IMPLICATIONS OF GRAVITATIONAL ANOMALIES WITHIN SEDIMENTARY BASINS OF NORTHWEST WASHINGTON

This study presents preliminary models based on new and existing gravity and magnetic data for two regions in the Puget Sound, Washington, area: the Bellingham basin and the Muckleshoot basin. The principle goals of the project are to determine whether and how faults mapped outside of these basins continue beneath their thick sedimentary cover.

In the Bellingham basin, cross-sectional models focus on the Drayton-Harbor magnetic lineament and the Birch Bay fault. Late Holocene displacements have been observed along these structures along the western Washington coast, and analyses of magnetic data and Lidar data suggest that these faults extend eastward into the basin. Preliminary models are consistent with the inland continuation of the Birch Bay fault towards Bellingham. The new gravity data also suggest that the Bellingham basin may consist of smaller subbasins, rather than one large basin, as previously mapped.

In the Muckleshoot basin, cross-sectional models focus on the possible connection of the east-trending Tacoma fault (west of the basin) and the northwest-trending White River fault, mapped to the east of the Muckleshoot basin. A connection between these two fault systems would have significant implications for hazard estimates in terms of the length and size of these seismogenic structures.