Abstract

The southern end of the Cascadia subduction zone (CSZ) in northwestern California poses a high seismic hazard. This study uses the Quaternary stratigraphy of the bays and estuaries to reconstruct coseismic subsidence caused by strong to great earthquakes. We used lithology, macrofossils, and microfossils to estimate the amount of relative sea‐level change at contacts caused by coseismic subsidence. Our paleoseismic record contains evidence of four to six earthquakes over the past 2000 years. Using the pattern and magnitude of submergence and other paleoseismic information (trenches and other sites), we determine whether the earthquakes were local or regional. We found that the record contained evidence for both smaller strong to major earthquakes on local structures (Mw 6.5–7.2) and larger regional subduction‐zone‐related great earthquakes (Mw>8.2). We compared our record to other records from Oregon and Washington and found three earthquakes likely caused by the rupture of the entire CSZ around approximately 230–270 (the A.D. 1700 event), 1150–1400, and 1750–1900 cal B.P. In addition, two other local earthquakes likely occurred around 500–600, 1000–1250, and possibly 1500–1650 cal B.P.

Online Material: Table of radiocarbon ages obtained in the study.

You do not currently have access to this article.