a continuum of stress strength and slip in the cascadia subduction zone movement of the down going oceanic plate in subduction zones is accommodated by earthquakes slow slip and free slip with increasing depth analysis of accompanying tremor reveals a continuum of slow slip events in the cascadia subduction zone which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone

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

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

effects of material property variations on slip estimates for subduction interface slow slip events we investigate the influence of elastic heterogeneity on geodetic inversions of slow slip events by inverting for slip distributions of four events along the hikurangi margin new zealand we generate green s functions using a finite element code in conjunction with a new zealand wide seismic velocity model to assign elastic properties we find that these heterogeneous models typically require 20 less slip than homogeneous models in cases where the slip is deep or there is reasonable geodetic coverage above the slipping region in cases where the slip is shallow and mostly offshore and there is little geodetic coverage directly above the slipping region the heterogeneous models can predict significantly larger amounts of slip 42 in our study these changes in the predicted amounts of slip have important implications for quantifying slip budgets accommodated by slow slip at subduction zones worldwide

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

simple physical model for the probability of a subduction zone earthquake following slow slip events and earthquakes application to the hikurangi megathrust new zealand

resonant slow fault slip in subduction zones forced by climatic load stress global positioning system gps measurements at subduction plate boundaries often record fault movements similar to earthquakes but much slower occurring over timescales of 1 week to 1 year these slow slip events have been observed in japan1 2 cascadia3 4 5 6 7 mexico8 9 alaska10 and new zealand11 the phenomenon is poorly understood but several observations hint at the processes underlying slow slip although slip itself is silent seismic instruments often record coincident low amplitude tremor in a narrow 1 5 cycles per second frequency range12 also modelling of gps data3 7 9 and estimates of tremor location13 indicate that slip focuses near the transition from unstable stick slip to stable friction at the deep limit of the earthquake producing seismogenic zone perhaps most intriguingly slow slip is periodic at several locations with recurrence varying from 6 to 18 months depending on which subduction zone or even segment is examined4 5 6 9 here i show that such periodic slow fault slip may be a resonant response to climate driven stress perturbations fault slip resonance helps to explain why slip events are periodic why periods differ from place to place and why slip focuses near the base of the seismogenic zone resonant slip should initiate within the rupture zone of future great earthquakes suggesting that slow slip may illuminate fault properties that control earthquake slip

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

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

the frictional hydrologic metamorphic and thermal habitat of shallow slow earthquakes slow earthquakes have recently been discovered on the shallow parts of some subduction zones a review of the conditions under which these quakes form reveals that they could be common in most subduction zones globally

large scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge seismic waves can trigger further fault slip analysis of seismic and geodetic data shows that seismic waves from the 2016 kaik ura new zealand earthquake were amplified by subduction zone sediments triggering slow fault slip up to 600 km away

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

quake clamps down on slow slip using continuous gps cgps data from the hikurangi subduction zone in new zealand we show for the first time that stress changes induced by a local earthquake can arrest an ongoing slow slip event sse the cgps data show that the slip rate in the northern portion of the 2013 2014 kapiti sse decreased abruptly following a nearby intraslab earthquake we suggest that deceleration of the kapiti sse in early 2014 occurred due to a tenfold increase in the normal stress relative to shear stress in the sse source induced by the nearby mw 6 3 earthquake consistent with expectations of rate and state friction our observation of an abrupt halting slowing of the sse in response to stress changes imposed by a local earthquake has implications for the strength of fault zones hosting sses and supports the premise that static stress changes are an important ingredient in triggering or delaying fault slip

time dependent modeling of slow slip events and associated seismicity and tremor at the hikurangi subduction zone new zealand we present a time dependent slip model of 12 slow slip events sses occurring in the hikurangi margin of new zealand during 2010 and 2011 this model is obtained by inverting daily gps solutions from geonet s continuous gps network on the north island and northern south island we compare the properties of these sses to observations in japan cascadia and mexico and find that hikurangi sses have comparatively large amounts of slip up to 27 cm high slip rates up to 1 4 cm d and a large range of depths 10 40 km durations 7 270 days and sizes mw 5 9 6 9 we further investigate the relationship between the cape turnagain sse and an associated seismic swarm and find that observations are consistent with stress triggering outside the slowly slipping region however other explanations cannot be ruled out we also compare slip during the long term manawatu sse with the tremor epicenters found by ide 2012 and note that tremor locations are offset in the downdip direction relative to the slipping region similar to observations in the bungo channel of japan and guerrero mexico

velocity and slip dependent weakening in simulated fault gouge implications for multimode fault slip in addition to the velocity dependence of friction slip dependence may play a major role before and during earthquake slip in fault zones we performed laboratory friction experiments on simulated fault gouges measuring both the velocity and slip dependence of friction in velocity step tests the pure velocity dependent component of friction measured over short displacements shows both velocity strengthening and velocity weakening friction depending on the amount of slip considered however we observe that increases in sliding velocity can induce slip weakening behavior which overwhelms the velocity dependence resulting in large overall weakening especially at rates 1 m s on natural tectonic faults this suggests that a velocity perturbation such as coseismic rupture propagating onto a fault patch could induce instability via large slip weakening therefore a fault which is experiencing a transient slip or slow earthquakes may be more easily induced to slip coseismically if a dynamic rupture from large earthquake propagates onto the fault

intense foreshocks and a slow slip event preceded the 2014 iquique mw 8 1 earthquake the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 the moment magnitude mw 8 1 iquique earthquake of 1 april 2014 broke a highly coupled portion of this gap to understand the seismicity preceding this event we studied the location and mechanisms of the foreshocks and computed global positioning system gps time series at stations located on shore seismicity off the coast of iquique started to increase in january 2014 after 16 march several mw 6 events occurred near the low coupled zone these events migrated northward for 50 kilometers until the 1 april earthquake occurred on 16 march on shore continuous gps stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred

earthquake nucleation on rough faults earthquake nucleation is currently explained using rate and state stability analysis which successfully models the behavior of laboratory simulated faults with constant thickness gouge layers however roughness is widely observed on natural faults and its influence on earthquake nucleation is little explored here we conduct frictional sliding experiments with different roughness on granite samples at upper crustal conditions 30 200 mpa we observe a wide range of behaviors from stable sliding to stick slip depending on the combination of roughness parameters and normal stress stick slip is repeatedly observed in velocity strengthening regimes and increases in normal stress stabilize slip these features are not fully predicted by current stability analysis we derive a new instability criterion that matches our observations based on fracture energy considerations and the size of weak patches created by fault roughness

modelling of short interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable stable transition regime summary recent high resolution observations of crustal movements have revealed silent slip events sses with propagation velocities of around 10 15 km d 1 and with intervals of 3 14 months along the deeper parts of the cascadia and nankai subduction zones this study develops 2 d and 3 d models of these short interval sses considering the frictional behaviour that was confirmed experimentally by shimamoto for the unstable stable transition regime to represent this frictional behaviour a small cut off velocity to an evolution effect is introduced in a rate and state dependent friction law when the cut off velocity to the evolution effect is significantly smaller than that to a direct effect steady state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities at the deeper cascadia and nankai subduction interfaces the pore pressure is inferred to be high because of the dehydration of materials in the descending plate under conditions where the pore fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small short interval sses with propagation velocities and slip velocities of 4 8 km d 1 and 2 4 10 7 m s 1 respectively can be reproduced the propagation velocity of short interval sses is in proportion to the slip velocity the results also show that during the nucleation process of large earthquakes the occurrence of short interval sses becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone our results suggest that monitoring of short interval sses might be useful for forecasting the main earthquakes

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

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

release of mineral bound water prior to subduction tied to shallow seismogenic slip off sumatra plate boundary fault rupture during the 2004 sumatra andaman subduction earthquake extended closer to the trench than expected increasing earthquake and tsunami size international ocean discovery program expedition 362 sampled incoming sediments offshore northern sumatra revealing recent release of fresh water within the deep sediments thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial induced temperature increases in the past 9 million years complete dehydration of silicates is expected before plate subduction contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction shallow slip offshore sumatra appears driven by diagenetic strengthening of deeply buried fault forming sediments contrasting with weakening proposed for the shallow tohoku oki 2011 rupture but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

physical and transport property variations within carbonate bearing fault zones insights from the monte maggio fault central italy the physical characterization of carbonate bearing normal faults is fundamental for resource development and seismic hazard here we report laboratory measurements of density porosity vp vs elastic moduli and permeability for a range of effective confining pressures 0 1 100 mpa conducted on samples representing different structural domains of a carbonate bearing fault we find a reduction in porosity from the fault breccia 11 7 total and 6 2 connected to the main fault plane 9 total and 3 5 connected with both domains showing higher porosity compared to the protolith 6 8 total and 1 1 connected with increasing confining pressure p wave velocity evolves from 4 5 km s to 5 9 km s in the fault breccia is constant at 5 9 km s approaching the fault plane and is low 4 9 km s in clay rich fault domains we find that while the fault breccia shows pressure sensitive behaviour a reduction in permeability from 2 10 16 m2 to 2 10 17 m2 the cemented cataclasite close to the fault plane is characterized by pressure independent behaviour permeability 4 10 17 m2 our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone in situ vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels m 1 earthquakes are less favoured in agreement with earthquake depth distribution during the l aquila 2009 seismic sequence that occurred on carbonates

effect of depth dependent shear modulus on tsunami generation along subduction zones estimates of the initial size of tsunamis generated by subduction zone earthquakes are significantly affected by the choice of shear modulus at shallow depths analysis of over 360 circum pacific subduction zone earthquakes indicates that for a given seismic moment source duration increases significantly with decreasing depth bilek and lay 1998 1999 under the assumption that stress drop is constant the increase of source duration is explained by a 5 fold reduction of shear modulus from depths of 20 km to 5 km this much lower value of shear modulus at shallow depths in comparison to standard earth models has the effect of increasing the amount of slip estimated from seismic moment determinations thereby increasing tsunami amplitude the effect of using depth dependent shear modulus values is tested by modeling the tsunami from the 1992 nicaraguan tsunami earthquake using a previously determined moment distribution ihmle 1996a we find that the tide gauge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution because excitation of seismic waves also depends on elastic heterogeneity it is important particularly for the inversion of short period waves that a consistent seismic tsunami shear modulus model be used for calculating slip distributions

modeling short and long term slow slip events in the seismic cycles of large subduction earthquakes 1 slow slip events sses occur in the deeper extents of areas where large interplate earthquakes are expected in subduction zones such as the nankai region of japan and the cascadia region of north america in the nankai region sses are divided into long and short term sses depending on their duration and recurrence interval we modeled and examined the occurrence of long and short term sses and changes in their behavior during the seismic cycles of large interplate earthquakes in these numerical simulations we adopted a rate and state dependent friction law with cutoff velocities and assumed that the distribution of pore fluid controls the recurrence interval of both long and short term sses the recurrence intervals of reproduced short term sses decrease during a long term sse as observed in western shikoku in the nankai region the recurrence intervals of both types of sses become shorter in the later stages of interseismic periods large interplate earthquakes nucleate between the region where sses occur and the locked region of the large earthquakes as suggested from observations of the 1944 tonankai earthquake our numerical results suggest that the stress buildup process in a seismic cycle affects the recurrence behavior of sses

triggering of tremor and inferred slow slip by small earthquakes at the nankai subduction zone in southwest japan the correlation of earthquakes with tremor and slow slip has not been clearly quantified we investigate 12 year earthquake and tremor catalogs for southwest japan and find that nearby small intraslab earthquakes are weakly correlated with tremor in particular the intraslab earthquakes with magnitudes 2 7 tend to be followed by tremor more often than expected at random by a factor of 2 to 6 the excess number of tremor before earthquakes is not as significant although marginally more than expected the underlying physical mechanism of the observed triggering of tremor and inferred slow slip by earthquakes is most likely to be the dynamic stress changes several to several tens of kilopascals rather than the much smaller static stress changes the rate of triggering of tremor by earthquakes is similar to although somewhat lower than rates observed for similar amplitude stress changes due to the lower frequency teleseismic surface waves and tidal stressing

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law

earthquake rupture propagation inferred from the spatial distribution of fault rock frictional properties abstract the frictional properties of fault rocks that rupture during earthquakes are expected to affect the nucleation and propagation of seismic slip but these properties and their variation under in situ earthquake conditions are typically unknown here we present experimental results on the variation of frictional properties of fault rocks from the alhama de murcia fault amf in se spain which ruptured in the 2011 lorca earthquake in the epicentral area the fault zone is characterized by the presence of abundant phyllosilicate rich gouges that surround more competent fractured lenses of the phyllitic basement measurements of lineaments and orientations of r shears in the gouges are consistent with cmt solutions of the 2011 lorca earthquake suggesting that the bulk of displacement on the amf was accommodated within similar gouges at depth in order to evaluate the frictional properties of the fault rocks of the amf we performed rotary shear experiments under hydrothermal conditions of samples obtained from surface outcrops of the amf zone progressively simulating deeper levels in the crust by stepping temperature effective normal stress and fluid pressure a negative velocity dependence of friction expressed as a negative value of the rate and state friction rsf parameter a b and which is a prerequisite for the nucleation of an instability was observed only for samples derived from competent lenses under hydrothermal conditions gouge derived samples exhibited only velocity strengthening properties i e positive values of a b which increase with deeper conditions in particular with increasing effective normal stress combined with our outcrop observation of the anastomosing nature of gouges surrounding more competent lenses of fractured protolith our results suggest that the 2011 lorca earthquake nucleated in the competent lenses followed by propagation of slip into the frictionally weaker gouges the inferred upward propagation direction of the 2011 lorca earthquake is consistent with propagation of the rupture into the velocity strengthening gouges which at shallower levels provide a smaller barrier to seismic slip due to lower values of effective normal stress and a b our results suggest that the spatial variation of the frictional properties along the amf was an important factor controlling the nucleation and propagation of seismic slip which together with the shallow hypocenter close to the city of lorca led to serious damage we infer that understanding of such variations in frictional properties may significantly improve seismic hazard evaluations in tectonically active regions

slow slip on the northern hikurangi subduction interface new zealand 1 in october 2002 a surface displacement episode of 20 30 mm magnitude was observed over a 10 day period on two continuous global positioning system gps instruments near gisborne north island new zealand we interpret this to result from slow slip on the northern hikurangi subduction interface using ten years of regional campaign gps 1995 2004 and recent continuous gps data we estimate the recurrence interval for similar events to be 2 3 yrs in november 2004 a similar slow slip event occurred within this recurrence period the 2002 event can be modeled by 18 cm of slow slip near the down dip end of the seismogenic zone on the subduction interface offshore of gisborne the campaign gps data show that the 2002 slow slip event had little effect on regional strain patterns

future cascadia megathrust rupture delineated by episodic tremor and slip 1 a suite of 15 episodic tremor and slip events imaged between 1997 and 2008 along the northern cascadia subduction zone suggests future coseismic rupture will extend to 25 km depth or 60 km inland of the pacific coast rather than stopping offshore at 15 km depth an ets derived coupling profile accurately predicts gps measured interseismic deformation of the overlying north american plate as measured by 50 continuous gps stations across western washington state when extrapolated over the 550 year average recurrence interval of cascadia megathrust events the coupling model also replicates the pattern and amplitude of coseismic coastal subsidence inferred from previous megathrust earthquakes here for only the washington state segment of the cascadia margin this translates into an mw 8 9 earthquake with significant moment release close to its metropolitan centers

teleseismic search for slow precursors to large earthquakes some large earthquakes display low frequency seismic anomalies that are best explained by episodes of slow smooth deformation immediately before their high frequency origin times analysis of the low frequency spectra of 107 shallow focus earthquakes revealed 20 events that had slow precursors 95 percent confidence level 19 were slow earthquakes associated with the ocean ridge transform system and 1 was a slow earthquake on an intracontinental transform fault in the east african rift system these anomalous earthquakes appear to be compound events each comprising one or more ordinary fast ruptures in the shallow seismogenic zone initiated by a precursory slow event in the adjacent or subjacent lithosphere

earthquakes triggered by slow slip at the plate interface in the hikurangi subduction zone new zealand 1 an earthquake swarm in the crust of the subducted plate in the southern hikurangi subduction zone in aprilmay 2004 was most likely triggered by slow slip at the plate interface directly downdip during the previous year double difference relocation indicates that the swarm represents incremental slip on adjacent patches of the same fault a subsequent ml 5 5 earthquake in january 2005 occurred on a separate deeper fault sub parallel to that of the 2004 swarm but separated from it by about 1 km this suggests a rapid decrease in mechanical damage in the slab with depth below the plate interface the normal faulting observed in both earthquake sequences represents unbending of the subducted plate such unbending may be diagnostic of deformation on the deeper edge of a strongly coupled region of the plate interface resulting from adjacent slow slip citation reyners m and s bannister 2007 earthquakes triggered by slow slip at the plate interface in the hikurangi subduction zone new zealand geophys res lett 34 l14305 doi 10 1029 2007gl030511

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

the 2006 aseismic slow slip event in guerrero mexico new results from gps 1 crustal deformation measurements from guerrero mexico were made with continuous gps instrumentation this network spans 75 km along the coast and a 275 km transect from the coast to mexico city which is perpendicular to the middle america trench a large aseismic slip transient occurred from april to december 2006 yielding horizontal displacements of nearly 6 cm in the direction opposite to that recorded interseismically this transient slip episode closely follows a series of m5 deep intraplate events that occurred in the northwestern portion of the guerrero region both horizontal and vertical displacements are inverted for the guerrero gps network to determine slip along the subduction interface using a simple fault model that accommodates changes in the subduction zone geometry it is found that slip is concentrated on interior fault patches and propagated to the southeast this slip event has a minimum equivalent mw of 7 5

diverse slow slip behavior at the hikurangi subduction margin new zealand 1 the installation of a continuous gps cgps network in new zealand since 2002 has enabled the discovery of 15 slow slip events at the hikurangi subduction margin our analysis and interpretation of the cgps data reveal a marked diversity in characteristics of slow slip events sses in the north island with durations varying from 6 days to 1 5 years equivalent moment release between mw 6 3 7 2 and recurrence intervals of repeating sses on the order of 2 years to more than 5 years the duration and magnitude characteristics of hikurangi sses appear to be related to the depth where they occur the deepest longest duration and largest sses occur at the southern hikurangi margin near the downdip limit of deep down to 40 km depth strong interseismic coupling the shallowest shortest duration smallest and most frequent sses occur at the northern and central hikurangi margin near the downdip limit of unusually shallow 10 15 km depth interseismic coupling the direction of slip on the interface in the hikurangi sses is consistent with slip partitioning at the hikurangi subduction margin we also show that moment accumulation rates on the interface for the time periods between sses are 40 higher than for the period of more than a decade averaging through the sses indicating that sses compose a major portion of the overall moment release budget of the hikurangi subduction interface

fluid budgets along the northern hikurangi subduction margin new zealand the effect of a subducting seamount on fluid pressure we estimate fluid sources around a subducted seamount along the northern hikurangi subduction margin of new zealand using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data calculated fluid sources are input into an independent fluid flow model to explore the key controls on overpressure generation to depths of 12 km in the thermomechanical models sediment transport through and beneath the wedge is calculated assuming a pressure sensitive frictional rheology the change in porosity pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration our calculations yield more precise information about source locations in time and space than previous averaged estimates for the hikurangi margin the volume of fluid release in the wedge is smaller than previously estimated from margin averaged calculations 14 m3 yr 1 m 1 and is exceeded by fluid release from underlying subducting sediment 16 m3 yr 1 m 1 clay dehydration contributes only a small quantity of fluid by volume 2 m3 yr 1 m 1 from subducted sediment but the integrated effect is still significant landward of the seamount fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid flow models using subducted sediment permeability derived from porosity and testing two end members for decollement permeability models in which the decollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment however if the subduction interface becomes impermeable with depth significant fluid overpressure develops in subducting sediment landward of the seamount the location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration shallow slow slip events sses landward of the seamount are caused by anomalous fluid pressures alternatively it may result from frictional effects of changing clay content along the subduction interface

episodic slow slip events and rate and state friction 1 there are several ways of generating episodic slow slip events in models of rate and state friction here i explore the possibility that they arise on velocity weakening faults whose length is tuned in some sense unlike spring block sliders which have a unique critical stiffness for instability elastically deformable faults have multiple length scales stiffnesses relevant to nucleation slow slip events occur when the fault is large enough to initiate an event but too small for that event to reach dynamic instability the available window of lengths depends upon the effective fracture energy of the growing nucleation zone for the aging evolution law this fracture energy increases rapidly with slip speed and the range of permissible lengths increases nearly as b b a where a and b are the coefficients of the velocity and state dependence of the frictional strength this range becomes very large for faults that are near velocity neutral a b however existing lab data firmly favor the slip law as a predictor of nucleation style and for this law the effective fracture energy increases much less rapidly with slip speed the range of fault lengths capable of hosting slow slip events in this case seems too small to explain why they are common to so many subduction zones slow slip can be stabilized over an exceedingly large range of fault lengths if the fracture energy increases with slip speed much more rapidly than for the aging law an appealing mechanism is inelastic dilation of the fault zone coupled with pore pressure reduction and diffusive recovery

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

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

episodic aseismic earthquake precursors shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves we know however that the earth allows failure over a wide range of timescales creep events occur on the san andreas fault system and for some earthquakes the high speed rupture is accompanied by a slower process1 5 here we report on episodic slow events which occurred before the japan sea earthquake of magnitude 7 7 on 26 may 1983 a sacks evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five month period before and immediately following the earthquake none has been detected after the large aftershocks the observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main shock fault plane such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes

slow slip events and the 2016 te araroa mw 7 1 earthquake interaction northern hikurangi subduction new zealand following a sequence of three slow slip events sses on the northern hikurangi margin between june 2015 and august 2016 a mw 7 1 earthquake struck 30 km offshore of the east cape region in the north island of new zealand on the 2nd september 2016 nz local time the earthquake was also followed by a transient deformation event sse or afterslip northeast of the north island closer to the earthquake source area we use data from new zealand s continuous global positioning system cgps networks to invert for the sse slip distribution and evolution on the hikurangi subduction interface our slip inversion results show an increasing amplitude of the slow slip towards the te araroa earthquake foreshock and mainshock area suggesting a possible triggering of the mw 7 1 earthquake by the later stage of the slow slip sequence we also show that the transient deformation following the te araroa earthquake ruptured a portion of the hikurangi trench northeast of the north island further north than any previously observed hikurangi margin sses our slip inversion and the coulomb stress calculation suggest that this transient may have been induced as a response to the increase in the static coulomb stress change downdip of the rupture plane on the megathrust these observations show the importance of considering the interaction between slow slip events seismic and aseismic events not only on the same megathrust interface but also on faults within the surrounding crust

slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip hikurangi subduction margin new zealand abstract travel times recorded onshore from active source marine seismic surveys were used to determine hikurangi forearc wavespeeds ray path midpoints sample forearc crust above the shallow v p from the seabed to 4 km depth consistent with rapidly deposited clastic sediments and near constant v p 5 0 0 2 km s at depths of 4 10 km within the forearc the northern region has slow slip events at shallow depths of c 5 15 km beneath the volume sampled by our analysis average travel times at offsets of 20 80 km are 1 s more than at equivalent offsets in the south and v p increases with depth within the forearc from 3 5 0 1 km s at 4 km depth to 4 5 0 2 km s at 10 km depth we interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously high porosity 10 with hydrostatic reference compaction models to show effective stress is low in both the north 27 10 mpa and south 36 14 mpa in the south pore space and conduits have a high chance of being localized on faults or sand layers the wedge is clearly compressionally critical and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework the observed wedge geometry and our estimate of fluid pressure within the wedge 0 87 0 05 suggest a weak overpressured subduction thrust in the north we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment a pore space geometry characterized by pervasively fractured rock and mudstone and a lower mean stress due to its stable and non critical wedge geometry we consider both end member assumptions of convergent and tensile failure the latter of which reveals the possibility of near lithostatic fluid pressure on the north hikurangi subduction interface although this remains unproven our observations are consistent with high fluid pressures encountered in petroleum wells at depths

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

source parameters of large historical 1918 1962 earthquakes south island new zealand we have studied 18 earthquakes of m w 5 5 within the north island new zealand region to determine fault parameters for earthquake hazards and regional tectonic studies since most large m w 6 5 earthquakes in the north island occurred prior to 1961 these events provide important information to supplement studies of more recent seismicity our results indicate that no large plate interface earthquakes have occurred in the central and southern north island for the past 80 years although six strike slip earthquakes of m w 6 8 occurred within the australian upper plate during an extremely active 25 year period that began in 1917 only the m w 7 2 strike slip events were associated with surface faulting indicating the difficulties that may arise in attempting to identify active faults within this region two earthquakes m w 6 9 7 1 off the northeastern north island in 1947 appear to have occurred along the plate interface and were associated with local tsunamis having runup heights of up to 10 m two m w 6 8 events also occurred within the pacific lower plate highlighting the hazards related to intraslab events slip vectors for the earthquakes studied suggest that the majority of transcurrent motion along the plate margin is accommodated within the australian plate similar to the results obtained from studies of more recent smaller earthquakes pure thrusting occurs along the plate interface and t axes of intraslab events indicate downdip tension in the pacific plate

madden julian oscillation in a climate model with a well resolved stratosphere 1 this work evaluates how well a coupled ocean atmospheric climate model with a well resolved stratosphere captures the observed madden julian oscillation mjo signal in the upper troposphere and lower stratosphere the model is the centro euro mediterraneo per i cambiamenti climatici cmcc coupled modeling system cmcc cms with 95 atmospheric levels cmcc cms produces mjo composites of precipitation that are very similar to those of the observations and the era 40 reanalysis product furthermore model precipitation is found to propagate eastward near the observed speed there is also strong agreement in the tropics and subtropics in the temperature and flows at 100 and 200 hpa upon comparing the composites based on the full 340 years of model output and those for the past 20 years of the model output with those of the era 40 reanalysis at the 100 hpa level the temperature anomalies in the model output and reanalysis propagate eastward at about 7 m s 1 at the equator 25 n and 65 n longitude and height cross sections at 25 n suggest vertically propagating rossby waves up to near the 10 hpa level in both the model and observations latitude and height cross sections of temperature and zonal and meridional winds indicate strong vertical propagation in the tropics in both the model and reanalysis overall the cmcc cms model shows mjo characteristics in the tropical and subtropical troposphere and lower stratosphere that are in very good agreement with observed analyses

a large slow slip event on the central hikurangi subduction interface beneath the manawatu region north island new zealand 1 we present continuous gps observations of a surface deformation episode in the manawatu region of the north island new zealand lasting 18 months from january 2004 to june 2005 we invert vertical and horizontal displacements of gps sites during the event for slow slip on the hikurangi subduction thrust we find that up to 350 mm of slip occurred on the thrust in the transition zone between strong interseismic coupling and aseismic creep if the entire slip in the manawatu slow slip event had occurred instantaneously it would have resulted in an mw 7 0 earthquake given the large amount of slip we expect that the recurrence interval for similar slow slip events may be ten years or more

variability of near term probability for the next great earthquake on the cascadia subduction zone the threat of a great m 9 earthquake along the cascadia subduction zone is evidenced by both paleoseismology data and current strain accumulation along the fault on the basis of recent information on the characteristics of this subduction system we estimate the conditional probabilities of a great earthquake occurring within the next 50 years and their variabilities the most important vari ation is associated with the existence of episodic slow slip on the deep portion of the subduction interface we show that these events modulate the conditional probability dramatically over their 14 month cycle during the 2 week slow slip events the weekly probability of a great earthquake is about 30 to 100 times as high as it is during any week of the rest of the year near term probabilities also vary significantly with the assumed distribution of earthquake recurrence intervals under a reference scenario of unimodal distribution of recurrence intervals about 500 600 years the 50 year conditional probability is low 0 12 at a 95 confidence interval how ever under a tantalizing bimodal distribution hypothesis this probability could be either four times as high 6 45 or four times as low less than 1 depending on whether the current interval is short 350 years or long 850 years

shear behavior of dfdp 1 borehole samples from the alpine fault new zealand under a wide range of experimental conditions the alpine fault is a major plate boundary fault zone that poses a major seismic hazard in southern new zealand the initial stage of the deep fault drilling project has provided sample material from the major lithological constituents of the alpine fault from two pilot boreholes we use laboratory shearing experiments to show that the friction coefficient of fault related rocks and their precursors varies between 0 38 and 0 80 depending on the lithology presence of pore fluid effective normal stress and temperature under conditions appropriate for several kilometers depth on the alpine fault 100 mpa 160 c fluid saturated a gouge sample located very near to the principal slip zone exhibits 0 67 which is high compared with other major fault zones targeted by scientific drilling and suggests the capacity for large shear stresses at depth a consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence critical nucleation patch lengths estimated using representative values of the friction velocity dependent parameter a b and the critical slip distance d c combined with previously documented elastic properties of the wall rock may be as low as 3 m this small value consistent with a seismic moment m o 4 1010 for an m w 1 earthquake suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3 7 km

simultaneous long term and short term slow slip events at the hikurangi subduction margin new zealand implications for processes that control slow slip event occurrence duration and migration 1 we document a sequence of simultaneous short term and long term slow slip events sses at the hikurangi subduction zone during the 2010 2011 period the sequence of short term events each 2 3 weeks in duration ruptured much of the shallow plate interface 15 km at central and northern hikurangi over a 6 month period was accompanied by microseismicity and involved patchy irregular migration of sse slip we suggest that the patchy migration of the short term sse is due to large scale 100 3500 km heterogeneities on the plate interface related to seamount subduction and sediment subduction and or underplating this is in contrast to a 2010 2011 long term sse at the central hikurangi margin which evolved steadily over 1 5 years and ruptured much of the plate interface between 20 and 70 km depth we suggest that the occurrence of long term versus short term sses at hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface the long term sse sequence began 1 year before the short term sequence coulomb stress change models suggest that the long term sse may have triggered initiation of the subsequent short term sse sequence initiation of the short term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior alternatively it could be characteristic of a region undergoing partial interseismic coupling this is in contrast to sses observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior

newly observed deep slow slip events at the central hikurangi margin new zealand implications for downdip variability of slow slip and tremor and relationship to seismic structure 1 we present previously undocumented deep 25 45 km depth moderate duration 2 3 months slow slip events sses directly downdip of known shallow 15 km depth short term 2 3 weeks sses at the central hikurangi margin new zealand the moderate duration sses produce 2 6 mm of horizontal displacement at continuous global positioning system stations in 2006 and 2008 similar to cascadia sses they involve 2 5 cm of slip over 5000 km2 of the subduction interface tremor occurs just downdip of the deep sse source area we also present results from a large shallow 12 km sse in 2013 just updip of the 2006 2008 sses the location of deep sses between shallow short term sses and tremor indicates strong downdip variation of subduction interface slip behavior at central hikurangi the deep sses occur where qp and vp vs data suggest an accumulation of fluid rich underplated sediment at the interface consistent with the idea that sses occur under high fluid pressure

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

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

integrating field observations and fracture mechanics models to constrain seismic source parameters for ancient earthquakes most of our understanding of earthquake rupture comes from interpretation of strong ground motion seismograms however near rupture tip fields of stress and particle motions are difficult to resolve in particular the decay of frictional resistance from a peak value at the leading tip of the rupture to a residual kinetic value and subsequent healing characterizes the earthquake process yet the nature of this evolution in situ is still unclear knowledge of this coseismic frictional constitutive behavior has been supplemented by laboratory experiments yet scaling laboratory experiments to natural faults is non trivial because these experiments do not exactly reproduce the boundary conditions governing natural earthquakes field investigations of exhumed faults provide the spatial resolution needed to integrate remote seismological observations laboratory experiments and numerical models of the rupture process for natural ruptures here we build on previous work that showed that the orientation of pseudotachylyte injection veins found along the gole larghe fault zone in granitoid rocks of the italian alps can be used to infer rupture directivity and velocity we demonstrate that the length of these veins can be used to further constrain the rupture size slip weakening distance stress drop and fracture energy the results are consistent with seismological observations and recent friction experiments incorporating rapid accelerations placing constraints on coseismic frictional evolution in granitoid rocks

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

foreshocks during the nucleation of stick slip instability 1 we report on laboratory experiments which investigate interactions between aseismic slip stress changes and seismicity on a critically stressed fault during the nucleation of stick slip instability we monitor quasi static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike slip fault 2 m long and 0 4 m deep in a saw cut sample of sierra white granite with 14 piezoelectric sensors we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture we observe localized aseismic fault slip in an approximately meter sized zone in the center of the fault while the ends of the fault remain locked clusters of high frequency foreshocks mw 6 5 to 5 0 can occur in this slowly slipping zone 5 50 ms prior to the initiation of dynamic rupture their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault the meter sized nucleation zone is generally consistent with theoretical estimates but source radii of the foreshocks 2 to 70 mm are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur we propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections which would typically slip aseismically may radiate seismic waves if they are rapidly stressed fault behavior of this type may provide physical insight into the mechanics of foreshocks tremor repeating earthquake sequences and a minimum earthquake source dimension

precursory changes in seismic velocity for the spectrum of earthquake failure modes temporal changes in seismic velocity during the earthquake cycle have the potential to illuminate physical processes associated with fault weakening and connections between the range of fault slip behaviors including slow earthquakes tremor and low frequency earthquakes1 laboratory and theoretical studies predict changes in seismic velocity prior to earthquake failure2 however tectonic faults fail in a spectrum of modes and little is known about precursors for those modes3 here we show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults we systematically altered the stiffness of the loading system to reproduce the transition from slow to fast stick slip and monitored ultrasonic wave speed during frictional sliding we find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure which suggests that real time monitoring of active faults may be a means to detect earthquake precursors

seismic reflection character of the hikurangi subduction interface new zealand in the region of repeated gisborne slow slip events summary we use seismic reflection data to map the geometry and character of the subduction interface in the gisborne area of the hikurangi subduction margin new zealand which experiences repeated shallow 15 km slow slip events the reflection character and geometry in this area is highly variable which we interpret to be related to the subduction of seamounts and underthrust sediments three zones of high amplitude interface reflectivity hrz 1 2 and 3 are interpreted to be the result of fluid rich sediments that have been entrained with subducting seamounts the interface above the hrz zones is shallower than the surrounding areas by 2 4 km due to the warping of the interface to accommodate seamount subduction these zones of high amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008 we hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high amplitude reflectivity due to 1 the enhanced underthrusting of fluid rich sediment 2 reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and 3 potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations from our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events

spectrum of slip behaviour in tohoku fault zone samples at plate tectonic slip rates the tohoku earthquake ruptured a fault that also generates slow slip events laboratory experiments on rock samples from the fault show that this spectrum of slip behaviours is a natural consequence of shearing at slow plate convergence rates

the minimum scale of grooving on faults at the field scale nearly all fault surfaces contain grooves generated as one side of the fault slips past the other grooves are so common that they are one of the key indicators of principal slip surfaces here we show that at sufficiently small scales grooves do not exist on fault surfaces a transition to isotropic roughness occurs at 4 500 mm although the scale of the transition can vary even between locales on a single fault the aspect ratio of the roughness at the transition is well defined for a given fault we interpret the transition between grooved and ungrooved scales as a transition in deformation mode of asperities on the slip surface grooves can form when a hard indenter slides past a softer surface at small scales the asperities appear to yield plastically and therefore do not generate grooves as hard indenters the plastic yielding can be a consequence of the high shear strains required to deform the surfaces at small scales where the aspect ratio roughness is high the transition to plastic yielding is predicted to occur at a specific aspect ratio for each fault as observed the new observation both shows a limit to one of the most commonly observed features of faults and suggests a change in the mode of failure of faults as a function of scale 2016 the authors

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

keys and pitfalls in mesoscale fault analysis and paleostress reconstructions the use of angelier s methods whereas most of the stress inversion methods using fault slip data only minimize the angle between the measured striation and a computed shear stress to find the best fitting reduced stress tensor angelier 1990 proposed an alternative method named invd that also takes into account the relative shear stress magnitude which allows the fault to move using artificial datasets and particular fault geometries we compare this method with one of the classical methods based on the minimization of the shear slip angles r4dt angelier 1984 and we show that in most cases the new method has improved the quality of the results furthermore as proposed by angelier we point out that the quality of the stress inversion primarily depends on the quality of the field data we give advice and warn about some pitfalls concerning determination of sense of slip on fault planes recognition of successive faulting events and their chronology drawer or wedge faults stress permutations faults in vertical bedding we also argue that in case of tilted sequences fault diagrams should not be presented without bedding planes but we show that stress inversions when realized with caution and with the correct method can havemuch more applications than reconstructing stress fields like for determining the paleo horizontal the nature and the sense of motion of large faults the chronology and age of large structures

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