The Oregon Seismic Safety Policy Advisory Commission (OSSPAC) has completed a comprehensive review of Oregon’s buildings, lifelines and community priorities to assess how to protect lives and commerce and facilitate recovery from a possible, M=9.0, Cascadia earthquake and tsunami. The “Oregon Resilience Plan” (Executive Summary) maps an ambitious, fifty-year program of policy and investment priorities for the state to offset a predicted $32 billion loss and potentially high mortality rates in coastal Oregon from a large, subduction-zone earthquake and tsunami.
The last megathrust earthquake in the Pacific northwest was in January, 1700, just over 300 years ago. Geological evidence indicates that such great earthquakes have occurred at least seven times in the last 3,500 years, a return interval of 400 to 600 years (Pacific Northwest Seismic Network- PNSN). John J. Clague, “Evidence for large earthquakes at the Cascadia Subduction Zone” (AGU – Review of Geophysics, January 1997) observed that very large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The last earthquake or series of earthquakes is believed to have ruptured the entire 1000-km length of the subduction zone; if this was a single earthquake, it probably exceeded M=9.0. The Cascadia zone earthquake and tsunami recurrence intervals are uncertain because of difficulties in identifying and dating ancient earthquakes. In southwestern Washington state, intervals for the seven most recent earthquakes average about 500 years, but range from less than 200 years to 700–1300 years. It is believed that part of the plate boundary in the subduction border is locked and accumulating elastic strain that will be released during a future large earthquake. Oregon has been assessing its seismic vulnerabilities, especially to a Cascadia subduction event, since at least the 1980s. Assessments provided simulated strong ground motions for two likely Cascadia mega-earthquake scenarios, detailed highway bridge retrofit and strengthening planning and reviews of multi-jurisdictional tsunami warning systems. However, after the 2011 Tohoku (Japan) great earthquake and tsunami provided dramatic lessons for the Pacific Northwest in North America, this OSSPC initiative integrates seismic resilience planning for the State of Oregon under one updated report and starkly lays out the large investment decisions required to achieve greater resiliency. In addition to local and state planning efforts, in the U.S.A., the federal agency, NOAA, is charged with tsunami coastal hazard mitigation, inundation mapping and tsunami forecast. See for example NOAA Technical Memorandum OAR PMEL-135.