influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica an isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath filchner ronne ice shelf investigating the role of tides on sub ice shelf circulation and ice shelf basal mass balance including tidal forcing causes a significant intensification in the sub ice shelf circulation with an increase in melting 3 fold and refreezing 6 fold the net melt rate and seawater flux through the cavity approximately doubles with tidal forcing the spatial pattern and magnitude of basal melting and freezing generally match observations the 0 22 m a 1 net melt rate is close to satellite derived estimates and at the lower end of oceanographic values the ice shelf water outflow mixes with shelf waters forming a cold 1 9 degrees c dense overflow 0 83 sv that spills down the continental slope these results demonstrate that tidal forcing is fundamental to both ice shelf ocean interactions and deep water formation in the southern weddell sea citation makinson k p r holland a jenkins k w nicholls and d m holland 2011 influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica geophys res lett 38 l06601 doi 10 1029 2010gl046462

modeling the jiyang depression northern china using a wave field extrapolation finite difference method and waveform inversion we study the 2d seismic velocity structure of the south edge of the jiyang depression in the bohai bay basin northern china using recordings from 28 broadband seismic stations and a method combining an shear horizontal sh forward synthetic calculation and waveform inversion the forward synthetic cal culations are performed by a finite difference fd method with its computational domain localized in the basin area and input motions at the base of the model ex trapolated from the displacement recorded at a nearby hard rock station in waveform inversion the shapes of the strata are varied to search for a best fitting model based on the fit of relative timing amplitude and phase of the synthetics to the seismic data numerical experiments using synthetic data and test models demonstrate the validity resolution and sensitivity of our technique with this technique we obtain two cross sections of the south edge of the jiyang depression the bohai bay basin northern china the waveform inversions using the seismic data from four teleseis mic events show consistent results for both cross sections those inversion results are also in agreement with a geological cross section available in the region our models estimate that the south edge of the jiyang depression is juxtaposed against the luxi uplift and its structures our inversion results also reveal a blind fault in the region and features suggesting major extensions in paleogene

the relation between ground acceleration and earthquake source parameters theory and observations abstract a simple relation between the root mean square rms of the ground acceleration and earthquake spectral or source parameters is introduced in which 0 is the low frequency displacement spectral plateau f 0 is the corner frequency is an attenuation parameter and t is the data interval this result uses the omega square model for far field radiation and accounts for site specific attenuation the main advantage of the new relation with respect to that of hanks 1979 is that it relaxes the simplifying assumption that the spectral corner frequency is much smaller than the maximum corner frequency resulting from attenuation and that the spectrum may be approximated as being perfectly flat between the two frequencies the newly proposed relation is tested using a composite dataset of earthquake records from japan california mexico and taiwan excellent agreement is found between observed and predicted ground acceleration for any combination of corner frequencies thus use of the above relation enables the extrapolation of ground motion prediction equation inferred from the frequent small magnitude earthquakes to the rare large magnitudes this capacity is extremely useful near slow slip plate boundaries where the seismic moment release rates are low

waveform inversion for modeling three dimensional crust structure with topographic effects abstract this paper proposes a waveform inversion analysis for three dimensional 3d crust structure modeling considering the effect of surface topography in order to compute the sensitivity functions we use a 3d finite element method fem based on a hybrid grid which can improve the accuracy in surface and interface modeling and reduce the computation cost of modeling and analysis the applicability of the method is examined by modeling the interface between a sedimentary layer and seismic bedrock we then demonstrate the advantage of our method by comparing 3d models with and without consideration of surface topography the results confirm the existence of significant surface topography effects and show that our method can accurately estimate 3d crust structures even with such topographic effects

correlation of change in magnetotelluric parameters before and after sikkim himalayan earthquakes associated with stress variation abstract mt stations that were recorded before the moderate 5 3 mw 2006 sikkim himalaya earthquake had been reoccupied after 2007 mw 5 0 and 2011 mw 6 9 seismic activities we analyzed the broadband mt data 0 001 1000 s within the 60 km radius from epicenters location and within the focal depth range of 9 30 km the observed change in apparent resistivity in the period range of 0 001 10 s has shown a considerable post seismic decrease of the order 10 30 the 2d geoelectric model constructed with post seismic acquired data also reveals the feature of a decrease in resistivity c1 the relative change decrease of observed apparent resistivity obs after 2006 2007 and 2011 earthquakes find a correlation with combined stress drop 11 18 2 mpa values estimated by arms approach the correlation is further strengthened by a similar change in relative apparent resistivities 2d model derived from 2d model with stress drop integrated stress drop values attribute to the accumulated apparent stresses a 0 23 that reveals the area might be subjugated to 0 6 1 2 mpa stress before 2006 1 1 5 mpa stress before 2007 and 1 7 2 9 mpa before 2011 earthquakes occurrence development of micro cracks and generation of aqueous fluids is possible due to such stressed and slow rate of relentless thrust shear environment within the area of crust therefore resistivity could have been decreased with a fluid transition from compression to dilatancy zones at different depth levels before each earthquake and further decreased upon subsequent stress drops associated with strong ground motions we interpreted that maximum stress might be dropped in dilatational zones oblique to the maximum compression stress directions along n200e n s and nnw sse before 2006 2007 and 2011 earthquakes and resulted in a considerable decrease in apparent resistivity along e w te mode i e yx component

a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation 1 an isopycnic coordinate ocean circulation model has been applied to the southern weddell sea including the cavity beneath filchner ronne ice shelf with the aim of investigating the buoyancy forced circulation on the continental shelf buoyancy forcing is associated with both the annual growth and decay of sea ice and the interaction between ice shelf and ocean in the model a generally anti cyclonic circulation develops beneath the ice shelf so that new shelf waters entering the cavity in the west emerge colder and fresher in the east the outflow contributes to a dense current that spills off the continental shelf and descends the slope oceanographic observations from the region are consistent with this picture and highlight the overflow as a major source of weddell sea bottom water

modeling the basal melting and marine ice accretion of the amery ice shelf the basal mass balance of the amery ice shelf ais in east antarctica is investigated using a numerical ocean model the main improvements of this model over previous studies are the inclusion of frazil formation and dynamics tides and the use of the latest estimate of the sub ice shelf cavity geometry the model produces a net basal melt rate of 45 6 gt year 1 0 74 m ice year 1 which is in good agreement with reviewed observations the melting at the base of the ice shelf is primarily due to interaction with high salinity shelf water created from the surface sea ice formation in winter the temperature difference between the coldest waters created in the open ocean and the in situ freezing point of ocean water in contact with the deepest part of the ais drives a melt rate that can exceed 30 m of ice year 1 the inclusion of frazil dynamics is shown to be important for both melting and marine ice accretion refreezing frazil initially forms in the supercooled water layer adjacent to the base of the ice shelf the net accretion of marine ice is 5 3 gt year 1 comprised of 3 7 gt year 1 of frazil accretion and 1 6 gt year 1 of direct basal refreezing

ocean circulation beneath filchner ronne ice shelf from three dimensional model results a high resolution three dimensional ocean circulation model is applied to the cavity beneath filchner ronne ice shelf fris the model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands the surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses in the former case leading to freezing while melting at the ice shelf base results in the latter case the pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath fris an anticyclonic circulation around the korff and henry ice rises with melting west of korff ice rise and freezing on the eastern side and north of henry ice rise is the main source for an ice pumping mechanism that produces the observed large marine ice body in the central filchner ronne ice shelf the estimates for net melting for realistic conditions at the open ocean boundary are 40 50 km 3 yr 1 indicating that ice shelf ocean interaction is an important contribution to the mass balance of the ice shelf

a model of ice shelf ocean interaction with application to the filcher ronne and ross ice shelves a simple analytical model has been developed to study the formation of ice shelf water isw isw is assumed to flow as a buoyancy driven layer underneath the ice shelf a relation between potential temperature and salinity in the isw layer is calculated from the mass and energy balance this temperature salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates the model results are obtained without making any assumptions about entrainment and melt rates the model is in good agreement with observations under the ronne ice shelf and it indicates that isw in the filchner depression is formed from western shelf water wsw with salinity higher than 34 75 practical salinity units such high salinity water is only observed in the ronne depression in the western part of the continental shelf this implies a circulation of wsw under the filchner ronne ice shelf from the ronne depression into the filchner depression similarly the model shows that the isw observed under j9 at the ross ice shelf has been formed from low salinity shelf water lssw from the eastern parts of the ross sea continental shelf lssw must therefore circulate under the eastern parts of the ross ice shelf

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

variability of ground motions in southern california data from the 1995 to 1996 ridgecrest sequence seismograms from the 1995 to 1996 ridgecrest california earthquake sequence recorded by the trinet digital seismic network provide high quality waveforms from sites throughout southern california including sites in markedly heterogeneous areas like the los angeles area sedimentary basins synthetic seismograms calculated by the reflectivity method with various 1d models are used as a baseline to measure the variability of amplitudes throughout southern california regardless of the model used there is greater variability in the amplitudes from basin site records than from rock site records rock soil and basin sites are all rather insensitive to radiation pattern nodes at the three frequency bands investigated 0 1 to 0 2 hz 0 2 to 0 4 hz and 0 4 to 0 8 hz this complicates the analysis because the nodes create singularity points in the distribution of ratios of observed and synthetic amplitudes when stations near nodal planes are removed the surface waves observed at most rock sites have peak amplitudes within a factor of 2 of synthetic waveform amplitudes peak amplitude of the surface waves observed at the soil and basin stations are more variable with the bulk of the distribution of data synthetic amplitude ratios less than 3 and a few outliers greater than 5 these outliers occur at the higher frequency bands soil and basin sites are also more often larger than the synthetics higher median values most outliers can be explained by applying a water level of 50 to the radiation pattern this reduces the scatter in the distributions to about the same extent as removing data within 10 of nodes thus most of the outliers are sites that are insensitive to the nodes not sites that are larger than the overall data distribution

numerical simulation of the filchner overflow the plume of ice shelf water isw flowing into the weddell sea over the filchner sill contributes to the formation of antarctic bottom water the filchner overflow is simulated using a hydrostatic primitive equation three dimensional ocean model with a 0 5 2 sv isw influx above the filchner sill the best fit to mooring temperature observations is found with influxes of 0 5 and 1 sv below a previous estimate of 1 6 0 5 sv based on sparse mooring velocities the plume first moves north over the continental shelf and then turns west along slope of the continental shelf break where it breaks up into subplumes and domes some of which then move downslope other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner the next western ridge is crossed by the plume through several paths despite a number of discrepancies with observational data the model reproduces many attributes of the flow in particular we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow where the temperature fluctuations are determined by the motion of the domes past the moorings our sensitivity studies show that while thermobaricity plays a role its effect is small for the flows considered smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges an increase in the bottom drag or viscosity leads to slowing down and hence thickening and widening of the plume

modeling tidal current profiles and vertical mixing beneath filchner ronne ice shelf antarctica abstract one of the warmest water masses beneath filchner ronne ice shelf fris is dense high salinity shelf water hssw that flows into the sub ice shelf cavity from the ice front and occupies the lower portion of the water column a one dimensional turbulence closure ocean model has been applied to this high latitude sub ice shelf environment to demonstrate that tidal currents mix hssw vertically through the water column and cause melting at the bottom of the ice shelf significantly fris lies near the critical latitude for the semidiurnal tide where the coriolis frequency equals the tidal frequency resulting in a strongly depth dependent tidal current and thick boundary layers using the model the effect of the critical latitude stratification and the polarization of the tidal current ellipse on boundary layer structure and subsequent vertical mixing are examined the model shows that stratification significantly affects how the shape of the tidal current ellipse varies with depth and that both

coda waves a review the analysis and interpretation of coda waves have received increasing attention since the early seventies in the past few years interest in this subject has spread worldwide and the study of high frequency seismic coda waves has become a very important seismological topic as a conclusion of the studies accomplished in this time coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves p ands waves play a particularly important role in this interaction the process introduces an attenuation which added to the intrinsic absorption gives the observed apparent attenuation therefore coda waves constitute a thumbprint left by the heterogeneities on the seismograms coda waves offer decisive information about the mechanism of how scattering and attenuation take place this review describes coda waves in detail and summarizes the work done in this subject to 1986 the relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed particular attention has been given to the application of coda waves to estimate source and medium parameters the state of the art of the temporal variations of coda wave shape and the possible use of these variations as an earthquake precursor also are presented care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering

far source long period displacement response spectra in the po and venetian plains italy from 3d wavefield simulations abstract design elastic displacement spectra for seismic codes and zoning studies of seismic hazard for long period structures are becoming of utmost importance in modern earthquake engineering the increase of displacement based design approaches together with the availability of good quality digital strong motion data allow the creation of reliable spectral shapes for seismic design of up to 10 s nevertheless to characterize the site amplification effects during large levels of shaking additional studies are needed especially in deep sedimentary basins and plains that can be strongly excited by large amplitude long period waves data within deep basins is rare because accelerometric seismic networks are normally deployed on rock or stiff soil and modeling of waveforms is hence necessary for bridging this gap focusing on the numerical modeling of the 3d wavefield in 1d flat layered and 3d structures in the po and venetian plains northern italy and on earthquakes with magnitudes m 6 6 and hypocentral distances greater than 50 km in this study we 1 estimate the displacement response spectra drs from 1 to 10 s in the po and venetian plains 2 compare the drs from numerical simulations using flat layered and heterogeneous 3d models against the currently available uniform hazard drs for italy and 3 evaluate the influence of deep sediments on drs shape and thus on seismic codes finally 1d and 3d simulated drs are compared with drs obtained from recordings in the kanto basin where similar long period displacement is observed

seismic structure of the bohai bay basin northern china implications for basin evolution as part of an extensive seismic research program 33 portable broadband seismic stations were deployed along a line crossing the bohai bay basin northern china three teleseismic events were selected to constrain the seismic structure along a 280 km profile across the western edge of the basin we determined the basin structure that described the observed shear horizontal sh wave field the synthetic sh wave was calculated using a finite difference fd method with its computational domain localized in the basin area and input motions at the base of the model extrapolated from the displacement recorded at a nearby hard rock station synthetic seismographs calculated for the models match the observations well in both waveform and travel time numerical tests indicate that the structural features of the preferred models are well resolved the analysis of relations between structures and stratigraphic units along the cross sections allows multiple deformational events in the basin to be inferred in conjunction with a profile across the southern edge that has been presented previously by zhao et al 14 l zhao t y zheng w w xu modeling the jiyang depression northern china using a wave field extrapolation fd method and waveform inversion bull seismol soc am 94 2004 988 1001 the results reveal basin wide extension with local inversion features in the bohai bay basin d 2005 elsevier b v all rights reserved

modeling tidal currents beneath filchner ronne ice shelf and on the adjacent continental shelf their effect on mixing and transport a depth averaged tidal model has been applied to the southern weddell sea the model domain covers the southern continental shelf including the ocean cavity beneath filchner ronne ice shelf reasonable agreement with the available current meter data has been achieved our results confirm that in areas with shallow water and large topographic gradients tidal oscillations with peak velocities up to 1 m s 1 play a significant role in the vertical mixing and transport of water masses the estimated energy dissipation beneath filchner ronne ice shelf due to surface friction is 25 gw approximately 1 of the world s total tidal dissipation tidally induced lagrangian residual currents converging at the ice front an area of strong mixing draw together water masses from the continental shelf and sub ice shelf cavity the model indicates that lagrangian residual currents have fluxes of up to 250 000 m3 s 1 and speeds of over 5 cm s 1 along the ice front with over 350 000 m3 s 1 being exchanged between the sub ice shelf cavity and adjacent continental shelf these currents are particularly efficient in ventilating the sub ice shelf cavity within 150 km of ronne ice front such strong tidal mixing will significantly modify the properties of water masses that flow through this region particularly to the west of berkner island the model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of ronne ice front shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

revised ml determination for crustal earthquakes in taiwan the local magnitude scale m l is an empirically derived scale anchored to a zero magnitude reference event richter defined the amplitude of ground shaking at an epicentral distance of 100 km for a zero magnitude earthquake in southern california the crustal attenuation characteristics of taiwan result in a relatively high ml when compared to moment magnitude mw we therefore define a revised ml scale for crustal earthquakes in the taiwan region that is more consistent with mw using observed peak ground shaking and mw we determine a new log a0 curve of ground shaking versus hypocentral distance r for a zero magnitude earthquake and station correction factors the log a0 curve determined in this study is log a r 0 332 1 568 log r 0 280 0 using this new log a0 curve and station corrections the new ml is more consistent with mw with a 0 2 magnitude unit uncertainty the new log a0 curve has a value of 2 80 at the distance of 100 km compared to the anchor point of log a0 3 at the same distance as defined by richter for southern california this means that the current ml estimates in taiwan which use richter s definition average 0 2 magnitude units larger than their mw the station correction factors also determined are large from 0 40 to 0 52 magnitude units the use of station corrections in routine seismic network operation in taiwan will improve magnitude estimates this is particularly important for smaller events when recording stations may be pre dominantly on either hard rock or soft soil sites which could lead to under or overestimates of the magnitude by up to half a magnitude unit

circulation and melting beneath george vi ice shelf antarctica introduction george vi ice shelf sandwiched between the western coast of palmer land and the eastern coast of alexander island is the largest and most studied of the west antarctic peninsula ice shelves it covers an area of approximately 25 000 km2 and is underlain by circumpolar deep water cdw with temperatures in excess of 1oc giving rise to rapid basal melting bishop and walton 1981 lennon et al 1982 the maximum ice thickness of about 500 m occurs about 70 km from the southern ice front where a ridge of thick ice extends across george vi sound near 70ow see figure 1 effectively dividing the upper water column into northern and southern regions the northern ice front which faces marguerite bay appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades lucchitta and rosanova 1998 a timeframe over which much of the nearby wordie ice shelf disintegrated doake and vaughan 1991 there is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate paren and cooper 1986 conditions in the south where the ice front faces into ronne entrance are colder and the ice front position appears to be steady the vast majority of the flow into the ice shelf comes from palmer land but basal melting is sufficient to remove most of this so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts potter et al 1984 there is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early holocene before reforming sugden and clapperton 1981 bentley et al 2005

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

high latitude oceanography using the autosub autonomous underwater vehicle a combination of measurements from autosub an autonomous underwater vehicle and ship based instruments are used to describe the oceanographic conditions beneath fimbul ice shelf antarctica the data show an intricate oceanographic regime that is suggestive of variability at seasonal or longer time scales results from a numerical model of the flow beneath the ice shelf lend support to a strong seasonal variability and data from instruments moored beneath the ice shelf show substantial temporal variations the autosub data indicate temperatures within the cavity somewhat lower than our model results would suggest thus our modeled melt rates average of 0 85 m yr21 might be overestimates although they are substantially lower than from previously published model studies 4 9 and 1 9 m yr21 the contribution of ice shelf melt in the freshwater balance of the waters upstream of the climatically important filchner ronne ice shelf needs to be reassessed

mixing on the continental slope of the southern weddell sea shipboard hydrography and current profiles collected in 2003 and time series from moored current meters deployed in late 1990s are analyzed to study the variability of mixing in the southeastern weddell sea profiles of eddy diffusivity k are inferred from finescale shear vertical derivative of horizontal velocity and strain vertical derivative of isopycnal displacement variance using parameterizations which relate the internal wave energy to the dissipation rate at small scales the highest mixing rates are seen near the bottom where the eddy diffusivities are elevated by one order of magnitude from those in the interior and exceed 10 4 m 2 s 1 the observations show latitudinal variability in k particularly near the bottom where k significantly increases near 74 28 s the critical latitude for lunar semi diurnal m2 tides in this region the critical latitude coincides with near critical topography on the upper continental slope a situation which favors generation of m2 internal waves consistent with the results from finescale shear and strain parameterizations which indicate highest bottom diffusivities near the critical latitude independent analysis of current time series from moored instruments shows a thickening of the frictional bottom boundary layer near the critical latitude semidiurnal tidal dynamics at the upper continental slope together with the critical latitude effects lead to mixing that might significantly affect the regional heat budget and the circulation in the study area

a structural vp model of the salton trough california and its implications for seismic hazard we present a high resolution three dimensional p wave seismic veloc ity model of the sedimentary basin in the salton trough southern california and use the model for spectral element method sem wave propagation and ground motion simulations to quantitatively assess seismic hazard in the region the basin geometry is defined by a surface representing the top of crystalline basement which was constrained by seismic refraction profiles and free air gravity data sonic logs from petroleum wells in the imperial valley and isovelocity surfaces defined by seismic refraction studies were used to define p wave velocity within the sedimentary basin as a function of two variables 1 absolute depth and 2 depth of the underlying crystalline basement surface cbs this velocity function was used to populate cells of a three dimensional spatial array voxet defining the p wave velocity structure in the basin the new model was then resampled in a computational mesh used for earthquake wave propagation and strong ground motion simulations based upon the sem komatitsch et al 2004 simulation of the 3 november 2002 mw 4 2 yorba linda earthquake demonstrates that the new model provides accurate simulation of strong ground motion amplification effects in the salton trough sedimentary basin offering substantial improvements over previous models a hypothetical mw 7 9 earthquake on the southern san andreas fault was then simulated in an effort to better understand the seismic hazard associated with the basin structure these sim ulations indicate that great amplification will occur during large earthquakes in the region due to the low seismic velocity of the sediments and the basin shape and depth

effect of critical latitude and seasonal stratification on tidal current profiles along ronne ice front antarctica 1 the ice front region of ronne ice shelf lies near the critical latitude of the semidiurnal m2 tide the principal tidal constituent in the southern weddell sea here the coriolis frequency almost equals the m2 tidal frequency resulting in a strong dependence of the m2 tidal currents on depth and stratification and a boundary layer that can occupy the entire water column using data from four long term moorings along ronne ice front we confirm the presence of strongly depth dependent semidiurnal tidal currents and their sensitivity to changes in stratification the time series show dramatic seasonal changes in tidal current profiles and significant interannual variability during periods of stratification the amplitude of the semidiurnal tides in the mid water column shows a twofold increase and despite being several kilometers offshore from the ice front the tidal currents clearly show a second boundary layer originating from the adjacent ice shelf base together these two boundary layers occupy most of the water column up to 600 m deep until intense sea ice formation and the production of high salinity shelf water erodes the vertical stratification during winter when homogeneous conditions prevail a single bottom boundary layer occupies the entire water column at some locations this strong seasonality and sensitivity of the m2 tidal current to stratification highlights the difficulties of interpreting current data from short term moorings while demonstrating that it is the best indicator for characterizing changes in stratification after direct observations of density variations

basin structure estimation by waveform modeling forward and inverse methods we introduce a technique for using broadband seismograms recorded from earthquakes at local and regional distances to refine basin structure for the region outside the basin we assume a one dimensional 1d crustal model and analytical techniques grt to propagate the energy from sources to the basin edge where the motions are then interfaced with a 2d finite difference algorithm wen and helmberger 1996 we parameterize the basin section by isovelocity layers with linear dipping segments between control points the control point depths are allowed to vary to improve the modeling of waveform data of stations inside the basin the comparison between data and synthetics is qualified by a fitness function defined by two factors the timing shift required for best alignment and the correlation coefficient the procedure was applied to a strong motion waveform profile across the extended los angeles basin produced by the 1992 landers california earthquake to refine the velocity structure using sensitivity testing and forward modeling only the correlation coefficient and amplitude were used because absolute timing was unknown the procedure was extended to a direct waveform inversion by employing a conjugate gradient approach which uses numerical derivatives numerical tests using the new inversion process with synthetic data demonstrate that it is possible to recover a detailed basin structure if a sufficient amount of high quality data exists

a coupled sea ice mixed layer pycnocline model for the weddell sea a dynamic thermodynamic sea ice model is coupled to a one dimensional model of the oceanic mixed layer and pycnocline and is applied to the weddell sea this model prognostically determines the vertical oceanic heat flux from the mixed layer dynamics in contrast to earlier sea ice modeling where the oceanic heat flux was prescribed in addition to the standard simulation polynya and paleoclimate experiments were performed to investigate the effects of sea ice dynamics furthermore the mixed layer pycnocline model is compared to the original kraus turner approach

propagation and vertical structure of the tidal flow in nares strait the southward freshwater flux through nares strait is an important component of the arctic s freshwater budget on short time scales flow through the strait is dominated by the tides and tidal dynamics may be important for the magnitude of the freshwater flux over longer periods here we build upon our existing knowledge of the tides in the region by exploring their propagation and vertical structure using data from four bottom mounted acoustic doppler current profilers deployed in nares strait between 2003 and 2006 we observe that propagating barotropic semidiurnal tidal waves interact to create a standing wave pattern explaining the abnormally large tidal amplitudes that are observed in this region in the along strait direction semidiurnal tidal currents exhibit strong variations with depth in contrast the diurnal tides propagate northward through the strait as progressive waves and the tidal currents are broadly depth invariant proximity of nares strait to the semidiurnal critical latitude and the topographical restriction imposed by the steep side wall of ellesmere island are primary drivers behind the observed vertical variability in the upper part of the water column baroclinic activity increases the tidal current amplitude by up to 25 in the across strait direction a two layer structure exists in both the diurnal and semidiurnal tidal flow with a phase lag of approximately a quarter of a tidal cycle across the strait for the semidiurnal tide our results suggest that strong vertical motion exists against the side walls of nares strait as the across strait flow interacts with the steeply sloping bathymetry

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

evidence of deep and bottom water formation in the western weddell sea abstract during ice station polarstern ispol r v polarstern cruise ant xxii 2 november 2004 january 2005 hydrographic and tracer observations were obtained in the western weddell sea while drifting closely in front of the larsen ice shelf these observations indicate recently formed weddell sea bottom water which contains significant contributions of glacial melt water in its upper part and high salinity shelf water in its lower layer the formation of this bottom water cannot be related to the known sources in the south the filchner ronne ice shelf we show that this bottom water is formed in the western weddell sea most likely in interaction with the larsen c ice shelf by applying an optimum multiparameter analysis omp using temperature salinity and noble gas observations helium isotopes and neon we obtained mean glacial melt water fractions of about 0 1 in the bottom water on sections across the weddell gyre farther north melt water fractions are still on the order of 0 04 using chlorofluorocarbons cfcs as age tracers we deduced a mean transit time between the western source and the bottom water found on the slope toward the north 9 3 years this transit time is larger and the inferred transport rate is small in comparison to previous findings but accounting for a loss of the initially formed bottom water volume due to mixing and renewal of weddell sea deep water a formation rate of 1 1 0 5 sv in the western weddell sea is plausible this implies a basal melt rate of 35 19 gt year or 0 35 0 19 m year at the larsen ice shelf this bottom water is shallow enough that it could leave the weddell basin through the gaps in the south scotia ridge to supply antarctic bottom water these findings emphasize the role of the western weddell sea in deep and bottom water formation particularly in view of changing environmental conditions due to climate variability which might induce enhanced melting or even decay of ice shelves

wintertime water mass modification near an antarctic ice front abstractunder ice measurements by seals carrying a miniaturized conductivity temperature depth ctd instrument fill an important gap in existing observations here the authors present data from an instrumented weddell seal that spent eight consecutive months february september foraging in close proximity to the filchner ice shelf thus providing detailed information about the evolution of mixed layer hydrography during the austral autumn and winter the resultant time series of hydrography shows strong seasonal water mass modification dominated by an upper ocean 0 300 m salinity increase of 0 31 corresponding to 3 1 m sea ice growth and the development of a 500 m thick winter mixed layer observations furthermore highlight a gradual salinity increase in a slow 3 5 cm s 1 southward flow on the continental shelf toward the site and suggest that the inferred ice production is better considered as a regional average rather than being purely local no clear seasonality is observed in the properties

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

hydrography and circulation in the filchner depression weddell sea antarctica cold and dense ice shelf water isw emerging from the filchner ronne ice shelf cavity in the southwestern weddell sea flows northward through the filchner depression to eventually descend the continental slope and contribute to the formation of bottom water new ship born observations of hydrography and currents from filchner depression in january 2013 suggest that the northward flow of isw takes place in a middepth jet along the eastern flank of the depression thus questioning the traditional view with outflow along the western flank this interpretation of the data is supported by results from a regional numerical model which shows that isw flowing northward along the eastern coast of berkner island turns eastward and crosses the depression to its eastern side upon reaching the filchner ice front the ice front represents a sudden change in the thickness of the water column and thus a potential vorticity barrier transport estimates of northward isw flux based on observations ranges from 0 2 to 1 0 sv

the oceanic environment beneath the northwest ronne ice shelf antarctica oceanographic data have been obtained from beneath the ronne ice shelf at a site on the southeastern side of a sea floor trough the ronne depression the data consist of 5 d of conductivity temperature depth ctd profiling and a long term temperature record from a thermistor cable the water column was found to be composed of two layers a cold buoyant layer of lce shelf water isw overlying a layer of unmodified western shelf water wsw a change in the water column was observed on the last day of profiling the wsw layer thinning a cold intrusion appearing directly above the wsw and the isw becoming colder in addition on three profiles the isw cooled further and increased in salinity becoming apparently statically unstable the temperature record from the thermistor cable confirms that the isw layer occasionally becomes colder with greater variation in temperature than seen on the ctd profiles these low temperatures and high salinities arc interpreted as indicating that water containing ice crystals in suspension has been advected beneath the site

p wave attenuation with implications for earthquake early warning several widely implemented and tested earthquake early warning algorithms employ empirical equations that relate earthquake magnitudes with ground motion peak amplitudes and hypocentral distances this approach is effective to the extent that the offline dataset available for setting the fitting coefficients in those equations is of sufficient quality and quantity however to address the problem of having a limited dataset it is instructive to gain physical understanding of the main factors controlling the p wave attenuation in this study theoretical expressions are derived that relate the root mean square rms of the p wave displacement d rms and velocity v rms to the seismic moment stress drop and hypocentral distance the theoretical attenuation laws are then validated against observed attenuation using earthquake data from southern california and japan good agreement is found between observed and predicted ground motions the similar ground motion attenuation in california and japan suggests that the attenuation laws are similarly applicable for the two regions and implies that they may also be implemented in other regions without having to go through a lengthy calibration phase because d rms is more strongly dependent on the seismic moment than v rms use of the attenuation law for d rms yields better magnitude prediction than that of v rms it is shown that the d rms to v rms ratio is proportional to the characteristic length of the rupture and that the stress drop is a function of the seismic moment and the cube of d rms v rms this result paves the way for a new stress drop determination scheme that is totally independent of previously used approaches finally it is shown that the rms of the ground motions are proportional to their peak values

estimation of three dimensional boundary shape of the osaka sedimentary basin by waveform inversion summary we present a method for estimating the boundary shape of a 3 d basin velocity structure by waveform inversion and we apply the method to real seismic data observed in the osaka sedimentary basin in japan the initial model is the 3 d osaka basin velocity structure model developed by iwata et al 2008 basin and crustal velocity structure models for the simulation of strong ground motions in the kinki area japan j seism 2 223 234 based on bedrock depth information obtained from various geophysical exploration surveys we adopt representation of the boundary shape of the seismic bedrock using a 2 d cubic b spline function the spline coefficients are estimated by minimizing the l2 norm of the difference between the observed and synthetic waveforms the target of the inversion is the set of velocity waveforms in the period range of 3 10 s of the real seismic data observed in the osaka basin during two regional mw5 0 earthquakes that occurred 60 90 km to the east and southeast of the osaka basin they contain later phases with large amplitudes and long durations we first perform a series of synthetic tests to validate the method and examine the effect of the choice of time window in the waveform inversion we show that the later phases of the ground motion have a larger influence on the inversion and are affected by the structure of a wider area relative to the body wave portion we then apply the method to the real data in two successive steps step 1 involves the body wave portion only and step 2 involves the body wave and later phases we propose a multi event inversion method in which ground motion data from two earthquakes with different azimuths are solved simultaneously the updated basin velocity structure model obtained in our inversion analysis reproduces the ground motion better than the initial model at most of the stations the bedrock depth of the updated model is smaller than that of the initial model in the central part of the basin which is consistent with the bedrock depth information inferred from deep borehole data

modeling antarctic ice shelf responses to future climate changes and impacts on the ocean 1 we investigate basal melting of all antarctic ice shelves by a circumpolar ice shelf sea ice ocean coupled model and estimate the total basal melting of 770 944 gt yr under present day climate conditions we present a comparison of the basal melting with previous observational and modeling estimates for each ice shelf heat sources for basal melting are largely different among the ice shelves sensitivities of the basal melting to surface air warming and to enhanced westerly winds over the antarctic circumpolar current are investigated from a series of numerical experiments in this model the total basal melting strongly depends on the surface air warming but is hardly affected by the change of westerly winds the magnitude of the basal melting response to the warming varies widely from one ice shelf to another the largest response is found at ice shelves in the bellingshausen sea followed by those in the eastern weddell sea and the indian sector these increases of basal melting are caused by increases of circumpolar deep water and or antarctic surface water into ice shelf cavities by contrast basal melting of ice shelves in the ross and weddell seas is insensitive to the surface air warming because even in the warming experiments there is high sea ice production at the front of the ice shelves that keeps the water temperature to the surface freezing point weakening of the thermohaline circulation driven by antarctic dense water formation under warming climate conditions is enhanced by basal melting of ice shelves

propagational corrections for basin structure landers earthquake transfer functions appropriate for correcting for propagational distortion caused by basin structures 2d given a hard rock 1d response are developed the transfer functions are generated theoretically using a hybrid method where the finite difference technique is employed only in the basin this allows for efficient computation and the ability to separate path contributions due to body waves and surface waves the procedure is used to explain some strong motions observed in the los angeles basin relative to the hard rock site pasadena during the 1992 landers earthquake

thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean the circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity which at least provides the potential for melting and freezing processes applying a three dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier we found an important parameter controlling the interaction between these two systems idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction because of the sudden decrease in water column thickness since the water column thickness and the coriolis force determine the characteristics for geostrophic flow separated circulation systems arise for the open ocean and the ice shelf cavity only in areas where constant water column thickness and from the oceanographic point of view constant f h contours can be observed across the barrier an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf this is only the case at lateral sloping sidewalls or at deep depressions which can be found for example in the southern weddell sea in all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier

basal melt seasonal water mass transformation ocean current variability and deep convection processes along the amery ice shelf calving front east antarctica despite the amery ice shelf ais being the third largest ice shelf in antarctica the seasonal variability of the physical processes involved in the ais ocean interaction remains undocumented and a robust observational oceanographic based basal melt rate estimate has been lacking here we use year long time series of water column temperature salinity and horizontal velocities measured along the ice shelf front from 2001 to 2002 our results show strong zonal variations in the distribution of water masses along the ice shelf front modified circumpolar deep water mcdw arrives in the east while in the west ice shelf water isw and dense shelf water dsw formed in the mackenzie polynya dominate the water column baroclinic eddies formed during winter deep convection down to 1100 m drive the inflow of dsw into the ice shelf cavity our net basal melt rate estimate is 57 4 25 3 gt yr 1 1 0 4 m yr 1 larger than previous modeling based and glaciological based estimates and results from the inflow of dsw 0 52 0 38 sv 1 sv 106 m3 s 1 and mcdw 0 22 0 06 sv into the cavity our results highlight the role of the mackenzie polynya in the seasonal exchange of water masses across the ice shelf front and the role of the isw in controlling the formation rate and thermohaline properties of dsw these two processes directly impact on the ice shelf mass balance and on the contribution of dsw isw to the formation of antarctic bottom water

magnitude estimation using the first three seconds p wave amplitude in earthquake early warning 1 pd is the peak amplitude of displacement in the first three seconds after the arrival of the p wave we investigated the attenuation of pd with the hypocentral distance r in southern california as a function of magnitude m and obtained the following relationship log pd 3 463 0 729 m 1 374 log r 0 305 given an earthquake location determined by the p wave arrival times at stations close to the epicenter this relationship can be used to define a so called pd magnitude of earthquakes our result shows that for earthquakes in southern california the pd magnitudes agree with the catalog magnitudes with a standard deviation of 0 18 for events less than magnitude 6 5 therefore pd is a robust measurement for estimating the magnitudes of earthquakes and has practical application in earthquake early warning systems citation wu y m and l zhao 2006 magnitude estimation using the first three seconds p wave amplitude in earthquake early warning geophys res lett 33 l16312 doi 10 1029 2006gl026871

dynamic path selection along branched faults experiments involving sub rayleigh and supershear ruptures building upon previous laboratory earthquake experiments of dynamic shear rupture growth taking place along faults with simple kinks new and complex fault geometries involving cohesively held fault branches are studied asymmetric impact at the specimen boundaries controls the incoming shear ruptures which are manipulated to propagate at either sub rayleigh or supershear velocities high speed photography and dynamic photoelasticity are used with a model material homalite 100 to monitor incoming and outgoing rupture propagation acceleration deceleration or arrest at the vicinity of the branch location differences and similarities of rupture velocity history between cases involving faults with either simple kinks or branches on the one hand and sub rayleigh and supershear incoming ruptures on the other are highlighted and explained results of the experiments show a clear general bias toward large branch inclination smaller branch angles appearing to be overshadowed and suppressed by the stress field associated with the main fault of great interest also is the sustenance of rupture propagation along a branch by the mach cone when the initial rupture is supershear driven generally higher rupture speeds favors larger arrays of branching angles to be triggered a companion analysis by templeton et al 2009 featuring detailed numerical simulations of these experiments provides further insight into the observed phenomena

water mass modification over the continental shelf north of ronne ice shelf antarctica 1 we use new data from the southern weddell sea continental shelf to describe water mass conversion processes in a formation region for cold and dense precursors of antarctic bottom water the cruises took place in early 1995 1998 and 1999 and the time series obtained from moored instruments were up to 30 months in length starting in 1995 we obtained new bathymetric data that greatly improve our definition of the ronne depression which is now shown to be limited to the southwestern continental shelf and so cannot act as a conduit to northward flow from ronne ice front large scale intrusions of modified warm deep water mwdw onto the continental shelf occur along much of the shelf break although there is only one location where the mwdw extends as far south as ronne ice front high salinity shelf water hssw produced during the winter months dominates the continental shelf in the west during summer ice shelf water isw exits the subice cavity on the eastern side of the ronne depression flows northwest along the ice front and reenters the cavity at the ice front s western limit during winter the isw is not observed in the ronne depression north of the ice front the flow of hssw into the subice cavity via the ronne depression is estimated to be 0 9 0 3 sv when combined with inflows along the remainder of ronne ice front reported elsewhere sufficient heat is transported beneath the ice shelf to power an average basal melt rate of 0 34 0 1 m yr 1

oceanographic conditions south of berkner island beneath filchner ronne ice shelf antarctica we have made oceanographic measurements at two sites beneath the southern filchner ronne ice shelf hot water drilled access holes were made during january 1999 allowing conductivity temperature depth ctd profiling and the deployment of instrument moorings the ctd profiles show that the entire water column is below the surface freezing point we estimate the summer flux of water between the two sites to be 2 106 m3 s 1 the summer potential temperature salinity properties of the water column suggest that this flow is part of a recirculation in the deepest part of the subice shelf cavity and the filchner depression the recirculation is driven by a combination of the melting of deep basal ice and the freezing that results from the depressurization of the cold buoyant water as it ascends the ice shelf base the source of the water was high salinity shelf water hssw produced in the ronne depression this is the water that provides the external heat necessary for the strong melting at the deep grounding lines in the vicinity of foundation ice stream instruments moored at the drill sites show that during the winter hssw formed on the berkner shelf flows beneath the ice shelf and largely displaces the recirculating water from the two sites this provides an externally driven through flow that is warmer nearer the surface freezing point and slower than the internal recirculation and which is low enough in density to escape the filchner depression

a geology based 3d velocity model of the los angeles basin sediments abstract seismic hazard studies of the los angeles basin area require a realistic seismic velocity model we use geologic information about depth to crystalline basement depths to sedimentary horizons uplift of sediments and surface geology in a velocity depth age function to construct a three dimensional velocity model in earthquake location tests the model predicts travel times satisfactorily and in earthquake ground motion simulations the model correctly determines the timing and amplitude of late arriving waves

modeling the seismic response of basin edge structure by a sequential waveform method a sequential waveform method is developed to simulate the seismic response of basin edge structure excited by a plane incident p wave the full procedure involves a a previous parameterization of the investigated model using the seismic wave velocities and depths of the sedimentary stratifications b an input motion determined from the records at stations installed on hard rock c forward computation of the p sv elastic wave field by means of a two dimensional finite difference fd method d the optimization of the model vector using simulated annealing technique and comparing the simulated seismic response of the tested structure with the observed wave field e the correction of the initial model by trial and error by testing the differences between synthetics and observed data and f the final solution obtained by iteration using the conjugate gradient algorithm the search of an optimal basin edge model has been parallel processed by varying the shapes and velocities of strata on the basis of the fitting of relative timing amplitude and phase between the output and the observed data the input motion and sensitivity have been checked and the validity of the method has been demonstrated by numeric analysis using the teleseismic records generated by 7 earthquakes recorded at 26 broadband seismic stations we have studied the seismic velocity structure of the southern edge of the jiyang depression located in the bohai bay basin northern china two cross sections show an agreement between the velocity results and the geological sections available in the region in addition we obtain evidence of three hidden faults under the sections and features that suggest major extensions at the paleogene

current measurements near ronne ice shelf implications for circulation and melting we present the first year long current meter records ever obtained near the floating filchner ronne ice shelf in the weddell sea the currents are steered along the ice front but in the lower layer where the bottom topography is descending toward the west the current has a component toward the ice front of about 3 cm s 1 during winter the temperature stayed near the surface freezing point while the salinity increased indicating that ice was formed and brine released the seasonal variation in salinity was 0 15 0 05 psu corresponding to the formation of 1 2 m of ice on a shelf depth of 400 m the transport of high salinity shelf water hssw into the ice shelf cavity was found to be of the order 0 5 106 m3 s 1 the production of this water due to oscillating tides and off shelf winds was found to be of the same order of magnitude in contact with glacial ice at great depths and because of the depression of the freezing point the hssw is transformed to ice shelf water isw by cooling and melting processes the melting rate was estimated to 1 1011 ton yr 1 this corresponds to the melting of 0 2 m ice per year if the melting is evenly distributed over the filchner ronne ice shelf if the melting is concentrated along a path from the berkner shelf around the berkner island to the filchner depression then melting rates up to 7 m yr 1 must be expected a comparison of hssw characteristics in the ronne depression our winter observations on the berkner shelf and the isw flowing out of the filchner depression indicates that very little water passes through the cavity from the ronne to the filchner depression it appears that most of the isw originating from processes on the berkner shelf escapes the cavity in the filchner depression this leaves the berkner shelf as the important source of isw and subsequently of the weddell sea bottom water formed from isw

eddy driven exchange between the open ocean and a sub ice shelf cavity the exchange between the open ocean and sub ice shelf cavities is important to both water mass transformations and ice shelf melting here we use a high resolution 500 m numerical model to investigate to which degree eddies produced by frontal instability at the edge of a polynya are capable of transporting dense high salinity shelf water hssw underneath an ice shelf the applied surface buoyancy flux and ice shelf geometry is based on ronne ice shelf in the southern weddell sea an area of intense wintertime sea ice production where a flow of hssw into the cavity has been observed results show that eddies are able to enter the cavity at the southwestern corner of the polynya where an anticyclonic rim current intersects the ice shelf front the size and time scale of simulated eddies are in agreement with observations close to the ronne ice front the properties and strength of the inflow are sensitive to the prescribed total ice production flushing the ice shelf cavity at a rate of 0 2 0 4 106 m3 s 1 depending on polynya size and magnitude of surface buoyancy flux eddy driven hssw transport into the cavity is reduced by about 50 if the model grid resolution is decreased to 2 5 km and eddies are not properly resolved

seasonal ventilation of the cavity beneath filchner ronne ice shelf simulated with an isopycnic coordinate ocean model 1 the ocean cavity beneath filchner ronne ice shelf is observed to respond to the seasonal cycle of water mass production on the continental shelf of the southern weddell sea here we use a numerical model to investigate the propagation of newly formed shelf waters into the cavity we find that the model reproduces the most distinctive features of the observed seasonality and offers a plausible explanation for those features the most saline shelf waters are produced in the far west where the inflow to the cavity peaks twice each year the major peak occurs during the short period around midwinter when convection reaches full depth and the densest waters are generated once the surface density starts to decline dynamic adjustment of the restratified water column leads to a gradual fall in the salinity at depth and a secondary peak in the inflow that occurs in summer at the western coast beneath the ice shelf the arrival of the wintertime inflow at the instrumented sites is accompanied by a rapid warming while the slower decline in the inflow leads to a more gradual cooling water brought in by the secondary summer peak flows mainly to the eastern parts of the cavity here the seasonality is suppressed because the new inflows mix with older waters that recirculate within a topographic depression this pooling of waters in the east where the primary outflow of ice shelf water is generated dampens the impact of seasonality on the local production of weddell sea bottom water

new oceanographic data from beneath ronne ice shelf antarctica oceanographic data have been obtained via an access hole made through ronne ice shelf the site which is the third in a series of similar studies lies 17 km west of korff ice rise where 825 m of ice overlies a 485 m deep water column measurements included conductivity and temperature profiles and an instrument mooring was deployed for long term measurements of currents temperature and conductivity at the sea floor there was a 150 m layer of well mixed water with a potential temperature and salinity of 1 97 c and 34 72 the water cooled and freshened towards the ice shelf base ultimately reaching 2 41 c and 34 51 the hydrographic and water current data imply a flow into the deepest part of the sub ice shelf cavity of about 200 000 m s 1 of the deeper relatively warm water which would be able to power an average basal melt rate of 0 2 m a 1 for the western portion of ronne ice shelf

spectral analysis of k net and kik net data in japan part ii on attenuation characteristics source spectra and site response of borehole and surface stations abstract in this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the k net and kik net networks in japan in order to derive attenuation characteristics source spectra and site response for this purpose we use a total of more than 67 000 s wave records from 2178 earthquakes m jma 2 7 8 obtained at 1555 stations at the earth s surface and more than 29 000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m attenuation characteristics are investigated in five separate regions showing that crustal q depicts lower values in central compared to southern japan and a significant frequency dependence is observed in every region the source spectra follow the 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones while strong amplification effects dominate the site contributions for the surface sensors those for the borehole sensors are characterized by smaller variability nevertheless consistent with observations from deconvolution of borehole surface recording pairs downgoing wave effects are visible in the site contributions for many borehole stations finally the site amplification functions obtained at the surface are compared with surface to borehole s b and horizontal to vertical h v spectral ratios showing that the s b ratios generally provide better estimates of the horizontal amplification than the h v ratios due to amplification of the vertical component of ground motion

deep and bottom water of the weddell sea s western rim oceanographic observations from the ice station weddell 1 show that the western rim of the weddell gyre contributes to weddell sea bottom water a thin 300 meters highly oxygenated benthic layer is composed of a low salinity type of bottom water overlying a high salinity component this complex layering disappears near 66 s because of vertical mixing and further inflow from the continental margin the bottom water flowing out of the western rim is a blend of the two types additionally the data show that a narrow band of warmer weddell deep water hugged the continental margin as it flowed into the western rim providing the continental margin with the salt required for bottom water production

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

the seismic response of the los angeles basin california using strong motion data recorded in the los angeles region from the 1992 mw 7 3 landers earthquake we have tested the accuracy of existing three dimensional 3d velocity models on the simulation of long period 2 sec ground motions in the los angeles basin and surrounding san fernando and san gabriel valleys first the overall pattern and degree of long period excitation of the basins were identified in the observations within the los angeles basin the recorded amplitudes are about three to four times larger than at sites outside the basins amplitudes within the san fernando and san gabriel valleys are nearly a factor of 3 greater than surrounding bedrock sites then using a 3d finite difference numerical modeling approach we analyzed how variations in 3d earth structure affect simulated waveforms amplitudes and the fit to the observed patterns of amplification significant differences exist in the 3d velocity models of southern california that we tested magistrale et al 1996 graves 1996a hauksson and haase 1997 major differences in the models include the velocity of the assumed background models the depth of the los angeles basin and the depth location and geometry of smaller basins the largest disparities in the response of the models are seen for the san fernando valley and the deepest portion of the los angeles basin these arise in large part from variations in the structure of the basins particularly the effective depth extent which is mainly due to alternative assumptions about the nature of the basin sediment fill the general ground motion characteristics are matched by the 3d model simulations validating the use of 3d modeling with geologically based velocity structure models however significant shortcomings exist in the overall patterns of amplification and the duration of the long period response the successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models whether geologically or tomographically derived the differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input scenario earthquake source models

the determination of source mechanisms of small earthquakes and revised models of local crustal structure by moment tensor inversion summary we describe a procedure for retrieving the source mechanism of small earthquakes and a revised model of crustal structure in heterogeneous media based on interactive inversion of high frequency seismograms and demonstrate its application to three local earthquakes the earthquakes are located along the rukwa rift south western tanzania and were recorded by a local network in the rift due to strong lateral crustal heterogeneity in the area the procedure involves a single station interactive inversion for moment tensor solution and modelling of the crustal structure along the propagation path between the source and the station based on this source solution and the a priori crustal structure synthetic seismograms are generated and compared with observed displacement seismograms at other stations where the event has been recorded deviations between the calculated and observed seismograms are indicated by time shifts and amplitude mismatches on prominent phases these are employed to correct the phase arrivals used to invert for the partial multistation source solution the moment tensor inversion of high frequency seismogram data in this study is based on a point source approach the process includes generation of synthetic seismograms using the frequency wavenumber integration technique for an elastic horizontally layered medium moment tensor inversion is carried out by minimizing the l2 norm of the difference between synthetic and observed displacement data under a deviatoric condition results for the three small earthquakes show good agreement between single station and multistation source solutions the inversions for these events indicate dominantly dip slip mechanisms with seismic moment magnitudes in the range 2 0 mw 3 3

wintertime ocean conditions over the southern weddell sea continental shelf antarctica 1 during the austral winter of 2007 a weddell seal tagged with a miniaturized conductivity temperature depth ctd instrument travelled over the central southern weddell sea continental shelf the instrument yielded 750 ctd profiles 250 of them to the sea floor the data show a full depth flow of water onto the shelf via a sill at the shelf break 74 s 44 w the warmth from the core of the flow was able to maintain the surface mixed layer above the freezing point resulting in a band of reduced ice production an estimate of the on shelf flux suggests that this flow accounts for most of the estimated 3 sv of water draining from the southern weddell sea continental shelf

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

reply to comment on the relation between ground acceleration and earthquake source parameters theory and observations by itzhak lior and alon ziv by j enrique luco

interannual variability and ventilation timescales in the ocean cavity beneath filchner ronne ice shelf antarctica 1 multiyear time series of ocean current and temperatures from beneath filchner ronne ice shelf antarctica demonstrate both seasonal and interannual variability the seasonal signal is visible at all measurement sites although it was swamped for a 2 year period 1999 2001 when extraordinarily light sea ice cover in the southern weddell sea during the 1997 1998 austral summer caused an anomalously large pulse of high salinity shelf water to flush beneath the ice shelf the pulse was observed twice at an instrumented site near the berkner island coast once on its way to the filchner depression and once after the signal had propagated around the depression and returned to the site as an anomalously large pulse of ice shelf water the timings of the signal allow an estimate of 24 30 months for the flushing timescales of the sub ice shelf ocean cavity indicating that the cavity is highly responsive to external forcing a timescale for the full ventilation of the cavity of 4 5 years is obtained from the length of time the sub ice shelf conditions take to return to their original state a timescale significantly shorter than previous estimates

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

oxygen 18 and helium as tracers of ice shelf water and water ice interaction in the weddell sea oxygen 18 and helium isotope data from stations located in the filchner depression on the continental slope of the southern weddell sea and in the central weddell sea are presented and discussed the 18o and 4he signals imprinted on the water circulating under the filchner ronne ice shelf ice shelf water or isw due to melting of glacial ice at the base of the ice shelf are traced across the sill separating the filchner depression from the weddell sea low 18o values are correlated with high 4he concentrations in the isw found in the filchner depression minimum 18o values 0 8 maximum 4he concentrations about 4 7 10 8 cm3 stp g 1 the fraction of glacial meltwater contained in the isw found in the filchner depression is estimated to about 6 to 7 the 18o and helium isotope data from the overflowing shelf water component observed on the continental slope confirm the hypothesis that isw contributes significantly to the weddell sea bottom water wsbw on the basis of a inultiparameter water mass analysis it is discussed which fraction of the wsbw originates from isw and which other shelf waters potentially could contribute to wsbw

notes on southern ocean hydrography sea ice and bottom water formation abstract the southern ocean circulation and sea ice distribution is briefly described the formation of extremely cold bottom water in the weddell sea and its relation to the floating ronne filchner ice shelves is discussed it is shown that a concentrated swift eroding bottom current with anomalous low 18 o ratio transports the cold and dense ice shelf water from the shelf towards large depths comments are made on possible implications of this process for the large scale deep water circulation and for the interpretation of sediment cores

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

source parameters of the sierra madre earthquake from regional and local body waves we inverted the three component long period data recorded by the terrascope array for the june 28 1991 sierra madre california event to determine the seismic moment and source orientation of the mainshock ml 5 8 remarkably four of the six stations were located 159 2 0 7 km from the epicenter variations in absolute traveltime were smaller than 2 and 3 for pn and sn whereas the surface waves displayed greater variation similarities in the waveforms and the small variance in traveltime suggest that a common green s function can be used to invert the data we used the whole waveforms excluding the fundamental surface waves at the more distant stations to invert for source parameters the results of the inversion indicated that this event was predominately a thrust type earthquake where the strike rake and dip were determined to be 235 74 and 50 respectively the seismic moment was determined to be 2 5 1024 dyne cm the source duration was found to be about 1 0 seconds by direct measurement of the s wave recorded at the pasadena station located 21 km from the epicenter considering circular rupture we obtain a stress drop of 460 bars

modeling the influence of the weddell polynya on the filchner ronne ice shelf cavity abstractopen ocean polynyas in the weddell sea of antarctica are the product of deep convection which transports warm deep water wdw to the surface and melts sea ice or prevents its formation t

modeling ice shelf water overflow and bottom water formation in the southern weddell sea 1 descent of ice shelf water isw overflowed from the filchner depression in the southern weddell sea and consequent weddell sea bottom water wsbw production is investigated through high resolution numerical experiments the model reproduces the part of isw which interacts with the submarine ridge near 36 w the baroclinic instability wave developed near the exit of the filchner depression leads to intermittent outflow of isw and the intermittent passage of isw origin dense water induces 6 day period oscillation of velocity at the east side of the ridge which coincides well with mooring data the existence of the ridge induces two different mechanisms for cross isobath transport of the isw origin dense water a strong downslope current at the east side of the ridge and offshore transport by eddies which are generated when a gravity current makes a detour around the tip of the ridge over a wide region to the west of the ridge in a sensitivity experiment where thermobaricity is removed from the equation of state there is almost no transport of the isw origin water below 3000 m depth wsbw production rate as a result of descent of the isw origin dense water in the model domain is about 0 15 sv