FINDING ACTIVE FAULTS IN A GLACIATED AND URBAN LANDSCAPE: THE SOUTHERN WHIDBEY ISLAND FAULT, WASHINGTON

In the heavily vegetated and glaciated Puget Lowland, we have applied Lidar (laser terrain mapping) and potential field geophysics to identify at least six seismically active crustal fault zones that together accommodate permanent shortening of the Cascadia forearc. Most of the Holocene surface-rupturing fault scarps revealed by Lidar tend to closely follow linear aeromagnetic anomalies that reflect underlying basement structure and help map the full extent of the faults.

Recent studies of the southern Whidbey Island fault (SWIF) provide an example. The SWIF was mapped previously using borehole data, potential-field anomalies, and seismic-reflection surveys on Whidbey Island and surrounding waterways. Gravity inversions, aeromagnetic mapping, and seismic tomography suggest that the SWIF extends at least 100 km southeastward from Vancouver Island to the Washington mainland. Abrupt subsidence and uplift of coastal marshes across the fault on Whidbey Island indicate that the SWIF experienced a MW 6.5 to 7.0 earthquake at about 3 ka. The southeastward projection of the SWIF makes landfall between the cities of Seattle and Everett, where linear, low-amplitude magnetic anomalies coincide with the hypothesized projection. High-resolution lidar surveys and aerial photography illuminate subtle (1-5 m) scarps on late Pleistocene surfaces at a number of locations along the aeromagnetic lineaments, suggesting that some of the lineaments are caused by fault contacts. The most prominent of the anomalies extends 16 km and passes within about 27 km of downtown Everett. Susceptibility measurements and a ground-magnetic survey indicate that faulted Pleistocene glacial deposits may contribute to these low-amplitude aeromagnetic anomalies, and spectral analysis suggests that fault offsets extend into underlying Eocene strata. In 2004, four trenches were excavated across two of the lidar scarps; all show evidence of Holocene deformation. Combined paleoseismic evidence suggests that at least three and probably four earthquakes have occurred on the SWIF during the last 16.4 ka. Lidar and high-resolution aeromagnetic mapping, followed by targeted paleoseismic studies, help constrain the length, age, and three-dimensional characteristics of the SWIF and other active faults in the Cascadia forearc.