the geography of semidiurnal mode 1 internal tide energy loss 1 the semidiurnal mode 1 internal tide receives 0 1 0 3 tw from the surface tide and is capable of propagating across ocean basins the ultimate fate of mode 1 energy after long distance propagation is poorly constrained by existing observations and numerical simulations here global results from a two dimensional semi analytical model indicate that topographic scattering is inefficient at most locations deeper than 2500 m next results from a one dimensional linear model with realistic topography and stratification create a map of mode 1 scattering coefficients along the continental margins on average mode 1 internal tides lose about 60 of their energy upon impacting the continental margins 20 transmits onto the continental shelf 40 scatters to higher modes and 40 reflects back to the ocean interior these analyses indicate that the majority of mode 1 energy is likely lost at large topographic features e g continental slopes seamounts and mid ocean ridges where it may drive elevated turbulent mixing

tropical storm induced near inertial internal waves during the cirene experiment energy fluxes and impact on vertical mixing 1 near inertial internal waves niw excited by storms and cyclones play an essential role in driving turbulent mixing in the thermocline and interior ocean storm induced mixing may be climatically relevant in regions like the thermocline ridge in the southwestern indian ocean where a shallow thermocline and strong high frequency wind activity enhance the impact of internal gravity wave induced mixing on sea surface temperature the cirene research cruise in early 2007 collected ship borne and mooring vertical profiles in this region under the effect of a developing tropical cyclone in this paper we characterize the niw field and the impact of these waves on turbulent mixing in the upper ocean niw packets were identified down to 1000 m the maximum depth of the measurements we estimated an niw vertical energy flux of up to 2 5 mw m 2 within the pycnocline which represents about 10 of the maximum local wind power input a non negligible fraction of the wind power input is hence potentially available for subsurface mixing the impact of mixing by internal waves on the upper ocean heat budget was estimated from a fine scale mixing parameterization during the first leg of the cruise characterized by little niw activity the average heating rate due to mixing was 0 06 c month 1 in the thermocline 23 24 kg m 3 isopycnals during the second leg characterized by strong niw energy in the thermocline and below this heating rate increased to 0 42 c month 1 indicative of increased shear instability along near inertial wave energy pathways

broadband seismic noise analysis of the himalayan nepal tibet seismic experiment we analyze background noise to assess the effects of experimental temporary seismic vault construction and to determine the time needed for noise at sites to stabilize after deployment noise as part of the himalayan nepal tibet seismic experiment himnt we calculated power spectral densities of background noise for each component of each broadband seismometer deployed in nepal and tibet and then compared them with the high noise model and low noise model of peterson 1993 all segments from day and night local time windows were included in the calculation without parsing out earthquakes noise levels were considerably higher for the nepal stations than the tibet stations in particular in the high frequencies the most northern of the stations in tibet show unexpected day night differences of 10 20 db for the vertical components which is significantly greater than the day night differences of the horizontal components however all the stations show noise levels concordant with microseism peak levels power spectral density estimates show moderate noise levels with all stations falling within the high and low bounds of peterson 1993 except for the southern nepal stations which were built above ground and exceed the high noise model at frequencies 1 0 hz the time needed for noise at a site to stabilize was 1 5 days after deployment

coastline retreat via progressive failure of rocky coastal cliffs despite much research on the myriad processes that erode rocky coastal cliffs accurately predicting the nature location and timing of coastline retreat remains challenging and is confounded by the apparently episodic nature of cliff failure the dominant drivers of coastal erosion marine and subaerial forcing are anticipated to increase in the future so understanding their present and combined efficacy is fundamental to improving predictions of coastline retreat we captured change using repeat laser scanning across 2 7 104 m2 of near vertical rock cliffs on the uk north sea coast over 7 yr to determine the controls on the rates patterns and mechanisms of erosion for the first time we document that progressive upward propagation of failure dictates the mode and defines the rate at which marine erosion of the toe can accrue retreat of coastline above this is a failure mechanism not conventionally considered in cliff stability models propagation of instability and failure operates at these sites at 10 yr time scales and is moderated by local rock mass strength and the time dependence of rock fracture we suggest that once initiated failure propagation can operate ostensibly independently to external environmental forcing and so may not be tightly coupled to prevailing subaerial and oceanographic conditions our observations apply to coasts of both uniform and complex lithology where failure geometry is defined by rock mass strength and structure and not intact rock strength alone and where retreat occurs via any mode other than full cliff collapse

landslide reconstruction using seismic signal characteristics and numerical simulations case study of the 2017 6 24 xinmo landslide abstract landslide modeling based on seismic signal characteristics is an effective method to study landslide evolution the scientific challenge lies in how to systematically characterize and reconstruct the entire landslide evolution process using the recorded seismic data we selected the 2017 6 24 xinmo landslide as a case study empirical mode decomposition emd is used to analyze the seismic signals from 14 seismic stations near the landslide area numerical simulations using two dimensional discrete element methods dem are performed to simulate landslide dynamics using the calculated kinetic energy profile after analyzing the short time fourier transform stft and fast fourier transform fft characteristics of the signal the landslide process is reconstructed and compared with the dem simulation the landslide process is observed to consist of a stationary stage slipping stage transition stage entrainment transportation stage and a deposition stage this is the first study that uses seismic signals to identify a transition stage which is a feature that is difficult to constrain from field survey and analysis alone landslide development and movement in the transition stage are discussed considering the sedimentology of the study area this study provides improved theoretical guidance for the reconstruction and characterization of landslide processes by seismic signals

broadband seismic background noise at temporary seismic stations observed on a regional scale in the southwestern united states background noise power spectral density psd estimates for 54 pass cal colorado plateau rio grande rift great plains seismic transect la ristra stations were computed using data from 1999 to 2000 at long periods 0 01 0 1 hz typical vertical noise levels are approximately 12 db higher than the nearby global seismic network gsn borehole station anmo but horizontal power spec tral density psd noise levels are approximately 30 db higher long period noise levels exhibit essentially no spatial correlation along the la ristra transect indi cating that local thermal or atmosphere driven local slab tilt is the dominant source of noise in this band between 0 1 and 0 3 hz typical noise levels are dominated by naturally occurring microseismic noise and are essentially identical to those observed at anmo at short periods 0 3 8 hz typical noise levels along the network exceed anmo levels by approximately 15 db with the highest levels corresponding to proximity to cultural noise sources no significant day night variations were observed in the microseismic band however both low and high frequency noise levels show an increase of up to 8 db in median midday versus midnight noise levels we find that the major shortcomings of these shallow passcal style temporary vaults rela tive to a gsn style borehole installation are increased susceptibility to long period horizontal 20 sec noise and to surface noise sources above approximately 2 hz although the high frequency near surface noise field is unavoidable in shallow vaults we suggest that increased understanding and mitigation of local tilt effects in shallow vaults offers the possibility of significantly improving the long period noise environment

subharmonic energy transfer from the semidiurnal internal tide to near diurnal motions over kaena ridge hawaii abstractnonlinear energy transfers from the semidiurnal internal tide to high mode near diurnal motions are documented near kaena ridge hawaii an energetic generation site for the baroclinic tide data were collected aboard the research floating instrument platform flip over a 35 day period during the fall of 2002 as part of the hawaii ocean mixing experiment home nearfield program energy transfer terms for a psi resonant interaction at midlatitude are identified and compared to those for near inertial psi close to the m2 critical latitude bispectral techniques are used to demonstrate significant energy transfers in the nearfield between the low mode m2 internal tide and subharmonic waves with frequencies near m2 2 and vertical wavelengths of o 120 m a novel prefilter is used to test the psi wavenumber resonance condition which requires the subharmonic waves to propagate in opposite vertical directions depth time maps of the interactions formed by directly estimating the energy transfer ter

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

subglacial discharge at tidewater glaciers revealed by seismic tremor subglacial discharge influences glacier basal motion and erodes and redeposits sediment at tidewater glacier termini discharge drives submarine terminus melting affects fjord circulation and is a central component of proglacial marine ecosystems however our present inability to track subglacial discharge and its variability significantly hinders our understanding of these processes here we report observations of hourly to seasonal variations in 1 5 10 hz seismic tremor that strongly correlate with subglacial discharge but not with basal motion weather or discrete icequakes our data demonstrate that vigorous discharge occurs from tidewater glaciers during summer in spite of fast basal motion that could limit the formation of subglacial conduits and then abates during winter furthermore tremor observations and a melt model demonstrate that drainage efficiency of tidewater glaciers evolves seasonally glaciohydraulic tremor provides a means by which to quantify subglacial discharge variations and offers a promising window into otherwise obscured glacierized environments

internal tide generation by abyssal hills using analytical theory 1 internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers state of the art topographic products barely resolve scales smaller than 10 km in the deep ocean on these scales abyssal hills dominate ocean floor roughness the impact of abyssal hill roughness on internal tide generation is evaluated in this study the conversion of m2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the shuttle radar topography mission srtm30 plus bathymetric product at 1 120 resolution with and without the addition of synthetic abyssal hill roughness internal tide generation by abyssal hills integrates to 0 1 tw globally or 0 03 tw when the energy flux is empirically corrected for supercritical slope i e 10 of the energy flux due to larger topographic scales resolved in standard products in both cases the abyssal hill driven energy conversion is dominated by mid ocean ridges where abyssal hill roughness is large focusing on two regions located over the mid atlantic ridge and the east pacific rise it is shown that regionally linear theory predicts an increase of the energy flux due to abyssal hills of up to 100 or 60 when an empirical correction for supercritical slopes is attempted therefore abyssal hills unresolved in state of the art topographic products can have a strong impact on internal tide generation especially over mid ocean ridges

incoherent signature of internal tides on sea level in idealized numerical simulations the nonpersistent phase relationship between internal tides and astronomical forcings also known as incoherence has been identified as a major question in the context of future wide swath satellite altimetry this study addresses this issue using a novel set of numerical experiments where a plane wave low mode internal tide propagates through a turbulent mesoscale eddy field these experiments demonstrate the emergence of internal tide incoherence as the eddy turbulence is strengthened in strongly turbulent situations the internal tide signature on sea level forms complex interference patterns with large amplifications of the initial internal wave these patterns evolve more rapidly than the signature of the turbulent eddy field on sea level the implications of such idealized numerical simulations for wide swath altimetry are discussed

seismic monitoring of an alpine mountain river the canfranc underground laboratory lsc excavated under the central pyrenees is mainly devoted to the study of phenomena which needs cosmic silence it also hosts a geodynamical facility named geodyn which holds an accelerometer a broadband seismometer and two high resolution laser strainmeters during the routine processing of the seismic data we detected an unusual spectral signature in the 2 10 hz frequency band which does not correspond to the typical sources of seismic noise and which can also be recognized in the strain records after checking against meteorological and hydrological data we can relate those signals to variations in the discharge by the aragon river an alpine style river in the southern pyrenees located about 400 m from the lsc geodyn facility four main episodes have been identified since early 2011 each lasting 1 2 to 6 8 days additionally a limited number of shorter episodes have also been detected three types of river generated seismic events have been identified related respectively to moderate rainfall snowmelt and flooding events associated to severe storms each of those types has distinctive characteristics which allow monitoring the hydrological events from the analysis of seismic and deformation data a few previous studies have already described the seismic noise close to rivers with larger discharge or in small scale experimental settings and we are showing here that the so called fluvial seismology can be useful to study the hydrological evolution of alpine style streams and may have a potential interest for the civil authorities in charge of the management of hydrological basins

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

a comparison of the high frequency 1 hz surface and subsurface noise environment at three sites in the united states abstract surface and subsurface high frequency 1 hz noise data were recorded using nearly identical instrumentation at three widely separated sites in the united states amarillo texas datil new mexico and pinedale wyoming for extended periods of time under varying wind conditions while the sites are geologically distinct the near surface noise environments have many common features that we believe may be due in large part to the presence of a surficial layer of highly attenuative unconsolidated material at each site noise levels seen at or near the surface 5 m or less are much higher up to 30 db and much more variable power range up to 44 db than those seen at depth the smallest range was 9 db for 1951 m at amarillo the greatest gains in noise level reduction are realized within the first 100 m and probably much shallower

high frequency analysis of seismic background noise as a function of wind speed and shallow depth we used a deep 1500 m cased borehole near the town of datil in west central new mexico to study high frequency 1 hz seismic noise characteristics the remote site had very low levels of cultural noise but strong winds winter and spring made the site an excellent candidate to study the effects of wind noise on seismograms along with a three component set of surface sensors teledyne geotech gs 13 a vertical borehole seismometer gs 28 was deployed at a variety of depths 5 43 and 85 m to investigate signal and noise variations wind speed was measured with an anemometer event triggered and time triggered data streams were recorded on a reftek 72 02 data acquisition system located at the site our data show little cultural noise and a strong correlation between wind speed and seismic background noise the minimum wind speed at which the seismic background noise appears to be influenced varies with depth 3 m sec at the surface 3 5 m sec at 43 m in depth and 4 m sec at 85 m in depth for wind speed below 3 to 4 m sec we observe omni directional background noise that is coherent at frequencies below 15 hz this coherence is destroyed when wind speeds exceed 3 to 4 m sec we use a test event md 1 6 and superimposed noise to investigate signal to noise ratio snr improvement with sensor depth for the low q valley fill of the datil borehole dbh site we have found that snr can be improved by as much as 20 to 40 db between 23 and 55 hz and 10 to 20 db between 10 and 20 hz by deploying at a 43 m depth rather than at the surface at the surface there is little signal above noise in the 23 to 55 hz frequency band for wind speeds greater than 8 m sec thus high frequency signal information that is lost at the surface can be recorded by deploying at the relatively shallow depth of 40 m because we observe only minor further reductions in seismic background noise sbn at deeper depths 40 m is likely to be a reasonable deployment depth for other high frequency monitoring sites in similar environmental and geologic conditions

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

internal tide generation and destruction by shoaling internal tides 1 internal tide generation is usually predicted from local topography surface tides and stratification however internal tides are often observed to be unrelated to local spring neap forcing appearing intermittently in 3 5 day bursts here we suggest a source of this intermittency by illustrating how remotely generated shoaling internal tides induce first order changes in local internal tide generation theory numerical simulations and observations show that pressure perturbations associated with shoaling internal tides can correlate with surface tide velocities to generate or destroy internal tides where shoaling internal tides have random phase such as on the new jersey slope time averaged internal tide generation is unaffected but instantaneous internal tide generation varies rapidly altering internal tide energy and possibly affecting nonlinear internal waves across shelf transport and mixing where shoaling internal tides are phase locked to the local surface tide such as in double ridge systems time averaged internal tide generation is affected and may result in resonance

estimates of m2 tidal energy dissipation from topex poseidon altimeter data most of the tidal energy dissipation in the ocean occurs in shallow seas as has long been recognized however recent work has suggested that a significant fraction of the dissipation perhaps 1 tw or more occurs in the deep ocean this paper builds further evidence for that conclusion more than 6 years of data from the topex poseidon satellite altimeter are used to map the tidal dissipation rate throughout the world ocean the dissipation rate is estimated as a balance between the rate of working by tidal forces and the energy flux divergence computed using currents derived by least squares fitting of the altimeter data and the shallow water equations such calculations require dynamical assumptions in particular about the nature of dissipation to assess sensitivity of dissipation estimates to input assumptions a large suite of tidal inversions based on a wide range of drag parameterizations and employing both real and synthetic altimeter data are compared these experiments and monte carlo error fields from a generalized inverse model are used to establish error uncertainties for the dissipation estimates owing to the tight constraints on tidal elevation fields provided by the altimeter area integrals of the energy balance are remarkably insensitive to required dynamical assumptions tidal energy dissipation is estimated for all major shallow seas excluding individual polar seas and compared with previous model and data based estimates dissipation in the open ocean is significantly enhanced around major bathymetric features in a manner consistent with simple theories for the generation of baroclinic tides

application of an internal tide generation model to baroclinic spring neap cycles 1 a numerical internal tide generation model is used to study the behavior of baroclinic spring neap cycles in open basins and channels the motivation for this study comes from an observation on tidal currents in faeroe shetland channel which is briefly described a surprisingly large baroclinic semidiurnal tidal signal is found in the lower part of the water column above the slope on the shetland side concurring with barotropic neap tides the numerical model results indeed show that the baroclinic spring neap cycle may have a phase shift with respect to the barotropic spring neap cycle and that the phase of the baroclinic cycle may vary strongly within short distances it is also shown that even small changes in the background conditions e g stratification can provoke a large phase shift in the baroclinic cycle a possible connection to intermittency is discussed

ground vibrations produced by rock motions and debris flows 1 in this study we compare ground vibrations generated by various rocks and debris flows at the same channel to explore the origin of ground vibrations caused by debris flows in field experiments we identified the ground vibrations caused by motions of individual rocks ground vibrations were detected by geophones and analyzed in the time frequency domains we found that ground vibrations caused by individual rocks were 10 150 hz and that larger stones generate ground vibrations with lower peak frequencies a sphere packing model of granular rocks was used to estimate the propagation speed of ground vibrations this was found to be underestimated when compared with our experimental data the signal decay rate was obtained under the assumption that ground vibrations propagate as exponentially attenuating cylindrical waves ground vibrations produced by a debris flow at ai yu zi creek nan tou taiwan on 2 july 2004 revealed that the debris flow forefront produced vibrations lower than 50 hz and ranged between 50 and 100 hz after the front had passed when the main front was closest to the sensor the frequency spectrum covered a wide range from 10 to 250 hz the mean velocity of the debris flow front was 13 3 m s frequencies of ground vibrations caused by individual rocks are within the frequency range of ground vibrations captured during the debris flow confirming that one of the main sources of ground vibration caused by debris flows is the interaction of rocks or boulders with the channel bed

monitoring the monsoon in the himalayas observations in central nepal june 2001 abstract the monsoon himalayan precipitation experiment mohprex occurred during june 2001 along the south slopes of the himalayas in central nepal radiosondes were launched around the clock from two sites one in the marsyandi river basin on the eastern footslopes of the annapurna range and one farther to the southwest near the border with india the flights supported rainfall and other hydrometeorological observations including surface winds from the marsyandi network that has been operated in this region since the spring of 1999 the thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon with total column moisture and instability maximized just before rainfall peaks coinciding with the appearance of a monsoon depression over central india the onset of the monsoon in this region was characterized by a weeklong weakening of the upper level westerlies and an increase in moisture and convective instability the vertical structure of

generation of internal waves in the deep ocean by tides 1 a direct computation of the tidal generation of internal waves over the global ocean is presented it is based on linear wave theory and high resolution data for the bottom topography the geographical distribution of the energy flux from tides to internal waves is determined with a spatial resolution of a few kilometers the total flux over the area with a depth greater than 500 m is found to be 1 2 tw the greatest uncertainties of the computation are due to unresolved topography and to nonlinear effects caused by supercritical bottom slope

focusing of internal tides by near inertial waves the refraction of internal waves by lower frequency near inertial waves has been predicted theoretically but never observed before here we report observations of semi diurnal internal tides generated by the rough topography of the central indian ridge in the presence of a strong lower frequency near inertial wave field generated by a tropical storm the semi diurnal internal tide energy is trapped within upward propagating bands with a periodicity close to the inertial period a ray tracing model suggests that this trapping results from the internal tide refraction by the shear associated with near inertial waves this yields a strong increase of the internal tide energy and shear in space time regions where the background flow focuses the rays leading to the formation of caustics this mechanism may increase vertical mixing generated by baroclinic tides in the vicinity of mid ocean ridges in tropical regions

seasonal variations of seismic velocities in the san jacinto fault area observed with ambient seismic noise s u m m a r y we observe seasonal seismic wave speed changes dv v in the san jacinto fault area and investigate several likely source mechanisms velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations we study the interrelation between dv v records frequency dependent seismic noise properties and nearby environmental data of wind speed rain ground water level barometric pressure and atmospheric temperature the results indicate peak to peak seasonal velocity variations of 0 2 per cent in the 0 5 2 hz frequency range likely associated with genuine changes of rock properties rather than changes in the noise field phase measurements between dv v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations the other considered environmental effects produce secondary variations that are superimposed on the thermal based changes more detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth

the effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station seismic investigations are mainly limited by seismic noise two microbarometers have been installed in the seismic vault of two different geoscope stations one at ssb and the other at tam all vertical components and most of the horizontal components show a significant correlation with pressure in order to correct the seismic signals from the atmospheric pressure noise a transfer function between the pressure data and the seismic data is inverted results show that after correction the noise levels reached on the horizontal components are similar between the two stations and the vertical components display noise levels below the low noise model as defined by peterson 1993 this technique reduces part of the noise and allows detection of small earthquakes and a better extraction of normal modes the analysis of the lowest normal modes of the earth excited by the m s 8 2 macquarie island earthquake is given to illustrate the perspectives of the method

thermal structure and exhumation history of the lesser himalaya in central nepal the lesser himalaya lh consists of metasedimentary rocks that have been scrapped off from the underthrusting indian crust and accreted to the mountain range over the last 20 myr it now forms a significant fraction of the himalayan collisional orogen we document the kinematics and thermal metamorphism associated with the deformation and exhumation of the lh combining thermometric and thermochronological methods with structural geology peak metamorphic temperatures estimated from raman spectroscopy of carbonaceous material decrease gradually from 520 550 c below the main central thrust zone down to less than 330 c these temperatures describe structurally a 20 50 c km inverted apparent gradient the ar muscovite ages from lh samples and from the overlying crystalline thrust sheets all indicate the same regular trend i e an increase from about 3 4 ma near the front of the high range to about 20 ma near the leading edge of the thrust sheets about 80 km to the south this suggests that the lh has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm yr for a convergence rate of about 20 mm yr this implies underthrusting of the indian basement below the himalaya by about 15 mm yr the structure metamorphic grade and exhumation history of the lh supports the view that since the mid miocene the himalayan orogen has essentially grown by underplating rather than by frontal accretion this process has resulted from duplexing at a depth close to the brittle ductile transition zone by southward migration of a midcrustal ramp along the main himalayan thrust fault and is estimated to have resulted in a net flux of up to 150 m 2 yr of lh rocks into the himalayan orogenic wedge the steep inverse thermal gradient across the lh is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

propagation of low mode internal waves through the ocean abstract the baroclinic tides play a significant role in the energy budget of the abyssal ocean although the basic principles of generation and propagation are known a clear understanding of these phenomena in the complex ocean environment is only now emerging to advance this effort a ray model is developed that quantifies the effects of spatially variable topography stratification and planetary vorticity on the horizontal propagation of internal gravity modes the objective is to identify baroclinic shoals where wave energy is spatially concentrated and enhanced dissipation might be expected the model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field the refraction of tidally generated internal waves at the hawaiian ridge is examined using an ensemble of mesoscale background realizations derived from weekly ocean topography experiment topex poseidon altimetric measurements the path of mode 1 is only slightly affected by typical cur

the effective elastic thickness of the india plate from receiver function imaging gravity anomalies and thermomechanical modelling summary the range and the meaning of the effective elastic thickness eet in continental areas have been subject to controversy over the last two decades here we take advantage of the new data set from the hi climb seismological experiment to re estimate the eet of the india plate along a south north profile extending from the ganges basin to central tibet receiver functions give a high resolution image of the base of the foreland basin at 5 km depth and constrain the crustal thickness which increases northwards from 35 km beneath the indo gangetic plain to 70 km in southern tibet together with available data sets including seismic profiles seismological images from both indepth and himnt experiments deep well measurements and bouguer anomaly profiles we interpret this new image with 2 d thermomechanical modelling solutions using different type of crustal and mantle rheologies we find that 1 the eet of the india plate decreases northwards from 60 80 to 20 30 km as it is flexed down beneath himalaya and tibet due to thermal and flexural weakening 2 the only resistant layer of the india plate beneath southern tibet is the upper mantle which serves as a support for the topographic load and 3 the most abrupt drop in the eet located around 200 km south of the mft is associated with a gradual decoupling between the crust and the mantle we show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region and they are in favour of a rheology in which the mantle is strong

spatial and temporal variability of global ocean mixing inferred from argo profiles 1 the influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water mass transformation stratification maintenance and the overturning circulation however the distribution of ocean mixing is not well described by sparse ship based observations since this mixing is both spatially patchy and temporally intermittent we use strain information from argo float profiles in the upper 2 000 m of the ocean to generate over 400 000 estimates of the energy dissipation rate indicative of ocean mixing these estimates rely on numerous assumptions and do not take the place of direct measurement methods temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans they corroborate previous observations linking elevated dissipation rates to regions of rough topography we also observe heightened estimated dissipation rates in areas of high eddy kinetic energy as well as heightened diffusivity in high latitudes where stratification is weak the seasonal dependence of mixing is observed in the northwest pacific suggesting a wind forced response in the upper ocean

the surface expression of semidiurnal internal tides near a strong source at hawaii part ii interactions with mesoscale currents abstract observations of semidiurnal surface currents in the kauai channel hawaii are interpreted in the light of the interaction of internal tides with energetic surface intensified mesoscale currents the impacts on internal tide propagation of a cyclone of 55 km diameter and 100 m vertical decay scale as well as of vorticity waves of 100 km wavelength and 100 200 m vertical decay scales are investigated using 3d ray tracing the doppler shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation using the local buoyancy frequency associated with the background currents through thermal wind or gradient wind balance the m2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of o 1000 m are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents despite the lack of scale separation between the internal waves and backgroun

on the interaction between internal tides and wind induced near inertial currents at the shelf edge 1 numerical calculations are performed at the shelf edge to examine the role of nonlinear processes in transferring energy from the m2 tide and wind induced currents close to the inertial frequency f into a wave at the sum of their frequencies termed the fm2 frequency a numerical model of the hebrides shelf edge represented by a cross section initially with idealized topography and subsequently with realistic topography is used in these calculations results show that in the near coastal ocean currents at the fm2 frequency are primarily due to coupling between wind induced inertial oscillations and the m2 internal tide a major source is associated with vertical shear in the inertial oscillations and the vertical velocity due to the internal tide a secondary source is due to the nonlinear momentum advection term in the case in which eddy viscosity is computed from a turbulence energy model shear across the thermocline is larger than when a constant viscosity is used the reduction in shear with a constant viscosity reduces the role of the nonlinear term involving vertical shear and hence the magnitude of the fm2 current in the region of the thermocline increasing the wind stress leads to a deeper thermocline and hence the location of maximum fm2 current in the water column changing the vertical stratification influences the intensity of the inertial oscillations in the surface layer and the distribution of the internal tide and hence changes the pattern of fm2 currents results with realistic topography confirm the major conclusions

seismic array constraints on reach scale bedload transport measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management a 700 m reach of the trinity river northern california usa a large gravel bed river was instrumented with an array of 76 seismographs during a dam controlled ood and gravel augmentation to investigate the potential for out of stream monitoring the temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area following gravel augmentations the seismic array reveals a period of enhanced transport that spans most or all of the reach for 7 10 h neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array sensitivity to along stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload ux constrained by time lapse bathymetry insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array observed peak seismic power is 1 5 of that predicted by a model of saltation over exposed bedrock our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport

time variable refraction of the internal tide at the hawaiian ridge abstractthe interaction of the dominant semidiurnal m2 internal tide with the large scale subtidal flow is examined in an ocean model by propagating the tide through an ensemble of background fields in a domain centered on the hawaiian ridge the background fields are taken from the simple ocean data assimilation soda ocean analysis at 2 month intervals from 1992 through 2001 tides are computed with the primitive equation z coordinate harmonic analysis of tides pez hat model by 14 day integrations using soda initial conditions and m2 tidal forcing variability of the tide is found to occur primarily as the result of propagation through the nonstationary background fields rather than via generation site variability generation of incoherent tidal variability is mapped and shown to occur mostly in association with waves generated at french frigate shoals scattering near the musicians seamounts to the north of the ridge the phase coherent internal tide loses energy at a domain average rate of 2 mw m

spectral analysis of seismic noise induced by rivers a new tool to monitor spatiotemporal changes in stream hydrodynamics to 20 db relative to m s 2 hz for all the stations located along a steep 30 km long narrow and deeply incised channel of the trisuli river a major trans himalayan river the early summer increase in high frequency energy is modulated by a 24 h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening a detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the trisuli river seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations the observed 24 h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region river induced seismic noise is partly generated by stream turbulence but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level this pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport this points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations which remain difficult with classical approaches

on noise reduction in vertical seismic records below 2 mhz using local barometric pressure we propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mhz the method consists in subtracting the locally recorded scaled atmospheric pressure signal from the gravity record the scale factor is frequency independent and can be linearly estimated from the data while such a pressure correction was previously only used in the analysis of tidal records it is equally efficient at the low frequency end of the normal mode band and we use gravity recordings of the big 1994 shikotan and bolivian quakes to demonstrate this point our examples also show that this correction permits to achieve noise levels well below the new low noise model nlnm peterson 1993 the success of the correction also sheds some light on the physical processes responsible for the generation of low frequency seismic noise at quiet observatories