anisotropy and transverse isotropy a number of authors in the exploration literature have written about anisotropy but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis this note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical normal to the free surface and more general anisotropy with an azimuthal variation of properties these differences are important now that effective azimuthal shear wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks

ventifacts at the pathfinder landing site about half of the rocks at the mars pathfinder ares vallis landing site appear to be ventifacts rocks abraded by windborne particles comparable resolution images taken by the imager for mars pathfinder imp camera and the viking landers show that ventifacts are more abundant at the pathfinder site the ventifacts occur in several forms including rocks with faceted edges finger like projections elongated pits flutes grooves and possible rills the trends of elongated pits flutes grooves and rills cluster at 280 330 clockwise from north and generally dip 10 30 away from their trend direction these orientations are indicative of southeast to northwest winds and differ from the trend of wind tails at the landing site the direction of local wind streaks and predictions of the global circulation model all of which indicate northeast to southwest winds the disparity between these data sets strongly suggests that local circulation patterns have changed since the abrasion of the ventifacted rocks the greater number of ventifacts at the pathfinder site compared to either of the viking sites is most easily explained as being due to a larger supply of abrading particles composed of either sand sized grains or indurated dust aggregates and higher surface roughness which should increase the momentum of saltating grains the pathfinder ventifacts may have formed shortly after the deposition of outflow channel sediments nearly 2 gry ago when a large local supply of abrading particles should have been abundant and atmospheric conditions may have been more conducive to rock abrasion from saltating grains based on how ventifacts form on earth the several ventifact forms seen at the pathfinder site and their presence on some rocks but not on others are probably due to local airflow conditions original rock shape exposure duration rock movement and to a lesser extent rock lithology the abundance of ventifacts at the pathfinder site together with other evidence of weathering indicates that unaltered rock surfaces are rare on mars

erosion rates on mars and implications for climate change constraints from the pathfinder landing site the observation that the mars pathfinder landing site looks very similar to its appearance after it was deposited by catastrophic floods around 1 8 3 5 ga allows quantitative constraints to be placed on the rate of change of the site since that time the abundance of erosional features such as an exhumed former soil horizon sculpted wind tails ripplelike and other tag deposits and ventifacts fluted and grooved rocks all suggest the site has undergone net deflation or loss of 3 7 cm of material the presence of barchan dunes and ventifacts argues for erosion by saltating crystalline sand size particles entrained in the wind most ventifacts probably formed soon after the catastrophic flood which likely introduced a large fresh supply of sand size particles distributed across the rocky plain the strongest winds blew toward the northwest during this time resulting in the sculpting of ventifacts deflation of the surface collections of dunes within big crater and other lows and possibly preferentially eroding small crater rims the predominant wind direction changed to blow toward the southwest similar to today these winds further deflated the surface completed the deposition of sand size material in dunes and ultimately trapped these dunes in lows the erosional features observed by pathfinder indicate extremely low long term deflation rates of 0 01 0 04 nm yr since the end of the hesperian 1 8 3 5 ga similar to less precise rates of 1 nm yr based on the preservation of craters at the viking 1 and pathfinder landing sites short term redistribution rates deposition and removal of atmospheric dust at the pathfinder landing site and preexisting dust and sand at other locations on mars are up to 10 5 nm yr estimates of erosion rates on mars show a rapid decrease by 3 6 orders of magnitude from 10 2 10 4 nm yr in noachian terrains characterized by rimless flat floored craters and valley networks to the exceedingly slow rates 10 2 10 nm yr operating during the hesperian and amazonian noachian erosion rates comparable to low continental denudation rates on earth are consistent with erosion by running water and perhaps a more clement climate the rapid decrease in erosion rates is consistent with a major climatic change during the noachian at the tail end of heavy bombardment and a cold dry desiccating climate similar to today s since that time

large wind ripples on mars a record of atmospheric evolution wind blowing over sand on earth produces decimeter wavelength ripples and hundred meter to kilometer wavelength dunes bedforms of two distinct size modes observations from the mars science laboratory curiosity rover and the mars reconnaissance orbiter reveal that mars hosts a third stable wind driven bedform with meter scale wavelengths these bedforms are spatially uniform in size and typically have asymmetric profiles with angle of repose lee slopes and sinuous crest lines making them unlike terrestrial wind ripples rather these structures resemble fluid drag ripples which on earth include water worked current ripples but on mars instead form by wind because of the higher kinematic viscosity of the low density atmosphere a reevaluation of the wind deposited strata in the burns formation about 3 7 billion years old or younger identifies potential wind drag ripple stratification formed under a thin atmosphere

s local wave seismic anisotropy in the forearc above the subducted nazca plate between 33 s and 34 5 s s wave splitting from local earthquakes within the nazca plate that are deeper than the interplate seismogenic zone enabled the determination of the fast velocity direction and the lag time t in the forearc of the overriding plate data were collected from 20 seismic stations most of which were temporary deployed between 33 5 s and 34 5 s and included part of the normal subduction section to the south and part of the transitional section to flat subduction to the north the fast velocity direction has a complex pattern with three predominant directions northwest southeast north south and northeast southwest and relatively high t a quality evaluation of the highest measurements enabled us to identify possible cycle skipping in some of the measurements which could be responsible for the large observed lag time we consider that most of the anisotropy that was observed in the forearc is probably located in the mantle wedge and a minor part is located in the crust the complex pattern of splitting parameters when the anisotropy is associated at the mantle wedge could be the result of three dimensional variations in the subducting nazca plate at these latitudes also similarities between the splitting parameters and the principal compressional stress direction from pliocene and quaternary rocks suggest that the anisotropy in the crust could originate by tectonic local stress

degradation of the organic molecules in the shallow subsurface of mars due to irradiation by cosmic rays 1 detection of the organic matter on mars is one of the main goals of the future martian landing missions yet the degradation of organic molecules by cosmic ray irradiation on mars is often ignored we calculate the radiation dose accumulation rates from solar and galactic cosmic rays at various depths in the shallow martian subsurface we demonstrate that a 1 billion year outcrop on mars accumulates the dosage of 500 mgy in the top 0 2 cm and 50 mgy at 5 10 cm depths we show that the preservation of ancient complex organic molecules in the shallow 10 cm depth subsurface of rocks could be highly problematic if the exposure age of a geologic outcrop would exceed 300 myr we demonstrate that more simple organic molecules with masses 100 amu should have a good chance to survive in the shallow subsurface of rocks implications to the sampling strategy for the oncoming martian missions are discussed

transverse aeolian ridges tars on mars aeolian processes are probably the dominant ongoing surface modification process on mars as demonstrated by observations of dunes ripples yardangs ventifacts wind streaks dust devils and dust storms large dark dunes ldds are particularly common and were first recognized in the 1960 s from mariner data however another population of smaller often brighter ripple like aeolian bed forms that are morphologically distinct from ldds has been observed in recent higher resolution images calculations of saltation path length under martian conditions suggest these small bed forms could have formed as small barchanoid dunes or as mega ripples also known as granule ripples these features have therefore been called transverse aeolian ridges tars as this label has no genetic inference we have begun a study of tars that examines all high resolution mars orbiter camera data 1 5 to 8 m pixel resolution in a 45o longitude wide pole to pole swath we aim to determine the geographic distribution of tars assess whether they are active and to identify possible sediment sources we have already made some key observations first we present a new classification of tar deposits based on crest ridge morphology of individual forms and the size and degree of topographic control of the entire sediment mass second we present associations of tars with ldds slope streaks dust devils tracks and small impact craters that provide clues as to whether tars are currently active finally we present preliminary pole to pole survey results for the northern hemisphere that show clusters of tar deposits in the low latitudes suggesting possible unique sediment sources in these regions

anisotropy detected in shallow clays using shear wave splitting in a vsp survey1 shear waves can provide valuable information about seismic anisotropy on entering an anisotropic medium a shear wave generally splits shear wave splitting into a fast and a slow quasi shear wave with polarizations fixed by the elastic properties of the medium and direction of travel if the medium contains planar discontinuities with common normals the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities measuring this polarization using a vsp geometry with oriented three component geophones in the borehole offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth such a shear wave vsp was carried out in an uncased 0 3 m diameter borehole drilled to a depth of 120 m in the north of the netherlands the upper 80 m of the sequence consisting of a glacial till and sands and clays of pleistocene age was studied the clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured in our shallow vsp experiment shear wave splitting and therefore anisotropy was identified at various geophone depths for one source offset hodograms showed a consistent polarization of the fast shear wave component over a large depth interval under the assumption that the anisotropy was caused by planar discontinuities with common normals this polarization direction gives the strike of the fissures in this interval the polarization direction of the fast s wave did not correspond exactly with the strike which was obtained from geological information on the fissures the geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole the discrepancy however could be explained in terms of dipping fissures and such a dip was confirmed by the geological and geotechnical information the orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned this study thus illustrates a practical application of shear wave splitting observed in shallow shear wave vsp for geology and geotechnical engineering

rate of wind abrasion on mars estimation of the rate of aeolian abrasion of rocks on mars requires knowledge of 1 particle flux 2 susceptibilities to abrasion of various rocks and 3 wind frequencies on mars fluxes and susceptibilities for a wide range of conditions were obtained in the laboratory and combined with wind data from the viking meteorology experiment assuming an abundant supply of sand sized particles estimated rates range up to 2 1 10 2 cm of abrasion per year in the vicinity of viking lander 1 this rate is orders of magnitude too great to be in agreement with the inferred age of the surface based on models of impact crater flux the discrepancy in the estimated rate of abrasion and the presumed old age of the surface cannot be explained easily by changes in climate or exhumation of ancient surfaces we consider the primary reason to be related to the agents of abrasion either windblown grains are in very short supply or the grains are ineffective as agents of abrasion at least some sand sized 100 m grains appear to be present as inferred from both lander and orbiter observations high rates of abrasion occur for all experimental cases involving sands of quartz basalt or ash however previous studies have shown that sand is quickly comminuted to silt and clay sized grains in the martian aeolian regime experiments also show that these fine grains are electrostatically charged and bond together as sand sized aggregates laboratory simulations of wind abrasion involving aggregates show that at impact velocities capable of destroying sand aggregates form a protective veneer on the target surface and can give rise to extremely low abrasion rates

reflection seismology over azimuthally anisotropic media recent surveys have shown that azimuthal anisotropy due most plausibly to aligned fractures has an important effect on seismic shear waves previous work had discussed these effects on vsp data the same effects are seen in surface recording of reflections at small to moderate angles of incidence the anisotropic effects one different polarization components of vertically traveling shear waves permit the recognition and estimation of very small degrees of azimuthal anisotropy of order 2 1 percent as in an interferometer anisotropic effects on traveltime yield estimates of anisotropy which are averages over large depth intervals often raw field data must be corrected for these effects before the reflectors may be imaged two variations of a rotational algorithm to determine the principal time series are derived anisotropic effects on moveout lead to abnormal moveout unless the survey line is parallel to the fractures anisotropic effects on reflection amplitude permit the recognition and estimation of anisotropy hence fracture intensity differences at the reflecting horizon i e with high vertical resolution

core log integration studies in hole a of taiwan chelungpu fault drilling project summary taiwan chelungpu fault drilling project tcdp was initiated to understand the physical mechanisms involved in the large displacements of the 1999 taiwan chi chi earthquake continuous measurements of cores including laboratory work and a suite of geophysical downhole logs including p and s wave sonic velocity gamma ray electrical resistivity density temperature electrical borehole images and dipole shear sonic imager were acquired in hole a over the depth of 500 2003 m integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks a total of 10 subunits were divided on the basis of geophysical characteristics generally formation velocity and temperature increase with depth as a result of the overburden and thermal gradient respectively gamma ray resistivity formation density shear velocity anisotropy and density derived porosity are primarily dependent on the lithology zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from dipole shear imager dsi are associated with the appearance of fractures steep bedding and shear zones the fast shear wave azimuth is in good agreement with overall dip of the bedding approximately 30 towards se and maximum horizontal compressional direction particularly in the kueichulin formation showing strong shear wave velocity anisotropy bedding parallel fractures are prevalent within cores whereas minor sets of high angle nnw sse trending with n and s dipping fractures are sporadically distributed the fault zone at depth 1111 m fza1111 is the chi chi earthquake slip zone and could be a fluid conduit after the earthquake the drastic change in fast shear wave polarization direction across the underlying non active sanyi thrust at depth 1710 m reflects changes in stratigraphy physical properties and structural geometry

observations of an aeolian landscape from surface to orbit in gale crater abstract landscapes derived solely from aeolian processes are rare on earth because of the dominance of subaqueous processes in contrast aeolian derived landscapes should typify mars because of the absence of liquid water the long exposure times of surfaces and the presence of wind as the default geomorphic agent using the full range of available orbital and mars science laboratory rover curiosity images wind formed features in gale crater were cataloged and analyzed in order to characterize the aeolian landscape and to derive the evolution of the crater wind regime over time inferred wind directions show a dominance of regional northerly winds over geologic time scales but a dominance of topography driven katabatic winds in modern times landscapes in gale crater show a preponderance of aeolian features at all spatial scales interpreted processes forming these features include first cycle aeolian abrasion of bedrock pervasive deflation organization of available sand into bedforms abundant cratering and gravity driven wasting all of which occur over a background of slow physical weathering the observed landscapes are proposed to represent a spectrum of progressive surface denudation from fractured bedrock to retreating bedrock capped mesas to remnant hills capped by bedrock rubble to desert pavement plains this model of landscape evolution provides the mechanism by which northerly winds acting over 3 ga excavated tens of thousands of cubic kilometers of material from the once sediment filled crater thus carving the intra crater moat and exhuming mount sharp aeolis mons the current crater surface is relatively sand starved indicating that potential sediment deflation from the crater is greater than sediment production and that most exhumation of mount sharp occurred in the ancient geologic past

aeolian processes and dune morphology in gale crater abstract datasets at resolutions many times greater than previously available were used to study aeolian features within gale crater high resolution thermal inertia data allowed for detailed particle size estimation with the data sufficient to resolve dunefields a wide range of grain sizes have now been identified in the gale crater dunefields ranging from medium to very coarse sand high resolution imaging science experiment hirise and themis vis data allowed for detailed analysis of the dune morphology and slip faces which shows that the dunes have responded to topographic influences on prevailing wind directions under a present day wind regime this result was corroborated by a regional mesoscale model for the crater under dust storm conditions the central mound and smaller scale crater floor topography has altered the prevailing wind regime and dune patterns aeolian activity has thus played and continues to play an important role in shaping many of the present surface features of gale crater the arrival of a future lander mission such as the mars science laboratory would be able to sample these surface features directly and add a wealth of data to the understanding of gale crater

interpretation of synthetic common depth point gathers for a single anisotropic layer analysis of synthetic traveltime gathers shows that anisotropy may have a large enough effect on p sh and sv propagation to alter significantly the interpretation of the subsurface below the anisotropic layers consequently if anisotropy exists below a seismic line it is important to estimate the anisotropic parameters correctly we discuss the effects of anisotropy on seismic waves and present a method for estimating the elastic constants of a transversely isotropic layer from p and sh arrival time gathers the technique may be extended to more general anisotropic symmetries by analyzing gathers from several azimuths to illustrate the possible effect of anisotropy on exploration surveys p sh and sv velocity variations are calculated for several types of anisotropic sedimentary fabrics alignments due to bedding shale lithology and dry parallel cracks may have similar velocity variations fabrics with other configurations of cracks may still possess overall transversely isotropic symmetry but t

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

assessing the mineralogy of the watershed and fan deposits of the jezero crater paleolake system mars we present results from geomorphic mapping and visible to near infrared spectral analyses of the jezero crater paleolake basin and its associated watershed the goal of this study is to understand the provenance of the sedimentary deposits within this open basin lake using a source to sink approach two fan deposits located within the basin have distinct visible to near infrared mineralogic signatures measured by the compact reconnaissance imaging spectrometer for mars crism the northern fan is spectrally characterized by a mixture of mg rich carbonate and olivine while the western fan is characterized by fe mg smectite e g saponite or nontronite with variable amounts of mg rich carbonate and olivine in isolated exposures the watersheds of these deposits contain a variety of geomorphic units that are likely to have supplied sediment to the jezero crater paleolake as the fluvial valleys that fed the basin incise these units the geomorphic units include exposures of fe mg smectite olivine and mg rich carbonate bearing terrain we show that the difference in fan deposit mineralogy is a function of the areal exposure of the major geomorphic units within their watersheds this indicates that the spectrally dominant aqueous alteration minerals in the fan deposits are primarily detrital or transported in nature and did not form in situ we conclude that the aqueous alteration of the units in the watershed occurred prior to the fluvial activity that carved the valleys of the jezero crater paleolake system and that the two periods of aqueous activity are not genetically related

measurements of seismic anisotropy and fracture compliances in synthetic fractured media ultrasonic velocities were measured on a stack of synthetic material plexiglass at low 90 120 khz long wavelength range and high 480 khz short wavelength range frequencies plates were pressed together with uniaxial normal stress these measurements were repeated under different uniaxial pressures for two cases 1without circular rubber inclusions between synthetic fractures and 2with circular rubber inclusions between fractures calculation of anisotropy parameters and also normal and shear interface fracture compliances using linear calculus show reasonable variations anisotropy and compliances decreases as overburden pressure increases suggesting that anisotropy is caused by compliances at higher frequencies synthetic fractures seem to be stiffer than at lower frequencies for the long and short wavelength ranges similar variations were observed for anisotropy parameters and fracture compliances vs measurements in different directions show that for the case without rubber inclusions there is a good approximation of vti medium whereas for the case with inclusions there is a significant difference between vs travelling in directions parallel and perpendicular to the plates

carving intracrater layered deposits with wind on mars crater basins on mars host thick sedimentary sequences which record the environments of early mars these basin fills commonly exhibit mound morphologies thought to arise from aeolian erosion of initially crater filling strata this study presents transport based models explaining how mounds could be carved by wind wind tunnel experiments generated morphologies similar to those observed on mars and numerical modeling of flow over a crater using large eddy simulation les demonstrated a positive feedback between topographic focusing of flow and erosion potential observations of yardangs dunes and wind streaks all proxies for wind direction largely agree with model results where mound strata origins have been interpreted basal subaqueous deposits are overlain by aeolian deposits this stratigraphic progression culminating in wind driven excavation is consistent with a global desiccation event the occurrence of sedimentary mounds only on noachian terrain argues that this event was related to late noachian climatic change

sedimentological evidence for a deltaic origin of the western fan deposit in jezero crater mars and implications for future exploration abstract we examine the stratigraphic architecture and mineralogy of the western fan deposit in the jezero crater paleolake on mars to reassess whether this fan formed as a delta in a standing body of water as opposed to by alluvial or debris flow processes analysis of topography and images reveals that the stratigraphically lowest layers within the fan have shallow dips

regional wave propagation in turkey and surrounding regions this paper was published by the american geophysical union agu copyright 2000 agu see also http www agu org journals gl gl0003 1999gl008375 pdf 1999gl008375 pdf http atlas geo cornell edu turkey publications gok et al 2000 htm

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

bulk mineralogy of the ne syrtis and jezero crater regions of mars derived through thermal infrared spectral analyses abstract we investigated the area to the northwest of the isidis impact basin hereby referred to as nw isidis using thermal infrared emission datasets to characterize and quantify bulk surface mineralogy throughout this region this area is home to jezero crater and the watershed associated with its two deltaic deposits in addition to ne syrtis and the strong and diverse visible near infrared spectral signatures observed in well exposed stratigraphic sections the spectral signatures throughout this region show a diversity of primary and secondary surface mineralogies including olivine pyroxene smectite clays sulfates and carbonates while previous thermal infrared investigations have sought to characterize individual mineral groups within this region none have systematically assessed bulk surface mineralogy and related these observations to visible near infrared studies we utilize an iterative spectral unmixing method to statistically evaluate our linear thermal infrared spectral unmixing models to derive surface mineralogy all relevant primary and secondary phases identified in visible near infrared studies are included in the unmixing models and their modeled spectral contributions are discussed in detail while the stratigraphy and compositional diversity observed in visible near infrared spectra are much better exposed and more diverse than most other regions of mars our thermal infrared analyses suggest the dominance of basaltic compositions with less observed variability in the amount and diversity of alteration phases these results help to constrain the mineralogical context of these previously reported visible near infrared spectral identifications the results are also discussed in the context of future in situ investigations as the nw isidis region has long been promoted as a region of paleoenvironmental interest on mars

seismic attenuation tomography of the tonga fiji region using phase pair methods the anelastic structure of the region surrounding the tonga slab and lau back arc spreading center in the southwest pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs two differential attenuation methods determine t over the frequency band 0 1 to 3 5 hz for earthquakes in the tonga slab the s p method measures the difference in spectral decay between p and s waves arriving at the same station the p p method measures the difference in spectral decay for p waves with different paths through the upper mantle eight hundred sixty phase pairs are used to invert for two dimensional 1 q structure using a nonnegative least squares algorithm a grid search method determines the q q ratio most compatible with both the s p and p p differential measurements the highest attenuation q 90 is found within the upper 100 km beneath the active portions of the lau basin extending westward to the lau ridge these regions probably delineate the source region for the back arc spreading center magmas expected to be within the upper 100 km based on petrological considerations the high attenuation regions also correlate well with zones of low p wave velocity determined by regional velocity tomography somewhat lower attenuation is found beneath the fiji plateau than beneath the lau basin the entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km the slab is imaged as a region of low attenuation q 900 material a q q ratio of 1 75 provides the best fit between the s p and p p data sets upon inversion spectral stacking shows no frequency dependence within the frequency band analyzed

ancient martian aeolian processes and palaeomorphology reconstructed from the stimson formation on the lower slope of aeolis mons gale crater mars reconstruction of the palaeoenvironmental context of martian sedimentary rocks is central to studies of ancient martian habitability and regional palaeoclimate history this paper reports the analysis of a distinct aeolian deposit preserved in gale crater mars and evaluates its palaeomorphology the processes responsible for its deposition and its implications for gale crater geological history and regional palaeoclimate whilst exploring the sedimentary succession cropping out on the northern flank of aeolis mons gale crater the mars science laboratory rover curiosity encountered a decametre thick sandstone succession named the stimson formation unconformably overlying lacustrine deposits of the murray formation the sandstone contains sand grains characterized by high roundness and sphericity and cross bedding on the order of 1 m in thickness separated by sub horizontal bounding surfaces traceable for tens of metres across outcrops the cross beds are composed of uniform thickness cross laminations interpreted as wind ripple strata cross sets are separated by sub horizontal bounding surfaces traceable for tens of metres across outcrops that are interpreted as dune migration surfaces grain characteristics and presence of wind ripple strata indicate deposition of the stimson formation by aeolian processes the absence of features characteristic of damp or wet aeolian sediment accumulation indicate deposition in a dry aeolian system reconstruction of the palaeogeomorphology suggests that the stimson dune field was composed largely of simple sinuous crescentic dunes with a height of ca 10 m and wavelengths of ca 150 m with local development of complex dunes analysis of cross strata dip azimuths indicates that the general dune migration direction and hence net sediment transport was towards the north east the juxtaposition of a dry aeolian system unconformably above the lacustrine murray formation represents starkly contrasting palaeoenvironmental and palaeoclimatic conditions stratigraphic relationships indicate that this transition records a significant break in time with the stimson formation being deposited after the murray formation and stratigraphically higher mount sharp group rocks had been buried lithified and subsequently eroded

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

boulder fall activity in the jezero crater mars jezero crater is the landing site for the mars 2020 perseverance rover we report 63 boulder fall tracks within the crater using high resolution imaging science experiment hirise images the boulder tracks have both fresh and faded morphologies similar to those reported elsewhere on mars but reported for the first time in jezero crater we combine observations from 16 boulder tracks on the western delta deposit with 47 in the surrounding regions to infer possible process es of boulder destabilization which can be tested with rover observations this newly found hazard should be taken into account for rover operations boulders associated with tracks are geologically recent falls so it is possible that the surfaces of these boulders may provide an opportunity to sample material less exposed to radiation than other rocks at the martian surface and could be ideal targets to analyze for organics

mineralogy of an ancient lacustrine mudstone succession from the murray formation gale crater mars the mars science laboratory curiosity rover has been traversing strata at the base of aeolis mons informally known as mount sharp since september 2014 the murray formation makes up the lowest exposed strata of the mount sharp group and is composed primarily of finely laminated lacustrine mudstone intercalated with rare crossbedded sandstone that is prodeltaic or fluvial in origin we report on the first three drilled samples from the murray formation measured in the pahrump hills section rietveld refinements and fullpat full pattern fitting analyses of x ray diffraction patterns measured by the msl chemin instrument provide mineral abundances refined unit cell parameters for major phases giving crystal chemistry and abundances of x ray amorphous materials our results from the samples measured at the pahrump hills and previously published results on the buckskin sample measured from the marias pass section stratigraphically above pahrump hills show stratigraphic variations in the mineralogy phyllosilicates hematite jarosite and pyroxene are most abundant at the base of the pahrump hills and crystalline and amorphous silica and magnetite become prevalent higher in the succession some trace element abundances measured by apxs also show stratigraphic trends zn and ni are highly enriched with respect to average mars crust at the base of the pahrump hills by 7 7 and 3 7 times respectively and gradually decrease in abundance in stratigraphically higher regions near marias pass where they are depleted with respect to average mars crust by more than an order of magnitude in some targets the mn stratigraphic trend is analogous to zn and ni however mn abundances are close to those of average mars crust at the base of pahrump hills rather than being enriched and mn becomes increasingly depleted moving upsection minerals at the base of the pahrump hills in particular jarosite and hematite as well as enrichments in zn ni and mn are products of acid sulfate alteration on earth we hypothesize that multiple influxes of mildly to moderately acidic pore fluids resulted in diagenesis of the murray formation and the observed mineralogical and geochemical variations the preservation of some minerals that are highly susceptible to dissolution at low ph e g mafic minerals and fluorapatite suggests that acidic events were not long lived and that fluids may not have been extremely acidic ph 2ph 2 alternatively the observed mineralogical variations within the succession may be explained by deposition in lake waters with variable eh and or ph where the lowermost sediments were deposited in an oxidizing perhaps acidic lake setting and sediments deposited in the upper pahrump hills and marias pass were deposited lake waters with lower eh and higher ph

stratigraphy and paleohydrology of delta channel deposits jezero crater mars abstract the jezero crater open basin lake contains two well exposed fluvial sedimentary deposits formed early in martian history here we examine the geometry and architecture of the jezero western delta fluvial stratigraphy using high resolution orbital images and digital elevation models dems the goal of this analysis is to reconstruct the evolution of the delta and associated shoreline position the delta outcrop contains three distinct classes of fluvial stratigraphy that we interpret from oldest to youngest as 1 point bar strata deposited by repeated flood events in meandering channels 2 inverted channel filling deposits formed by avulsive distributary channels and 3 a valley that incises the deposit we use dems to quantify the geometry of the channel deposits and estimate flow depths of 7 m for the meandering channels and 2 m for the avulsive distributary channels using these estimates we employ a novel approach for assessing paleohydrology of the formative channels in relative terms this analysis indicates that the shift from meandering to avulsive distributary channels was associated with an approximately four fold decrease in the water to sediment discharge ratio we use observations of the fluvial stratigraphy and channel paleohydrology to propose a model for the evolution of the jezero western delta the delta stratigraphy records lake level rise and shoreline transgression associated with approximately continuous filling of the basin followed by outlet breaching and eventual erosion of the delta our results imply a martian surface environment during the period of delta formation that supplied sufficient surface runoff to fill the jezero basin without major drops in lake level but also with discrete flooding events at non orbital e g annual to decadal timescales

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

frosted granular flow a new hypothesis for mass wasting in martian gullies abstract recent gully deposits on mars have been attributed to both wet and dry mass wasting processes in this paper frosted granular flow fgf is presented as a new hypothesis for recent mass wasting activity in martian gullies fgf is a rare type of granular flow observed on a talus slope in the province of quebec canada hetu b van steijn h vandelac p 1994 geogr phys quat 48 3 22 frost reduces dynamic inter particle friction enabling flows to mobilize onto relatively low slope gradients 25 30 compared to those involving dry granular flow of the same material 35 41 resulting erosional and depositional features include straight to sinuous channels levees and digitate to branching arrangements of terminal deposits similar features are commonly found in association with geologically young gully systems on mars based on terrestrial observations of fgf processes the minimum criteria required for their occurrence on mars include i readily mobilized unconsolidated sediment at the surface ii an upper slope gradient at or near the angle of repose iii frost accumulation at the surface and iv triggering by rock fall all four conditions appear to be met in many areas on present day mars though triggering mechanisms may vary compared to terrestrial fgfs which are lubricated by thin liquid films at inter particle contacts those occurring on mars are more likely lubricated by vaporization of co 2 and small amounts of h 2 o frost that becomes incorporated in the translating mass some recent mass wasting activity in martian gullies therefore could be interpreted as the product of fgf

fluvial sedimentary deposits on mars ancient deltas in a crater lake in the nili fossae region 1 new spacecraft observations in the nili fossae region of mars reveal two valley networks 80 and 200 km long that each formed distributary fans as they entered an ancient 40 km diameter impact crater an outlet channel on the eastern crater rim lying at an elevation above the fans suggests these fans formed as subaqueous deltas in a crater lake water flowing in the valley networks entered the crater and deposited the fans and ponded to a maximum depth of about 450 m the lake then overtopped and breached the crater rim incising it and dropping the lake level by 100 m geological evidence combined with the volume of water necessary to fill the crater suggests the duration of flow through this system was prolonged the existence of the deposits described here implies that the early martian environment supported overland flow and ponding of water for an extended period of time

three dimensional attenuation structure beneath the northeastern japan arc estimated from spectra of small earthquakes abstract a fine scale three dimensional attenuation structure beneath the northeastern japan arc has been obtained using a joint inversion for source parameters site response and q values data used for this study are p wave spectra of microearthquakes occurring in the subducting pacific plate the subducting plate is found to have higher q values than the overlying mantle there is an along arc variation in the distribution of low q zones in the crust and the mantle wedge within the study area for the southern part of the study area low q zones are distributed in the crust beneath the volcanic front they extend into the mantle wedge and become deep toward the west that is toward the backarc region on the other hand the northern part of the study area has low q zones at two depth ranges at shallower depths just beneath the volcanic front and at the deeper part of the mantle wedge beneath the backarc region the region without active volcanoes along the volcanic front has relatively high mantle q values seismic activity in the lower plane of the double planed deep seismic zone is high beneath this relatively high q region of the mantle wedge spatial distribution of low q zones found in the present study is nearly consistent with that of the low velocity zones estimated from studies of travel times tomography partial melting zones in the mantle wedge whose existence has been suggested from the study of seismic velocity structure and laboratory experiments are located in the low q zones obtained in the present study these results suggest that spatial distribution of q values is closely related to heterogeneous distribution of temperature in the mantle wedge

mars global digital dune database mgd 3 global dune distribution and wind pattern observations abstract the mars global digital dune database mgd 3 is complete and now extends from 90 n to 90 s latitude the recently released south pole sp portion mc 30 of mgd 3 adds 60 000 km 2 of medium to large size dark dune fields and 15 000 km 2 of sand deposits and smaller dune fields to the previously released equatorial eq 70 000 km 2 and north pole np 845 000 km 2 portions of the database bringing the global total to 975 000 km 2 nearly all np dunes are part of large sand seas while the majority of eq and sp dune fields are individual dune fields located in craters despite the differences between mars and earth their dune and dune field morphologies are strikingly similar bullseye dune fields named for their concentric ring pattern are the exception possibly owing their distinctive appearance to winds that are unique to the crater environment ground based wind directions are derived from slipface sf orientation and dune centroid azimuth dca a measure of the relative location of a dune field inside a crater sf and dca often preserve evidence of different wind directions suggesting the importance of local topographically influenced winds in general however ground based wind directions are broadly consistent with expected global patterns such as polar easterlies intriguingly between 40 s and 80 s latitude both sf and dca preserve their strongest though different dominant wind direction with transport toward the west and east for sf derived winds and toward the north and west for dca derived winds

clay minerals in delta deposits and organic preservation potential on mars clay rich sedimentary deposits are often sites of organic matter preservation and have therefore been sought in mars exploration however regional deposits of hydrous minerals including phyllosilicates and sulphates are not typically associated with valley networks and layered sediments that provide geomorphic evidence of surface water transport on early mars the compact reconnaissance imaging spectrometer for mars crism has recently identified phyllosilicates within three lake basins with fans or deltas that indicate sustained sediment deposition eberswalde crater holden crater and jezero crater here we use high resolution data from the mars reconnaissance orbiter mro to identify clay rich fluvial lacustrine sediments within jezero crater which has a diameter of 45 km the crater is an open lake basin on mars with sedimentary deposits of hydrous minerals sourced from a smectite rich catchment in the nili fossae region we find that the two deltas and the lowest observed stratigraphic layer within the crater host iron magnesium smectite clay jezero crater holds sediments that record multiple episodes of aqueous activity on early mars we suggest that this depositional setting and the smectite mineralogy make these deltaic deposits well suited for the sequestration and preservation of organic material

a habitable fluvio lacustrine environment at yellowknife bay gale crater mars the curiosity rover discovered fine grained sedimentary rocks which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy this aqueous environment was characterized by neutral ph low salinity and variable redox states of both iron and sulfur species carbon hydrogen oxygen sulfur nitrogen and phosphorus were measured directly as key biogenic elements by inference phosphorus is assumed to have been available the environment probably had a minimum duration of hundreds to tens of thousands of years these results highlight the biological viability of fluvial lacustrine environments in the post noachian history of mars

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

study of the crustal thickness and the subducting lithosphere in greece from gravity data the area of greece has experienced a complex tectonic history dominated by the subduction of the african plate beneath eurasia in this study the variations of the crustal thickness in the area of greece were obtained by means of the multiple source werner deconvolution mswd method applied to gravity data thicknesses of 40 49 km are estimated beneath the hellenides mountain belt to the west eastward thinning of the crust to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the aegean region these results are in good agreement with recent seismological results demonstrating that the mswd method successfully treated the problem using the crustal model we derived we computed the gravity effect of the crust and extracted it from the bouguer anomaly we also extracted the gravity effect of the subducting lithosphere from the bouguer anomaly producing a residual map where most of the original gravity variation has been successfully removed the remaining anomalies appear related to near surface features and an area of low velocity mantle in the central aegean sea

stress induced upper crustal anisotropy in southern california 1 we use an automated method to analyze shear wave splitting from local earthquakes recorded by the southern california seismic network between 2000 and 2005 the observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression hmax suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern california is regional stress however at other stations fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock splitting delay times range widely within 0 2 s these upper crustal anisotropy observations together with previous studies of sks shear wave splitting surface waves and receiver functions suggest different mechanisms of anisotropy at different depths under southern california anisotropy in the upper crust appears to be in response to the current horizontal maximum compression hmax which differs from the cause of anisotropy in the lower crust and mantle we also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays

the environmental significance of ventifacts a critical review abstract ventifacts wind abraded clasts or rock surfaces are common features of many terrestrial hot and cold deserts coastal periglacial and mountain environments as well as on mars and have a long history of investigation this review paper discusses some of the main themes in terrestrial ventifact research including their formation and geomorphic controls the varied morphological forms of ventifacts in terrestrial environments and their environmental significance as indicators of past and present wind direction and sediment mobility are critically evaluated future research directions are identified

the effect of the aspect ratio on crack induced anisotropy1 media containing aligned cracks show anisotropy with respect to elastic wave propagation there are several models describing the wave propagation in cracked media most of them only valid for cracks with small aspect ratios one of these models crampin s model is compared with a model valid for all aspect ratios nishizawa s model the elastic constants and the group velocities are compared for both dry and liquid filled inclusions with aspect ratios ranging from 0 0001 flat cracks up to 1 spheres the difference between both models is small for small aspect ratios but becomes larger for increasing aspect ratios at a crack density of 0 05 both models give within an error of 5 the same results for aspect ratios up to 0 3 therefore crampin s model can be applied to a large range of cracked media even if the aspect ratio of the inclusions is not small the variation of the anisotropy as a function of the aspect ratio can be studied using thomsen s dimensionless parameters e and y they show how inclusions with large aspect ratios result in elliptical anisotropy

long term wind erosion on mars abstract wind erosion and deposition are powerful agents of surface change on mars erosion is sensitive to the atmospheric density so feedback between orbit variations and atmospheric density can enhance the sensitivity of erosion rates to orbital parameters we use statistics derived from a 1 gyr integration of the spin axis of mars coupled with runs of the nasa ames 3 d general circulation model gcm at a variety of orbital conditions and pressures to explore this feedback we find that wind erosion in the gcm is associated with two factors baroclinic zone winds and strong cross equator solstice flows both of these factors are influenced by topography producing an asymmetry in the erosion pattern between the north and the south the erosion model averaged over 1 gyr produces potential erosion rates of 1 7 10 5 m yr 1 in the north and 1 0 10 5 m yr 1 in the south which increase by an order of magnitude in an early 28 mbar atmosphere the stability of these potential erosion patterns over geological time indicates that the lowland regions of mars are continuously eroded and that wind is capable of revealing ancient sedimentary deposits or eroding substantial deposits that may have otherwise been preserved

seismic wave attenuation in the upper mantle beneath the aegean region a detailed study of p and s waves from earthquakes located along the hellenic arc and recorded by the greek seismological network shows an abnormal distribution of the seismic waves in the central aegean sea the data indicate a zone of anomalously high seismic wave attenuation in the upper mantle beneath the cyclades plateau and the inner part of the volcanic arc several independent observations suggest the presence of magma beneath the cyclades massif

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

radiation patterns of a shear wave vibrator in near surface shale two shallow surface to borehole field tests were conducted to investigate the radiation patterns of a shear wave vibrator in near surface shale in the in line direction for which baseplate motion is transverse to the source geophone raypath the vibrator produces a broadband high amplitude signal of the sh type and virtually no signal of the sv or p types the amplitude of the sh radiation is approximately constant as a function of polar angle but it falls off as the cosine of the azimuthal angle measured with respect to the in line direction in the broadside direction the vibrator produces a strong uncontaminated signal of the sv type the radiation pattern of the sv signature is lobate in the polar plane and peaks at an incident angle of 30 to 35 degrees in the horizontal plane the sv radiation varies as the sine of the azimuthal angle measured with respect to the in line direction the field tests were conducted on a permian shale in oklahoma and an eocene shale in east texas comparison between sh and sv wave surfaces indicates that both shales behave approximately like transversely isotropic solids with a vertical axis of symmetry theoretical wave surfaces are fit to the observed data by using elastic moduli determined from the field measurements and by making the assumption that the near surface shale at each field site is a single homogeneous inherently transversely isotropic material the analyses indicate that horizontal sh group velocity is about 30 percent greater than horizontal sv group velocity in the permian shale and about 25 percent greater in the eocene shale

modelling orthorhombic anisotropic effects for reservoir fracture characterization of a naturally fractured tight carbonate reservoir onshore texas usa in this study we present a step by step theoretical modelling approach using established seismic wave propagation theories in anisotropic media to generate unique anisotropic reflection patterns observed from three dimensional pure mode pressure 3d pp full azimuth and full offset seismic reflection data acquired over a naturally fractured tight carbonate field onshore texas usa our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns from the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shear wave splitting and variation in p wave velocity with offset and azimuth amplitude variation with azimuth avaz analysis also confirmed multi crack sets induced anisotropy which is characteristic of orthorhombic symmetry evident as multiple bright and dim amplitude azimuth directions as well as complete reversal of bright amplitude to dim amplitude azimuth direction as the angle of incidence increases from near 15 to mid 30 offsets finally we fitted the generated p wave velocity into an ellipse to determine the intensity and orientation n26e of the open crack set as well as the direction of the minimum in situ stress axis n116e within the reservoir the derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir which was on production decline despite reservoir studies that indicate un depleted reserves

ventifacts on earth and mars analytical field and laboratory studies supporting sand abrasion and windward feature development abstract terrestrial ventifacts rocks that have been abraded by windblown particles are found in desert periglacial and coastal environments on mars their abundance suggests that aeolian abrasion is one of the most significant erosional processes on the planet there are several conflicting viewpoints concerning the efficacy of potential abrasive agents principally sand and dust and the relationships between wind direction and ventifact form our research supported by a review of the literature shows that sand rather than dust or other materials is the principle abrasive agent on earth and mars relative to dust sand delivers about 1000 the energy onto rock surfaces on a per particle basis even multiple dust collisions will do little or no damage because the stress field from the impact is much smaller than the spacing of microflaws in the rock the abrasion profiles of terrestrial ventifacts are consistent with a kinetic energy flux due to saltating sand not airborne dust furthermore scanning electron microscope images reveal surfaces that are fractured and cleaved by sand grain impact with respect to their distribution ventifacts are found in regions that contain sand or did so in the past but are not found where only dust activity occurs contrary to some published reports our evidence from field studies analytical models and wind tunnel and other experiments indicates that windward not leeward abrasion is responsible for facet development and feature formation pits flutes and grooves leeward abrasion is confined to fluvial conditions in which the high viscosity and density of water are able to entrain sand size material in vortices therefore ventifacts and abraded terrain provide an unambiguous proxy for the direction of the highest velocity winds and can be used to reconstruct palaeowind flow

shear waves in lithosphere studies on the territory of the u s s r abstract thousands of kilometers of dss profiles were compiled with three component stations in the u s s r the results showed that the usual shots in holes or water basins normally create not only p waves but shear waves as well the most favourable condition for their generation is the presence of a sharp seismic boundary near the source for example the basement surface the s wave field as a rule contains all the types of reflected and refracted waves that are found in the p field the difference is in lower frequencies they are lower by one third somewhat higher intensity and greater variability of amplitudes of shear waves when recording the s waves the major information is obtained from the velocity relation v p v s with depth and laterally it reveals that three major factors are affecting this relation the degree of rock fissuring the composition of rocks and the temperatures at depth reduction of fissuring with depth i e with greater distances from the source results in an overall drop of as the composition of the uppermost crust changes the urals increases in blocks composed of basic rocks this value shows little changes on the baltic and ukrainian shields on the siberian platform first increases with depth from 1 71 up to 1 76 probably due to the dilatancy effect and then it decreases to values less than 1 7 in the lower crust and upper mantle in western siberia grows with depth reaching 1 79 in the lower crust it is somewhat reduced in the mantle but still above 1 7 this can be ascribed apparently to a higher temperature regime of the lower crust in western siberia in many regions a splitting of the shear waves of different polarization is observed due to velocity anisotropy this was found for the first refracted waves in the urals and for reflected waves from the m boundary on the siberian platform in both cases anisotropy is associated with the crust on the ukrainian and baltic shields the difference in the velocities of sh and sv waves was not recorded

wind driven erosion and exposure potential at mars 2020 rover candidate landing sites aeolian processes have likely been the predominant geomorphic agent for most of mars history and have the potential to produce relatively young exposure ages for geologic units thus identifying local evidence for aeolian erosion is highly relevant to the selection of landing sites for future missions such as the mars 2020 rover mission that aims to explore astrobiologically relevant ancient environments here we investigate wind driven activity at eight mars 2020 candidate landing sites to constrain erosion potential at these locations to demonstrate our methods we found that contemporary dune derived abrasion rates were in agreement with rover derived exhumation rates at gale crater and could be employed elsewhere the holden crater candidate site was interpreted to have low contemporary erosion rates based on the presence of a thick sand coverage of static ripples active ripples at the eberswalde and southwest melas sites may account for local erosion and the dearth of small craters moderate flux regional dunes near mawrth vallis were deemed unrepresentative of the candidate site which is interpreted to currently be experiencing low levels of erosion the nili fossae site displayed the most unambiguous evidence for local sand transport and erosion likely yielding relatively young exposure ages the downselected jezero crater and northeast syrtis sites had high flux neighboring dunes and exhibited substantial evidence for sediment pathways across their ellipses both sites had relatively high estimated abrasion rates which would yield young exposure ages the downselected columbia hills site lacked evidence for sand movement and contemporary local erosion rates are estimated to be relatively low

the geometry of the wadati benioff zone and lithospheric kinematics in the hellenic arc abstract accurate locations of 961 shallow and intermediate depth earthquakes which occurred between 1956 and 1995 in the hellenic arc 34 39 n 19 28 e are used to define the plate boundaries in this area reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc an ocean continent type of interaction occurs on a curved surface which is defined by the shallow branch 20 100 km of the wadati benioff zone the intersection of this zone with the earth s surface is a curve which follows the convex outer side of the sedimentary arc western peloponnese west of cythera south coast of crete east coast of rhodes and dips at low angle 30 to the aegean sea coupling between the subducted oceanic crust and the overriding of the aegean lithospheric plate takes place on this surface the deep branch 100 180 km of the wadati benioff zone is dipping freely without coupling at a high angle 45 beneath the south aegean trough and the volcanic arc the high shallow seismicity h 20 km which is observed in the southwestern convex outer side of the arc ionian sea is attributed to the fast southwestward motion of the aegean plate location of strong deep earthquakes h 100 km in the fore arc area of the southwestern part of the hellenic arc west of cythera indicates that oceanic crust is destroyed in this part of the hellenic trench due to roll back of the descending lithospheric slab

martian fluvial conglomerates at gale crater observations by the mars science laboratory mast camera mastcam in gale crater reveal isolated outcrops of cemented pebbles 2 to 40 millimeters in diameter and sand grains with textures typical of fluvial sedimentary conglomerates rounded pebbles in the conglomerates indicate substantial fluvial abrasion chemcam emission spectra at one outcrop show a predominantly feldspathic composition consistent with minimal aqueous alteration of sediments sediment was mobilized in ancient water flows that likely exceeded the threshold conditions depth 0 03 to 0 9 meter average velocity 0 20 to 0 75 meter per second required to transport the pebbles climate conditions at the time sediment was transported must have differed substantially from the cold hyper arid modern environment to permit aqueous flows across several kilometers

the rock abrasion record at gale crater mars science laboratory results from bradbury landing to rocknest ventifacts rocks abraded by wind borne particles are found in gale crater mars in the eastward drive from bradbury landing to rocknest they account for about half of the float and outcrop seen by curiosity s cameras many are faceted and exhibit abrasion textures found at a range of scales from submillimeter lineations to centimeter scale facets scallops flutes and grooves the drive path geometry in the first 100 sols of the mission emphasized the identification of abrasion facets and textures formed by westerly flow this upwind direction is inconsistent with predictions based on models and the orientation of regional dunes suggesting that these ventifact features formed from very rare high speed winds the absence of active sand and evidence for deflation in the area indicates that most of the ventifacts are fossil features experiencing little abrasion today

high frequency shear wave propagation across the hellenic subduction zone abstract this study investigates shear wave propagation across the hellenic subduction zone using high quality waveforms of 71 intermediate depth events 40 160 km depth the data have been recorded by a permanent seismic network consisting of 22 broadband three component stations that cover most parts of greece shear waves can be efficiently transmitted with low attenuation q s 1100 through the descending slab which extends laterally over much of the forearc area with a northwest southeast strike the slab also appears to be continuous downdip in the depth range 60 160 km even though it was found that a gap smaller than 10 km would not affect shear wave efficiency significantly two highly attenuating areas have been detected one in the central southeast aegean corresponding to the mantle wedge and the other in the northern aegean probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area these results provide additional constraints for the geometry of the hellenic subduction zone and the geodynamic setting of the aegean region

pervasive aeolian activity along rover curiosity s traverse in gale crater mars the nasa mars science laboratory msl rover curiosity has safely landed near a 35 km long dark dune field in gale crater on mars this dune field crosses the landing site from the northeast to the southwest and lies along curiosity s traverse to aeolis mons here we present the first evidence of recent aeolian activity in the form of ripple and dune migration and further estimate wind directions within the dune field through analysis of ripple and dune morphologies and the mars regional atmospheric modeling system mrams we measured a minimum ripple migration rate of 0 66 m per earth year and dune migration rate of 0 4 m per earth year in the southwest portion of the field a strongly bidirectional ripple crestline orientation nearly orthogonal dune slipfaces and linear seif or oblique dunes indicate a bidirectional wind regime with winds mainly coming from the ene and from the northwest however mrams results indicate primary winds from the ene our constraints on the wind regime provide the unique opportunity to use ground measurements from msl to test the accuracy of winds predicted from orbital data

scarp orientation in regions of active aeolian erosion on mars abstract the morphologies of wind formed features on mars provide diagnostic information about ancient and modern surface winds aeolian erosional features include decimeter scale ventifacts and kilometer scale yardangs but intermediate scale erosional features are less well understood understanding aeolian erosion may be critical to identifying ancient martian biosignatures cosmogenic radiation destroys complex organic molecules during prolonged exposure at the martian surface but outcrops freshly re exposed by aeolian erosion provide potential sites where biosignatures could have been protected and made recently available for sampling wind driven scarp retreat has been cited as the cause for young exposure ages measured in gale crater upcoming exploration by the mars 2020 rover will focus on jezero crater another location of extensive aeolian erosion and meter scale scarps this work is motivated by the hypothesis that retreating scarps on mars may prefer orientations that reflect the direction of erosive winds we mapped scarps in jezero and gale craters and compared their orientation distributions with local wind regimes interpreted from other aeolian indicators no strong correlation between wind direction and scarp orientation was identified the near random distribution of scarp orientations suggests that in the locations studied the dominant processes controlling scarp orientation are either processes that do not prefer an orientation e g impact or thermal fracturing processes or that turbulent flow structures form at the scale of scarp topography and obscure the regional scale signals of erosion with scarp scale eddies and flow deflection

three term taylor series for t2 x2 curves of p and s waves over layered transversely isotropic ground the arrival time curve of a reflection from a horizontal interface beneath a homogeneous isotropic layer is a hyperbola in the x t domain if the subsurface is one dimensionally inhomogeneous horizontally layered or if some or all of the layers are transversely isotropic with vertical axis of symmetry the statement is no longer strictly true though the arrival time curves are still hyperbola like in the case of transverse isotropy however classical interpretation of these curves fails interval velocities calculated from t 2 x 2 curves do not always approximate vertical velocities and therefore cannot be used to calculate depths of reflectors to study the relationship between velocities calculated from t 2 x 2 curves and the true velocities of a transversely isotropic layer we approximate t 2 x 2 curves over a vertically inhomogeneous transversely isotropic medium by a three term taylor series and calculate expressions for these terms as a function of the elastic parameters it is shown that both inhomogeneity and transverse isotropy affect slope and curvature of t 2 x 2 curves for p waves the effect of transverse isotropy is that the t 2 x 2 curves are convex upwards for sv waves the curves are convex downwards for sh waves transverse isotropy has no effect on curvature

three dimensional seismic attenuation structure beneath the aegean region and its tectonic implication abstract the three dimensional attenuation structure beneath the aegean sea and the surrounding regions is determined by inversion of seismic intensity data a large number of seismic intensity data have been accumulated in a uniform scale in the aegean region where the seismic activity is much higher than that of the other mediterranean regions nearly 7000 seismic intensity data from 50 earthquakes that have occurred in these regions are used to determine seismic attenuation structure and source acceleration of the earthquakes the resultant structure reveals a remarkable contrast of attenuation in the top layer depth 0 40 km low q is dominant in the aegean sea while high q is dominant in the surrounding land areas except for the southwestern turkey the low q regions correspond to areas of neogene quaternary grabens where the high seismicity of shallow earthquakes appears in the upper mantle high q zones corresponding to the subducting african plate dominate along the hellenic arc the irregular shape of high q zones might reflect the split or disintegration of the african slab some low q spots corresponding to the distribution of volcanoes exist along the volcanic arc the low q spots might correspond to diapirs causing the subduction volcanism

complex patterns of fluid and melt transport in the central andean subduction zone revealed by attenuation tomography abstract we present a high resolution 3 d model of p wave attenuation q p 1 for the central andean subduction zone data from 1500 mostly intermediate depth 60 250 km earthquakes recorded at three temporary seismic networks covering the forearc arc and backarc around 23 s were used for tomographic inversion the forearc is characterised by uniformly high q p values indicating low temperature rocks in accordance with low surface heat flow values prominent low q p anomalies are found beneath the magmatic arc and the backarc in the crust and mantle continuous regions of low q p connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge where they induce melting explain our observations we propose that low q p regions indicate source and ascent pathways of metamorphic fluids and partial melts ascent of fluids and melts as imaged by seismic q p are not vertical as is often implicitely assumed instead sources of fluids are located at different depth levels and ascent paths are complex and exhibit significant variation within the study area the largest quaternary backarc volcano cerro tuzgle is fed by mantle melts which are imaged as a plume of low q p material that reaches to the strong earthquake cluster at 200 km depth

periodic bedrock ridges on mars 1 evidence for sediment transport and erosion by wind is widespread over the surface of mars today and was likely a major geomorphic process for much of its geological past although martian surface features resembling aeolian dunes and ripples have been recognized since the mariner and viking missions such features have been interpreted previously as active indurated or exhumed sedimentary forms here we report evidence based on high resolution imaging science experiment images that show some megaripple forms are eroded into cohesive substrate rather than being composed of loose granular material or fossilized dunes exposure of stratigraphic continuity within layered cohesive material extending crest to trough through features with mean wavelengths of 18 to 51 m demonstrates the primarily erosional formation of what we term periodic bedrock ridges pbrs hence some surfaces on mars previously considered to be covered by wind deposited material are actually wind carved exposures that offer windows into martian history pbrs lack the distinctive streamlining associated with wind parallel yardangs and comparison of pbr orientation to yardangs megayardangs and active sedimentary dunes in the same vicinity confirm that these pbrs formed transverse to prevailing winds observed wavelengths of pbrs are comparable to those predicted by a simple model for erosional wavelengths of periodic transverse bed forms owing to the spacing of flow separations within the flow recognition of these transverse aeolian erosional forms brings up the question of how widespread martian pbrs are and how many have been misinterpreted as active or indurated fossilized sedimentary dunes