a seiche hazard study for lake union seattle washington abstract large seismic waves can be a result of earthquakes on local regional or distant fault zones occasionally setting bodies of water into oscillation seismic seiching is a recurrent phenomenon in the state of washington and elsewhere e g berninghausen 1969 barberopoulou et al 2004 cassidy et al 2005 barberopoulou et al 2006 to investigate and characterize wave motion in lakes during strong ground shaking various scenarios of seiche generation are considered for lake union in seattle washington as subjected to a variety of ground motions both relatively simple harmonic and more complex earthquake excitation synthetic and real data are used in the modeling to isolate any effects resulting from a predominantly east west or north south motion or the contribution from parts of the shoreline ground motions are first separated into their horizontal components motions along latitude or longitude before they are combined this approach shows that long linear shorelines are the largest contributors to water wave heights depending on the shape of the lake enhanced wave heights can also be a result of focusing the wave motion in the lake is found to be predominantly east west even when the ground motions are in the north south direction the results of this study suggest that coastal structures can be vulnerable due to resonant excitation initiated from earthquakes on water bodies far from tsunamigenic sources

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

subsurface geometry and evolution of the seattle fault zone and the seattle basin washington the seattle fault a large seismically active east west striking fault zone under seattle is the best studied fault within the tectonically active puget lowland in western washington yet its subsurface geometry and evolution are not well constrained we combine several analysis and modeling approaches to study the fault geometry and evolution including depth converted deep seismic reflection images p wave velocity field gravity data elastic modeling of shoreline uplift from a late holocene earthquake and kinematic fault restoration we propose that the seattle thrust or reverse fault is accompanied by a shallow antithetic reverse fault that emerges south of the main fault the wedge enclosed by the two faults is subject to an enhanced uplift as indicated by the boxcar shape of the shoreline uplift from the last major earthquake on the fault zone the seattle basin is interpreted as a flexural basin at the footwall of the seattle fault zone basin stratigraphy and the regional tectonic history lead us to suggest that the seattle fault zone initiated as a reverse fault during the middle miocene concurrently with changes in the regional stress field to absorb some of the north south shortening of the cascadia forearc kingston arch 30 km north of the seattle fault zone is interpreted as a more recent disruption arising within the basin probably due to the development of a blind reverse fault manuscript received 23 august 2001

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

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

the skipjack island fault zone an active transcurrent structure within the upper plate of the cascadia subduction complex abstract the skipjack island fault zone has been mapped in the san juan islands between vancouver island canada and the washington state mainland usa a decade ago interpretation of multibeam sonar seafloor imagery revealed that skipjack island an east west striking sedimentary bedrock outcrop was a fault controlled structural feature a major fault separates skipjack island from a deformed sedimentary bedrock outcrop on the seafloor to the north recently the skipjack island fault zone s morphology and extent have been explored both to the west and east of the island using multibeam echosounder bathymetry seismic reflection profiles and sediment cores the character of the skipjack island fault zone is well defined locally by the interpreted seismic reflection profiles which show active faults that displace sediments deposited since the last glacial maximum the central part of the fault zone near skipjack island appears as a near vertical structure in the shallow subsurface that has been subjected to left lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault the result of drag from fault motion interpretations of recent geophysical data suggest an extension of the skipjack island fault zone farther to the west where it trends towards and possibly connects with the fulford fault on vancouver island and to the east where it either terminates north of lummi island or continues eastward onto the mainland the skipjack island fault zone is interpreted to be the northern boundary of the san juan archipelago with the devils mountain fault zone being the southern boundary both fault zones represent the longest continuous faults of the san juan archipelago this newly mapped fault zone and its zone of deformation to the north may represent the northern boundary of the cascadia forearc potential sesimicity of the skipjack island fault zone and its proximity to the steep unstable northeastern side of orcas island and the southern front of the fraser river delta poses a geohazads problem in the form of earthquakes landslides submarine slides and tsunamis

sedimentary basin effects in seattle washington ground motion observations and 3d simulations seismograms of local earthquakes recorded in seattle exhibit surface waves in the seattle basin and basin edge focusing of s waves spectral ratios of s waves and later arrivals at 1 hz for stiff soil sites in the seattle basin show a dependence on the direction to the earthquake with earthquakes to the south and southwest producing higher average amplification earthquakes to the southwest typically produce larger basin surface waves relative to s waves than earthquakes to the north and northwest probably because of the velocity contrast across the seattle fault along the southern margin of the seattle basin s to p conversions are observed for some events and are likely converted at the bottom of the seattle basin we model five earthquakes including the m 6 8 nisqually earthquake using 3d finite difference simulations accurate up to 1 hz the simulations reproduce the observed dependence of amplification on the direction to the earthquake the simulations generally match the timing and character of basin surface waves observed for many events the 3d simulation for the nisqually earthquake produces focusing of s waves along the southern margin of the seattle basin near the area in west seattle that experienced increased chimney damage from the earthquake similar to the results of the higher frequency 2d simulation reported by stephenson et al 2006 waveforms from the 3d simulations show reasonable agreement with the data at low frequencies 0 2 0 4 hz for the nisqually earthquake and an m 4 8 deep earthquake west of seattle

pumice in the interglacial whidbey formation at blowers bluff central whidbey island wa usa abstract a new 40ar 39ar age of 128 9 ka and chemical analyses of pumice layers from interglacial alluvium at blowers bluff whidbey island wa show that the deposits are part of the whidbey formation a widespread mainly subsurface unit glass chemistry of the dated dacitic pumice does not match any analyzed northern cascade source but upper pleistocene dacites from glacier peak and early pleistocene silicic rocks from the kulshan caldera are chemically similar the chemistry of pumiceous dacite in younger units including the latest pleistocene partridge gravel is similar to that of the dated material the deep troughs of the modern northern puget lowland must have been filled during deposition of the whidbey formation allowing volcanic rich sediment to reach what is now whidbey island topographic analysis of lidar images demonstrates that extensive erosion occurred during latest pleistocene ice retreat the partridge gravel likely records subglacial fluvial erosion along an ice tunnel and ice marginal deposition into adjacent marine waters pumice in the partridge gravel probably was reworked from stratigraphically and topographically lower deposits including those at blowers bluff

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

site response and attenuation in the puget lowland washington state simple spectral ratio ssr and horizontal to vertical h v site response estimates at 47 sites in the puget lowland of washington state document significant attenuation of 1 5 to 20 hz shear waves within sedimentary basins there amplitudes of the horizontal components of shear wave arrivals from three local earthquakes were used to compute ssrs with respect to the average of two bedrock sites and h v spectral ratios with respect to the vertical component of the shear wave arrivals at each site ssr site response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 hz and decreasing spectral amplification with increasing frequency above 6 hz ssrs at nonbasin sites show a variety of shapes and larger resonance peaks we attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata computing the frequency independent depth dependent attenuation factor q s int from the ssr spectral decay between 2 and 20 hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata 7 km depth h v site responses show less spectral decay than the ssr responses but contain many of the same resonance peaks we hypothesize that the h v method yields a flatter response across the frequency spectrum than ssrs because the h v reference signal vertical component of the shear wave arrivals has undergone a degree of attenuation similar to the horizontal component recordings correcting the ssr site responses for attenuation within the basins by removing the spectral decay improves agreement between ssr and h v estimates

vented sediments and tsunami deposits in the puget lowland washington differentiating sedimentary processes the sandy deposits produced by tsunamis and liquefaction share many sedimentary features and distinctions between the two are important in seismically active coastal zones both types of deposits are present in the wetlands bordering puget sound where one or more earthquakes about 1100 years ago caused both tsunami flooding and sediment venting this co occurrence allows an examination of the resulting deposits and a comparison with tsunami and liquefaction features of modern events vented sediments occur at four of five wetland field localities and tsunami deposits at two in comparison with tsunami deposits vented sediments in this study and from other studies tend to be thicker although they can be thin vented sediments also have more variable thickness at both outcrop and map scale are associated with injected dykes and contain clasts derived from underlying deposits further vented sediments tend to contain a greater variety of sedimentary structures and these structures vary laterally over metres tsunami deposits compared with vented sediments are commonly thinner fine and thin landward more consistently have more uniform thickness on outcrop and map scales and have the potential of containing coarser clasts up to boulders for both tsunami deposits and vented sediments the availability and grain size of source material condition the characteristics of the deposit in the cases presented in this paper both foraminifera and diatom assemblages within tsunami deposits and vented sediments consisted of brackish and marine species and no distinction between processes could be made based on microfossils in summary this study indicates a need for more careful analysis and mapping of coastal sediments associated with earthquakes to avoid misidentification of processes and misevaluation of hazards

basin architecture and density structure beneath the strait of georgia british columbia georgia basin is located within one of the most seismically active and populated areas on canada s west coast over the last decade geological investigations have resolved important details concerning the basin s shallow structure and composition yet until recently relatively little was known about deeper portions of the basin in this study new seismic velocity information is employed to develop a 3 dimensional density model of the basin comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales at smaller scales several differences between model and observed gravity fields are recognized analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin fill and underlying basement specifically four regions with thick accumulations of unconsolidated pleistocene and younger sediments which were not resolved in the velocity model are id

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

long term shoreline retreat rates on whidbey island washington usa abstract long term retreat rates of puget sound s unconsolidated sediment shorelines have been difficult to quantify and little systematic research has been completed to constrain retreat in this area we put forward a new application of cosmogenic 10be exposure dating to assess long term shoreline retreat on whidbey island wa by dating lag boulders exposed on the shore platform as the shoreline erodes production of 10be in shoreline boulders is modulated by both tidal submergence and topographic shielding from the retreating bluff by modeling the combined effect of these variables on 10be production the timing of exposure can be determined and used to calculate long term 103 104 yr bluff retreat rates in rare cases retreat rates are underestimated due to inherited 10be within the study area average retreat rates ranged between 0 and 8 cm yr 1 our results demonstrate the utility of cosmogenic nuclides for determining long term shoreline retreat rates in areas with thick sediment cover where large numbers of samples can be collected and where the pre depositional history of the boulders is uncomplicated

stratigraphic and microfossil evidence for late holocene tsunamis at swantown marsh whidbey island washington abstract four muddy sand sheets occur within a tidal marsh peat at swantown on the west coast of whidbey island washington the two largest sand sheets pinch out about 100 m inland and became thinner and finer grained landward all four sand sheets contain marine microfossils and have internal stratification they record repeated inundation of the marsh over a short time period by distinct pulses of sediment laden ocean water consistent with deposition by a tsunami wave train the layers have been radiocarbon dated to 1160 1350 1400 1700 1810 2060 and 1830 2120 cal yr b p the overlap in age between the two youngest layers and inferred great earthquake events at the cascadia plate boundary some 250 km to the west suggests they were emplaced by tsunamis from this source area the two older layers do not correlate with plate boundary events they may be products of tsunamis caused by earthquakes on local faults in the strait of juan de fuca or by submarine landslides in this area

coeval brittle and ductile deformation beneath the late wisconsinan puget lobe washington state usa late wisconsinan glacial sediments exposed on whidbey island and camano island puget sound washington state usa were deposited in a proglacial shallow marine outwash environment during northward retreat of the puget lobe of the cordilleran ice sheet sediments mainly comprise massive and cross bedded sand and gravels and rhythmically bedded clay and silt fine sand couplets interbedded with diamictons that were deposited by a range of mass flows of different viscosities although sediment stratigraphy and ice advance retreat patterns are well established for the puget lobe brittle and ductile deformation structures within and separating these sediment units are less well understood these structures record the nature of ice bed interactions taking place in subglacial and proglacial environments this study examines evidence for these processes and environments key deformation structures identified include open to overturned folds normal and reverse faults clastic dikes and hydrofractures and passive loading structures evidence for coeval development of ductile and brittle deformation structures shows the close relationship between porewater changes sediment rheology and sediment system responses to changes in strain caused by ice bed interactions

the southern whidbey island fault an active structure in the puget lowland washington information from seismic reflection profiles outcrops boreholes and potential field surveys is used to interpret the structure and history of the southern whidbey island fault in the puget lowland of western washington this northwest trending fault comprises a broad as wide as 6 11 km steep northeast dipping zone that includes several splays with inferred strike slip reverse and thrust displacement transpressional deformation along the southern whidbey island fault is indicated by along strike variations in structural style and geometry positive flower structure local unconformities out of plane displacements and juxtaposition of correlative sedimentary units with different histories the southern whidbey island fault represents a segment of a boundary between two major crustal blocks the cascade block to the northeast is floored by diverse assemblages of pre tertiary rocks the coast range block to the southwest is floored by lower eocene marine basaltic rocks of the crescent formation the fault probably originated during the early eocene as a dextral strike slip fault along the eastern side of a continental margin rift bending of the fault and transpressional deformation began during the late middle eocene and continues to the present oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation evidence for quaternary movement on the southern whidbey island fault includes 1 offset and disrupted upper quaternary strata imaged on seismic reflection profiles 2 borehole data that suggests as much as 420 m of structural relief on the tertiary quaternary boundary in the fault zone 3 several meters of displacement along exposed faults in upper quaternary sediments 4 late quaternary folds with limb dips of as much as 9 5 large scale liquefaction features in upper quaternary sediments within the fault zone and 6 minor historical seismicity the southern whidbey island fault should be considered capable of generating large earthquakes m s 7 and represents a potential seismic hazard to residents of the puget lowland

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

structural variation along the devil s mountain fault zone northwestern washington the eastern juan de fuca strait is subject to long term north south oriented shortening the observed deformation is interpreted to result from the northward motion of the oregon block which is being driven north by oblique subduction of the oceanic juan de fuca plate seismic data acquired during the seismic hazards investigation in puget sound survey are used with coincident first arrival tomographic velocities to interpret structural variation along the devil s mountain fault zone in the eastern juan de fuca strait the primary fault of the devil s mountain fault zone developed at the northern boundary of the everett basin during north south oriented tertiary compression interpretation of seismic reflection data suggests that based on their similar geometry including the large magnitude of pre tertiary basement offset the primary fault of the devil s mountain fault west of 122 95 w and the utsalady point fault represent the main fault of the tertiary devil s mountain fault zone the tertiary

holocene faulting in the bellingham forearc basin upper plate deformation at the northern end of the cascadia subduction zone 1 the northern cascadia forearc takes up most of the strain transmitted northward via the oregon coast block from the northward migrating sierra nevada block the north south contractional strain in the forearc manifests in upper plate faults active during the holocene the northern most components of which are faults within the bellingham basin the bellingham basin is the northern of four basins of the actively deforming northern cascadia forearc a set of holocene faults drayton harbor birch bay and sandy point faults occur within the bellingham basin and can be traced from onshore to offshore using a combination of aeromagnetic lineaments paleoseismic investigations and scarps identified using lidar imagery with the recognition of such holocene faults the northernmost margin of the actively deforming cascadia forearc extends 60 km north of the previously recognized limit of holocene forearc deformation although to date no holocene faults are recognized at the northern boundary of the bellingham basin which is 15 km north of the international border there is no compelling tectonic reason to expect that holocene faults are limited to south of the international border

seismic amplification within the seattle basin washington state insights from ships seismic tomography experiments recent observations indicate that the seattle sedimentary basin under lying seattle and other urban centers in the puget lowland washington amplifies long period 1 5 sec weak ground motions by factors of 10 or more we computed east trending p and s wave velocity models across the seattle basin from seismic hazard investigations of puget sound ships experiments to better characterize the seismic hazard the basin poses the 3d tomographic models which resolve features to a depth of 10 km for the first time define the p and s wave velocity structure of the eastern end of the basin the basin which contains sedimentary rocks of eocene to holocene is broadly symmetric in east west section and reaches a maximum thickness of 6 km along our profile beneath north seattle a comparison of our velocity model with coincident amplification curves for weak ground motions pro duced by the 1999 chi chi earthquake suggests that the distribution of quaternary deposits and reduced velocity gradients in the upper part of the basement east of seattle have significance in forecasting variations in seismic wave amplification across the basin specifically eastward increases in the amplification of 0 2 to 5 hz energy correlate with locally thicker unconsolidated deposits and a change from crescent formation basement to pre tertiary cascadia basement these models de fine the extent of the seattle basin the seattle fault and the geometry of the basement contact giving insight into the tectonic evolution of the seattle basin and its influence on ground shaking