Quaternary Geology and Seismic Hazard of the Sierra Madre and Associated Faults, Western San Gabriel Mountains

Abstract

This detailed study of a 40-km-long section of the Sierra Madre and associated fault zones in the central transverse Ranges, along the south side of the San Gabriel Mountains, is aimed at providing information for evaluating the seismic hazard that these faults pose to the heavily populated area immediately to the south. Evidence on the location of fault strands and the style and timing of fault movements during the Quaternary was obtained from detailed geologic mapping, aerial-photograph interpretation, alluvial stratigraphy, structural and stratigraphic relations in some 33 trench excavations at critical localities, and subsurface data. We present a time-stratigraphic classification for the Quaternary deposits in the study area, based on soil development, geomorphology, and contact relations among the alluvial units. We distinguish four units, with approximate ages, as follows: unit 4, about 200,000 yr to middle Quaternary; unit 3: about 11,000 to 200,000 yr; unit 2; about 1,000 to 11,000 yr; and unit 1; younger than about 1,000 yr. We use this classification to evaluate on a semi-quantitative basis the evidence for fault activity in the study area and to infer the relative seismicity of different segments of the Sierra Madre fault zone during the Quaternary. Alluvial-fan development (particularly fanhead incision and the ages of alluvial-fan deposits) also gives clues as to relative seismicity. The most active segment of the Sierra Madre fault zone within the study area is the westernmost section, adjacent to the faults that broke during the 1971 San Fernando, Calif., earthquake. The age of activity, as indicated by the occurrence of Holocene faulting, decreases toward the east. Along the Sierra Madre fault, through La Canada, Altadena, Sierra Madre, and Duarte, is abundant evidence of late Pleistocene faulting. Total vertical displacement is more than 600 m, but there is no evidence for Holocene fault movement. These observations suggest that the presently applicable recurrence interval between major earthquakes in the central and eastern sections of the Sierra Madre fault zone is longer than about 5,000 yr. The local magnitude (M_L) of the largest credible earthquake that could occur on the Sierra Madre fault zone in the study area is estimated at 7, on the grounds that the fault zone is probably limited mechanically by subdivision into separate arcuate segments about 15 km long. The Raymond fault, which branches southwestward from the Sierra Madre fault in the eastern part of the study area, shows well-defined evidence of a late Quaternary history of repeated fault movements. Displacements of alluvial strata observed in trench excavations across the fault give evidence of five major seismic events, whose times of occurrence can be estimated from radiometric dating at approximately 36,000, 25,000, 10,000-2,200 (two events), and 2,200-1,500 yr B.P. Further evidence suggests at least three more faulting events in the past 29,000 yr, for which specific dates cannot be determined. Because some additional events probably remain undetected, we infer that an average recurrence interval of about 3,000 yr, with an average vertical displacement of 0.4 m per event, is applicable to the Raymond fault in its present state, as indicated by its history of movement over the past 36,000 yr. This level of activity is distinctly higher than that found for the Sierra Madre fault zone in the central and eastern parts of the study area. If the entire 15-km length of the Raymond fault would rupture in a single event, as seems likely, a maximum credible earthquake of M_L 6 3/4 can reasonably be assumed.

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