Tsunami history of an Oregon coastal lake reveals a 4600 yr record of great earthquakes on the Cascadia subduction zone

Abstract

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.

Sand Sheets on a Beach-Ridge Plain in Thailand: Identification and Dating of Tsunami Deposits in a Far-Field Tropical Setting

“…Moist environments such as beach ridge swales and coastal lakes preserve tsunami deposits in other environments (e.g. Pinegina et al, 2003;Kelsey et al, 2005) and in Thailand the equivalent environments are beach ridge swales and mangrove swamps. In our fieldwork we learned that the 2004 tsunami deposits are not well preserved in mangrove environment, mainly because of burrowing by crabs.…”

“…Paleoseismological and paleotsunami records, if correctly identified and dated, can extend the chronology of earthquake and tsunami of the area for several thousand of years beyond the historic time (e.g. Kelsey et al, 2005). To improve understanding of the risk from large Indian Ocean tsunamis, there is a need to study tsunami deposits in order to extend the tsunami record in this region.…”

“…Paleotsunami deposits are often identified in lowenergy coastal environments, such as tidal marsh, back-barrier marshes and coastal lakes. Most publications on paleotsunami studies during the past two decades are from temperate zones such as the north Atlantic (Bondevik et al, 1997;Bondevik et al, 2003;Dawson et al, 1988), the circum-Pacific shores of the Americas and Asia (Atwater & Moore, 1992;Atwater et al, 2005;Bourgeois et al, 2006;Cisternas et al, 2005;Fiedorowicz & Peterson, 2002;Kelsey et al, 2005;Minoura & Nakata, 1994;Nanayama et al, 2003;Tuttle et al, 2004) Goff et al, 2001;Goff et al, 1998;Grauert et al, 2001). In many cases tsunami studies occur in conjunction with other evidence for large earthquakes such as the study of coastal geological evidence of past tsunamis and associated post seismic uplift in Japan by Sawai et al (2009b).…”

Proximal Records of Paleotsunami Runup in Barrage Creek Floodplains from Late-Holocene Great Earthquakes in the Central Cascadia Suduction Zone, Oregon, USA

“…Investigations of the associated nearfield paleotsunami inundations moved from the coastal marshes to overland inundation recorded in lakes, beach plains, back-barrier wetlands, and barrage lakes (Hutchinson et al, 1997;Kelsey et al, 2005;Schlichting & Peterson, 2006). The geologic records of paleotsunami sand sheet deposition that are located landward of foredunes, beach ridges, and other unstable coastal barriers provide minimum estimates of runup height .…”

地層中に存在する古津波堆積物の調査

“…22 Nanayama et al, 2003;, 17 Sawai, 2002, 300 , 1987869 Sawai et al, 2008, 1999;, 2012John Clague , Hutchinson et al, 1997Clague et al, 1999Clague et al, , 2000López, 2012 Bradly Lake 4600 Kelsey et al, 2005 British Virgin Atwater et al, 2010;Reinhardt et al, 2011, Bondevik et al, 19971 Atwater, 1987Atwater et al, 2005Pinegina et al, 2003, 2012 10 m , Goff et al, 2004;Shi et al, 19951992Higman and Bourgeois, 2008, Paris et al, 2007Sawai et al, 2009b , suspension ; foredune , 1994 Sawai et al, 2009a, Kench et al, 2006Moore et al, 2007Moore et al, , 2004Moore et al, , , 1994Moore and Moore, 1984;Goto et al, 2007Horton et al 20112010Nanayama and Shigeno 2006Donato et al, 2007, Paris et al, 2010Tanaka et al, 2012Hemphill-Haley, 19951996;, 1996;, 2002;, 2004;…”