iceberg calving flux and mass balance of the austfonna ice cap on nordaustlandet svalbard 1 satellite radar interferometry 60 mhz airborne ice penetrating radar data and visible band satellite imagery were used to calculate the velocity structure ice thickness and the changing ice marginal extent of austfonna 8000 km2 and 2500 km3 the largest ice cap in the eurasian arctic ice cap motion is generally less than about 10 m a 1 except where faster flowing curvilinear features with velocities of several tens to over 200 m a 1 are present most drainage basins of austfonna have undergone ice marginal retreat over the past few decades at an average of a few tens of meters per year integrating margin change around the whole ice cap gives a total area loss of about 10 km2 a 1 iceberg flux from the marine margins of austfonna is about 2 5 0 5 km3 a 1 water equivalent about 45 of the total calving flux from the whole svalbard archipelago when mass loss by iceberg production is taken into account the total mass balance of austfonna is negative by between about 2 5 and 4 5 km3 a 1 this iceberg flux represents about 33 5 of total annual mass loss from austfonna with the remainder lost through surface ablation iceberg flux should be included in calculations of the total mass balance of the many large arctic ice caps including those located in the russian and canadian arctic that end in tidewater the neglect of this term has led to underestimates of mass loss from these ice caps and thus to underestimates of the contribution of arctic ice caps to global sea level rise

variations of apparent stresses and stress drops prior to the earthquake of 6 may 1984 mb 5 8 in the adak seismic zone abstract on 6 may 1984 a m b 5 8 earthquake occurred within a subregion of the adak seismic zone that had shown clustering of m b 4 5 earthquakes during the last 3 yr this region has been examined for premonitory variations in stress drop and apparent stresses calculated from the s wave spectra of 69 microearthquakes m d 13 nm decrease with decreasing moment

stress drop for three m 4 3 4 7 1992 1994 parkfield ca earthquakes in a previous paper fletcher and spudich 1998 we determined the slip distribution for the three m 4 3 to 4 7 parkfield events that occurred between oct 20 1992 and dec 20 1994 the rupture characteristics of these events are of particular interest because they are the largest events to occur at parkfield since the 1966 mainshock to further investigate the rupture process of these events we compute the static stress change or stress drop caused by the slip using a boundary integral method from quin and das 1989 adapted for the forward modeling case of computing stresses from a static slip distribution we find that the initial phase at or very near the hypocenter has the largest stress change in all cases this peak has a value of about 3 8 mpa for the oct 20 1992 event which also has the smallest magnitude of the three at 4 3 and about 45 mpa for the other nov 14 1993 and 60 mpa for the dec 20 1994 event the rest of the stress release is comparatively small and smooth for the nov 14 1993 event which is the deepest event and the most impulsive the pattern of stress release of the other two events oct 20 1992 and dec 20 1994 is more complex with a greater spatial extent the spatial distribution of stress is approximately similar to the spatial distribution of slip but with the larger initial peak in stress drop at the hypocenter

the response of black rapids glacier alaska to the denali earthquake rock avalanches 1 we describe the impact of three simultaneous earthquake triggered rock avalanches on the dynamics of black rapids glacier alaska by using spaceborne radar imagery and numerical modeling we determined the velocities of the glacier before and after landslide deposition in 2002 by using a combination of ers 1 ers 2 tandem radarsat 1 and alos palsar synthetic aperture radar data ice velocity above the debris covered area of the glacier increased up to 14 after the earthquake but then decreased 20 by 2005 within the area of the debris sheets mean glacier surface velocity increased 44 within 2 years of the landslides at the downglacier end of the lowest landslide where strong differential ablation produced a steep ice cliff velocities increased by 109 over the same period by 2007 ice velocity throughout the debris area had become more uniform consistent with a constant ice flux resulting from drastically reduced ablation at the base of the debris without further analysis we cannot prove that these changes resulted from the landslides because black rapids glacier displays large seasonal and interannual variations in velocity however a full stokes numerical ice flow model of a simplified glacier geometry produced a reversal of the velocity gradient from compressional to extensional flow after 5 years which supports our interpretation that the recent changes in the velocity field of the glacier are related to landslide induced mass balance changes

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

the earthquake prediction experiment at parkfield california since 1985 a focused earthquake prediction experiment has been in progress along the san andreas fault near the town of parkfield in central california parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years the most recent a magnitude 6 event in 1966 the probability of another moderate earthquake soon appears high but studies assigning it a 95 chance of occurring before 1993 now appear to have been over simplified the identification of a parkfield fault segment was initially based on geometric features in the surface trace of the san andreas fault but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths on the other hand geodetic measurements are consistent with the existence of a locked patch on the fault beneath parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake a magnitude 4 7 earthquake in october 1992 brought the parkfield experiment to its highest level of alert with a 72 hour public warning that there was a 37 chance of a magnitude 6 event however this warning proved to be a false alarm most data collected at parkfield indicate that strain is accumulating at a constant rate on this part of the san andreas fault but some interesting departures from this behavior have been recorded here we outline the scientific arguments bearing on when the next parkfield earthquake is likely to occur and summarize geophysical observations to date

influence of ionospheric electron density fluctuations on satellite radar interferometry evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry sri obtained with the radarsat ers and jers 1 satellites fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar sar imagery which can be detected using sri measurements of azimuth shift in sri can help to differentiate ionospheric from tropospheric propagation problems and to understand better the impact of the ionosphere on spaceborne sar further sri azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances

combining airborne photographs and spaceborne sar data to monitor temperate glaciers potentials and limits monitoring temperate glacier activity has become more and more necessary for economical and security reasons and as an indicator of the local effects of global climate change remote sensing data provide useful information on such complex geophysical objects but they require specific processing techniques to cope with the difficult context of moving and changing features in high relief areas this paper presents the first results of a project involving four laboratories developing and combining specific methods to extract information from optical and synthetic aperture radar sar data two different information sources are processed namely 1 airborne photography and 2 spaceborne c band sar interferometry the difficulties and limitations of their processing in the context of alpine glaciers are discussed and illustrated on two glaciers located in the mont blanc area the results obtained by aerial triangulation techniques provide digital terrain models with an accuracy that is better than 30 cm which is compatible with the computation of volume balance and useful for precise georeferencing and slope measurement updating the results obtained by sar differential interferometry using european remote sensing satellite images show that it is possible to measure temperate glacier surface velocity fields from october to april in one day interferograms with approximately 20 m ground sampling this allows to derive ice surface strain rate fields required to model the glacier flow these different measurements are complementary to results obtained during the summer from satellite optical data and ground measurements that are available only in few accessible points

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

three dimensional surface velocities of storstrommen glacier greenland derived from radar interferometry and ice sounding radar measurements non steady state vertical velocities of up to 5 m y 1 exceed the vertical surface parallel flow components over much of the ablation area of storstrommen a large outlet glacier from the east greenland ice sheet neglecting a contribution to the vertical velocity of this magnitude results in substantial errors up to 20 also on the south north component of horizontal velocities derived by satellite synthetic aperture radar interferometry insar measurements in many glacier environments the steady state vertical velocity component required to balance the annual ablation rate is 5 to 10 m y 1 or more this indicates that the surface parallel flow assumption may be problematic also for glaciers in steady state here we derive the three dimensional surface velocity distribution of storstrommen by using the principle of mass conservation to combine insar measurements from ascending and descending satellite tracks with airborne ice sounding radar measurement of ice thickness the results are compared to insar velocities previously derived by using the assumption of surface parallel flow and to velocities obtained by inin situ gps measurements the velocities derived by using the principle of mass conservation are in better agreement with the gps velocities than the previously calculated velocities derived with the assumption of surface parallel flow

remote sensing of the albedo and detection of the slush line on the greenland ice sheet advanced very high resolution radiometer images of a section of the greenland ice sheet western part between 64 5 and 70 5 n with a size of 667 n s by 700 w e km were processed in order to retrieve the surface albedo the images were for the summer seasons of 1990 1997 inclusive and results were compared to simultaneous mass balance measurements carried out along the transect 67 n the main findings are as follows first many images show a distinct transition from low spatial variability of the albedo at higher elevations to higher spatial variability of the albedo at lower elevations it is argued that these transitions most likely correspond to transitions at the surface slush line in short from the area that is uniformly covered by snow to a mosaic of snow patches slush and ice second 70 of the interannual variations in the mean specific mass balance can be explained by interannual variations in the maximum elevation of the slush line i e the slush limit however a higher percentage 82 can be explained by interannual variations in the sum of positive degree days third in 3 years with a relatively large amount of melt the inland migration of the slush line stopped or slowed considerably down at a fixed distance from the ice sheet margin we believe that this so called maximum slush line coincides with an abrupt change in the density profile the maximum slush line forms an important limitation for the use of satellite data to monitor interannual variations in the mass balance its position is a climate indicator and was determined for the entire section of the greenland ice sheet investigated

steady state three dimensional ice flow over an undulating base first order theory with linear ice rheology on considere un ecoulement stationnaire d epaisseur constante en utilisant le rapport d amplitude des oscillations basales et de l epaisseur de la glace en tant que parametre de perturbation on etablit et on resout les equations du premier ordre des perturbations de vitesse et de pression

ice sheet change detection by satellite image differencing abstract differencing of digital satellite image pairs highlights subtle changes in near identical scenes of earth surfaces using the mathematical relationships relevant to photoclinometry we examine the effectiveness of this method for the study of localized ice sheet surface topography changes using numerical experiments we then test these results by differencing images of several regions in west antarctica including some where changes have previously been identified in altimeter profiles the technique works well with coregistered images having low noise high radiometric sensitivity and near identical solar illumination geometry clouds and frosts detract from resolving surface features the etm sensor on landsat 7 ali sensor on eo 1 and modis sensor on the aqua and terra satellite platforms all have potential for detecting localized topographic changes such as shifting dunes surface inflation and deflation features associated with sub glacial lake fill drain events or grounding line changes availability and frequency of modis images favor this sensor for wide application and using it we demonstrate both qualitative identification of changes in topography and quantitative mapping of slope and elevation changes

a method for monitoring glacier mass balance using satellite albedo measurements application to vatnajskull iceland we compare satellite albedo images of vatnajokull iceland with mass balance measurements for the years 1991 99 we find that the equilibrium line is mostly not visible when it is located above its position of the previous year s equilibrium line detection is further hindered by clouds and a gradual transition between ice and firn or snow consequently firn line elevation at the end of the melting season is not particularly useful for estimating the annual mass balance instead we propose to study the mean albedo of the entire ice cap throughout the melting season so that all available information about the surface albedo is taken into account the mean net potential global radiation which can be estimated from the mean surface albedo alone both depends on and influences summer melt it also depends on winter precipitation and integrated over the melting season is found to relate linearly to the specific mass balance b r 0 87 and 0 94 for different outlets of vatnajokull b can be estimated quantitatively when this relation is known and qualitatively when it is not the uncertainty in the satellite derived value of b is 0 5 0 8 m w e which for vatnajokull corresponds to about 27 of the interannual variability of b

spectral magnitudes magnitude spectra and earthquake quantification the stability issue of the corner period and of the maximum magnitude for a given earthquake abstract p wave magnitude spectra of a foreshock mainshock aftershock sequence on 7 and 8 may 1986 in the aleutian islands are determined using broad band seismograms recorded at the grf array the magnitude spectra are compared for different stations of the array the maximum magnitude is stable with a standard deviation below 1 the reliability of other source parameters fault length seismic moment and stress drop derived from the magnitude spectrum m f depends on the determination of the corner period different methods to derive the corner period are compared best suited and most stable is the one based on the formula t c 0 e s f df 0 e s f f df where es f 102mf 14 is the energy density spectrum of the p wave and mf the spectral p wave magnitude of the earthquake at the respective period

shortwave broadband black sky surface albedo estimation for arctic sea ice using passive microwave radiometer data 1 a new remote sensing method to estimate the optical broadband black sky surface albedo using passive microwave radiometer data has been developed in this research the advanced very high resolution radiometer avhrr based shortwave broadband black sky albedos processed using the algorithms of the new surface broadband albedo cm saf product sal and the advanced microwave scanning radiometer amsr e microwave data from the national snow and ice data center boulder colorado have been employed to analyze the correspondence between avhrr based optical albedo and microwave brightness temperature amsr e frequencies of 6 9 18 7 and 36 5 ghz have been tested by fitting a third degree polynomial curve to the sal albedo and the amsr e data points the best correlation occurs in 6 9 ghz vertical polarization brightness temperature r2 0 92 to illustrate and compare the spatial variabilities of the sal and amsr e albedos of the sea ice during the melting and refreezing period maps of 13 weeks have been prepared of both albedos from june to august 2007 the albedo time series from amsr e and avhrr data are calculated for the combined sea ice and open water cover for the northern hemisphere as a whole and for six specific sea ice regions the arctic ocean the kara and barents seas the greenland sea the labrador sea hudson bay and the canadian archipelago the standard errors between the optical and passive microwave estimates varied from about 0 001 greenland sea to 0 04 canadian archipelago being 0 013 on the average in absolute units the relative standard errors are then smaller than 5 in most of the regions

development and validation of a snow albedo algorithm for the modis instrument a prototype snow albedo algorithm has been developed for the moderate resolution imaging spectroradiometer modis it complements existing modis products by providing albedo measurements for areas mapped as snow on a global daily basis by modis cloud detection and atmospheric correction are accomplished using existing modis products models of the bidirectional reflectance of snow created using a discrete ordinate radiative transfer disort model are used to correct for anisotropic scattering effects over non forested surfaces initial algorithm validation is undertaken through comparisons with broadband albedo measurements made at the u s national oceanic and atmospheric administration noaa surface radiation budget network surfrad site in fort peck mt in situ surfrad albedo measurements are compared to daily modis snow albedo retrievals for the period 21 26 november 2000 created from five narrow to broadband albedo conversion schemes the prototype modis algorithm produces reasonable broadband albedo estimates maximum daily differences between the five modis broadband albedo retrievals and in situ albedo are 15 daily differences between the best modis broadband estimate and the measured surfrad albedo are 1 8 however no single conversion scheme consistently provides the closest albedo estimate further validation and algorithm development using data from north america and greenland is ongoing

quantification of everest region glacier velocities between 1992 and 2002 using satellite radar interferometry and feature tracking quincey d j luckman a benn d 2009 quantification of everest region glacier velocities between 1992 and 2002 using satellite radar interferometry and feature tracking journal of glaciology 55 192 596 606 sponsorship knowledge transfer project no 3742

atmospheric propagation heterogeneities revealed by ers 1 interferometry spaceborne synthetic aperture radar sar repeat pass interferometry is a technique based on the exploitation of the interference pattern of two sar images acquired in similar geometric conditions at two different dates this measure contains geometric atmospheric topographic and land deformation information in this paper we focus on the atmospheric information first we show theoretically that an adequate processing is sufficient to remove homogeneous atmospheric effects and that heterogeneities as tropospheric turbulences rain falls and ionospheric phenomena can be detected with sar interferometry then we interpret several observations and we present interferometric images of cumulus and turbulences due to relief sar interferometry could be a new remote sensing tool for the study of tropospheric turbulences at a high horizontal resolution 40 m

estimating the mass balance of vatnajokull iceland from noaa avhrr imagery we investigate the possibility of obtaining the mass balance of vatnajo kull iceland from us national oceanic and atmospheric administration noaa advanced very high resolution radiometer avhrr albedo images for the years 1991 2002 de ruyter de wildt and others 2002 demonstrated that the mean potential absorbed radiation averaged over the melting season correlates well with the mean specific mass balance and that can be estimated from the evolution of the surface albedo here we improve the retrieval method of de ruyter dewildt and others 2002 by introducing the more realistic 6s atmospheric transfer model and by adding the latest narrowto broadband ntb albedo conversion equations bidirectional reflectance distribution functions for both ice and snow are used where appropriate we show that the choice of the ntb conversion equations greatly influences the calculated measured mass balance correlates well with if enough cloud free images can be found through the year and if the spatial variation in measured mass balances is high enough the correlation coefficient for all drainage basins combined is 0 92 with a residual standard deviation of 0 18mw e we present an estimated mass balance series for the whole of vatnajo kull based on our findings switching between different avhrr instruments over time may cause serious errors in the calculated mass balance

measuring snow and glacier ice properties from satellite satellite remote sensing is a convenient tool for studying snow and glacier ice allowing us to conduct research over large and otherwise inaccessible areas this paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing these methods have been improving with the use of new satellite sensors like the synthetic aperture radar sar during the last decade leading to the development of new and powerful methods such as sar interferometry for glacier velocity digital elevation model generation of ice sheets or snow cover mapping some methods still try to overcome the limitations of present sensors but future satellites will have much increased capability for example the ability to measure the whole optical spectrum or sar sensors with multiple polarization or frequencies among the methods presented are the satellite derived determination of surface albedo snow extent snow volume snow grain size surface temperature glacier facies glacier velocities glacier extent and ice sheet topography in this review emphasis is put on the principles and theory of each satellite remote sensing method an extensive list of references with an emphasis on studies from the 1990s allows the reader to delve into specific topics

forecast model for moderate earthquakes near parkfield california earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation in the forecast procedure proposed here repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters the early part of the model simulation corresponds to the available field data and the subsequent part constitutes an estimate of future faulting and ground deformation in particular the time location and size of unstable faulting are estimates of the pending earthquake parameters the forecast accuracy depends on the model realism and parameter resolution the forecast procedure is applied to fault creep and trilateration data measured near parkfield california where at least five magnitude 5 5 to 6 earthquakes have occurred regularly since 1881 the last in 1966 the quasi static model consists of a flat vertical plane embedded in an elastic half space spacially variable fault slip of strike slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle strain softening fault the field data are consistent with these approximate values of patch parameters radius of 3 km patch center 5 km deep and 8 km southeast of the 1966 epicenter and maximum brittle strength of 26 bars fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21 8 year recurrence interval however the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake as predicted by the model

geometric calibration of ers satellite sar images geometric calibration of the european remote sensing ers satellite synthetic aperture radar sar slant range images is important in relation to mapping areas without ground reference points and also in relation to automated processing the relevant sar system parameters are discussed and calibrated by using the european space agency esa transponders at flevoland the resulting accuracy of the slant range images corresponds to 10 m horizontally on the ground the results are verified by using runway intersections and corner reflectors surveyed with differential gps techniques based on a seven year ers 1 and a four year ers 2 time series the long term stability is found to be sufficient to allow a single calibration covering the entire mission period a descending and an ascending orbit tandem pair of the esa calibration site on flevoland suitable for calibration of ers sar processors is described to allow other researchers to geometrically calibrate their processing systems

ice sheet motion and topography from radar interferometry both topography and motion information are present in repeat pass ers 1 interferograms over ice sheets the authors demonstrate that the topography is separable from the surface displacement field when a sequence of radar images are available if the velocity field is constant over the time span of observation the topography can be derived from differential interferograms formed from sequential observations with this measurement a pure displacement field can then be obtained by removal of the topographic contribution to the interferometric phase at each pixel further they discuss how the vertical and horizontal components of displacement affect the interferometrically derived motion field they illustrate their approach with four successive 3 day repeat ers 1 images of a flow feature in northeastern greenland

a spatially adjusted elevation model in dronning maud land antarctica based on differential sar interferometry in this paper a new digital elevation model dem is derived for the ice sheet in western dronning maud land antarctica it is based on differential interferometric synthetic aperture radar sar from the european remote sensing 1 2 ers 1 2 satellites in combination with icesat s geoscience laser altimeter system glas a dem mosaic is compiled out of 116 scenes from the ers 1 ice phase in 1994 and the ers 1 2 tandem mission between 1996 and 1997 with the glas data acquired in 2003 that served as ground control using three different sar processors uncertainties in phase stability and baseline model resulting in height errors of up to 20 m are exemplified atmospheric influences at the same order of magnitude are demonstrated and corresponding scenes are excluded for validation of the dem mosaic covering an area of about 130 000 km2 on a 50 m grid independent icesat heights 2004 2007 ground based kinematic gps 2005 and airborne laser scanner data als 2007 are used excluding small areas with low phase coherence the dem differs in mean and standard deviation by 0 5 10 1 1 1 6 4 and 3 1 4 0 m from icesat gps and als respectively the excluded data points may deviate by more than 50 m in order to suppress the spatially variable noise below a 5 m threshold 18 of the dem area is selectively averaged to a final product at varying horizontal spatial resolution apart from mountainous areas the new dem outperforms other currently available dems and may serve as a benchmark for future elevation models such as from the tandem x mission to spatially monitor ice sheet elevation

integrated satellite interferometry tropospheric noise gps estimates and implications for interferometric synthetic aperture radar products interferometric synthetic aperture radar insar like other astronomic and space geodetic techniques is limited by the spatially and temporally variable delay of electromagnetic waves propagating through the neutral atmosphere statistical analysis of these variations from a wide variety of instruments reveals a power law dependence on frequency that is characteristic of elementary kolmogorov turbulence a statistical model for a major component of the delay fluctuations the wet component has previously been developed by treuhaft and lanyi 1987 for very long baseline interferometry a continuous global positioning system gps network is now in place in southern california that allows estimation of along with geodetic parameters the total delay due to the atmosphere above each site on a subhourly basis these measurements are shown to conform to the treuhaft and lanyi tl statistical model both temporally and spatially the tl statistical model is applied to the problem of insar and used to produce the covariance between two points separated in time and or space the error due to the atmospheric variations for sar products such as topography and surface deformation is calculated via propagation of errors there are two methods commonly cited to reduce the effect of atmospheric distortion in products from sar interferometry stacking and calibration stacking involves averaging independent interferograms to reduce the noise calibration involves removing part or all of the delay using data from an independent source such as total zenith delay estimates from continuous gps networks despite the relatively poor spatial density of surface measurements calibration can be used to reduce noise if the measurements are sufficiently accurate reduction in tropospheric noise increases with increasing number of measurement points and increasing accuracy up to a maximum of n where n is the number of points stacking and calibration are shown to be complementary and can be used simultaneously to reduce the noise to below that achievable by either method alone

continuous swarm like seismicity the wanganui new zealand earthquakes summary during the last 40 years of seismic observation earthquakes have occurred persistently near wanganui in the western part of the north island displaying continuous swarm like seismicity altogether 445 shallow earthquakes of magnitude above 3 5 have been found for the period 1940 1980 and only three shocks reached magnitude slightly above 5 a large majority of small shocks are located within the compact central zone and their composite focal mechanisms seem to indicate extensional normal faulting fewer but larger events are scattered outside the central zone and the mechanism of the three largest shocks implies compressional thrust faulting a continuous decrease in source size and increase in stress drop as well as a longterm decreasing tendency in b values and increasing tendency in the rate of occurrence are observed small earthquakes in the central zone might be considered as a response to local stresses probably of thermal origin and the larger shocks as a response to stress redistribution in the central zone and to stresses resulting from sub duction processes at the boundary between the indian and pacific plates the time variations of the source parameters and b values might imply that stress accumulation on a large scale is in progress in this case the wanganui activity would be situated at the northern limit of the seismic gap

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

bidirectional anisotropic reflectance of snow and sea ice in avhrr channel 1 and channel 2 spectral regions ii correction applied to imagery of snow on sea ice for pt i see ibid vol 37 no 1 p 543 54 1999 advanced very high resolution radiometer avhrr images acquired over the arctic ocean often show strong bidirectional reflectance of snow on sea ice the observed anisotropic reflectance characteristics of these images are consistent with the theoretical analysis presented in part i of this study the anisotropic reflectance at the top of atmosphere toa is simulated by radiative transfer modeling and the results show a good model observation agreement based on these modeling results a method was developed to correct the effect of anisotropic reflectance on avhrr images in channels 1 and 2 results show the method is effective and efficient comparisons of toa snow reflectance before and after anisotropic correction show that the systematic and large variation of snow reflectance across the scan lines of an image due to satellite viewing illumination geometry can either be eliminated or greatly reduced by applying the correction algorithm

analysis of peak ground motion in terms of a model of inhomogeneous faulting measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors with local magnitudes ranging from 0 8 to 6 4 have been analyzed in terms of a model of inhomogeneous faulting the fault model involves the failure of a circular asperity of radius ri surrounded by a previously faulted annular region of outer radius r0 the failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1 ri followed by lower frequency radiation proportional to 1 r0 as static equilibrium is regained over the larger region for r0 ri 1 both v and a are associated primarily with the failure of the asperity but also depend on the large scale source parameters specifically v r0 r 0 10 r0 ri 0 15 and a r 0 30 r0 ri 2 0 45 where is the shear wave velocity is the overall stress drop is the modulus of rigidity and is density the terms involving r0 ri correspond to the high frequency radiation associated with the failure of the asperity and the other terms indicate the peak parameters due to the broad scale readjustment observations of peak ground motion for events with seismic moments ranging from 5 1016 to 1026 dyn cm indicate that r0 ri is normally in the range of 1 10 and appears to be independent of earthquake size the inhomogeneous fault model also yields convenient expressions for the small scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip

a low cost glacier mapping system an old portable 60 mhz radar has been upgraded with a new digital data processing and acquisition system and a new antenna construction enabling a fast and low cost installation on atwin otter aircraft augmented by a laser altimeter and kinematic global positioning system gps the system has the capability of acquiring accurate data on location and ice surface elevation and adequate quality data on ice thickness the system has been applied successfully in mapping the nioghalvfjerdsfjorden glacier northeast greenland in spite of the difficult conditions with melting water on the glacier surface the measurements from the floating part of the glacier have been evaluated by comparison of radar data with laser altimeter and in situ measurements

assessment of the surface mass balance along the k transect greenland ice sheet from satellite derived albedos this paper explores the potential of using satellite derived albedos to estimate the surface mass balance of the kangerlussuaq transect k transect greenland ice sheet we first retrieved surface albedos from advanced very high resolution radar data by using among other techniques a new cloud detection algorithm based on the relation between brightness temperature and surface elevation we then computed the satellite derived mass balance bsat from the mean albedo for the transect by taking fixed values for atmospheric transmissivity and the longwave and turbulent fluxes we found that bsat explains 71 of the variance in 13 years of stake mass balance measurements bm our method also provides good estimates of the magnitude of the interannual variability in bm the performance of the method degrades considerably without correction for anisotropic reflection at the surface and recalibration of the satellite sensors with dry snow at the top of the ice sheet sensitivity tests indicate that the method s performance is hardly sensitive to uncertainties in parameters therefore we expect that the method could be successfully applied on other glaciers and parts of ice sheets and ice caps especially where accumulation rates are relatively small we show that the investigated method performs best just below the mean equilibrium line altitude

rupture process of the ms 6 6 superstition hills california earthquake determined from strong motion recordings application of tomographic source inversion abstract we analyze strong motion recordings of the m s 6 6 superstition hills earthquake to determine the timing location spatial extent and rupture velocity of the subevents that produced the bulk of the high frequency 0 5 to 4 hz seismic energy radiated by this shock the earthquake can be characterized by three principal subevents the largest ones occurring about 3 and 10 sec after initiation of rupture timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the superstition hills fault the two largest subevents display different directivity effects we apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance based on a one dimensional fault model the azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the superstition hills fault at a velocity about equal to the p wave velocity an alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent but it is not supported by the aftershock distribution the third subevent ruptured to the southeast along an 8 km long portion of the superstition hills fault at about the shear wave velocity this rupture propagation caused the relatively large accelerations and velocities observed in strong motion records for stations southeast of the hypocenter the long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development the southern half of the superstition hills fault did not generate significant high frequency strong ground motion although it showed substantial co seismic surface displacement the subevents are situated along the same northern portion of the fault where most of the aftershocks are located the locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth

are reservoir induced earthquakes of magnitude 5 0 at koyna india preceded by pairs of earthquakes of magnitude 4 0 foreshocks of koyna india earthquakes of magnitude 5 0 are investigated using data from the poona wwssn seismic station situated at a distance of 120 km from koyna schemes for estimating earthquake magnitude on the spz component seismogram at poona have been developed these schemes are applicable to earthquakes of magnitude 2 0 occurring in the koyna region earthquakes of magnitude 5 on 13 september 1967 10 december 1967 and 17 october 1973 in the koyna region were each preceded by two earthquakes of magnitude 4 0 occurring within the preceding 15 days in comparison the activity of september 1980 was different an earthquake of magnitude 5 0 occurred on 2 september and two more 5 0 earthquakes occurred on 20 september these earthquakes were not preceded by pairs of 4 0 events all events of magnitude 4 0 during 1974 through 1976 were singular no two such events occurred within 15 days of one another there appears to be a 50 per cent probability of occurrence of a magnitude 5 0 earthquake if two earthquakes of magnitude 4 0 are closely spaced in time within 15 days in the koyna region

classification of glacier zones in western greenland using albedo and surface roughness from the multi angle imaging spectroradiometer misr the greenland ice sheet has been the subject of mass balance and melt measurements over the past two decades and its margins have shown significant thinning in recent years surface characteristics of the ice sheet margins are strongly modified by the annual process of snow accumulation and melt in this work we explore spatial and temporal relationships between near infrared albedo and surface roughness from the multi angle imaging spectroradiometer misr aboard nasa s terra satellite our study area is a region in western greenland in the vicinity of jakobshavn glacier and its upland drainage basin we compute near infrared albedo from misr data using imagery over the april to september period for 2000 through 2005 near infrared albedo is inversely related to both snow age and melt intensity we map surface roughness using the normalized difference angular index ndai applied to misr atmospherically corrected surface hemispherical directional reflectance in previous work the ndai has been correlated with surface roughness on the scale of about 70 m we have further substantiated that relationship here the ndai and albedo images for each of the six years are used in an isodata unsupervised classification classification results for the individual years show year to year differences which appear to depend on the number and temporal distribution of images with minimal cloud cover as well as interannual differences in ice sheet surface properties the year 2003 which had the greatest number of images with minimal cloud cover shows good correspondence with previously mapped glacier zones however we do not have an isodata class that corresponds to the dry snow zone and we see greater differentiation between zones at lower elevations within each isodata class distinct relationships between near infrared albedo and surface roughness emerge and in all cases but one there is an inverse temporal relationship between albedo and roughness the one class that shows a direct relationship between albedo and surface roughness is hypothesized to be the superimposed ice zone while still somewhat preliminary these results suggest that concurrent measurements of near infrared albedo and surface roughness have significant potential for ice sheet surface characterization as well as for ongoing monitoring

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

avhrr surface temperature and narrow band albedo comparison with ground measurements for the greenland ice sheet an ice surface temperature retrieval algorithm for the greenland ice sheet was developed using noaa 11 thermal radiances from channels 4 and 5 temperature pressure and humidity profiles cloud observations and skin temperatures from the swiss federal institute of technology eth camp located at the equilibrium line altitude at 49 deg17 min w 69 deg 34 min n were used in the lowtran 7 model through a statistical analysis of daily clear sky profiles the coefficients that correct for the atmospheric effects were determined for the eth camp field season may to august surface temperatures retrieved by this method were then compared against the in situ observations with a maximum difference of 0 6 k the noaa 11 narrow band planetary albedo values for channels 1 and 2 were calculated using pre launch calibration coefficients scattering and absorption by the atmosphere were modelled with lowtran 7 then narrow band albedo values for the avhrr visible and near infrared channels were compared with in situ high resolution spectral reflectance measurements in the visible band 580 680 nm avhrr derived narrow band albedo and the in situ measurements corrected with radiative transfer model lowtran 7 showed a difference of less than 2 for the near infrared channel 725 1100 nm the difference between the measured and modelled narrow band albedo was 14 these discrepancies could be either the result of inaccurate aerosol scattering modelling lack of the in situ observation or the result of sensor drift due to degradation

the impact of cloud cover on the net radiation budget of the greenland ice sheet energy balancemodelsdrivenbyradiationandturbulentheatfluxeshave beenwidelyappliedto predictingtheresponse ofthegreenland ice sheetto climatechange however a lackof knowledge of the temporal and spatial distribution of cloudamount and type has necessitated the use of parameterizations or statistical models of cloud cover this deficiency results in large uncertainties in both shortwave and longwave radiation fluxes stereo matching of nadir and forward viewalongtrack scanning radiometer 2 atsr 2 image pairs has been shown to be a reliable method of retrieving cloud top height and further cloud properties can be derived from thermal imagery allowing classification into cloud type a1year cloud record for a transect across southern greenland derived from stereo matching is presented here and comparisons are made with climate re analysis data and ground observations the cloud cover data were used in a simple radiative transfer model and the impact of clouds on the net radiation fluxes was found to be considerable different cloud scenarios produced up to 40wm 2 difference in net radiation balance in theablationzone wherethealbedoislowerandmost variable thesensitivity to cloud cover fraction was less marked but the higher spatial resolution of the atsr 2 cloud record was reflected by a much more varied trend in radiation balance whether the net radiation balance increases or decreases with increased cloud cover was foundto be a function of the cloud amount andtype and also the surface albedo the sensitivity of the model to a 5 change in cloud amount was found to be comparable to a1k change in temperature this clearlydemonstratestheimportanceofreliable quantitativeclouddatainmass balanceand otherglaciological studies

comparison of avhrr derived and in situ surface albedo over the greenland ice sheet abstract this paper discusses a methodology for computing the clear sky surface albedo over the greenland ice sheet by using advanced very high resolution radiometer avhrr global area coverage visible and near infrared reflectances this approach is then used to map the monthly changes in albedo over the entire greenland ice sheet during the spring and summer months the methodology includes corrections for the intervening atmosphere by using the 6s radiative transfer model additional corrections for the anisotropic nature o f snow reflectance and the conversion from a marrow into a broadband albedo are made based on in site measurements from the swiss federal institute of technology university of colorado camp at 69 34 n 49 18 w comparison with surface albedo field ineasureinents indicates that before the onset of snoic melt agreement is good between the satellite derived and measured surface albedo after melt begins it is difficult to compare the satellite values with the field data owing to melt water ponding on the ice sheet surface with appropriate modifications the 6s radiative transfer model is suitable for atmospherically correcting avhrr visible and near infrared radiances atmospherically polar regions

shear wave anisotropy in the parkfield varian well vsp abstract a vertical seismic profile vsp survey was run to 1334 m depth in the instrumented varian well 1 4 km from the san andreas fault trace at parkfield california to test the sensor string shortly after its permanent installation the cable subsequently failed near the 1000 m level so the test survey represents the deepest data acquired in the study a shear wave vibrator source was used at three ofsets and two orthogonal orientations and the data have been processed for p and s wave velocities and for s wave velocity anisotropy velocities are well determined 3 3 and 1 9 km sec respectively at the deeper levels and the s waves are seen clearly to be split by anisotropy below about 400 m some 8 per cent velocity difference is apparent between polarizations parallel to and perpendicular to the san andreas fault faster and slower respectively and the difference seems to decrease with distance from the fault suggesting that the cause may be the fabric of the fault zone repeated surveys at the 1000 m depth are being conducted as part of the parkfield monitoring program

seasonal variation of ice ablation at the margin of the greenland ice sheet and its sensitivity to climate change qamanârssup sermia west greenland monthly ice ablation was measured at the margin of the greenland ice sheet for june july and august over 7 years 1980 86 the total winter ablation september may has also been measured and a simple ablation temperature model used to assign ablation values to individual months under the present climate the most ablation occurred in june august on average 81 of annual ablation moderate ablation took place in may and september 17 and very little ablation occurred in october april 2 the effect of climate change on ice ablation is simulated using the ablation model to recalculate ablation for higher temperatures summer ice ablation increases with temperature in the model but there is proportionally greater increase for may and september whereas the period from october to april is presently so cold that even a temperature rise of 5 c will hardly increase ablation the difference in annual ice ablation caused by future climate change will therefore depend upon the seasonal distribution of the temperature change changes in precipitation and accumulation will further modify the seasonal variation of ablation

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

effects of frictional heating on the thermal hydrologic and mechanical response of a fault the mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating in this study we formulate a fault model which incorporates the effects of frictional heating on the thermal hydrologic and mechanical response of a small patch of the failure surface this model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant the problem has two time scales a characteristic slip duration and a characteristic time for thermal pressurization the slip duration is set by the fault geometry the characteristic time for thermal pressurization is set by the slip rate the friction coefficient and the thermal and hydraulic characteristics of the medium the response of the fault depends on the relative magnitude of these two times results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response for earthquakes occurring across zones of low porous medium compressibility 10 9 pa 1 and permeability 10 18 m2 the characteristic time for thermal pressurization is small in this case frictional heating can cause fluid pressures to approach lithostatic values the shear strength to approach zero and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault moderate slip events where shear strains exceed two cause substantial strain weakening and consequently large stress drops accelerations and displacements thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones both the dynamic stress drop and total displacement decrease for zones with larger compressibility permeability or width if the compressibility or permeability exceeds 10 8 pa 1 or 10 14 m2 or the shear strain is less than one then the effects of frictional heating may be negligible and the fault will exhibit no strain weakening characteristics consequently the patch acts as barrier that halts or resists further fault motion extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface explaining many of the features of active faulting e g barriers nonuniform slip rupture stoppage random ground accelerations strong motions and frequency magnitude relations

neglecting ice atmosphere interactions underestimates ice sheet melt in millennial scale deglaciation simulations dynamic and thermodynamic interactions between the atmosphere and underlying ice sheets are generally not represented in the traditional one way boundary condition forcing used to drive ice sheet models this shortcoming is investigated through a series of idealized millennial scale deglaciation simulations designed to isolate the mechanisms regulating the deglaciation timescale of the laurentide ice sheet sensitivity experiments indicate that the conventional use of one way non interactive atmospheric forcing fields leads to an unrealistically insensitive melt response in the ice sheet model even when atmospheric carbon dioxide is set to modern preindustrial levels and earth s angle of obliquity is set to its early holocene value a more realistic deglaciation timescale is obtained only through the application of a new two way interactive asynchronous ice atmosphere coupling scheme and a seasonal ice albedo parameterization that accounts for the observed darkening of ice in the moist summertime ablation zone

three dimensional glacial flow and surface elevation measured with radar interferometry outlet glaciers which serve to drain ice from ice sheets seem to be dynamically less stable in north greenland than in south greenland1 2 3 storstrommen a large outlet glacier in northeastern greenland which surged between 1978 and 1984 ref 2 has been well studied in general neither glacier surge mechanisms nor the geographical distribution of the surges are well known conventional satellite radar interferometry can provide large scale topography models with high resolution4 and can measure the radar line of sight component of ice flow vectors5 but cannot map full vector flow fields here we present an interferometry method that combines observations from descending and ascending satellite orbits which assuming ice flow parallel to the topographic surface allows us to use the differing view angles to estimate full three dimensional surface flow patterns the accuracy of our technique is confirmed by the good agreement between our radar based flow model and in situ global positioning system gps reference data at storstrommen radar measurements such as these made regularly and at high spatial density have the potential to substantially enhance our understanding of glacier dynamics and ice sheet flow as well as improve the accuracy of glacier mass balance estimates

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

the surface albedo of the greenland ice sheet satellite derived and in situ measurements in the sondre stromfjord area during the 1991 melt season the temporal and spatial variation in the surface albedo of the greenland ice sheet during the ablation season of 1991 is investigated the study focuses on an area east of sondre stromfjord measuring 200 km by 200 km and centred at 67 5 n 48 13 w the analysis is based on satellite radiance measurements carried out by the advanced very high resolution radiometer avhrr the broad band albedo is estimated from the albedos in channel 1 visible and channel 2 near infrared the results are calibrated with the surface albedo of sea and dry snow satellite derived albedos are compared with gimex ground measurements at three stations there is a high degree of consistency in temporal variation at two of the three stations large systematic differences are attributed to albedo variations on subpixel scale in the course of the ablation season four zones appear each parallel to the ice edge it is proposed that these are in order of increasing altitude i clean and dry ice ii ice with surface water iii superimposed ice and iv snow an extensive description of these zones is given on the basis of the situation on 25 july 1991 zones i iii and iv reveal fairly constant albedos 0 46 0 65 and 0 75 on average whereas zone ii is characterised by an albedo minimum 0 34 survey of the western margin of the greenland ice sheet up to 71 n shows that the zonation occurs between 66 and 70 n

the potential of satellite radar interferometry and feature tracking for monitoring flow rates of himalayan glaciers abstract the stagnation of himalayan glaciers will only be exacerbated by a warming climate increasing hazards associated with glacier lake development frontal recession is a poor indicator of negative balance in this region where most glaciers are debris covered but reduced glacier flow rates provide a valuable warning of stagnation in situ measurements of velocity are understandably rare and remote sensing measurements have hitherto been limited by severe topography and lack of suitable data in this paper we compare two techniques to measure flow rates in everest region glaciers firstly satellite radar interferometry sri has recently become more viable because of improved digital elevation data and precision orbit models which now allow snapshots of velocities to be retrieved from ers imagery in the mid 1990s without ground control information secondly feature tracking between synthetic aperture radar sar images is found to be successful even on the small glaciers of this region because of the patterning associated with debris cover this technique extends through the 1990s and beyond the time frame over which velocities can be measured using the examples of the khumbu glacier flowing south into nepal and the kangshung glacier flowing east into china from the everest massif we compare sri and sar feature tracking in ers images consider the errors in these techniques and examine the potential to monitor flow rates across the region the techniques are found to be highly complementary and appropriate for monitoring flow rates in this region suggesting that routine monitoring of himalayan glaciers is possible the kangshung glacier appears to sustain relatively high flow rates of up to 10 cm day which along with a convex topographic profile indicates a healthy glacier not exhibiting significant negative mass balance in comparison much of the khumbu glacier appears to have stagnated the surface velocity at gorak shep is only 20 of a field based measurement in the 1970 s and the concave surface profile confirms the downwasting associated with a debris covered glacier in recession

influence of fault bends on ruptures data on slip along the san andreas fault have been examined to understand the influence of a bend in the fault system on the extent of slip release a correlation of slip along the fault in the great 1906 and 1857 earthquakes with fault geometry shows that maximum slip occurred near a major change in the fault strike and may be a function of this change the data appear to suggest that in general it takes about 10 years of strain accumulation to negotiate a 1 change in fault strike assuming a slip rate of about 3 cm yr seismic hazard along the san andreas fault may therefore also be a function of fault geometry

accuracy of three dimensional glacier surface velocities derived from radar interferometry and ice sounding radar measurements we present a method for analyzing the errors involved in measuring three dimensional glacier velocities with interferometric radar we address the surface parallel flow assumption and an augmented approach with a flux divergence fd term the errors in an interferometric ers 1 2 satellite radar dataset with ascending and descending orbit data covering storstrommen glacier northeast greenland are assessed the fd error assessment is carried out on airborne 60 mhz ice sounding radar data from the same area a simple model of an interferometric radar system is developed and analyzed the error sources considered include phase noise atmospheric distortions baseline calibration errors a dry snow layer and the stationary flow assumption used in differential interferometry the additional error sources in the analysis of fd errors are noise bias and unknown variations of the ice thickness and approximations of the ice flow model the example glacier is now building up following a surge the analysis shows that in the case study presented the errors are small enough to justify the use of both the estimated surface parallel flow term of the vertical velocity and the estimated fd term of the vertical velocity

validation of avhrr and modis derived albedos of snow and ice surfaces by means of helicopter measurements we describe the validation of surface albedos of snow and glacier ice as derived from advanced very high resolution radiometer avhrr and moderate resolution imaging spectrometer modis satellite data for this purpose we measured surface albedos from a helicopter over vatnajokull iceland and the kangerlussuaq transect western part of the greenland ice sheet in thematic mapper tm bands 2 and 4 and avhrr bands 1 and 2 and converted these values to measured albedos in three modis bands relative to other validation methods our helicopter measurements have the advantages of larger spatial coverage and of almost direct measurements in satellite sensor spectral bands we found the smallest differences between the satellite derived and helicopter albedos for the kangerlussuaq transect for avhrr data a mean difference of 0 01 in both bands with the satellite in near nadir position and for two modis images a mean difference of 0 00 0 02 for bands 2 and 4 and 0 03 for band 1 for two avhrr images of vatnajokull we found mean differences of up to 0 06 differences are primarily due to errors in the satellite derived albedos which in turn are mainly caused by errors in the calibration coefficients of the satellite sensors and insufficient knowledge of the angular distribution of the radiation reflected by snow and ice satellite data obtained from view zenith angles larger than 50 55 appeared to be unsuitable

assessment of greenland albedo variability from the advanced very high resolution radiometer polar pathfinder data set the advanced very high resolution radiometer polar pathfinder app data set is used to examine the variability of the surface albedo over greenland analysis of the app albedo record from 1981 to 1998 show anomalously low albedo during 1995 and 1998 over most of the ice sheet as compared with the other years the low albedo encountered during these years suggests that the ice sheet experienced considerable melt in 1995 and 1998 particularly near the western margin of the ice sheet conversely anomalously high albedos were found in 1992 as a result of colder temperatures and hence less melt following the eruption of mount pinatubo the relationship between the annual north atlantic oscillation nao index and the mean summer albedo from all the stations reveals a positive correlation of 0 44 and a positive correlation of 0 55 for the southern part of the ice sheet therefore variations in the mean summer albedo over greenland can in part be explained by variations in the nao such that during periods of intensification of the normal mode of the nao the mean summer albedo is above normal trend analysis reveals an overall downward trend in surface albedo from 1981 to 1998 which agrees with recent trends in melt and precipitation however the trend was found not to be statistically significant but rather influenced by the low albedo in recent years

present and past ice sheet mass balance simulations for greenland and the tibetan plateau net annual mass balance was evaluated for greenland and the tibetan plateau using the meteorological forcings from the ncep reanalysis and two gcms foam1 0 and csm1 4 for modern climate and for different time periods extending back to the beginning of the holocene 11 000 years ago for the climate models the ice sheet budget calculations using the degree day methodology were performed on a finer grid than the model output by interpolating monthly precipitation and surface temperature and correcting the latter to account for the gcm s smoothed topography the computed net mass balance for greenland in the present day is positive and it ranges between 290 300 mm water equivalent w e year for the two models values close to the ncep estimate of 250 mm year the past climate simulations show that the greenland mass balance has become slightly more positive since the beginning of the holocene the tibetan plateau s present day area average net mass balance is negative and ranges between 1200 and 2000 mm w e year for the two models values bracketing the ncep estimate of 1700 mm year although the balance is positive over small regions of the plateau consistent with the existence of small ice caps and glaciers the calculated past mass balance shows an increasingly less negative value for foam from 11 000 years ago towards the present and expansion of the positive mass balance areas mainly due to decreased snow ablation as the summertime insolation decreases with the changes in orbital forcing in csm the opposite trend occurs but changes are smaller and less systematic the result from foam shows that the likelihood of ice sheets developing on the tibetan plateau may have increased since 11000 years ago which is consistent with some glacial records

narrowband to broadband albedo conversion for glacier ice and snow based on aircraft and near surface measurements this article presents albedo measurements of snow and glacier ice at vatnajokull iceland and the kangerlussuaq transect greenland radiative fluxes were measured in the broadband and in four narrowbands namely thematic mapper tm bands 2 and 4 and advanced very high resolution radiometer avhrr bands 1 and 2 the incoming fluxes were measured near the ground and the outgoing fluxes from a helicopter extracts of the data collected over snow ice supraglacial moraines supraglacial lakes and tundra are discussed using the data sets from iceland and greenland and data sets with entirely ground based albedo measurements from the morteratschgletscher switzerland and scharffenbergbotnen antarctica new equations for narrowband to broadband ntb conversion were developed they have a residual standard deviation of 0 011 for tm and 0 008 for avhrr and can be applied without having to classify the surface the helicopter data are also used to develop criteria for distinguishing different types of surfaces which are needed for the application of bidirectional reflectance distribution functions brdfs snow can be distinguished from ice by defining a threshold for a single narrowband albedo the ratio of the albedos in tm4 and tm2 and to a lesser extent the ratio of the albedos in avhrr2 and avhrr1 may serve as a proxy for the surface characteristics of glacier ice in terms of concentrations of water debris and dust

p wave spectra for ml 5 foreshocks aftershocks and isolated earthquakes near parkfield california abstract wood anderson seismograms recorded at mount hamilton mhc 185 km 327 santa barbara sbc 180 km 158 and tinemaha tin 240 km 56 provide data for comparing p wave spectra for two immediate 17 min foreshocks one early 55 hr foreshock two aftershocks and two isolated parkfield earthquakes all are m l 5 0 shocks with epicenters within 7 km of the common epicenter of the 1934 and 1966 parkfield main shocks the set of events is well suited for testing the hypothesis that foreshocks are high stress drop sources calculated stress drops are controlled by source directivity at azimuths aligned with the fault break at mhc and sbc p wave radiation from the three foreshocks is focused along one fault trace azimuth suggesting that foreshock sources are characterized by pronounced unilateral rupture expansion at tin broadside to the fault where directivity has minimum effect on calculated relative stress drop the two immediate foreshocks are higher stress drop sources the early foreshock is a low to average stress drop source indicating the possibility that stress concentration is a rapidly occurring phenomenon in rupture nucleation alternatively the stress field is highly variable on the scale of 2 to 3 km in the focal region of an impending earthquake with a rupture length of 20 to 30 km

temporal and spatial variation of the surface albedo of the morteratschgletscher switzerland as derived from 12 landsat images this investigation uses landsat images from 12 days in 1999 and 2000 to study the spatial and temporal variation in surface albedo of a glacier with a rugged topography morteratschgletscher switzerland our retrieval method considers all processes that substantially influence the relationship between the satellite signal and the surface albedo the correction for the anisotropy of the reflected radiation field of ice and snow ranges up to 0 10 depending on wavelength band solar zenith angle and surface type we analyzed the uncertainties in the retrieval method and mainly expect errors in satellite derived albedos for areas with large variation in topography and high albedos the latter is due to application of bidirectional reflectance distribution function brdf parameterizations for the anisotropic correction to albedos outside the parameterizations validity on average satellite derived albedos exceed the measured surface albedo by 0 03 the glacier tongue is characterized by bands of low and high albedo relating to ice with higher and lower concentrations of debris the ice albedo shows no dependence on altitude except at 2000 2200 m a s l it increases during summer which is likely associated with rainfall as concluded from a comparison between summer rainfall and measured albedos

combining sar interferometry and the equation of continuity to estimate the three dimensional glacier surface velocity vector until now an assumption of surface parallel glacier flow has been used to express the vertical velocity component in terms of the horizontal velocity vector permitting all three velocity components to be determined from synthetic aperture radar interferometry we discuss this assumption which neglects the influence of the local mass balance and a possible contribution to the vertical velocity arising if the glacier is not in steady state we find that the mass balance contribution to the vertical surface velocity is not always negligible as compared to the surface slope contribution moreover the vertical velocity contribution arising if the ice sheet is not in steady state can be significant we apply the principle of mass conservation to derive an equation relating the vertical surface velocity to the horizontal velocity vector this equation valid for both steady state and non steady state conditions depends on the ice thickness distribution replacing the surface parallel flow assumption with a correct relationship between the surface velocity components requires knowledge of additional quantities such as surface mass balance or ice thickness

apparent stresses stress drops and amplitude ratios of earthquakes preceding and following the 1975 hawaii ms 7 2 main shock abstract s wave spectra for approximately 300 earthquake signals 2 8 m l 3 9 were obtained from hand digitized wood anderson records from hilo hawaii all sources were located within part of the rupture volume of the 1975 hawaii m s 7 2 earthquake the epicentral distance to hilo was about 45 3 km the backazimuth was 180 10 south and the sources were located at depths of 9 1 4 km the apparent stress and brune stress drop based on 3 sec windows of the s waves were examined for possible temporal and spatial dependence most of the data showed a remarkable constancy of the average apparent stress and stress drop per event as a function of time no measurable change in average signal content took place at the time of the main shock a statistically significant difference in the average stress parameters per event was found between neighboring subvolumes radius 5 km of the 1975 aftershock zone a volume previously proposed as asperity based on seismicity pattern interpretation contained earthquakes with higher apparent stresses of 9 0 8 8 bars compared to the neighboring volume with 4 2 6 1 bars seventy eight spectra obtained from a second station showed the same difference between the two volumes as that found at station hilo suggesting that the signal differences should be interpreted in terms of source differences in addition amplitude ratios of sv p waves measured at hilo showed that a systematic difference of 10 in fault plane orientation existed during years before the 1975 main shock this agrees with the rotation of the fault plane during the main rupture reported elsewhere we hypothesize that heterogeneities along the fault plane with dimensions of about 5 km cause changes in fault plane dip differences in average stress release per earthquake strong contrasts in seismicity patterns and differences in aseismic strain release by fault creep these same heterogeneities in the fault plane apparently also control the location of major earthquakes and the complexity of their ruptures

estimation of ice sheet motion using satellite radar interferometry method and error analysis with application to humboldt glacier greenland satellite radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets we examine the sources of error in interferometrically derived ice motion measurements including those errors due to inaccurate estimates of the interferometric baseline several simulations are used to assess baseline accuracy in terms of tie point error and the number and distribution of tie points these results give insight into how best to select tie points and also demonstrate the level of accuracy that can be achieved examination of two representative cases likely to occur in mapping ice sheet motion leads to the conclusion that with adequate tie point information ice velocity can be measured accurately to within a few meters per year a method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed finally we estimate the single component velocity field for an area on humboldt glacier northern greenland using interferograms formed from ers i sar images we estimate that these velocity measurements are accurate to within 2 3 m year 1

physical and optical properties of snow covering arctic tundra on svalbard snow thickness duration of snow coverage and amount of ice covering the soil are crucial for the development of biota in the arctic tundra environment the snow thickness and optical properties control the amount of photosynthetically active radiation par that is available for vegetation a late snow cover may prevent birds from nesting on the ground furthermore ice at the snow soil interface can be an obstacle for grazing of svalbard reindeer and affect the microfauna population snow and ice thickness and the physical and optical properties of snow covering arctic tundra were measured on the broggerhalvoya peninsula on western svalbard in spring of 1997 additionally thicknesses of ground covering ice were measured in spring of 1998 the initial maximum thickness of snow in the observed areas varied from 0 4 to 0 9 m the snow around ny alesund began to disappear by the beginning of june with the entire snow pack disappearing within 2 3 weeks at the bottom of the snow pack there was a soil covering ice layer between 0 05 and 0 1 m thick we obtained radiation and reflectance parameters spectral albedo attenuation of par and global radiation as well as physical properties of snow e g temperature and density over six weeks from early may to late june electrolytic conductivity of melted snow samples from snow pits shows clearly different conductivity for different stratigraphic sections within the snow pack in early june later on these contrasts disappeared as internal ice layers melted and the snow pack underwent percolation the albedo maximum before melt onset exceeded 0 9 whereas in the later phase of melting snow surfaces exhibited significantly lower albedo due to metamorphosis thinning and blackening by soil particle contamination however even an apparently clean snow surface had about 30 lower albedo in mid june than in mid may observations from under snow par measurements are verified using a physically based radiative transfer model this comparison indicates that scattering features that are smaller than the bulk grain size may contribute significantly to the interaction between radiation and snowpack

insar for estimation of changes in snow water equivalent of dry snow this paper describes the theoretical relation between interferometric phase and changes in snow water equivalent swe and show results from experiments using ers 1 2 tandem data the main scattering contribution from a dry snow cover is from the snow ground interface however the radar wave will be refracted in the snow thus only small changes in the snow properties between two interferometric synthetic aperture radar sar images will change the interferometric phase this phase change is shown to introduce a significantly increase in the digital elevation model dem height error although no effects are observed on the degree of coherence the phase change is also shown to affect the differential interferometric results and may wrongly be interpreted as range displacement the presented theory and results implies that light snowfall and or small changes in snow properties between interferometric sar insar image acquisitions may introduce significant height errors in dem derived from glaciers ice sheets or bare ground even in the case of high degree of coherence thus meteorological observations in addition to degree of coherence must be considered when generating dem in areas covered with snow or where snow fall is likely to have occurred

modelling aftershock migration and afterslip of the san juan bautista california earthquake of october 3 1972 abstract the san juan bautista earthquake of october 3 1972 m l 4 8 located along the san andreas fault in central california initiated an aftershock sequence characterized by a subtle but perceptible tendency for aftershocks to spread to the northwest and southeast along the fault zone the apparent dimension of the aftershock zone along strike increased from about 7 10 km within a few days of the earthquake to about 20 km eight months later in addition the mainshock initiated a period of accelerated fault creep which was observed at 2 creep meters situated astride the trace of the san andreas fault within about 15 km of the epicenter of the mainshock the creep rate gradually returned to the preearthquake rate after about 3 yrs both the spreading of the aftershocks and the rapid surface creep are interpreted as reflecting a period of rapid creep in the fault zone representing the readjustment of stress and displacement following the failure of a stuck patch or asperity during the san juan bautista earthquake numerical calculations suggest that the behavior of the fault zone is consistent with that of a material characterized by a viscosity of about 3 6 10 14 p although the real rheology is likely to be more complicated in this model the mainshock represents the failure of an asperity that slips only during earthquakes aftershocks represent the failure of second order asperities which are dragged along by the creeping fault zone

glaciar perito moreno patagonia climate sensitivities and glacier characteristics preceding the 2003 04 and 2005 06 damming events mass balance and climate sensitivity of glaciar perito moreno gpm one of the main outlet glaciers of hielo patagonico sur southern patagonia icefield were investigated field measurements were carried out from 1995 to 2003 including ice ablation and velocity at stakes seismic profiling bathymetry of the lake near the calving fronts and meteorological data the database was complemented by satellite observations to derive the motion field by interferometric data map glacier boundaries and snowlines from multi year time series of radar images and obtain glacier topography from the shuttle radar topography mission in september 2003 gpm started to dam the southern arm of lago argentino resulting in a maximum rise of the lake level of 9 35 m before the dam burst in march 2004 the ice dam formed again in august 2005 bursting in march 2006 analysis of mass fluxes indicates no long term trend in mass balance this behaviour contrasting with most retreating glaciers in the vicinity can be attributed to a particular glacier geometry monthly climate sensitivity characteristics for glacier mass balance were derived using a degree day model showing similar importance of both temperature and precipitation also the reconstruction of the mass balance for the last 50 years from local climate data shows a near steady state condition for gpm with some small fluctuations such as a slightly positive balance after 1998 that may have triggered the minor advance leading to damming events in 2003 and 2005