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I11. Environmental Setting, Impacts, and Mitigations J. Geology,Soils and Seismicity ismicity The Central Valley of California is considered an area of relatively low seismicity compared to other areas of the state,however,historic earthquakes have been centered in San Joaquin County, and the area is subject to the effects of major earthquakes on active regional faults. Faults within San Joaquin County,or within the region,would have the greatest potential for affecting the project site,although the probability of activity on the various faults shown in Figure III.J.1 and listed in Table III.J.2 varies widely and cannot be easily quantified. The closest identified fault to the project site is the Tracy-Stockton Fault. This fault crosses San Joaquin County from southwest, near Tracy,to the northeast near Linden. The position of this fault is known only from oil well log data,since no surface trace has been mapped. Subsurface data indicate that no appreciable movement has occurred on the fault for five million years or more. The closest large,regional faults are those of the southern portion of the Foothill Fault Zone(including the Melones and Bear Mountains fault zones)to the east of the project area (Figure III.J.1). This portion of the Foothill Fault Zone has been considered inactive, and the UU.S.G.S. monitoring in the vicinity of the New Melones Dam since 1972 has not found evidence of micro-seismic activity(City of Stockton, 1990). San Joaquin County has been subjected to the effects of several historical earthquakes,including some with epicenters apparently in the county. At least 15 earthquakes prior to 1900 and 5 earthquakes between 1900 and 1949 produced Modified Mercalli intensities of V or greater in Stockton(Toppozada and others, 1981;Toppozada and Parke, 1982). On April 10, 1881,an earthquake was felt near Linden(possible epicenter region)with an estimated Modified Mercalli intensity of VII (City of Stockton, 1990). Although direct correlations between intensity and magnitude are not precise,the quake was estimated to be equivalent to Richter magnitude 5. Two other smaller earthquakes(Richter magnitude 4)occurred in September 1940,centered about five miles south of Linden. It is not certain whether or not these epicenters lie along a northeastern extension of the Tracy-Stockton Fault. No maximum credible earthquake(MCE) has been assigned to this fault;however the City of Stockton acknowledges that there is the possibility that an active fault,capable of generating at least a 5.0 magnitude earthquake is located in this vicinity(City of Stockton, 1990). III.J.6