holocene fault scarps near tacoma washington usa airborne laser mapping confirms that holocene active faults traverse the puget sound metropolitan area northwestern continental united states the mapping which detects forest floor relief of as little as 15 cm reveals scarps along geophysical lineaments that separate areas of holocene uplift and subsidence along one such line of scarps we found that a fault warped the ground surface between a d 770 and 1160 this reverse fault which projects through tacoma washington bounds the southern and western sides of the seattle uplift the northern flank of the seattle uplift is bounded by a reverse fault beneath seattle that broke in a d 900 930 observations of tectonic scarps along the tacoma fault demonstrate that active faulting with associated surface rupture and ground motions pose a significant hazard in the puget sound region

upper crustal structure in puget lowland washington results from the 1998 seismic hazards investigation in puget sound a new three dimensional 3 d model shows seismic velocities beneath the puget lowland to a depth of 11 kin the model is based on a tomographic inversion of nearly one million first arrival travel times recorded during the 1998 seismic hazards investigation in puget sound ships allowing higher resolution mapping of subsurface structures than previously possible the model allows us to refine the subsurface geometry of previously proposed faults e g seattle hood canal southern whidbey island and devils mountain fault zones as well as to identify structures tacoma lofall and sequim fault zones that warrant additional study the largest and most important of these newly identified structures lies along the northern boundary of the tacoma basin we informally refer to this structure here as the tacoma fault zone although tomography cannot provide information on the recency of motion on any structure holocene earthquake activity on the tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north the tomography reveals four large west to northwest trending low velocity basins tacoma seattle everett and port townsend separated by regions of higher velocity ridges that are coincident with fault bounded uplifts of eocene crescent formation basalt and pre tertiary basement the shapes of the basins and uplifts are similar to those observed in gravity data gravity anomalies calculated from the 3 d tomography model are in close agreement with the observed anomalies in velocity cross sections the tacoma and seattle basins are asymmetric the basin floor dips gently toward a steep boundary with the adjacent high velocity uplift locally with a velocity overhang that suggests a basin vetgent thrust fault boundary crustal fault zones grow from minor folds into much larger structures along strike inferred structural relief across the tacoma fault zone increases by several kilometers westward along the fault zone to lynch cove where we interpret it as a zone of south vergent faulting overthrusting tacoma basin in contrast structural relief along the seattle fault zone decreases west of seattle which we interpret as evidence that the n s directed compression is being accommodated by slip transfer between the seattle and tacoma fault zones together the tacoma and seattle fault zones raise the seattle uplift one of a series of east west trending pop up structures underlying puget lowland from the black hills to the san juan islands

fault interaction may generate multiple slip vectors on a single fault surface slip vectors of multiple orientations on a single fault surface may be explained by the interaction of crustal scale faults in response to nearby earthquakes field observations of at least five sets of striae on a single planar fault surface can be reproduced in numerical experiments these experimental results suggest that slip vectors having variable orientations may reflect a highly transient local stress state in an otherwise constant regional stress field our results suggest that stress inversion techniques should be applied with caution to areas with complex regional scale fault patterns

little ice age subsidence and post little ice age uplift at juneau alaska inferred from dendrochronology and geomorphology abstract application of dendrochronology and geomorphology to a recently emerged coastal area near juneau alaska has documented a little ice age lia sea level transgression to 6 2 m above current sea level the rise in relative sea level is attributed to regional subsidence and appears to have stabilized by the mid 16th century based on a sea cliff eroded into late pleistocene glaciomarine sediments land began emerging between a d 1770 and 1790 coincident with retreat of regional glaciers from their lia maximums this emergence has continued since then paralleling regional glacier retreat total juneau uplift since the late 18th century is estimated to be 3 2 m the rate of downward colonization of newly emergent coastline by sitka spruce during the 20th century closely parallels the rate of sea level fall documented by analysis of local tide gauge records 1 3 cm yr regional and glacier bay lia loading and unloading are inferred to be the primary mechanisms driving subsidence and uplift in the juneau area climate change rather then regional tectonics has forced relative sea level change over the last several hundred years

erosion by pyroclastic flows on lascar volcano chile pyroclastic flows generated in the 19 20 april 1993 eruption of lascar volcano chile produced spectacular erosion features scree and talus were stripped from the channels and steep slopes on the flanks of the volcano exposed bedrock and boulders suffered severe abrasion producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders erosional furrows developed with wavelengths of 0 5 2 m and depths of 0 1 0 3 m furrows commonly nucleated downstream of large boulders or blocks which are striated on the upstream side and thereby produced crag and tail structures erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes the pyroclastic flows are inferred to have segregated during movement into lithic rich and pumice rich parts lithic rich deposits occur on slopes up to 14 whereas pumice rich deposits occur only on slopes less than 4 and mainly at the margins and distal parts of the 1993 fan the lithic rich deposits contain large up to 1 m lithic clasts eroded from the substrate and transported from the vent whereas pumice rich deposits contain only small typically 2 cm lithic clasts these observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena the erosive features distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions in some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s however shorter striae at different angles impact marks segregation of the deposits into pumice and lithic rich parts and mixing of bedrock derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows

active tectonics of the seattle fault and central puget sound washington implications for earthquake hazards we use an extensive network of marine high resolution and conventional industry seismic reflection data to constrain the location shallow structure and displacement rates of the seattle fault zone and crosscutting high angle faults in the puget lowland of western washington analysis of seismic profiles extending 50 km across the puget lowland from lake washington to hood canal indicates that the west trending seattle fault comprises a broad 4 6 km zone of three or more south dipping reverse faults quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault a the northernmost fault which forms the boundary between the seattle uplift and seattle basin analysis of growth strata deposited across fault a indicate minimum quaternary slip rates of about 0 6 mm yr slip rates across the entire zone are estimated to be 0 7 1 1 mm yr the seattle fault is cut into two main segments by an active north trending high angle strike slip fault zone with cumulative dextral displacement of about 2 4 km faults in this zone truncate and warp reflections in tertiary and quaternary strata and locally coincide with bathymetric lineaments cumulative slip rates on these faults may exceed 0 2 mm yr assuming no other crosscutting faults this north trending fault zone divides the seattle fault into 30 40 km long western and eastern segments although this geometry could limit the area ruptured in some seattle fault earthquakes a large event ca a d 900 appears to have involved both segments regional seismic hazard assessments must 1 incorporate new information on fault length geometry and displacement rates on the seattle fault and 2 consider the hazard presented by the previously unrecognized north trending fault zone

tsunami history of an oregon coastal lake reveals a 4600 yr record of great earthquakes on the cascadia subduction zone bradley lake on the southern oregon coastal plain records local tsunamis and seismic shaking on the cascadia subduction zone over the last 7000 yr thirteen marine incursions delivered landward thinning sheets of sand to the lake from nearshore beach and dune environments to the west following each incursion a slug of marine water near the bottom of the freshwater lake instigated a few year to several decade period of a brackish 4 salinity lake four additional disturbances without marine incursions destabilized sideslopes and bottom sediment producing a suspension deposit that blanketed the lake bottom considering the magnitude and duration of the disturbances necessary to produce bradley lake s marine incursions a local tsunami generated by a great earthquake on the cascadia subduction zone is the only accountable mechanism extreme ocean levels must have been at least 5 8 m above sea level and the cumulative duration of each marine incursion must have been at least 10 min disturbances without marine incursions require seismic shaking as well over the 4600 yr period when bradley lake was an optimum tsunami recorder tsunamis from cascadia plate boundary earthquakes came in clusters between 4600 and 2800 cal yr b p tsunamis occurred at the average frequency of 3 4 every 1000 yr then starting 2800 cal yr b p there was a 930 1260 yr interval with no tsunamis that gap was followed by a 1000 yr period with 4 tsunamis in the last millennium a 670 750 yr gap preceded the a d 1700 earthquake and tsunami the a d 1700 earthquake may be the fi rst of a new cluster of plate boundary earthquakes and accompanying tsunamis local tsunamis entered bradley lake an average of every 390 yr whereas the portion of the cascadia plate boundary that underlies bradley lake ruptured in a great earthquake less frequently about once every 500 yr therefore the entire length of the subduction zone does not rupture in every earthquake and bradley lake has recorded earthquakes caused by rupture along the entire length of the cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary the tsunami record from bradley lake indicates that at times most recently 1700 yr b p overlapping or adjoining segments of the cascadia plate boundary ruptured within decades of each other

a modern analogue for plant colonization of palaeotsunami sands in cascadia british columbia canada the sediment stratigraphy of a modern analogue for pioneer plant colonization of palaeotsunami sands near tofino british columbia canada shows that a sand outcrop is continuous with a buried sand stratum deposited by tsunamis of the ad 1700 cascadia earthquake plants growing on the sand outcrop form a low marsh community that includes triglochin maritimum salicornia virginica spergularia canadensis puccinellia pumila and plantago maritima in contrast low marshes on muddy substrates include only carex lyngbyei although carex lyngbyei colonizes low marsh on mud it is restricted to middle marsh or higher elevation communities on sand with two way indicator species analysis twinspan and detrended corre spondence analysis dca plant cover abundance clearly distinguishes sandy from muddy low marsh a finding that suggests that particle size influences plant colonization twinspan and dca show that pollen spectra of the sandy marsh surface resemble fossil pollen of sediments that immediately over

coseismic temporal changes of slip direction the effect of absolute stress on dynamic rupture abstract we investigate the dynamics of rupture at low stress level we show that one main difference between the dynamics of high and low stress events is the amount of coseismic temporal rake rotation occurring at given points on the fault curved striations on exposed fault surfaces and earthquake dislocation models derived from ground motion inversion indicate that the slip direction may change with time at a point on the fault during dynamic rupture we use a 3d boundary integral method to model temporal rake variations during dynamic rupture propagation assuming a slip weakening friction law and isotropic friction the points at which the slip rotates most are characterized by an initial shear stress direction substantially different from the average stress direction over the fault plane we show that for a given value of stress drop the level of initial shear stress i e the fractional stress drop determines the amount of rotation in slip direction we infer that seismic events that show evidence of temporal rake rotations are characterized by a low initial shear stress level with spatially variable direction on the fault possibly due to changes in fault surface geometry and an almost complete stress drop our models motivate a new interpretation of curved and cross cutting striations and put new constraints on their analysis the initial rake is in general collinear with the initial stress at the hypocentral zone supporting the assumptions made in stress tensor inversion from first motion analysis at other points on the fault especially away from the hypocenter the initial slip rake may not be collinear with the initial shear stress contradicting a common assumption of structural geology on the other hand the later part of slip in our models is systematically more aligned with the average stress direction than the early slip our modeling suggests that the length of the straight part of curved striations is usually an upper bound of the slip weakening distance if this parameter is uniform over the fault plane and the direction of the late part of slip of curved striations should have more weight in the estimate of initial stress direction

viscoelastic cycle model of interseismic deformation in the northwestern united states summary we apply a viscoelastic cycle model to a compilation of gps velocity fields in order to address the kinematics of deformation in the northwestern united states a viscoelastic cycle model accounts for time dependent deformation following large crustal earthquakes and is an alternative to block models for explaining the interseismic crustal velocity field building on the approach taken in pollitz et al we construct a deformation model for the entire western united states based on combined fault slip and distributed deformation and focus on the implications for the mendocino triple junction mtj cascadia megathrust and western washington we find significant partitioning between strike slip and dip slip motion near the mtj as the tectonic environment shifts from northwest directed shear along the san andreas fault system to east west convergence along the juan de fuca plate by better accounting for the budget of aseismic and seismic slip along the cascadia subduction interface in conjunction with an assumed rheology we revise a previous model of slip for the m 9 1700 cascadia earthquake in western washington we infer slip rates on a number of strike slip and dip slip faults that accommodate northward convergence of the oregon coast block and northwestward convergence of the juan de fuca plate lateral variations in first order mechanical properties e g mantle viscosity vertically averaged rigidity explain to a large extent crustal strain that cannot be rationalized with cyclic deformation on a laterally homogeneous viscoelastic structure our analysis also shows that present crustal deformation measurements particularly with the addition of the plate boundary observatory can constrain such lateral variations

location structure and seismicity of the seattle fault zone washington evidence from aeromagnetic anomalies geologic mapping and seismic reflection data a high resolution aeromagnetic survey of the puget lowland shows details of the seattle fault zone an active but largely concealed east trending zone of reverse faulting at the southern margin of the seattle basin three elongate east trending magnetic anomalies are associated with north dipping tertiary strata exposed in the hanging wall the magnetic anomalies indicate where these strata continue beneath glacial deposits the northernmost anomaly a narrow elongate magnetic high precisely correlates with magnetic miocene volcanic conglomerate the middle anomaly a broad magnetic low correlates with thick nonmagnetic eocene and oligocene marine and fluvial strata the southern anomaly a broad complex magnetic high correlates with eocene volcanic and sedimentary rocks this tripartite package of anomalies is especially clear over bainbridge island west of seattle and over the region east of lake washington although attenuated in the intervening region the pattern can be correlated with the mapped strike of beds following a northwest striking anticline beneath seattle the aeromagnetic and geologic data define three main strands of the seattle fault zone identified in marine seismic reflection profiles to be subparallel to mapped bedrock trends over a distance of 50 km the locus of faulting coincides with a diffuse zone of shallow crustal seismicity and the region of uplift produced by the m 7 seattle earthquake of a d 900 930

were they all giants perspectives on late holocene plate boundary earthquakes at the northern end of the cascadia subduction zone abstract the relative magnitude of plate boundary earthquakes at the northern end of the cascadia subduction zone was assessed from the temporal concordance between the ages of coseismically buried late holocene soils in southwest washington their counterparts in central and southern cascadia offshore turbidites and paleoseismic deposits on the west coast of vancouver island only three of the seven buried soils in southwest washington that can be reliably traced as buried soils or paleotsunami deposits in the coastal lowlands of south central and southern cascadia have well dated counterparts in northern cascadia the three wide ranging events date from cascadia earthquakes y 250 cal bp u 1260 cal bp and n 2520 cal bp all three likely ruptured the entire plate margin and therefore potentially qualify as giants m w 9 deposits that may derive from tsunamis generated by earthquakes s 1570 cal bp l 2870 cal bp and j 3360 cal bp can also be found in northern cascadia but the ages of these deposits are not yet well enough constrained to determine whether they are coeval with their southern counterparts earthquake w 850 cal bp appears to be present in the northern cascadia paleoseismic record but yields considerably older ages than in central cascadia and may be missing from southernmost cascadia the onshore record of an offshore turbidite t2 displays a similar spatio temporal pattern to that of earthquake w

holocene earthquake records from the cascadia subduction zone and northern san andreas fault based on precise dating of offshore turbidites abstract we present preliminary evidence for a 10 000 year earthquake record from two major fault systems based on sediment cores collected along the continental margins of western north america new stratigraphic evidence from cascadia demonstrates that 13 earthquakes ruptured the entire margin from vancouver island to at least the california border since the eruption of the mazama ash 7700 years ago the 13 events above this prominent stratigraphic marker have an average repeat time of 600 years and the youngest event 300 years ago coincides with the coastal record we also extend the record of past earthquakes to the base of the holocene at least 9800 years ago during which 18 events correlate along the same region the sequence of holocene events in cascadia appears to contain a repeating pattern of events a tantalizing first look at what may be the long term behavior of a major fault system the northern california margin cores show a cyclic record of turbidite beds that may represent holoce

tectonic evolution of the cenozoic olympic subduction complex washington state as deduced from fission track ages for detrital zircons

transpressive duplex and flower structure dent fault system nw england abstract revised mapping along the dent fault northwest england has improved the resolution of folds and faults formed during variscan late carboniferous sinistral transpression a nne trending east down monocline comprising the fell end syncline and taythes anticline was forced in carboniferous cover above a reactivated precursor to the dent fault within the lower palaeozoic basement the taythes anticline is periclinal due to interference with earlier acadian folds the steep limb of the monocline was eventually cut by the west dipping dent fault the hangingwall of the dent fault was dissected by sub vertical or east dipping faults together forming a positive flower structure in cross section and a contractional duplex in plan view the footwall to the dent fault preserves evidence of mostly dip slip displacements whereas strike slip was preferentially partitioned into the hangingwall faults this pattern of displacement partitioning may be typical of transpressive structures in general the faults of the taythes duplex formed in a restraining overlap zone between the dent fault and the rawthey fault to the west the orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly kinematic constraints provided by the dent and neighbouring variscan faults yield a nnw sse regional shortening direction in this part of the variscan foreland

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

integration of paleoseismic data from multiple sites to develop an objective earthquake chronology application to the weber segment of the wasatch fault zone utah we present a method to evaluate and integrate paleoseismic data from multiple sites into a single objective measure of earthquake timing and recurrence on discrete segments of active faults we apply this method to the weber segment ws of the wasatch fault zone using data from four fault trench studies completed between 1981 and 2009 after systematically reevaluating the stratigraphic and chro nologic data from each trench site we constructed time stratigraphic oxcal models that yield site probability density functions pdfs of the times of individual earth quakes we next qualitatively correlated the site pdfs into a segment wide earthquake chronology which is supported by overlapping site pdfs large per event displace ments and prominent segment boundaries for each segment wide earthquake we computed the product of the site pdf probabilities in common time bins which emphasizes the overlap in the site earthquake times and gives more weight to the narrowest best defined pdfs the product method yields smaller earthquake timing uncertainties compared to taking the mean of the site pdfs but is best suited to earth quakes constrained by broad overlapping site pdfs we calculated segment wide earthquake recurrence intervals and uncertainties using a monte carlo model five surface faulting earthquakes occurred on the ws at about 5 9 4 5 3 1 1 1 and 0 6 ka with the exception of the 1 1 ka event we used the product method to define the earthquake times the revised ws chronology yields a mean recurrence interval of 1 3 kyr 0 7 1 9 kyr estimated two sigma 2 range based on interevent recurrence these data help clarify the paleoearthquake history of the ws including the important question of the timing and rupture extent of the most recent earthquake and are essential to the improvement of earthquake probability assessments for the wasatch front region

great cascadia earthquakes and tsunamis of the past 6700 years coquille river estuary southern coastal oregon cascadia subduction zone earthquakes dropped tidal marshes and low lying forests to tidal flat elevations 12 times in the last 6700 cal yr b p at the coquille river estuary in southwestern oregon the youngest buried soil preserved in tidal marsh deposits near the estuary mouth records the a d 1700 earthquake that ruptured the entire cascadia margin eleven other buried marsh and upland soils found in tributary valleys of the estuary provide repeated evidence for rapid lasting relative sea level rise interpreted as coseismic subsidence additional stratigraphic criteria supporting a coseismic origin for soil burial include lateral soil correlation over hundreds of meters fossil diatom assemblages that indicate a maximum of 1 2 3 0 m of submergence and sand deposits overlying buried soils consistent with earthquakeinduced tsunamis that traveled 10 km up the estuary twelve earthquakes occurred in the last 6500 6720 cal yr b p recurring on average every 570 590 yr intervals between earthquakes varied from a few hundred years to over 1000 comparisons of the coquille record to earthquake histories from adjacent sites in oregon southwestern washington and northwestern california suggest that at least two earthquakes in the last 4000 yr did not rupture the entire length of the subduction zone an earthquake 760 1140 cal yr b p in southwestern washington may have ruptured as far south as coos bay but probably stopped before it reached the coquille estuary because no buried soil records the event and tidal marsh conditions were set to record an earthquake an earthquake limited to a southern segment of the cascadia margin 1940 2130 cal yr b p probably did not rupture north of the coquille estuary an analysis of relative sea level histories from either side of the coquille fault failed to find conclusive evidence for late holocene vertical deformation however we cannot preclude recent upper plate faulting if the fault is active as geomorphic features suggest then constraints on the highest possible elevation of mean tide level allow a maximum vertical slip rate of 0 2 0 4 mm yr in the past 6200 6310 cal yr b p

seismic reflection images beneath puget sound western washington state the puget lowland thrust sheet hypothesis seismic reflection data show that the densely populated puget lowland of western washington state is underlain by subhorizontal paleogene and neogene sedimentary rocks deformed by west and northwest trending faults and folds from south to north beneath the lowland features seen on the seismic data include the horizontally stratified 3 5 km thick tacoma sedimentary basin the seattle uplift with south dipping 20 strata on its south flank and steeply 50 to 90 north dipping strata and the west trending seattle fault on its north flank the 7 5 km thick northward thinning seattle sedimentary basin the antiformal kingston arch and the northwest trending transpressional southern whidbey island fault zone swif interpreting the uplifts as fault bend and fault propagation folds leads to the hypothesis that the puget lowland lies on a north directed thrust sheet the base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic crescent formation its edges may be right lateral strike slip faults along the base of the cascade range on the east and the olympic mountains on the west our model suggests that the seattle fault has a long term slip rate of about 0 25 mm year and is large enough to generate a m7 6 to 7 7 earthquake

surficial slip and rupture geometry on the beichuan fault near hongkou during the mw 7 9 wenchuan earthquake chinasurficial slip and rupture geometry on beichuan fault near hongkou during the wenchuan earthquake

coseismic subsidence in the 1700 great cascadia earthquake coastal estimates versus elastic dislocation models seismic hazard assessments for a cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data this paper tests the consistency of the models for the 1700 great cascadia earthquake with compiled coastal coseismic subsidence as estimated from paleoelevation studies coastal estimates have large uncertainties but show a consistent pattern greatest coseismic subsidence 1 2 m occurred in northern oregon southern washington subsidence elsewhere was 0 1 m elastic dislocation models constrained by interseismic geodetic and thermal data are used to predict the coseismic subsidence for two likely strain accumulation periods of i 800 and ii 550 yr of plate convergence and for uniform megathrust slip of 10 20 30 and 50 m the former two models provide a better and equally good fit predicted subsidence is in broad agreement with marsh estimates discrepancies exist however at the ends of the subduction zone in the south misfit may be due to breakup of the gorda plate the discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there consistent with long term net uplift in excess of eustatic sea level rise the coseismic slip magnitude estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates is consistent with the m 9 earthquake indicated by japanese tsunami records the coseismic slip was greatest in northern oregon southern washington declining to the north and south

ages and stratigraphy of lower and middle tertiary sedimentary and volcanic rocks of the central cascades washington application to the tectonic history of the straight creek fault in the central cascade range of washington three structural blocks of early tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the straight creek fault east of the fault in the teanaway river block the early eocene fluviatile feld spathic sandstone of the swauk formation is interbedded with largely dacitic volcanic rocks of the silver pass volcanic member zircon fission track ages on the silver pass are about 52 m y overlying the tightly folded swauk and silver pass is the teanaway formation a gently dipping accumulation of basalt andesite rare dacite and rhyolite that yields middle eocene about 47 m y whole rock k ar ages conformably overlying the teanaway is the roslyn formation a coal bearing fluviatile feld spathic sandstone of probable middle to late eocene age the late eocene and oligocene naches formation exposed west of the straight creek fault in the cabin creek block is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite the basal guye sedimentary member of the naches formation underlies and is interbedded with the mount catherine rhyolite member in the snoqualmie pass area on the basis of zircon fission track ages on silicic tuffs and whole rock k ar ages on basalt the basal part of the naches is about 40 to 44 m y old the formation is tightly folded and complexly faulted along the straight creek fault the straight creek fault intersects the olympic wallowa lineament in the strongly deformed manastash river block in this block fluviatile coal bearing feldspathic sandstone of the manastash formation is overlain by principally dacitic volcanic rocks of the taneum formation overlying the taneum is the basalt of frost mountain we correlate this threefold sequence feldspathic sandstone dacite basalt with the sequence in the teanaway river block small patches of rhyolitic ash flow tuff dated at 33 m y late oligocene and interbedded feldspathic sandstone overlie the roslyn and teanaway formations in the teanaway river block and are probably correlative with the late oligocene wenatchee formation exposed in the chiwaukum graben east of the teanaway river block as well as with the late oligocene volcanic rocks 30 m y along the cascade crest to the southwest these latter rocks are little deformed and unconformably overlie the naches formation the ages and depositional record in the three blocks indicate a tertiary history of dominantly vertical movement along the straight creek fault and its southeasterly splays that merge with the olympic wallowa lineament the horst and graben structure of the blocks as well as enechelon fold axes in the swauk formation suggests some post early eocene right lateral shear along the fault although there is no direct evidence of lateral offset vertical movement significant since early eocene swauk deposition followed late eocene naches deposition but tapered off by late oligocene time and ceased by miocene time when the fault was intruded by the 25 m y old snoqualmie batholith and other plutons however structures with olympic wallowa lineament trends appear to have influenced folding of the miocene yakima basalt subgroup

mount mazama eruption calendrical age verified and atmospheric impact assessed geochemical identification of mount mazama ash in the greenland ice sheet project 2 gisp2 ice core gives a calendrical age of 7627 150 cal yr b p 5677 150 b c for the eruption thus providing a more accurate early holocene stratigraphic time line than previously available the gisp2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 mt spread over an 6 yr period following the eruption of mount mazama taking into account the likelihood of some tropospheric aerosol transport to greenland realistic estimates of the resulting atmospheric optical depth range from 0 6 to 1 5 these values may have produced a temperature depression of 0 6 to 0 7 c at mid to high northern latitudes for 1 3 yr after the eruption these results indicate that the 5677 b c eruption of mount mazama was one of the most climatically significant volcanic events of the holocene in the northern hemisphere we also calculate a maximum stratospheric cl release of 8 1 mt by the eruption which may have led to substantial stratospheric ozone depletion

on the ignition of geostrophically rotating turbidity currents two models of a geostrophically rotating turbidity current are examined to compare predictions for ignition with the catastrophic state both models describe the current as a tube of sediment laden water traversing along and down a uniform slope the first four equation model neglects the energy required to lift the sediment from the seabed into suspension the second five equation model rectifies this shortcoming by introducing a turbulent kinetic energy equation and coupling the bottom stress to turbulence in the plume these models can be used to predict the ignition path and sediment deposition of a geostrophically rotating turbidity current the criteria for ignition in the four equation model can be described by a surface in three dimensional phase space for a non entraining current this surface lies near the geostrophic equilibrium state for a turbidity current occurring in the greenland sea velocities above 0 053 m s 1 or volumetric concentrations of sediment above 2 7 10 5 lead to ignition in general if the tube is started pointing downslope then ignition is more likely than if it is initially directed alongslope however there exists a set of initial conditions in which the current ignites if started along or downslope but deposits if started at an intermediate angle the five equation model requires a larger initial velocity greater than 1 6 m s 1 to ignite than does the four equation model ignition is determined qualitatively by the geostrophic state and the initial normal froude number solutions show a tendency to travel further alongslope during ignition reflecting the restriction that the energy budget places on the sediment load a qualitative difference to phase space in the five equation model is the existence of a region in which the tube has insufficient energy to support the sediment turbulence dies rapidly in this region and so the sediment is deposited almost immediately

precipitation landsliding and erosion across the olympic mountains washington state usa abstract in the olympic mountains of washington state landsliding is the primary surface process by which bedrock and hillslope regolith are delivered to river networks however the relative importance of large earthquakes versus high magnitude precipitation events to the total volume of landslide material transported to valley bottoms remains unknown in part due to the absence of large historical earthquakes to test the hypothesis that erosion is linked to precipitation approximately 1000 landslides were mapped from google earth imagery between 1990 and 2015 along a 15 km wide 85 km long 1250 km2 swath across the range the volume of hillslope material moved by each slide was calculated using previously published area volume scaling relationships and the spatial distribution of landslide volume was compared to mean annual precipitation data acquired from the prism climate group for the period 1981 2010 statistical analysis reveals a significant correlation r 0 55 p

the lower limb and mechanics of walking in australopithecus sediba the discovery of a relatively complete australopithecus sediba adult female skeleton permits a detailed locomotor analysis in which joint systems can be integrated to form a comprehensive picture of gait kinematics in this late australopith here we describe the lower limb anatomy of au sediba and hypothesize that this species walked with a fully extended leg and with an inverted foot during the swing phase of bipedal walking initial contact of the lateral foot with the ground resulted in a large pronatory torque around the joints of the foot that caused extreme medial weight transfer hyperpronation into the toe off phase of the gait cycle late pronation these bipedal mechanics are different from those often reconstructed for other australopiths and suggest that there may have been several forms of bipedalism during the plio pleistocene

tectonic deformation in western washington from continuous gps measurements the analysis of 3 years of continuous data from 7 permanent gps stations along the western washington section of the cascadia subduction zone indicates that the direction of the observed horizontal velocities with respect to station drao nominally representing the stable north american continent is roughly parallel to the relative plate convergence direction of the juan de fuca and north america plates and that their magnitude decreases away from the trench most of this deformation can be attributed to inter seismic strain accumulation due to the locking of the thrust interface when the dislocation model predictions are subtracted from the observed velocities there is evidence for an additional n s oriented contraction at a rate of 4 mm yr over a distance of 250 km this signal presumably represents a more long term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western washington that are characterized by predominantly n s oriented maximum principal stress

radiocarbon evidence for extensive plate boundary rupture about 300 years ago at the cascadia subduction zone the cascadia subduction zone a region of converging tectonic plates along the pacific coast of north america has a geological history of very large plate boundary earthquakes1 2 but no such earthquakes have struck this region since euro american settlement about 150 years ago geophysical estimates of the moment magnitudes mw of the largest such earthquakes range from 8 ref 3 to 9 1 2 ref 4 radiocarbon dating of earthquake killed vegetation can set upper bounds on earthquake size by constraining the length of plate boundary that ruptured in individual earth quakes such dating has shown that the most recent rupture or series of ruptures extended at least 55 km along the washington coast within a period of a few decades about 300 years ago5 here we report 85 new 14c ages which suggest that this most recent rupture or series extended at least 900 km between southern british columbia and northern california by comparing the 14c ages with written records of the past 150 years we conclude that a single magnitude 9 earthquake or a series of lesser earthquakes ruptured most of the length of the cascadia subduction zone between the late 1600s and early 1800s and probably in the early 1700s

a tsunami about 1000 years ago in puget sound washington water surged from puget sound sometime between 1000 and 1100 years ago overrunning tidal marshes and mantling them with centimeters of sand one overrun site is 10 kilometers northwest of downtown seattle another is on whidbey island some 30 kilometers farther north neither site has been widely mantled with sand at any other time in the past 2000 years deposition of the sand coincided to the year or less with abrupt probably tectonic subsidence at the seattle site and with landsliding into nearby lake washington these findings show that a tsunami was generated in puget sound and they tend to confirm that a large shallow earthquake occurred in the seattle area about 1000 years ago

crustal earthquake triggering by pre historic great earthquakes on subduction zone thrusts triggering of earthquakes on upper plate faults during and shortly after recent great m 8 0 subduction thrust earthquakes raises concerns about earthquake triggering following cascadia subduction zone earthquakes of particular regard to cascadia was the previously noted but only qualitatively identified clustering of m 6 5 crustal earthquakes in the puget sound region between about 1200 900 cal years b p and the possibility that this was triggered by a great cascadia thrust subduction thrust earthquake and therefore portends future such clusters we confirm quantitatively the extraordinary nature of the puget sound region crustal earthquake clustering between 1200 900 cal years b p at least over the last 16 000 we conclude that this cluster was not triggered by the penultimate and possibly full margin great cascadia subduction thrust earthquake however we also show that the paleoseismic record for cascadia is consistent with conclusions of our companion study of the global modern record outside cascadia that m 8 6 subduction thrust events have a high probability of triggering at least one or more m 6 5 crustal earthquakes

active shortening of the cascadia forearc and implications for seismic hazards of the puget lowland 1 margin parallel shortening of the cascadia forearc is a consequence of oblique subduction of the juan de fuca plate beneath north america strike slip thrust and oblique crustal faults beneath the densely populated puget lowland accommodate much of this north south compression resulting in large crustal earthquakes to better understand this forearc deformation and improve earthquake hazard assessment we here use seismic reflection surveys coastal exposures of pleistocene strata potential field data and airborne laser swath mapping to document and interpret a significant structural boundary near the city of tacoma this boundary is a complex structural zone characterized by two distinct segments the northwest trending eastern segment extending from tacoma to carr inlet is formed by the broad 11 5 km southwest dipping 11 20 rosedale monocline this monocline raises crescent formation basement about 2 5 km resulting in a moderate gravity gradient we interpret the rosedale monocline as a fault bend fold forming above a deep thrust fault within the rosedale monocline inferred quaternary strata thin northward and form a growth triangle that is 4 1 to 6 6 km wide at its base suggesting 2 3 mm yr of slip on the underlying thrust the western section of the 40 km long north dipping tacoma fault extending from hood canal to carr inlet forms the western segment of the tacoma basin margin structural relief on this portion of the basin margin may be several kilometers resulting in steep gravity and aeromagnetic anomalies quaternary structural relief along the tacoma fault is as much as 350 400 m indicating a minimum slip rate of about 0 2 mm yr the inferred eastern section of the tacoma fault east of carr inlet crosses the southern part of the seattle uplift has variable geometry along strike and diminished structural relief the tacoma fault is regarded as a north dipping backthrust to the seattle fault so that slip on a master thrust fault at depth could result in movement on the seattle fault the tacoma fault or both

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

great earthquakes and tsunamis of the past 2000 years at the salmon river estuary central oregon coast usa four buried tidal marsh soils at a protected inlet near the mouth of the salmon river yield definitive to equivocal evidence for coseismic subsidence and burial by tsunami deposited sand during great earthquakes at the cascadia subduction zone an extensive landward tapering sheet of sand overlies a peaty tidal marsh soil over much of the lower estuary limited pollen and macrofossil data suggest that the soil suddenly subsided 0 3 1 0 m shortly before burial regional correlation of similar soils at tens of estuaries to the north and south and precise 14c ages from one salmon river site imply that the youngest soil subsided during the great earthquake of 26 january a d 1700 evidence for sudden subsidence of three older soils during great earthquakes is more equivocal because older soil stratigraphy can be explained by local hydrographic changes in the estuary regional 14c correlation of two of the three older soils with soils at sites that better meet criteria for a great earthquake origin is consistent with the older soils recording subsidence and tsunamis during at least two great earthquakes pollen evidence of sudden coseismic subsidence from the older soils is inconclusive probably because the amount of subsidence was small 0 5 m the shallow depths of the older soils yield rates of relative sea level rise substantially less than rates previously calculated for oregon estuaries manuscript received 10 october 2003

interseismic uplift rates for western oregon and along strike variation in locking on the cascadia subduction zone 1 we quantify the spatial pattern of uplift rate in western oregon and northernmost california using tidal and leveling records to better understand the pattern of interseismic locking on the cascadia subduction zone we extend relative sea level time series of the six primary noaa tide gauges to include all observations from 1925 to 2006 previously unidentified bench mark instability biases portions of tidal records by 1 6 mm a 1 before correction we determine precise relative uplift rates at the six tidal sites with an adjustment that includes rates of differenced time series as additional constraints our analysis of national geodetic survey leveling data between tide gauges corrects errors in 1941 leveling and 184 secondary ties double the number of highest quality uplift rate estimates relative uplift rates from leveling are adjusted to the tidal rates accounting for uncertainties in both data types tidal and leveling uplift rates agree within error for all but one of the coastal segments where we infer systematic leveling error affects the 1988 line uplift rates are made absolute using an interval and location specific geocentric sea level rise rate of 2 3 0 2 mm a 1 total propagated one sigma errors for the absolute uplift rates of bench marks are 0 4 mm a 1 along strike changes in uplift rate near 45 n and 42 8 n require two distinct changes in locking depth as inferred from elastic dislocation modeling the along strike changes in locking on this portion of the cascadia subduction zone interface correlate to the western and southern extent of the mafic siletzia block in the fore arc

the endocast of mh1 australopithecus sediba the virtual endocast of mh1 australopithecus sediba obtained from high quality synchrotron scanning reveals generally australopith like convolutional patterns on the frontal lobes but also some foreshadowing of features of the human frontal lobes such as posterior repositioning of the olfactory bulbs principal component analysis of orbitofrontal dimensions on australopith endocasts mh1 sts 5 and sts 60 indicates that among these mh1 orbitofrontal shape and organization align most closely with human endocasts these results are consistent with gradual neural reorganization of the orbitofrontal region in the transition from australopithecus to homo but given the small volume of the mh1 endocast they are not consistent with gradual brain enlargement before the transition

numerical simulation of powder snow avalanches la simulation numerique d une avalanche de neige poudreuse maseguichi japon est proposee la profondeur de neige frai che apparai t comme un parametre determinant au declenchement de ce type d avalanche

paleomagnetic correlation of late holocene earthquakes among estuaries in washington and oregon 1 paleomagnetic directions of estuarine mud provide additional evidence that individual earthquakes or rapid series of earthquakes caused widespread coseismic land level changes during the past 2000 years in western washington and oregon most of the paleomagnetic measurements were made on mud dating from the first decades after coseismic subsidence from plate boundary earthquakes at the cascadia subduction zone mud deposited soon after the a d 1700 cascadia earthquake has similar remanent directions among all five sites k 171 sampled along 80 km of pacific coast between grays harbor and the mouth of the columbia river likewise internally consistent directions were obtained along this stretch of coast from mud deposited soon after a plate boundary earthquake or earthquake series in a d 340 410 and soon after another such event in a d 680 720 also analyzed were remanent magnetizations of mud deposited shortly before or shortly after land level changes from seismicity in the north america plate beneath puget sound a mean direction for sites on the snohomish river delta near everett from the time of an earthquake on the seattle fault in a d 900 930 is statistically identical 95 confidence level to a mean direction in mud that was uplifted in a d 800 1000 at potentially correlative sites near tacoma and olympia the paleomagnetic direction from everett for the upper plate earthquake of a d 900 930 differs substantially from that for a plate boundary earthquake in a d 810 1190 this difference implies that the upper plate earthquake preceded the plate boundary earthquake by a century or two on the basis of comparisons of their paleomagnetic poles with a previously reconstructed path of geomagnetic paleosecular variation in western north america

finding concealed active faults extending the southern whidbey island fault across the puget lowland washington 1 the southern whidbey island fault zone swif as previously mapped using borehole data potential field anomalies and marine seismic reflection surveys consists of three subparallel northwest trending strands extending 100 km from near vancouver island to the northern puget lowland east of puget sound the swif makes landfall between the cities of seattle and everett but is concealed beneath a thick mantle of young glacial deposits and vegetation a 20 km wide northwest trending swath of subparallel low amplitude aeromagnetic anomalies crosses this region of the puget lowland and is on strike with the swif the most prominent aeromagnetic anomaly the cottage lake lineament extends at least 18 km and lies approximately on strike with the swif on whidbey island subtle scarps and topographic lineaments on pleistocene surfaces visible on high resolution lidar topography at a number of locations along the swif lie on or near these magnetic anomalies in the field scarps exhibit northeast side up and vertical relief of 1 to 5 m excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation most likely above buried reverse oblique faults excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation in total paleoseismological evidence suggests that the swif produced at least four earthquakes since deglaciation about 16 400 years ago the most recent less than 2700 years ago

termites and necrophagous insects associated with early pleistocene gelasian australopithecus sediba at malapa south africa abstract this study focuses on two early pleistocene australopithecus sediba hominin specimens and associated fauna from malapa south africa these specimens have been interpreted as having fallen through a shaft opening into a cave where they died and likely mummified before being washed into a lower chamber in order to better understand the taphonomy of the fossils and to identify the invertebrate agent s responsible for damage we examined the fossils using high resolution imagery and studied the site and associated breccia blocks in the laboratory for traces of invertebrate activity we also conducted bone modification experiments with termites and hide beetles results show the presence of blowfly puparia within the cranium of the juvenile male mh1 indicating that flies visited the facial orifices of the freshly decomposing corpse blowflies which have a requirement to oviposit in daylight imply that the body was exposed to sunlight for part of the day a shaft opening that admitted sunlight is supported by the presence of a fossil fungus garden associated with mh2 fungus gardens occur subsurface and sprout mushrooms in areas with dappled light analysis of the fossil surfaces revealed a pattern comprising 14 damage types that were made on relatively fresh bone a bone modification experiment conducted with resident termites at the site showed that they created the same suite of microscopic surface features as those observed on the fossils the features are mostly the result of harvesting symbiotic fungi from fresh bones and mineral rich coatings from fossils damage resulting from the activity of hide beetles is similar to that of termites apart from invasive edge gnawing produced by the beetles this research highlights the importance of insect activity in the modification of fossils in the cradle of humankind world heritage site area and calls for consideration of this agent at other fossil sites in the region

tree ring analysis in the reconstruction of slope instabilities associated with earthquakes and precipitation the crimean mountains ukraine the crimean mountains in ukraine constitute a seismically active region that is abundant in various types of mass movement the limestone escarpments that rise above the black sea coast in the vicinity of the town of yalta are particularly subject to considerable rockfall landslide and debris flow processes that often endanger infrastructure and human life one of these very active slopes is the area of the ancient taraktash rockslide characterised by active block type movements lateral spreading toppling and incipient sliding rockfalls ongoing sliding activities and debris flows the aims of this study were i to employ tree ring analysis to reconstruct a record of slope instabilities mainly block type movements rockfalls landslides and debris flows within the taraktash slope deformation over a period of more than two centuries and ii to establish the degree of correlation between periods of enhanced slope activity and both historical earthquakes and climatic data the temporal frequency of significant slope processes was addressed through a tree ring analysis that took 738 increment cores from 255 crimean pine trees pinus nigra ssp pallasiana the tree ring record reveals periods of increased activity of block type movements rockfalls and landslides during the major earthquakes in 1790 1875 1927 and 1986 although the slope processes studied are predominantly driven by seismic activity their intensity and duration are also dependent on climatic factors this is especially true for landslides and debris flows which occurred mainly in years characterised by high precipitation totals

renewal of tidal forests in washington state after a subduction earthquake in a d 1700 abstract with few exceptions today s tidal trees near washington s pacific coast postdate an earthquake that lowered the region by 1 m or more the earthquake which occurred in a d 1700 is the most recent to have ruptured much of the plate boundary at this central part of the cascadia subduction zone because of the coseismic subsidence lowland forests became tidal flats where thousands of trees died most of the trees killed were sitka spruce picea sitchensis in the centuries since the earthquake tidal deposits have built new land that has been colonized by new sitka spruce all but several tens of the region s tidal spruce consequently postdate 1700 as shown by counts of annual rings in 121 of the largest spruce in tidal forests at copalis river grays harbor and willapa bay forests began to return to each of these estuaries in the early 1700s and spread seaward in the late 1700s and 1800s annual rings in the oldest of the trees thus record a large fraction of the earthquake recurrence interval that began with the 1700 earthquake

gps deformation in a region of high crustal seismicity n cascadia forearc we estimate the rate of crustal deformation in the central and northern cascadia forearc based on a combination of existing global positioning system gps velocity data along the cascadia subduction zone gps strain rates and velocities show that the northwestern washington southwestern british columbia region is currently shortening at 3 3 5 mm yr 31 in a n s direction in good agreement with inference from crustal earthquake statistics on the longterm the shortening rate is 5 6 mm yr 31 providing that the subduction related interseismic loading of the margin is purely elastic compared to the velocity of the oregon forearc with respect to north america v 7m m yr 31 this indicates that most of the forearc motion is accommodated in the puget georgia basin area corresponding to the main concentration of crustal seismicity the difference between the current and long term shortening rates may be taken up during subduction megathrust earthquakes thus these events could produce a sudden increase of n s compression in the puget sound region and could trigger major seattle fault type crustal earthquakes published by elsevier science b v

australopithecus sediba at 1 977 ma and implications for the origins of the genus homo newly exposed cave sediments at the malapa site include a flowstone layer capping the sedimentary unit containing the australopithecus sediba fossils uranium lead dating of the flowstone combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments provides a tightly constrained date of 1 977 0 002 million years ago ma for these fossils this refined dating suggests that au sediba from malapa predates the earliest uncontested evidence for homo in africa

prehistoric rock avalanches in the olympic mountains washington rock avalanches blocked streams in the olympic mountains southwest of puget sound during the past few thousand years limiting radiocarbon ages indicated that three or four of six avalanches occurred from 1000 to 1300 years ago or shortly thereafter most of the dates were from the outer preserved rings of trees drowned behind avalanche dams these three or four avalanches may be coeval not only with one another but also with abrupt tectonic deformation in western washington no rock avalanches in the olympic mountains are known to have resulted from storms or earthquakes during the past century the avalanches strengthen the case that a large prehistoric earthquake occurred in the puget sound region

abrupt uplift within the past 1700 years at southern puget sound washington shorelines rose as much as 7 meters along southern puget sound and hood canal between 500 and 1700 years ago evidence for this uplift consists of elevated wave cut shore platforms near seattle and emerged peat covered tidal flats as much as 60 kilometers to the southwest the uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform the tidal flats also emerged abruptly they changed into freshwater swamps and meadows without first becoming tidal marshes where uplift was greatest it adjoined an inferred fault that crosses puget sound at seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago the uplift and probable fault slip show that the crust of the north america plate contains potential sources of damaging earthquakes in the puget sound region

evidence for earthquake induced subsidence about 1100 yr ago in coastal marshes of southern puget sound washington buried forest and high marsh soils indicate abrupt changes in relative sea level at four coastal localities in southern puget sound at little skookum inlet and red salmon creek douglas fir stumps in growth position are buried by salt marsh peat at localities along mcallister creek and the nisqually river high marsh soils are buried by tidal flat mud localized liquefaction coincided with submergence of the high marsh soil at mcallister creek dramatic changes in seed and diatom assemblages across these contacts confirm rapid submergence at little skookum inlet and red salmon creek salt marsh peat immediately above a buried forest soil contains diatoms indicative of low marsh and tidal flat environments at mcallister creek and nisqually river low marsh and tidal flat diatoms are abundant in laminated mud directly over high marsh peat inferences from modern analogs indicate at least 1 m of subsidence at each site and possibly up to 3 m at skookum inlet abrupt burial of lowland soils in southern puget sound is best explained by coseismic subsidence some of the submergence may be the result of coseismic compaction and postearthquake settlement widespread buried soils large amounts of subsidence coeval submergence across a wide area and ground shaking at the time of subsidence all point to a large earthquake between 1150 and 1010 cal yr b p in southern puget sound as the most likely cause of subsidence

prediction of ignitive turbidity currents in scripps submarine canyon swift highly erosive continuous turbidity currents are known to be triggered in scripps submarine canyon during storms a relatively modest agitation due to edge waves can generate currents at least as swift as 1 9 m s 1 pantin 1979 and parker 1982 have described a mechanism by which a turbidity current can self accelerate by entraining sediment from the bed parker termed the mechanism ignition herein two models of steady spatially developing turbidity currents are developed and applied to scripps submarine canyon fairly specific predictions are obtained for the onset of ignitive acceleration and for the downstream development of the turbidity currents the results are in general agreement with available measurements in the same canyon it is speculated that ignitive turbidity currents constitute a major mechanism by which sand submarine canyons are eroded and by which sand is delivered to the abyssal plain

great earthquakes of variable magnitude at the cascadia subduction zone comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate boundary ruptures of varying length implying great earthquakes of variable magnitude at the cascadia subduction zone inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis and relative amounts of coseismic subsidence and heights of tsunamis written records of a tsunami in japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 january 1700 cascadia stratigraphic evidence dating from about 1600 cal yr b p similar to that for the 1700 earthquake implies a similarly long rupture with substantial subsidence and a high tsunami correlations are consistent with other long ruptures about 1350 cal yr b p 2500 cal yr b p 3400 cal yr b p 3800 cal yr b p 4400 cal yr b p and 4900 cal yr b p a rupture about 700 1100 cal yr b p was limited to the northern and central parts of the subduction zone and a northern rupture about 2900 cal yr b p may have been similarly limited times of probable short ruptures in southern cascadia include about 1100 cal yr b p 1700 cal yr b p 3200 cal yr b p 4200 cal yr b p 4600 cal yr b p and 4700 cal yr b p rupture patterns suggest that the plate boundary in northern cascadia usually breaks in long ruptures during the greatest earthquakes ruptures in southernmost cascadia vary in length and recurrence intervals more than ruptures in northern cascadia

radiocarbon age constraints on rates of advance and retreat of the puget lobe of the cordilleran ice sheet during the last glaciation abstract calibrated radiocarbon dates of organic matter below and above till of the last fraser glaciation provide limiting ages that constrain the chronology and duration of the last advance retreat cycle of the puget lobe in the central and southeastern puget lowland seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17 420 90 cal yr b p which is the closest limiting age for arrival of the glacier near the latitude of seattle a time distance curve constructed along a flowline extending south from southwestern british columbia to the central puget lowland implies an average glacier advance rate of ca 135 m yr the glacier terminus reached its southernmost limit ca 16 950 yr ago and likely remained there for ca 100 yr in the vicinity of seattle where the glacier reached a maximum thickness of 1000 m ice covered the landscape for ca 1020 yr postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16 420 cal yr b p and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum the average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes

land level changes from a late holocene earthquake in the northern puget lowland washington an earthquake probably generated on the southern whidbey island fault zone caused 1 2 m of ground surface uplift on central whidbey island 2800 3200 yr ago the cause of the uplift is a fold that grew coseismically above a blind fault that was the earthquake source both the fault and the fold at the fault s tip are imaged on multichannel seismic refection profiles in puget sound immediately east of the central whidbey island site uplift is documented through contrasting histories of relative sea level at two coastal marshes on either side of the fault late holocene shallow crustal earthquakes of mw 5 6 5 7 pose substantial seismic hazard to the northern puget lowland

detection limits of tidal wetland sequences to identify variable rupture modes of megathrust earthquakes abstract recent paleoseismological studies question whether segment boundaries identified for 20th and 21st century great m8 earthquakes persist through multiple earthquake cycles or whether smaller segments with different boundaries rupture and cause significant hazards the smaller segments may include some currently slipping rather than locked in this review we outline general principles regarding indicators of relative sea level change in tidal wetlands and the conditions in which paleoseismic indicators must be distinct from those resulting from non seismic processes we present new evidence from sites across southcentral alaska to illustrate different detection limits of paleoseismic indicators and consider alternative interpretations for marsh submergence and emergence we compare predictions of coseismic uplift and subsidence derived from geophysical models of earthquakes with different rupture modes the spatial patterns of agreement and misfits between model predictions and quantitative reconstructions of coseismic submergence and emergence suggest that no earthquake within the last 4000 years had a pattern of rupture the same as the mw 9 2 alaska earthquake in 1964 from the alaska examples and research from other subduction zones we suggest that if we want to understand whether a megathrust ruptures in segments of variable length in different earthquakes we need to be site specific as to what sort of geological based criteria eliminate the possibility of a particular rupture mode in different earthquakes we conclude that coastal paleoseismological studies benefit from a methodological framework that employs rigorous evaluation of five essential criteria and a sixth which may be very robust but only occur at some sites 1 lateral extent of peat mud or mud peat couplets with sharp contacts 2 suddenness of submergence or emergence and replicated within each site 3 amount of vertical motion quantified with 95 error terms and replicated within each site 4 syncroneity of submergence and emergence based on statistical age modelling 5 spatial pattern of submergence and emergence 6 possible additional evidence such as evidence of a tsunami or liquefaction concurrent with submergence or emergence we suggest that it is possible to consider detection limits as low as 0 1 0 2 m coseismic vertical change

use of fault striations and dislocation models to infer tectonic shear stress during the 1995 hyogo ken nanbu kobe earthquake dislocation models of the 1995 hyogo ken nanbu kobe earthquake derived by yoshida et al 1996 show substantial changes in direction of slip with time at specific points on the nojima and rokko fault systems as do striations we observed on exposures of the nojima fault surface on awaji island spudich 1992 showed that the initial stress that is the shear traction on the fault before the earthquake origin time can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points from yoshida s slip model we calculated dynamic stress changes on the ruptured fault surfaces to estimate errors we compared the slip velocities and dynamic stress changes of several published models of the earthquake the differences between these models had an exponential distribution not gaussian we developed a bayesian method to estimate the probability density function pdf of initial stress from the striations and from yoshida s slip model striations near toshima and hirabayashi give initial stresses of about 13 and 7 mpa respectively we obtained initial stresses of about 7 to 17 mpa at depths of 2 to 10 km on a subset of points on the nojima and rokko fault systems our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near hirabayashi and ogura on awaji island our results indicate that the nojima fault slipped at very low shear stress and fractional stress drop was complete near the surface and about 32 below depths of 2 km our results at depth depend on the accuracy of the rake rotations in yoshida s model which are probably correct on the nojima fault but debatable on the rokko fault our results imply that curved or cross cutting fault striations can be formed in a single earthquake contradicting a common assumption of structural geology

improved experimental evidence for autosuspension abstract field evidence theoretical analysis and numerical simulation all attest to the importance of autosuspension as a major geophysical process important features of the submarine landscape notably the excavation of submarine canyons and the deposition of the distinctive turbidite sequences on the continental rise and in ocean basins find convincing explanations of their modes of formation in terms of autosuspension theory the final element required to confirm autosuspension as a real physical phenomenon is experimental evidence above all that the energy required to maintain suspension is supplied by the gravitational power of the flow itself and by no external process early attempts to reproduce autosuspension in the laboratory have been unconvincing or have only achieved limited success the laboratory experiments described in the present paper were therefore undertaken to improve and extend the available experimental evidence of autosuspension the methodology used represents a significant advance in terms of technique and instrumentation allowing collection of considerably more and higher quality data the new data reinforce the confidence with which autosuspension may be regarded as an important factor in the transport of sediment from continental areas to eventual deposition in the deeper marine environment

long term perspectives on giant earthquakes and tsunamis at subduction histories of earthquakes and tsunamis inferred from geological evidence aid in anticipating future catastrophes this natural warning system now influences building codes and tsunami planning in the united states canada and japan particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records under favorable circumstances paleoseismology can thus provide long term advisories of unusually large tsunamis the extraordinary indian ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks such variation in rupture mode known from written history at a few subduction zones is also characteristic of earthquake histories inferred from geology on the pacific rim

erratum to surficial slip and rupture geometry on the beichuan fault near hongkou during the mw 7 9 wenchuan earthquake china worldwide occurrence and documentation of reverse type ruptures are sparse near hongkou the wenchuan rupture passes through the broad baisha river valley and provides excellent opportunities to trace the surface faulting in fine details for 13 km distance one of the longest continuous sections along the entire rupture in this paper we present the results of our mapping of the surface rupture in this reach based on the discontinuities in slip and geometry the rupture was divided into four sections for convenience from west to east the shenxi gou the miaoba the gaoyuan and the bajiao miao sections respectively the vertical offset is large in the shenxi gou and the bajiao miao sections locally reaching 5 6 m in maxima and generally low in the miaoba section 1 2 m or less in most places the slip gradient for vertical offset is generally 10 3 locally up to 10 1 similar to that in well documented strike slip ruptures near gaoyuan village the surface rupture con sists of two subparallel branches with the northern one exhibiting right lateral slip with minor southeast side up thrusting while the southern one is almost pure south east side up thrusting this pattern mimics the incomplete slip partitioning of oblique thrusting on parallel faults but at a local scale in addition the sense of vertical throw on these two strands is opposite to the general northwest side up thrusting of the wenchuan rupture we propose that it is likely due to the inheritance at shallow depth from the southeast dipping geological faults and that old fault zone structures can have a strong effect on the dynamic rupture by guiding the rupture propagation onto paths of preexisting though locally unfavorable dipping fault planes we also discuss the cross cutting slickenside striations observed near bajiao miao which indicated temporal rake rotation during dynamic rupture and their geological and seismological implications

paleoseismic signature in late holocene sediment cores from saanich inlet british columbia abstract this paper explores the paleoseismic record potentially preserved in the upper 40 m of hydraulic piston cores collected in 1996 at two sites in saanich inlet british columbia during ocean drilling program odp leg 169s the odp cores are missing 1 2 m of water rich sediment directly underlying the seafloor but this sediment is preserved in shorter piston cores collected in 1989 and 1991 the upper part of the odp cores consists of rhythmically laminated varved marine mud with intercalated massive beds interpreted to be debris flow deposits some of the debris flow deposits are linked to past earthquakes including the 1946 vancouver island earthquake m7 2 a great m8 9 plate boundary earthquake at the cascadia subduction zone in january 1700 and a large crustal or plate boundary earthquake about 1000 yr ago earthquakes may also be responsible for debris flows in about ad 1600 1500 1250 1150 850 450 350 180 and bc 200 220 500 900 and 1050 if so the average recurrence interval for moderate to large earthquakes which trigger debris flows in saanich inlet is about 150 yr this recurrence interval is broadly consistent with the frequency of moderate to large earthquakes in the region during the historical period debris flows however can also be triggered by non seismic processes making it difficult to assemble a complete earthquake record from the saanich inlet cores we propose that extensive debris flow deposits emplaced by single large failures or many smaller coincident failures probably have a seismic origin

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

first paleoseismic evidence for great surface rupturing earthquakes in the bhutan himalayas the seismic behavior of the himalayan arc between central nepal and arunachal pradesh remains poorly understood due to the lack of observations concerning the timing and size of past major and great earthquakes in bhutan we present here the first paleoseismic study along the himalayan topographic front conducted at two sites in southern central bhutan paleoseismological excavations and related oxcal modeling reveal that bhutan experienced at least two great earthquakes in the last millennium one between the seventeenth and eighteenth century and one during medieval times producing a total cumulative vertical offset greater than 10 m along with previous studies that reported similar medieval events in central nepal sikkim and assam our investigations support the occurrence of either i a series of great earthquakes between a d 1025 and a d 1520 or ii a single giant earthquake between a d 1090 and a d 1145 in the latter case the surface rupture may have reached a total length of 800 km and could be associated with an earthquake of magnitude mw 8 7 9 1

response of a small oregon estuary to coseismic subsidence and postseismic uplift in the past 300 years the sixes river estuary south coastal oregon sits above the locked portion of the cascadia subduction zone which intermittently releases in subduction zone earthquakes one such cascadia earthquake 300 years ago caused subsidence and a tsunami at the sixes estuary the subsidence raised the river9s base level resulting in an 3 km upstream shift of the head of tide of the estuary at the upper end of the expanded estuary more than 4 m of overbank sediment was deposited in the first decades or century after subsidence subsequent incision through the overbank deposits accompanied the gradual emergence of the estuary and attendant downstream shift of the head of tide as relative sea level fell in response to interseismic uplift

paleoseismic evidence for late holocene tectonic deformation along the saddle mountain fault zone southeastern olympic peninsula washington abstract trench and wetland coring studies show that northeast striking strands of the saddle mountain fault zone ruptured the ground about 1000 years ago generating prominent scarps three conspicuous subparallel fault scarps can be traced for 15 km on light detection and ranging lidar imagery traversing the foothills of the southeast olympic mountains the saddle mountain east fault the saddle mountain west fault and the newly identified sund creek fault uplift of the saddle mountain east fault scarp impounded stream flow forming price lake and submerging an existing forest thereby leaving drowned stumps still rooted in place stratigraphy mapped in two trenches one across the saddle mountain east fault and the other across the sund creek fault records one and two earthquakes respectively as faulting juxtaposed miocene age bedrock against glacial and postglacial deposits although the stratigraphy demonstrates that reverse motion generated the scarps slip indicators measured on fault surfaces suggest a component of left lateral slip from trench exposures we estimate the postglacial slip rate to be 0 2 mm yr and between 0 7 and 3 2 mm yr during the past 3000 years integrating radiocarbon data from this study with earlier saddle mountain fault studies into an oxcal bayesian statistical chronology model constrains the most recent paleoearthquake age of rupture across all three saddle mountain faults to 1170 970 calibrated years cal b p which overlaps with the nearby m w 7 5 1050 1020 cal b p seattle fault earthquake an earlier earthquake recorded in the sund creek trench exposure dates to around 3500 cal b p the geometry of the saddle mountain faults and their near synchronous rupture to nearby faults 1000 years ago suggest that the saddle mountain fault zone forms a western boundary fault along which the fore arc blocks migrate northward in response to margin parallel shortening across the puget lowland

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

late holocene earthquakes on the toe jam hill fault seattle fault zone bainbridge island washington five trenches across a holocene fault scarp yield the first radiocarbon measured earthquake recurrence intervals for a crustal fault in western washington the scarp the first to be revealed by laser imagery marks the toe jam hill fault a north dipping backthrust to the seattle fault folded and faulted strata liquefaction features and forest soil a horizons buried by hanging wall collapse colluvium record three or possibly four earthquakes between 2500 and 1000 yr ago the most recent earthquake is probably the 1050 1020 cal calibrated yr b p a d 900 930 earthquake that raised marine terraces and triggered a tsunami in puget sound vertical deformation estimated from stratigraphic and surface offsets at trench sites suggests late holocene earthquake magnitudes near m7 corresponding to surface ruptures 36 km long deformation features recording poorly understood latest pleistocene earthquakes suggest that they were smaller than late holocene earthquakes postglacial earthquake recurrence intervals based on 97 radiocarbon ages most on detrital charcoal range from 12 000 yr to as little as a century or less corresponding fault slip rates are 0 2 mm yr for the past 16 000 yr and 2 mm yr for the past 2500 yr because the toe jam hill fault is a backthrust to the seattle fault it may not have ruptured during every earthquake on the seattle fault but the earthquake history of the toe jam hill fault is at least a partial proxy for the history of the rest of the seattle fault zone

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

evidence for liquefaction identified in peeled slices of holocene deposits along the lower columbia river washington peels made from 10 geoslices beneath a riverbank at washington9s hunting island 45 km inland from the pacific coast aid in identifying sand that liquefied during prehistoric earthquakes of estimated magnitude 8 9 at the cascadia subduction zone each slice was obtained by driving sheetpile and a shutter plate to depths of 6 8 m the resulting sample as long as 8 m had a trapezoidal cross section 42 55 cm by 8 cm the slicing created few artifacts other than bending and smearing at slice edges each slice is dominated by well stratified sand and mud deposited by the tidal columbia river nearly 90 of the sand is distinctly laminated the sand contains mud beds as thick as 0 5 m and at least 20 m long and it is capped by a mud bed that contains a buried soil that marks the 1700 cascadia earthquake of estimated magnitude 9 every slice intersected sills and dikes of fluidized sand and many slices show folds and faults as well sills which outnumber dikes mostly follow and locally invade the undersides of mud beds the mud beds probably impeded diffuse upward flow of water expelled from liquefied sand trapped beneath mud beds this water flowed laterally destroyed bedding by entraining fluidizing sand and locally scoured the overlying mud horizontal zones of folded sand extend at least 10 or 20 m and some contain low angle faults many of the folds probably formed while sand was weakened by liquefaction the low angle faults may mark the soles of river bottom slumps or lateral spreads as many as four great cascadia earthquakes in the past 2000 yr contributed to the intrusions folds and faults this subsurface evidence for fluid escape and deformation casts doubt on maximum accelerations that were previously inferred from local absence of liquefaction features at the ground surface along the columbia river the geosliced evidence for liquefaction abounds not only beneath banks riddled with dikes but also beneath banks in which dikes are absent such dike free banks of the columbia river if interpreted without study of postdepositional structures in deposits beneath them provide insufficient basis for setting upper bounds on the strength of shaking from great cascadia earthquakes online material data from outcrop surveys vibracores and penetrometer tests tabular summary of depositional and postdepositional features in geoslices

two post glacial earthquakes on the saddle mountain west fault southeastern olympic peninsula washington abstract two earthquakes ruptured the western strand of the saddle mountain fault after the cordilleran ice sheet began its retreat from the puget sound lowland about 17 ka the fault system includes multiple northeast trending thrust faults on the southeastern olympic peninsula delineated on light detection and ranging topography by prominent east side up scarps that deform vashon till and extend over 6 7 km the cargill creek trench on the saddle mountain west fault exposed faulted drift and postglacial colluvium below a 1 2 m high scarp steeply dipping northeast striking faults form a 10 m wide zone beneath the scarp structural relations in the trench indicate that two earthquakes produced at least 1 m of vertical separation additional folding or faulting beyond the trench raised the scarp to a total height of 1 7 m aligned cobbles dipping approximately 40 to the north attest to earthquake related tilting a cobble firmly lodged in till and split by a secondary fault indicates that hanging wall deformation involved a dextral component of slip however slip on the master fault may have involved a left lateral component based on slip indicators at adjacent trench sites in the region a single charcoal age and estimates of the onset of ice retreat limit the time of the oldest earthquake to 17 7 7 ka the most recent event occurred after 1 7 ka and may be the same earthquake that dammed price lake about 1100 yr ago known from related studies empirical relations suggest that the most recent earthquake was about m 7 0 2 and produced an estimated total slip of 1 3 1 7 m the total postglacial slip rate ranges from 0 1 to 0 3 m k yr these results support the hypothesis that the saddle mountain fault is part of a larger fault system that accommodates transpressive strain as the puget lowland crust migrates around the eastern flank of the olympic mountains