rayleigh wave anelastic attenuation across the bermuda rise and the volcanic caribbean arch summary average fundamental mode group velocities and attenuation coefficients of rayleigh waves have been obtained in a region of the western north atlantic comprising the bermuda rise the volcanic caribbean arch and normal seafloor greater than 65myr in age the average rayleigh wave attenuation coefficients decrease from a value of 7 78 10 4km at a period of 15s to a value of 0 21 lo km at a period of 100s attenuation values and qr factors obtained are characteristic of a young oceanic region and they do not correspond to a typical region greater than 65 myr in age it is apparent from the data that the high attenuation values reported are due to the effect that the back arc basin behind the caribbean arch has on rayleigh wave attenuation inversion theory applied to the attenuation data yields a qj model presenting the same features as those corresponding to extension zones such as the spreading centres of the atlantic and pacific oceans the low q region in the model is situated between depths of around 15 and 100 km with the maximum qp value 17 x occurring at about 50 km depth

to what degree can historical seismicity records assist in seismic microzonation abstract it is well known that historical data of seismic effects in urban areas can be useful in seismic microzonation but we must consider how to make the most of available documentary sources and assess to what degree historical information can help us from this starting point this paper deals in a more objective manner with the potential contribution of macroseismic site data suggesting the compilation of an ad hoc questionnaire in this way the two main aims of this paper can be achieved to make the most of available documentary sources and to quantify the usefulness of macroseismic information for microzonation analysis via a site normalised utility and reliability parameter snurp this second aim resolves for example the question of priority in resource allocation for microzonation site surveys on the whole the proposed approach will provide an easily used tool to deal with historical accounts especially for professionals in the engineering geology field

controls on turbulent mixing in a strongly stratified and sheared tidal river plume abstractconsiderable effort has been made to parameterize turbulent kinetic energy tke dissipation rate e and mixing in buoyant plumes and stratified shear flows here a parameterization based on kunze et al is examined which estimates e as the amount of energy contained in an unstable shear layer ri ric that must be dissipated to increase the richardson number ri n2 s2 to a critical value ric within a turbulent decay time scale observations from the tidal columbia river plume are used to quantitatively assess the relevant parameters controlling e over a range of tidal and river discharge forcings observed e is found to be characterized by kunze et al s form within a factor of 2 while exhibiting slightly decreased skill near ri ric observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification this is found to be appropriate in energetic regimes when the bulk averaged richardson number rib is less than

shallow structure of part of northwestern iberia from short period rayleigh wave observations propagation of 0 2 2 5 s short period rayleigh waves across a part of northwestern iberia is investigated analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot point provides group velocities and attenuation coefficients of the fundamental mode first higher mode group velocity has been obtained for only one of the studied cases the studied region has been divided into two subrogions the northwestern and the southeastern side from the shot point inversion procedures applied to the velocity and attenuation data yield the shear velocity and the q 1 models up to a depth of about 1 5 km for the nw and 2 4 for the se side from the shear wave obtained models corresponding to the nw and to the se sides it is possible to infer the existence of some degree of lateral variation of s velocity the nw side presents velocities varying between 1 8 km s 1 at the surface and 3 3 km s 1 at 1 2 km depth the velocities in the se model for the same depths are 2 2 and 3 4 km s 1 respectively using only the fist six traces of the se side it is possible to infer a low velocity region located between 0 4 km and 0 7 km depth it seems that the velocities are higher in the complex geological region of the se side than in the nw one which is in agreement with p velocity models found by others from the tentative anelastic attenuation coefficients q 1 models have been inferred for both sides of the seismic refraction profile although the obtained q 1 models are somehow different in shape the maximum q 1 value is about 0 05 in both cases

turbulence transition in stratified atmospheric and oceanic shear flows reynolds and prandtl number controls upon the mechanism 1 we provide detailed analyses of the role of secondary instabilities in the transition to turbulence of stratified shear flows at the high reynolds and prandtl numbers appropriate to the atmosphere and ocean our focus is upon a series of recent observations suggestive of a marked change in the mechanism of transition in this laboratory inaccessible regime our results demonstrate the joint dependance of the transition mechanism on these two nondimensional parameters we discuss the primary transition mechanisms of shear aligned convective instability stagnation point instability and shear instabilities of the braid in detail

lobe cleft instability in the buoyant gravity current generated by estuarine outflow gravity currents represent a broad class of geophysical flows including turbidity currents powder avalanches pyroclastic flows sea breeze fronts haboobs and river plumes a defining feature in many gravity currents is the formation of three dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades the prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a nonslip boundary recent studies suggest that frontal dynamics change as the flow scale increases but no measurements have been made that sufficiently resolve the flow structure in full scale geophysical flows here we use thermal infrared and acoustic imaging of a river plume to reveal the three dimensional structure of lobes and clefts formed in a geophysical gravity current front the observed lobes and clefts are generated at the front in the absence of a nonslip boundary contradicting the prevailing explanation the observed flow structure is consistent with an alternative formation mechanism which predicts that the lobe scale is inherited from subsurface vortex structures

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

prominent magsat anomalies over india abstract the present study confirms the well known pogo and magsat anomalies showing a broad magnetic high on the indian shield and a broad low over the himalayas a characteristic difference in magnetic signatures over the peshawar himalaya a broad high the punjab himalaya a broad low and the kumaun himalaya north of sarda depression a sharp and deeper low indicates the existence of different blocks in the himalayas the short wavelength low over the rifted region of the narmada son lineament can be interpreted as a reflection of an ancient suture zone interestingly the surface bouguer gravity picture shows an inverse correlation with magsat anomalies over peninsular india and a positive correlation over the himalayan region quantitative model estimates from the observed satellite magnetic and surface gravity data have revealed compositional changes in the peninsular india and thickening of magnetic crust in the himalayas

large artificial water reservoirs in the vicinity of the himalayan foothills and reservoir induced seismicity there are 11 large reservoirs exceeding a capacity of 1 km3 and or the height of the dam being 100 m or more in the vicinity of the himalayan foothills nine of these have been fully impounded only a couple of these reservoirs are adequately instrumented for seismic surveillance while the rest have inadequate instrumentation however there is no incidence of reservoir induced seismicity reported from any of these nine sites nonoccurrence of induced earthquakes is comprehended primarily due to the thrust fault environment generally prevalent in the himalayan foothills which is not conducive for reservoir induced seismicity nevertheless a few normal and strike slip faults oblique and parallel to the general trend of the himalayas have been reported where reservoir induced seismicity could occur all of these dams are located in a highly seismic area where earthquakes of magnitude exceeding 7 have occurred in their vicinity in the past normally occurring earthquakes therefore pose a much more severe threat to himalayan reservoirs than do reservoir induced seismicity

turbulence and internal waves at the equator part ii details of a single event abstract in the low richardson number shear flow above the pacific equatorial undercurrent a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves the observed dissipation rates within the overturning wave were so high that if they were representative of the volume averaged rate the total wave energy would have been dissipated within a single buoyancy period the chaotic structure and temperature fluctuations with horizontal scales less than 2 m of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing similar high horizontal wavenumber waves believed to be shear instability waves have been observed in low richardson number regions of the midlatitude seasonal thermocline although the equatorial waves have a horizontal wavelength appropriate for shear instability waves t

role of overturns in optimal mixing in stratified mixing layers turbulent mixing plays a major role in enabling the large scale ocean circulation the accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years primarily on the basis of adequately high resolution numerical simulations we add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes we study the inherently intermittent and time dependent nature of wave induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small scale isotropic turbulence grows the anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales we show that the characteristics of turbulent mixing depend on the relative time evolution of the ozmidov length scale compared to the so called thorpe overturning scale which represents the scale containing available potential energy upon which turbulence feeds and grows we find that when the mixing is most active and efficient since stirring by the largest overturns becomes optimal in the sense that it is not suppressed by ambient stratification we argue that the high mixing efficiency associated with this phase along with observations of in oceanic turbulent patches together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected this neglect arising through the assumption of a clear separation of scales between the background mean flow and small scale quasi isotropic turbulence leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density such an exclusion may well be significant if the mechanism of shear induced turbulence is assumed to be representative of at least some turbulent events in the ocean while our results are based upon simulations of shear instability we show that they are potentially more generic by making direct comparisons with data from ocean and lake observations which represent a much wider range of turbulence inducing physical processes

seasonality of deep cycle turbulence in the eastern equatorial pacific abstractthe seasonal cycles of the various oceanic and atmospheric factors influencing the deep cycle of turbulence in the eastern pacific cold tongue are explored moored observations at 140 w have shown seasonal variability in the stratification velocity shear and turbulence above the pacific equatorial undercurrent euc in boreal spring the thermocline and euc shoal and turbulence decreases marginal instability clustering of the local gradient richardson number around the critical value of 1 4 evident throughout the rest of the year has not been detected during spring while the daily averaged turbulent energy dissipation in the euc is weakest during the spring it is not clear whether the diurnal fluctuations that define the deep cycle cease large eddy simulations are performed using climatological initial and boundary conditions representative of january april july and october deep cycle turbulence is evident in all cases the mechanism remains the same and the maximum turbulence leve

q structure beneath the tibetan plateau from the inversion of love and rayleigh wave attenuation data abstract the fundamental mode love and rayleigh waves generated by ten earthquakes and recorded across the tibet plateau at que lah ndi nil kbl shl chg sng and hkg are analysed love and rayleigh wave attenuation coefficients are obtained at time periods of 5 120 s using the spectral amplitudes of these waves for 23 different paths love wave attenuation coefficient varies from 0 0021 km 1 at a period of 10 s to 0 0002 km 1 at a period of 90 s attaining two maxima at time periods of 10 and 115 s and two minima at time periods of 25 and 90 s the rayleigh wave attenuation coefficient also shows a similar trend the very low value for the dissipation factor q obtained in this study suggests high dissipation across the tibetan paths backus gilbert inversion theory is applied to these surface wave attenuation data to obtain average q 1 models for the crust and uppermost mantle beneath the tibetan plateau independent inversion of love and rayleigh wave attenuation data shows very high attenuation at a depth of 50 120 km q 10 the simultaneous inversion of the love and rayleigh wave data yields a model which includes alternating regions of high and low q 1 values this model also shows a zone of high attenuating material at a depth of 40 120 km the very high inferred attenuation at a depth of 40 120 km supports the hypothesis that the tibetan plateau was formed by horizontal compression and that thickening occurred after the collision of the indian and eurasian plates

objective regionalization of rayleigh wave dispersion data by clustering algorithms an application to the mediterranean basin abstract the main target of the present study is an objective and automated regionalization of rayleigh wave dispersion data for the mediterranean basin without a priori seismotectonic constraints and to determine the corresponding regional shear velocity structures the database used is formed by almost 200 rayleigh wavetrains corresponding to 42 regional events with surface wave magnitude greater than 4 5 recorded at the mednet very broad band stations in the mediterranean area path averaged group velocities for the rayleigh wave fundamental mode are derived for each available epicentre station trajectory crossing the mediterranean basin after this a principal component analysis and a clustering process are applied to local group velocities obtained for 13 different periods from 10 to 70 s in order to classify the mediterranean basin into several homogeneous regions the stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear velocity structures down to a depth of 150 160 km the characteristics of these areas and their possible correlation with the main seismotectonic features of the mediterranean region are discussed the regional models reveal significant lateral changes in the elastic structure with the main differences concerning particularly the upper 35 40 km within this depth range low shear velocities varying from 2 8 to 3 9 km s 1 characterize the eastern mediterranean whereas higher velocities ranging from 3 0 to 4 2 km s 1 are deduced for the western mediterranean these results suggest a thicker crust in the eastern part but with a greater thickness of sedimentary layers however for depths of between 80 and 110 km lower shear velocities are obtained in the western part while higher shear velocities are derived for the eastern mediterranean sea in the aegean sea greece the south of italy sicily and tunisia this velocity pattern suggests an averaged thicker lithosphere under the latter areas as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions this thicker lithosphere can be related to processes associated with the convergence of the eurasian and african plates and subduction under the calabrian and hellenic arcs

thermal effects of thrust faulting abstract calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements if rates are sufficiently high e g plate tectonic rates the thrust sheets sufficiently thick 5 10 and 15 km are modelled here the distances moved sufficiently large and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field regions where large scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow the theoretical effects are presented as a series of maximum temperature vs depth and heat flow vs time plots

trains of large kelvin helmholtz billows observed in the kuroshio above a seamount trains of large kelvin helmholtz kh billows within the kuroshio current at 230 m depth off southeastern taiwan and above a seamount were observed by shipboard instruments the trains of large kh billows were present in a strong shear band along the 0 55 m s 1 isotach within the kuroshio core they are presumably produced by flow interactions with the rapidly changing topography each individual billow resembling a cat s eye had a horizontal length scale of 200 m a vertical scale of 100 m and a timescale of 7 min near the local buoyancy frequency overturns were observed frequently in the billow cores and the upper eyelids the turbulent kinetic energy dissipation rates estimated using the thorpe scale had an average value of o 10 4 w kg 1 and a maximum value of o 10 3 w kg 1 the turbulence mixing induced by the kh billows may exchange kuroshio water with the surrounding water masses

convective instability in sedimentation linear stability analysis 1 convective sedimentation in a stably stratified saltwater is studied using the linear stability analysis convective sedimentation is known to occur due to the double diffusive mechanism and the settling driven mechanism in this study a semi empirical closure of sediment diffusivity based on the long range hydrodynamics effect is adopted the sediment phase can act as either the slow or fast diffusing agent in the double diffusive system for the given salt diffusivity moreover the settling driven effect is proportional to the square of the sediment diameter via stoke settling law we consider sediment concentration grain size in the upper freshwater layer to be in the range of 0 1 to 39 4 g l 2 to 60 m which is on top of a saltwater layer with salinity 35 ppt linear stability analysis allows us to identify the dominant mechanism that triggers the instability the growth rate and the resulting characteristic finger width model results suggest that for fine sediment with grain diameter smaller than 10 m settling velocity 0 09 mm s double diffusive mechanism controls the instability and the resulting sediment finger size is of millimeter scale for the given threshold of growth rate of o 0 01 s 1 the minimum sediment concentration is about 8 15 g l for grain size greater than or around 10 m the settling driven mechanism dominates and instabilities occur at sediment concentration as low as o 0 1 g l with centimeter scale fingers our findings may contribute to a better understanding on the observed rapid sediment removal in the plume of small mountainous rivers

mixing by shear instability at high reynolds number 1 shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere the transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate reynolds number it involves rolling up i e overturning of the density structure within the cores of the instabilities in contrast measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high reynolds number does not primarily occur by overturning in the cores rather it results from secondary shear instabilities within the zones of intensified shear separating the cores this regime is not likely to be observed in the relatively low reynolds number flows of the laboratory or in direct numerical simulations but it is likely a common occurrence in the ocean and atmosphere

laboratory investigation of the impact of lateral spreading on buoyancy flux in a river plume abstractthe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments the vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry piv and planar laser induced fluorescence plif lateral spreading is determined in the unconfined experiments based on plan view imaging using the optical thickness method otm the authors find that lateral spreading dramatically modifies the plume structure the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface whereas the channelized plume profiles are uniform near the surface lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions however the local turbulent buoyancy flux in the spreading experiments i

a deterministic seismic hazard map of india and adjacent areas a seismic hazard map of the territory of india and adjacent areas has been prepared using a deterministic approach based on the computation of synthetic seismograms complete with all main phases the input data set consists of structural models seismogenic zones focal mechanisms and earthquake catalogues there are few probabilistic hazard maps available for the indian subcontinent however this is the first study aimed at producing a deterministic seismic hazard map for the indian region using realistic strong ground motion modelling with the knowledge of the physical process of earthquake generation the level of seismicity and wave propagation in anelastic media synthetic seismograms at a frequency of 1 hz have been generated at a regular grid of 0 2 x 0 2 by the modal summation technique the seismic hazard expressed in terms of maximum displacement d m a x maximum velocity v m a x and design ground acceleration dga has been extracted from the synthetic signals and mapped on a regular grid over the studied territory the estimated values of the peak ground acceleration are compared with the observed data available for the himalayan region and are found to be in agreement many parts of the himalayan region have dga values exceeding 0 6 g the epicentral areas of the great assam earthquakes of 1897 and 1950 in northeast india represent the maximum hazard with dga values reaching 1 2 1 3 g the peak velocity and displacement in the same region is estimated as 120 177 cm s 1 and 60 90 cm respectively

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

on nonhydrostatic coastal model simulations of shear instabilities in a stratified shear flow at high reynolds number the nonhydrostatic surface and terrain following coastal model nhwave is utilized to simulate a continually forced stratified shear flow in a straight channel which is a generic problem to test the existing nonhydrostatic coastal models capability in resolving shear instabilities in the field scale the resolved shear instabilities in the shear layer has a reynolds number of about 1 4 106 which is comparable to field observed value using the standard smagorinsky closure with a grid size close to the ozmidov length scale simulation results show that the resolved energy cascade exceeds 1 order of magnitude and the evolution and turbulent mixing characteristics are predicted well two different approaches are used to estimate the turbulent dissipation rate namely using the resolved turbulent energy spectrum and the parameterized subgrid turbulent dissipation rate and the predicted results provide the upper and lower bounds that encompass the measured values model results show significantly higher turbulence in braids of shear instabilities which is similar to field observations while both the subgrid turbulent dissipation rate and resolved vorticity field can be used as surrogates for measured high acoustic backscatter signals simulation results also reveal that the surface velocity divergence convergence is an effective identifier for the front of the density current and the shear instabilities to guide future numerical studies in more realistic domains an evaluation on the effects of different grid resolutions and subgrid viscosity on the resolved flow field and subgrid dissipation rate are discussed

extremely long kelvin helmholtz billow trains in the romanche fracture zone in the atlantic ocean the densest water mass antarctic bottom water aabw can only cross the mid atlantic ridge from its southwestern to northeastern basins in limited because deep conduits at the southwestern entrance of one of these the equatorial romanche fracture zone aabw crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep at the sill slope the rapidly flowing aabw causes shear induced turbulent mixing with the overlying water masses we present an excerpt of 1 hz sampled half yearlong moored observations from 99 high resolution temperature sensors that demonstrate and quantify the turbulence details on top of quasi steady shear flow an internal tide modulates the mixing together they constitute a means for an extremely long train of 250 consecutive kelvin helmholtz billows in a day that vary between 5 and 100 m in vertical scale

transitional continental oceanic structure beneath the norwegian sea from inversion of surface wave group velocity data summary we have analysed the fundamental mode of love and rayleigh waves generated by 12 earthquakes located in the mid atlantic ridge and jan mayen fracture zone using the multiple filter analysis technique we isolated the rayleigh and love wave group velocities for periods between 10 and 50 s the surface wave propagation paths were divided into five groups and average group velocities calculated for each group the average group velocities were inverted and produced shear wave velocity models that correspond to a quasi continental oceanic structure in the greenland norwegian sea region although resolution is poor at shallow depth we obtained crustal thickness values of about 18 km in the norwegian sea area and 9 km in the region between svalbard and iceland the abnormally thick crust in the norwegian sea area is ascribed to magmatic underplating and the thermal blanketing evect of sedimentary layers maximum crustal shear velocities vary between 3 5 and 3 9 km s 1 for most paths an average lithospheric thickness of 60 km was observed which is lower than expected for oceanic type structure of similar age we also observed low shear wave velocities in the lower crust and upper mantle we suggest that high heat flow extending to depths of about 30 km beneath the surface can account for the thin lithosphere and observed low velocities anisotropy coeycients of 1 5 per cent in the shallow layers and 7 per cent in the upper mantle point to the existence of polarization anisotropy in the region

broadband acoustic quantification of stratified turbulence high frequency broadband acoustic scattering techniques have enabled the remote high resolution imaging and quantification of highly salt stratified turbulence in an estuary turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors the acoustic frequencies used span 120 600 khz which for the highly stratified and dynamic estuarine environment correspond to wavenumbers in the viscous convective subrange 500 2500 m 1 the acoustically measured spectral levels are in close agreement with spectral levels measured with closely co located micro conductivity probes the acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim bladdered fish the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure the direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt stratified estuary

thermoelastic bending of the lithosphere implications for basin subsidence summary thermoelastic stresses are capable of producing significant lithospheric deflection a systematic approach to modelling this effect is presented in which the lithosphere is presumed to behave as a thin elastic or viscoelastic plate on a fluid substrate and lateral variations in basal heat flow induce both vertical buoyant loads and thermoelastic bending moments the amount of uplift or subsidence produced by a given heat source depends on a number of factors including the strength duration and lateral extent of the thermal anomaly and the thickness density rigidity and viscosity of the plate the mechanical response of the plate is characterized by two distinct length scales one for shearing the other for bending and the deformation produced depends critically on the scaled width of the heat source the plate acts as a low pass spatial filter in response to thermal loads but has a narrow band pass filter response to applied moments the temporal response to an abrupt change in basal temperature or heat flow also depends on the ratio of the thermal diffusion time for the plate versus the viscoelastic relaxation time of the material a rapid localized increase in basal heat flow will initially produce a central depression and a peripheral uplift thus while it is often assumed that the main depositional phase in platform basins is associated with the cooling and contraction of the lithosphere following an earlier thermal doming event it is shown that if conditions are right a basin may also form in direct response to the initial heating event either way subsequent thermal equilibration and viscous stress relaxation will tend to modify the initial deflection profile

where large deep ocean waves break underwater topography like seamounts causes the breaking of large internal waves with associated turbulent mixing strongly affecting the redistribution of sediment here ocean turbulence is characterized and quantified in the lowest 100 m of the water column at three nearby sites above the slope of a deep ocean seamount moored high resolution temperature sensors show very different turbulence generation mechanisms over 3 and 5 km horizontal separation distances at the steepest slope turbulence was 100 times more energetic than at the shallowest slope where turbulence was still 10 times more energetic than found in the open ocean away from topography the turbulence on this extensive slope is caused by slope steepness and nonlinear wave evolution but not by bottom friction critical internal tide reflection or lee wave generation

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

elastic anelastic regional structures for the iberian peninsula obtained from a rayleigh wave tomography and a causal uncoupled inversion the elastic and anelastic structure of the lithosphere and asthenosphere of the iberian peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of rayleigh wave fundamental mode the database consists of surface wavetrains recorded at the broadband stations located in the iberian peninsula on the occasion of the iliha project path averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and subsequently local dispersion curves were computed according to the yanovskaya ditmar formulation first a principal component analysis pca and the average linkage al clustering algorithm are applied to these local values in order to classify the iberian peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves without previous seismotectonic constraints second averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere this purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients the main features of the homogeneous regions are discussed by taking as reference the hercynic alpine and neogene domains of the iberian peninsula and two questions affecting the reliability of the elastic anelastic models are revised first the coherence of the shear velocity and q 1 models obtained by causal uncoupled inversion for each region is analysed second the influence of the causal phase and group velocities on the shear velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non causal inversion

topographic site effects a comparison of two nonreference methods abstract we present an experimental study of topographic site effects the data we use come from an experiment carried out during the summer of 1989 in epire northern greece ten digital stations recorded a total of 68 small earthquakes a recent article chavez garcia et al 1996 presented a comparison between site effects determined using horizontal to vertical spectral ratios hvsr for this data set and theoretical modeling in this note we compare the topographic site effects determined using hvsr with another independent experimental estimate a generalized inversion scheme gis neither hvsr nor gis depend on the availability of a reference site we obtain a very good agreement between both estimates of topographic site effects for both horizontal components our results support the use of hvsr to determine topographic site effects

a new insight into seismic attenuation characteristics of northwest himalaya and its surrounding region implications to structural heterogeneities and earthquake hazards abstract the frequency dependent attenuation characteristics of p waves q s waves q and coda waves qc in north west nw himalaya and its surrounding region have been estimated using 515 micro to moderate earthquakes 2 5 mw 5 0 recorded from january 2008 to november 2015 for a central frequency range fc varying from 1 5 to 12 hz average attenuation relationships of qc q and q found as qc 126 94 37 9 f 1 122 0 06 q 65 127 5 7 f 1 1261 0 03 and q 93 855 3 05 f 1 1145 0 01 for the region it is found that the nature and extent of seismic attenuation are different for different seismotectonic belts consisted of various kinds of fault systems distributed into the indo gangetic plain igp the sub himalaya sh the lesser himalaya lh and the higher himalaya hh geotectonic segments beneath nw himalaya and its surrounding region material property heterogeneity beneath the subsurface earth is playing an important role in influencing the degree of release of seismic energy in nw himalaya and its surrounding region because of varying attenuative property in different geotectonic segments q 1 c hh q 1 c igp q 1 c sh q 1 c lh this study may help in better understanding of structural heterogeneities that control the nature and extent of seismogenesis which in turn may help in assessing seismic hazards through estimates of strong ground motion by computing earthquake source parameters our schematic model for nw himalaya and its surrounding region shed an important light on attenuative characteristics of different tectonic blocks associated with both local and regional fault systems that dictate the source characteristics and potential of earthquake hazards of the region

shear wave velocity tomography of the lithosphere asthenosphere system beneath the mediterranean area abstract the purpose of this study is deriving a 3d tomographic image of the shear wave velocity structure of the lithosphere asthenosphere system of the mediterranean region from rayleigh wave fundamental mode dispersion data the database consists of almost 200 wavetrains corresponding to regional events recorded at the mednet very broad band stations placed in the mediterranean area on the basis of the path averaged group velocities derived for each epicentre station trajectory local group velocities over the area covered by the seismic paths have been obtained for the 10 90 s period range by means of yanovskaya s formulation for laterally heterogeneous media according to the resolution of these local velocities a grid of 1 of latitude per 1 of longitude has been defined over the mediterranean area and a local group velocity dispersion curve has been then assigned to each gridpoint the stochastic inversion of almost 450 local dispersion curves permits deriving the respective 1d shear velocity structures from which the elaboration of a 3d tomographic image of the lithosphere asthenosphere system of the mediterranean region is straightforward as expected taking into account the complexity of this area the shear velocity model reveals significant lateral changes in the crust and uppermost mantle elastic structure as a main feature low shear velocities are deduced for most part of the central and eastern mediterranean basin at shallow depths down to 35 40 km whereas significantly higher velocities are obtained for the basins of the western mediterranean at this depth range this velocity pattern suggests a thicker crust in the centraleastern part with a greater thickness of sedimentary layers on account of the lower velocities deduced for the uppermost levels on the contrary for depths 80 km low shear velocities are obtained towards the west while the highest shear velocities are derived for the eastern mediterranean sea the aegean sea greece the centre and the south of the italian peninsula and the adriatic sea sicily and tunisia the velocity pattern at this depth range suggests a deeper beginning of the asthenosphere varying between 100 and 150 km under these areas while a shallower top around 70 km depth is found beneath the western mediterranean sea the areas where a thicker lithosphere has been deduced depict the main contact between the eurasian and african plates this feature being related to the collision and subduction processes linked to the convergence of both lithospheric plates

earthquakes and deformational structures seismites in holocene sediments from the himalayan andaman arc india soft sediment deformational structures from the himalayan arc caused by episodic earthquake recurrences during the holocene recent are produced by the ongoing tectonic processes these small scale subtle structures occur along some major thrusts in the garhwal himalaya and vary from normal faults in the lower zone to compressional structures in the middle zone along the main central thrust in the tons valley the latter includes isoclinal angular kink and disharmonic folds with discrete displacements along their axial surfaces thrusts with decollement listric and overstep thrusts in duplex style and load slumping the upper zone is marked by very low angle thrusts with left lateral displacement and associated displacement shears d riedel shears r and thrust shears p as well as small scale sporadic kinks three zones reveal changing tectonic character from gravity tectonics in the lower zone to compression in both the middle and upper zones with the maximum principal stress 1 oriented n60 to n110 respectively along the tons thrust post depositional structures include drag along normal fault zone characterized by d r and p shears and conjugate fractures that provide localized orientation of stresses 1 at 75 n60 2 at 12 n280 and 3 at 12 n290 earthquake induced liquefaction and subtle variations in physiomechanical properties of soft sediments have primarily controlled the deformational pattern due to localized tectonic stresses ground and sediment deformations during two earthquakes in the himalayan andaman arc provide additional support for the seismogenic origin of such structures preserved in loose or consolidated sediments

anelasticity of the crust and upper mantle beneath the pacific ocean from the inversion of observed surface wave attenuation summary backus gilbert inversion theory is applied to a new set of surface wave attenuation data to obtain average q 1 models for the crust and upper mantle beneath the pacific ocean the favoured model is obtained from the inversion of rayleigh wave data alone it includes an increase in q 1 or decrease in q at depths corresponding to the thickness of the litho sphere as determined from surface wave velocity studies a thick zone of high q 1 or low q in the upper mantle and decreasing q 1 deeper in the mantle the mean q 1 values for the lithosphere are lower than those previously determined for the lower crust beneath continents simultaneous inversions of the love and rayleigh wave data yield models which include thin alternating regions of high and low q 1 values independent inversions of the love and rayleigh wave data yield models which differ in their anelastic properties at depths between 20 and 120km these latter two types of models probably reflect systematic errors in the love wave data due to higher mode interference or complexities along the propagation path

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

shear wave velocity structure over the eastern indian subcontinent abstract fundamental mode surface wave generated by twelve earthquakes which occurred in northeastern india and nearby regions and were recorded at the shillong shl and chiengmai chg seismic stations are used to determine the shear wave velocity structure beneath these regions the frequency time analysis method is used to determine the group velocities for periods ranging from 4 to 50 s for fundamental mode rayleigh and love waves a high shear wave velocity 4 2 4 7 km s 1 is estimated in the lower 30 34 km of the crust crustal thickness is found to vary from 36 to 56 km with an overlying 2 4 km sedimentary thickness of shear wave velocity of 2 55 2 75 km s 1 the shear wave velocity in the upper 10 km of crust is 3 5 3 7 km s 1 below the sedimentary layer the inferred high shear velocity for the lower crust beneath these regions suggests an oceanic affinity and they are different from the indian shield structure the lower crust must have an oceanic origin as derived from the reminiscent of a certain oceanic lithosphere the available data do not permit us to estimate the upper mantle structure precisely however it shows a higher upper mantle shear velocity of 5 1 5 3 km s 1 this may be indicative of active tectonism beneath these regions in the uppermost mantle the moho boundary is not sharply defined here the increase in crustal thickness may be due to the collision of the indian and eurasian plates in the north and subduction of the indian plate beneath the burmese arc in the east

northwest pacific fundamental mode rayleigh wave group velocity and its relationship with tectonic structures the fundamental mode rayleigh wave generated by 27 earthquakes and recorded at 11 digital broadband seismic stations belonging to the ocean hemisphere project network in the northwest pacific region are analyzed to estimate the rayleigh wave group velocity for 160 paths at periods from 10 to 50 sec the inverted group velocity results suggest a quasi continental oceanic transitional structure of the north west pacific ocean with 21 km as an average crustal thickness the rayleigh wave group velocity tomography results for short time periods 10 50 sec showed a good correlation with known geological and tectonic features and were consistent with the tectonic complexities of the region slow velocity anomalies at varying periods correspond to ridges basins trenches and volcanic fronts of the northwest pacific ocean while the fast velocity anomalies show the effects of plate interactions at relatively longer periods 30 50 sec a triple junction formed by the subduction of the phillipine sea and pacific plates beneath the eurasian plate is imaged by a high velocity zone in this study

observations of the frontal region of a buoyant river plume using an autonomous underwater vehicle to characterize the transitional region from the near field to far field of a river plume entering coastal waters we conducted four surveys using an autonomous underwater vehicle auv to target the outflow of the new river inlet north carolina during maximum ebb tide the utilization of a mobile sensor to synoptically observe current velocity data in tandem with natural river plume tracers e g colored dissolved organic matter salinity was essential in understanding the mechanisms driving the observed circulation and mixing patterns within these waters we find that this region is regularly impacted by two primary processes 1 the interaction of an old dredged channel plume with the main discharge and 2 the recirculation of the discharge plume by an eddy that persistently forms between the old channel and main discharge location wind driven processes in the nearshore can enhance the interaction of these two plumes resulting in unstable regions where mixing of the merged plume with the receiving waters is accelerated we also conduct comparisons between auv velocity observations from two surveys and their corresponding velocity outputs from a parallelized quasi 3 d model we conclude that the ability to observe the estuarine outflow transitional region at near synoptic temporal scales and resolutions discussed in this paper is key in providing the mechanisms driving local circulation which is essential for proper parameterization of high resolution numerical coastal models

estimation of crustal qβ in the ganga basin using teleseismic broad band sh waveforms for the 1988 nepal india border earthquake abstract the average value of the intrinsic shear wave quality factor q has been estimated for the upper 55 km of the earth 47 km of crustal and upper 8 km of the upper mantle thickness in the border region between the ganga basin and himalaya in bihar we have used the teleseismic broad band sh and s sh waveforms of the 1988 nepal india border earthquake 6 4 m b for this purpose the frequency independent average value of q estimated in the range of 0 2 1 5 hz is found to be 1250 158

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

some elements of continental subduction along the himalayan front abstract detachment tectonics in which the subducting basement is not internally affected by thrusting and a steady state system in which the pattern of deformation and topography are invariant uplift is balanced by erosion and the material can experience subsequent phases of deformation by migrating through the system are proposed to be the fundamental mechanisms of convergence at the himalayan arc both surface and subsurface data are more consistent with these concepts than with more familiar concepts widely used in current models intracrustal thrusting and evolutionary tectonics where superimposed phases of deformation are interpreted as distinct phases the belt of intermediate magnitude thrust earthquakes the topographic front at the high himalaya and the main central thrust mct are associated with the same fundamental element the basement thrust front btf which traces a small circle in the central portion of the himalayan arc fault plane solutions indicate thrusting in the radial direction of this arc this radial convergence at the btf implies that tibet is extending laterally at a rate similar to the rate of convergence across the btf this extension cannot be unidirectional and must be parallel to the btf if the circular shape of the btf is invariant

rayleigh wave attenuation and its variation across the atlantic ocean summary fundamental mode rayleigh walies generated by several earthquakes situated on great circle paths traversing pairs of seismograph stations in american and european coastal regions and on atlantic islands have been analysed to obtain anelastic attenuation coefficients group and phase velocities and specific quality factors at periods between 15 and 100s inversions of the dispersion and attenuation data have yielded shear velocity and shear wave internal friction qj models of the upper mantle for regions of varying age in the atlantic ocean the attenuation coefficients and internal friction values can be related to the age of the atlantic ocean floor older regions being characterized by lower internal friction or higher q values a lowq zone is prominent in the upper mantle of all regions less than 65 myr in age but is poorly developed in older regions in contrast to the attenuation results no systematic variations of rayleigh or shear velocities with age can be resolved using the dispersion data of this study a comparison of internal friction models beneath the atlantic with those beneath the pacific indicates that for regions of similar age internal friction is smaller or q larger beneath the atlantic than beneath the pacific especially in the younger regions immediately contiguous with the ridges in particular the upper mantle low velocity low q zone is less prominent beneath the atlantic the fact that spreading rates of the lithospheric plates are also smaller in the atlantic suggests that internal friction and the rate of long term creep in the mantle are closely related

observation and analysis of shear instability in the fraser river estuary 1 we investigate the occurrence of shear instability in the fraser river estuary instabilities observed with an echo sounder are compared with a linear stability analysis based on observed velocity and density profiles we find that each set of observed instabilities coincides with an unstable mode predicted by the taylor goldstein equation each of these instabilities occurs in a region where the gradient richardson number is less than the critical value of 1 4 both the taylor goldstein predictions and the echo soundings indicate the instabilities are concentrated either above or below the density interface this one sidedness is in contrast to the archetypal kelvin helmholtz instability although the dominant source of mixing in the estuary appears to be caused by shear instability when the tide produces strong near bed velocities small scale overturning due to boundary layer turbulence is apparent throughout the depth

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

anelasticity of the crust and upper mantle beneath north and central india from the inversion of observed love and rayleigh wave attenuation data the fundamental mode love and rayleigh waves generated by earthquakes occurring in kashmir nepal himalaya northeast india and burma and recorded at hyderabad new delhi and kodaikanal seismic stations are analysed love and rayleigh wave attenuation coefficients are obtained at time periods of 15 100 seconds using the spectral amplitude of these waves for 23 different paths along northern across burma to new delhi and central across kashmir nepal himalaya and northeast india to hyderabad and kodaikanal india love wave attenuation coefficients are found to vary from 0 0003 to 0 0022 km 1 for northern india and 0 00003 km 1 to 0 00016 km 1 for central india similarly rayleigh wave attenuation coefficients vary from 0 0002 km 1 to 0 0016 km 1 for northern india and 0 00001 km 1 to 0 0009 km 1 for central india backus and gilbert inversion theory is applied to these surface wave attenuation data to obtainq 1 models for the crust and uppermost mantle beneath northern and central india inversion of love and rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20 80 km with lowq for northern india similarly inversion of love and rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 km the inferred lowq value at mid crustal depth high attenuating zone in the model for northern india can be by underthrusting of the indian plate beneath the eurasian plate which has caused a low velocity zone at this shallow depth the gradual increase ofq 1 from shallow to deeper depth shows that the lithosphere asthenosphere boundary is not sharply defined beneath central india but rather it represents a gradual transformation which starts beneath the uppermost mantle the lithospheric thickness is 100 km beneath central india and below that the asthenosphere shows higher attenuation a factor of about two greater than that in the lithosphere the very lowq can be explained by changes in the chemical constitution taking place in the uppermost mantle

continental collision tectonics evidence from the himalaya and the neighbouring regions abstract the evolution of the himalaya and the tectonics of neighbouring regions are generally believed to be due to continent to continent collision of the eurasian and the indian plates during the last decade many new data have been obtained and examined and a suite of models has been developed to explain the evolution of the himalaya and the neighbouring regions these models could broadly be classified into three categories the subdivision of the himalaya into five main structural units is now well accepted a tectogenetic diagram of the himalaya is briefly discussed ophiolites are reported to occur all along the indus tsangpo suture zone results of geochemical analysis of an ophiolite sequence from dras in kashmir himalaya are presented surface wave dispersion and attenuation investigations have revealed the crustal and upper mantle structure below the himalaya and tibet plateau the finding of low velocity layers is consistent with the concept that horizontal compression has been responsible for creating these weak zones and the inferred elevated temperatures in the vicinity of the moho discontinuity below tibet earthquake focal mechanism solutions and interpretation of landsat imagery indicate that convergence of the indian plate with eurasia continues and is accommodated partly by crustal shortening and partly by lateral mass movement at the northwest and the northeast margins of the indian plate palaeomagnetic data from the himalaya belt and central asia can be interpreted in support of molnar and tapponnier s model of continental collision

identification of amplified damage zones in palermo sicily italy during the earthquakes of the last three centuries the damage scenario for the city of palermo has been reconstructed for the three most damaging earthquakes that have occurred since the eighteenth century 1 september 1726 5 march 1823 15 january 1940 research has been carried out on the original written documents mostly kept in the archives of administrative and clerical institutions in sicily the chance to collate and compare the information coming from the different coeval sources provides an assurance as to the reliability of the collected data and optimizes the information framework all the damage episodes have been pinpointed in the ancient urban pattern of the city the historic urban area has been divided up using a grid with square module geometry each grid element being 100 100 m in size the resulting damage distribution shows a systematic amplification of effects in some of the grid elements in order to formulate an interpretation of the damage anomalies in terms of near surface geology a geographic information system gis has been used to handle a geological database of about 2000 geo referenced soil profiles in palermo this approach has provided us with a quick 2d and 3d visualization of the upper layer structure in the study area as expected a close correlation was found between the zones of major damage and the thickness of recent deposits characterized by weak cohesiveness and high compressibility the damage level increases at sites where thickness exceeds a 12 m threshold the strongest effects occur on water saturated deposits thicker than 22 m in terms of the macroseismic effects a transition from the hardest oligo miocene numidian flysch and calcarenites to the softest ground recent alluvial deposits and man made fill present in the historic center of palermo implies a difference in mercalli cancani sieberg mcs intensity by one to two degrees during the same earthquake an m 5 6 earthquake occurred off the palermo coast on 6 september 2002 subsequent to our analysis of the macroseismic data the instrumental data from this most recent shock provide a posteriori experimental evidence of significant ground shaking variations between the different geological formations this validates the macroseismic results and indicates that research and applications based on crossing different datasets can be successfully performed for urban areas using gis technology manuscript received 25 june 2002

mapping ocean outfall plumes and their mixing using autonomous underwater vehicles this paper reports on our experiences developing methods to survey an ocean outfall discharge plume using an autonomous underwater vehicle auv the mobility of an auv provides a significant advantage in surveying discharge plumes over traditional boat based methods and when combined with optical and oceanographic sensors provides a capability for both detecting plumes and assessing their mixing in both the near and far fields unique to this study is the optical sensing of of the presence of colored dissolved organic matter cdom in the discharge plume for quantitatively assessing the dilution ratio of the plume water this capability is demonstrated for two publicly operated treatment works potws located offshore san diego california that have significantly different environmental settings and operating parameters the first is the south bay ocean outfall sboo operated by the international boundary water commission ibwc whose discharge is located in approximately 30m deep water at a flow rate of 28 million gallons per day mgd the second discharge is the point loma ocean outfall ploo which discharges at approximately 90m with a nominal volume flow rate of 250 mgd the methodologies for planning the auv missions to sample the discharge plume characterizing the plume using the signals measured by the onboard oceanographic and optical sensors and use of those data in predictive models is outline the results suggest that even in variable oceanic conditions properly planned missions for auvs equipped with appropriate sensors can accurately characterize and track ocean outfall plumes with high spatial resolution more effectively than traditional methods

collision tectonics in the himalaya as evidenced by the indus and shyok rock assemblages abstract the himalayan orogeny is closely associated with the tectonic developments which occurred along the indus suture zone during the last decade many new data on sediment stratigraphy and volcanic plutonic suites have been obtained these data have been interpreted in terms of plate tectonics the meso tethys was generated following the permo triassic rifting and spreading during the jurassic the tethys ocean is postulated in accordance with geophysical data to have been consumed during northward subduction along the indus suture resulting in the himalayan orogeny through collision between india and eurasia however the nature of tectonic processes and their chronology appear to be different in the western and eastern himalayan segments in the western himalaya subduction in an island arc setting and eocene collision are evident mainly by the occurrence of the island arc dras volcanics the sangeluma group srikantia and rajdhan 1980 trench deposits ophiolitic blocks older 60 m y calc alkaline plutons and blueschist facies rocks the indus group srikantia and rajdhan 1980 continental deposits and the younger

anelasticity of the crust and upper mantle beneath the eurasian continent and the nearby regions from the inversion of love and rayleigh wave attenuation data summary the fundamental mode love and rayleigh waves generated by five earthquakes which occurred in central asia and were recorded at 20 35 wwssn stations throughout the world are analysed the love and rayleigh wave attenuations are determined at the time period of 10 120s for the continental and oceanic paths across eurasia and nearby regions using the spectral amplitudes of these waves the large standard deviation observed in the attenuation coefficients of surface waves may be due to the regional variation of the attenuative properties of the crust and upper mantle beneath eurasia the continental rayleigh wave attenuation coefficients are smaller by a factor of up to 2 compared to oceanic paths the continental rayleigh wave attenuation coefficient attains two minima of 0 1 5 x and 0 1 1 x 10 3km 1 at the time periods of 35 and 80 s respectively and two maxima of 0 58 and 0 3 x 10 3km at time periods of 10 and 100s respectively a similar trend is observed for the oceanic paths as well backus and gilbert inversion theory is applied to these love and rayleigh waves attenuation data to obtain qj models for the crust and upper mantle beneath eurasia and the nearby regions independent inversion of continental love and rayleigh waves attenuation data yields the thickness of the lithosphere to be 90 km for the oceanic paths the thickness of the lithosphere is estimated to be 110 km the base of the lithosphere is identified as the depth at which qf increases sharply qj in the asthenosphere is greater to a minimum of a factor of 2 as compared to the lithosphere

site amplification at five locations in san francisco california a comparison of s waves codas and microtremors abstract we compare microtremor data to weak motion s wave and coda recordings at sites in san francisco in order to clarify the range of applicability of microtremor data to ground motion prediction we also compare s wave results to coda results for each type of data we compute spectral ratios of motions from two soil rock station pairs and from an uphole downhole pair in the marina district we compute horizontal vertical ratios nakamura9s method at a soil site a rock site and the surface and borehole instruments in the station pair analyses microtremor data show amplifications at the same fundamental frequency as s waves but the frequencies of other peaks do not agree the amplification at frequencies higher than 2 hz is greater in the microtremor data station pair ratios of coda data generally show spectral peaks occurring at the same frequencies but with levels varying from one to four times the amplification from s wave ratios nakamura9s method of analyzing microtremors agrees better with s wave station pair results than the microtremor station pair method over a limited frequency band that varies from station to station

site effects and microzonation in the city of thessaloniki greece comparison of different approaches ten seismological stations were installed for 3 months in the city of thessaloniki greece to record earthquakes and ambient noise over areas of different geotechnical characteristics about 40 local earthquakes were recorded with a good signal to noise ratio in more than two stations we analyze the records with three different techniques a the classical spectral ratios to a reference station b the receiver functions and c the horizontal to vertical h v spectral ratios on noise recordings the results are compared in terms of predominant frequencies and amplification levels the three methods are found equally able to reveal the fundamental frequency regarding local site amplification they provide significantly different results with a general trend of the two h v ratio methods b and c to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique a the results are then used in relation with the microzonation of the city of thessaloniki a good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock finally we compare our observations with the damage distribution in the city which was hit by a magnitude 6 5 earthquake in 1978 the amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity

anelasticity of the crust and upper mantle of south america from the inversion of observed surface wave attenuation fundamental mode rayleigh and love waves generated by several earthquakes situated along great circle paths between pairs of seismograph stations have been analysed to obtain coefficients of attenuation group velocities phase velocities and specific quality factors in the period range 18 80s in two regions of the south american continent one set of paths crosses the shield region which lies on the eastern coast and another set traverses the mountainous region inland the average attenuation coefficient values are clearly higher in the tectonically active western region throughout the entire period range than in the eastern or shield region inversion of the attenuation data yielded shear wave internal friction q 1 models as a function of depth in the crust and upper mantle in both regions a low q zone below the lithosphere is prominent in both regions the results show that substantial variations of q occur in the two regions of south america the q values were found to be inversely related to the heat flow values or to the temperature

the quest for an unambiguous parameterization of mixing efficiency in stably stratified geophysical flows estimates of turbulent mixing in geophysical settings typically depend on the efficiency at which shear driven turbulence mixes density across isopycnals to date however no unifying parameterization of diapycnal mixing efficiency exists due to the variability of natural flows and also due to certain ambiguities that arise from descriptions based on a single parameter here we highlight important ambiguities of some common single parameter schemes in the context of a multiparameter framework that considers the independent effects of shear buoyancy and viscosity parameterizations based on the gradient richardson number ri the turbulent froude number frt and the buoyancy reynolds number reb are considered the diagnostic ability of these parameters is examined using published data from both direct numerical simulations and field observations

anelastic structure of the iberian peninsula obtained from an automated regionalization algorithm and stochastic inversion abstract an automated regionalization of a reduced domain of the earth s surface without a priori seismotectonic information and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of rayleigh waves our database is formed by path averaged attenuation coefficients derived from fundamental mode rayleigh waves crossing the iberian peninsula the wavetrains were recorded at the broad band stations installed some years ago in the iberian peninsula for the iliha project before the sequential application of the algorithms the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10 120 s period range all these maps were obtained by applying yanovskaya s formulation for laterally heterogeneous media to the set of path averaged attenuation coefficients mentioned above after that we divided the iberian peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm we then obtained by stochastic inversion the respective anelastic structure down to a depth of 225 km for the six homogeneous regions the coefficients vary from 1 0 to 2 0 10 3 km 1 and the q 1 structures ranging from 110 10 3 to 40 10 3 suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth respectively a shallow beginning of the asthenosphere is detected for the sixth homogeneous region this range is very similar to the depths deduced from previous tomographic studies of the iberian peninsula based on rayleigh wave phase and group velocities finally correlations obtained between the regions and some seismotectonic characteristics of the peninsula are discussed it is noteworthy that we cannot associate each of the six homogeneous regions with a single hercynian alpine or neogene domain

electrical structure across the indus tsangpo suture and shyok suture zones in nw himalaya using magnetotelluric studies 1 magnetotelluric studies in the nw himalayan region have shown anomalously high conductance of about 20 000 siemens in the crust beneath the indus tsangpo suture its and the adjoining tso morari dome in the depth range of 1 20 km high heat flow and high attenuation of the seismic waves in the himalayan crust together with the observed high conductance indicate presence of wide spread partial melt generated from the subducted indian crust the ladakh batholith appears as a resistive block to the north of its a moderately conductive zone demarcates the ladakh batholith from karakoram batholith to the north the similarity in the resistive structure with the results reported from the tibetan region by wei et al 2001 about 1500 km to the east is rather significant indicative of a two dimensional nature of the himalayan collision belt at least to the first order

observations of small scale secondary instabilities during the shoaling of internal bores on a deep ocean slope abstractthe rockall bank area located in the northeast atlantic ocean is a region dominated by topographically trapped diurnal tides these tides generate up and downslope displacements that can be locally described as swashing motions on the bank using high spatial and time resolution of moored temperature sensors the transition toward the upslope flow cooling phase is described as a rapid upslope propagating bore likely generated by breaking trapped internal waves buoyant anomalies are found during the bore propagation likely resulting from small scale instabilities the imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing i e young and have high mixing potential

crust and upper mantle velocity structure beneath north and central india from the phase and group velocity of rayleigh and love waves abstract crust and upper mantle velocity structure is determined using the fundamental mode rayleigh and love waves for 23 different paths across north and central india the earthquakes occurring in kashmir nepal himalaya burma and nearby regions and recorded at hyderabad new delhi and kodaikanal seismic stations are used for dispersion studies frequency time analysis ftan using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13 71 s the single station technique is used to estimate rayleigh and love wave phase velocities at time periods of 15 75 s velocity structure down to a depth of 120 km is determined using both rayleigh and love wave phase and group velocities the crustal thickness is found to be 50 km with 4 km sedimentary thickness for the northern india across burma to new delhi path and 44 km for the central india across kashmir nepal himalaya and northeast india to hyderabad and kodaikanal path a low velocity zone v p 8 4 km s v s 4 15 km s is found from 100 km below the surface velocity structure shows that the shield structure of the peninsula extends up to northernmost india below sub himalayan regions with an overlying sedimentary thickness of 4 km the lower value of observed group velocity and higher phase velocity across northern and central india may be caused by the presence of varying sedimentary thickness in the northernmost part of india the indo gangetic plain below the sub himalayan region the observed lower value of rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a p wave velocity of 4 6 km s an s wave velocity of 2 5 km s a density of 3 24 g cm3 and a thickness of 3 km centred above the moho

improvements in the knowledge of the anelastic structure of the iberian peninsula from seismic data of the iliha project summary attenuation coefficients of rayleigh waves propagating across the iberian peninsula and quality factor models were obtained for different two station great circle paths the data used were long period wave trains contained in the seismograms provided by the iliha array installed in the peninsula in the 1980s more than 300 seismograms were analysed to obtain 14 sets of similar paths consisting of either paths totally included in one of the four pure tectonic regions or mixed paths crossing several regions we used the two station method to determine attenuation parameters r and then an inversion method to derive q 1 models the attenuation coefficients of seismic energy in the 10 120 s range are in the range 2 8 10 3 0 1 10 3 km 1 and correlate well with both previous results obtained from analogue wwssn and most seismogeotectonic characteristics of iberia the q models obtained also agree with a few models for iberia determined earlier and they show significant variations from one region to another as a general feature the values are higher in the active seismotectonic regions typical maximum of 2 6 10 3 km 1 than in the more stable ones typical maximum of 1 4 10 3 moreover the maximum of q 1 in the active regions 145 10 3 120 10 3 is higher than in the stable ones 40 10 3 100 10 3 in general both the regionalized attenuation coefficients of rayleigh waves and the quality factor models exhibit a good agreement with previously published lg wave attenuation and coda q distribution patterns in iberia

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

structure and generation of turbulence at interfaces strained by internal solitary waves propagating shoreward over the continental shelf abstract detailed observations of the structure within internal solitary waves propagating shoreward over oregon s continental shelf reveal the evolving nature of interfaces as they become unstable and break creating turbulent flow a persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake this is demonstrated to be due to enhanced density microstructure increased small scale strain ahead of the wave trough compresses select density interfaces thereby locally increasing stratification this is followed by a sequence of overturning high density microstructure and turbulence at the interface which is coincident with the high acoustic backscatter the richardson number estimated from observations is larger than 1 4 indicating that the interface is stable however density profiles reveal these preturbulent interfaces to be o 10 cm thick much thinner than can be resolved with shipboard velocity measurements by as

strain deformation in the northern indian ocean abstract seismicity focal mechanisms and moment tensors are used to estimate the strain deformation for earthquakes which occurred during the last 80 yrs in the northern indian ocean up to 38 s the shear strain deformation values are 0 47 10 8 near the owen s fracture zone 0 26 10 8 in the north carlsberg ridge 0 51 10 8 in the south carlsberg ridge 4 92 10 8 in the chagos laccadive ridge 0 21 10 8 in the rodriguez fracture zone 0 9 10 8 in the southwest mid indian ocean ridge 0 16 10 8 in the southeast mid indian ocean ridge 0 15 10 8 in the ninety east ridge and 2 11 10 8 in the sunda arc the high shear strain obtained in the area 6 s 10 s and 65 e 72 e suggests that the earthquakes here are characterized by high creep strength 20 kbars of material in the vicinity of the earthquake epicentral regions the slip rates obtained from the seismic moment are found to vary from 0 30 to 9 85 mm yr in different parts in the northern indian ocean the maximum slip rate is obtained for the chagos laccadive ridge moment tensor values obtained in different parts of the northern indian ocean suggest that the e w extension and the n s compression are predominant vertical compression crustal thinning is observed in the western areas whereas vertical extension crustal thickening occurs in eastern parts of the northern indian ocean the tension and compression strain deformation values estimated in different parts of the northern indian ocean support the hypothesis of ocean floor spreading and continental collision of the indian and eurasian plates the principal stress directions obtained from the moment tensor of earthquakes show that the compressive stress field is acting approximately in the alignment of the ridge and that the extensional stress is perpendicular to the ridge axis vertical compressions are more prominent near the owen s fracture zone in the chagos laccadive ridge and in the southwest mid indian ocean ridge whereas vertical extension is prominent in the sunda arc and the ninety east ridge

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

crustal q structure in the united states from multi mode surface waves determination of q structures are made using a new method which matches observed surface wave spectra with theoretical spectra computed for earthquakes with known fault plane solutions the importance of the method lies in its ability to utilize relatively short paths between a single source and receiver both of which are preferably located within a single geological province the method has been applied to study the regions of the eastern united states the colorado plateau and the basin and range province q models have been obtained through trial and error procedures and with an inversion scheme developed for that purpose models for each region obtained from these two procedures are in good agreement q values in the upper crust as obtained from inversions are 275 for the eastern united states 160 for the colorado plateau and 85 for the basin and range province q values in the lower crust although not well determined must be much higher than upper crustal values in all three regions the conclusions are consistent with those of previous work in which q values in the upper crust of a broad region of the western united states are lower than those of the eastern united states these new results show that substantial variations of q occur in different regions of the western united states the upper crustal q values show an inverse relationship to heat flow values

attenuation characteristics of body waves for the bilaspur region of himachal lesser himalaya the attenuation characteristics around bilaspur region of the himachal lesser himalaya have been estimated adopting extended coda normalization method and using a data set of 41 local events 0 5 m l 2 9 that occurred in the region from may 2013 to march 2014 the frequency dependent relations governing the quality factors of p waves q and s waves q in the frequency range from 1 5 to 24 hz are q 43 4 f 1 30 0 04 and q 79 6 f 1 25 0 02 the average estimates of q and q are found to vary from 71 and 125 at 1 5 hz to 2901 and 4243 at 24 hz respectively the q and q estimates are compared to the similar estimates obtained for the other seismically active regions of the himalaya it is found that for the various himalayan regions the q estimates at 1 hz vary between 22 for the kumaon himalaya and 97 for the northwest himalaya whereas q estimates range between 63 for the garhwal himalaya and 127 for the northwest himalaya for the bilaspur region the q q ratio is greater than unity and varies between 1 84 and 1 45 in the frequency range from 1 to 24 hz the region specific attenuation relations can be adopted for estimating earthquake source parameters simulating strong ground motion and assessing seismic hazard for the bilaspur region of himachal lesser himalaya

q structure beneath the north and central indian ocean from the inversion of observed love and rayleigh wave attenuation data abstract the fundamental mode love and rayleigh waves generated by 57 earthquakes which occurred in the north and central indian ocean extending to 40 s and recorded at indian seismograph and other wwssn stations such as how shl vis mdr hyb kod chg trd poo bom goa ndi nil and que are analysed love and rayleigh wave attenuation coefficients are estimated at periods of 15 100 s using the spectral amplitude of these waves for 98 different paths across the bay of bengal fan the arabian fan and the north and central indian ocean the large standard deviations observed in the surface wave attenuation coefficients may be a result of regional variation of the attenuative properties of the crust and upper mantle beneath these regions love wave attenuation coefficients are found to vary from 0 000 03 to 0 000 45 km 1 for the bay of bengal fan from 0 000 03 to 0 000 85 km 1 for the arabian fan and from 0 000 03 to 0 000 35 km 1 for the north and central indian ocean similarly rayleigh wave attenuation coefficients vary from 0 000 03 to 0 0004 km 1 for the bay of bengal fan from 0 000 06 to 0 0007 km 1 for the arabian fan and from 0 000 03 to 0 0007 km 1 for the north and central indian ocean backus and gilbert inversion theory is applied to these surface wave attenuation data to obtain average q 1 models for the crust and upper mantle beneath the bay of bengal the arabian fan and the north and central indian ocean inversion of love and rayleigh wave attenuation data shows a high attenuation zone centred at a depth of 120 km q 125 for the bay of bengal fan similarly a high attenuation zone q 40 70 occurs at a depth of 60 160 km for the arabian fan at 100 160 km q 115 for the indian ocean off ninetyeast ridge and at 80 160 km q 80 for the indian ocean across the ninetyeast ridge the q 1 models show a lithosphere thickness of 120 km beneath the bay of bengal fan similarly lithosphere thickness of 70 100 and 80 km is estimated beneath the arabian fan and the indian ocean off ninetyeast ridge and across ninetyeast ridge respectively the base of the lithosphere is identified as the depth at which there is a significant increase in the q 1 value which attains its maximum value in the asthenosphere the thinning of indian lithosphere beneath the arabian fan suggests high temperature below moho depth 60 km from surface which has caused a high attenuation zone at this shallow depth

hydraulics and mixing in a laterally divergent channel of a highly stratified estuary author posting american geophysical union 2017 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 122 2017 4743 4760 doi 10 1002 2016jc012455

shear velocity and shear attenuation models inverted from the world wide and pure path average data of mantle rayleigh waves 0s25 to 0s80 and fundamental spheroidal modes 0s2 to 0s24 summary global and regional structures of the earth s mantle are studied original phase velocities and q 1 of rayleigh waves are converted to equivalent free oscillation periods and q 1 global average upper mantle shear velocity models are derived from the inversions of the world wide average eigenperiods corrected for attenuative dispersion resolution analysis shows that the thickness and velocity of the lithosphere cannot be determined from our data set inverted models have a well developed low velocity zone in the depth range between 70 and 280 km global average q 1 models for the whole mantle are derived from the world wide average q 1 of mantle rayleigh waves and fundamental spheroidal modes the resolution analysis indicates that q 1 at depths shallower than about 100 km cannot be determined by our data set and that the depth resolution for q 1 of the lower mantle is poor all inverted q 1 models are characterized by a well developed high q 1 zone in the depth range between about 100 and 300 km in the upper mantle q has a minimum value of about 90 at depths around 200 km begins to increase at a depth of about 300 km rapidly increases in the depth range from 300 to 400 km and gradually increases at depths between 400 and 800 km q of the lower mantle is about 330 the boundary between the high q 1 upper mantle and low q 1 lower mantle is sharper than has been suggested in the past inverted q 1 models predict qscs 226 to 250 the earth s surface is divided into mountainous oceanic ridge shield and trench and marginal sea regions and a regionalized shear velocity model of the upper mantle is derived the inversions of the pure path eigenperiods without correction for attenuative dispersion show that strong lateral variations in shear velocity exist at depths shallower than 250 km in the upper mantle a low velocity zone for shear waves exists in the upper mantle beneath all the tectonic regions except perhaps the shield region if the sn velocity of the starting model is equal to or higher than 4 7 km s 1 the shield data require a moderate low velocity zone data from the mountainous region require a much lower shear velocity than the shield region at depths shallower than 250 km the oceanic ridge and trench and marginal sea models have a well developed low velocity zone the shear velocity model for the trench and marginal sea region is characterized by a velocity increase at depths between 250 and 400 km this increase in velocity is interpreted as a consequence of the subducted lithospheric slabs beneath many of our trench and marginal sea regions regional differences in rayleigh wave q 1 are found for modes from 0s25 to 0s45 since the resolving power of this data set is limited we could not locate the depths at which regional variations of q 1 exist since the oceanic q 1 for modes from 0s45 to 0s78 appear to be stable the oceanic q 1 model is derived this model has a well developed high q 1 zone q has a minimum value of about 110 at depths between 200 and 300 km and rapidly increases to a high value of about 250 at a depth of about 400 km

internal wave turbulence in the llobregat prodelta nw mediterranean under stratified conditions a mechanism for sediment waves generation abstract an array of 76 high resolution temperature sensors at 0 5 m intervals between 5 and 42 5 m off the bottom was moored near the barcelona harbor buoy in 81 m water depth between october 2013 and april 2014 the mooring was located just seaward of an extensive sediment wave area developed in the llobregat river prodelta with 1 m high crests parallel to the coast and 50 100 m wavelengths in the nw mediterranean the thermal stratification reaches its maximum penetration through the water column in autumn until it is broken by winter convection such a deep stratification affects large scale sub inertial slope currents which are mostly confined to the upper half of the water column by the hampered vertical exchange of frictional turbulence and supports near bottom internal waves between the inertial and buoyancy frequencies observed onshelf propagating frontal bores most likely interact with the sediment waves and contribute to their generation as they are trailed by considerable shear induced turbulence and high frequency internal waves close to the buoyancy frequency that have wavelengths matching those of the sediment waves the bores are either driven by near inertial or 3 7 day periodic sub inertial motions just following a brief period of large convective instability at the end of the offshelf flow phase