mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

mount logan ice core record of tropical and solar influences on aleutian low variability 500 1998 a d continuous high resolution paleoclimate records from the north pacific region spanning the past 1500 years are rare and the behavior of the aleutian low alow pressure center the dominant climatological feature in the gulf of alaska remains poorly constrained here we present a continuous 1500 year long calibrated proxy record for the strength of the wintertime december march alow from the mount logan summit pr col 5200 m asl ice core soluble sodium time series we show that ice core sodium concentrations are statistically correlated with north pacific sea level pressure and zonal wind speed our alow proxy record reveals a weak alow from circa 900 1300 a d and 1575 1675 a d and a comparatively stronger alow from circa 500 900 a d 1300 1575 a d and 1675 a d to present the mount logan alow proxy record shows strong similarities with tropical paleoclimate proxy records sensitive to the el nino southern oscillation and is consistent with the hypothesis that the medieval climate anomaly was characterized by more persistent la nina like conditions while the little ice age was characterized by at least two intervals of more persistent el nino like conditions the mount logan alow proxy record is significantly p 0 05 correlated and coherent with solar irradiance proxy records over various time scales with stronger solar irradiance generally associated with a weaker alow and la nina like tropical conditions however a step like increase in alow strength during the dalton solar minimum circa 1820 is associated with enhanced walker circulation furthermore rising co2 forcing or internal variability may be masking the twentieth century rise in solar irradiance

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust

slip rate dependent melt extraction at oceanic transform faults crustal thickness differences between oceanic transform faults and associated mid ocean ridges may be explained by melt migration and extraction processes slow slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers indicating relative thin crust in the transform domain whereas at intermediate spreading and fast spreading ridges transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers indicating thick crust in the transform domain we present numerical models reproducing these observations and infer that melt can be extracted at fast slipping transforms but not at slow slipping ones melt extraction is modeled as a three step process 1 melt moves vertically through buoyancy driven porous flow enhanced by subvertical dissolution channels 2 melt accumulates in and travels along a decompaction channel lining a low permeability barrier at the base of the thermal boundary layer 3 melt is extracted to the surface when it enters a melt extraction zone a melt extraction width of 2 4 km and a melt extraction depth of 15 20 km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle our conclusions are supported by the different degrees of magmatic activities exhibited at fast slipping and slow slipping transforms as reflected in geological features geochemical signals and seismic behaviors we also constrain that the maximum lateral distance of crust level dike propagation is about 50 70 km

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

olivine friction at the base of oceanic seismogenic zones 1 we investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault triaxial compression tests were conducted on dry olivine powder grain size 60 m at effective confining pressures between 50 and 300 mpa using argon as a pore fluid temperatures between 600 c and 1000 c and axial displacement rates from 0 06 to 60 m s axial strain rates from 3 10 6 to 3 10 3 s 1 yielding shows a negative pressure dependence consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments a combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes sample strengths were pressure dependent and nearly independent of temperature localized shear zones formed in initially homogeneous aggregates early in the experiments the frictional response to changes in loading rate is well described by rate and state constitutive laws with a transition from velocity weakening to velocity strengthening at 1000 c microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 c consistent with the focal depths of earthquakes in the oceanic lithosphere

variations in earthquake rupture properties along the gofar transform fault east pacific rise mid ocean ridge transform faults experience more foreshocks than continental faults yet the mainshock rarely ruptures the entire fault analysis of seismic data from the gofar transform fault at the east pacific rise indicates that the foreshock region has different material properties from the mainshock region and acts as a barrier to rupture propagation on a global scale seismicity on oceanic transform faults that link mid ocean ridge segments is thermally controlled1 2 however temperature cannot be the only control because the largest earthquakes on oceanic transform faults rupture only a small fraction of the area that thermal models predict to be capable of rupture3 4 5 instead most slip occurs without producing large earthquakes3 4 6 when large earthquakes do occur they often repeat quasiperiodically7 8 moreover oceanic transform faults produce an order of magnitude more foreshocks than continental strike slip faults7 9 here we analyse a swarm of about 20 000 foreshocks recorded on an array of ocean bottom seismometers which occurred before a magnitude 6 0 earthquake on the gofar transform fault east pacific rise we find that the week long foreshock sequence was confined to a 10 km long region that subsequently acted as a barrier to rupture during the mainshock the foreshock zone is associated with a high porosity and undergoes a 3 decrease in average shear wave speed during the week preceding the mainshock we conclude that the material properties of fault segments capable of rupturing in large earthquakes differ from those of barrier regions possibly as a result of enhanced fluid circulation within the latter we suggest that along strike variations in fault zone material properties can help explain the abundance of foreshocks and the relative lack of large earthquakes that occur on mid ocean ridge transform faults

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

implications of fault constitutive properties for earthquake prediction the rate and state dependent constitutive formulation for fault slip characterizes an exceptional variety of materials over a wide range of sliding conditions this formulation provides a unified representation of diverse sliding phenomena including slip weakening over a characteristic sliding distance dc apparent fracture energy at a rupture front time dependent healing after rapid slip and various other transient and slip rate effects laboratory observations and theoretical models both indicate that earthquake nucleation is accompanied by long intervals of accelerating slip strains from the nucleation process on buried faults generally could not be detected if laboratory values of dc apply to faults in nature however scaling of dc is presently an open question and the possibility exists that measurable premonitory creep may precede some earthquakes earthquake activity is modeled as a sequence of earthquake nucleation events in this model earthquake clustering arises from sensitivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and assigns physical interpretation to aftershock parameters the seismicity formulation predicts large changes of earthquake probabilities result from stress changes two mechanisms for foreshocks are proposed that describe observed frequency of occurrence of foreshock mainshock pairs by time and magnitude with the first mechanism foreshocks represent a manifestation of earthquake clustering in which the stress change at the time of the foreshock increases the probability of earthquakes at all magnitudes including the eventual mainshock with the second model accelerating fault slip on the mainshock nucleation zone triggers foreshocks

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

upper mantle earthquakes beneath the arafura sea and south aru trough implications for continental rheology 1 the extent and controls of long term elastic strength and seismicity in the upper continental lithospheric mantle uclm are controversial topics in continental tectonics one key issue is the scarcity of uclm earthquakes even where the uclm is likely to be colder than 600 c the rarity of these earthquakes could be because the uclm generally relatively hydrous causing it to deform aseismically even when colder than 600 c unless it is deforming at exceptionally high strain rates alternatively the uclm could be relatively anhydrous and potentially seismogenic at temperatures below 600 c in which case the rarity of uclm earthquakes may be because areas where the uclm is colder than 600 c have such a thick seismogenic layer and such a cool mantle root that they deform exceptionally slowly the identification and study of uclm earthquakes allows us to distinguish between these possibilities here we show that two earthquakes occurred in the uclm beneath the epicontinental arafura sea both earthquakes occurred where the uclm is probably cooler than 600 c and one of these earthquakes lies 25 km below the moho in an region where there is no evidence of unusually high strain rates there at least it is probable that the uclm is relatively anhydrous and seismogenic at temperatures below 600 c we also find evidence that regions where the uclm is colder than 600 c also have a seismogenic lower crust this results in a single extremely strong layer comprising the entire crust and the uclm down to the 600 c isotherm

constructing constitutive relationships for seismic and aseismic fault slip for the purpose of modeling natural fault slip a useful result from an experimental fault mechanics study would be a physically based constitutive relation that well characterizes all the relevant observations this report describes an approach for constructing such equations where possible the construction intends to identify or at least attribute physical processes and contact scale physics to the observations such that the resulting relations can be extrapolated in conditions and scale between the laboratory and the earth the approach is developed as an alternative but is based on ruina 1983 and is illustrated initially by constructing a couple of relations from that study in addition two example constitutive relationships are constructed these describe laboratory observations not well modeled by ruina s equations the unexpected shear induced weakening of silica rich rocks at high slip speed goldsby and tullis 2002 and fault strength in the brittle ductile transition zone shimamoto 1986 the examples provided as illustration may also be useful for quantitative modeling

towards a general state variable constitutive relation to describe granular deformation triaxial compression experiments were performed on cylindrical quartz sand packs to explore the universality of statevariable constitutive relations used to describe granular deformation samples were subjected to relaxation creep experiments similar to slide hold slide tests from friction studies for relaxation creep tests a constant rate of axial shortening was interrupted by time intervals for which the applied shortening rate was set to zero during holds loadforce decayed exponentially with time and the total stress relaxation scales with the logarithm of hold time consistent with friction tests on reloading after holds stress monotonically increases to the pre hold steady state levels without displaying the restrengthening of static friction often observed from shear tests on granular layers we find that the existing rate and state variable friction laws can model the transient strength behavior during relaxation creep and subsequent reloading by using simple assumptions about the deformation of cylindrical sand packs in triaxial compression by analogy with the rate and state friction constitutive relations we propose a generalized state variable expression for granular deformation in simple and pure shear although additional experiments under a variety of load boundary geometries and load paths will be needed such a general constitutive relation may ultimately describe granular deformation under a broad range of conditions e g for large variations in loading rate shear stress normal stress mean stress and may be applicable to a variety of research problems that involve deformation within granular media d 2005 elsevier b v all rights reserved

a microphysical interpretation of rate and state dependent friction for fault gouge the evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip nature of earthquake stress drop and earthquake nucleation laboratory based rate and state dependent friction rsf laws can describe changes in fault strength during slip but the connections between fault strength and the mechanisms that dictate the mode of failure from aseismic creep to earthquake rupture remain poorly understood the empirical nature of rsf laws remains a drawback to their application in nature here we analyze an extensive data set of friction constitutive parameters with the goal of illuminating the microphysical processes controlling rsf we document robust relationships between 1 the initial value of sliding or kinetic friction 2 rsf parameters and 3 the time rates of frictional strengthening aging we derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by 1 an activation energy that controls the rate of asperity growth by plastic creep and 2 an inverse relationship between material hardness and the activation volume of plastic deformation collectively our results illuminate the physics expressed by the rsf parameters and which describe the absolute value of frictional strength and its dependence on time and slip rate moreover we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain scale asperity contacts

shear enhanced compaction during non linear viscous creep of porous calcite quartz aggregates abstract in this paper we extend the previous studies of semi brittle flow of synthetic calcite quartz aggregates to a range of temperatures and effective pressures where viscous creep occurs triaxial deformation experiments were performed on hot pressed calcite quartz aggregates containing 10 20 and 30 wt quartz at confining pressure of 300 mpa pore pressures of 50 290 mpa temperatures of 673 1073 k and strain rates of 3 0 10 5 s 8 3 10 5 s and 3 0 10 4 s starting porosity varied from 5 to 9 we made axial and volumetric strain measurements during the mechanical tests pore volume change was measured by monitoring the volume of pore fluid that flows out of or into the specimen at constant pore pressure yield stress increased with decreasing porosity and showed a dependence on effective pressure thus the yield stress versus effective pressure can be described as a yield surface with negative slope that expands with decreasing porosity and increasing strain hardening gradually approaching the envelope of strength at 10 strain which has a positive slope creep of porous rock can be modeled to first order as an isolated equivalent void in an incompressible nonlinear viscous matrix an incremental method is used to calculate the stress strain curve of the porous material under a constant external strain rate the numerical simulations reproduce general trends of the deformation behavior of the porous rock such as the yield stress decreasing with increasing effective pressure and significant strain hardening at high effective pressure the drop of yield stress with increasing porosity is modeled well and so is the volumetric strain rate which increases with increasing porosity

fracture induced pore fluid pressure weakening and dehydration of serpentinite abstract we investigate the strength deformation processes and pore fluid weakening during localized shear of antigorite serpentine recent work has shown that some phyllosilicates including antigorite undergo a reverse transition from ductile to localized deformation at the pressure temperature conditions of deep slow slip and tremor in subduction zones here we investigate the processes that lead to and occur during localized deformation because high pore fluid pressure is hypothesized to control the location and style of fault slip at these conditions we investigate the role of pore fluids on these deformation processes we present the results of undrained general shear experiments on antigorite rich serpentinite deformed to varying strains at 500 c 1 gpa pressure and with 0 to 2 wt added pore water at all fluid conditions the serpentinite exhibits strain hardening during distributed deformation and subsequent strain weakening associated with the formation of a prominent shear fracture zone the magnitude of strain weakening correlates with increasing pore water content we evaluate two end member scenarios for how the effective stress influences strength during localized deformation and find that either an increase in fluid pressure or increase in the parameter in the effective stress law can explain the weakening at all fluid conditions we also find evidence for localized dehydration of antigorite within the fracture zones at pressures and temperatures where antigorite is considered stable although the extent of the reaction did not measurably affect fault strength in our experiments at the time scales of in situ deformation in the earth reaction weakening and associated pore fluid pressurization may occur

dependence of seismic coupling on normal fault style along the northern mid atlantic ridge while normal faults are essential in shaping the seafloor formed at slow spreading mid ocean ridges information on their behavior on short seismic cycle time scales is limited here we combine catalogs of hydro acoustically and teleseismically recorded earthquakes to characterize the state of seismic coupling along the northern mid atlantic ridge mar between 12 and 35on along this portion of the mar axis tectonic extension is either taken up by steep conjugate faults that outline well defined ridge parallel abyssal hills or dominantly by a large offset detachment fault on one side of the axis we investigate variations in seismicity and seismic moment release rates across thirty ridge sections that can be clearly characterized either as abyssal hill or detachment bearing we find that detachment bearing sections are associated with significantly greater seismicity and moment release rates than abyssal hill bearing sections but show variability that may reflect the along axis extent of individual detachment faults overall the measured seismic moment release rates fail to account for the long term fault slip rates this apparent seismic deficit could indicate a mixed mode of fault slip where earthquakes only account for 10 30 of offset build up at abyssal hill faults while the rest is accommodated by some form of transient aseismic creep we find this seismic coupling fraction to be significantly greater 40 60 at individual detachment systems which is somewhat at odds with the common inference that detachment faults can sustain long lived localized strain because they are weak we therefore propose alternative interpretations for seismic coupling based on dynamic friction theory this article is protected by copyright all rights reserved

stress rotation across the cascadia megathrust requires a weak subduction plate boundary at seismogenic depths abstract the mendocino triple junction region is the most seismically active part of the cascadia subduction zone the northward moving pacific plate collides with the subducting gorda plate causing intense internal deformation within it here we show that the stress field rotates rapidly with depth across the thrust interface from a strike slip regime within the subducting plate reflecting the pacific plate collision to a thrust regime in the overriding plate we utilize a dense focal mechanism dataset including observations from the cascadia initiative ocean bottom seismograph experiment to constrain the stress orientations to quantify the implications of this rotation for the strength of the plate boundary we designed an inversion that solves for the absolute stress tensors in a three layer model subject to assumptions about the strength of the subducting mantle our results indicate that the shear stress on the plate boundary fault is likely no more than about 50 mpa at 20 km depth regardless of the assumed mantle strength we infer a relatively weak megathrust fault with an effective friction coefficient of 0 to 0 2 at seismogenic depths such a low value for the effective friction coefficient requires a combination of high fluid pressures and or fault zone minerals with low inherent friction in the region where a great earthquake is expected in cascadia

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

a rare great earthquake on an oceanic fossil fracture zone first motion solution found in this study and has a strike consistent with the fracture zone north of the bend the southern fault plane with strike 178 d ip 54 and rake 65 has a strike consistent with the portion of the fracture zone to the south of the bend and has a dip which can be explained by the style of deformation that the region is undergoing the centroid moment tensor solution of the broad band model is calculated and found to be consistent with the region of low misfit in mantle wave solution space the broad band solution has a moment of 1 53 10 21 nm mw 8 1 again consistent with the mantle wave data slip propagated bilaterally with an approximate rupture velocity of 3k ms 1 80 per cent of local shear wave speed the rupture front is less well resolved to the south of the epicentre than to the north the majority 75 per cent of moment originated from slip on the northern fault the broad band data requires significant slip below the oceanic moho with as much as 70 per cent of moment due to slip in the brittle uppermost mantle in the preferred model

measuring earthquake source parameters in the mendocino triple junction region using a dense obs array implications for fault strength variations abstract subduction zones produce earthquakes on a set of faults that operate under a wide variety of conditions resulting from considerable variations in depth temperature rock type and fluid pressure these variations likely lead to variation in the stress levels that drives particular earthquakes and that in turn effects the magnitude of seismic shaking they produce in the mendocino triple junction mtj region intraplate faults within the mantle of the subducting plate fail regularly in energetic earthquakes while the adjacent thrust interface of the cascadia subduction zone remains seismically quiet despite the likelihood that it operates at much lower levels of stress and strength in 2012 as part of the cascadia initiative community experiment an ocean bottom seismometer obs array was deployed in the mtj area providing unusually dense data covering both the inter and intra plate earthquakes combining these data with onshore networks we detect and relocate 1137 earthquakes with a three dimensional velocity model we perform detailed spectral and time domain analysis to study variations in earthquake source properties between the different types of faults we observe a wide variability of stress drops and systematic lateral and depth variations in the earthquake source spectra resulting from the different types of tectonic fault systems in this region intraplate faults within the subducted oceanic mantle the mendocino transform plate boundary fault and the thrust interface of the cascadia subduction zone some of the depth variability of source spectra can be explained by the expected increase in rupture velocity with depth however the overall variation in stress drop estimates is consistent with the highest stress drop earthquakes occurring in the depth range predicted by strength envelopes moreover the earthquakes in the vicinity of the thrust interface likely including some within the subducted oceanic crust show clearly lower stress drops and depletion of high frequency radiation for their size compared to events in the oceanic mantle and along the transform fault at a given depth our results suggest that the variability in earthquake source spectra that can be expected in the cascadia subduction zone is systematic and that the thrust interface operates at a lower stress level than the core of the subducted plate a few kilometers away

is the vertebrate defined permian triassic boundary in the karoo basin south africa the terrestrial expression of the end permian marine event the end permian extinction records the greatest ecological catastrophe in earth history the vertebrate fossil record in the karoo basin south africa has been used for more than a century as the standard for understanding turnover in terrestrial ecosystems recently claimed to be in synchrony with the marine crisis workers assumed that systematic turnover at the dicynodon assemblage zone boundary followed by the appearance of new taxa directly above the base of the lystrosaurus assemblage zone is the continental expression of the end permian event and recovery to test this hypothesis we present the first highprecision age on strata close to the inferred permian triassic boundary a u pb isotope dilution thermal ionization mass spectrometry zircon age of 253 48 0 15 ma early changhsingian is from a silicified ash layer 60 m below the current vertebrate defined boundary at old lootsberg pass southern south africa this section yields newly discovered plants and vertebrates and is dominated by a normal polarity signature our collective data suggest that the dicynodon lystrosaurus assemblage zone boundary is stratigraphically higher than currently reported and older than the marine extinction event therefore the turnover in vertebrate taxa at this biozone boundary probably does not represent the biological expression of the terrestrial end permian mass extinction the actual permian triassic boundary in the karoo basin is either higher in the katberg formation or is not preserved the currently accepted model of the terrestrial ecosystem response to the crisis both in this basin and its extension globally requires reevaluation

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

geology of the earthquake source an introduction earthquakes arise from frictional stick slip instabilities as elastic strain is released by shear failure almost always on a pre existing fault how the faulted rock responds to applied shear stress depends on its composition environmental conditions such as temperature and pressure fluid presence and strain rate these geological and physical variables determine the shear strength and frictional stability of a fault and the dominant mineral deformation mechanism to differing degrees these effects ultimately control the partitioning between seismic and aseismic deformation and are recorded by fault rock textures the scale invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake related deformation here we emphasize that the seismological character of a fault is highly dependent on fault geology and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record

scaling relations for seismic cycles on mid ocean ridge transform faults 1 mid ocean ridge transform faults rtfs have thermal structures that vary systematically with tectonic parameters resulting in predictable seismic characteristics and clear seismic cycles we develop a scaling relation for repeat time tr of the largest expected earthquake mc tr 1 2 3cmc1 3at1 4v 1 where is the shear modulus is the stress drop cmc is a constant at is the area above 600 c and v is the slip rate we identify repeating mc earthquakes by measuring differential arrival times of first orbit rayleigh waves to determine centroid offsets between pairs of events comparing our observations of tr 5 14 years for earthquakes on gofar and blanco rtfs with predictions from our scaling relation we can constrain rtf stress drops specific tests of this scaling relation are proposed for earthquakes on blanco gofar discovery and clipperton rtfs which are all expected to have large ruptures in the next few years

simulations of tremor related creep reveal a weak crustal root of the san andreas fault 1 deep aseismic roots of faults play a critical role in transferring tectonic loads to shallower brittle crustal faults that rupture in large earthquakes yet until the recent discovery of deep tremor and creep direct inference of the physical properties of lower crustal fault roots has remained elusive observations of tremor near parkfield ca provide the first evidence for present day localized slip on the deep extension of the san andreas fault and triggered transient creep events we develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near parkfield is consistent with triggered fault creep governed by laboratory derived friction laws between depths of 20 35 km on the fault simulated creep and observed tremor northwest of parkfield nearly ceased for 20 30 days in response to small coseismic stress changes of order 104 pa from the 2003 m6 5 san simeon earthquake simulated afterslip and observed tremor following the 2004 m6 0 parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust these creep responses require very low effective normal stress of 1 mpa on the deep san andreas fault and near neutral stability frictional properties expected for gabbroic lower crustal rock

low temperature plasticity of olivine during high stress deformation of peridotite at lithospheric conditions an experimental study abstract deformation experiments on natural peridotite from the almklovdalen complex norway were carried out in a griggs type apparatus at temperatures of 20 300 and 600 c confining pressures of 1 0 to 2 5 gpa and strain rates of 3 10 6 to 8 10 5 s 1 the experiments yield maximum differential stresses in the range of 1 0 to 2 9 gpa falling mostly between byerlee s law and goetze s criterion and thus indicating semi brittle behaviour whereas strength of samples deformed at 20 c increases significantly with increasing confining pressure a systematic pressure dependence of strength is not obvious at 300 and 600 c the intracrystalline deformation features of the main constituent olivine were analysed by light and electron microscopic techniques sem ebsd tem deformation microstructures systematically vary with temperature but are insensitive to confining pressure samples deformed at 20 c reflect predominantly brittle failure by intragranular microcracks and shear zones microstructures from samples deformed at higher temperatures show evidence of low temperature plasticity of olivine in the form of pronounced undulatory extinction associated with high dislocation densities pile up of dislocations leads to the formation of either fracture arrays at 300 c or deformation lamellae parallel 100 and cellular structures at 600 c indicating intragranular work hardening a gradual increase in glide controlled crystal plastic deformation of olivine at increasing temperature is interpreted to be responsible for the variation in mechanical behaviour and microstructural characteristics the mechanical data and microstructural observations consistently suggest a temperature for the transition from the strength controlling dominance of brittle to crystal plastic deformation mechanisms close to 600 c the tested peridotite samples show a lower strength than quartzite samples at comparable experimental conditions possibly related to crystallographic differences of olivine and quartz the agreement between microfabrics of experimentally and naturally deformed peridotites demonstrates the importance of low temperature plasticity of olivine during high stress deformation at lithospheric conditions related in particular to seismic activity in the mantle

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

laterally propagating slow slip events in a rate and state friction model with a velocity weakening to velocity strengthening transition we investigate the behavior of simulated slow slip events using a rate and state friction model that is steady state velocity weakening at low slip speeds but velocity strengthening at high slip speeds our simulations are on a one dimensional line fault but we modify the elastic interactions to mimic the elongate geometry frequently observed in slow slip events simulations exhibit a number of small events as well as periodic large events the large events propagate approximately steadily along strike and stress and slip rate decay gradually behind the propagating front their recurrence intervals can be determined by considering what is essentially an energy balance requirement for long distance propagation it is possible to choose the model parameters such that the simulated events have the stress drops slip velocities and propagation rates observed in cascadia

seismic rupture on an oceanic continental plate boundary strike slip earthquakes along the queen charlotte fairweather fault abstract we investigate the 2013 m w 7 5 craig alaska earthquake and nearby seismicity to understand better how temperature and composition may control the depth of seismic rupture along a strike slip fault offsetting contrasting lithosphere types the queen charlotte fairweather qcf fault lies between the oceanic lithosphere of the pacific plate and the accreted insular superterrane of the north american continent we use point source and finite fault modeling of teleseismic body waves to characterize the focal mechanism and the depth extent of seismic rupture of five m w 5 9 7 5 earthquakes four of the five earthquakes are consistent with rupture on the qcf fault we find that these four earthquakes have centroid depths between 11 and 18 km 3 km and aftershocks with an order of magnitude less than typical continental earthquakes finite fault modeling of the 2013 craig earthquake favors bilateral rupture along a 150 km fault with a depth range of slip between 5 and 25 km with faster rupture 4 5 km s to the north than the south 1 km s these results suggest that the transition of brittle to ductile deformation along this section of the pacific north american plate boundary is thermally controlled by a more mafic rheology than average continental crust exhibiting behavior consistent with that of an oceanic strike slip fault online material figures of best fitting point source focal mechanism and waveform fits dip sensitivity directivity tests aftershock distributions and finite fault results

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

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

direct measurement of asperity contact growth in quartz at hydrothermal conditions earthquake recurrence requires interseismic fault restrengthening which results from solid state deformation in room temperature friction and indentation experiments in contrast exhumed fault zones show solution transport processes such as pressure solution and contact overgrowths influence fault zone properties in the absence of fluid flow overgrowths are driven by gradients in surface curvature where material is dissolved diffuses and precipitates at the contact without convergence normal to the contact to determine the rate of overgrowth for quartz we conducted single contact experiments in an externally heated pressure vessel convergence was continuously monitored using reflected light interferometry through a long working distance microscope contact normal force was constant with an initial effective normal stress of 1 7 mpa temperature was between 350 and 530 c and water pressure was constant at 150 mpa two control experiments were conducted one dry at 425 c and one bimaterial sapphire at 425 c and 150 mpa water pressure no contact growth or convergence was observed in the controls for wet single phase contacts growth was initially rapid and then decreased with time no convergence was observed fluid inclusions indicate that the contact is not uniformly wetted the contact is bounded by small regions of high aperture reflecting local free face dissolution as the source for the overgrowth the apparent activation energy is 125 kj mol extrapolation predicts rates of contact area increase orders of magnitude faster than in dry room temperature and hydrothermal friction experiments suggesting that natural strength recovery near the base of the seismogenic zone could be dominated by contact overgrowth

a reassessment of outer rise seismicity and its implications for the mechanics of oceanic lithosphere we use body waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope these data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone our results provide new constraints on the faulting of oceanic lithosphere at the outer rise which is important for understanding the transmission of plate driving forces through the subduction system in all cases shallow normal faulting earthquakes are observed at the top of the plate and are separated in depth from any deeper thrust faulting earthquakes no temporal variation associated with large thrust faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise the transition depth from trench normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in plane stresses and bending stresses resulting principally from slab pull combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature we place upper bounds on the strength of the lithosphere at the outer rise which is required to be 300 mpa for a constant yield stress model or governed by an effective coefficient of friction of 0 3

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

the relationship between seismicity and fault structure on the discovery transform fault east pacific rise there is a global seismic moment deficit on mid ocean ridge transform faults and the largest earthquakes on these faults do not rupture the full fault area we explore the influence of physical fault structure including step overs in the fault trace on the seismic behavior of the discovery transform fault 4s on the east pacific rise one year of microseismicity recorded during a 2008 ocean bottom seismograph deployment 24 377 0 ml 4 6 earthquakes and 24 years of mw 5 4 earthquakes obtained from the global centroid moment tensor catalog are correlated with surface fault structure delineated from high resolution multibeam bathymetry each of the 15 5 4 mw 6 0 earthquakes that occurred on discovery between 1 january 1990 and 1 april 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross correlation technique microseismicity was relocated using the hypodd relocation algorithm the western fault segment of discovery dw is composed of three zones of varying structure and seismic behavior a zone with no large events and abundant microseismicity a fully coupled zone with large earthquakes and a complex zone with multiple fault strands and abundant seismicity in general microseismicity is reduced within the patches defined by the large repeating earthquakes while the extent of the large rupture patches on dw correlates with physical features in the bathymetry step overs in the primary fault trace are not observed at patch boundaries suggesting along strike heterogeneity in fault zone properties controls the size and location of the large events

mode of slip and crust mantle interaction at oceanic transform faults oceanic transform faults connecting offset mid ocean spreading centres rupture quasi periodically in earthquakes up to about magnitude m 7 0 that are often preceded by foreshocks in addition to seismic slip a large portion of slip takes place as aseismic creep which likely influences initiation of large earthquakes although oceanic transform faults are one of the major types of plate boundaries the exact mode of slip and interaction between the seismic and aseismic motion remains unclear here we present a detailed model of the mode of slip at oceanic transform faults based on data acquired from a recent temporary deployment of ocean bottom seismometers at the blanco transform fault and existing regional and teleseismic observations in the model the crustal part of the fault is brittle and fully seismically coupled while the fault in the mantle shallower than the depth of the 600 c isotherm creeps partially and episodically the creep activates small asperities in the mantle that produce seismic swarms both mantle and the crustal zones release most of the plate motion strain during large magnitude earthquakes large earthquakes appear to be preceded by a brief episode of shallow mantle creep accompanied by seismic swarms which explains the observation of foreshocks and shows that mantle creep likely influences initiation of large seismic events earthquakes in the crust and mantle at transform faults are distinct yet coupled with seismic swarms in the mantle apparently preceding large earthquakes according to ocean bottom seismic monitoring of the blanco transform fault

micromechanisms of creep in clay rich gouge from the central deforming zone of the san andreas fault we report the strength and constitutive behavior of gouge sampled from the central deforming zone cdz of the san andreas fault layers of flaked cdz gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 10 10s 1 and within an order of magnitude of in situ rates deformation conditions correspond to the in situ effective normal stress 100 mpa and temperature 65 to 120 c at the sampling depth of 2 7 km gouge was sheared dry and with brine pore fluid at 25 mpa pore pressure dry gouge is stronger and more rate strengthening than brine saturated gouge brine saturated cdz gouge strengthens with increasing strain rate and decreasing temperature and the dependencies of strength on strain rate and temperature increase at rates below 5 10 9s 1 at strain rates greater than 5 10 9s 1 the rate dependence is consistent with previous studies on the cdz gouge conducted at even higher rates the increase in rate dependence below 5 10 9s 1 indicates a change in the rate controlling deformation mechanism the magnitude of the friction rate dependence parameter a and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates we hypothesize a micromechanical model for the cdz gouge whereby a transition from fracture and delamination accommodated frictional flow to crystal plasticity accommodated frictional flow occurs with decreasing strain rate

precise relative locations for earthquakes in the northeast pacific region we report that double difference methods applied to cross correlation measured rayleigh wave time shifts are an effective tool to improve epicentroid locations and relative origin time shifts in remote regions we apply these methods to seismicity offshore of southwestern canada and the u s pacific northwest occurring along the boundaries of the pacific and juan de fuca including the explorer plate and gorda block plates the blanco mendocino revere dellwood nootka and sovanco fracture zones host the majority of this seismicity largely consisting of strike slip earthquakes the explorer juan de fuca and gorda spreading ridges join these fracture zones and host normal faulting earthquakes our results show that at least the moderate magnitude activity clusters along fault strike supporting suggestions of large variations in seismic coupling along oceanic transform faults our improved relative locations corroborate earlier interpretations of the internal deformation in the explorer and gorda plates north of the explorer plate improved locations support models that propose northern extension of the revere dellwood fault relocations also support interpretations that favor multiple parallel active faults along the blanco transform fault zone seismicity of the western half of the blanco appears more scattered and less collinear than the eastern half possibly related to fault maturity wemore use azimuthal variations in the rayleigh wave cross correlation amplitude to detect and model rupture directivity for a moderate size earthquake along the eastern blanco fault lastly the observations constrain the seismogenic zone geometry and suggest a relatively narrow seismogenic zone width of 2 to 4 km less

frictional behavior of oceanic transform faults and its influence on earthquake characteristics 1 we use a three dimensional strike slip fault model in the framework of rate and state dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults otfs gabbro friction data under hydrothermal conditions are mapped onto otfs using temperatures from 1 a half space cooling model and 2 a thermal model that incorporates a visco plastic rheology non newtonian viscous flow and the effects of shear heating and hydrothermal circulation without introducing small scale frictional heterogeneities on the fault our model predicts that an otf segment can transition between seismic and aseismic slip over many earthquake cycles consistent with the multimode hypothesis for otf ruptures the average seismic coupling coefficient c is strongly dependent on the ratio of seismogenic zone width w to earthquake nucleation size h c increases by four orders of magnitude as w h increases from 1 to 2 specifically the average c 0 15 0 05 derived from global otf earthquake catalogs can be reached at w h 1 2 1 7 further in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms which is also consistent with observations to match these observations over this narrow range of w h requires an increase in the characteristic slip distance dc as the seismogenic zone becomes wider and normal stress is higher on long transforms earthquake magnitude and distribution on the gofar and romanche transforms are better predicted by simulations using the visco plastic model than the half space cooling model

stability and localization of rapid shear in fluid saturated fault gouge 1 linearized stability analysis field observations of major earthquake fault zones show that shear deformation is often confined to principal slipping zones that may be of order 1 100 m wide located within a broader gouge layer of order 10 100 mm wide this paper examines the possibility that the extreme strain localization observed may be due to the coupling of shear heating thermal pressurization and diffusion in the absence of a stabilizing mechanism shear deformation in a continuum analysis will collapse to an infinitesimally thin zone two possible stabilizing mechanisms studied in this paper are rate strengthening friction and dilatancy for rate strengthening friction alone a linear stability analysis shows that uniform shear of a gouge layer is unstable for perturbations exceeding a critical wavelength using this critical wavelength we predict a width for the localized zone as a function of the gouge properties taking representative parameters for fault gouge at typical centroidal depths of crustal seismogenic zones we predict localized zones of order 5 40 m wide roughly consistent with field and experimental observations for dilatancy alone linearized strain rate perturbations with a sufficiently large wavelength will undergo transient exponential growth before decaying back to uniform shear the total perturbation strain accumulated during this transient strain rate localization is shown to be largely controlled by a single dimensionless parameter e which is a measure of the dilatancy of the gouge material due to an increase in strain rate

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

thermal structure of oceanic transform faults we use three dimensional fi nite element simulations to investigate the temperature structure beneath oceanic transform faults we show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions with the thickest lithosphere located at the center of the transform these earlier studies used simplifi ed rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere we show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology this temperature structure is consistent with a wide range of observations from ridge transform environments including the depth of seismicity geochemical anomalies along adjacent ridge segments and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion

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

a thin elastic core can control large scale patterns of lithosphere shortening abstract peak lithospheric strength should reside in the rocks that under the applied stress cannot either creep due to low temperature or break due to high confining pressure the greatest resistance comes from dry olivine pyroxene rich upper mantle lowermost crust at moho conditions 400 600 c and 1 gpa we have conducted laboratory experiments to investigate the importance of the unbreakable core of the lithosphere in between its brittle and ductile parts and conclude that it can control the large scale lithospheric deformation pattern under shortening regardless of the thickness of the unbreakable core it acts as a restraining layer that is easily flexed but is unstretchable this eliminates large scale brittle faulting or homogeneous thickening as available shortening modes and results in irregular wrinkling of the unbreakable layer we discuss geodynamic implications of our laboratory experiments and advocate studies of large scale buckling of the lithosphere as a relevant shortening mode

experimental deformation of olivine single crystals at lithospheric temperatures 1 rheological properties of mantle minerals and rocks at temperatures t appropriate to much of earth s lithosphere have remained poorly constrained even though past experimental studies on olivine single crystals and polycrystalline aggregates have quantified the high temperature creep mechanisms t 1200 c consequently we have performed deformation experiments on crystals of san carlos olivine at lower temperatures from 900 to 1200 c in triaxial compression along the 101 c direction the experiments were carried out at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus at differential stresses of 100 to 500 mpa several samples were deformed at constant displacement rate and others at constant load in order to provide insight into possible effects of work hardening under the deformation conditions investigated little evidence of work hardening was observed the data follow a power law dependence on stress as in previous high temperature deformation studies the samples were however considerably weaker than predicted by the experimentally determined high temperature constitutive equation for olivine crystals of this orientation from the study of bai et al 1991 the mechanical behavior correlates instead with the weaker of the two mechanisms flow laws that contribute to the high temperature constitutive equation thus our experiments demonstrate that published high temperature constitutive equations overestimate the strength of lithospheric mantle and that the transition to low temperature creep occurs at lower temperatures than previously inferred

two stage red sea rifting inferred from mantle earthquakes in neoproterozoic lithosphere abstract we use earthquake geothermometry measured heat flow and structural constraints from p wave receiver functions to model the thermal evolution of the lithosphere beneath harrat lunayyir we suggest that the lithosphere thinned to its present 60 km thickness in a second stage of lithospheric thinning at 15 12 ma following initial red sea extension at 27 ma harrat lunayyir is an active volcanic field located in the arabian shield 150 km east of the red sea rift axis in the lithospheric mantle beneath harrat lunayyir we locate 64 high frequency earthquakes at depths of 42 48 km all with m l 2 5 these brittle failure earthquakes must have nucleated at relatively low temperatures based upon global maximum nucleation depths and temperature dependent deformation experimental results therefore the mantle earthquakes show that the upper mantle lithosphere is not in thermal equilibrium with the shallow 60 km underlying asthenosphere our thermal modeling indicates that the lithosphere beneath harrat lunayyir thinned to its current 60 km thickness at 12 2 ma as constrained by thermal modeling of 1 surface heat flow 2 the depth to the mid crustal brittle ductile transition and 3 the depth to the upper mantle brittle ductile transition