RECENT HIGH-RESOLUTION SEISMIC REFLECTION STUDIES OF ACTIVE FAULTS IN THE PUGET LOWLAND

In the past four years, new high-resolution seismic surveys have filled in gaps in our understanding of active structures beneath the Puget Lowland, western Washington State. Although extensive regional and high-resolution marine seismic surveys have been fundamental to understanding the tectonic framework of the area, these marine profiles lack coverage beneath water bodies that large ships cannot navigate and beneath city streets underlain by late Pleistocene glacial deposits that are missing from the waterways. Recent land surveys and profiles in restricted waterways across structures can therefore bridge the gap between paleoseismic and marine geophysical studies, and test elements of models proposed by regional-scale geophysical studies. Results from these surveys have: 1) documented several meters of vertical displacement on at least 2 separate faults in the Olympia area; 2) clarified the relationship between the LIDAR scarps and observed structures across the Tacoma fault zone; 3) provided a window into structures beneath the north portion of the western Tacoma fault zone, north of previous marine profiles; 4) documented deformation along the Seattle fault near the Vasa Park trench; 5) extended the Seattle fault zone well east of Lake Sammamish; and 6) documented multiple fault strands in and south of the Seattle fault zone south of Bellevue. The results better constrain interpretations of paleoseismic investigations of past earthquakes on these faults and provide targets for future paleoseismic studies.