SU0004094 - Laserfiche WebLink

Home Browse Search

4. ENVERONMENTAL ANALYSIS - 4.7 GEOLOGY,SOILS.AND SEISMICITY Significance after Mitigation: Less-than-significant. r Impact 4.7.4: In the event of a major earthquake,seismic ground shaking could potentially injure people and cause collapse or structural damage to proposed structures. This would ,r be a less-than-significant impact with mitigation. During the life of the project,there is a potential that ground shaking from a moderate to strong r earthquake would be felt at the proposed RMC Pacific Vemalis Quarry site. Ground shaking from a moderate to strong earthquake generated from an earthquake fault in the San Francisco Bay or San Joaquin/Sacramento Regions could generate ground accelerations at the proposed project site that could reach 0.5 g and therefore cause damage to structures,unsecured equipment and engineered and non-engineered quarry slopes. As discussed previously in this chapter,the potential for surface fault rupture or other secondary effects of earthquake ground shaking, such as liquefaction, are not considered a significant impact to the proposed quarry operation. The difficulty in assessing earthquake risk at the proposed RMC Pacific Vernalis Quarry is partly due to the lack of historic data regarding earthquakes. Simply stated,there has not been an earthquake in recent history strong enough to assess the ground motion performance of active surface quarry operations in the Vernalis-San Joaquin region. The strong earthquake on the ` northern Hayward Fault Zone,for instance,which many experts anticipate in the next 30 years, could result in substantially more ground shaking than the Bay Area experienced during the Loma _ Prieta earthquake in 1989. The Loma Prieta event is the most recent moderate to strong earthquake to affect the Bay Area-San Joaquin region. This event caused considerable damage to the Bay Area with an epicenter located approximately 140 miles southeast of San Francisco. The Loma Prieta event is the largest earthquake to affect the Bay Area-San Joaquin Delta Region since the Great Earthquake of 1906. Some scientists believe that the Bay Area and San Joaquin Delta region is overdue for the next large earthquake. The USGS has determined a 62 percent likelihood of a strong to moderate earthquake occurring in the Bay Area in the next 30 years (USGS WG02,2003). As discussed in the analysis of Impact 4.7.1, above, Geomatrix evaluated the stability of temporary and permanent slopes using a slope stability model. Typically,these models are sufficiently conservative and although not able to predict actual performance of a slope during an earthquake,they are able to provide engineers with an adequate estimation of dynamic stability. Seismologists also use other methods to estimate the ground acceleration on a particular site using information on earthquake fault characteristics,historic seismicity,and underlying geology. Based on information provided by the Geomatrix study(Geomatrix,2002),and by other published data(i.e.,CGS, 1991;CGS, 2000),earthquake ground shaking could originate from causative faults in the Bay Area or San Joaquin-Sacramento Valley region,including the Great ` Valley thrust fault,the San Joaquin fault,the Midway-Black Butte fault,the Vernalis fault, or the Marsh Creek-Greenville fault. r. RMC Pacific veroalis Quarry Mining and Reclamation Project 4.7-25 PSA/203015 Mach 2004 Administnuce Dmf EIR