Glendale (PDF) - Hazard Mitigation Web Portal - State of California

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – EarthquakesSan Andreas Fault Zone: As discussed previously, the San Andreas fault is the principal boundarybetween the Pacific and North American plates, and as such, it is considered the “Master Fault”because it has frequent (geologically speaking), large, earthquakes, and it controls the seismic hazardin southern California. The fault extends over 750 miles (1,200 kilometers), from near CapeMendocino in northern California to the Salton Sea region in southern California. At its closestapproach, the San Andreas fault is approximately 24 miles (38 km) north of Glendale.Large faults, such as the San Andreas fault, are generally divided into segments in order to evaluatetheir future earthquake potential. The segments are generally defined at discontinuities along thefault that may affect the rupture length. In central and southern California, the San Andreas faultzone is divided into five segments named, from north to south, the Cholame, Carrizo, Mojave, SanBernardino Mountains, and Coachella Valley segments (Working Group on California EarthquakeProbabilities - WGCEP, 1995). Each segment is assumed to have a characteristic slip rate (rate ofmovement averaged over time), recurrence interval (time between moderate to large earthquakes),and displacement (amount of offset during an earthquake). While this methodology has some valuein predicting earthquakes, historical records and studies of prehistoric earthquakes show that it ispossible for more than one segment to rupture during a large quake or for ruptures to overlap intoadjacent segments.The last major earthquake on the southern portion of the San Andreas fault was the 1857 Fort Tejon(Mw 7.8) event. This is the largest earthquake reported in California. The 1857 surface rupturebroke the Cholame, Carrizo, and Mojave segments, resulting in displacements of as much as 27 feet(9 meters) along the rupture zone. Peak ground accelerations in the Glendale area as a result of the1857 earthquake are estimated to have been as high as 0.18g. Rupture of these fault segments as agroup, during a single earthquake, is thought to occur with a recurrence interval of between 104 and296 years. Map 6.3 shows the seismic intensities that would be expected in the southern Californiaareas if a repeat of the 1857 earthquake occurred.The closest segment of the San Andreas fault to Glendale is the Mojave segment, locatedapproximately 29 miles to the northeast of the City Center area. This segment is 83 miles (133 km)long, extending from approximately Three Points southward to just northwest of Cajon Creek, atthe southern limit of the 1857 rupture (WGCEP, 1995). Using a slip rate of 30±8 millimeters peryear (mm/yr) and a characteristic displacement of 4.5±1.5 meters (m), the Working Group onCalifornia Earthquake Probabilities (WGCEP, 1995) derived a recurrence interval of 150 years forthis segment. The Mojave segment is estimated to be capable of producing a magnitude 7.1earthquake, which could result in peak ground accelerations in the Glendale area of about 0.13g.The WGCEP (1995) calculated that this segment has a 26 percent probability of rupturing sometimebetween 1994 and 2024.2006 PAGE 6 - 17