a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

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

frictional melting processes in planetary materials from hypervelocity impact to earthquakes frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip within planetary materials which are mainly natural ceramics frictional melting occurs at high strain rates typically 10 2 s 1 and at slip velocities greater than 0 1 m s 1 t he pathway to friction melting is controlled by the mechanical properties of a rock s constituent minerals especially fracture toughness minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt the product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix this cools to form the rock type known as pseudotachylyte and at even higher strain rates it forms shock veins in meteorites and in impact craters which may contain high pressure mineral polymorphs the generation of melt on sliding surfaces can lubricate earthquake faults facilitate the post shock modification of impact craters and make landslides more hazardous

pseudotachylite generated as a result of a drilling burn in abstract material similar to naturally occurring pseudotachylite was generated in proterozoic lava as a result of frictional overheating of a diamond drill dh 5000 impregnated crown at a depth of 2 4 km below surface bit rotation equation to slip rates of about 1 m s is thought to have caused severe overheating to last for 3 10 s estimates show that melting occurred in the temperature range 800 1100 c under wet conditions and at comparatively low effective normal and shear stresses of about 1 9 107 and 9 7 106 pa respectively associated breccia formed as a result of thermal decrepitation rather than dynamic comminution and as a corollary it is suggested that some cataclasites associated with pseudotachylite bearing faults may also form by the mechanism this implies that the presence of both cataclasite and pseudotachylite on a fault plane does not necessarily indicate reactivation

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

selective melting processes as inferred from experimentally generated pseudotachylytes abstract high speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1 0 1 5 mpa and at slip rates up to 2 m s using a rotary shear high speed frictional testing machine the experimentally generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub angular to rounded fragments these locally occur as injected network veins in the fractured rock x ray diffraction analysis has revealed the presence of as much as 40 70 wt glass the mineral contents of clasts in the experimental pseudotachylyte in granite as determined by x ray diffraction analysis indicate that quartz is the most resistant to melting biotite the least and feldspar is intermediate thus the sio2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting

rate weakening friction characterizes both slow sliding and catastrophic failure of landslides abstract catastrophic landslides cause billions of dollars in damages and claim thousands of lives annually whereas slow moving landslides with negligible inertia dominate sediment transport on many weathered hillslopes surprisingly both failure modes are displayed by nearby landslides and individual landslides in different years subjected to almost identical environmental conditions such observations have motivated the search for mechanisms that can cause slow moving landslides to transition via runaway acceleration to catastrophic failure a similarly diverse range of sliding behavior including earthquakes and slow slip events occurs along tectonic faults our understanding of these phenomena has benefitted from mechanical treatments that rely upon key ingredients that are notably absent from previous landslide descriptions here we describe landslide motion using a rate and state dependent frictional model that incorporates a nonlocal stress balance to account for the elastic response to gradients in slip our idealized one dimensional model reproduces both the displacement patterns observed in slow moving landslides and the acceleration toward failure exhibited by catastrophic events catastrophic failure occurs only when the slip surface is characterized by rate weakening friction and its lateral dimensions exceed a critical nucleation length h h that is shorter for higher effective stresses however landslides that are extensive enough to fall within this regime can nevertheless slide slowly for months or years before catastrophic failure our results suggest that the diversity of slip behavior observed during landslides can be described with a single model adapted from standard fault mechanics treatments

frictional melting of sedimentary rock during high speed diamond drilling an analytical sem and tem investigation abstract high speed drilling at 900 rpm with a downhole motor driven natural diamond drag bit has resulted in the extreme alteration of sedimentary rock in an offshore canadian wildcat well these effects as revealed in the rock cuttings retrieved from the drilling fluid at surface include discolouration darkening cataclasis slickensiding and partial fusion calcareous siltstones are the most susceptible to alteration especially melting whereas sandstone and limestone cuttings mainly exhibit fracturing and surface glazing analytical scanning electron microscopy reveals that the calcareous siltstones have undergone decalcification via decarbonation and the loss of cao to the drilling fluid decarbonation combined with dehydration resulted in the vesiculation and explosive fragmentation of certain cuttings analytical transmission electron microscopy shows that glass is present in the calcareous siltstone cuttings most of the modifications exhibited by the sedimentary rocks can be attributed to the effects of high strain rate deformation and frictional heating at the bit rock interface that melting took place is supported by thermal calculations which indicate that for a coefficient of friction of 0 5 temperatures of approximately 1200 c were realized at diamond rock contacts such high temperatures can accelerate the wear of diamond and its substrate and hence shorten bit life as well as obliterate potentially important geological information in the rock cuttings

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

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

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

magnetic fabric constraints on friction melt flow regimes and ore emplacement direction within the south range breccia belt sudbury impact structure abstract magnetic fabric analysis and petrography have been carried out on 1100 samples from the south range breccia belt srbb a 45 km long arc of huronian supergroup breccia fragments and crystallized friction melt pseudotachylyte sub concentric to the 1 85 ga sudbury impact structure optical petrography and analytical sem analyses show the pseudotachylyte to consist of fine grained quartz biotite and ilmenite with minor plagioclase apatite zircon pyrite and pyrrhotite alternating field and thermal demagnetization spectra indicate that the principal magnetic carrier in the matrix is pyrrhotite while clasts can contain either pyrrhotite or magnetite mean magnetic susceptibilities in the matrix are generally on the order of 10 4 si two ams anisotropy of magnetic susceptibility fabrics have been determined in the matrix the most common is an oblate sub vertical foliation sub parallel to the local strike of the srbb with anisotropies p ranging from 1 08 to 1 20 the second ams fabric is a weakly to strongly prolate sub vertical lineation with generally higher p values of 1 2 to 1 55 both fabrics are coaxial to the petrofabric in the matrix huronian clasts exhibit scattered heterogeneous ams fabrics distinct from those found in the srbb matrix the strongly prolate t 0 73 and anisotropic p 1 55 ams fabric found directly over the giant frood stobie ni cu pge deposit hosted within the srbb suggests that sulfide droplets were injected and or squeezed into the matrix above the deposit samples containing pyrrhotite from the edges of the deposit do not exhibit this magnetic signature similar prolate moderately anisotropic lineations are also found further to the east of the frood stobie mine and appear to be associated with a magnetotelluric conductor at depth it is suggested that these lineations are due to injection of material from depth into the srbb during failure of the inner rim of the sudbury impact structure during the crater modification stage the oblate magnetic fabric found within the srbb is consistent with frictional comminution and melting of the wall rock lithologies during collapse of the transient cavity and subsequent slumping of the crater walls however subsequent deformation of the sudbury structure by late penokean folding and thrusting to the nw could produce a similar oblate fabric tight clustering of the maximum principal axes of the magnetic susceptibility could be interpreted as being consistent with tectonometamorphic transposition of the matrix petrofabric as well as generation by slumping and friction melt flow during failure of the hanging wall further work is needed on undeformed impact structures of similar size to sudbury in order to constrain the petro and magnetic fabrics produced during rim collapse additional work in the westerly trending srbb along the edge of the creighton pluton would help to determine if the dominant ne sw trending matrix fabric is regional in origin or is controlled by the current orientation of portions of the srbb

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

mechanics of grain size reduction in fault zones 1 recent observations of nanometer scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive especially if significant shear heating is produced during an earthquake commercial crushing and grinding operations encounter a grind limit near 1 mm below which particles deform plastically rather than fracturing a fragmentation model and low temperature plasticity mechanics indicate that it is not possible to produce under compressive loading and short timescale significantly smaller particles at any strain rate however shock loading and subcritical crack growth can produce nanometer scale fragments in compression under tensile loading the fragment size is determined by a competition between the nucleation of cracks and stress relaxation in their neighborhoods therefore higher tensile strain rates produce smaller fragments the ultimate limit is determined by the availability of elastic strain energy which does not place a significant constraint on the minimum grain size grain growth kinetics suggests that survivability of grains is very temperature sensitive a 10 nm quartz fragment will double its size in 0 1 s at 1000c in 20 s at 800c in 14 h at 600c and in 10 years at 400c the observation of grains smaller than 10 nm places meaningful constraints on the dynamic fields and permeability of the fault zone during a large earthquake microstructural analysis of the grains and rock damage may be used to infer whether fragmentation occurred under macroscopic tension or compression

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

the south range breccia belt of the sudbury impact structure a possible terrace collapse feature the south range breccia belt srbb is an arcuate 45 km long zone of sudbury breccia in the south range of the 1 85 ga sudbury impact structure the belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of huronian supergroup 2 49 2 20 ga nipissing diabase 2 2 ga and proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters the srbb matrix is composed of a fine grained 100 m assemblage of biotite quartz and ilmenite with trace amounts of plagioclase zircon titanite epidote pyrite chalcopyrite pyrrhotite and occasionally chlorite the srbb hosts the frood stobie vermilion and kirkwood quartz diorite offset dykes the former being associated with one of the largest ni cu pge sulphide deposits in the world optical petrography and whole rock geochemistry concur with previous studies that have suggested that the matrix of the srbb is derived from comminution and at least partial frictional melting of the wall rock huronian supergroup lithologies rare earth element ree data from all sampled lithologies associated with the srbb exhibit crustal signatures when normalized to c1 chondrite values additionally ree data from the quartz diorites disseminated sulphides in sudbury breccia and a sample of an aphanitic biotite hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites quartz gabbro and granophyre units of the sudbury igneous complex sic which suggests that these lithologies are representative of bulk sic melt we suggest that the srbb was formed by high strain rate 1 m s gravity driven seismogenic slip of the inner ring of the sudbury impact structure during postimpact crustal readjustment crater modification stage failure of the hanging wall may have facilitated the injection of bulk sic melt into the srbb along with the ni cu pge sulphides of the frood stobie deposit postimpact penokean 1 9 1 7 ga tectonism particularly northwest directed shearing along the south range shear zone and associated thrust faulting could account for the present subvertical orientation of the srbb and the apparent lack of a connection at depth with the sic

early pleistocene lake formation and hominin origins in the turkana omo rift abstract prior research has correlated the formation of plio pleistocene lakes in east africa to global regional climate changes and interpreted the lacustrine basins as significant settings of hominin evolution paleo lake lorenyang from the turkana omo rift is important to these issues as its marginal deposits contain some of if not the earliest currently known specimens of acheulian stone tools and african homo erectus magnetostratigraphic and sedimentological evidence indicates that the oldest preserved paleo lake lorenyang deposits are dated at 2 148 2 128 ma and derive from the nw turkana basin predating those from the omo basin by 100 kyr and the ne turkana basin by 190 kyr apparently the lake expanded asynchronously in the rift potentially due to a volcano tectonic influence on the location of drainage networks depositional slopes or topographic elevation differences within and between the basins at the time of flooding the onset of the lake temporally coincides with the eruption of basalt lava flows dated to 2 2 2 0 ma that blocked the southeast outlet of the turkana basin this provides a plausible mechanism for hydrologic closure and lacustrine basin formation through volcano tectonic impounding it also points to a non climatic cause for the initial formation of paleo lake lorenyang at 2 14 ma first appearances for african h erectus 1 87 ma and acheulian tools 1 76 ma in the turkana omo rift postdate the lake s initial formation by about 270 kyr and 380 kyr respectively such timing differences contrast with studies that correlate all three to the 400 kyr eccentricity maximum at 1 8 ma although the turkana omo rift is just one example it does provide alternative insights to views that link climate hominin evolution and lake formation in east africa

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

fractal dimension of molten surfaces as a possible parameter to infer the slip weakening distance of faults from natural pseudotachylytes high velocity friction experiments on gabbro and monzodiorite using a rotary shear high velocity friction apparatus have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening eventually reaching a nearly steady state the melting surface at the host rock molten layer interface is initially very flat but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the gibbs thomson effect this change in the melting surface topography can be quantitatively expressed by the fractal dimension d as determined by the divider method from about 1 0 near the peak friction to around 1 1 near the steady state friction the ultimate fractal dimension at steady state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting a systematic correlation of d with mechanical behavior of the fault during frictional melting may provide a way of estimating slip weakening distance and heat production rate at steady state friction by measuring d for natural pseudotachylytes on slip surfaces with different displacements the weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength the experimental studies point to ways to estimate these difficult quantities for natural faults

viscosity determinations of some frictionally generated silicate melts implications for fault zone rheology at high strain rates analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths the data show that the matrices of the friction melts which cool to form pseudotachylytes are relatively basic and hydrous even when their protoliths are intermediate to acid this is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points not the formation of minimum melts through equilibrium mineral interaction this means that hydrous ferromagnesian minerals e g micas and amphiboles melt preferentially to form the liquid matrix while feldspars and especially quartz more readily survive as clasts pseudotachylytes generated by frictional melting are therefore not bulk melts and as clast melt suspensions they cannot be considered as simple newtonian fluids the calculated viscosities of the friction melts are low for example at 1200 c most friction melts possess zero shear suspension viscosities of 102 104 dpa s 1 dpa s 1 p this is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature these viscosities are maximum determinations because as clast melt suspensions friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates i e during slip on a fault surface contrary to earlier suggestions where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding this work shows that most pseudotachylytes are partial melts possessing low viscosities the formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation this would be manifest as an increase in high frequency seismic wave radiation and vibrational