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Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington

Geological Society of America Bulletin

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Abstract

Fossil diatoms from four stratigraphic sections along the tidal Niawiakum River, southwestern Washington, provide an independent paleoecological test of a relative sea-level rise that has been attributed to subsidence during an inferred earthquake in the Cascadia subduction zone about 300 yr ago. Diatom assemblages in a buried soil and overlying mud indicate a sudden and lasting shift from marshes and forests near or above highest tides to mud flats and incipient tidal marshes, with a progressive return to high-level tidal marshes by sediment aggradation and, perhaps, gradual tectonic uplift. The maount of coseismic submergence required to generate the paleoecological changes observed at these sites could have ranged from a minimum of 0.8-1.0m to a maximum of ~3.0m. The following tsunami extended farther landward than was previously inferred from the stratigraphy. These data rule out proposed alternatives to the coseismic subsidence model - that is, climatically induced sea-level rise, temporary submergence caused by storms - and support the hypothesis that a great earthquake struck southwestern Washington 300 yr ago. -from Author

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington
Series title:
Geological Society of America Bulletin
Volume
107
Issue:
3
Year Published:
1995
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geological Society of America Bulletin
First page:
367
Last page:
378