Glendale (PDF) - Hazard Mitigation Web Portal - State of California
Glendale (PDF) - Hazard Mitigation Web Portal - State of California
Glendale (PDF) - Hazard Mitigation Web Portal - State of California
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Natural <strong>Hazard</strong>s <strong>Mitigation</strong> PlanCity <strong>of</strong> <strong>Glendale</strong>, <strong>California</strong>Section 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 southern<strong>California</strong> with steep slopes have a high likelihood <strong>of</strong> being impacted by landslides.Primary Ground Rupture Due to Fault Movement typically results in a relatively smallpercentage <strong>of</strong> 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 <strong>of</strong> this hazard through building andfoundation design. Therefore, the primary mitigation measure is to avoid active faults by settingstructures back from the fault zone. Application <strong>of</strong> this measure is subject to requirements <strong>of</strong> theAlquist-Priolo Earthquake Fault Zoning Act and guidelines prepared by the <strong>California</strong> GeologicalSurvey – previously known as the <strong>California</strong> Division <strong>of</strong> Mines and Geology.History <strong>of</strong> Earthquake Events in Southern <strong>California</strong>To 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 <strong>of</strong>instrumentation, the detection and record <strong>of</strong> earthquakes are based on observations and felt reports,and are dependent upon population density and distribution. Since <strong>California</strong> was sparsely populatedin the 1800s, our record <strong>of</strong> 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 <strong>of</strong> the tremendously damaging potential <strong>of</strong> earthquakes in southern <strong>California</strong>. Morerecently, two M7.3 earthquakes struck southern <strong>California</strong>, 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 <strong>California</strong>’s population centers has the potential toplace millions <strong>of</strong> people at risk.Since seismologists started recording and measuring earthquakes, there have been tens <strong>of</strong> thousands<strong>of</strong> recorded earthquakes in southern <strong>California</strong>, most with a magnitude below 3.0. Plate H-3 (inAppendix H) shows the historical seismicity in the immediate vicinity <strong>of</strong> <strong>Glendale</strong>. The map showsthat small earthquakes, <strong>of</strong> magnitude between 1 and 3, have occurred historically in the area, but thatno moderate to large earthquakes have occurred beneath <strong>Glendale</strong> in historical times. Nevertheless,these recordings show that only the easternmost portion <strong>of</strong> southern <strong>California</strong> may be beyond thereach <strong>of</strong> a damaging earthquake. Table 6-2 lists the moderate to large historical earthquake eventsthat have affected southern <strong>California</strong>. The most significant <strong>of</strong> these events based on their felt effectsin <strong>Glendale</strong> are summarized in the next pages.2006 PAGE 6 - 10