SURFACE FAULTING: A PARADIGM SHIFT IN THE PUGET SOUND REGION

Environmental impact statements for large construction projects in the Puget Sound region formerly considered only earthquake hazards from strong ground motion, liquefaction, and lateral spreading. Surface faulting was recognized as a potential hazard in 1992, when the Seattle fault was first identified as active. However, poor exposure and heavy vegetative cover prevented mapping surface fault traces with sufficient precision to target geotechnical excavations. Beginning in 1999, the availability of regional LiDAR surveys collected by the Puget Sound LiDAR Consortium changed all that. The U.S. Geological Survey developed an exploration strategy whereby possible crustal faults are identified by regional aeromagnetic surveys followed by examination of LiDAR images. A bare-ground topographic image calculated from raw LiDAR data is “viewed” from a low-incidence light angle, highlighting subtle scarps and providing precise targets for subsequent field work and paleoseismological trench excavations. This strategy has successfully identified at least eight Holocene faults, including the Seattle, Tacoma, Southern Whidbey Island, Saddle Mountain, and Utsalady Point faults. Although Washington adopted IBC 2003, the state lacks legislation comparable to California's Alquist-Priolo Act, which prohibits the placement of certain structures for human occupancy across Holocene surface faults: mitigation by avoidance. Advances in structural and geotechnical engineering show that strict avoidance of smaller surface faults with minor ground displacement may not be necessary. But to successfully mitigate against fault displacements, a detailed understanding of displacement magnitude and kinematics is required, which is not always available at every site. Even without legislation, the City of Woodinville, which lies within the Southern Whidbey Island fault zone, has incorporated aeromagnetic and LiDAR lineaments into its GIS maps to guide future development.