changes in snow distribution and surface topography following a snowstorm on antarctic sea ice snow distribution over sea ice is an important control on sea ice physical and biological processes we combine measurements of the atmospheric boundary layer and blowing snow on an antarctic sea ice floe with terrestrial laser scanning to characterize a typical storm and its influence on the spatial patterns of snow distribution at resolutions of 1 10 cm over an area of 100 m x 100 m the pre storm surface exhibits multi directional elongated snow dunes formed behind aerodynamic obstacles newly deposited dunes are elongated parallel to the predominant wind direction during the storm snow erosion and deposition occur over 62 and 38 of the area respectively snow deposition volume is more than twice that of erosion 351 m3 vs 158 m3 resulting in a modest increase of 2 1 cm in mean snow depth indicating a small net mass gain despite large mass relocation despite significant local snow depth changes due to deposition and erosion the statistical distributions of elevation and the two dimensional correlation functions remain similar to those of the pre storm surface pre and post storm surfaces also exhibit spectral power law relationships with little change in spectral exponents these observations suggest that for sea ice floes with mature snow cover features under conditions similar to those observed in this study spatial statistics and scaling properties of snow surface morphology may be relatively invariant such an observation if confirmed for other ice types and conditions may be a useful tool for model parameterizations of the sub grid variability of sea ice surfaces

laboratory earthquakes the sub rayleigh to supershear rupture transition we report on the experimental observation of spontaneously nucleated supershear rupture and on the visualization of sub rayleigh to supershear rupture transitions in frictionally held interfaces the laboratory experiments mimic natural earthquakes the results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquake events

physical structural and isotopic characteristics and growth processes of fast sea ice in lutzow holm bay antarctica a sea ice ocean study was conducted off queen maud land and enderby land antarctica from 1990 to 1991 by the japanese antarctic research expedition observations of multiyear land fast sea ice were made in lutzow holm bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes the snow depth in the bay reached large values of 1 0 to 1 5 m during the winter season at offshore locations from the analysis of ice thickness measurements it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months on the basis of ice core structure salinity and stable isotopic composition we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute these processes were the primary contributors to sea ice growth and characteristics only where the snow accumulation was large in areas of low snow accumulation there was no surface growth superimposed ice formation on sea ice in antarctica has not been reported previously evidence for snow cover melting which is a prerequisite for superimposed ice formation was also found

sensitivity studies with a sea ice mixed layer pycnocline model in the weddell sea the sensitivity of a dynamic thermodynamic sea ice model coupled to a one dimensional mixed layer pycnocline model to variations of dynamic and thermodynamic model parameters is investigated furthermore the modifications of the model results due to the inclusion of a prognostic snow cover and the implementation of simplified sea ice rheologies are investigated in these comparisons special emphasis is placed upon the ice ocean boundary conditions buoyancy fluxes and the mixed layer properties

the association of micro earthquake clusters with mapped faults in the dead sea basin about 1100 micro earthquakes that occurred along the dead sea basin during the past 25 years express ongoing seismic activity along mostly obscured fault segments we apply seismological and statistical methods in order to associate seismic activity with geological and geophysical data and to locate and characterize active fault segments in the basin three regional 1 d velocity models and the double difference method were used for the relocation of seismic events geostatistical analysis shows that in first order the micro earthquakes reflect faulting along segments of the main longitudinal fault while seismic quiescence along historically active zones indicates locked segments and stress concentration based on waveform similarity spatial and temporal proximity we define earthquake clusters that comprise about 30 of the relocated seismicity and two thirds of the total seismic moment about 80 of the clusters are associated with n s trending faults in two cases the clusters indicate a migration of the seismicity in time and space one of which preceded the largest earthquake in our catalog m5 2 in addition to the associated longitudinal activity several clusters appear to represent faulting in east west orientation which is interpreted here as a secondary fault system

winter snow cover on sea ice in the weddell sea measurements of snow thickness temperature salinity density and stratigraphy acquired during the 1992 winter weddell gyre study are presented results indicate that the winter snow cover on sea ice in the weddell sea is extremely variable extreme fluctuations in antarctic synoptic conditions air temperature precipitation humidity and wind speed occur during the austral winter they result in unique modifications and additions to the snow layer during the aging process and act to stabilize an otherwise easily wind redistributed shallow snow cover and develop well packed drift features the latter occur even over relatively undeformed areas of sea ice and have a significant localized effect on the snow thickness distribution significant variability in snow grain size mean 2 73 3 12 mm and density 0 32 0 09 g cm 3 is observed as a result of cyclical switches between high and low temperature gradient metamorphism multiple icy layers indicate multiple thaw freeze events one such event occurred during a 3 day station during which the air temperature rose by 22 c in 12 hours to approximately 0 c this paper also examines mechanisms for flooding of the snow ice interface including snow loading even where the latter is not a factor the layer of snow immediately above the snow ice interface is commonly damp and saline 10 limitations in the data set are discussed and comparisons are drawn with other experiments

the mechanisms of sea ice melt pond formation and evolution 1 a series of observations were made on melting first year landfast arctic sea ice near barrow alaska to explore the seasonal evolution of melt pond coverage observations of pond coverage albedo and ice properties are combined with terrestrial lidar measurements of surface topography and meltwater balance to quantitatively identify the timing and role of mechanisms driving pond coverage the formation of interposed fresh ice is found to eliminate meltwater percolation through early pond formation and allow widespread ponding well above sea level pond drainage to sea level occurs principally by horizontal meltwater transport over the ice surface to macroscopic flaws freeboard loss caused by buoyancy decline as the ice thins controls pond growth late in the melt season after percolation begins the majority of the macroscopic flaws that drain melt ponds to sea level are observed to develop from brine drainage channels within the ice a simple thermodynamic model of meltwater percolation illustrates that fresh meltwater inflow causes pores in the ice to either shrink and freeze shut or enlarge based on initial size and ice temperature this threshold behavior of pore diameter controls both the blockage of smaller pores with interposed ice and the enlargement of larger brine drainage channels to allow meltwater drainage the results identify links between the temporal evolution of pond coverage and ice temperature salinity and thickness providing new opportunities to realistically parameterize ponds and summer ice albedo within sea ice models

ers 1 radar and field observed characteristics of autumn freeze up in the weddell sea ers 1 satellite microwave radar data are analyzed to investigate changes in sea ice characteristics during a period when a drifting ice camp was deployed in the weddell sea antarctica synthetic aperture radar and scatterometer data are calibrated and geolocated to derive a time series of c band backscatter coefficient corresponding with simultaneous surface measurements during the austral autumn freeze up thermistor strings were implanted in the snow and ice at a number of local and regional sites surface measurements at these sites indicate that up to 50 of the surface of ice floes surviving the summer were flooded with an unconsolidated saline slush layer at the snow ice interface consisting of approximately half seawater and half ice of meteoric origin the slush was typically 5 30 cm thick and covered by a 20 to 50 cm thick dry snow layer results show that the microwave radar backscatter characteristics of this perennial ice region responded sensitively to changes in air temperature and corresponding changes in turbulent flux of heat at the surface of the sea ice at ice concentrations exceeding 95 the modulation of the regional backscatter coefficient by wind speed and direction was negligible warm summer conditions persisted for around 2 weeks after ice camp deployment on february 7 day 38 with air temperatures of around 3 to 5 c prior to the onset of autumn freeze up from february 26 day 57 onward cooling began and snow ice growth proceeded as air temperatures fell to below 20 c altogether between february 7 day 38 and march 15 1992 day 75 the backscatter coefficient time series measured by each radar indicated that fell by several decibels during the freezing and transformation of the layer of saturated saline basal snow into snow ice this change is caused by a reduction in the permittivity and thus the scattering intensity of the basal snow as a function of the disappearance by freezing of the saltwater saturated layer these results suggest the possibility of monitoring the timing and autumn freeze up transition of regional ice signatures as a means of quantifying the proportion of flooded perennial sea ice

implications of fault constitutive properties for earthquake prediction the rate and state dependent constitutive formulation for fault slip characterizes an exceptional variety of materials over a wide range of sliding conditions this formulation provides a unified representation of diverse sliding phenomena including slip weakening over a characteristic sliding distance dc apparent fracture energy at a rupture front time dependent healing after rapid slip and various other transient and slip rate effects laboratory observations and theoretical models both indicate that earthquake nucleation is accompanied by long intervals of accelerating slip strains from the nucleation process on buried faults generally could not be detected if laboratory values of dc apply to faults in nature however scaling of dc is presently an open question and the possibility exists that measurable premonitory creep may precede some earthquakes earthquake activity is modeled as a sequence of earthquake nucleation events in this model earthquake clustering arises from sensitivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and assigns physical interpretation to aftershock parameters the seismicity formulation predicts large changes of earthquake probabilities result from stress changes two mechanisms for foreshocks are proposed that describe observed frequency of occurrence of foreshock mainshock pairs by time and magnitude with the first mechanism foreshocks represent a manifestation of earthquake clustering in which the stress change at the time of the foreshock increases the probability of earthquakes at all magnitudes including the eventual mainshock with the second model accelerating fault slip on the mainshock nucleation zone triggers foreshocks

geoacoustic inversion of broadband data by matched beam processing this paper describes results of geoacoustic inversion using broadband signals from an experiment carried out at a site near the south florida ocean measurement centre in the florida straits m sequence coded pulse trains at different center carrier frequencies from 100to3200hz were recorded at a vertical line array at a distance around 10km geoacoustic inversion was carried out to determine the feasibility of inverting the environmental parameters from this long range propagation experiment the received signal at lower frequencies below 400hz consisted of a dominant water column signal and a secondary arrival delayed by 0 4s the secondary signal was spatially filtered by beamforming the array data and the beam data were inverted by matched beam processing in the time domain combined with an adaptive simplex simulated annealing algorithm the estimated values of compressional wave speed and density were in good agreement with ground truth values from sediment cores the inverted shear wave speed appea

range dependent seabed characterization by inversion of acoustic data from a towed receiver array the mapex2000 experiments were conducted in the mediterranean sea in march 2000 to determine seabed properties using a towed acoustic source and receiver array towed systems are advantageous because they are easy to deploy from a ship and the moving platform offers the possibility for estimating spatially variable range dependent seabed properties in this paper seabed parameters are determined using a matched field geoacoustic inversion approach with measured towed array data previous research has successfully applied matched field geoacoustic inversion techniques to measured acoustic data however in nearly all cases the inverted data were collected on moored vertical receiver arrays results here show that seabed parameters can also be extracted by inverting acoustic measurements from a towed array of receivers and these agree with those inverted using data received simultaneously on a vertical array these findings imply that a practical technique could be developed to map range dependent seabed parameters over large areas using a towed acoustic system an example of such a range dependent inversion is given using measurements from the mapex2000 experiments

evolution of first year and second year snow properties on sea ice in the weddell sea during spring summer transition observations of snow properties superimposed ice and atmospheric heat fluxes have been performed on first year and second year sea ice in the western weddell sea antarctica snow in this region is particular as it does usually survive summer ablation measurements were performed during ice station polarstern ispol a 5 week drift station of the german icebreaker rv polarstern net heat flux to the snowpack was 8 w m 2 causing only 0 1 to 0 2 m of thinning of both snow cover types thinner first year and thicker second year snow snow thinning was dominated by compaction and evaporation whereas melt was of minor importance and occurred only internally at or close to the surface characteristic differences between snow on first year and second year ice were found in snow thickness temperature and stratigraphy snow on second year ice was thicker colder denser and more layered than on first year ice metamorphism and ablation and thus mass balance were similar between both regimes because they depend more on surface heat fluxes and less on underground properties ice freeboard was mostly negative but flooding occurred mainly on first year ice snow and ice interface temperature did not reach the melting point during the observation period nevertheless formation of discontinuous superimposed ice was observed color tracer experiments suggest considerable meltwater percolation within the snow despite below melting temperatures of lower layers strong meridional gradients of snow and sea ice properties were found in this region they suggest similar gradients in atmospheric and oceanographic conditions and implicate their importance for melt processes and the location of the summer ice edge

asperity failure control of stick slip along brittle faults stick slips are spontaneous unstable slip events during which a natural or man made system transitions from a strong sticking stage to a weaker slipping stage stick slips were proposed by brace and byerlee science 153 990 992 1966 as the experimental analogue of natural earthquakes we analyze here the mechanics of stick slips along brittle faults by conducting laboratory experiments and by modeling the instability mechanics we performed tens of shear tests along experimental faults made of granite and gabbro that were subjected to normal stresses up to 14 3 mpa and loading velocities of 0 26 617 m s we observed hundreds of spontaneous stick slips that displayed shear stress drops up to 0 66 mpa and slip velocities up to 14 1 mm s the pre shear and post shear fault surface topography were mapped with atomic force microscopy at pixel sizes as low as 0 003 m2 we attribute the sticking phase to the locking of touching asperities and the slipping phase to the brittle failure of these asperities and found that the fault asperities are as strong as the inherent strength of the host rock based on the experimental observations and analysis we derived a mechanical model that predicts the relationships between the measured stick slip properties stress drop duration and slip distance and asperity strength

seasonal variations in the properties and structural composition of sea ice and snow cover in the bellingshausen and amundsen seas antarctica sixty three ice cores were collected in the bellingshausen and amundsen seas in august and september 1993 during a cruise of the r v nathaniel b palmer the structure and stable isotopic composition 18 o 16 o of the cores were investigated in order to understand the growth conditions and to identify the key growth processes particularly the contribution of snow to sea ice formation the structure and isotopic composition of a set of 12 cores that was collected for the same purpose in the bellingshausen sea in march 1992 are reassessed frazil ice and congelation ice contribute 44 and 26 respectively to the composition of both the winter and summer ice core sets evidence that the relatively calm conditions that favour congelation ice formation are neither as common nor as prolonged as the more turbulent conditions that favour frazil ice growth and pancake ice formation both frazil and congelation ice layers have an av erage thickness of 0 12 m in winter evidence that congelation ice and pancake ice thicken primarily by dynamic processes the thermodynamic development of the ice cover relies heavily on the formation of snow ice at the surface of floes after sea water has flooded the snow cover snow ice layers have a mean thickness of 0 20 and 0 28 m in the winter and summer cores respectively and the contribution of snow ice to the winter 24 and summer 16 core sets exceeds most quantities that have been reported previously in other antarctic pack ice zones the thickness and quantity of snow ice may be due to a combination of high snow accumulation rates and snow loads environmental conditions that favour a warm ice cover in which brine convection between the bottom and top of the ice introduces sea water to the snow ice interface and bottom melting losses being compensated by snow ice formation layers of superimposed ice at the top of each of the summer cores make up 4 6 of the ice that was examined and they increase by a factor of 3 the quantity of snow entrained in the ice the accumulation of superimposed ice is evidence that melting in the snow cover on antarctic sea ice floes ran reach an advanced stage and contribute a significant amount of snow to the total ice mass

spring snow conditions on arctic sea ice north of svalbard during the norwegian young sea ice n ice2015 expedition snow is crucial over sea ice due to its conflicting role in reflecting the incoming solar energy and reducing the heat transfer so that its temporal and spatial variability are important to estimate during the norwegian young sea ice n ice2015 campaign snow physical properties and variability were examined and results from april until mid june 2015 are presented here overall the snow thickness was about 20 cm higher than the climatology for second year ice with an average of 55 27 cm and 32 20 cm on first year ice the average density was 350 400 kg m 3 in spring with higher values in june due to melting due to flooding in march larger variability in snow water equivalent was observed however the snow structure was quite homogeneous in spring due to warmer weather and lower amount of storms passing over the field camp the snow was mostly consisted of wind slab faceted and depth hoar type crystals with occasional fresh snow these observations highlight the more dynamic character of evolution of snow properties over sea ice compared to previous observations due to more variable sea ice and weather conditions in this area the snowpack was isothermal as early as 10 june with the first onset of melt clearly identified in early june based on our observations we estimate than snow could be accurately represented by a three to four layers modeling approach in order to better consider the high variability of snow thickness and density together with the rapid metamorphose of the snow in springtime

thin sea ice thick snow and widespread negative freeboard observed during n ice2015 north of svalbard in recent years sea ice conditions in the arctic ocean changed substantially toward a younger and thinner sea ice cover to capture the scope of these changes and identify the differences between individual regions in situ observations from expeditions are a valuable data source we present a continuous time series of in situ measurements from the n ice2015 expedition from january to june 2015 in the arctic basin north of svalbard comprising snow buoy and ice mass balance buoy data and local and regional data gained from electromagnetic induction em surveys and snow probe measurements from four distinct drifts the observed mean snow depth of 0 53 m for april to early june is 73 above the average value of 0 30 m from historical and recent observations in this region covering the years 1955 2017 the modal total ice and snow thicknesses of 1 6 and 1 7 m measured with ground based em and airborne em measurements in april may and june 2015 respectively lie below the values ranging from 1 8 to 2 7 m reported in historical observations from the same region and time of year the thick snow cover slows thermodynamic growth of the underlying sea ice in combination with a thin sea ice cover this leads to an imbalance between snow and ice thickness which causes widespread negative freeboard with subsequent flooding and a potential for snow ice formation with certainty 29 of randomly located drill holes on level ice had negative freeboard

yellow shark spring 1995 inversion results from sparse broadband acoustic measurements over a highly range dependent soft clay layer in may 1995 saclantcen performed broadband 200 800 hz acoustic measurements in the giglio basin off the coast of italy as part of the yellow shark inversion experiments in this paper inversion of sparse broadband transmission loss tl measurements is investigated to determine bottom properties in strongly range dependent rd situations commonly encountered in shallow water the data are from the elba formiche transect where water depth varies from 65 m at the acoustic projector location to approximately 120 m at the 4 element vertical arrays deployed at ranges of 8 16 24 32 and 40 km along the transect the experimental site has a soft clay layer bottom which varies in thickness from 3 to 10 m with a sound speed less than the water column a modal analysis including coupling effect is given to explain the frequency bands for which high tl was observed as a function of range and depth the tl measurements were inverted by matching the rd fields with model results using parabolic equation modeling bottom geoacoustic parameters were varied in a marching search to fit the tl measured at the five ranges the experimental results demonstrate that rd bottom properties such as sound speed and thickness of the slow clay layer can be obtained from broadband tl measurements sparsely distributed in range and depth

the effect of snow on antarctic sea ice simulations in a coupled atmosphere sea ice model the effect of a snow cover on sea ice accretion and ablation is estimated based on the zero layer version sea ice model of semtner and is examined using a coupled atmosphere sea ice model including feedbacks and ice dynamics effects when snow is disregarded in the coupled model the averaged antarctic sea ice becomes thicker when only half of the snowfall predicted by the atmospheric model is allowed to land on the ice surface sea ice gets thicker in most of the weddell and ross seas but thinner in east antarctic in winter with the average slightly thicker when twice as much snowfall as predicted by the atmospheric model is assumed to land on the ice surface sea ice also gets much thicker due to the large increase of snow ice formation these results indicate the importance of the correct simulation of the snow cover over sea ice and snow ice formation in the antarctic our results also illustrate the complex feedback effects of the snow cover in global climate models in this study we have also tested the use of a mean value of 0 16 wm 1 k 1 instead of 0 31 for the thermal conductivity of snow in the coupled model based on the most recent observations in the eastern antarctic and bellingshausen and amundsen seas and have found that the sea ice distribution changes greatly with the ice becoming much thinner by about 0 2 m in the antarctic and about 0 4 m in the arctic on average this implies that the magnitude of the thermal conductivity of snow is of considerable importance for the simulation of the sea ice distribution an appropriate value of the thermal conductivity of snow is as crucial as the depth of the snow layer and the snowfall rate in a sea ice model the coupled climate models require accurate values of the effective thermal conductivity of snow from observations for validating the simulated sea ice distribution under the present climate conditions

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

snow contribution to first year and second year arctic sea ice mass balance north of svalbard the salinity and water oxygen isotope composition 18o of twenty nine first year fyi and second year syi arctic sea ice cores total length 32 0 m from the drifting ice pack north of svalbard were examined to quantify the contribution of snow to sea ice mass five cores total length 6 4 m were analyzed for their structural composition showing variable contribution of 10 30 by granular ice in these cores snow had been entrained in 6 to 28 of the total ice thickness we found evidence of snow contribution in about three quarter of the sea ice cores when surface granular layers had very low 18o values snow contributed 7 5 9 7 to sea ice mass balance on average including also cores with no snow using 18o mass balance calculations in syi cores snow fraction by mass 12 7 16 3 was much higher than in fyi cores 3 3 4 4 while the bulk salinity of fyi 4 9 was distinctively higher than for syi 2 7 we surmise that oxygen isotopes and salinity profiles can give information on the age of the ice and allows to distinguish between fyi and syi or older ice in the area north of svalbard this article is protected by copyright all rights reserved

nonlinear inversion for ocean bottom properties high resolution methods based on simulated annealing and full wave sound propagation models are developed for nonlinear inversion for ocean bottom properties simulated annealing is used to search the high dimensional parameter space of ocean bottoms for the parameter set corresponding to the best replica field the parabolic equation method is used to solve range dependent inversion problems for data taken by lynch et al from a range dependent region of the gulf of mexico j acoust soc am 89 648 665 1991 this approach achieves excellent agreement between the theoretical and measured acoustic pressures the recovered sediment parameters suggest that a sound speed boundary layer exists in the upper part of the sediment and that the depth of an interface in the sediment is range dependent for locally range independent problems inversion is performed in wave number space large efficiency gains are possible with this approach because the number of wave number samples required for inversion is mu

modeling of periodic great earthquakes on the san andreas fault effects of nonlinear crustal rheology we analyze the cycle of great earthquakes along the san andreas fault with a finite element numerical model of deformation in a crust with a nonlinear viscoelastic rheology the viscous component of deformation has an effective viscosity that depends exponentially on the inverse absolute temperature and nonlinearly on the shear stress the elastic deformation is linear crustal thickness and temperature are constrained by seismic and heat flow data for california the models are for anti plane strain in a 25 km thick crustal layer having a very long vertical strike slip fault the crustal block extends 250 km to either side of the fault during the earthquake cycle that lasts 160 years a constant plate velocity p 2 17 5 mm yr 1 is applied to the base of the crust and to the vertical end of the crustal block 250 km away from the fault the upper half of the fault is locked during the interseismic period while its lower half slips at the constant plate velocity the locked part of the fault is moved abruptly 2 8 m every 160 years to simulate great earthquakes the results are sensitive to crustal rheology models with quartzite like rheology display profound transient stages in the velocity displacement and stress fields the predicted transient zone extends about 3 4 times the crustal thickness on each side of the fault significantly wider than the zone of deformation in elastic models models with diabase like rheology behave similarly to elastic models and exhibit no transient stages the model predictions are compared with geodetic observations of fault parallel velocities in northern and central california and local rates of shear strain along the san andreas fault the observations are best fit by models which are 10 100 times less viscous than a quartzite like rheology since the lower crust in california is composed of intermediate to mafic rocks the present result suggests that the in situ viscosity of the crustal rock is orders of magnitude less the rock viscosity determined in the laboratory

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

west antarctic peninsula sea ice in 2005 extreme ice compaction and ice edge retreat due to strong anomaly with respect to climate in september october 2005 the juxtaposition of low and high pressure anomalies at 130 degrees w and 60 degrees w respectively created strong and persistent northerly airflow across the west antarctic peninsula wap this had a major impact on regional sea ice conditions with extreme ice compaction in the bellingshausen and east amundsen seas 60 degrees w 130 degrees w but divergence in the west amundsen and east ross seas this resulted in the former in a highly compact marginal ice zone and ice cover mean modeled ice thicknesses of 5 m and an earlier than average maximum extent mid august while rapid ice retreat in late winter spring created a major negative ice extent anomaly compact ice persisted in the subsequent summer other effects were anomalies in air temperature of 1 degrees c to 5 degrees c and precipitation rates to 2 5 mm d the patterns in late 2005 are consistent with the occurrence of a weak la nina and a near neutral southern annular mode with a quasi stationary zonal wave three pattern dominating hemispheric atmospheric circulation once a compact ice edge was created it took only one additional week of strong winds to solidify the pack in place conditions in 2005 are analyzed in the context of 1979 2005 and compared with the springs of 1993 1997 1999 2001 and 2004 a statistically significant increase of the northerly 10 m wind component between 110 degrees w and 125 degrees w occurred in the septembers of 1979 2005 no clear trends occur in other spring months this work underlines the key importance of ice dynamics in recent changes in the wap sea ice regime

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

winter snow cover variability on east antarctic sea ice copyright 1998 by the american geophysical union analysis of the first detailed data set of snow characteristics collected over east antarctic sea ice in winter confirms that on small scales snow on antarctic sea ice is highly variable in both thickness and properties high amplitude cyclical variability in atmospheric forcing related to the passage of storms is responsible for the high degree of textural heterogeneity observed changes in snow properties were examined over a 3 week period during which a largely icy snow cover formed at near freezing temperatures metamorphosed to snow in which facetted crystals and depth hoar dominated as the air temperature plummeted even on flat ice significant localized thickening of snow occurs in the form of barchan dunes although we observed great variability in snow thickness and properties on local scales overall snow thickness distribution and the complex textural assemblage of snow types are similar from region to region similar observations were made by sturm et al 1998 in west antarctica large scale similarities are also apparent in mean snow density grain size and bulk snow salinity although high variability is again found across individual floes rapid depth hoar formation is a ubiquitous process that greatly affects the density texture grain size and effective thermal conductivity of the snow cover the observed heterogeneity results in varying snow effective thermal conductivities the mean bulk effective thermal conductivity computed from the proportion of observed snow types is 0 164 w m 1 k 1 significantly lower than values typically used in large scale sea ice modeling but similar to that derived by sturm et al 1998 in a near simultaneous experiment in the bellingshausen and ross seas it varies from 0 097 to 0 383 w m 1 k 1 in different snow pits the findings support those of sturm et al 1998 that periodic flooding and subsequent snow ice formation which are also ubiquitous processes effectively diminish the degree to which basal snow processes create inhomogeneities in the snow pack

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

slip complexity in earthquake fault models we summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes for models of smooth faults between elastically deformable continua it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state that results in a finite nucleation size or coherent slip patch size h models of smooth faults using numerical cell size properly small compared to h show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self similar power law gutenberg richter frequency size statistics this conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences in contrast some models of locally heterogeneous faults with quasi independent fault segments represented approximately by simulations with cell size larger than h so that the model becomes inherently discrete do show small event complexity of the gutenberg richter type models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h 0 and hence always fall into the inherently discrete class we suggest that the small event complexity that some such models show will not survive regularization of the constitutive description by inclusion of an appropriate length scale leading to a finite h and a corresponding reduction of numerical grid size

numerical simulations of the profile properties of undeformed first year sea ice during the growth season a simulation scheme is developed that estimates salinity profiles for first year sea ice during the growth season as a function of the growth history of the ice the model considers the dependence of the initial ice salinity on ice growth velocity and sea water salinity and also the subsequent drainage of brine from the ice the equation for ice growth assumes a linear temperature profile within the ice and is driven by surface heat balance equations that are based on smoothed climatic data for the central arctic basin the estimated salinity profiles are in good agreement with natural profiles although temperature and salinity profiles depend upon the time of the year when ice growth is initiated the brine volume profiles which they specify are essentially a unique function of ice thickness a conclusion that holds even when the insulative effects of snow are considered the temperature and brine volume profiles are then utilized to calculate the ice strength and elastic modulus profiles which in turn specify the composite mechanical properties of the ice sheets through the application of a theory developed by assur 1967 significant differences which are largest for thin ice sheets are observed between ice sheet properties as calculated using composite plate theory and properties calculated from uniform plate theory and average ice properties these results provide a justification for the practice common within the ice modeling community of parameterizing the mechanical behavior of pack ice on the basis of the ice thickness distribution

development structure and composition of land fast sea ice in the northern baltic sea the structural and stable oxygen isotopic properties of sea ice samples collected on land fast sea ice in the northern baltic sea at 60 2 65 7 n 21 3 26 9 e were used to estimate the contribution of snow ice in the total sea ice thickness and the fraction of snow in the snow ice layers and in the total sea ice thickness the textural analysis of the sea ice cores revealed a general structure of granular surface layer and a columnar basal layer using different isotopic criteria to distinguish snow ice layers we estimate that snow ice layers contributed on average 24 32 of the total ice thickness in the study region transition ice intermediate granular columnar ice also contributed significantly to the total ice thickness the snow in the snow ice layers contributed on average 18 21 of the total sea ice thickness by mass a significant contribution when compared to other regions with sea ice cover the methods used could be improved to give more accurate estimates especially by improving knowledge of the temporal variability of the isotropic properties of the snow incorporated into the sea ice cover however our results can be regarded is plausible estimates for the contribution of snow ice and snow to sea ice growth in the study area

crystal structure stable isotopes δ18o and development of sea ice in the ross amundsen and bellingshausen seas antarctica the crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern ross sea the amundsen sea and the western bellingshausen sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern pacific sector of the southern ocean the isotope data indicate that a moderate amount of snow contributes to the development of the sea ice however even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice nevertheless it seems clear that much of the snow is contained in granular snow ice that results from seawater flooding of floes and the base of the snow cover the snow cover in the ross amundsen region was as much as 2 m deep and supported by 7 to 8 m thick floes primarily composed of frazil ice in the bellingshausen region the snow cover and the floes were thinner than in the ross amundsen region the bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice in addition in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores the data indicate that the sea ice develops by multiple mechanisms in a turbulent environment

ice mass balance buoys a tool for measuring and attributing changes in the thickness of the arctic sea ice cover recent observational and modeling studies indicate that the arctic sea ice cover is undergoing significant climate induced changes affecting both its extent and thickness the thickness or more precisely the mass balance of the ice cover is a key climate change indicator since it is an integrator of both the surface heat budget and the ocean heat flux accordingly efforts are underway to develop and deploy in situ observing systems which when combined with satellite remote sensing information and numerical models can effectively monitor and attribute changes in the mass balance of the arctic sea ice cover as part of this effort we have developed an autonomous ice mass balance buoy imb which is equipped with sensors to measure snow accumulation and ablation ice growth and melt and internal ice temperature plus a satellite transmitter the imb is unique in its ability to determine whether changes in the thickness of the ice cover occur at the top or bottom of the ice cover and hence provide insight into the driving forces behind the change since 2000 imbs have been deployed each spring from the north pole environmental observatory and in several other areas including a few in the beaufort sea and central basin at this point the collective time series is too short to draw significant and specific conclusions regarding interannual and regional variability in ice mass balance comparisons of available data indicate that ice surface ablation is greater in the beaufort region 67 80 cm relative to the north pole 0 30 cm consistent with a longer period of melt in the more southerly location ablation at the bottom of the ice 22 cm maximum ice thickness 235 cm and maximum snow depth 28 cm were comparable in the two regions

the 1995 off etorofu earthquake joint relocation of foreshocks the mainshock and aftershocks and implications for the earthquake nucleation process abstract prominent foreshock activity preceded the mw 7 9 off etorofu earthquake in the kurile islands on 3 december 1995 using a modified joint hypocenter determination method hurukawa 1995 we simultaneously relocated foreshocks the mainshock and aftershocks to study the foreshock activity in detail in view of the nucleation process of a large earthquake the distribution of relocated earthquakes and their focal mechanisms suggest that the 1995 off etorofu earthquake was an interplate earthquake at the boundary between the north american and pacific plates its source area overlaps with those of the 1958 off etorofu mw 8 3 and the 1963 off urup mw 8 5 earthquakes nine days before the occurrence of the mainshock a first immediate foreshock of magnitude 6 4 occurred at the deepest point of the foreshock area this event was followed by many foreshocks including three additional m 6 events which occurred east and southeast of the first foreshock the foreshock area expanded toward the trench axis with a velocity of several to several tens centimeters per second the number of events per day increased daily and the final size of the foreshock area was about 80 30 km eventually the rupture of the mainshock started at the deepest point of the foreshock area these observational facts are consistent with recent theoretical studies and laboratory experiments in which foreshocks are regarded as the rupture of localized asperities in a broad weak zone where the nucleation of the large earthquake started

accelerated seismic release and related aspects of seismicity patterns on earthquake faults observational studies indicate that large earthquakes are sometimes preceded by phases of accelerated seismic release asr characterized by cumulative benioff strain following a power law timeto failure relation with a term tf t m where tr is the failure time of the large event and observed values of m are close to 0 3 we discuss properties of asr and related aspects of seismicity patterns associated with several theoretical frameworks the subcritical crack growth approach developed to describe deformation on a crack prior to the occurrence of dynamic rupture predicts great variability and low asymptotic values of the exponent m that are not compatible with observed asr phases statistical physics studies assuming that system size failures in a deforming region correspond to critical phase transitions predict establishment of long range correlations of dynamic variables and power law statistics before large events using stress and earthquake histories simulated by the model of ben zion 1996 for a discrete fault with quenched heterogeneities in a 3 d elastic half space we show that large model earthquakes are associated with nonrepeating cyclical establishment and destruction of long range stress correlations accompanied by nonstationary cumulative benioff strain release we then analyze results associated with a regional lithospheric model consisting of a seismogenic upper crust governed by the damage rheology of lyakhovsky et al 39 over a viscoelastic substrate we demonstrate analytically for a simplified 1 d case that the employed damage rheology leads to a singular power law equation for strain proportional to t f t 1 3 and a nonsingular power law relation for cumulative benioff strain proportional to t f t 1 3 a simple approximate generalization of the latter for regional cumulative benioff strain is obtained by adding to the result a linear function of time representing a stationary background release to go beyond the analytical expectations we examine results generated by various realizations of the regional lithospheric model producing seismicity following the characteristic frequency size statistics gutenberg richter power law distribution and mode switching activity we find that phases of asr exist only when the seismicity preceding a given large event has broad frequency size statistics in such cases the simulated asr phases can be fitted well by the singular analytical relation with m 1 3 the nonsingular equation with m 0 2 and the generalized version of the latter including a linear term with m 1 3 the obtained good fits with all three relations highlight the difficulty of deriving reliable information on functional forms and parameter values from such data sets the activation process in the simulated asr phases is found to be accommodated both by increasing rates of moderate events and increasing average event size with the former starting a few years earlier than the latter the lack of asr in portions of the seismicity not having broad frequency size statistics may explain why some large earthquakes are preceded by asr and other are not the results suggest that observations of moderate and large events contain two complementary end member predictive signals on the time of future large earthquakes in portions of seismicity following the characteristic earthquake distribution such information exists directly in the associated quasi periodic temporal distribution of large events in portions of seismicity having broad frequency size statistics withrandom or clustered temporal distribution of large events the asr phases have predictive information the extent to which natural seismicity may be understood in terms of these end member cases remains to be clarified continuing studies of evolving stress and other dynamic variables in model calculations combined with advanced analyses of simulated and observed seismicity patterns may lead to improvements in existing forecasting strategies

ice processes and surface ablation in a shallow thermokarst lake in the central qinghai tibetan plateau the qinghai tibetan plateau qtp is characterized by a cold climate and a large number of lakes the long ice season necessitates study of the widespread ice covers in the region an unprecedented multidisciplinary field campaign was conducted on lake ice processes in the central qtp during the period 2019 13 the study lake generally froze up in late october or early november and broke up in mid or late april with a maximum ice thickness of 50 70 cm the mass balances at both ice surface and bottom were measured continuously significant ice surface sublimation ablation was detected and accounted for up to 40 of the whole ice thickness over the ice season a simple heat transfer model was developed for the surface ice loss the calculated values were in good agreement with the observations they also indicated that atmospheric conditions including low air humidity and prevailing strong winds are the primary drivers of the ice surface sublimation

foreshock and aftershock sequence of the 1998 m 5 0 sendai northeastern japan earthquake and its implications for earthquake nucleation we investigated foreshock and aftershock sequences of a m 5 0 earthquake that occurred on 15 september 1998 near the nagamachi rifu fault an active fault crossing the urban area of sendai city northeastern japan the focal mechanism of this event was a reverse fault type with a nearly horizontal p axis parallel to the plate conversion direction aftershock distribution located by the homogeneousstation method delineates the fault plane of this event dipping to the northwest at an angle of 30 in the depth range of 10 12 km the location of the fault plane and the slip direction thus estimated suggest that this event was a slip at a deep portion of the nagamachi rifu fault three days prior to the mainshock many foreshocks with similar waveforms started to occur very close to the mainshock hypocenter we estimated relative locations of these similar foreshocks based on crossspectrum analyses of waveforms recorded at nearby stations they are located at the deepest portion of the mainshock fault plane and just above the mainshock hypocenter migration of the foreshocks is observed from deeper to shallower depths cumulative coulomb stress changes caused by preceding events show negative values at hypocenters of similar waveform foreshocks our results do not support the cascade model for the process of foreshock generation nucleation phase prior to the high speed rupture of the mainshock is observed in seismograms at nearby stations the initial point of the nucleation phase is located on a deeper extension of the foreshock area the high speed breakout point is located in the area where a large amount of seismic moment release was estimated from waveform inversions these observations probably reflect an underlying aseismic nucleation process preceding the main dynamic rupture of the m 5 0 event

seasonal evolution of an ice shelf influenced fast ice regime derived from an autonomous thermistor chain ice shelves strongly interact with coastal antarctic sea ice and the associated ecosystem by creating conditions favorable to the formation of a sub ice platelet layer the close investigation of this phenomenon and its seasonal evolution remains a challenge due to logistical constraints and a lack of suitable methodology in this study we characterize the seasonal cycle of antarctic fast ice adjacent to the ekstrom ice shelf in the eastern weddell sea we used a thermistor chain with the additional ability to record the temperature response induced by cyclic heating of resistors embedded in the chain vertical sea ice temperature and heating profiles obtained daily between november 2012 and february 2014 were analyzed to determine sea ice and snow evolution and to calculate the basal energy budget the residual heat flux translated into an ice volume fraction in the platelet layer of 0 18 0 09 which we reproduced by a independent model simulation and agrees with earlier results manual drillings revealed an average annual platelet layer thickness increase of at least 4 m and an annual maximum thickness of 10 m beneath second year sea ice the oceanic contribution dominated the total sea ice production during the study effectively accounting for up to 70 of second year sea ice growth in summer an oceanic heat flux of 21 w m 2 led to a partial thinning of the platelet layer our results further show that the active heating method in contrast to the acoustic sounding approach is well suited to derive the fast ice mass balance in regions influenced by ocean ice shelf interaction as it allows subdiurnal monitoring of the platelet layer thickness

tomographic inversion of geoacoustic properties in a range dependent shallow water environment this paper presents a matched field tomographic method to estimate the geoacoustic properties of the ocean bottom for a range dependent medium in shallow water the inversion method has been developed in order to interpret experimental data from the haro strait primer sea trial this experiment was carried out in june 96 and used low frequency broadband signals that were received on three vertical line arrays inversion of the data is particularly difficult because of the complex bathymetry of the haro strait experimental site for this inversion a range dependent ray code was developed to solve the forward problem allowing an arbitrarily layered bottom environment the inversion scheme is based on modeling the propagation time and the amplitude of the recorded data and a simple new cost function is proposed the signal ray paths are identified automatically using a simple process that compares calculated and measured travel times data from multiple source positions are used to invert the range dependence of the geoacoustic model the environment is separated into segments and within each segment the inversion is carried out layer by layer for a multilayer geoacoustic model starting with the topmost layer the range dependent thickness and sound speed are estimated via a monte carlo method inversion results are presented for synthetic and experimental data from the haro strait sea trial

shear zones formed along long straight traces of fault zones during the 28 june 1992 landers california earthquake surface rupturing during the 28 june 1992 landers california earthquake east of los angeles accommodated right lateral offsets up to about 6 m along segments of distinct en echelon fault zones with a total length of 80 km the offsets were accommodated generally not by faults distinct slip sur faces but rather by shear zones tabular bands of localized shearing along simple stretches of fault zones at landers the rupture is characterized by telescoping of shear zones and intensification of shearing broad shear zones of mild sheafing containing narrow shear zones of more intense sheafing containing even nar rower shear zones of very intense sheafing which may contain a fault thus the ground ruptured across broad belts of shearing with clearly defined subparallel walls oriented nw each broad belt consists of a broad zone of mild sheafing extending across its entire width 50 to 200 m and much nan ower a few meters wide shear zones that accommodate most of the offset of the belt and are por trayed by en echelon tension cracks in response to right lateral shearing the slices of ground bounded by the tension cracks rotated in a clockwise sense producing left lateral sheafing and the slices were forced against the walls of the shear zone producing thrusting even narrower shear zones formed within the narrow shear zones although these probably are guides to right lateral fault segments below the surface rupturing during the earthquake is characterized not by faulting but by the formation of shear zones at various scales

observations of flooding and snow ice formation in a thinner arctic sea ice regime during the n ice2015 campaign influence of basal ice melt and storms seven ice mass balance instruments deployed near 83 n on different first year and second year ice floes representing variable snow and ice conditions documented the evolution of snow and ice conditions in the arctic ocean north of svalbard in january march 2015 frequent profiles of temperature and thermal diffusivity proxy were recorded to distinguish changes in snow depth and ice thickness with 2 cm vertical resolution four instruments documented flooding and snow ice formation flooding was clearly detectable in the simultaneous changes in thermal diffusivity proxy increased temperature and heat propagation through the underlying ice slush then progressively transformed into snow ice flooding resulted from two different processes i after storm induced breakup of snow loaded floes and ii after loss of buoyancy due to basal ice melt in the case of breakup when the ice was cold and not permeable rapid flooding probably due to lateral intrusion of seawater led to slush and snow ice layers at the ocean freezing temperature 1 88 c after the storm the instruments documented basal sea ice melt over warm atlantic waters and ocean to ice heat flux peaked at up to 400 w m 2 the warm ice was then permeable and flooding was more gradual probably involving vertical intrusion of brines and led to colder slush and snow ice 3 c the n ice2015 campaign provided the first documentation of significant flooding and snow ice formation in the arctic ice pack as the slush partially refroze snow ice formation may become a more frequently observed process in a thinner ice arctic

loading mechanism and scaling relations of large interplate earthquakes we have done a numerical simulation of tectonic loading at transform plate boundaries with a lithosphere asthenosphere coupling model subject to steady relative plate motion in this model a seismic fault is represented by a locked patch with a finite length on an infinitely long vertical dislocation surface cutting the entire lithosphere through the numerical simulation we have obtained the following results stress accumulation on the fault is partly due to viscous drag at the base of the lithosphere base loading and partly due to dislocation pile ups at horizontal edges of the fault edge loading if the viscosity of the asthenosphere is less than 1018 pa s the base loading is not effective since its rate exponentially decays very soon if the viscosity of the asthenosphere is greater than 1020 pa s both types of loading are effective and then the stress accumulation rate in the early stage of loading is formally written as d dt vpl a bl where vpl is a rate of relative plate motion and l is a fault length when the fault length l is small the effect of edge loading the second term is dominant and the stress accumulation rate is in proportion to the inverse of l when the fault length l is large the effect of base loading the first term becomes dominant and the stress accumulation rate is independent of l it is well known that the general l cubed dependence of seismic moment m0 breaks for large interplate earthquakes this break in the moment length relation can be ascribed to difference in loading mechanism between small and large earthquakes from the results of numerical computation if interplate earthquakes have the same stress drop regardless of their fault lengths we may derive a l squared dependence of m0 for large interplate earthquakes and a linear l dependence of m0 for very large interplate earthquakes

a method to automatically determine sea level for referencing snow freeboards and computing sea ice thicknesses from nasa icebridge airborne lidar abstract the nasa icebridge flights have obtained critical observations for earth s polar ice since icesat stopped collecting data in 2009 this study develops an automatic method in processing icebridge airborne topographic mapper atm altimeter l1b data one elevation per 3 4 m horizontally to derive a local sea level height for referencing snow freeboards and then computing sea ice thicknesses four 30 km l1b profiles a b c and d flown on october 21 2009 over the bellingshausen sea in antarctica are selected the local sea level reference is first obtained by visual examination of atm l1b heights over leads or thin ice identified on images simultaneously acquired from the digital mapping system camera called manual selection this sea level reference is then used as ground truth to validate sea level heights derived by automatic calculations using five thresholds of 2 1 0 5 0 2 and 0 1 of the lowest l1b data the l1b 0 2 method gives a similar sea level height as from the l1b manual selection by mean absolute difference of 0 01 0 06 m the sea level heights demonstrate a near linear gradient of 0 01 m km to 0 03 m km within each 30 km l1b profile along the flight track from section a to d the resulting mean snow freeboards are 0 59 m 0 67 m 0 53 m and 0 60 m on sections a b c and d respectively three empirical equations and the buoyancy equation with zero ice freeboard assumption all give similar statistics in ice thickness estimation with mean ice thicknesses of 1 91 m for section a 2 16 m for b 1 76 m for c and 1 94 m for d the sea level over leads cannot be accurately resolved from the atm l2 data 60 m x 80 m horizontal averaging however by using a sea level reference obtained from the l1b data the atm l2 data can achieve reasonable ice thickness estimates with mean absolute difference of only 0 10 m compared to thickness derived from the l1b data

range dependent geoacoustic inversion of vertical line array data using matched beam processing this paper describes the results of range dependent geoacoustic inversion using vertical line array data obtained from the 4th matched acoustic properties and localization experiment conducted in the east sea of korea the narrowband multitone continuous wave signal from the towed source was analyzed to estimate the range dependent geoacoustic properties along the radial track the primary approach is based on the sectorwise inversion scheme the inversion region up to 7 5 km from the vertical line array was divided into several segments and the subinversions for each segment were performed sequentially to reduce the dominance of low angle arrivals which bears little information for the bottom segment in question matched beam processing with beam filtering was used for the cost function the performance of proposed algorithm was tested using simulated data for an environment representative of the experimental site the inversion results for the experimental data were consistent with the geophysical database and were validated from matched field source localization using frequencies different from those used in the inversion

experimental observation of stick slip instability fronts summary stick slip dynamic instability is a key mechanism governing frictional processes from microscale physics to earthquake faults and landslides yet challenging questions are still open about its nucleation and propagation dynamics here we present novel observations on laboratory experimental faults where spontaneously nucleating fractures are produced describing 1 an initial quasistatic stable rupture front propagating at about 5 per cent the shear wave velocity vs accelerating to subshear and then to intersonic velocity 2 the arisal of a higher degree of complexity when the friction to prestress ratio is increased on the sliding surface the complex behaviour includes stop and go sequences irregular propagation and rerupturing episodes within short time intervals implying rapid restrengthening of the surface and the formation of self healing pulses reproducing experimentally for the first time a behaviour observed on seismic faults

rupture speed dependence on initial stress profiles insights from glacier and laboratory stick slip a r t i c l e i n f o a b s t r a c t slow slip events are now well established in fault and glacier systems though the processes controlling slow rupture remain poorly understood the whillans ice plain provides a window into these processes through bi daily stick slip seismic events that displace an ice mass over 100 km long with a variety of rupture speeds observed at a single location we compare the glacier events with laboratory experiments that have analogous loading conditions both systems exhibit average rupture velocities that increase systematically with the pre rupture stresses with local rupture velocities exhibiting large variability that correlates well with local interfacial stresses the slip events in both cases are not time predictable but clearly slip predictable local pre stress may control rupture behavior in a range of frictional failure events including earthquakes 2014 the authors published by elsevier b v this is an open access article under the cc by nc nd license http creativecommons org licenses by nc nd 3 0

near source ground motions from simulations of sustained intersonic and supersonic fault ruptures we examine the long period near source ground motions from simulations of m 7 4 events on a strike slip fault using kinematic ruptures with rupture speeds that range from subshear speeds through intersonic speeds to supersonic speeds the strong along strike shear wave directivity present in scenarios with subshear rupture speeds disappears in the scenarios with ruptures propagating faster than the shear wave speed furthermore the maximum horizontal displacements and velocities rotate from generally fault perpendicular orientations at subshear rupture speeds to generally fault parallel orientations at supersonic rupture speeds for rupture speeds just above the shear wave speed the orientations are spatially heterogeneous as a result of the random nature of our assumed slip model at locations within a few kilometers of the rupture the time histories of the polarization of the horizontal motion provide a better diagnostic with which to gauge the rupture speed than the orientation of the peak motion subshear ruptures are associated with significant fault perpendicular motion before fault parallel motion close to the fault supershear ruptures are associated with fault perpendicular motion after significant fault parallel motion consistent with previous studies we do not find evidence for prolonged supershear rupture in the long period 2 sec ground motions from the 1979 imperial valley earthquake however we are unable to resolve the issue of whether a limited portion of the rupture approximately 10 km in length propagated faster than the shear wave speed additionally a recording from the 2002 denali fault earthquake does appear to be qualitatively consistent with locally supershear rupture stronger evidence for supershear rupture in earthquakes may require very dense station coverage in order to capture these potentially distinguishing traits

scaling of the shear rupture process from nucleation to dynamic propagation implications of geometric irregularity of the rupturing surfaces a series of systematic high resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces on the basis of the experimental results it has been discussed theoretically how consistently scale dependent physical quantities inherent in shear rupture are scaled the experiments led to conclusive results that the nucleation process consists of two phases phase i an initial quasi static phase and phase ii a subsequent accelerating phase and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces in phase i the rupture grows at a slow steady speed which is independent of the rupture growth length l in contrast during phase ii the rupture develops at accelerating speeds v which increase with an increase in l obeying a power law v vs l c n where vs is the shear wave velocity c is the characteristic length representing the geometric irregularity of the fault surfaces and and n are constants 8 87 10 29 and n 7 31 scale dependency of scale dependent physical quantities including the nucleation zone size and its duration is commonly ascribed to scale dependency of the slip dependent constitutive law parameter dc which is in turn governed by c it has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip dependent law

probabilities for jumping fault segment stepovers 1 seismic hazard analysis relies heavily on the segmentation of faults the ability of ruptures to break multiple segments has a big impact on estimated hazard current practice for estimating multiple segment breakage relies on panels of experts voting on their opinions for each case here we explore the probability of elastodynamic ruptures jumping segment stepovers in numerical simulations of segmented fault systems we find a simple functional form for the probability of jumping a segment stepover as a function of stepover distance an exponential falloff with distance we suggest this simple parameterization of jumping probabilities combined with sparse observational data to fix the lengthscale parameter as a new approach to estimating multisegment earthquake hazard

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

nucleation and early seismic propagation of small and large events in a crustal earthquake model 1 earthquake nucleation and early seismic propagation are studied in a two dimensional strike slip fault model with depth variable properties the fault is governed by the dieterich ruina rate and state friction law we use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by lapusta et al 2000 we find that for decreasing values of the characteristic slip distance of the friction law small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone small and large events have very similar nucleation phases in our simulations here by nucleation phase we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic seismically detectable event moment acceleration to which velocity seismograms are proportional in early stages of seismic propagation exhibits irregular fluctuations in the form of speedups and slowdowns in the moment release rate consistently with observations as reported by ellsworth and beroza 1995 our simulations show that such irregular moment acceleration can at least in part be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity strengthening fault regions which inhibit seismic slip index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 3220 mathematical geophysics nonlinear dynamics 7230 seismology seismicity and seismotectonics 3230 mathematical geophysics numerical solutions 8164 tectonophysics stresses crust and lithosphere keywords earthquake nucleation phase event clustering irregular moment release stress concentrations rate and state friction earthquake sequences

the effect of ocean heat flux on seasonal ice growth in young sound northeast greenland the seasonal ice cover plays an important role in the climate system limiting the exchange of heat and momentum across the air water interface among other factors sea ice is sensitive to the ocean heat flux in this study we use in situ oceanographic sea ice and meteorological data collected during winter 2013 2014 in young sound ys fjord in northeast greenland to estimate the ocean heat flux to the landfast ice cover during the preceding ice free summer incident solar radiation caused sea surface temperatures of up to 5 6 c subsequently this heat was transferred down to the intermediate depths but returned to the surface and retarded ice growth throughout winter two different approaches were used to estimate the ocean heat fluxes i a residual method based on a 1 d thermodynamic ice growth model and ii a bulk parameterization using friction velocities and available heat content of water beneath the ice the average heat flux in the inner ys varied from 13 w m 2 in october december to less than 2 w m 2 in january may an average heat flux of 9 w m 2 was calculated for the outer ys moreover we show that the upward heat flux in the outer fjord is strongly modulated by surface outflow which produced two maxima in heat flux up to 18 24 w m 2 during 26 december to 27 january and from 11 february to 14 march by may 2014 the upward ocean heat flux reduced the landfast ice thickness by 18 and 24 in the inner and outer ys respectively

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

spontaneous complex earthquake rupture propagation the historical development of spontaneous rupture propagation starting from the landmark paper of griffith in 1920 through to the late 1980s is traced with particular emphasis on the work carried out at mit in the 1970s by k aki and his co workers numerical applications of kostrov s method for planar shear cracks were developed by hamano das and aki simultaneously at mit madariaga considered the radiated field of a dynamic shear crack the further development of these ideas for example three dimensional spontaneous planar faulting models continued through the 1980s major insight into the maximum possible rupture speeds for earthquakes developed with the acceptance of the theoretical possibility of supersonic rupture speeds for faults with cohesion and friction the theoretical developments spurring the search for such observations for earthquake ruptures possible mechanisms by which faults stop were elucidated it was shown that a propagating rupture can jump over barriers for cracks with a cohesive zone at its tip complex faulting models namely the barrier and asperity models and their associated radiated field developed in the late 1980s it was shown that dynamic or transient asperities can develop during the complex rupturing process even seemingly relatively simple physical situations can lead to such complex rupturing processes that the usual idea of rupture velocity needs to be abandoned in those cases some of the work initiated by aki and his co workers such as the details of the transition from sub rayleigh to super shear speeds in inplane shear mode and the behavior of the cohesive zone size as the crack extends still remains the subject of research today

predictive modeling of the seismic cycle of the greater san francisco bay region the seismic cycle for the san francisco bay region is synthesized by a model combining the pre and post 1906 seismic histories the long term acceleration of seismic release seismic moment benioff strain release or event count in the seismic cycle and the shorter term accelerations preceding the larger earthquakes within that cycle are modeled using an empirical predictive technique called time to failure analysis in which rate of seismic release is proportional to an inverse power of the remaining time to failure the exponent of time to failure in the accelerating sequences appears to be scale invariant and the length of the full cycle is estimated at 269 50 years the 1989 loma prieta earthquake which is the culmination of the first subcycle in the present long term seismic cycle should have been predictable with an uncertainty of 2 years in time and 0 5 in magnitude although the specific location at loma prieta was not predictable by this technique if our model is correct and if the loma prieta earthquake is the culmination of a subcycle the san francisco bay region should be entering a relatively long 20 50 years period of seismic quiescence above magnitude 6 a great earthquake such as the 1906 san francisco event would appear to be more than a century in the future

ice platelets below weddell sea landfast sea ice basal melt of ice shelves may lead to an accumulation of disc shaped ice platelets underneath nearby sea ice to form a sub ice platelet layer here we present the seasonal cycle of sea ice attached to the ekstrom ice shelf antarctica and the underlying platelet layer in 2012 ice platelets emerged from the cavity and interacted with the fast ice cover of atka bay as early as june episodic accumulations throughout winter and spring led to an average platelet layer thickness of 4m by december 2012 with local maxima of up to 10 m the additional buoyancy partly prevented surface flooding and snow ice formation despite a thick snow cover subsequent thinning of the platelet layer from december onwards was associated with an inflow of warm surface water the combination of model studies with observed fast ice thickness revealed an average ice volume fraction in the platelet layer of 0 25 0 1 we found that nearly half of the combined solid sea ice and ice platelet volume in this area is generated by heat transfer to the ocean rather than to the atmosphere the total ice platelet volume underlying atka bay fast ice was equivalent to more than one fifth of the annual basal melt volume under the ekstrom ice shelf

parameter estimation using multifrequency range dependent acoustic data in shallow water the estimation of all forward model parameters geometric geoacoustic and ocean sound speed by the inversion of acoustic field observations is considered the data was taken at a mildly range dependent shallow water site in the mediterranean sea the inversion is based on data from a vertical array and carried out using information at multiple frequencies global optimization using a directed monte carlo search based on genetic algorithms and the bartlett objective function is used all geometric parameters are well determined a range dependent geoacoustic model is determined and the ocean sound speed is estimated comparisons of the observed pressure field as a function of depth and the predicted field show good agreement the use of observations at multiple frequencies provides considerable stability for the estimated parameters optimization of only geometric and geoacoustic parameters in a range independent environment is found to be satisfactory at the lower frequencies 165 175 hz but for the h

the dynamics of the onset of frictional slip the way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies by measuring the shear and normal stresses along the interface together with the subsequent rapid real contact area dynamics we find that the ratio of shear stress to normal stress can locally far exceed the static friction coefficient without precipitating slip moreover different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio these results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction selection and arrest of different modes of earthquakes

surface properties and processes of perennial antarctic sea ice in summer ice core and snow data from the amundsen bellingshausen and weddell seas antarctica show that the formation of superimposed ice and the development of seawater filled gap layers with high algal standing stocks is typical of the perennial sea ice in summer the coarse grained and dense snow had salinities mostly below 0 1 per mil a layer of fresh superimposed ice had a mean thickness ranging from 0 04 0 12 m 0 04 to 0 08 m thick gap layers extended downwards from 0 02 to 0 14 m below the water level these gaps were populated by diatom standing stocks up to 439 g l chlorophyll a we propose a comprehensive heuristic model of summer processes where warming and the reversal of temperature gradients cause major transformations in snow and ice properties the warming also causes the re opening of incompletely frozen slush layers caused by flood freeze cycles during winter alternatively superimposed ice forms at the cold interface between snow and slush in the case of flooding with negative freeboard combined these explain the initial formation of gap layers by abiotic means alone the upward growth of superimposed ice above the water level competes with a steady submergence of floes due to bottom and internal melting and accumulation of snow

salinity profiles of antarctic sea ice field data and model results in order to describe and compare sea ice salinity distributions which are of interest in the study of physical and biological ice characteristics as well as ice ocean interaction we have computed composite profiles and fitted third degree polynomials to salinity data of 129 cores from the weddell sea four characteristic profile shapes have been recognized and described through the polynomial coefficients c s i and type profiles the field data have been compared with ideal salinity profiles generated by a simulation scheme based on thermodynamic growth under climatological conditions representative of the weddell sea composite salinity profiles agree well with simulations irrespective of the growth mechanism i e frazil or congelation suggesting that ice properties which depend on salinity or porosity may evolve independent of ice growth conditions analysis of salinity and 18o data in conjunction with the simulations demonstrates that apart from meteorological conditions controlling growth velocity and temperature distribution within the ice flooding and upward brine expulsion are important in raising top salinities as frequently observed low top salinities are typical of ice that has undergone retexturing and differential desalination salinity decreases towards the bottom of a floe are a result of high oceanic heat fluxes and snow cover effects as a consequence of these processes the bulk salinity of weddell sea ice cannot be described by a simple linear age thickness relationship as characteristic of arctic first year ice

superimposed ice formation and surface energy fluxes on sea ice during the spring melt freeze period in the baltic sea the development of land fast sea ice and overlying snow was monitored during a 4 week period until the snow cover had completely disappeared at a site in the gulf of bothnia baltic sea 63 57 n 19 85 e the meteorological and radiative boundary conditions were continuously recorded during the observation period a 15 cm thick snow layer on the ice was transformed into a 7 cm thick granular ice layer superimposed ice on the ice surface contributing significantly about 11 to the total ice thickness approximately 1 cm w e of the snow was sublimated neither snow ice formation nor basal ice growth was significant during the same period the salinity and isotopic 18 o composition of the ice indicated that prior to the experiment a 7 cm layer of superimposed ice had already formed hence superimposed ice layers contributed 22 of the total ice thickness by the time all snow had disappeared the advancing spring decrease in surface albedo diurnal cycle in the incoming solar radiation and synoptic scale changes in the cloud cover and the air ice turbulent heat fluxes caused variations in the heat budget of the snowpack superimposed ice formation due to refreezing of meltwater occurred during most nights of the study period and the most important refreezing periods were under such synoptic conditions that the air and snow surface temperatures also remained below zero during daytime in contrast to typical summer conditions in polar oceans low snow surface temperatures acted as the primary heat sink for the refreezing of meltwater

slip sequences in laboratory experiments resulting from inhomogeneous shear as analogs of earthquakes associated with a fault edge faults are intrinsically heterogeneous with common occurrences of jogs edges and steps we therefore explore experimentally and theoretically how fault edges may affect earthquake and slip dynamics in the presented experiments and accompanying theoretical model shear loads are applied to the edge of one of two flat blocks in frictional contact that form a fault analog we show that slip occurs via a sequence of rapid rupture events that initiate from the loading edge and are arrested after propagating a finite distance each successive event extends the slip size transfers the applied shear across the block and causes progressively larger changes of the contact area along the contact surface resulting from this sequence of events a hard asperity is dynamically formed near the loaded edge the contact area beyond this asperity is largely reduced these sequences of rapid events culminate in slow slip events that precede a major unarrested slip event along the entire contact surface we suggest that the 1998 m5 0 sendai and 1995 off etorofu earthquake sequences may correspond to this scenario our work demonstrates qualitatively how the simplest deviation from uniform shear loading may significantly affect both earthquake nucleation processes and how fault complexity develops