by Bill Steele
The Last Cascadia Great Earthquake and Tsunami;
313 Years and Ticking
Thousands of coastal residents settled in for the night on January 26th 1700 when the ground began to shake. For most, the first signs were subtle, dogs barked nervously as the primary or “P” wave vibrations passed by. The earthquake became unmistakable when the “S” (secondary or shear) waves arrived at village after village traveling at about 6 kilometers a second as the entire Cascadia Subduction Zone ruptured. The 1000 km long fault rupture propagated from its origin at about 3 km a second, generating fresh seismic waves as the fault continued to unzip and slip. Assuming the rupture began in Northern California, it likely took over 5 minutes break the entire fault to northern Vancouver Island.
The earthquake that released about 1500 times the seismic energy than the 2001 M 6.8 Nisqually Earthquake, and can be seen as a connected series of large earthquakes at least one of which produced very low frequency waves with 10s of meters of displacement, and a dramatic popping up of the sea floor that lifted a great column of water. That uplifted column of water then collapsed producing a series of tsunami waves that would batter the coastline through the night and cross the pacific basin. Though all people west of the Cascades were disrupted by the shaking and some injured by falling logs and possessions, it was the water that likely claimed the hundreds of lives lost that night when villages were overtopped by tsunami generated floods.