correction to strong ferrimagnetic resonance signal and magnetic susceptibility of the nojima pseudotachylyte in japan and their implication for coseismic electromagnetic changes 1 pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement i examine the nojima pseudotachylyte found along the nojima fault which caused the 1995 kobe earthquake m 7 2 in japan using the electron spin resonance technique the nojima pseudotachylyte has a strong ferrimagnetic resonance fmr signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides this fmr signal appears by heating the surrounding fault gouge that is the source material of the nojima pseudotachylyte furthermore the magnetic susceptibilities of the nojima pseudotachylyte are 10 2 10 3 times larger than those of the surrounding fault gouge and are proportional to the fmr signal intensities here i reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the fmr signal the results indicate that the instantaneous magnetization of the nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction

associations between burial diagenesis of smectite chemical remagnetization and magnetite authigenesis in the vocontian trough se france results of a paleomagnetic rock magnetic geochemical and petrographic study on jurassic and cretaceous carbonates in the vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization crm carried by magnetite where smectite has altered to other clay minerals limestones are characterized by a prefolding secondary normal polarity magnetization throughout the basin the magnetization is interpreted to be a crm based on low burial depths which cannot cause thermoviscous resetting where significant smectite is still present the crm is absent weakly developed and where the clays show no evidence for burial alteration the units are characterized by a primary magnetization crm intensity also varies with the amount of smectite and burial isothermal anhysteretic and natural remanent magnetization intensities increase where smectite has altered both stratigraphically and geographically this is interpreted to indicate magnetite authigenesis associated with clay diagenesis superparamagnetic magnetite is more dominant in highly altered units based on the results of low temperature experiments all sections away from the alps have 87sr 86sr values that are similar to coeval seawater and stable isotopes of carbon and oxygen show no sign of alteration orogenic type fluids therefore are not a likely agent of remagnetization near the alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the crm through a reversal a postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic type fluids we conclude that burial diagenesis of smectite is the likely cause for the development of the widespread crm in the vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere

analysis of the internal structure of a carbonate damage zone implications for the mechanisms of fault breccia formation and fluid flow abstract a segment of the salzach ennstal fault zone talhof fault eastern alps shows evidence for joint nucleation by layer parallel shear causing the formation of joint bounded slices oriented at high angles 65 85 with respect to the shear zone boundary szb subsequent slice rotation resulted in joint reactivation as antithetic shears slice kinking and breaking up of the individual slices into smaller fragments the latter process due to the longitudinal constraint of slices with impeded shear zone widening marked the transition to cataclasite formation and fault core evolution during shear localization cataclasites were subsequently cemented and underwent continuous shear deformation by re fracturing cement precipitation from fluids therefore played a fundamental role in the evolution of the fault zone with a cyclic change between an open and a closed permeability system during fault evolution stable isotope compositions 13 c 18 o of fault rock cements indicate a continuous equilibration between protolith derived fragments and cements precipitated from those fluids this points to limited fluid amounts only temporally replenished by meteoric water and a hydraulic gradient that directed fluid flow from the damage zone towards the fault core

modification of streaming potential by precipitation of calcite in a sand water system laboratory measurements in the ph range from 4 to 12 spontaneous potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential which is usually positive in the flow direction i e zeta potential of the rock is negative the water rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals this work addresses the study of calcite precipitation in a sand composed of 98 quartz and 2 calcite using streaming potential measurements the precipitation of calcite as a secondary mineral phase inferred by high calcite saturation indices and by a fall in permeability has a significant effect on the electrokinetic behaviour leading to a significant reduction in the zeta potential in absolute value and even a change in sign the measured decrease in zeta potential from 16 mv to 27 4 mv takes place as the ph rises from 4 to 7 while it remains constant at 25 1 mv as the ph increases from 8 to 10 5 for ph higher than 10 5 calcite precipitates and is expected to coat the quartz surface the measured zeta potential vary from 17 to 8 mv for ph ranging from 10 6 to 11 7 depending on the amount of precipitated calcite indicated by the decrease in permeability the observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of sp anomalies in order to determine fluid circulations even at ph lower than 9 if calcite is widely present as a secondary mineral phase since the electrical surface potential of calcite depends also on co2 partial pressure and ca2 therefore sp anomalies as measured in hydrothermal field without mineralogical analyses of hydrothermal deposits and without geochemical fluid survey should be interpreted with caution

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

mass removal and clay mineral dehydration rehydration in carbonate rich surface exposures of the 2008 wenchuan earthquake fault geochemical evidence and implications for fault zone evolution and coseismic slip 1 we report variations in the mineralogical geochemical and isotopic 13c 18o composition of fault rocks sampled in transects across the zhaojiagou and pingxi exposures of the wenchuan earthquake or longmenshan fault zone where the gouge rich fault core and principal slip surface cuts through carbonate rich strata pervasive fluid infiltration was found to modify the mineralogical and geochemical architecture of the fault zones studied enrichment depletion patterns element partitioning and a very large implied volume loss are quite different from those characterizing faults in granites and clastic sedimentary rocks and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution an increasing enrichment in smectite observed toward the principal slip surface a high abundance of elements such as ba mg and f the deposition of minerals such as barite and fluorapatite as well as the distinct depletion in 13c in vein material consistently suggest reactions involving a hydrothermal fluid originating at depth illitization of black gouges caused by coseismic frictional heating was found to be widespread we propose that coseismic frictional heating along with the action of postseismic hydrothermal fluids controlled the transformation and distribution of smectite and illite within the fault core of the longmenshan fault zone the coseismic dewatering reactions are expected to have been more extensive at depth possibly helping generate excess pore pressure assisting dynamic slip weakening during the wenchuan earthquake

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

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

rock fragmentation and particle size in crushed zones by faulting abstract the size distribution of fragments in the crushed zone adjacent to a fault and that of grains in a zone of very fine grained particles the fault gouge was determined for two different faults in central japan one fault has a wide ca 100 m zone of rock fragmentation between the fault gouge zone and the basement rock formation dimensions of the 50 largest rock fragments were measured in the field at locations within a continuous outcrop of this fault at some places bulk samples were also collected and the particle fragment and grain size distributions were determined in the laboratory the relationship between the logarithmic mean of the sizes of the largest 50 fragments and their distances from the fault suggests that size reduction of fragments is due to bulk crushing away from the fault and to surface grinding near the fault the other fault has no crushed zone of basement rocks but does have a 5 m wide zone of fault gouge ten samples of ca 200 g each and three bulk samples of ca

magnetic properties of single and multi domain magnetite under pressures from 0 to 6 gpa 1 using novel experimental methods we measured the acquisition of isothermal remanent magnetization direct field demagnetization and alternating field demagnetization of multi domain md and single domain sd magnetite under hydrostatic pressures to 6 gpa we find that the saturation remanence of md magnetite increases 2 8 times over initial non compressed values by 6 gpa while its remanent coercivity remains relatively constant for sd magnetite remanent coercivity and saturation remanence vary little from 0 to 1 gpa increase markedly from 1 to 3 gpa then plateau above 3 gpa these new findings suggest that by 3 gpa sd magnetite either undergoes a magnetic phase transition or that it reaches an optimal magnetic state where magnetostriction and or magnetocrystalline energy constants attain some threshold state without reorganization of the pre existing magnetic lattices similar behavior is not observed in md magnetite likely due to domain wall effects

fault gouge evolution in highly overconsolidated claystones processes of fault gouge evolution in hard claystones start with cataclasis that produces a soft clay gouge which fills the space between the initial fragments this re seals the initially highly porous and permeable damage zone without diagenetic processes we studied this process in shales and sandstones deformed in normal faults after uplift to shallow crustal depths in the ardennes and eifel detailed mapping shows four different lithotypes from intact shale in the wall rock broken wall rock fragments with various amounts of clay to a fully reworked gouge these lithotypes reflect different evolutionary stages the similar mineralogy of the different lithotypes suggests that the main process involved is mechanical fragmentation with minor contribution by diagenetic changes or weathering accompanied by large changes in microstructure porosity and transport properties while the wall rock has a very low porosity and permeability initial deformation results in a much higher porosity with large pores the reworked clay gouge also has a high porosity but its pore size corresponds to a lower permeability and higher capillary seal capacity in upper crustal fault zones in strong brittle lithologies this early re sealing in sections consisting of claystone will lead to local barriers to along fault fluid flow

strath terraces of jinshaan canyon yellow river and quaternary tectonic movements of the ordos plateau north china studies of the strath terraces in jinshaan canyon where the yellow river cuts through the ordos plateau in the continental interior of china provide an opportunity to solve some of the critical problems associated with quaternary tectonic movement of the plateau six strath terraces indicate that five episodes of regional epeirogenic uplift affected the ordos plateau during the quaternary the local tectonic deformation is superimposed on the regional epeirogenic uplift expressed either as faulting or as non uniform uplift induced by local arching the results have important implications for understanding the relationship between the regional epeirogenic uplift of the ordos plateau and the uplift of the qingzang plateau time lines of climate change in the regional epeirogenic uplift landscape and the genesis of strath terraces associated with the through flowing plateau drainage in the continental interior

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

fracture zone conditions on a recently active fault insights from mineralogical and geochemical analyses of the hirabayashi nied drill core on the nojima fault southwest japan which ruptured in the 1995 kobe earthquake abstract an 1800 m deep borehole into the nojima fault zone was drilled at nojima hirabayashi japan after the 1995 hyogo ken nanbu kobe earthquake three possible fracture zones were detected at depths of about 1140 1300 and 1800 m to assess these fracture zones in this recently active fault we analyzed the distributions of fault rocks minerals and chemical elements in these zones the central fault plane in the shallowest fracture zone was identified by foliated blue gray gouge at a depth of 1140 m the degree of fracturing was evidently greater in the hanging wall than in the footwall minerals detected in this zone were quartz orthoclase plagioclase and biotite as in the parent rock granodiorite and also kaolinite smectite laumontite stilbite calcite ankerite and siderite which are related to hydrothermal alteration biotite was absent in both the hanging wall and footwall across the central fault plane but it was absent over a greater distance from the central fault plane in the hanging wall than in the footwall major element compositions across this zone suggested that hydrothermal alteration minerals such as kaolinite and smectite occurred across the central fault plane for a greater distance in the hanging wall than in the footwall similarly h2o and co2 had higher concentrations in the hanging wall than in the footwall this asymmetrical distribution pattern is probably due to the greater degree of wall rock fracturing and associated alteration in the hanging wall we attributed the characteristics of this zone to fault activity and fluid rock interactions we analyzed the other fracture zones along this fault in the same way in the fracture zone at about 1300 m depth we detected the same kinds of hydrothermal alteration minerals as in the shallower zone but they were in fewer samples we detected relatively little h2o and co2 and little evidence for movement of the major chemical elements indicating little past fluid rock interaction in the fracture zone at about 1800 m depth h2o and co2 were very enriched throughout the interval as in the fracture zone at about 1140 m depth however smectite was absent and chlorite was present indicating the occurrence of chloritization which requires a temperature of more than 200 c only smectite can form under the present conditions in these fracture zones the chloritization probably occurred in the past when the fracture zone was deeper than it is now these observations suggest that among the three fracture zones that at about 1140 m depth was the most activated at the time of the 1995 hyogo ken nanbu kobe earthquake

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

high magnetic susceptibility produced by thermal decomposition of core samples from the chelungpu fault in taiwan abstract we carried out thermomagnetic susceptibility analyses of fault rocks from core samples from hole b of the taiwan chelungpu fault drilling project tcdp to investigate the cause of high magnetic susceptibilities in the fault core test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 c and by high velocity frictional tests before magnetic analyses thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 c in the heating cycle and showed three step increases at 600 to 550 c and at 300 c during the cooling cycle these behaviors are consistent with the presence of pyrite siderite and chlorite suggesting that tcdp gouge originally included these minerals which contributed to the generation the magnetic susceptibility by thermomechanical reactions the change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core color measurement results indicate that thermal decomposition by frictional heating took place under low oxygen conditions at depth which prevented the minerals from oxidizing to reddish hematite this finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility a kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite our results show that decomposition of pyrite to pyrrhotite siderite and to some extent chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the chelungpu fault zone

spatial variability in microscopic deformation and composition of the punchbowl fault southern california implications for mechanisms fluid rock interaction and fault morphology abstract we examine the distribution and nature of microstructures geochemistry and mineralogy along two traverses across the punchbowl fault southern california to determine the morphology and deformation mechanisms of the fault zone in schistose rocks the punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right slip protolith in the study area consists of the pelona schist which is primarily a quartz albite muscovite actinolite schist with thin to medium banding and rare metabasalts the traverses are 1 3 km apart and were conducted at a site with a single fault strand and a site where both principal strands of the fault are exposed the fault zone thickness is a function of the type of measurement that is used to define it for the single strand site analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40 m thick damaged zone adjacent to the fault core the damaged zone is marked by an increase in veins thin cataclasite bands inter and intragranular fracturing and alteration relative to the country rock brittle grain size reduction occurs in a zone 10 m thick as measured from the fault core which consists of a continuous 10 cm thick very fine grained cataclasite that experienced repeated alteration vein injection and grain size reduction whole rock geochemical analyses of the fault related rocks suggest that the geochemically defined fault zone is less than 10 m thick volume loss at the site with one fault strand appears to have been small the dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz chlorite albite epidote ultracataclasite the composition of the fault core is variable and locally influenced by one of the adjacent protoliths the examination of the two fault strand sites shows that two damaged zone fault core structures are present the region between the two strands experienced a greater degree of deformation than the protolith but the total deformation is much less than immediately adjacent to the fault cores the total thickness of the damaged zone around the two strands is less than 200 m the fault core enveloped by a damaged zone morphology as well as the textures of the fault core rocks are similar to rocks associated with the north branch san gabriel fault which formed in crystalline rocks with a total displacement of 22 km fault thickness is less in the pelona schist than in the crystalline rocks perhaps owing to more efficient strain localization in the schists thus the faults of the san andreas system may be thinner in regions where schists or franciscan rocks are the protolith but the main fault core may be a constant feature of the fault zone transformation induced weakening is less important in this part of the punchbowl fault since the protolith has a large amount of mica the structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures and indicates that some parts of the san andreas fault system may consist of multiple slip surfaces each with a damaged zone that together may create a fault zone 100 m thick and in which slip is localized to zones meters to decimeters thick

rock magnetic properties of fault rocks from the rupture of the 2008 wenchuan earthquake china and their implications preliminary results from the zhaojiagou outcrop beichuan county sichuan abstract rock magnetic measurements were conducted on fault rocks from the zhaojiagou outcrop at leigu town beichuan county sichuan province china located on the yingxiu beichuan fault the main fault ruptured in the 2008 wenchuan ms 8 0 earthquake hysteresis measurements indicate that fault rocks contain very low concentrations of ferrimagnetic minerals and their magnetic behavior is dominated by para diamagnetic components temperature dependent magnetic susceptibility increased significantly above 400 c in the heating run magnetic behavior reveals the presence of siderite lepidocrocite chlorite and smectite as the possible major fe bearing minerals in fault rocks their thermal decomposition to ferrimagnetic minerals e g magnetite and or maghemite could be responsible for the high magnetic susceptibility during the thermal treatment a kinetic model analysis indicates that frictional heating could increase the temperature within the fault zone up to 320 908 c at depth of 590 m and 335 952 c at depth of 618 3 m respectively during the seismic slip of the wenchuan earthquake depending on shear stress such high temperatures are enough to drive thermal decomposition of fe bearing minerals in fault rocks in particularly in fault gouge to form ferrimagnetic magnetite and or maghemite this is the most likely mechanism for the reported high magnetic susceptibility anomalies within the fault zone in the pilot hole of wenchuan earthquake fault scientific drilling project wfsd 1

differences in magnetic properties of fragments and matrix of breccias from the rupture of the 2008 wenchuan earthquake china relationship to faulting abstract rock magnetic and mineralogical analyses were performed on fragments and matrix of six fault breccias from the zhaojiagou outcrop of yingxiu beichuan fault which was the main fault ruptured during the 2008 wenchuan mw 7 9 earthquake at leigu town beichuan county sichuan province china the matrix which is generally enriched in dolomite feldspar and clay minerals but depleted in calcite has much higher low field magnetic susceptibility and magnetization and lower coercivity in comparison with fragments and bulk samples magnetic behavior of the bulk samples and fragments is dominated by dia paramagnetic components in contrast small amounts of partially oxidized magnetite and lepidocrocite are present in the matrix a simple conceptual model associating faulting related effects was proposed to explain these significant differences in magnetic properties of fragments and matrix during coseismic slip intense shear may crush the pre existing magnetic grains in fault rocks into finer ones while stress and frictional heating to a little extent induce magnetic changes and thermochemical alterations of magnetic mineralogy in fault breccias respectively during interseismic periods meteoric fluids would infiltrate and percolate into fault zone and cause dissolution precipitation and recrystallization of fe bearing minerals as the diverse permeability structure and grain size of fragments and matrix these effects would modify their magnetic mineralogy at various levels consequently faulting related effects especially the fluid movements taking place over many previous earthquake cycles would be the most likely reasons for the observed different magnetic properties in fragments and matrix it further proposes that magnetic studies of fault breccias would provide clues to help understand seismic faulting and history of fault activity

structural analysis of the coseismic shear zone of the 2008 mw 7 9 wenchuan earthquake china abstract field investigations reveal that the surface rupture of the 2008 m w 7 9 wenchuan earthquake china occurred along a pre existing shear zone in the longmen shan thrust belt structural analyses of the coseismic fault zone and fault rocks show that i the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of 5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults ii the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense consistent with that indicated by the coseismic surface rupture and iii coseismic slip was largely localized to within a narrow fault gouge zone of

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

statistical discrimination of foreshocks from other earthquake clusters summary when earthquake activity begins it may be a foreshock sequence to a larger earthquake a swarm or a simple main shock aftershock sequence this paper is concerned with the conditional probability that it will be foreshock activity of a later larger earthquake depending on the occurrence pattern of some early events in the sequence the earthquake catalogue of the japan meteorological agency 1926 1993 m 24 is decomposed into a large number of clusters in time and space in order to compare statistical features of foreshocks with those of swarms and aftershocks using such a data set ogata utsu katsura 1995 revealed some discriminating features of foreshocks relative to the other types of clusters for example the events closer proximity in time and space and a tendency towards chronologically increasing magnitudes which encouraged us to construct models which forecast the probability of the earthquakes being foreshocks specifically the probability is a function of the history of magnitude differences spans between origin times and distances between epicentres within a cluster for purposes of illustration the models were fitted to the early part of the data 1926 1975 and the validity of the forecasting procedure was checked on data from the later period 1976 1993 two procedures for evaluating the performance of the probability forecast are suggested furthermore for the case where only a single event is available i e either it is the first event in a cluster or an isolated event we also forecast the probability of the event being a foreshock as a function of its geographic location then the validity of the forecast is demonstrated in a similar manner finally making use of the multi element prediction formula we show that the forecasting performance is enhanced by the joint use of the information in the location of the first event and that in the subsequent interevent history in the cluster

ferrimagnetic resonance signal produced by frictional heating a new indicator of paleoseismicity 1 high speed fault slips during earthquakes may generate sufficient frictional heat to produce fused fault rocks such as pseudotachylyte we have carried out high speed slip tests using natural fault gouge to judge whether or not frictional heating universally occurs during seismic fault slips in our shearing tests natural fault gouge is put between two cylindrical silica glasses and sheared under a fixed axial stress of 0 61 mpa despite such a low stress near the earth s surface a darkened cohesive material resembling pseudotachylyte is made from the fault gouge along the edge of a circular shear plane when shearing at a high speed of 1500 rpm the maximum slip rate reaches 1 96 m s at the edge electron spin resonance measurements reveal that the darkened cohesive material has a strong ferrimagnetic resonance fmr signal which is derived from bulky trivalent iron ions in ferrimagnetic iron oxides fe2o3 the fmr signal is produced by the thermal dehydration of antiferromagnetic iron oxides feooh in the fault gouge this may be applicable to the detection of past heating during seismic fault slip we thus attempt to reconstruct the temperature of frictional heat generated on the nojima fault plane in the 1995 kobe earthquake m 7 3 by inversion using the fmr signal the computer simulation indicates that the frictional heat generated on the nojima fault plane at 390 m depth may have attained 390 c during the 1995 kobe earthquake the temperature in the fault plane may have returned to its initial state after 1 year this result suggests that a heat flow anomaly generated by faulting may be difficult to detect

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records