Saticoy & Wells Community Plan & Development ... - City Of Ventura

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Saticoy & Wells Community Plan and Code EIRSection 4.6 Geological Hazards4.6 GEOLOGICAL HAZARDSThis section discusses potential seismic and geologic hazards in the Saticoy & Wells ProjectArea.4.6.1 Settinga. Regional (Structural) Geology. California is divided geologically into severalphysiographic or geomorphic provinces, including the Sierra Nevada range, the Central (Great)Valley, the Transverse Ranges, the Coast Ranges, and others. The Project Area lies within theTransverse Range geomorphic province of California. The Transverse Range includes VenturaCounty and portions of Los Angeles, San Bernardino, and Riverside counties.The Transverse Range was formed at the intersection of two tectonic plates: the Pacific to thewest and the North American plate. The compressive and shearing motions between thetectonic plates resulted in a complex system of active strike-slip faults, reverse faults, thrustfaults and related folds (bends in rock layers). Locally, the Transverse Ranges are characterizedby east-west trending mountains and faults. Major basins and ranges in the Transverse Rangesinclude the Ventura basin and the San Gabriel and San Bernardino Mountains.The Saticoy & Wells Project Area is located in the Ventura basin, which is drained primarily bythe Santa Clara River. The Ventura Basin is one of the most active tectonic regions in the world.b. Seismic Hazards. The Project Area lies in a highly active earthquake region ofsouthern California and thus is subject to various seismic and geologic hazards, includingground shaking, surface rupture, and landslides. Each potential geological hazard is describedbelow.Seismically Induced Ground Shaking. Faults produce comprehensive damage in twoways: ground shaking and surface rupture. Seismically induced ground shaking covers a widearea and is greatly influenced by the distance of the site to the seismic source, soil conditions,and depth to groundwater. Surface rupture is limited to very near the fault. Other hazardsassociated with seismically induced ground shaking include earthquake-triggered landslidesand liquefaction.Alquist-Priolo (A-P) Earthquake Fault Zones encompass surface traces of active faults that havepotential for future surface fault rupture. A-P Fault Zones are designated within 500 feet from aknown fault trace. Per the Alquist-Priolo legislation, no structure for human occupancy ispermitted on the trace of an active fault. The term “structure for human occupancy” is definedas any structure used or intended for supporting or sheltering any use or occupancy, which isexpected to have a human occupancy rate of more than 2,000 person-hours per year. Ifdevelopment is proposed within an A-P Fault Zone, a geologic study must be conducted fordevelopments of four units or more to determine the location of the fault trace. Based on thefindings in the geologic study, all structures for human occupancy must be set back a minimumof 50 feet from the fault trace because, unless proven otherwise, an area within 50 feet of anactive fault is presumed to be underlain by active traces of the fault.4.6-1City of Ventura
Saticoy & Wells Community Plan and Code EIRSection 4.6 Geological HazardsThe U.S. Geological Survey defines active faults as those that have had surface displacementwithin Holocene time (about the last 11,000 years). Holocene surface displacement can berecognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughsand aligned saddles, sag ponds, and the existence of steep mountain fronts. Potentially activefaults are those that have had surface displacement during Quaternary time, within the last 1.6million years. Inactive faults have not had surface displacement within the last 1.6 millionyears. A fault is a plane or surface in the earth along which failure has occurred and materialson opposite sides have moved relative to one another in response to the accumulation andrelease of stress. Faults that are known to have moved in recent history (the last 200 years) areconsidered historically active. Faults that have exhibited signs of activity during the last 11,000years are considered active, and faults that have exhibited signs of activity within 11,000 yearsto 2 to 3 million years ago are considered potentially active. Ground surface displacementalong a fault, although more limited in area than the ground shaking associated with it, canhave disastrous consequences when structures are located across or near the fault zone.Amounts of movement during an earthquake can range up to tens of feet. Fault displacementmay also occur gradually, not as a result of earthquakes, but as the nearly imperceptiblecontinual movement known as creep. Creep can produce the rupture or bending of buildings,fences, railroads, streets, pipelines, curbs, and other linear structures.Faults in the Project Area. Potentially active faults within in the Project Area include theCountry Club Fault and McGrath Fault. The Country Club Fault is a northwest-southeasttrending zone in the eastern portion of the City between Kimball Road and Wells Road to thewest and east, and Telegraph and Telephone Roads to the north and south. This fault isconsidered potentially active; however, it was evaluated in 1976 and was not designated as anAlquist-Priolo Special Studies Zone. The McGrath Fault runs along the Santa Clara River on thesouthern boundary of the Project Area. Other faults within the vicinity of the Project Areainclude the Ventura-Foothill Alquist-Priolo Zone, and Oak Ridge. Areas on or around activeand potentially active fault traces are potentially subject to surface rupture. These faults mayproduce damaging ground shaking and are shown on Figure 4.6-1.Effects of Seismicity. Table 4.6-1 shows the estimated maximum earthquake that mayoccur due to activity along the most significant faults that could affect the Saticoy & WellsProject Area. It includes active regional faults such as the San Andreas and the Anacapa thatare known to produce tremors sufficient in magnitude to affect large areas.In the event of a strong earthquake (magnitude 6.0 to 7.5) originating in southern VenturaCounty or a major earthquake (8.0 magnitude) along the San Andreas Fault, damage to manyexisting structures could be severe and some loss of life could occur.b. Landslides. A landslide is the perceptible downslope movement of earth mass. It ispart of the continuous, natural, gravity-induced movement of soil, rock and debris. Landslidingcan range from downslope creep of soil and rock material to sudden failure of entire hillsides.Landslides include rockfalls, slumps, block glides, mudslides, debris flows, and mud flows.4.6-2City of Ventura
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