New determination of the shape of the Seattle basin, Washington from gravity and magnetic data: Implications for seismic ground motion and crustal faults

Abstract:

The greater Seattle urban area overlies a large basin subject to amplification of seismic waves during earthquakes. The depth and shape of the Seattle basin was last determined via inversion of regional gravity data in 2001; since that time, we have collected over 2000 new gravity data points across the basin. Two dimensional modeling of these gravity data with existing aeromagnetic data reveal a boundary between the two major basement rock types, the basaltic Crescent Formation (a part of Siletzia) to the west and the metamorphic western mélange belt to the east, passing beneath the middle of the basin. Our most surprising results include a westward dip for this boundary across the Seattle uplift, as opposed to an eastward dip across the Kingston arch, and steeply-dipping, deeply-rooted slices of non-magnetic Crescent included within Siletzia under the Puget lowland. We explain these results with one tectonic story: amalgamation of Siletzia with North America during the Eocene involved subduction-related duplication in a fold and thrust belt style within the Crescent and its likely obduction over the western mélange belt near the Seattle uplift. The new data also show local gravity gradients that correlate with steeply-dipping, neotectonic faults that traverse the eastern end of the Seattle basin. Prominent examples include the northwest-striking, right-lateral Rattlesnake and Whidbey Island fault systems, as well as a possible new fault striking northwest through the Lake Sammamish/Bellevue area. A simple assessment of gravity anomalies for the depth of the basin suggest it is deeper and wider to the east of Seattle than to the west. However, including the basement boundary in the gravity inversion and utilizing measured rock densities which show the western mélange belt is less dense than the Crescent Formation, we find a basement/sediment contact that is a consistent depth at a large scale across much of the basin and even shallows to the east in some areas. The new data also define local minima in basin depth that could complicate wavefields passing through it. The eventual goal of this work is generation of new simulations of ground motion amplification within the basin from both subduction zone events and crustal earthquakes on previously known and the newly-characterized faults.