Resolution No. 4814 - Laserfiche WebLink

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10.2 Seismic Hazards for Lane County and the Eugene/SpringfieM [~etro Area Earthquakes in Western Oregon, and throughout the world, occur predominantly because of plate tectonics - the relative movement of plates of oceanic and continental rocks that make up the rocky surface of the earth. Earthquakes can also occur because of volcanic activity and due to other geologic processes. The earthquake hazard for the three counties addressed in the regional multi-hazard mitigation plan for Benton, Lane and Linn Counties is reviewed in detail in Chapter 3 of the Regional Al~ Hazard Mitigation P~an, Phase Two Technical Appendix, Benton, Lane and Linn Counties Oregon, Seismic Loss Potential (2001). The main findings of that chapter are briefly summarized here. The Cascadia Subduction Zone is a geologically complex area off the Pacific Northwest coast from Northern California to British Columbia. In simple terms, several pieces of oceanic crust (the Juan de Fuca Plate, Gorda Plate and other smaller pieces) are being subducted (pushed under) the crust of North America. This subduction process is responsible for most of the earthquakes in the Pacific Northwest as well as for creating the volcanoes in the Cascades. Figure 10-1 (from the Regional All Hazard Mitigation Plan, Phase Two Technical Appendix) shows the geologic (plate~tectonic) setting for Oregon. There are three source regions for earthquakes that can affect the Eugene/Springfield Metro Area: 1) ~lnterface" earthquakes on the boundary between the subducting oceanic plates and the North American plate, 2) "intraslab" or "intraplate' earthquakes within the subducting oceanic plates, and 3) "crustal" earthquakes within the North American Plate. The geographic and geometric relationships of these earthquake source zones are shown in Figure 10-2 (from the Phase Two Technical Appendix). A full discussion of current understanding of these subduction zone and crustal earthquakes is given in Chapter 3 of the Phase Two Technical Appendix. The "interface" earthquakes on the Cascadia Subduction Zone may have magnitudes of 8 or greater, with probable recurrence intervals of 500 to 800 years. The last major earthquake in this source region probably occurred in the year 1700, based on current interpretations of Japanese tsunami records. Such earthquakes are the great Cascadia Subduction Zone eadhquake events that have received attention in the popular press. These earthquakes occur about 20 to 60 kilometers (12 to 40 miles) offshore from the Pacific Ocean coastline. Ground shaking from such earthquakes would be very strong near the coast and moderately strong ground shaking would be felt throughout Lane County, with the level of shaking decreasing towards eastern Lane County. Public Review Draft: August 4, 2004 10-3