a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

escape hypothesis for the stikine block comparison of stratigraphic faunal and paleomagnetic characteristics of the stikine terrane of british columbia with other terranes in the cordilleran collage reveals broad similarities with a group of terranes that formed a volcanic belt marginal to north america in paleozoic and early mesozoic time unlike stikine these terranes lie inboard of another belt of terranes that represents an early mesozoic subduction complex or melange belt thus in british columbia the marginal volcanic belt is apparently doubled in the columbia embayment region the marginal volcanic and melange belts are missing and rocks of the outermost major component of the collage the wrangellia superterrane are juxtaposed directly against cratonic rocks we propose that stikine is the missing fragment from the columbia embayment its northward tectonic escape was driven by the early stages of collision of the wrangefia superterrane with north america in middle to late jurassic time the escaped fragment was then trapped between melange and more northerly later arriving parts of the wrangellia superterrane in early to mid cretaceous time

e i corrected paleolatitudes for the sedimentary rocks of the baja british columbia hypothesis paleomagnetic inclinations from sediments of the western terranes of canada are consistently too shallow for their reconstructed paleogeographic positions two contradicting explanations for these discrepancies are 1 terranes have been displaced northward with respect to the stable american craton by several thousands of kilometres between the late cretaceous 75 ma and the eocene 50 ma and 2 sedimentary inclination error has caused a shallow bias in the paleomagnetic directions here we apply the elongation inclination e i method to paleomagnetic data sets from sedimentary rocks of supposedly allochtonous terranes of western north america to correct for inclination flattening our results indicate that the paleomagnetic directions from the continental silverquick sediments 95 92 ma of southern british colombia are not seriously affected by inclination error because the magnetic signal most likely concerns a chemical remanent magnetisation crm in contrast the marine sediments of the nanaimo group 84 72 ma of vancouver island region appear seriously affected by inclination flattening f 0 7 and the e i corrected mean inclinations are about 98 steeper than the original data we arrive at corrected inclinations paleolatitudes of i 578 k 388n for the silverquick and i 558 k 368n for the nanaimo sediments our corrected paleolatitudes indicate that the canadian terranes were indeed located adjacent to the baja californian margin during the late cretaceous thus supporting the baja bc hypothesis d 2005 elsevier b v all rights reserved

a viscoplastic shear zone model for deep 15 50 km slow slip events at plate convergent margins abstract a key issue in understanding the physics of deep 15 50 km slow slip events d sse at plate convergent margins is how their initially unstable motion becomes stabilized here we address this issue by quantifying a rate strengthening mechanism using a viscoplastic shear zone model inspired by recent advances in field observations and laboratory experiments the well established segmentation of slip modes in the downdip direction of a subduction shear zone allows discretization of an interseismic forearc system into the 1 frontal segment bounded by an interseismically locked megathrust 2 middle segment bounded by episodically locked and unlocked viscoplastic shear zone and 3 interior segment that slips freely the three segments are assumed to be linked laterally by two springs that tighten with time and the increasing elastic stress due to spring tightening eventually leads to plastic failure and initial viscous shear this simplification leads to seven key model parameters that dictate a wide range of mechanical behaviors of an idealized convergent margin specifically the viscoplastic rheology requires the initially unstable sliding to be terminated nearly instantaneously at a characteristic velocity which is followed by stable sliding i e slow slip the characteristic velocity which is on the order of

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

slow slip near the trench at the hikurangi subduction zone new zealand the range of fault slip behaviors near the trench at subduction plate boundaries is critical to know as this is where the world s largest most damaging tsunamis are generated our knowledge of these behaviors has remained largely incomplete partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries here we present detailed seafloor deformation observations made during an offshore slow slip event sse in september and october 2014 using a network of absolute pressure gauges deployed at the hikurangi subduction margin offshore new zealand these data show the distribution of vertical seafloor deformation during the sse and reveal direct evidence for sses occurring close to the trench within 2 kilometers of the seafloor where very low temperatures and pressures exist

a cusp catastrophe model of mid long term landslide evolution over low latitude highlands of china abstract based on a model describing a certain landslide case and catastrophe theory we derived a cusp catastrophe model and corresponding inversion method to study mid long term landslide evolution according to data of landslides precipitation and socioeconomic development from 1976 to 2008 the cusp catastrophe model describing this landslide evolution across a low latitude highland area in china is obtained with the least squares method results of the model indicate that human activity determines landslide intensity local precipitation also impacts yearly landslide intensity to some extent and controls the time when a strong and abrupt change in landslides occurs during the period 1976 2008 there was an abrupt decrease of landslide intensity during 1994 1995 and an abrupt increase during 1995 1996 since then there have been frequent landslides in the low latitude highland with greater intensity all these factors provide a scientific basis for formulating a contingency plan regarding landslide disasters

numerical simulations on megathrust rupture stabilized under strong dilatancy strengthening in slow slip region 1 episodic slow slip events sses typically involve a few millimeters to centimeters of slip over several days to months at depths near or further downdip of megathrust seismogenic zones despite its widespread presence in subduction margins it remains unknown how sses interact with the seismogenic zone and affect megathrust ruptures here i construct a 2 d thrust fault model governed by rate state friction to investigate how fault dilatancy influences the amplitude and spatial distribution of coseismic slip afterslip and sses model results illustrate that under strong dilatancy and high pore pressure around the friction stability transition coseismic rupture stops at the onset of sses modeled sses have lower velocities longer recurrence intervals and durations and larger slip amounts as dilatancy becomes stronger demonstrating a transition from short term to long term type of sse behavior these results qualitatively explain the range of spatial distributions of sses and megathrust ruptures observed or inferred in natural subduction zones furthermore the relative depths of sses and megathrust afterslip may serve as an indicator of dilatancy effectiveness

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

recurring and triggered slow slip events near the trench at the nankai trough subduction megathrust the discovery of slow earthquakes has revolutionized the field of earthquake seismology defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults we report on a family of recurring slow slip events sses on the plate interface immediately seaward of repeated historical moment magnitude mw 8 earthquake rupture areas offshore of japan the sses continue for days to several weeks include both spontaneous and triggered slip recur every 8 to 15 months and are accompanied by swarms of low frequency tremors we can explain the sses with 1 to 4 centimeters of slip along the megathrust centered 25 to 35 kilometers km from the trench 4 to 10 km depth the sses accommodate 30 to 55 of the plate motion indicating frequent release of accumulated strain near the trench

paleomagnetism magnetic anisotropy and mid cretaceous paleolatitude of the duke island alaska ultramafic complex we report paleomagnetic results from layered igneous rocks that imply substantial post mid cretaceous poleward motion of the insular superterrane western canadian cordillera and southeast alaska relative to north america the samples studied are from the stratiform zoned ultramafic body at duke island which intruded rocks of the alexander terrane at the south end of the southeastern alaska archipelago at about 110 ma thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the curie temperature of magnetite this remanence is likely carried by low ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts the duke island intrusion exhibits a well developed gravitational layering that was deformed during initial cooling but below 540 c into folds that plunge moderately to the west southwest the characteristic remanence clearly predates this early folding and is therefore primary the fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site mean directions are corrected by restoring the layering to estimated paleohorizontal all samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion and we show that it has no significant or systematic effect on the site mean directions of remanence at least some of the anisotropy derives from secondary magnetite formed during partial serpentinization the mean paleomagnetic inclination 56 10 corroborates paleomagnetic results from five coeval silicic plutons of the canadian coast plutonic complex to the south and southeast and implies 3000 km 1300 km of poleward transport relative to the north american craton between mid cretaceous and middle eocene time the insular superterrane and coast plutonic complex shared a common paleolatitude history with more poleward transport than coeval inboard terranes

characterizing an uncharacteristic ets event in northern cascadia 1 gps and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern cascadia in november 2006 accompanying a brief episode of seismic tremor the event is much smaller in area and duration than other well known ets events in northern cascadia but is strikingly similar to typical ets events at the nankai subduction zone the 30 45 km depth range and the 2 3 cm slip magnitude as interpreted for this event appear to be common to most ets events in these two subduction zones regardless of their sizes we infer that the nankai type small ets events must be abundant at cascadia and that ets events at the two subduction zones are governed by a similar physical process

measurements of the cretaceous paleolatitude of vancouver island consistent with the baja british columbia hypothesis a previously unsampled outcrop of gently dipping or flat lying upper cretaceous sedimentary strata in the vancouver island region which contains unaltered aragonitic mollusk fossils yielded a stable remanent magnetization that is biostratigraphically consistent with cretaceous magnetochrons 33r 33n and 32r these results characterized by shallow inclinations indicate an upper cretaceous paleolatitude of about 25 3 degrees north which is equivalent to that of modern day baja california these findings are consistent with the baja british columbia hypothesis which puts the insular superterrane well south of the oregon california border in the late cretaceous

anisotropic viscoplasticity explains slow slip m 0 t scaling at convergent plate margins abstract in this study we quantify the mechanisms that govern two related observations with regard to deep 15 50 km subduction zone aseismic slow slip events sses i the linear scaling relationship between seismic moments m0 and event durations t and ii the direction dependent slow slip rupture speeds geological observations suggest that deep subduction slow slip shear zones are anisotropic and viscoplastic the anisotropy is due to the presence of dip parallel mafic lineaments created by seamount subduction whereas the viscoplasticity is due to deformation of mixed brittle mafic and ductile felsic materials we postulate that a dip parallel slip direction mafic lineament in an overall felsic slow slip shear zone acts as a stress guide which localizes initial slow slip rupture in the dip direction subsequent stress concentration along the dip parallel edges of the early ruptured lineament leads to along strike rupture with the rupture front propagating through the felsic shear zone the second phase slip area expansion maintains a constant dip parallel rupture zone length inherited from the length of the early ruptured mafic lineament by combining an energy balance equation with a two phase rupture model outlined above we obtain the first analytical expression of the observed linear scaling law in the form of m0 c0t where c 0 4 1 z g 2 l 2 s d gh z e cos v fw 2 l z s 2 d 2 gh 2 4 1 g l z zlg v fw v a 2 the observed value of this empirical constant is between 1011 5 and 1013 5 j s in the above expression l h z g e s and d are length depth thickness dip angle shear rigidity effective viscosity and effective coefficients of static and dynamic friction of the slow slip shear zone 1 is surface energy density of the initially ruptured mafic lineament is overriding plate density vfw and va are subducting plate and slow slip velocities and g is surface gravity our model based on the assumed shear zone anisotropy successfully predicts fast 100 km h dip parallel rupture along high viscosity 1020 pa s mafic lineaments and slow 2 10 km day strike parallel rupture through low viscosity 1017 pa s felsic materials during a deep subduction slow slip event

paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look the spences bridge group is a mid cretaceous 104 ma volcanic succession in the southern intermontane belt of the canadian cordillera 50 5 n 121 w it comprises the pimainus formation mafic to felsic lava volcaniclastic and interbedded epiclastic rocks and the overlying spius formation andesitic lava flows including previous work we have 55 sites distributed among 15 localities representing most of the 3000 m thickness forty seven sites 286 oriented cores 457 specimens mainly andesites yielded acceptable data the beds are gently to moderately tilted partly due to synvolcanic deformation samples taken from a sequence of flows at any one locality have with one exception well grouped magnetization directions polarities are all normal as expected for rocks laid down in the cretaceous normal superchron declinations always are clockwise of that expected of cratonic north america indicating 60 rotation of the spences bridge group as a whole however declinations differ from locality to locality implying relative interlocality rotations about vertical axes hence inclination only analysis has been used to estimate dispersion mean inclination and paleolatitude minimum dispersion for the spius formation 27 sites was achieved after 80 untilting but the changes between 80 and 100 untilting are insignificant indicating that magnetization was acquired predominantly before tilting by contrast minimum dispersion for the pimainus formation 20 sites was achieved after 50 untilting indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying spius formation polished thin section studies show that magnetite in the pimainus formation has undergone extensive low temperature hydrothermal alteration whereas magnetite in the spius formation shows little alteration throughout the range of tilt correction from 0 to 100 the mean inclinations of both formations were less than expected from observations obtained from mid cretaceous rocks of cratonic north america the best estimate of paleolatitude from the spius formation is 50 8 5 0 p 0 05 which is 9 5 5 7 less than would be expected had the rocks had been rigidly attached to north america this corresponds to displacement from the south of 1100 600 km displacement between the northern intermontane belt and craton probably was accommodated along major strike slip faults northern rocky mountain trench finlay pinchi etc in the south our results require a major dextral fault the intra quesnellia fault to be situated during late cretaceous or paleocene time within or marginal to the omineca belt along which about 1000 km of dextral motion occurred this could be a southern extension of the pinchi fault whose trace is now obscured by eocene extension and tectonic denudation the results also indicate that the largest tectonic discontinuity in the canadian cordillera occurs not to the east of the intermontane belt as commonly assumed but to the west because the displacements relative to cratonic north america observed from the intermontane belt are only about one third of those observed from the coast belt

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

metamorphic origin of ore forming fluids for orogenic gold bearingquartz vein systems in the north american cordillera constraints froma reconnaissance study of δ15n δd and δ18o the western north american cordillera hosts a large number of gold bearing quartz vein systems from the mother lode of southern california through counterparts in british columbia and southeastern alaska to the klondike district in central yukon these vein systems are structurally controlled by major fault zones which are often reactivated terrane bounding sutures that formed in orogens built during accretion and subduction of terranes along the continental margin of north america mineralization ages span mid jurassic to early tertiary and encompass much of the evolution of the cordilleran orogen nitrogen contents and 15n values of hydrothermal micas from veins are between 130 and 3 500 ppm and 1 7 to 5 5 per mil respectively these values are consistent with fluids derived from metamorphic dehydration reactions within the phanerozoic accretion subduction complexes which have 15n values of 1 to 6 per mil the 18o values of gold bearing vein quartz from different locations in the cordillera are between 14 6 and 22 2 per mil but are uniform for individual vein systems the d values of hydrothermal micas are between 110 and 60 per mil ore fluids have calculated 18o values of 8 to 16 per mil and d values of 65 to 10 per mil at an estimated temperature of 300 c d values of ore fluids do not show any latitudinal control these results indicate a deep crustal source for the ore forming fluids most likely of metamorphic origin low dh2o values of 120 to 130 per mil for a hydrous muscovite from the sheba vein in the klondike district reflect secondary exchange between recrystallizing mica and meteoric waters collectively the n h and o isotope compositions of ore related hydrothermal minerals indicate that the formation of these gold bearing veins involved dilute aqueous carbonic and nitrogen bearing fluids that were generated from metamorphic dehydration reactions at deep crustal levels these data are not consistent with either mantle derived fluids or granitoid related magmatic fluids nor do they support a model involving deeply circulated meteoric water

a paleomagnetic constraint on the late cretaceous paleoposition of northwestern baja california mexico we present palaeomagnetic data for cretaceous sedimentary rocks from the el rosario formation of northwestern baja california which imply a northward displacement of 15 0 3 8 and a 30 8 4 2 clockwise rotation for the baja peninsula relative to the north american craton since late cretaceous time paired irm acquisition x ray diffraction and mossbauer spectra indicate that fine grained magnetite is the dominant magnetic mineral present with goethite and other more oxidized phases rare to absent biochemical evidence constrains the amount of diagenesis undergone by the sediments we subjected oriented samples to both progressive alternating field and thermal demagnetization and in general found univectorial magnetizations yielding a mean inclination of 44 6 and declination of 5 3 with an 95 of 2 0 the corresponding paleolatitude for these values is 26 3 1 6 most of the samples were of normal polarity which is consistent with their deposition within the upper campanian chron 33n we collected samples from younger maastrichtian sediments which yield consistent reversed directions

generation mechanism of slow earthquakes numerical analysis based on a dynamic model with brittle ductile mixed fault heterogeneity 1 various characteristics have been discovered for small slow earthquakes occurring along subduction zones which are deep nonvolcanic tremor low frequency earthquakes lfes and very low frequency earthquakes vlfs in this study we model these slow earthquakes using a dynamic model consisting of a cluster of frictionally unstable patches on a stable background the controlling parameters in our model are related to the patch distribution and the viscosity of both the patches and the background by decreasing patch density or increasing viscosity we observed the transition in rupture propagation mechanism that is from fast elastodynamic interactions characterized by an elastic wave propagation to slow diffusion limited by viscous relaxation times of traction on fault patches and or background some sets of these geometrical and frictional parameters collectively explain the moment rate functions source spectra and scaled energy of observed slow earthquakes in addition we successfully explain both parabolic and constant velocity migrations in the case of the diffusion limited rupture therefore the observed various characteristics of tremor lfes vlfs and potentially slow slip events may be essentially explained by our simple model with a few parameters describing source structures and frictional properties of brittle ductile transition zones along plate boundaries

jurassic cretaceous basins along the canadian coast belt their bearing on pre mid cretaceous sinistral displacements the dominantly plutonic canadian coast belt is flanked on the northwest by a late jurassic early cretaceous intra or back arc basin in the south it contains strata of the same age deposited in an accretionary fore arc complex east of a coeval arc this distribution is ascribed to pre mid cretaceous sinistral movement of part of the jurassic cretaceous north american plate margin arc to a position west of the accretionary fore arc complex

episodic tremor and slip on a frictional interface with critical zero weakening in elastic solid summary non volcanic tremor nvt and related relatively weak and slow slip events termed episodic tremor and slip ets are observed below the seismogenic sections of numerous subduction zones and several major strike slip faults these events have various characteristics that distinguish them from regular tectonic earthquakes here we show that a frictional fault in elastic solid with a region below the seismogenic zone having on average a critical near zero weakening during slip provides a simple unified explanation for the diverse observed phenomenology of nvt ets the results imply that nvt ets have little predictive power on the occurrence of large events in the overriding seismogenic zone additional model predictions that may be tested with future high resolution observations are fractal slip distributions and failure areas discrete power law frequency moment statistics with exponent 3 2 and exponential tapering overall scale invariant potency moment magnitude time histories triggered periodic nvt with evolving size and rate correlated with the stressing rate of the periodic triggering mechanisms and parabolic or exponential source time functions for event sizes measured by duration or moment potency

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

thermo poro mechanics of chemically active creeping faults 3 the role of serpentinite in episodic tremor and slip sequences and transition to chaos during the last decade knowledge over episodic tremor and slip ets events has increased dramatically owing to the widespread installation of gps and seismic networks the most puzzling observations are i the periodic nature of slow seismic events ii their localization at intermediate depths estimated 15 40 km and iii the origin of the nonvolcanic fluids that are responsible for the tremor activity we reconcile these observations using a first principles approach relying on physics continuum mechanics and chemistry of serpentinite in the megathrust interface the approach reproduces the gps sequences of 17 years of recording in cascadia north america as well as over 10 years in the hikurangi trench of new zealand we show that strongly endothermic reactions such as serpentinite dehydration are required for ets events we report that in this tectonic setting it is its chemical reaction kinetics not the low friction that marks serpentinite as a key mineral for stable self sustained oscillations we find that the subduction zone instabilities are driven from the ductile realm rather than the brittle cover even when earthquakes in the cover perturb the oscillator it relaxes to its fundamental mode such a transition from stable oscillations to chaos is witnessed in the ets signal of nz following the m6 8 2007 seismic event which triggered a secondary mode of oscillations lasting for a few years we consequently suggest that the rich dynamics of ductile modes of failure may be used to decipher the chaotic time sequences underpinning seismic events

tidal modulation of slow slip in cascadia 1 several studies have shown that the seismic tremor in episodic tremor and slip is tidally modulated suggesting a sensitivity to the rather small tidal stresses we address whether the slip rate in slow slip events is also tidally modulated by examining data from six borehole strainmeters in northwest washington and southern vancouver island we simultaneously fit data from multiple stations and from slow slip events occurring over a 3 year interval from january 2007 to june 2009 as we are unable to extract a meaningful signal from a single record we find modulation of the strain rate with a 12 4 h period that of the tide with the largest amplitude that is significant at the 99 level the amplitude of this modulation suggests that the slip rate during slow slip events oscillates on average 25 above and below its mean value during a tidal cycle tidal modulation estimates at three other periods are significant with more than 70 probability the phase of maximum strain rate in the 12 4 h m2 period coincides with the phase of the maximum tremor rate taken from a catalog in an overlapping region comparison with a simple tidal loading model shows that the phase of maximum strain rate in the m2 period may occur at the maximum shear stress or up to 90 before it depending on the location of slip in the subduction zone

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

anatomy of a megathrust the 2010 m8 8 maule chile earthquake rupture zone imaged using seismic tomography abstract knowledge of seismic velocities in the seismogenic part of subduction zones can reveal how material properties may influence large ruptures observations of aftershocks that followed the 2010 m w 8 8 maule chile earthquake provide an exceptional dataset to examine the physical properties of a megathrust rupture zone we manually analysed aftershocks from onshore seismic stations and ocean bottom seismometers to derive a 3 d velocity model of the rupture zone using local earthquake tomography from the trench to the magmatic arc our velocity model illuminates the main features within the subduction zone we interpret an east dipping high p wave velocity anomaly 6 9 km s as the subducting oceanic crust and a low p wave velocity we assess the role played by physical heterogeneity in governing megathrust behaviour greatest slip during the maule earthquake occurred in areas of moderate p wave velocity 6 5 7 5 km s where the interface is structurally more uniform at shallow depths high fluid pressure likely influenced the up dip limit of seismic activity the high velocity bodies lie above portions of the plate interface where there was reduced coseismic slip and minimal postseismic activity the northern velocity anomaly may have acted as a structural discontinuity within the forearc influencing the pronounced crustal seismicity in the pichilemu region our work provides evidence for how the ancient geological structure of the forearc may influence the seismic behaviour of subduction megathrusts

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

paleogeographic trends in late triassic reef ecology from northeastern panthalassa the late triassic was a pivotal period in reef evolution but the majority of information about reef ecology during this time comes from buildups in the alps e g the tethys ocean recent studies of reefs in western north america have recognized unique ecologies along the eastern margin of the panthalassa ocean although there are numerous twenty five localities with putative reef builders only four buildups had syndepositional relief and a rigid framework i e true reefs the most paleo northern true reefs were microbial patch reefs with only a few large skeletal bioconstructors hypercalcified sponges and spongiomorphs built the mid latitude reefs with secondary microbial encrustation and branching phaceloid retiophyllia corals corals are the primary bioconstructors in panthalassa s most paleo equatorial reefs and calcareous microbes are sparse when all reefal deposits are analyzed the n s gradient is also present with microbial and bivalve deposits in the north sponge coral deposits in the mid latitudes and coral deposits near the equator this ecological gradient is not apparent in the tethys ocean tethyan reefs thrived in oligotrophic tropical waters without strong latitudinal gradients by contrast paleoceanographic considerations suggest that cool nutrient rich waters swept south along the western north american borderlands in the late triassic the eastern boundary current is interpreted to have created a strong north south differentiation of environments in northeastern ne panthalassa that was manifested in both the biotic and abiotic characteristics of eastern panthalassic reefs reefs from equatorial panthalassa are similar to tethyan reefs warm water photozoan coral reef structures whereas higher paleo latitude reefs from panthalassa are interpreted as cool water heterozoan buildups with abundant calcareous microbes diminutive biocalcifiers and few large framework building corals

gps constraints on 34 slow slip events within the cascadia subduction zone 1997 2005 1 refinements to gps analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the cascadia subduction zone from 1997 through 2005 timing of transient onset is determined with wavelet transformation of geodetic time series thirty continuous stations are included in this study ranging from northern california to southwestern british columbia our improvements in analysis better resolve the largest creep events and also identify many smaller events at 48 5 n latitude a 14 month average recurrence interval has been observed over eight events since 1997 farther north along vancouver island a host of smaller events with a distinct 14 month periodicity also occurs in southern washington state some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence along central oregon an 18 month recurrence is evident while in northern california an 11 month periodicity continues through 2005 we invert gps offsets of the 12 best recorded events for thrust slip along the plate interface using a cross validation scheme to derive optimal smoothing parameters these 12 events have equivalent moment magnitudes between 6 3 and 6 8 and have 2 3 cm of slip unlike other subduction zones no long duration events are observed and cumulative surface deformation is consistently less than 0 6 cm the many newly resolved smaller transient events in cascadia show that slow slip events occur frequently with gps best capturing only the largest events it is likely that slow slip events occur more frequently at levels not detectable with gps

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

dilatant strengthening as a mechanism for slow slip events 1 the mechanics of slow slip events sse in subduction zones remain unresolved we suggest that sse nucleate in areas of unstable friction under drained conditions but as slip accelerates dilatancy reduces pore pressure p quenching instability competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast we model sse with 2 d elasticity rate state friction and a dilatancy law where porosity evolves toward steady state ss over distance dc and ss 0 ln v v0 v is slip speed we consider two diffusion models membrane diffusion md is approximated by p p tf where p and p are shear zone and remote pore pressure and tf is a characteristic diffusion time homogeneous diffusion hd accurately models fault normal flow with diffusivity chyd for md linearized analysis defines a boundary 1 a b between slow and fast slip where f0e b p f0 a and b are friction parameters and is compressibility when 1 a b slip speeds remain quasi static for hd ep eh p defines dilatancy efficiency where h is shear zone thickness and v is plate velocity sse are favored by large eh and low effective stress the ratio ep to thermal pressurization efficiency scales with 1 p so high p favors sse consistent with seismic observations for ep 10 3 transient slip rates repeat times average slip and stress drops are comparable to field observations model updip propagation speeds are comparable to those observed along strike many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress model sse accommodate only a fraction of plate motion the remaining deficit must be accommodated during coseismic or postseismic slip

magnetostratigraphy of upper cretaceous strata of the sacramento valley california a magnetic polarity zonation has been developed for nine stratigraphic sections of the upper cretaceous portion of the great valley group in the sacramento valley of california collectively the sections include five biostratigraphic range zones based on ammonites and other mollusks the zones are all of santonian or campanian age and each section contains at least two zones a total of 681 fully oriented samples was collected from 252 horizons spanning more than 3 km of section the samples were subjected to alternating field demagnetization after it was found that the rocks were not amenable to thermal demagnetization rock magnetic studies were used to determine that the principal magnetic carrier is magnetite the paleomagnetic measurements reveal the existence of an interval of reversed polarity that is consistently associated with one of the zones namely the baculites chicoensis zone given the biostratigraphic control the reversed interval can be unambiguously correlated with the first reversed interval following the cretaceous long normal interval on the magnetic polarity time scale this reversed interval corresponds to marine magnetic anomaly 33r the correlation of the re versed interval implies that the base of the chicoensis zone is nearly coincident with the santonian campanian boundary in california and that it is not significantly time transgressive between the shallow water facies on the east side of the sacramento valley and the deep water facies on the west side furthermore the magnetostratigraphy supports the conclusion that all of the zones underlying the chicoensis zone are pre campanian in age a result which is consistent with other lines of evidence the biostratigraphy of these sections in california can now be compared directly to that of the paleomagnetic stratotype section in gubbio italy

stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake yan hu 1 roland burgmann 1 naoki uchida 2 paramesh banerjee 3 and jeffrey t freymueller 4 university of california berkeley berkeley ca united states graduate school of science tohoku university japan earth observatory of singapore nanyang technological university singapore singapore university of alaska fairbanks fairbanks ak united states corresponding author yan hu earth and planetary science university of california berkeley 307 mccone hall berkeley ca 94720 usa yhu seismo berkeley edu this article has been accepted for publication and undergone full peer review but has not been through the copyediting typesetting pagination and proofreading process which may lead to differences between this version and the version of record please cite this article as doi 10 1002 2015jb012508 2015 american geophysical union all rights reserved

tilt and northward offset of cordilleran batholiths resolved using igneous barometry considerable controversy surrounds the suggestion based on palaeomagnetic evidence that large segments of the north american cordillera travelled long distances parallel to the coast during the latest cretaceous and early tertiary periods well after the amalgamation of exotic terranes discordant palaeomagnetic data from mid cretaceous plutonic rocks of the peninsular ranges batholith of coastal southern california and baja and the mount stuart batholith of the cascade range in washington play a pivotal role in the controversy the discordant data were originally interpreted to reflect northward transport of 1 000 km relative to cratonal north america after the batholiths cooled through their magnetic blocking temperatures1 5 more recently it has been argued that the discordances arise from local tilting of batholiths rather than northward offset6 7 here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting our results indicate that the peninsular ranges and mount stuart batholiths have undergone northward offsets of 1 000 450 and 2 900 700 km respectively and also significant tilting

paleomagnetic evidence for displacement from the south of the coast plutonic complex british columbia the mid cretaceous spuzzum and porteau plutons of the coast plutonic complex of british columbia have two magnetizations a and b the a magnetization eight sites 83 specimens d 30 3 i 56 7 95 4 9 paleolatitude 37 5 n paleopole 65 0 n 14 9 w a95 6 2 is considered to have been acquired in the age range 105 90 ma this result differs from the field established for cratonic north america in this time range the difference could be caused either by previously undetected tilting about a horizontal axis of the plutons or by their rotation about a vertical axis and lateral displacement relative to the craton previously observed mid cretaceous magnetizations from other rock units from the western canadian cordillera and the cascades of washington united states are similarly discordant with respect to the craton this similarity over such a large area indicates that although local undetected tilting could be partly responsible it is unlikely to be the prime cause and we argue ther

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

creep events at the brittle ductile transition 1 we present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones we assume that creep events at the brittle ductile transition bdt are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material these events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids we show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust a ductile shear zone and a creeping zone of fractures at the bdt the time scale of the events varies between seconds to thousands of years depending on the viscous elastic and brittle plastic properties of the fractured bdt the shear zone and the crust the nature of the events depends on the aspect ratio of the shear zone thickness hw to the length of the fractured zone w we find that thick shear zones with small fractures at the bdt are stiff and generate creep oscillations thin shear zones with well connected fractures over a large width have very small stiffness and are well lubricated they generate slow creep events or steady creep event the former are similar to transient slip events and the latter to creep at the far field tectonic rates the viscosity of the shear zone w enhances lubrication if it is small and stiffness if it is large

a unified gps based earthquake catalog for the sumatran plate boundary between 2002 and 2013 we have compiled the first self consistent gps based earthquake catalog for the sumatran plate boundary using continuous daily position time series from the sumatran gps array sugar we document 30 earthquakes which occurred within or outside the sugar network from august 2002 through the end of 2013 and we provide estimates of both vertical and horizontal coseismic offsets associated with 1 m9 2 3 m8 6 m7 19 m6 and 1 m5 9 earthquakes as well as postseismic decay amplitudes and times associated with 9 m 7 earthquakes and 1 m6 7 earthquake for most of the previously studied earthquakes our geodetic catalog provides more complete coseismic displacements than those published showing consistent patterns of motion across a large range of distances for many of the moderate to large earthquakes we publish their coseismic displacements for the first time providing new constraints on their locations and slip distributions for the postseismic time series we have tackled the challenge of separating the signals for individual events from the overlapping effects of many other earthquakes as a result we have obtained either new or much longer time series than previously published based on our long time series we find logarithmic decay times ranging from several days to more than 20 years and sometimes a second decay time is needed suggesting that when studying large to great sumatran earthquakes we need to consider multiple postseismic mechanisms our geodetic catalog provides rich spatial and temporal sumatran earthquake cycle information for future studies of the physics and dynamics of the sumatran plate boundary

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

deformation cycles of subduction earthquakes in a viscoelastic earth subduction zones produce the largest earthquakes over the past two decades space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes the short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle piecing together geodetic snapshots from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short term years and long term decades and centuries viscous behaviour of the mantle traditional views based on elastic models such as coseismic deformation being a mirror image of interseismic deformation are being thoroughly revised snapshots of subduction zones using space geodesy reveal that the viscous behaviour of the mantle controls crustal deformation requiring the revision of traditional elastic models for earthquake risk assessment space geodesy the use of satellites to monitor earth has revolutionized our view of crustal deformation between consecutive earthquakes however the brevity of such measurements means that studies must be done by comparing multiple subduction zones at different stages of the earthquake cycle by piecing together geodetic snapshots from the sumatra chile and cascadia subduction zones this review presents a unifying picture in which deformation is controlled by the viscous behaviour of the mantle in both the short term years and the long term decades to centuries

velocity weakening behavior of plagioclase and pyroxene gouges and stabilizing effect of small amounts of quartz under hydrothermal conditions 1 we investigated properties of frictional sliding of plagioclase pyroxene and their mixture gouges with a small amount of hornblende biotite and quartz as accessory minerals under hydrothermal conditions with an effective normal stress of 200 mpa pore pressure of 30 mpa and temperatures from 100 c to 600 c axial loading rate was stepped between 0 001 and 0 0001 mm s to acquire the rate dependence both plagioclase and pyroxene gouges showed velocity weakening behavior in the whole temperature range except the velocity strengthening behavior of pyroxene at 200 c for temperatures above 400 c both plagioclase and pyroxene gouges showed oscillatory slips as a result of small dc values of 3 4 m which make the critical stiffness rise remarkably and approach the system stiffness above 300 c the direct effect of plagioclase shows an increasing trend with temperature indicating control of the deformation process by thermally activated mechanisms as the difference of a and b values here are only 20 at most this trend also applies to the evolution effect our analytical derivation based on the theory of pressure solution shows a log linear contact area growth with time that corresponds to an evolution effect and estimations based on this encompass the plagioclase data though the identification of actual mechanisms is not easy finally it is found that a little quartz 3 5 added to the plagioclase 60 62 pyroxene 35 mixture has a strong stabilizing effect leading to a transition from velocity weakening to velocity strengthening these results may help constrain the depth range of seismic slips on deep faults in the lower crust of gabbroic composition

fault spacing in continental strike slip shear zones abstract strike slip shear zones with sub parallel arrays of evenly spaced faults are widely observed in nature but the controls on the spacing between major faults are unclear we analyze a 2 d model and develop a scaling law relating the fault spacing to structural and rheological parameters in the continental crust we find that fault spacing positively correlates with brittle layer thickness viscous lower crust thickness and strength contrast between active faults and surrounding intact blocks and is inversely correlative with lower crust viscosity this is corroborated for either a zero shear traction decoupled or a prescribed velocity coupled basal boundary condition in the 2 d analytical solution the zero shear traction boundary condition represents low viscosities in the lowermost crust or the topmost mantle that may decouple deformations from mantle flow the prescribed velocity boundary condition emphasizes basal drag tractional forces imparted on the lower crust by a strong mantle for a viscous layer that is thicker than half of its average fault spacing models with either of the boundary conditions produce the same results otherwise a thinner viscous layer with a linear velocity condition tends to produce smaller fault spacings than a no shear model all else being equal these theoretical models are comparted to data from shear zones in california the marlborough fault zone in new zealand and central tibet modeling indicates that the effective viscosity of the viscous layer underlying the brittle layer in all of the selected areas is 2 10 20 to 4 10 21 pa s the subducted oceanic plate attached to the lower crust of the eastern marlborough fault zone also appears to influence fault spacing in the overriding plate

slip weakening models of the 2011 tohoku oki earthquake and constraints on stress drop and fracture energy we present 2d dynamic rupture models of the 2011 tohoku oki earthquake based on linear slip weakening friction we use different types of available observations to constrain our model parameters the distribution of stress drop is determined by the final slip distribution from slip inversions as three groups of along dip slip distribution are suggested by different slip inversions we present three slip weakening models in each model we assume uniform critical slip distance eastward from the hypocenter but several asperities with smaller critical slip distance westward from the hypocenter the values of critical slip distance are constrained by the ratio of deep to shallow high frequency slip rate power inferred from back projection source imaging our slip weakening models are consistent with the final slip slip rate rupture velocity and high frequency power ratio inferred for this earthquake the average static stress drop calculated from the models is in the range of 4 5 7 mpa though large spatial variations of static stress drop exist to prevent high frequency radiation in the region eastward from the hypocenter the fracture energy needed there is in the order of 10 mj m2 and the average up dip rupture speed cannot exceed 2 km s the radiation efficiency calculated from our models is higher than that inferred from seismic data suggesting the role of additional dissipation processes we find that the structure of the subduction wedge contributes significantly to the up dip rupture propagation and the resulting large slip at shallow depth

probing the transition between seismically coupled and decoupled segments along an ancient subduction interface the transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip as well as the focus site of episodic tremor and slip features exhumed remnants of the former alpine subduction zone found in the swiss alps allow analyzing fluid and deformation processes near the transition zone region 30 40 km paleodepth the dent blanche thrust dbt is a lower blueschist facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge we report field observations from the dbt region where multiple several tens of meters thick foliated cataclastic networks are interlayered within the basal dbt mylonites petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions 400 500 c 1 1 1 3 gpa during subduction of the tethyan seafloor in eocene times 42 48 ma some of these networks exhibit mutual crosscutting relationships between mylonites foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep whole rock chemical compositions in situ 40ar 39ar age data of recrystallized phengite and sr isotopic signatures reveal that dbt rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the dbt from the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates 10 12 s 1 accompanied by fluctuation of supralithostatic and quasi lithostatic fluid pressures 1 0 95 dbt brittle plastic deformation switches highlight the diversity of deformation processes and fluid rock interactions in the transition zone region of the subduction interface

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

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

campanian maastrichtian stage boundary in southern california resolution and implications for large scale depositional patterns detailed paleomagnetic study of upper cretaceous predominantly deep sea fan deposits from the point loma formation san diego california situated within the allochthonous peninsular ranges terrane has produced a magnetic polarity stratigraphy with one reversed magnetozone foraminiferal and megafossil primarily molluscan occurrences demonstrate that these strata are late campanian to early maastrichtian in age and that the reversed magnetozone is almost certainly basal maastrichtian chron 32r recognition of this chron provides a direct correlation of the north pacific province biostratigraphic zonations to the standard cretaceous stages in europe the identification of chron 32r indicates that for the available biostratigraphic zonations the pacific slope megafossil campanian maastrichtian stage boundary shows the best accord with the standard european stage boundary improved age relations for these strata suggest that their large scale depositional patterns were strongly influenced by both local active plate margin tectonics and global eustatic factors during the late cretaceous

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

experimental demonstration of a semi brittle origin for crustal strain transients plate tectonic motions cause faults to slip during earthquakes and through creep laboratory shear experiments on semi brittle rock analogue materials suggest that such slip could occur via growth of a series of fractures

intermittent tremor migrations beneath guerrero mexico and implications for fault healing within the slow slip zone slow slip events exhibit significant complexity in slip evolution and variations in recurrence intervals behavior that varies systematically with recurrence interval is likely to reflect different extents of fault healing between these events here we use high resolution tremor catalogs beneath guerrero mexico to investigate the mechanics of slow slip we observe complex tremor propagation styles including rapid tremor migrations propagating either along the main tremor front or backward reminiscent of those in northern cascadia we also find many migrations that originate well behind the front and repeatedly occupy the same source region during a tremor episode similar to those previously reported from shikoku japan these migrations could be driven by slow slip in the surrounding regions with recurrence intervals possibly modulated by tides the propagation speed of these migrations decreases systematically with time since the previous migration over the same source area tremor amplitudes seem consistent with changes in the propagation speeds being controlled primarily by changes in the slip speeds one interpretation is that the high propagation speeds and inferred high slip speeds during the migrations with short recurrence intervals are caused by incomplete healing within the host rock adjacent to the shear zone which could lead to high permeability and reduced dilatant strengthening of the fault gouge similar processes may operate in other slow slip source regions such as cascadia

episodic slow slip events in the japan subduction zone before the 2011 tohoku oki earthquake abstract we describe two transient slow slip events that occurred before the 2011 tohoku oki earthquake the first transient crustal deformation which occurred over a period of a week in november 2008 was recorded simultaneously using ocean bottom pressure gauges and an on shore volumetric strainmeter this deformation has been interpreted as being an m6 8 episodic slow slip event the second had a duration exceeding 1 month and was observed in february 2011 just before the 2011 tohoku oki earthquake the moment magnitude of this event reached 7 0 the two events preceded interplate earthquakes of magnitudes m6 1 december 2008 and m7 3 march 9 2011 respectively the latter is the largest foreshock of the 2011 tohoku oki earthquake our findings indicate that these slow slip events induced increases in shear stress which in turn triggered the interplate earthquakes the slow slip event source area on the fault is also located within the downdip portion of the huge coseismic slip area of the 2011 earthquake this demonstrates episodic slow slip and seismic behavior occurring on the same portions of the megathrust fault suggesting that the faults undergo slip in slow slip events can also rupture seismically

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers

paleomagnetism and tectonics of the cretaceous mt stuart batholith of washington translation or tilt abstract the mt stuart batholith is a composite pluton of late cretaceous age that intrudes the crystalline north cascades terrane of northwestern washington its paleomagnetic direction d 10 0 i 45 5 95 4 9 is markedly different from the direction expected for cretaceous rocks from northwestern washington d 330 5 i 73 0 which means that the mt stuart batholith either has moved relative to the north american interior or has been tilted through a substantial angle or both since it acquired its magnetism either tilt or translation are possible geometrically but translation is more likely because 1 local geology apparently does not support tilt in the required direction 2 it probably is not possible for a panel of rock the size of the mt stuart batholith to tilt through the necessary angle ca 35 after its deeper parts have cooled sufficiently to retain remanent magnetization 3 the sense of paleomagnetic discordance found in the mt stuart batholith clockwise rotation of declination flattened inclination is identical to that found in nearly every other allochthonous terrane in the western north american cordillera including every other cretaceous batholith studied except the sierra nevada

testing terrane transport an inclusive approach to the baja b c controversy the baja british columbia hypothesis holds that a large segment of the western edge of northern north america baja b c was situated alongside california and northern mexico in middle cretaceous time was displaced northward in the late cretaceous and paleocene by north oblique convergence of the kula plate with north america and arrived near its present location by the early eocene a consistent body of paleomagnetic data supports this hypothesis however doubt persists and various crucial tests of a geologic nature have been proposed one such test concerns the provenance of zircons in cretaceous sedimentary basins of baja b c in this paper we use both paleomagnetic data and zircon occurrences to reconfirm the baja b c hypothesis we first argue that the only truly crucial tests yet performed have been paleomagnetic and that all such tests have been positive second we show that detrital zircon data from the upper cretaceous nanaimo group although not a crucial test provide valuable paleogeographic information available data demonstrate a change in detrital zircon provenance in the nanaimo group that closely matches the position of these rocks predicted by the baja b c hypothesis detrital zircons in the nanaimo group suggest a change from a southwestern north american source rich in grenville and 1 4 1 5 ga rocks to an increasing contribution from older parts of the craton such as the wyoming province together detrital zircons and paleomagnetic inclinations allow us to assemble a detailed schedule of northward tectonic transport of the baja b c terranes