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DISCUSSION OF ENVIRONMENTAL ATTRIBUTES t. GEOLOGYIEARTH A. Description 1. Geology & Soils Tracy lies in the upland valley portion of the central valley geomorphic province. From the edge of the Delta, the upland valley rises gradually in elevation toward the west and the southwest. Most of the area lies at elevations between sea level and 100 feet (30 meters) above sea level. The upland valley, flat and featureless in most places, is broken by the bottomiands of the major rivers and the smaller streams. It has been extensively leveled and graded in the course of cultivation and urban development. 2. Seismic Activity Faults which have historically been the source of earthquakes felt in Tracy include the San Andreas, Calaveras, Hayward, Midland, Green Valley, and Tracy-Stockton. The Tracy area, itself, has a low to moderate seismic history. In the past it has been subject mainly to ground motions from several earthquakes at moderate to great distance, perhaps up to more than 100 miles. None of these earthquakes had a magnitude greater than 3.9 on the Richter Scale within Tracy. Subsurface data indicates that no appreciable movement has occurred on the Tracy-Stockton Fault for five million years or more. However, inconclusive evidence of activity was found at the eastern edge of the fault in 1881 and 1940. Therefore, it is not certain whether this is an active fault and experts indicate the possibility of a 5.0 magnitude earthquake along this fault. 3. Soil The soils on the upland valley consist of alluvium produced through the erosion of rocks in the surrounding mountains and foothills and transported to the valley floor by rivers and streams. The two most widely used soil classification systems in California are the Storie Index, developed by the University of California, and the Land Use Capacity System, developed by the USDA Soil Conservation Service. Both of these systems classify soils based on their usefulness for agriculture. 17