| At the Cascadia Subduction Zone in Washington and Oregon, the Juan de Fuca plate subducts beneath the North American plate at a rate of about 4 cm/year. Earthquake hazards in the region are associated with plate interface, intraslab, and crustal earthquakes. All have the potential to cause significant physical and monetary damage, and, as population and infrastructure continue to grow, the possibility of more extensive damage from seismicity in the region also grows. This study examines and models regional and teleseismic seismograms for historic, intraslab events of M > 5.5 occurring within the Pacific Northwest region to better determine their source locations, source mechanisms, and rupture histories. It focuses on the seismograms of four intraslab earthquakes that occurred in western Washington between 1939 and 1965. Digital seismograms of recent (post-1990) intraslab events with well-determined focal depths and mechanisms, and that were recorded in the same locations as the historic events, were used to aid in the interpretation of the historical seismograms. Some of the recent events were used as empirical Green's functions to more accurately determine the rupture processes of the historic earthquakes. My results revise the original depths and relative magnitudes previously published for the historical events. I also find evidence for directivity associated with the 1949 Olympia event that may be the cause of increased intensity observed to the south. Finally, I observe similarities between two event pairs: (1) the 1939 South Puget Sound and 1965 Sea-Tac events; and (2) the 1949 Olympia and 2001 Nisqually events. These similarities have implications for hazards in the Seattle-Olympia region because they may indicate characteristic earthquakes periodically rupturing along the same faults. |
