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

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesalso common. The 1989 Loma Prieta and Northridge earthquakes showed that reactivation ofexisting deep-seated landslides also occurs (Spittler et al., 1990; Barrows et al., 1995).A combination of geologic conditions leads to landslide vulnerability. These include high seismicpotential; rapid uplift and erosion resulting in steep slopes and deeply incised canyons; highlyfractured and folded rock; and rock with inherently weak components, such as silt or clay layers.The orientation of the slope with respect to the direction of the seismic waves (which can affect theshaking intensity) can also control the occurrence of landslides.Several areas in Glendale have been identified as vulnerable to seismically induced slope failure (seePlate H-5). The mountainous region along the northern reaches of the city (the San GabrielMountains) is susceptible to slope failure due to the steep terrain. The crystalline bedrock that cropsout in the northern and central portions of the San Rafael Hills is locally highly fractured andweathered. In steep areas, strong ground shaking can cause slides or rockfalls in this material. Slopefailures can also occur in the western and central portions of the city, in the Verdugo Mountains,where locally steep terrain is combined with fractured igneous and metamorphic rock units.Numerous small landslides can be expected to occur in these areas in response to an earthquake onthe Sierra Madre, the Verdugo or other nearby faults. For a more detailed assessment of potentialslope instability in the Glendale area, refer to Section 9 of this report.Ridgetop Fissuring and ShatteringLinear, fault-like fissures occurred on ridge crests in a relatively concentrated area of rugged terrainin the Santa Cruz Mountains during the Loma Prieta earthquake. Shattering of the surface soils onthe crests of steep, narrow ridgelines occurred locally in the 1971 San Fernando earthquake, but waswidespread in the 1994 Northridge earthquake. Ridgetop shattering (which leaves the surfacelooking as if it was plowed) by the Northridge earthquake was observed as far as 22 miles away fromthe epicenter. In the Sherman Oaks area, severe damage occurred locally to structures located at thetops of relatively high (greater than 100 feet), narrow (typically less than 300 feet wide) ridgesflanked by slopes steeper than about 2.5:1 (horizontal:vertical). It is generally accepted that ridgetopfissuring and shattering is a result of intense amplification or focusing of seismic energy due to localtopographic effects (Barrows et al., 1995).Ridgetop shattering can be expected to occur in the topographically steep portions of the SanGabriel Mountains north of Glendale, in the Verdugo Mountains, and locally in the San Rafael Hills.These areas are for the most part undeveloped, so the hazard associated with ridgetop shattering isrelatively low. However, above ground storage tanks, reservoirs and utility towers are often locatedon top of ridges, and during strong ground shaking, these can fail or topple over, with the potentialto cause widespread damage to development downslope (storage tanks and reservoirs), ordisruptions to the lifeline systems (utility towers).Vulnerability AssessmentThe effects of earthquakes span a large area, and large earthquakes occurring in the southernCalifornia area would be felt throughout the region. However, the degree to which earthquakes arefelt, and the damages associated with them may vary. At risk from earthquake damage are largestocks of old buildings and bridges; many hazardous materials facilities; extensive sewer, water, andnatural gas pipelines; earthen dams; petroleum pipelines; and other critical facilities, not to mentionprivate property and businesses. Secondary earthquake hazards, such as liquefaction and earthquakeinducedlandslides, can be just as devastating as the ground shaking.Damage to the extensive building stock in the area is expected to vary. Older, pre-1945 steel frame2006 PAGE 6 - 31
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesstructures may have unreinforced masonry such as bricks, clay tiles and terra cotta tiles as claddingor infilling. Cladding in newer buildings may be glass, infill panels or pre-cast panels that may failand generate a band of debris around the building exterior (with considerable threat to pedestrians inthe streets below). Structural damage may occur if the structural members are subject to plasticdeformation which can cause permanent displacements. If some walls fail while others remain intact,torsion or soft-story problems may result. Overall, modern steel frame buildings have been expectedto perform well in earthquakes, but the 1994 Northridge earthquake broke many welds in thesebuildings, a previously unanticipated problem.Buildings are often a combination of steel, concrete, reinforced masonry and wood, with differentstructural systems on different floors or different sections of the building. Combination types thatare potentially hazardous include: concrete frame buildings without special reinforcing, precastconcrete and precast-composite buildings, steel frame or concrete frame buildings with unreinforcedmasonry walls, reinforced concrete wall buildings with no special detailing or reinforcement, largecapacity buildings with long-span roof structures (such as theaters and auditoriums), largeunengineered wood-frame buildings, buildings with inadequately anchored exterior cladding andglazing, and buildings with poorly anchored parapets and appendages (FEMA, 1985). Additionaltypes of potentially hazardous buildings may be recognized after future earthquakes.Mobile homes are prefabricated housing units that are placed on isolated piers, jackstands, ormasonry block foundations (usually without any positive anchorage). Floors and roofs of mobilehomes are usually plywood, and outside surfaces are covered with sheet metal. Mobile homestypically do not perform well in earthquakes. Severe damage occurs when they fall off theirsupports, severing utility lines and piercing the floor with jackstands.In addition to building types, there are other factors associated with the design and construction ofthe buildings that also have an impact on the structures’ vulnerability to strong ground shaking.Some of these conditions are discussed below:• Building Shape - A building’s vertical and/or horizontal shape can be important. Simple,symmetric buildings generally perform better than non-symmetric buildings. During anearthquake, non-symmetric buildings tend to twist as well as shake. Wings on a buildingtend to act independently during an earthquake, resulting in differential movements andcracking. The geometry of the lateral load-resisting systems also matters. For example,buildings with one or two walls made mostly of glass, while the remaining walls are made ofconcrete or brick, are at risk. Asymmetry in the placement of bracing systems that provide abuilding with earthquake resistance, can result in twisting or differential motions.• Pounding - Site-related seismic hazards may include the potential for neighboring buildingsto "pound," or for one building to collapse onto a neighbor. Pounding occurs when there islittle clearance between adjacent buildings, and the buildings "pound" against each other asthey deflect during an earthquake. The effects of pounding can be especially damaging if thefloors of the buildings are at different elevations, so that, for example, the floor of onebuilding hits a supporting column of the other. Damage to a supporting column can result inpartial or total building collapse.Damage to the region’s critical facilities and infrastructure need to be considered and planned for.Critical facilities are those parts of a community's infrastructure that must remain operational afteran earthquake. Critical facilities include schools, hospitals, fire and police stations, emergencyoperation centers, and communication centers. Plate H-12 shows the locations of the City’s firestations, police stations, schools, and other critical facilities. A vulnerability assessment for thesefacilities involves comparing the locations of these facilities to the hazardous areas identified in the2006 PAGE 6 - 32
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NOTES:This map is intended for gene
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City of GlendaleUpdateHazard Mitiga
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Similar to Safety ElementIncludesEa
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HospitalsPartnershipsWithin City an
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Impact on CityMost significant area
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