The depth of the basins is a result of the Sound being part of the Cascadia subduction zone , where the terranes accreted at the edge of the Juan de Fuca Plate are being subducted under the North American Plate . There has not been a major subduction zone earthquake here since the magnitude nine Cascadia earthquake ; according to Japanese records , it occurred 26 January 1700 . Lesser Puget Sound earthquakes with shallow epicenters , caused by the fracturing of stressed oceanic rocks as they are subducted , still cause great damage . The Seattle Fault cuts across Puget Sound , crossing the southern tip of Bainbridge Island and under Elliott Bay . To the south , the existence of a second fault , the Tacoma Fault , has buckled the intervening strata in the Seattle Uplift
Cascadia earthquake
1700 Cascadia earthquake
Puget Sound