An earthquake history derived from stratigraphic and microfossil evidence of relative sea-level change at Coos Bay, southern coastal Oregon

Alan R. Nelson, Anne E. Jennings, Kaoru Kashima

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Abstract

Much of the uncertainty in determining the number and magnitude of past great earthquakes in the Cascadia subduction zone of western North America stems from difficulties in using estuarine stratigraphy to infer the size and rate of late Holocene relative sea-level changes. A sequence of interbedded peaty and muddy intertidal sediment beneath a small, protected tidal marsh in a narrow inlet of Coos Bay, Oregon, records ten rapid to instantaneous rises in relative sea level. Each rise is marked by a contact that records an upward transition from peaty to muddy sediment. But only two contacts, dating from about 1700 and 2300 yr ago, show the site-wide extent and abrupt changes in lithology and foraminiferal and diatom assemblages that can be used to infer at least half a meter of sudden coseismic subsidence. Although the characteristics of a third, gradual contact do not differ from those of some contacts produced by nonseismic processes, regional correlation with other similar sequences and high-precision 14C dating suggest that the third contact records a great plate-boundary earthquake about 300 yr ago. A fourth contact formed too slowly to have been caused by coseismic subsidence. Because lithologic and microfossil data are not sufficient to distinguish a coseismic from a nonseismic origin for the other six peatmud contacts, we cannot determine earthquake recurrence intervals at this site. Similar uncertainties in great earthquake recurrence and magnitude prevail at similar sites elsewhere in the Cascadia subduction zone, except those with sequences showing changes in fossils indicative of > 1 m of sudden subsidence, sand sheets deposited by tsunamis, or liquefaction features.

Original languageEnglish
Pages (from-to)141-154
Number of pages14
JournalBulletin of the Geological Society of America
Volume108
Issue number2
DOIs
Publication statusPublished - Feb 1996

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microfossil
sea level change
earthquake recurrence
subsidence
earthquake
subduction zone
history
recurrence interval
earthquake magnitude
plate boundary
tsunami
liquefaction
sediment
marsh
lithology
stratigraphy
diatom
Holocene
sea level
fossil

All Science Journal Classification (ASJC) codes

  • Geology

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An earthquake history derived from stratigraphic and microfossil evidence of relative sea-level change at Coos Bay, southern coastal Oregon. / Nelson, Alan R.; Jennings, Anne E.; Kashima, Kaoru.

In: Bulletin of the Geological Society of America, Vol. 108, No. 2, 02.1996, p. 141-154.

Research output: Contribution to journalArticle

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