AN EARTHQUAKE HISTORY DERIVED FROM STRATIGRAPHIC AND MICROFOSSIL EVIDENCE OF RELATIVE SEA-LEVEL CHANGE AT COOS-BAY, SOUTHERN COASTAL OREGON

Much of the uncertainty in determining the number and magnitude of pas t great earthquakes in the Cascadia subduction zone of western North A merica stems from difficulties in using estuarine stratigraphy to infe r the size and rate of late Holocene relative sea-level changes, A seq uence of interbedded peaty and muddy intertidal sediment beneath a sma ll, protected tidal marsh in a narrow inlet of Coos Bay, Oregon, recor ds ten rapid to instantaneous rises in relative sea level, Each rise i s 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-nide extent and abrupt changes in lithology and foraminiferal and diatom assemblages that can be used to infer at lea st half a meter of sudden coseismic subsidence, Although the character istics of a third, gradual contact do not differ from those of some co ntacts produced by nonseismic processes, regional correlation with oth er similar sequences and high-precision C-14 dating suggest that the t hird contact records a great plate-boundary earthquake about 300 yr ag o, A fourth contact formed too slowly to have been caused by coseismic subsidence, Because lithologic and microfossil data are not sufficien t to distinguish a coseismic from a nonseismic origin for the other si x peat-mud contacts, we cannot determine earthquake recurrence interva ls at this site, Similar uncertainties in great earthquake recurrence and magnitude prevail at similar sites elsewhere in the Cascadia subdu ction zone, except those with sequences showing changes in fossils ind icative of >1 m of sudden subsidence, sand sheets deposited by tsunami s, or liquefaction features.