Resolution No. 4793 - Laserfiche WebLink

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STUDY AREA CHARACTERISTICS Liquefaction Liquefaction is a phenomenon in which shaking of a saturated so/~ causes its material properties to change so that it behaves as a liquid. Lateral spreading ks a liquefaction- induced hazard that involves the displacement of essentially intact blocks of soil either downslope or toward a free face, such as a river channel. In the Eugene-Springfield area the lateral spread hazard ranges from none to moderate, with a moderate lateral-spread hazard within the Holocene meander belts of the Willamette and McKenzie Rivers. Soils that liquefy tend to be young, unconsolidated, water-saturated silts and sands with low clay content. Older (Pleistocene) gravels with a thin veneer of silt (outside the meander belt) or young (Holocene) sand and gravel (~rtside the meander belt) underlie that part of the Eugene-Springfield area potentially subject to liquefaction. Gravel will liquefy only under exceptional conditions. Within the modem meander belts of the Willamette and McKenzie Rivers, shear wave velocity measurements indicate that the youngest sands and gravels may liquefy. This urdt averages about 5 meters (approximately 16 feet) in thickness with a range of 0-8 meters (approximately 0-26 feet) (DOGAMI, 2000). A seismic vulnerability evaluation was performed in 1996 to provide a preliminary opinion on major seksrrdc weaknesses and deficiencies at the WPCF. The report revealed that the potential for liquefaction or other permanent ground deformation is low for all structures except the plant ouffall (see Section 2.2.4). Landslides tn the Eugene-Springfield area, slope instability from strong shaking could be a significant threat. However, the movement and characteristics of existing landslides are highly variable, ranging from active movement to stable. Although most earthquake-induced landslides occur in materials not previously involved in sliding (Keefer, 1984), it would require numerous site-specific studies to analyze landslide hazards in the area and to understand the nature of each of the landslides that currently exist in the Eugene- Springfield area. However, hazard characterizations for the area are provided on the Relative Earthquake Hazard Map of the Eugene-Springfield Metropolitan Area (Figure 2.2.3-2). Seismic Hazard I~ap Areas prone to seism_~c hazards in the Eugene-Springfield area are identified on the Relative Earthquake Hazard Map of the Eugene-Springfield Metropolitan Areaa (Figure 2.2.3-2) (DOGAMI, 2000). The map depicts the relative risk of earthquake damage that results from local geologic conditions. The composite hazard map was developed by combining single hazard maps for ground motion amplification, and slope instability. The single component maps were developed to show geographic patterns of stronger earthquake effects for two likely sources. Zones that are expected to have the most pronounced damage in any moderate or larger earthquake are shown on the map as having the greatest hazard. 2 The map depicts earthquake hazard zones that are based on limited geologic and geophysical data. The map is not a substitute for site-specific investigations by qualified practitioners. At any point in the map area, site-specific investigations may give results that differ from those shown on the map. For a complete understanding of the earthquake hazard, consultation of the following DOGAMI publication is also recommended: Madin, I.P., and Mabey, M.A., 1996, Earthquake Hazard Maps for Oregon: Oregon Department of Geology and Mtneral Industries Geological Map Series GMSIO0. MWMC_2.0_REV23 DOC 2d ~