structural and stratigraphie features and ers 1 synthetic aperture radar backscatter characteristics of ice growing on shallow lakes in nw alaska winter 1991 1992 changes in ers 1 c band synthetic aperture radar sar backscatter intensity from ice growing on shallow tundra lakes at three locations in nw alaska are described ice core analysis shows that at all lakes on the coast at barrow the ice whether floating or frozen to the bottom includes an inclusion free layer overlying a layer of ice with tubular bubbles oriented parallel to the direction of growth the clear ice may also be overlain by a discontinuous layer of bubbly snow ice backscatter is low 16 to 22 db at the time of initial ice formation probably due to the specular nature of the upper and lower ice surfaces causing the radar pulse to be reflected away from the radar as the ice thickens during the autumn backscatter rises steadily once the ice freezes to the lake bottom regardless of the presence of forward scattering tubular bubbles low backscatter values of 17 to 18 db are caused by absorption of the radar signal in the lake bed for ice that remains afloat all winter the ice water interface and the tubular bubbles combine presumably via an incoherent double bounce mechanism to cause maximum backscatter values of the order of 6 to 7 db the saturates at 6 to 7 db before maximum ice thickness and tubular bubble content are attained a simple ice growth model suggests that the layer of ice with tubular bubbles need be only a few centimeters thick midway through the growth season to cause maximum backscatter from floating ice during the spring thaw a previously unreported backscatter reversal is observed on the floating and grounded portions of the coastal lakes but not on the lakes farther inland this reversal may be related to the ice surface topography and wetness plus the effects of a longer cooler melt period by the coast time series of backscatter variations from shallow tundra lakes are a record of 1 the development of tubular bubbles in the ice and by association changes in the gas content of the underlying water and 2 the freezing of ice to the bottoms of the lakes and therefore lake bathymetry and water availability sar is also able to detect the onset of lake ice growth in autumn and the initiation of the spring thaw and thus has potential for monitoring high latitude lake ice growth and decay processes in relation to climate variability

the role of advective heat transport in talik development beneath lakes and ponds in discontinuous permafrost 1 regions of warm thin discontinuous permafrost have been observed to be experiencing rapid changes in lake and pond dynamics in recent decades even though surface water and groundwater interactions are thought to play a significant role in heat transport in these regions the effect of these interactions on permafrost remains largely unquantified in order to examine the influence of groundwater flow on permafrost dynamics we modeled the development of a sub lake talik under permafrost conditions similar to those observed in the southern central seward peninsula region of alaska using a numerical solution that couples heat transport and groundwater flow including the effect of water phase changes on soil permeability and latent heat content a comparison of model simulations with and without near surface subpermafrost groundwater flow indicates that stable permafrost thicknesses are 2 to 5 times greater in the absence of groundwater flow simulations examining the thermal influence of lakes on underlying permafrost suggest that a through going talik can develop in a matter of decades and that the incorporation of advective heat transport reduces the time to complete loss of ice beneath the lake by half relative to heat transport by conduction alone this work presents the first quantitative assessment of the rates of sub lake permafrost response to thermal disturbances such as talik development in systems with near surface groundwater flow the results highlight the importance of coupled thermal and hydrologic processes on discontinuous permafrost dynamics

paleoclimatic forcing of magnetic susceptibility variations in alaskan loess during the late quaternary visual matches and statistical tests suggest correlations between marine isotope curves retrodictive solar insolation at lat 65 n and magnetic susceptibility profiles through late quaternary age alaskan loess sections the susceptibility changes largely appear to reflect variability in magnetite content due to climatically controlled changes in wind intensity and competence magnetic susceptibility profiles through massive loess can provide stratigraphic context for intercalated paleosols and tephras a prominent paleosol correlated with marine isotope stage 5 occurs several metres above the old crow ash in loess sections indicating that this important tephra is older than suggested by thermoluminescence dates and may have been deposited ca 215 25 ka

compaction law for cretaceous sandstones of alaska s north slope lithic arenites and wackes of the national petroleum reserve in alaska contain a significant volume of ductile grains and have suffered substantial physical compaction with burial as determined by stratigraphic studies these rocks were buried to maximum depths of 500 to 4 400 m initially the grain fraction grain volume as a percent of total rock volume must have been near 59 but increased to over 90 at depth statistical analysis reveals that the grain fraction gf is primarily a function of burial depth in meters d percent ductile grains such as phyllite rock fragments p and matrix content m multiple regression suggests the following linear equation gf 90 0 23p 0 72m 0 0018d simple grain rearrangement operates to a depth of 550 800 m and accounts for a 25 increase in grain fraction at greater depths compaction is by plastic deformation however the increase in grain fraction appears to fall off considerably below 1 000 m and compaction due to plastic deformation increases grain fraction on average by only 7 early quartz cementation never progressed beyond a beginning stage and thus was ineffective at preventing compaction on the other hand clay matrix drastically impedes compaction grain fraction for lithic wackes with an abundant matrix never exceeds 83 the compaction law derived in this study may prove useful in further exploration for petroleum reservoirs for example lithic arenites with a low ductile content should retain an intergranular porosity of 6 to a burial depth of 2 000 m

algorithms for removing surface water signals from surface nuclear magnetic resonance infiltration surveys abstractsurface nuclear magnetic resonance surface nmr is a geophysical method that directly detects water and can be used to determine the depth profile of water content within the subsurface although surface nmr has proven useful for investigating groundwater in the saturated zone its use to study the vadose zone is still in development a recent study for the south avra valley storage and recovery project savsarp demonstrated that surface nmr can be used to monitor infiltrating water associated with aquifer storage and recovery a water resource management method in which surface water is stored in local aquifers during wet periods for use during dry periods however one of the major issues associated with using surface nmr to monitor infiltrating water is the influence of large bodies of surface water we have examined the effect that large bodies of surface water have on the surface nmr signal and we have developed three algorithms the a priori late signal and long signal inversion lsi a

detecting unfrozen sediments below thermokarst lakes with surface nuclear magnetic resonance 1 a talik is a layer or body of unfrozen ground that occurs in permafrost due to an anomaly in thermal hydrological or hydrochemical conditions information about talik geometry is important for understanding regional surface water and groundwater interactions as well as sublacustrine methane production in thermokarst lakes due to the direct measurement of unfrozen water content surface nuclear magnetic resonance nmr is a promising geophysical method for noninvasively estimating talik dimensions we made surface nmr measurements on thermokarst lakes and terrestrial permafrost near fairbanks alaska and confirmed our results using limited direct measurements at an 8 m deep lake we observed thaw bulb at least 22 m below the surface at a 1 4 m deep lake we detected a talik extending between 5 and 6 m below the surface our study demonstrates the value that surface nmr may have in the cryosphere for studies of thermokarst lake hydrology and their related role in the carbon cycle

microwave backscatter from arctic lake ice and polarimetric implications polarimetric synthetic aperture radar satellite and ground based ku and x band scatterometer measurements are used to explore the scattering mechanism for ice in shallow arctic lakes wherein strong radiometric responses are seen for floating ice and low returns are evident where the ice has grounded scatterometer measurements confirm that high backscatter is from the ice water interface whereas polarimetric decomposition suggests that the dominant scattering mechanism from that interface is single bounce using fresnel equations a simple model for surface bounce from the ice water interface is proposed and its predictions are supported by experimental parameters such as co pol phase difference co pol ratio and the results of rigorous numerical modeling despite early research suggesting double bounce scattering from columnar air bubbles and the ice water interface as the dominant scattering mechanism in shallow lakes this paper strongly supports a single bounce model

a full scale field experiment 1978 1995 on the growth of permafrost by means of lake drainage western arctic coast a discussion of the method and some results on 13 august 1978 a lake on the western arctic coast was artificially drained in a multidisciplinary experiment on the growth of permafrost on the unfrozen bottom of the drained lake a bowl shaped talik unfrozen basin with a maximum depth of about 32 m underlay the lake bottom prior to drainage in the first winter after drainage downward freezing started on the exposed lake bottom and upward freezing from permafrost beneath the talik after drainage the soft lake bottom sediments hardened from water loss and freeze thaw consolidation gradual thinning of the active layer at many sites was accompanied by ground uplift and the growth of aggradational ice downward and upward freezing has resulted in solute rejection freezing point depressions pore water expulsion from the freezing of the saturated lake bottom sands and convective heat transfer from groundwater flow in an open hydrologie system the increasingly saline intrapermafrost groundwater flowing at an increasingly negative temperature be

a pleistocene sand sea on the alaskan arctic coastal plain a ridge and thermokarst basin landscape that is strikingly portrayed in landsat winter imagery consists of large pleistocene dunes that have been modified by younger eolian activity and thermokarst processes this is the most extensive area of large stabilized dunes yet reported in the north american arctic the landscape is of particular interest because it has been proposed as a first order analog for martian fretted terrain recognition of the large dunes permits a new interpretation for linear and curvilinear trends visible in landsat summer imagery

longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern pacific abstract several long range explosion seismology experiments have been conducted in the northwestern pacific basin where one of the oldest oceanic lithospheres is postulated to exist the experiments were conducted from 1974 to 1980 highly sensitive ocean bottom seismographs which had been developed for longshot experiments were used the lengths of the profiles ranged from 1000 to 1800 km and the directions were chosen to provide wide azimuthal coverage one of the aims of this series of experiments was to test the existence of velocity anisotropy on a large regional scale the results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4 7 the anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom however the magnitude of the anisotropy may vary with depth the azimuth of the maximum velocity is 150 160 clockwise from north and coincides with the fossil direction of spreading of the pacific plate whereas it differs from the present direction of plate motion by 30 the azimuth does not seem to depend on depth in the direction of maximum velocity the lithosphere is basically two layered 8 0 8 2 and 8 6 km s 1 the depth of the interface is 50 60 km beneath the sea floor

hydrogeomorphic processes of thermokarst lakes with grounded ice and floating ice regimes on the arctic coastal plain alaska thermokarst lakes cover 20 of the landscape throughout much of the alaskan arctic coastal plain acp with shallow lakes freezing solid grounded ice and deeper lakes maintaining perennial liquid water floating ice thus lake depth relative to maximum ice thickness 1 5 2 0 m represents an important threshold that impacts permafrost aquatic habitat and potentially geomorphic and hydrologic behaviour we studied coupled hydrogeomorphic processes of 13 lakes representing a depth gradient across this threshold of maximum ice thickness by analysing remotely sensed water quality and climatic data over a 35 year period shoreline erosion rates due to permafrost degradation ranged from 0 2 m year in very shallow lakes 0 4 m up to 1 8 m year in the deepest lakes 2 6 m this pattern of thermokarst expansion masked detection of lake hydrologic change using remotely sensed imagery except for the shallowest lakes with stable shorelines changes in the surface area of these shallow lakes tracked interannual variation in precipitation minus evaporation p el with periods of full and nearly dry basins shorter term 2004 2008 specific conductance data indicated a drying pattern across lakes of all depths consistent with the long term record for only shallow lakes our analysis suggests that grounded ice lakes are ice free on average 37 days longer than floating ice lakes resulting in a longer period of evaporative loss and more frequent negative p el these results suggest divergent hydrogeomorphic responses to a changing arctic climate depending on the threshold created by water depth relative to maximum ice thickness in acp lakes copyright 2011 john wiley sons ltd

seismic anisotropy beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

experimental investigations of the wettability of clays and shales 1 wettability in argillaceous materials is poorly understood yet it is critical to hydrocarbon recovery in clay rich reservoirs and capillary seal capacity in both caprocks and fault gouges the hydrophobic or hydrophilic nature of clay bearing soils and sediments also controls to a large degree the movement of spilled nonaqueous phase liquids in the subsurface and the options available for remediation of these pollutants in this paper the wettability of hydrocarbons contacting shales in their natural state and the tendencies for wettability alteration were examined water wet oil wet and mixed wet shales from wells in australia were investigated and were compared with simplified model shales single and mixed minerals artificially treated in crude oil the intact natural shale samples preserved with their original water content were characterized petrophysically by dielectric spectroscopy and nuclear magnetic resonance plus scanning electron optical and fluorescence microscopy wettability alteration was studied using spontaneous imbibition pigment extraction and the sessile drop method for contact angle measurement the mineralogy and chemical compositions of the shales were determined by standard methods by studying pure minerals and natural shales in parallel a correlation between the petrophysical properties and wetting behavior was observed these correlations may potentially be used to assess wettability in downhole measurements

multiecho scheme advances surface nmr for aquifer characterization 1 surface nuclear magnetic resonance nmr is increasingly used as a method to noninvasively characterize aquifers this technology follows a successful history of nmr logging applied over decades to estimate hydrocarbon reservoir properties in contrast to logging however surface methods have utilized relatively simple acquisition sequences from which pore scale properties may not be reliably and efficiently estimated we demonstrate for the first time the capability of sophisticated multiecho measurements to rapidly record a surface nmr response that more directly reflects aquifer characteristics specifically we develop an adaptation of the multipulse carr purcell meiboom gill cpmg sequence widely used in logging to measure the t2 relaxation response in a single scan we validate this approach in a field surface nmr data set and by direct comparison with an nmr log adoption of the cpmg marked a landmark advancement in the history of logging nmr we have now realized this same advancement in the surface nmr method

numerical simulation of permafrost thermal regime and talik development under shallow thaw lakes on the alaskan arctic coastal plain 1 thaw lakes are one of the most obvious manifestations of the hydrological system at work in the tundra regions of the alaskan arctic coastal plain but the extent of the role of thaw lakes in arctic land atmosphere interactions and feedback has yet to be fully understood this study uses a two dimensional heat transfer model with phase change under a cylindrical coordinate system to simulate the long term influence of shallow thaw lakes on the thermal regime of permafrost and talik development on the alaskan arctic coastal plain on the basis of previous studies of permafrost and thaw lakes at barrow alaska a series of simulation cases was conducted using different combinations of long term mean lake bottom temperature and lake depth the simulated results indicate that shallow thaw lakes are a significant heat source to permafrost and talik for a thaw lake with a long term mean lake bottom temperature of greater than 0 0 c a talik forms under the thaw lake the maximum talik thicknesses vertical distance from the ground surface to the permafrost surface are 28 0 43 0 and 53 2 m 3000 years after the formation of a shallow thaw lake with long term mean lake bottom temperatures of 1 0 2 0 and 3 0 c respectively talik development rate is very high in the first several years after a thaw lake formation and decreases gradually with time no talik forms below a thaw lake with a long term mean lake bottom temperature equal to or lower than 0 0 c but the temperature of permafrost below the thaw lake increases with time three thousand years after the formation of a thaw lake with a long term mean lake bottom temperature of greater than or equal to 2 0 c ground temperature increases of more than 0 5 c occur as far as 300 m from the lake shore and as deep as about 400 m below the ground surface it is concluded that variation of long term mean lake bottom temperature has a significant influence on permafrost thermal regime and talik development continued monitoring for thaw lake bottom temperature and ground temperature under shallow thaw lakes is needed to further improve the simulation

shifting balance of thermokarst lake ice regimes across the arctic coastal plain of northern alaska 1 the balance of thermokarst lakes with bedfast and floating ice regimes across arctic lowlands regulates heat storage permafrost thaw winter water supply and over wintering aquatic habitat using a time series of late winter synthetic aperture radar sar imagery to distinguish lake ice regimes in two regions of the arctic coastal plain of northern alaska from 2003 2011 we found that 18 of the lakes had intermittent ice regimes varying between bedfast ice and floating ice conditions comparing this dataset with a radar based lake classification from 1980 showed that 16 of the bedfast ice lakes had shifted to floating ice regimes a simulated lake ice thinning trend of 1 5 cm yr since 1978 is believed to be the primary factor driving this form of lake change the most profound impacts of this regime shift in arctic lakes may be an increase in the landscape scale thermal offset created by additional lake heat storage and its role in talik development in otherwise continuous permafrost as well as increases in over winter aquatic habitat and winter water supply

polarities of p and s waves and shear wave splitting observed from the bucaramanga nest colombia the bucaramanga nest colombia is an intense seismic source centered at 6 48 n 73 10 w and 161 km depth beneath the eastern andes during a 3 week period 161 microearthquakes mb 4 3 were recorded by a 260 by 140 km array of 12 three component university of wisconsin uw seismographs centered above the nest eighty eight percent of the events recorded were from the nest providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths shear wave splitting analysis shows that the average polarization direction of the first arriving shear waves from the nest events at nine of the seismographs is n12 e 18 after corrections based on the amount of discrepancy between the observed average p wave azimuth and the expected event to station azimuth average s wave polarization is n6 e 15 variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system shear wave splitting observed from 17 events along the wadati benioff zone outside the nest shows that the time delays increase with increasing depth and defines a wedge shaped anisotropic volume above the subducting plate the anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip the anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the wadati benioff zone a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab

evidence for formation of a flexural backarc basin by compression and crustal thickening in the central alaska peninsula the north aleutian basin is a large cenozoic sedimentary basin in the northern part of the central alaska peninsula and the southern bering shelf the gravity anomaly pattern the geometry and the structure of the basin suggest that if formed by downward flexure of the backarc lithosphere basin modeling suggests that the flexure was driven by the emplacement of surface and subsurface loads having densities comparable to those of oceanic crust and mantle rocks at approximately the position of the present day volcanic arc and forearc the authors suggest that the inferred loads consist of tectonically thickened mafic crustal materials lying beneath the arc and forearc of the central alaska peninsula the crustal thickening may have occurred within a dominantly transpressional regime resulting from oblique convergence between the north american and pacific plates during the cenozoic

evolution of lakes and basins in northern alaska and discussion of the thaw lake cycle 1 we evaluated the development of lake basins on the central coastal plain of northern alaska on the basis of topographic profiles soil and ground ice surveys radiocarbon dating photogrammetric analysis and regional comparisons our analysis reveals that lake evolution is much more complex and less cyclic than theorized by previous investigations in the area we studied there was insufficient ground ice in the oldest terrain to form thaw lakes the aggradation of ice in the margins of drained lake basins was insufficient to heave the surface up to near original topographic conditions and the process occurs at too slow a rate for a thaw lake cycle to develop within the holocene accordingly we revised the conceptual model of lake and basin development to be consistent with the patterns and process we observed on the extensive sand sheets underlying most of the coastal plain developmental stages include 1 initial flooding of depressions to form primary lakes 2 lateral erosion with sorting and redistribution of sediments 3 lake drainage as the stream network expands 4 differential ice aggradation in silty centers and sandy margins 5 formation of secondary thaw lakes in the heaved centers of ice rich basins and infilling of ponds along the low margins and 6 basin stabilization

time dependent morphology of thaw lakes and taliks in deep and shallow ground ice 1 the shape and depth of thaw lake basins depends on lake age on whether the talik thaw bulb is at steady state and on the distribution of ice in the ground to investigate implications of this broad hypothesis we use a numerical model of conductive heat transfer phase change and thaw subsidence of ice rich sediment beneath a lake in cross section for ground thermal properties with lake temperatures and dimensions consistent with measurements modeled talik depth approximately increases with except in lakes in deep ground ice which deepen more rapidly because of consolidation on thawing in deep ground ice environments basins achieve depths of 20 m in 5 ka expanding lakes with disequilibrium taliks have deep basins with broad 100 m wide inclined margins in shallow ground ice settings either original epigenetic ice or in permafrost that has reformed in drained basins lakes are 3 m deep and flat bottomed unaffected by preexisting topography and ground ice variations first generation lakes in deep ground ice are rounder and grow larger in area than later generation lakes these predictions are consistent with gps sonar and remote sensing measurements of bathymetry and plan view shape of first and later generation lakes in substrates with deep syngenetic ground ice pleistocene loess northern seward peninsula alaska and shallow epigenetic ice yukon arctic coastal plain

airborne electromagnetic imaging of discontinuous permafrost 1 the evolution of permafrost in cold regions is inextricably connected to hydrogeologic processes climate and ecosystems permafrost thawing has been linked to changes in wetland and lake areas alteration of the groundwater contribution to streamflow carbon release and increased fire frequency but detailed knowledge about the dynamic state of permafrost in relation to surface and groundwater systems remains an enigma here we present the results of a pioneering 1 800 line kilometer airborne electromagnetic survey that shows sediments deposited over the past 4 million years and the configuration of permafrost to depths of 100 meters in the yukon flats area near fort yukon alaska the yukon flats is near the boundary between continuous permafrost to the north and discontinuous permafrost to the south making it an important location for examining permafrost dynamics our results not only provide a detailed snapshot of the present day configuration of permafrost but they also expose previously unseen details about potential surface groundwater connections and the thermal legacy of surface water features that has been recorded in the permafrost over the past 1 000 years this work will be a critical baseline for future permafrost studies aimed at exploring the connections between hydrogeologic climatic and ecological processes and has significant implications for the stewardship of arctic environments

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

three dimensional inversion of regional p and s arrival times in the east aleutians and sources of subduction zone gravity highs free air gravity highs over forearcs represent a large fraction of the power in the earth s anomalous field yet their origin remains uncertain seismic velocities as indicators of density are estimated here as a means to compare the relative importance of upper plate sources for the gravity high with sources in the downgoing plate p and s arrival times for local earthquakes recorded by a seismic network in the eastern aleutians are inverted for three dimensional velocity structure between the volcanic arc and the downgoing plate a three dimensional ray tracing scheme is used to invert the 7974 p and 6764 s arrivals for seismic velocities and hypocenters of 635 events one dimensional inversions show that station p residuals are systematically 0 25 0 5 s positive at stations 0 30 km north of the aleutian volcanic arc indicating slow material while residuals at stations 10 30 km south of the arc are 0 1 0 25 s negative both features are explained in three dimensional inversions by velocity variations at depths less than 25 35 km tests using a one dimensional or a two dimensional slab starting model show that below 100 km depth velocities are poorly determined and trade off almost completely with hypocenters for earthquakes at these depths the locations of forearc velocity highs in the crust of the upper plate correspond to the location of the gravity high between the trench and volcanic arc free air anomalies calculated from the three dimensional velocity inversion result match observed gravity for a linear density velocity relationship between 0 1 and 0 3 mg m 3 km s 1 when a 50 km thick slab is included with a density of 0 055 0 005 mg m 3 values outside these ranges do not match the observed gravity the slab alone contributes one third to one half of the total 75 150 mgal amplitude of the gravity high but predicts a high that is much broader than is observed the inclusion of upper plate velocity anomalies predicts the correct width of the anomaly 100 150 km where the anomaly is most positive because the forearc gravity high is continuous along the entire aleutian arc and is found in most arcs globally high upper plate forearc velocities are suspected to be a common feature of the upper plate of most subduction zones the forearc mass excesses appear to be sustained by upward regional flexure of the upper plate that partly balances the depression of the lower plate at the trench thus elevating high density and high velocity material thus a part of the downward flexure of the subducting plate is regionally compensated by shallow positive mass anomalies in the upper plate and the strength of the upper plate helps generate the forearc gravity highs

transient electromagnetic surveys for the determination of talik depth and geometry beneath thermokarst lakes thermokarst lakes are prevalent in arctic coastal lowland regions and sublake permafrost degradation and talik development contributes to greenhouse gas emissions by tapping the large permafrost carbon pool whereas lateral thermokarst lake expansion is readily apparent through remote sensing and shoreline measurements sublake thawed sediment conditions and talik growth are difficult to measure here we combine transient electromagnetic surveys with thermal modeling backed up by measured permafrost properties and radiocarbon ages to reveal closed talik geometry associated with a thermokarst lake in continuous permafrost to improve access to talik geometry data we conducted surveys along three transient electromagnetic transects perpendicular to lakeshores with different decadal scale expansion rates of 0 16 0 38 and 0 58 m year we modeled thermal development of the talik using boundary conditions based on field data from the lake surrounding permafrost and a borehole independent of the transient electromagnetics a talik depth of 91 m was determined from analysis of the transient electromagnetic surveys using a lake initiation age of 1400 years before present and available subsurface properties the results from thermal modeling of the lake center arrived at a best estimate talk depth of 80 m which is on the same order of magnitude as the results from the transient electromagnetic survey our approach has provided a noninvasive estimate of talik geometry suitable for comparable settings throughout circum arctic coastal lowland regions