coastline uplift in oregon and washington and the nature of cascadia subduction zone tectonics coastline deformation resulting from great shallow thrust earthquakes can provide information concerning the paleoseismicity of a subduction zone and thus information on the nature of potential seismicity the cascadia subduction zone is different from most other subduction zones in that it has been quiescent with respect to great earthquakes for at least the past 200 yr the washington oregon coastline also differs from most other coastlines associated with subduction zones in its lack of uplifted holocene shoreline features and low overall rate of late quaternary uplift 0 2 0 6 mm yr the uplift differences suggest that repeated great earthquakes have not occurred along the cascadia subduction zone at least during the late holocene alternatively if the plate interface has generated earthquakes the differences may be explained by longer recurrence intervals for great earthquakes smaller magnitude earthquakes or a mechanism that does not result in uplift of the coastline where expected

volcanism and aseismic slip in subduction zones the spatial and temporal relationship of volcanism to the occurrence of large earthquakes and convergent plate motion is examined the number of volcanic eruptions per year in a convergent zone is found to be linearly related to the aseismic slip component of plate motion if the aseismic slip rate is low coupling between converging plates is strong then the primary manifestation of tectonic activity is the occurrence of large earthquakes with only infrequent volcanic activity if however the aseismic slip rate is high coupling is weak then there are few large earthquakes and volcanism is the principal manifestation of tectonic activity this model is consistent with the spatial distribution of large earthquakes and active volcanoes in the circum pacific area it is tested by examining the extent of volcanic activity in the rupture zones of the 1952 1973 sequence of earthquakes in the japan kurile islands area the number of volcanic euptions along these zones during the interval between large earthquakes is used to compute the aseismic slip rates for these segments based on the relationship developed in this study the aseismic slip rates so computed agree with those determined from the earthquake history of the area and rates of plate motion themore agreement suggests that in the interval between large earthquakes the aseismic plate motion is manifested in a specific number of volcanic eruptions therefore in areas with adequate historial data it should be possible to use the model developed in this study to monitor volcanic eruptions for long term prediction of large earthquakes less

teleseismic time functions for large shallow subduction zone earthquakes broadband vertical p wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum pacific in the last 45 yr most of the records studied come from a common instrument the pasadena california benioff 1 90 seismometer propagation and instrument effects are deconvolved from the p wave records using a damped least squares inversion to obtain the teleseismic source time function the inversion has the additional constraint that the time function be positive everywhere the period band over which the time functions are considered reliable is from 2 5 to 50 sec fourier displacement amplitude spectra computed for each of the 1 90 p wave trains indicate spectral slopes measured between 2 and 50 sec of 1 0 to 2 25 with an average value of 1 5 these values assume an average attenuation of t 1 0 the seismic moments derived from the p wave time functions compare well with other published values for earthquakes having moments smaller than 2 5 10 28 dyne cm m w 8 2 because the 1 90 seismometer has little response at very long periods this technique underestimates the moments of the very largest events the time functions are characterized using five parameters 1 spectral slope between 2 and 50 sec 2 roughness of the time function 3 multiplicity of sources 4 pulse widths of individual sources and 5 overall signal duration the 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere convergence rates and maximum size of earthquakes comparing the time function parameters with age rate and m w of the subduction zone does not yield obvious global trends however most of the subduction zones do behave characteristically and can be grouped accordingly

testing model simulations of the thermocline depth variability in the tropical atlantic from 1982 through 1984 abstract the multivariate model toning procedure of frankignoul et al has been extended to the general time series case thus allowing to test ocean model ability at simulating the interannual variability the method aims at distinguishing between model inadequacies and data uncertainties model performances are assessed from a misfit evaluated in a space of strongly reduced dimension with basis vectors issued from a double application of common principal component analysis the testing procedure has been used to investigate the ability of cane s linear multimode model at simulating the evolution of the 20 c isotherm depth in the equatorial atlantic during the 1982 1984 focal sequal experiment using monte carlo techniques and five different drag laws 25 equally plausible wind stress fields were constructed to represent the wind stress uncertainties consistently with the sample means and variances of the original ship measurements even during this well sampled period the forcing uncertainties were sub

sudden probably coseismic submergence of holocene trees and grass in coastal washington state growth position plant fossils in coastal washington state imply a suddenness of holocene submergence that is better explained coseismic lowering of the land than be decade or century long rise of the sea these fossils include western red cedar and sitka spruce whose death probably resulted from estuarine submergence close to 300 years ago rings in eroded bark free trunks of the red cedar show that growth remained normal within decades of death rings in buried bark bearing stumps of the spruce further show normal growth continuing until the year of death other growth position fossils implying sudden submergence include the stems and leaves of salt marsh grass entombed in tide flat mud close to 300 years ago and roughly 1 700 and 3 100 years ago the preservation of these stems and leaves shows that submergence and initial burial outpaced decomposition which appears to take just a few years in modern salt marshes in some places the stems and leaves close to 300 year old are surrounded by sand left by an extraordinary landward directed surge probably a tsunami from a great thrust earthquake on the cascadia subduction zone

juan de fuca plate aseismic subduction at 1 8 cm yr volcanic activity in the cascades in historic times suggests that the juan de fuca plate is underthrusting aseismically at about 1 8 cm yr this rate of underthrusting is identical to the rate computed from sediment studies

on the ability of the lodyc gcm to simulate the thermocline depth in the equatorial atlantic using a multivariate model testing procedure that distinguishes between model inadequacies and data uncertainties we investigate the ability of the lodyc gcm to simulate the evolution of the 20 c isotherm depth during the 1982 1984 focal sequal experiment in the equatorial atlantic two different versions of the model are considered the ri version which has a richardson number dependent parameterization of vertical mixing and the new tke version which uses a local estimation of the turbulent kinetic energy to parameterize vertical mixing some effects of the forcing uncertainties are considered by forcing the tke version with three equally plausible wind stress fields whose differences are consistent with the measurement and sampling errors and the drag coefficient indeterminacy the resulting uncertainties in the model response are substantial and can be as large as the differences between simulations with the two gcm versions which stresses the need to take the forcing uncertainties into account although only one ri run is available it is shown that the tke parameterization significantly improves the representation of the equatorial upwelling and the simulation of the depth of the thermocline in the eastern atlantic however there remain significant differences with the observations which cannot be explained by the forcing uncertainties that were considered the two model versions perform better in the equatorial wave guide than in the 12 n 12 s domain and they are better distinguished over large domains than along sections which shows that a global multivariate view point must be used in model reality comparisons finally a comparison with a linear multimode model emphasizes the need for greater model complexity to properly simulate the equatorial upwelling and the thermocline variability in the tropical atlantic

the great jalisco mexico earthquakes of 1932 subduction of the rivera plate we have analyzed seismograms of the 3 june 1932 ms 8 2 and the 18 june 1932 ms 7 8 jalisco earthquakes and their aftershocks at manzanillo mnz guadalajara gum and tacubaya tac the aftershocks locations the first motions at mnz and the isoseismic maps of the two main shocks strongly suggest that a the 3 june 1932 earthquake initiated nw of but close to mnz and propagated nw for an estimated length of rupture of 220 km b the 18 june 1932 earthquake nucleated sw of mnz offshore and perhaps ruptured a length of about 60 km and c the width of rupture was approximately 80 km there is no evidence of events occurring se of mnz even up to 1 1 2 yr after the first main shock a gap of about 60 km remains between the aftershock areas of the 1932 jalisco and the 1973 colima earthquakes whose seismic potential is unknown although the boundary between the rivera and cocos plates is uncertain there is little doubt that the 1932 earthquakes broke the shallow part of the rivera subduction zone our results may help in quantifying seismic potential of tectonically similar areas such as the juan de fuca subduction zone in the nw united states

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

the calm before the quake with the exception of alaska the cascadia subduction zone extending 1 200 km from northern california to vancouver island is the largest tectonically active fault system in north america although is widely accepted that the north american continental plate is converging at a rate of 2 5 4 5cm yr 1 modern seismic activity has been minimal along the cascadia subduction zone are the pacific oceanic plates sliding effortlessly below the continental margin or are the plates welded together gradually accumulating elastic strain to be released in another cataclysmic earthquake sometime in the coming centuries there is geological evidence much debated at a recent meeting that there have been six massive earthquakes energy magnitude m w 8 5 9 5 see box overleaf in this region in the past 3 600 years most recently sometime in 1680 90

seismicity structure and tectonics of the wellington region new zealand summary the wellington region lies on the border between the pacific and australian plates the former being subducted the plate interface lies at shallow depth under the region 10 60 km so that detailed and long term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible hypocentres for events recorded by a 12 station seismic network for the period 1978 1982 7231 events have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival time data from 129 local events for both hypocentres and velocity the hypocentres show that the subducting pacific plate is defined by a nw dipping zone of relatively intense activity the plate interface as given by the upper envelope of this activity is offset vertically by 7 km on average along a nw se line through cook strait parallel to the direction of absolute motion of the pacific plate rather than to the direction of convergence the strike of the interface at deeper levels also changes across this line this distortion may partly reflect effects of the transition to continent continent type convergence further south within the subducting pacific plate the seismic activity defines two parallel zones separated by about 20 km the lower zone being much less active and perhaps merging with the upper zone at a depth of 80 90 km composite focal mechanisms of events in the upper zone imply predominantly down dip tension with a compression axis perpendicular to the plate interface the mechanism in the lower zone also implies down dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate these two types of mechanism correlate well with those found previously for events at depths of 150 km or more thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region the lithosphere acting as a stress guide there is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface the seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust which here is probably considerably thicker than normal there is no evidence for a substantial increase in velocity down

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

paleoseismicity of the cascadia subduction zone evidence from turbidites off the oregon washington margin cores from cascadia deep sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the mount mazama eruption 6845 50 radiocarbon yrbp turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel there being only 13 turbidite deposits in the lower main channel since the mazama eruption instead of the twice as many expected if the tributaries had behaved independently in addition to the cascadia channel 13 post mazama turbidites have been deposited in the astoria canyon and at two sites off cape blanco sample locations that span 580 km of the oregon washington margin pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly every 590 170 years on average the best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the cascadia subduction zone the variability of turbidite timing is similar to that for great earthquake cycles the thickness of the topmost pelagic layer suggests the last event was 300 60 years ago from three places along the margin but this number may be a biased underestimate it is however consistent with the youngest sudden subsidence event on the washington coast the turbidite data demonstrate that the near term hazard of a great earthquake on the cascadia subduction zone is of the order of 2 10 in the next 50 years

the north equatorial countercurrent observed during the programme francais ocean climat dans l atlantique equatorial experiment in the atlantic ocean july 1982 to august 1984 observations from programme francais ocean climat dans l atlantique equatorial focal cruises in 1982 and 1984 have confirmed that in the central atlantic ocean the north equatorial countercurrent necc has a well defined seasonal regime with a very reduced flux during spring the main core of the necc always lies between 4 n and 6 n focal observations have also shown that a long term change of necc may occur and contribute to the development of an el nino type event in the eastern atlantic ocean

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

an assessment of the megathrust earthquake potential of the cascadia subduction zone the active tectonic setting of the southwest coast of canada and the pacific northwest coast of the united states is dominated by the cascadia subduction zone the zone can be divided into four segments where oceanic lithosphere is converging independently with the north american plate the winona and the explorer segments in the north the larger juan de fuca segment that extends into both canada and the united states and the gorda segment in the south the oceanic lithosphere entering the cascadia subduction zone in all segments is extremely young less than 10 ma of the other six zones around the pacific where young 20 ma lithosphere is being subducted five have had major thrust earthquakes megathrust events on the subduction interface in historic time an estimation based on potential area of rupture gives maximum possible earthquake magnitudes along the cascadia subducting margin of 8 2 for the winona segment 8 5 for the explorer segment 9 1 for the juan de fuca segment and 8 3 for the so

a linear analysis of equatorial atlantic ocean thermocline variability abstract observed variations in the atlantic ocean s equatorial thermocline are compared at four locations with simulations using an analytical reduced gravity model the comparison shows the essential features of the seasonal wind forced thermocline response to be accounted for by a linear superposition of equatorial long waves evolving basinwide tending to bring the zonal pressure gradient into balance with the wind stress a frequency response function is derived whose properties provide a basis for discussing the large scale features of the equatorial atlantic ocean s seasonal cycle for example its evolution along the equator the maximum upwelling region observed in the gulf of guinea and the secondary upwelling season also observed there clarification is also given to the issue of remote versus local forcing for these features

equatorial atlantic ocean temperature and current variations during 1983 and 1984 the equatorial regions of the earth s oceans are climatically sensitive ones because of the zonal sea surface temperature contrasts observed there1 equatorial sea surface temperature normally varies on an annual cycle with the prevailing trade winds but deviations from this cycle may have significant global implications as occurred for example during the 1982 83 pacific ocean el nino southern oscillation event2 understanding the annual and interannual variability of the tropical oceans has therefore been the goal of several measurement programmes in the atlantic ocean the seasonal response of the equatorial atlantic sequal experiment and the programme francais ocean et climat dans l atlantique equatorial focal have provided a basin wide and synoptic data set over two annual cycles we present here results from surface moored current meters which were one of several fixed and shipborne measurement systems employed by sequal and focal we will describe the evolution of the upper ocean thermal and zonal velocity component variations in relation to forcing by the trade winds show differences observed along the equator at 28 w and 4 w and compare the oceans responses at these locations during 1983 and 1984 the synoptic data realizations of these years differed from climatology and these differences are related to the rapidly varying nature and intensity of the wind stress in a given year changes in wind stress from year to year result in interannual variability as a modulated annual cycle and 1984 a year of weak winds relative to 1983 offers a case in point the zonal sea surface temperature gradient vanished along the equator in 1984 during the season when it normally would have been a maximum

strain accumulation in western washington the juan de fuca plate is subducted beneath the north american plate off the coast of washington at a rate of about 40 mm yr n68 e the average principal strain rates extension reckoned positive measured in northwestern washington are as follows olympic peninsula 25 km south of port angeles from 1982 through 1990 e 1 0 011 0 027 strain yr n31 e 6 6 and e 2 0 092 0 028 strain yr n59 w 6 6 and near seattle from 1972 through 1985 e 1 0 027 0 019 strain yr n22 e 6 4 and e 2 0 036 0 013 strain yr n68 w 6 4 both strain measurements are consistent with uniaxial contraction in the direction of plate convergence uplift rates inferred from tide gage recordings are about 4 mm yr on the pacific coast and near 0 mm yr farther inland near seattle these deformation rates are consistent with a model of the cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward the limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth 20 km of the isotherm 450 c that defines the brittle ductile transition small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths the principal strain rates measured from 1972 through 1983 in the back arc region near richland washington are e 1 0 016 0 013 strain yr n03 w 34 and e 2 0 024 0 013 strain yr n87 e 34

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

late holocene rupture of the northern san andreas fault and possible stress linkage to the cascadia subduction zone we relate the late holocene northern san andreas fault nsaf paleo seismic history developed using marine sediment cores along the northern california continental margin to a similar dataset of cores collected along the cascadia margin including channels from barclay canyon off vancouver island to just north of mon terey bay stratigraphic correlation and evidence of synchronous triggering imply earthquake origin and both temporal records are compatible with onshore paleoseis mic data in order to make comparisons between the temporal earthquake records from the nsaf and cascadia we refine correlations of southern cascadia great earth quakes including the land paleoseismic record along the nsaf during the last 2800 yr 15 turbidites including one likely from the great 1906 earthquake establish an average repeat time of 200 yr similar to the onshore value of 240 yr the combined land and marine paleoseismic record from the southern cascadia subduction zone includes a similar number of events during the same period while the average recurrence interval for full margin cascadia events is 520 yr the southern cascadia margin has a repeat time of 220 yr similar to that of the nsaf thirteen of the 15 nsaf events were preceded by cascadia events by 0 80 yr averaging 25 45 yr as compared to 80 400 yr by which cascadia events follow the nsaf based on the temporal association we model the coseismic and cumulative post seismic deformation from great cascadia megathrust events and compute related stress changes along the nsaf in order to test the possibility that cascadia earth quakes triggered the penultimate and perhaps other nsaf events the coulomb fail ure stress cfs resulting from viscous deformation related to a cascadia earthquake over 60 yr does not contribute significantly to the total cfs on the nsaf however the coseismic deformation increases cfs on the northern san andreas fault nsaf b y up to about 9 bars offshore of point delgada most likely enough to trigger that fault to fail in north to south propagating ruptures

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

coefficients for sea surface wind stress heat flux and wind profiles as a function of wind speed and temperature surface layer coefficients for wind profiles wind stress and heat flux in typical open sea conditions are briefly reviewed businger dyer flux gradient relationships and a charnock wind stress formula fit the empirical data and are dimensionally consistent these have been solved by an iterative method and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data

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

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

seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america a quantitative reappraisal the seismic potential of the chilean and southern peruvian margins of south america is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years 1984 2004 long term estimates of seismic potential or the conditional probability of recurrence within a specified period of time are based on 1 statistical analysis of historic repeat time data using weibull distributions and 2 deterministic estimates of recurrence times based on the time predictable model of earthquake recurrence both methods emphasize the periodic nature of large and great earthquake recurrence and are compared with estimates of probability based on the assumption of poisson type behavior the estimates of seismic potential presented in this study are long term forecasts only as the temporal resolution or standard deviation of both methods is taken to range from 15 to 25 of the average or estimated repeat time at present the valparaiso region of central chile 32 35 s has a high potential or probability of recurrence in the next 20 years coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971 1973 average repeat times also suggest this area is due for a great shock within the next 20 years flanking segments of the chilean margin coquimbo illapel 30 32 s and talca concepcion 35 38 s presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years in contrast the rupture zone of the great 1960 earthquake 37 46 s has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years in the north the seismic potentials of the mollendo arica 17 18 s and arica antofagasta 18 24 s segments which last ruptured during great earthquakes in 1868 and 1877 are also high but poorly constrained

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

pole of rotation analysis of present day juan de fuca plate motion convergence rates between the juan de fuca and north america plates are calculated by means of their relative present day pole of rotation subduction rates as measured normal to the strike of the washington oregon trench are predicted to range from about 40 mm yr under washington to 20 mm yr under oregon an integral part of this analysis is the formulation of a numerical inversion scheme which calculates both the pole of rotation between two plates and the associated errors this method is used to determine the euler pole for pacific juan de fuca this pole can then be vectorially added to previously published poles for north america pacific and hot spot pacific to obtain north america juan de fuca and hot spot juan de fuca respectively the errors associated with these resultant poles are also determined by propagating the errors of the two summed angular velocity vectors with the assumption that hot spots are fixed with respect to a mantle reference frame if is computed that the average absolute velocity of the juan de fuca plate is approximately 15 mm yr thereby making it the slowest moving of the oceanic plates

seismicity and focal mechanisms associated with the subduction of the juan de fuca plate beneath the olympic peninsula washington abstract two and a half years of data from an expanded telemetered seismic network on the olympic peninsula reinforce the conclusion that the juan de fuca plate is subducting beneath washington however the rate of seismicity is presently much lower than typical rates at other convergent margins the located earthquakes can be divided into two groups on the basis of depth at the coast a shallow zone extends to a depth of approximately 20 km while a deep zone begins at 25 km on the eastern edge of the peninsula the shallow zone has thickened to 30 km and the depth to the top of the deep zone has increased to 45 km the rate of crustal seismicity is much lower on the peninsula than in the adjacent puget sound region the deep zone is 5 to 10 km thick and dips to the east at an angle of about 11 the deep events appear to be occurring in the upper part of the subducting oceanic lithosphere focal mechanisms were constructed for 16 events in the suite of deep events with magnitudes ranging from 1 6 to 4 3 the solutions show primarily normal faulting with the p axes normal to the slab and the t axes displaying a range of angles within the slab with the average t axis in the downdip direction small events with down dip tension in the downgoing slab are consistent with the slab sinking under its own weight several strike slip events may be caused by a shearing within the slab these features are consistent with ongoing subduction of the juan de fuca plate beneath north america