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

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – EarthquakesEarthquake-induced landslides and rockfalls are secondary earthquake hazards that occur fromground shaking. Gravity inexorably pulls hillsides down, and earthquake shaking enhances this ongoingprocess. Landslides and rockfalls can destroy the roads, buildings, utilities, and other criticalfacilities necessary to respond and recover from an earthquake. Many communities in southernCalifornia with steep slopes have a high likelihood of being impacted by landslides.Primary Ground Rupture Due to Fault Movement typically results in a relatively smallpercentage of the total damage in an earthquake, yet being too close to a rupturing fault can result inextensive damage. It is difficult to safely reduce the effects of this hazard through building andfoundation design. Therefore, the primary mitigation measure is to avoid active faults by settingstructures back from the fault zone. Application of this measure is subject to requirements of theAlquist-Priolo Earthquake Fault Zoning Act and guidelines prepared by the California GeologicalSurvey – previously known as the California Division of Mines and Geology.History of Earthquake Events in Southern CaliforniaTo better understand earthquake hazards, scientists study past earthquakes by looking at theirrecords, or by studying the effects that past earthquakes had on the ground surface and the builtenvironment. Historical earthquake records are either from the instrumental period (since about1932, when the first seismographs were deployed), or pre-instrumental. In the absence ofinstrumentation, the detection and record of earthquakes are based on observations and felt reports,and are dependent upon population density and distribution. Since California was sparsely populatedin the 1800s, our record of pre-instrumental earthquakes is relatively incomplete. However, twovery large earthquakes, the Fort Tejon in 1857 (M7.9) and the Owens Valley in 1872 (M7.6) areevidence of the tremendously damaging potential of earthquakes in southern California. Morerecently, two M7.3 earthquakes struck southern California, in Kern County (1952) and Landers(1992), and a M7.1 earthquake struck the Mojave Desert (Hector Mine, in 1999). The damage fromthese five large earthquakes was limited because they occurred in sparsely populated areas. Asimilarly sized earthquake closer to southern California’s population centers has the potential toplace millions of people at risk.Since seismologists started recording and measuring earthquakes, there have been tens of thousandsof recorded earthquakes in southern California, most with a magnitude below 3.0. Plate H-3 (inAppendix H) shows the historical seismicity in the immediate vicinity of Glendale. The map showsthat small earthquakes, of magnitude between 1 and 3, have occurred historically in the area, but thatno moderate to large earthquakes have occurred beneath Glendale in historical times. Nevertheless,these recordings show that only the easternmost portion of southern California may be beyond thereach of a damaging earthquake. Table 6-2 lists the moderate to large historical earthquake eventsthat have affected southern California. The most significant of these events based on their felt effectsin Glendale are summarized in the next pages.2006 PAGE 6 - 10