Characterizing Subsurface Structural Features of the Bellingham Basin by Gravity and Magnetic Modeling

Abstract

This study uses gravity and magnetic data modeling to identify the geometry of the Bellingham basin, Washington, and its deformational history. The study also provides additional insight into three newly identified Holocene-active faults, Drayton Harbor, Birch Bay, and Sandy Point, that are linked with ongoing basin deformation. The models resulting from this analysis show the Bellingham basin as an arcuate-shaped, east-west-trending basin. The extent of the basin to the west is not revealed owing to a lack of offshore data. The Cascade Mountains form the basin’s eastern boundary. On the east side of the basin, the Boulder Creek fault forms the basin’s northern boundary, which may have been reactivated by ongoing crustal block rotation. Wavelength filtering of magnetic field data show distinct magnetic lineations, or “contacts,” that are assumed to result from juxtaposed rock units. Several of the contacts align with the Drayton Harbor, Birch Bay and Sandy Point faults, and suggest that the faults continue into the basin. Based on cross-sectional modeling and the filtered maps, the study postulates minimum lengths of 30, 30, and 24 km for the Drayton Harbor, Birch Bay, and Sandy Point faults, respectively. In this study, the Sandy Point and Birch Bay faults have been modeled as reverse faults with the northern side up. Drayton Harbor, although mapped as south-side up in previous work along the western shoreline, does not exhibit significant offset in its projected position onto the cross-sectional models. This result suggests that the Drayton Harbor fault does not extend far into the basin, or alternatively, the sense of slip along the fault is predominantly strike-slip as it continues into the basin. Epicenters of recorded earthquakes in the Bellingham basin appear closely aligned with the Birch Bay and Sandy Point faults, supporting the idea that these faults are currently active. Similarly, the moderately large (M = 5.0) 1990 Deming, WA, earthquake, which occurred ~ 7 km south of the Boulder Creek fault, along with scattered seismicity throughout the deep basin, provides evidence of ongoing deformation related to crustal block rotation and north-south directed compression. Results from this study and previous work argue that these crustal faults within the Bellingham basin should be considered in assessing seismic hazard in the Puget Sound region.