments result from the mesoscale stirring of large scale temperature and salinity gradients by geostrophic turbulence resulting in a direct cascade of thermohaline variance to small scales this hypothesis is investigated as follows measurements from natre are used to generate mean temperature salinity and shear profiles the mean stratification and shear are used as the background state in a high resolution horizontally homogeneous quasigeostrophic model the mean state is baroclinically unstable and the model produces a vigorous eddy field temperature and salinity are stirred laterally in each density layer by the geostrophic velocity and vertical advection is by the ageostrophic velocity the simulated temperature salinity diagram exhibits fluctuations at depths ju

targeted lagrangian sampling of submesoscale dispersion at a coastal frontal zone 1 the potential impact of rapidly evolving submesoscale motions on relative dispersion is at the forefront of physical oceanography posing challenges for both observations and modeling a persistent coastal front driven by river outflows in the north western mediterranean sea is targeted by two observational cruises conducted in the summer of 2010 the frontal zone is sampled using drifters launched with a multi scale strategy consisting of modules of triplets released on either side of the front by small boats this experiment is original in that the submesoscale range of 100 m to 1000 m is directly targeted and the results are expected to provide guidance for practical applications such as prediction of the initial spreading of pollutants and biogeochemical tracers the influence of submesoscale motions on relative dispersion is quantified using both particle mean square separation as a function of time and scale dependent finite size lyapunov exponents fsle our main finding is the identification of a local dispersion regime with values reaching as high as 20 days 1 at drifter pair separation distances of 100 m this value is more than an order of magnitude greater than that obtained by drifters in the offshore ligurian current the ligurian sea circulation is modeled using a fully realistic regional ocean modeling system roms with 1 60 horizontal resolution it is found that the numerical model significantly underestimates the relative dispersion at submesoscales indicating the need for particle dispersion parameterizations for unresolved processes

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

petrology and geochronology of mesoproterozoic basement of the mount rogers area of southwestern virginia and northwestern north carolina implications for the precambrian tectonic evolution of the southern blue ridge province this study was supported by awards from the educational geologic mapping program administered by the u s geological survey usgs to george washington university gwu the research also benefited from additional support provided by the usgs appalachian landscape project