mineral assemblage anomalies in the slip zone of the 1999 taiwan chi chi earthquake ultrafine particles preserved only in the latest slip zone we determined mineral assemblages of samples from the taiwan chelungpu fault and from milling and heating experiments by using x ray diffraction and scanning and transmission electron microscopy the fault system contains three dominant fault zones the shallowest of which slipped during the 1999 chi chi earthquake the quartz and clay mineral contents of the primary slip zone were low and it contained partly amorphous ultrafine particles several tens of nanometers up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns whereas nearly 100 weight percent of those in surrounding samples could be the unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake because weak diffraction intensities are caused from mineral lattice distortion granulation and amorphous coatings such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault

a detailed seismic anisotropy study during the 2011 2012 unrest period in the santorini volcanic complex abstract the santorini volcanic complex svc is an area in the southern aegean greece which has been characterized by low seismicity rates for the last decades especially in the santorini caldera where they have been very low until 2010 this pattern changed completely in february 2011 when intense microseismic activity was initiated within the caldera during the manual analysis of the events the shear wave splitting phenomenon was observed revealing the existence of an anisotropic upper crust in the svc area a detailed anisotropy study has been conducted using 231 events within the shear wave window that fulfilled the selection criteria the polarization direction of the fast shear wave the time delay between the two split shear waves and the source polarization direction were calculated after visual inspection using both the polarigram and the hodogram representations this procedure applied for eight local stations resulted in the determination of 340 splitting parameters the obtained mean anisotropy directions are not homogeneous revealing a complex regime in the activated area nevertheless these results are explained by the ape model related to the stress sensitive behavior of fluid saturated microcracked rocks a detailed analysis of the temporal evolution of both the time delay and anisotropy direction was carried out the time delays measured in the band 1 window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity as well as to the major m w 5 1 and 5 2 events which took place about 40 km sw of santorini on 26 and 27 january 2012 respectively on the other hand no significant temporal variations or 90 flips of the s fast polarization direction were observed

changes in shear wave splitting at the 2014 barðarbunga seismic crisis and dyke intrusion in iceland compared with earthquakes and other eruptions abstract we use shear wave splitting sws above small earthquakes to monitor stress variations before the 2014 bar arbunga seismic crisis and dyke intrusion in central iceland sws is sensitive to stress induced variations in the internal geometry of the distributions of stress aligned fluid saturated microcracks pervading almost all crustal rocks this allows stress accumulation before both impending earthquakes and impending volcanic eruptions to be monitored and potentially the events stress forecast we compare the behaviour of sws before bar arbunga with the behaviour before large earthquakes and other volcanic episodes where sws has been monitored on the basis of the duration of stress changes we classify volcanic episodes into three broad classes during analysis of sws before bar arbunga temporal variation in polarizations and another retrospective stress forecast earthquake were recognized

clay mineralogy and geochemistry investigations in the host rocks of the chelungpu fault taiwan implication for faulting mechanism abstract the chelungpu fault with northward propagating ruptures was ruptured as a result of the m w 7 6 earthquake which struck central taiwan on 21st september 1999 to understand the consequences of clays in fault gouges we examined the clay mineralogy and major element geochemistry of the host rocks of the chelungpu fault from four outcrops shallow boreholes at fengyuan 455 3 m in depth nantou 211 9 m in depth and a deep borehole of the taiwan chelungpu fault drilling project 2003 m in depth results revealed different degrees of chemical weathering the intensity as documented by differences in the smectite content could be further understood through the relative clay percentage of smectite illite chemistry index and illite crystallinity these mineralogical proxies combined with the chemical index of alteration cia and the intensity of chemical weathering indicate that the degree of chemical weathering is a function of depth i e the most severe on the surface and the mildest in the tcdp samples the mineralogical and geochemical data obtained in this study also suggest that chemical weathering rather than leaching seems to be the main driving force for the phase changes of clays the amount of smectite produced by chemical weathering varies with depth and it suggests that the previous idea of weak fault caused by the presence of smectite in the fault zone will be risky without considering weathering processes the observations of clay mineralogy and major element geochemistry in this study indicate that the presence of smectite in the outcrops may not play a significant role during faulting and suggest that fault weakening as a result of the presence of smectite cannot be applied to the chelungpu fault

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

temporal variation of seismic velocity and anisotropy before the 2009 mw 6 3 l aquila earthquake italy we describe a set of seismological observations of the foreshock sequence preceding the 6 april 2009 m w 6 3 l9aquila earthquake the dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock approaching the earthquake we observed clear variations in the seismic wave propagation properties the elastic properties of rocks in the fault region underwent a sharp change about a week before the earthquake from our observations we infer that a complex sequence of dilatancy diffusion processes takes place and that fluids play a key role in the fault failure process

shear wave splitting in the crust in north china stress faults and tectonic implications summary this paper uses data from the capital area seismograph network in north china nc currently the largest regional seismograph network in china to investigate the compressional stress orientations from the polarizations of faster shear waves pfs a parameter of shear wave velocity anisotropy data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust the results show that the regional compressional stress direction is ene wsw to nearly ew in nc which agrees with seismic mechanisms and drilling and gps data predominantly pfs are also nearly ew in nc and are influenced by faults geology and tectonics pfs orientations at stations on seismically active faults are consistent with the strike of strike slip faults and indicate large seismically active faults breaking or nearly breaking the free surface pfs orientations are influenced by both regional compressional stress and by nearby faults it is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network shear wave splitting may also be used to indicate hitherto unknown faults

initiation movement and run out of the giant tsaoling landslide what can we learn from a simple rigid block model and a velocity displacement dependent friction law abstract tsaoling landslide is the largest and best documented landslide among several large landslides induced by the 1999 taiwan chi chi earthquake pliocene sedimentary rocks of about 125 mm 3 in volume slid along very flat bedding planes dipping by 14 with an average speed of 35 40 m s for about 1650 m before hitting the bank of the chinshui river during the landslide detailed analysis of dtms before and after the earthquake using a gis software leads to an accurate determination of the locations of the centroids of landslide mass revealing the horizontal and vertical displacements of the 2524 m and 524 m respectively those displacements and landslide mass give an apparent friction coefficient of 0 21 and the release of the potential energy of 1 6 10 15 j we conducted rotary shear high velocity friction experiments on fault gouge from bedding parallel faults under semi wet conditions and at 3 mpa normal stress corresponding to the overburden pressure of the landslide mass we also compiled reported data on the frictional properties on shale powders and fault gouge from the landslide site under both dry and wet conditions and proposed a velocity displacement dependent friction law that can describe most experimental data newmark analysis of landslide motion with six scenarios for different landslide materials and conditions assuming a simple rigid block sliding and using measured frictional parameters revealed that the landslide did not occur with dry frictional properties and that the landslide occurred at 38 39 s with accumulated displacements of 0 62 m 1 09 m and reached at the river bank at 82 87 s after the generation of chi chi earthquake at its epicenter those timings are consistent with high frequency signals at 32 40 s and at 76 s recorded at a nearby seismic station and with a survivor s witness that the landslide initiated 10 s after he felt strong ground motion possible s wave arrival at 25 2 s slip weakening is essential in initiating the landslide and low friction coefficient 0 08 0 1 allowed high speed of the landslide possible the landslide was caused by a few peaks of northeast oriented strong accelerations of the ground motion frictional work during the sliding of the landslide mass was estimated to be of about 23 of potential energy and the rest of the released energy is likely to have been consumed during the stopping phase of the landslide after hitting the river bank in complex processes such as fragmentation heat dissipation and spreading of the landslide deposits

pyrite alteration and neoformed magnetic minerals in the fault zone of the chi chi earthquake mw7 6 1999 evidence for frictional heating and co seismic fluids 1 during an earthquake physical and chemical transformations lead to alteration and formation of minerals in the gouge layer altered and neoformed minerals can be used as tracers of some earthquake processes in this study we investigate pyrite and magnetic minerals within the host chinshui siltstone and the 16 cm thick gouge this gouge hosts the principal slip zone of chi chi earthquake mw7 6 1999 in the chinshui siltstone pyrite framboids of various sizes and euhedral pyrite are observed the magnetic mineral assemblage comprises stoichiometric magnetite greigite and fine grained pyrrhotite the pyrite content is generally reduced in the gouge compared to the wall rock the magnetic mineral assemblage in the gouge consists of goethite pyrrhotite and partially oxidized magnetite the pyrrhotite goethite and some magnetite are neoformed pyrrhotite likely formed from high temperature decomposition of pyrite 500 c generated during co seismic slip of repeated earthquakes goethite is inferred to have formed from hot aqueous co seismic fluid 350 c in association with the 1999 chi chi event elevated fluid temperatures can also explain the partial alteration of magnetite and the retrograde alteration of some pyrrhotite to pyrite we suggest that characterization of neoformed magnetic minerals can provide important information for studying earthquake slip zones in sediment derived fault gouge

fracture surface energy of the punchbowl fault san andreas system the energetics of earthquake rupture slip weakening and rupture surface formation are poorly understood by measuring the energy associated with fracture damage along the punchbowl fault in the devil s punchbowl los angeles county park part of the san andreas system chester et al show that the fracture surface energy for rupture along a mature fault of this type accounts for only a small fraction of the energy released during large earthquakes as determined by seismologists this suggests that mechanisms such as melt lubrication and thermal pressurization are responsible for slip weakening fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening1 2 3 fracture energy has been estimated from seismological and experimental rock deformation data4 5 6 7 8 yet its magnitude mechanisms of rupture surface formation and processes leading to slip weakening are not well defined8 9 10 here we quantify structural observations of the punchbowl fault a large displacement exhumed fault11 12 in the san andreas fault system and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake if fracture energy is attributed entirely to the production of fracture surfaces then all of the fracture surface area in the punchbowl fault could have been produced by earthquake displacements totalling 1 km but this would only account for a small fraction of the total energy budget and therefore additional processes probably contributed to slip weakening during earthquake rupture

pore pressure migration along a normal fault system resolved by time repeated seismic tomography in this study we outline the space and time variations of body wave velocities during one of the best documented series of normal faulting earthquakes the 1997 umbria marche sequence in central italy we show the first ever observations of rock fracturing and fluid overpressure propagation along a fault system by using space time resolved variations of vp vs anomalies four dimensional variations that accompany earthquake migration and precede large aftershocks the vp vs increase observed before the mid october earthquakes was related to a pore pressure increase on fluid filled cracks in the volume around the fault we also document that such variations are measurable only by using s and p wave arrival times

calculable fluid rock interactions this paper introduces a model of rock deformation anisotropic poro elasticity or ape where the response of fluid saturated rock to changing conditions prior to fracturing can be calculated the driving mechanism for deformation is fluid migration along pressure gradients between neighbouring intergranular microcracks and pores at different orientations to the stress field the parameters that control changes to microcrack geometry also control the splitting birefringence of seismic shear waves so that changes in deformation can be directly monitored by analysing the shear wave splitting which is observed in almost all rocks analysis of shear wave splitting in the earth9s crust and ape modelling show that distributions of mostly intergranular cracks in the crust are always geometrically close to fracturing with the implication that shear wave splitting is sensitive to comparatively minor changes of stress and minor changes of in situ conditions this has important implications for the state of criticality of the rockmass and as a consequence changes in shear wave splitting have been observed before larger earthquakes on those few occasions when suitable source receiver geometry coincides with appropriate seismic activity ape also has implications for monitoring changing conditions in reservoirs during hydrocarbon recovery

a stress forecast earthquake with hindsight where migration of source earthquakes causes anomalies in shear wave polarisations abstract the stress aligned shear wave splitting observed throughout most of the crust is a three dimensional function of the effective elastic anisotropy of distributions of fluid saturated grain boundary cracks and preferentially oriented pores fluid saturated microcracks are highly compliant and crack geometry responds readily to changes in stress monitoring with shear wave splitting allows the accumulation of stress before earthquakes to be recognised and the time and magnitude of the impending earthquake to be stress forecast such increases in stress have been observed some 15 times with hindsight and one successful real time stress forecast here we report a further with hindsight stress forecast in northern iceland where increases of time delays implying stress accumulation are observed before a 4 9 earthquake at two seismic stations at epicentral distances of 50 km and 92 km there are however two anomalies changes in shear wave polarisations at the nearest station appear to be due to migration of shear wave source earthquakes in a uniform anisotropic symmetry into ray path directions with different anisotropic shear wave polarisations such directional changes in shear wave polarisations are occasionally observed above small earthquakes and may be misinterpreted as structural or temporal changes in anisotropy a further anomaly is that the station at 92 km displays variations in time delays but a station just 4 km farther away on the other side of a major fault appears to be in the stress shadow of the fault and does not display temporal variations despite having adequate source data

coseismic fluid rock interactions at high temperatures in the chelungpu fault chemical and isotopic variations in crushed material along the chelungpu fault in taiwan are suggestive of interactions at high temperatures with coseismically generated aqueous fluids high fluid pressures along the fault zone during the magnitude 7 6 chi chi earthquake in 1999 could have reduced friction and fault strength aqueous fluids are thought to have an essential role in faulting and the dynamic propagation of earthquake rupture fluid overpressure can affect earthquake nucleation1 2 and in a process termed thermal pressurization pore fluid pressure produced by frictional heating can reduce the effective normal stress acting on the fault surface3 4 5 this may lead to a marked reduction in fault strength during slip however the coseismic presence of fluids within slip zones and the role of fluids in dynamic fault weakening is still a matter of debate here we present compositions of major and trace elements as well as isotope ratios of core samples representing relatively undamaged as well as very fine grained deformed material from three active zones of the chelungpu fault taiwan depth profiles across the most intensely sheared bands that range in thickness from 2 15 cm exhibit sharp compositional peaks of fluid mobile elements and of strontium isotopes we suggest that high temperature fluids 350 c derived from heating of sediment pore fluids during the earthquake interacted with material within the fault zone and mobilized the elements the coseismic presence of high temperature fluids under conditions of low hydraulic diffusivity6 within the fault zone is favourable for thermal pressurization this effect may have caused a dynamic decrease of friction along the chelungpu fault during the 1999 magnitude 7 6 chi chi earthquake

frictional melting of clayey gouge during seismic fault slip experimental observation and implications clayey gouges are common in fault slip zones at shallow depths thus the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths we conducted rotary shear tests on clayey gouge 41 wt clay minerals at a seismic slip rate of 1 3 m s here we report that the gouge was melted at 5 mpa of normal stress and room humidity conditions the initial local melting was followed by melt layer formation clay minerals e g smectite and illite and plagioclase were melted and quenched to glass with numerous vesicles both flash heating and bulk temperature increases appear to be responsible for the melting this observation of clayey gouge melting is comparable to that of natural faults e g chelungpu fault taiwan due to heterogeneous fault zone properties e g permeability frictional melting may be one of the important processes in clayey slip zones at shallow depths

a review of retrospective stress forecasts of earthquakes and eruptions changes in shear wave splitting sws monitor stress induced changes to the geometry of the stress aligned fluid saturated microcracks pervading almost all sedimentary igneous and metamorphic rocks in the earth s crust and upper mantle changes in sws implying stress accumulation and stress relaxation suggesting crack coalescence before large earthquakes have been observed retrospectively in the rock mass surrounding large or larger earthquakes in one case the time magnitude and fault plane of a m 5 earthquake in sw iceland was successfully stress forecast 3 days before it occurred similar characteristic behaviour of shear wave splitting has been observed retrospectively before 17 other earthquakes and before three volcanic eruptions these retrospective stress forecasts have been published in different formats in different journals for clarification this paper redraws all observations of stress accumulation and stress relaxation in a consistent normalised format that allows the overall similarities in behaviour to be recognised before earthquakes and volcanic eruptions such behaviour inconsistent with conventional sub critical geophysics confirms the compliance of the new geophysics of a critically microcracked earth where the microcracks are so closely spaced that they verge on failure and hence are critical systems that impose a range of fundamentally new properties on conventional sub critical geophysics one of the implications of new geophysics is that there are similarities in the behaviour of stress before earthquakes and volcanic eruptions the normalised formats show such similarities and include the opportunity to stress forecast both earthquakes and eruptions

temperature estimates of coseismic heating in clay rich fault gouges the chelungpu fault zones taiwan abstract to investigate the coseismic frictional temperature in seismogenic fault zones we examine the characteristics of clays in the chelungpu fault zones with isothermal heating experiments scanning electron microscope coupled to an energy dispersive spectrometer sem edx and thermogravimetry analysis tga in the tcdp case taiwan chelungpu fault drilling project three fault zones of the chelungpu fault system were identified at the depth of 1111 m 1153 m and 1222 m described as fz1111 fz1153 and fz1222 hereafter respectively the clay mineral assemblages of fz1111 show evidence of melting and the temperature in a 2 cm band within the black gouge zone is estimated to be from 900 c to 1100 c by comparing the sem images of in situ natural samples with those of heated materials and the finding of no recrystallization of kaolinite amorphous aluminosilicates spinel in the fault samples the clay mineral assemblages of fz1153 suggested that kaolinite has been broken down by the thermal decomposition dehydroxylation but chlorite has not the clay characteristics and results of sem edx and tga constrain the faulting temperature from 500 c to 900 c with a spatial distribution up to 1 3 m the clay characteristics of fz1222 indicated that clays were changed by experiencing high temperature acid fluids instead of thermal decomposition dehydroxylation processes and that the temperature is localized in 2 cm and ranges from 350 c to 500 c the lowest temperature among three fault zones the estimates of temperature ranges and thermal anomaly intervals among three fault zones provide important information and constraints on the physical and chemical processes coseismic dynamic weakening mechanism and earthquake energy budget in the future

slip weakening distance of faults during frictional melting as inferred from experimental and natural pseudotachylytes the slip weakening distance d c over which a fault weakens during seismogenic fault motion is one of most important parameters controlling fault instability a paradox over the last decade has been the source of the large difference between laboratory measured values of d c using conventional friction experiments 10 5 10 3 m and seismically determined d c values for natural earthquakes 10 1 10 0 m d c is a difficult quantity to estimate for natural faults but this study attempts to determine the order of d c values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes experiments at a constant slip velocity of 0 85 m sec and a normal stress of 1 4 mpa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening slip weakening a reduction in frictional strength with displacement and the growth of the molten layer an increase in the thickness of the molten layer with displacement are both characterized by exponential changes with nearly the same characteristic distances the comparison suggests that the order of d c can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement sibson s 1975 data from the outer hebrides exhibit a similar pseudotachylyte thickness versus fault displacement curve with a characteristic distance of 0 38 m this is the same order of magnitude as seismically inferred values of d c the effect of frictional melting is a possible solution for the d c paradox although it is not the only possible solution since frictional melting along large faults is rather uncommon

smaller source earthquakes and improved measuring techniques allow the largest earthquakes in iceland to be stress forecast with hindsight summary a group of three earthquakes in 2000 june in sw iceland included the two largest earthquakes in iceland in the past 30 yr previously temporal changes in shear wave splitting had not been recognized before these earthquakes as there were too few small earthquakes to provide adequate shear wave data and they were not stress forecast even with hindsight these large earthquakes were subject to a special investigation by the european community funded prepared project during which the seismic catalogue was extended to include smaller magnitude earthquakes this more detailed data set together with a semi automatic programme for measuring the parameters of shear wave splitting greatly increased the number of time delay measurements the new measurements displayed the typical temporal variations before larger earthquakes as seen elsewhere a long term increase in time delays interpreted as stress accumulation before the earthquake and a decrease interpreted as crack coalescence immediately prior to the earthquake the logarithms of the durations of both the implied accumulation of stress and the crack coalescence have the same self similar relationships to earthquake magnitude as found elsewhere in iceland this means that in principle the time and magnitude of the larger earthquakes could have been stress forecast in real time had the smaller source earthquakes of the extended catalogue and the improved measuring procedures been available at the time

evidence of frictional melting from disk shaped black material discovered within the taiwan chelungpu fault system 1 the taiwan chelungpu fault drilling project penetrated three fault zones as the chelungpu fault system which slipped during the 1999 chi chi earthquake discovering disk shaped black material bm disk within the middle and lower fault zones in hole b the microscopic features of the bm disks indicated that they were pseudotachylytes and they showed high magnetic susceptibility possibly the result of intense shearing or high temperature conditions inorganic carbon content of the bm disks was low possibly because of thermal decomposition of carbonate minerals the high temperatures might be related to frictional heating during the earthquake implying that the bm disks were produced under intense shearing with frictional heating that reached melting temperature because the disks which provide the only evidence of melting pre date the 1999 earthquake we concluded that frictional melting did not occur during the earthquake

ultrafine spherical quartz formation during seismic fault slip natural and experimental evidence and its implications abstract in recent works on the determination of pseudotachylyte within the principal slip zone psz of the chelungpu fault taiwan we demonstrated that frictional melting occurred at shallow depths during the 1999 mw 7 6 chi chi earthquake thus the characteristics of melts are of paramount importance to investigate processes controlling dynamic fault mechanics during seismic slips we conducted rock friction experiments on siltstone recovered from the taiwan chelungpu fault drilling project tcdp at a slip rate of 1 3 m s and a normal stress of 1 mpa here we not only target to characterize experimental pseudotachylyte and evaluate the associated frictional behavior but also compare it with natural frictional melts of tcdp our results show that 1 initial shear stress drop was related to the generation of low viscosity melt patches 2 the evolution of shear stress in the postmelting regime was congruent with frictional melt rheology and 3 the slip strengthening presumably resulted from the extremely low content of water of the frictional melt in particular the state of art of in situ synchrotron analyses x ray diffraction and transmission x ray microscope determine the presence of ultrafine spherical quartz usq grains 10 nm to 50 nm in the glassy matrices presumably produced at high temperature our observations confirm that the usq grains formed in rock friction experiments do occur in natural faults we surmise the usq is the result of frictional melting on siltstone and represents the latest slip zones of the chelungpu fault and further infer that the viscous melts may terminate seismic slips at shallow crustal conditions

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

a few earthquake conundrums resolved several fundamental questions conundrums about earthquakes and rocks are inexplicable in terms of conventional sub critical geophysics these questions have become so familiar that they are now generally accepted as the way earthquakes and rocks behave and are not recognised as presenting conceptual difficulties these conundrums are resolved by a new understanding of fluid rock deformation where fluid saturated microcracks in almost all rocks are so closely spaced they verge on failure and hence are highly compliant critical systems which impose a range of new properties on conventional sub critical geophysics this new understanding of fluid rock deformation this new geophysics allows earthquakes to be stress forecast and has implications and applications to many solid earth developments

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

experimental investigation of flash weakening in limestone abstract flash heating and weakening could operate during earthquake nucleation and propagation we performed 27 friction experiments in a compression torsion apparatus on ring shaped limestone samples at sub seismic to seismic slip rates 340 mm s centimetric displacements and normal stresses of 8 mpa friction decreases dramatically at slip rates of 50 150 mm s flash weakening was contemporaneous with a peripheral temperature rise of 90 c measured with an infrared camera the peripheral temperature yields a lower limit to the slipping zone temperature the decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by 1 flash heating or 2 mechanically activated reactions however x ray powder diffraction and raman spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone instead white light interferometry and field emission scanning electron microscope observations reveal the presence of a smooth sliding surface made of nanometric particles the mechanical data can be fit by the rate and state dependent friction model or by a quadratic model the latter proposed for powder lubrication we conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and for the conditions investigated here in nature

shear wave splitting changes associated with the 2001 volcanic eruption on mt etna summary the time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 july 17 august 9 2001 flank eruption on mt etna sicily normalized time delays measured by singular value decomposition show a systematic increase starting several days before the onset of the eruption on several occasions before the eruption the polarization directions of the shear waves at station mnt closest to the eruption show 90 flips where the faster and slower split shear waves exchange polarizations the last 90 flip being 5 days before the onset of the eruption the time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a relaxation phenomena due to crack coalescence this behaviour has many similarities to that observed before a number of earthquakes elsewhere

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

clay clast aggregates in gouges new textural evidence for seismic faulting 1 spherical aggregates named clay clast aggregates ccas have been reported from recent investigations on retrieved clay bearing fault gouges from shallow depth seismogenic faults and rotary shear experiments conducted on clay bearing gouge at seismic slip rates the formation of ccas appears to be related to the shearing of a smectite rich granular material that expands and becomes fluidized we have conducted additional high velocity rotary shear experiments and low velocity double shear experiments we demonstrate that a critical temperature depending on dynamic pressure temperature conditions is needed for the formation of ccas this temperature corresponds to the phase transition of pore water from liquid to vapor or to critical which induced gouge pore fluid expansion and therefore a thermal pressurization of the fault a detailed examination by energy dispersive x ray spectrometry edx sem element mapping sem and transmission electron microscopy tem shows strong similar characteristics of experimental and natural ccas with a concentric well organized fabric of the cortex and reveals that their development may result from the combination of electrostatic and capillary forces in a critical reactive medium during the dynamic slip weakening accordingly the occurrence of ccas in natural clay rich fault gouges constitutes new unequivocal textural evidence for shallow depth thermal pressurization and consequently for past seismic faulting

experimental investigation of frictional melting of argillite at high slip rates implications for seismic slip in subduction accretion complexes 1 discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite rich argillite derived slip zones during subduction earthquakes we conducted high velocity friction experiments on argillite at a slip rate of 1 13 m s and normal stresses of 2 67 13 33 mpa experiments show slip weakening followed by slip strengthening slip weakening is associated with the formation and shearing of low viscosity melt patches the subsequent slip strengthening occurred despite the reduction in shear strain rate due to the growth thickening of melt layer suggesting that the viscosity of melt layer increased with slip microstructural and chemical analyses suggest that the viscosity increase during the slip strengthening is not due to an increase in the volume fraction of solid grains and bubbles in the melt layer but could be caused primarily by dehydration of the melt layer our experimental results suggest that viscous braking can be efficient at shallow depths of subduction accretion complexes if substantial melt dehydration occurs on a timescale of seismic slip melt lubrication can possibly occur at greater depths within subduction accretion complexes because the ratio of viscous shear to normal stress decreases with depth argillite derived natural pseudotachylytes formed at seismogenic depths in subduction accretion complexes are more hydrous than the experimentally generated pseudotachylytes and may be evidence of nearly complete stress drop

faulting processes in active faults evidences from tcdp and safod drill core samples abstract the microstructures mineralogy and chemistry of representative samples collected from the cores of the san andreas fault drill hole safod and the taiwan chelungpu fault drilling project tcdp have been studied using optical microscopy tem sem xrd and xrf analyses safod samples provide a transect across undeformed host rock the fault damage zone and currently active deforming zones of the san andreas fault tcdp samples are retrieved from the principal slip zone psz and from the surrounding damage zone of the chelungpu fault substantial differences exist in the clay mineralogy of safod and tcdp fault gouge samples amorphous material has been observed in safod as well as tcdp samples in line with previous publications we propose that melt observed in tcdp black gouge samples was produced by seismic slip melt origin whereas amorphous material in safod samples was formed by comminution of grains crush origin rather than by melting dauphine twins in quartz grains of safod and tcdp samples may indicate high seismic stress the differences in the crystallographic preferred orientation of calcite between safod and tcdp samples are significant microstructures resulting from dissolution precipitation processes were observed in both faults but are more frequently found in safod samples than in tcdp fault rocks as already described for many other fault zones clay gouge fabrics are quite weak in safod and tcdp samples clay clast aggregates ccas proposed to indicate frictional heating and thermal pressurization occur in material taken from the psz of the chelungpu fault as well as within and outside of the safod deforming zones indicating that these microstructures were formed over a wide range of slip rates

clay mineral reactions caused by frictional heating during an earthquake an example from the taiwan chelungpu fault 1 to understand the chemical reactions of clay minerals in a fault zone during an earthquake we analyzed the clay minerals in the chelungpu fault which slipped during the 1999 chi chi earthquake x ray diffraction spectroscopy showed that kaolinite and smectite contents were lower in the black gouge zone than in the surrounding gray gouge breccia or fracture damaged zones we applied a chemical kinetics approach to examine whether dehydroxylation of kaolinite and dehydration of interlayer water dehydroxylation and illitization of smectite occurred during coseismic frictional heating and found that the first two reactions could complete under the temperature time profile of the chi chi earthquake reconstructed by a previous study because dehydration of smectite interlayer water and dehydroxylation of kaolinite would have completed 3 6 and 8 2 s after the beginning of slip the resulting release of water might have affected the frictional mechanism during the earthquake

stress forecasting not predicting earthquakes a paradigm shift after 120 years of unsuccessful endeavor a paradigm shift is required before earthquakes can be predicted the most sensitive diagnostic of low level changes of stress in in situ rock variations in microcrack geometry can be monitored by analyzing shear wave splitting the suggested paradigm shift is that instead of investigating the source zone we monitor stress accumulation before earthquakes at possibly substantial distances from the source characteristic temporal variations of shear wave time delays have been observed in retrospect before 14 earthquakes worldwide on one occasion when changes were recognized early enough the time magnitude and fault break of an m 5 earthquake in southwest iceland were successfully stress forecast in a narrow time magnitude window such stress accumulation can be theoretically modeled and is believed to be at least partially understood when sufficient shear wave source earthquakes are available increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs this is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break the new result confirming these ideas and justifying the paradigm shift is that logarithms of the durations of both increases and decreases in time delays are found to be proportional self similar to the magnitudes of impending earthquakes

particle size and energetics of gouge from earthquake rupture zones surprisingly considering the forces that we see at the surface not all the energy involved in an earthquake is transformed into ground motion much of it is consumed by frictional heating and rock pulverization an analysis of pulverized rock or gouge collected from the san andreas fault in california and from a rupture zone at 2 km depth in a south african gold mine contradicts the commonly held view that gouge surface energy is a negligible component of earthquake energy balance the analysis reveals unexpected diminutive particles in gouge samples indicating that pulverization not frictional heating is the major consumer of earthquake energy at depth this might explain the heat flow anomaly seen in the san andreas fault system and perhaps some other long standing puzzles in earthquake energetics grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales1 2 3 4 and most slip along mature faults is observed to have been localized within gouge zones5 6 this fine grain gouge is thought to control earthquake instability3 6 7 8 and thus understanding its properties is central to an understanding of the earthquake process7 9 here we show that gouge from the san andreas fault california with 160 km slip and the rupture zone of a recent earthquake in a south african mine with only 0 4 m slip display similar characteristics in that ultrafine grains approach the nanometre scale gouge surface areas approach 80 m2 g 1 and grain size distribution is non fractal these observations challenge the common perception that gouge texture is fractal10 11 and that gouge surface energy is a negligible contributor to the earthquake energy budget3 9 12 we propose that the observed fine grain gouge is not related to quasi static cumulative slip but is instead formed by dynamic rock pulverization during the propagation of a single earthquake

clay mineral anomalies in the fault zone of the chelungpu fault taiwan and their implications 1 the taiwan chelungpu fault drilling project tcdp hole a recovered continuous core samples across the rupture zone of the 1999 chi chi earthquake mw7 6 studying in situ chemical properties sequentially from fresh fault zone materials of the chelungpu fault provides insight into possible faulting mechanisms distinct anomalies of mineral assemblages at the 1111 m fault zone of tcdp hole a are found to be 1 a decrease in clay content within the primary slip zone psz and 2 a significant decline of illite disappearance of chlorite and kaolinite and spike in smectite within the psz meanwhile features relating to melting or amorphous material in the psz have been observed by sem and tem the results suggest that the psz might have experienced generation of glassy materials such as pseudotachylyte by the expense of clay minerals due to strong shear heating then prompt alteration of pseudotachylyte into smectite characteristics of clay minerals and images obtained from electronic microscopes in the psz thus imply that pseudotachylyte possibly developed during the 1999 chi chi earthquake but quickly altered into smectite this particular phenomenon may explain why pseudotachylyte is rarely found in exhumed hydrated fault zones

the new geophysics new geophysics of critically microcracked rock is a new understanding of fluid rock deformation that matches much of the behaviour of in situ rock observed throughout the earth s crust and uppermost mantle the behaviour of rock in new geophysics is self similar can be monitored calculated predicted even in principle controlled prevails in almost all rocks and has butterfly effect sensitivity all of these remarkable features except controllability have been observed in some cases many times however new geophysics is innovative controversial and currently ridiculed and violently opposed schopenhauer stages 1 and 2 and is difficult to get published via consensus driven peer review consequently in the last 10 years various aspects of new geophysics have been published in some 20 different journals and conference proceedings making it difficult to access by the general reader this review presents a synopsis of what is believed to be a fundamental advance in solid earth geoscience

earthquake prediction a critical review summary earthquake prediction research has been conducted for over 100 years with no obvious successes claims of breakthroughs have failed to withstand scrutiny extensive searches have failed to find reliable precursors theoretical work suggests that faulting is a non linear process which is highly sensitive to unmeasurably fine details of the state of the earth in a large volume not just in the immediate vicinity of the hypocentre any small earthquake thus has some probability of cascading into a large event reliable issuing of alarms of imminent large earthquakes appears to be effectively impossible key word earthquake prediction

thermal history estimation of the taiwan chelungpu fault using rock magnetic methods 1 to investigate the cause of high magnetic susceptibility in disk shaped black materials bm disks within core samples of hole b the taiwan chelungpu fault drilling project we carried out magnetic analyses of samples from the bm disks and surrounding gouge zones frequency dependence of low temperature magnetic susceptibility and thermal demagnetization of low temperature remanent magnetization revealed lower amounts of superparamagnetic grains in the bm disks than in the surrounding gouges magnetic grain size reduction by shearing is therefore not a plausible explanation for the high magnetic susceptibility of the bm disks thermomagnetic analyses of the gouge samples showed that above 400 c magnetization could be increased by thermal decomposition of paramagnetic minerals whereas analyses of the bm disks showed a much smaller increase in magnetization thus formation of ferrimagnetic magnetite or maghemite by thermal decomposition of paramagnetic minerals was inferred to be the cause of the high magnetic susceptibility of the bm disks indicating that the bm disks but not the gouges have experienced temperatures of at least 400 c

coseismic thickness of principal slip zone from the taiwan chelungpu fault drilling project a tcdp a and correlated fracture energy abstract direct observations of the physical structures of the seismogenic zones of active faults are rare due to the difficulty in reaching the fault zone at depth current geological evidences mostly from the surface suggest that principal slip zone psz accommodated most shear displacement and was the place where physico chemical processes occurred during an individual coseismic event and the thickness of psz is a few millimeter to tens of centimeter wide however the actual thickness of psz of a large earthquake a key parameter of seismology in understanding energy dissipation and rupture processes remains largely unknown the chelungpu fault that ruptured during the 1999 mw 7 6 chi chi earthquake taiwan was drilled to a depth of 2003 m providing a unique opportunity to sample an active fault that slipped in a recent large earthquake the psz corresponding to the 1999 chi chi earthquake was well characterized within cores at a borehole depth of 1111 m from the taiwan chelungpu fault drilling project a tcdp a here we determine the interval of clay anomaly that resulted from frictional melting thermal decomposition process by state of art in situ synchrotron xrd analysis providing very high spatial resolution for mineralogy combined with the interval of the presence of vesicles from microstructural observation the thickness of chi chi psz is estimated to be 1 mm thus the correlated contribution of surface fracture energy to earthquake breakdown work at least in this locality is quantified to be 1 9 the huge remaining part of the breakdown work seems to be turned into heat associated with fault dynamic processes during the 1999 chi chi earthquake

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

comment on systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory changes by yungfeng liu ta liang teng and yehuda ben zion liu et al 2004 systematically analyze the parameters of shear wave splitting before and after the 1999 chi chi earthquake in taiwan they claim that they see no evidence for the temporal changes in shear wave time delays which some of us use to monitor the buildup of stress before earthquakes in fact the only diagram where one would expect to see the type of temporal changes observed elsewhere figure 14a in liu et al 2004 does show two types of temporal variations of time delays here we fit least squares lines to these temporal changes and show that the duration earthquake magnitude relationships of these various changes agree very well with those seen elsewhere thus the behavior before the chi chi earthquake lends further support to the use of shear wave splitting to stress forecast the times and magnitudes of impending earthquakes

observations of stress relaxation before earthquakes summary theory and observations suggest that seismic shear wave splitting caused by fluid saturated stress aligned microcracks directly monitors low level deformation before fracturing faulting and earthquakes occur in the past it had been assumed that the accumulation of stress before earthquakes continued until stress was released by faulting at the time of the earthquake however new data and reappraisal of existing data sets now suggests that the stress begins to relax and cracks close from tens of minutes to months before the earthquake actually occurs with the logarithm of the duration of the relaxation proportional to the magnitude of the impending earthquake the duration of the relaxation appears to be directly correlated with earthquake magnitude and may have implications for the earthquake source process and the ability to predict earthquakes

exploring fault rocks at the nanoscale abstract this paper aims to demonstrate the potential role of micro nanoscale investigations in the study of fault rocks given that a multiscale and multidisciplinary approach linking structural geology petrology and mineralogy from the outcrop to the unit cell scale is essential to the comprehensive characterization of geological materials to explore fault rocks at the sub micrometer scale a fundamental contribution is provided by the transmission electron microscope tem which offers a real time combination of high resolution images diffraction patterns and chemical data for extremely small volumes providing accurate mineralogical and nanostructural determinations this paper focuses on a few selected examples that are particularly appropriate for tem investigation in particular 1 grain size determinations on ultrafine grained fault rocks 2 detection and characterization of poorly crystalline minerals e g clays and amorphous phases within fault rocks 3 identification of deformation induced mineral reactions occurring within fault zones e g dolomite decarbonation and serpentine dehydration and 4 observations of recurrent deformation nanotextures in phyllosilicates tem investigation can provide an unexpected amount of data much of which cannot be obtained by conventional techniques and it can therefore significantly contribute to an understanding of the physico chemical conditions for faulting and fault mechanics

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

in situ stress state from walkaround vsp anisotropy in the kumano basin southeast of the kii peninsula japan to reveal the stress state within the kumano basin which overlies the nankai accretionary prism we estimated seismic anisotropy from walkaround vertical seismic profiling vsp data recorded at site c0009 during integrated ocean drilling program iodp expedition 319 we obtained the following anisotropic parameters 1 p wave velocity anisotropy derived from azimuthal normal moveout nmo velocity analysis 2 p wave amplitude variation with azimuth and 3 axes of symmetry of s wave splitting azimuthal variations of p wave velocity by ellipsoidal fitting analysis showed that p wave velocity anisotropy within sediments of the kumano basin was 5 both the directions of fast p wave velocity and strong amplitude are aligned with the convergence vector of the philippine sea plate furthermore s wave splitting analysis indicated that s wave polarization axes were parallel to and normal to the direction of plate subduction these results indicate that the maximum horizontal stress at site c0009 in the kumano basin is in the direction of plate subduction the horizontal differential stress estimated from the p wave velocity anisotropy 2 7 5 5 mpa indicates that the maximum horizontal stress is similar in magnitude to or a little higher than the vertical stress

high magnetic susceptibility of fault gouge within taiwan chelungpu fault nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from hole b tcdp 1 the taiwan chelungpu fault drilling project tcdp was undertaken in 2002 to investigate the faulting mechanism of the 1999 taiwan chi chi earthquake hole b penetrated the chelungpu fault and recovered core samples from between 948 42 m and 1352 60 m depth three zones marked 1136mfz 1194mfz and 1243mfz were recognized in the core samples as active fault zones within the chelungpu fault multi sensor core logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones even though the fault zone that slipped during the 1999 earthquake has not been identified higher magnetic susceptibilities indicate that frictional heating has taken place in the chelungpu fault

frictional strength of fault gouge in taiwan chelungpu fault obtained from tcdp hole b abstract three fault zones fzb1136 fzb1194 and fzb1243 are found in core samples in hole b borehole penetrating through the chelungpu fault central taiwan that activated at the 1999 taiwan chi chi earthquake each fault zone is composed of a black gouge zone bgz and a gray gouge zone ggz surrounded by a fault breccia zone bz and or fracture damaged zone fz we conducted tri axial friction experiments on samples collected from subzones of the above three fault zones under the 1 km depth ambient condition with axial shortening rates of 0 1 to 10 m s to investigate frictional strength and its velocity dependence we found that the coefficient of friction is 0 3 for the bgz at fzb1136 1136bgz which is lower than those at other fault zones this result indicates that fault slip at the chi chi earthquake occurred at the 1136bgz the estimated frictional strength profile across fzb1136 was that the frictional strength was the lowest the coefficient of friction was 0 3 for the bgz and increased up to 0 5 for undeformed host rock with increasing distance from the bgz presence of the intermediate frictional strength zone consisting of the ggz and the bz suggests a development of off fault damage zone where deformation occurred not only in the bgz but also in the intermediate zone this feature should be important to estimate total fracture energy released during earthquakes we also suggest that coefficients of friction and the velocity dependences of the samples are closely related to localization of shear deformation and clay content respectively

shear wave splitting and earthquake forecasting seismic shear wave splitting sws monitors the low level deformation of fluid saturated microcracked rock we report evidence of systematic sws changes recorded above small earthquakes monitoring the accumulation of stress before earthquakes that allows the time and magnitude of impending large earthquakes to be stress forecast the effects have been seen with hindsight before some 15 earthquakes ranging in magnitude from an m1 7 seismic swarm event in iceland to the ms7 7 chi chi earthquake in taiwan including a successfully stress forecast of a m5 0 earthquake in sw iceland characteristic increases in sws time delays are observed before large earthquakes which abruptly change to deceases shortly before the earthquake occurs there is a linear relationship between magnitudes and logarithms of durations of both increases and decreases in sws time delays before large impending earthquakes however suitably persistent swarms of small earthquakes are too scarce for routine stress forecasting reliable forecasting requires controlled source cross hole seismics between neighbouring boreholes in stress monitoring sites sms it would be possible to stress forecast damaging earthquakes worldwide by a global network of sms in real time

crustal anisotropy in the forearc of the northern cascadia subduction zone british columbia summary this paper aims to identify sources and variations of crustal anisotropy from shear wave splitting measurements in the forearc of the northern cascadia subduction zone of southwest british columbia over 20 permanent stations and 15 temporary stations were available for shear wave splitting analysis on 4500 event station pairs for local crustal earthquakes results from 1100 useable shear wave splitting measurements show spatial variations in fast directions with margin parallel fast directions at most stations and margin perpendicular fast directions at stations in the northeast of the region crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress however studies have also shown anisotropy can be complicated by crustal structure southwest british columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults to use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure we determine the source of anisotropy at each station by comparing fast directions from shear wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion most stations show agreement between the fast direction and the maximum horizontal compressive stress this suggests that anisotropy is related to stress aligned fluid filled microcracks based on extensive dilatancy anisotropy these stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress and in the future may be used as a monitoring and mapping tool

a successfully stress forecast earthquake summary a m 5 earthquake in iceland has been successfully stress forecast by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress modified microcracking reaches fracture criticality within the stressed volume where strain is released local investigations suggested the approximate location of the forecast earthquake we report the criteria on which this stress forecast was based

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

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

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

spatial slip distribution of the september 20 1999 chi chi taiwan earthquake mw7 6 inverted from teleseismic data the teleseismic waveforms of the mw7 6 september 20 1999 chi chi earthquake were examined to obtain the quick information on the fault rupture process the deconvolution results show the fault ruptured from south to the north and revealed the west movement of the hanging wall to the footwall on the eastern dipping plane the spatial slip distribution shows that the earthquake was mainly composed by a large asperity with a dimension of about 45km 15km the maximum slip was about 8 m located at about 45 km to the north of the epicenter the slip distributions obtained in this study have a good agreement with the observed surface breaks the comparable static and the dynamic stress drops of about 11 mpa in the large slip region indicate that the melting fluid pressurization might have taken place in this earthquake it reduces the dynamic friction and results in the large slip observed

characterization of slip zone associated with the 1999 taiwan chi chi earthquake x ray ct image analyses and microstructural observations of the taiwan chelungpu fault abstract to characterize the fault related rocks within the chelungpu fault we performed x ray computed tomography ct image analyses and microstructural observations of hole b core samples from the taiwan chelungpu fault drilling project we identified the slip zone associated with the 1999 chi chi earthquake within the black gouge zone in the shallowest major fault zone by comparison with previous reports the slip zone was characterized by low ct number cataclastic or ultracataclastic texture and high possibility to have experienced a mechanically fluidized state taking these characteristics and previous reports of frictional heating in the slip zone into consideration we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake

distribution of recent benthic foraminifera along continental slope of the sunda shelf south china sea the distribution of live rose bengal stained and dead benthic foraminiferal distribution 150 mu m was investigated in sediment surface samples 0 1 cm from the winter upwelling region off borneo on the continental slope of the sunda shelf and from the continental slope of southern vietnam shelf the benthic foraminiferal faunas of both areas are highly diverse h s 2 91 4 25 for living and 4 01 4 38 for dead a canonical correspondence analysis proved that bathyal assemblages are most strongly influenced by the decreasing rates of organic carbon flux with increasing water depth resulting in a pronounced depth related succession of assemblages correlation between estimated c org flux and factor 1 loadings of the cca for modem dead foraminiferal assemblages is very good r 2 0 96 thus a local transfer function y 5 0289x 6 8142 for estimating paleo c org flux rates from the down core samples has been successfully developed for the continental slope of the sunda shelf four benthic foraminiferal biofacies were recognised within the studied depth ranges upper bathyal biofacies siphotextularia foliosa cibicidoides robertsonianus well supplied and well oxygenated middle bathyal biofacies uvigerina auberiana nuttallides rugosus located within the oxygen minimum zone uppermost lower bathyal biofacies lagenammina difflugiformis uvigerina peregrina and lower bathyal biofacies paratrochammina challengeri parrelloides bradyi from the relatively well oxygenated but food limited environment c 2009 elsevier b v all rights reserved

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

temporal changes in shear wave splitting at an isolated swarm of small earthquakes in 1992 near dongfang hainan island southern china summary an isolated swarm of small earthquakes occurred in 1992 near dongfang on hainan island southern china the institute of geophysics state seismological bureau of china monitored the swarm with five dcs 302 digital accelerometers for three months from 1992 june 1 18 earthquakes with magnitudes m l ranging from 1 8 to 3 6 were well located by five stations and shear wave splitting varying azimuthally was analysed on 27 seismic records from these events the mean polarization azimuth of the faster shear wave was wnw time delays between the split shear waves at two stations varied with time and space the time delays at one station fell abruptly after earthquakes of magnitudes 3 1 and 3 6 but did not change significantly at the second station this behaviour is consistent with the delay time changes being caused by changes in the aspect ratio of vertical liquid filled eda cracks thus the variation in shear wavesplitting time delay could be due to changes in crustal stress related to nearby smallmagnitude earthquake activity the connection between earthquake activity and crustal stress variation measured by shear wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake however our data cannot be considered as definite evidence for such precursors

spatial and temporal forecasting of large earthquakes in a spring block model of a fault summary we study a recently proposed statistical physics model of earthquake dynamics that includes stress relaxation in the plates as a fundamental ingredient the model is known to reproduce manyrealisticfeaturesofseismicphenomena suchas thegutenberg richterlawfortheevent size distribution the omori law for aftershocks and an overall velocity weakening dependence of the average friction force here we analyse the dynamics of the model in detail in order to investigate to what extent the occurrence of large events in the model can be anticipated we systematically find that large events occur in fault patches where strain accumulation has exceeded some threshold value the spatial extent of these patches which correlate with the magnitude offorthcoming events canbe calculated ifthe strainstateofthe systemissupposed tobe known inaddition wefind that some large events are preceded by well defined precursor activity thisallows inafractionofcases tocomplementtheforecastofmagnitudeandspatial location with a sensible prediction of time of occurrence although our work is exclusively limited to the numerical model analysed we argue that it gives new breath to earthquake forecast techniques that combine the historical analysis of seismic activity with a search of appropriate precursor activity

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

criticality in earthquakes good or bad for prediction abstract through the analysis of the correlation functions in simulations of an earthquake model the critical properties of the system are studied simulations are performed in a more realistic modification of the olami feder christensen model of earthquakes and result in uncorrelated avalanches distributed following a power law with weak signs of foreshocks and aftershocks the spatial autocorrelation function of the system and other structural variables are computed in every step of the simulation the spatial autocorrelation between points separated from each other by a constant distance equal to 1 4 and 1 8 of the linear size of the system shows large variations temporally correlated with the time series of avalanche size i e spatial correlation values are in average very high before a large earthquake very small after a large earthquake and they evolve between these two states however the temporal average of the spatial autocorrelation over the whole simulation shows values close to zero result that is in contradiction with the idea that the correlation length is of the same order as the linear size of the system diverging in an infinite system which is the main signature of a critical scenario by averaging the autocorrelation in smaller time windows the critical properties of temporal states can be used as an indication of upcoming catastrophic events the structural variables are also correlated with the occurrence of large avalanches suggesting the possibility of monitoring these variables in order to achieve prediction

the effect of particle size on the rheology of liquid solid mixtures with application to lava flows results from analogue experiments 1 we investigate the effect of crystal size on the rheology of basaltic magmas by means of a rheometer and suspensions of silicon oil with natural magmatic crystals of variable size from 63 to 0 5 mm and volume fraction from 0 03 to 0 6 at constant finer suspensions display higher viscosities than coarser ones shear thinning flow index n 0 1 0 2 and is more pronounced stronger departure from the newtonian behavior in finer suspensions maximum packing and average crystal size displays a nonlinear positive correlation while yield stress develops at 0 2 0 3 irrespective of the crystal size we incorporate our results into physical models for flow of lava and show that with respect to lava flows containing coarser crystals those with smaller crystals are expected to 1 flow at lower velocity 2 have a lower velocity gradient and 3 be more prone to develop a region of plug flow our experimental results explain the observation that phenocryst bearing and microlite bearing lavas at etna volcano italy show smooth pahoehoe and rough aa surfaces respectively

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

earthquakes can be stress forecast in 1997 geller et al wrote earthquakes cannot be predicted because scale invariance is ubiquitous in self organized critical systems and the earth is in a state of self organized criticality where small earthquakes have some probability of cascading into a large event physically however large earthquakes can only occur if there is sufficient stress energy available for release by the specific earthquake magnitude this stress dependence can be exploited for stress forecasting by using shear wave splitting to monitor stress accumulation in the rock mass surrounding impending earthquakes the technique is arguably successful but because of the assumed unpredictability requires explicit justification before it can be generally accepted avalanches are also phenomena with self organized criticality recent experimental observations of avalanches in 2 d piles of spherical beads show that natural physical phenomena with self organized criticality such as avalanches and earthquakes can be predicted the key to predicting both earthquakes and avalanches is monitoring the matrix material not monitoring impending source zones

detection of multiple fracture sets using observations of shear wave splitting in microseismic data the hydrocarbon industry is moving increasingly towards tight sandstone and shale gas resources reservoirs that require fractures to be produced economically therefore techniques that can identify sets of aligned fractures are becoming more important fracture identification is also important in the areas of coal bed methane production carbon capture and storage ccs geothermal energy nuclear waste storage and mining in all these settings stress and pore pressure changes induced by engineering activity can generate or reactivate faults and fractures p and s waves are emitted by such microseismic events which can be recorded on downhole geophones the presence of aligned fracture sets generates seismic anisotropy which can be identified by measuring the splitting of the s waves emitted by microseismic events the raypaths of the s waves will have an arbitrary orientation controlled by the event and geophone locations meaning that the anisotropy system may only be partly illuminated by the available arrivals therefore to reliably interpret such splitting measurements it is necessary to construct models that compare splitting observations with modelled values allowing the best fitting rock physics parameters to be determined commonly splitting measurements are inverted for one fracture set and rock fabrics with a vertical axis of symmetry in this paper we address the challenge of identifying multiple aligned fracture sets using splitting measured on microseismic events we analyse data from the weyburn ccs eor reservoir which is known to have multiple fracture sets and from a hydraulic fracture stimulation where it is believed that only one set is present we make splitting measurements on microseismic data recorded on downhole geophone arrays our inversion technique successfully discriminates between the single and multiple fracture cases and in all cases accurately identifies the strikes of fracture sets previously imaged using independent methods borehole image logs core samples microseismic event locations we also generate a synthetic example to highlight the pitfalls that can be encountered if it is assumed that only one fracture set is present when splitting data are interpreted when in fact more than one fracture set is contributing to the anisotropy

plate wide deformation before the sumatra andaman earthquake rock is weak to shear stress and the energy released by the 26th december 2004 m 9 2 sumatra andaman earthquake the largest earthquake for four decades must have accumulated over enormous volumes of crust and mantle certainly plate wide possibly world wide here we report evidence for plate wide stress accumulation changes in seismic shear wave splitting monitor stress induced changes in the geometry of the microcrack distributions in almost all rocks in the earth s crust such changes observed in iceland show stress accumulation beginning at least four years before the sumatra andaman earthquake these changes were recognised as monitoring stress accumulation before an impending large earthquake and 10 stress forecasts were emailed to iceland meteorological office for some 29 months forecasting an impending large earthquake the remarkable sensitivity of critical systems of microcrack geometry to miniscule changes of stress had not been recognised at that time and the stress accumulation was expected to lead to a m 7 earthquake somewhere in iceland only now is it recognised that the changes in shear wave splitting were monitoring stress accumulation which would eventually lead to the sumatra andaman earthquake at a distance of some 10 500 km on the opposite side of the eurasian plate this extreme sensitivity confirms the critical nature of fluid saturated stress aligned microcracks in the earth s crust

developing stress monitoring sites using cross hole seismology to stress forecast the times and magnitudes of future earthquakes abstract a new understanding of rockmass deformation suggests that changing stress in the crust modifies the geometry of the distributions of fluid saturated grain boundary cracks and pores pervading almost all rocks in the crust these stress aligned micro cracks cause the widely observed splitting of seismic shear waves which are sensitive to the details of the microcrack geometry this means that analysing shear wave splitting along appropriate ray paths above small earthquakes can monitor the build up of stress before large earthquakes this allowed the time and magnitude of an m 5 earthquake in iceland to be successfully stress forecast such forecasting using small earthquakes as the source of shear waves is possible only on those rare occasions when the comparatively severe restrictions on the geometry of source receiver and large earthquake allow shear wave splitting to be analysed along appropriate ray paths this paper suggests that in the absence of such pronounced local seismicity and optimum recording geometry the time and magnitude of future earthquakes can be estimated by analysing shear wave splitting in controlled source cross well seismology between three boreholes we suggest that stress forecasting at such stress monitoring sites smss is the best deterministic option for reliable forewarning of large earthquakes near any earthquake vulnerable township or vibration sensitive installation a preliminary stress monitoring site is currently being set up in northern iceland

vp vs ratio and shear wave splitting in the nankai trough seismogenic zone insights into effective stress pore pressure and sediment consolidation to estimate variation of stress state and sediment consolidation in the nankai plate subduction zone off southwest japan we measured the p wave to s wave velocity ratio vp vs and s wave splitting along the seismic line extending from the trench to the seismogenic zone for this purpose we used active source seismic data recorded by multicomponent ocean bottom seismometers obs because it is difficult to identify the ps converted reflection waveforms for each of the geological boundaries in this deep offshore region we focused on the more easily identified pps refracted waveforms that register the conversion of the up going p waves to s waves at the igneous crust surface we estimated the average vp vs ratio within the sedimentary section by using the time lag between the p refracted waves and pps converted waves this vp vs ratio changes abruptly at the trough axis i e the deformation front of the accretionary prism arguably because of compaction associated with the accretion process we observ

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

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

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

changes in shear wave splitting before the 2010 eyjafjallajokull eruption in iceland summary we use shear wave splitting sws above microearthquakes to monitor stress variations before the 2010 march and april flank and summit eruptions of eyjafjallaj okull volcano in iceland sws time delays before eyjafjallaj okull show characteristic variations similar to those seen before earthquakes the time delays display a nearly linear increase before the eruption an abrupt change of slope and a rapid nearly linear decrease until the flank eruption begins similar variations before earthquakes are interpreted as stress accumulation increases and stress relaxation decreases as microcracks coalesce onto the eventual fault plane the changes in sws before eyjafjallaj okull are interpreted as a similar stress accumulation increase as magma penetrates the crust and stress relaxation decrease as microcracks coalesce onto the magma conduit prior to magma release we suggest that the remarkable similarity between stress changes before eruption and earthquake is strong evidence for the new geophysics of a critically microcracked crust