ACTIVE DEFORMATION OFFSHORE SOUTHWESTERN SAN JUAN ISLAND, WASHINGTON: IMPLICATIONS FOR SEISMIC HAZARD ASSESSMENT

The discovery of Quaternary seismic activity along the Devils Mountain fault and associated structures, located just south of the San Juan Islands, highlights a need for high-resolution bathymetric imagery to help identify and characterize seafloor morphology. Because seismic activity along the Devils Mountain fault could reactivate older faults in the area, including the Late Cretaceous San Juan Thrust system, detailed mapping of these fault systems can help constrain the regional seismic hazard. Previously, the location and subsurface geometry of a major crustal boundary separating the San Juan Thrust system from the Wrangellia terrane offshore southwestern San Juan Island remained unclear. We combine seismic reflection, aeromagnetic, earthquake, and new high-resolution multibeam imagery to map and characterize structures in this region. Multibeam imagery constrains locations of a number of previously inferred NW-trending faults associated with the active Devils Mountain fault, characterized by 10 to 50 m of vertical seafloor relief. Interpretation of a Geological Survey of Canada seismic reflection line suggests Holocene deformation on a number of these faults.

The subsurface location of the Haro fault, which we interpret as a terrane boundary separating the San Juan Thrust system from Wrangellia, is inferred based on changes in magnetic anomaly patterns across the boundary visible in shallow-focused aeromagnetic data. The subsurface geometry of the terrane boundary is inferred using a spectral technique that estimates important source depths from magnetic anomalies, then filters the anomalies in order to emphasize those depths. Offshore southwestern San Juan Island, the Haro fault is oriented parallel to active NW-trending faults associated with the Devils Mountain fault system, suggesting that the Haro fault has been or will be reactivated. Seismic activity along the western Devils Mountain fault is relatively low, but a cluster of shallow (<30 km) offshore earthquakes suggests some stress is being transferred to NW-trending structures offshore southwestern San Juan Island. Large Holocene earthquakes have occurred on NW-trending structures in the broader Devils Mountain fault zone to the east. This active deformation likely represents a significant seismic hazard for northern Puget Sound.