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Seismic or hydrodynamic control of rapid late-Holocene sea-level rises in southern coastal Oregon, USA?

US Geological Survey, MS 966, PO Box 25046, Denver, CO 80225 USA

Yoko Ota

Department of Geography, Senshu University, Ikuta Campus, 1-1, Higashimita 2-Chrome, Tama-ku Kawasaki-shi, Kanagawa 214, Japan

Masatomo Umitsu

Department of Geography, University of Nagoya, Chikusa-ku, Nagoya 464-01, Japan

Kaoru Kashima

Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 810-81, Japan

Yoshiaki Matsushima

Kanagawa Prefectural Museum of Natural History, 499, Iryuda, Odawara, 250, Japan

Intertidal stratigraphy has been instrumental in demonstrating the hazard posed by great earthquakes at the Cascadia subduction zone, but inferring an earthquake history from interbedded sequences of peat and mud is complicated by many factors that influence sedimentation and relative sea-level change on both tectonic and nontectonic coasts. Rapid-to-sudden rises in relative sea level marked by sharp contacts between intertidal peat and overlying mud or sand may reflect coseismic coastal subsidence and tsunami deposition or, alternatively, nonseismic hydrodynamic changes in estuaries. Reconnaissance coring at 16 sites in the marshes fringing a narrow, protected tidal inlet of Coos Bay, supplemented by diatom and ¹⁴C analyses at four sites, reveals a stratigraphic record too fragmentary and ambiguous to distinguish seismic from hydrodynamic causes for more than three of the 10 rises in relative sea-level identified. Only three sharp contacts have the wide extent and evidence of substantial (>0.5 m) submergence that distinguish them from similar contacts produced by nonseismic processes. Correlation with stratigraphic sequences at other estuaries shows that the fringing marshes suddenly subsided and were partially buried by tsunami sand during a great plate-boundary earthquake about 300 years ago. Similar contacts were produced by earthquakes about 1500–1800 years ago, and perhaps about 2400–2700 years ago. Other earthquakes with substantially less subsidence may also have occurred, but evidence is too ambiguous to reconstruct a more complete history.

Key Words: Paleoseismology • earthquake hazard • Cascadia subduction zone • intertidal diatoms • sea-level changes • salt-marsh stratigraphy • tsunami deposits • Oregon • North America

The Holocene, Vol. 8, No. 3, 287-299 (1998)
DOI: 10.1191/095968398668600476


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