Geologic map of the greater Portland metropolitan area and surrounding region, Oregon and Washington

Abstract

Mills, Oregon, earthquake (Madin and others, 1993; Thomas and others, 1996). Great offshore earthquakes as large as M 9 on the Cascadia Megathrust have shaken the metro area every 500 years or so, most recently in 1700 (Atwater and others, 2015). The intensity and duration of shaking during great earthquakes likely varies from place to place, depending on the nature of underlying geologic materials (Mabey and others, 1997; Ma and others, 2012). Giant floods have periodically inundated the metro area, from the cataclysmic ice-age Missoula floods about 20,000-15,000 years ago (age in calendar years;