THE CASCADIA SUBDUCTION ZONE EARTHQUAKE(S) FROM ABOUT 600 CAL YR BP: A FULL SUBDUCTION ZONE RUPTURE, A SERIES OF RUPTURES, OR A NORTHERN SEGMENT RUPTURE?

Tsunami deposits from Discovery Bay, along the Strait of Juan de Fuca, span the last 2,500 years. The penultimate tsunami deposit at Discovery Bay, bed 2, has a revised radiocarbon age of 560-630 cal yr BP (1320-1390 A.D.). This age overlaps with the ages of tsunami deposits from several sites on Vancouver Island to the north, and tsunami deposits and coastal subsidence as far south as central Oregon. Additional evidence for a strong shaking around 600 cal yr BP includes submarine slope failures from Vancouver Island inlets and Puget Sound; and lacustrine slope failures in Lake Washington in Seattle, and Lake Crescent on the Olympic Peninsula. Despite the evidence for a large earthquake or earthquakes, there is no known evidence of coastal subsidence or tsunami from this time in any of the southwest Washington coastal estuaries. These estuaries record seven subduction zone earthquakes over the last 3,500 years, including drowned trees that were used to date the 1700 A.D. Cascadia earthquake. One of these trees on the Columbia River lived through the time of Bed 2, and two other trees, from Grays Harbor and Willapa Bay, were alive through at least part of the time range during which bed 2 was deposited. If there was not significant coastal subsidence in these areas, the earthquake(s) from 600 cal yr BP was probably smaller than the 1700 A.D. earthquake. The offshore record of turbidite T2, with a modeled age range that overlaps Discovery Bay bed 2, has been interpreted as a full subduction zone event. Whether there was a single earthquake that ruptured the entire subduction zone, a series of smaller segment ruptures with varying amounts of coastal subsidence, or a single event limited to the northern part of the subduction zone is an unresolved question important to earthquake hazard evaluation.