SR 06:209 - Laserfiche WebLink

'\ T 1ql1f:.f';JC"tinn In the City of Pleasanton, about 12,000 acres are in areas of moderate, high or very high liquefaction susceptibility. See Figure V -4 for a detailed liquefaction susceptibility map for Pleasanton. Liquefaction is a specialized form of ground failure caused by earthquake ground motion. It is a "quicksand" condition, occurring in water-saturated, unconsolidated, and relatively clay-free sands and silts caused by hydraulic pressure (from ground motion) forcing apart soil particles, and forcing those into quicksand-like liquid suspension. In the process, ground materials that are normally firm but wet take on the characteristics of liquids. The potential for liquefaction depends on soil conditions and groundwater levels, which may fluctuate. Susceptibility to this hazard is greatest when groundwater tables are high. 4 T ';Jtpr';Jl c:prp';lrling The type of displacement of major concern associated with liquefaction is lateral spreading because it involves displacing large blocks of soil down gende slopes or towards stream channels. Lateral spreading could occur along the artoyos where surface materials consist of young alluvial and fluvial deposits.3 An occurrence of lateral spreading due to seismic activity is also most likely in conjunction with heavy rainfall. .c; Othpr Sp1o;:.m1C" H';I'Z';Jrrlo;:. Other seismic hazards with less potential impact on the Planning Area include surface faulting, lurch cracks,4 rock falls, differential settlement, and seiches. Catastrophic events - such as failure of the Del Valle Reservoir Dam and subsequent flooding of the Pleasanton Planning Area - are considered to have an extremely low potential for occurrence. Earthquake Fault Zones The Alquist-Priolo Hazards Act passed by the State legislature ill 1972 (renamed the Alquist-Priolo Earthquake Poult Zoning Act in 1993) established earthquake fault zones along faults considered by the State Division of Mines and Geology to be active or potentially active. An active fault is considered one which has experienced surface displacement within the last 11,000 years, while a potentially active fault is a fault which has moved during the past two to three million years but not proven to have moved within the past 11,000 years. The State Geologist designates "seismic hazard zones" and the State issues earthquake fault zone maps to assist cities and counties in avoiding the hazard of surface fault rupture. The earthquake fault zone 3 Alluvial deposits are water-transported sediments such as clays. sands. and/or gravels. fluvial deposits are those produced by action of. or found in. a river or stream. 4 Lurch cracks - or fissures - do not result from surface faulting. They form when weak or unconsolidated soil is subjected to intense shaking and is incapable of responding elastically. or when the boundaries of contrasting surface materials respond differently to ground motion. Public Safety 082906, dean V-8 City Council 8/29/2006