Curt D. Peterson

Summary of Coastal Geologic Evidence for Past Great Earthquakes at the Cascadia Subduction Zone

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

Evidence for eight great earthquake-subsidence events detected with ground-penetrating radar, Willapa barrier, Washington

A new approach to detect Holocene subduction-zone earthquakes combines the results from ground-penetrating radar (GPR), Vibracores, and accelerator mass spectrometry (AMS) dates from a barrier spit located west of Willapa Bay, southwest Washington. GPR data show a 10-m-thick facies of beach sand within which we identify, and Vibracores confirm, beach-parallel, wave-eroded, buried scarps mantled with multiple beds of magnetite. The eight GPR-detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subsidence during earthquakes associated with the Juan de Fuca plate subducting under the North American plate. Of these scarps, four have been AMS dated at 300, 1110, 2540, and 4250 (radiocarbon) yr B.P. No datable material has yet been found for the other four radar-detected scarps, but we interpolate and extrapolate dates of 1800, 3400, 5000, and 5800 yr B.P.

Mapped Overland Distance of Paleotsunami High-Velocity Inundation in Back-Barrier Wetlands of the Central Cascadia Margin, U.S.A.

Investigations of back‐barrier, open‐coastal plain settings have been used to establish minimum inundation distances of prehistoric tsunamis produced by great subduction zone earthquakes in the central Cascadia margin. Distinctive sand sheets were characterized at four localities within the central Cascadia margin, a shoreline distance of about 250 km. The sand sheets vary in thickness from 0.2 to 25 cm. They thin in the landward direction and consist of well‐sorted beach sand that fines upsection. Many of the sand sheets include capping layers of organic‐rich detritus, as well as assimilated mud rip‐up clasts and soil litter. Marine diatoms and bromine (i.e., marine tracers) were used to confirm marine surge origins for the anomalous sand sheets. Radiocarbon dating of the sand sheets demonstrates correspondence with reported great Cascadia earthquake events at 0.3, ∼1.1, ∼1.3, ∼1.7, and ∼2.5 Ka. One sand sheet mapped at all four localities is dated at 600–950 calibrated radiocarbon years before present. This interpreted paleotsunami event does not correspond to a central Cascadia rupture, so it is tentatively assigned to a far‐field source. Minimum overland inundation distances of the near field (Cascadia tsunami) at the four study localities range from 0.3 to 1.3 km, with a mean inundation for all sand sheets of 0.5 km.

Sediment Composition and Hydrography in Six High-Gradient Estuaries of the Northwestern United States

ABSTRACT Small estuaries of the northwestern United States are fed by high-gradient streams which range widely in mean fluvial-discharge rates. Sediment composition in these estuaries differs with grain size and with the relative abundances of river- and beach-derived sand. In order to determine to what degree sediment composition is related to the relative influence of tidal and river flow in these active-margin estuaries, the modern sediment compositions in six Pacific Northwest estuaries were analyzed by standard grain-size and heavy-mineral techniques. The average textural and sand-source compositions for each bay were calculated on a percent surface-area basis. Estuarine sediment compositions are compared and found to be correlated with a hydrographic parameter HR (mean tidal-p ism volume: mean fluvial discharge rate 6 hours) computed for each bay. Both % mud and % beach sand increase as the dimensionless hydrographic ratio (HR) increases. Unusual results in two of the estuaries are attributed to man-made alterations in one estuary and to eolian transport of beach sand into the other estuary.

Tsunami Runup Survey along the Southeast Indian Coast

The 26 December Indian Ocean tsunami was an extraordinary event in the history of natural hazards. It severely affected many countries surrounding the Indian Ocean: Indonesia, Thailand, Malaysia, Myanmar, Bangladesh, India, Sri Lanka, the Maldives, and African countries. Unlike the previous tsunami events in the last 40 years, the seriously affected areas are so vast that a traditional ground-level tsunami survey covering all the necessary areas by a single survey team was impractical. This destructive event will undoubtedly provide many opportunities to explore both basic and applied research in tsunami science and engineering fields and will lead to better preparedness for future disasters. A tsunami runup survey was conducted that spans Vedaranniyam (10° 23.5′ N) to Vodarevu (15° 47.6′ N)—more than 600 km of the southeast Indian coast—which suffered from the distant tsunami, whose source was more than 1,500 km away.

Coseismic Subsidence and Paleotsunami Run-Up Records from Latest Holocene Deposits in the Waatch Valley, Neah Bay, Northwest Washington, U.S.A.: Links to Great Earthquakes in the Northern Cascadia Margin

ABSTRACT Peterson, C.D.; Cruikshank, K.M.; Darienzo, M.E.; Wessen, G.C.; Butler, V.L., and Sterling, S.L., 2013. Coseismic subsidence and paleotsunami run-up records from latest Holocene deposits in the Waatch Valley, Neah Bay, northwest Washington, U.S.A.: links to great earthquakes in the northern Cascadia Margin. Representative shallow cores (1–2-m depth) from the Waatch Valley (n = 10) and from Neah Bay back-barrier wetlands (n = 7) record four coseismic subsidence events and associated paleotsunami inundations during the last 1300 years in the North Central Cascadia Margin. Three of the subsidence events (SUB1, SUB2b, and SUB3) correlate to reported great earthquakes dated at AD 1700, about 1.1 ka, and about 1.3 ka. An additional subsidence horizon (SUB2a), which is newly discovered in the study area, might correlate to a widely reported paleotsunami inundation, dated between 0.7 and 0.9 ka in the study region. The magnitudes of paleosubsidence in the Waatch Valley are modest (about 0.5−1.0 m), as based on macofossil evidence of abrupt wetland burial. Paleotsunami origins of the four landward thinning sand sheets are confirmed by the presence of ocean diatom taxa and beach sand grains. Long wave run-up in the low-gradient Waatch floodplain ranged from 2.5 to 4.5 km up-valley distance from the present tidal inlet shoreline. Paleotsunami overtopping of the Neah Bay barrier ridge (6–8-m elevation North American Vertical Datum of 1988 [NAVD88]) provides the first estimates of paleotsunami minimum run-up height at the entrance to the Juan de Fuca Strait.

Correlation of Tephra Marker Beds in Latest Pleistocene and Holocene Fill of the Submerged Lower Columbia River Valley, Washington and Oregon, U.S.A

Abstract Peterson, C.; Minor, R.; Gates, E.B.; Vanderburgh, S., and Carlisle, K., 2012. Correlation of tephra marker beds in latest Pleistocene and Holocene fill of the submerged Lower Columbia River Valley, Washington and Oregon, U.S.A. Seven tephra layers in the submerged fill of the Lower Columbia River Valley (LCRV) are radiocarbon dated (0.5–13 ka in age). They are correlated to reported eruption sources in the adjacent Cascade Range volcanic arc. The tephras provide a stratigraphic framework for the transgressive depositional filling of the tidal Columbia River system (∼200 km in length). The thickest and most continuous tephra marker beds in the LCRV are from the Mount Mazama set-O eruption at 7.7 ka. The eruption source for this tephra (1–10 m thick) is confirmed by glass geochemistry and radiocarbon dating of borehole samples from depths of −13 to −38 m (North American Vertical Datum of 1988 [NAVD88]). The Mazama set-O tephra conveniently divides the depositional sequences of the Early Holocene from the Late Holocene in the deep valley fill (50–120 m axial valley depth). A much deeper tephra, at −84 m, is attributed to Mount St. Helens set-S/J eruptions at ∼13 ka, which confirms the end of the Glacial Lake Missoula Flooding in the LCRV. Late Holocene tephra marker beds in the LCRV include the Mount St. Helens set-J/P eruptions (2.5–4.0 ka), set-W/T eruptions (∼AD 1480), and an intermediate-depth tephra, attributed to a Mount St. Helens or Mount Adams eruption (∼1.3 ka). The Late Holocene tephras date the ages of tidal flats and extensive floodplains throughout the length of the LCRV. The Late Holocene marker beds provide the first widespread constraints on wetland sedimentation rates and preservation potentials for archaeology sites in the LCRV.

Accommodation Space Controls on the Latest Pleistocene and Holocene (16–0 ka) Sediment Size and Bypassing in the Lower Columbia River Valley: A Large Fluvial–Tidal System in Oregon and Washington, USA

ABSTRACT Peterson, C.D.; Gates, E.B.; Minor, R., and Baker, D.L., 2013. Accommodation space controls on the latest Pleistocene and Holocene (16–0 ka) sediment size and bypassing in the Lower Columbia River Valley: a large fluvial–tidal system in Oregon and Washington, USA. In this study, we establish the roles that increasing basin accommodation space have on sediment size and bypassing in the transgressive fill (16–0 ka) in the submerged Lower Columbia River Valley (LCRV). The antecedent forearc valley (225 km in length, 4–8 km in width, and 60–115 m in axial valley depth) is characterized by high sediment supply rates (10–15 million t y−1) but no delta at its mouth to the Pacific Ocean. Core sample sediment textures (N = 1600) are analyzed from 3000 m of borehole sections in 58 representative boreholes to characterize the ancestral valley fill: 57% sand, 17% muddy sand, 12% sandy mud, and 14% mud (total fill volume = 77 km3). Decreasing mud endmember texture from between 30 and 60% (early Holocene) to 18% (late Holocene) is directly related to (1) declining rates of increasing basin accommodation space and (2) increasing sediment bypassing through the LCRV. Both conditions result from decreasing rates of sea level rise (15.0–1.5 mm y−1) in Holocene time. The overall coarsening of sediments, both upvalley and upsection, is controlled by declining rates of increasing basin accommodation space (5.66 million–0.57 million m3 y−1) during middle to late Holocene time. Differences between middle and late Holocene sediment accumulation rates yield bedload bypassing rates of 5– 6 million m3 y−1, under assumed conditions of constant sediment supply rates in middle to late Holocene time.

Distal Run-up Records of Latest Holocene Paleotsunami Inundation in Alluvial Flood Plains: Neskowin and Beaver Creek, Oregon, Central Cascadia Margin, West Coast U.S.A.

Abstract Paleotsunami records in two localities of the central Cascadia margin, Neskowin and Beaver (West Coast, U.S.A., Northeast Pacific Ocean coast), are extended landward to distal flood plain settings. Three paleotsunami sand sheets are correlated to Cascadia subduction zone earthquakes, between 0.3 and ∼1.3 ka in age. One older paleotsunami layer (2960–3220 cal YBP) is apparent in some deeper core sites from the Beaver Creek locality. Marine sand (22%–100%) and marine diatoms (40%–100%) from the distal sand sheets distinguish the catastrophic marine inundations from creek floods. The greatest inundations are correlated to two Cascadia paleotsunami events, #3 at ∼1.3 ka and an older event between ∼2.6 and ∼3.2 ka, based on radiocarbon dating and great earthquake sequence. The best-preserved records are from paleotsunami #3, which reached 4.1 km in overland inundation up the North Beaver flood plain (3 m elevation North American Vertical Datum). At the Neskowin locality, a sand sheet from the #3 paleotsunami was traced to 8.3 m elevation in the Hawk flood plain. Adjusting for paleosea level at 1.3 ka, we estimate that the #3 paleotsunami run-up height reached 9 m at a landward distance of 1.0 km in Neskowin. The paleotsunami sand sheets in Neskowin and Beaver represent the maximum recorded distal run-up for Cascadia paleotsunami reported to date. The potential for preservation of marine surge deposits in alluvial flood plains should greatly extend the geologic record of prehistoric inundations in other susceptible coastlines.

Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA

The Seaside beach ridge plain was inundated by six paleotsunamis during the last ~2500 years. Large runups (adjusted >10 m in height) overtopped seawardmost cobble beach ridges (7 m elevation) at ~1.3 and ~2.6 ka before present. Smaller paleotsunami (6–8 m in height) likely entered the beach plain interior (4-5 m elevation) through the paleo-Necanicum bay mouth. The AD 1700 Cascadia paleotsunami had a modest runup (6-7 m height), yet it locally inundated to 1.5 km landward distance. Bed shear stresses (100–3,300 dyne ) are estimated for paleotsunami surges (0.5–2 m depths) that flowed down slopes (0.002–0.017 gradient) on the landward side of the cobble beach ridges. Critical entrainment shear stresses of 1,130–1,260 dyne were needed to dislodge the largest clasts (26–32 cm diameter) in paleotsunami coulees that were cut (100–200 m width) into the landward side of the cobble ridges.