Resolution No. 4814 - Laserfiche WebLink

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The 'intraslab" earthquakes, which are also called "intraplate" earthquakes, occur within the subducting oceanic plate. These earthquakes may have magnitudes up to about 7.5, with probable recurrence intervals of about 500 to 1000 years (recurrence intervals are poorly determined by current geologic data). These earthquakes occur quite deep in the earth, about 30 or 40 kilometers (18 to 25 miles) below the surface with epicenters that would likely range from near the Pacific Ocean coast to about 50 kilometers (30 miles) inland. Thus, epicenters from these types of earthquakes could be located in Lincoln County or western Lane County or possibly in western Benton County. Ground shaking from such earthquakes would be very strong near the epicenter and moderately strong ground shaking would be felt throughout all of Lane County, with the level of shaking decreasing towards eastern Lane County. "Crustal" earthquakes within the North American plate are possible on faults mapped as active or potentially active as well as on unmapped (unknown) faults. Historically observed crustal earthquakes in Northern Oregon from 1841 to 1986 are shown in Figure 3-3 from the Regional Mitigation Plan Phase Two Technical Appendix. During this time period, several dozen, mostly small, earthquakes have occurred in Lane County. Section 3.3 of the Phase Two Technical Appendix discusses the current geologic knowledge about crustal faults in the three county area. Several faults have been mapped in the area, especially in Benton and Lane Counties. In the Willamette Valley, the geological processes of erosion and deposition have obliterated most of the possible evidence for past surface fault ruptures. In the three county area, the mapped faults are generally considered to be inactive, with no evidence for activity within the past 11,000 years. Based on the historical seismicity in Western Oregon and on analogies to other geologically similar areas, small to moderate earthquakes up to M5 or M5.5 are possible almost anyplace in Western Oregon, including almost anyplace in Lane County. Such earthquakes would be mostly much smaller than the Scotts Mills earthquake up to about the magnitude of that 1993 earthquake. The possibility of larger crustal earthquakes in the M6+ range cannot be ruled out. However, in the absence of known, mapped faults, the probability of such events is likely to be very Iow. Because the probability of large crustal earthquakes (M6 or greater) affecting Lane County is so low and because any damage in smaller crustal earthquakes is likely to be minor and very localized, crustal earthquakes are not considered significant for hazard mitigation planning purposes. Therefore, our analysis focuses on the larger, much more damaging earthquakes arising from the Cascadia Subduction Zone. Public Review Draft: August 4, 2004 10-6