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

heat flow strong near fault seismic waves and near fault tectonics on the central san andreas fault the main san andreas fault strikes subparallel to compressional folds and thrust faults its fault normal traction is on average a factor of 1 2 thr 1 thr2 thr where thr is the coefficient of friction for thrust faults times the effective lithostatic pressure a useful upper limit for thr of 0 6 where is 3 12 is obtained from the lack of heat flow anomalies by considering off fault convergence at a rate of 1 mm yr for 10 km across strike if the fault normal traction is in fact this high the well known heat flow constraint of average stresses of 10 20 mpa during strike slip on the main fault becomes more severe only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault the spatial dimension of the high stress rupture tip zone is 10 m for 3 12 and for comparison 100 m for 1 high dynamic stresses during shaking occur within these distances of the fault plane in terms of scalars fine scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress plate scale slip causes stresses to build up near geometrical irregularities of the fault plane strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities there also is a mild tendency for near fault material to extrude upward slip on minor thrust faults causes the normal traction on the main fault to be spatially variable

shallow creep along the 1999 izmit earthquake rupture turkey from gps and high temporal resolution interferometric synthetic aperture radar data 2011 2017 characterizing the spatiotemporal evolution of creep is essential to constrain fault slip budget and understand creep mechanism studies based on interferometric synthetic aperture radar and global positioning system gps satellite observations until 2012 have shown that the central segment of the 17 august 1999 mw 7 4 izmit earthquake on the north anatolian fault began slipping aseismically following the event in the present study we combine new interferometric synthetic aperture radar time series based on terrasar x and sentinel 1a b radar images acquired over the period 2011 2017 with near field gps measurement campaigns performed every 6 months from 2014 to 2016 the mean velocity fields reveal that creep on the central segment of the 1999 izmit fault rupture continues to decay more than 19 years after the earthquake in overall agreement with models of postseismic afterslip decaying logarithmically with time for a long period of time along the fault section that experienced supershear velocity rupture during the izmit earthquake creep continues with a rate up to 8 mm year a significant transient accelerating creep is detected in december 2016 on the sentinel 1 time series near the maximum creep rate location associated with a total surface slip of 10 mm released in 1 month only additional analyses of the vertical velocity fields show a persistent subsidence on the hanging wall block of the golcuk normal fault that also ruptured during the izmit earthquake our results demonstrate that afterslip processes along the north anatolian fault east southeast of istanbul are more complex than previously proposed as they vary spatiotemporally along the fault

constraints on the stress state of the san andreas fault with analysis based on core and cuttings from san andreas fault observatory at depth safod drilling phases 1 and 2 stress state constraints on the saf

crystallographic controls on the frictional behavior of dry and water saturated sheet structure minerals 1 we compare the frictional strengths of 17 sheet structure mineral powders measured under dry and water saturated conditions to identify the factors that cause many of them to be relatively weak the dry coefficient of friction ranges upward from 0 2 for graphite leveling off at 0 8 for margarite clintonite gibbsite kaolinite and lizardite the values of dry correlate directly with calculated 001 interlayer bond strengths of the minerals this correlation occurs because shear becomes localized along boundary and riedel shears and the platy minerals in them rotate into alignment with the shear planes for those gouges with dry 0 8 shear occurs by breaking the interlayer bonds to form new cleavage surfaces where dry 0 8 consistent with byerlee s law the interlayer bonds are sufficiently strong that other frictional processes dominate the transition in dry friction mechanisms corresponds to calculated surface energies of 2 3 j m2 adding water causes to decrease for every mineral tested except graphite if the minerals are separated into groups with similar crystal structures wet increases with increasing interlayer bond strength within each group this relationship also holds for the swelling clay montmorillonite whose water saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure water in the saturated gouges forms thin structured films between the plate surfaces the polar water molecules are bonded to the plate surfaces in proportion to the mineral s surface energy and wet reflects the stresses required to shear through the water films

the role of metasomatic alteration on frictional properties of subduction thrusts an example from a serpentinite body in the franciscan complex california abstract fluid rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone in order to understand the effect of ultramafic components on the degree of interplate coupling we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic melange of the franciscan complex central california field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists respectively forming a ductile shear zone simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses n eff of 60 180 mpa and temperatures t of 20 400 c using a hydrothermal ring shear machine the serpentinite is frictionally strong steady state friction coefficient ss 0 5 0 6 and exhibits both velocity weakening and velocity strengthening behavior the ss of the argillite and tremolite schist increases up to 0 7 and 0 6 respectively with increasing n eff and t while a b transitions from positive to negative with increasing t at a given experimental condition and a b values are lower and higher for the tremolite schist than for the argillite in contrast the talc schist exhibits very low friction ss 0 1 0 2 and velocity strengthening behavior at all experimental conditions tested our results demonstrate that si metasomatism along the argillite serpentinite contact results in reduced friction and stabilized slip forming frictionally weak velocity strengthening regions in the megathrust zone

thirty five year creep rates for the creeping segment of the san andreas fault and the effects of the 2004 parkfield earthquake constraints from alignment arrays continuous global positioning system and creepmeters we present results from differential global positioning system gps surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the san andreas fault surveys of four alignment arrays from the central creeping segment yield 33 to 36 year average minimum slip rates of 21 26 mm yr these rates are consistent with previous alignment array surveys spanning a 10 year period and with rates determined by creepmeters indicating approximate steady state creep along the central creeping segment for at least 35 years motion between continuous gps sites that span the central creeping segment is 28 2 0 5 mm yr for two sites that are 1 km apart and 33 6 1 mm yr for two sites that are 70 km apart slip rates therefore increase with distance from the creeping segment of the san andreas fault all rates reported here are significantly slower than the 39 2 mm yr rate predicted for motion between the sierra nevada great valley block and the pacific plate repeat surveys of three alignment arrays following the 2004 parkfield earthquake demonstrate that its coseismic and short term postseismic offsets decrease rapidly with distance from the epicenter from 150 mm to 15 mm to 5 mm at respective distances of 9 36 and 54 km to the northwest continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed

mineralogical characterization of protolith and fault rocks from the safod main hole 1 washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the san andreas fault observatory at depth safod both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology p t conditions deformation mechanisms and fluid composition and abundance regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults in addition both smectites occurring as mixed layer phases and serpentine minerals are found in association with active strands of the san andreas fault that were intersected during drilling although their rheological influence is not yet fully known faults containing these mineralogical phases are prime candidates for continuous coring during phase 3 of safod drilling in the summer of 2007

earthquake recurrence and magnitude and seismic deformation of the northwestern okhotsk plate northeast russia 1 recorded seismicity from the northwestern okhotsk plate northeast asia is currently insufficient to account for the predicted slip rates along its boundaries due to plate tectonics however the magnitude frequency relationship for earthquakes from the region suggests that larger earthquakes are possible in the future and that events of mw 7 5 which should occur every 100 350 years would account for almost all the slip of the plate along its boundaries due to eurasia north america convergence we use models for seismic slip distribution along the bounding faults of okhotsk to conclude that relatively little aseismic strain release is occurring and that larger future earthquakes are likely in the region our models broadly support the idea of a single okhotsk plate with the large majority of tectonic strain released along its boundaries

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

comparative mineral chemistry and textures of safod fault gouge and damage zone rocks abstract creep in the san andreas fault observatory at depth safod drillhole is localized to two foliated gouges the central deforming zone cdz and southwest deforming zone sdz the gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of mg smectite clays that formed as a result of shearing enhanced reactions between the serpentinite and quartzofeldspathic rocks the cdz takes up most of the creep and exhibits differences in mineralogy and texture from the sdz that are attributable to its higher shearing rate in addition a 0 2 m wide sector of the cdz at its northeastern margin ne cdz is identical to the sdz and may represent a gradient in creep rate across the cdz the sdz and ne cdz have lower clay contents and larger porphyroclasts than most of the cdz and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the cdz matrix clays in the sdz and ne cdz are saponite and corrensite whereas the rest of the cdz lacks corrensite saponite is younger than corrensite reflecting clay crystallization under declining temperatures and clays in the more actively deforming portions of the cdz have better equilibrated to the lower temperature conditions

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

paleomagnetism and u pb geochronology of the 2 17 ga biscotasing dyke swarm ontario canada evidence for vertical axis crustal rotation across the kapuskasing zone u pb dates on baddeleyite yield ages of 2167 8 2 2 and 2171 6 1 2 ma on two northeast trending dykes west of the kapuskasing zone in ontario canada these ages identify the dykes as belonging to the biscotasing dyke swarm east of the kapuskasing zone which was previously dated at 2166 7 1 4 ma by u pb on baddeleyite and zircon the new dates show that the biscotasing swarm was emplaced over an area of at least 300 000 km2 much larger than hitherto suspected and in a geologically short period of time of about 5 million years a comparison of paleomagnetic data from biscotasing and 2 45 ga matachewan dykes on either side of the kapuskasing zone suggests that the western half of the superior province has rotated about 10 20 counterclockwise relative to the eastern half across the kapuskasing zone this movement may have been accompanied by rifting farther north which ultimately led to the paleoproterozoic embayment underlying hudson bay that gives the superior province its characteristic butter

the 2015 mw 7 1 earthquake on the charlie gibbs transform fault repeating earthquakes and multimodal slip on a slow oceanic transform the 2015 mw 7 1 earthquake on the charlie gibbs transform fault along the mid atlantic ridge is the latest in a series of seven large earthquakes since 1923 we propose that these earthquakes form a pair of quasi repeating sequences with the largest magnitudes and longest repeat times for such sequences observed to date we model teleseismic body waves and find that the 2015 earthquake ruptured a distinct segment of the transform from the previous 1998 earthquake the two events display similarities to earthquakes in 1974 and 1967 respectively we observe large oceanic transform earthquakes to exhibit characteristic slip behavior initiating with small slip near the ridge and propagating unilaterally to significant slip asperities nearer the center of the transform these slip distributions combined with apparent segmentation support multimode slip behavior with fault slip accommodated both seismically during large earthquakes and aseismically in between

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

strengths of serpentinite gouges at elevated temperatures serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones to test these hypotheses we have measured the strength of chrysotile lizardite and antigorite rich serpentinite gouges under hydrothermal conditions with emphasis on chrysotile which has thus far received little attention at 25 c the coefficient of friction of chrysotile gouge is roughly 0 2 whereas the lizardite and antigorite rich gouges are at least twice as strong the very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content when the adsorbed water is removed chrysotile is as strong as pure antigorite gouge at room temperature heating to 200 c causes the frictional strengths of all three gouges to increase limited data suggest that different polytypes of a given serpentine mineral have similar strengths thus deformation induced changes in polytype should not affect fault strength at 25 c the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities consistent with previous studies at temperatures up to 200 c however chrysotile strength is essentially independent of velocity at low velocities overall chrysotile has a restricted range of velocity strengthening behavior that migrates to higher velocities with increasing temperature less information on velocity dependence is available for the lizardite and antigorite gouges but their behavior is consistent with that outlined for chrysotile the marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth the velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip but in the presence of a hydrostatic fluid pressure gradient all varieties of serpentine are too strong to explain the apparent weakness of faults such as the san andreas

inferences drawn from two decades of alinement array measurements of creep on faults in the san francisco bay region we summarize over 20 years of monitoring surface creep on faults of the san andreas system in the san francisco bay region using alinement arrays the san andreas fault is fully locked at five sites northwest from san juan bautista the southern end of the 1906 earthquake rupture that is no creep 1 mm yr is observed likewise the san gregorio rodgers creek and west napa faults show no creep the measured creep rate on the calaveras paicines fault from hollister southward is either 6 or 10 mm yr depending on whether the arrays cross all of the creeping traces northward of hollister the central calaveras creep rate reaches 14 2 mm yr but drops to 2 mm yr near calaveras reservoir where slip transfers to the southern hayward fault at a maximum creep rate of 9 mm yr at its south end however the hayward fault averages only 4 6 mm yr over most of its length the northern calaveras fault now creeping at 3 4 mm yr steps right to the concord fault which has a similar rate 2 5 3 5 mm yr which is slightly slower than the 4 4 mm yr rate on its northward continuation the green valley fault the maacama fault creeps at 4 4 mm yr near ukiah and 6 5 mm yr in willits the central and southern segments of the calaveras fault are predominantly creeping whereas the hayward northern calaveras and maacama faults are partly locked and along with the rodgers creek and san andreas have high potential for major earthquakes manuscript received 6 november 2002

the relationship between seismic and aseismic slip on the philippine fault on leyte island bayesian modeling of fault slip and geothermal subsidence delineating seismic and aseismic slip on faults allows the exploration of the complex relationship between these different modes of slip further quantifying them helps in the assessment of seismogenic potential we present a distributed slip model of the rate of aseismic slip along the leyte island section of the philippine fault and make comparisons with the extent of seismic slip from the latest significant earthquakes in july 2017 we derived both coseismic and aseismic slip distributions from kinematic inversions of synthetic aperture radar interferometric insar observations using a probabilistic bayesian framework velocity maps from stacking and time series analysis of alos interferograms spanning 4 years 2007 2011 show a step change in ground deformation right on the fault inverting for slip at depth reveals along strike variations of aseismic slip rate over 100 km aseismic slip on the surface reaches rates of more than 3 cm yr equivalent to the long term slip rate 3 3 0 2 cm yr over the same period a 20 km segment in tongonan appears to be locked this segment ruptured in mw 6 5 and mw 5 8 earthquakes on 6 and 10 july 2017 respectively as constrained by sentinel 1 and alos 2 insar data seismic slip appears to be restricted within the tongonan segment with up to 152 21 cm of left lateral displacement the slip budget and complementarity between the extents of interseismic and coseismic slip suggest that a seismogenic asperity exists in tongonan the presence of active hydrothermal systems and rate strengthening materials provide physical conditions that can promote aseismic slip

effects of topography on pyroclastic density current runout and formation of coignimbrites laboratory experiments of dilute mixtures of warm talc powder in air simulate dilute pyroclastic density currents pdcs and show the effects of topography on current runout buoyancy reversal and liftoff into buoyant plumes the densimetric and thermal richardson froude stokes and settling numbers for our experiments match those of natural pdcs the laboratory currents are fully turbulent although the experiments have lower reynolds numbers than pdcs in sum our experiments are dynamically similar to natural currents comparisons of currents traversing flat topography or encountering barriers show that runout distance is not significantly reduced for currents that traverse barriers with height

orientation of the stress field surrounding the creeping section of the san andreas fault evidence for a narrow mechanically weak fault zone we determine how the orientations of the principal stresses change with distance from the creeping section of the san andreas fault saf our data are 2392 earthquake focal mechanisms which are grouped according to distance from the saf into 33 bins parallel to fault strike the width of the bins are dictated by the spatial organization and density of the seismicity only events with m 1 5 depth 2 km and well constrained focal mechanisms are used the focal mechanisms are inverted simultaneously for the orientations and relative amplitudes of the principal stresses using two different inversion programs a bootstrap resampling method provides confidence limits on the stress orientations our general result is that the maximum horizontal compression lies mostly at high angles to the saf except in a narrow zone on the fault no wider than 1 3 km in which it appears to lie at a smaller angle 45 from the fault strike in particular the maximum horizontal compression lies at a high angle to the saf trend immediately adjacent to the fault this stress state is most readily interpreted as due to a narrow mechanically weak saf in this region the narrowness of this zone contrasts with the region wider than 20 30 km of rotated stresses found surrounding the saf in southern california hardebeck and hauksson 1999 suggestive of different mechanical behaviors of the saf in these two regions this approach may provide a way to define the width of the mechanically weak part of faults

co seismic and or a seismic microstructures of jfast 343 core samples from the japan trench abstract the microstructures mineralogy and chemistry of four representative samples collected from cores extracted from the japan trench during integrated ocean drilling project expedition 343 the japan trench fast drilling project jfast have been studied using optical microscopy tem sem xrf xrd and microprobe analyses the samples provide a transect from relatively undeformed marine sediments in the hanging wall to the undeformed footwall material crossing the thrust interface between the pacific and north american plate where the fault slipped during the march 2011 tohoku oki earthquake our preliminary results suggest that the low strength of jfast fault gouge material is caused by the high amount of clay minerals 60 smectite 14 illite clay minerals in the decollement gouge sample are partly replaced by newly formed manganese oxide which precipitated from hydrothermal fluids dauphine twins were found in quartz grains of the decollement sample suggesting local high stress possible during seismic loading other microstructures cannot be assigned unambiguously to co seismic or a seismic faulting processes the observed scaly clay fabric is consistent with observations in many other plate boundary fault zones significant grain size reduction was found in the fault decollement zone sample but a change in lithology of the fault material cannot be ruled out microstructures typical for a seismic deformation like dissolution precipitation features e g dissolved grain boundaries mineral alteration occur in all jfast core samples but more frequently in the decollement sample

fabric induced weakness of tectonic faults 1 mature fault zones appear to be weaker than predicted by both theory and experiment one explanation involves the presence of weak minerals such as talc however talc is only a minor constituent of most fault zones and thus the question arises what proportion of a weak mineral is needed to satisfy weak fault models existing studies of fault gouges indicate that 30 of the weak phase is necessary to weaken faults a proportion not supported by observations here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt talc provided the talc forms a critically aligned through going layer observations of foliated fault rocks in mature large offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults

surface fault slip associated with the 2004 parkfield california earthquake surface fracturing occurred along the san andreas fault the subparallel southwest fracture zone and six secondary faults in association with the 28 september 2004 m 6 0 parkfield earthquake fractures formed discontinuous breaks along a 32 km long stretch of the san andreas fault sense of slip was right lateral only locally was there a minor 1 11 mm vertical component of slip right lateral slip in the first few weeks after the event early in its afterslip period ranged from 1 to 44 mm our observations in the weeks following the earthquake indicated that the highest slip values are in the middle mountain area northwest of the mainshock epicenter creepmeter measurements indicate a similar distribution of slip surface slip along the san andreas fault developed soon after the mainshock field checks in the area near parkfield and about 5 km to the southeast indicated that surface slip developed more than 1 hr but generally less than 1 day after the event slip along the southwest fracture zone developed coseismically and extended about 8 km sense of slip was right lateral locally there was a minor to moderate 1 29 mm vertical component of slip right lateral slip ranged from 1 to 41 mm surface slip along secondary faults was right lateral the right lateral component of slip ranged from 3 to 5 mm surface slip in the 1966 and 2004 events occurred along both the san andreas fault and the southwest fracture zone in 1966 the length of ground breakage along the san andreas fault extended 5 km longer than that mapped in 2004 in contrast the length of ground breakage along the southwest fracture zone was the same in both events yet the surface fractures were more continuous in 2004 surface slip on secondary faults in 2004 indicated previously unmapped structural connections between the san andreas fault and the southwest fracture zone further revealing aspects of the structural setting and fault interactions in the parkfield area