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Geotechnicai Engineering and Geological Hazard Study Lincoln Elementary School Modernization Proiect Manteca, California Page 1 4.2 A,"% -.AL GEOI Sediments deposited by streams draining the western slopes of the Sierra Nevada Mountains and eastern slopes of the Coast Range have formed gently sloping alluvial fans, which compose the surface and near- suace soils across most of the Central Valley. Surface soils at the site are attributed to dune sand deposits overlying the Modesto Formation of the late Quaternary Period, 11,000 to 40,000 years before the present. These deposits are underlain by non -marine sediments of Holocene (Riverbank, Turlock Lake, Laguna) to Tertiary age (Mehrten, Valley Springs) and, at greater depth, by the late Jurassic to Cretaceous marine and non -marine sediments of the Great Valley Sequence (Wagner and others, 1987). The United States Department of Agriculture (USDA) mapped the surface soil type at the site as Delhi — Urban land complex, 100 percent, which is equivalent to silty sand in the Unified Soil Classification System. However, fill and grading activities may have altered the surface conditions during development of the Site. The geologic distribution of near -surface deposits in the vicinity of the Site is shown on is shown on Figure 4 — Geologic Map, Appendix A. 4.3 FAULTING AND SEISMICITY I he Lincoln Elementary School site is located in a moderately seismic region of California's Central Valley. The locations of significant faults relative to the site are shown on Figure 5 — Regional Fault Map, Appendix A. USGS data files of historic earthquakes indicate that 95 earthquakes of estimated magnitude MW5.0 or greater and 19 earthquakes of M,,,.6.0 or greater, and 2 earthquakes of estimated magnitude M.7.0 or greater have occurred within 161 km (100 miles) of the site since 1836. Among historic earthquakes, the 1906 earthquake on the San Andreas Fault located about 97 km (59 miles) west of the site is likely to have caused the strongest shaking. Effects of the 1906 earthquake were reported to be minimal in the Tracy, Byron, and Stockton areas in the report by the State Geologist. The more recent Loma Prieta earthquake (M,N 6.9 in 1989) occurred about 105 km (65 miles) to the southwest. The closest historical earthquake (MW5.0 in 1980) occurred about 45 km (28 miles) to the west near Livermore on the Greenville fault. The historical earthquake that may also have caused damage at the site (M,,.5.8 in 1866) occurred about 23 km (14 miles) to the northwest near the Great Valley thrust fault zone. A number of major active strike -slip faults belonging to the San Andreas Fault system trend northwest through the San Francisco Bay Area to the west of the site. According to the segmentation model developed by the Working Group on Northern California Earthquake Potential (1996), the Great Valley thrust fault zone, a system of northwest -trending concealed ("blind") thrust faults, lies approximately 23 km (14 miles) southeast of the site at an estimated depth of 7 km beneath the east foothills of the Coast Range Mountains. The trace of the vertical projection of the Great Valley Fault is shown on Figure 5. Since faulting within the Great Valley thrust fault zone does not typically rupture the ground surface, this fault system has only recently been recognized as a potential source of earthquakes. The Great Valley thrust fault zone was responsible for the M,,6.7 Coalinga earthquake of 1983 and is considered the probable source of the twin Vacaville -Winters earthquakes of 1892 of similar magnitude, as well as perhaps five other historic earthquakes of Mw6 or greater. No known active or potentially active faults cross the Lincoln Elementary School site, and the site is not located in a Fault -Rupture Hazard Zone as established by the Alquist-Priolo Earthquake Fault Zoning Act (Hart, 1994 and 2007). Therefore, ground rupture from faulting is not considered a significant hazard. The site is also not in an area currently covered by the Seismic Hazard Mapping Act which includes landslide and liquefaction hazards. Nevertheless, the site is near a number of major active faults capable of generating strong earthquakes. Active and potentially active faults considered capable of causing strong ground motion at the site are listed in the following table along with both respective distances to the site W