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V GEOMATRIX `+ 4.0 SITE GEOLOGY AND SUBSURFACE CONDITIONS �. The site is located near the western margin of the San Joaquin Valley. The San Joaquin Valley, along with the Sacramento Valley to the north, comprise the Central Valley of California, an asymmetric synclinal trough filled with up to 5 '/z miles of Jurassic to Quaternary (last 180 million years) age sediments. Locally, the City of Tracy in underlain by Quaternary age (last 2 million years) alluvial fan deposits eroded from the highlands to the southwest. The San Joaquin Valley and the Coast Ranges is within a broad zone of dominantly right- lateral strike slip deformation along the Pacific-North American plate boundary. A small component of compressional deformation across the plate boundary is accommodated by folding and thrust faulting between and to the east of major right-lateral strike slip faults (Wong and others, 1988). The site lies to the east of the major mapped active plate boundary faults, such as the San Andreas, Hayward and Calaveras faults. V The Vernalis fault is the closest potentially active seismic source; it is located about 2.8 miles northeast of the site. Although it is a subsurface reverse or thrust fault that poses no surface faulting hazard, and does not displace beds of Holocene age (last 11,000 years), the Vernalis fault is probably an active element within the Coast Ranges-Sierra Nevada Boundary Zone (CRSNBZ). In this regard, it may be similar to compressional faults that may have generated the Vacaville-Winters earthquakes (Magnitude 6%s, 4/19/1892 and Magnitude 61/4, 4/22/1892) `r along a segment of the CRSNBZ to the north near Vacaville or the Coalinga earthquake ' (Magnitude 6.4, 5/211983)to the south near Coalinga(Geomatrix Consultants, 1993). a. The boring drilled to explore the site's shallow subsurface soil and groundwater conditions encountered clayey sand/sandy clay to a depth of about 23 feet below the ground surface. Underlying these clayey soils, gravel of unknown thickness was encountered to the bottom of i the boring (i.e., 26% feet). In general, the blowcounts measured in the boring indicate that the clayey soils are soft to medium stiff. The relative density of the gravel could not be meaningfully evaluated from the information obtained from the boring; the single blowcount measurement obtained in the gravel may be influenced by these coarse-grained soils. Groundwater was first encountered in the boring at a depth of about 12 feet below the ground surface. It should be noted that the actual groundwater level at the site may be higher than that recorded at the time of drilling and may fluctuate during the year. I;\Projectl5 000s153131LE W IS-MAN.DOC 4 W