RADIOCARBON EVIDENCE FOR EXTENSIVE PLATE-BOUNDARY RUPTURE ABOUT 300 YEARS AGO AT THE CASCADIA SUBDUCTION ZONE

THE Cascadia subduction zone, a region of converging tectonic plates a long the Pacific coast of North America, has a geological history of v ery large plate-boundary earthquakes(1,2), but no such earthquakes hav e struck this region since Euro-American settlement about 150 years ag o. Geophysical estimates of the moment magnitudes (M(w)) of the larges t such earthquakes range from 8 (ref. 3) to 9 1/2 (ref. 4). Radiocarbo n dating of earthquake-killed vegetation can set upper bounds on earth quake size by constraining the length of plate boundary that ruptured in individual earthquakes. Such dating has shown that the most recent rupture, or series of ruptures, extended at least 55 km along the Wash ington coast within a period of a few decades about 300 years ago(5). Here we report 85 new C-14 ages, which suggest that this most recent r upture (or series) extended at least 900 km between southern British C olumbia and northern California. By comparing the C-14 ages with writt en records of the past 150 years, we conclude that a single magnitude 9 earthquake, or a series of lesser earthquakes, ruptured most of the length of the Cascadia subduction zone between the late 1600s and earl y 1800s, and probably in the early 1700s.