Applying Research Data and Results: A Case History of a Cascadia Great Earthquake and Tsunami

We are developing case histories of relevant natural disasters to provide students a means of learning fundamental earth system science and applying their understanding to mitigating future disasters. We will present the case history of the 9PM, January 26, 1700 great earthquake (M9.1) and tsunami that devastated native coastal populations and drowned old forests when the coastline dropped as much as three meters. Students explore the fascinating story of how we know when this event occurred—down to the day and hour—despite the fact that there are no written records. The case history consists of multiple investigations: a comparison of a Native American oral history of the 1700 event to the 2004 M9.3 Sumatran earthquake and tsunami, an excellent analogue for Cascadia. Patterns of deformation leading up to and after the Sumatran event are compared to geodetic and seismic data for Cascadia. The analogy is extended to the 1960 M9.4 Chilean and the 1964 M9.2 Alaskan earthquakes and tsunamis before exploring the geologic and geophysical data that has documented the tsunami history of Cascadia. The students explore evidence of 18 catastrophic earthquakes and tsunamis that affected the entire Cascadian margin over the last 10,000 years, at intervals from about 200 to 1500 years, with an average of 570 years. With knowledge that these events occur frequently in Cascadia and vivid images of the devastation they can cause, the exploration turns to two of the most vulnerable cities in Cascadia. Seaside Oregon is a small city on the coast that would be inundated by a local tsunami within 20 minutes of a great subduction zone earthquake and Seattle – the largest city in the Pacific Northwest. Development of evacuation plans from Seaside and analysis of the impact on critical infrastructure in Seattle bring the reality of the hazard to light.