Aspects of the Quaternary Geology of the San Fernando Valley, California
J. C. Tinsley III
Earthquake Hazards Team, U.S. Geol Survey, Menlo Park, CA
The San Fernando Valley (SFV) is a structurally complex, sedimentologically diverse, and tectonically evolving late Tertiary- Quaternary basin situated within the Transverse Ranges of southern California. The SFV contains the headwaters of the Los Angeles River and its tributaries. Prior to the advent of flood control, the valley floor was composed of an active suite of alluvial fans and floodplains divisible in terms of provenance into eastern and western parts. East of I-405, powerful streams emanating from Pacoima and Big Tujunga canyons drain the petrologically complex western San Gabriel Mountains and deposit coarse, highly permeable alluvium that contains generally high-quality ground water and a deep water table. The more shallow western part derives mainly from Tertiary and pre- Tertiary sedimentary rocks, and is underlain by less permeable, finegrained deposits containing persistent shallow ground water and poorer water quality. Adjudication of water rights (1958–1961) provided a window on the Quaternary geology and a comprehensive evaluation of the attributes of the valley’s late Quaternary fill pertaining to the recharge, storage, transmittal, and yield of ground water from local and imported sources. Home of the 1971 San Fernando and the 1994 Northridge earthquakes, the SFV experienced nearrecord levels of strong ground motion in 1994 that caused widespread damage from strong shaking and ground failure. Ground motions affected lifelines and infrastructure regionally, in patterns reflecting not only source directivity, but which often reflected subsurface site conditions that are, in turn, functions of sedimentary provenance, Holocene depositional history and process, and persistence of shallow ground water. Post-earthquake investigations conducted for engineering and other scientific pursuits continue to explore the valley fill and its underpinnings, and to provide new insights into the Valley’s structural framework and earthquake potential, mainly involving the Northridge Hills fault, the Mission Hills fault, the Verdugo fault, and other valley-bounding structures.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California