Source Investigation and Comparison of the 1939, 1946, 1949 and 1965 Earthquakes, Cascadia Subduction Zone, Western Washington

Over the past 65 years intraslab earthquakes have caused the most significant damage in the western Washington region. This study examines regional and teleseismic seismograms for four historic, suspected intraslab events of M > 5.5 occurring within the Cascadia Subduction zone in 1939 (South Seattle), 1946 (Puget Sound), 1949 (Olympia) and 1965 (Sea-Tac) to better determine the source locations, mechanisms and rupture histories of these events. Our study is aided by digital seismograms of post-1990 intraslab events with well-determined focal depths and focal mechanisms that were recorded in the same locations as the historic events. Thus the recent events were used as empirical Greens functions to study the historic events. Our results suggest that the 1946 earthquake is not an intraslab event, that the 1939 event closely resembles the 1965 event, and that the 1949 event is similar to the 2001 Nisqually earthquake, although the 1949 event appears to have ruptured toward the south, causing significantly more damage than the Nisqually event. These results suggest that earthquakes periodically rupture along the same or similarly oriented faults within the subducting slab.