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Geologic Hazards on the Oregon Coast -
Earthquakes and tsunamis documented at southern Oregon coast

From the Spring, 2002 issue of Oregon Geology

Research by California scientists Harvey Kelsey and Eileen Hemphill-Haley of Humboldt State University and Robert C. Witter of William Lettis and Associates, Walnut Creek, has found evidence of 11 large, tsunami-producing earthquakes that occurred off the Pacific Northwest coast in the past 6,000 years.

The research was centered on 28 cores from wetland sediments in an abandoned-meander wetland of the lower Sixes River, just inland from Cape Blanco in southwest Oregon, and the scientists reported on it in the March, 2002 issue of the Geological Society of America Bulletin (v. 114, no. 3, p. 298-314).

Ten suddenly submerged wetland soils were identified, covered first with sand and then with tidal mud, documenting abrupt tectonic subsidence of at least 0.5 m and, in some cases, as much as 2.4 m.

Supported by other research, such as diatom biostratigraphy, radiocarbon dating, and the discovery of shaking-induced liquefaction features in sand bodies, this evidence points to 10 subduction-zone earthquakes, each of them accompanied by a tsunami that spread sand from the beach as much as 3 km inland.

An additional buried soil at the nearby mouth of the Sixes River documents the 11th and youngest earthquake, that of A.D. 1700.

The 11 earthquakes and tsunamis took place roughly between 300 and 5,400 years ago, with an average occurrence interval of about 510 years
(Note: Scientists estimate the chance in the next 50 years of a great subduction zone earthquake is between 10 and 20 percent, assuming that the recurrence is on the order of 400 ± 200 years. The last great subduction zone earthquake occurred about 300 years ago).

Comparison with similar evidence from sites in southwest Washington and the Coos Bay and Coquille River areas suggests that not all these earthquakes ruptured the entire Cascadia Subduction Zone from southern Washington to southern Oregon. ¨

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