DETECTION OF LARGE PREHISTORIC EARTHQUAKES IN THE PACIFIC-NORTHWEST BY MICROFOSSIL ANALYSIS

Geologic and palynological evidence for rapid sea level change similar to 3400 and similar to 2000 carbon-14 years ago (3600 and 1900 calendar years ago) has been found at sites up to 110 kilometers apart in southwestern British Columbia. Submergence on southern Vancouver Island and slight emergence on the mainland during the older event are consistent with a great (magnitude M greater than or equal to 8) earthquake on the Cascadia subduction zone. The younger event is characterized by submergence throughout the region and may also record a plate-boundary earthquake or a very large crustal or intraplate earthquake. Microfossil analysis can detect small amounts of coseismic uplift and subsidence that leave little or no lithostratigraphic signature.