seismicity of the teton southern yellowstone region wyoming abstract epicenter patterns focal depths and focal mechanisms for earthquakes occurring between 1973 and 1981 in the teton jackson hole southern yellowstone region are presented in this study available earthquake information recorded prior to 1980 was combined with the results obtained from two microearthquake surveys operated in 1980 and 1981 earthquakes used in this joint analysis met rigid standards for location accuracy and hence the resulting data set provides the most complete accurate and up to date information on seismicity in this region the majority of earthquakes in the region do not appear to be associated with mapped traces of quaternary faults the teton fault appears to be active at the small earthquake level along some of its segments although several segments including the north central segment which exhibits the greatest prehistoric displacement appear to be quiescent seismicity in the gros ventre range may be related to reactivation of older basement structures fault plane solutions in the region show predominately normal faulting with extension in an east west to northwest southeast direction geologic seismic moment rates of 1 1 10 24 dyne cm yr for the teton region and 4 0 10 23 dyne cm yr for the teton fault were estimated using available geologic information on mapped faults from the limited available data a return period of 130 to 155 yr for a magnitude 6 5 to 7 5 earthquake is predicted for the teton region while the teton fault has a predicted return period of 800 to 1800 yr for a magnitude 7 5 earthquake a regional strain rate of 6 9 10 9 yr is also obtained

a 3d shear wave velocity model of the upper mantle beneath china and the surrounding areas we present a three dimensional model of shear wave velocity for the upper mantle of china and the surrounding region by analyzing 50 338 vertical component multi mode rayleigh wave seismograms recorded at 144 permanent and more than 300 temporary broadband stations in and around china the procedure involves combination of 1 d path average models obtained by modeling each rayleigh waveform up to the 4th higher mode in a tomographic inversion scheme the dense station network and the use of multi mode analysis help to achieve a lateral resolution of a few hundred kilometers down to 400 km depth the seismic lithosphere as it is defined by the crust and the high velocity mantle lid is to the first order thin in east china and thick in the west with a high velocity lid extending down to about 200 km depth beneath much of the tibet pamir plateau beneath india the thickness of the seismic lithosphere gradually increases from 100 km in south india to more than 150 km in north india where it underthrusts the tibetan plateau to approximately the jinsha river suture high velocity lid extending down to 100 150 km depth is also observed in the tarim basin sichuan basin and ordos block in the eastern part of the north china craton the seismic lithosphere is probably close to or thinner than 70 km adjacent to these areas the high velocity lid in the eastern yangtze craton and south china fold system extends down to 70 80 km depth a large scale subhorizontal high velocity body is observed at depths of 150 350 km beneath the entire east china cratonic areas this high velocity body might be the remnant of a delamination process which resulted in the decratonization of the north china and the yangtze cratons

moment magnitude local magnitude calibration for earthquakes off canada s west coast local magnitude m l values of earthquakes off canada9s west coast are known to be underestimated by at least 0 5 magnitude units compared with other magnitude scales moment magnitude m w derived from moment tensor analysis provides the most robust estimate of the magnitude of earthquakes moment tensor analysis of regional seismic data in western canada is now possible due to the installation of more than 40 three component broadband stations in western canada the u s pacific northwest and southeast alaska moment tensor solutions are now possible down to m 4 0 more than 230 regional moment tensor solutions have been calculated off canada9s west coast at the geological survey of canada for 1995 2002 these solutions along with 14 previous solutions by oregon state university in 1994 1995 and 13 harvard solutions for 1984 1993 allow a systematic m w m l calibration for earthquakes in this region the study area extends from the queen charlotte islands region in the north to the area off the west coast of southern vancouver island at the northern end of the study area where there is little oceanic crust in the source receiver travel path m w is systematically larger than m l by 0 28 0 08 magnitude units at the southern end of the study area where there is a significant amount of ocean crust in the source receiver travel path m w is systematically larger than m l by 0 62 0 08 magnitude units calibration of m l with m w will allow the western canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis

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

seismicity in the st elias region of northwestern canada and southeastern alaska abstract low level monitoring in the st elias region between september 1978 and march 1981 has identified two principal zones of seismicity that are consistent with the historical pattern of seismicity at higher magnitude levels the most active zone follows the plate boundary along the coast of southeast alaska a recurrence relation based on completeness over varying time intervals is log n m 0 74 m 3 77 a less active zone is observed generally following the duke river and dalton segments of the denali fault system through the southwest yukon instead of continuing into chatham strait this trend of seismicity turns south into the glacier bay region to connect with the fairweather fault north of cross sound the seismicity and high rate of uplift observed in the glacier bay region is probably the result of convergence across the fairweather fault north of cross sound the recurrence relation for the denali zone is log n m 0 90 m 3 77 a similiar recurrence curve is found for activity with magnitudes down to about 2 0 observed over the 39 month low level monitoring program the rate of activity in the denali zone appeared to decrease for a period of about 1 yr following the 28 february 1979 m 7 2 st elias earthquake the rate of aftershock activity was also found to vary with the b value decreasing throughout the sequence only minor seismicity is observed northeast of the denali fault system in the southwest yukon and east of chatham strait in northwest british columbia a prominent cluster east of juneau near the british columbia border may be related to volcanic activity a 5 week microearthquake survey over a 40 km segment of the denali fault system found activity in a highly faulted zone about 15 km wide at depths of less than about 15 km a composite p nodal solution indicates motion on either east west or north south striking planes neither of which are consistent with the general trend of the denali system in the area an average slip rate of no more than about 1 mm yr is estimated from the seismicity for the denali fault system through the southwest yukon an estimate of the minimum present accumulated elastic strain suggests the potential for a magnitude 6 1 2 earthquake

seismic structure in the southeastern china using teleseismic receiver functions abstract the southeastern margin of china is an ideal area to study the modern plate interactions the previous models however are poorly constrained with respect to the geometry due to the sparse coverage here we present new observations from a temporary array with 20 stations in southeastern china we isolated converted seismic phases in the p and s wave coda to generate receiver function from the structure of the crust and upper mantle by using h k stacking and common conversion point stacking the geometry of discontinuities above was imaged along the southeastern chinese coastline the ccp section shows moho dips gently northeastward with a mean depth of 30 km the 410 and 660 km discontinuities are at the depth close to iasp91 model this means that we have not seen any significant anomaly from moho and mantle transition zone that corresponded with the geodynamics of plate subducting instead we found that the crust was ruptured by min river fault to depth of the moho we provide a 2d map of moho topography by combining our crustal thickness to that from permanent stations active seismic and obs offshore profiles the map is consistent with previous studies and shows a crust that thins coastward and southwestward in the rate of 1 5 km per 100 km we image the lithosphere asthenosphere boundary at a depth of 60 to 70 km by using s wave to combine with the p wave our crustal structure suggests that min river fault certainly plays an important role in adjusting regional stress field induced by plate interactions in the study area our lithospheric thickness is more consistent with a very strongly attenuated continental lithosphere given the close proximity to the coast this might suggest that the lithospheric mantle transitions to oceanic before the crust does alternatively we may infer that the continental lithosphere has been thermally eroded in this region

kinematic models of dewatering accretionary prisms steady state two dimensional analytical solutions are derived for sediment velocity and rate of fluid expulsion in a thickening accretionary prism with smooth upper and lower surfaces two different kinematic assumptions lead to two similar sets of solutions in one case the sediment flow lines are assumed to diverge uniformly landward in the other case the horizontal velocity of any given sediment column is assumed uniform by further assuming zero horizontal darcy velocity in the prism expressions can be obtained for the approximate vertical fluid velocity and pore fluid pressure fields results from applying these solutions to the cascadia accretionary prism off vancouver island show the importance of modeling the effects of lateral as well as vertical porosity variation on the hydrogeology of the prism these solutions provide a means to estimate fluid flow and pore pressure distribution for prisms with realistic geometry the rate of fluid expulsion can also be incorporated into numerical models as the fluid source term

the winona basin structure and tectonics a compilation of published and new geophysical data from the winona basin off northern vancouver island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made the basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin the crust beneath the basin is young 1 5 ma increasing in age from southeast to northwest and of normal oceanic thickness it is virtually non magnetic however probably because of its having been rapidly buried by turbidite sedimentation subsidence of the basin and uplift of the paul revere ridge began in the early pleistocene ca 1 8 ma and since that time up to 8 km of turbidite sediments has accumulated in the basin the nature of the fanning of the deposits suggests that the basin has been kept full throughout its history the minimum average supply rate necessary to accomplish this is about 70 106 mg year 1 this pleistocene ave

deformation rates estimated from earthquakes in the northern cordillera of canada and eastern alaska 1 we estimate deformation rates from earthquake catalog statistics to investigate current strain distribution resulting from the collision of the yakutat block with north america the collision is accommodated over a broad region of alaska and nw canada in a variety of tectonic regimes our estimates have significant uncertainties but the results are generally in agreement with other deformation rate estimates gps and geological data and provide useful first order constraints on local long term tectonic deformation and seismic hazard we estimate that 50 75 of relative plate motion is taken up by earthquakes on the yakutat north america boundaries with lateral crustal extrusion other far field strain and possibly aseismic creep accommodating the remainder we calculate dextral strike slip of 4 5 10 2 mm a on the denali fault system in alaska remarkably similar to late pleistocene holocene rates from geomorphic offsets right lateral motion essentially bypasses the denali fault in easternmost alaska and westernmost yukon passing instead along the totschunda and duke river fault systems right lateral transpression of 5 10 mm a suggests that the alaskan fore arc does not have fully block like behavior between the denali and tintina kaltag faults in alaska 10 20 mm a of dextral shear is accommodated by several ne trending bookshelf faults significant seismic deformation in cases higher than expected from gps data is indicated in the ne cordillera where preexisting faults have a strong control on the pattern and mechanisms of current deformation

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

seismicity surveys with ocean bottom seismographs off western canada three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the juan de fuca ridge system off western canada nearly 100 events were located with estimated accuracies generally better than 10 km all lying on or near the en echelon ridge transform fault plate boundaries as defined in this area by the magnetic anomalies the seafloor morphology and by other geophysical data the depths of 12 events were determined to lie between 2 and 6 km below the top of the crust the seismograms exhibit clear p and s wave arrivals along with phases that involve p to s and sometimes s to p conversion probably at the base of the sediments beneath the instruments the event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station the magnitude estimates permit the determination of rough magnitude frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data the mean p wave velocity in the uppermost mantle from the earthquake datamore recorded by the sea floor arrays is 7 6 km s sup 1 and the mean v sub p v sub s ratio is 1 71 or a poisson s ratio of 0 24 less

petrogenesis of the late yanshanian mantle derived intrusions in southeastern china response to the geodynamics of paleo pacific plate subduction abstract several late yanshanian syenitic and gabbroic rocks from the coastal area of southeastern se china have been studied to determine zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings the systematic la icp ms zircon u pb dating results indicate that they can be divided into two main stages the early stage 141 118 ma and the late stage 98 86 ma which temporally correspond to the regional lower and upper volcanic series respectively the early stage syenitic and gabbroic rocks are accompanied by vast rhyolitic and dacitic volcanic rocks with minor granites their parental magma was derived from an enriched mantle metasomatized by subducted sedimentary materials the late stage syenitic rocks mainly occur as the central intrusion of the caldera with ages close to the accompanied volcanics forming a caldera related volcanic intrusive ring complex they were produced by magma mixing between depleted asthenosphere melts and subduction related enriched mantle melts it is suggested that the late yanshanian tectonic settings of se china transformed from a compressional environment to an extensional environment corresponding to the transformation from forward to rollback subduction of the paleo pacific plate at approximately 110 ma

new probabilistic strong seismic ground motion maps of canada new probabilistic strong seismic ground motion maps of canada are derived by applying the cornell mcguire method to 32 earthquake source zones in canada and adjacent regions peak horizontal acceleration and peak horizontal velocity are mapped to depict strong ground motion in the frequency ranges near 5 hz and near 1 hz respectively some earthquake source zones are well defined on the basis of both tectonics and average seismicity rates but a lack of understanding of the near field effects of the large earthquakes prohibits accurate estimates of ground motion in their vicinity some source zones have no known large scale crustal features or zones of weakness that can explain the seismicity and must therefore be defined simply as concentrations of seismic activity with no geological or geophysical controls on the boundaries other source zones lack information on low magnitude seismicity that would be representative of longer periods of time a significant source of uncertainty in the ground motion estimates is introduced by the uncertainty in strong ground motion attenuation relations particularly in the near field of large earthquakes and particularly in eastern canada where no strong motion data are available for earthquakes larger than magnitude 5 valid estimates can be made of relative levels of probabilistic strong ground motion on a regional basis at moderate probabilities however the simple peak ground motion parameters are not sufficient for engineering design purposes in the immediate vicinity of large earthquakes a comparison of the new probabilistic ground motion estimates at the canada united states border with equivalent estimates made in the united states reveals that peak acceleration contours match well in the british columbia washington state yukon alaska and lake ontario border regions canadian peak velocities are a factor of two higher near the british columbia washington state border canadian peak accelerations are higher in the border regions of eastern canada influenced by the charlevoix zone and in regions of the alaska panhandle british columbia border influenced by the active queen charlotte and fairweather fault zones the new ground motion estimates are used to produce new peak acceleration and peak velocity zoning maps to replace the 1970 peak acceleration zoning map in the national building code of canada the acceleration and velocity maps provide independent ground motion reference levels for small rigid structures and taller flexible structures respectively compared to the 1970 map with four zones the new maps with seven zones have a finer subdivision of zoning in moderate risk areas and additional zones in the high risk areas a change in probability of exceedence from 0 01 per annum on the 1970 map to 10 per cent in 50 yr on the new maps and the addition of velocity as a zoning parameter have necessitated changes in the seismic design provisions these revisions will be incorporated into the 1985 edition of the code

seismic hazard in western canada from gps strain rates versus earthquake catalog 1 probabilistic seismic hazard analyses psha are commonly based on frequency magnitude statistics from 50 100 yearlong earthquake catalogs assuming that these statistics are representative of the longer term frequency of large earthquakes we test an alternative psha approach in continental western canada including adjacent areas of northwestern u s a using regional strain rates derived from 179 global positioning system gps horizontal velocities gps strain rates are converted to earthquake statistics seismic moment rates and ground shaking probabilities in seismic source zones using a logic tree method for uncertainty propagation median gps based moment rates and shaking estimates agree well with those derived from earthquake catalogs in only two zones puget sound and mid vancouver island in most other zones median gps based moment rates are 6 150 times larger than those derived from earthquake catalogs shaking estimates 2 5 times larger although the gps based and catalog estimates commonly agree within their 67 uncertainties this discrepancy may represent an under sampling of long term moment rates and shaking by earthquake catalogs in some zones however a systematic under sampling is unlikely over our entire study area although not demonstrated with a high confidence level long term regional aseismic deformation may account for a significant part of the gps catalog discrepancy and in some areas represent as much as 90 of the total deformation budget in order to integrate gps strain rates in psha models seismic versus aseismic partitioning of long term deformation needs to be quantified and understood in terms of the underlying mechanical processes

current tectonics of northern cascadia from a decade of gps measurements 1 global positioning system gps measurements to study regional deformation were initiated in northern cascadia in the late 1980s and early 1990s on the basis of a decade of gps data we derive a crustal velocity field for nw washington sw british columbia the permanent and campaign gps velocities are defined with respect to north america in the itrf2000 reference frame velocity uncertainties are estimated using a model of time series noise spectra this new velocity field is the basis for interpretation of the tectonics of the northern cascadia subduction system gps velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models our data confirm that the upper part of the megathrust is nearly fully locked an exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition the landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models this could be a consequence of a small 0 3 mm yr long term motion of the southern vancouver island forearc with respect to north america or of a concentration of interseismic strain across the elastically weaker cascadia volcanic arc in northern vancouver island our velocity field supports the existence of an independent explorer microplate currently underthrusting underneath north america at least up to brooks peninsula further north gps velocities indicate transient and or permanent deformation of northernmost vancouver island related to the interaction with the explorer microplate and possibly with the queen charlotte transform margin

temporal and spatial variability of sea level and volume flux in the murderkill estuary abstract the instantaneous sea level determined at two sites in the murderkill estuary a tributary of delaware bay results from the superposition of temporal variability operating over different time and spatial scales over the relatively short tidal time scales the semidiurnal tides that represent the dominant tidal constituents in lower delaware bay show a modest increase in tidal amplitudes from the bay mouth lewes delaware up to bowers beach the mouth of the murderkill estuary however as the tides propagate into the murderkill estuary the semidiurnal constituents undergo heavy attenuation resulting in a 48 reduction in tidal amplitude from bowers to frederica approximately the extent of saline intrusion the diurnal tide on the other hand experiences only a 25 reduction in amplitude the limited tidal asymmetry that is observed may be a result of interaction between flows in the tidal channel and the adjacent salt marsh at longer time scales the subtidal sea level experiences no attenuation the murderkill estuary thus behaves like a low pass filter to preferentially damp out high frequency sea level forcing from lower delaware bay the subtidal volume flux in the murderkill is highly coherent with the time rate of change of sea level indicating that the murderkill basically co oscillates with delaware bay in a standing wave fashion over the subtidal time scale this remote coupling controls more than 90 of the variance in subtidal sea level in the estuary the surface slopes in the lower bay and the murderkill estuary are closely correlated with winds along the orientation of the two waterways consistent with the effect of local wind on subtidal sea level

seismic evidence for rotating mantle flow around subducting slab edge associated with oceanic microplate capture tectonic plate reorganization at a subduction zone edge is a fundamental process that controls oceanic plate fragmentation and capture however the various factors responsible for these processes remain elusive we characterize seismic anisotropy of the upper mantle in the explorer region at the northern limit of the cascadia subduction zone from teleseismic shear wave splitting measurements our results show that the mantle flow field beneath the explorer slab is rotating anticlockwise from the convergence parallel motion between the juan de fuca and the north america plates re aligning itself with the transcurrent motion between the pacific and north america plates we propose that oceanic microplate fragmentation is driven by slab stretching thus reorganizing the mantle flow around the slab edge and further contributing to slab weakening and increase in buoyancy eventually leading to cessation of subduction and microplate capture

earthquake scaling the effect of a viscoelastic asthenosphere summary viscous creep in the crust and mantle can have important consequences for earthquake patterns in time and space here numerical models of repeating earthquake rupture are used to investigate how viscoelasticity can affect size distributions of repeated earthquakes on vertical planar fault segments the earthquake model is based on exact solutions of static 3 d elasticity theory viscous flow beneath the seismogenic crust is modelled as a maxwell relaxation process two different fault strength models are used a smooth rate and state dependent friction model and a strongly heterogeneous asperity model in a previous paper it was shown that the characteristic scale of fault segmentation is proportional to the vertical width of a seismogenic fault for both the heterogeneous and smooth friction models viscous relaxation of the substrate modifies the spatio temporal distribution of cumulative slip and slip events for the heterogeneous models the resulting quake size distributions are independent of the viscous relaxation time in contrast for the smooth models it is shown that the characteristic event size is inversely proportional to the relaxation time of the substrate these results imply a connection between the spatial complexity of fault zones and the maximum earthquake size a fault can sustain whereas earthquake sizes on complex fault zones depend only weakly on the viscous rheology beneath the seismogenic crust the sizes of large events on relatively simple faults can be substantially increased due to viscous relaxation and partial decoupling of the seismogenic crust from the deeper crust and mantle

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

rates and uncertainties on seismic moment and deformation in eastern canada 1 on the basis of earthquake statistics from the canadian earthquake hazard model we estimate the rates of seismic moment and deformation in seismic zones of eastern canada we use a logic tree approach to derive median values and 66 confidence intervals for models based on earthquake cluster distributions and geological and tectonic structures typical moment rates are low 0 1 1 1017 n m yr 1 equivalent to a magnitude mw 7 earthquake every 350 3500 years in each seismic zone in a few zones such as baffin bay moment rates reach 2 10 1017 n m yr 1 expressed in terms of relative velocity between zone boundaries seismic deformation rates are typically 0 2 0 5 mm yr 1 for a few regions of high seismicity such as baffin bay charlevoix or grand banks the relative velocities reach 1 2 mm yr 1 strain rates are typical of the intraplate environment 10 13 10 10 yr 1 with values locally as high as 10 8 yr 1 these rates give some insight into geodynamic aspects of the northeastern north america plate assuming that the seismic deformation rates represent long term deformation of the lithosphere they provide some constraint to the amplitude and variability of intraplate strain on a shorter timescale seismic strain rates can be compared with postglacial rebound predictions and might help constrain some aspects of these models because they reflect the seismic energy released over the last few hundred years our estimates in combination with geodetic measurements can also help constrain models of earthquake dynamics in eastern north america

gps crustal strain postglacial rebound and seismic hazard in eastern north america the saint lawrence valley example 1 we present global positioning system gps measurements that constrain the amplitude pattern and origin of crustal deformation in the saint lawrence valley quebec one of the most seismically active regions of eastern north america the gps network shows coherent southeastward motion of 0 6 0 2 mm yr 1 relative to north america and uplift of 2 6 0 4 mm yr 1 network average horizontal strain rates are mostly ese wnw shortening at 1 7 1 0 10 9 yr 1 the shortening rate across the charlevoix seismic zone is about twice as big as the regional average these measurements are consistent with both postglacial rebound pgr models and the deformation style indicated by earthquake focal mechanisms although the gps data do not discriminate between various models of crustal deformation they provide important constraints on large earthquake recurrence assuming that the gps strain estimates are representative of seismic moment release they constrain the maximum magnitude of truncated gutenberg richter recurrence statistics in the charlevoix seismic zone to mx 7 8 0 6 one m 7 earthquake per 400 1300 years the remarkable agreement between the gps strain rates seismic catalogue statistics and pgr predictions suggests that in charlevoix most of the pgr driven crustal strain may be released by large m 7 earthquakes in the rest of the saint lawrence valley pgr strain rates are significantly larger than seismic strain rates suggesting either that pgr deformation remains mostly elastic or that large events are more frequent than indicated by small earthquake statistics i e characteristic earthquakes

reply to w gawthrop s comments on the lompoc california earthquake november 4 1927 m 7 3 and its aftershocks

comparison of seismicity parameters in different subduction zones and its implications for the cascadia subduction zone the cascadia subduction zone is a young subduction zone with associated volcanic activity but with no history of large thrust earthquakes associated with it as is characteristic of other subduction zones this has led to questions about the potential for great earthquakes associated with this subduction the seismic potential associated with this subduction has been investigated in this study by examining and comparing the number of shallow earthquakes with magnitude 6 which have occurred in different subduction zones since 1910 this examination shows that all subduction zones exhibit a higher level of activity than cascadia furthermore two of these zones with similar rates of plate motion and age at subduction the rivera plate and southwest japan have not only experienced great earthquakes but show a higher rate of activity than the cascadia zone the activity rate in subduction zones with predominantly aseismic slip is also higher than the activity rate in the cascadia zone earthquake activity in the rupture zones of great earthquakes both before and after the occurrence of great earthquakes is higher than the activity rate in the cascadia zone these results therefore suggest that the low rate of activity in the cascadia zone is unusual and supports the idea that the subduction here is different and that perhaps the potential for great earthquakes cannot be extrapolated simply on the basis of certain similarities between the cascadia zone and other subduction zones with a history of great earthquakes the low rate of earthquake activity in the cascadia zone can however be explained on the basis of the combined effect of low rate of plate motion significant aseismic subduction and young age at subduction the activity rate in the cascadia zone is consistent with the activity rates in other subduction zones if it is assumed that much of the plate motion is accommodated aseismically the seismic slip rate associated with this subduction zone is a small fraction of the rate of plate motion and therefore even if the potential for great earthquakes may exist the return period for great earthquakes will be much longer 2 or 3 times than in other zones with similar characteristics such as the rivera plate or southwest japan

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

significant geometric variation of the subducted plate beneath the northernmost cascadia subduction zone and its tectonic implications as revealed by the 2014 mw 6 4 earthquake sequence abstract at the northernmost extent of the cascadia subduction zone the explorer plate subducts at approximately 2 cm yr less than half the rate of the juan de fuca plate to the south the boundary between these two plates is known as the nootka fault zone which is one of the focuses of the seafloor earthquake array japan canada cascadia experiment seajade during this survey an m w 6 4 earthquake occurred on 24 april 2014 this event and the subsequent aftershocks referred to as the nootka sequence reveal an approximately 40 km long subducted fault within the explorer plate to the north of the nootka fault zone we infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the nootka fault zone the depth distribution and focal mechanisms of the aftershocks indicate significant margin parallel deformation of the subducting plate the subduction interface at the nootka sequence fault has been deflected downward to the northwest from a depth of approximately 15 25 km over a distance of 25 km we propose two possible scenarios that are modified from previously suggested slab tear model with induced margin parallel mantle flow to explain the significant deformation of the young warm subducting explorer plate to the northwest of this change in slab geometry a lack of seismic activity above the plate interface indicates that the explorer plate has partially decoupled from the overriding north america plate we conclude that the geometric variation separating the southern explorer plate from the north along with decoupling and a possible intraslab tear may be a significant combination to resist the propagation of a megathrust rupture across this boundary

investigating the lithospheric velocity structures beneath the taiwan region by nonlinear joint inversion of local and teleseismic p wave data slab continuity and deflection the interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the taiwan region whether and in what form the eurasian plate subducts deforms under taiwan island is critical to the debate of tectonic models although an east dipping high velocity anomaly down to a depth below 200 km has been reported previously its detailed morphology remains uncertain and leads to different interpretations with a two step strategy of nonlinear joint inversion the slab images of the eurasian plate were retrieved in a geometry that is hyperthin in the south becoming massive and steeper in the central and severely deformed in the north the possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn moreover the slab deflection found at 23 2 n latitude seems to correspond to where the nonvolcanic tremors and recent nw se striking structures have occurred in southern taiwan

strain rates in northwestern italy from spatially smoothed seismicity 1 this work presents seismic strain rate maps for the western alps and northern apennines northern italy as derived from an earthquake catalog collecting both historical and instrumental data strain rates are calculated on the basis of the rate of seismic moment release using the anderson method unlike previous applications which determined the total strain rate associated with specific seismogenic sources we have employed an innovative zoneless approach based on a spatially smoothed seismicity method in addition a monte carlo simulation procedure is applied to allow for uncertainty in the input data e g magnitude to moment conversion seismogenic thickness maximum earthquake magnitude strain rate maps are developed by summing the moments of the earthquakes reported in the catalog and by using two different earthquake recurrence relations our results indicate that deformation rates are quite high ranging from about 2 to 12 10 9 yr 1 in the northern apennines and from 0 5 to 6 10 9 yr 1 in the western alps these values however are 1 order less than those derived from global positioning system measurements suggesting that a portion of the recent deformation in northwestern italy is related to aseismic processes the discrepancies between seismic and geodetic strain rates may also indicate that the record of seismicity may not provide a sufficient time window for assessment of secular rates of moment release or secular deformation rates and rates of recurrence of large magnitude earthquakes in the study area

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

queen charlotte fault zone microearthquakes from a temporary array of land stations and ocean bottom seismographs a temporary array of land and ocean bottom seismograph stations was used to accurately locate microearthquakes on the queen charlotte fault zone which occurs along the continental margin of wester

the lithosphere of north china craton from surface wave tomography abstract fundamental mode rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east asia this article expounds the features of lithosphere structure in different sub regions of north china and discusses their implications for the tectonic evolution and present tectonics of north china craton ncc the study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust the active nw trending zhangjiakou bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid in addition to the well known structural difference between western and eastern ncc we are able to resolve clearly the lithospheric features of sub regions in eastern ncc the southern north china basin and north yellow sea are characterized by a high velocity crust and upper mantle lid down to 70 80 km depth while the northern north china basin has a low velocity crust and the velocity below moho to about 130 km is rather uniform and low there is also a general trend of velocity decrease in the upper mantle from west to east the velocity images bear the imprints of subduction and collision that took place in the periphery of ncc since mesozoic and shed some light on the mechanism of mesozoic lithospheric thinning of eastern ncc

a fractal approach to probabilistic seismic hazard assessment abstract the definition of a fractal distribution is that the number of objects events n with a characteristic size greater than r satisfies the relation n r d where d is the fractal dimension the applicability of a fractal relation implies that the underlying physical process is scale invariant over the range of applicability of the relation the empirical frequency magnitude relation for earthquakes defining a b value is a fractal relation with d 2b accepting the fractal distribution the level of regional seismicity can be related to the rate of regional strain and the magnitude of the largest characteristic earthquake high levels of seismic activity indicate either a large regional strain or a low magnitude maximum characteristic earthquake or both if the regional seismicity has a weak time dependence the approach can be used to make probabilistic seismic hazard assessments as in many fractal applications the basic approach has been used in the past however the association of a fractal behavior with scale invariance provides a rational basis for the extrapolation of frequency magnitude statistics in order to make predictions of seismic hazards

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

earthquake rate analysis abstract earthquake rate analysis explores the quantitative relationship between earthquake occurrence rates and geological deformation rates one application is the estimation of tectonic deformation rates from seismicity for comparison with tectonic models another application is the use of geological models to help quantify seismic hazards seismicity is described by a catalog of the largest earthquakes in the region the contribution of small earthquakes towards causing geological deformation is insignificant although the occurrence rate of small events may be predictable from the occurrence rate of large events by a b value relationship the critical information about each earthquake is its seismic moment or better still its seismic moment tensor when moment is not available directly it can be estimated from the earthquake magnitude although this increases uncertainties considerably the average annual moment from all the earthquakes in the catalog is the seismic moment rate uncertainties in seismic estimates of moment rate arise from the short time period of observations from the magnitude moment relationship from the shape of the distribution function giving number of earthquakes as a function of magnitude and from uncertainties in the physical properties of the earth near the source geological models for deformation rates are described by slip rates on individual faults or by regional strain rates a regional strain rate can be derived as a limiting case where there are numerous individual faults such a strain rate is a particularly useful concept where information about each of the faults is limited in the case of either faulting or regional strain the average seismic moment rate is proportional to the deformation rate geological uncertainties arise from difficulties in estimating slip rates or strain rates from fault creep and other aseismic deformation from uncertainty in the part of the fault that fails during earthquakes and boundaries of the deforming region and from uncertainty about the earth s physical properties the seismic moment rate is the critical factor for relating the earthquake occurrence rate and the geological deformation rate using this intermediate parameter one can infer earthquake occurrence rates from deformation rates or infer deformation rates from earthquake occurrence rates generally it is found that deformation rates and earthquake occurrence rates are consistent within uncertainties where both are well constrained they are generally consistent within a factor of two or three

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

evidence for precursory changes in the frequency magnitude b value summary a clear case has been established for a significant increase in the slope of the frequency magnitude relation for periods of up to six years before a number of earthquakes in new zealand and california in a region of about 200 km diameter in central new zealand the b value increased from near unity to more than 2 0 at the end of 1968 and remained abnormally high until the beginning of 1975 an earthquake of magnitude 6 0 followed in 1977 january within 50 km of the location which had registered the highest b value the anomaly was detected in contour maps of b values windowed in space and time then resolved more precisely in time using the cusum technique it was statistically significant at a level higher than 99 99 per cent compared with the average rate of moment release in this region there was a deficit of 1 3 x 10 nm during the anomalous period which is equivalent to an earthquake of magnitude 6 08 this phenomenon of a high b value has been found before all shallow new zealand earthquakes of magnitude 6 0 or more since 1965 wherever the seismicity rate and detection threshold were adequate to resolve it further the only statistically significant occurrences of high b value were those which were followed by large earthquakes two of magnitude 6 8 and 6 0 and a swarm of equivalent magnitude 5 3 the phenomenon has also been found before the san fernando and coalinga earthquakes in california both of which were thrust events but it is not apparent before two large californian earthquakes with strike slip mechanisms observations of high b value are consistent with reported instances of quiescence in background seismicity in fact what has been interpreted as quiescence may probably be more accurately described as a localized high b value the duration of the anomaly is not a simple function of magnitude different strain rates in the regions examined may account for this the available data suggest that careful monitoring of the b value may provide a tool for medium term earthquake prediction although perhaps not in all tectonic environments

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

layered deformation in the taiwan orogen the underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins we report three dimensional seismic anisotropic tomography of taiwan that shows a nearly 90 rotation of anisotropic fabrics across a 10 to 20 kilometer depth consistent with the presence of two layers of deformation the upper crust is dominated by collision related compressional deformation whereas the lower crust of taiwan mostly the crust of the subducted eurasian plate is dominated by convergence parallel shear deformation we interpret this lower crustal shearing as driven by the continuous sinking of the eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen the two layer deformation clearly defines the role of subduction in the formation of the taiwan mountain belt

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

upper mantle structure and dynamics beneath southeast china we applied teleseismic tomography to 5671 relative travel time residuals from 257 teleseismic events recorded by 69 seismic stations to determine the 3d p wave velocity structure of the upper mantle under southeast se china our results show prominent low p wave velocity low vp anomalies under se china which may reflect the remnant magma chambers and channels of the late mesozoic igneous rocks which may be reheated by the upwelling mantle flow from the lower mantle driven by the deep subduction in east asia during the cenozoic high vp anomalies are revealed in the upper mantle to the east of taiwan which represent the subducted eurasian plate our result also suggests the break off of the subducted eurasian plate caused by its interaction with the philippine sea plate under central and north taiwan the slab break off may have created a mantle window through which the asthenospheric flow arises causing the high heat flow and rapid uplift in the taiwan orogen

geodetic strain measurements in washington two new geodetic measurements of strain accumulation in the state of washington for the interval 1972 1979 are reported near seattle the average principal strain rates are 0 07 0 03 strain yr n 19 w and 0 13 0 02 strain yr n71 e and near richland south central washington the average principal strain rates are 0 02 0 01 strain yr n36 w and 0 04 0 01 strain yr n54 e extension is taken as positive and the uncertainties quoted are standard deviations a measurement of shear strain accumulation dilatation not determined in the epoch 1914 1966 along the north coast of vancouver island by the geodetic survey of canada indicates a marginally significant accumulation of right lateral shear 0 06 0 03 rad yr across the plate boundary n40 w strike although there are significant differences in detail these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the juan de fuca plate the observed accumulation of strain implies that large shallow thrust earthquakes should be expected off the coast of washington and british columbia however this conclusion is not easily reconciled with either observations of elevation change along the washington coast or the focal mechanism solutions for shallow earthquakes in washington

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

the crust of cathaysia age assembly and reworking of two terranes abstract u pb dating and hf isotope data from detrital zircons have been used to analyse the crustal evolution of the eastern and western parts of the cathaysia block in se china zircons from the oujiang river in eastern cathaysia indicate that the basement is dominantly paleoproterozoic 1850 1870 ma 2100 2400 ma in age with minor archean components it was extensively reworked in jurassic cretaceous time 100 155 ma to produce the widespread yanshanian magmatic suite both the 1850 1870 ma event and the yanshanian magmatism show wide ranges in hf isotope composition consistent with mixing between crustal and juvenile magmas marked downstream changes in the relative proportions of zircon age populations emphasize the care required in using detrital zircon data to estimate continental growth rates zircons from the north river indicate that the crust of western cathaysia was generated mainly during neoproterozoic time although it contains some archean 2500 3500 ma to mesoproterozoic components this crust was strongly reworked during caledonian ca 450 ma indosinian ca 240 ma and early yanshanian ca 160 ma thermal events there is little evidence for juvenile crustal growth in any of these events although the yanshanian granitoids of the eastern and western parts of the cathaysia give similar mean hf model ages consistent with published nd model ages this is a coincidence they have different crustal sources the distinct patterns of crustal evolution suggest that eastern and western cathaysia may represent separate microcontinents accreted to the older yangtze craton and transposed by extensive strike slip faults

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

the structure of the crust and uppermost mantle beneath south china from ambient noise and earthquake tomography summary two years of continuous recordings of ambient seismic noise observed at 354 stations in south china from 2009 to 2010 are used to estimate rayleigh wave group and phase velocity maps from 6 to 40 s period these results are merged with rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame eikonal tomography generates the dispersion maps which by monte carlo inversion are used to estimate a 3 d vsv model of the crust and upper mantle down to a depth of 150 km across all of south china with attendant uncertainties a clear image emerges of the west yangtze block a region of the western yangtze craton characterized by relatively thick crust 40 km overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the yangtze craton in the archean and may present a present day obstacle to the eastward expansion of tibet the west yangtze block contrasts with the thinner crust 30 km and mantle lithosphere 70 80 km beneath the eastern yangtze craton and south china foldbelt these observed differences are consistent with processes associated with flat slab subduction in the mesozoic that may have eroded the lithosphere of the eastern yangtze craton and the south china foldbelt

three dimensional p velocity structures of the lithosphere beneath taiwan from the analysis of taiger and related seismic data sets 1 the convergence of the philippine sea and the eurasian plates in the taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities the geological processes underlying the collision and its relation to subduction are the primary targets of taiger taiwan integrated geodynamic research project newly acquired passive and active sources data on land supplemented by ocean bottom as well as permanent seismic network data are used to derive a new 3 d tomographic velocity model using the 7 5 km sec contour as a marker for crustal deformation two trend parallel roots one under the central range with a maximum depth of 55 km and the other under the coastal range at 40 km are found to extend from southern to central 24 n taiwan between the two roots and lying approximately beneath the longitudinal valley the 7 5 km sec contour rises to 25 km depth in the upper mantle a high velocity zone east dipping and its upper surface coinciding with a wadati benioff zone in the south 22 8 n becomes near vertical and ill defined in the north 23 9 n the crustal deformation as defined by the roots and the along trend variations of the upper mantle structures under taiwan provide key information for the orogeny with the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play

stress origins and earthquake potentials in cascadia focal mechanism solutions for shallow earthquakes throughout the cascadia plate system indicate that the primary regional stress is northerly compression even though the juan de fuca plate generally is thought to be subducting n50 e this compressional stress is pervasive throughout the gorda juan de fuca explorer plate system and much of the adjoining section of north american plate modeling using a discrete element code shows that this stress primarily is due to the pacific plate being driven into the gorda block and juan de fuca plate at the mendocino and blanco fracture zones this plate collision causes compression of the offshore plate system northward through the 45 w trending vancouver island and leads to transfer of northerly compression into the overriding plate the northerly compression in the cascadia plate system has caused large crustal earthquakes in the regions of vancouver and northern california and may be capable of causing large earthquakes also in and offshore of washington and oregon several independent lines of evidence confirm that the cascadia subduction interface is coupled strongly and that subduction primarily must be accompanied by significant earthquakes rather than occurring aseismically for numerous earthquakes within the subducted plate focal mechanisms indicate extensional stresses that trend downdip these earthquakes reflect plate extension that is due to sinking of the more deeply subducted plate and to plate motion being resisted at the shallow subduction interface the slab pull force of the sinking plate also can lead to earthquakes at the shallow subduction interface and the potential exists for m 7 5 8 0 subduction earthquakes to occur at segments of the subduction boundary at washington and vancouver and less frequently so at segments of the subduction boundary at oregon the in plate driving forces of the offshore cascadia plate system do not appear to contribute significantly to regional stresses stresses and earthquakes at the cascadia province primarily result from the action of the pacific plate on the gorda juan de fuca explorer plate system a transfer of this action into the overriding north american plate and the independent sinking and slab pull force of the subducted oceanic lithosphere

source characteristics of hypothetical subduction earthquakes in the northwestern united states historic earthquake sequences on subduction zones that are similar to the cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern united states based on systematic comparisons of several physical characteristics including physiography and seismicity subduction zones that are deemed most similar to the cascadia subduction zone are those in southern chile southwestern japan and colombia these zones have all experienced very large earthquake sequences and if the cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length then earthquakes of very large size m w 8 1 2 must be considered circumstantial evidence is presented that suggests but does not prove that large subduction earthquakes along the cascadia subduction zone may have an average repeat time of 400 to 500 yr

collisional tectonics between the eurasian and philippine sea plates from tomography evidences in southeast china abstract the upper mantle structure of southeast china is important for us to understand the deformation and mantle dynamics process associated with the interaction between the eurasian plate and philippine sea phs slab we determined a detailed three dimensional p wave velocity vp structure of the crust and upper mantle down to 400 km depth beneath southeast china by applying teleseismic tomography to 6869 high quality p wave arrival times the data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in southeast china our images show that the high vp phs slab subducts toward the north along the ryukyu trench at the latitude of about 24 n and extends down to 350 km depth and even more high vp anomalies are imaged in the upper mantle under central and southern taiwan which represent the subducted eurasian plate break off eurasian plate at a big angle subducting eastward is revealed under central taiwan at depths from the upper mantle to 400 km while continuous eurasian plate under south taiwan is mainly imaged from the moho down to 400 km depth a torn mantle window within the eurasian continent beneath central and northern taiwan created by the northward motion of the philippine sea plate is the upwelling path of the asthenosphere the tomographic images also show the low vp anomalies spread widely under the coastal areas of mainland china and taiwan strait the structure of the crust and upper mantle suggests that the mountain building process in the central part of taiwan is mainly attributed to the subduction collision tectonics at the boundary between the eurasian continental lithosphere and the subducting oceanic lithosphere of the phs slab

prediction of strong ground motion in the puget sound region the 1965 seattle earthquake spatial variations in strong ground motion in the 1965 seattle washington earthquake ms 6 5 can be accurately reproduced using three dimensional ray tracing techniques synthetic accelerograms calculated at nearly 1600 receivers spread over the puget sound region indicate that peak ground accelerations are significantly affected by subsurface structure as well as by near receiver soil properties the large synthetic accelerations computed for the harbor island downtown seattle area appear to be due to subsurface focusing of rays within a deep sedimentary basin and are consistent with the observed isoseismal distribution in extreme circumstances such as a bedrock site adjacent to a site situated upon a soft sedimentary lens peak accelerations may vary by nearly an order of magnitude predicted values of peak ground acceleration as a function of distance reproduce both the trend and scatter in data from subduction zone earthquakes of comparable depth in japan

the queen charlotte islands refraction project part i the queen charlotte fault zone the active margin between the continental north american plate and oceanic pacific plate west of the queen charlotte islands was the site of an extensive onshore offshore seismic refraction project

a three dimensional vp vs and vp vs crustal structure in fujian southeast china from active and passive source experiments abstract fujian southeastern china has experienced multistage tectonic activities since the neoproterozoic era and is currently influenced by collision between the eurasian and philippine sea plates topography fault zones and patterns of seismicity are the imprints of tectonic evolution historically there have been several catastrophic earthquakes in the southeastern part of fujian to understand the crustal structure related to the fault zones we performed vp vs and vp vs travel time tomography using joint inversion of active and passive sources a total of 75 827 and 31 044 arrivals of p and s waves respectively from 33 explosions and 2543 earthquakes are used in our study as a result seismicity has indicated that two ne strike seismogenic zones the zhenghe dapu and changle zhaoan fault zones are currently active low vp vs ratios in inland fujian imply that the crust is mainly composed of felsic rocks as part of the eurasian continental crust which is consistent with geological observations at the surface based on vp tomography the thickness of the crust along the coastline is shallower than that on land which is related to higher heat flow and the bouguer anomaly this shallow crust phenomenon near the coastline could be related to the regional extensional stress the remaining structure of the back arc extension that stretched the continental crust during the mesozoic era or and the cenozoic extension due to south china sea opening in taiwan strait

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