Great Cascadia Earthquake Research Articles

Post-glacial stratigraphy and late Holocene record of great Cascadia earthquakes in Ozette Lake, Washington, USA

Ozette Lake is an ~100-m-deep coastal lake located along the outer coast of the Olympic Peninsula (Washington, USA); it is situated above the locked portion of the northern Cascadia megathrust but also relatively isolated from active crustal faults and intraslab earthquakes. Here we present a suite of geophysical and geological evidence for earthquake-triggered mass transport deposits (MTDs) and related turbidite deposition in Ozette Lake since ca. 14 ka. Comprehensive high-resolution bathymetry data, seismic reflection profiles, and sediment cores are used to characterize the post-glacial stratigraphic framework and examine paleoseismic evidence in the lacustrine sediments. Stacked sequences of MTDs along the steep eastern flanks of the lake appear to grade basin-ward from thick, chaotic, blocky masses to thin, parallel-bedded turbidite beds. The discrete turbidite event layers are separated by fine-grained (silt and clay) lake sedimentation. The event layers are observed throughout the lake, but the physical characteristics of the deposits vary considerably depending on proximity to primary depocenters, steep slopes, and subaqueous deltas. A total of 30–34 event deposits are observed in the post-glacial record. Radiometric dating was used to reconstruct a detailed sedimentation history over the last ~5.5 k.y., develop an age model, and estimate the recurrence (365–405 yr) for the most recent 12 event layers. Based on sedimentological characteristics, temporal overlap with other regional paleoseismic chronologies, and recurrence estimates, at least 10 of the dated event layers appear to be sourced from slope failures triggered by intense shaking during megathrust ruptures; the recurrence interval for these 10 events is 440–560 yr. Thus, Ozette Lake contains one of the longest and most robust geological records of repeated shaking along the northern Cascadia subduction zone.

A new chronology for tsunami deposits prior to the 1700 CE Cascadia earthquake from Vancouver Island, Canada

Coastal deposits at Tofino, Ucluelet, and Port Alberni in Vancouver Island along the Cascadia subduction zone were re-examined to improve the earthquake history of the southwest coast of Canada. We found sand sheets interbedded within peat and mud, suggesting deposition by strong flows in a low-energy environment. Based on limiting maximum and minimum ages derived from plant macrofossils, the age of one of the sand sheets below the tsunami deposits of the great Cascadia earthquake in 1700 CE was estimated to be 1330–1430 CE. Onshore paleoseismic evidence has been documented in Vancouver Island, northern Washington, and northern Oregon during this period. However, the newly constrained age is between those of coseismic subsidence Y and W events in southern Washington, which have been recognized as the 1700 CE and the penultimate Cascadia earthquakes, respectively. Moreover, the new age partly overlaps with the age of offshore paleoseismic evidence for T2, interpreted to have originated from the penultimate Cascadia earthquake, based on offshore turbidite records. The new chronology prior to the 1700 CE Cascadia tsunami deposit from Vancouver Island contributes to a better understand of the timing of the penultimate Cascadia earthquake.

Sediment gravity flows triggered by remotely generated earthquake waves

AbstractRecent great earthquakes and tsunamis around the world have heightened awareness of the inevitability of similar events occurring within the Cascadia Subduction Zone of the Pacific Northwest. We analyzed seafloor temperature, pressure, and seismic signals, and video stills of sediment‐enveloped instruments recorded during the 2011–2015 Cascadia Initiative experiment, and seafloor morphology. Our results led us to suggest that thick accretionary prism sediments amplified and extended seismic wave durations from the 11 April 2012 Mw8.6 Indian Ocean earthquake, located more than 13,500 km away. These waves triggered a sequence of small slope failures on the Cascadia margin that led to sediment gravity flows culminating in turbidity currents. Previous studies have related the triggering of sediment‐laden gravity flows and turbidite deposition to local earthquakes, but this is the first study in which the originating seismic event is extremely distant (> 10,000 km). The possibility of remotely triggered slope failures that generate sediment‐laden gravity flows should be considered in inferences of recurrence intervals of past great Cascadia earthquakes from turbidite sequences. Future similar studies may provide new understanding of submarine slope failures and turbidity currents and the hazards they pose to seafloor infrastructure and tsunami generation in regions both with and without local earthquakes.

Multiple Reoccupations after Four Paleotsunami Inundations (0.3–1.3 ka) at a Prehistoric Site in the Netarts Littoral Cell, Northern Oregon Coast, USA

Netarts Bay is the setting of one of the largest concentrations of late prehistoric Native American settlements on the tectonically active Oregon coast. A prehistoric site (35TI74) exposed by sea cliff erosion in 1998 at the south end of the Netarts littoral cell contained a stratigraphic record of activity by prehistoric Native Americans and interbedded paleotsunami deposits. The nearfield paleotsunamis were produced by great Cascadia earthquakes (Mw8.5 ± 0.5) emanating from the Cascadia Subduction Zone offshore, as previously documented in episodically buried tidal marsh surfaces at the south end of Netarts Bay and in many other estuaries in the region. The cultural deposits at 35TI74 reflect multiple reoccupations by Native Americans at this barrage creek flood plain site between paleotsunami runup events (≥7.5 m elevation) at 1.3 ka, 1.1 ka, 0.8–0.9 ka, and 0.3 ka. The conditions that led to the exposure of 35TI74 are traced to erosion of a protective beach dune ramp at the south end of the Netarts littoral cell, placing the site within reach of progressive storm‐surf erosion. The diminishing geoarchaeological record at 35TI74 represents a microcosm of the sea cliff erosional processes that threaten vulnerable prehistoric shoreline archaeological sites along much of the Pacific Northwest Coast.

Holocene Earthquake History of Cascadia: A Quantitative Test

Abstract Motivated by the Holocene history of great Cascadia earthquakes proposed by Goldfinger et al. (2012), we developed a simple model by which we can examine this further. Combining their observation that the plate boundary ruptures in four characteristic segment types with an empirical area versus seismic moment relation and an assumption of constant stress drop, we obtain a model for the along‐strike slip distribution for each earthquake type. Summing the slips in each segment over the number of earthquake of all types produces estimates of Holocene plate convergence that quantitatively agree with independent estimates obtained from geodetic and geologic data. Because the width of the fully coupled subduction interface decreases to the south, the assumption of constant average stress drop in earthquakes requires that their slip also decreases to the south. To satisfy the plate convergence constraints, this requires shorter and increasingly numerous earthquakes to finger in toward the south, just as observed in the Goldfinger et al. (2012) paleoseismic record. We conclude that the Goldfinger et al. (2012) history is quantitatively consistent with plate tectonic constraints, and that the Cascadia subduction zone is fully coupled seismically. The seismic flux release rate averaged over the Holocene is estimated as 2.7×10 9 m 3 /yr.

Re-Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia

Research Article| November 26, 2013 Re‐Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia N. M. Beeler; N. M. Beeler U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Evelyn Roeloffs; Evelyn Roeloffs U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Wendy McCausland Wendy McCausland U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Author and Article Information N. M. Beeler U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Evelyn Roeloffs U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Wendy McCausland U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2014) 104 (1): 128–144. https://doi.org/10.1785/0120120022 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. M. Beeler, Evelyn Roeloffs, Wendy McCausland; Re‐Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia. Bulletin of the Seismological Society of America 2013;; 104 (1): 128–144. doi: https://doi.org/10.1785/0120120022 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Mazzotti and Adams (2004) estimated that rapid deep slip during typically two week long episodes beneath northern Washington and southern British Columbia increases the probability of a great Cascadia earthquake by 30–100 times relative to the probability during the ∼58 weeks between slip events. Because the corresponding absolute probability remains very low at ∼0.03% per week, their conclusion is that though it is more likely that a great earthquake will occur during a rapid slip event than during other times, a great earthquake is unlikely to occur during any particular rapid slip event. This previous estimate used a failure model in which great earthquakes initiate instantaneously at a stress threshold. We refine the estimate, assuming a delayed failure model that is based on laboratory‐observed earthquake initiation. Laboratory tests show that failure of intact rock in shear and the onset of rapid slip on pre‐existing faults do not occur at a threshold stress. Instead, slip onset is gradual and shows a damped response to stress and loading rate changes. The characteristic time of failure depends on loading rate and effective normal stress. Using this model, the probability enhancement during the period of rapid slip in Cascadia is negligible (<10%) for effective normal stresses of 10 MPa or more and only increases by 1.5 times for an effective normal stress of 1 MPa. We present arguments that the hypocentral effective normal stress exceeds 1 MPa. In addition, the probability enhancement due to rapid slip extends into the interevent period. With this delayed failure model for effective normal stresses greater than or equal to 50 kPa, it is more likely that a great earthquake will occur between the periods of rapid deep slip than during them. Our conclusion is that great earthquake occurrence is not significantly enhanced by episodic deep slip events. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Are great Cascadia earthquakes recorded in the sedimentary records from small forearc lakes?

Abstract. Here we investigate sedimentary records from four small inland lakes located in the southern Cascadia forearc region for evidence of earthquakes. Three of these lakes are in the Klamath Mountains near the Oregon–California border, and one is in the central Oregon Coast range. The sedimentary sequences recovered from these lakes are composed of normal lake sediment interbedded with disturbance event layers. The thickest of these layers are graded, and appear to be turbidites or linked debrites (turbidites with a basal debris-flow deposit), suggesting rapid deposition. Variations in particle size and organic content of these layers are reflected in the density and magnetic susceptibility data. The frequency and timing of these events, based on radiocarbon ages from detrital organics, is similar to the offshore seismogenic turbidite record from trench and slope basin cores along the Cascadia margin. Stratigraphic correlation of these anomalous deposits based on radiocarbon ages, down-core density, and magnetic susceptibility data between lake and offshore records suggest synchronous triggering. The areal extent and multiple depositional environments over which these events appear to correlate suggest that these deposits were most likely caused by shaking during great Cascadia earthquakes.

Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates

AbstractPast earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3‐D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along‐strike slip heterogeneity, with multiple patches of high‐moment release separated by areas of low‐moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high‐quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.

Recurrence intervals of major paleotsunamis as calibrated by historic tsunami deposits in three localities: Port Alberni, Cannon Beach, and Crescent City, along the Cascadia margin, Canada and USA

We review geologic records of both historic and prehistoric tsunami inundations at three widely separated localities that experienced significant damage from the 1964 Alaskan tsunami along the Cascadia margin. The three localities are Port Alberni, Cannon Beach, and Crescent City, representing, respectively, the north, central, and south portions of the study area (1,000 km in length). The geologic records include anomalous sand sheets from marine surges that are hosted in supratidal peaty mud deposits. Paleotsunami sand sheets that exceed the thickness, continuity and/or extent of the 1964 historic tsunami are counted as major paleotsunami inundations. Major paleotsunamis (6–7 in number) at each locality during the last 3,000 years demonstrate mean recurrence intervals of 450–540 years, and within-cluster intervals (three events each) of 270–460 years. It has been 313 years since the last major paleotsunami from a great Cascadia earthquake in AD 1700. We compare the dated sequences of major paleotsunami inundations to the nearest regional records of coastal coseismic subsidence in Willapa Bay in the central margin, Waatch/Neah Bay in the northern margin, and Coquille in the southern margin. Similar numbers of events from both types of records suggest that the major paleotsunamis are locally derived (near-field) from ruptures of the Cascadia margin megathrust fault zone, rather than from transoceanic tsunamis (far-field) originating at other subduction zones around the Pacific Rim. Given the catastrophic hazard of the near-field Cascadia margin tsunamis, we propose a basic rule for reminding the general public of the need for self-initiated evacuation following a great earthquake at the Cascadia margin.

Modern foraminifera, δ13C, and bulk geochemistry of central Oregon tidal marshes and their application in paleoseismology

We assessed the utility of δ13C and bulk geochemistry (total organic content and C:N) to reconstruct relative sea-level changes on the Cascadia subduction zone through comparison with an established sea-level indicator (benthic foraminifera). Four modern transects collected from three tidal environments at Siletz Bay, Oregon, USA, produced three elevation-dependent groups in both the foraminiferal and δ13C/bulk geochemistry datasets. Foraminiferal samples from the tidal flat and low marsh are identified by Miliammina fusca abundances of >45%, middle and high marsh by M. fusca abundances of <45% and the highest marsh by Trochamminita irregularis abundances >25%. The δ13C values from the groups defined with δ13C/bulk geochemistry analyses decrease with an increasing elevation; −24.1±1.7‰ in the tidal flat and low marsh; −27.3±1.4‰ in the middle and high marsh; and −29.6±0.8‰ in the highest marsh samples. We applied the modern foraminiferal and δ13C distributions to a core that contained a stratigraphic contact marking the great Cascadia earthquake of AD 1700. Both techniques gave similar values for coseismic subsidence across the contact (0.88±0.39m and 0.71±0.56m) suggesting that δ13C has potential for identifying amounts of relative sea-level change due to tectonics.

Evidence for earthquake triggering of large landslides in coastal Oregon, USA

Landslides are ubiquitous along the Oregon coast. Many are large, deep slides in sedimentary rock and are dormant or active only during the rainy season. Morphology, observed movement rates, and total movement suggest that many are at least several hundreds of years old. The offshore Cascadia subduction zone produces great earthquakes every 300–500years that generate tsunami that inundate the coast within minutes. Many slides and slide-prone areas underlie tsunami evacuation and emergency response routes. We evaluated the likelihood of existing and future large rockslides being triggered by pore-water pressure increase or earthquake-induced ground motion using field observations and modeling of three typical slides. Monitoring for 2–9years indicated that the rockslides reactivate when pore pressures exceed readily identifiable levels. Measurements of total movement and observed movement rates suggest that two of the rockslides are 296–336years old (the third could not be dated). The most recent great Cascadia earthquake was M 9.0 and occurred during January 1700, while regional climatological conditions have been stable for at least the past 600years. Hence, the estimated ages of the slides support earthquake ground motion as their triggering mechanism. Limit-equilibrium slope-stability modeling suggests that increased pore-water pressures could not trigger formation of the observed slides, even when accompanied by progressive strength loss. Modeling suggests that ground accelerations comparable to those recorded at geologically similar sites during the M 9.0, 11 March 2011 Japan Trench subduction-zone earthquake would trigger formation of the rockslides. Displacement modeling following the Newmark approach suggests that the rockslides would move only centimeters upon coseismic formation; however, coseismic reactivation of existing rockslides would involve meters of displacement. Our findings provide better understanding of the dynamic coastal bluff environment and hazards from future subduction-zone earthquakes.

The first ocean floor evidence of great Cascadia earthquakes

It can be argued that Japan's giant 11 March 2011 Tohoku earthquake and its tsunami should not have come as geophysical surprises. GPS had shown that the entire subduction zone along the Japan Trench behaves as one enormous unit rather than segmented sections that rupture with different frequencies and strengths [Suwa et al., 2006]. Historical documents and coastal sand deposits had shown a history of Sendai area tsunamis larger than those of recent centuries [Sawai et al., 2008]. Such hindsight brings to mind the Cascadia subduction zone, with its history of great earthquakes that, despite being literally in the backyards of many Pacific Northwest geologists and geophysicists, went unrecognized until late in the twentieth century. In 1969, at the dawn of the development of plate tectonic theory, scientists lacked a record of significant historical earthquakes along the U.S. northwestern coast and had no reason to believe that there were any seismic hazards in the region. What was known was that deep‐sea cores taken offshore in 1965–1967 contained extensive turbidity current deposits— graded beds of sand and silt— interbedded with pelagic clays, sequences that by 1969 I had just finished studying for my doctoral dissertation. At the time, those turbidites (the sediments deposited by turbidity currents or submarine mudflows) posed a great enigma. What process or disturbance could have generated these recurring turbidity currents?

Tsunamis and Floods in Coos Bay Mythology

Tsunamis and Floods in Coos Bay Mythology byPatricia Whereat Phillips The principal evidence for major prehistoricearthquakes associatedwith subduction comes from investigations of estuarine marsh sediments buried by sand layers. These deposits suggest that portions ofthe coast subsided abruptly and were then overwashed by extreme tsunamis, or "tidal waves," that swept over the area and deposited the sand_ The number of these layers ... indicate that catastrophic earthquakes have occurred along the Northwest coast at least six times in the past 7,000 years, at intervals ranging from 300 to 600 years, with the last having occurred about 300 years ago. ? Paul Komar1 TSUNAMIS ARE TERRIFYING anddestructive events but relatively infre quent from the standpoint of a human lifetime. It isno wonder, then, that tsunamis appear in themythology of coastal peoples. To my knowledge, no one has looked in depth at a particular Oregon coastal people's language and theirmythology to assess the scope and effectof these events from the point of view ofNative people. Iwill focus on themythology of the Coos Bay Indians and on theHanis language (along with Milluk, itwas one of the two languages thatwas spoken on Coos Bay), as it is the one I am the most familiar with. Great Cascadia Earthquakes and Tsunamis 181 The linguistic data on this subject are unfortunately quite limited. The sole phrase recorded in theHanis language for earthquake is tVta lishtats. It translates literally as 'earth shaking', from tl'ta (or ti'da) for 'earth, land, ground' and the verb root lish- 'to shake, tomove'.2 Although there are a handful ofCoos texts thatmention tsunamis, there are none explicitly about earthquakes. In various interviews with ethnologists and linguists, however, some mention of earthquakes has been recorded. In a 1942 interview with linguist John P.Harrington, Hanis Coos informant John Waters said, "My [grandmother] toldwhen years ago therewas a big earthquake that caused cracks in the ground."3 In 1881, when George Bissell was working on a vocabu lary listwith an unidentified Lower Umpqua woman, he listed s? hwass as aword for earthquake and noted next to iton his schedule that "y?ohy for a caving in."4The woman volunteering 'caving in' along with 'earthquake' hints that she had heard accounts of earthquakes. Several words and phrases describe high and low tides, which is to be expected in the vocabulary of a coastal people. Iwas able to find only one word inHanis Coos that can be translated as 'flood', maatl, but therewas no further definition of the semantic domain for theword. The word was recorded only once, as the title of a tsunami story told by James Buchanan, from theHanis village Wu'allach (near Empire). Maatl probably refers to flooding in general and not specifically tsunamis. More often in theHanis language, phrases are used todescribe tsunamis, as 'when theflood time came therewas no ebb tide', 'now thewater rushed in',and 'thewater ran ashore far across the land'. Iwill consider these descriptions within the context of the narratives inwhich they appear. Althoughdozens ofCoos myths (bothin Hanis andMilluk) and other narratives have been recorded, they represent only a small portion of the narratives that have existed. Most of the narratives that have come down to us are primarily from twoHanis-speaking informants ? JimBuchanan, who worked principally with Henry Hull St. Clair and Leo Frachtenberg in the first decade of the twentieth century, and Annie Minor Peterson, who worked with Melville Jacobs in 1933and 1934.Annie Peterson, who was born on Coos Bay nearWu'laench (Cooston) but raised at theYachats reservation, also spoke Milluk, the language spoken at Cape Arago and South Slough on Coos Bay. The traditional narratives can be divided loosely between myths and historical narratives. Throughout the Northwest, Indians recognized this division of their stories. As Melville Jacobs noted, Long ago, [Franz] Boas and others pointed to the larger Pacific Northwest's natives own contrast of myths versus tales. Boas wrote correctly that this larger region s myths 182 OHQ vol. 108, no. 2 Courtesy ofCoos Historical andMaritimeMuseum, 995-D185 Coos elderAnnieMiner Peterson displays her traditional regalia and baskets in Empire,Oregon, in 1914. comprised recitalsofhappenings dated inan earlyerawhen things were ratherdiffer ent from what they were in the more recent era that...

Native American Vulnerability and Resiliency to Great Cascadia Earthquakes

Native American Vulnerability and Resiliency to Great Cascadia Earthquakes

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.

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.

Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon

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Coastal flooding and beach retreat from coseismic subsidence in the central Cascadia margin, USA

Abstract Prehistoric great earthquakes (Mw 8-9) in the central Cascadia margin have produced coastal subsidence (0-2 m) that has resulted in chronic lowland flooding and catastrophic beach erosion. Geologic records of wetland burial in 13 bays from Washington and Oregon are used with simple calculations to estimate likely coastal flooding and beach retreat following a great Cascadia earthquake. Plant macrofossils and peat-to-mud ratios recorded in core logs discriminate between forest, marsh, colonizing marsh, and mud-flat tidal settings. Transitions between these settings, i.e., across abrupt burial contacts, demonstrate either 0+ or -0.5, 1+ or -0.5, or 2+ or -0.5 m of paleosubsidence. Paleosubsidence from the last Cascadia event (AD 1700) decreases from 2+ or -0.5 m in the eastern reaches of south-west Washington bays to 0+ or -0.5 m in the western reaches of central Oregon bays. First-order estimates of post-subsidence flooding hazards are based on the predicted regional subsidence added to current 10- and 100-year flood elevations. At least 525 km of bay shorelines are threatened by chronic flooding following coseismic subsidence. Catastrophic beach retreat is estimated from Bruun's Rule to range from 50 to 300 m depending on assumed depth of closure, measured beach-berm heights, and predicted coseismic subsidence along the margin. Shorelines that are susceptible to catastrophic beach retreat from coseismic subsidence total at least 250 km in longshore distance.

Relative sea-level change inferred from intertidal sediments beneath marshes on Vancouver Island, British Columbia

Abstract Relative sea-level change at the time of, and since, the most recent great earthquake at the Cascadia subduction zone is estimated from intertidal sediments at three marshes on western Vancouver Island, British Columbia. We compare the elevation of the pre-earthquake surface, which is marked by a tsunami sand sheet, with the modern depositional elevation range of the sediment type upon which the sand was deposited. At a site south of the Nootka fault zone, which is the northern boundary of the subducting Juan de Fuca plate, tidal mud overlies the pre-earthquake marsh surface. The stratigraphy at this site indicates 0.2–1.6 m of coseismic submergence and 1.1 m of subsequent emergence. In contrast, two sites to the north lack obvious stratigraphic evidence for coseismic land-level change and record between 0.1 and 1.7 m of post-earthquake submergence. These results indicate a difference in tectonic environment across the Nootka fault zone and suggest that plate-boundary rupture during the last great Cascadia earthquake probably did not extend north of central Vancouver Island.

Precise measurements help gauge Pacific Northwest's Earthquake potential

Except for the recent rumblings of a few moderate earthquakes and the eruption of Mt. St. Helen's, all has been relatively quiet on the Pacific Northwestern front. The Cascades region in the Pacific Northwest, a sporadically active earthquake and volcanic zone, still has great seismic potential [Atwater, 1987], as comparisons with other subduction zones around the world have shown [Heaton and Kanamori, 1984]. Recent tsunami propagation models [Satake, 1996] and tree ring studies suggest that the last great Cascadia earthquake occurred in the winter of 1700 A.D. and had a magnitude of −8.9. The North Cascades or Wenatchee earthquake followed in 1872. With an estimated magnitude greater than 7, it was the largest earthquake in the written history of Washington and Oregon.