stability and localization of rapid shear in fluid saturated fault gouge 2 localized zone width and strength evolution field and laboratory observations indicate that at seismic slip rates most shearing is confined to a very narrow zone just a few tens to hundreds of microns wide and sometimes as small as a few microns rice et al 2014 analyzed the stability of uniform shear in a fluid saturated gouge material they considered two distinct mechanisms to limit localization to a finite thickness zone rate strengthening friction and dilatancy in this paper we use numerical simulations to extend beyond the linearized perturbation context in rice et al 2014 and study the behavior after the loss of stability neglecting dilatancy we find that straining localizes to a width that is almost independent of the gouge layer width suggesting that the localized zone width is set by the physical properties of the gouge material choosing parameters thought to be representative of a crustal depth of 7 km this predicts that deformation should be confined to a zone between 4 and 44 m wide next considering dilatancy alone we again find a localized zone thickness that is independent of gouge layer thickness for dilatancy alone we predict localized zone thicknesses between 1 and 2 m wide for a depth of 7 km finally we study the impact of localization on the shear strength and temperature evolution of the gouge material strain rate localization focuses frictional heating into a narrower zone leading to a much faster temperature rise than that predicted when localization is not accounted for since the dynamic weakening mechanism considered here is thermally driven this leads to accelerated dynamic weakening

the proportionality between relative plate velocity and seismicity in subduction zones the magnitude and rate of seismicity differ between subduction zones calculations of background seismicity rates based on a global model of subduction zone seismicity reveal a positive correlation between relative plate velocity and background seismicity yet only the seismically quieter zones seem capable of generating magnitude 9 earthquakes

detailed observations of california foreshock sequences implications for the earthquake initiation process we find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes in this study we examine in detail the evolution of six california foreshock sequences the 1986 mount lewis ml 5 5 the 1986 chalfant ml 6 4 the 1986 stone canyon ml 4 7 the 1990 upland ml 5 2 the 1992 joshua tree mw 6 1 and the 1992 landers mw 7 3 sequence typically uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution however the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations we use these results to identify small scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock in general these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer instead the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process fault zone heterogeneity may also be important in controlling the number of foreshocks i e the stronger the heterogeneity the greater the number of foreshocks the size of the nucleation region as measured by the extent of the foreshock sequence appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase we also find evidence for slip localization as predicted by some models of earthquake nucleation

holocene turbidites record earthquake supercycles at a slow rate plate boundary ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment especially where recurrence times overstep historical records we show that most turbidites emplaced at the africa eurasia plate boundary off west algeria over the past 8 k y correlate across sites fed by independent sedimentary sources requiring a regional trigger correlation with paleoseismic data inland and ground motion predictions support that m 7 earthquakes have triggered the turbidites the bimodal distribution of paleo events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults 13 paleo earthquakes underpin clusters of 3 6 events with recurrence intervals of 300 600 yr separated by periods of quiescence of 1 6 k y without major events on other faults over the study area this implies broad phases of strain loading alternating with phases of strain release our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

interseismic coupling and asperity distribution along the kamchatka subduction zone 1 gps measurements of interseismic horizontal surface velocities reveal the degree of kinematic coupling of the plate boundary thrust along the kamchatka subduction zone from about 51 degrees to 57 degrees n latitude inversions for the distribution of aseismic slip rate along the similar to 15 degrees nw dipping underthrust suggest a nonslipping plate interface in southern kamchatka above similar to 50 km depth along the segment that ruptured in the m w 9 1952 earthquake north of similar to 53 degrees n the subduction interface experiences significant aseismic slip consistent with the lower seismic moment release in m 8 5 earthquakes along this portion of the subduction zone the gps velocities are consistent with a boundary element forward model in which historic earthquake rupture zones are represented as locked asperities surrounded by a zero shear stress subduction interface loaded by plate convergence models in which the complete rupture zones of historic earthquakes are considered locked greatly overpredict the degree of kinematic coupling reducing the area of the locked model asperities to the central 25 area of historic rupture zones fits the data well suggesting that large earthquakes involve small fully locked core asperities surrounded by conditionally stable portions of the plate interface areas of low aseismic slip rate appear to be roughly correlated with areas of low isostatic gravity anomalies over offshore forearc basins while less coupled portions of the kamchatka subduction zone coincide with high gravity anomalies offshore of two peninsulas possibly related to the subduction of the emperor meji seamount chain and the kruzenstern fracture zone

kinetics of pressure solution at halite silica interfaces and intergranular clay films pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during aliagenesis yet the mechanisms and kinetics of this process remain highly controversial to better understand the fundamental factors controwing the rates of pressure solution at the grain to grain scale we conducted experiments in which convex halite lenses were pressed against fiats of fused silica in brine fluid pressures were maintained at 0 1 mpa temperatures and mean contact normal stresses ranged from 8 3 o to 90 2oc and 0 5 to 13 5 mpa respectively the geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another convergence were monitored using reflected light interferometry and transmitted light photomicrography convergence occurred when halite and silica lenses were pressed together in brine halite silica experiments no undercutting was observed and dry control experiments indicated negligible dislocation creep convergence rates in experiments at 50 2oc ranged from 0 01 to 0 05 m d depending on mean normal stress and contact spot radius the data are consistent with intergranular pressure solution ips rate limited by diffusion through an intergranular film with a very high diffusion coefficient 10 5 10 7 cm2 s the data further suggest that the diffusion coefficient and or thickness of this film increases with decreasing normal stresses at least for normal stresses less than about 4 mpa as no island channel boundary structures were observed we propose that this film consists of a continuous layer of strongly adsorbed i e structured water that is maintained between the halite and silica lenses during deformation convergence rates in similar experiments conducted at a constant load of 0 11 n but at 8 3 o 50 2 o and 90 2oc were approximately constant at a given normal stress and contact spot size the cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation an experiment was also conducted in which a halite lens was pressed against a fused silica fiat coated with an 0 8 m thick film of na montmorillonite in brine this clay film produced an approximately fivefold increase in convergence rates over those observed in a halite silica experiment conducted without clay at the same load and temperature the strong sensitivity of ips rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine grained clay rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust

earthquake supercycles inferred from sea level changes recorded in the corals of west sumatra records of relative sea level change extracted from corals of the mentawai islands sumatra imply that this 700 kilometer long section of the sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years the moment magnitude 8 4 earthquake of september 2007 represents the first in a series of large partial failures of the mentawai section that will probably be completed within the next several decades

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

time dependent cracking and brittle creep in crustal rocks a review abstract rock fracture under upper crustal conditions is driven not only by applied stresses but also by time dependent chemically activated subcritical cracking processes these subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales a macroscopic manifestation of time dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses a phenomenon known as brittle creep here we review the available experimental evidence for brittle creep in crustal rocks and the various models developed to explain the observations laboratory experiments have shown that brittle creep occurs in all major rock types and that creep strain rates are extremely sensitive to the environmental conditions differential stress confining pressure temperature and pore fluid composition even small changes in any of these parameters produce order of magnitude changes in creep strain rates and times to failure three main classes of brittle creep model have been proposed to explain these observations phenomenological statistical and micromechanical statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep however no current model can predict quantitatively all of the observed features of brittle creep experimental data are limited by the timescale over which experiments are realistically feasible clearly an extension of the range of available laboratory data to lower strain rates and the development of new modelling approaches are needed to further improve our current understanding of time dependent brittle deformation in rocks

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

fault zone architecture and deformation processes within evaporitic rocks in the upper crust 1 recently in the northern apennines geophysical data have identified the triassic evaporites te anhydrites and dolomites as the source region of the major extensional earthquakes of the area m 6 in order to characterize fault zone architecture and deformation processes within the te we have studied exhumed evaporite bearing normal faults within the upper crust the structure of large displacement 100 m normal faults is given by 1 a zoned fault core with a wider portion of fault parallel foliated ca sulphates ductile deformation overprinted by an inner fault core ifc of localized brittle deformation and 2 wide dolostones to absent ca sulphates damage zones of fault fracture patterns fault rock assemblage within the ifc is characterized by fault breccia gouge and cataclasites of different grain size most of the deformation within the ifc is localized along thin and fault parallel principal slip surfaces pss made of dolomite rich fine grained cataclasite sem analyses show an evolution from ca to st to gypsum rich mineralization due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation the development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones progressive deformation within the fault core leads to the development of fault parallel dolomite rich cataclastic layers their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the ifc

small scale faulting in the upper cretaceous of the groningen block the netherlands 3d seismic interpretation fault plane analysis and regional paleostress abstract over the last years field based studies have shown that fault surfaces can exhibit a considerable self affine topography it is reasonable to assume that similar undulations are also present in fault interpretations from 3d reflection seismic data however both the interpretation uncertainty and geophysical resolution limits hinder their analysis this study analyses a set of small scale non reactivated faults in the upper cretaceous chalk group upper ommelanden formation of the nw part of the groningen block the netherlands in a high quality pre stack depth migrated 3d seismic data set the studied faults are fully contained inside the chalk group in an area located between the major tectonic bounding faults of the nw groningen block over 200 faults with offsets in the order of 30 50 m were interpreted across an area of ca 150 km 2 showing a clear preferential orientation for strike dip and dip direction detailed interpretations and 3d fault plane analyses show undulations on the fault plane we show that these undulations are not an interpretation or gridding artefact and interpret these to indicate direction of fault slip these results were used to calculate a paleostress tensor using all faults to calculate a single stress tensor for the entire study area by numerical dynamic analysis based on the orientation position and a thickness analysis it is interpreted that these faults formed due to the tectonic reactivation of salt structures in the latest cretaceous the calculated paleostress state shows a general nw se extension with a vertical maximum principle stress and a stress ratio of about 0 3 indicating that the studied faults are not the result of dewatering this interpretation agrees both with a nearby salt tectonic reconstruction as well as field based paleostress results from the uk belgium and france a first look at other surveys from the dutch sector indicates that similar faults are present in other areas with different orientations we propose that a dedicated analysis of these faults across on and offshore europe would allow extending the stress map of the late cretaceous into areas where the chalk is not outcropping

low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress at subduction zones deep parts of the fault can slip slowly generating tremor analysis of tremor in cascadia reveals increasing sensitivity of slip to tidal stresses over several days implying that the fault is weak and weakens as it slips

earthquake synchrony and clustering on fucino faults central italy as revealed from in situ cl 36 exposure dating we recover the holocene earthquake history of seven seismogenic normal faults in the fucino system central italy we collected 800 samples from the well preserved limestone scarps of the faults and modeled their 36cl concentrations to derive their seismic exhumation history we found that 30 large earthquakes broke the faults in synchrony over the last 12 ka the seven faults released strain at the same periods of time 12 9 ka 5 3 ka and 1 5 1 ka on all faults the strain accumulation and release occurred in 3 6 ka supercycles each included a 3 5 ka phase of slow 0 5 2 mm yr strain accumulation in relative quiescence followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in 1 2 ka the large earthquakes repeated every 0 5 0 3 ka during the paroxysmal phases and every 4 3 0 9 ka between those phases earthquakes on the northern faults produced twice larger surface slips 2 m and had larger magnitudes mw 6 2 6 7 than those on the southern faults on most faults the relative strain level was found to control the amount of slip and the time of occurrence of the next large earthquake faults entered a phase of clustered activity once they had reached a specific relative strain threshold the tre monti fault is identified as the most prone to break over the next century our data document earthquake synchrony and clustering at a broader space and time scale than has been reported to date

towards inferring earthquake patterns from geodetic observations of interseismic coupling ultimately seismotectonic studies seek to provide ways of assessing the timing magnitude and spatial extent of future earthquakes ample observations document the spatial variability in interseismic coupling defined as a degree of locking of a fault during the period of stress build up between seismic events fully or nearly locked fault patches are often surrounded by aseismically creeping areas however it is unclear how these observations could help assess future earthquakes here we simulate spontaneous seismic and aseismic fault slip with a fully dynamic numerical model our simulations establish the dependence of earthquake rupture patterns and interseismic coupling on spatial variations of fault friction we consider the long term evolution of slip on a model fault where two seismogenic locked segments are separated by an aseismically slipping patch where rupture is impeded we find that the probability for a large earthquake to break through the rupture impeding patch is correlated with the interseismic coupling averaged over this patch in addition the probability that an earthquake breaks through the rupture impeding patch and interseismic coupling are both related to fault friction properties through a single non dimensional parameter our study opens the possibility of predicting seismic rupture patterns that a fault system can produce on the basis of observations of its interseismic coupling and suggests that regions of low interseismic coupling may reveal permanent barriers to large earthquakes

the 1964 prince william sound earthquake joint inversion of tsunami and geodetic data the 1964 prince william sound alaska earthquake mw 9 2 ruptured a large area beneath the continental margin of alaska from prince william sound to kodiak island a joint inversion of tsunami waveforms and geodetic data consisting of vertical displacements and horizontal vectors gives a detailed slip distribution two areas of high slip correspond to seismologically determined areas of high moment release the prince william sound asperity with average slip of 18 m and the kodiak asperity with average slip of 10 m the average slip on the fault is 8 6 m and the seismic moment is estimated as 6 3 1022 n m or over 75 of the seismic moment determined from long period surface waves

global frequency of magnitude 9 earthquakes for decades seismologists have sought causal relationships between maximum earthquake sizes and other properties of subduction zones with the underlying notion that some subduction zones may never produce a magnitude 9 or larger event the 2004 andaman m w 9 2 earthquake called into question such ideas given multicentury return times of the greatest earthquakes ignorance of those return times and our very limited observation span i suggest that we cannot yet make such determinations present evidence cannot rule out that any subduction zone may produce a magnitude 9 or larger earthquake based on theoretical recurrence times i estimate that one to three m9 earthquakes should occur globally per century and the past half century with five m9 events reflects temporal clustering and not the long term average

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

preslip and cascade processes initiating laboratory stick slip recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a cascade up process to explore such a case in the laboratory we study the initiation of dynamic rupture stick slip of a smooth saw cut fault in a 76 mm diameter cylindrical granite laboratory sample at 40 120 mpa confining pressure we use a high dynamic range recording system to directly compare the seismic waves radiated during the stick slip event to those radiated from tiny m 6 discrete seismic events commonly known as acoustic emissions aes that occur in the seconds prior to each large stick slip the seismic moments focal mechanisms locations and timing of the aes all contribute to our understanding of their mechanics and provide us with information about the stick slip nucleation process in a sequence of 10 stick slips the first few microseconds of the signals recorded from stick slip instabilities are nearly indistinguishable from those of premonitory aes in this sense it appears that each stick slip begins as an ae event that rapidly 20 s grows about 2 orders of magnitude in linear dimension and ruptures the entire 150 mm length of the simulated fault we also measure accelerating fault slip in the final seconds before stick slip we estimate that this slip is at least 98 aseismic and that it both weakens the fault and produces aes that will eventually cascade up to initiate the larger dynamic rupture

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

the nature and importance of phyllonite development in crustal scale fault cores an example from the median tectonic line japan abstract like many large crustal scale faults the median tectonic line mtl in sw japan has a long history of movement having been active predominantly as a strike slip fault since the mid cretaceous fault rock exposures in the core of the mtl preserve a history of deformation at a range of mid to shallow crustal depths ryoke mylonites 1 4 km north of the main contact record deeper level cretaceous top to the south sinistral movements the remainder of the fault zone core is surprisingly narrow exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn tectonic fluid influx over a range of deformation conditions exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite cataclasite foliated cataclasite phyllonite breccia gouge this sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone this leads to the replacement of load bearing phases such as feldspar by fine grained foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite the grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid assisted stress induced diffusive mass transfer mechanisms comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the mtl will lead to long term weakening provided the network of phyllosilicate rich fault rocks are able to form an interconnected thin layer of weak material on kilometre to tens of kilometre length scales

slow slip phenomena in cascadia from 2007 and beyond a review recent technological advances combined with more detailed analyses of seismologic and geodetic observations have fundamentally changed our understanding of the ways in which tectonic stresses arising from plate motions are accommodated by slip on faults the traditional view that relative plate motions are accommodated by a simple cycle of stress accumulation and release on locked plate boundary faults has been revolutionized by the serendipitous discovery and recognition of the significance of slow slip phenomena mostly in the deeper reaches of subduction zones the cascadia subduction zone located in the pacific northwest of the conterminous united states and adjacent canada is an archetype of exploration and learning about slow slip phenomena these phenomena are manifest as geodetically observed aseismic transient deformations accompanied by a previously unrecognized class of seismic signals although secondary failure processes may be involved in generating the seismic signals the primary origins of both aseismic and seismic phenomena appear to be episodic fault slip probably facilitated by fluids on a plate interface that is critically stressed or weakened in cascadia this transient slip evolves more slowly and over more prolonged durations relative to the slip in earthquakes and it occurs between the 30 and 40 km depth contours of the plate interface where information was previously elusive although there is some underlying organization that relaxes nearly all the accrued plate motion stresses along the entirety of cascadia we now infer that slow slip evolves in complex patterns indicative of propagating stress fronts our new understanding provides key constraints not only on the region where the slow slip originates but also on the probable characteristics of future megathrust earthquakes in cascadia herein we review the most significant scientific issues and progress related to understanding slow slip phenomena in cascadia and highlight some of their societal implications we provide a comprehensive review from the big picture as inferred from studies of regional scale monitoring data to the details revealed by innovative focused experiments and new instrumentation we focus on what has been learned largely since 2007 when several major investments in monitoring and temporary deployments dramatically increased the quality and quantity of available data

40ar 39ar ages and palaeomagnetism of transitionally magnetized volcanic rocks in the society islands french polynesia raiatea excursion in the upper gauss chron summary volcanic rock samples from several sites in the society islands french polynesia were found to be transitionally magnetized to better characterize these geomagnetic events we performed laser heating 40ar 39ar dating and additional palaeomagnetic measurements on these samples the measurements give virtual geomagnetic pole vgp latitudes of 0 7 52 4 n and virtual dipole moment vdm values of 0 433 0 682 1022 a m2 which are typical for the transitional geomagnetic field these transitional events occurred at 2 77 0 02 ma raiatea 3 20 0 03 ma tahaa and 4 40 0 01 ma maupiti considering the reported radioisotopic ages for known geomagnetic events it is suggested the first event is a previously unknown geomagnetic excursion raiatea excursion while the latter two probably correspond to known polarity transitions upper boundary ages of the mammoth reversed subchron and the nunivak normal subchron the existence of the proposed raiatea excursion is supported by the contemporaneous palaeointensity low recognized in the relative palaeointensity stack from the equatorial pacific

diagenetic compaction experiments on simulated anhydrite fault gouge under static conditions faults that crosscut subsurface co2 storage systems offer potential leakage pathways especially if fault reactivation and dilation occur after reactivation however newly formed fault gouge is expected to gradually compact and seal as a function of time to estimate the time scale on which this occurs the processes that control compaction must be understood we performed uniaxial compaction experiments on simulated anhydrite fault gouge to investigate the deformation mechanisms that operate under postslip conditions in faulted anhydrite caprocks this involved constant stress 5 12 mpa and stress stepping experiments 5 7 5 10 mpa performed at 80 c under dry and wet conditions on fault gouge samples prepared from crushed natural anhydrite sieved into different grain size fractions in the range 20 500 m dry samples showed little to no compaction creep whereas wet samples i e flooded with presaturated caso4 solution showed rapid compaction our mechanical data and microstructural observations on wet samples suggest that for fine grain sizes 50 m and low stresses gouge compaction is controlled by diffusion controlled pressure solution with increasing grain size and stress fluid assisted subcritical microcracking becomes dominant pressurizing solution flooded samples with co2 15 mpa led to no significant effect on compaction rates in fine grained material but it decreased compaction rates in coarse samples since fine grain sizes are expected in reactivated faults we infer that pressure solution will dominate in anhydrite cap rocks with extrapolation of our lab data to reservoir conditions suggesting sealing time scales of a few decades

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

a friction to flow constitutive law and its application to a 2 d modeling of earthquakes establishment of a constitutive law from friction to high temperature plastic flow has long been a challenging task for solving problems such as modeling earthquakes and plate interactions here we propose an empirical constitutive law that describes this transitional behavior using only friction and flow parameters with good agreements with experimental data on halite shear zones the law predicts steady state and transient behaviors including the dependence of the shear resistance of fault on slip rate effective normal stress and temperature it also predicts a change in velocity weakening to velocity strengthening with increasing temperature similar to the changes recognized for quartz and granite gouge under hydrothermal conditions a slight deviation from the steady state friction law due to the involvement of plastic deformation can cause a large change in the velocity dependence we solved seismic cycles of a fault across the lithosphere with the law using a 2 d spectral boundary integral equation method revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep including slow slip prior to earthquakes seismic slip followed by creep is consistent with natural pseudotachylytes overprinted with mylonitic deformation overall fault behaviors during earthquake cycles are insensitive to transient flow parameters the friction to flow law merges christmas tree strength profiles of the lithosphere and rate dependency fault models used for earthquake modeling on a unified basis strength profiles were drawn assuming a strain rate for the flow regime but we emphasize that stress distribution evolves reflecting the fault behavior a fault zone model was updated based on the earthquake modeling

frequency size distribution of competent lenses in a block in matrix melange imposed length scales of brittle deformation 1 this study explores the frequency size distribution of competent lenses phacoids in the chrystalls beach complex a melange shear zone within the otago schist accretion collision assemblage new zealand the distribution of phacoids within a cleaved pelitic matrix follows a power law distribution with exponent 1 18 d 2 37 2 d at outcrop scale whereas distributions at thin section scale have higher d than the outcrop they were sampled from d relates to volume fraction and distribution of competent material where high d correlates with small aspect ratio phacoids and high matrix volume fraction in outcrops which experienced mixed continuous discontinuous deformation d 1 4 0 2 it is suggested that d 1 4 indicates dominant continuous deformation whereas d 1 4 is indicative of localized deformation on shear discontinuities in this melange d therefore appears a function of dominant deformation style actively deforming melange shear zones likely exhibit a range of seismic styles and earthquake magnitudes at length scales limited by lithological layering and shear zone thickness a hypothesis is suggested where strongly coupled shear zone segments may be distinguished by a high volume fraction of competent material low d while aseismically creeping regions are characterized by a high proportion of incompetent rock high d as the b value in the gutenberg richter frequency magnitude relationship also tends to be high in coupled and low in creeping fault segments it may be that d is analogous to b and the b value is also related to the ratio of incompetent competent material and dominant fault zone deformation style

strain localization driven by thermal decomposition during seismic shear field and laboratory observations show that shear deformation is often extremely localized at seismic slip rates with a typical deforming zone width on the order of a few tens of microns this extreme localization can be understood in terms of thermally driven weakening mechanisms a zone of initially high strain rate will experience more shear heating and thus weaken faster making it more likely to accommodate subsequent deformation fault zones often contain thermally unstable minerals such as clays or carbonates which devolatilize at the high temperatures attained during seismic slip in this paper we investigate how these thermal decomposition reactions drive strain localization when coupled to a model for thermal pressurization of in situ groundwater building on rice et al 2014 we use a linear stability analysis to predict a localized zone thickness that depends on a combination of hydraulic frictional and thermochemical properties of the deforming fault rock numerical simulations show that the onset of thermal decomposition drives additional strain localization when compared with thermal pressurization alone and predict localized zone thicknesses of 7 and 13 m for lizardite and calcite respectively finally we show how thermal diffusion and the endothermic reaction combine to limit the peak temperature of the fault and that the pore fluid released by the reaction provides additional weakening of 20 40 of the initial strength

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

a large slow slip event and the depth of the seismogenic zone in the south central alaska subduction zone abstract we modeled crustal deformation before during and after a slow slip event sse on the plate interface in southern alaska we divided the gps time series into three time periods before 1998 1998 to 2001 and 2001 2002 the first and last periods represent the steady deformation while the middle period represents the sse we found a stationary zone of 100 slip deficit in the shallow part of the seismogenic zone to depths of 25 30 km which we identify with the main asperity that ruptured in 1964 earthquake the sse occurs downdip of the asperity on a section that accumulated some slip deficit during the steady time periods during the sse 20 000 sq km of the plate interface slipped 10 cm for a cumulative moment magnitude of m w 7 2 the area of the sse did not have a significant slip in the 1964 earthquake compared to the slip of the main asperity we propose that between earthquakes this zone fails repeatedly in sses and remains at relatively low shear stress compared to the main asperity up dip of it

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

lack of precursory slip to the 1999 hector mine california earthquake as constrained by insar we looked for evidence of interseismic strain occurring between the 1992 landers earthquake and the 1999 hector mine earthquake near the lavic lake and bullion faults by using interferometric synthetic aperture radar insar interferograms covering the hector mine epicentral region were studied for possible slip along the bullion and lavic lake faults by both visual inspection and a matched filter technique intended to emphasize slip located at the nucleation point some indications of possible deformation associated with the 5 july 1992 m l 5 4 pisgah event was observed but high decorrelation prevented a conclusive determination we have seen no evidence for precursory slip in the epicentral region up to 30 days before the hector mine event we estimated that the slip equivalent to a m w 4 5 event would have been observable in the months before the hector mine event and this places an upper bound on the long term precursory slip had it occurred we have noted that insar is well suited for detecting precursory slip in general due to the high spatial resolution and the lack of ground instrumentation required but that the detection level depends on the depth and orientation of the slip back

geologic heterogeneity can produce aseismic slip transients 1 slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones 10 103 m composed of rocks with marked differences in mechanical properties here we model quasi dynamic rupture along faults composed of material mixtures characterized by different rate and state dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting keeping other parameters fixed the relative proportions of velocity weakening vw and velocity strengthening vs materials control the sliding character stable slow or dynamic along the fault the stability boundary between slow and dynamic is accurately described by linear analysis of a double spring slider system with vw and vs blocks our results place bounds on the volume fractions of vw material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones

tohoku earthquake a surprise abstract we consider three questions related to the 2011 tohoku mega earthquake 1 why was the event size so grossly underestimated by japan s national hazard map 2 how should we evaluate the chances of giant earthquakes in subduction zones 3 what is the repeat time for magnitude 9 earthquakes off the tohoku coast the maximum earthquake size is often guessed from the available history of earthquakes a method known for its significant downward bias we show that historical magnitudes systematically underestimate this maximum size of future events but the discrepancy shrinks with time there are two quantitative methods for estimating the corner magnitude in any region a statistical analysis of the available earthquake record and the moment conservation principle however for individual zones the statistical method is usually ineffective in estimating the maximum magnitude only the lower limit can be evaluated the moment conservation technique which we prefer matches the tectonic deformation rate to that predicted by earthquakes with a truncated or tapered magnitude frequency distribution for subduction zones the seismic or historical record is insufficient to constrain either the maximum or corner magnitude however the moment conservation principle yields consistent estimates for all the subduction zones the corner magnitude is of the order 9 0 9 7 moreover moment conservation indicates that variations in estimated corner magnitude among subduction zones are not statistically significant another moment conservation method applied at a point on a major fault or plate boundary also suggests that magnitude 9 events are required to explain observed displacement rates at least for the tohoku area the global rate of magnitude 9 earthquakes in subduction zones predicted from statistical analysis of seismicity as well as from moment conservation is about five per century five actually happened

long term perspectives on giant earthquakes and tsunamis at subduction histories of earthquakes and tsunamis inferred from geological evidence aid in anticipating future catastrophes this natural warning system now influences building codes and tsunami planning in the united states canada and japan particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records under favorable circumstances paleoseismology can thus provide long term advisories of unusually large tsunamis the extraordinary indian ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks such variation in rupture mode known from written history at a few subduction zones is also characteristic of earthquake histories inferred from geology on the pacific rim

foreshock sequence of the 1992 landers california earthquake and its implications for earthquake nucleation the june 28 1992 landers california earthquake mw 7 3 was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events in this study we examine the geometry and temporal development of the foreshocks using high precision locations based on cross correlation of waveforms recorded at nearby stations by aligning waveforms rather than trying to obtain travel time picks for each event independently we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals we perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically with the improved locations the geometry of the foreshock sequence becomes clear the landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane the nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step this geometry is confirmed by the focal mechanisms of the foreshock sequence which are right lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault and are rotated clockwise for foreshocks that occur within the step the extent of the foreshock sequence is approximately 1 km both vertically and horizontally modeling of the coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other this result is particularly clear for the mw 4 4 immediate foreshock since stress transfer in the sequence appears not to have played a significant role in its development we infer an underlying aseismic nucleation process probably aseismic creep other studies have shown that earthquake nucleation may be controlled by fault zone irregularities this appears to be true in the case of the landers earthquake although the size of the irregularity is so small that it is not detectable by standard location techniques

rheological transitions facilitate fault spanning ruptures on seismically active and creeping faults the apparent stochastic nature of earthquakes poses major challenges for earthquake forecasting attempts physical constraints on the seismogenic potential of major fault zones may greatly aid in improving seismic hazard assessments but the mechanics of earthquake nucleation and rupture are obscured by the enormous complexity that natural faults display in this study we investigate the mechanisms behind giant earthquakes by employing a microphysically based seismic cycle simulator this microphysical approach is directly based on the mechanics of friction as inferred from laboratory tests and can explain a broad spectrum of fault slip behaviour we show that exceptionally large fault spanning earthquakes are governed by different micro scale mechanisms than regular small earthquakes more importantly the stress driven transition from ductile creep to granular flow facilitates the nucleation of giant earthquakes on faults that are otherwise seismically quiet this microphysically based approach offers new opportunities for investigating long term seismic cycle behaviour of natural faults

compaction creep of quartz sand at 400 600 c experimental evidence for dissolution controlled pressure solution intergranular pressure solution ips is an important compaction and deformation mechanism in quartzose rocks but the kinetics and rate controlling process remain unclear the aim of the present study is to test microphysical models for compaction creep by ips against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution confining pressure 300 mpa pore water pressure 150 250 mpa temperature 400 600 c microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions for porosities down to 15 the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz suggesting that the rate limiting mechanism of ips was dissolution the model also predicts ips rates within one order of magnitude of those measured in previous experiments at 150 350 c and thus seem robust enough to model sandstone compaction in nature such applications may not be straightforward however as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence ips rates in sandstones

interactions and triggering in a 3 d rate and state asperity model we present a 3 d continuous quasi dynamic rate and state model of multiple seismic asperities forced by surrounding aseismic creep and motivated by observations of coplanar multiplets our model allows to study the physics of interactions among a set of asperities first we show that the amount of interactions and clustering characterized by the omori law and interevent time distribution depends on how far the system is from a critical density of asperities which is related to the friction properties of the barriers separating the sources this threshold controls the ability of a population of asperities to destabilize the creeping barriers between them and therefore determines whether dynamic sequences including several asperities in the same event might occur in agreement with what is expected from observed magnitude frequency distributions therefore the concept of critical density of asperity provides a mechanical interpretation of statistical properties of seismicity as an illustration we used our numerical results in the specific case of parkfield in the period preceding the mw 6 2004 earthquake in order to infer the steady state friction parameter a b characterizing the creep of this part of the san andreas fault we estimate a value of a b that locally exceeds 0 001 which is in the upper range of what has already been proposed for the postseimic period of the mw 6 2004 parkfield earthquake

large nucleation before large earthquakes is sometimes skipped due to cascade up implications from a rate and state simulation of faults with hierarchical asperities 1 does a large earthquake have a large quasi static preparation a hierarchical asperity model in which a large tough patch patch l contains smaller fragile patches patch s enables a large earthquake to start with only small preparation because of cascade up rupture growth we realized such a model in a rate and state framework by heterogeneous distributions of the state evolution distance and its consequences are investigated by earthquake sequence simulations focus is put on elementary processes the interaction between two scales one patch l containing one patch s is simulated with their size ratio as a parameter if patch s is larger than the nucleation size of patch l the system falls into a limit cycle consisting of only one earthquake that starts with patch s nucleation and grows dynamically to rupture entire patch l if patch s is considerably smaller than the nucleation size of patch l small earthquakes never dynamically cascade up and the large earthquakes are initiated by large quasi static nucleation in between large earthquakes start in various ways by large nucleation dynamic cascade up or delayed cascade up in the final stage of quasi static nucleation the preseismic moment release rate increases roughly inversely proportional to the time to failure tf with its amplitude depending on the nucleation size for a patch s rupture to cascade up strength in the adjacent region must have been reduced manifested by a higher creep velocity before the patch s nucleation starts following 1 tf acceleration large nucleation sometimes has a precursory small earthquake characterized by larger afterslip than nonprecursory ones

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data