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

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

the rupture process and aftershock distribution of the 6 april 1992 ms 6 8 earthquake offshore british columbia abstract on 6 april 1992 a magnitude 6 8 m s earthquake occurred in the triple junction region at the northern end of the cascadia subduction zone this was the largest earthquake in at least 75 yr to occur along the 110 km long revere dellwood wilson rdw transform fault and the first large earthquake in this region recorded by modern broadband digital seismic networks it thus provides an opportunity to examine the rupture process along a young p and sh waveforms reveals that this was a complex rupture sequence consisting of three strike slip subevents in 12 sec the initial rupture occurred 5 to 6 km to the sw of the seafloor trace of the rdw fault at 50 55 n 130 46 w the dominant subevent occurred 2 to 3 sec later and 4 3 km beneath the seafloor trace of the rdw fault and a third subevent occurred 5 sec later 18 km to the nnw suggesting a northwestward propagating rupture the aftershock sequence extended along a 60 to 70 km long segment of the rdw fault with the bulk of the activity concentrated 30 to 40 km to the nnw of the epicenter consistent with this interpretation the well constrained mechanism of the initial rupture strike dip slip 339 90 168 and of the largest aftershock 165 80 170 are rotated 15 to 20 clockwise relative to the seafloor trace of the rdw fault but are parallel to the pacific north america relative plate motion vector in contrast the mechanisms of the dominant subevent 326 87 172 and the long period solution derived from surface waves aligns with the rdw fault this suggests that small earthquakes m 26 dyne cm a stress drop of 60 bars and an average slip of 1 2 m this represents only 21 yr of strain accumulation implying that there is either a significant amount of aseismic slip along the rdw fault or that much of the strain accumulation manifests itself as deformation within the dellwood and winona blocks or along the continental margin

seismicity normal faulting and the geomorphological development of the gulf of corinth greece the corinth earthquakes of february and march 1981 three major destructive earthquakes of m s 6 7 6 4 6 4 occurred in the eastern part of the gulf of corinth in february and march 1981 associated normal faulting was observed on both the north and the south sides of the gulf examination of teleseismic local seismic surface faulting and geomorpbological data suggests that the first and second of these shocks activated major north dipping normal faults these faults control the topography and bathymetry and are related to the recent uplift and subsidence of the coastline it is probable that the first earthquake occurred on a faultwhich outcrops underwater and the second on a fault which is visible on land it appears that the third shock activated an antithetic normal fault dipping southwards on the north side of the gulf and that the overall structure of the gulf of corinth graben is asymmetric the northern antithetic fault clearly demonstrates the geomorphological changes associated with developing young normal faulting in addition we suggest that observations of recent coastline movement provide a powerful tool for identifying major active fault systems and predicting their historical role in the paleogeography of an area

pacific north america plate boundary reorganization in response to a change in relative plate motion offshore canada 1 the transition from subduction in cascadia to the transform queen charlotte fault along western canada is often drawn as a subduction zone yet recent studies of gps and earthquake data from northern vancouver island are not consistent with that model in this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of 1 microplate subduction and 2 reorganization of the preexisting strike slip plate boundary we focus on the critical region of outer queen charlotte sound and the adjacent offshore on much of the continental shelf several million years of subsidence above thin crust are a counterindicator for subduction an undated episode of compression uplifted the southernmost shelf but subsidence patterns offshore show that recent subduction is unlikely to be responsible previously unremarked near vertical faults and a mix of extensional and compressional faults offshore indicate that strike slip faulting has been a significant mode of deformation seismicity in the last 18 years is dominantly strike slip and shows large amounts of moment release on the revere dellwood fault and its overlap with the queen charlotte fault the relative plate motion between the pacific and north american plates rotated clockwise 6 ma and appears to have triggered formation of an evolving array of structures we suggest that the paleo queen charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed revere dellwood fault propagated to the northwest

global detection and location of seismic sources by using surface waves we develop an algorithm for the detection and location of seismic sources using intermediate period 35 150 sec surface waves recorded on a global array of stations continuous vertical seismic waveforms from the global network are collected and a 4 4 global grid of target locations is defined for each target location and each station a surface wave propagation operator is deconvolved from the seismogram to restore any source pulse present the envelope of the seismogram is calculated and cross correlated with a theoretical source pulse shape the resulting waveforms are stacked to improve signal to noise characteristics and the quality strength and timing of the potential detection are determined when a successful event detection is made a finer grid is applied to locate the event with greater precision we apply the algorithm systematically for the period 1993 2003 and catalog the events approximately 2000 events are detected and located for each year and 98 of shallow events in the harvard centroid moment tensor cmt catalog are detected and located by the new algorithm a comparison of 9482 events common to the two catalogs allows the detection strength to be calibrated against the cmt moment magnitude all detected events have estimated moment magnitudes m w 4 5 in each year approximately 100 events not listed in other global seismicity catalogs are detected and located many of these events lie along the ridge transform plate boundaries in the southern hemisphere and may be regular earthquakes that have gone undetected because of poor station coverage other events located in areas where global and regional networks provide good coverage are potentially anomalous and may have escaped detection as a result of their unusual source properties

geodetic constraints to the kinematics of the kapareli fault reactivated during the 1981 gulf of corinth earthquakes abstract comparison of historical and of post seismic triangulation data is used to model vertical crustal movements in the vicinity of the kapareli fault or the alkynonides earthquakes north fault one of the two antithetic normal faults which reactivated during the 1981 gulf of corinth ms 6 7 earthquakes this fault is characterized by a much smaller geomorphological signature than the south or perachora fault of the same seismic sequence analysis of geodetic data on the basis of polynomial filtering and elastic dislocation modelling as well as analysis of structural and coastal change data permits us to conclude that the upper bound in the uncertainty level of most of the available elevation changes is 20 30 cm usually lower than the corresponding dislocation signal in addition the available geodetic data have a systematic pattern and are consistent with structural data for this reason they permit more precise constraints on the geometry and the role of the kapareli fault or the alkyonides earthquakes north fault its total length is estimated about 17 km about 50 longer than its surface trace about 30 40 cm subsidence of its hanging wall as well as at least 15 cm maximum uplift of its footwall is also inferred this new evidence suggests that although in the long term the kapareli fault may represent a rather secondary antithetic fault to the alkyonides earthquakes south perachora fault during the 1981 earthquakes it probably had a more important structural role

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

multiple event relocation of historic earthquakes along blanco transform fault zone ne pacific 1 blanco transform fault zone btfz is part of the juan de fuca ridge system located 200 500 km west of the oregon coastline individually located epicenters reported by the international seismological center isc and national earthquake information center along the btfz are not well correlated spatially with the active transform boundary more accurate locations of 120 mb 5 0 earthquakes relative to each other have been obtained using a joint relocation process that yields improved epicenter locations relative to each other six of these earthquakes have been independently located using data from the u s navy s sosus hydrophone array enabling specification of a static correction to improve absolute locations for all 120 relocated epicenters relative to the btfz the final locations average 34 6 15 2 km toward azimuth 196 28 relative to the original isc locations the 95 confidence ellipses of most of the relocated epicenters fall on or very near active structural features along the btfz

plate boundary deformation between the pacific and north america in the explorer region abstract one of the consequences of plate tectonics is that a spreading ridge will eventually approach a subduction zone the problem whether the possible break up of the approaching ridge will lead to the development of independent micro plates or not is still unresolved some 4 million years ago the interaction between the juan de fuca ridge and the cascadia subduction zone resulted in ridge fragmentation in the explorer region there are two proposed post miocene kinematic models one that proposes the presence of a micro plate and the other that treats the region as a transform deformation zone or so called pseudo plate we use earthquake strain rates derived from 74 events since 1948 to estimate a long term velocity field for the region by comparing this result with the predicted velocity fields for both models we try to discriminate between the two the earthquake strain rates indicate the presence of a transform deformation zone between the north american nam and pacific pac plates the velocity field derived from the inversion of the earthquake strain rates indicates that seismic activity takes up 50 30 1 of the pac nam relative motion nuvel 1a demets et al 1994 effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions geophys res lett 21 2191 2194 within the seismogenic layer of this zone and is indistinguishable in direction from the nuvel 1a pac nam model the presence of this explorer transform zone is consistent with the strain rate and velocity field for the pseudo plate model and indicates that seismicity defines a new plate boundary zone between the pacific and north american plates earthquake derived strain rates are low along the nootka transform which accommodates relative motion between the juan de fuca and north american plates the cause for the absence of significant seismic slip along this transform is unclear and may be closely linked to the fact that the adjacent cascadia subduction zone is locked the micro plate model which we reject predicts sw ne convergence in the eastern explorer region and this is inconsistent with the earthquake strain rates as well as with surface fault observations

aftershocks of the 2012 off coast of sumatra earthquake sequence abstract aftershocks of the 2012 off coast of sumatra earthquake sequence exhibit a complex and diffuse spatial distribution the first order complexity in aftershock distribution is clear and well beyond the influence of typical earthquake location uncertainty the sequence included rupture of multiple fault segments spatially separated we use surface wave based relative centroid locations to examine whether at the small scale the distribution of the aftershocks was influenced by location errors surface wave based relative location has delineated precise oceanic transform fault earthquake locations in multiple regions however the relocated aftershocks off the coast of sumatra seldom align along simple linear trends that are compatible with the corresponding fault strikes as estimated for the gcmt catalog the relocation of roughly 60 moderate earthquake epicentroids suggests that the faulting involved in the 2012 earthquake aftershock sequence included strain release along many short fault segments statistical analysis and temporal variations of aftershocks show a typical decay of the aftershocks but a relatively low number of aftershocks as is common for intraplate oceanic earthquakes coulomb stress calculations indicate that most of the moderate magnitude aftershocks are compatible with stress changes predicted by the large event slip models the patterns in the aftershocks suggest that the formation of the boundary and eventual localization of deformation between the indian and australian plate is a complicated process

the 1978 earthquake sequence near thessaloniki northern greece summary this paper presents a detailed study of teleseismically and locally recorded foreshocks and aftershocks of the 1978 thessaloniki earthquake sequence and discusses the relations between the locations and mechanisms of these shocks and the three dimensional geometry of faulting in the source region sixteen teleseismically recorded events were relocated using a relative relocation method and positioned geographically using the accurate locally determined location of the largest aftershock all these shocks had depths shallower than 15 km the spatial distribution of the foreshocks showed that the activity was initiated at the central part of the epicentral region progressed westwards and for about 20 days clustered in the region of the imminent mainshock no teleseismically recorded foreshock was located west of the mainshock location in less than 1 hr after the mainshock large aftershocks occurred west of it and in later stages spanned the entire western part of the epicentral region no teleseismically recorded aftershocks were located east of the mainshock locally recorded aftershocks were accurately located and provided a more detailed picture of the seismic deformation they occurred at depths between 3 and 12 km and in three distinct clusters separated by well defined gaps the largest foreshock on may 23 had a moment of 5 6 1024dyne cm and a mean displacement of 31cm the mainshock on june 20 had a moment of 5 2 1025dyne cm a strain drop of 4 0 10 5 static stress drop of 12 bar and a mean displacement of 64 cm these values are for one of the two possible interpretations of the aftershock distribution which suggests that the largest earthquakes of the sequence occurred on two different but adjacent fault segments the first event 1 on may 23 occurred along the central fault segment and was followed 28 days later by the second event 2 on june 20 along a western fault segment finally 61 days later another smaller earthquake event 3 occurred along another fault east of event 1

relict caves as evidence of landscape and aquifer evolution in a deeply dissected carbonate terrain southwest edwards plateau texas u s a abstract caves lying well above valley floors and within narrow ridges of carbonate rocks in the deeply dissected southwest edwards plateau texas provide considerable information about the geomorphic history of the region and the evolution of the edwards trinity aquifer in the sonora area horizontally extensive cave passages occur at three distinct levels along massive burrowed dolomitic beds in the cretaceous segovia formation passages lying beneath a resistant caprock consist of long corridors barren of speleothems however in the vicinity of the caprock edge passages are modified by collapse and stoping mazework and speleothem deposition conduits are generally oriented down the regional dip and guided by dominant ne trending fractures upper cave levels developed first but lower passages were excavated as piezometric surfaces dropped in response to valley cutting cavern collapses blocked flowpaths in places and ensuing floodwaters enlarged passages formed maze sections and promoted development of diversion routes around these constrictions collapse at the caprock edge has enhanced retreat of valley slopes and headward erosion of streams in the langtry area caves developed in the cretaceous devils river formation are up to 113 m deep the devils river formation is a stratigraphically homogeneous high purity limestone in which fractures are few but vertically extensive incision of the rio grande and nearby streams has locally exposed up to 70 m of the devils river formation inducing steep hydraulic gradients that have promoted bathyphreatic circulation and development of deep cave systems horizontal passages are relatively short and are connected by vertical shafts and steeply inclined corridors the two localities have experienced similar climates and dissection in topography but the relationship of stream incision to geologic structure stratigraphy and lithology has differed significantly enough to produce long horizontal caves within the shallow phreatic zone around sonora and deep vertical caves in the deep phreatic zone around langtry

active deformation of the gorda plate constraining deformation models with new geophysical data the gorda plate the southernmost fragment of the larger juan de fuca plate system is an example of a nonrigidly deforming tectonic accommodation zone or buffer plate absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion here we present a new structural analysis of the plate based on full plate bathy metric coverage augmented by seismic reflection data and earthquake moment tensors and locations we interpret internal deformation of the gorda plate as an asymmetrical flexural slip buckle with a vertical axis utilizing reactivation of spreading ridge fabric normal faults as strike slip faults newly formed second generation faults crosscutting the structural grain overprint the reactivated structures the spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone this model based on fault constraints has allowed investigation of ridge plate subduction interac tions and suggests that spreading rate variations along the gorda ridge may be con trolled by internal deformation of the plate rather than the reverse as previously hypothesized

recent vertical movements across the atalandi fault zone central greece repeated leveling data collected over a 14 km long traverse crossing the atalandi fault that last broke in 1894 revealed small amplitude motions for the interval 1969 1984 these motions are significant against random errors and are probably free of systematic errors in addition no local or superficial ground effect may account for the observed elevation changes which correlate with the tectonic pattern in the area as they show a relative downward motion of the hanging wall of a normal fault this motion is similar to the co seismic slip geomorphological observations and observations of submerged archaeological sites on the other hand corroborate long term motions which are unlikely to be explained as cumulative co seismic slip and are possibly similar to the short term motions leveling data reveal

mode of seismic moment release at transform faults abstract seismic moments of oceanic transform fault tf events being the largest ones at various tfs and covering a wide range of spreading rates are obtained by the moment tensor and the scalar moment inversion methods for the p wave first motion amplitudes on wwssn long period records the seismic moment of the largest event at each tf shows a negative correlation with the spreading rate and a positive correlation with the elapse time during which the ocean floor newly created at the ridge passes over ridge offset the seismic moment has a poor correlation with geometrical parameters such as tf length and cumulative moment expected along the tfs in the next step cumulative seismic moments are deduced for three tfs using rayleigh wave excitations relative to those of the largest events to infer seismic moments of small tf events ratios between the cumulative moments and the moments expected by simple plate kinematics amount to about 5 6 at the eltanin fracture zone spreading rate is 9 cm yr 29 at the tasman fracture zone 7 cm yr and 35 at the george v fracture zone 7 cm yr it is clear that the faster the spreading rate is the larger is the amount of tf movement released aseismically this leads to the conclusion that hot mantle materials are supplied by fast moving convection currents and that the mode of the seismic moment release at tfs is principally governed by the thermal state in the uppermost mantle beneath the spreading center

uppermantle anisotropy and the oceanic lithosphere rayleigh wave and love wave dispersion data for oceanic paths cannot be satisfied by an isotropic uppermantle and it is incorrect to invert these two datasets independently for separate isotropic structures available phase and group velocity data are inverted for oceanic structure as a function of age taking into account anelastic dispersion sphericity and anisotropy the resulting models are quite different from previous results which ignore the above effects and the extra parameters involved in proper anisotropic inversion the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age and an anisotropic low velocity zone with an age dependent anisotropy the lid or seismic lithosphere reaches a maximum thickness of 50 km which is about one half the thickness obtained in previous isotropic or pseudo isotropic inversions the seismic lithosphere therefore may be comparable in thickness to the elastic or flexural lithosphere raising the possibility that both are controlled by effects other than temperature stress and time such as mineralogy crystal orientation or partial melting the velocities in the low velocity zone are higher than inferred by previous surface wave studies

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

precise relative earthquake magnitudes from cross correlation abstract we present a method to estimate precise relative magnitudes using cross correlation of seismic waveforms our method incorporates the intercorrelation of all events in a group of earthquakes as opposed to individual event pairings relative to a reference event this method works well when a reliable reference event does not exist we illustrate the method using vertical strike slip earthquakes located in the northeast pacific and panama fracture zone regions our results are generally consistent with the global centroid moment tensor catalog which we use to establish a baseline for the relative event sizes online material supplementary description and formulation of uncertainty and tables of relative magnitudes

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

coastal wetlands as recorders of earthquake subsidence in the aegean a case study of the 1894 gulf of atalantiearthquakes central greece abstract earthquakes may have a major impact on the morphology and evolution of coasts in tectonically active areas in the area of the gulf of atalanti central greece a damaging earthquake sequence in 1894 is reported to have caused nearshore slumping tsunami induced flooding and decimetre to metre scale coastal subsidence the earthquakes caused major changes to coastal configuration in particular the separation of the gaiduronisi peninsula from the mainland detailed stratigraphical microfossil and radiometric 210pb and 137cs analyses of coastal wetlands around the gulf of atalanti show that these environments preserve evidence of sudden lasting subsidence contemporaneous with the 1894 events with foraminiferal assemblages at a site on the mainland opposite the island indicating an abrupt change from supratidal soil to intertidal marine conditions the elevation change indicated by this terrestrial to marine transition is 30 80 cm the coastal stratigraphy in the atalanti area however lacks laterally extensive sharp soil mud contacts or tsunami deposits of the type used in identifying past earthquake subsidence events at plate boundary settings despite the major earthquake induced changes in coastal configuration the stratigraphical signature of the 1894 earthquakes is relatively subtle and is consequently difficult to distinguish from other rapid coastal changes e g storm events or barrier breaching without high resolution dating control and reference to documentary records for this reason coastal wetland stratigraphies are only likely to be useful in identifying pre historical or poorly documented earthquakes in the aegean at sites where metre scale coseismic elevation changes and or major tsunami inundation have occurred in addition the resolution of the biostratigraphic data reported here is insufficient to distinguish between the contrasting fault rupture models for the 1894 events the coastal elevation changes recorded in these wetlands however are best accounted for by a slip of 1 m on a multi segment fault

tectonics and sedimentation in the gulf of corinth and the zakynthos and kefallinia channels western greece abstract single channel seismic reflection profiling data are presented from the gulf of corinth and from the continental margin outside the gulf of patras different tectonic styles are exhibited in the two regions the gulf of corinth is occupied by a complex asymmetrical graben structure with major faults trending nearly perpendicular to the local tectonic zones of the internal hellenides the gulf is interpreted to have formed during the quaternary in response to approximately north south extensional stress the deep floor of the gulf is underlain by a sequence of turbidites up to 1 km thick affected by normal growth faulting that extends up to the sea floor the sea areas inside the ionian islands of zakynthos and kefallinia zakynthos and kefallinia channels occupy a broad structural depression that trends parallel to the local tectonic zones of the external hellenides and is situated on the continental side of the strophades continental margin rise the channels contain local nw se trending structural basins zakynthos and kefallinia basins whose geometry is controlled by reverse faults or thrusts with associated diapirism in triassic evaporites hydra bank is interpreted as a tilted fault block with a diapiric core and its faulted western margin can be traced into the zone of thrusting and diapirism in southeast kefallinia at the boundary between the ionian and pre apulian paxos zones of the external hellinides the zakynthos and kefallinia structural basins are filled with a variety of sediments including prodeltaic deposits in front of the modern delta complex of the acheloos river slumped masses and debris flow deposits and in the zakynthos basin occasional turbidites the boundary separating zones of extensional and compressional focal mechanisms for shallow earthquakes crosses the gulf of patras the inner part of which exhibits wnw ese trending graben tectonics apparently related to the gulf of corinth tectonic province onshore neotectonic studies around the gulf of patras are required in order to understand the nature and cause of the westward tectonic transition from wnw ese graben structures in the gulfs of corinth and patras to nw se oriented zones of subsidence with associated compressional tectonics and diapirism in the zakynthos and kefallinia channels

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

a comparison of epicenter relocation techniques a case study for the juan de fuca plate abstract the master event relocation and hypocentroid techniques are compared in an effort to determine their relative performance in evaluating the location of epicenters in a region hampered with recording station location bias and a poorly understood seismic velocity structure the study areas chosen are the explorer and gorda plates two small lithospheric subplates of the oceanic juan de fuca plate which show a high degree of apparent intraplate seismicity previous studies have indicated systematic eastward errors in the location of epicenters due to the lack of recording stations to the west both techniques produce similar results in defining plate boundary vs intraplate seismicity however noteworthy differences exist the selection of a well constrained master event has a clear effect on the alignment of relocated epicenters with plate boundaries an event on the gorda plate located by ocean bottom seismometer allows for a relocation of epicenters about the gorda rise using a more widely recorded master event on the explorer plate produces parallelism of the events with the plate boundaries but an additional uniform shift in the relocated epicenters is required to align seismicity with the boundaries location of the master event with respect to tectonic features of the plate appears to affect the results probably owing to differences in lithospheric conditions and their effect on wave travel paths through the plate a comparison of results of the two techniques on intraplate seismicity on the gorda plate shows opposing direction vectors for the relocations the hypocentroid technique is less affected by changing conditions within the plate as events are clustered according to location the relocations are still affected by the station location bias as the events are relocated in relation to the weighted center of the event cluster the technique produces greater definition of the trends of the relocated epicenters however in this situation an additional shift of the epicenters is required for alignment with the boundaries

geodetic constraints on the fault pattern of the 1978 thessaloniki northern greece earthquake ms 6 4 summary repeated measurement of a 110 km long high precision spirit levelling traverse indicates that between 1958 and 1978 about 25 cm of subsidence occurred along a distance of 30 km in the mygdonian quaternary graben near thessaloniki northern greece observed displacements are significant against random and systematic errors and most are unrelated to near surface effects and can be assigned to the 1978 thessaloniki destructive earthquakes the available data do not permit a formal inversion but an elastic dislocation analysis indicates that the main shock of the seismic sequence ms 6 4 was probably associated with a ne dipping wnw trending blind normal fault marking the southwestern margin of the mygdonian graben in agreement with seismological and structural data there also exists evidence for incipient faulting in the central southern part of the mygdonian graben

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

geodetically controlled taphrogenesis in back arc environments three examples from central and northern greece abstract repeated leveling data collected in the vicinity of three tectonic grabens in the hellenic back arc area reveal a tendency to continuation of taphrogenesis processes that started in the quaternary or earlier these processes are associated with earthquakes of short or long recurrence intervals corinthiakos tens of years atalandi hundreds of years although in most cases the geodetic displacement signal is small i suggest that it reflects real motions because first the measurement specifications are very strict and second geologic and other data confirm geodetically derived displacements

relocation of earthquakes west of vancouver island british columbia 1965 1983 in the tectonically complex region of young plate interaction west of vancouver island 360 earthquakes have been relocated the earthquakes occurred in the years 1965 1983 when the canadian seismograph network in the region did not significantly change configuration and are in the magnitude range 3 5 a traveltime model was derived and applied to arrival times for a selected limited set of station phase combinations time corrections for these combinations were derived from joint hypocentre locations of earthquakes in specific regions using independently located reference events an algorithm for routine location of offshore earthquakes in this region is suggested the correlation between seismicity and mapped bathymetrical features is strong along the revere dellwood transform fault and the northern segments of the explorer ridge transform fault system considerable seismicity occurs inside the explorer plate indicating internal deformation the sovanco and nootka shear zones the southern border

precise relative earthquake location using surface waves 1 earthquake locations provide a fundamental tool for seismological investigations while dense seismic networks can provide robust locations accuracy and precision of these locations suffer outside dense networks this is particularly true in offshore areas where location analysis relies heavily on distant seismic observations we present a method for estimating precise relative seismic source epicentroid locations using surface waves several reasons including lower velocities and strength of the signal at distance make use of surface waves for event location appealing we focus on the panama fracture zone region and relocate 81 strike slip earthquakes to produce tectonically consistent epicentroid locations the resulting pattern of earthquakes more clearly delineates recently active regional structures than original body wave locations the mean shift between the us geological survey national earthquake information center epicenter and our epicentroids is about 14 km the median is about 11 km and typical origin time changes are generally less than 2 s we find that north of 6 5 n the plate boundary motion is split across two roughly north south striking structures the panama and balboa fracture zones for the last 36 years slip along these two structures roughly matches slip along the panama fracture zone to the south from 4 5 n to 6 25 n but the balboa fracture zone has roughly three times the moment than the northern panama fracture zone our analyses show that observed rayleigh wave signal to noise ratios for moderate to large shallow earthquakes are suitable for applying the procedure and that rayleigh wave observations form a self consistent set of constraints on the relative location of earthquake centroids

kinematics and deformation of central and southwestern greece from historical triangulation data and implications for the active tectonics of the aegean abstract changes in the coordinates of 38 stations 32 of which were common to three historical triangulation surveys 1889 1905 1927 1930 and 1950 1970 in central greece were computed based on conventional adjustment techniques and the assumption of a nearly fixed baseline length some of the calculated coordinate changes are significant against the a posteriori network adjustment errors and are likely to indicate tectonic motions consistent with those deduced from geological and geophysical data and other geodetic studies comparison of the first survey network with gps data more explicitly the historical geodetic data confirm the rotational character of the deformation in the area but they show that the strain pattern is likely to have changed after 1930 between 1890 and 1930 strain was changing smoothly as if no strain discontinuities existed while the peloponnesus was under contraction from the northeast probably reflecting accommodation of strain from the arc significant left lateral shear in the gulf of corinth and n s extension in the whole of the study area were observed only after 1930 estimates of strain are consistent with those deduced from comparison of historical triangulation and gps data corrected for the scale error that exists in the terrestrial geodetic data in greece and estimates of strain from seismological data the change in the kinematic pattern after 1930 seems to be confirmed by changes in the shear strain computed directly from changes in observed angles between stations and from changes in the seismicity rates such changes although not unusual in the geological record are not easy to explain but may reflect elastic rebound effects in area with an extremely complicated tectonic fabric or hysteresis in the accommodation of e w compression from the arc by n s stretching

continuous gps monitoring of elastic strain in the northern cascadia subduction zone previous monitoring and modeling of crustal deformation across the northern cascadia margin at vancouver is has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes the recent establishment of the western canada deformation array wcda a network of continuous gps trackers in southwestern british columbia provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region 17 months of continuous data from the three longest running wcda sites penticton victoria holberg indicate 1 a 7 mm yr easterly motion of victoria relative to penticton victoria is located in the forearc at the southeastern end of vancouver is 230 km from trench axis whereas penticton is located behind the arc 520 km from the trench axis and assumed fixed on the stable north american plate 2 a 3 mm yr northwesterly motion and 10 mm yr uplift with a large uncertainty of holberg relative to penticton holberg is located on the northern most part of vancouver is 50 km from the trench and near the plate triple junction and 3 the presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to penticton the landward motion of victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction thrust earthquake as constrained by previous deformation data although not as well resolved the motion of holberg roughly parallel to the margin is significantly different and is inconsistent with simple subduction models the non linear variations in measured position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal effects

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

rupture processes of the 2013 2014 minab earthquake sequence iran we constrain epicentroid locations magnitudes and depths of moderate magnitude earthquakes in the 2013 2014 minab sequence using surface wave cross correlations surfacewave spectra and teleseismic body wave modelling we estimate precise relative locations of 54 mw 3 8 earthquakes using 48 409 teleseismic intermediate period rayleigh and lovewave cross correlation measurements to reduce significant regional biases in our relative locations we shift the relative locations to align the mw 6 2 main shock centroid to a location derived from an independent insar fault model our relocations suggest that the events lie along a roughly east west trend that is consistent with the faulting geometry in the gcmt catalogue the results support previous studies that suggest the sequence consists of leftlateral strain release but better defines the main shock fault length and shows that most of the mw 5 0 aftershocks occurred on one or two similarly oriented structures we also show that aftershock activity migrated westwards along strike away from the main shock suggesting that coulomb stress transfer played a role in the fault failure we estimate the magnitudes of the relocated events using surface wave cross correlation amplitudes and find good agreement with the gcmt moment magnitudes for the larger events and underestimation of small event size by catalogue ms in addition to clarifying details of the minab sequence the results demonstrate that even in tectonically complex regions relative relocation using teleseismic surface waves greatly improves the precision of relative earthquake epicentroid locations and can facilitate detailed tectonic analyses of remote earthquake sequences

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

locating earthquakes with surface waves and centroid moment tensor estimation traditionally p wave arrival times have been used to locate regional earthquakes in contrast the travel times of surface waves dependent on source excitation and the source parameters and depth must be determined independently thus surface wave path delays need to be known before such data can be used for location these delays can be estimated from previous earthquakes using the cut and paste technique ambient seismic noise tomography and from 3d models taking the chino hills event as an example we show consistency of path corrections for 10 s love and rayleigh waves to within about 1 s obtained from these methods we then use these empirically derived delay maps to determine centroid locations of 138 southern california moderate sized 3 5 m w 5 7 earthquakes using surface waves alone it appears that these methods are capable of locating the main zone of rupture within a few 3 km accuracy relative to southern california seismic network locations with 5 stations that are well distributed in azimuth we also address the timing accuracy required to resolve non double couple source parameters which trades off with location with less than a km error required for a 10 compensated linear vector dipole resolution

monitoring the earthquake source process in north america with the implementation of the usgs national earthquake information center prompt assessment of global earthquakes for response system pager rapid determination of earthquake moment magnitude is essential especially for earthquakes that are felt within the contiguous united states we report an implementation of moment tensor processing for application to broad seismically active areas of north america this effort focuses on the selection of regional crustal velocity models codification of data quality tests and the development of procedures for rapid computation of the seismic moment tensor we systematically apply these techniques to earthquakes with reported magnitude greater than 3 5 in continental north america that are not associated with a tectonic plate boundary using the 0 02 0 10 hz passband we can usually determine with few exceptions moment tensor solutions for earthquakes with m w as small as 3 7 the threshold is significantly influenced by the density of stations the location of the earthquake relative to the seismic stations and of course the signal to noise ratio with the existing permanent broadband stations in north america operated for rapid earthquake response the seismic moment tensor of most earthquakes that are m w 4 or larger can be routinely computed as expected the nonuniform spatial pattern of these solutions reflects the seismicity pattern however the orientation of the direction of maximum compressive stress and the predominant style of faulting is spatially coherent across large regions of the continent

quaternary elevation change of the gulf of corinth in central greece dramatic elevation changes have occurred since early pleistocene time in and around the gulf of corinth sedimentary basin in central greece which is actively extending at a maximum rate of ca 2 mm a 1 shoreline deposits younger than 1 million years are found at up to ca 1200 m elevation south of the gulf with sediment from an older plio pleistocene basin above 1750 m the gulf floor in the hanging wall of normal faults along its southern coast has subsided up to ca 1650 m on the same timescale however other hanging wall localities such as the corinth isthmus east of the gulf have uplifted instead during the past million years the gulf has alternated between marine and lacustrine environments in response to glacially induced sea level variations this study establishes its chronology of marine and lacustrine terraces for the past half million years allowing lateral variations in uplift to be investigated to explain the observed elevation changes it is suggested that ca 0 9 million years ago the previously lacustrine gulf became flooded by ca 80 m during an interglacial marine highstand the resulting ca 0 8 mpa increase in lithostatic pressure at the base of the brittle upper crust caused southward lower crustal flow to beneath the older plio pleistocene basin isostatic adjustment uplifted this basin creating unstable topography that enabled rapid erosion this lower crustal flow which since been maintained by the sediment load in the gulf that results from this erosion also causes the hanging wall uplift of the corinth isthmus and other anomalous localities the gulf of corinth is thus an important case study locality for revealing the existence of lower crustal flow during continental extension

seismic properties of the eltanin transform system south pacific abstract we present a compilation of the seismic properties of the eltanin transform fault tf system compelled by the recent discovery of the hollister ridge and the possibility of a change of plate kinematics pattern in the region the hollister ridge is a major volcanic system located on the western flank of the pacific antarctic ridge par immediately south of the eltanin fracture zones fzs we find or confirm two anomalous characteristics the occurrence of seven normal faulting events on the transform segments expressing extension across the plate boundary in the azimuth n26 w and more than 90 deficiency in the seismic moment released during strike slip events on the transforms as compared to the rate expected from kinematic models other seismic properties are typical of the seismicity of a fast spreading mid ocean ridge mor system in particular we could not document a single teleseismically recorded event on the hollister ridge earthquakes are confined to narrow tfs with no activity present on the ridge segments the transform events have regular frequency moment statistics and we could not document any significantly slow sources these seismic properties generally support conventional plate tectonics models such as nuvel 1 and cannot be reconciled with a proposed reorientation of the pacific plate 4 ma ago

the 2 march 2016 wharton basin mw 7 8 earthquake high stress drop north south strike slip rupture in the diffuse oceanic deformation zone between the indian and australian plates the diffuse deformation zone between the indian and australian plates has hosted numerous major and great earthquakes during the seismological record including the 11 april 2012 m w 8 6 event the largest recorded intraplate earthquake on 2 march 2016 an m w 7 8 strike slip faulting earthquake occurred in the northwestern wharton basin in a region bracketed by north south trending fracture zones with no previously recorded large event nearby despite the large magnitude only minor source finiteness is evident in aftershock locations or resolvable from seismic wave processing including high frequency p wave backprojections and love wave directivity analysis our analyses indicate that the event ruptured bilaterally on a north south trending fault over a length of up to 70 km with rupture speed of 2 km s and a total duration of 35 s the estimated stress drop 20 mpa is high comparable to estimates for other large events in this broad intraplate oceanic deformation zone

plate boundary adjustments of the southernmost queen charlotte fault abstract a revision of the plate boundary configuration offshore southern haida gwaii which is the locus of the 2012 m w 7 8 thrust earthquake is based on a re evaluation of regional geophysical data and leads to recognition of fault segments critical for seismic hazard analysis off western canada the strike slip queen charlotte fault qcf and revere dellwood fault rdf constitute the transpressive plate boundary between the pacific and north american plates the southernmost 80 km of the qcf strikes at the highest angle 20 to the relative plate motion yet geomorphology adjacent to the fault is the most subdued synthesizing geomorphology seismicity sidescan sonar gravity and seismic reflection profiles indicates that the plate boundary is evolving to a less transpressive configuration than previously interpreted the rdf is 100 km longer than previously mapped it has propagated northward and overlaps the qcf by 120 km propagation is defined by strike slip earthquakes 3 9 m w m w 7 8 and the 1949 m 8 1 earthquakes

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

seismic cycles and earthquake predictability on east pacific rise transform faults the concept of a seismic cycle where the stress on a fault repeatedly builds up over a long period of time and then is rapidly released in a large earthquake influences studies of both the basic physics of faulting and applied research aimed at estimating earthquake hazards this hypothesis suggests that large earthquakes might be quasi periodic and that the probability of a particular portion of a fault rupturing twice in quick succession should be low however this basic hypothesis has been difficult to verify owing to the long repeat times of the largest earthquakes on most faults east pacific rise epr transform faults are an advantageous location to evaluate the seismic cycle hypothesis owing to their fast slip rates and the moderate size m w 6 of their largest earthquakes using surface wave based determinations of the relative separations between earthquake centroids i document 16 pairs of m w 5 5 events that had overlapping ruptures the distribution of interevent times for these pairs is tightly clustered around 5 yr with a coefficient of variation 0 2 indicating that quasi periodicity may be prevalent for the largest events on these faults moreover i find no pairs of overlapping m w 5 5 6 2 earthquakes separated by less than 50 cm of elapsed plate motion indicating that the two basic features of the seismic cycle hypothesis are evident in the timing of large epr transform mainshocks i have also confirmed earlier results demonstrating a high degree of short term predictability of epr mainshocks by combining teleseismic and hydroacoustic earthquake catalogs thus there appears to be a high degree of both short and long term predictability on epr transforms

earthquake scaling relations for mid ocean ridge transform faults 1 a mid ocean ridge transform fault rtf of length l slip rate v and moment release rate can be characterized by a seismic coupling coefficient ae at where ae v is an effective seismic area and at l3 2v 1 2 is the area above an isotherm tref a global set of 65 rtfs with a combined length of 16 410 km is well described by a linear scaling relation 1 ae at which yields 0 15 0 05 for tref 600 c therefore about 85 of the slip above the 600 c isotherm must be accommodated by subseismic mechanisms and this slip partitioning does not depend systematically on either v or l rtf seismicity can be fit by a truncated gutenberg richter distribution with a slope 2 3 in which the cumulative number of events n0 and the upper cutoff moment mc dcac depend on at data for the largest events are consistent with a self similar slip scaling dc ac1 2 and a square root areal scaling 2 ac at1 2 if relations 1 and 2 apply then moment balance requires that the dimensionless seismic productivity 0 0 atv should scale as 0 at 1 4 which we confirm using small events hence the frequencies of both small and large earthquakes adjust with at to maintain constant coupling rtf scaling relations appear to violate the single mode hypothesis which states that a fault patch is either fully seismic or fully aseismic and thus implies ac ae the heterogeneities in the stress distribution and fault structure responsible for relation 2 may arise from a thermally regulated dynamic balance between the growth and coalescence of fault segments within a rapidly evolving fault zone

multiple event relocation of earthquakes near the gorda rise mendocino fracture zone intersection earthquakes along the southern edge of the gorda plate and the mendocino fracture zone that were relocated jointly generally tend to move 25 to 35 km southwest of their pde and isc locations this has the effect of reducing the number of events occurring deep in the plate and increasing the number of events located on the gorda rise as well as revealing a tightly grouped linear zone of activity extending along the mendocino fracture zone the resulting spatial pattern of seismicity is more consistent with recent models of the tectonics of the gorda plate and the mendocino fracture zone

epicentral location based on rayleigh wave empirical green s functions from ambient seismic noise summary a new method to locate the epicentre of regional seismic events is developed with strengths and limitations complementary to existing location methods this new technique is based on applying empirical green s functions egfs for rayleigh waves between 7 and 15 s period that are determined by cross correlation of ambient noise time series recorded at pairs of seismic receivers the important advantage of this method in comparison with standard procedures based on use of body wave traveltimes is that it does not rely on an earth model rather it is based on interpolating the egfs to arbitrary hypothetical event locations the method is tested by locating well known ground truth crustal events in the western united states magnitude 4 earthquakes in california magnitude 3 earthquakes in utah and a mine collapse as well as locating seismic stations as virtual events in these applications location errors average less than 1 km but are expected to vary with event mechanism and depth numerical simulations show that the method optimizes when source depth is less than 1 km or more than 5 km and when the source mechanism is nearly purely strike slip thrust or normal

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

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

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

the seismology of greece abstract a review and a bibliography are given of the seismological literature for the area of greece in the period from 1950 up to the present time the aim is to incorporate all aspects of seismology starting with instrumental installations passing on to earthquake catalogues statistics and effects and ending up with structural properties and seismotectonics the purpose is to provide a service to every geophysicist who has an interest in this tectonically complex area and particularly to the numerous researchers engaged in geophysical expeditions to greece

master event relocation of gorda block earthquakes implications for deformation we have used the master event technique to relocate 73 earthquakes in the northern half of the gorda block the revised epicenters indicate a systematic eastward bias of 30 km in epicenters as published in the isc bulletin the new seismicity pattern reflects a more symmetric distribution of earthquakes about the gorda rise and a reduced degree of seismicity within the gorda block although the level of intraplate activity has apparently been reduced a significant number of events 10 20 still appears to be related to intraplate deformation further relocation studies coupled with focal mechanism determinations are necessary to better constrain deformation models for this region

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

upper crustal seismic structure of the endeavour segment juan de fuca ridge from traveltime tomography implications for oceanic crustal accretion the isotropic and anisotropic p wave velocity structure of the upper oceanic crust on the endeavour segment of the juan de fuca ridge is studied using refracted traveltime data collected by an active source three dimensional tomography experiment the isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers oscs at the segment ends these low velocities are indicative of pervasive tectonic fracturing and persist off axis recording the history of ridge propagation near the segment center velocities within the upper 1 km show ridge parallel bands with low velocities on the outer flanks of topographic highs these features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault tilted blocks that form the abyssal hill topography on axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures seismic anisotropy is high on axis but decreases substantially off axis over 5 10 km 0 2 0 4 ma this decrease coincides with an increase in seismic velocities resolved at depths 1 km and is attributed to the infilling of cracks by mineral precipitation associated with near axis hydrothermal circulation the orientation of the fast axis of anisotropy is ridge parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the oscs

episodic uplift deduced from holocene shorelines in the perachora peninsula corinth area greece four raised shorelines between 1 1 m and 3 5 m have been identified at two localities on the perachora peninsula and were dated by ams 14c to between 6400 and 1500 calibrated years ago uplift movements seem to have occurred in increments of 0 8 0 3 m with a return period of about 1600 years the last uplift coincided with a regional tectonic paroxysm which has already been documented in several areas of the eastern mediterranean the average uplift rate was probably faster during the holocene than the average since the last interglacial period no important vertical displacement occurred in this area at the time of or after the 1981 earthquake but a new episode of coseismic uplift may be in preparation

segmentation of the blanco transform fault zone from earthquake analysis complex tectonics of an oceanic transform fault 1 the blanco transform fault zone btfz forms the 350 km long pacific juan de fuca plate boundary between the gorda and juan de fuca ridges nearby broadband seismic networks provide a unique framework for a detailed long term seismotectonic study of an entire oceanic transform fault otf system we use regional waveforms to determine 129 earthquake source parameters combined with 28 harvard moment tensors they represent the largest waveform derived otf source parameter data set joint epicenter determination removes the northeasterly routine location bias projecting seismicity onto the btfz we determine along fault seismic slip rate variations earthquake source parameters and morphology indicate several transform segments separated by extensional step overs the eastern segment from gorda ridge to gorda depression is a pull apart basin the longest transform 150 km following blanco ridge from the gorda to cascadia depression is seismically very active seismically fully coupled has a wider seismic zone 9 km than other btfz transform segments and accommodates the largest mw 6 4 6 5 btfz earthquakes interpretation of cascadia depression as spreading ridge is supported by plate motion parallel normal faulting t axes spreading is currently tectonic 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling a short transform connects to the pull apart surveyor depression widely spread seismicity along the western btfz reflects complex morphology indicating ongoing plate boundary reorganization along short narrow width subparallel faults seismic coupling is low in extensional 15 compared to transform areas 35 100 implying different mechanical properties centroid depth variations are consistent with seismic slip cutoff near 600 c

tectonic evolution of the explorer northern juan de fuca region from 8 ma to the present new magnetic anomaly data in conjunction with sea beam and seamarc ii data has been used to schematically model the tectonic development of the explorer northern juan de fuca region with a series of five ridge offset propagators during the last 8 m y the sovanco fracture zone played a major role in the evolution of this area since its origin as a spreading center offset between the explorer and juan de fuca ridges approximately 7 4 ma between this time and 2 5 ma the transform migrated southwards due to southward propagation of explorer ridge and lengthened by the addition of three propagating ridge offsets as well as by asymmetric spreading to the east on explorer ridge since the independence of explorer plate at 4 ma there has been clockwise rotation of the explorer spreading center relative to the juan de fuca ridge in response to forces acting at the convergent margin the sovanco fracture zone also underwent clockwise reorientation and in the process developed into a broad zone of shear the tectonic deformation associated with this rotation distorted the previously existing n s magnetic anomaly pattern explorer seamount a major geological feature in the region is modeled as a presently extinct southern spreading segment of the explorer ridge system which was isolated by the northward migration of the west end of the sovanco fracture zone the recent 1 ma tectonic evolution of explorer ridge involved both northward and southward propagation of southern explorer ridge as well as an eastward ridge jump during the brunhes ih the northern portion of the study area the axis of spreading jumped 40 km to the nw around 0 3 ma from the eastern explorer deep to the western explorer rift

ephemeral plate tectonics at the queen charlotte triple junction three plate boundaries the queen charlotte transform the cascadia subduction zone and the juan de fuca ridge meet in a complex triple junction offshore vancouver island some interpretations of the plate tectonics of the region have included the explorer ridge the dellwood knolls and the tuzo wilson volcanic field as part of the spreading system requiring up to three triple junctions in the region new seabeam bathymetry interpreted with existing regional data sets indicate that the dellwood knolls and tuzo wilson volcanic field are not independent plate boundaries but the result of leaky transform tectonics between two overlapping transform faults seismicity shows that the explorer plate is being deformed by pacific north american relative motion as a new transform plate boundary forms and cuts off the explorer ridge the system is evolving to a single triple junction at the northern terminus of the juan de fuca ridge where it meets the nootka deformation zone thus the explorer microplate which was spawned 5 ma is an ephemeral adjustment to mechanical difficulties at the triple junction this new model implies that the explorer subduction zone is no longer active ephemeral oceanic microplates have also existed at pacific north american triple junctions off southern california and baja california

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

active backstop faults in the mentawai region of sumatra indonesia revealed by teleseismic broadband waveform modeling abstract two earthquake sequences that affected the mentawai islands offshore of central sumatra in 2005 mw 6 9 and 2009 mw 6 7 have been highlighted as evidence for active backthrusting of the sumatran accretionary wedge however the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks we refine the locations and focal mechanisms of medium size events mw 4 5 of these two earthquake sequences through broadband waveform modeling in addition to modeling the depth phases for accurate centroid depths we use teleseismic surface wave cross correlation to precisely relocate the relative horizontal locations of the earthquakes the refined catalog shows that the 2005 and 2009 backthrust sequences in mentawai region actually occurred on steeply 60 degrees landward dipping faults masilo fault zone that intersect the sunda megathrust beneath the deepest part of the forearc basin contradicting previous studies that inferred slip on a shallowly seaward dipping backthrust static slip inversion on the newly proposed fault fits the coseismic gps offsets for the 2009 mainshock equally well as previous studies but with a slip distribution more consistent with the mainshock centroid depth 20 km constrained from teleseismic waveform inversion rupture of such steeply dipping reverse faults within the forearc crust is rare along the sumatra java margin we interpret these earthquakes as unsticking of the sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger sunda lithosphere alternatively the reverse faults may have originated as pre miocene normal faults of the extended continental crust of the western sunda margin our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults

seismicity of oceanic and continental rifts a geodynamic approach abstract two major kinds of divergent structures oceanic and intracontinental rifts were compared in principal seismic and tectonic characteristics first the role of main components of the mid oceanic ridges mor was estimated for the whole earth we considered two levels of the mor segmentation the first order structures are the segments of mor between triple junctions and the second order structures are a transform faults and rift parts of mor the seismic catalogues neic and cmt were used to assess the seismic moment release the seismic moment release was calculated another way using the global plate tectonic model nuvel 1 and brune s formulae comparison of these two values shows that the seismic coupling coefficient varies from 1 to 10 for most of mor and is always higher for transform faults most of the deformation therefore is aseismic slip most seismicity of mor is confined to transform faults the energy contribution of transform faults is one to two orders magnitude higher than that of the rift and increases with the spreading rate there is a strong correlation between the seismic moment release of strike slip faults and their total lengths the correlation shows that the seismic moment release depends on the total transform area and confirms the simple thermal model of transform seismicity that was given by burr and solomon the seismic moment release and the spreading rate have opposite patterns for the rifts there is an inverse correlation between the seismic moment of normal faults and spreading velocity while it seems for transforms that these parameters are independent finally these results show that the seismicity of transforms and rifts depends first of all on the thermal structure of oceanic lithosphere in the case of continental rifts one can distinguish in the degree of seismic activity depending on the stage of rifting hence analysis of the continental seismicity requires the consideration of factors of a geological evolution that play practically no role in the case of oceanic lithosphere the comparison of geological and seismic data for the east african region has allowed us to outline the regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to mature oceanic rift in the evolutionary series intracontinental incipient rift intracontinental mature rift intercontinental rift oceanic slow spreading rift oceanic fast spreading rift there is a gradual decrease of the role of rifts sensu stricto as tension structures and increase of the role of strike slip faults epicenters concentrate along major faults as well the level of seismic energy becomes lower although the rate of deformation increases

empirical measurements of tectonic moment release in nuclear explosions from teleseismic surface waves and body waves summary a method of source parameter estimation based principally on observations of teleseismic long period surface waves is developed and applied to 71 underground nuclear explosions at the balapan shagan river test site in kazakhstan first we quantify the observed radiation patterns of love and rayleigh waves noting that such data place only three linear constraints on the five parameters of the standard point source model for each explosion the three constraints are the strengths of the isotropic sin2 and cos2q components of the observed pattern where is azimuth the five source parameters are the isotropic moment m the doublecouple moment adt and the strike dip and rake angles qs 6 a these last four parameters characterize the inferred faulting tectonic release surface waves which propagate from different seismic events in a geographically tight cluster to a distant station are similar in shape and for very shallow sources the propagation operator can be described by a single wave shape a differential propagation delay and a single amplitude factor containing the effects of attenuation scattering focusing and geometrical spreading the inverse problem is formulated which iteratively solves simultaneously for the path amplitude factors and the three radiation pattern parameters for all events using observed seismograms as the empirical wave shapes we fit 1110 relative measurements of rayleigh and love wave amplitudes to determine path amplitude factors to 29 globally distributed stations and radiationpattern parameters for the 71 explosions second we present a new method for interpreting such radiation patterns in previous studies two of the five source parameters have typically been fixed by making the very restricted assumption of pure dip slip faulting at 45 dip the assumption leads to estimates of strike that are mostly in the north west south east direction at balapan and to estimates of m that correlate very poorly with estimates based upon short period signals p lg however now that excellent yield estimates have become available we can avoid the assumption of pure dip slip faulting instead we can estimate mi from p and lg wave data thus adding a fourth constraint to help in interpreting the teleseismic surface wave amplitudes the outcome is a method for estimating the strike as of faulting associated with tectonic release for each explosion and for estimating the two combinations m sin 6 cos a and mo sin 26 sin a which respectively are the strike slip and dip slip components of the tectonic release the strike directions of inferred faulting at balapan are found to lie in the range 90 120 which conforms better with limited geological information on faulting in the region than does the trend that has long been inferred from assuming pure dip slip faulting we find that without additional information about individual explosions we are not able to provide a high precision isotropic moment or yield estimate from teleseismic surface waves

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

seismicity and rates of relative motion on the plate boundaries of western north america summary the consistency of earthquake data and plate tectonic models and other estimates of fault slip may be tested by estimating rates of motion from the earthquakes using the concept of seismic moment the contribution of individual events may simply be summed but a generally better estimate of long term average slip rate is obtained by integration over magnitudefrequency of occurrence relations estimates of fault motion rates associated with earthquakes are possible within about a factor of 2 using this approach if the major sources of uncertainty are given careful consideration e g incompleteness and inaccuracies in the earthquake data empirical moment magnitude relations and the effect of their stochasticity fault widths or depth extents the recurrence relations maximum magnitudes and the form of truncation at the maximum magnitude a formulation for the recommended magnitude density truncation is developed application of the latter method to the earthquake data of the offshore transform faults of the juan de fuca ridge system the queen charlotte fault zone and the northern vancouver island area in each case gives good agreement with rates from plate tectonic models for the southern san andreas fault and gulf of california area there is also good agreement with previous estimates obtained by summing the contributions of individual events however the displacement rate in the margin convergence zone of southern british columbia washington and oregon computed from the seismicity is at least a factor of 10 lower than from plate models and from other convergence estimates and primarily aseismic slip is suggested the fault motions as a function of time computed from the seismicity records have also been plotted and compared to the longterm average rates the plots permit estimates of the minimum present accumulated elastic strain and show if there are any temporal relations among earthquake displacements on different fault zones

earthquake rate slip rate and the effective seismic thickness for oceanic transform faults of the juan de fuca plate system summary the earthquake rate average fault slip rate and the effective seismic thickness have been examined for the revere dellwood wilson sovanco nootka blanco and mendocino transform faults bordering the juan de fuca plate system seismicity statistics are related to the rate of slip along a given fault due to earthquakes using the concept of seismic moment there are significant sources of uncertainty including the incompleteness and limited history of the earthquake catalogue the variety of magnitude definitions which can only be related empirically empirical moment magnitude relations and the effect of their stochasticity uncertainty in fault lengths and the effective seismic thickness the recurrence relation and the determination of maximum magnitude and how the recurrence relation is truncated at maximum magnitude nonetheless this method has been used successfully to provide estimates of deformation in good agreement with those from plate models an agreement between the deformation rate predicted by seismicity statistics for the fault zones and observed deformation from gps and other geophysical data is used to show the soundness of the method and parameters used the least constrained parameter is the effective seismic thickness thus the effect of a 2 3 6 5 and 10 km thick zone is investigated for each fault the selection of a thin effective seismic layer of about 3 km can consistently explain most of the deformation in the region as being seismically accommodated the upper mantle is inferred to be aseismic which is consistent with evidence of its serpentinization beneath these faults the similarity of the deformation estimates based on seismicity and those from plate models shows a remarkable consistency in these rates over a significant temporal range from tens to millions of years