high resolution study of layering within the percolation and soaked facies of the greenland ice sheet within the percolation and soaked facies of the greenland ice sheet the relationship between radar derived internal reflection horizons and the layered structure of the firn column is unclear we conducted two small scale ground penetrating radar gpr surveys in conjunction with 10 m firn cores that we collected within the percolation and soaked facies of the greenland ice sheet the two surveys were separated by a distance of 50 km and 340 m of elevation leading to 40 days of difference in the duration of average annual melt at the higher site 1997 m a s l which receives less melt we found that internal reflection horizons identified in gpr data were largely laterally continuous over the grid however stratigraphic layers identified in cores could not be traced between cores over any distance from 1 5 to 14 0 m thus we found no correlation between firn core stratigraphy observed directly and radar derived internal reflection horizons at the lower site 1660 m a s l which receives more melt we found massive ice layers 0 5 m thick and stratigraphic boundaries that span 15 m horizontally some ice layers and stratigraphic boundaries correlate well with internal reflection horizons that are laterally continuous over the area of the radar grid internal reflection horizons identified at 1997 m a s l are likely annual isochrones but the reflection horizons identified at 1660 m a s l are likely multi annual features we find that mapping accumulation rates over long distances by tying core stratigraphy to radar horizons may lead to ambiguous results because 1 there is no stratigraphic correlation between firn cores at the 1997 m location and 2 the reflection horizons at the 1660 m location are multi annual features

unusual radar echoes from the greenland ice sheet airborne radar images of part of the greenland ice sheet reveal icy terrain whose radar properties are unique among radar studied terrestrial surfaces but resemble those of jupiter s icy galilean satellites the 5 6 and 24 centimeter wavelength echoes from the greenland percolation zone like the 3 5 and 13 centimeter wavelength echoes from the icy satellites are extremely intense and have anomalous circular and linear polarization ratios however the detailed subsurface configurations of the galilean satellite regoliths where heterogeneities are the product of prolonged meteoroid bombardment are unlikely to resemble that within the greenland percolation zone where heterogeneities are the product of seasonal melting and refreezing

active and passive microwave signatures of antarctic firn by means of field measurements and satellite data the angular dependence and polarization behaviour of back scattering and emission of polar firn at 5 2 ghz and 10 3 ghz were measured during an oversnow traverse in dronning maud land antarctica the signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast in permanently dry snow areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like polarized backscattering coefficients this behaviour is confirmed by the analysis of c band scatterometer measurements of the active microwave instrument aboard the european space agency s ers 1

southern san andreas fault seismicity is consistent with the gutenberg richter magnitude frequency distribution the magnitudes of any collection of earthquakes nucleating in a region are generally observed to follow the gutenberg richter gr distribution on some major faults however paleoseismic rates are higher than a gr extrapolation from the modern rate of small earthquakes would predict this along with other observations led to the formulation of the characteristic earthquake hypothesis which holds that the rate of small to moderate earthquakes is permanently low on large faults relative to the large earthquake rate wesnousky et al 1983 schwartz and coppersmith 1984 we examine the rate difference between recent small to moderate earthquakes on the southern san andreas fault ssaf and the paleoseismic record hypothesizing that the discrepancy can be explained as a rate change in time rather than a deviation from gr statistics we find that with reasonable assumptions the rate changes necessary to bring the small and large earthquake rates into alignment agree with the size of rate changes seen in epidemic type aftershock sequence modeling where aftershock triggering of large earthquakes drives strong fluctuations in the seismicity rates for earthquakes of all magnitudes the necessary rate changes are also comparable to rate changes observed for other faults worldwide these results are consistent with paleoseismic observations of temporally clustered bursts of large earthquakes on the ssaf and the absence of m 7 earthquakes on the ssaf since 1857

two basin filling boxes the convection induced by turbulent buoyant plumes in a rectangular filling box baines and turner 1969 is extended to situations where the lower depths of the box are divided into two basins by a vertical barrier the barrier restricts horizontal spreading and allows the interbasin exchange of water to occur only in the common region above the barrier height one and two plume systems are considered in the two plume case it is assumed that each of the plumes arises from an independent buoyancy source above each basin the resulting two basin circulation is characterized by different stratifications in each basin and by an overflow from one basin into the other the spreading depths of the plumes and ambient stratification depend on the barrier height relative basin sizes and in the two plume case the relative strengths of the two buoyancy fluxes these results may be of relevance to multibasin seas and oceans where the circulation produced by near surface sources of intermediate and deep water is influenced by underwater ridges

cryosphere applications of nscat data though designed to measure vector winds over the ocean new imaging techniques facilitate the use of nasa scatterometer data nscat in cryosphere studies nscat provides data of unprecedented coverage resolution and quality which when coupled with the scatterometer image reconstruction with filtering sirf algorithm enables images of spl sigma sup o at resolutions approaching 8 km over stationary targets such images are useful in ice mapping and classification and multidecadal studies are possible by comparison with seasat scatterometer sass data the utility of nscat data in polar ice studies is illustrated through a review of two cryosphere applications of nscat data 1 sea ice mapping and tracking and 2 ice sheet change in greenland and antarctica the wide swath and frequent revisit coupled with incidence and azimuth angle diversity makes nscat data very effective in mapping the extent of sea ice in greenland snow and ice facies are clearly delineated on the basis of the seasonally dependent radar backscattering cross section due to sensitivity of radar backscatter to diagenetic changes occurring at and beneath the surface comparison of nscat and sass data enables study of change in greenland between 1978 and 1996

500 2000 mhz brightness temperature spectra of the northwestern greenland ice sheet an ultra wideband radiometer has been developed to measure subsurface properties of the cryosphere including ice sheets and sea ice the radiometer measures brightness temperature spectra from 0 5 to 2 ghz using 12 channels each of which measures scene brightness temperatures over an 88 mhz bandwidth resolved into 0 24 mhz intervals the instrument was flown over northwestern greenland in september 2016 and acquired the first wideband low frequency brightness temperature spectra over the ice sheet and coastal region the results reveal strong spatial and spectral variations that correlate well with the physical properties of the surface encountered along the flight path which started over ocean then passed the rock near the coast and then up onto the ablation wet percolation and dry snow zones of the interior ice sheet in particular strong spectral responses in percolation and dry snow zones are observed and plausibly explained by varying the distribution of horizontal density layers and isolated icy bodies in the upper portion of the firn the success of the airborne deployment of the instrument and subsequent implementation of algorithms to limit radio frequency interference in unprotected bands is motivating continued airborne investigations as well as stimulating research into the feasibility of a spaceborne instrument

detection of snowmelt regions on the greenland ice sheet using diurnal backscatter change snowmelt regions on greenland ice are mapped daily with the seawinds wideswath ku band 13 4 ghz scatterometer on the quikscat satellite the approach exploits the high temporal resolution of seawinds quikscat data for the melt mapping using diurnal backscatter change independent of the absolute calibration the results reveal several pronounced melting and refreezing events and effects of topography are evident in the melt patterns the spatial resolution is sufficient to identify melt features on the sukker toppen iskappe west of the main ice sheet an anomalous warming event caused by downward mixing of warm air is detected in late september 1999 over the west flank of the southern greenland ice sheet time series images of melt regions are presented over the period from summer to the fall freeze up the satellite observations are verified with in situ measurements from the greenland climate network stations

filling box flows in porous media we report upon a theoretical and experimental investigation of a porous medium filling box flow by specifically examining the details of the laminar descending plume and its outflow in a control volume having an impermeable bottom boundary and sidewalls the plume outflow initially comprises a pair of oppositely directed gravity currents the gravity currents propagate horizontally until they reach the lateral sidewalls at y pm l the flow then becomes of filling box type with a vertically ascending first front separating discharged plume fluid below from ambient fluid above the flow details are described analytically by first deriving a new similarity solution for darcy plumes with mathit pe o 1 where mathit pe is the peclet number from the similarity solution so obtained we then derive expressions for the plume volume flux and mean reduced gravity as functions of the vertical distance from the source regarding the plume outflow a similarity solution adopted from huppert woods j fluid mech vol 292 1995 pp 55 69 describes the height and front speed of the gravity currents whereas a semi implicit finite difference scheme is used to predict the first front elevation versus time and horizontal distance as with high reynolds number filling box flows that studied here is an example of a coupled problem the gravity current source conditions are prescribed by the plume volume flux and mean reduced gravity conversely discharged plume fluid may be re entrained into the plume be it soon or long after reaching the bottom impermeable boundary to corroborate our model predictions analogue laboratory experiments are performed with fresh water and salt water as the working fluids our experiments consider as independent variables the porous medium bead diameter and the plume source volume flux and reduced gravity predictions for the gravity current front position and height compare favourably against analogue measured data good agreement is likewise noted when considering either the mean elevation or the profile of the first front results from this study may be adopted in modelling geological plumes for example our equations can be used to predict the time required for discharged plume fluid to return to the point of injection in the case of aquifers closed on the sides and below by impermeable boundaries

retrieving snowpack properties and accumulation estimates from a combination of sar and scatterometer measurements this study combines two satellite radar techniques low resolution c ku band scatterometer and high resolution c band synthetic aperture radar sar for glaciological studies in particular mass balance estimations three parameters expressing the mean backscattering and its dependency on azimuth and incidence angle are used to describe and classify the antarctic ice sheets backscattering behavior simple linear regression analyses are carried out between ground truth accumulation data and the sar backscattering coefficient along continuous profile lines from this we parameterize the accumulation rate separately for certain snow pack regimes we find that sar data can be used to map mass balance changes however only within limited areas applying this method therefore generally requires accurate ground truth for regional calibration together with additional information regarding mean air temperature or elevation this investigation focuses on the area of dronning maud land antarctica we present the first high resolution accumulation map based on sar data for the surrounding area of the epica deep ice core drilling site kohnen which is compared to reliable ground truth records as well as to a surface mass balance map interpolated from these at low resolution

growth of greenland ice sheet measurement measurements of ice sheet elevation change by satellite altimetry show that the greenland surface elevation south of 72 degrees north latitude is increasing the vertical velocity of the surface is 0 20 0 06 meters per year from measured changes in surface elevations at 5906 intersections between geosat paths in 1985 and seasat in 1978 and 0 28 0 02 meters per year from 256 694 intersections of geosat paths during a 548 day period of 1985 to 1986

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

greenland snow accumulation estimates from satellite radar scatterometer data data collected by the c band ers 2 wind scatterometer escat the ku band adeos 1 nasa scatterometer nscat and the ku band seawinds on quikscat qscat satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the greenland ice sheet microwave radar backscatter images of greenland are derived using the scatterometer image reconstruction sir method at 3 day intervals over the periods 1991 1998 and 1996 1997 for escat and nscat respectively the backscatter coefficient normalized to 40 incidence a and gradient in backscatter b in the range 20 60 are compared with historical snow accumulation data and recent measurements made in the program for arctic regional climate assessment parca shallow snow pits empirical relationships derived from these comparisons reveal different exponential relationships between c and ku band a values and dry snow zone mean annual accumulation q frequency difference images between overlapping scatterometer images suggest that c band data are more sensitive to snow layering and buried inhomogeneities whereas ku band data are more sensitive to volume scattering from recently accumulated snow direct comparisons between nscat b values and in situ q measurements show a linear relationship between ln q and b with a negative rank correlation of r 0 8 the root mean square residual in fitting regression line equation ln q 3 08 17 83b to the data is 0 05 m snow water equivalent this simple ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between seasat 1978 and nscat 1996 additional comparisons between nscat and recent qscat 1999 data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet recent acceleration in the increase in intensity of scattering is observed in the percolation zone suggesting increased melting between 2000 and 3000 m elevation in the southern half of the ice sheet

spatial and temporal variability in the snowpack of a high arctic ice cap implications for mass change measurements interpretation of ice mass elevation changes observed by satellite altimetry demands quantification of the proportion of elevation change which is attributable to variations in firn densification detailed stratigraphic logging of snowpack structure and density was carried out at 1k m intervals along a 47 km transect on devon ice cap canada in spring pre melt and autumn during after melt 2004 and 2006 to characterize seasonal snowpack variability across the full range of snow facies simultaneous meteorological measurements were gathered spring pre melt snowpacks show low variability over large spatial scales with low magnitude changes in density the end of summer autumn density profiles show high variability in both 2004 and 2006 with vastly different melt regimes generating dissimilar patterns of ice layer formation over the two melt seasons dye tracing experiments from spring to autumn 2006 reveal that vertical and horizontal distribution of meltwater flow within and below the annual snowpack is strongly affected by the pre existing often subtle stratigraphic interfaces in the snowpack rather than its bulk properties strong interannual variability suggests that using a simple relationship between air temperature elevation and snowpack densification to derive mass change from measurements of elevation change across high arctic ice caps may be misguided melt timing and duration are important extrinsic factors governing snowpack densification and ice layer formation in summer rather than averaged air temperatures

importance of seasonal and annual layers in controlling backscatter to radar altimeters across the percolation zone of an ice sheet radar altimeters are one of the main tools for measuring elevation changes across the antarctic and greenland ice sheets and larger ice caps a ground based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the greenland ice sheet this radar is a high bandwidth system operating in the ku band the same frequency as several satellite altimeters measurements were made over an elevation range of 1795 to 2350 m along with snow pit and shallow core studies these measurements demonstrate the spatial and temporal variations in the backscatter relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone the extent of percolation will affect elevation estimates made by radar altimeters

fundamental questions of earthquake statistics source behavior and the estimation of earthquake probabilities from possible foreshocks abstract estimates of the probability that an m l 4 8 earthquake which occurred near the southern end of the san andreas fault on 24 march 2009 would be followed by an m 7 mainshock over the following three days vary from 0 0009 using a gutenberg richter model of aftershock statistics reasenberg and jones 1989 to 0 04 using a statistical model of foreshock behavior and long term estimates of large earthquake probabilities including characteristic earthquakes agnew and jones 1991 i demonstrate that the disparity between the existing approaches depends on whether or not they conform to gutenberg richter behavior while gutenberg richter behavior is well established over large regions it could be violated on individual faults if they have characteristic earthquakes or over small areas if the spatial distribution of large event nucleations is disproportional to the rate of smaller events i develop a new form of the aftershock model that includes characteristic behavior and combines the features of both models this new model and the older foreshock model yield the same results when given the same inputs but the new model has the advantage of producing probabilities for events of all magnitudes rather than just for events larger than the initial one compared with the aftershock model the new model has the advantage of taking into account long term earthquake probability models using consistent parameters the probability of an m 7 mainshock on the southernmost san andreas fault is 0 0001 for three days from long term models and the clustering probabilities following the m l 4 8 event are 0 00035 for a gutenberg richter distribution and 0 013 for a characteristic earthquake magnitude frequency distribution our decisions about the existence of characteristic earthquakes and how large earthquakes nucleate have a first order effect on the probabilities obtained from short term clustering models for these large events

earthquake frequency distribution and the mechanics of faulting the level of intraplate seismicity in japan generally shows a positive correlation with the density of quaternary faulting in southwest japan where intraplate seismicity is concentrated on land rates of seismic moment release m 0 are similar when calculated from either the 400 year historical record of seismicity or geologically determined slip rates of quaternary faults a data set of 18 earthquakes with seismic moments m0 ranging from 0 01 to 3 1027 dyn cm shows a relationship between rupture lengthl and m0 log m0 23 5 1 94 log l when seismic moment on each quaternary fault is assumed to occur in discrete events every t m0 m 0g years where m0 is estimated for a rupture extended over the entire fault length and m 0g is proportional to the slip rate of each quaternary fault the moment frequency distribution of earthquakes log n a b log m0 predicted from the geologic record is virtually identical to that seen with the 400 year record of seismicity in contrast if it is assumed that earthquakes on each fault occur according to the gutenberg richter relation we obtain poor agreement with the observed seismicity thus while regional seismicity satisfies the relation log n a b log m0 or equivalently log n a b log m where m is magnitude it appears that seismicity on individual faults does not this further implies that the primary factor that leads to the magnitude frequency distribution in regional seismicity studies is the relative distribution of the slip rates and lengths of preexisting faults

statistics of small earthquakes and frequency of occurrence of large earthquakes along the mexican subduction zone abstract the validity of the gutenberg richter g r frequency magnitude relation over a wide range of magnitudes is usually accepted and the statistics of small earthquakes is often extrapolated to predict the frequency of occurrence of large earthquakes we take noaa and pde data from 1963 to 1981 related to the mexican subduction zone h 65 km and convert m b to m s in the range of 3 3 m b 5 7 for this conversion we use regression of m s on m b for mexican and central american events as well as an m s m b regression based on worldwide data wyss and habermann 1982 we also take into account the standard error in m s this may be important since the number of events decreases as m b increases the m s data of 18 5 yr is normalized to 75 5 yr and is compared to the statistics of observed large earthquakes m s 6 4 for the period 1906 to 1981 it is found that the g r relation in the range 4 5 m s 6 0 when extrapolated grossly underestimates the observed frequency of large earthquakes m s 6 5 for the oaxaca and jalisco regions there is strong evidence that the plate interface in the oaxaca region consists of a few 3 or 4 large fault zones each of which cyclically ruptures over its entire dimension without giving rise to smaller events in the manner implied by the g r relation the result is of profound importance in seismic risk analysis and in the understanding of fault mechanics this phenomenon has been reported for only a few regions e g turkey bath 1981 japan wesnousky et al 1983 it may have escaped detection in other regions due to a variety of causes the most important being the long recurrence periods of large earthquakes

analytical solutions for leakage rates through abandoned wells 1 disposal of waste fluids via injection into deep saline aquifers is practiced in a variety of industries injection takes place in sedimentary basins that often have a history of oil and gas exploration and production which means that wells other than those used for waste disposal may exist in the vicinity of the injection site these existing wells provide possible pathways for leakage of waste fluids toward the shallow subsurface and the land surface for single phase flows of liquids with essentially constant properties the equations governing the system are linear and solutions may be written using the superposition principle because leakage through existing wells produces a time varying flux rate the solution of the governing equations involves convolution integrals previous solutions have addressed the problem of one injection well one existing passive well and a simple geometry of two aquifers separated by an aquitard by use of laplace transforms even for this simple case inversion of the transform is difficult solutions involving more than one passive well have not been developed nor have solutions been developed for more than two aquifers and one aquitard realistic injection cases often involve layered systems with multiple aquifers and aquitards as well as multiple passive wells sometimes numbering in the hundreds solutions for the general case of multiple aquifers and wells may be developed through introduction of approximations to the well function and appropriate simplification of the convolution integral such a solution is computationally simple comparison to solutions using the full laplace transform solution indicates that the new solution procedure produces excellent results application of the new solution to a case of multiple passive wells shows that the cumulative leakage flux in the passive wells is not a simple sum of the single well case owing to leakage induced drawdown around the passive wells in addition application to the case of multiple aquifers and aquitards demonstrates the importance of leakage into intervening aquifers as a mechanism to mitigate leakage into shallow zones a process referred to as the elevator model the new analytical solution provides a tool to analyze practical injection problems and forms a foundation on which more complex solutions such as those involving injection of a nonaqueous fluid into a deep brine formation may be based

on varying b values with depth results from computer intensive tests for southern california summary the gutenberg richter b value is thought to reflect the stress conditions in the crust therefore spatial and or temporal variations of the b value can provide important information regarding crustal tectonics we investigate the variation of b value with depth in seven selected areas of southern california a previous study provided a detailed mapping of the variations of b with depth in california our study is less systematic than this study our approach is more similar to the regional one used by mori abercrombie in comparison to these previous studies our investigation indicates that the variability of b is often not statistically significant and that the decrease of b with depth should be interpreted with caution the seismic catalogues used are subsets of a set of about 100 000 seismic events recorded by the southern california seismic network scsn and relocated by richards dinger shearer we study the performance of utsu s test compared to bootstrap tests for comparison of b values the results of our investigation also raise the question of the relevancy of utsu s test when comparing b values both simulations and real cases show that the utsu s test is biased towards rejection of the null hypothesis in favour of the hypothesis that b values are significantly different

observation and characterization of radar backscatter over greenland characterization of the microwave signature of the greenland snow surface enables delineation of the different snow facies and is a tool for tracking the effects of climate change a new empirical observation model is introduced that uses a limited number of parameters to characterize the snow surface based on the dependence of radar backscatter on incidence angle azimuth angle spatial gradient and temporal rate of change the individual model parameters are discussed in depth with examples using data from the nasa scatterometer nscat and from the c band european remote sensing ers satellite advanced microwave instrument in scatterometer mode the contribution of each model term to the overall accuracy of the model is evaluated the relative contributions of the modeled dependencies vary by region two studies illustrating applications of the model are included first interannual changes over the greenland ice sheet are investigated using nine years of ers data surface changes are observed as anomalies in the spl sigma spl deg model parameters second intraannual variations of the surface are investigated these changes are observed in the average backscatter normalized to a given observation geometry the results indicate that the model can be used to obtain a more complete understanding of multiyear change and to obtain low variance high temporal resolution observations of intraannual changes the model may be applied for increased accuracy in scatterometer synthetic aperture radar sar and wide angle sar studies

draining viscous gravity currents in a vertical fracture we consider the flow of instantaneous releases of a finite volume of viscous fluid in a narrow vertical fracture or hele shaw cell when there is a still narrower vertical crack in the horizontal base of the cell the predominant motion is over the horizontal surface but fluid also drains through the crack progressively diminishing the volume of the current in the fracture when the crack is shallow on the scale of the current it saturates immediately with the draining fluid in this case we obtain an exact analytical solution for the motion when the crack is deeper and does not saturate immediately we calculate numerically the motion of the fluid in both the fracture and the crack in each case the current advances to a finite run out length and then retreats we describe both phases of the motion and characterize the run out length in terms of the controlling parameters

emptying filling boxes the fluid mechanics of natural ventilation this paper describes the fluid mechanics of the natural ventilation of a space connected to a large body of stationary ambient fluid the flows are driven by buoyancy differences between the interior and exterior fluids connections with the ambient fluid are high level and low level openings two main forms of ventilation are identified mixing ventilation and displacement ventilation mixing ventilation occurs when the incoming ambient fluid mixes with the fluid within the space as is the case if dense fluid enters through a high level inlet in this case vertical stratification is weak displacement ventilation occurs when dense fluid enters at low levels and displaces the lighter fluid within the space out through high level openings a strong stable stratification develops in this case and there is little mixing between the incoming fluid and that in the interior both of these modes of ventilation are studied theoretically and the results are compared with laboratory experiments transient draining flows which occur when a space initially contains fluid of a density different from the ambient are examined the presence of internal sources of buoyancy allows steady states to be established and the effects of point line and vertically distributed sources are studied these steady states are extensions of filling box models with the addition of continuous exchange of fluid with the environment outside the space a major result of this work is that the form of the stratification within the space depends on the entrainment caused by the convective elements plumes produced by the buoyancy sources but is independent of the strength of the sources the strength of the stratification and the magnitudes of the velocities do however depend on the source strength the effects of opening size s and configurations are determined and criteria for producing a particular stratification within the space are established applications of this work to the ventilation of buildings are presented

a ground based radar backscatter investigation in the percolation zone of the greenland ice sheet abstract satellite radar altimeters and scatterometers deployed over ice sheets experience backscatter from the surface and from within the snowpack termed surface and volume backscatter respectively in order to assess the errors in satellite altimeter measurements it is vital to know where the return is originating from in the snowpack this return can vary spatially and temporally seasonal variations in the volume backscatter can be a major complicating factor in the radar return from the percolation zone ground based step frequency radar was deployed in the percolation zone of the greenland ice sheet at 1945 m elevation 69 51n 47 15w previous measurements in this area made by scientists from the byrd polar research centre and the university of kansas undertaken prior to summer melt events have shown the strongest backscatter from ice features at around 1 m depth buried beneath the previous end of summer surface in autumn 2004 radar measurements in the ku band with bandwidths of 1 and 8 ghz were made alongside detailed stratigraphic observations within a 1 km 2 site the radar results revealed no continuous reflecting horizons in the upper 3 5 m of the firn shallow cores and snowpits also indicated that there were no spatially continuous stratigraphic horizons across the study site an average electromagnetic wave velocity of 2 11 0 05 10 8 m s 1 was determined for the upper metre of the firn surface and volume backscatter at vertical incidence were calculated using a standard model the contribution of the surface backscatter to the total backscatter was on average 6 db higher than that of the volume backscatter however at the higher 8 ghz bandwidth the strongest return frequently originated not from the surface but from within the upper 30 cm of the snowpack most probably from thin ice layers at 1 ghz bandwidth these ice layers were not always resolved their return merged with the surface return causing it to broaden with the peak and leading edge moving down modelling using density and thickness measurements from shallow cores and snowpits showed that the backscatter from these shallow thin ice layers could be stronger than the surface return owing to constructive interference from the top and base of the layers

use of c band ground penetrating radar to determine backscatter sources within glaciers the question of penetration of synthetic aperture radar sar signals at c band frequency into polar glaciers is addressed by comparing ground penetrating radar gpr and sar backscatter signatures profiles of the kongsvegen glacier svalbard were obtained with a c band gpr the received signal is converted to the equivalent radar cross section using the standard radar equation thus mapping the effective scattering sources within the glacier at this frequency the depth of the observed scattering sources is greatest in the superimposed ice where layers are clearly seen to a depth of approximately 14 m the very high scattering properties of the upper firn layers preclude layers deeper than approximately 6 m from being imaged integrating the radar cross sections over the depth gives a single backscatter value that we compare with the backscatter coefficient scattering cross section per unit area of the processed sar data for the same profile the comparison indicates that for coincidentally acquired gpr and sar data the radar cross section measured by the gpr does represent the features that contribute to the sar signal

faceted crystal formation in the northeast greenland low accumulation region snow pit analysis in the low accumulation region in northeast greenland indicated a mean annual accumulation of 12 1 6 cm it also revealed numerous faceted layers that were not related to the summer horizon of previous years in general the faceted crystals were found under a wind crust these crusts appear to originate during the winter snow and air temperature data from automatic weather stations suggest that the katabatic storms are responsible for the formation of the wind crusts and the faceted layers frequent abrupt temperature changes up to 30 c were observed within two days due to turbulent mixing of cold air inversions the near surface air temperature variations are correlated with wind speed the latter leading the former by 6 9 hours for increasing wind and 1 4 hours for decreasing wind because of low accumulation katabatic storms affect the same snow surface for several months causing repeated air temperature fluctuations this leads to strong vapor pressure gradients and faceted crystal growth in the top snow layer on average we found two faceted layers in addition to the summer layer per year for a 4 year record in addition to its climatological significance in the snow record the formation of these layers is especially important to understand for the interpretation of microwave satellite observations of the firn

measuring geophysical parameters of the greenland ice sheet using airborne radar altimetry this paper presents radar altimeter scattering models for each of the diagenetic zones of the greenland ice sheet aafe radar altimeter waveforms obtained during the 1991 and 1993 nasa multi sensor airborne altimetry experiments over greenland reveal that the ku band return pulse changes significantly with the different diagenetic zones these changes are due to varying amounts of surface and volume scattering in the return waveform in the ablation and soaked zones where surface scattering dominates the aafe return geophysical parameters such as rms surface height and rms surface slope are obtained by fitting the waveforms to a surface scattering model waveforms from the percolation zone show that the sub surface ice features have a much more significant effect on the return pulse than the surrounding snowpack model percolation waveforms created using a combined surface and volume scattering model and an ice feature distribution obtained during the 1993 field season agree well with actual aafe waveforms taken in the same time period using a combined surface and volume scattering model for the dry snow zone return waveforms the rms surface height and slope and the attenuation coefficient of the snowpack are obtained these scattering models not only allow geophysical parameters of the ice sheet to be measured but also help in the understanding of satellite radar altimeter data

fine resolution radar altimeter measurements on land and sea ice satellite radar altimeter ra measurements are important for continued monitoring of rapidly changing polar regions in 2010 the european space agency launched cryosat 2 carrying siral a ku band ra with objectives of determining the thickness and extent of sea ice and the topography of the ice sheets one difficulty with ku band radar surveys over snow and ice is unknown penetration of ra signal into snow cover improving our understanding of the interactions of ra signals with snow and ice is needed to produce accurate elevation products to this end we developed a low power ultrawideband 12 18 ghz ra for airborne surveys to provide fine resolution measurements capable of detecting both scattering from the surface and layers within sea ice and ice sheets these measurements provide a means of identifying the dominant scattering location of lower resolution ra measurements comparable to satellite based instruments we generated two products a full bandwidth waveform fbw to identify scattering targets at fine resolution and a reduced bandwidth waveform rbw to represent conventional ra measurements retrackers are used to generate height estimates over various surface conditions for comparisons over ice sheets the leading edge tracker provided consistent ice surface elevation measurements between the fbw and rbw results however there were significant differences between the results from the centroid tracker over sea ice the location of the dominant return between the results from snow covered sea ice is highly variable this paper provides an overview of ra surveys in polar regions a description of the cresis system and a discussion of the results

radar measurements of melt zones on the greenland ice sheet surface based microwave radar measurements were performed at a location on the western flank of the greenland ice sheet here firn metamorphosis is dominated by seasonal melt which leads to marked contrasts in the vertical structure of winter and summer firn this snow regime is also one of the brightest radar targets on earth with an average backscatter coefficient of 0 db at 5 3 ghz and an incidence angle of 25 by combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter s snow pack this observation has important implications for monitoring and understanding changes in ice sheet volume using space borne microwave sensors

the effect of a fissure on storage in a porous medium we consider the two dimensional buoyancy driven flow of a fluid injected into a saturated semi infinite porous medium bounded by a horizontal barrier in which a single line sink representing a fissure some distance from the point of injection allows leakage of buoyant fluid our studies are motivated by the geological sequestration of carbon dioxide co2 and the possibility that fissures in the cap rock may compromise the safe long term storage of co2 a theoretical model is presented that accounts for leakage through the fissure using two parameters which characterize leakage driven both by the hydrostatic pressure within the overriding fluid and by the buoyancy of the fluid within the fissure we determine numerical solutions for the evolution of both the gravity current within the porous medium and the volume of fluid that has escaped through the fissure as a function of time a quantity of considerable practical interest is the efficiency of storage which we define as the amount of fluid remaining in the porous medium relative to the amount injected this efficiency scales like t 1 2 at late times indicating that the efficiency of storage ultimately tends to zero we confirm the results of our model by comparison with an analogue laboratory experiment and discuss the implications of our two dimensional model of leakage from a fissure for the geological sequestration of co2

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

seismic evidence for a thinner mantle transition zone beneath the south pacific superswell broadband seismograms recorded by the seismic stations deployed on oceanic islands in the south pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the south pacific where a large scale hot plume might ascend from the core mantle boundary cmb by stacking s waves reflected at the mantle transition zone discontinuities we find clear signals associated with the 410 km and 660 km discontinuities the transition zone thickness which is constrained from the travel time difference between the reflected waves from the 410 km and 660 km discontinuities is observed to be approximately 15 km thinner beneath the south pacific superswell than that beneath other regions the result suggests that the transition zone temperature beneath french polynesia is approximately 100 200 k higher than the surrounding mantle

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

emptying filling boxes free turbulent versus laminar porous media plumes we examine the transient evolution of a negatively buoyant laminar plume in an emptying filling box containing a uniform porous medium in the long time limit unicode stix x1d70f rightarrow infty the box is partitioned into two uniform layers of different densities however the approach towards steady state is characterized by a lower contaminated layer that is continuously stratified the presence of this continuous stratification poses non trivial analytical challenges we nonetheless demonstrate that it is possible to derive meaningful bounds on the range of possible solutions particularly in the limit of large unicode stix x1d707 where unicode stix x1d707 represents the ratio of the draining to filling time scales the validity of our approach is confirmed by drawing comparisons against the free turbulent plume case where unlike with porous media plumes an analytical solution that accounts for the time variable continuous stratification of the lower layer is available baines turner j fluid mech vol 37 1969 pp 51 80 germeles j fluid mech vol 71 1975 pp 601 623 a separate component of our study considers time variable forcing where the laminar plume source strength changes abruptly with time when the source is turned on and off with a half period unicode stix x0394 unicode stix x1d70f the depth and reduced gravity of the contaminated layer oscillate between two extrema after the first few cycles different behaviour is seen when the source is merely turned up or down for instance a change of the source reduced gravity leads to a permanent change of interface depth which is a qualitative point of difference from the free turbulent plume case

seismic potential of acapulco san marcos region along the mexican subduction zone recently seismic quiescence based on p d e and i s c data has been reported in the acapulco san marcos region 16 17 2 n 98 5 99 7 w mcnally 1981 in order to assess the seismic potential of this region we have estimated seismic moments and slips associated with 1957 ms 7 5 and 1907 ms 8 0 events which occurred in this area historical data suggest that earthquakes occurred in this region in 1845 ms 7 9 and probably also in 1820 ms 7 6 we find preference for the time predictable model in the data set which if valid forecasts the next large earthquake between now and 1993 with 1985 as the most likely year although this forecast can not be taken as definitive since it is based on several assumptions each of which are reasonable but uncertain the implications of reported quiescence and the results of this paper suggest a careful monitoring of the area

seismicity in a model governed by competing frictional weakening and healing mechanisms summary observations from laboratory field and numerical work spanning a wide range of space and time scales suggest a strain dependent progressive evolution of material properties that control the stability of earthquake faults the associated weakening mechanisms are counterbalanced by a variety of restrengthening mechanisms the efficiency of the healing processes depends on local material properties and on rheologic temperature and hydraulic conditions we investigate the relative effects of these competing non linear feedbacks on seismogenesis in the context of evolving frictional properties using a mechanical earthquake model that is governed by slip weakening friction weakening and strengthening mechanisms are parametrized by the evolution of the frictional control variable the slip weakening rate r using empirical relationships obtained from laboratory experiments in our model weakening depends on the slip of an earthquake and tends to increase r following the behaviour of real and simulated frictional interfaces healing causes r to decrease and depends on the time passed since the last slip results from models with these competing feedbacks are compared with simulations using non evolving friction compared to fixed r conditions evolving properties result in a significantly increased variability in the system dynamics we find that for a given set of weakening parameters the resulting seismicity patterns are sensitive to details of the restrengthening process such as the healing rate b and a lower cutoff time t c up to which no significant change in the friction parameter is observed for relatively large and small cutoff times the statistics are typical of fixed large and small r values respectively however a wide range of intermediate values leads to significant fluctuations in the internal energy levels the frequency size statistics of earthquake occurrence show corresponding non stationary characteristics on time scales over which negligible fluctuations are observed in the fixed r case the progressive evolution implies that except for extreme weakening and healing rates faults and fault networks possibly are not well characterized by steady states on typical catalogue time scales thus highlighting the essential role of memory and history dependence in seismogenesis the results suggest that an extrapolation to future seismicity occurrence based on temporally limited data may be misleading due to variability in seismicity patterns associated with competing mechanisms that affect fault stability

accuracy of satellite altimeter elevations over the greenland ice sheet the accuracy of geosat satellite altimetry over the greenland ice sheet is evaluated by comparing the measured heights to radar elevations from the airborne greenland aerogeophysics project at the center of the ice sheet where the ice surface is nearly level surface comparisons show a fit at the 1 to 3 m level as expected but even at moderately sloping ice regions 0 3 0 6 satellite altimetry mean errors in the range of 10 to 35 m are observed these errors are found for slope corrected and waveform retracked data so most previous accuracy estimates of current satellite altimetry ice sheet elevations in regions of sloping or undulating ice appear to be too optimistic

quantifying vulnerability of antarctic ice shelves to hydrofracture using microwave scattering properties abstract recent ice shelf disintegrations on the antarctic peninsula and subsequent increases in ice sheet mass loss have highlighted the importance of tracking ice shelf stability with respect to surface melt ponding and hydrofracture in this study we use active microwave scatterometry in time series to estimate melt season duration and winter backscatter levels as a proxy for relative concentration of refrozen ice lenses in antarctic ice shelf firn we demonstrate a physical relationship between melt days and firn ice backscatter using scatterometry and field data from greenland and apply the observed relationship to derive and map a vulnerability index for antarctica s ice shelves the index reveals that some remaining antarctic peninsula ice shelves have already reached a firn state that is vulnerable to hydrofracture we also show that the progression of an ice shelf towards vulnerability is affected by many factors such as surface mass balance internal stresses and ice shelf geometry

sources of backscatter at 5 3 ghz from a superimposed ice and firn area revealed by multi frequency gpr and cores we investigate the major sources of backscatter at 5 3 ghz within the superimposed ice and firn areas of a polythermal glacier two ground penetrating radar systems an 800 mhz impulse system and a polarimetric 5 3 ghz frequency modulated continuous wave system are used to acquire along glacier profiles in the accumulation area of kongsvegen svalbard the 800 mhz response is used to map reflection horizons in the glacier using cores from the superimposed ice and firn areas the causes of these reflection horizons in terms of snow firn and ice layers are investigated superimposing the reflection horizons on the co polarized and cross polarized 5 3 ghz profile we are able to determine how the 5 3 ghz frequency responds to the different media scattering at rough interfaces and volume scattering occur in the superimposed ice area and are apparently caused by air bubble number size and distribution in the firn the strongest return originates from below the previous summer surface consistent with previous findings at approximately the same depth strong incoherent scattering begins the rapid decrease in coherent reflections indicates the significance of scattering in the firn

investigations of meltwater refreezing and density variations in the snowpack and firn within the percolation zone of the greenland ice sheet the mass balance of polythermal ice masses is critically dependent on the proportion of surface generated meltwater that subsequently refreezes in the snowpack and firn in order to quantify this effect and to characterize its spatial variability we measured near surface 26 resulting in a 32 increase in net accumulation this seasonal densification increased at lower elevations rising to 47 10km closer to the ice sheet margin at 1860 m a s l density depth profiles from nine sites within 1 km 2 at similar to 1945ma s l reveal complex stratigraphies that change over short spatial scales and seasonally we conclude that estimates of massbalance change cannot be calculated solely from observed changes in surface elevation but that near surface densification must also be considered however predicting spatial and temporal variations in densification may not be straightforward further the development of complex firn density profiles both masks discernible annual layers in the near surface firn and ice stratigraphy and is likely to introduce error into radar derived estimates of surface elevation

snow and ice facies variability and ice layer formation on canadian arctic ice caps 1999 2005 1 time series of enhanced resolution data from the seawinds scatterometer aboard quikscat were used to map the distribution of snow and ice surface facies and ice layer formation in the percolation zone on ice caps in the queen elizabeth islands during the period 1999 2005 iterative self organizing data analysis classification of the mean postfreeze up biweekly average 0 signal for the 7 year period resulted in the delineation of four snow and ice surface facies interpreted as the percolation saturation superimposed ice and glacier ice zones analysis with national centers for environmental prediction national center for atmospheric research reanalysis reveals that changes in geopotential height in the troposphere 700 500 and 300 hpa and air temperature 700 hpa level are positively negatively correlated with area changes in the glacier ice percolation and saturation zones and changes in facies boundary elevation the change in biweekly averaged backscatter following the freeze up periods between successive autumns was used to map changes in the distribution of ice layers formed by meltwater percolation and refreezing in the snowpack within the percolation zone strongly positive air temperature anomalies at the 700 hpa level in 2001 and 2005 are consistent with extensive ice layer formation in the percolation zones of all ice caps such large interannual changes in ice layer formation are likely associated with large changes to the density profile of the snowpack and may be associated with surface elevation changes that are unrelated to changes in surface mass balance

shuttle imaging radar sir c x sar reveals near surface properties of the south patagonian icefield shuttle imaging radar c x band synthetic aperture radar sir c x sar views of the south patagonian icefield in southern chile and argentina demonstrate the ability of spaceborne multiparameter radar to detect climatically driven intra annual changes in the snow and ice conditions on glaciers the sir c x sar system aboard space shuttle endeavor acquired images during two 11 day missions in april and october 1994 the radar signatures of differing snow and ice conditions are distinctive and homogeneous over large areas of the ice fields the signatures are characterized mainly by 1 backscatter amplitude levels 2 relative amplitudes of the c and l bands wavelengths of 5 7 and 24 cm respectively and 3 polarization properties indicative of volumetric or surface scattering the radar signatures are interpreted by correlating the radar characteristics with elevation of the snow and ice surfaces and with changes in the meteorological conditions we are able to define four radar glacier zones zone a a relatively dry snow zone with dominant c band returns zone b a moderately wet snow zone with dominant l band returns zone c a wet snow zone with weak returns in all bands and zone d bare ice and or heavily crevassed surfaces with strong returns in all bands the spatial changes in the radar glacier zones between april and october are consistent with colder temperatures recorded in october producing drier snow conditions at lower elevations than in april

natural ventilation in interconnected chambers ventilation of adjacent connected chambers forced in one chamber by an isolated point source of buoyancy is investigated there are floor and ceiling level external openings in the forced and unforced chambers respectively while the partition between the chambers has both a floor and ceiling level opening the flow evolves on the time scale over which the volume flux associated with the plume at the ceiling would fill both chambers the steady state in the forced chamber is analogous to the single chamber flow described by linden lane serff smeed j fluid mech vol 212 1990 p 309 with a well mixed buoyant upper layer which is deeper than in the single chamber flow due to the extra pressure drop at the upper interior opening the steady state in the unforced chamber inevitably exhibits vertical stratification and depends on the transient flow all the opening areas and the relative plan area of the two chambers as is verified by laboratory experiments when the upper interior opening is relatively large the buoyant layer in the unforced chamber is deeper than the buoyant layer in the forced chamber which contradicts model predictions based on the assumption that the layers are always well mixed

relating microwave backscatter azimuth modulation to surface properties of the greenland ice sheet azimuth dependence of a normalized radar cross section over the greenland ice sheet is modeled with a simple surface scattering model the model assumes that azimuth anisotropy in surface roughness at scales of 3 300 m is the primary mechanism driving the modulation to evaluate the contribution of azimuth anisotropy in surface roughness to the radar backscatter the model is compared to models based on isotropic surface roughness the models are inverted to estimate snow surface properties using measurements from the c band european remote sensing satellite advanced microwave instrument in scatterometer mode results indicate that the largest mesoscale rms surface slopes are found in the lower portions of the dry snow zone estimates of the preferential direction in surface roughness are highly correlated with katabatic wind fields over greenland which is consistent with wind formed sastrugi as the dominant mechanism causing azimuth modulation of the maximum improvement of the azimuth modulation surface model compared to its isotropic counterparts occurs in the lower regions of the dry snow zone where the azimuth variability of is the largest in regions with azimuth modulation over 1 db the mean root mean square error estimate of the azimuth dependent surface scattering model is 0 46 db compared with 0 70 db for similar models using isotropic roughness

from glacier facies to sar backscatter zones via gpr we present a comparison between data acquired with frequency modulated ground penetrating radar gpr and satellite synthetic aperture radar sar both radars are polarimetric and operate at a center frequency of 5 3 ghz the field site is the polythermal glacier kongsvegen svalbard along glacier gpr profiles cover the ablation area and the accumulation area where the latter consists of superimposed ice si and firn the glacier facies are clearly identifiable on the gpr profiles although we show that the copolarized response is better for distinguishing different ice zones whereas the si firn boundary is most obvious in the cross polarized response a calibrated backscatter coefficient is calculated for the gpr data and compared with the sar backscatter coefficient the sar zones are in very good agreement with the gpr derived glacier facies we show that in the ablation area the sar response is dominated by backscatter from the previous summer surface in the si and firn areas it is dominated by sources below the previous summer surface

backscatter model for the unusual radar properties of the greenland ice sheet a number of planetary objects exhibit large radar reflectivity and polarization ratios and more recently a similar behavior has been observed over a vast portion of the earth s surface the percolation facies of the greenland ice sheet surface based ranging radar data and snow stratigraphy studies demonstrated that the radar properties of that portion of greenland are caused by enhanced scattering from massive large solid ice bodies buried in the top few meters of the dry cold clean snowy surface of the ice sheet and created by seasonal melting and refreezing events from author

interpretation of ssm i measurements over greenland multispectral brightness temperature t sub b measurements over greenland are obtained from the special sensor microwave imager ssm i which are flown aboard the dmsp satellites this paper examines the different spectral characteristics over greenland throughout the year although snow covers the vast majority of greenland the southern regions rarely exhibit the spectral characteristics associated with snowcover i e t sub b decreases at higher frequencies in fact the ssm i polarization and frequency measurements over southern greenland are more indicative of water than a snow covered surface i e t sub b increases at higher frequencies a simplified physical model is developed to help explain the anomalous measurements over southern greenland model results indicate that high frequency radiation is mainly scattered by snow grains residing above the subsurface ice layers whereas low frequency radiation is scattered throughout a much greater depth since low frequencies are scattered throughout a greater volume they are depressed relative to high frequencies and the typical snowcover signature is absent

structure of southeastern antarctic peninsula ice shelves and ice tongues from synthetic aperture radar imagery examination of synthetic aperture radar data collected over the southeastern antarctic peninsula shows that features sometimes mapped as ice shelves are more likely composed of numerous ice tongues interspersed within a matrix of fast ice and icebergs the tongues are formed by the seaward extension of numerous small mountain glaciers that drain from the antarctic peninsula once afloat the tongues intermingle with a matrix of fast ice and brash examination of 1997 radarsat 1 image mosaics shows that southeastern antarctic peninsula composite ice shelves covered an area of about 3500 km 2 like ice tongues around the rest of antarctica these features are highly fragmented and likely to be susceptible to mechanical failure one such composite shelf located between new bedford and wright inlets was observed to decrease in area by 1200km 2 between 1997 and 2000

characteristics and small scale variability of gpr signals and their relation to snow accumulation in greenland s percolation zone we investigate snowpack properties at a site in west central greenland with ground penetrating radar gpr supplemented by stratigraphic records from snow pits and shallow firn cores gpr data were collected at a validation test site for cryosat t05 on the expglaciologiques internationales au groenland egig line over a 100 m 100 m grid and along 1 km sections at fre quencies of 500 and 800 mhz several internal reflection horizons irhs down to a depth of 10 m were tracked irhs are usually related to ice layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season warm conditions in the following melt season can change these characteristics by percolating meltwater in cold conditions smaller melt volumes at the surface can lead to faint irhs the absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data our gpr derived depth of the 2003 summer surface of 1 48 m measured in 2004 is confirmed by snow pit observations the distribution of irh depths on a 1 km scale reveals a gradient of increasing accumu lation to the northeast of about 5 cm w e km 1 we find that point measurements of accumulation in this area are representative only over several hundred metres with uncertainties of about 15 of the spatial mean

leakage from gravity currents in a porous medium part 2 a line sink we consider the propagation of a buoyancy driven gravity current in a porous medium bounded by a horizontal impermeable boundary the current is fed by a constant flux injected at a point and leaks through a line sink at a distance from the injection point this is an idealized model of how a fault in a cap rock might compromise the geological sequestration of carbon dioxide the temporal evolution of the efficiency of storage defined as the instantaneous ratio of the rate at which fluid is stored without leaking to the rate at which it is injected is of particular interest we show that the efficiency of storage decays like t 2 5 for times t that are long compared with the time taken for the current to reach the fault this algebraic decay is in contrast to the case of leakage through a circular sink neufeld et al j fluid mech vol 2010 where the efficiency of storage decays more slowly like 1 ln t the implications of the predicted decay in the efficiency of storage are discussed in the context of geological sequestration of carbon dioxide using parameter values typical of the demonstration project at sleipner norway we show that the efficiency of storage should remain greater than 90 on a time scale of millennia provided that there are no significant faults in the cap rock within about 12 km of the injection site

statistics of characteristic earthquakes statistical methods are used to test the characteristic earthquake hypothesis several distributions of earthquake size seismic moment frequency relations are described based on the results of other researchers as well as my own tests evidence of the characteristic earthquake hypothesis can be explained either by statistical bias or statistical artifact since other distributions of earthquake size provide a simpler explanation for available information the hypothesis cannot be regarded as proven

detection and monitoring of stratigraphic markers and temperature trends at the greenland ice sheet project 2 using passive microwave remote sensing data satellite passive microwave sensors provide a sensitive means of studying ice sheet surface processes that assists ice core interpretation and can extend local observations across regional scales analysis of special sensor microwave imager ssm i brightness temperature tb data supports ice core research in two specific ways first the summer hoar complex layers used to date the holocene portion of the greenland ice sheet project 2 ice core can be defined temporally and spatially by ssm i 37 ghz vertically v and horizontally h polarized b ratio v h trends second comparison of automatic weather station temperatures to ssm i 37 ghz v tb data shows that they are an effective proxy temperature record in this region also the tb data can be correlated with proxy temperature trends from stable isotope ratio 18o and d profiles from snow pits and this allows the assignment of dates to specific snow depths

circular polarization radar properties of high altitude ice western kunlun shan and central andes full polarization synthetic aperture radar images of high altitude glaciers and ice fields in the western kunlun shan of tibet and the central andes near santiago chile made with the shuttle imaging radar display regions with circular polarization ratios c in excess of unity the mountainous topography at both locales allows reconstruction of the angular scattering behavior the behavior is very similar to that displayed by greenland s percolation zone glaciologic evidence from the tibetan site confirms ice pipes and lenses embedded in the firn which have been identified as the scatterers producing c 1 in greenland this demonstrates that the cold snow percolation facies radar signature exists outside of greenland the greenland radar behavior had been suggested as an analogue for that of the icy galilean satellites we like previous workers find that the near nadir backscatter is too specular to be considered analogous to that of the jovian moons we also present a crevasse region in tibet that exhibits c 1 and a possible detection of the ice substrate of a rock covered glacier in the andes

an accumulation map for the greenland dry snow facies derived from spaceborne radar 1 an important component of ice sheet mass balance calculations is the snow accumulation rate traditionally determined across the ice sheet from interpolated ice core data we derive an accumulation rate map within the greenland dry snow facies using c band radar backscatter and the mean annual temperature values for the backscatter were obtained from the ers 1 synthetic aperture radar mosaic acquired during september november 1992 an empirical equation based on elevation and latitude is used to determine the mean annual temperature we examine the influence of accumulation rate and mean annual temperature on c band 5 3 ghz radar backscatter using a forward model which incorporates snow metamorphosis and radar backscatter components our model is run over a range of accumulation and temperature conditions consistent with the greenland dry snow facies on the basis of our modeled results a look up table is generated which uniquely maps the measured radar backscatter and mean annual temperature to accumulation rate our results compare favorably 15 4 average difference with 124 in situ accumulation rate measurements falling within our study area we attribute some of the differences to 1 unreliable in situ data points which were based on a single year s accumulation and 2 a systematic underestimate of accumulation rate by our forward model in the low accumulation regions associated with the northern portion of our map with questionable data points and those above 77 n removed the average differences were less than 10 3 as determined from 73 ice cores

a global index earthquake approach to probabilistic assessment of extremes 1 this paper applies recent innovations in flood frequency analysis to regional series of earthquake magnitudes in the global centroid moment tensor cmt catalog from 1976 to 2005 probability plot correlation coefficient hypothesis tests and l moment goodness of fit evaluations reveal that the gumbel gum distribution provides a good approximation to the probability distribution function pdf of series of annual maximum am earthquake magnitudes homogeneity tests based on the theory of l moments further reveal that broad regions of the globe are homogeneous in the sense that the am observations of earthquake magnitudes are well approximated by a gum pdf with fixed upper moments the homogeneity of global earthquake data across broad tectonic environments enables us to pool data into a regional pdf of earthquake magnitudes termed an index earthquake distribution research in hydrology has shown that frequency analysis based on pooling of data using an analogous index flood method are much more accurate than frequency analysis based on site or region specific data the index earthquake distribution is a dimensionless gum distribution with fixed scale parameter so only the mean earthquake magnitude must be estimated for a region to define the frequency distribution of large earthquakes we show how the degree of spatial homogeneity of earthquake magnitudes across broad tectonic environments can be exploited to yield improved estimates of the risk posed by extreme earthquake magnitudes

melt season duration and ice layer formation on the greenland ice sheet 2000 2004 1 time series of enhanced resolution quikscat images were used to map the extent and duration of surface melt and the distribution of ice layer formation on the greenland ice sheet in the period 2000 2004 apart from the bare ice and the mixed pixels 10 along the ice sheet margin where melt cannot be determined reliably annual melt duration and melt anomaly maps were produced across the ice sheet 90 for each summer over the 5 year period the mean melt duration for the ice sheet ranged from 14 3 to 20 5 d the proportion of the ice sheet that experienced melting in a given year ranged from 44 2 to 79 2 extensive melt in 2002 was caused by a single melt event of a few days duration this event was associated with the intrusion of a ridge of high pressure from the north atlantic ocean that brought warm air onto the ice sheet the change in biweekly averaged backscatter between the freeze up periods in the current and previous falls was used to identify changes in the distribution of ice layers in snow between successive melt seasons the year 2002 had the maximum melt extent and duration and ice layer formation extended to higher elevations than in other years interannual changes in the distribution of ice layer formation may be associated with changes in surface height that are not necessarily indicative of mass balance changes mapping of ice layer formation is thus potentially useful for interpreting both real and apparent height changes measured by altimetry

automated delineation of dry and melt snow zones in antarctica using active and passive microwave observations from space this paper presents the algorithms and analysis results for delineating snow zones using active and passive microwave satellite remote sensing data with a high resolution radarsat synthetic aperture radar sar image mosaic dry snow zones percolation zones wet snow zones and blue ice patches for the antarctic continent have been successfully delineated a competing region growing and merging algorithm is used to initially segment the sar images into a series of homogeneous regions based on the backscatter characteristics and texture property these image regions are classified into different snow zones the higher level of knowledge about the areal size of and adjacency relationship between snow zones is incorporated into the algorithms to correct classification errors caused by the sar image noise and relief induced radiometric distortions mathematical morphology operations and a line tracing algorithm are designed to extract a vector line representation of snow zone boundaries with the daily passive microwave special sensor microwave imager ssm i data dry and melt snow zones were derived using a multiscale wavelet transform based method the analysis results respectively derived from radarsat sar and ssm i data were compared and correlated the complementary nature and comparative advantages of frequently repeated passive microwave data and spatially detailed radar imagery for detecting and characterizing snow zones were demonstrated

the magnitude distribution of earthquakes near southern california faults we investigate seismicity near faults in the southern california earthquake center community fault model we search for anomalously large events that might be signs of a characteristic earthquake distribution we find that seismicity near major fault zones in southern california is well modeled by a gutenberg richter distribution with no evidence of characteristic earthquakes within the resolution limits of the modern instrumental catalog however the b value of the locally observed magnitude distribution is found to depend on distance to the nearest mapped fault segment which suggests that earthquakes nucleating near major faults are likely to have larger magnitudes relative to earthquakes nucleating far from major faults

ice flow dynamics of the greenland ice sheet from sar interferometry synthetic aperture radar sar interferograms produced from esa s ers 1 satellite provide the first synoptic view of ice flow dynamics of the western sector of the greenland ice sheet glacial motion is detected in the radar ranging direction at millimetric scales across a complete sequence of snow accumulation and melting regimes despite significant variations in their radar scattering properties ice flow evolves from a slow regular motion at the higher elevations at lower elevations motion is strongly convoluted by meter scale undulations in surface topography which have a unique interferometric signature that enables a novel approach for retrieving flow direction inferred flow directions combined with surface displacements in the radar ranging direction yield ice velocity estimates that are within 6 of in situ measurements gathered along a 40 km survey line application of repeat pass sar interferometry to the entire greenland ice sheet should enable precise mapping of its ice flow dynamics at an unprecedented level of spatial detail

on the variation of seismic hazard along the san andreas fault recurrence probabilities of earthquakes were evaluated at 33 locations along the san andreas fault using gumbel s method two minima in the mode of the largest annual earthquakes occur one north of san francisco centered around 38 75 n latitude and the other centered in the carizzo plain area at 35 25 n latitude the latter is offset by an unusually large ratio of large to small earthquakes so that only one minimum around 38 75 n is observed in the expected 100 yr earthquake size this broad minimum closely coincides with where the san andreas fault broke in 1906 and may be only temporary there appears to be a slight systematic tendency for the observed largest 50 year earthquake to become larger than the expected 50 yr earthquake as the size of the area being studied is reduced the preferred explanation for this is that the pattern of size as a function of frequency becomes nonlinear when the size of the area studied is less than the focal area of the largest earthquakes

penetration depths inferred from interferometric volume decorrelation observed over the greenland ice sheet radar interferometry provides a novel way to study the subsurface of glaciers interferometric correlation since the two complex images that comprise the interferogram have slightly different incidence angles at each point on the ground a decorrelating phase noise with statistics related to the scattering medium is present in the interferogram the amount of surface decorrelation is increased by volume scatter the larger the vertical extent of the scatterers contributing to the radar echo the greater the decorrelation will be by modeling this effect the authors can estimate radio wave penetration depths within the upper layers of the glacier or ice sheet observations of the greenland ice sheet using ers data yield penetration depths one way 1 e point for power that range from 12 to 35 m due to the contribution of volume scatter the critical interferometer baseline is decreased and the authors find for the greenland data the baseline must be restricted to be less than 300 m the authors also compare penetration depths measured within the dry snow zone with those found in the percolation zone and coastal areas they find that as expected the rocky coastal areas evidence minimal penetration interestingly the penetration depths that the authors measure in the percolation zone spl sim 23 m indicate a large degree of volume scatter which is contrary to earlier results that found the scattering in the percolation zone dominated by structures in the first few meters this discrepancy may be due to unmodeled scattering behavior or the radar return may indeed include significant contributions from scatterers far beneath the surface