Surface Faulting: A New Paradigm for the Pacific Northwest

Abstract

The past twenty years have witnessed three major paradigm shifts in our understanding of earthquake hazards in the Pacific Northwest. The first paradigm shift was the recognition in the 1980's that the Cascadia subduction zone is capable of great earthquakes. The locked part of the Cascadia subduction zone lacks instrumental seismicity, but geological evidence shows that it can produce an earthquake of MW 9, larger than any expected for California south of Cape Mendocino. In response to the geological evidence, building codes have been upgraded, tsunami inundation maps have been prepared for coastal areas from northern California to northern Washington, and earthquake hazard maps have been constructed for northwestern cities, including Victoria, British Columbia. Earthquake awareness has increased among the general public and within many private companies. Earthquake awareness has increased among the general public and within many private companies in the Pacific Northwest. The second paradigm shift, the alignment of seismic provisions in building codes to reflect greatly increased scientific understanding, has been subtle but of overarching societal importance. This alignment began in the early 1990's when Washington, under the leadership of the Structural Engineers Association of Washington, expanded the Uniform Building Code seismic zone III to include the Mount St. Helens seismic zone, and Oregon adopted a greatly revamped designation of seismic zone maps. This trend toward direct use of scientific results in the building code has accelerated in the last two years, with the State of Washington adopting the International Building Code (IBC). The IBC uses the U.S. Geological Survey (USGS) probabilistic hazard map as its basis, thus aligning current best science with engineering practice. Oregon may follow suit; currently 32 states have adopted the IBC. The IBC literally brings USGS hazard calculations onto the desktops of structural engineers. Despite the paradigm shifts related to Cascadia …