estimating melt onset over arctic sea ice from time series multi sensor sentinel 1 and radarsat 2 backscatter information on the timing of melt onset over sea ice is important for understanding the arctic s changing climate the daily temporal resolution of passive microwave brightness temperatures provides the most widely utilized observations to detect melt onset but are limited to a spatial resolution of 25 km wide swath synthetic aperture radar sar imagery provides a much higher spatial resolution 20 100 m but melt onset detection remains challenging because of i insufficient temporal resolution to facilitate accurate melt onset detection ii inconsistent viewing geometries and iii limited image availability across the arctic here we construct high temporal resolution composite gamma nought backscatter products 1 day 1 2 day and 2 4 day using sentinel 1 and radarsat 2 over a close to seamless revisit region located in northern canadian arctic and greenland for estimating melt onset over arctic sea ice in 2016 and 2017 we employ the necessary radiometric terrain flattening and local resolution weighting techniques to generate normalised backscatter over the entire study region removing restrictions limiting analysis to a single sensor or track s swath width by integrating both ascending and descending passes into the composite products results indicate that higher temporal resolution multi sensor composite gamma nought products 1 day that make use of the most imagery provide a robust temporal evolution of the backscatter this allows for more representative estimates of melt onset as it is easier to separate the melt onset threshold from winter variability that is otherwise a considerable challenge for sar based melt onset algorithms because of inconsistent temporal resolution multi sensor composite gamma naught melt onset detection is in good agreement with melt onset estimates derived from the advance scatterometer ascat backscatter values and passive microwave brightness temperatures over homogenous sea ice regions but very noticeable improvement was found within narrow channels and regions with more heterogeneous sea ice in anticipation of the availability of data from even more sar satellites with the launch of the radarsat constellation mission the multi sensor composite gamma nought approach presented here may offer the most robust approach to estimate the timing of melt onset over sea ice across the arctic using high spatiotemporal resolution sar

electric and magnetic field changes observed during a seismic swarm in pollino area southern italy abstract over the last few years seismic activity in the pollino area a sector of the calabro lucanian apennines in southern italy known as a seismic gap has been very weak however in 2011 the seismicity gradually intensified culminating in an earthquake of m w 5 0 occurred on 25 october 2012 the depth of the 2011 2012 earthquake hypocenters ranges between 2 and 10 km the seismicity results in two separate clusters and traces a north northwest south southeast fracture more evident in the western sector in this area an mt station was installed on 26 september 2012 by the institute of methodologies for environmental analysis national research council of italy imaa cnr italy at about 50 km from another mt station operating since 2003 in the agri valley tramutola southern italy such a seismic swarm occurred in the pollino area more than 3600 events in last two years with local magnitude m l 0 1 it has provided a rare opportunity to study the earthquake related temporal patterns of electromagnetic em signals and is potentially informative about ongoing seismogenic processes in this study we present several cases of em field variations associated with the passage of seismic waves the maximum amplitude of the electrical signals registered at the two mt sites and the earthquake magnitude are related by an attenuation factor that depends on the distance between the hypocenter and the mt station furthermore at the two mt sites the maximum electrical anomalies seem to be more appreciable predominantly in different directions indicating a certain directivity in the propagation of the electric field a deep analysis of em time series recorded during the mainshock m w 5 0 was performed in particular by applying time frequency misfit criteria based on the continuous wavelet transform we compared the electric field with seismic recordings and we found a good waveform similarity between signals moreover we also found an em signal that significantly anticipates the theoretical first p wave arrival at the tramutola mt station

seismoelectromagnetic effect associated with the izmit earthquake and its aftershocks in order to investigate the resistivity structure beneath the northern and the southern branches of the western part of the north anatolian fault zone nafz which has been known as a seismic gap area we started wideband magnetotelluric mt measurements on 27 july 1999 along a north south profile crossing the northern branch near izmit and the southern branch near iznik when the izmit kocaeli earthquake m w 7 4 occurred on 17 august 1999 mt fields were being measured at five sites four of which happened to be located near the surface rupture zone associated with the mainshock and large variations in the mt fields were observed in association with seismic waves we propose motional electromagnetic induction in the electrically conducting crust which vibrates under the earth s magnetic field as one of the possible generation mechanisms for such mt field variations for more detailed studies of such a mechanism we installed a wideband mt instrument and a three component seismometer at two of the four sites on 16 september 1999 and made simultaneous measurements during several aftershocks the estimate of the order of the magnitude of motional electromagnetic response derived from the mt fields and ground motion observed during a large aftershock m 4 5 supports our claim that the seismo dynamo effect is a plausible mechanism for variations in the mt fields associated with the seismic waves manuscript received 31 july 2000

integrated pan arctic melt onset detection from satellite active and passive microwave measurements 2000 2009 1 an integrated pan arctic melt onset data set is generated for the first time by combining estimates derived from active and passive microwave satellite data using algorithms developed for the northern high latitude land surface ice caps large lakes and sea ice the data set yields new insights into the spatial and temporal patterns of mean melt onset date mmod and the associated geographic and topographic controls for example in the terrestrial arctic tree fraction and latitude explain more than 60 of the variance in mmod with the former exerting a stronger influence on mmod than the latter elevation is also found to be an important factor controlling mmod with most of the arctic exhibiting significant positive relationships between mmod and elevation with a mean value of 24 5 m d 1 melt onset progresses fastest over land areas of uniform cover or elevation 40 80 km d 1 or both and slows down in mountainous areas on ice caps and in the forest tundra ecotones over sea ice melt onset advances very slowly in the marginal seas while in the central arctic the rate of advance can exceed 100 km d 1 comparison of the observed mmod with simulated values from the third version of the canadian coupled global climate model showed good agreement over land areas but weaker agreement over sea ice particularly in the central arctic where simulated mmod is about 2 3 weeks later than observed because of a cold bias in simulated surface air temperatures over sea ice

sea ice response to atmospheric and oceanic forcing in the bering sea abstract a coupled sea ice ocean model is developed to quantify the sea ice response to changes in atmospheric and oceanic forcing in the bering sea over the period 1970 2008 the model captures much of the observed spatiotemporal variability of sea ice and sea surface temperature sst and the basic features of the upper ocean circulation in the bering sea model results suggest that tides affect the spatial redistribution of ice mass by up to 0 1 m or 15 in the central eastern bering sea by modifying ice motion and deformation and ocean flows the considerable interannual variability in the pattern and strength of winter northeasterly winds leads to southwestward ice mass advection during january may ranging from 0 9 1012 m3 in 1996 to 1 8 1012 m3 in 1976 and averaging 1 4 1012 m3 which is almost twice the january may mean total ice volume in the bering sea the large scale southward ice mass advection is constrained by warm surface waters in the south that melt 1 5 1012 m3 of ice in mainly t

pocket beach sediment a field investigation of the geodynamic processes of coarse clastic beaches on the maltese islands central mediterranean abstract research on pocket beaches is generally lacking and merits more intensive investigative efforts in view of the importance of coastal landscapes especially along the mediterranean coastline this paper is a first as a geomorphological study of pocket coarse clastic beaches in the maltese islands shedding light on beach sediment dynamics and related spatio temporal controls between and within beaches four beaches along the coastline of the maltese islands were selected as study areas with distinct geological lithologies coastal configurations and wave exposures systematic sampling was done with field data collected on beach morphology sediment size and shape properties to account for both seasonal and post storm trends sediment tracer experiments were also carried out to determine the major transport pathways and recoveries of gravel beach sediments wave exposure geological background and coastal configuration were found to be important components in coarse clastic pocket beach response to forcings this study re affirms that microtidal pocket beaches constitute individual environments which require a rigorous and representative systematic sampling approach based on sediment behaviour in both the longshore and cross shore dimensions this study demonstrates the need to further investigate the different spatio temporal processes in microtidal pocket beaches including aspects related to intrinsic sediment dynamics

atmospheric forcing on the barents sea winter ice extent abstract the atmospheric forcing on the barents sea ice extent during winter december february djf has been investigated for the period 1967 2002 the time series for the sea ice extent is updated and includes the winter of 2005 which marks a new record low in the wintertime barents sea ice extent and a linear trend of 3 5 decade 1 in the ice extent was found covariability between the barents sea ice extent and the atmospheric mean seasonal flow and the synoptic cyclones has been discussed separately for the mean flow linear correlations and regression analysis reveal that anomalous northerly southerly winds prevail in the nordic seas during winters with extensive sparse barents sea ice extent some of the variability in the mean flow is captured by the north atlantic oscillation nao however the wintertime link between the barents sea ice extent and the nao is moderate by studying the cyclone activity in the high latitude northern hemisphere using a dataset of individual cyclones two

geometry kinematics and deformation rates along the active normal fault system in the southern apennines implications for fault growth throw rates throws and kinematic data have been collected from a localised system of active normal faults in the southern apennines italy to assess how its growth history differs from that for a distributed fault system in the central apennines both show evidence for fault interaction in that 1 faults located centrally along strike have higher throws and throw rates than distal faults and 2 cumulative throw and throw rate profiles summed across strike show central maxima with values decreasing to zero at fault system tips however although throw rates increased through time in the central apennines we are unable to resolve such changes if they exist for the southern apennines specifically throw rates derived from offsets of 18 ka geomorphic surfaces are consistent with total throws in the southern apennines when extrapolated back through time but overestimate total throws in the central apennines by a factor of 2 4 suggesting an increase in throw rate through time for the latter we discuss why some fault systems appear to have throw rates that are constant through time whilst others accelerate during growth interaction

temporal variation of seismic velocity and anisotropy before the 2009 mw 6 3 l aquila earthquake italy we describe a set of seismological observations of the foreshock sequence preceding the 6 april 2009 m w 6 3 l9aquila earthquake the dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock approaching the earthquake we observed clear variations in the seismic wave propagation properties the elastic properties of rocks in the fault region underwent a sharp change about a week before the earthquake from our observations we infer that a complex sequence of dilatancy diffusion processes takes place and that fluids play a key role in the fault failure process

variability in ice phenology on great bear lake and great slave lake northwest territories canada from seawinds quikscat 2000 2006 abstract the temporal evolution of the backscatter coefficient sigma nought from quikscat was evaluated for monitoring ice phenology on great bear lake 66 n 121 w and great slave lake 61 40 n 114 w northwest territories canada results indicated that from quikscat can be used to detect melt onset water clear of ice and freeze onset dates on both lakes an ice phenology algorithm was then developed to assess the spatiotemporal variability on both lakes from quikscat for the period 2000 2006 results showed that for great slave lake the average melt onset date occurred on year day yd 123 the average water clear of ice date was on yd164 and the average freeze onset date was on yd330 on great bear lake the average melt onset date occurred on yd139 the average water clear of ice date was yd191 and the average freeze onset date was yd321 ice cover remained present for at least five weeks longer on great bear lake than on great slave lake and most of the difference can be explained by earlier ice melt on great slave lake spatially on great bear lake melt onset took place first in the eastern arm water clear of ice occurred first in southeastern and western arms and freeze onset appeared first in the northern arm and along the shorelines on great slave lake melt onset began first in the central basin and then progressed to the northern and eastern arms later in the season the central basin of great slave lake cleared earlier than the periphery due to the discharge from the slave river freeze onset on great slave lake occurred first within the east arm closely followed by the north and west arms and then finally in the centre of the main basin

diagnosis of the record minimum in arctic sea ice area during 1990 and associated snow cover extremes the arctic sea ice cover exhibited its record minimum area during 1990 characterized by extensive ice free conditions during august along the siberian coast these reductions are consistent with warm windy conditions in may and continued warmth in june promoting early melt and reductions in ice concentration followed in august by strong coastal winds forcing a final breakup and retreat of the pack ice the unusually warm arctic conditions in 1990 are part of a larger scale temperature anomaly pattern linking the sea ice anomaly to accompanying record minima in eurasian snow cover

observation of perennial arctic sea ice melt and freeze up using passive microwave data the onset of spring melt and autumn freeze up of arctic sea ice are shown through comparison with coincident ers 1 synthetic aperture radar sar and surface air temperature data to produce significant and detectable changes in the passive microwave signature melt onset is marked by a sharp rise in brightness temperatures resulting from the increased emissivity of moisture in the snow cover freeze up is accompanied by a sharp drop in brightness temperatures because of volume scattering by bubbles in the upper ice layer an algorithm is developed to detect melt and freeze up from passive microwave data and is applied to the scanning multichannel microwave radiometer smmr and special sensor microwave imager ssm i data from 1979 to 1996 the mean accuracy of melt and freeze up dates determined from the ssm i is estimated to be 2 and 3 days respectively the reduced temporal sampling of the smmr degrades the accuracy by 1 day on average melt onset varies from late may in the southern beaufort sea to mid june in the central arctic north of the canadian archipelago while freeze up varies between mid september and mid august the shortest melt season appears to occur north of the canadian archipelago rather than at the north pole which is consistent with the fact that minimum summer air temperatures are centered over greenland the mean standard deviations of the interannual variability of melt and freeze up are 6 8 and 11 4 days respectively

comparing l and c band synthetic aperture radar estimates of sea ice motion over different ice regimes abstract estimating sea ice motion from synthetic aperture radar sar imagery at c band is the most reliable approach because of its high spatial resolution and ever increasing temporal resolution given the multiple current and upcoming sar platforms however there is still uncertainty in sar derived sea ice motion depending on the ice type and its thermodynamic state there have been suggestions mostly theoretical that use of l band sar and its inherent longer wavelength 15 30 cm and subsequent increased penetration capability could be beneficial for estimating sea ice motion especially during the melt season here we estimate and analyze sea ice motion for 9 pairs of c and l band sar imagery from radarsat 2 palsar 1 and palsar 2 located in the canadian arctic over a variety of sea ice types at different thermodynamic states results show that the increased signal penetration of l band sar into multi year ice myi during the melt season facilitates the detection of more motion vectors with stronger cross correlation coefficients compared to c band sar over newly formed ice and dry first year ice the reduced sensitivity to surface scattering and richer texture from l band sar imagery facilitates the detection of more motion vectors with stronger cross correlation coefficients compared to c band sar over dry myi l band provided stronger cross correlation coefficients but c band detected more motion vectors with a more representative spatial distribution with arctic sea ice continuing shift from a multi year to first year dominated icescape coupled with an increasing melt season length l band sar s ability to provide improved sea ice motion estimates during both the melt and freeze up time periods could prove even more useful in the coming decades

waveform joint inversion of seismograms and electrograms for moment tensor characterization of fracking events electromagnetic signals have been observed in association with fracking experiments in the laboratory and in the field we have developed a seismoelectric forward modeling approach to produce synthetic seismograms and electrograms generated by fracking events using the finite element method with perfect matched layer boundary conditions the poroelastodynamic equations are solved in the frequency domain using a formulation based on the solid phase displacement and the pore pressure these results are used to compute the electrical field disturbances of electrokinetic nature three types of electrical signals are generated type i disturbance is associated with the seismic source itself type ii disturbance corresponds to seismoelectric conversions and type iii corresponds to coseismic signals this model is applied to simulate the seismic and electrical signals corresponding to the occurrence of a fracking event in a two layers system we perform a stochastic joint inversion of the seismograms and electrograms using the adaptive metropolis algorithm ama to obtain the posterior probability density functions of the parameters characterizing the seismic source assuming that the velocity model is perfectly known the joint waveform inversion is performed on synthetic noise free data and the ama algorithm is successful in retrieving the true values of the unknown parameters the proposed approach is then tested on the same synthetic data after being contaminated with 15 random noise with respect to the maximum amplitude of the signals the model parameters are better determined for the joint inversion of seismic and electrical data by comparison with the inversion of the seismic time series alone we also propose a deterministic tomographic algorithm that is successful in locating the in situ source current density distribution for types i and ii anomalies from the electrical data alone

is the dipole anomaly a major driver to record lows in arctic summer sea ice extent recent record lows of arctic summer sea ice extentare found to be triggered by the arctic atmospheric dipoleanomaly da pattern this local second leading mode ofsea level pressure slp anomaly in the arctic produced astrong meridional wind anomaly that drove more sea ice outof the arctic ocean from the western to the eastern arcticinto the northern atlantic during the summers of 1995 1999 2002 2005 and 2007 in the 2007 summer the daalso enhanced anomalous oceanic heat flux into the arcticoceanviaberingstrait whichacceleratedbottomandlateralmelting of sea ice and amplified the ice albedo feedback acoupled ice ocean model was used to confirm the historicalrecord lows of summer sea ice extent

spatial and temporal variability in snowmelt onset over arctic sea ice climate models suggest surface warming in the arctic will be rapid and pronounced implying substantial changes in snowmelt onset are likely this research therefore examines spatial and temporal variability in passive microwave derived snowmelt onset dates over arctic sea ice the objectives are to understand better the regional characteristics of snowmelt and to document whether the snowmelt onset record shows signs of climate change snowmelt onset dates are derived with scanning multichannel microwave radiometer and special sensor microwave imager brightness temperature data and they are subsequently stratified into 13 regions to analyze spatial and temporal variability results illustrate significant spatial variability in snowmelt onset with the median annual snowmelt onset date in one region of the arctic typically being statistically different from most other regions the examination of temporal variability also shows large interannual differences in the median snowmelt onset date in most regions additionally trends towards earlier snowmelt onset are documented in the vvest central arctic lincoln sea beaufort sea and canadian arctic archipelago regions

variations in snowpack melt on the greenland ice sheet based on passive microwave measurements a simple microwave emission model is used to simulate 37 ghz brightness temperatures associated with snowpack melt conditions for locations across the greenland ice sheet the simulated values are utilized as threshold values and compared to daily gridded smmr and ssm i passive microwave data in order to reveal regions experiencing melt the spatial extent of the area classified as melting is examined on a daily monthly and seasonal may august basis for 1979 91 the typical seasonal cycle of melt coverage shows melt beginning in late april a rapid increase in the melting area from mid may to mid july a rapid decrease in melt extent from late july through mid august and cessation of melt in late september seasonal averages of the daily melt extents demonstrate an apparent increase in melt coverage over the 13 year period of approximately 3 8 annually significant at the 95 confidence interval this increase is dominated by statistically significant positive trends in melt coverage during july and august in the west and southwest of the ice sheet we find that a linear correlation between microwave derived melt extent and a surface measure of ablation rate is significant in june and july but not august so caution must be exercised in using the microwave derived melt extents in august nevertheless knowledge of the variability of snowpack melt on the greenland ice sheet as derived from microwave data should prove useful in detecting climate change in the arctic and examining the impact of climate change on the ice sheet

recent changes in the arctic melt season melt season duration melt onset and freeze up dates are derived from satellite passive microwave data and analyzed from 1979 to 2005 over arctic sea ice results indicate a shift towards a longer melt season particularly north of alaska and siberia corresponding to large retreats of sea ice observed in these regions although there is large interannual and regional variability in the length of the melt season the arctic is experiencing an overall lengthening of the melt season at a rate of about 2 weeks decade sup 1 in fact all regions in the arctic except for the central arctic have statistically significant at the 99 level or higher longer melt seasons by greater than 1 week decade sup 1 the central arctic shows a statistically significant trend at the 98 level of 5 4 days decade sup 1 in 2005 the arctic experienced its longest melt season corresponding with the least amount of sea ice since 1979 and the warmest temperatures since the 1880s overall the length of the melt season is inversely correlated with the lack of sea ice seen in september north of alaska and siberia with a mean correlation of 0 8

influence of the arctic circumpolar vortex on the mass balance of canadian high arctic glaciers abstract variability in july mean surface air temperatures from 1963 to 2003 accounted for 62 of the variance in the regional annual glacier mass balance signal for the canadian high arctic a regime shift to more negative regional glacier mass balance occurred between 1986 and 1987 and is linked to a coincident shift from lower to higher mean july air temperatures both the interannual changes and the regime shifts in regional glacier mass balance and july air temperatures are related to variations in the position and strength of the july circumpolar vortex in years when the july vortex is strong and its center is located in the western hemisphere positive mass balance anomalies prevail in contrast highly negative mass balance anomalies occur when the july circumpolar vortex is either weak or strong without elongation over the canadian high arctic and its center is located in the eastern hemisphere the occurrence of westerly positioned july vortices has decreased by 40 since 1987 the associat

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

recent changes in arctic sea ice melt onset freezeup and melt season length 1 in order to explore changes and trends in the timing of arctic sea ice melt onset and freezeup and therefore melt season length we developed a method that obtains this information directly from satellite passive microwave data creating a consistent data set from 1979 through present we furthermore distinguish between early melt the first day of the year when melt is detected and the first day of continuous melt a similar distinction is made for the freezeup using this method we analyze trends in melt onset and freezeup for 10 different arctic regions in all regions except for the sea of okhotsk which shows a very slight and statistically insignificant positive trend 0 4 d decade 1 trends in melt onset are negative i e toward earlier melt the trends range from 1 0 d decade 1 for the bering sea to 7 3 d decade 1 for the east greenland sea except for the sea of okhotsk all areas also show a trend toward later autumn freeze onset the chukchi beaufort seas and laptev east siberian seas observe the strongest trends with 7 d decade 1 for the entire arctic the melt season length has increased by about 20 days over the last 30 years largest trends of over 10 d decade 1 are seen for hudson bay the east greenland sea the laptev east siberian seas and the chukchi beaufort seas those trends are statistically significant at the 99 level

space time clustering of seismicity in california and the distance dependence of earthquake triggering 1 using two recent high resolution earthquake catalogs i examine clustering in california seismicity by plotting the average rate of earthquakes as a function of both space and time from target events of m 2 to 5 comparisons between pre and post target event activity can be used to resolve earthquake to earthquake triggering associated with target events of different magnitudes the results are more complicated than predicted by computer simulations of earthquake triggering that begin with background events occurring at random times in particular at least some of the temporal clustering of seismicity at short scales 0 1 to 5 km does not appear to be caused by local earthquake triggering but instead reflects an underlying physical process that temporarily increases the seismicity rate such as is often hypothesized to drive earthquake swarms earthquake triggering for m 4 5 earthquakes is only resolvable in average seismicity rates at times less than about one day and to distances of less than about 10 km and its linear density decreases as r 1 5 to r 2 5 significantly steeper than some previous studies have found

modeling the microwave emission of bubbly ice applications to blue ice and superimposed ice in the antarctic and arctic passive microwave remote sensing is extensively used in polar regions to study the cryosphere to better understand the measured signal above continental ice covered areas our objective is to estimate the microwave emission of bubbly ice surfaces using a physically based multilayer electromagnetic model i e the dense media radiative transfer multilayer model dmrt ml this model accounts for ice layers with variable amounts of bubbles each layer is fully described by its temperature density thickness and air bubble radius simulations are performed using in situ data from two distinct sites one in antarctica on a coastal blue ice area near the cap prud homme cph station in adelie land and the other on the barnes ice cap bic located on baffin island in the arctic on this ice cap superimposed ice with seasonal snow cover about 1 m thick was observed in both cases several ice parameters were measured or estimated and the others were optimized results of the dmrt ml simulations are compared with in situ surface based radiometer sbr measurements at 11 19 and 37 ghz at both horizontal and vertical polarizations results show that dmrt ml is able to reproduce the microwave emission of different ice types with good accuracy when accounting for ice bubbles final hbox rmse 7 37 hbox k and 8 42 k for cph and bic respectively compared with rmse ranging from 15 k to 40 k without bubbles comparisons between sbr measurements and satellite data for the bic also show good agreement hbox rmse 4 1 hbox k for 19 and 37 ghz both polarizations

detection of melt onset over the northern canadian arctic archipelago sea ice from radarsat 1997 2014 abstract we present an algorithm to detect melt onset over arctic sea ice using high resolution synthetic aperture radar sar images from radarsat the algorithm is based on the temporal evolution of the sar backscatter coefficient using an ice type specific threshold approach that also corrects for incidence angle variation using 4457 radarsat images the algorithm was applied over sea ice in the northern canadian arctic archipelago that low spatial resolution microwave sensors have difficulty resolving thus generating a new time series of melt onset from 1997 to 2014 over the 1997 2014 period the mean annual average melt onset date in the northern caa was on year day 164 mid june with a standard deviation of 4 days no significant trend for melt onset date was found over the 18 year period an earlier later melt onset was associated with increased decreased solar energy absorption r 0 64 significant at 95 confidence and subsequently associated in lighter heavier september sea ice coverage in the northern caa r 0 62 significant at 95 confidence radarsat estimates of melt onset were found to be in good agreement but more robust compared to passive microwave and scatterometer estimates over the northern caa

a five year record of summer melt on eurasian arctic ice caps abstract climatologies and annual anomaly patterns 2000 04 of melt season duration and dates of melt onset freeze up on eurasian arctic ice masses were derived from quick scatterometer quikscat backscatter data severnaya zemlya russia has later melt onset earlier freeze up and shorter melt seasons than svalbard norway novaya zemlya russia in all three archipelagos 2001 was the longest melt season and 2000 was the shortest anomalously long short melt seasons on svalbard were associated with negative positive sea ice concentration anomalies along the north coast in june and august annual mean melt duration was strongly correlated with the mean june august ncep ncar reanalysis 850 hpa air temperature allowing reconstruction of melt durations for the period of 1948 2005 the 2000 04 pentad had the second or third longest mean melt duration of all pentads in the 1950 2004 epoch while the 1950 54 pentad probably had the longest integration of these results with previous results from gre

recent seismicity and crustal stress field in the lucanian apennines and surrounding areas southern italy seismotectonic implications abstract we analyzed the instrumental seismicity of southern italy in the area including the lucanian apennines and bradano foredeep making use of the most recent seismological data base available so far p and s wave arrival times recorded by the italian national seismic network rsnc operated by the istituto nazionale di geofisica e vulcanologia ingv were re picked along with those of the saptex temporary array deployed in the region in the period 2001 2004 for some events located in the upper val d agri we also used data from the eni agip oil company seismic network we examined the seismicity occurred during the period between 2001 and 2006 considering 514 events with magnitudes m 2 0 we computed the v p v s ratio obtaining a value of 1 83 and we carried out an analysis for the one dimensional 1d velocity model that approximates the seismic structure of the study area earthquakes were relocated and for well recorded events we also computed 108 fault plane solutions finally using 58 solutions the most constrained we computed regional stress field in the study area earthquake distribution shows three main seismic regions the westernmost lucanian apennines characterized by high background seismicity mostly with shallow hypocenters the easternmost below the bradano foredeep and the murge with deeper and more scattered seismicity and finally the more isolated and sparse seismicity localized in the sila range and in the offshore area along the northeastern calabrian coast focal mechanisms computed in this work are in large part normal and strike slip solutions and their tensional axes t axes have a generalized ne sw orientation the denser station coverage allowed us to improve hypocenters determination compared to those obtained by using only rsnc data for a better characterization of the crustal and subcrustal seismicity in the study area

long term monitoring of ulf electromagnetic fields at parkfield california 1 electric and magnetic fields in the 10 4 1 0 hz band were monitored at two sites adjacent to the san andreas fault near parkfield and hollister california from 1995 to 2007 a data window 2002 2005 enclosing the 28 september 2004 m6 parkfield earthquake was analyzed to determine if anomalous electric or magnetic fields or changes in ground conductivity occurred before the earthquake the data were edited removing intervals of instrument malfunction leaving 875 days in the 4 year period frequent spikelike disturbances were common but were not more frequent around the time of the earthquake these were removed before subsequent processing signal to noise amplitude spectra estimated via magnetotelluric processing showed the behavior of the ultralow frequency fields to be remarkably constant over the period of analysis these first order plots make clear that most of the recorded energy is coherent over the spatial extent of the array three main statistical techniques were employed to separate local anomalous electrical or magnetic fields from the dominant coherent natural fields transfer function estimates between components at each site were employed to subtract the dominant field and look deeper at the residual fields the data were decomposed into principal components to identify the dominant coherent array modes and the technique of canonical coherences was employed to distinguish anomalous fields which are spatially broad from anomalies which occur at a single site only and furthermore to distinguish anomalies present in both the electric and magnetic fields from those present in only one field type standard remote reference apparent resistivity estimates were generated daily at parkfield a significant seasonal component of variability was observed suggesting local distortion due to variations in near surface resistance in all cases high levels of sensitivity to subtle electromagnetic effects were demonstrated but no effects were found that can be reasonably characterized as precursors to the parkfield earthquake

melt season duration on canadian arctic ice caps 2000 2004 1 the extent and duration of summer melt on ice caps in the queen elizabeth islands qei nunavut canada in 2000 2004 were mapped using enhanced resolution quikscat qscat scatterometer images the mean melt duration depends mainly on surface elevation and distance from baffin bay over most ice caps inter annual variations in melt duration and the variation in melt duration with elevation are closely related to variations in the july 500 hpa height over the qei pressure related variability in the vertical gradient of near surface air temperature appears to be a major control on the inter annual variations in average melt duration

upper crustal structure seismicity and pore pressure variations in an extensional seismic belt through 3 d and 4 d vp and vp vs models the example of the val d agri area southern italy 1 we use local earthquake tomography and background seismicity to investigate static and transient features of the crustal velocity structure in the val d agri southern apennines italy one of the regions in central mediterranean with the highest seismogenic potential the upper crust is dominated by two broad high velocity anticlines of the buried apulia carbonate platform ramping on two parallel high angle thrusts interpreted as preexisting inverted normal faults the deep core of the anticlines consists of very high vp up to 6 9 km s and low vp vs rocks suggesting the involvement of the apulian crystalline basement in the apennine belt these results provide valuable constraints on the apennine belt tectonic evolution supporting a thick skinned interpretation for the pliocene terminal phase of the compressional tectonics the geometry of the val d agri quaternary basin is controlled by these inherited compressive features whereas the presently active extensional tectonics barely reworked the structure we find inconsistency between the structure of the apulia carbonate platform and the location and geometry of the quaternary normal faults mapped at the surface this suggests either the immaturity of the normal faults or their secondary role in accommodating the extension we observe spatiotemporal 4 d changes of vp and vp vs models defining transient variations of pore fluid pressure in the upper crust a strong change in the vp vs ratio heralds a raise in the seismicity rate that can be related to large water level changes in a nearby artificial lake this evidence is consistent with a mechanism of reservoir induced seismicity by fluid pressure increase and pore pressure diffusion the 4 d velocity variations are confined in the shallow portion of the upper crust 3 6 km depth where fluids are stored in a highly fractured medium pore pressure fluctuations can affect the strength of fault segments favoring seismicity rate changes along the active faults and possibly promoting large future earthquakes

geophysical applications of electrokinetic conversion in volume 1 of geophysics 1936 r r thompson of humble oil and refining company proposed that the coupling of seismic and electrical energy can be used as an exploration tool subsequent efforts were directed toward using electric field antennas as geophones small scale studies concluded that antennas are not as sensitive as conventional geophones and their response is highly variable depending on the local soil conditions by 1959 martner and sparks of pan american petroleum reported in geophysics a systematic study of seismoelectric coupling using explosives at various depths their results were the first to show that the conversion of seismic to electromagnetic energy at the water table could be detected using surface antennas no studies have systematically explored the full potential of electroseismic methods using the same level of experimental sophistication used in seismic exploration

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

seismomagnetic effects from the long awaited 28 september 2004 m 6 0 parkfield earthquake precise measurements of local magnetic fields have been obtained with a differentially connected array of seven synchronized proton magnetometers located along 60 km of the locked to creeping transition region of the san andreas fault at parkfield california since 1976 the m 6 0 parkfield earthquake on 28 september 2004 occurred within this array and generated coseismic magnetic field changes of between 0 2 and 0 5 nt at five sites in the network no preseismic magnetic field changes exceeding background noise levels are apparent in the magnetic data during the month week and days before the earthquake or expected in light of the absence of measurable precursive deformation seismicity or pore pressure changes obser vations of electric and magnetic fields from 0 01 to 20 hz are also made at one site near the end of the earthquake rupture and corrected for common mode signals from the ionosphere magnetosphere using a second site some 115 km to the northwest along the fault these magnetic data show no indications of unusual noise before the earthquake in the ulf band 0 01 20 hz as suggested may have preceded the 1989 ml 7 1 loma prieta earthquake nor do we see electric field changes similar to those suggested to occur before earthquakes of this magnitude from data in greece uni form and variable slip piezomagnetic models of the earthquake derived from strain displacement and seismic data generate magnetic field perturbations that are con sistent with those observed by the magnetometer array a higher rate of longer term magnetic field change consistent with increased loading in the region is apparent since 1993 this accompanied an increased rate of secular shear strain observed on a two color edm network and a small network of borehole tensor strainmeters and increased seismicity dominated by three m 4 5 5 earthquakes roughly a year apart in 1992 1993 and 1994 models incorporating all of these data indicate increased slip at depth in the region and this may have played a role in the final occurrence of the 28 september 2004 m 6 0 parkfield earthquake the absence of electric and magnetic field precursors for this and other earthquakes with m 5 7 3 elsewhere in the san andreas fault system indicates useful prediction of damaging earthquakes seems unlikely using these electromagnetic data

a major seismogenic fault in a silent area the castrovillari fault southern apennines italy summary large historical earthquakes in italy define a prominent gap in the pollino region of the southern apennines geomorphic and palaeoseismological investigations in this region show that the castrovillari fault cf is a major seismogenic source that could potentially fill the southern part of this gap the surface expression of the cf is a complex 10 13 km long set of prominent scarps trenches across one scarp indicate that at least four surface faulting earthquakes have occurred along the cf since late pleistocene time each producing at least 1 m of vertical displacement the length of the fault and the slip per event suggest m 6 5 7 0 for the palaeoearthquakes preliminary radiocarbon dating coupled with historical considerations imply that the most recent of these earthquakes occurred between 380 bc and 1200 ad and probably soon after 760 ad no evidence for this event has been found in the historical record we estimate a minimum recurrence interval of 1170 years and a vertical slip rate of 0 2 0 5 mm yr for the cf which indicates that the seismic behaviour of this fault is comparable to other major seismogenic faults of the central southern apennines the lack of mention or the mislocation of the most recent event in the historical seismic memory of the pollino region clearly shows that even in italy which has one of the longest historical records of seismicity a seismic hazard assessment based solely on the historical record may not be completely reliable and shows that geological investigations are critical for filling possible information gaps

the role of snow on the thermal dependence of microwave backscatter over sea ice our understanding of snow distribution in the polar regions is severely restricted owing to the heterogeneity both in space and time of this solid precipitate processes such as vapor and mass fluxes across the interface are to a large extent controlled by the presence and geophysical state of the snow cover on sea ice numerous studies have shown the importance of snow cover in ecosystem processes and particularly in photosynthetically active radiation extinction researchers are currently exploiting developments in electromagnetic interaction theory in an attempt to measure snow thickness distributions remotely in this paper we investigate the dependence of radar backscatter on snow thickness over smooth first year ice we use data from the seasonal sea ice monitoring and modeling site located in the canadian archipelago results show that the thermodynamics of the snow cover affect wave propagation attenuation and scattering through the control that brine volume exerts on interfacial characteristics of the snow and ice layers the effect is subtle and specific to certain ranges of salinity surface roughness and thickness of sea ice we describe the phenomenon responsible for this effect using a microwave model consistently for both complex effective permittivities and backscatter coefficients coupled to a one dimensional model we validate the physical principles using in situ field data we then discuss the potential of synthetic aperture radar in estimating snow thickness distributions under these specific conditions using both observed and modeled microwave scattering

developing clear sky cloud and cloud shadow mask for producing clear sky composites at 250 meter spatial resolution for the seven modis land bands over canada and north america abstract a new technology was developed at the canada centre for remote sensing ccrs for generating canada wide and north america continental scale clear sky composites at 250 m spatial resolution for all seven modis land spectral bands b1 b7 the modis level 1b mod02 swath level data are used as input to circumvent the problems with image distortion in the mid latitude and polar regions inherent to the global sinusoidal sin projection utilized for the standard modis data products the modis 500 m land bands b3 to b7 are first downscaled to 250 m resolution using an adaptive regression and normalization scheme for compatibility with the 250 m bands b1 and b2 a new method has been developed to produce the mask of clear sky cloud and cloud shadow at 250 m resolution it shows substantial advantages in comparison with the modis 250 m standard cloud masks the testing of new cloud mask showed that it is in reasonable agreement with the modis 1 km standard product once it is aggregated to 1 km scale while the cloud shadow detection looks more reliable with the new methodology nevertheless more quantitative analyses of the presented scene identification technique are required to understand its performance over the range of input scenes in various seasons the new clear sky compositing scheme employs a scene dependent decision matrix it is demonstrated that this new scheme provides better results than any others based on a single compositing criterion such as maximum ndvi or minimum visible reflectance to account for surface bi directional properties two clear sky composites for the same time period are produced by separating backward scattering and forward scattering geometries which separate pixels with the sun satellite relative azimuth angles within 90 270 and outside of this range comparison with landsat imagery and with modis standard composite products demonstrated the advantage of the new technique for screening cloud and cloud shadow and generating high spatial resolution modis clear sky composites the new data products are mapped in the lambert conformal conic lcc projection for canada and the lambert azimuthal equal area laea projection for north america presently this activity is limited to modis terra due to known problems with band to band registration and noisy swir channels on modis aqua

arctic sea ice retreat in 2007 follows thinning trend the minimum of arctic sea ice extent in the summer of 2007 was unprecedented in the historical record a coupled ice ocean model is used to determine the state of the ice and ocean over the past 29 yr to investigate the causes of this ice extent minimum within a historical perspective it is found that even though the 2007 ice extent was strongly anomalous the loss in total ice mass was not rather the 2007 ice mass loss is largely consistent with a steady decrease in ice thickness that began in 1987 since then the simulated mean september ice thickness within the arctic ocean has declined from 3 7 to 2 6 m at a rate of 0 57 m decade 1 both the area coverage of thin ice at the beginning of the melt season and the total volume of ice lost in the summer have been steadily increasing the combined impact of these two trends caused a large reduction in the september mean ice concentration in the arctic ocean this created conditions during the summer of 2007 that allowed persistent winds to push the remaining ice from the pacific side to the atlantic side of the basin and more than usual into the greenland sea this exposed large areas of open water resulting in the record ice extent anomaly

mechanical and electrical response due to fluid pressure equilibration following an earthquake 1 the mechanical and electrical response in a uniform porous crust are determined following a shear dislocation earthquake on an internal slip surface a uniform crust is studied because many exact analytical relations hold between the various response fields in this case the initial stress field that is created immediately following the earthquake subsequently relaxes through time as the fluid pressure equilibrates by fluid flow maps of the electric field generated due to the fluid flow electrokinetic coupling are presented using conservative estimates of the various parameters involved electric fields significantly larger than the diurnal magnetotelluric fields are generated quantitative results for how various components of the stress tensor change due to the fluid equilibration are also presented the coulomb stress can easily change by 100 and more in the months following an earthquake due to the fluid pressure equilibration