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

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 2 – Community ProfileThe most striking geologic features of the Glendale area are the Verdugo and San GabrielMountains, ranges that form a dramatic backdrop to the southern and northern portions,respectively, of the city. These rugged, geologically young uplands consist of a series ofpredominantly east-west trending mountain ranges and intervening valleys. The mountains aremade up of various bedrock types, including igneous (granite and diorite) and metamorphic(gneiss and quartzite) rocks (see Plate H-2: Geologic Map of Glendale, in Appendix H). Asthese bedrock types weather and break down over time, they form unconsolidated sedimentsconsisting of silt-, sand-, and gravel-sized pieces of granite and gneiss. These unconsolidatedsediments are transported into the intervening valleys by running water, gravity, and wind,where they are deposited on the floodplain and fans as alluvium and fan material. This processhas been occurring for thousands of years, and some of the earliest alluvium (referred to as olderalluvium), which crops out in the La Cañada Valley and the piedmont surface south of theVerdugo Mountains, has developed secondary clay minerals that make the deposits weaklyconsolidated, and with a slightly red color as a result of some of the iron minerals turning torust.Other Significant Geologic FeaturesThe city of Glendale, like most of the Los Angeles Basin, overlies or is near to several knownfaults capable of producing damaging earthquakes. The major faults that have the potential toaffect the city of Glendale include the Sierra Madre, Verdugo, Hollywood, Raymond, PuenteHills Blind Thrust, San Andreas, San Gabriel, Newport-Inglewood, and Palos Verdes faults(refer to Section 6, see Map 2-2, and Plate H-4 in Appendix H). The San Gabriel Mountains arebeing uplifted along the Sierra Madre fault zone, whereas the Verdugo Mountains are beinguplifted along the Verdugo fault. Uplift of these mountains accelerated in mid-Pleistocene time,about 500,000 years ago, continues today, and is one of the fastest in the world, in the context ofgeologic time.The Los Angeles Basin experiences many small tremors every year, but its history has beenshaped by several relatively infrequent, but powerful earthquakes. The first historicalearthquake was recorded in 1769, when the Portola expedition was camped next to the SantaAna River in what is now the city of Orange, but earthquakes undoubtedly have shaken the areafor millennia. Other more recent earthquakes were recorded in 1812, 1857, 1933 (Long Beach),1987 (Whittier), and 1994 (Northridge). The 1857 Fort Tejon event was a large magnitude 8+earthquake on the San Andreas fault that caused only minor damage because the epicentral areawas largely unpopulated. A similar-sized earthquake today would result in thousands ofcasualties and billions of dollars in property loss. Given that paleoseismological researchindicates that great earthquakes (i.e., M8.0+) occur on the San Andreas fault at intervalsbetween 45 and 332 years, with an average interval of 140 years, another similar M8 earthquakeon the San Andreas fault is considered likely in the not-too-distant future. This fact aloneshould encourage local governments to strengthen their infrastructure and prepare for “the BigOne.” Furthermore, as we will discuss in this document, there are other lesser-known faultscloser to Glendale that have the potential to cause more damage to the city than the moredistant San Andreas fault. The earthquake hazard to the Los Angeles basin and the citiestherein is severe.2006 PAGE 2 - 4