The Bellingham basin, spanning onshore and offshore regions of northwest Washington (United States) and southwest British Columbia (Canada), is deforming under north-south shortening in the north Cascadia forearc. Accommodating the regional strain are Holocene-active faults within the basin that have been previously mapped both onshore by paleoseismic investigations and inferred through LiDAR imagery and aeromagnetic anomalies. In this study, we add new gravity measurements to an existing database and interpret previously acquired aeromagnetic and legacy marine seismic reflection data to define better the structure of the southern Bellingham basin and to assess Holocene and recently active faults within it. A regional-residual separation method reveals prominent northwest- and northeast-oriented contacts created by magnetic field gradients that align with fault traces, documented Holocene fault displacement, and basin structures discovered through geologic mapping and exploratory wells. One of these magnetic contacts is associated with the Sandy Point fault and extends northwest toward a cluster of seismicity near Point Roberts. The magnetic contact suggests that the fault may be longer than previously thought and thus capable of producing a larger earthquake and posing a greater earthquake hazard to the cities of Bellingham (Washington), Vancouver (British Columbia), and surrounding communities. The study also presents a model for the possible interplay among the Holocene-active Boulder Creek fault, Lummi Island fault, and Macaulay Creek thrust fault, the latter of which was responsible for the 1990 M 5 Deming earthquake and was the likely source of previous historic earthquakes.
Bhattacharya et al. (Fri,) studied this question.