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CHAPTER 8.0 SEISMICITY A. ENVIRONMENTAL SETTING The= site,which lies within the Sacramento-San Joac uin Valley,is located within the central portion of the tectonic province known as the Sierran block. The interior of this province is less seismically active than the boundaries, which are associated with known faults and zones of intense deformation. These active faults include the San Andreas fault,the Hayward fault,the Calaveras fault, and a portion of the Telsa fault, west of the province; the Big Pine,Garlock and White Wolf faults to the south; and the Sierra Frontal system,which includes the Sierra Nevada fault, to the east. These faults along the margins of the block are the primary sources of major seismic events that could potentially cause ground shaking at the site. Seismic events within the Sierran block are characteristically infrequent,of moderate to low grade magnitude, and associated with small to no surface displacement (Woodward-Clyde Consultants, 1978; Bryant, 1983). Three regional faults within the Sierran block are: the Stockton fault; the buried Midland fault zone; and the Bear Mountain fault zone, none of which are shown as active on the State fault map (Jennings, 1977). These faults are discussed below: • The Stockton fault is covered by a few thousand feet of continental sediments, suggesting that there has been no Quaternary movement along this fault,which is in agreement with the State fault map. • The Midland fault has been classified as inactive as recently as 1987 (Harwood and Helley, 1987) because it does not extend through materials deposited since the early Oligocene epoch, about 35 million years ago. However, it is increasingly postulated that blind thrust faulting is occurring at depth along much of the western edge of the Central Valley, and that the Midland fault may be part of that activity. Some possibility of activity on the Midland fault or a nearby blind thrust fault,therefore,should be recognized,although such activity would quite probably have a very long return period. • The Bear Mountain fault zone has been associated with the Oroville earthquakes in 1975; therefore, Bolt (1985) suggests that the very low probability that a moderate earthquake might occur on the Bear Mountain fault zone be considered in assessing the seismic intensity of the Stockton area. Most of the ground shaking experienced in the Stockton area was the result of earthquakes occurring outside the boundaries of the Sierran block. Seismic intensities in the site vicinity are conservatively estimated as V to VI on the Modified Mercalli scale (Woodward Clyde Consultants, 1987;Bolt, 1985). Ground surface accelerations in the area are estimated at less than 0.018 for more than 90 percent of the historical earthquakes. The highest maximum ground acceleration was estimated as 0.12g for one f of the historical events. These estimates were based on deterministic analysis of the historical earthquakes using the attenuation equation by Donovan and Bornstein(1975). 8-1