seismic and aseismic slip indicators in serpentinite gouge structures observed within experimentally deformed serpentinite gouges provide information that may be used to identify the seismic behavior of natural fault zones in laboratory friction experiments serpentinite exhibits two modes of behavior one can only result in stable fault creep and the other may result in stable slip but has the potential for earthquakes the microstructures that form during these experiments reflect the deformation style of the serpentinite distributed deformation results from aseismic fault creep and localized deformation results from conditions favorable for seismic slip distributed deformation produces a crystallographic preferred orientation of the serpentine grains s foliation localized deformation forms riedel shears similar structures occur within a natural serpentinite shear zone from monterey county california and suggest a history of stable fault creep with intermittent seismic events

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

effect of quartz inclusions on plastic flow in marble to investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks we produced synthetic marbles with a grainsize of 5 25 m containing quartz of similar size with volume fractions of 0 5 1 and 20 4 vol and between 3 to 9 porosity these rocks were deformed in triaxial compression at a strain rate of 3 x 10 5 s 1 confining pressure of 200 mpa and 600 c marbles containing 20 4 vol quartz were almost 5 times stronger than those containing no quartz the work hardening rate at given strain increased with quartz content marbles containing 20 4 vol quartz work hardened up to 10 strain whereas in single phase samples work hardening was negligible beyond 3 strain the dislocation density in the calcite increased with increasing quartz content the data are consistent with models for the strength of ceramic matrix composites or with the hypothesis that strength is increased by decreasing porosity

local observations of the onset of a large earthquake 28 june 1992 landers california the landers earthquake mw 7 3 of 28 june 1992 had a very emergent onset the first large amplitude arrivals are delayed by about 3 sec with respect to the origin time and are preceded by smaller scale slip other large earthquakes have been observed to have similar emergent onsets but the landers event is one of the first to be well recorded on nearby stations we used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release and to determine the slip function of the 3 sec nucleation process relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2 5 sec three station array analysis of the p wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals indicating that the large amplitude source was several kilometers deeper than the initial onset an ml 2 8 foreshock located close to the hypocenter was used as an empirical green s function to correct for path and site effects from the first 3 sec of the mainshock seismogram the resultant deconvolution produced a slip function that showed two subevents preceding the main energy release an mw 4 4 followed by an mw 5 6 these subevents do not appear anomalous in comparison to simple moderate sized earthquakes suggesting that they were normal events which just triggered or grew into a much larger earthquake if small and moderate sized earthquakes commonly detonate much larger events this implies that the dynamic stresses during earthquake rupture are at least as important as long term static stresses in causing earthquakes and the prospects of reliable earthquake prediction from premonitory phenomena are not improved

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

ultramafic exposures and the gravity signature of the lithosphere near the fifteen twenty fracture zone mid atlantic ridge 14 16 5 n abstract we analyze geological and geophysical data near the fifteen twenty fracture zone to constrain the processes of magmatic accretion and tectonic extension in the area characterized by extensive outcrops of ultramafic rocks at the seafloor close to the fracture zone 60 km the crust is thicker associated with circular or elongated gravity lows bull s eyes axis parallel faults and abyssal hills and no peridotite outcrops consistent with a more homogeneous and magmatic crust simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry we explain these discrepancies primarily a by the entrapment of melt in the lithosphere at deep levels particularly near the transform b by along axis melt migration away from the transform and c by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite the along axis variations in melt supply lithospheric thickness and alteration of the lithosphere can result in a change of rheological structure and therefore in fault pattern as observed along the ridge axis identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor these detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere the origin and evolution of these structures is yet unconstrained but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere and that once formed they localize strain due to the presence of weak serpentinites along the fault planes

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

spatial and temporal variations in crustal production at the mid atlantic ridge 25 n 27 30 n and 0 27 ma we use high resolution multibeam bathymetry shipboard gravity side scan sonar images and magnetic anomaly data collected on conjugate flanks of the mid atlantic ridge at 25 n 27 30 n and out to 27 ma crust to investigate the crustal evolution of the ridge substantial variations in crustal structure and thickness are observed both along and across isochrons along isochrons within spreading segments there are distinct differences in seafloor morphology and gravity derived crustal thickness between inside and outside corners inside corners are associated with shallow depths thin crust and enhanced normal faulting while outside corners have greater depths thicker crust and more limited faulting across isochrons systematic variations in crustal thickness are observed at two different timescales one at 2 3 myr and another at 10 myr and these are attributed to temporal changes in melt supply at the ridge axis the shorter term variations mostly are in phase between conjugate ridge flanks although the actual crustal thickness can be significantly different on the two flanks at any given time we observe no correlation between crustal thickness and spreading rate thus during periods of low melt supply tectonic extension must increase to accommodate the full plate separation rate this extension commonly is concentrated in long lived faults on only one side of the axial valley resulting in strong across axis asymmetries in crustal thickness and seafloor morphology the thin crust flank has few volcanic features and exhibits elevated blocky topography with large offset often irregular faults while the conjugate thicker crust flank shows shorter offset regular faulting and common volcanic features the variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle

influence of temperature pressure and fluid salinity on the distribution of chlorine into serpentine minerals abstract serpentinization produces serpentine minerals that have abundant water and fluid mobile elements e g ba cs and cl the dehydration of serpentine minerals produces chlorine rich fluids that may be linked with the genesis of arc magmas however the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained we performed serpentinization experiments at 80 500 c and pressures from vapor saturated pressures to 20 kbar on peridotite orthopyroxene and olivine with

the 1996 earthquakes in sulawesi indonesia we study the rupture process of the 1 january mw 7 9 16 july mw 6 6 and 22 mw 7 july 1996 earthquakes in sulawesi indonesia a teleseismic body waveform inversion of very broadband records shows that these events occurred beneath the accretionary prism the main shock was due to a well determined fault with a strike of 53 n and a very shallow dip of 7 from its source time function duration 30 sec and the aftershock distribution we estimate a rupture area of 90 60 km2 and an average slip of 1 80 m the surface displacement computed with our best model fits well the displacement vector at the only available gps station at tomini the tsunami generated by the mainshock had an approximate source radius of 45 km which roughly agrees with the rupture size estimated above the earthquakes took place in a relay zone between the trench and the palu koro transcurrent fault they ruptured a shallow dipping thrust fault which corresponds to the subduction interface under the north sulawesi arm the slip vectors of all the events have a nnw orientation parallel to the direction of convergence between the north sulawesi arm and the celebes sea among the peculiarities of the rupture process we found that the july 22 aftershock was the only event to have a well defined precursor 1 8 sec before the main p wave onset

direct geologic evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n comment and reply comment macleod et al 2002 1 made a strong case that subhorizontal corrugated surfaces adjacent to the slowly spreading mid atlantic ridge near 15 45 n are exhumed fault surfaces that have been uncovered by normal faulting associated with oceanic lithosphere divergence see also cann et al 1997

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

controls on the location of compressional deformation on the nw european margin the distribution of cenozoic compressional structures along the nw european margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling and with the distribution of high velocity lower crust and or partially serpentinized upper mantle detected by seismic experiments only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle notably partially serpentinized upper mantle is well documented beneath the central part of the southern rockall basin but compressional features are sparse in that area where compressional structures have formed but the upper mantle is not serpentinized simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling the presence of pre existing weak zones associated with the properties of the gouge and overpressure in fault zones and local stress magnitude and orientation are important contributing factors

a very broad band inversion method for the recovery of earthquake source parameters abstract waveform studies of the earthquake source have traditionally been performed in narrow bands of the seismic spectrum waveform matching of p and sh waves emphasizes frequencies near and above the corner frequency normal mode and surface wave analyses on the other hand are performed on data which contain information only below the corner frequency a new method is presented which formally includes the result of a long period centroid moment tensor analysis in an inversion of broad band body wave phases a parametrized source model is used which allows for the recovery of a detailed source time function as well as the direction and magnitude of rupture propagation the technical details and the performance of the method are demonstrated in an analysis of the october 28 1983 borah peak idaho earthquake in particular it is shown how the combination of the long period and broad band data sets in the inversion improves the resolution of both the focal mechanism and the scalar moment the very broad band method is applied in an analysis of the september 19 1985 michoacan m w 8 0 earthquake two concentrations of moment release are identified in the source time function separated by 25 30 s in time the spatial distribution of moment is well explained both by a model of unilateral rupture towards the east with a propagation velocity of 2 4 km s 1 and by two point sources separated by 95 km it is shown that the p wave data are largely insensitive to the scalar moment and that source time functions obtained without consideration of the total moment release exaggerate the heterogeneity of slip on the fault plane source time functions which do contain the total moment show that a large fraction of the moment was released smoothly over the whole duration 80 s of the event the results suggest that the fault plane was characterized by two regions of high stress drop but that a large portion of slip occurred on surrounding weak portions of the plane

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

an integrated approach for identifying aquifers in transcurrent fault systems the najd shear system of the arabian nubian shield groundwater potentials were assessed for major transcurrent fault systems in basement complex terrains using the najd transcurrent shear system of the arabian nubian shield as a test case spatial analysis within a geographical information system environment of relevant data sets field remote sensing geophysical and digital elevation revealed a spatial correlation between existing well locations and distribution of najd transcurrent shear system structural elements i e faults shear zones melange dykes within highly fractured domains suggesting a causal relationship specifically at 1 intersection of najd brittle shear zones and fault systems 2 intersections of two or more fault systems 3 intersections of najd related dykes with the main streams within catchment areas and 4 within tectonic melange for each of the former aquifer types aquifer specific field and satellite based characteristics were identified and used in conjunction with complementary data e g watershed morphology precipitation soil moisture to target aquifers of similar settings postulated well locations were then successfully tested and refined using very low frequency technologies results could potentially highlight the important role of major transcurrent fault systems in providing settings conducive for hosting and transmitting groundwater and could potentially serve as a replicable model for regional assessment of groundwater potentials in similar tectonic and hydrologic settings

new paleomagnetic constraints on the cenozoic tectonic evolution of the north arm of sulawesi indonesia abstract extensive paleomagnetic sampling 34 sites was carried out in cenozoic formations of the north arm of sulawesi between 120 and 124 e consistent results obtained between 120 and 122 e suggest the existence of a unique post miocene clockwise rotation of about 20 25 of the western part of the north arm this rotation probably occurred during the northward drift of the north arm along the palu matano sinistral transcurrent fault between 122 5 and 124 e both clockwise and counterclockwise rotations with various amplitudes from 6 to 85 are encountered and these very likely correspond to individual microblock rotations due to pervasive deformation along shear zones this result is consistent with the complex fault system of the gorontalo and kotamobagu shear zones that have been recognized on radar imagery and by spot image analyses

drilling constraints on lithospheric accretion and evolution at atlantis massif mid atlantic ridge 30 n expeditions 304 and 305 of the integrated ocean drilling program cored and logged a 1 4 km section of the domal core of atlantis massif postdrilling research results summarized here constrain the structure and lithology of the central dome of this oceanic core complex the dominantly gabbroic sequence recovered contrasts with predrilling predictions application of the ground truth in subsequent geophysical processing has produced self consistent models for the central dome the presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100 220 kyr and not as a single magma pulse isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth below 800 m alteration is restricted to narrow zones surrounding faults veins igneous contacts and to an interval of locally intense serpentinization in olivine rich troctolite hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range deformation of the sequence was remarkably localized despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation presumably during unroofing via detachment faulting both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent southern ridge of the massif there the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70 of recovered samples we develop a working model of the evolution of atlantis massif over the past 2 myr outlining several stages that could explain the observed similarities and differences between the central dome and the southern ridge

microearthquake activity lithospheric structure and deformation modes at an amagmatic ultraslow spreading southwest indian ridge segment while nascent oceanic lithosphere at slow to fast spreading mid ocean ridges mor is relatively well studied much less is known about the lithospheric structure and properties at ultraslow mors here we present microearthquake data from a 1 year ocean bottom seismometer deployment at the amagmatic oblique supersegment of the ultraslow spreading southwest indian ridge a refraction seismic experiment was performed to constrain upper lithosphere p velocities and results were used to construct a 1d velocity model for earthquake location earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures frequent earthquake activity extends to 31 km beneath the seafloor indicating an exceptionally thick brittle lithosphere and an undulating brittle ductile transition that implies significant variations in the along axis thermal structure of the lithosphere we observe a strong relation between petrology microseismicity distribution and topography along the ridge axis peridotite dominated areas associate with deepest hypocenters vast volumes of lithosphere that deforms aseismically as a consequence of alteration and the deepest axial rift valley areas of basalt exposure correspond to shallower hypocenters shallower and more rugged axial seafloor focal mechanisms deviate from pure extension and are spatially variable earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges instead the seismicity band sharply terminates to the south immediately beneath the rift boundary considering the deep alteration large steep boundary faults might be present but are entirely aseismic

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

lacode a lagrangian two dimensional thermo mechanical code for large strain compressible visco elastic geodynamical modeling abstract we present lacode lagrangian compressible deformation a matlab solver for the stokes equations for compressible non newtonian visco elastic flow in two dimensions based on a lagrangian formulation of the finite element method the incompressible boussinesq approximation is a widespread assumption in numerical models of lithospheric deformation thus potentially masking a significant contribution of mechanisms linked to volumetric changes that occur in the asthenospheric mantle and the lithosphere lacode employs a compressible formulation of the stokes equations designed to address such volume changing processes first we provide a description of the equations governing the deformation of earth rocks and detailed overview of the algorithm its numerical implementation treatment of the non linearities rising from the compressible formulation and ineleastic deformation and the remeshing algorithm that tracks and transfers the physical fields from a highly distorted mesh to a high quality one lacode is then benchmarked by comparing numerical results to analytical solutions for the bending of a thin elastic beam under a constant uniform load flow around a rigid inclusion rayleigh taylor instability stress build up in a visco elastic maxwell body and couette flow with viscous heating the rayleigh taylor instability test is further used to demonstrate the accuracy of the remeshing algorithm the importance of including volumetric strain for geodynamic processes is illustrated by two numerical experiments i volumetric strain inducing phase changes in amagmatic slow spreading ridges and ii subducting slabs

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

crustal structure of a rifted oceanic core complex and its conjugate side at the mar at 5 s implications for melt extraction during detachment faulting and core complex formation we present results of a seismic refraction experiment which determines the crustal and upper mantle structure of an oceanic core complex occ and its conjugate side located south of the 5 s ridge transform intersection at the mid atlantic ridge the core complex with a corrugated surface has been split by a change in location of active seafloor spreading resulting in two massifs on either side of the current spreading axis we applied a joint tomographic inversion of wide angle reflected and refracted phases for five intersecting seismic profiles the obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness an abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall the observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface the thickness of the high velocity body is constrained by pmp reflections to vary along and across axis between 3 and 5 km the thickest crust is associated with the central phase of detachment faulting at the higher elevated northern portion of the massif beneath the breakaway of the occ the crust is 2 5 km thick and reveals significantly lower velocities this implies that the fault initially exhumed low velocity material overlying the gabbro plutons in contrast crust formed at the conjugate side during occ formation is characterized by an up to 2 km thick seismic layer 2 overlying a 1 7 km thick seismic layer 3 obtained upper mantle velocities range from 7 3 to 7 9 km s 1 and seem to increase with distance from the median valley however velocities of 7 3 7 5 km s 1 beneath the older portions of the occ may derive from deep fluid circulation and related hydrothermal alteration which may likely be facilitated by the subsequent rifting our velocity models reveal a strongly asymmetric velocity structure across the ridge axis associated with the accretion of gabbros into the footwall of the detachment fault and upper crustal portions concentrated at the conjugate side our results do not support a substantial increase in the axial ridge s melt supply related to the final phase of detachment faulting hence the footwall rifting at 5 s may be a generic mechanism of detachment termination under very low melt conditions as predicted by recent numerical models of tucholke et al

a geophysical study of oceanic core complexes and surrounding terrain mid atlantic ridge 13 n 14 n 1 we describe a geophysical study of oceanic core complexes occ and surrounding seafloor on the mid atlantic ridge at 13 n 14 n and off axis to 1 9 myr data include a detailed deep towed side scan sonar magnetic field and bathymetry survey supplemented by concurrent sea surface bathymetry magnetic field and gravity measurements using side scan and bathymetry we infer areas and relative ages of seafloor volcanism revealing a complex pattern of melt accretion across the median valley including close to its walls we estimate tectonic and magmatic extension throughout the area and find that average tectonic extension since chron 2 on plates containing occs is up to three times that on their conjugates deep towed magnetic data reveal asymmetric spreading faster on occ containing plates and crustal magnetization that is highly heterogeneous on a scale of 5 km suggesting that exhumed domes of occs have highly variable lithologies perhaps comprising both serpentinized peridotite and gabbro improved fits to magnetic data are provided by models incorporating 45 of occ footwall rotation an axial zone of normal magnetization of presumed brunhes epoch has highly variable width and amplitude with parts of the ridge axis displaying very low or apparently reversed magnetization gravity requires that occs have dense cores capped by lower density zones several kilometers thick gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of occ formation and show that waxing magmatism may terminate occs

migration of seismicity and earthquake interactions monitored by gps in se asia triple junction sulawesi indonesia 1 global positioning system gps measurements made in sulawesi indonesia from 1992 to 1999 detected coseismic and transient postseismic deformation related to the 1 january 1996 mw 7 9 earthquake on the north sulawesi minahassa trench these motions are superimposed on the long term secular motion 40 mm yr of the left lateral palu fault in central sulawesi and continued for about 1 5 2 years following the earthquake a string of earthquakes of magnitude 6 migrated along the minahassa trench from west to east subsequently two earthquakes of magnitude 6 occurred on or near the palu fault migrating toward the south modeling the increase in coulomb stress generated by the successive earthquakes agrees with the hypothesis of interacting events an unclamping effect possibly due to fluid migration in the palu area is also suggested by the stress computations and the detected gps displacements

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

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

the 1992 cape mendocino earthquake broadband determination of source parameters the april 25 1992 cape mendocino earthquake ms 7 1 has renewed speculation about the nature of subduction along the cascadia subduction zone and the associated seismic hazard this event may represent the first large m 6 thrust event along the entire cascadia subduction zone in historic times last 200 years we analyze long period surface waves and broadband body waves in order to estimate the mainshock source parameters we also examine broadband body waves from the nearby 1991 honeydew earthquake m 6 in order to assess the contributions of both rupture complexity and unmodeled source and receiver structure in the cape mendocino waveforms from both body and surface wave inversions as well as forward modeling of body waves we estimate a best double couple mechanism for the cape mendocino earthquake strike 330 10 dip 12 2 rake 75 15 seismic moment 1 93 1019 n m and mw 6 8 this mechanism contains a significant component of slip in the estimated direction of gorda north america plate convergence although this earthquake had suitable geometry for relieving strain accumulated by gorda north america plate convergence we cannot resolve whether it occurred on the interplate megathrust or on a fault within the overriding accretionary prism we find evidence for southwest offshore rupture toward an azimuth of 240 in addition we find evidence for early aftershocks in both the teleseismically recorded body waves and in locally recorded strong motions we model one aftershock delayed 13 s from the mainshock with a mechanism that is different from the mainshock mechanism but is consistent with the north south trending horizontal compression found offshore within the gorda plate we postulate that this aftershock and two additional large strike slip aftershocks that ruptured the gorda plate within 24 hours of the mainshock were caused by the transfer of stress accumulated across the cascadia subduction zone and accretionary prism far offshore to the gorda plate where it reduced the normal stress across nw se oriented faults triggering failure the complexity of fault interactions near the mendocino triple junction needs to be understood before potential seismic hazards of the southern cascadia subduction zone can be quantified

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

frictional properties of olivine at high temperature with applications to the strength and dynamics of the oceanic lithosphere 1 faulting and brittle deformation of mantle rocks occurs in many tectonic settings such as oceanic transform faults oceanic detachment faults subduction zones and continental rifts however few data exist that directly explore the frictional properties of peridotite rocks improved constraints on the brittle deformation of peridotite is important for a more complete understanding of the rheological properties of the lithosphere furthermore our comparatively detailed understanding of plastic deformation in olivine allows us to explore the possible role of thermally activated intracrystalline deformation mechanisms in macroscopically brittle processes it has been hypothesized and some experimental data indicate that plastic yielding by dislocation glide low temperature plasticity determines the direct effect in the rate and state frictional constitutive formulation plastic flow may also have important implications for the blunting or necking at asperity contacts that influences the time and or displacement dependent friction evolution effect and frictional healing we present results from saw cut experiments on fine grained synthetic olivine fault gouge conducted in a gas medium deformation apparatus in the temperature range of 400 1000 c with 100 mpa confining pressure we conducted velocity stepping tests to explore the rate and temperature dependence of sliding stability we also conducted slide hold slide experiments to investigate the time and temperature dependence of fault zone restrengthening frictional healing the mechanical data and microstructural observations allow us to explore the role of thermally activated processes in frictional sliding the data indicate systematic temperature dependenceof rate and state variables that can be attributed to plastic yielding at grain to grain contacts we explore the implications of such temperature dependent behavior for controlling the base of the seismogenic zone in the oceanic lithosphere and we seek insight into possible mechanistic models for the interactions between fracture and flow that could lead to improved constraints on the strength of the lithosphere

field survey of the east java earthquake and tsunami of june 3 1994 a field survey of the june 3 1994 east java earthquake tsunami was conducted within three weeks and the distributions of the seismic intensities tsunami heights and human and house damages were surveyed the seismic intensities on the south coasts of java and bali islands were small for an earthquake with magnitude m 7 6 the earthquake caused no land damage about 40 minutes after the main shock a huge tsunami attacked the coasts several villages in east java province were damaged severely and 223 persons perished at pancer village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami the relationship between tsunami heights and distances from the source shows that the hatori s tsunami magnitude was m 3 which seems to be larger for the earthquake magnitude but we should not consider this an extraordinary event because it was pointed out by hatori 1994 that the magnitudes of tsunamis in the indonesia philippine region generally exceed 1 2 grade larger than those of other regions

simulation of the submarine landslide tsunami on 28 september 2018 in palu bay sulawesi island indonesia using a two layer model the strike slip earthquake on 28 september 2018 mw 7 5 along the palu koro fault on sulawesi island has raised concerns about the potential impact of tsunamis generated by submarine landslides in palu bay indonesia the horizontal displacement of the palu koro fault generated landslide tsunamis that covered palu bay creating wave related hazards along the coastal area based on the unusual amount of information on this tsunami this study investigated possible sources using the available preliminary data the generation of comparatively small tsunamis by coseismic seafloor deformation is omitted and only tsunamis generated by submarine subaerial landslides are analyzed in this study two layer modeling soil and water based on the shallow water equation was used to simulate the tsunami propagation in the bay with severe moderate and minor impacts the accuracy of the model was validated based on the waveform at the pantoloan tidal gauge and trace data the tsunami heights from a combination of small to large submarine landslides could reach up to 3 0 7 0 m along the palu shores this model focused on studying the effects of the tidal level on coastal inundation in palu bay using the 2018 palu tsunami event as a benchmark scenario to demonstrate the capabilities of the model one result shows that regardless of the tidal level the 2018 palu tsunami which occurred during high tide will always result in flooding with a maximum tsunami height of up to 7 0 m above mean sea level the main results suggest two causes for this tsunami event the tsunami source and the topography first the model requires one large source at the bay entrance to reproduce the arrival time approximately 5 min and the large wave observed at the pantoloan gauge to reproduce the later waves small sources in the bay s1 s6 and minor large sources l2 and l3 are needed second the datum correction for the terrain is changed to improve the accuracy of the water level additionally the removal of buildings from the topography is important to achieve highly accurate flow depths and to obtain an inundation area close to the real situation the impacts along the coastline of palu bay from peak waves can be used to identify tsunami hazards in the area in the future

determining the sula block kinematics in the triple junction area in indonesia by gps summary the point of convergence of the eurasian philippine and australian plates is situated adjacent to the island of sulawesi indonesia the relative plate velocities are estimated by nuvel1 to be 7 to 9 cm yr 1 the complex tectonic mechanism of the triple junction has been studied over a two year period in the course of the geodyssea southeast asian project the gps investigations concentrate on measurements of both the sulawesi eastern indonesia part of the inter regional geodyssea network and a local subnetwork on sulawesi motions derived using data from the subnetwork con rm what the results of the inter regional geodyssea network have suggested that is that current deformation is high and there are distinct deformation domains in the study area on sulawesi the tectonic mechanism of the triple junction has been analysed using a rigid microblock model the triple junction area can best be interpreted as a headland of the australian plate de ected by its collision with the philippine plate thereby identifying the driving forces of the current deformation the northern part is dominated by the sula domain which shows clockwise rotation to the south it is connected to the australian plate by an ensemble of microblocks undergoing counterclockwise rotation in addition to the above our tectonic model permits the determination of the local in uence of two large earthquakes m 7 8 1996 january 1 and m 7 0 1996 july 22 on the motion of the station tomini north sulawesi more observations and a denser gps network are planned in order to study the behaviour of the palu koro fault the main fault on the western limit of the sula block

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

the early geodynamic evolution of mars type planets abstract it is not clear whether mars once possessed active tectonics yet the question is critical for understanding the thermal evolution of mars and the origin and longevity of its early dynamo to address these issues we have coupled mantle flow simulations together with parameterized core evolution models to simulate the early evolution of mars like planets and constrain the influence of early mobile lid tectonics on core evolution we have explored a wide parameter suite encapsulating a range of uncertainties in initial conditions rheological parameters and surface strength we present successful models that experience early mobile lid behaviour with a later transition into a stagnant lid mode which reproduce core dynamo histories similar to the magnetic history of early mars

gps compared to long term geologic motion of the north arm of sulawesi geodetic data acquired in the north arm of sulawesi are compared with geologic and palaeomagnetic data from north sulawesi and the celebes sea the objective of this study is to assess whether the long term motion deduced from palaeomagnetic and geological studies is coherent with the extrapolation of the current motions measured by gps palaeomagnetic data that have been collected in the north arm of sulawesi indicate that since 5 ma about 20 to 25o of semi rigid rotation occurred about a pole located north of manado this suggests 200 to 250 km of left lateral displacement along the palu koro fault with a displacement rate of 4 cm year a rather similar displacement on the palu fault is derived using the distribution of the asymmetric magnetic anomalies of the celebes seafloor the distribution implies that 200 to 250 km of oceanic crust was subducted at the north sulawesi trench another marker for the rotation is derived from the opening of the gulf of tomini and the nw migration of the calc alkaline subduction related volcanism the far field observation by gps of 4 cm year of left lateral strike slip motion over 17 months and the 5 year average of 3 4 0 3 cm year on a transect across the palu fault fit well with the geological observations that indicate a motion of 4 to 5 cm year of the north arm we therefore conclude that the current rates deduced from gps measurements approximate the long term rates and may hence be extrapolated over a few million years 1998 elsevier science b v all rights reserved

serpentinization of oceanic peridotites 1 a high sensitivity method to monitor magnetite production in hydrothermal experiments 1 a new method using the magnetic properties of magnetite fe 3 o 4 was developed to monitor experimental serpentinization the saturation remanent magnetization signal jrs was measured during the course of experiments designed to react san carlos olivine mg 0 91 fe 0 09 2 sio 4 with water at 250 to 350 c and 500 bars at the end of the experiments the ratio with saturation magnetization jrs js ratio allowed to convert each successive jrs measurement into an in situ amount of magnetite produced by the serpentinization reaction water weight loss was also measured on the end product to determine the final degree of serpentinization the application of this procedure to a series of experiments performed at 300 c 500 bars for various run duration 9 to 514 days and starting olivine grain size 1 to 150 mm shows a linear relationship between magnetite production and reaction progress this relationship can be safely transposed to other experimental conditions using thermochemical modeling and or the fe content of the product phases we show that this high sensitivity magnetic method is a powerful tool to precisely monitor serpentinization kinetics in fe bearing systems it represents in addition a new indirect mean for monitoring the production of hydrogen which is bound to magnetite production rate citation malvoisin b j carlut and f brunet 2012 serpentinization of oceanic peridotites 1 a high sensitivity method to monitor magnetite production in hydrothermal experiments

a new assessment of the abundance of serpentinite in the oceanic crust the gabbro model of the composition of the oceanic crust has prevailed for more than two decades but serpentinites are common along much of the mid atlantic ridge and elsewhere leading to renewed debate over the abundance of serpentinites in crust created at slow spreading ridges based on data from both oceanic and ophiolite samples it has also been suggested that serpentinites are not distinguishable from meta gabbros and diabases by their seismic properties we have made new statistical analyses of the relationship between vs and vp in oceanic gabbro and diabase samples and serpentinized peridotites and compared them to in situ s wave and p wave velocities derived from expanding spread profiles esps acquired in the atlantic our findings demonstrate 1 that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites but 2 over the range of typical layer 3 seismic velocities the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites and 3 that in situ seismic velocities are in excellent agreement with the properties of the gabbros while the shear velocities in serpentinites are significantly lower partially serpentinized peridotite is certainly present but not abundant in normal oceanic crust

spatial heterogeneity of the mantle wedge structure and interplate coupling in the ne japan forearc region 1 we performed 3 d seismic tomography in the forearc region of the northeastern japan subduction zone using both onshore and offshore seismic station data we obtained the vp vs and vp vs structures around the plate boundary with high spatial resolution the position of the plate boundary as defined by relocated hypocenters coincides with the sharp velocity boundary between the oceanic crust and the mantle wedge the mantle wedge above the coseismic slip area of the 1978 and 2005 off miyagi interplate earthquakes m 7 is characterized by high vp and vs but low vp vs off fukushima however where large earthquakes rarely occur we found a high vp vs anomaly at the tip of the mantle wedge the spatial distribution of serpentinized mantle wedge limits the spatial extent of the strongly coupled area on the plate boundary and thus can explain the difference in seismic activity between the off miyagi and off fukushima regions

plastic deformation and development of antigorite crystal preferred orientation in high pressure serpentinites we have inferred the deformation mechanisms of antigorite by high resolution ebsd mapping of samples from cerro del almirez ultramafic massif betic cordillera se spain textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600 630 c and 1 6 1 9 gpa deformation was followed by static annealing of antigorite at ca 680 c the crystal preferred orientation cpo of antigorite is characterised by a strong alignment of 001 poles normal to the foliation plane and a weaker but clear parallelism between 100 axes and the macroscopic lineation defined by the elongation of magnetite aggregates analysis of misorientations across subgrains shows predominance of 010 rotation axis consistent with activation of the 100 001 slip system however tilt subgrain boundaries subparallel to 100 probably formed by edge dislocations of this system are subsidiary most subgrain boundaries are subparallel to 001 planes they are interpreted as 001 twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins modelling of the evolution of the antigorite cpo using a lower bound approach and considering different deformation regimes and sets of basal and non basal slip systems has shown that intensities of 100 and 010 maxima reflect the relative strength of the antigorite 100 001 and 010 001 systems activation of other basal or non basal slip systems does not change significantly the cpo patterns but results in less concentrated cpo 3d transpression models better reproduce the cpo of natural antigorite serpentinites we propose that the widespread occurrence of strong cpo in high pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on hk0 001 together with the activation of twinning implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge

mantle wedge structure in the miyagi prefecture forearc region central northeastern japan arc and its relation to corner flow pattern and interplate coupling 1 we applied three dimensional 3 d seismic tomography to both active source data and local earthquake data recorded at onshore and offshore seismic stations in the miyagi prefecture forearc region of central northeastern japan to obtain a detailed seismic velocity model of the mantle wedge the velocity model showed a clear boundary that divides the mantle wedge into an oceanward high velocity toe and a landward low velocity region that extends trenchward from beneath currently active arc volcanoes the top of the seaward dipping boundary is about 20 30 km landward from the coastline and its bottom is approximately beneath the coastline we consider that this velocity boundary represents the trenchward limit of corner flow and the high velocity toe represents a stagnant mantle wedge the high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences we also identified a swarm of active microseismicity in the mantle wedge well above the seismogenic plate boundary this activity may be related to dehydration of a detached seamount

fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission abstract three dry silicate rocks gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature these rocks exhibited brittle fracture behavior up to the highest confining pressure the change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission the existence of the high pressure brittle fracture phase is proposed the fracture strength increased with increase of confining pressure the increasing rate of strength was lowered at a value of confining pressure at about 0 8 gpa on gabbro at about 1 0 gpa on dunite and at about 1 5 gpa on eclogite at lower confining pressures than the above value the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased at the higher confining pressures however the acoustic emission rate did not increase rapidly before final fracture and stayed constant to the fracture the similar behavior was shown on the granite studied previously it is interesting that the frictional strength forms the boundary between low and high pressure brittle fracture phases

deformation of antigorite serpentinite at high temperature and pressure abstract deformation experiments were conducted in a griggs type apparatus to constrain the rheology of antigorite serpentinite at high temperatures and pressures samples deformed at temperatures of 400 625 c and pressures of 0 85 1 5 gpa experienced semi brittle deformation resulting in localization on faults at 5 to 25 strain samples deformed at 300 c exhibited transitional behavior the stresses measured for our samples are in good agreement with recent experiments conducted in the deformation dia apparatus however our results indicate that steady state ductile flow is not achieved in contrast with interpretations based on the deformation dia experiments extrapolation of our data to geologic conditions suggests that serpentinites may be weak enough during localized frictional sliding to account for slab wedge decoupling inferred from geologic observations experiments conducted above the thermal stability of antigorite at 700 c and 1 5 gpa show evidence for distributed deformation this observation does not support the hypothesis that intermediate depth earthquakes result directly from dehydration embrittlement in addition samples exhibit velocity strengthening from 300 to 700 c a frictional behavior that inhibits earthquake nucleation

mineral precipitation as a mechanism of fault core growth abstract faults vary in structural style from simple planes to complex systems composed of fault cores and damage zones increased fault complexity results from the interaction of mechanical and chemical processes including fracture growth shear and linkage and mineral dissolution and precipitation although water rock interaction is traditionally associated with fault rock weakening and shear localization we investigate processes of fault core widening by water rock interactions that resulted in quartz precipitation we combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the dixie comstock epithermal gold deposit nevada usa mineralized portions of the fault contain strong thick silicified fault cores and wide weak damage zones with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement dilation and cementation we present a model of fault zone evolution in which the hydrothermal regimes favoring either alteration weakening or precipitation strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems alteration weakening favors localization of the fault into thinner clay rich low permeability fault cores precipitation strengthening promotes thick strong and low permeability fault cores with mineralization embrittlement enhancing transient permeability following coseismic failure

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

very broadband seismic analysis of the 1992 flores indonesia earthquake mw 7 9 the december 12 1992 flores island earthquake m w 7 9 which caused a destructive tsunami is located in the back arc of the transition zone between the sunda and banda arcs a region in active collision with the australian continental margin we study the earthquake using four seismological techniques spanning a period range of 1 1000 s and infer that the flores event may be characterized as a shallow back arc thrust with fault plane strike 70 30 dip 28 10 and rake 80 to 140 linear trade off with strike a rupture consisting of three discrete pulses with a total duration of 70 s centroid time 26 3 s and an estimated moment release between 7 5x10 20 and 8 0x10 20 n m from the slip vector we infer that compressional forces play an important role in the deformation mechanism of the back arc north of flores island body waves constrain the hypocentral depth to be 16 km with rupture mainly propagating updip to shallow depth and bilaterally along strike with most subevents located toward the northeast evidence for rupture directivity from surface wave deconvolutions is inconclusive but the overall source function is consistent with body wave results our body wave finite fault model is in rough agreement with two models proposed to explain tsunami run up observations normal mode modeling of w phase observations at periods longer than 70 s shows that slow slip is negligible from this and the slip distribution determined from the body wave analysis we infer that the seismic source is an unlikely candidate to explain anomalously high tsunami run ups measured on northeastern flores island

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

new evidence for the serpentinization of the palaeozoic basement of southeastern sicily from joint 3 d seismic velocity and attenuation tomography

exhumation of the ultrahigh pressure tso morari unit in eastern ladakh nw himalaya a case study exhumation processes of the ultra high pressure uhp tso morari dome nw himalaya are investigated using structural petrological and geochronological data the uhp tso morari unit is bounded by the low grade metamorphic indus suture zone to the ne and mata unit to the sw three deformation phases d1 d2 and d3 are observed only d3 is common to the uhp unit and the surrounding units in the uhp unit the first deformation phase d1 produced upright folds under eclogitic conditions 20 kbar 580 60 c d1 is overprinted by d2 structures related to a nw se trending open anticline this phase is characterized by blueschist mineral associations and corresponds to the quasi isothermal decompression from a depth of 90 km eclogitic conditions up to 30 40 km the final exhumation phase of the tso morari unit is dominated by tectonic denudation and erosion d3 associated with a slight temperature increase radiochronological analyses indicate that the uhp exhumation process began during the eocene exhumation was fast during d1 d2 and slowed down through d3 in oligocene time the change in the deformation style from d1 d2 to d3 in the tso morari unit coincides with changes in the exhumation rates and in the metamorphic conditions these changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge to the vertical uplift of the tso morari unit across the upper crust

dependence of mid ocean ridge morphology on spreading rate in numerical 3 d models abstract the morphology of natural mid ocean ridges changes significantly with the rate of extension full spreading rate on earth varies over more than one order of magnitude ranging from less than 10 mm yr at the gakkel ridge in the arctic ocean to 170 mm yr at the east pacific rise the goal of this study is to reproduce and investigate the spreading patterns as they vary with extension rate using 3 d thermomechanical numerical models the applied finite difference marker in cell code incorporates visco plastic rheology of the lithosphere and a crustal growth algorithm the evolution of mid ocean ridges from nucleation to a steady state is modelled for a wide range of spreading rates with increasing spreading rate four different regimes are obtained a stable alternating magmatic and amagmatic sections 10 mm yr b transient features in asymmetrically spreading systems 20 mm yr c stable orthogonal ridge transform fault patterns 40 mm yr and d stable curved ridges 60 mm yr modelled ultraslow and slow mid ocean ridges share key features with natural systems abyssal hills and oceanic core complexes are the dominant features on the flanks of natural slow spreading ridges numerically very similar features are produced both generated by localised asymmetric plate growth controlled by a spontaneous development of large offset normal faults detachment faults asymmetric accretion in our models implies a lateral migration of the ridge segment which might help explaining the very large offsets observed at certain transform faults in nature

teleseismic body wave analysis of the 9 october 1995 mw 8 0 colima jalisco mexico earthquake and its largest foreshock and aftershock the source process of the october 9 1995 colima jalisco mexico earthquake mw 8 0 and its largest foreshock and aftershock were determined from teleseismic body waves using a least squares inversion scheme the three events are shallow dipping thrust fault earthquakes in agreement with the relative plate motions for rivera north america and cocos north america plate boundaries neither the foreshock nor the largest aftershock show significant differences in focal mechanism nor centroidal depth with respect to the average mechanism of the mainshock the rupture of the mainshock began near the foreshock hypocenter and propagated with a variable rupture velocity towards the nw the source time function for this event shows four distinct episodes of energy release about 50 of the moment was released at the northern end of the rupture between 90 and 110 km from the epicenter this rupture geometry is in agreement with crustal deformation observed inland using gps measurements however the scalar seismic moment of the main shock obtained in this study mo 1 84 1020 is smaller with respect to the lower frequency cmt results reported by harvard mo 1 15 1021

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

magnetic signatures of serpentinization at ophiolite complexes we compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes pindos greece oman chenaillet france and the mid atlantic ridge near the kane fracture zone mark the pindos and oman sites show low susceptibility and remanence k 250 3008c occurring at the axis i e chenaillet similar to serpentinites from magmatically poor mid ocean ridges from lower temperature serpentinization 200 2508c likely occurring off axis and possibly during obduction i e pindos and oman at both settings serpentinization can result in significant hydrogen release

the sulu sea a back arc basin setting within a neogene collision zone abstract the southeast sulu sea is a small oceanic basin that opened in middle miocene time in a back arc setting along the sundaland margin during subduction of the celebes sea plate beneath the sulu arc this back arc extension post dates compressive tectonism present in this back arc area in early middle miocene time progressive and still incomplete closing of this basin initiated in late miocene time is the result of collision of exotic terranes with the sundaland margin this collision also induced flipping of the sulu subduction zone the neogene geodynamic evolution of the sulu sea provides a modern example of how young oceanic crust can be incorporated within an orogenic belt

structure and rifting evolution of the northern newfoundland basin from erable multichannel seismic reflection profiles across the southeastern margin of flemish cap we present the results from processing and interpreting five lines from the 1992 erable multichannel seismic reflection experiment extending from the southeastern margin of flemish cap into the northern newfoundland basin these profiles reveal significant along strike variations in the rifting styles experienced by flemish cap in the southwest a 100 km wide transition zone is identified between thinned continental crust and thin oceanic crust similar to the conjugate galicia bank and iberian margins this transition zone contains a section of deep basement adjacent to a series of shallower ridges and is interpreted as exhumed serpentinized mantle along strike towards the northeast this transition zone pinches out completely within 100 km and is replaced by thin oceanic crust directly adjacent to thinned continental crust by interpreting nearby seismic profiles and profiles on the conjugate margins using the same classification criteria we construct regional maps of the distribution of crustal domains on both sides of the north atlantic these maps reveal significant variations in rifting style on the conjugate margins and along strike of each margin and also highlight the role of ancient transfer zones in compartmentalizing these rifting variations into four distinct regions we propose that the limited localization of shallow topographically high serpentinized peridotite ridges on the newfoundland iberia and flemish cap galicia bank conjugate margins was directly related to an increase in the rate of extension following the separation of flemish cap and galicia bank which exhumed deeper less serpentinized mantle

serpentine minerals discrimination by thermal analysis this paper reports a complete set of tg dtg and dta data coupled with emitted gas analysis for well constrained almost pure serpentine samples serpentine dehydroxylation takes place between 550 and 800 c with dtg and dta peak temperatures progressively decreasing from antigorite 720 and 715 c respectively to lizardite 708 and 714 c polygonal serpentine 685 and 691 c and chrysotile 650 and 654 c antigorite has an additional diagnostic signal at 740 760 c always absent in the other serpentines and dependent on antigorite superperiodicity t shift of 20 c from 36 to 49 a modulation wavelength a sharp exothermic peak occurs at extremely constant temperatures 820 c independently from the starting serpentine structure the high t mineral assemblage is always represented by forsterite and enstatite based on the observed relationships between serpentine structures and dtg dta dehydroxylation temperatures thermal analysis may represent a useful tool for serpentine identification particularly in the case of natural massive samples where different varieties are mutually intermixed the accurate definition of serpentine mineralogy would have obvious implications in both geological petrological and health related issues

ultramafic and gabbroic exposures at the mid atlantic ridge geological mapping in the 15 n region abstract the outcrops of mantle derived ultramafic rocks in the 15 n region are the most extensive yet reported for the mid atlantic ridge north of the fifteen twenty fracture zone these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall ultramafic rocks also crop out extensively south of the fifteen twenty fracture zone based on dredging and on a morphological analysis of the bathymetric map we propose that ultramafic outcrops may be common in the crust formed between 14 30 n and 15 50 n during at least the past 2 4 m y moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops diving observations also show that the ultramafic rocks are capped in stratigraphic contact by a thin layer of basalt this suggests that these rocks were tectonically emplaced at the axial seafloor or very close to it then uplifted in the footwall of the faults that bound the axial valley the occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region instead of one master shear zone there would be a complex array of cross cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds

monitoring of the palu koro fault sulawesi by gps years of gps measurements across the palu koro fault sulawesi indonesia state left lateral strike slip 3 4 cm yr with a small normal component 0 4 cm yr the measurements on intermediate points of the transect show that the fault is locked at a depth estimated around 8 16 km by a very simple model the best measured baseline linking the two most separate sites watatu and toboli shows very interestingly a constant rate of 2 6 cm yr from 1992 to 1995 and an increased rate of 6 3 cm yr since then this increase is due to an earthquake on january 1 1996 in the minahasa trench presumably affecting station toboli s position by 2 cm of co seismic and 2 cm of post seismic displacement which is added to its long term velocity we interpret this phenomenon as additional stress loading on the palu fault driven by the slip during the earthquake 100 km to the north east

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

scardec a new technique for the rapid determination of seismic moment magnitude focal mechanism and source time functions for large earthquakes using body wave deconvolution accurate and fast magnitude determination for large shallow earthquakes is of key importance for post seismic response and tsumami alert purposes when no local real time data are available which is today the case for most subduction earthquakes the first information comes from teleseismic body waves standard body wave methods give accurate magnitudes for earthquakes up to mw 7 7 5 for larger earthquakes the analysis is more complex because of the non validity of the point source approximation and of the interaction between direct and surface reflected phases the latter effect acts as a strong high pass filter which complicates the magnitude determination we here propose an automated deconvolutive approach which does not impose any simplifying assumptions about the rupture process thus being well adapted to large earthquakes we first determine the source duration based on the length of the high frequency 1 3 hz signal content the deconvolution of synthetic double couple point source signals depending on the four earthquake parameters strike dip rake and depth from the windowed real data body wave signals including p pcp pp sh and scs waves gives the apparent source time function stf we search the optimal combination of these four parameters that respects the physical features of any stf causality positivity and stability of the seismic moment at all stations once this combination is retrieved the integration of the stfs gives directly the moment magnitude we apply this new approach referred as the scardec method to most of the major subduction earthquakes in the period 1990 2010 magnitude differences between the global centroid moment tensor cmt and the scardec method may reach 0 2 but values are found consistent if we take into account that the global cmt solutions for large shallow earthquakes suffer from a known trade off between dip and seismic moment we show by modelling long period surface waves of these events that the source parameters retrieved using the scardec method explain the observed surface waves as well as the global cmt parameters thus confirming the existing trade off for some well instrumented earthquakes our results are also supported by independent studies based on local geodetic or strong motion data this study is mainly focused on moment determination however the scardec method also informs us about the focal mechanism and source depth and can be a starting point to study systematically the complexity of the stf

paleomagnetic evidence for clockwise rotation of the northern arm of sulawesi indonesia abstract paleomagnetic results from the northern arm of sulawesi show that the arm has been subjected to a clockwise rotation of more than 90 and that its rotational motion began no later than the middle miocene the mean direction showing a normal polarity at the eocene to the early miocene is d 98 0 and i 6 9 a declination value of d 50 1 obtained from miocene rocks indicates a transition stage of the rotational motion the datum from plio pleistocene volcanics is d 4 6 and i 9 3 this suggests that the rotational motion terminated before the initiation of volcanic activity during the plio pleistocene

structural and petrological constraints on the tectonic evolution of the garnet lherzolite facies higashi akaishi peridotite body sanbagawa belt sw japan 1 the higashi akaishi peridotite body in the sanbagawa metamorphic belt is a unique example of a kilometer scale body including garnet lherzolite facies rocks from an oceanic subduction type orogen in this body four distinct deformational phases d1 d2 d3 and d4 can be defined the tectonic significance of d1 is unclear microstructural observations and application of garnet orthopyroxene geothermobarometry suggest that d2 took place during high temperature subduction up to depths of 100 km from 2 3 gpa to above 2 8 gpa at temperature of 700 800 c or more d3 represents a major phase of exhumation after exhumation the higashi akaishi body became juxtaposed with the adjacent besshi and eclogite units at a depth of around 35 km 1 gpa the common history of the two units after d3 is shown by the intertectonic growth of tremolite between d3 and d4 that corresponds to a stage of intertectonic growth of plagioclase in the besshi and eclogite units both the spatial distribution and the estimated pressure temperature p t conditions of this intertectonic phase of mineral growth in the different units are in excellent agreement the development of an antigorite schistosity during the late stage of d2 indicates an anticlockwise p t path with cooling at depths of around 100 km the association of the antigorite bearing fabric and the onset of exhumation after the d2 stage suggest that the density decrease due to antigorite formation may have been sufficient to trigger the exhumation of dense ultramafic body from uhp conditions in subduction zones syn d2 water introduction and the anticlockwise p t path are expected when convection in the mantle wedge brings peridotite toward a subduction boundary through mantle wedge or when peridotite is dragged down along a subduction boundary that is progressively cooling immediately after initiation of subduction

miocene to recent extension in nw sulawesi indonesia the malino metamorphic complex mmc in the western part of the north arm of sulawesi indonesia has previously been suggested to be a metamorphic complex exhumed in the early middle miocene this idea was based on limited k ar and 40ar 39ar age data but no structural data were presented to provide evidence for the mechanism of exhumation here we present new field observations micro structural analyses and a revised stratigraphy of nw sulawesi based on new age data to provide better constraints on the timing and mechanism of exhumation the data presented here suggest that the mmc is a metamorphic core complex which underwent lithospheric extension during the early middle miocene although the mmc experienced significant extension there is no evidence that it was exhumed during this time there is no contact between the mmc and the pliocene ongka volcanics contradicting a previously inferred unconformable contact pliocene undeformed granitoids intruding the mmc indicate the complex was still at depth during their emplacement furthermore pliocene and pleistocene cover sequences do not contain metamorphic detritus a second phase of extensional uplift was accommodated by brittle faulting from the late miocene pliocene onwards during which the mmc was exhumed this extension is widespread as indicated by synchronous exhumation of the adjacent palu metamorphic complex in west sulawesi and rapid subsidence offshore in gorontalo bay it is linked to northward slab rollback of the southward subducting celebes sea since the pliocene gps data show rapid northward motion of the north arm of sulawesi with respect to the celebes sea indicating that this process is ongoing at present day

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

assessment of scardec source parameters of global large mw 7 5 subduction earthquakes rapid and accurate source characterizations of large shallow subduction earthquakes are key for improved tsunami warning efforts we assess the quality of source parameters of large magnitude mw 7 5 shallow subduction earthquakes of the past 20 yr determined using scardec a recent fully automated broad band body wave source inversion technique for the fast estimation of the moment magnitude depth focal mechanism and source time functions of global events we find that scardec source parameters agree well with those reported in the global centroid moment tensor gcmt catalogue with only the fault dip angle showing a tendency for steeper scardec dip values than gcmt we investigate this discrepancy through independent validation tests of the source models by i testing how well they explain data not used in their construction notably low frequency normal mode data and ii assessing the data fit using 3 d forward modelling tools more sophisticated than those used to build the source models specifically we use a spectral element method for a 3 d earth model we find that scardec source parameters explain normal mode data reasonably well compared to gcmt solutions in addition for the 3 d earth model used in our experiments scardec dip angles explain body wave data similarly or slightly better than gcmt moreover scardec dip angles agree well with results from individual earthquake studies in the literature and with geophysical constraints for different subduction zones our results show that scardec is a reliable technique for rapid determinations of source parameters of large mw 7 5 subduction earthquakes since the scardec method provides realistic source time functions allowing the fast identification of long duration tsunami earthquakes it is complementary to existing methods routinely used for earthquake monitoring and suitable for ocean wide tsunami warning purposes

thermal decomposition of serpentine during coseismic faulting nanostructures and mineral reactions abstract this paper reports a detailed characterization of an antigorite bearing serpentinite deformed at seismic slip rate 1 1 m s in a high velocity friction apparatus micro nanostructural investigation of the slip zone 200 m thick revealed a zonal arrangement with a close juxtaposition of horizons with significantly different strength respectively consisting of amorphous to poorly crystalline phases with bulk anhydrous composition close to starting antigorite and of highly crystalline assemblages of forsterite and disordered enstatite 200 nm in size and in polygonal like nanotextures the slip zone also hosts micro nanometre sized cr magnetite grains aligned at low angle with respect to the slipping surface and inherited from the host serpentinite overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820 1200 c in agreement with flash temperature theory responsible for serpentine complete dehydration and amorphization followed by crystallization of forsterite and enstatite under post deformation static conditions the results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite hosted faults through the seismic cycle

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

microblock rotations and fault coupling in se asia triple junction sulawesi indonesia from gps and earthquake slip vector data the island of sulawesi eastern indonesia is located within the triple junction of the australian philippine and sunda plates and accommodates the convergence of continental fragments with the sunda margin we quantify the kinematics of sulawesi by modeling gps velocities and earthquake slip vectors as a combination of rigid block rotations and elastic deformation around faults we find that the deformation can be reasonably described by a small number of rapidly rotating crustal blocks relative to the sunda plate the southwestern part of sulawesi makassar block rotates anticlockwise at 1 4 myr the northeastern part of sulawesi the bangai sula domain comprises three blocks the central north sula block moves toward the nnw and rotates clockwise at 2 5 myr the northeastern manado block rotates clockwise at 3 myr about a nearby axis and east sulawesi is pinched between the north sula and makassar blocks along the boundary between the makassar block and the sunda plate gps measurements suggest that the trench accommodates 15 mm yr of slip within the makassar strait with current elastic strain accumulation the tectonic boundary between north sula and manado blocks is the gorontalo fault moving right laterally at about 11 mm yr and accumulating elastic strain the 42 mm yr relative motion between north sula and makassar blocks is accommodated on the palu koro left lateral strike slip fault zone the data also indicate a pull apart structure in palu area where the fault shows a transtensive motion and may have a complex geometry involving several active strands sulawesi provides a primary example of how collision can be accommodated by crustal block rotation instead of mountain building

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data