LAND-BASED SEISMIC IMAGING OF THE TACOMA FAULT ZONE, WASHINGTON STATE

Recent land-based shallow seismic reflection images from the Tacoma fault zone, west of the city of Tacoma in western Washington State, show complex structures. The east-trending Tacoma fault zone forms the boundary between the Seattle uplift to the north and the Tacoma basin to the south, and is known from gravity and aeromagnetic anomalies that extend 50 km across the central Puget Lowland. Previously published tomographic data suggests there is as much as 6 km of post-Eocene uplift of the hanging wall of the Tacoma fault. We acquired two north-south seismic profiles in July, 2005 to define the character of the Tacoma fault zone near Case and Carr Inlets. The 8.5 km-long Carney Lake profile is located just east of Case Inlet and traverses two fault scarps imaged by LIDAR (LIght Detecting And Ranging) data. This profile shows flat-lying strata of the Tacoma basin, and folded and faulted Tertiary and older strata forming a broad anticline within and north of the fault zone. The 9 km Bethel-Burley profile, located near Gig Harbor and Carr Inlet east of the Carney Lake profile, shows a prominent antiform near the north end of the profile, accentuated by a strong, south-dipping reflector. The strength of this reflector suggests that it marks the top of Eocene volcanic basement rocks. The south-dipping strata imaged in the southern portion of this profile can be correlated with strata on previously published marine seismic profiles in Carr Inlet. The antiform in the Eocene rocks appears to broaden to the east, with the main Tacoma fault strand being less obvious or missing on the easternmost profile. These seismic reflection profiles suggest that the north edge of the Tacoma basin is bounded by south-dipping, faulted and folded Eocene rocks. South-dipping basement rocks cut by faults supports an interpretation of deformation in the hanging wall above a major thrust or reverse fault.