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

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesthat crop out in the Verdugo Mountains from the alluvial fan deposits to the southwest. The fault isprobably coincident with the sharp break in slope along the southwestern edge of the VerdugoMountains, where many of the alluvial fans that emanate from the mountains merge together to formthe gently southwest-facing alluvial surface between the mountains and the Los Angeles River. Inolder aerial photographs of the area, Dolan and Tucker (1999) interpreted several small scarps thatcould represent the last surface rupturing event on this fault, but these scarps have all beenobliterated by development. In fact, the inferred trace of the Verdugo fault is covered with buildingsand roads along almost its entire length, which makes it difficult to find suitable field study areaswhere the fault can be exposed and studied.To date, there has been only one study in Glendale that attempted to locate and date the most recentsurface rupturing events on this fault. This study, conducted in Brand Park (Dolan and Tucker,1999) may have constrained the location of the fault zone in the area, but the actual fault trace couldnot be identified due to the discontinuous nature of the alluvial fan deposits that they encountered,and because the trench excavated was too unstable to be entered safely. Dolan and Tucker (1999)proposed that the trace of the Verdugo fault in this area is approximately 300 feet (90 m) farther tothe north of where it is inferred by Dibblee (1991), extending in a southeasterly direction throughthe area between the Tea House and the Dr.’s House at Brand Park. Unfortunately, Dolan andTucker (1999) could not confirm the fault location elsewhere due to landscaping and previousground surface modifications at the park (for parking lots and playing fields) that precluded thepossibility of excavating another trench.Previous investigators (Byer, 1968) also identified a wide zone of faulting farther to the north thatconsists of laterally discontinuous fault planes that generally dip to the northeast. Locally, theyobserved minor shearing of the terrace deposits, which suggested to them relatively youthfulmovement on the fault. This zone of faulting is identified in Plate H-4 with cross-hatchures. Thiszone of faulting may not be the most recent fault trace, but there are insufficient data to determinewhether or not these faults are active. Therefore, this fault zone should be investigated in the futureif development is proposed in the area.Although the most recently active traces of the Verdugo fault are not well located, mostinvestigators agree that the Verdugo fault is active and therefore has the potential to generate futuresurface-rupturing earthquakes. Earlier investigators suggested that this fault is primarily a thrustfault, responsible for uplift of the Verdugo Mountains (R.T. Frankian & Associates, 1968; Weber etal., 1980; Weber, 1980), but more recently, it is thought that the fault displays primarily left-lateralstrike-slip movement (Walls et al., 1998; Dolan, personal communication, 2002). A fault hazardmanagement zone that includes the inferred trace of the fault as mapped by Dibblee (1991), but iswider to the north, to include the break in slope and the zone of faulting mapped by Byer (1968) isproposed. As with the fault hazard management zone for the Rowley fault, geological studies shouldbe conducted for sites within the Verdugo fault hazard management zone if new development orsignificant redevelopment is proposed.The Eagle Rock fault crosses the southwestern part of Pasadena and the northernmost portion ofLos Angeles, including along a 2-mile stretch of the Ventura (134) Freeway, where it separatescrystalline bedrock on the north from sedimentary rock on the south (see Plates H-2 and H-4). Theportion of the Eagle Rock fault east of the San Rafael Hills was originally termed the “San Rafaelfault” by Weber (1980), who suggested the fault was active in late Quaternary time. This conclusionwas based on the presence of linear topographic features across the Pleistocene alluvial fan surfaceeast of the San Rafael Hills. Farther to the southeast, the fault appears to join the Raymond fault,however the exact location of the eastern terminus of the Eagle Rock fault is not well defined, and itsgeomorphology in this area is much more subdued than that of the Raymond fault. Consequently,Weaver and Dolan (2000) concluded that a connection with the Raymond fault could not be2006 PAGE 6 - 27
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesestablished with certainty. To the west, the Eagle Rock fault lies on trend with the Verdugo fault,although in the subsurface, based on gravity data, Weber (1980) suggests that there may be a step orbend between the two fault zones. Although very little is known about the Eagle Rock fault, giventhat it appears to be related to active faults in the area, such as the Verdugo fault, it should beconsidered potentially active, subject to further study. For example, although the Eagle Rock faultmay not be capable of generating an earthquake, it may break co-seismically with movement on theVerdugo fault. A fault hazard management zone for this fault has been recommended in the Pasadenaarea, similar to that for the Sierra Madre and Verdugo faults. Extension of this zone betweenPasadena and Glendale is recommended, but the limits of this zone are predominantly outside theCity of Glendale.The Scholl Canyon faults were mapped by Byer (1968), and Envicom (1975) suggested that thisfault zone connects the Verdugo fault in the west to the Eagle Rock fault in the east. However, morerecent mapping by Dibblee (1989b) does not even show these faults, and there are no data availableto indicate that these fault traces, if even present, are active.The York Boulevard fault is a short, northeast trending fault first mapped by Lamar (1970), andmore recently by Dibblee (1989a, 1989b) in the Adams Hill area of southern Glendale. According toLamar (1970) the fault does not offset older, Pleistocene-age deposits, and is therefore not active.However, the York Boulevard fault does appear to separate the Raymond fault from the Hollywoodfault, in an area where according to Weber (1980) there is step or bend in the fault zones at depth.Alternatively, the York Boulevard fault may be the eastern extension of the Hollywood fault. Basedon these relationships, and given that both the Raymond and Hollywood faults are active, Envicom(1975) suggested that the York Boulevard fault may be active also. Given its length, the YorkBoulevard fault is not likely to generate an earthquake, but it may move co-seismically with anearthquake on the Hollywood fault. Therefore, a hazard management zone for this fault is proposed,where geological studies to locate and characterize the fault would be required prior to developmentof a critical facility.The eastern terminus of the Hollywood fault has been mapped along the southwesternmost cornerof the City of Glendale (see Map 6.1 and Plate H-4). This fault has been shown to be active in theLos Angeles and West Hollywood areas, where recently obtained data indicate that this fault breaksin infrequent, but large magnitude earthquakes. In the West Hollywood area, the inferred location ofthe fault along Sunset Boulevard has been proven to be incorrect; the fault is farther south, in thevalley. However, in the Los Angeles area, the fault does appear to be at the mountain front. Thefault has been well located in the Hollywood Hills, just to the west of Glendale, by Yerkes (1967) andDibblee (1991b), but as it extends across the Los Angeles River and into the Glendale area, itslocation is less well defined. Given that this fault is considered active, the inferred location of thefault in Glendale is herein included in a fault hazard management zone. Because of its location in thefloodplain of the Los Angeles River, where shallow ground water and deep Holocene sediments areanticipated, geologic studies to locate this fault may prove to be difficult and expensive, requiring theuse of deep boreholes rather than trenching.A few other minor, unnamed faults have been mapped both in the San Rafael Hills and in theVerdugo Mountains (see Plate H-4). These faults appear to be confined to the older bedrock units,with no impact on the younger terrace and alluvial deposits, and are therefore not considered active.Fault hazard management zones for these faults are not considered warranted, however, geologistsstudying these areas should continue to look for evidence of Holocene movement on these faults. Asnew data are developed and verified by third-party reviewers, Plate H-4 should be amended to reflectany changes in the location, recency of activity and need for future studies on these faults.2006 PAGE 6 - 28
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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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