focal mechanisms and stress variations in the caucasus and northeast turkey from constraints of regional waveforms abstract the continental collision between arabia and eurasia created large strike slip faults in turkey as well as mountains in the caucasus and the volcanic plateau between them in this study we use regional waveforms of a new seismic array deployed between 2008 and 2012 to constrain the focal mechanisms and depths of small to moderate sized earthquakes occurring in the western part of the central caucasus and northeast turkey the distribution of aftershocks and the twelve focal mechanisms involved in the sequence of the 2009 earthquake in racha are clearly a reactivation of a deeper segment of the 1991 m7 racha rupture zone the deeper segment is not well connected to the shallower decollement separating the basement and sedimentary basin the earthquakes we determined in northeastern turkey and southern georgia are related to the strike slip fault system we further combined all of the reliably determined focal mechanisms over the last 30 years to investigate the current stress status of the crust in three areas racha in the western greater caucasus javakheti near the lesser caucasus and in northeast turkey our results show that the directions of maximum compressional stress consistently fall within 2 to 14 n throughout the entire study region this appears to be controlled by the continental collision nonetheless the minimum compression switches from vertical in the greater caucasus to the east west direction in northeastern turkey due to the westward extrusion of the anatolia block which is driven partly by the hellenic subduction the transition of the stress field is close to the javakheti volcanic plateau in the lesser caucasus where the relative magnitude between the principal stresses appears to be strongly variable

evidence for tilted toroidal circulations in cumulus intense vortical circulations often organized in counterrotating vortex pairs were detected in midcontinental cumulus congestus over southeast wyoming in july 2003 the sampled clouds developed in dry environments and at cold temperatures and were a few kilometers in depth and width observations were obtained with the wyoming cloud radar from aboard the wyoming king air research aircraft dualdoppler analyses of the data yielded high resolution 30 45 m depictions of the horizontal components of air motions across vigorously growing clouds the vortices found in the horizontal cross sections are interpreted as components of the toroidal circulations in thermals when those are tilted because of the effect of ambient cross flow this configuration also leads to a partial stabilization of the vertical trajectory of the updraft by opposing the drag by the ambient wind additionally dry air intrusions were seen to accompany these features when the vortices developed near the cloud outer boundaries recirculation of hydrometeors occurred when the vortices were adjacent to in cloud downdrafts these features are also evident in the radar reflectivity patterns in general gradients of velocities and vorticity values in horizontal planes are comparable to those found in vertical planes

the owles iop2b lake effect snowstorm dynamics of the secondary circulation abstractintense lake effect snowfall results from a long lake axis parallel llap precipitation band that often forms when the flow is parallel to the long axis of an elongated body of water such as lake ontario the intensity and persistence of the localized precipitation along the downwind shore and farther inland suggests the presence of a secondary circulation that helps organize such a band and maintain it for some time as the circulation is advected inland unique airborne vertical plane dual doppler radar data are used here to document this secondary circulation in a deep well organized llap band observed during intensive observing period iop 2b of the ontario winter lake effect systems owles field campaign the circulation centered on a convective updraft intensified toward the downwind shore and only gradually weakened inland the question arises as to what sustains such a circulation in the vertical plane across the llap band wrf model simulations indicate that the primary llap band a

dynamics of the cumulus cloud margin an observational study aircraft observations of shallow to moderately deep cumulus clouds are analyzed with the purpose of describing the typical horizontal structure of thermodynamic and kinematic parameters near the cumulus margin from the cloud center into the ambient clear air the cumuli were sampled in a broad range of environments in three regions the tropical atlantic ocean in winter the sonoran desert during the monsoon and the arid high plains of wyoming in summer the composite analysis of 1624 cumulus penetrations shows thattheverticalmassflux temperature buoyancy thebuoyancyflux andtheturbulentkineticenergyalltend toreachaminimumnearthecloudedge mostofthesevariables andalsotheliquidwatercontent thedroplet concentration and the mean droplet size generally decrease in value from within the cumulus toward the cloud edge slowly atfirst and rapidly close to the cloud edge thesefindingscorroborate recent observational and modeling studies and provide further evidence for significant evaporative cooling in laterally entraining and detraining eddies in the cloud margin a transition zone within 200 m or 10 of the cloud diameter ofthecloudedge thiscoolingexplainsthetendencyfordownwardaccelerating buoyantlydrivensubsidence in the cloud margin

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

a high resolution dual doppler technique for fixed multiantenna airborne radar a new technique is presented for the analysis of data collected by pairs of aircraft ac mounted fixed radar beams directed 30deg apart leading to two dimensional wind field syntheses in the horizontal and vertical planes the technique was applied to the 95 ghz wyoming cloud radar wcr configurations aboard the university of wyoming king air and the national center for atmospheric research c 130 research ac the construction of the data grid allows to closely follow the ac flight attitude and thus the beam scanned surfaces with a resolution between 30 and 45 m a weighted least squares method is employed to solve the velocity inverse decomposition problem for every valid grid cell the effect on the solution of the nonmeasured cross plane wind component is minimized by using an external estimate of the ambient wind the error sources are discussed and maximum expected values for the velocity field errors are provided with reference to the radar system the data collection process and specific aspects of the target under investigation these uncertainties 1 2 m s are quantified for two case studies that show the potential of the technique in retrieving the kinematic field in transects of cumulus and stratus clouds the technique produces smooth fine scale kinematic fields although each velocity point is the result of an independent calculation and no binding constraint is imposed among neighboring points as is done in other dual doppler techniques

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

oxygen isotope fractionation during the freezing of sea water the dependence of oxygen isotope fractionation on ice growth rate during the freezing of sea water is investigated based on laboratory experiments and field observations in mcmurdo sound antarctica the laboratory experiments were performed in a tank filled with sea water with sea ice grown under calm conditions at various room temperatures ranging from 58c to 208c in mcmurdo sound the ice growth rate was monitored using thermistor probes for first year landfast ice that grew to 2 m in thickness combining these datasets allows for the first time examination of fractionation at a wide range of growth rates from 0 8 10 7 to 9 3 10 7 ms 1 in the analysis a stagnant boundary layer model is parameterized using these two independent datasets as a result the optimum values of equilibrium pure ice fractionation factor and boundary layer thickness are estimated it is suggested that a regime shift may occur at a growth rate of 2 0 10 7 ms 1 a case study on sea ice in the sea of okhotsk where the growth rate is modeled by coupling the thermodynamic properties of the sea ice with meteorological data demonstrates the utility of the fitted models

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

a study of thermals in cumulus clouds air motions in the thermals contained within shallow florida cumulus clouds were observed to be similar to the circulation observed in laboratory thermals there was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud widespread inward flow towards the centre of the cloud and a narrow but strong updraught was occasionally observed reminiscent of the tail found at the rear of the laboratory thermals a region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1d divergence were strongest and complete holes were observed on two occasions although horizontal wind shear was generally weak it was significant in a few cases causing the flow pattern in the cloud to be asymmetric ascending regions of cloud with high values of liquid water content cloud cores were commonly observed at all altitudes but generally the percentage of clouds measured with high liquid water content decreased with altitude the observations of airflow and liquid water content structure in warm cumulus clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base but erodes as it ascends copyright 2005 royal meteorological society

onset of subduction as the cause of rapid pliocene quaternary subsidence in the south caspian basin backstripping a representative stratigraphic column from the northwest of the south caspian basin shows that 2 4 km of tectonic subsidence has occurred since ca 5 5 ma at 1 5 10 times the rate recorded in typical foreland basins roughly half the basin s sedimentary thickness of 20 km has accumulated in this time while the upper part of the succession has begun to deform by buckle folding the south caspian basement appears to be either thick oceanic crust or thinned high velocity continental crust seismicity and gravity data indicate that the basement is in the initial stages of subduction under the middle caspian region to the north we propose that basement subduction began ca 5 5 ma to create the major pliocene quaternary subsidence subduction may have been triggered by a regional reorganization in the arabia eurasia collision at that time possibly following the construction of the turkish iranian plateau

stress accumulation and increased seismic risk in eastern turkey abstract unlike the north anatolian fault zone which has produced 11 large earthquakes since 1939 the east anatolian fault zone eafz has been relatively quiescent in the last century when compared to historical records and has therefore accumulated significant stresses along its length determination of the location and likely magnitude of a future probable earthquake along the eafz is of interest both because of this history of large earthquakes m 8 and the density of population in the area here we calculate stress evolution along the fault zone due to both seismic and tectonic loading since 1822 a sequence of 10 well constrained historical earthquakes is selected and the resulting stresses are calculated summed with tectonic loading stresses and resolved onto the mapped active faults we identify two areas of particular seismic risk one of which might be expected to yield a large event our results are sensitive to the previous history of large earthquakes in the region and indicate a need for detailed investigations to constrain the exact rupture geometries of previous earthquakes on these segments

effects of coulomb stress change on mw 6 earthquakes in the caucasus region abstract coulomb stress variations may trigger earthquakes while the stress transfer process is not clear in this paper coulomb stress changes are investigated for the sequences of strong earthquakes in the caucasus region which followed the 1988 mw6 8 in spitak the 1991 mw6 9 in racha and the 2000 mw6 2 in baku our results show that the mainshocks induced variations in the stress field where most of the seismic events occurred coulomb stress increases 2 bars corresponding nearly to places of aftershocks while the stress falls above 10 bars for racha mainshock the calculated coulomb stress loaded about 5 bars at the end of rupture plane and the stress falls 9 bars for spitak mainshock our stress pattern shows a correlation within the areas of coulomb stress caused by the combination of seismicity correlations between the coseismic coulomb stress changes and the observed spatial patterns of the aftershocks created by the mainshock are illustrated by the stress variations over both optimally oriented and specified fault planes an apparent correlation between the mainshock stress changes and the detected spatial pattern of the aftershocks is found confirming the utility of the stress maps in constraining the expected locations of the forthcoming aftershocks and mitigating earthquake hazards

a similarity theory for unsaturated downdrafts within clouds abstract recent observations of cumulus clouds strongly support the hypothesis of squires 1958 that much of the mixing within such clouds is associated with downward propagating currents initiated near their tops a similarity theory is here proposed to describe the properties of such currents the use of similarity is defended on the basis of the observed and predicted scale of the downdrafts the theory suggests that downward propagating unsaturated thermals are pervasive throughout all but the largest convective clouds and that quasi steady unsaturated downdraft plumes may exist in the lower portions of cumulonimbi in addition to providing a reasonable explanation for the microstructure of and liquid water distribution within cumulus clouds the theory appears to account for certain severe convective phenomena including down bursts a new but related cloud instability is proposed to account for the occurrence of mamma

geodynamics seismicity and seismic hazards of the caucasus abstract being a part of ongoing continental collision between the arabian and eurasian plates the caucasus region is a remarkable site of moderate to strong seismicity where devastating earthquakes caused significant losses of lives and livelihood in this article we survey geology and geodynamics of the caucasus and its surroundings magmatism and heat flow active tectonics and tectonic stresses caused by the collision and shortening gravity and density models and overview recent geodetic studies related to regional movements the tectonic development of the caucasus region in the mesozoic cenozoic times as well as the underlying dynamics controlling its development are complicated processes it is clear that the collision is responsible for a topographic uplift inversion and for the formation of the fold and thrust belts of the greater and lesser caucasus tectonic deformations in the region is influenced by the wedge shaped rigid arabian block indenting into the relatively mobile region and producing near n s compressional stress and seismicity in the caucasus regional seismicity is analysed with an attention to sub crustal seismicity under the northern foothills of the greater caucasus which origin is unclear whether the seismicity associated with a descending oceanic crust or thinned continental crust recent seismic tomography studies are in favour of the detachment of a lithospheric root beneath the lesser and greater caucasus the knowledge of geodynamics seismicity and stress regime in the caucasus region assists in an assessment of seismic hazard and risk we look finally at existing gaps in the current knowledge and identify the problems which may improve our understanding of the regional evolution active tectonics geodynamics shallow and deeper seismicity and surface manifestations of the lithosphere dynamics among the gaps are those related to uncertainties in regional geodynamic and tectonic evolution e g continental collision and associated shortening and exhumation lithosphere structure deformation and strain stress partitioning and to the lack of comprehensive datasets e g regional seismic catalogues seismic gravity and geodetic surveys

detailed dual doppler structure of kelvin helmholtz waves from an airborne profiling radar over complex terrain part i dynamic structure abstractkelvin helmholtz kh waves are remarkably common in deep stratiform precipitation systems associated with frontal disturbances at least in the vicinity of complex terrain as is evident f

toggling of seismicity by the 1997 kagoshima earthquake couplet a demonstration of time dependent stress transfer 1 two m 6 well recorded strike slip earthquakes struck just 4 km and 48 days apart in kagoshima prefecture japan in 1997 providing an opportunity to study earthquake interaction aftershocks are abundant where the coulomb stress is calculated to have been increased by the first event and they abruptly stop where the stress is dropped by the second event this ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks whereas the dynamic coulomb stresses which should only promote seismicity appear to play a secondary role if true the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution however adding the stress changes from the two kagoshima events does not fully capture the ensuing seismicity such as its rate change temporal decay or migration away from the ends of the ruptures we therefore implement a stress transfer model that incorporates rate state friction in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip slip rate and elapsed time since the last event the model reproduces the temporal response of seismicity to successive stress changes including toggling decay and aftershock migration nevertheless the match of observed to predicted seismicity is quite imperfect due perhaps to inadequate knowledge of several model parameters however to demonstrate the potential of this approach we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks taking advantage of the large statistical sample the small shocks afford not surprisingly such probabilities are highly time and location dependent during the first decade after the main shocks the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones

microburst modelling and scaling a microburst can be modeled by releasing a volume of fluid that is slightly heavier than the ambient fluid allowing it to fall onto a horizontal surface vorticity develops on the sides of this parcel as it descends and causes it to roll up into a turbulent vortex ring which impinges on the ground such a model exhibits many of the features of naturally occurring microbursts which are a hazard to aviation in this paper this model is achieved experimentally by releasing a volume of salt water into fresh water from a cylindrical dispenser when care is taken with the release the spreading rate of the surface outflow is measurable and quite repeatable despite the fact that the flow is turbulent an elementary numerical approximation to this model based on inviscid vortex dynamics has also been developed a scaling law is proposed which allows experiments with different fluid densities to be compared with each other and with the numerical results more importantly the scaling law makes it possible to compare the model results with real microbursts

radar observations of updrafts downdrafts and turbulence in fair weather cumuli abstract observations from a 94 ghz radar are used to define the vertical structure of marine fair weather cumuli doppler spectra obtained from the radar provide mean vertical velocities as well as detailed spectral shapes that can be used to infer small scale vertical velocity shear illuminate cloud microphysical processes and provide estimates of turbulence dissipation rates these new observations facilitate the analysis and understanding of in cloud circulations and the physical processes involved since the cloud boundaries and dimensions are mapped along with the internal structure of the clouds coincident observations from a 915 mhz radar wind profiler were used to further define the turbulence structure in and around the clouds the observations document the detailed vertical and horizontal dimensions of updraft and downdraft circulations in the clouds observed the two cumuli studied in detail have similar circulation patterns an updraft core surrounded by downdrafts although the clouds ha

precursory stress changes before large earthquakes on a new physical law for earthquakes abstract in this article we first review the progress in our understanding of tectonic stresses in the earth s crust achieved in last three to four decades we briefly review the literature on roughly four groups of studies 1 world s stress map project wsm 2 fault slip data and paleostress analyses 3 coulomb stress transfer and 4 theories of the generalized continua with emphasis on the cosserat continuum based on these studies we develop a new numerical algorithm to calculate tectonic strain rate tectonic stress and normalized tectonic shear stress fields from the earthquake focal mechanisms we show that this field changes over time and is therefore far from being constant further on we focus our efforts on the analysis of the relationship between the magnitudes of the earthquakes and the normalized tectonic shear stress we discover a new and previously unknown physical law which we call the maximum magnitude law mm law we discuss further the origin and physical background of this law and its importance in earthquake studies

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

present day kinematics in the eastern mediterranean and caucasus from dense gps observations abstract the eastern mediterranean and caucasus are located among the eurasian african and arabian plates and tectonic activities are very complex in this paper the kinematics and strain distribution in these regions are determined and investigated from dense gps observations with over 1000 stations and longer observations the elastic block model is used to constrain present day plate motions and crustal deformation the relative euler vectors between the nubian arabian caucasus anatolian and central iranian plates are estimated the arabian eurasia anatolian eurasia nubian eurasia caucasus eurasia and central iranian euler vectors are 0 584 0 1 myr 1 0 825 0 064 myr 1 0 35 0 175 myr 1 0 85 0 086 myr 1 and 0 126 0 016 myr 1 the strain rate in the east mediterranean and caucasus has been estimated from the gps velocity field the results show that the thrust dominated areas the eastern mediterranean middle east caucasus and zagros have negative dilatation and the western anatolia region has positive 2d dilatation rate with significant rotation the west anatolian shows the extension in nw se with about 150 199 nstrain yr in the w e direction the central anatolia shows compression rate below 50 nstrain yr and extensional strain rate adjacent to east anatolian fault and dead sea fault is about 0 100 nstrain yr the contraction strain rate is higher in zagros and caucasus between 100 150 nstrain yr and contraction orientation is along the ne sw direction in caucasus the north part of iran shows less contraction rate below 50 nstrain yr but north east zagros mountains tabriz fault and chalderan fault show extensional rate between 50 110 nstrain yr and principal axes rotation in the n s direction the maximum contraction observed in the kopek dag is about 100 194 nstrain yr and orientated in the ne sw direction east zagros mountain and makran subduction zone have a large clockwise rotation with 70 85 nradian and principal axes remains mostly along the n s direction the observed extension is along n s by about 0 100 nstrain yr with counter clockwise rotation in dead sea fault the sinai block shows shortening rate in the range of 0 100 nstrain yr

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

significance of stress transfer in time dependent earthquake probability calculations 1 a sudden change in stress is seen to modify earthquake rates but should it also revise earthquake probability data used to derive input parameters permit an array of forecasts so how large a static stress change is required to cause a statistically significant earthquake probability change to answer that question effects of parameter and philosophical choices are examined through all phases of sample calculations drawing at random from distributions of recurrence aperiodicity pairs identifies many that recreate long paleoseismic and historic earthquake catalogs probability density functions built from the recurrence aperiodicity pairs give the range of possible earthquake forecasts under a point process renewal model consequences of choices made in stress transfer calculations such as different slip models fault rake dip and friction are tracked for interactions among large faults calculated peak stress changes may be localized with most of the receiving fault area changed less than the mean thus to avoid overstating probability change on segments stress change values should be drawn from a distribution reflecting the spatial pattern rather than using the segment mean disparity resulting from interaction probability methodology is also examined for a fault with a well understood earthquake history a minimum stress change to stressing rate ratio of 10 1 to 20 1 is required to significantly skew probabilities with 80 85 confidence that ratio must be closer to 50 1 to exceed 90 95 confidence levels thus revision to earthquake probability is achievable when a perturbing event is very close to the fault in question or the tectonic stressing rate is low

the ice thickness distribution across the atlantic sector of the antarctic ocean in midwinter during the first leg of the winter weddell sea project antarktis v 2 cruise of f s polarstern the entire width of the antarctic sea ice zone was traversed in the vicinity of 0 longitude in the period july 18 to september 10 1986 ice thicknesses were measured by direct drilling and by helicopter profiling using an exstar 100 mhz impulse radar system in addition aerial photography of the ice cover was done from 100 to 2000 m altitude using a 70 mm aerial camera mounted in the helicopter the results of the point measurements drilling are reported in this paper together with an indication of how the radar and photography data will be used to extend them so as to yield area averaged ice thickness distributions it was found that the main ice type across the entire width of the ice cover was consolidated pancake ice occurring in vast floes this formed out of a 250 km wide band at the advancing ice edge which comprised a concentrated field of individual pancakes in a matrix of frazil ice preferred thicknesses of undeformed floes were 40 60 cm of ice covered with 5 15 cm of snow the individual pancakes attained almost all of this thickness before consolidation subsequent congelation growth was slow estimated at 0 4 cm d 1 the floes contained much small scale roughness on the upper and lower surfaces due to rafting of pancakes at the time of consolidation but pressure ridging was modest except in the far south a few very thick 8 11 m multiyear floes were observed embedded in the pack at latitudes beyond 66 s

the caucasus an actual example of the initial stages of continental collision abstract a field study of the neotectonics of the georgian caucasus is combined with geophysical geodetic and geological information in order to obtain a mechanical model of actual deformation the analysis of satellite photos together with topographical and geological maps shows that the great caucasus is bisected by a nne trending sinistral strike slip fault zone the western great caucasus is thrusted to the south over the rioni basin while on the other hand its northern flank is a monocline that descends gently into the russian platform active volcanism is present from elbruz to kazbeg the eastern great caucasus is limited by thrusts to the north and to the south and intermediate structures are disposed in a fan like shape no recent volcanism is observed the eastern limit of the chain is a nnw trending dextral strike slip fault zone seismicity from a local network illustrates the activity of the thrusts and gives evidence for the strike slip faults microtectonic data confirm the recent activity and the sinistral character of the borjomi kazbeg strike slip fault the strike slip faults bordering the eastern caucasus may be seen as the limits of a plastic wedge in front of the arabian plate at an early stage of continental collision

evolution of a supercooled ice shelf water plume with an actively growing subice platelet matrix we use new observations in western mcmurdo sound combined with longitudinal hydrographic transects of the sound to identify a northward flowing ice shelf water isw plume exiting the cavity of the mcmurdo ross ice shelf we estimate the plume s net northward transport at 0 4 0 1 sv carving out a corridor approximately 35 km wide aligned with the victoria land coast basal topography of the mcmurdo ice shelf is such that the plume is delivered to the surface without mixing with overlying warmer water and is therefore able to remain below the surface freezing temperature at the point of observation beneath first year ice thus the upper ocean was supercooled by up to 50 mk at the surface due to pressure relief from recent rapid ascent of the steep basal slope the 70 m thick supercooled layer supports the growth and maintenance of a thick semirigid and porous matrix of platelet ice which is trapped by buoyancy at the ice ocean interface continued growth of individual platelets in supercooled water creates significant brine rejection at the top of the water column which resulted in convection over the upper 200 m thick homogeneous layer by examining the diffusive nature of the intermediate water between layers of isw and high salinity shelf water we conclude that the isw plume must have originated beneath the ross ice shelf and demonstrate that it is likely to expand eastward across mcmurdo sound with the progression of winter

sensitivity of static stress calculations to the earthquake slip distribution 1 the apparent strong correlation between coulomb stress changes and the spatial distribution of aftershocks suggests the possibility of making near real time estimations of areas at risk of experiencing off fault aftershocks in order to do this in practice a number of issues must first be addressed including the extent to which the main shock slip must be known in detail before a meaningful stress map can be constructed here we investigate this issue by constructing a time ordered sequence of slip solutions for the landers earthquake computing coulomb stress changes for each solution and quantitatively comparing the stress field with the observed aftershocks by 1 resolving the coulomb stress change onto the aftershock nodal planes and calculating the percentage of events consistent with triggering and 2 constructing a two dimensional map of coulomb stress and computing the correlation coefficient between the positive and negative areas and the locations of the aftershocks we find that slip solutions based on empirical relations and either focal mechanism or moment tensor data produce stress fields inconsistent with the observed spatial distribution of aftershocks whereas slip solutions incorporating the correct rupture geometry but greatly simplified slip produce stress fields consistent with the aftershock distribution when very near fault events are excluded we further find that resolving stress perturbations onto earthquake nodal planes and computing the percentage of events experiencing positive coulomb stress provides a poor measure of success because of the limited range of structures on which events occur and the compatibility of the main shock stress field with these structures our results support the hypothesis that coulomb stress changes affect the spatial distribution of aftershocks and suggest that meaningful calculations of coulomb stress can be made as soon as an earthquake s rupture geometry is well constrained

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

composite vertical structure of vertical velocity in nonprecipitating cumulus clouds abstractvertical transects of doppler vertical velocity data obtained from an airborne profiling millimeter wave cloud radar are composited for a large number of cumulus clouds cu at various stages of their life cycle to examine typical circulations patterns the cu clouds range in depth between 500 and 6000 m and are generally nonprecipitating they were sampled on board the university of wyoming king air over a mountain in southern arizona during the summer monsoon and over the high plains of southeastern wyoming the composite analysis shows clear evidence of an updraft downdraft dipole in the upper cloud half consistent with a horizontal vortex ring a single cloud scale toroidal circulation emerges notwithstanding the complex finescale structure with multiple vortices commonly evident in individual transects of cu clouds the stratification of all cu samples as a function of their buoyancy and mean vertical velocity shows that the vortex ring pattern tends to be more pronounced in positively

observations of platelet ice growth and oceanographic conditions during the winter of 2003 in mcmurdo sound antarctica 1 platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice water interface related to the proximity of ice shelves it is known to form in supercooled seawater this study details the conditions for its growth during the austral winter of 2003 in mcmurdo sound antarctica a key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column monitored using the strength of the backscattered signal from an acoustic doppler current profiler as a proxy generally these crystals appeared from mid may as water near the interface became supercooled this near surface supercooling appears to be related to a simultaneous abrupt change in the structure of the upper 250 m of the water column from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature near surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density over the course of winter of the surface mixed layer created by salt rejection during ice growth these processes allowed cold water trapped by buoyancy in a band at the base of the mixed layer in early winter to gain access to the ice water interface by midwinter this band of cold water probably had its origin beneath the mcmurdo ice shelf

variability in the distribution of fast ice and the sub ice platelet layer near mcmurdo ice shelf variability in the volume of supercooled ice shelf water outflow in mcmurdo sound is reflected in the thickness and distribution of fast ice and the sub ice platelet layer beneath ground based electromagnetic induction and drill hole surveys of the distribution and thickness of ice shelf influenced fast ice and the sub ice platelet layer in mcmurdo sound were carried out in late spring of 2011 2013 2016 and 2017 in 2011 and 2017 thicker sub ice platelet layers of up to 7 5 and 6 m were observed respectively fast ice formation throughout the winters of 2011 and 2017 was influenced by a higher occurrence of strong southerly wind events and resultant activity of the ross sea polynya in contrast lower wind conditions in 2016 led to largely undisturbed sea ice growth and anomalously extensive fast ice coverage a thinner sub ice platelet layer of up to 4 m was observed in 2016 in 2011 and 2017 substantial and variable sub ice platelet layers were detected in a region of exchange of water masses between the ross sea and the mcmurdo ross ice shelf cavity which were not observed in 2013 and 2016 we hypothesize that a higher frequency of strong southerly wind events resultant polynya activity and high salinity shelf water production over winter accelerates circulation and increases melting in the proximal shallow mcmurdo ice shelf and the deeper ross ice shelf regions of the conjoined cavity the outflow of supercooled ice shelf water and sub ice platelet layer formation in mcmurdo sound are consequently promoted

forecasting the evolution of seismicity in southern california animations built on earthquake stress transfer 1 we develop a forecast model to reproduce the distribution of main shocks aftershocks and surrounding seismicity observed during 1986 2003 in a 300 310 km area centered on the 1992 m 7 3 landers earthquake to parse the catalog into frames with equal numbers of aftershocks we animate seismicity in log time increments that lengthen after each main shock this reveals aftershock zone migration expansion and densification we implement a rate state algorithm that incorporates the static stress transferred by each m 6 shock and then evolves coulomb stress changes amplify the background seismicity so small stress changes produce large changes in seismicity rate in areas of high background seismicity similarly seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates thus a key constituent of the model is the background seismicity rate which we smooth from 1981 to 1986 seismicity the mean correlation coefficient between observed and predicted m 1 4 shocks the minimum magnitude of completeness is 0 52 for 1986 2003 and 0 63 for 1992 2003 a control standard aftershock model yields 0 54 and 0 52 for the same periods four m 6 0 shocks struck during the test period three are located at sites where the expected seismicity rate falls above the 92 percentile and one is located above the 75 percentile the model thus reproduces much but certainly not all of the observed spatial and temporal seismicity from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades finally we offer a m 5 earthquake forecast for 2005 2015 assigning probabilities to 324 10 10 km cells

teleseismic body wave analysis of the 1988 armenian earthquake long period and broadband body waves from 14 digital seismic stations are used to investigate the rupture process of the december 7 1988 earthquake near spitak armenia ussr the inversion of these data gives the following centroidal source parameters strike 299 degree dip 64 degree rake 151 degree depth 6 3 km and seismic moment 1 5 times 10 sup 19 nm indicating that on average the earthquake had a strike slip mechanism with a substantial reverse component the mechanism with a substantial reverse component the broadband waveforms however show significant complexity they are best fit with a source model that includes three sub events very similar in size but with distinct focal mechanisms and locations rupture apparently initiated as a shallow reverse fault at a point of maximum bending on a right lateral strike slip fault and then extended bilaterally first towards the southeast and then towards the west this interpretation agrees with the aftershock distribution and fault lineations observed on landsat images

signatures of supercooling mcmurdo sound platelet ice near ice shelves around antarctica the ocean becomes supercooled and has been observed to carry small suspended ice crystals our measurements demonstrate that these small crystals are persistently present in the water column beneath the winter fast ice and when incorporated in sea ice they reduce the mean grain size of the sea ice cover by midwinter larger ice crystals below the ice water interface are observed to form a porous sub ice platelet layer with an ice volume fraction of 0 25 0 06 the magnitude and direction of the oceanic heat flux varied between 5 6 wm 2 upwards and 15 10 wm 2 downwards in may but by september it settled between 6 2 and 11 2 w m 2 the negative values imply that the ocean acts as a heat sink which is responsible for the growth of 12 of the ice thickness between june and september this oceanic contribution should not be ignored in models of antarctic sea ice thickness close to an ice shelf

the structure of thermals in cumulus from airborne dual doppler radar observations abstract a newly developed technique for airborne dual doppler observations with the wyoming cloud radar is used to characterize the velocity fields in vertical planes across cumulus turrets the clouds sampled were continental in nature with high bases near 0 c and with depths of 2 3 km clear evidence was found that the clouds evolved through sequences of bubbles or thermals with well defined toroidal circulations or vortex rings the ring core and tube diameters were about 200 600 m leading to turret sizes of 1 2 km in the horizontal the largest updraft speeds were observed in the ring centers but regions of turbulent ascending air extended behind the thermals to distances comparable with the toroid sizes vertical shear of ambient winds when present led to a tilting of the updrafts and toroids patterns in the reflectivity and velocity fields indicated regions of major intrusions into the thermals accompanied by entrainment of ambient air or recycling of larger hydrometeors depending on

modelling antarctic fast ice growth an existing thermodynamic ice growth model semtner 1976 has been tested for its ability to predict the growth of fast ice in mcmurdo sound antarctica significant discrepancies between observed and predicted ice thicknesses were found to occur primarily due to the presence of sub ice platelets and the formation of a snow ice layer although these ice growth processes are not well enough understood to permit rigorous physical modelling it is shown that fairly simple modifications to the model greatly improve the accuracy of the thickness predictions and serve to highlight the importance of these processes in the antarctic fast ice environment surface flooding and snow ice formation are assumed to occur immediately upon the establishment of a positive hydrostatic water level and a surface temperature in excess of a critical value above which interconnecting channels in the ice matrix permit the flow of water to the surface the presence of the sub ice platelet layer is assumed to increase columnar ice growth at a rate proportional to the volume fraction of ice in the platelet layer a simple technique but one that permits estimates of platelet enhanced growth without detailed knowledge of oceanographic conditions the resulting model predictions are in close agreement with measurements of fast ice growth and decay in mcmurdo sound however data suitable for testing the model over a complete range of conditions and over multi year cycles are not available at the present time

dynamics of the cloud environment interface and entrainment in small cumuli two dimensional simulations in the absence of ambient shear abstract we employ a two dimensional numerical model with interacting nested domains to simulate the evolution of a small nonprecipitating cumulus cloud in the absence of shear grid nesting permits the use of a realistic boundary layer forcing to initiate cloud growth and at the same time the specification of very high spatial resolution in the vicinity of the cloud the finest mesh employed in this study 5 m gives about 160 points across the base of the cloud initially the model produces a cloud which has a smooth upper surface about eight to nine minutes after the onset of condensation nodes appear on the upper cloud boundary these nodes have a characteristic tangential length scale which is small compared to the width of the cloud base in one of our simulations a down draft forms above the center of the cloud top and penetrates into the interior of the cloud the entrainment of this unsaturated air reduces the liquid water content of the cloud below the adiabatic value and curtails growth o

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

atmospheric boundary layer structure and drag coefficients over sea ice this paper estimates the air sea drag coefficient for first year ice from recent aircraft measurements and reconciles the range of observed drag coefficients 103cd 1 2 3 7 referenced to 10 m for all sea ice types based on ice roughness and seasonal meteorology for the purpose of sea ice modeling it is necessary to define an effective drag coefficient which relates regional stress to regional wind because sea ice is heterogeneous on scales less than 20 km regional stress is influenced by the distribution of surface roughness the buoyancy flux from quasi periodic leads and external atmospheric conditions principally the inversion height 103cd is 1 3 1 5 for smooth ice but is much greater for nonflat surfaces for wind speeds greater than 5 m s and air temperatures below freezing the effective 103cd is 2 5 3 0 for nearly continuous pack ice such as first year ice in seasonal ice zones and central arctic basin the range of values of 103cd is 3 0 3 7 for unstable boundary layers typical of off ice winds in the marginal ice zone miz or even greater if the ice has been broken by a recent storm cd values at the lower end of these ranges are associated with low inversion heights these coefficients are confirmed by 103cd of 2 9 2 5 and 3 1 for first year sea ice calculated from gust probe data collected by the noaa p 3 aircraft interior to the inner miz in the bering sea during the marginal ice zone experiment mizex west in february 1983 and 103cd of 2 6 for the arctic calculated from arctic ice dynamics joint experiment aidjex aircraft data for february 1976 the effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 103cd of 2 2 calculated from noaa p 3 gust probe data over a 40 km track of 0 4 ice concentration in the outer miz of the greenland sea in june 1984 during mizex 84 the relation of surface wind and stress to the geostrophic wind for shallow inversion heights typical of high latitudes is reviewed with a turbulent closure atmospheric boundary layer model the winter arctic is typified by low inversions 100 m low geostrophic drag coefficients and large geostrophic surface wind turning angles with low functional dependence on surface roughness seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics

active tectonics of the turkish iranian plateau 1 this paper examines how active faulting in the turkey iran caucasus region accommodates the arabia eurasia collision and the velocity field observed by gps the overall shortening across the zone is in general spatially separated partitioned into right lateral strike slip in the turkish iranian plateau and thrusting in the greater caucasus a band of counterclockwise rotating nw se right lateral strike slip faults accommodates a nw se gradient in ne directed velocity relative to eurasia between the black and caspian seas a nnw sse band of previously unrecognized oblique normal faults is present on the turkey iran border we estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 2 ma at present rates this implies a reorganization of deformation in the collision zone at that time after the initial collision at 12 ma probably in response to mantle induced dynamic uplift

growth rate and salinity profile of first year sea ice in the high arctic this paper describes the growth of sea ice and the salinity profiles observed in eclipse sound near pond inlet baffin island canada during the winter of 1977 78 a numerical method of calculation has been developed to incorporate the variations in snow conditions and physical properties of ice and snow during the growth season it is shown that the growth rate can be predicted reasonably well it is also shown that the vertical salinity profile in the ice towards the end of the season provides a record of previous climatological conditions a dependence has been shown between the predicted growth rate and the measured salinity

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

observed platelet ice distributions in antarctic sea ice an index for ocean ice shelf heat flux antarctic sea ice that has been affected by supercooled ice shelf water isw has a unique crystallographic structure and is called platelet ice in this paper we synthesize platelet ice observations to construct a continent wide map of the winter presence of isw at the ocean surface the observations demonstrate that in some regions of coastal antarctica supercooled isw drives a negative oceanic heat flux of 30 wm 2 that persists for several months during winter significantly affecting sea ice thickness in other regions particularly where the thinning of ice shelves is believed to be greatest platelet ice is not observed our new data set includes the longest ice ocean record for antarctica which dates back to 1902 near the mcmurdo ice shelf these historical data indicate that over the past 100 years any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements

thermodynamics of slush and snow ice formation in the antarctic sea ice zone snow over antarctic sea ice is often flooded by brine or seawater particularly in spring forming slush and snow ice here we evaluate the representation of the thermodynamics of slush and snow ice formation in large scale sea ice models using laboratory experiments nacl solutions poured into grated ice in an isolated container scaling analysis highlights latent heat as the main term of the energy budget the temperature of the new sea ice immediately after flooding is found very close to the saltwater freezing point whereas its bulk salinity is typically 20 g kg large scale sea ice models faithfully represent such physics yet the uncertainty on the origin of flooding saltwater impacts the calculated new ice temperature because of the different salinities of seawater and brine the laboratory experiments also suggest a potential limitation to the existing physical representations of flooding for brine fractions 60 ice crystals start floating upon saltwater natural sea ice observations suggest that the isolated system assumption holds for a few hours at most after which rapid heat and salt exchanges mostly destroy the initial flooding signature on temperature and salinity a small footprint on ice salinity remains however natural snow ice is found 3 5 g kg more saline than other forms of sea ice

measurements of ice shelf water beneath the front of the ross ice shelf using gliders measurements made by an underwater glider deployed near the ross ice shelf were used to identify the presence of ice shelf water isw which is defined as seawater with its potential temperature lower than its surface freezing point temperature properties logged by the glider included in situ temperature electrical conductivity pressure gps location at surfacings and time for most of the first 30 recorded dives of its deployment evidence suggests the glider was prevented from surfacing due to being under the ice shelf for dives under the ice shelf farthest from the ice shelf front isw layers of varying thicknesses and depth locations were observed between 2 m thick centred at 231 m depth to 93 m thick centred at 360 m for dives under the ice shelf close to the ice shelf front either no isw was observed or isw layers were centred at shallower depths 116 127 m thicker isw layers e g up to 250 m thickness centred at 421 m were observed for some glider dives in open water in front of the ross ice shelf no in situ supercooling water colder than the pressure dependent freezing point temperature was observed

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

active thrust front of the greater caucasus the april 29 1991 racha earthquake sequence and its tectonic implications although fault bounded thrust sheest are common in the geological record seismic evidence for their motion is sparse the april 29 1991 racha earthquake m s 7 0 the largest instrumentally recorded earthquake in the greater caucasus is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion using data from a deployment of program for array seismic studies of the continental lithosphere passcal digital seismographs and various other instrumental we locate 1952 aftershocks occurring between may 7 and june 30 1991 the aftershocks form a zone 70 km long and 10 25 km wide striking e w following the racha ridge at the southern boundary of the greater caucasus thrust system teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks the solutions show thrust faulting with centroid depths of 3 10 km comparable to depths of locally recorded aftershocks 2 12 km the shallow dipping nodal plane the aftershock distribution and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping nne at 20 o 31 o bounding the southern slope of the greater caucasus this fault system thrusts the greater caucasus structures south over the dzhirula basement massif the inferred fault geometry suggests that the active fault is either a detachment between sediments and dzhirula basement or cuts through the basement at shallow depths the 1500 m high racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes hence the 1991 earthquake sequence shows that the western greater caucasus is accommodating plate convergence at a rate possibly comparable to the eastern greater caucasus a few millimeters per year along strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area no strong evidence for transfer structures could be found along strike suggesting that differences in collisional style between the western and eastern greater caucasus may reflect differences in mechanical properties rather than differences in convergence rate a june 15 1991 event and its aftershocks southeast of the primary aftershock zone along strike show fault planes and slip vectors rotated 41 o clockwise from the mainshock this rotation is consistent with an along strike change in direction of the thrust front near 44 o e longitude and demonstrates strong local structural or topographic control on slip direction the rotation requires along strike shortening within the greater caucasus thrust system at a rate comparable to the rate of thrusting