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KLEINFELDER The reevaluation of the Foothills fault system consisted, in part,of trenching across known lineaments of the Bear Mountains and Melones faults. Trenches excavated in the vicinity of Rawhide Flat, near Sonora, California, indicate offset within the older soil horizons. Additional trenching in areas to the north indicate possible offset of the Mio-Pliocene Mehrten Formation by the Bear Mountains fault. Using methods derived by H. Bolton Seed and I.M. Idriss, Table 1-1, below was complied for bedrock accelerations to be expected in the site area. TABLE 1-1 ESTIMATED PEAK BEDROCK ACCELERATION Estimated Peak Closest Maximum Maximum Bedrock Acceleration Distance Credible Probable (g) Based on Maximum of Fault to Fault Earthquake* Earthquake* Credible Earthquake Site (mi.) San Andreas 8.3 8.0 0.06 80 Sierra Nevada 8.3+ 8.0 <0.05 >100 Hayward 7.5 7.0 0.07 60 Calaveras 7.5 7.0 0.12 45 Midland 7.0 6.5 0.14 30 Foothills System 6.5 0.28 12 *Richter Magnitude The information shown on Table 1-1 for the Melones and Bear Mountains faults represent the main fault trace on which the maximum credible earthquake is most likely to occur. This information includes only faults that are designated Holocene or later. The above analysis indicates there is a potential for peak bedrock accelerations as high as 0.28g, based on a maximum credible Richter magnitude 6.5 earthquake occurring on the Bear Mountains fault approximately 12 miles from the site. Based on past history, the potential for a 6.5 magnitude event on the Bear Mountains fault is very low4. There does not appear to be Holocene faults (faults younger than 10,000 years before present) mapped in. the immediate vicinity of the Foothill Landfill facility. However, according to review of the Earthquake Epicenter Map of California3 it appears the Stockton Fault may extend to within one mile south of the Foothill Landfill. FOOTHILL 5