REGULATING AGAINST SURFACE RUPTURE

Science and society in the Pacific Northwest have made two great advances in earthquake hazard mitigation: (1) recognition of hazard from earthquakes on the subduction zone, slab, and crust, especially on the Seattle fault, and (2) upgrading building codes to address strong ground motion. In recent years, led by the USGS, laser aerial mapping available through the Puget Sound LiDAR Consortium has allowed the identification of surface rupture even where the ground is densely vegetated. This allows paleoseismic investigations to determine the likelihood of additional surface rupture based on past fault history. Surface rupture causes hazards different from those from strong shaking, requiring a different approach to mitigation and regulation. Why is this important? Earthquakes in Taiwan and Turkey in 1999 and Pakistan in 2005 resulted in tens of thousands of deaths that could have been avoided if development had taken into account surface faulting that could have been mapped prior to those earthquakes. Legislation at the county level in Utah and the state level in California (Alquist-Priolo and Seismic Hazard Mapping acts) requires, prior to permitting, special geological studies of faults that are well defined and have evidence of Holocene activity. In most cases, the mitigation strategy is avoidance of surface ruptures. However, where expected displacements are small, geotechnical and structural-engineering strategies may allow development on active faults where faults are well located based on paleoseismic trenching and high-resolution seismic surveys, and their anticipated displacements are known to be small. The States of Washington and Oregon lack regulation against surface rupture, and as a result, construction of a large critical facility near Woodinville, Washington, within the active Southern Whidbey Island fault zone, was permitted and is being built without a full evaluation of potential surface-rupture hazard.