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36 <br /> As noted, the peak ground accelerations shown in the above table are based <br /> on Campbell' s 1981 unconstrained attenuation relationship. This relationship <br /> statistically relates the averages of the peak ground accelerations in the x and <br /> y directions to the distance of instrumented sites from the actual earthquake <br /> fault rupture surface. These attenuation curves then take the statistical <br /> average of the data points for the various magnitude earthquakes to develop the <br /> relationship of acceleration versus distance. It is important to recognize that <br /> Campbell's attenuation relationship, as well as all other similar relationships, <br /> are based on a limited amount of data points for sites close to large earth- <br /> quakes. <br /> As listed above, the peak ground acceleration expected at the site is 0.2 g. <br /> Such an acceleration may be caused by a maximum credible earthquake along the San <br /> Joaquin fault, 5 miles from the site. The greatest ground acceleration from a <br /> maximum probable earthquake is 0.1 g. Such an acceleration may be caused by a <br /> Magnitude 8 earthquake located 60 miles from the site along the San Andreas <br /> fault. <br /> San Joaquin County is considered by the County Building Department as being <br /> in U.B.C. Seismic Zone 3. Structures in the county must be built to at least UBC <br /> Zone 3 specifications. <br /> Site Period <br /> We have estimated the characteristic site period, Ts, using the criteria <br /> presented in UCB Standard No. 23-1. Computations to determine site period were <br /> performed using the equivalent single-layer and multi-layer methods as described <br /> in UBC. For our computations we used shear wave velocity values based on actual <br /> site blow count data, material properties and layer thicknesses to the depths <br /> drilled. We also have assumed that: (a) structures and equipment are founded on <br /> in place materials or on piles, or (b) the site to be improved by removal and <br /> replacement, or by in-place DDC treatment; . <br /> For the in-situ condition or the pile foundation options, a site period of <br /> about 0.45 sec was computed. This site period should be increased to 0.5 seconds <br /> in conformance to UBC recommendations. However, the presence of glass compli- <br /> cates the conditions at this site. Owing to this complication, if equipment and <br /> structures are founded on unimproved materials or piles, the design also should <br /> be checked assuming a site period of 0.7 sec. <br /> McClelland <br /> 1144A/CC-36 ::' <br />