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

earthquakes good tidings tidal stresses in the earth s crust don t seem to influence earthquakes water wells on the other hand seem strangely sensitive to seismic activity explanations are now proposed

long range triggered earthquakes that continue after the wave train passes 1 large earthquakes can trigger distant earthquakes in geothermal areas some triggered earthquakes happen while the surface waves pass through a site but others occur hours or even days later does this prolonged seismicity require a special mechanism to store the stress from the seismic waves that differs from ordinary aftershock mechanisms these questions have driven studies of long range triggering since the phenomenon s discovery here i attempt to answer the questions by examining the statistics of triggered sequences two separate observations are consistent with the prolonged sequences being simply local aftershocks of earthquakes triggered early in the wave train first the sequences obey omori s law over both short 1 hour and longer 5 day time intervals secondly the number of observed triggered earthquakes in the first hour after the wave train can be predicted from the number of earthquakes triggered during the wave train even the very vigorous 10 day triggering at long valley from the 1992 landers mw 7 3 earthquakes can be interpreted as the aftershocks of either a local m 4 1 earthquake or an equivalent combination of several smaller mainshocks therefore long range triggering does not need to include a mechanism to produce sustained stresses other than the process that generates aftershocks of the earthquakes that occur while the wave train is passing

shear and normal load perturbations on a two dimensional continuous fault 2 dynamic triggering 1 the influence of normal and shear stress static perturbations on a strike slip fault is addressed on the basis of a two dimensional continuous and quasi dynamic model friction along the fault plane is described using a rate and state friction law with depth variable properties normal and shear stress perturbations result in similar effects in terms of earthquake triggering iftm s is constant t ands being the amplitude of the shear and normal stress fluctuations respectively and m being a constant which can be interpreted as the static friction coefficient on the fault in a coulomb failure model therefore the coulomb stress changecff tm s is a useful tool to account simultaneously for normal and shear stress variations in our model we also show that when estimating the clock advance or clock delay of an earthquake the simple coulomb failure model is at first order in good agreement with our results during the first 90 of the earthquake cycle however it differs significantly during the last 10 due to the sharp velocity increase predicted by the rate and state friction law before rupture this suggests that as long as static variations of stress are concerned realistic fault models using rich laboratory based friction laws like rate and state friction laws may lead topredictions fairly close to the ones made using one of the simplest failure model i e the coulomb failure model this may explain why coulomb stress change computations although often based on drastic approximations have been able in many occasions to explain earthquake triggering sequences index terms 7209 seismology earthquake dynamics and mechanics 7215 seismology earthquake parameters 7260 seismology theory and modeling keywords earthquake triggering static triggering coulomb stress change rate and state friction laws clock advance delay

observing earthquakes triggered in the near field by dynamic deformations we examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments our analysis focuses on seismicity at close range within the aftershock zone complementing published studies of long range triggering our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction therefore we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically with a majority of triggered earthquakes in the direction of rupture propagation bilaterally propagating ruptures also may be directive and we propose simple criteria for assessing their directivity we compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes m 5 7 to m 8 1 that occurred in california and idaho in the united states the gulf of aqaba syria guatemala china new guinea turkey japan mexico and antarctica nine of these mainshocks had clearly directive unilateral ruptures of these nine seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction the two exceptions include an earthquake preceded by a comparable magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results similar but weaker correlations were found for the bilaterally rupturing earthquakes we studied although the static stress change also may trigger seismicity it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus for several of the directive earthquakes we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change manuscript received 1 march 2002

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

evidence of earthquake triggering by the solid earth tides article i nfo clearevidence forearthquake triggeringbythe earth tides hasremained elusive for morethan a century using the largest global earthquake catalog available the neic catalog with 442412 events we observe a clear correlation with 99 confidence between the phase of the solid earth tide and the timing of seismic events earthquakes occur slightly more often at the time of ground uplift by the earth tide i e when normal stresses are reduced within the lithosphere we observe that this phase distribution anomaly is larger for smaller and shallowerearthquakes although earthquakes inregionswithdominantlynormalandstrike slip faulting seem to exhibit more tidal triggering than regions dominated by thrust faulting there is no statistically significant evidence for a focal mechanism dependence on earthquake triggering finally we show here that it is highly probablethat the observedtriggeringiscaused bythe solid earth tide ratherthan byloading fromthe oceanor atmospheric tides although an additional impact due to loading from ocean tides is possible and probable we cannot detect it here because the earthquake database is not sufficiently complete and homogeneous more smallmagnitudeearthquakesinoceanicareasareneeded ourresultsareconsistentwiththeideaofadamped

exploring links between arctic amplification and mid latitude weather 1 this study examines observed changes 1979 2011 in atmospheric planetary wave amplitude over northern mid latitudes which have been proposed as a possible mechanism linking arctic amplification and mid latitude weather extremes we use two distinct but equally valid definitions of planetary wave amplitude termed meridional amplitude a measure of north south meandering and zonal amplitude a measure of the intensity of atmospheric ridges and troughs at 45 n statistically significant changes in either metric are limited to few seasons wavelengths and longitudinal sectors however in summer we identify significant increases in meridional amplitude over europe but significant decreases in zonal amplitude hemispherically and also individually over europe and asia therefore we argue that possible connections between arctic amplification and planetary waves and implications of these are sensitive to how waves are conceptualized the contrasting meridional and zonal amplitude trends have different and complex possible implications for midlatitude weather and we encourage further work to better understand these

structure seismicity and thermal regime of the queen charlotte transform margin 1 this study examines structural seismicity thermal and current deformation constraints on the tectonic regime of the queen charlotte margin located off the northwest coast of canada the margin is primarily a transcurrent boundary between the oceanic pacific plate and the continental north america plate but plate motion models indicate 20 of current oblique convergence that initiated 5 myr the estimated 120 km of convergence has been variously explained as accommodated by underthrusting or by crustal shortening the near vertical queen charlotte transcurrent fault and lack of wadati benioff earthquakes argue against underthrusting however structural models for the margin from receiver function and refraction studies show little evidence of crustal thickening the pattern of landward decreasing heat flow gravity anomalies an accretionary prism landward of the trench or trough down bowing ocean crust and sediments horizons and a variety of other geophysical evidence support underthrusting receiver function analysis on teleseismic data indicates a 10 km thick low velocity layer at 35 km depth dipping 20 to the east which is interpreted as underthrusting oceanic crust the heat flow across the fault zone rapidly decreases landward in agreement with predictions from a finite element thermal model of underthrusting

the role of barents sea ice in the wintertime cyclone track and emergence of a warm arctic cold siberian anomaly abstractsea ice variability over the barents sea with its resultant atmospheric response has been considered one of the triggers of unexpected downstream climate change for example east asia has experienced several major cold events while the underlying temperature over the arctic has risen steadily to understand the influence of sea ice in the barents sea on atmospheric circulation during winter from a synoptic perspective this study evaluated the downstream response in cyclone activities with respect to the underlying sea ice variability the composite analysis including all cyclone events over the nordic seas revealed that an anticyclonic anomaly prevailed along the siberian coast during light ice years over the barents sea this likely caused anomalous warm advection over the barents sea and cold advection over eastern siberia the difference in cyclone paths between heavy and light ice years was expressed as a warm arctic cold siberian wacs anomaly the lower baroclinicity over the barents se

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

crustal thickness beneath the queen charlotte basin canada results of a seismically constrained gravity inversion a gravity inversion routine was used to calculate crustal thickness variations in the queen charlotte basin western canada first a crustal bouguer anomaly was derived using a digital well constrained depth to basement map and densities determined from well logs and rock samples this anomaly was subsequently inverted for crustal thickness assuming a single density contrast at depth and an average crustal thickness provided by seismic refraction data the results restricted to that portion of the basin for which there are good quality data show crustal thickness variations of the order of 12 km regions of thin crust are located off northern vancouver island 20 km along the axis of hecate strait and in northern queen charlotte sound 24 km thicker crust is associated with central queen charlotte ridge and northeastern hecate strait tests show these trends in crustal thickness prevail for all reasonable ranges of input parameters calculated residual gravity anomalies contain information useful for mapping the distribution of volcanics interbedded with sediments in subbasins and for refining estimates of depth to basement derived from seismic data alone our results are for the most part in excellent agreement with seismically derived estimates of moho depth they show minimum crustal thickness beneath regions of maximum sediment accumulation supporting an extensional origin for the basin they indicate that this extension was quite heterogeneous and yield minimum estimates which vary from 13 in some parts of the basin to as much as 65 in others the results can be used to discriminate between models on the tertiary evolution of the basin the observed trends are not predicted by the lithospheric flexure or simple margin perpendicular extension models but are consistent with a model of crustal thinning by distributed shear in a transtensional regime

dynamic triggering by strong motion p and s waves evidence from the 1999 chi chi taiwan earthquake high frequency band pass filtering of acceleration records from the 1999 chi chi taiwan earthquake m w 7 6 resolves the continuous signal into a series of relatively short duration discrete energy bursts we hypothesize that these bursts originate near the individual stations as small shallow events that have been dynamically triggered by the p and s waves generated by the chi chi mainshock however we cannot rule out the possibility that they originate from some nonseismic source associated with the surface such as buildings or trees bursts are observed only during the seismic signal and not in the pre or postsignal noise the bursts are not likely to be generated by the instruments because groupings of three colocated individual instruments record identical bursts also we show that the bursts are not due to the band pass filtering of instrumentally generated step functions the hypothesis that they are local events is supported by the observation of bursts in the 40 hz frequency band at distances up to 170 km from the epicenter if the bursts originated on the chi chi fault plane as hypothesized by chen et al 2006 based on their analysis of recordings within 20 km from the chelungpu fault then they should not be observable at this distance assuming any reasonable value of crustal attenuation assuming a local origin we estimate an average local event magnitude of m w 0 2 and a source receiver distance of approximately 1 km we extended our analysis to lower stress levels by analyzing records from a smaller m w 5 3 event that was recorded by many of the same instruments used in the chi chi analysis for this event bursts are observed only on the accelerograms from stations relatively close to the mainshock hypocenter analysis of the combined data set from both mainshocks suggests a stress threshold for triggering in the range 0 03 0 05 mpa for s wave triggering and 0 0013 0 0033 mpa for p wave triggering consistent with prior observations of surface wave triggering

strain dependent partial slip on rock fractures under seismic frequency torsion author s saltiel s bonner bp ajo franklin jb abstract published 2017 this article is a u s government work and is in the public domain in the usa measurements of nonlinear modulus and attenuation of fractures provide the opportunity to probe their mechanical state we have adapted a low frequency torsional apparatus to explore the seismic signature of fractures under low normal stress simulating low effective stress environments such as shallow or high pore pressure reservoirs we report strain dependent modulus and attenuation for fractured samples of duperow dolomite a carbon sequestration target reservoir in montana blue canyon dome rhyolite a geothermal analog reservoir in new mexico and montello granite a deep basement disposal analog from wisconsin we use a simple single effective asperity partial slip model to fit our measured stress strain curves and solve for the friction coefficient contact radius and full slip condition these observations have the potential to develop into new field techniques for measuring differences in frictional properties during reservoir engineering manipulations and estimate the stress conditions where reservoir fractures and faults begin to fully slip

a new observation of dynamically triggered regional seismicity earthquakes in greece following the august 1999 izmit turkey earthquake the m w 7 4 izmit turkey earthquake triggered widespread regional seismicity in greece over a study region extending from 400 km to nearly 1000 km away from the epicenter small events began immediately after the passage of the mainshock surface waves suggesting that the transient stresses of the seismic waves were the trigger the increase in cataloged earthquakes in ordinary continental crust is a new observation and is statistically significant at the 95 level unlike the previous example of distant triggering during the landers earthquake the activated seismicity occurred entirely in non volcanic areas the greek sites were triggered by waves with amplitudes at least a factor of 3 lower than the observed triggering threshold for imperial valley we speculate that dynamic triggering on a regional scale results in countrywide episodes of increased seismicity or superswarms in regions with low triggering thresholds such as greece

transpression between two warm mafic plates the queen charlotte fault revisited the queen charlotte fault is a transpressive transform plate boundary between the pacific and north american plates offshore western canada previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate the latter has been the preferred model both plates are warm and mafic and have similar mechanical structures new multichannel seismic reflection data show a near vertical queen charlotte fault down to the first water bottom multiple significant subsidence east of the queen charlotte fault a large melange where the fault is in a compressive left step and faulting which involves oceanic basement gravity modeling of profiles indicates that moho varies fairly smoothly across the plate boundary isostatic anomalies indicate that the pacific plate is flexed downward adjacent to the queen charlotte fault upward flexure of north america along with crust thickened relative to crust in the adjacent basin creates topography known as the queen charlotte islands combined with other regional studies these observations suggest that the plate boundary is a vertical strike slip fault and that transpression is taken up within each plate

seismic velocity structure across the 2013 craig alaska rupture from aftershock tomography implications for seismogenic conditions abstract the 2013 craig alaska mw 7 5 earthquake ruptured along 150 km of the queen charlotte fault qcf a right lateral strike slip plate boundary fault separating the pacific and north american plates regional shear wave analyses suggest that the craig earthquake rupture propagated in the northward direction faster than the s wave supershear theoretical studies suggest that a bimaterial interface such as that along the qcf which separates oceanic and continental crust with differing elastic properties can promote supershear rupture propagation we deployed short period ocean bottom seismometers obs as a part of a rapid response effort less than four months after the craig earthquake mainshock during a 21 day period 1 133 aftershocks were recorded by 8 obs instruments aftershock spatial distribution indicates that the base of the seismogenic zone along the qcf approaches 25 km depth consistent with a thermally controlled fault rheology expected for igneous rocks at oceanic transform faults the spatial distribution also provides supporting evidence for a previously hypothesized active strand of the qcf system within the pacific plate tomographic traveltime inversion for velocity structure indicates a low velocity vp and vs zone on the pacific side of the plate boundary at 5 20 km depths where neogene pacific crust and upper mantle seismic velocities average 3 11 slower than the north american side where the paleozoic north american crust is seismically faster our results suggest that elastic properties along the studied portion of the qcf are different than those of a simple oceanic continental plate boundary fault in our study region velocity structure across the qcf while bimaterial does not support faster material on the west side of the fault which has been proposed as one possible explanation for northward supershear propagation during the craig earthquake instead we image low velocity material on the west side of the fault explanations could include that part of the rupture was subshear or that fault damage zone properties or fault smoothness are more important controls on supershear rupture than a bimaterial contrast

basement controlled structure and evolution of the queen charlotte basin west coast of canada abstract fault block tectonics usually associated with intracontinental areas characterizes the western part of the north america plate immediately adjacent to the pacific plate the long lived basement supported mosaic of fault bounded blocks controlled the evolution of this part of the west coast of canada since at least the late jurassic manifesting itself strongly in the tertiary this conclusion is based on recently collected geological and geophysical data gravity and magnetic maps and seismic profiles reveal orthogonal fault networks across the region in the tertiary queen charlotte sedimentary basin and on its pre tertiary rim the faults were reactivated and block movements sometimes inverted in the cenozoic similarity of fault networks across the region suggests inheritance of many structures in the basin from earlier time passage of the region over a mantle hot spot in the miocene may have been a partial cause of renewed block movements crosscutting relationships of large faults and their displacement histories onshore preclude regional strike slip faulting in the tertiary tectonic extension within the basin was small maximum 10 the previous model of formation of the queen charlotte basin by continental rifting sensu mckenzie 1978 is negated evolution of the region was not affected fundamentally by plate interactions at the adjacent continental margin in particular by the change from subduction to strike slip regime at 42 ma a similar structural style existed in the area before and after that event

an abrupt transition in the mechanical response of the upper crust to transpression along the queen charlotte fault the queen charlotte fault qcf is a major strike slip fault that forms the boundary between the pacific and north american plates from 51 to 58 n near 53 2 n the angle of oblique convergence predicted by the mid ocean ridge velocity morvel interplate pole of rotation decreases from 15 in the south to 15 in the north south of 53 2 n the convergent component of plate motion results in the formation of a 40 km wide terrace on the pacific plate west of qcf and earthquakes with thrust mechanisms including the 2012 haida gwaii earthquake sequence are observed north of 53 2 n in the primary rupture zone of the m 8 1 strike slip earthquake of 1949 the linear terrace disappears and topography of the continental slope west of the qcf is characterized by a complex pattern of ridges and basins that trend obliquely to the primary trace of the qcf deformation within the pacific plate appears to occur primarily through strike slip faulting with a minor thrust component on secondary synthetic faults the orientations of these secondary faults as determined from seismic reflection and bathymetric data are consistent with the reactivation of faults originally formed as ridge parallel normal faults and as thrust faults formed parallel to the qcf south of the bend at 53 2 n and subsequently translated to the north we suggest that an oblique convergence angle of 15 represents a critical threshold separating distinct crustal responses to transpression this result is consistent with theoretical and analog strain models of transpressive plate boundaries the sharpness of this transition along the qcf in contrast to purely continental transform boundaries may be facilitated by the relatively simple structure of oceanic crust and the presence of pre existing optimally oriented faults in the young pacific plate

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

effects of load point velocity on frictional instability behavior abstract triaxial experiments were conducted on sawcut westerly granite specimens sandwiched with a simulated gouge layer of ultrafine quartz over a wide range of normal stresses from 25 to 250 mpa tens and sometimes hundreds of repeated stick slip events were observed to persist over a total slip displacement up to 10 mm a transition from stick slip to stable sliding was observed as the result of cumulative slip displacement the total slip displacement over which repeated stick slip instabilities occurred before the transition to stable sliding decreases with increasing normal stress the stress drop amplitude decreases with increasing load point velocity the experimentally determined stress drop amplitude was also found to increase as a function of the time of stationary contact the dependence of stress drop amplitude on load point velocity and on time of stationary contact can be characterized empirically by a power law preliminary data suggest that the two effects are coupled the seismological implications of the experimental data are discussed at relatively high normal stresses above 250 mpa no systematic dependence of stress drop amplitude on load point velocity was observed an increase in gouge particle size stabilizes the frictional instability behavior a mixed mode of velocity weakening and velocity strengthening behavior was occasionally observed

a link between reduced barents kara sea ice and cold winter extremes over northern continents the recent overall northern hemisphere warming was accompanied by several severe northern continental winters as for example extremely cold winter 2005 2006 in europe and northern asia here we show that anomalous decrease of wintertime sea ice concentration in the barents kara b k seas could bring about extreme cold events like winter 2005 2006 our simulations with the echam5 general circulation model demonstrate that lower troposphere heating over the b k seas in the eastern arctic caused by the sea ice reduction may result in strong anti cyclonic anomaly over the polar ocean and anomalous easterly advection over northern continents this causes a continental scale winter cooling reaching 1 5 c with more than three times increased probability of cold winter extremes over large areas including europe our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it being in qualitative agreement with the simulated large scale atmospheric circulation realignment furthermore our results suggest that high latitude atmospheric circulation response to the b k sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then again back to cyclonic type of circulation as the b k sea ice concentration gradually reduces from 100 to ice free conditions we present a conceptual model which may explain the nonlinear local atmospheric response in the b k seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area

atmospheric impact of arctic sea ice loss in a coupled ocean atmosphere simulation the atmospheric response to an idealized decline in arctic sea ice is investigated in a novel fully coupled climatemodel experiment in this experimenttwo ensembles of single year model integrations are performed starting on 1 april the approximate start of the ice melt season by perturbing the initial conditions of sea ice thickness sit declines in both sea ice concentration and sit which result in sea ice distributions that are similar to the recent sea ice minima of 2007 and 2012 are induced in the ice loss regions there are strong 3k local increasesin sea surface temperature sst additionally there are remoteincreases in sst in the central north pacific and subpolar gyre in the north atlantic over the central arctic there are increases in surface air temperature sat of 8k due to increases in ocean atmosphere heatfluxes there are increases in sat over continental north america that are in good agreement with recent changes as seen by reanalysis data it is estimated that up to two thirds of the observed increase in sat in this region could be related to arctic sea ice loss in early summer there is a significant but weak atmospheric circulation response that projects onto the summer north atlantic oscillation nao in early summer and early autumn there is an equatorward shift of the eddy driven jet over the north atlantic as a result of a reduction in the meridional temperature gradients in winter there is no projection onto a particular phase of the nao

nucleation and triggering of earthquake slip effect of periodic stresses abstract results of stability analyses for spring and slider systems with state variable constitutive properties are applied to slip on embedded fault patches unstable slip may nucleate only if the slipping patch exceeds some minimum size subsequent to the onset of instability the earthquake slip may propagate well beyond the patch it is proposed that the seismicity of a volume of the earth s crust is determined by the distribution of initial conditions on the population of fault patches that nucleate earthquake slip and the loading history acting upon the volume patches with constitutive properties inferred from laboratory experiments are characterized by an interval of self driven accelerating slip prior to instability if initial stress exceeds a minimum threshold this delayed instability of the patches provides an explanation for the occurrence of aftershocks and foreshocks including decay of earthquake rates by time 1 a population of patches subjected to loading with a periodic component results in periodic variation of the rate of occurrence of instabilities the change of the rate of seismicity for a sinusoidal load is proportional to the amplitude of the periodic stress component and inversely proportional to both the normal stress acting on the fault patches and the constitutive parameter a 1 that controls the direct velocity dependence of fault slip values of a 1 representative of laboratory experiments indicate that in a homogeneous crust correlation of earthquake rates with earth tides should not be detectable at normal stresses in excess of about 8 mpa correlation of earthquakes with tides at higher normal stresses can be explained if there exist inhomogeneities that locally amplify the magnitude of the tidal stresses such amplification might occur near magma chambers or other soft inclusions in the crust and possibly near the ends of creeping fault segments if the creep or afterslip rates vary in response to tides observations of seismicity rate variations associated with seasonal fluctuations of reservoir levels appear to be consistent with the model

global ubiquity of dynamic earthquake triggering surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes this suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment earthquakes can be triggered by local changes in the stress field static triggering1 2 3 4 5 6 7 due to nearby earthquakes or by stresses caused by the passage of surface rayleigh and love waves from a remote large earthquake dynamic triggering8 9 10 11 12 13 14 15 16 17 18 however the mechanism frequency controlling factors and the global extent of dynamic triggering are yet to be fully understood because rayleigh waves involve compressional and dilatational particle motion volumetric changes as well as shearing whereas love waves only involve shearing triggering by either wave type implies fundamentally different physical mechanisms here we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes the low amplitude signals of such earthquakes would normally be masked by high amplitude surface waves our analysis reveals that out of 15 earthquakes studied of magnitude m greater than 7 0 that occurred after 1990 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the love and rayleigh waves we conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event

triggered creep as a possible mechanism for delayed dynamic triggering of tremor and earthquakes the passage of radiating seismic waves generates transient stresses in the earth s crust that can trigger slip on faults far away from the original earthquake source the triggered fault slip is detectable in the form of earthquakes 1 3 and seismic tremor 4 7 however the significance of these triggered events remains controversial 8 9 in part because they often occur with some delay long after the triggering stress has passed here we scrutinize the location and timing of tremor on the san andreas fault between 2001 and 2010 in relation to distant earthquakes we observe tremor on the san andreas fault that is initiated by passing seismic waves yet migrates along the fault at a much slower velocity than the radiating seismic waves we suggest that the migrating tremor records triggered slow slip of the san andreas fault as a propagating creep event we find that the triggered tremor and fault creep can be initiated by distant earthquakes as small as magnitude 5 4 and can persist for several days after the seismic waves have passed our observations of prolonged tremor activity provide a clear example of the delayed dynamic triggering of seismic events fault creep has been shown to trigger earthquakes 10 12 and we therefore suggest that the dynamic

seismic wave triggering of nonvolcanic tremor episodic tremor and slip and earthquakes on vancouver island 1 we explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on vancouver island british columbia during and immediately after the arrival of large amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes we identify tremor triggered by four of the teleseismic earthquakes the close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure it is particularly susceptible to triggering of further events the amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor tremor and energy radiated from regional local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller more frequent events we confidently identify triggered local earthquakes following only one teleseism that with the largest amplitude and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates we also note an interesting possible correlation between large teleseismic events and episodic tremor and slip ets episodes whereby ets events that are late and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large distant event while ets events that are early or on time are not

pulse of the seafloor tidal triggering of microearthquakes at 9 50 n east pacific rise 1 unequivocal evidence of tidal triggering is observed for microearthquakes 0 4 to 2 0 ml recorded between october 2003 to april 2004 near 9 50 n on the east pacific rise epr although semidiurnal tidal stress changes are small 99 9 nonrandom temporal distribution with events occurring preferentially near times of peak extension due to the proximity of this site to an ocean tidal node where changes in sea surface height are minimal periodic stress changes are dominated by the solid earth tide in contrast previous studies on the juan de fuca ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide the modulation of 9 50 n microearthquakes by small amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on axis magmatic and hydrothermal processes

basement and regional structure along strike of the queen charlotte fault in the context of modern and historical earthquake ruptures the queen charlotte fault qcf is a dextral transform system located offshore of southeastern alaska and western canada accommodating 4 4 cm yr of relative motion between the pacific and north american plates oblique convergence along the fault increases southward and how this convergence is accommodated is still debated using seismic reflection data we interpret offshore basement structure faulting and stratigraphy to provide a geological context for two recent earthquakes an m w 7 5 strike slip event near craig alaska and an m w 7 8 thrust event near haida gwaii canada we map downwarped pacific oceanic crust near 54 n between the two rupture zones observed downwarping decreases north and south of 54 n parallel to the strike of the qcf bending of the pacific plate here may have initiated with increased convergence rates due to a plate motion change at 6 ma tectonic reconstruction implies convergence driven pacific plate flexure beginning at 6 ma south of a 10 bend the qcf which is currently at 53 2 n and lasting until the plate translated past the bend by 2 ma normal faulted approximately late miocene sediment above the deep flexural depression at 54 n topped by relatively undeformed pleistocene and younger sediment supports this model aftershocks of the haida gwaii event indicate a normal faulting stress regime suggesting present day plate flexure and underthrusting which is also consistent with reconstruction of past conditions we thus favor a pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading in addition mapped structures indicate two possible fault segment boundaries along the qcf at 53 2 n and at 56 n

remotely triggered microearthquakes and tremor in central california following the 2010 mw 8 8 chile earthquake 1 we examine remotely triggered microearthquakes and tectonic tremor in central california following the 2010 mw 8 8 chile earthquake several microearthquakes near the coso geothermal field were apparently triggered with the largest earthquake ml 3 5 occurring during the large amplitude love surface waves the chile mainshock also triggered numerous tremor bursts near the parkfield cholame section of the san andreas fault saf the locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from coso but can be identified by applying high pass or matched filters both triggered tremor along the saf and the ml 3 5 earthquake in coso are consistent with frictional failure at different depths on critically stressed faults under the coulomb failure criteria the triggered tremor however appears to be more phase correlated with the surface waves than the triggered earthquakes likely reflecting differences in constitutive properties between the brittle seismogenic crust and the underlying lower crust

the october 28 2012 mw 7 8 haida gwaii underthrusting earthquake and tsunami slip partitioning along the queen charlotte fault transpressional plate boundary the pacific north american plate boundary is undergoing predominantly right lateral strike slip motion along the queen charlotte and fairweather transform faults the queen charlotte fault qcf hosted the largest historical earthquake in canada the 1949 m s 8 1 strike slip earthquake which ruptured from offshore northern haida gwaii several hundred kilometers northwestward on january 5 2013 an m w 7 5 strike slip faulting event occurred near the northern end of the 1949 rupture zone along central and southern haida gwaii the relative plate motion has 20 oblique convergence across the left stepping plate boundary there had been uncertainty in how the compressional component of plate motion is accommodated the october 28 2012 m w 7 8 haida gwaii earthquake involved slightly 20 oblique thrust faulting on a shallow 18 5 northeast dipping fault plane with strike 320 parallel to the qcf consistent with prior inferences of pacific plate underthrusting beneath haida gwaii the rupture extended to shallow depth offshore of moresby island beneath a 25 30 km wide terrace of sediments that has accumulated in a wedge seaward of the qcf the shallow thrusting caused seafloor uplift that generated substantial localized tsunami run up and a modest far field tsunami that spread across the northern pacific prompting a tsunami warning beach closure and coastal evacuation in hawaii although ultimately tide gauges showed less than 0 8 m of water level increase the mainshock rupture appears to have spread with a 2 3 km s rupture velocity over a length of 150 km with slip averaging 3 3 m concentrated beneath the sedimentary wedge the event was followed by a substantial aftershock sequence in which almost all of the larger events involve distributed intraplate normal faulting extending 50 km oceanward from the qcf the highly oblique slip partitioning in southern haida gwaii is distinctive in that the local plate boundary parallel motion on the qcf may be accommodated either by infrequent large strike slip ruptures or by aseismic creep as seems to be the case for deeper oblique relative plate motion beneath haida gwaii while the sedimentary terrace accumulates plate boundary perpendicular compressional strain that releases in almost pure thrust faulting earthquakes seaward of the qcf

earthquake triggering by static and dynamic stress changes 1 in this study we aim to understand the effect of static and dynamic stress changes in promoting earthquake failures on secondary faults toward this goal we solve the equation of motion of a spring slider dynamic system including inertia and using rate and state dependent constitutive laws we separately investigate the dynamic response of this fault analog system to a sudden stress change represented either as a stress step or as a stress pulse which are used to model permanent static and transient dynamic stress perturbations the induced earthquake failure does not occur immediately at the application of the coseismic stress change but it is delayed in time we define this time interval as the triggering delay for a given stress perturbation we analyze the dependence of triggering delays on different system conditions and constitutive parameters our results clearly show that the effects of static and dynamic stress changes are quite different while a static stress change is able to advance as well as to delay an induced instability depending on its sign a dynamic stress pulse is only able to promote a nearly instantaneous failure provided its amplitude is positive and large enough with respect to the direct effect of friction in other words dynamic stress changes can only cause nearly instantaneous failures without any relevant triggering delay these results should be considered in interpreting the seismicity rate changes caused by large earthquakes at least as long as seismic events are interpreted as sliding instabilities obeying rate and state dependent friction laws

dynamic triggering of small local earthquakes in the central himalaya we present the first observation of remote dynamic triggering of local micro earthquakes in central himalaya caused by the teleseismic waves from the 2007 m8 5 sumatra earthquake that occurs 4 500 km away we find small local earthquakes in the kumaon garhwal himalaya triggered by teleseismic long period surface waves interestingly an elevated level of seismicity persists for a week or so after the arrival of the teleseismic waves the teleseismic waves impart 9 kpa of peak dynamic stresses suggesting that the himalayan faults in this area are sensitive to small stress changes this heightened and protracted seismicity indicates that the transient dynamic stresses may have triggered a secondary process such as slow slip that may be responsible for the persistence of this earthquake sequence the region is thought to be close to a large damaging earthquake in the near future rajendran rajendran 2005 this study provides improved constraints on the factors controlling the earthquake cycle

slip behavior of the queen charlotte plate boundary before and after the 2012 mw 7 8 haida gwaii earthquake evidence from repeating earthquakes the queen charlotte plate boundary near haida gwaii b c includes the dextral strike slip queen charlotte fault qcf and the subduction interface between the downgoing pacific and overriding north american plates in this study we present a comprehensive repeating earthquake catalogue that represents an effective slip meter for both structures the catalogue comprises 712 individual earthquakes 0 3 mw 3 5 arranged into 224 repeating earthquake families on the basis of waveform similarity and source separation estimates from coda wave interferometry we employ and extend existing relationships for repeating earthquake magnitudes and slips to provide cumulative slip histories for the qcf and subduction interface in 6 adjacent zones within the study area between 52 3 n and 53 8 n we find evidence for creep on both faults however creep rates are significantly less than plate motion rates which suggests partial locking of both faults the qcf exhibits the highest degrees of locking south of 52 8 n which indicates that the seismic hazard for a major strike slip earthquake is highest in the southern part of the study area the october 28 2012 mw 7 8 haida gwaii thrust earthquake occurred in our study area and altered the slip dynamics of the plate boundary the qcf is observed to undergo accelerated right lateral slip for 1 2 months following the earthquake the subduction interface exhibits afterslip thrust motion that persists for the duration of the study period i e 3 years and 2 months after the haida gwaii earthquake afterslip is greatest 5 7 8 4 cm yr on the periphery of the main rupture zone of the haida gwaii event

influence of low arctic sea ice minima on anomalously cold eurasian winters 1 influence of low arctic sea ice minima in early autumn on the wintertime climate over eurasia is investigated observational evidence shows that significant cold anomalies over the far east in early winter and zonally elongated cold anomalies from europe to far east in late winter are associated with the decrease of the arctic sea ice cover in the preceding summer to autumn seasons results from numerical experiments using an atmospheric general circulation model support these notions the remote response in early winter is regarded as a stationary rossby wave generated thermally through an anomalous turbulent heat fluxes as a result of anomalous ice cover over the barents kara seas in late autumn which tends to induce an amplification of the siberian high causing colder conditions over the far east the late winter cold anomalies over eurasia are also reproduced in our experiment which is associated with the negative phase of the north atlantic oscillation

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

tectonics and structure of the queen charlotte fault zone haida gwaii and large thrust earthquakes abstract this article provides a summary of the structure and tectonic history of the queen charlotte transform fault zone off western canada as background to understanding the 2012 m w 7 8 thrust earthquake off haida gwaii there was margin subduction prior to the eocene the fault zone then became the mainly transcurrent pacific north america boundary there was mid tertiary oblique extension then 15 20 oblique convergence from 6 ma to the present that resulted in underthrusting and subduction initiation the total underthrusting has been too small for benioff wadati seismicity or arc volcanics but is indicated by 1 a trench the queen charlotte trough into which the oceanic plate bows downward and an offshore flexural bulge the oshawa rise 2 the queen charlotte terrace an accretionary sedimentary prism 3 seismic receiver function delineation of the underthrusting pacific plate 4 heat flow decreasing landward as predicted for underthrusting 5 low gravity offshore and high onshore consistent with underthrusting and 6 late tertiary uplift and erosion of the west coast of the islands oblique convergence is partitioned into nearly margin normal underthrusting i e m w 7 8 event relative to the terrace which is moving along the margin and margin parallel on the queen charlotte strike slip fault just off the coast that produced the 1949 m s 8 1 earthquake landward on the mainland global positioning system data suggest slow coast parallel shear with no historical seismicity the convergence rate decreases to the north of haida gwaii off dixon entrance but large thrust earthquakes are possible to the south underthrusting of the winona basin margin also could generate large earthquakes

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

warm season subseasonal variability and climate extremes in the northern hemisphere the role of stationary rossby waves abstractthis study examines the nature of boreal summer subseasonal atmospheric variability based on the new nasa modern era retrospective analysis for research and applications merra for the period 1979 2010 an analysis of the june july and august subseasonal 250 hpa meridional wind anomalies shows distinct rossby wave like structures that appear to be guided by the mean jets on monthly subseasonal time scales the leading waves the first 10 rotated empirical orthogonal functions reofs of the 250 hpa wind explain about 50 of the northern hemisphere wind variability and account for more than 30 60 of the precipitation surface temperature variability over a number of regions of the northern middle and high latitudes including the u s northern great plains parts of canada europe and russia the first reof in particular consists of a rossby wave that extends across northern eurasia where it is a dominant contributor to monthly surface temperature and precipitation variability and

the 11 april 2012 east indian ocean earthquake triggered large aftershocks worldwide although strong remote aftershocks are exceedingly rare their rate increased fivefold during the six days following the 2012 east indian ocean earthquake perhaps as a result of the strike slip nature of the 2012 event or a build up of close to failure nucleation sites

dynamic triggering of shallow earthquakes near beijing china summary we perform a comprehensive analysis of dynamic triggering around the babaoshan and huangzhuang gaoliying faults near beijing china the triggered earthquakes are identified as impulsive seismic arrivals with clear p and s waves in 5 hz high pass filtered three component velocity seismograms during the passage of large amplitude body and surface waves of large teleseismic earthquakes we find that this region was repeatedly triggered by four earthquakes in east asia including the 2001 mw 7 8 kunlun 2003 mw 8 3 tokachi oki 2004 mw 9 2 sumatra and 2008 mw 7 9 wenchuan earthquakes in most instances the microearthquakes coincide with the first few cycles of the love waves and more are triggered during the large amplitude rayleigh waves such an instantaneous triggering by both the love and rayleigh waves is similar to recent observations of remotely triggered non volcanic tremor along major plate boundary faults and can be explained by a simple coulomb failure criterion we are able to locate five of the earthquakes triggered by the kunlun and tokachi oki earthquakes these events occurred at shallow depth 5 km above the background seismicity near the boundary between nw striking babaoshan and huangzhuang gaoliying faults and the fangshan pluton we suggest that these triggered earthquakes occur near the transition between the velocity strengthening and weakening zones in the top few kilometres of the crust and are likely driven by relatively large dynamic stresses on the order of few tens of kpa

precise tremor source locations and amplitude variations along the lower crustal central san andreas fault 1 we precisely locate 88 tremor families along the central san andreas fault using a 3d velocity model and numerous p and s wave arrival times estimated from seismogram stacks of up to 400 events per tremor family maximum tremor amplitudes vary along the fault by at least a factor of 7 with by far the strongest sources along a 25 km section of the fault southeast of parkfield we also identify many weaker tremor families which have largely escaped prior detection together these sources extend 150 km along the fault beneath creeping transitional and locked sections of the upper crustal fault depths are mostly between 18 and 28 km in the lower crust epicenters are concentrated within 3 km of the surface trace implying a nearly vertical fault a prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6 0 parkfield earthquake

seismicity in idaho and montana triggered by the denali fault earthquake a window into the geologic context for seismic triggering we present a case study of dynamically triggered seismicity in idaho and western montana from the 2002 mw 7 9 denali fault earthquake to investigate the relationship between measured geological discriminants and propensity for trig gering we first establish triggering we find events that are not reported in the advanced national seismic system catalog in idaho and yellowstone following the denali fault earthquake by filtering broadband waveforms an ml 4 6 earthquake is discovered near new meadows idaho during the passage of the rayleigh waves and another earthquake probably located near yellowstone we find that central western idaho and yellowstone have statistically significant seismicity increases by applying a b test to the cataloged events in the 48 hr after the denali fault earthquake in idaho and surrounding regions we also find that pine idaho may have triggered events but the measurement is not robust because of uncertainty in the background seismicity rate both central western idaho and yellowstone triggered previously during the 1992 landers earthquake we then try to determine the local geological conditions necessary for triggered seismicity to occur geothermal regions with high total dissolved solids and background seismicity appear to be favorable conditions to trigger earthquakes in the study

evidence for underthrusting beneath the queen charlotte margin british columbia from teleseismic receiver function analysis summary the queen charlotte fault zone is the transpressive boundary between the north america and pacific plates along the northwestern margin of british columbia two models have been suggested for the accommodation of the 20 mm yr 1 of convergence along the fault boundary 1 underthrusting 2 internal crustal deformation strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the queen charlotte islands the modelling reveals a 10 km thick low velocity zone dipping eastward at 28 interpreted to be underthrusting oceanic crust the oceanic crust is located beneath a thin 28 km eastward thickening 10 continental crust

dynamically triggered slip leading to sustained fault gouge weakening under laboratory shear conditions we investigate dynamic wave triggered slip under laboratory shear conditions the experiment is composed of a three block system containing two gouge layers composed of glass beads and held in place by a fixed load in a biaxial configuration when the system is sheared under steady state conditions at a normal load of 4 mpa we find that shear failure may be instantaneously triggered by a dynamic wave corresponding to material weakening and softening if the system is in a critical shear stress state near failure following triggering the gouge material remains in a perturbed state over multiple slip cycles as evidenced by the recovery of the material strength shear modulus and slip recurrence time this work suggests that faults must be critically stressed to trigger under dynamic conditions and that the recovery process following a dynamically triggered event differs from the recovery following a spontaneous event

transient triggering of near and distant earthquakes we demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes we assume that earthquake triggering is a stick slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring slider model and a rate and state dependent friction constitutive law a critical triggering threshold is implicit in such a model formulation our first hypothesis is that transient loads lead to clock advances i e transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased i e triggering may be delayed these simple clock advance models predict complex relationships between the triggering delay the clock advance and the transient characteristics the triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied this implies that the stress required to bring about failure does not depend linearly on loading time even when the fault is loaded at a constant rate the timing of instability also depends nonlinearly on the transient loading rate faster rates more rapidly hastening instability this implies that higher frequency and or longer duration seismic waves should increase the amount of clock advance these modeling results and simple calculations suggest that near tens of kilometers small moderate earthquakes and remote thousands of kilometers earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load i e accumulated strain energy would have been relieved via other mechanisms we test this using two new seismicity models that 1 are inherently unstable but slide at steady state conditions under the background load and 2 are conditionally stable such that instability occurs only for sufficiently large perturbations for the new seismicity models very small amplitude transients trigger instability relative to the clock advance models the unstable steady state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude as in the clock advance models we were unable to generate delayed triggering with conditionally stable models for both new seismicity models the potential for triggering is independent of when the transient load is applied or equivalently of the prestress unlike in the clock advance models in these models a critical triggering threshold appears to be inversely proportional to frequency further advancement of our understanding will require more sophisticated quantitative models and observations that distinguish between our qualitative yet distinctly different model predictions

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

seismology dynamic triggering of earthquakes the landers earthquake of 1992 triggered a series of earthquakes in other parts of california focusing attention on the mechanism of remote triggering laboratory experiments on granular material under pressure suggest that triggering is a result of seismic waves impinging on a fault and inducing elastic nonlinear behaviour of the fault core accompanied by instantaneous weakening and failure the resulting softening to weakening model fits in well with field observations of real earthquakes after an earthquake numerous smaller shocks are triggered over distances comparable to the dimensions of the mainshock fault rupture although they are rare at larger distances here we analyse the scaling of dynamic deformations the stresses and strains associated with seismic waves with distance from and magnitude of their triggering earthquake and show that they can cause further earthquakes at any distance if their amplitude exceeds several microstrain regardless of their frequency content these triggering requirements are remarkably similar to those measured in the laboratory for inducing dynamic elastic nonlinear behaviour which suggests that the underlying physics is similar1 2

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

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

the queen charlotte islands refraction project part ii structural model for transition from pacific plate to north american plate discussion the oceanic continental boundary west of the queen charlotte islands is marked by the active queen charlotte fault zone motion along the fault is predominantly dextral strike slip but relative plate motion and other studies indicate that a component of convergence between the oceanic pacific plate and the continental north american plate presently exists this convergence could be manifest through different types of deformation oblique subduction crustal thickening or lateral distortion of the plates in 1983 a 330 km offshore onshore seismic refraction profile extending from the deep ocean across the islands to the mainland of british columbia was recorded to investigate i structure of the fault zone and associated oceanic continental boundary and ii lithospheric structure beneath the islands and hecate strait to define the regional transition from pacific plate to north american plate and thus the nature of the convergence two dimensional ray tracing and synthetic seismogram modelling of many

potential for earthquake triggering from transient deformations 1 we report on laboratory experiments in which stick slipping shear surfaces are subject to transient stressing to simulate earthquake triggering by seismic waves granular layers and bare granite surfaces were sheared in a servo controlled deformation apparatus in double direct shear the seismic waves from an earthquake and tectonic load were simulated by superimposing a loading rate sinusoid on a constant shear loading rate the dependence of triggered stick slip failure on fault stress state and architecture was analyzed fault architecture was evaluated by varying gouge layer thickness 2 6 mm and studying bare granite surfaces we compare the shortened recurrence times for faults under transient loading conditions to the consistent recurrence intervals under constant loading rate our results imply that triggering depends on oscillation amplitude and frequency as well as properties of the fault larger amplitude dynamic stresses reduce stick slip recurrence intervals for granular layers whereas failure times for granite surfaces are uncorrelated with oscillation amplitude granular layers have shorter recurrence rates at higher frequency whereas the recurrence intervals of granite surfaces are lengthened or unaffected by high frequency oscillations higher frequencies can inhibit failure when fault slip exceeds a critical distance dc prior to peak velocity and encourages failure if dc is achieved postpeak velocity increasing velocity temporarily strengthens faults whereas velocity reduction further weakens and promotes failure as predicted by the rate and state friction laws our results may explain variations in earthquake triggering thresholds and imply that high frequency thresholds may not be constant as has been previously proposed

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

possible precursory signals in damage zone foreshocks foreshocks may provide a precursory signal of an impending earthquake but their role in nucleation of the mainshock is unclear one way to further our understanding of foreshock failure mechanisms is to determine where they occur in the fault zone however earthquake locations commonly include uncertainties large enough to allow rupture on either the main fault interface or on subsidiary fractures within a surrounding damage zone here we obtain precise earthquake locations with 10 m uncertainty for foreshocks and aftershocks of an mw5 0 near prague ok usa repeating earthquakes imply that some precursory slow slip occurred before the mainshock in addition we show that foreshocks initially rupture faults and fractures throughout the 300 meter thick fault damage zone and later localize onto a narrower zone 100 m thick nearer the mainshock hypocenter focal mechanisms corroborate that foreshocks occur in the surrounding damage zone as well as on the mainshock rupture interface these results highlight that earthquake nucleation is most likely a complex feedback between frictional failure processes on the fault interface and deformation in the surrounding damaged rock not just nucleation on a single surface

frequency dependence of delayed and instantaneous triggering on laboratory and simulated faults governed by rate state friction earthquake triggering by transient stresses is commonly observed however some aspects remain unexplained the first is the often observed delay between the triggered earthquakes and the triggering waves and the second is the unexpected effectiveness of transient stressing in the seismic frequency band previous theoretical and laboratory studies have suggested that seismic transients should have little impact on faults if the duration of the transient is smaller than the timescale for nucleation of slip we reexamine the dynamics of stress triggering during stick slip sliding on a laboratory fault and make three important observations that pertain to earthquake triggering 1 delayed triggering clock advance occurs for both bare granite surfaces and granular gouge prior to the onset of instantaneous triggering 2 triggering occurs much earlier in the stick slip cycle than expected for a simple coulomb stress threshold 3 shorter period higher stressing rate pulses are more effective at triggering than longer period pulses of the same stress amplitude we use numerical simulations to show that rate state friction can explain each of the observed features but not all three simultaneously only the ruina slip law for state evolution in which faults must slip to heal can reproduce early onset and stressing rate dependent triggering the laboratory and numerical experiments show that faults can remain relatively weak over much of the seismic cycle and that the triggered response depends on a competition between healing and weakening during triggered slip transient stressing at seismic frequencies may be more effective at triggering earthquakes than previously recognized