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MWMC FACILmES PLAN (FEMA, 1997), and the USGS National Seismic Hazard Mapping Project database. One of the values generally used to determine an earthquake's relation to building damage is peak ground acceleration (PGA). According to USGS, a PGA of 0.10 g (g equals the acceleration as a result of gravity) may be the approximate threshold of damage to older (pre-1965) dwellings or dwellings not made to resist earthquakes. In comparison, some post-1985 dwellings, built to California earthquake standards, have experienced severe shaking (0.60 g) with only chimney damage and damage to dwelling contents. The PGA at the site corresponding to a 10 percent probability of exceedance in 50 years (approximately 475-year mean return period) is approximately 0.18 to 0.19 g at the bedrock surface. This value of PGA on rock is an average representation of the acceleration most likely to occur at the site for all seismic events (crustal, intraplate, or subduction) for the 475- year return period. TABLE 2.2.3-2 Modified Mercalli Scale MWMC Facilities Plan, Eugene-Springfield MM Accel. Intensity %g Description of Intensity Level I 0.1 Not felt except by a very few under especially favorable circumstances. II 0.2 Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many III 0.3 people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, IV 0.7 windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects V 1.5 overturned. Pendulum clocks may stop. Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. VI 3 Damage slight. Damage negligible in building of good design and construction; slight to moderate in well- VII 7 built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars. Damage slight in specially designed structures; considersble in ordinary substantial VIII 15 buildings, with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Damage considerable in specially designed structures; well-designed frame structures IX 32 thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Some well-built wooden structures destroyed; most masonry and frame structures with X 70 foundations destroyed. Rails bent. XI - Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII - Damage total. Lines of sight and level distorted. Objects thrown into the air. Note: The approximate values of acceleration shown are not part of the definition of MM intensity. The values are from Richter, C., Elementary Seismology, W.H. Freeman, 1958. 2-10 MWMC 2.0 REV23.DOC