ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

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

kelvin helmholtz waves in precipitating midlatitude cyclones abstractthis study examines kelvin helmholtz kh waves observed by dual polarization radar in several precipitating midlatitude cyclones during the olympic mountains experiment olympex field cam

structures and high velocity frictional properties of the pingxi fault zone in the longmenshan fault system sichuan china activated during the 2008 wenchuan earthquake abstract this paper reports on the internal structures of the pingxi fault zone near the northeastern end of the coseismic surface ruptures associated with the 2008 wenchuan earthquake the fault zone consists of a 50 80 cm wide fault core and several meter wide fault breccia zones the fault core contains yellowish gouge yg gray blackish gouge gbg black gouge bg and gray blackish fault breccia a coseismic slip during the wenchuan earthquake occurred within gbg in a slip zone of 24 9 mm in width high velocity friction experiments were conducted at normal stresses n of 0 45 2 50 mpa and slip rates v of 0 0014 2 09 m s yg and gbg exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient p to steady state friction coefficient ss over a slip weakening distance dc specific fracture energy eg for yg and gbg was 0 37 2 02 mj m2 of the same order of magnitude as the fracture energy estimated for natural earthquakes empirical equations describing the changes in p ss and dc in terms of n and v indicate that eg decreases with increasing n so that higher normal stress experiments will lead to eg lower than seismic values thus our experiments probably reproduced deeper seismic fault motion under dry conditions with respect to eg frictional behavior of bg is characterized by a sharp initial weakening subsequent strengthening to the second peak friction and slip weakening towards the steady state friction the initial weakening and strengthening seem to be caused by the formation of a very sharp slip zone 5 m in width and subsequent formation of a widely deformed zone a comparison of our data with reported results for other outcrops indicates that the high velocity frictional properties of the longmenshan fault system may be quite uniform the observed dramatic high velocity weakening must have promoted dynamic rupture propagation during the wenchuan earthquake

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

an observational study of primary and secondary billows in the free atmosphere a case study is presented showing the kinematic structure of a train of large amplitude billows associated with a strongly sheared layer of high static stability in the upper troposphere the observations were obtained using a high power microwave doppler radar with a range resolution of 30 m a beamwidth resolution almost as good as this was achieved at the ranges of interest using a 25 m paraboloid aerial the resulting detailed measurements revealed the kinematic structure of secondary billows of wavelength 350m growing on the back of primary billows of wavelength 4 2km the secondary billows were regions of moderate turbulence the outer scale of which was small compared with the radar resoluton streamers of cirrus ice particles were present in the region of interest and these served as tracers showing how the billows perturbed the flow over a considerable depth above and below the primary billows

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

coseismic microstructures of experimental fault zones in carrara marble abstract experimental fault zones developed in carrara marble that were deformed at seismic slip rates 1 18 1 30 m s 1 using a high velocity rotary shear apparatus exhibit very low friction friction coefficient as low as 0 06 at steady state due to nanoparticle lubrication of the decomposition product lime the fault zones show a layered structure a central slip localization layer 5 60 m thick of lime nanograins mantled by gouge layers 5 150 m thick and a plastically deformed layer 45 500 m thick between the wall rock and gouge layer in the marginal portion of cylindrical specimens calcite grains of the wall rock adjacent to the slip zone deform by dislocation glide when subjected to frictional heating and a lower strain rate than that of the principal slip zone the very fine 2 5 m calcite grains in the gouge layer show a foam structure with relatively straight grain boundaries and 120 triple junctions this foam structure is presumed to develop by welding at high temperature and low strain once slip is localized along the central layer we suggest that a seismic event can be inferred from deformed marbles given i the presence of welded gouge with foam structure in a fault zone where wall rocks show no evidence of thermal metamorphism and ii a thin plastically deformed layer immediately adjacent to the principal slip zone of a cataclastic fault zone

properties of embedded convection in warm frontal mixed phase cloud from aircraft and polarimetric radar paper associated with the cwvc clouds water vapour and climate dataset held on the ceda archive

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

earthquake rupture at focal depth part i structure and rupture of the pretorius fault tautona mine south africa we analyze the structure of the archaean pretorius fault in tautona mine south africa as well as the rupture zone that recently reactivated it the analysis is part of the natural earthquake laboratory in south african mines nelsam project that utilizes the access to 3 6 km depth provided by the mining operations the pretorius fault is a 10 km long oblique strike slip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in tautona mine we identify here three structural zones within the fault zone 1 an outer damage zone 100 m wide of brittle deformation manifested by multiple widely spaced fractures and faults with slip up to 3 m 2 an inner damage zone 25 30 m wide with high density of anastomosing conjugate sets of fault segments and fractures many of which carry cataclasite zones and 3 a dominant segment with a cataclasite zone up to 50 cm thick that accommodated most of the archaean slip of the pretorius fault and is regarded as the principal slip zone psz this fault zone structure indicates that during its archaean activity the pretorius fault entered the mature fault stage in which many slip events were localized along a single psz the mining operations continuously induce earthquakes including the 2004 m2 2 event that rejuvenated the pretorius fault in the nelsam project area our analysis of the m2 2 rupture zone shows that 1 slip occurred exclusively along four pre existing large quasi planer segments of the ancient fault zone 2 the slipping segments contain brittle cataclasite zones up to 0 5 m thick 3 these segments are not parallel to each other 4 gouge zones 1 5 mm thick composed of white rock flour formed almost exclusively along the cataclasite host rock contacts of the slipping segments 5 locally new fresh fractures branched from the slipping segments and propagated in mixed shear tensile mode 6 the maximum observed shear displacement is 25 mm in oblique normal slip the mechanical analysis of this rupture zone is presented in part ii heesakkers et al earthquake rupture at focal depth part ii mechanics of the 2004 m2 2 earthquake along the pretorius fault tautona mine south africa 2011 this volume

spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles morphology energetics and nonisentropic mixing activity the significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic scale baroclinic wave life cycles along the subtropical jet stream using a nonhydrostatic anelastic mesoscale model subject to a free slip surface boundary condition a variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture episodic overturnings along the surface fronts wrinkling of the tropopause and injection of tropospheric air into the stratosphere the evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the base component of potential energy that cannot be converted into kinetic energy the structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity this activity is found to be primarily longitudinal extending from the surface to the lower stratosphere to form a three dimensional spiral in the synoptic scale cyclone and along the fronts however an exceptional region also exists along the warm front where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small scale turbulence in all simulations the net transfer of mass and heat across the tropopause is from troposphere to stratosphere the maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere the same climatological choice leads to a minimum net irreversible mixing which occurs primarily at earth s surface

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

proglacial lake sediment records of holocene climate change in the western cordillera of peru abstract sediment records from proglacial lakes between 9 and 10 s in the western cordillera of the peruvian andes document the waxing and waning of alpine glaciers since the end of the lateglacial stage these records from the southern tropical andes provide supporting evidence that the early holocene between 12 and 8 ka was relatively warm and dry and the middle holocene between 8 and 4 ka was marked by a shift to cooler and possibly wetter conditions in certain regions leading to glacial advances although there were multiple periods of brief ice advances that interrupted the overall trend glaciers in multiple valleys generally retreated from 4 0 ka through the medieval climate anomaly 1 0 0 7 ka this late holocene pattern of ice retreat occurred during a period when lake level studies and both lacustrine and speleothem stable isotopic records indicate wetter conditions relative to the middle holocene suggesting that higher temperatures contributed to the pattern of ice retreat following this period of glacial retreat multiple proxy records suggest that the start of the little ice age 0 6 0 1 ka was a colder and wetter time throughout much of the tropical andes there appear to be two primary synoptic scale climatic controls on temperature and precipitation linked to insolation dynamics that drive changes in ice cover in the southern tropical andes during the holocene 1 the strength of the south america summer monsoon which is linked to northern hemisphere temperatures and the mean position of the intertropical convergence zone over the atlantic and 2 sea surface temperature distributions in the tropical pacific ocean and its influence on atmospheric temperature precipitation and circulation patterns

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

detailed dual doppler structure of kelvin helmholtz waves from an airborne profiling radar over complex terrain part i dynamic structure abstractkelvin helmholtz kh waves are remarkably common in deep stratiform precipitation systems associated with frontal disturbances at least in the vicinity of complex terrain as is evident f

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

kelvin helmholtz waves in extratropical cyclones passing over mountain ranges kelvin helmholtz billows with horizontal scales of 3 4 km have been observed in midlatitude cyclones moving over the italian alps and the oregon cascades when the atmosphere was mostly statically stable with high amounts of shear and ri 0 25 in one case data from a mobile radar located within a windward facing valley documented a layer in which the shear between down valley flow below 1 2 km and strong upslope cross barrier flow above was large several episodes of kelvin helmholtz waves were observed within the shear layer the occurrence of the waves appears to be related to the strength of the shear when the shear attained large values an episode of billows occurred followed by a sharp decrease in the shear the occurrence of large values of shear and kelvin helmholtz billows over two different mountain ranges suggests that they may be important features occurring when extratropical cyclones with statically stable flow pass over mountain ranges

measurements of dissipation rate in frontal zones summary this paper demonstrates that it is possible to use measurements of the doppler spectrum width from a highresolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones using the high resolution chilbolton radar this method has been applied for the first time to infer the rate at which kinetic energy is dissipated by turbulence within mid and lower tropospheric frontal zones analysis of detailed measurements for one cold frontal region showed multiple shallow sheets of high dissipation rate with spatially averaged values around i x m2s 3 comparable values were also observed in a warm frontal zone the magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts in that the turbulence acted to destroy the frontal shear layer on a tirne scale of several hours these issues have particular relevance to the representation of fronts within future very high resolution numerical weather prediction models

radar observations of wind shear splitting within evolving atmospheric kelvin helmholtz billows summary the high resolution radar at chilbolton was used to measure the velocity structure of large amplitude kelvinhelmholtz billows within a precipitating warm frontal zone a deep layer of frontal shear was observed to split into thin layers of more concentrated shear displaying the characteristic braided structure previously reported within clear air radar reflectivity patterns and attributed to a splitting of the thermal stratification the billows were situated within a region of strong wind veer with height and they were orientated at right angles to the mean flow such that air parcels within them were tending to follow helical trajectories up the inclined frontal zone radar scans normal to the axes of the billows at different distances from their origin close to the position of the surface warm front provided an indication of the evolution of the split shear structure

fast slip with inhibited temperature rise due to mineral dehydration evidence from experiments on gypsum anomalously low heat flow around active faults has been a recurrent subject of debate over past decades we present a series of high velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large scale endothermic dehydration reaction the tests were performed at 1 mpa normal stress and a velocity of 1 3 m s 1 while measuring the temperature close to the sliding surface and the relative humidity around the sample the temperature close to the sliding surface is remarkably stable at 100 c during the dehydration reaction of gypsum microstructural and x ray diffraction investigations show that dehydration occurs at the very beginning of the test and progresses into the bulk as slip increases in the hottest parts of the sample anhydrite crystal growth is observed the half thickness of the dehydrated layer ranges from 160 m at 2 m slip to 5 mm at 68 m slip thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10 to 50 of the total mechanical work input the temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam we suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake this is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature

strain localization and the onset of dynamic weakening in calcite fault gouge abstract to determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates single slide and slide hold slide experiments were conducted on 2 3 mm thick layers of calcite gouge at normal stresses up to 26 mpa and slip rates up to 1 m s 1 microstructures were analyzed from short displacement 35 cm experiments stopped prior to and during the transition to dynamic weakening in fresh calcite gouge layers dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness strain is initially distributed across the full thickness of the gouge layer but within a few millimeters displacement the strain becomes localized to a boundary parallel high strain shear band c 20 m wide during the strengthening phase which lasts between 3 and 30 cm under the investigated conditions the shear band broadens to become c 100 m wide at peak stress the transition to dynamic weakening in calcite gouges is associated with the nucleation of micro slip surfaces dispersed throughout the c 100 m wide shear band each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite interpreted to result from grain welding driven by local frictional heating in the shear band by the end of dynamic weakening strain is localized to a single 2 3 m wide principal slip surface flanked by layers of recrystallized gouge calcite gouge layers re sheared following a hold period weaken nearly instantaneously much like solid cylinders of calcite marble deformed under the same experimental conditions this is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide reducing the effective gouge layer thickness to a few microns our results suggest that formation of a high strain shear band is a critical precursor to dynamic weakening in calcite gouges microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate the delayed onset of dynamic weakening in fresh calcite gouge layers particularly at low normal stresses may inhibit large coseismic slip at shallow crustal levels in calcite bearing fault zones

nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation crystal plasticity at sub seismic slip rates at 18 150 c a central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic seismic transitions in fault gouges and to identify microstructural indicators for such transitions we present new data on the slip stability of calcite fault gouges and on microstructural development down to the nanometer scale our experiments consisted of direct shear tests performed dry at slip rates of 0 1 10 m s at a constant normal stress of 50 mpa at 18 150 c the results show a transition from stable to potentially unstable slip above 80 c all samples recovered showed an optical microstructure characterized by narrow 30 40 m wide riedel and boundary shear bands marked by extreme grain comminution and a crystallographic preferred orientation cpo boundary shear bands sectioned using fib sem focused ion beam scanning electron microscopy revealed angular grain fragments decreasing from 10 to 20 m at the outer margins to 0 3 m in the shear band core where dense aggregates of nanograins also occurred transmission electron microscopy applied to foils extracted from boundary shears using fib sem combined with the optical cpo showed that these aggregates consist of calcite nanocrystals 5 20 nm in size with the 104 201 dislocation glide system oriented parallel to the shear plane and direction our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity because crystal plasticity is strongly thermally activated we infer that the transition to velocity weakening slip is likely due to enhanced crystal plasticity at 80 c this implies that tectonically active limestone terrains will tend to be particularly prone to shallow focus seismicity

reduction in bet surface area of nojima fault gouge with seismic slip and its implication for the fracture energy of earthquakes abstract a common view concerning the energetics of seismogenic fault motion is that at least part of the fracture energy is consumed in grain crushing in the fault zone and that this part may be estimated by grain size analysis of fault rocks we address this problem by conducting room dry friction experiments on nojima fault gouge at subseismic to seismic slip rates 0 009 1 31 m s and at normal stresses up to 3 64 mpa and by measuring the bet surface area of the gouge before and after the experiments where it cuts granite the nojima fault zone has bet surface area of about 65 10 6 m 2 per unit fault area 1 m 2 clayey and granular fault gouges composed mainly of quartz plagioclase kaolinite and smectite were collected from a granitic fault zone at a new outcrop in funaki awaji island southwest japan both clayey and granular gouges exhibit dramatic weakening at high slip rates specific bet surface areas of clayey and granular gouges decreased with increasing slip rate from 46 0 and 15 4 m 2 g before deformation to about 20 and 5 m 2 g after deformation 55 70 reduction respectively microstructural observations revealed that grain welding within the slip zones at high slip rates reduced grain surface area the energetics of seismic fault motion should be examined with broader views taking into account grain crushing grain welding decomposition and frictional melting

energy partition for grain crushing in quartz gouge during subseismic to seismic fault motion an experimental study abstract brunauer emmett teller bet surface area of quartz gouge was measured before and after shearing deformation at subseismic to seismic slip rates to determine how much of frictional work and fracture energy was consumed during grain crushing measurement of bet surface area provides more direct data on surface area change than grain size measurement does twenty four experiments on quartz gouge composed of ottawa sands or crushed quartz were conducted dry with room humidity or wet with added water at slip rates of 0 006 1 3 m s and normal stresses of 0 76 3 37 mpa using a high velocity friction apparatus and a low to high velocity friction apparatus about 1 mm thick gouge was placed between cylindrical specimens of belfast dolerite of 25 mm in diameter results show that grain crushing absorbed only 0 02 0 22 of frictional work and only 0 05 1 06 of fracture energy under dry and wet conditions thus grain comminution is unlikely to be an important energy sink at least for mature faults with well developed slip zone surface area of gouge decreases with deformation at seismic slip rates this and sem observations suggest partial sintering due to frictional heating thus a simple scenario of grain crushing as an energy sink does not hold for estimating the energy budget during earthquakes

multi sensor observations of a wave beneath an impacting rear inflow jet in an elevated mesoscale convective system observations of a wave generated below and propagating with an elevated mesoscale convective system mcs are presented the mcs and the wave tracked directly across the 90 km extent of the observational network deployed for the convective storm initiation project csip and maintained a coherent structure and velocity during this 1 5 hour period the rear inflow jet of the mcs was observed to impact upon but not penetrate the stable undercurrent that was located beneath the mcs apparently constricting the depth of the undercurrent which did not participate in the main convective updraught the wave crest was located approximately 15 km ahead of this constriction the overall wave led to between 200 and 1000 m of ascent and approximately 1500 m of descent however undulations were superimposed on the overall wave with a wavelength of approximately 7 km and a crest totrough amplitude of approximately 1000 m these undulations were associated with double maxima in surface pressure and the elevation of the observed low level wind anomaly where low level winds were backed by more than 90 t hese winds were never at rest relative to the system and the wave was therefore a gravity wave without stagnation the hydrostatic effect expected from the observed lifting in the wave was broadly consistent with the observed surface pressure perturbation of 2 hpa no cold pool outflow was ever observed instead the observed surface cooling of approximately 4 c was shown to be consistent with reduced solar insolation the generation of the observed wave structure and its implications for the maintenance of the mcs and numerical modelling of the system are discussed copyright c

dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates to investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite gouge at subseismic to seismic slip rates we conducted single velocity step and multiple velocity step friction experiments on an antigorite rich and lizardite rich serpentinite at slip rates v from 0 003 m s to 6 5 m s sliding displacements up to 1 6 m and normal stresses n up to 22 mpa for gouge and 97 mpa for bare surfaces nominal steady state friction values nss in gouge at v 1 m s are larger than in bare surfaces for all n tested and demonstrate a strong n dependence nss decreased from 0 51 at 4 0 mpa to 0 39 at 22 4 mpa conversely nss values for bare surfaces remained 0 1 with increasing n and v additionally the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces extrapolation of the normal stress dependence for nss suggests that the behavior of antigorite gouge approaches that of bare surfaces at n 60 mpa x ray diffraction revealed dehydration reaction products in samples that frictionally weakened microstructural analysis revealed highly localized slip zones with melt like textures in some cases gouge experiments and in all bare surfaces experiments for v 1 m s one dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed

fault lubrication and earthquake propagation in thermally unstable rocks experiments performed on dolomite or mg calcite gouges at seismic slip rates v 1 m s and displacements d 1 m show that the frictional coefficient decays exponentially from peak values m p 0 8 in the byerlee9s range to extremely low steady state values ss 0 1 attained over a weakening distance d w microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products periclase and lime or portlandite were produced in the slip zone during the transient stage d w these observations integrated with co 2 emissions data recorded during the experiments suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes e g decarbonation reactions t 550 c during steady state d d w shear strength is very low and not dependent upon normal stresses suggesting that pressurized fluids co 2 may have been temporarily trapped within the slip zone at this stage a continuous layer of nanoparticles is developed in the slip zone for d d w a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone at this stage dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone experimentally derived seismic source parameter w b i e breakdown work the energy that controls the dynamics of a propagating fracture 1 matches w b values obtained from seismological data of the a d 1997 m6 colfiorito italy earthquakes which nucleated in the same type of rocks tested in this study and 2 suggests similar earthquake scaling relationships as inferred from existing seismological data sets we conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms which are activated or inhibited by physicochemical reactions in the slip zone

polarimetric doppler radar observations of kelvin helmholtz waves in a winter storm abstractkelvin helmholtz waves were observed by the twin lakes oklahoma ktlx weather surveillance radar 1988 doppler wsr 88d the norman oklahoma koun polarimetric wsr 88d and the polarimetric collaborative adaptive sensing of the atmosphere casa radars on 30 november 2006 during a winter storm in central oklahoma the life cycle and structure of the waves are analyzed from the radar data and the nearby atmospheric conditions are examined the initial perturbations associated with the waves are first evident only in the radars radial velocity fields as the waves mature perturbations become discernable in the reflectivity factor z and spectrum width sw fields of both radars and in the differential reflectivity zdr and to a lesser extent the cross correlation coefficient hv fields of koun as the waves break and begin to dissipate the perturbations subside a dual doppler analysis is synthesized to examine the kinematic structure of the waves and to relate the polarimetric observation

observations of kelvin helmholtz instability at a cloud base with the middle and upper atmosphere mu and weather radars 1 using the very high frequency 46 5 mhz middle and upper atmosphere radar mur ka band 35 ghz and x band 9 8 ghz weather radars a kelvin helmholtz kh instability occurring at a cloud base and its impact on modulating cloud bottom altitudes are described by a case study on 8 october 2008 at the shigaraki mu observatory japan 34 85 n 136 10 e kh braids were monitored by the mur along the slope of a cloud base gradually rising with time around an altitude of 5 0 km the kh braids had a horizontal wavelength of about 3 6 km and maximum crest to trough amplitude of about 1 6 km nearly monochromatic and out of phase vertical air motion oscillations exceeding 3 m s 1 with a period of 3 min 20 s were measured by the mur above and below the cloud base the axes of the billows were at right angles of the wind and wind shear both oriented east north east at their altitude the isotropy of the radar echoes and the large variance of doppler velocity in the kh billows including the braids indicate the presence of strong turbulence at the bragg 3 2 m scale after the passage of the cloud system the kh waves rapidly damped and the vertical scale of the kh braids progressively decreased down to about 100 m before their disappearance the radar observations suggest that the interface between clear air and cloud was conducive to the presence of the dynamical shear instability by reducing static stability and then the richardson number near the cloud base downward cloudy protuberances detected by the ka band radar had vertical and horizontal scales of about 0 6 1 1 and 3 2 km respectively and were clearly associated with the downward air motions observed oscillations of the reflectivity weighted doppler velocity measured by the x band radar indicate that falling ice particles underwent the vertical wind motions generated by the kh instability to form the protuberances the protuberances at the cloud base might be either kh billow clouds or perhaps some sort of mamma reflectivity weighted particle fall velocity computed from doppler velocities measured by the x band radar and the mur showed an average value of 1 3 ms 1 within the cloud and in the protuberance environment

flash weakening of serpentinite at near seismic slip rates 1 to investigate the processes responsible for dynamic frictional weakening in antigorite serpentinite we conducted single velocity and velocity stepping friction experiments at slip rates v from 0 01 to 0 4 m s 1 sliding displacements up to 40 mm and a normal stress of 5 mpa single velocity experiments demonstrate an approximately 1 v dependence of friction on velocity above a characteristic weakening velocity vw 0 1 m s 1 consistent with theoretical predictions for flash heating and subsequent weakening of asperities velocity stepping experiments impose stepwise increases in slip rate show stepwise weakening at slip rates above vw scanning electron microscopy of experimental fault surfaces reveals nanoparticle gouge textures at raised sites 10 m in diameter in tests that exhibit dramatic weakening furthermore x ray diffraction analyses of fault gouge from the sliding surface document the formation of significant talc in these tests indicating that weakening temperatures reached 500 c 700 c in contrast no talc is observed in samples for which v remained significantly below vw the observed value for vw is consistent with published microphysical models for flash weakening when independent constraints on the thermal stability and contact strength of antigorite are considered finally while serpentinite displays velocity strengthening behavior at plate tectonic slip rates our results indicate that seismic ruptures propagating into serpentinized regions in the shallow lithosphere may trigger seismicity or slow earthquakes after limited amounts of displacement

a case study of a large patch of billows surmounted by elevated convection a region of large amplitude kelvin helmholtz billows of area 2000 km2 was observed in a region of moderate rain at the rear of a mesoscale convective system on a day during the convective storm initiation project in southern england the mesoscale convective system mcs was characterized by elevated convection above a layer of cool air separated by a sloping statically stable layer characterized by strong wind shear doppler radar observations showed that the large patch of billows was situated within this shear layer and that it persisted over the 2 h period of observation maintaining its position with respect to the mcs and producing surface pressure perturbations of 0 3 hpa potentially unstable air above the sloping shear layer reached its level of free convection while ascending above this layer the resulting elevated convection was in the form of cells whose spacing appeared to be influenced by the underlying billows results from large eddy model simulations are shown to support the hypothesis that although the large scale ascent alone would have triggered the elevated convection the billows exerted a secondary forcing effect on the convection copyright 2012 royal meteorological society

role of seismogenic processes in fault rock development an example from death valley california fault rocks developed along the mormon point turtleback of southern death valley suggest that a jog in the oblique slip death valley fault zone served as an ancient seismic barrier where dominantly strike slip ruptures were terminated at a dilatant jog dramatic spatial variations in fault rock thickness and type within the bend are interpreted as the products of 1 fault overshoot in which planar ruptures bypass the intersection of the two faults composing the bend and slice into the underlying footwall and 2 implosion brecciation in which coseismic ruptures arrested at a releasing bend in the fault lead to catastrophic collapse brecciation fluid influx and mineralization

airborne radar observations of a cold front during fastex1 abstract a detailed analysis using airborne doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented the precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core relative winds in contrast to previous schematic models it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation core the southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipitation the peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical vorticity in the boundary layer the overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled cla

release of potential shearing instability in warm frontal zones potential shearing instability pshi exists in the atmosphere when the richardson number in a stably stratified shear layer would be less than 1 4 if the layer were lifted to saturation sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated its richardson number is greater than 1 4 but pshi exists over a deep layer this paper presents a study of a warm frontal zone using observations from the high resolution doppler radar at chilbolton supported by radiosondes and output from the mesoscale version of the operational united kingdom meteorological office unified model very large amplitude kelvin helmholtz billows were observed to form where the pshi was realised following the onset of saturation the billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train the present version of the unified model does not parametrize vertical mixing due to shearing instability in the free atmosphere this may be an important omission

small scale precipitation elements in midlatitude cyclones crossing the california sierra nevada abstractradar data in some frontal systems passing over the sierra nevada of california show large variance on scales of 10 km the most prominent features are a few kilometers in scale and are similar to small scale precipitation cells embedded in fronts seen over other mountain ranges other frontal systems crossing the sierras are characterized by more uniform air motions updrafts in large variance storms have characteristics of shear induced turbulence although buoyant instability may also contribute large variance storms occur under stronger upstream winds and vertically integrated cross and along barrier moisture fluxes rain gauges indicate that large variance storms have precipitation greater than smaller variance storms stronger horizontal moisture fluxes may provide greater mean upslope condensation rates however it is hypothesized that accelerated microphysical processes are needed to most efficiently convert the condensate into precipitation that falls out on the lower slopes before be

radiography of a normal fault system by 64 000 high precision earthquake locations the 2009 l aquila central italy case study 1 we studied the anatomy of the fault system where the 2009 l aquila earthquake mw 6 1 nucleated by means of 64 k high precision earthquake locations spanning 1 year data were analyzed by combining an automatic picking procedure for p and s waves together with cross correlation and double difference location methods reaching a completeness magnitude for the catalogue equal to 0 7 including 425 clusters of similar earthquakes the fault system is composed by two major faults the high angle l aquila fault and the listric campotosto fault both located in the first 10 km of the upper crust we detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops we observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays the width of the l aquila fault zone varies along strike from 0 3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution up to 1 5 km at the fault tips with an increase in the geometrical complexity these characteristics similar to damage zone properties of natural faults underline the key role of aftershocks in fault growth and co seismic rupture propagation processes additionally we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological i e creeping discontinuity explaining how normal faults detach at depth