A Gravity Study of Holocene Active Structures in the Puget Lowland of Washington State
Type of Degreethesis
Geology and Geography
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This study presents models based on existing and new gravity and magnetic data for two regions in the Puget Sound, Washington, area: the Bellingham basin and the Muckleshoot basin. The principal goals of the project are to identify basin boundaries and determine whether and how faults mapped outside of these basins continue beneath their thick sedimentary cover. In the Bellingham basin, new gravity maps reveal that the north basin boundary is located further south than previously thought. Cross-sectional models presented in this study focus on two previously identified structures, the Drayton-Harbor magnetic lineament and the proposed Birch Bay fault. Late Holocene displacements along these structures have been observed by other workers on the western Washington coast, and analyses of magnetic and LiDAR data suggest that these faults extend eastward into the basin. Models are consistent with the inland continuation of the Birch Bay fault towards the city of Bellingham and also suggest that a previously unidentified fault or fold could exist nearby. Results indicate that the Bellingham basin may consist of smaller subbasins, rather than one large basin, as previously mapped. In the Muckleshoot basin, gravity maps support the previously defined basin boundaries. Magnetic data and new gravity data support the existence of at least two subbasins within the Muckleshoot that are likely separated by a fault. A 2-D cross-sectional model focuses 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 and the maximum magnitudes that could be generated by faulting.