Deep Earthquake Scenarios
Washington State has a rich history of deep earthquakes greater than magnitude 6 starting with the 1909 M 6.0 under the San Juan Islands. Due to small number of operating seismographs in the region it is difficult to have much confidence in the depth control of locations. Still the 1909 earthquake, a M 6.2 in Kitsap Co in 1939, the 1949 M 7.1 Olympia Earthquake, the 1965 M 6.5 under Federal Way and the 2001 M 6.8 Nisqually earthquake all contribute to our understanding of the impacts of deep earthquakes. Because they usually occur between 30 and 75 Kms depth, the shaking strength is somewhat attenuated or lessened at the surface compared to shallow crustal earthquakes though they impact a large area. Hundreds of millions of dollars worth of damage and a dozen lives were lost in these earthquakes.
Deep earthquakes can have occured from British Columbia to Northern California but in historic times only The Puget lowland has experienced them larger than Magniude 6. Due to the thickness of the decending Juan de Fuca Plate in which these ruptures occur, it is thought the magnitude of deep earthquakes is limited to ~ 7.5.
Due to this history, and the USGS estimate that there is an 84% probability of another greater than M 6 Cascadia Region deep earthquake occurring in the next 50 years, deep earthquake ground motion scenarios have been developed. These can be found in the Cascadia Deep Earthquakes publication by CREW and is available for downloading.