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SAN LEANDRO SHORELINE DEVELOPMENT DRAFT EIR <br />CITY OF SAN LEANDRO <br />GEOLOGY, SOILS, AND SEISMICITY <br />Geotechnical investigations have been conducted for construction of various buildings and other <br />improvements (proposed and existing) on the marina fill dikes.$ Borings drilled for these improvements <br />generally encountered 5 to 13 feet of fill underlain by 3 to 16 feet of Bay Mud, which was in turn <br />underlain by older, firmer alluvial, and estuarine deposits. <br />East of the historic shoreline the project site is underlain by alluvial sediments. These sediments typically <br />consist of interbedded clay and silt with some sand. Little or no Bay Mud is likely to be present in this <br />area. <br />Liquefaction <br />Liquefaction is a phenomenon where loose, saturated, non -cohesive soils such as silts, sands, and gravels <br />undergo a sudden loss of strength during earthquake shaking. The test borings performed at the site for <br />previous developments encountered potentially liquefiable sands and silts within the alluvial sediments <br />and in the dredged fill created from the alluvial sediments. These materials are intermixed with clays that <br />would not normally be subject to liquefaction. <br />Regional Faulting, Seismicity, and Seismic Hazards <br />Regional Faulting and Seismicity <br />Seismic activity within the Coast Ranges is generally associated with active faults of the San Andreas <br />system, which includes major active faults both east and west of the site, as shown in Figure 4.5-2. Over <br />the width of the San Francisco Bay Region, approximately 1.5 inches/year of relative horizontal movement <br />occurs between the North American and Pacific Plates9. This movement is partially accommodated by <br />creep and earthquakes occurring along active faults. The approximate distances and directions to major <br />active Bay Area faults are summarized in Table 4.5-2. <br />As indicated in the above table, the active fault nearest the site is the Hayward fault, which is located <br />approximately 3.8 miles northeast of the east end of Project site. The Hayward fault is a northwest - <br />trending zone approximately 51 miles long, which extends from southeastern San Jose, through the East <br />Bay communities, into San Pablo Bay. Beneath San Pablo Bay, faulting generally steps right (east) to the <br />Rodgers Creek fault. To the south, the Hayward fault merges with the Calaveras fault. <br />Although the City of San Leandro is listed as an area affected by the Alquist-Priolo Fault Zones, the Project <br />site is not in one of the Alquist-Priolo Earthquake Fault Zones. The closest such zone is along the Hayward <br />Fault approximately 3.8 miles northeast of the Project site. <br />8 Harding Lawson Associates, 1977, Geotechnical Investigation For Horatio's Restaurant; Gribaldo Jones & Associates, 1969, <br />Geotechnical Investigation For Tia Maria Restaurant; Woodward, Clyde, Sherard & Associates, 1965 &1972, Geotechnical <br />Investigations for Marina (Blue Dolphin) Restaurant; Earth Systems Consultants, 1884, Geotechnical Investigation For Launch <br />Ramp Restaurant, Geotechnical Investigation for Marina Launch Ramp, Treadwell and Rollo, 2005. <br />9 Working Group on California Earthquake Probabilities (WGCEP), 2008, "The Uniform California Earthquake Rupture <br />Forecast, Version 2 (UCERF 2): 2007 — 2036. U. S. Geological Survey Open File Report 2007-1437. <br />4.5-6 DECEMBER 2014 <br />