a large scale anomaly in enceladus microwave emission abstract the cassini spacecraft flew by enceladus on 6 november 2011 configured to acquire synthetic aperture radar imaging of most of the surface with the radar instrument the pass also recorded microwave thermal emission from most of the surface we report on global patterns of thermal emission at 2 17 cm based on this data set in the context of additional unresolved data both from the ground and from cassini the observed thermal emission is consistent with dielectric constants of pure water or methane ice but cannot discriminate between the two the emissivity is similar to those of other icy satellites 0 7 consistent with volume scattering the most intriguing result however is an anomaly in the thermal emission of enceladus leading hemisphere evidence presented here suggests the anomaly is buried at depths on the order of a few meters this anomaly is located in similar geographic location to anomalies previously detected with the cirs and iss instruments on mimas tethys and dione howett c j a et al 2011 icarus 216 221 226 howett c j a et al 2012 icarus 221 1084 1088 howett c j a et al 2014 icarus 241 239 247 schenk p et al 2011 icarus 211 740 757 but also corresponds with a geological feature on enceladus leading terrain crow willard e pappalardo r t 2011 global geological mapping of enceladus in epsc dps joint meeting 2011 p 635 simple models show that the crow willard and pappalardo crow willard e pappalardo r t 2011 global geological mapping of enceladus in epsc dps joint meeting 2011 p 635 model is a better fit to the data our best supported hypothesis is that the leading hemisphere smooth terrain is young enough 3 5 m a picture consistent with ground and space radar measurements which show no variation at 2 cm but an increase in albedo in the anomaly region at 13 cm

effects of prestress state and rupture velocity on dynamic fault branching 1 we consider a mode ii rupture which propagates along a planar main fault and encounters an intersection with a branching fault using an elastodynamic boundary integral equation formulation allowing the failure path to be dynamically self chosen we study the following questions does the rupture initiate along the branch does it continue is the extensional or compressional side most favored for branching does rupture continue on the main fault too failure is described by a slip weakening law for which the strength at any amount of slip is proportional to normal stress our results show that dynamic stresses around the rupture tip which increase with rupture velocity at locations off the main fault plane relative to those on it could initiate rupture on a branching fault as suggested by prior work whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity the most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults however it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the rayleigh velocity natural examples seem consistent with the simulations that we present

self healing slip pulses in dynamic rupture models due to velocity dependent strength abstract seismological observations of short slip duration on faults short rise time on seismograms during earthquakes are not consistent with conventional crack models of dynamic rupture and fault slip in these models the leading edge of rupture stops only when a strong region is encountered and slip at an interior point ceases only when waves from the stopped edge of slip propagate back to that point in contrast some seismological evidence suggests that the duration of slip is too short for waves to propagate from the nearest edge of the ruptured surface perhaps even if the distance used is an asperity size instead of the entire rupture dimension what controls slip duration if not dimensions of the fault or of asperities in this study dynamic earthquake rupture and slip are represented by a propagating shear crack for all propagating shear cracks slip velocity is highest near the rupture front and at a small distance behind the rupture front the slip velocity decreases as pointed out by heaton 1990 if the crack obeys a negative slip rate dependent strength relation the lower slip velocity behind the rupture front will lead to strengthening that further reduces the velocity and under certain circumstances healing of slip can occur the boundary element method of hamano 1974 is used in a program adapted from andrews 1985 for numerical simulations of mode ii rupture with two different velocity dependent strength functions for the first function after a slip weakening displacement the crack follows an exponential velocity weakening relation the characteristic velocity v 0 of the exponential determines the magnitude of the velocity dependence at dynamic velocities the velocity dependence at high velocity is essentially zero when v 0 is small and the resulting slip velocity distribution is similar to slip weakening if v 0 is larger rupture propagation initially resembles slip weakening but spontaneous healing occurs behind the rupture front the rise time and rupture propagation velocity depend on the choice of constitutive parameters the second strength function is a natural log velocity dependent form similar to constitutive laws that fit experimental rock friction data at lower velocities slip pulses also arise with this function for a reasonable choice of constitutive parameters slip pulses with this function do not propagate at speeds greater than the raleighwave velocity the calculated slip pulses are similar in many aspects to seismic observations of short rise time in all cases of self healing slip pulses the residual stress increases with distance behind the trailing edge of the pulse so that the final stress drop is much less than the dynamic stress drop in agreement with the model of brune 1976 and some recent seismological observations of rupture

radar detection of iapetus saturn s satellite iapetus has the largest albedo asymmetry of any natural satellite with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere 1 1 the sharp contrast on this synchronously rotating satellite suggests that exogenic materials such

constraining the heat flux between enceladus tiger stripes numerical modeling of funiscular plains formation abstract the cassini spacecraft s composite infrared spectrometer cirs has observed at least 5 gw of thermal emission at enceladus south pole the vast majority of this emission is localized on the four long parallel evenly spaced fractures dubbed tiger stripes however the thermal emission from regions between the tiger stripes has not been determined these spatially localized regions have a unique morphology consisting of short wavelength 1 km ridges and troughs with topographic amplitudes of 100 m and a generally ropy appearance that has led to them being referred to as funiscular terrain previous analysis pursued the hypothesis that the funiscular terrain formed via thin skinned folding analogous to that occurring on a pahoehoe flow top barr a c preuss l j 2010 icarus 208 499 503 here we use finite element modeling of lithospheric shortening to further explore this hypothesis our best case simulations reproduce funiscular like morphologies although our simulated fold wavelengths after 10 shortening are 30 longer than those observed reproducing short wavelength folds requires high effective surface temperatures 185 k an ice lithosphere or high viscosity layer with a low thermal conductivity one half to one third that of intact ice or lower and very high heat fluxes perhaps as great as 400 mw m 2 these conditions are driven by the requirement that the high viscosity layer remain extremely thin 200 m whereas the required conditions are extreme they can be met if a layer of fine grained plume material 1 10 m thick or a highly fractured ice layer 50 m thick insulates the surface and the lithosphere is fractured throughout as well the source of the necessary heat flux a factor of two greater than previous estimates is less obvious we also present evidence for an unusual color spectral character of the ropy terrain possibly related to its unique surface texture our simulations demonstrate that producing the funiscular ridges via folding remains plausible but the relatively extreme conditions required to do so leaves their origin open to further investigation the high heat fluxes required to produce the terrain by folding which equate to an endogenic blackbody temperature near 50 k should be observable by future nighttime cirs observations if funiscular deformation is occurring today

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

laboratory earthquakes along inhomogeneous faults directionality and supershear we report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch rupture is always found to be asymmetric bilateral in one direction rupture always propagates at the generalized rayleigh wave speed whereas in the opposite direction it is subshear or it transitions to supershear the lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in parkfield california and in north anatolia

evidence for melt lubrication during large earthquakes 1 rotary friction experiments have been performed on westerly granite at start velocities of 2 0 4 0 m s 1 under loads of 250 500 n until the system stalls up to 4 s interface temperature velocity and force were measured at intervals of 0 2 milliseconds the results show that kinetic friction increases in direct proportion to temperature up to a critical value tcr whereupon the system changes from boundary lubrication slip strengthening to hydrodynamic lubrication slip weakening with an associated decrease in friction and temperature for westerly granite tcr is 1150 c this corresponds to the melting point of feldspar which constitutes 50 of the rock if tcr is exceeded in natural slip systems which will depend on rock type this can result in the generation of friction melt melt lubrication can explain the scaled energy of large earthquakes being 10 to 100 times more than that of small earthquakes

new cassini radar results for saturn s icy satellites abstract cassini radar tracks on saturn s icy satellites through the end of the prime mission in 2008 have increased the number of radar albedo estimates from 10 ostro et al 2006 to 73 the measurements sample diverse subradar locations and for dione rhea and iapetus almost always use beamwidths less than half the target angular diameters thereby constraining the satellites global radar albedo distributions the echoes result predominantly from volume scattering and their strength is thus strongly sensitive to ice purity and regolith maturity the combination of the cassini data set and arecibo 13 cm observations of enceladus tethys dione rhea black et al 2007 and iapetus black et al 2004 discloses an unexpectedly complex pattern of 13 to 2 cm wavelength dependence the 13 cm albedos are generally smaller than 2 cm albedos and lack the correlation seen between 2 cm and optical geometric albedos enceladus and iapetus are the most interesting cases we infer from hemispheric albedo variations that the e ring has a prominent effect on the 13 cm radar lightcurve the uppermost trailing side regolith is too fresh for meteoroid bombardment to have developed larger scale heterogeneities that would be necessary to elevate the 13 cm radar albedo whereas all of enceladus is clean and mature enough for the 2 cm albedo to be uniformly high for iapetus the 2 cm albedo is strongly correlated with optical albedo low for the optically dark leading side material and high for the optically bright trailing side material however iapetus 13 cm albedo values show no significant albedo dichotomy and are several times lower than 2 cm values being indistinguishable from the weighted mean of 13 cm albedos for main belt asteroids 0 15 0 10 the leading side s optically dark contaminant must be present to depths of at least one to several decimeters so 2 cm albedos can mimic the optical dichotomy however it does not have to extend any deeper than that the fact that both hemispheres of iapetus look asteroid like at 13 cm means that coherent backscattering itself is not nearly as effective as it is at 2 cm since iapetus entire surface is mature regolith the wavelength dependence must involve composition not structure either the composition is a function of depth everywhere with electrical loss much greater at depths greater than a decimeter or two or the intrinsic electrical loss of some pervasive constituent is much higher at 13 cm than at 2 cm ammonia is a candidate for such a contaminant if ammonia s electrical properties do not depend on frequency and if ammonia is globally much less abundant within the upper one or two decimeters than at greater depths then coherent backscattering would effectively be shut down at 13 cm explaining the asteroid like 13 cm albedo

implications of fault constitutive properties for earthquake prediction the rate and state dependent constitutive formulation for fault slip characterizes an exceptional variety of materials over a wide range of sliding conditions this formulation provides a unified representation of diverse sliding phenomena including slip weakening over a characteristic sliding distance dc apparent fracture energy at a rupture front time dependent healing after rapid slip and various other transient and slip rate effects laboratory observations and theoretical models both indicate that earthquake nucleation is accompanied by long intervals of accelerating slip strains from the nucleation process on buried faults generally could not be detected if laboratory values of dc apply to faults in nature however scaling of dc is presently an open question and the possibility exists that measurable premonitory creep may precede some earthquakes earthquake activity is modeled as a sequence of earthquake nucleation events in this model earthquake clustering arises from sensitivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and assigns physical interpretation to aftershock parameters the seismicity formulation predicts large changes of earthquake probabilities result from stress changes two mechanisms for foreshocks are proposed that describe observed frequency of occurrence of foreshock mainshock pairs by time and magnitude with the first mechanism foreshocks represent a manifestation of earthquake clustering in which the stress change at the time of the foreshock increases the probability of earthquakes at all magnitudes including the eventual mainshock with the second model accelerating fault slip on the mainshock nucleation zone triggers foreshocks

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

increase of vulnerability of karst aquifers due to leakage from landfills karst aquifers are very vulnerable to various pollution sources among these landfills can contribute contaminants over long periods groundwater quality monitoring is required to assess the impact of a landfill leachate on underlying aquifer water or spring discharge tracer tests are useful for selecting the location and frequency of sampling three landfill sites in karst areas were studied with tracer tests additional insight was accomplished by the comparison of the tracer transfer and the breakthrough curves obtained from these tests with the results of two other tracer tests carried out in the same period in karst areas without a landfill to explain the differences observed the hypothesis of increased permeability below the landfill due to the presence of inorganic acids in leachates was further tested the results of detailed long term monitoring of contaminated drip water in the postojna cave were used to verify the hypothesis the analysis of the simultaneously increased content of calcium magnesium and contaminants in the drip water indicates a direct correlation between limestone dissolution and contaminants increased dissolution increases secondary porosity and thus permeability of the vadose zone resulting in higher vulnerability of underlying aquifers and springs in the vicinity landfills

europa ganymede and callisto new radar results from arecibo and goldstone observations of the icy galilean satellites conducted during 1987 1991 with the arecibo 13 cm system and the goldstone 3 5 cm system yield significant improvements in our knowledge of the satellites radar properties hardly any wavelength dependence is seen for either the total power radar albedo or the circular polarization ratio c for europa ganymede and callisto our 13 cm estimates of mean values and rms dispersions are 1 39 0 14 and 0 69 0 06 and c 1 53 0 03 1 43 0 06 and 1 17 0 04 radar albedo features are seen on each satellite evidence for c features is lacking except for indications of a weak hemispheric asymmetry for callisto that intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in c is consistent with predictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance the satellites albedo distributions overlap the most prominent radar features are tentatively identified with galileo regio and the valhalla basin estimates of echo doppler frequencies show callisto to be lagging its ephemeris by 200 50 km

acidic mine drainage in carbonate terrains geochemical processes and rates of calcite dissolution abstract in situ rates of calcite dissolution were measured by weight loss of single crystals placed in an east tennessee stream that is affected by acidic mine drainage the dissolution rates ranged from 2 6 10 8 to 7 9 10 8 mmol cm 2 s 1 over the entire ph range of 3 6 7 7 calcite dissolution rates were several orders of magnitude slower than observed in laboratory solutions of co 2 hcl h 2 o caco 3 and appeared to be inhibited by dissolved iron and possibly other solutes in the stream

three dimensional earthquake sequence simulations with evolving temperature and pore pressure due to shear heating effect of heterogeneous hydraulic diffusivity 1 a new methodology for three dimensional 3 d simulations of earthquake sequences is presented that accounts not only for inertial effects during seismic events but also for shear induced temperature variations on the fault and the associated evolution of pore fluid pressure in particular the methodology allows to capture thermal pressurization tp due to frictional heating in a shear zone one dimensional 1 d diffusion of heat and pore fluids in the fault normal direction is incorporated using a spectral method which is unconditionally stable accurate with affordable computational resources and highly suitable to earthquake sequence calculations that use variable time steps the approach is used to investigate the effect of heterogeneous hydraulic properties by considering a fault model with two regions of different hydraulic diffusivities and hence different potential for tp we find that the region of more efficient tp produces larger slip in model spanning events the slip deficit in the other region is filled with more frequent smaller events creating spatiotemporal complexity of large events on the fault interestingly the area of maximum slip in model spanning events is not associated with the maximum temperature increase because of stronger dynamic weakening in that area the region of more efficient tp has lower interseismic shear stress which discourages rupture nucleation there contrary to what was concluded in prior studies seismic events nucleate in the region of less efficient tp where interseismic shear stress is higher in our model hypocenters of large events do not occur in areas of large slip or large stress drop

on the transient behavior of frictional melt during seismic slip s n and g d t were supported by a european research council starting grant project acronym usems and by a progetti di eccellenza fondazione cassa di risparmio di padova e rovigo we are grateful to nick beeler and to an anonymous referee for their constructive reviews and their help to improve the clarity of the manuscript

laboratory observations of transient frictional slip in rock analog materials at co seismic slip rates and rapid changes in normal stress knowledge of frictional shear resistance and its dependency on slip distance slip velocity normal stress and surface roughness is fundamental information for understanding earthquake physics and the energy released during such events in view of this in the present study plate impact pressure shear friction experiments are conducted to investigate the frictional resistance in a rock analog material i e soda lime glass under interfacial conditions of relevance to fault rupture the results of the experiments indicate that a wide range of frictional slip conditions exist at the slip interface ranging from initial no slip and followed by slip weakening slip strengthening healing and seizure all during a single slip event the slip weakening phase is understood to be most likely due to thermal induced flash heating and incipient melting at asperity junctions while the slip strengthening slip healing phase is understood to be a result of coalescence and solidification of local melt patches on the slip interface in addition plate impact pressure shear normal stress change drop experiments are employed to probe the response of the slip interface due to sudden alterations in normal stress in particular the location timing of the stress drop is varied so as to investigate the behavior of the slip interface in its slip weakening slip strengthening healing phase or the seized phase in response to sudden drop in normal stress these experimental results provide a rich set of data to better understand the range of possible friction slip states that can be achieved and or critically examine existing dynamic friction models for fault slip behavior

the environmental hazards of locating wastewater impoundments in karst terrain abstract a wastewater storage lagoon failed due to the development of a sinkhole at a site in the lehigh river valley in allentown pennsylvania pa the polluted wastewater from the lagoon entered into the underlying aquifer and moved within a narrow pathway controlled by cracks fissures and solution channels within the karstified allentown formation of the cambrian period the allentown formation serves as the principal aquifer for the public water supply of the area to develop appropriate remedial measures a thorough understanding of the geologic setting was required therefore a geologic and hydrogeologic characterization of the area was completed aerial photography and satellite imagery interpretations were performed stratigraphic core holes were drilled and geophysically logged and the data correlated to define structural control and movement of ground water and pollutants a number of wells were drilled and constructed and water levels were monitored on a continuous basis to correlate with climatic changes and determine the direction of flow water samples were collected periodically and analyzed to delineate the vertical and lateral extent of migration of pollutants five saturated water bearing zones were identified within the bedrock based on the analysis of cores and interpretation of geophysical logs ground water in the lower zones is polluted the concentration of pollution increases with depth monitoring stations were established in the creek south of the site to measure flow rate several times during different seasons and at different reaches to determine the losing and gaining sections of the creek pumping tests were conducted to determine hydraulic characteristics of the aquifer based on the hydrogeologic model of the karstified aquifer flow regime and structural control a plan of action was defined and initiated to remediate the aquifer the ground water is being remediated using a pump and treat methodology the cleanup effort is continuous and the pollutant level is fluctuating with an overall declining trend the application of this technology has also created a pressure trough thereby controlling off the site migration of pollutants

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

infrared spectroscopic characterization of the low albedo materials on iapetus abstract iapetus one of the large satellites of saturn has been studied over the centuries for its signature brightness contrast light on one side and dark on the opposite it has recently been suggested that the dark material is a combination of native and exogenous materials with distinct histories we present an analysis of parts of the cassini regio the darkest region on the leading hemisphere of iapetus focusing on the hydrocarbon signature with a view to detect and investigate differences in the material s we find variations in the hydrocarbon bands with geographic location one type predominantly located on the leading hemisphere a comparison with the equivalent spectral features on phoebe and hyperion reveals a predictable resemblance between the leading hemisphere material and phoebe and an unexpected likeness between hyperion s darkest material and iapetus trailing hemisphere surface an analysis of the slope in the visible part of the spectrum is strongly affected by a rise in the continuum 0 35 0 65 m attributed to rayleigh scattering from nano size particles on the surface the continuum rise varies in strength with the albedo and h 2 o ice content and when it is properly accounted for the overall slope in all the identified spectral units is the same over the interval 0 35 2 3 m independent of albedo or ice abundance the interpretation of current and previous results offers two different scenarios illustrated by the presence of one vs two dark materials distributed over the iapetus surface we describe the scenarios and their implications the appearance of the aromatic and aliphatic absorption bands together in their measured relative strengths makes this spectral signature unique and thus enables the comparison among the three satellites

cassini radar observations of enceladus tethys dione rhea iapetus hyperion and phoebe cassini 2 2 cm radar and radiometric observations of seven of saturn s icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy galilean satellites average radar albedos decrease in the order enceladus tethys hyperion rhea dione iapetus and phoebe this sequence most likely corresponds to increasing contamination of near surface water ice which is intrinsically very transparent at radio wavelengths plausible candidates for contaminants include ammonia silicates metallic oxides and polar organics ranging from nitriles like hcn to complex tholins there is correlation of our targets radar and optical albedos probably due to variations in the concentration of optically dark contaminants in near surface water ice and the resulting variable attenuation of the high order multiple scattering responsible for high radar albedos our highest radar albedos for enceladus and tethys probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex i e mature enough for there to be high order multiple scattering within it at the other extreme phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering iapetus 2 2 cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side whereas 13 cm results reported by black et al black g j campbell d b carter l m ostro s j 2004 science 304 553 show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2 2 cm these iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths and if the leading side s optically dark contaminant is present to depths of at least one to several decimeters as argued by lanzerotti et al lanzerotti l j brown w l marcantonio k j johnson r e 1984 nature 312 139 140 a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of saturn s icy satellites given the hypersensitivity of water ice s absorption length to ammonia concentration an increase in ammonia with depth could allow efficient 2 2 cm scattering from within the top one to several decimeters while attenuating 13 cm echoes which would require a six fold thicker scattering layer if so we would expect each of the icy satellites average radar albedos to be higher at 2 2 cm than at 13 cm as is the case so far with rhea black g campbell d 2004 bull am astron soc 36 1123 as well as iapetus

dynamic weakening of gouge layers in high speed shear experiments assessment of temperature dependent friction thermal pressurization and flash heating 1 the rotary shear of gouge demonstrates dramatic weakening at coseismic slip rates but the inherent variation in shear rate with radius of the rotary shear configuration prohibits determining friction constitutive properties from measures of whole sample response here representative results of rotary shear high speed experiments including both constant velocity tests 0 1 1 3 m s and constant acceleration tests 0 1 3 m s 0 05 and 0 1 m s2 are analyzed using a thermal mechanical and hydrologic finite element method fem model to constrain friction constitutive properties and test hypotheses of dynamic weakening by thermal pressurization and flash heating the observed frictional behavior of room dry gouge can be explained by using a state variable friction constitutive relation in which the friction coefficient is inversely proportional to temperature and by employing a two mechanism constitutive formulation in which the friction coefficient increases with temperature temperature strengthening at low temperatures and decreases with temperature temperature weakening at higher temperatures water dampened gouge displays a transient weakening during the early stages of constant acceleration tests fem analysis indicates the weakening could reflect thermal pressurization of pore water provided both the permeability of the gouge layer and the sealing capacity of the teflon sleeve used to contain the gouge during shear contribute to restricting fluid flow microstructural observations indicate that the dynamic weakening coincides with slip localization and temperature increase by frictional heating which are conditions that favor weakening by flash heating at steady state the relationship between slip rate and coefficient of friction by the fem model analysis is consistent with predictions of micromechanical models for weakening by flash heating

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

constraining fault constitutive behavior with slip and stress heterogeneity 1 we study how enforcing self consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event we explore features of rupture dynamics that 1 lead to slip heterogeneity in earthquake ruptures and 2 maintain these conditions following rupture so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events our three dimensional finite element simulations of magnitude 7 events on a vertical planar strike slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop initial shear stress and breakdown work fracture energy are spatially heterogeneous in these models the breakdown work is on the order of mj m 2 which is comparable to the radiated energy these conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model an alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence which also makes them difficult to implement in numerical models we hypothesize that self consistent ruptures could also be produced by very narrow slip pulses propagating in a self sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kj m 2

the study of hydrodynamic behaviour of a complex karst system under low flow conditions using natural and artificial tracers the catchment of the unica river sw slovenia in this study a multi tracer test with fluorescent tracers was combined with time series analyses of natural tracers to characterize the dynamics of the solute transport through different recharge pathways and to study hydraulic behaviour of a binary karst system under low flow conditions fluorescent tracer testing included the introduction of uranine amidorhodamine g or naphthionate at three injection points sampling and monitoring took place at two karst springs malenscica unica and at two underground rivers pivka rak recharging the unica river at the polje of planina sw slovenia other monitored parameters included precipitation spring or underground river discharge water temperature and electrical conductivity water samples were collected and analyzed for total organic carbon mg2 so42 and no3 in the laboratory in the study area results of the tracer test suggest that contaminant transport in karst may be retarded for several weeks during low flow conditions followed by increases in contaminant concentrations after subsequent rainfall events based on interpretation of tracer concentration breakthrough curves low apparent dominant flow velocities i e between 5 8 and 22 8 m h through the well developed karst conduits and 3 6 m h through the prevailing vadose zone with a dominant influence of a diffuse recharge were obtained together with analyses of hydro chemographs the artificial tracing identified different origins of water recharging the studied aquifer during prolonged low water conditions the malenscica spring is mainly recharged from the karst aquifer and the unica spring by the sinking pivka river after more intensive rainfall events allogenic recharge from cerknica prevails in the malenscica spring while the unica spring drains mainly the allogenic water from the pivka valley these findings of alternating hydraulic connections and drainage areas due to respective hydrological conditions are important and should be considered when monitoring water quality implementing groundwater protection measures and optimizing future water exploitation

comparative application of two methods cop and paprika for groundwater vulnerability mapping in mediterranean karst aquifers france and spain a comparative test of two vulnerability mapping methods cop and paprika specifically dedicated to for karst aquifers was carried out on two mediterranean carbonate aquifers the vulnerability maps obtained for each aquifer present important differences to identify and determine the origin of these differences the results were statistically analyzed using sensitivity analysis coefficients of determination and scatter graphs in addition the global vulnerability gv parameter was used to measure the general vulnerability of the aquifer and to compare the results obtained this statistical analysis led us to conclude that the main cause of differences between these two methods used to assess aquifer vulnerability lie in the relative importance of the parameters employed in calculating the vulnerability index for the paprika method the variable related to infiltration slope and karst features has the most influence with less weight being assigned to the protective capacity of layers overlying the aquifer for the cop method the most influent variable is defined by the layers overlying the aquifer together with infiltration characteristics determined by the relative importance of different forms of infiltration in each aquifer the vulnerability mappings performed using the cop method present greater coherence with the known hydrogeological behavior of the study areas especially the spanish aquifers nevertheless further hydrogeological investigations are needed such as ones to validate the obtained vulnerability maps

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

effects of normal stress perturbations on the frictional properties of simulated faults 1 we report on laboratory experiments to investigate the frictional response of creeping faults to sudden changes in normal stress experiments were conducted on layers of quartz powder bare surfaces of westerly granite and layers of a 50 50 mixture of quartz powder and smectite clay powder the tests were carried out at room temperature and controlled humidity using a servo controlled double direct shear configuration normal stress perturbations corresponding to loading and unloading of tectonic fault zones were applied during steady sliding at constant loading rate from 3 to 1000 m s shear strain rates of 1 5 10 3 to 0 5 s 1 sudden changes in normal stress resulted in a linear elastic response of shear stress followed by a transient evolution of friction over a characteristic displacement the transient inelastic response is quantified as ln 0 where is the transient change in shear stress following a step change from initial normal stress 0 to final normal stress we find that is independent of sliding velocity and varies with ambient relative humidity and shear loading history for unloading we document a transition from stable to unstable behavior as a function of net slip in the range 3 to 30 mm shear strains of 1 5 to 15 increased humidity led to higher values of for pure quartz gouge but smaller for the quartz clay gouge the effects of shear displacement and humidity are discussed in the context of particle characteristics and gouge fabric development the extended rate and state dependent friction laws using one state variable and the ruina evolution law with normal stress variation describe our observations

how can the epikarst zone influence the karst aquifer hydraulic behaviour the role of an epikarst zone in the karst aquifer hydraulic behaviour was brought into focus in our studies referring to the catchment area of the hubelj spring sw slovenia this study points out the significance of effects of the fast preferential flow epiflow which is the main factor controlling solute contaminant transport towards the aquifer saturated zone the so called epikarstic hypothesis is verified on the basis of the most significant research results that are supported by the most important findings from the literature

dust and snow cover on saturn s icy moons the final analysis of the cassini radar observations of saturn s icy moons presented here shows that the exchange of material between the planet s dust rings and moons which is specific to the saturnian system plays a key role in the current state of the airless satellite regolith far from saturn the vast debris ring from phoebe progressively coats the leading side of iapetus with optically dark material reducing its radar brightness on the contrary close to the planet the extreme radar brightness of the innermost moons mimas enceladus and tethys that exceeds that of the galilean satellites is most likely related to enceladus s geysers and the e ring which brings ultra clean water ice to their surfaces the measured radar albedos and observed hemispheric dichotomies require at least a few decimeters thick snow cover and that the near surfaces of saturn s innermost moons contain especially efficient backscattering structures whose nature remains an outstanding problem

microwave dielectric constant of liquid hydrocarbons application to the depth estimation of titan s lakes 1 cassini radar reveals the surface of titan since flyby ta acquired on october 2004 the radar instrument discovered volcanic structures craters dunes channels lakes and seas in particular flyby t16 realized in july 2006 imaged tens of radar dark features close to titan s north pole they are interpreted as lakes filled with liquid hydrocarbons mainly methane a key material in the geologic and climatic history of titan in order to perform quantitative analysis and modeling of the radar response of titan s lakes the dielectric constant of liquid hydrocarbons is a crucial parameter in particular to estimate the radar wave attenuation we present here first measurements of the dielectric constant of lng liquefied natural gas mainly composed of methane at ku band 10 13 ghz we obtained a value 1 75 0 002j this value is used to model the radar backscattering of lakes observed during t16 flyby using a two layer scattering model we derive a relationship that is used to estimate a minimum depth for titan s lakes the proposed relationship is also coherent with the observation that the larger and then the deeper lakes are also the darker in radar images

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

dynamic shear rupture interactions with fault bends and off axis secondary faulting 1 on the basis of elastodynamic stress fields for singular crack and nonsingular slip weakening models of propagating rupture we develop preliminary answers to such questions as follows if a rupturing fault is intersected by another providing a possible bend in the failure path when will stressing be consistent with rupture along the bend what secondary fault locations and orientations in a damaged region bordering a major fault will be stressed to failure by the main rupture stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed especially near the limiting speed rayleigh for mode ii shear for mode iii whether a bend path once begun can be continued to larger scales depends on principal stress directions and ratios in the prestress field conditions should often be met in mode ii for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger scale stressing a basis for intermittent rupture propagation and spontaneous arrest secondary failure in the damage zone likewise increases markedly as the limiting speed is approached such may make the fracture energy much greater than for slip on a single surface the extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength for mode ii prestress controls whether activation occurs primarily on the extensional side which we show to be the typical case or on the compressional side too natural examples are consistent with the concepts developed

rhea s subsurface probed by the cassini radiometer insights into its thermal structural and compositional properties abstract during its 13 5 years of operation around the saturn system the microwave radiometer incorporated in the cassini radar observed rhea at 2 2 cm during 9 flybys with resolutions up to a tenth of rhea s radius we compare the antenna temperatures measured by this instrument to simulated data generated with the combination of thermal radiative transfer and emissivity models in order to derive new constraints on the thermal structural and compositional properties of the near subsurface of different regions of rhea we find that the cassini radiometer probes depths of 5 to 15 m on rhea that is 200 700 wavelengths implying a weakly absorbing regolith and therefore little contamination by non ice compounds in the south polar region local summer and fall observations constrains the maximum loss tangent to be 8 1 10 4 implying a contaminant volumetric fraction of 10 in the first 10 m the derived thermal inertias 60 mks in the south pole are higher than the ones measured in the thermal infrared 1 46 mks consistent with increasing compaction with depth over all of rhea current models relating surface microwave emissivity and backscatter cannot explain both the emissivities and the high radar backscatter recorded by the cassini radar suggesting the presence of especially efficient backscattering structures in the subsurface of rhea this interpretation is consistent with the very low derived dielectric constants 1 1 1 5 indicating that the subsurface structures are depolarizing in particular the ejecta blanket of the inktomi crater has an emissivity about 20 lower than its surroundings and is very radar bright the impact that formed this young crater must have excavated fresh water ice from the subsurface while also creating structures such as cracks reflecting centimetric wavelengths

spectrophotometry of the small satellites of saturn and their relationship to iapetus phoebe and hyperion abstract we have obtained broadband spectrophotometric observations of four of the recently discovered small satellites of saturn gladman et al 2001 nature 412 163 166 the new data enable an understanding of the provenance composition and interrelationships among these satellites and the other satellites of saturn particularly iapetus phoebe and hyperion temporal coverage of one satellite s21 tarvos was sufficient to determine a partial rotational lightcurve our major findings include 1 the satellites are red and similar in color comparable to d type asteroids some kbos iapetus and hyperion 2 none of the satellites including those from the phoebe group has any spectrophotometric relationship to phoebe and 3 s21 tarvos exhibits a rotational lightcurve although the data are not well constrained and more observations are required to fit a definitive period dust created by meteoritic impacts and ejected from these satellites and additional undiscovered ones may be the source of the exogenous material deposited on the low albedo side of iapetus recent work which states that the small irregular satellites of saturn have impacted phoebe at least 6 7 times in the age of the solar system nesvorny et al 2003 astron j 126 398 429 suggests that such collisions may have propelled additional material from both phoebe and the small irregular satellites toward iapetus the accretion of material from outer retrograde satellites may be a process that also occurs on callisto and the uranian satellites

effects of slip slip rate and shear heating on the friction of granite the stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction ss ln v this quantity can vary substantially with displacement temperature and slip rate to investigate the physical basis for this behavior and the possible influence of shear heating we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses n of 10 and 25 mpa and at room temperature we imposed step changes in slip rate within the range 10 2 to 103 5 m s and also monitored frictional heating with thermistors embedded in the granite the transient response of to slip rate steps was fit to a rate and state dependent friction law using two state variables to estimate the values of several parameters in the constitutive law the first 20 mm of slip shows rising friction and falling ss ln v further slip shows roughly constant friction ss ln v and parameter values suggesting that a steady state condition is reached on the fault surface at v 10 m s ss ln v 0 004 0 001 at higher rates the response is sensitive to normal stress at n 25 mpa granite shows a transition to effective velocity strengthening ss ln v 0 008 0 004 at the highest slip rates tested at 10 mpa granite shows a less dramatic change to ss ln v 0 at the highest rates the maximum temperature measured in the granite is 60 c at 25 mpa and 103 5 m s temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface the simplest interpretation of our measurements of ss ln v is that the granite is inherently velocity weakening ss ln v 0 mimics velocity strengthening these results have implications for the frictional behavior of faults during earthquakes high slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop for fluid saturated faults strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid

particle deposition on the saturnian satellites from ephemeral cryovolcanism on enceladus the geologically active south pole of enceladus generates a plume of micron sized particles which likely form saturn s tenuous e ring extending from the orbit of mimas to titan interactions between these particles and satellites have been suggested though only as very thin surficial phenomena we scrutinize high resolution images with a newly developed numerical shape model of helene and find that the leading hemisphere of helene is covered by thick deposits of e ring particles which occasionally collapse to form gully like depressions the depths of the resultant gullies and near absence of small craters on the leading hemisphere indicate that the deposit is tens to hundreds of meters thick the ages of the deposits are less than several tens of my which coincides well with similar deposits found on telesto and calypso our findings as well as previous theoretical work collectively indicate that the cryovolcanic activity currently occurring on enceladus is ephemeral

iapetus near surface thermal emission modeled and constrained using cassini radar radiometer microwave observations abstract since its arrival at saturn the cassini spacecraft has had only a few opportunities to observe iapetus saturn s most distant regular satellite these observations were all made from long ranges 100 000 km except on september 10 2007 during cassini orbit 49 when the spacecraft encountered the two toned moon during its closest flyby so far in this pass it collected spatially resolved data on the object s leading side mainly over the equatorial dark terrains of cassini regio cr in this paper we examine the radiometry data acquired by the cassini radar during both this close targeted flyby referred to as ia49 3 and the distant iapetus observations in the radar s passive mode the receiver functions as a radiometer to record the thermal emission from planetary surfaces at a wavelength of 2 2 cm on the cold icy surfaces of saturn s moons the measured brightness temperatures depend both on the microwave emissivity and the physical temperature profile below the surface down to a depth that is likely to be tens of centimeters or even a few meters combined with the concurrent active data passive measurements can shed light on the composition structure and thermal properties of planetary regoliths and thus on the processes from which they have formed and evolved the model we propose for iapetus microwave thermal emission is fitted to the ia49 3 observations and reveals that the thermal inertias sensed by the cassini radiometer over both cr and the bright mid to high latitude terrains namely ronceveaux terra rt in the north and saragossa terra st in the south significantly exceed those measured by cassini s cirs composite infrared spectrometer which is sensitive to much smaller depths generally the first few millimeters of the surface this implies that the subsurface of iapetus sensed at 2 2 cm wavelength is more consolidated than the uppermost layers of the surface in the case of cr a thermal inertia of at least 50 j m 2 k 1 s 1 2 and most probably 200 j m 2 k 1 s 1 2 is inferred this suggests a gradient in density with depth or more likely that the radiometer has probed the icy substrate underlying the dark layer furthermore the measured thermal emission is found to arise from the upper few meters of the subsurface which points to tholins rather than iron oxide compounds as the primary contaminants of the dark material we also find that although there is a latitudinal decrease probably related to the thinning of the dark layer away from the equator the cr region exhibits a high 2 2 cm emissivity 0 87 in average which is close to the emissivity of phoebe a putative source of the dark matter in the case of rt st model fitting points to a mean thermal inertia of 160 j m 2 k 1 s 1 2 along with the possible presence of an absorbing compound in the regolith of the bright terrains nevertheless this layer is transparent enough for the radiometer to capture the seasonal contrast between the northern and southern hemispheres lastly a global decline of the microwave emissivity with latitude is revealed it is probably indicative of a progressive increase of the water ice content in the near surface