seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

interplay between fluid flow and fault fracture mesh generation within underthrust sediments geochemical evidence from the chrystalls beach complex new zealand the chrystalls beach complex in the otago schist on the south island of new zealand is a melange comprising sheared trench fill sediments and fragments of oceanic crust it represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces the melange is cross cut by a fault fracture mesh comprising subvertical extension veins and subhorizontal slickenfibre coated shear surfaces both shear and extension veins have a crack seal microstructure indicating episodic growth shear veins are associated with pressure solution selvages along the shear surface whereas wall rock alteration is not observed adjacent to extension veins electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins and therefore these slickenfibres probably grew by local dissolution precipitation of silica on the contrary no depletion or addition of silica is detected around extension veins indicating these veins grew by precipitation from advecting fluids oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 18o 10 consistent with a fluid derived from low grade metamorphic dehydration reactions fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable melange with increasing metamorphic grade and decreasing porosity fault fracture mesh generation therefore involved localized shear assisted by dissolution precipitation creep and concomitant extension fracturing this led in turn to transient permeability associated with a fluid pressure drop allowing episodic vein growth this process may be analogous to geophysically observed episodic tremor and slow slip which also involves a mixture of deformation styles that put together achieve shear slip along the subduction thrust interface

mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

deformation processes at the down dip limit of the seismogenic zone the example of shimanto accretionary complex in order to constrain deformation processes close to the brittle ductile transition in seismogenic zone we have carried out a microstructural study in the shimanto accretionary complex japan the fossil equivalent of modern nankai accretionary prisms the hyuga tectonic melange was sheared along the plate interface at mean temperatures of 245 c 30 c as estimated by raman spectroscopy of carbonaceous material rscm it contains strongly elongated quartz ribbons characterized by very high fluid inclusions density as well as micro veins of quartz both fluid inclusion planes and micro veins are preferentially developed orthogonal to the stretching direction furthermore crystallographic preferred orientation cpo of quartz c axes in the ribbons has maxima parallel to the stretching direction recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons in such recrystallized quartz domains cpo of quartz c axes are orthogonal to foliation plane the evolution of deformation micro processes with increasing temperature can be further analyzed using the foliated morotsuka a slightly higher grade metamorphic unit 342 30 c by rscm from the shimanto accretionary complex in this unit in contrast to hyuga tectonic melange recrystallization of quartz veins is penetrative cpo of quartz c axes is concentrated perpendicularly to foliation plane these variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures above 300 c dislocation creep is dominant and results in intense quartz dynamic recrystallization in contrast below 300 c quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth in addition the geometry of the quartz ribbons with respect to the phyllosilicate rich shear zones shows that bulk rheology is controlled by quartz behavior consequently below 300 c the application of quartz pressure solution laws based on realistic geometry derived from hyuga microstructures results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

the role of trapped fluids during the development and deformation of a carbonate shale intra wedge tectonic melange mt massico southern apennines italy abstract numerous studies exist on exhumed tectonic melanges along subduction channels whereas in accretionary wedge interiors deformation mechanisms and related fluid circulation in tectonic melanges are still underexplored we combine structural and microstructural observations with geochemical stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins and u pb geochronological data to define deformation mechanisms and syn tectonic fluid circulation within the mt massico intra wedge tectonic melange located in the inner part of the central southern apennines accretionary wedge italy this melange developed by shear deformation at the base of a clastic succession deformation was characterized by disruption of the primary bedding mixing and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds recurrent cycles of mutually overprinting fracturing veining and pressure solution processes generated a block in matrix texture the geochemical signatures of syntectonic calcite veins suggest calcite precipitation in a closed system from warm 108 147 c paleofluids with 18o vlaues between 9 and 14 such as trapped pore waters after extensive 18o exchange with the local limestone host rock and or derived by clay dehydration processes at t 120 c the 3he 4he ratios in fluid inclusions are lower than 0 1 ra indicating that he was exclusively sourced from the crust we conclude that 1 intraformational rheological contrasts inherited trapped fluids and low permeability barriers such as marly shaly matrix can promote the generation of intra wedge tectonic melanges and the development of transient fluid overpressure 2 clay rich tectonic melanges developed along intra wedge decollement layers may generate low permeability barriers hindering the fluid redistribution within accretionary wedges

very low frequency earthquakes within accretionary prisms are very low stress drop earthquakes 1 we report characteristic p wave spectra and stress drops of very low frequency vlf earthquakes that occurred within the accretionary prism along the nankai trough southwestern japan although many vlf earthquakes showed no distinct phases on a raw seismogram a few showed slightly distinct phases the arrival times of some phases were consistent with the calculated p wave arrival times the corner frequencies of the vlf earthquakes were calculated using the stacked p wave spectra assuming an omega square spectrum stress drops were very low i e in the range of 0 1 10 kpa which corresponds to 0 1 1 of those of ordinary earthquakes such extremely low stress drops of the vlf earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

lower mantle structure from scs s differential travel times knowledge of the spectrum of structure in the lower mantle is crucial to our understanding of the dynamical evolution of the earth and in principle can be inferred from the analysis of global seismic data sets the long wavelength compressional velocity structure has usually been constrained through tomographic inver sion of the isc catalogue of p wave travel times1 3 whereas shear velocity has been inferred from waveform modelling of long period shear waves4 6 the models that have been produced are similar in only the largest scale features7 and explain little of the variance of the raw data used in their construction thus there is a legitimate concern that the current generation of global scale models give only a crude approximation to the largest scale structure and that there may be significant aliasing of short wavelength heterogeneity here we use scs s differential travel times to demonstrate that the three dimensional structure of the lower mantle is indeed dominated by continental scale features particularly convincing is the fact that the features are apparent in the raw data and are not the product of a complicated modelling procedure

a range of fault slip styles on progressively misoriented planes during flexural slip folding cape fold belt south africa flexural slip folds are distinctive of mixed continuous discontinuous deformation in the upper crust as folding is accommodated by continuous bending of layers and localized discontinuous slip along layer interfaces the mechanism of localized layer parallel slip and the stress and fluid pressure conditions at which flexural slip occurs are therefore distinctive of shear localization during distributed deformation in the prince albert formation mudstone sequence of the karoo basin the foreland basin to the cape fold belt folds are well developed and associated with incrementally developed bedding parallel quartz veins with slickenfibers oriented perpendicular to fold hinge lines locally cross cutting axial planar cleavage and showing hanging wall motion toward the fold hinge bedding parallel slickenfiber coated veins dip at angles from 18 to 83 implying that late increments of bedding parallel shear occurred along unfavorably oriented planes the local presence of tensile veins in mutually cross cutting relationship with bedding parallel slickenfiber coated veins indicate local fluid pressures in excess of the least compressive stress slickenfiber vein microstructures include a range of quartz morphologies dominantly blocky to elongate blocky but in places euhedral to subhedral the veins are commonly laminated with layers of quartz separated by bedding parallel slip surfaces characterized by a quartz phyllosilicate cataclasite crack seal bands imply incremental slickenfiber growth in increments from tens of micrometers to a few millimeters in some places whereas other vein layers lack evidence for incremental growth and likely formed in single slip events single slip events however also involved quartz growth into open space and are inferred to have formed by stick slip faulting overall therefore flexural slip in this location involved bedding parallel faulting along progressively misoriented weak planes with a range of slip increments

from soft sediment deformation to fluid assisted faulting in the shallow part of a subduction megathrust analogue the sestola vidiciatico tectonic unit northern apennines italy

syntectonic fluid flow and deformation mechanisms within the frontal thrust of a foreland fold and thrust belt example from the internal jura eastern france abstract structural mapping and microstructural analyses are combined with stable clumped and radiogenic isotopes to unravel deformation mechanisms and syntectonic fluid flow during upward propagation along the frontal thrust fuans thrust of the internal part of the jura fold and thrust belt the fuans thrust cuts through upper jurassic lower cretaceous carbonate deposits branches into two splays and generates a system of fault propagation folds folding was accomodated by layer parallel shortening and layer parallel shear which generated flexural slip along bedding surfaces stylolites and foliated fabric in marly limestone interbeds thrusting was characterized by cycles of mutually overprinting pressure solution and brittle processes such as frictional sliding and fracturing veining this generated foliated tectonites in marls and limestones without significant cataclasis of the host rock pressure solution and micrometer slip along slickenfibers are evidence of aseismic slip in agreement with the ongoing aseismic growth of anticlines in the jura fold and thrust while crackle like vein texture may account for episodic seismic fracturing geochemical data show that fuans thrust acted as a conduit for the downward infiltration of meteoric derived fluids which channeled along foliated tectonites during syntectonic fluid flow calcite mineralizations precipitated at temperatures between 54 c and 10 c showing an increased contribution of organic carbon from soils in colder paleofluids this indicates a progressive opening of the system characterized by a more efficient and extensive meteoric fluids circulation considering the highest recorded temperature of 54 c and assuming the modern geothermal gradient of 30 c km thrusting in the study outcrop initiated at a burial depth of at least 1 4 km

crack seal patterns records of uncorrelated stress release variations in crustal rocks statistical properties of crack seal veins are investigated with a view to assessing stress release fluctuations in crustal rocks crack seal patterns correspond to sets of successive parallel fractures that are assumed to have propagated by a subcritical crack mechanism in the presence of a reactive fluid they represent a time sequence record of an aseismic and anelastic process of rock deformation the statistical characteristics of several crack seal patterns containing several hundreds of successive cracks have been studied samples were collected in three different areas gold bearing quartz veins from abitibi in canada serpentine veins from the san andreas system in california and calcite veins from the apennine mountains in italy digitized pictures acquired from thin sections allow accurate measurement of crack seal growth increments all the samples show the same statistical behaviour regardless of their geological origin the crack seal statistical properties are described by an exponential distribution with a characteristic length scale and do not show any spatial correlation they differ from other fracture patterns such as earthquake data which exhibit power law correlations gutenberg richter relationship crack seal series represent a natural fossil record of stress release variations less than 50 bars in the crust that show a characteristic length scale associated with the resistance of rock to effective tension and no correlation in time

new perspectives on geological strain rates calculated from both naturally deformed and actively deforming rocks abstract a value of 10 14 s 1 is commonly cited as an average geological strain rate this value was first suggested for finite strain across an orogen but based on more limited information than the combined geophysical geological and experimental data now available on active and ancient rock deformation thus it is timely to review the data constraining strain rates in the continents and to consider the quantifiable range of crustal strain rates here where resolution allows both spatial and temporal strain rate variations are explored this review supports that a strain rate of 10 14 1 s 1 arises from geological estimates of bulk finite strains microstructural arguments combining laboratory derived piezometers and viscous flow laws however imply local rates that are orders of magnitude faster geodetic rates in contrast are typically 10 15 s 1 in actively deforming areas about an order of magnitude slower than the bulk rates estimated from geological observations this difference in estimated strain rates may arise from either low spatial resolution or the fact that surface velocity fields can not capture strain localisation in the mid to lower crust integration of geological and geodetic rates also shows that strain rates can vary in both space and time over both single and multiple earthquake cycles overall time averaged geological strain rates are likely slower than the strain rates in faults and shear zones that traverse the crust or lithosphere

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

heating during thrust faulting in the rocky mountains friction or fiction abstract measurement of the levels of organic maturation below and within shear zones of thrust faults in the rocky mountains has revealed no general thermal metamorphism that can be attributed to faulting with the exception of very localized areas with few exceptions the vitrinite reflectance values obtained are in the range expected if the maximum level of organic maturation was produced as a result of increasing temperature during progressive burial accompanying sedimentation only at marias pass evidence has been found to suggest additional maturation as a result of post orogenic burial below the thrust sheets anomalously high vitrinite reflectances obtained from the lewis thrust mcconnell thrust coleman thrust and two unnamed thrusts are restricted to very narrow films immediatly adjacent to or within the shear zone which considering any reasonable thermal conductivity indicates elevated temperatures were very short lived the anomalously high vitrinite reflectances within these films when compared to laboratory heated coals suggest temperatures in the order of 350 650 c were locally generated during faulting such high temperatures are considered to have been generated during stick slip faulting at macro asperities or at ramps on the fault plane where local and possibly transient high frictional stresses existed the absence of evidence for extensive frictional heating supports previous arguments that stable sliding and or low frictional stress must exist during thrusting

striation and slickenline development on quartz fault surfaces at crustal conditions origin and effect on friction fragments of optically flat silica discs embedded in synthetic gouge were deformed to examine the relationship between the development of striations and slickenlines and deformation mechanisms conditions and fault rheology experiments were performed under hydrothermal conditions in a rotary shear apparatus at 100 c or 450 c to shear strains of 2 02 8 25 slip hardening and softening prevail at low and high temperatures respectively in recovered samples disc fragment surfaces are decorated by fine gouge sometimes arranged in trails pits and scratch marks prominent grooves inferred slickenlines with constant orientation wavelength 10 m and amplitude 0 7 m are only observed on disc fragments deformed at 450 c some parts of the grooves below the original disc fragment surface contain scattered rounded beads of silica 200 nm diameter conversely close examination of pits in 100 c experiments reveals they contain angular particles 2 m diameter the 200 nm diameter crystalline quartz can precipitate at 450 c in only 250 s well within the time frame of the experiments but precipitation at 100 c would take at least 8 years no systematic dislocation arrays were observed in the quartz disc fragments but microfractures are sporadically present this indicates that at both temperatures brittle failure generated microfractures and microcomminution occurred where gouge particles impacted disc fragment surfaces these observations suggest formation of silica beads by precipitation from amorphous silica facilitates slip weakening smoothing of the fault surface parallel to the slip vector development of undulations perpendicular to the slip vector by a pressure solution creep mechanism weakening and maintenance of a constant slip direction

periodic pulsing of characteristic microearthquakes on the san andreas fault deep fault slip information from characteristically repeating microearthquakes reveals previously unrecognized patterns of extensive large amplitude long duration quasiperiodic repetition of aseismic events along much of a 175 kilometer segment of the central san andreas fault pulsing occurs both in conjunction with and independent of transient slip from larger earthquakes it extends to depths of 10 to 11 kilometers but may be deeper and it may be related to similar phenomena occurring in subduction zones over much of the study area pulse onset periods also show a higher probability of larger earthquakes which may provide useful information for earthquake forecasting

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

antitaxial crack seal vein microstructures and their relationship to displacement paths abstract the microstructures developed in an example of a syntectonic crack seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack seal vein growth this observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation it is demonstrated that crack seal processes can lead to the development of both irregular and laminated vein microstructures as well as fibrous microstructures the types of microstructures which develop in crack seal veins and the extent to which they reflect the opening paths of veins depend on a range of factors the most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures and the location of sites of material accretion during successive crack seal increments

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

high vp vs zone accompanying non volcanic tremors and slow slip events beneath southwestern japan abstract in southwestern japan the philippine sea plate subducts beneath the eurasian plate non volcanic tremors and slow slow slip events are observed beneath southwestern japan seismic tomography based on the data obtained from the dense seismic network revealed the velocity structure in and around the subducting philippine sea plate the oceanic crust of the philippine sea plate subducting from the suruga and nankai troughs can be imaged clearly beneath the tokai region and the kii peninsula non volcanic tremors and short term slow slip events may occur in the high vp vs zones owing to the fluid dehydrated from the subducting plate where the oceanic crust of the philippine sea plate encounters the serpentinized wedge mantle of the eurasian plate these tremors and short term slow slip events occur in the high vp vs zone at depths of 30 40 km beneath the tokai region the landward edge of the high vp vs zone beneath the kii peninsula and eastern shikoku region and the seaward edge of the high vp vs zone beneath the western shikoku region long term slow slip events were observed beneath the tokai region and the bungo channel and both these events are located on the seaward side of a high vp vs zone these long term slow slip events are considered to be caused at the plate boundary between the oceanic crust of the philippine sea plate and the lower crust of the eurasian plate by the high pore fluid pressure caused by the fluid dehydrated from the subducting plate the existence of the high vp vs lower crust of the eurasian plate may control the occurrence of the long term slow slip events

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

tectonics magmatism and paleo fluid distribution in a strike slip setting insights from the northern termination of the liquine ofqui fault system chile this study addresses the interplay between strain stress fields and paleo fluid migration in the southern andean volcanic zone svz the svz coexists with the margin parallel liquine ofqui fault system lofs and with nw striking andean transverse faults atf to tackle the role of different fault fracture systems on deformation distribution and magma fluid transport we map the nature geometry and kinematics of faults veins and dikes at various scales fault slip data analysis yields stress and strain fields from the full study area data base regional scale and fault zones representative of each fault system local scale regional scale strain analysis shows kinematically heterogeneous faulting local strain analyses indicate homogeneous deformation with ne trending shortening and nw trending extension at nne striking liquine ofqui master fault zones strain axes are clockwise rotated at second order fault zones with ene trending shortening and nnw trending stretching the atf record polyphasic deformation conversely stress field analysis at regional scale indicates a strike slip dominated transpressional regime with n64 e trending 1 and n30 w trending 3 deformation is further partitioned within the arc through nne striking dextral reverse faults ne striking dextral normal faults and nw striking sinistral reverse faults with normal slip activation the regional tectonic regime controls the geometry of ne striking dikes and volcanic centers ne striking faults record local stress axes that are clockwise rotated with respect to the regional stress field nne and ne striking faults are favorably oriented for reactivation under the regional stress field and show poorly developed damage zones conversely nw striking fault systems misoriented under the regional stress field show multiple fault cores wider damage zones and dense vein networks deformation driven by oblique subduction is partially partitioned into strike slip and shortening components the trench parallel component is mostly accommodated by ns striking right lateral faults of the lofs trench perpendicular shortening is accommodated by sinistral reverse atf and dextral reverse nne striking faults we conclude that the svz records a deformation history coeval with magma fluid migration

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part i meso and macro scopic scale there is considerable variability in the thickness structure and macroscopic deformation style within shale prone major thrust zones which also impacts the structural style in the overlying and sometimes the underlying section the properties and percentages of clay minerals and percentages of organic material quartz and calcite affect fault zone weakness and behaviour the distribution of competent units within a shale prone zone can also significantly affect fault zone architecture even over short strike distance of 10s to 100s of metres other influences include high pore fluid pressures and associated veins deformation of the fault zone may disrupt the original principal displacement zones and force them to relocate including switching lithologies calcite veins can become concentrated locally in the fault zone and become a focus of fault zone weakness we still need many more descriptions of shale prone fault zones to understand the potential variations in thrust sheet structural style caused by differences in shale detachment characteristics the variations in structural style in the hanging walls of shale detachments are broad and range between the sledge runner style in many appalachian examples and the extensive closely spaced imbricate style of the osen roa detachment southern norway such contrasts are related to several factors including thrust sheet mechanical stratigraphy thickness of the overburden lateral continuity of the detachment detachment mechanical properties composition pore fluid pressures presence absence of higher level detachments and strain rate in many structural locations and at many scales shale prone sequences will pile up to produce over thickened masses cleavage duplexes s c fabrics duplexes imbricates pseudo duplexes and vein formation can all contribute to the thickening at nested scales a mixture of brittle and solution diffusion processes overall cause the affected unit to exhibit flow like characteristics while the broad principles are now well established considerable work remains to be done understanding the full range of structural geometries associated with shale detachments what factors regarding the nature of the detachments affect overlying structural styles rates of deformation and the controls on deformation mechanisms within the thrust zones

scales of heterogeneity near the core mantle boundary we have mapped shear velocity variations near the base of the mantle with scs s differential travel times beneath the north atlantic there is a systematic variation in velocity over a distance of 1500 km or more with a pronounced low velocity anomaly centered roughly beneath madeira island beneath the northern indian ocean there is no similar long wavelength anomaly the average scs s residual beneath the indian ocean is 3 6 0 7 s more negative than the atlantic residual indicating faster average velocities near the base of the mantle

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

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

paleo stress estimates on ancient seismogenic faults based on frictional heating of coal 1 faulted coals from eastern kentucky and montana usa and the south wales coalfield uk show fault related vitrinite reflectance anomalies interpreted as due to frictional heating during seismic slip the vitrinite reflectance anomalies are converted to temperature using kinetic based software easy r for post seismic cooling rates shear stresses are calculated using lachenbruch s 1986 piezometer which depends only on the area under the thermal anomaly and fault displacement for maximum displacements of 1 3 meters the minimum shear stresses are 13 64 mpa and overlap with seismological estimates of stress drops during modern earthquakes the calculated local mean normal stress is 100 mpa resulting in coefficients of friction of 0 1 0 6 thrust and 0 2 0 4 normal faults which are lower than laboratory based values

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

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

deformation and hydrofracture in a subduction thrust at seismogenic depths the rodeo cove thrust zone marin headlands california we have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths the rodeo cove thrust zone which outcrops north of the golden gate bridge of san francisco imbricates two basalt chertsandstone sequences belonging to the marin headlands terrane franciscan complex the thrust outcrop is a 200 m thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop dominated by basaltic lithologies where zones of concentration of deformation have been mapped disruption is made by variably dense discrete fault systems synthetic to the main thrust r and p fractures these faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite which also constrain the depth of faulting 8 10 km t 200 250 c within the seismogenic zone the central part of the fault also features the densest system of carbonate fi lled veins veins occur in the broken formation matrix and fragments in both cases parallel to the foliation the veins are either folded truncated or pressure solved along the cleavage cementation and hardening of shear surfaces of the fault core may have caused the distribution as opposed to localization of subsequent slip events the fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone the crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and consequently that large transient variations in permeability and cohesion may occur the repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress or by difference in tensional strength parallel to and perpendicular to the foliation both of which would require extremely high fl uid pressure we interpret the features of the rodeo cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase

amphibolitic cu fe skarn deposits in the central coast of peru monterrosas eliana ra uacute l and condestable are the most important copper mines in the central coast of peru between lat 12 degrees 309 and 14 degrees 309 s their related geologic setting close age relation and similar mineralogy and geochemistry argue in favor of their grouping together as amphibolitic cu fe skarn deposits in the lower cretaceous continental margin 100 to 120 ma incipient rifting accompanied by batholithic gabbrodiorite intrusions at depth gave rise to an elongated trough in which submarine volcanic and sedimentary rocks accumulated skarn deposits developed in association with the intrusive activity are characterized by chalcopyrite concentrations with low ti magnetite and or pyrite in actinolite gangue minor constituents are amphiboles of hastingsitic and tschermakitic varieties apatite pyrrhotite and sodic scapolite copper ores are fracture controlled and hosted by albian gabbrodiorite plutons at monterrosas and eliana volcanogenic strata bound mantos as well as disseminated sulfide stringers and minor veins occur at ra uacute l and condestable in hauterivian to albian volcano sedimentary rocks geochronologic k ar data identify a lower cretaceous episode of metallogenesis for ra uacute l condestable and eliana geologically related iron deposits of hierro acari and marcona are dated as lower cretaceous and upper jurassic respectively petrography and major element chemistry of the gabbrodiorite suite at monterrosas and eliana indicate a low k tholeiitic character with calc alkaline affinities and high na 2 o k 2 o ratios late magmatic and hydrothermal activity developed in the roof zones of the plutons and formed amphibole magnetite chalcopyrite deposits mineralization at eliana involved introduction of cu fe and p along fractures and contacts to form ore shoots the scapolite alteration halo is characterized by si na and ca enrichment the hydrothermal deposits of ra uacute l formed mainly at subvolcanic levels with a dominant seawater component ripley and ohmoto 1977 1979 geologic correlation with eliana and monterrosas indicates that both magmatic and seawater components coincided to form amphibolitic cu fe skarn deposits which span the plutonic and volcanic environments lead isotope compositions of chalcopyrite pyrite and galena define a primitive trend for these skarn deposits compared to that of porphyry cu deposits in southern peru and chile and sn deposits in bolivia the isotopic contrast is interpreted as a result of rifting and extreme crustal thinning during the lower cretaceous along the continental margin of central and southern peru this tectonic and metallogenetic environment continued into northern and central chile where amphibolitic fe and cu deposits of the same age range are known

strength of strained two phase mixtures application to rapid creep and stress amplification in subduction zone melange aseismic creep may occur by distributed deformation in melange shear zones comprising weak matrix and stronger clast materials slow slip events or steady tectonic displacement can be distributed over 100 m thick shear zones if weak matrix controls bulk shear zone deformation we use 2 d numerical models to quantify the rheology of moderately strained shear strain 1 75 melange for various volumetric proportions of competent clasts melange deformation with 50 clasts is matrix dominated and can accommodate steady creep at higher clast proportions melange viscosity increases more than tenfold after small strains because strong clasts interact and form force chains clast shear stress is amplified above the imposed shear stress by a factor of 14 where force chains develop slow slip events may occur due to a temporary absence of force chains while localized regions of high shear stress generate coincident fracturing and potentially tremor events

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

the role of silica redistribution in the evolution of slip instabilities along subduction interfaces constraints from the kodiak accretionary complex alaska abstract quartz veins from a subhorizontal shear zone in the kodiak accretionary complex in southwest alaska record cycles of cracking and sealing contemporaneous with simple shear and analysis of the vein array in the context of silica dissolution diffusion and precipitation leads to the conclusion that the time needed to seal after a fracture event overlaps with the time scales associated with the recurrence of slip instabilities along active megathrusts the central belt of the kodiak accretionary complex is interpreted as a shear zone developed adjacent to a paleo decollement that was active in the late cretaceous based on large strain magnitudes recumbent isoclinal folds and regional cleavage patterns that depict rotation of the steep cleavage that characterizes most of the kodiak formation into the horizontal fabrics of the central belt within this regional shear zone crack seal veins composed of quartz with phyllosilicate inclusion bands are spaced at less than a cm but en echelon vein arrays are spaced at 500 cm these arrays indicate top to the southeast shear on southeast dipping brittle ductile shear zones vein textures indicate cracking and sealing of thin veins in the matrix but thicker veins in en echelon sets record initial cracking and sealing followed by periodic collapse of fracture porosity at a frequency that is lower than the frequency of crack seal cycles silica depletion zones in the wall rock adjacent to veins indicate local transport of silica by diffusion by combining the maximum time in which diffusion could dominate over advection with the time needed to precipitate the quartz associated with one crack seal band we show that a reasonable range of p or pressure difference between matrix and crack can produce crack seal bands in less than 10 days this time frame is less than the recurrence of earthquakes and similar to the duration of some slow earthquakes in nature therefore silica redistribution could play a role in modulating the frequency of plate boundary slip instabilities along convergent plate boundaries

the geologic record of deep episodic tremor and slip transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults best documented in subduction zones and associated with nonvolcanic tremor most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding the dilemma is that slow slip is generated at a wide range of crustal depths including at pressure temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones we present a model for how fracture and viscous flow within mid crustal shear zones can produce episodic creep transients our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of 500 c and 0 6 gpa during early orogenesis following late cretaceous subduction of a backarc ocean basin within these shear zones relatively strong lenses of metabasalt localized fluid filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss the spatial and temporal characteristics of the modeled creep events are similar to those of slow slip events observed in modern subduction zones we therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials

chemical origin of tectonic tremor tectonic tremor may ultimately be caused by in situ fluid overpressure generated by chemical reactions between a subducting slab and the mantle according to field and microstructural observations of a shear zone

probing the prodigious strain fringes from lourdes abstract we investigate the kinematics of classic sigmoidal strain fringes from lourdes france and review previous genetic models strain methods and strain rates for these microstructures displacement controlled quartz and calcite fibers within the fringes yield an average strain of 195 with the technique of ramsay and huber 1983 this agrees well with strains measured from boudinaged pyrite layers and calcite veins in the same rocks but conflicts with ca 675 strain in previous analogue models for the studied strain fringes produced by progressive simple shear we show that the detailed geometry and orientation of fiber patterns are insufficiently explained by simple shear but imply two successive differently oriented strain fields although all strain fringes have the same overall asymmetry considerable morphological variation resulted from different amounts of rotation of pyrite grains and fringes minor rotation led to sharply kinked fibers that record a ca 70 rotation of the kinematic frame larger up to 145 rotations accommodated by antithetic sliding on pyrite fringe contacts produced more strongly and smoothly curved fibers combined with published rb sr ages for the studied microstructures our new strain data indicate an average strain rate of 1 41 10 15 s 1 during ca 37 myr continuous growth

quartz vein formation by local dehydration embrittlement along the deep tremorgenic subduction thrust interface hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes including the namibian damara belt here subduction related deformation occurred at temperatures 550 c and vein geometry is consistent with plate interface shear low effective normal stresses and mixed mode deformation quartz vein 18o values relative to standard mean ocean water smow range from 9 4 to 17 9 n 30 consistent with precipitation from metamorphic fluids a dominant subset of quartz veins away from long lived high strain zones and basaltic slivers have 18o values in a smaller range of 14 9 1 requiring precipitation from a fluid with 18o of 12 1 at 470 550 c this uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime in active subduction zones brittle deformation within the plastic regime is inferred from observations of tectonic tremor a noise like seismic signal including overlapping low and very low frequency earthquakes which occurs below the seismogenic zone both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure could represent a mixture of shear and dilatant failure and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

distributed deformation along the subduction plate interface the role of tectonic melanges recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically however the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified tectonic melange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate in this work we examine several tectonic melange zones hyuga okitsu mugi from the shimanto belt an accretionary prism in southwest japan connecting to the active nankai subduction zone these tectonic melange zones have a block in matrix structure with lenses of sandstones and basalts within a metapelitic matrix and their deformation is distributed over zones of hundreds of meters in thickness in addition the examples of melange considered here are bounded by sharp faults some of them bearing pseudotachylyte layers so that distributed deformation within the melange and localized deformation on its boundary are juxtaposed distributed deformation involves the development of a foliation as well as of a pervasive network of macroscopic and microscopic shear zones along with slip on this network strain proceeds by fracturing and precipitation of quartz in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins the analysis of shear band kinematics shows in all three examples a dominant top to the trench sense of shear consistent with deformation along the plate boundary during subduction moreover most shear zones when foliation is restored back to syn subduction position are extensional structures finally the geometry and kinematics of the melange bounding faults as well as radiometric constraints show that in most cases the faults localized structures were formed during a later stage than melange internal deformation these findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths first there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation second rather than proposed models of underplating where all deformation is localized into the thrusts bounding the tectonic sheets we suggest that underplating was to a large extent accommodated by distributed deformation within the melange sheets this underplating model accounts for i the large amount of strain within the melange ii the absence of contractional structures during underplating iii thinning of the melange required by the network of extensional shear bands and stretched boudins third melanges appear as likely candidates for portions of the plate interface deforming by aseismic slip the seismic vs aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain which in turn depends on the efficiency of pressure solution to operate

fluid chemistry of veining associated with an ancient microearthquake swarm benmore dam new zealand steeply dipping strata in the vicinity of benmore dam otago new zealand are complexly deformed metasedimentary rocks of the torlesse supergroup middle triassic age over an exposed area 100 m wide 25 m high these strata are disrupted by a fault fracture mesh comprising conjugate coulomb shears interlinked by extensional and extensional shear fractures all formed in a common stress field and hosting quartz prehnite epidote calcite veining the combined effect of these structures is shortening perpendicular to beddings and subvertical extension so that in their present attitude they correspond to a set of conjugate thrust faults with associated extension fractures on the evidence of incremental vein textures the development of this distributed fault fracture mesh is interpreted as resulting from a fluid driven microearthquake swarm which postdated regional low grade metamorphism mechanical considerations suggest that the migrating hydrothermal fluids were significantly overpressured possibly to approximately lithostatic values if the mesh structure developed in its present attitude fluid inclusion microthermometric studies show that benmore vein quartz contains two phase aqueous inclusions with salinities between 1 4 and 2 9 wt nacl equivalent and homogenization temperatures t h between 189 and 217 c the assemblage quartz prehnite epidote suggests trapping temperatures t t of 280 c requiring the addition of an 70 c correction to t h values late calcite contains inclusions with noticeably lower salinity 0 0 0 9 wt nacl and t h values 129 175 c studies on quartz pumpellyite calcite veins from nearby lake aviemore show similar fluid inclusion salinity and t h values fluid inclusion gas analyses show all the vein samples to be dominated by h 2 o 99 3 99 9 mol with few other gases apparent including ch 4 0 5 n 2 0 1 co 2 0 1 and c 2 c 4 hydrocarbons cation and anion analyses when combined with the gas data show that nacl dominates the fluid inclusion salinities oxygen isotope results when combined with calculated t t values indicate that the water responsible for the deposition of benmore and aviemore quartz had 18 o compositions of 9 4 and 4 8 respectively calcite 13 c values between 25 3 and 38 0 are indicative of oxidation of ch 4 to co 2 as a result of hydrothermal fluids interacting with organic rich sediments fluid inclusion delta d h 2 o values for benmore range between 73 and 89 compared to 109 for the one aviemore sample this research has demonstrated that 1 water of meteoric origin probably from subantarctic latitudes penetrated to 6 km depth and underwent an oxygen isotope shift before depositing the benmore aviemore veins 2 the migrating hydrothermal fluids were likely overpressured well above hydrostatic to near lithostatic values if the mesh structure was active in its present orientation and 3 fluid migration was coupled to distributed brittle failure in the prevailing stress field self generating a permeable fault fracture mesh

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event