SHALLOW HOLOCENE FAULTING IN THE TACOMA FAULT ZONE, WESTERN WASHINGTON, FROM HIGH-RESOLUTION SEISMIC DATA

High-resolution, marine seismic reflection data were collected across the Tacoma fault zone, Washington State. The Tacoma fault is a north-dipping thrust or reverse fault at the south edge of the Seattle uplift; it is defined from gravity, magnetic, tomography and Light Detection And Ranging (LIDAR) data. We collected 110 km of single-channel, high-resolution marine seismic reflection data in Carr Inlet, Case Inlet, and Hood Canal. Survey lines crossed the trends of scarps seen on the LIDAR data with sub-bottom penetration of up to 130 m depth. We interpret strata imaged on the profiles as Pleistocene glacial deposits unconformably overlain by up to 10 m of subhorizontally stratified late Pleistocene and Holocene muds. In Carr Inlet, the easternmost waterway, profiles imaged Holocene faulting having 3-4 m of vertical displacement. An oblique crossing of this feature on another profile suggests a northwest trend, parallel to linear shorelines to the southeast. The seismic reflection data show other disruptions and abrupt terminations of gas-saturated muds that could mark other northwest trending faults. Evidence for faulting was less clear in Case Inlet, but the abrupt termination of a gas-saturated mud layer and a south-dipping segment of the erosional surface at the top of the glacial deposits align with the trends of scarps imaged by the LIDAR data several km to the west. A 300-m-wide, northeast trending ridge composed of glacial deposits could also be related to faulting, or it could be a glacial feature such as an esker having a trend transverse to the ice flow pattern. In Hood Canal, the westernmost waterway, several abrupt lateral changes in reflectivity and inflections in dip within the shallowest strata occur in the northeastern part of the inlet, where geologic and LIDAR data suggest faults may be crossing the waterway. Taken together, the shallow seismic profiles and LIDAR data suggests the Tacoma fault is a series of west- to northwest-trending faults in a zone about 7 km wide. Fault displacement may be distributed across this zone of shallow faulting, or the shallow faults may be related to folding and are not rooted in deeper faults. In either case, the shallow faults presumably moved during major earthquakes on the deeper fault.