Geology

Coastal Ups and Downs

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Science  17 Jun 2005:
Vol. 308, Issue 5729, pp. 1715
DOI: 10.1126/science.308.5729.1715d

A subduction zone extending along the northwest margin of North America, from northern California up to the Aleutians, is capable of generating giant earthquakes and tsunamis. One means for assessing the current hazard is to reconstruct records of past major tremblors. From a coastal lake in Oregon, Kelsey et al. have obtained a detailed and well-dated history of tsunamis stretching back 7000 years. The lake sediments reveal the periodic input of beach sand and saline water, which likely was associated with tsunami incursions. Tsunami-generating earthquakes clustered, with three to four occurring every 1000 years, and clusters were separated by earthquake-free periods lasting about 1000 years; the most recent large event, in 1700, followed a 700-year quiescent span.

Separately, Hawkes et al. show that noticeable changes in the foraminifera and diatom populations in tidal marshes near two recent large earthquakes along the Alaskan and Oregon coasts may reveal subtle subsidence that began several months before the earthquakes. Additional verification of this provocative signal is needed, including whether such changes are connected to recently recognized aseismic slip along these subduction zones. — BH

Geol. Soc. Am. Bull. 117, 1009; 996 (2005).

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