Resolution No. 4814 - Laserfiche WebLink

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The characteristics of the subduction zone earthquakes affecting Lane County are summarized in Table 10.4 below. The maximum magnitudes are estimated from the ~ength and width of the mapped fault plane or from similar earthquakes elsewhere in the Pacific Northwest (for the intraslab earthquakes). Recurrence intervals are based on current best estimates. Table 10.4 Seismic Sources Affecting Lane County Fau~t Maximum Probable Recurrence Magnitude Interval (years) Cascadla Subducbon Zone 8,5 500 to 800 (interface earthquake) Cascadla Subduction Zone 7.5 500 to 1000 (intraslab earthquake) Chapter 4 of the Regional Mitigation Plan Phase Two Technical Appendix analyses these two scenario earthquakes affecting the three counties: a Cascadia Subduction Zone interface earthquake (M8.5) and a Cascadia Subduction Zone intrasiab earthquake (M7.5)~ For each of these earthquakes, appropriate attenuation relationships are used to calculate the levels of ground shaking (peak ground acceleration, PGA) with distance from the earthquake. 10.3 Other Aspects of Seismic Hazards in Lane County Most of the damage in earthquakes occurs directly because of ground shaking which affects buildings and infrastructure. However, there are several other aspects of earthquakes that can result in very high levels of damage in localized sites: liquefaction, landslides, dam failures and tsunamis. 10,3.1 Soil Effects Liquefaction is a process where loose, wet sediments lose strength during an earthquake and behave similarly to a liquid. Once a soil ~iquefies, it will tend to settle and/or spread laterally. With even very slight slopes, liquefied soils tend to move sideways downhill (lateral spreading). Settling or lateral spreading can cause major damage to buildings and to buried infrastructure such as pipes and cables. Figure 3-6 in the Regional Mitigation Plan Phase Two Technical Appendix shows the areas in the three counties where liquefaction potential is high. These areas of high liquefaction potential largely follow the main river and stream drainage channels in the three county area; these are the areas with loose, wet sediments. Liquefaction does not occur in all such areas or in all earthquakes. However, in ~arger earthquakes with strong ground shaking and long duration shaking, liquefaction is likely in many of these high liquefaction potential areas. Settlements of a few inches or more and lateral spreads of a few inches to several feet are possible. Even a few inches of settlement 10-8