modeling the sar backscatter of linear dunes on earth and titan abstract we modeled the synthetic aperture radar sar response of linear dunes of the great sand sea in egypt using a single surface scattering term based on integral equation and physical optics models using multi frequency sir c x sar radar scenes and topography obtained from the shuttle radar topography mission srtm we were able to estimate reasonable values for the parameters describing the surface roughness of the dunes as the linear dunes of the great sand sea are relevant analogs for the linear dunes observed on titan by the cassini radar instrument these results were thus used as a starting point to simulate the radar response of titan s dunes as imaged by the radar instrument onboard the cassini spacecraft during the t8 flyby in october 2005 we show that a single surface scattering term is not sufficient to simulate the radar signal backscattered by the dunes on titan one has to add a diffuse scattering term indicating that titan s dunes are likely to have somewhat inhomogeneous internal structures related to porosity and or internal layering our results also indicate that the dunes on titan should be close to perfectly smooth possibly because of the formation of smaller ripples than on earth plus smoothing resulting from precipitation events

internal structure of aeolian dunes in abu dhabi determined using ground penetrating radar a ground penetrating radar survey of aeolian dunes in the al liwa area of abu dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures the interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles nnw sse orientated radar profiles approximately parallel to the prevailing wind direction show two sets of dipping reflectors which are interpreted as sets of cross stratification and second and third order bounding surfaces radar profiles orientated wsw ene across the prevailing wind direction are dominated by concave up reflectors which are interpreted as trough shaped scours and sets of trough cross stratification produced by oblique progradation of barchanoid dunes nested troughs with small sets of trough cross stratification within larger troughs may be due to reactivation following wind reversal or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes convex upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs overall there is a tendency for the larger second order bounding surfaces to dip downwind which confirms brookfield s ideas of the relative migration paths of dunes and draa

empirical observations regarding reverse earthquakes blind thrust faults and quaternary deformation are blind thrust faults truly blind active thrust faults pose a significant seismic hazard worldwide many of these faults include blind thrusts where the propagating fault tip does not reach the earth s surface and buried faults where the geomorphic expression of the fault is obscured by subsequent sedimentation and or erosion this raises the issue of whether conventional geologic geomorphic and paleoseismic methods can be used to identify and characterize thrust faults for the assessment of seismic hazards or whether these faults sometimes are truly blind we compiled a data base of 148 worldwide moderate to large magnitude thrust reverse earthquakes to evaluate whether or not the event occurred on a fault that could have been identified prior to the earthquake on the basis of recognizable quaternary surface deformation i e a pre existing fault or fold analysis of the data shows that interplate reverse earthquakes almost always are associated with pre existing quaternary deformation that was or could have been recognized prior to the earthquake in particular most interplate reverse earthquakes are associated with an active reverse fault at the surface and or an active anticline in contrast intraplate reverse earthquakes seldom occur on faults associated with pre existing recognizable surface deformation we conclude that thrust faults can be detected in interplate regions with careful quaternary geologic and geomorphic mapping furthermore the absence of quaternary surface deformation can be used to infer the absence of an underlying active blind thrust fault in interplate tectonic settings however the data show that quaternary geologic mapping techniques alone likely are insufficient to characterize blind thrusts in intraplate regions in these areas inclusion of a floating or random earthquake may be necessary to assess earthquake hazards

low frequency radar sounding investigations of the north amargosa desert nevada a potential analog of conductive subsurface environments on mars 1 theoretical estimates of low frequency radar sounding performance and its potential for mapping moist subsurface interfaces in conductive environments on mars are controversial with predictions of ultimate penetration depth ranging from a few meters to kilometers to address this issue we conducted a broadband electromagnetic field survey in which we combined ground penetrating radar gpr operating at multiple low frequencies with the transient electromagnetic method tem to investigate the dependence of radar penetration depth on ground resistivity surveys were performed in the frequency range 16 100 mhz at two locations on the northwest margin of the amargosa desert nevada where numerous mars analog investigations have been performed the surveys were conducted on a 20 m high homogenous sand dune and on the flanks of a 20 m high scoria cone and above a buried lava flow a wet alluvial interface was located at the bottom of each structure gpr detected the wet alluvium contact at the base of the sand dune but failed to penetrate to the same depth at the scoria cone under similar residual moisture content depths of investigation for both the scoria cone and the buried lava flow were limited to approximately 10 m owing to the presence of conductive inclusions in the first few meters which are below the radar resolution but dramatically decreased the dynamic of the radar backscattered echoes and hence the penetration depth absorption models constrained by the tem data are in good agreement with these observations depths of investigation varied weakly with frequency owing to substantial frequency independent absorption

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

the sir c x sar synthetic aperture radar system the spaceborne imaging radar c x band synthetic aperture radar sir c x sar was a joint united states german italian space agency imaging radar system successfully flown aboard the shuttle endeavor in april 1994 and again in september october 1994 the multifaceted sir c x sar represents a major technological step forward in radar remote sensing and is the first spaceborne multifrequency polarimetric sar the united states developed sir c operated at l and c band each with quad polarization the sir c antenna was an active phased array with the capability for electronic beam steering and multiple swath width illumination the german italian x sar operated at x band with a single polarization using a slotted waveguide antenna mechanically steerable in elevation sir c and x sar were designed to operate synchronously collecting data over common sites synchronously a total of 143 hours 93 terabits of sar data were recorded on tape

latitudinal and altitudinal controls of titan s dune field morphometry dune fields dominate 13 of titan s surface and represent an important sink of carbon in the methane cycle herein we discuss correlations in dune morphometry with altitude and latitude these correlations which have important implications in terms of geological processes and climate on titan are investigated through the microwave electromagnetic signatures of dune fields using cassini radar and radiometry observations the backscatter and emissivity from titan s dune terrains are primarily controlled by the amount of interdune area within the radar footprint and are also expected to vary with the degree of the interdunal sand cover using sar derived topography we find that titan s main dune fields shangri la fensal belet and aztlan tend to occupy the lowest elevation areas in equatorial regions occurring at mean elevations between 400 and 0 m relative to the geoid in elevated dune terrains we show a definite trend towards a smaller dune to interdune ratio and possibly a thinner sand cover in the interdune areas a similar correlation is observed with latitude suggesting that the quantity of windblown sand in the dune fields tends to decrease as one moves farther north the altitudinal trend among titan s sand seas is consistent with the idea that sediment source zones most probably occur in lowlands which would reduce the sand supply toward elevated regions the latitudinal preference could result from a gradual increase in dampness with latitude due to the asymmetric seasonal forcing associated with titan s current orbital configuration unless it is indicative of a latitudinal preference in the sand source distribution or wind transport capacity

cassini sar radiometry scatterometry and altimetry observations of titan s dune fields large expanses of linear dunes cover titan s equatorial regions as the cassini mission continues more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation sar radiometry scatterometry altimetry and with an increasing variety of observational geometries in this paper we report on cassini s radar instrument observations of the dune fields mapped through may 2009 and present our key findings in terms of titan s geology and climate we estimate that dune fields cover 12 5 of titan s surface which corresponds to an area of 10 million km 2 roughly the area of the united states if dune sand sized particles are mainly composed of solid organics as suggested by vims observations cassini visual and infrared mapping spectrometer and atmospheric modeling and supported by radiometry data dune fields are the largest known organic reservoir on titan dune regions are with the exception of the polar lakes and seas the least reflective and most emissive features on this moon interestingly we also find a latitudinal dependence in the dune field microwave properties up to a latitude of 11 dune fields tend to become less emissive and brighter as one moves northward above 11 this trend is reversed the microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion relative to that of the dune roughness and degree of sand cover in agreement with radiometry and scatterometry observations sar images suggest that the fraction of interdunes increases northward up to a latitude of 14 in general scattering from the subsurface volume scattering and surface scattering from buried interfaces makes interdunal regions brighter than the dunes the observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north thus allowing wave penetration in the underlying substrate altimetry measurements over dunes have highlighted a region located in the fensal dune field 5 latitude where the icy bedrock of titan is likely exposed within smooth interdune areas the hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and or an increase in the ground humidity toward the north which could be related to titan s asymmetric seasonal polar insolation alternatively it may indicate that either the wind pattern or the topography is less favorable for dune formation in titan s northern tropics

subsurface imaging in south central egypt using low frequency radar bir safsaf revisited we present the capabilities of low frequency radar systems to sound the subsurface for a site located in south central egypt the bir safsaf region this site was already intensively studied since the sir a and sir b orbital radars revealed buried paleodrainage channels our approach is based on the coupling between two complementary radar techniques the orbital synthetic aperture radar sar in c and l bands 5 3 and 1 25 ghz for imaging large scale subsurface structures and the ground penetrating radar gpr at 500 and 900 mhz for sounding the soil at a local scale we show that the total backscattered power computed from l band sar and 900 mhz gpr profiles can be correlated and we combined both data to derive the geological structure of the subsurface gpr data provide information on the geometry of the buried scatterers and layers while the analysis of polarimetric sar data provides information on the distribution of rocks in the sedimentary layers and at the interface between these layers the analysis of 500 mhz gpr data revealed some deeper structures that should be detected by lower frequency sars such as a p band system

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

overview of results of spaceborne imaging radar c x band synthetic aperture radar sir c x sar the spaceborne imaging radar c x band synthetic aperture radar sir c x sar was launched on the space shuttle endeavour for two ten day missions in the spring and fall of 1994 radar data from these missions are being used to better understand the dynamic global environment during each mission radar images of over 300 sites around the earth were obtained returning over a terabit of data sir c x sar science investigations were focused on quantifying radar s ability to estimate surface properties of importance to understanding global change and focused studies in geology ecology hydrology and oceanography as well as radar calibration and electromagnetic theory studies in addition the second flight featured an interferometry experiment where digital elevation maps were obtained by interfering data from the first and second shuttle flight and from successive days on the second flight sir c x sar data have been used to validate algorithms which produce maps of vegetation type and biomass snow soil and vegetation moisture and the distribution of wetlands developed with earlier aircraft data

mechanisms associated with the erosion of sand dune cliffs magilligan northern ireland a fifteen year history of coastal dune recession at magilligan northern ireland has revealed both time and space contrasts in processes and morphology since 1968 storm frequency has increased while dune retreat reached a peak 5 4 m in 1978 1979 three types of dune eroding events are noted resulting from particular wave wind tide combinations the spatial variability in dune scarping is associated with vegetation and soil development and slope failures range from granular avalanches to retrogressive rotational slumps

normal faults and their hanging wall deformation an experimental study we have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault the experimental results show that fault shape influences the style of secondary faulting and folding mostly antithetic normal faults form above concave upward fault bends whereas mostly synthetic normal faults form above low angle fault segments and convex upward fault bends beds dip toward the master normal fault above concave upward fault bends and away from the master normal fault above low angle fault segments and convex upward fault bends generally secondary faulting and folding are youngest at fault bends and become progressively older past fault bends hanging wall deformation patterns differ significantly when a basal plastic sheet imposes a constant magnitude displacement distribution on the master normal fault in models without a plastic sheet numerous secondary normal faults form in the hanging wall of the master normal fault most secondary normal faults propagate upward and consequently have greater displacement at depth in models with a plastic sheet few visible secondary normal faults develop most of these faults propagate downward and consequently have less displacement at depth hanging wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet the observed particle paths displacement distributions bedding dips and orientations of the principal strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous inclined simple shear accommodates the hanging wall deformation not all of our modeling observations however are consistent with this assumption specifically the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous inclined simple shear as the hanging wall deformation mechanism

geometric evolution of the chelungpu fault taiwan the mechanics of shallow frontal ramps and fault imbrication abstract the september 21 1999 mw 7 6 earthquake in taiwan ruptured along the north south trending east dipping chelungpu thrust fault that bounds the western foothills of the taiwan fold and thrust belt the near surface

filling the gap osl data expanding 14c chronologies of late quaternary environmental change in the libyan desert several decades of geomorphological and archaeological research in the libyan desert conducted by the cologne projects b o s and acacia have yielded a wealth of radiocarbon data from archaeological sites and over the last decade also 48 osl ages mainly from aeolian deposits this paper focuses on the combination of both data sets in order to obtain a synthesis of late quaternary occupational and environmental history in this region case studies demonstrate that despite a poorer time resolution of osl chronologies compared to 14 c data osl ages can be more consistent with the archaeological context particularly if the dating of charcoals poses problems due to poor quality or re deposition comparison of data sets from the egyptian sand seas and from the wadi howar sudan shows that in both cases osl and 14 c data cover complementary time slices on the regional scale however contradictory intervals of aeolian accumulation are documented which can be explained by the different environmental histories the results accentuate that interpretation of osl data has to be done in the context of the entire environmental systems including the effective geomorphological processes r 2007 elsevier ltd and inqua all rights reserved

star and linear dunes on mars abstract a field containing 11 star and incipient star dunes occurs on mars at 8 8 s 270 9 w examples of linear dunes are found in a crater at 59 4 s 343 w while rare dune varieties that form in bi and multidirectional wind regimes are not absent from the surface of mars the occurrence of both of these dune fields offers new insight into the nature of martian wind conditions and sand supply the linear dunes appear to have formed through modification of a formerly transverse aeolian deposit suggesting a relatively recent change in local wind direction the 11 dunes in the star dune locality show a progressive change from barchan to star form as each successive dune has traveled up into a valley into a more complex wind regime the star dunes corroborate the model of n lancaster 1989 progr physical geogr 13 67 91 1989 sedimentology 36 273 289 for the formation of star dunes by projection of transverse dunes into a complex topographically influenced wind regime the star dunes have dark streaks emanating from them providing evidence that the dunes were active at or near the time the relevant image was obtained by the viking 1 orbiter in 1978 the star and linear dunes described here are located in different regions on the martian surface unlike most star and linear dunes on earth both martian examples are isolated occurrences neither is part of a major sand sea previously published mars general circulation model results suggest that the region in which the linear done field occurs should be a bimodal wind regime while the region in which the star dunes occur should be unimodal the star dunes are probably the result of localized complication of the wind regime owing to topographic confinement of the dunes local topographic influence on wind regime is also evident in the linear dune field as there are transverse dunes in close proximity to the linear dunes and their occurrence is best explained by funneling of wind through a topographic gap in the upwind crater wall

numerical investigation of dip slip fault propagation effects on offshore seabed sediments abstract an enhanced understanding of the phenomenon of dip slip fault propagation through young unconsolidated offshore sediments is essential for the evaluation of the response of shallow offshore structures and subsea oil gas pipelines substantial and permanent seafloor deformation constitutes a severe geohazard for these marine structures and associated offshore elements components this paper addresses the response of seabed sand deposits subjected to underlying dip slip fault movement the resulting propagation of the faulting offset in seabed sediments is explored using 2d finite element modeling in abaqus while accounting for nonlinear soil behavior with strain softening a 2d plain strain fem seabed model is adopted to investigate the effect of different seabed soil properties and layer thicknesses on the extent and magnitude of seabed surface deformation induced by normal and reverse faults with different dip angles the results indicate that 1 the adoption of sand properties that depend on depth is necessary to realistically model the fault propagation mechanism particularly for fault propagation through thick and dense sand deposits 2 the presentation of fault propagation results in a normalized manner with respect to sand cover thickness does not eliminate the dependency on the actual layer thickness and 3 the deformation of the beds in the cover as a result of basement faulting can be modeled realistically using trishear kinematic approaches

combining ground penetrating radar surveys and optical dating to determine dune migration in namibia ground penetrating radar gpr profiles across a complex linear dune in the namib sand sea have been used to image sets of cross stratification and their bounding surfaces a combination of radar facies analysis and radar stratigraphy has been used to interpret the radar profiles and define a relative chronology thick sets of cross stratification indicate when the dune was most active whereas thin sets of cross stratification are interpreted to indicate the increased prevalence of wind reversals and lower rates of dune migration with bounding surfaces formed during periods of stabilization non deposition or erosion a drilling and dating campaign was designed on the basis of the dune stratigraphy as defined by the gpr survey sampling was targeted at large sets of cross stratification formed when the dunes were most active and avoiding bounding surfaces formed when the dune was stable or even eroded the results from optical dating give ages between 0 34 0 02 ka and 1 57 0 07 ka indicating a time averaged dune migration rate of 0 12 m a 1 over the past 1600 years

linear dunes on titan extensive dunes produced by wind transport of sand sized particles are common on earth and are also seen on mars and venus saturn9s moon titan is now shown to have similar dunes

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

1999 chi chi earthquake a case study on the role of thrust ramp structures for generating earthquakes the 21 september 1999 chi chi earthquake m w 7 6 occurred on east dipping shallow thrust faults that produced a high relief surface rupture extraordinary surface breaks appeared that could be clearly traced for about 100 km across many counties these thrust faults the chelungpu and shihkang are part of an active fold and thrust belt related to ongoing recent arc continent collision measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a westward direction n70 90 w on the chelungpu fault to a northwestward direction n30 40 w on the shihkang fault the stress field underwent a clockwise rotation of about 40 during the chi chi earthquake near rupture vertical displacements in the hanging wall of the shihkang fault have more cumulative displacement than on the chelungpu fault which is consistent with global positioning system gps measurements maximum vertical offset on the rupture was found to be about 10 m by the surficial rupture and gps measurements in addition analysis of crustal deformation by gps measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure our synthesis of geological and geodetic analyses shows the importance of ramp structures associated with thrust faults for generating large earthquakes and provides a general framework for understanding earthquake in fold and thrust belts large surficial coseismic uplift and strong asperities appear to be a function of fault ramp geometry our analysis also indicates that in general ramp structures in fold and thrust belts may have a high potential in generating large earthquakes manuscript received 14 november 2000

characteristics of surface ruptures associated with the chi chi earthquake of september 21 1999 abstract the chi chi earthquake which reactivated the chelungpu fault in central taiwan generated surface ruptures over 100 km long surface ruptures of the chi chi earthquake can be divided into the north south and the east west sections the north south section extends for about 84 km and the northern e w section 16 km the north south section consists of a continuous main rupture and some branches these are characterized by thrust motion with some strike slip components topographic surveys indicate that the ground commonly uplifted as a single continuous ramp step surface and deformation is mainly concentrated on the hanging wall fault slip data and gps measurements show that the hanging wall along the northern segment of the chelungpu fault moved northwest but that along the southern segment of the fault the hanging wall shifted to the southwest sandbox models suggest that the above dissimilarity in hanging wall movement direction is mainly due to the presence of the peikang basement high in the east west section the ruptures comprise a series of en echelon steep faults with right step or left step geometry and trend ene wsw to e w near the ta an river the surface rupture orientation shifts from ne sw to nw se and terminates at the shuangtung fault here the ruptures appear to be controlled by the 10 15 km wide nw trending sanyi puli shear zone clay modeling displays the development of e w trending ruptures when a n s trending fault meets a n30 w trending weak zone

slipfaceless whaleback dunes in a polar desert victoria valley antarctica insights from ground penetrating radar the lower victoria valley is one of the mcmurdo dry valleys antarctica and has the largest concentration of aeolian sand dunes on the continent aeolian bedforms include elongate slipfaceless dunes which have been called whaleback dunes these dunes are composed of coarse sand and capped by granule ripples ground penetrating radar gpr profiles across one of these dunes reveal low angle inclined sigmoid tangential and convex reflections that are interpreted as strata within the dune these dipping strata record the migration of the whaleback dune and show that they are not sand mantles as previously described but are actively migrating long wavelength low amplitude bedforms resembling zibar an 800 m profile along the axis of the dune reveals low angle dips from west to east showing that the dune has accreted towards the east gpr profiles collected at 100 m intervals transverse to the dune axis reveal sets of cross stratification dipping towards the south at the eastern end of the dune and towards the north at its western end these apparent dips are resolved to show that the dune has migrated from west to east driven by westerly foehn winds and katabatic winds blowing from the polar ice cap whilst at the same time it has been building obliquely towards the south accretion towards the north at the western end of the dune is attributed to reworking of the dune sand by easterly winds blowing inland from mcmurdo sound an unconformity within the dune suggests that the wind regime has varied in the past beneath the unconformity the direction of dune migration was from west to east and eastward extension of the dune continues at the present day however this is combined with accretion towards the north and west at the western end of the dune which is not recorded in the older sediments beneath the unconformity sand wedge structures that may have taken hundreds to thousands of years to form are found within the dune and suggest that arid conditions suitable for dune construction have persisted in victoria valley for a considerable time

exploring morphology layering and formation history of linear terrestrial dunes from radar observations implications for titan abstract understanding the morphology and internal layering of large aeolian dune fields through radar observations can provide unique insights into the climatic and geophysical conditions that led to their formation in this study we perform a large scale characterization of the morphology and internal layering of linear dunes in hyper arid areas on earth through utilizing multiple complementary radar datasets sir c imaging srtm interferometry derived elevations and radar sounding or ground penetrating radar gpr linear dune fields in the egyptian desert are of special interest due to their significance as planetary analogs to dunes on mars and saturn s largest moon titan satellite radar imagery and elevation data of the region show significant variance in the geomorphology of different dune fields in egypt in addition gpr probing of the first few meters suggests different inner settings in the layering of dunes of different ages in eastern and western egypt reflecting different paleo climatic regimes that led to their formation furthermore our radiometric analysis suggests that dunes with different inner layering arrangement also exhibit different radar backscatter returns as a function of their heights for relatively younger dunes with a homogeneous inner structure like the ones in eastern egypt in the qattaniya dune field we observe that sigma0 does not change as a function of the dune height for relatively older dunes in western egypt like the great sand sea northern siwa and southern dune fields we observed a linear correlation between sigma0 and the dune height thus surface properties of dunes like morphology and backscatter variation with height are related to inner characteristics like arrangement of internal layering relative ages and can be used to infer their depositional history linear dunes discovered in the equatorial regions of titan by the cassini huygens mission are morphologically very similar to these linear dune fields in the egyptian sahara hence assessing the variability of morphology and radar backscatter properties of titan s dunes as a function of their heights can help constrain the ambiguities associated with their internal structure and formation history and provide insights into titan s paleo wind regimes

true triaxial strength and deformability of the siltstone overlying the chelungpu fault chi chi earthquake taiwan 1 we conducted true triaxial compression tests to examine the effect of the intermediate principal stress 2 on brittle failure and deformability of the siltstone overlying the chelungpu fault taiwan for constant minimum stress 3 a pattern of increased strength 1 as a function of 2 unpredicted by common strength criteria was observed strength data are well fitted by a mogi modified nadai criterion relating the octahedral shear stress at failure to the mean normal stress on the induced shear fracture or fault the induced fault strikes subparallel to 2 and dips steeply in the direction of 3 significantly fault dip angle steadily increases as 2 rises for unchanged 3 sem inspection of tested specimens reveals only sporadic localized intergranular microcracks associated with the creation of the fault the modulus of elasticity and the onset of dilatancy exhibit a similar behavior to that of 1 as they increase with 2 when subjected to a constant 3

remote sensing and spatial analysis of aeolian sand dunes a review and outlook abstract for more than four decades remote sensing images have been used to document and understand the evolution of aeolian sand dunes early studies focused on mapping and classifying dunes recent advances in sensor technology and software have allowed investigators to move towards quantitative investigation of dune form evolution and pattern development these advances have taken place alongside progress in numerical models which are capable of simulating the multitude of dune patterns observed in nature the potential to integrate remote sensing rs spatial analysis sa and modeling to predict the future changes of real world dune systems is steadily becoming a reality here we present a comprehensive review of significant recent advances involving rs and sa our objective is to demonstrate the capacity of these technologies to provide new insight on three important research domains 1 dune activity 2 dune patterns and hierarchies and 3 extra terrestrial dunes we outline how several recent advances have capitalized on the improved spatial and spectral resolution of rs data the availability of topographic data and new sa methods and software we also discuss some of the key research challenges and opportunities in the application of rs and sa dune field including the integration of rs data with field based measurements of vegetation cover structure and aeolian transport rate in order to develop predictive models of dune field activity expanding the observational evidence of dune form evolution at temporal and spatial scales that can be used to validate and refine simulation models the development and application of objective and reproducible sa methods for characterizing dune field pattern and expanding efforts to quantify three dimensional topographic changes of dune fields in order to develop improved understanding of spatio temporal patterns of erosion and deposition overall our review indicates a progressive evolution in the way sand dunes are studied whereas traditional field studies of airflow and sand transport can clarify event based process form interactions investigators are realizing a synoptic perspective is required to address the response of dune systems to major forcings the integration and evolution of the technologies discussed in this review are likely to form a foundation for future advances in aeolian study

analysis of large aeolian wind blown bedforms using the shuttle radar topography mission srtm digital elevation data abstract dry land areas cover large parts of the land masses of earth of these a large portion is mantled by aeolian wind blown deposits and subjected to aeolian process i e the transport of sand or dust by wind one of the most prominent morphologies created by wind driven deposits are dunes and in their larger form draas or mega dunes these mega dunes tend to be widely spaced very large often forming a base on which smaller dunes form producing a compound dune pattern in february 2000 the shuttle radar topography mission onboard the space shuttle endeavour flew an interferometric radar system whose objective was to create a dem of nearly all earth land masses in this paper dem data from the c band 90 m data set and the x band 30 m data set were used to map and characterize the height and spacing of the large sand seas ergs on earth only the larger forms of dunes could be mapped reliably the dune spacing and heights extracted from the srtm dems tended to be in overall agreement with those reported in the literature in a comparison between the x band and the c band data it was found that the x band data are more sensitive to the smaller scale undulations on the compound dunes and better revealed the full height of the dunes

lateral migration of linear dunes in the strzelecki desert australia linear dunes in the strzelecki desert trend roughly south north sand transport which is toward the nne has caused the dunes to migrate eastward while they extend or migrate northward eastward lateral migration is evidenced by 1 asymmetrical shape of the dunes east facing slopes are several times as steep as west facing slopes 2 asymmetrical accumulation of loose recently transported sand relatively abundant on east facing slopes 3 asymmetrical outcropping of older semiconsolidated aeolian sand on the dune surface more abundant on west facing slopes and 4 east dipping foreset beds that underly the west facing flanks of some dunes dunes in the strzelecki desert are still active in the sense that sand is transported along and across many dune crests however the dunes are composed primarily of pleistocene strata indicating that the trend of the dunes was established before the holocene the obliquity of the dunes to the transport direction is not merely an aberration of the wind regime of the last few decades preferential accumulation of sand on east facing flanks indicates that the dunes migrated eastward several metres during the holocene moreover the west facing flanks of some dunes have experienced a minimum of tens of metres of erosion this asymmetric erosion and deposition were caused by dune obliquity and lateral migration that may have begun as early as the pleistocene dunes in the strzelecki desert and in the adjacent simpson desert display a variety of grossly different internal structures computer graphics experiments demonstrate that many of these differences in structure can be explained by different angles of climb of the dunes

why deposits of longitudinal dunes are rarely recognized in the geologic record dunes that are morphologically of linear type many of which are probably of longitudinal type in a morphodynamic sense are common in modern deserts but their deposits are rarely identified in aeolian sandstones one reason for non recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long term sand transport direction thereby depositing cross strata that have unimodal cross bed dip directions and consequently resemble deposits of transverse dunes dune parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along crest direction over the large dunes

dunes on titan observed by cassini radar thousands of longitudinal dunes have recently been discovered by the titan radar mapper on the surface of titan these are found mainly within 30 of the equator in optically near infrared and radar dark regions indicating a strong proportion of organics and cover well over 5 of titan s surface their longitudinal duneform interactions with topography and correlation with other aeolian forms indicate a single dominant wind direction aligned with the dune axis plus lesser off axis or seasonally alternating winds global compilations of dune orientations reveal the mean wind direction is dominantly eastwards with regional and local variations where winds are diverted around topographically high features such as mountain blocks or broad landforms global winds may carry sediments from high latitude regions to equatorial regions where relatively drier conditions prevail and the particles are reworked into dunes perhaps on timescales of thousands to tens of thousands of years on titan adequate sediment supply sufficient wind and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes

centrifuge modeling of reverse fault rupture propagation through single layered and stratified soil abstract many buildings have been built over active faults surface faulting as a main effect of earthquakes can cause damage such buildings properties of material over bedrock affect the pattern of rupture propagation through deposits and the width of the rupture zone on ground surface several researchers have primarily studied fault rupture propagation patterns on single layered granular soil however natural geologic deposits are rarely homogeneous and uniform but mostly consist of different layers in this study centrifuge model tests were conducted to investigate the patterns of reverse fault rupture propagation through single and double layered soil stratum it was observed that the soil type and sequence of layers affected the fault rupture propagation pattern as well as the surface rupture zone the results show that the width of deformation zone tends to decrease in stiffer soil in compare to softer soil these findings may help to produce a framework for predicting the occurrence and geometry of surface fault rupture zone that can help us to define a setback zone which is a major concern in building codes

radarclinometry of the sand seas of africa s namibia and saturn s moon titan abstract radarclinometry is a powerful technique for estimating heights of landforms in synthetic aperture radar sar images of planetary surfaces in particular it has been used to estimate heights of dunes in the sand seas of saturn s moon titan lorenz r d and 39 colleagues 2006 science 312 724 727 in this work we verify the technique by comparing dune heights derived from radarclinometry to known topography of dune fields in the namib sand sea of western africa we compared results from three different image grid spacings and found that 350 m pixel the same spacing at which the cassini radar data was processed is sufficient to determine dune height for dunes of similar morphometry to those of the namib sand sea at this grid spacing height estimates derived from radarclinometry are largely representative of though may underestimate by as much as 30 or overestimate by as much as 40 true dune height applying the technique to three regions on titan we estimate dune heights of 45 180 m and dune spacings of 2 3 3 3 km obtaining accurate heights of titan s dunes will help to constrain the total organic inventory on titan

the effect of the intermediate principal stress on fault formation and fault angle in siltstone abstract we conducted true triaxial compression tests on specimens prepared from two siltstone core sections one above and one below the chelungpu fault taiwan for different constant 2 and 3 magnitudes the maximum principal stress 1 was raised until a post failure stage was reached and a through going fault had developed despite differences between the properties of the two cores in all tests peak 1 increased as 2 was set at higher levels than 3 in contrast to mohr coulomb condition predictions the fault normal vector was aligned with the 3 direction and made an angle with 1 direction the angle which corresponds to fault dip in case of normal faulting increased monotonically with 2 for fixed 3 a variation that is also inconsistent with mohr coulomb theory the results of shear band localization theory are used with fault angles observed for axisymmetric compression and deviatoric pure shear to infer properties of the inelastic constitutive behavior these properties are significantly different for the two cores using them to predict for other deviatoric stress states yields good agreement with the observations for core ii and acceptable agreement for core i the results are used to predict the angle variation for constant mean normal stress decreases as the deviatoric stress state varies from axisymmetric extension to axisymmetric compression and at fixed deviatoric stress state decreases monotonically with increasing mean normal stress

sharad sounding radar on the mars reconnaissance orbiter 1 sharad shallow radar is a sounding radar provided by agenzia spaziale italiana asi as a facility instrument on the mars reconnaissance orbiter mission its 20 mhz center frequency and 10 mhz bandwidth complement the lower frequency relatively narrower bandwidth capability of the marsis sounding radar a joint italian u s team has guided the experiment development and is responsible for data analysis and interpretation the radar transmits signals at a 700 hz pulse repetition frequency prf and collects reflections from both the surface and near subsurface of mars vertical and horizontal resolutions are respectively 15 m free space and 3 6 km cross track by 0 3 1 km along track the scientific objective of sharad is to map in selected locales dielectric interfaces to at least several hundred meters depth in the martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials including competent rock soil water and ice a signal to noise ratio of 50 db for a specular surface return is achieved with 10 w of radiated power by using range and azimuth focusing in ground data processing preprocessed data as well as range and azimuth focused data will be formatted according to planetary data system pds standards and be made available from the asi science data center asdc and from the geosciences node of the planetary data system pds important targets for sharad include the polar layered deposits sedimentary stacks especially in terra meridiani buried channel systems buried impact craters volcanic complexes and shallow ice deposits in equilibrium with the atmosphere

geomorphic indicators of holocene winds in egypt s western desert abstract geomorphic mapping of egypt s western desert from landsat mss images reveals oriented aeolian landforms that record in part holocene winds wind directions reconstructed from these landforms indicate the dominance of n s airflow from 30 n to 20 n turning clockwise southward to ne sw conformable with modern circulation a second direction appears over western egypt w between 30 n and 26 n nw between 26 n and 20 n cross cutting aeolian landforms show that w nw winds are older than the n ne winds geomorphic evidence abundant south to 26 n and less abundant to 20 n also indicates that w and nw winds were early holocene palaeowesterlies some evidence also indicates that they extended eastward to at least 30 e perhaps to the red sea these winds steered moist atlantic mediterranean air masses to egypt sustaining early holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20 n upon aridification beginning after 5 kyr bp yardangs oriented west to east were eroded in early holocene basinal sediments in western egypt indicating that these winds continued there for 1 2 kyr until 3 4 kyr bp optically stimulated luminescence osl ages of surface sand sheet in southern egypt indicate that the present north south winds were established ca 3 4 kyr bp at the same time as the northern savanna boundary was stabilized at its present position

spaceborne and airborne imaging radar observations of sand dunes seasat and aircraft radar imagery of five areas of sand dunes in the southwestern united states and northwestern mexico have been studied and compared to landsat imagery and air photos radar imaging system parameters included two wavelengths 23 5 cm and 3 0 cm and incidence angles ranging from 0 through 70 horizontal polarization was used on both transmit and receive for all radar imagery resolution ranged from 12 m for the best aircraft imagery to about 40 m for seasat sand dunes studied behave as smooth surfaces for both 23 5 cm and 3 0 cm wavelength radars and the reflection of the radar energy from the dunes is specular measured surface irregularities of sand dunes are inadequate to cause either bragg or incoherent scattering of the incident radar beam in the 3 0 to 23 5 cm wavelength region this results in dunes that appear dark when imaged at incidence angles greater than the angle of repose because none of the dune faces are oriented properly normal to the incident radar beam to give a specular return to the antenna a scattering model based on specular return from randomly oriented flat dune faces was used and experimentally verified using aircraft data imaging radar resolution must be adequate to define the changing distribution of dune faces that characterize the dune morphology linear dune features with interdune spacing of less than 3 4 resolution elements are not recognizable distributed dune types such as barchans or stars must be more than about 10 resolution elements in each direction to be defined on the imagery illumination direction of the radar beam is important directional dune features must be oriented within about 60 of perpendicular to the radar illumination direction in order to be imaged if such features have considerable topographic relief the direction of radar illumination is less important because directional trends in unexplored regions cannot be predicted availability of radar imagery from two directions greatly facilitates interpretation of dune morphology and hence conclusions about causative wind regimes

relict drainages conical hills and the eolian veneer in southwest egypt applications to mars viking images indicate that no single process is sufficient to explain the origin and evolution of martian landforms landforms common to both earth and mars result from a complex interplay of geologic agents including water mass wasting wind and possibly ice the relict wadis of the gilf kebir plateau in southwest egypt strongly resemble fluvial channels on mars particularly those of the fretted terrain like the fretted channels on mars the egyptian wadis are dry flat floored and deeply incised in a plateau surface they have stubby theatre headed wide and deep tributaries that commonly join the trunk valleys at odd angles they apparently lack drainage basins and are separated by areas that appear on most of the available images to be undissected the wadis grade into adjacent low lying smooth plains mantled by eolian debris and dotted with myriads of conical and flat topped inselbergs that have obviously undergone mass wasting the unusual geomorphic characteristics of the gilf region are herein interpreted as the results of an extreme eolian and mass wasting overprint on a landscape originally carved by running water the relict networks of southwest egypt were initiated by streams that drained formerly extensive gilf highlands those highlands are inferred to have been a major tertiary watershed in north africa as quaternary dessication set in these rivers vanished and were replaced by ephemeral gullies cliff sapping wind abrasion and deflation have widened the gilf valleys and reduced many interfluves to clusters of flat topped and conical hills these now rise as inselbergs on the adjacent desert plains on which little evidence of former fluvial activity remains wind has deflated the surface and redistributed the finer particles from fluvial sediments and the debris of mass wasting into eolian sand sheets drifts and dunes dunes are migrating southward through the region along eolian thoroughfares that follow the apparent paths of ancient master streams toward distant basins of sand accumulation in the central sahara the fluvial and mass wasting origin of the particles in the eolian deposits the segregation of these materials according to grain size and the migration of those particles capable of saltation to areas of accumulation in lowland basins of the sahara are suggested as analogs for the formation and accumulation of sand sheets and dunes on the northern plains and in the polar erg on mars outliers of the martian plateau in the fretted terrain appear to have been dissected at least initially by channels whose upstream portions are incised in the uplands the martian wadis have many geomorphic peculiarities similar to those of the gilf wadis and like the egyptian features they have been attributed to mass wasting although basal sapping and removal of debris by wind have almost certainly modified the martian features their deep incision in the plateau and their inferred northward extensions on the northern plains seem to require not only initial downcutting by fairly energetic streams but also prolonged and long distance flow of water on mars as in southwest egypt running water is now nonexistent as suggested by milton 1973 the northern plains may be the most extensive of the landforms produced during a fluvial stage of martian history inferred integrated stream networks in the fretted terrain on mars as in the gilf plateau may be relics of earlier less arid climatic intervals when runoff collected and flowed with sufficient power to carve valleys in bedrock in this interpretation the uplands might once have stood as a watershed on a much less arid mars

return to thermal equilibrium of an intermittently drilled hole theory and experiment abstract in 1992 a 150 m deep borehole was drilled in ordovician shales in prague drilling lasted 1 month and was interrupted by frequent stoppages temperature recovery after drilling was observed until equilibrium was reached the maximum temperature disturbance in the upper part of the hole was 0 6 c 2 days after drilling and the temperature gradient was reduced to 14 1 mk m 1 at that time 17 8 0 6 mk m 1 in equilibrium to be able to appreciate the effect of stoppages in drilling the horner plot method was modified so that the discontinuity of drilling was respected it was demonstrated theoretically on two examples that neglect of discontinuity of the drilling might result in an error if the temperature undisturbed by drilling is estimated from non equilibrium temperature logs the horner plot method and the modified approach were applied to the non equilibrium temperature data estimates of the equilibrium temperature in the borehole were calculated and compared with the true measured equilibrium temperature if discontinuity of drilling was taken into account the deviation of the estimates from the equilibrium temperature was reduced more than twofold to less than 0 06 c in comparison with the horner plot method and the least squares temperature gradient determined from the estimates differed from the equilibrium gradient by less than its standar error we may conclude that the suggested method which includes the effect of discontinuous drilling and which was tested on measured data considerably improved the results in comparison with the horner plot technique

global pattern of titan s dunes radar survey from the cassini prime mission 1 we present a map of the orientation and extent of titan s sand dunes derived from the complete radar imaging dataset from the cassini prime mission the 16 000 dune segments we have mapped cover 8 of the titan s surface suggesting a total coverage of 20 are confined within 30 of the equator and show local and regional deviations of dune orientation of up to about 40 from due eastwards there is no obvious global longitudinal pattern although some divergence with latitude is apparent the most striking pattern is one of collimation by and divergence around bright and or high terrain obstacles 100 300 m high obstruct dunes when the local slope is 1 50 or steeper while slopes 1 200 or shallower cause dunes to thin out as they climb or to deviate around the obstacles

the sand seas of titan cassini radar observations of longitudinal dunes the most recent cassini radar images of titan show widespread regions up to 1500 kilometers by 200 kilometers of near parallel radar dark linear features that appear to be seas of longitudinal dunes similar to those seen in the namib desert on earth the ku band 2 17 centimeter wavelength images show 100 meter ridges consistent with duneforms and reveal flow interactions with underlying hills the distribution and orientation of the dunes support a model of fluctuating surface winds of 0 5 meter per second resulting from the combination of an eastward flow with a variable tidal wind the existence of dunes also requires geological processes that create sand sized 100 to 300 micrometer particulates and a lack of persistent equatorial surface liquids to act as sand traps

age and dynamics of linear dunes in the namib desert bristow c s duller g a t lancaster n 2007 age and dynamics of linear dunes in the namib desert geology 35 6 555 558 sponsorship american chemical society

detailed surface co seismic displacement of the 1999 chi chi earthquake in western taiwan and implication of fault geometry in the shallow subsurface the northern segment of the chelungpu fault shows an unusually large co seismic displacement from the event of the mw 7 6 chi chi earthquake in western taiwan part of the northern segment near the fengyuan city provides an excellent opportunity for characterizing active thrust related structures due to a dense geodetic benchmark network we reproduced co seismic deformation patterns of a small segment of this chelungpu fault using 924 geodetic benchmarks according to the estimated displacement vectors we identified secondary deformations such as local rigid block rotation and significant shortening within the hanging wall the data set also allows us to determine accurately a 3d model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip and the horizontal and vertical displacements at the surface the predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area the successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations

the chiuhsiungken fault a candidate to trigger a hazardous earthquake in western taiwan abstract the chiuhsiungken fault located along the boundary between the western foothills fold and thrust belt and the coastal plain in the chiayi yunlin area of taiwan has been identified as an active thrust through geological and geomorphic investigations young geomorphic features provided by shortening across the chiuhsiungken fault are well preserved and provide excellent examples to illustrate features provided by active thrust faulting historic seismicity records show that the deformation front in the chiayi yunlin area is very active and the fault slips at a rate of mm yr based on tectonic geomorphic investigation the chiuhsiungken fault has the potential to produce large earthquakes in the future the fault with a length about 15 km can be traced northward to the sinistral kuken transfer fault and southward to near the sinistral chuchi transfer fault the maximum potential magnitude of an earthquake triggered by the chiuhsiungken fault is estimated to be a m w 6 4 however the magnitude of the earthquake could be larger than expected if the adjacent faults rupture together with the chiuhsiungken fault

toward a better understanding of palaeoclimatic regimes that recharged the fossil aquifers in north africa inferences from stable isotope and remote sensing data abstract the isotopic composition of modern precipitation collected monthly from the international atomic energy agency iaea stations over north africa is enriched average d morocco 25 8 algeria 21 3 tunisia 20 6 libya 17 7 egypt 11 7 compared to fossil groundwater across north africa which shows progressive west to east depletion this indicates that precipitation from paleowind regimes was different from those prevailing in the present day late holocene however a few monthly records march 1982 december 1987 december 1991 march 2002 and april 2002 collected from iaea stations sidi barrani cairo rafah and el arish showed isotopic depletions d 56 to 43 7 approaching those of the fossil groundwater in egypt for each of the five events we investigated the direction of cloud propagation in relation to the isotopic composition of precipitation as it travels over the african continent and surroundings cloud propagation directions and air parcel flow paths were extracted from temporal every 30 min meteosat image products 1 infra red eumetsat 2 cloud motion winds cmw and 3 back trajectory models btm the inferred wind regimes for each of these events were then compared to those for the remaining egyptian records 160 that produced precipitation with enriched to intermediate isotopic compositions d 39 7 to 34 6 the extracted cloud propagation directions and the observed spatial variation in the isotopic compositions of the precipitating rain support a model that attributes previous wet climatic periods over north africa to the intensification of palaeowesterlies during glacial periods

spectroscopy morphometry and photoclinometry of titan s dunefields from cassini vims fine resolution 500 m pixel cassini visual and infrared mapping spectrometer vims t20 observations of titan resolve that moon s sand dunes the spectral variability in some dune regions shows that there are sand free interdune areas wherein vims spectra reveal the exposed dune substrate the interdunes from t20 are variously materials that correspond to the equatorial bright 5 m bright and dark blue spectral units our observations show that an enigmatic dark red spectral unit seen in t5 in fact represents a macroscopic mixture with 5 m bright material and dunes as its spectral endmembers looking more broadly similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes the presence of interdunes indicates that titan s dunefields are both mature and recently active the spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of titan various organics are consistent with the dunes measured reflectivity we measure a mean dune spacing of 2 1 km and find that the dunes are oriented on the average in an east west direction but angling up to 10 from parallel to the equator in specific cases where no interdunes are present we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m these results pave the way for future exploration and interpretation of titan s sand dunes

internal structure and surface geometry of longitudinal seif dunes abstract a mutual relationship exists between the morphology of longitudinal seif dunes and their dynamics and internal structure the basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank the magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line after heavy rain exposed the internal structure of the dune two kinds of depositional unit were found the main unit forms in the meandering area on both sides of the dune the other secondary unit is a narrow belt on both sides and parallel to the crest line the two units have different strikes because the laminae deposited on the dune are of the three primary stratification types climbing translatent strata sand flow cross strata and grainfall laminae the range of dips is wide 10 to 30 these dips accord with topographic profiles across the dune a model of the internal structure of a longitudinal dune has been developed from the field data the model shows the erosional and depositional areas along this linear dune

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

analyzing dune dynamics at the dune field scale based on multi temporal analysis of landsat tm images abstract studying dune dynamics at the dune field scale requires intensive fieldwork and a relatively huge dataset however this is not always possible with the high cost and the limited availability of data here we present a technique based on the analysis of multi temporal landsat tm images for studying the dynamics of different dune morphologies from five dune field sites around the world a pair of landsat tm images for each site has been used after performing several steps of image pre processing and enhancement i e image co registration radiometric calibration and histogram equalization rgb clustering is a commonly used algorithm for data compression and iso clustering in this study we implemented this technique as an unconventional change detection algorithm with 32 clustering partitions per band a bi temporal layerstack of the near infrared bands was the input for the rgb clustering process rgb clustering produced thematic maps that gave an immediate insight into the dune dynamics at the dune field scale given the availability of landsat images the technique gave the best results with dunes developed under uni directional wind pattern in addition it was also easier to detect migration of simple dunes than in compound or complex dunes these maps can be used as a bi temporal digital atlas of large dune systems and help in defining areas for detailed field measurements on dune dynamics and migration moreover it can be implemented in analyzing aeolian systems where field measurements are practically impossible i e large and extra terrestrial aeolian systems

evolution of fault patterns in clay experiments abstract the evolution of systems of strike slip faults from a single fault to intricate patterns is examined here in a series of clay experiments in the experiments plates of clay are loaded laterally under constant strain rate in plane strain with continuous monitoring of the stresses the experiments indicate that an individual fault appears as a linear trace about 1 mm in length that grows by in plane propagation of one or both ends next the fault evolves by out of plane propagation and interacts and coalesces with other subparallel faults this process generates a crooked nonlinear fault which at this point usually ceases to grow younger faults developing close to the older fault may cross and displace it the fault pattern in a sample changes in nature during a single run the early pattern is either domainal or conjugate depending on the initial fault distribution the second stage is usually domainal when subparallel faults develop close to each other during the final stage young faults cross old ones and generate conjugate patterns the relation between rheology and growth mechanisms of the faults is discussed faults in brittle elastic solids grow by out of plane propagation into zones of reduced normal stress whereas many of the faults in the present experiments grow by in plane propagation this propagation direction and the fault pattern evolution is interpreted here as indicating relaxation of nonuniform stresses at the proximity of the faults it is further argued that stress relaxation may be applicable to faulting of rocks in the upper crust

palaeoenvironmental reconstructions from linear dunefields recent progress current challenges and future directions abstract this paper reviews recent progress in the use of linear dunes as geoproxies of late quaternary environmental change summarises the challenges facing their use and explores some potential solutions to these challenges large areas of the swathes of linear dunes which occupy the continental interior of southern africa australia and parts of central asia and southern america currently have limited or negligible aeolian activity they have been recognised as offering potential information about past environments for more than a century but only with the widespread application of luminescence dating during the 1990s did they realistically start to offer the prospect of being an extensive dateable proxy of late quaternary palaeoenvironments and possibly palaeoclimates dating aeolian dune sands with luminescence methods is generally although not always relatively straightforward over the past twenty years a large number 1000 of luminescence ages have been added to the global dataset yet there has also been significant criticism of some of the rationale underpinning much of the interpretation of the records derived at the landscape scale developments of arguably equal importance have come from improved geomorphological understanding based on the wider availability of remotely sensed data and the paradigm of dunefield evolution as a self organising complex system current challenges are identified in three key regions incomplete understanding of how the process geomorphology of linear dunes affect the accumulation and preservation of sediment a lack of clarity regarding the temporal and spatial scale of the response in a dynamic environmental setting and uncertainty surrounding the drivers of changing rates of net accumulation solutions to these challenges lie within diverse research methodologies certainly further field study is required with improvement required in understanding system responses to changing environmental stimuli at scales from sedimentological to landscape in parallel the full implications of complex systems approaches to aeolian geomorphology for linear dunes lag behind the adoption of the concept to some other dune forms e g barchans the relationship between observed dune complexity and age from field data complicates previous suggestions that dune patterning may be purely a function of development time parallel to this lies the need to improve interpretation of the results of dune geochronological studies a suggestion is offered which aims to test the statistical significance of dune accumulation time series by comparison with a modelled system with unchanging external forcing despite the initial promise of linear dune records as a revolutionary source of palaeoenvironmental information in drylands the outcomes to date have been coarse although still valuable in resolution perhaps the most valuable realization has been that geomorphological understanding of these widespread landforms is incomplete

multifrequency and multipolarization radar scatterometry of sand dunes and comparison with spaceborne and airborne radar images understanding the unusual radar scattering characteristics of sand dunes is necessary in the analysis of radar images of aeolian landscapes of the earth and of other planets in this paper we report on airborne radar scatterometer data of sand dunes acquired at multiple frequencies and polarizations radar backscatter from sand dunes is very sensitive to the imaging geometry at small incidence angles the radar return is mainly due to quasi specular reflection from dune slopes favorably oriented toward the radar a peak return usually occurs at the incidence angle equal to the angle of respose for the dunes the peak angle is the same at all frequencies as computed from specular reflection theory at larger angles the return is significantly weaker the scatterometer measurements verified observations made with airborne and spaceborne radar images acquired over a number of dune fields in the united states central africa and the arabian peninsula the imaging geometry constraints indicate that possible dunes on other planets such as venus will probably not be detected in radar images unless the incidence angle is less than the angle of repose of such dunes and the radar look direction is approximately orthogonal to the dune trends

great sand sea and selima sand sheet eastern sahara geochronology of desertification the relation of playa sediments and associated archeological sites with longitudinal dunes allows estimation of ages for the two uppermost strata of the great sand sea active dune formation corresponds with interpluvial periods of hyperaridity dune stability corresponds with semiarid pluvial periods archeological sites associated with truncated paleosols in the selima sand sheet suggest a similar climatic relation and indicate that the isohyets of central sudan shifted at least 400 kilometers northward during the peak of pluvials

new airborne imaging radar observations of sand dunes kelso dunes california abstract high resolution multiple wavelength and polarization radar data of kelso dunes were acquired during the mojave field experiment they offer the opportunity to develop criteria for the selection of optimal wavelengths and polarizations for imaging of dune areas the backscatter from unvegetated dune surfaces at all wavelenghts c l and p band with direct hh vv polarization is largely controlled by dune topography and incidence angle these data especially p band provide the best images of dune topography the best discrimination between active and inactive dunes is obtained by cross polarized c band data cvh chv as a result of multiple scattering from vegetation and small scale topography e g shadow dunes

titan s surface from cassini radar sar and high resolution radiometry data of the first five flybys abstract the first five titan flybys with cassini s synthetic aperture radar sar and radiometer are examined with emphasis on the calibration and interpretation of the high resolution radiometry data acquired during the sar mode sar radiometry maps of the 2 cm wavelength brightness temperature are obtained coincident with the sar swath imaging with spatial resolution approaching 6 km a preliminary calibration shows that brightness temperature in these maps varies from 64 to 89 k surface features and physical properties derived from the sar radiometry maps and sar imaging are strongly correlated in general we find that surface features with high radar reflectivity are associated with radiometrically cold regions while surface features with low radar reflectivity correlate with radiometrically warm regions we examined scatterplots of the normalized radar cross section 0 versus brightness temperature finding differing signatures that characterize various terrains and surface features implications for the physical and compositional properties of these features are discussed the results indicate that volume scattering is important in many areas of titan s surface particularly xanadu while other areas exhibit complex brightness temperature variations consistent with variable slopes or surface material and compositional properties

new geomorphic data on the active taiwan orogen a multisource approach a multisource and multiscale approach of taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model dem side looking airborne radar slar and satellite spot imagery aerial photographs and control from independent field data this analysis enables us not only to present an integrated geomorphic description of the taiwan orogen but also to highlight some new geodynamic aspects well known major geological structures such as the longitudinal valley lishan pingtung and the foothills fault zones are of course clearly recognized but numerous previously unrecognized structures appear distributed within different regions of taiwan for instance transfer fault zones within the western foothills and the central range are identified based on analyses of lineaments and general morphology in many cases the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently in turn the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas examples are the conjugate pattern of strike slip faults within the central range and the oblique fold and thrust pattern of the coastal range furthermore neotectonic and morphologic analyses drainage and erosional surfaces have been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in taiwan such as for the longitudinal valley fault next at a more general scale numerical processing of digital elevation models resulting in average topography summit level or base level maps allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance finally a preliminary morphotectonic sketch map of taiwan combining information from all the sources listed above is presented