Special San Fernando Earthquake Edition

Effects of the San Fernando Earthquake

in the Oat Mountain Quadrangle

By R.B. Saul

Detailed geologic mapping in the southeast quarter of the Oat Mountain quadrangle was a part of the California Division of Mines and Geology's program of urban mapping between 1966 and 1968, in cooperation with the Los Angeles County Flood Control District and the Los Angeles County Engineer. This work revealed details of a segment of the Santa Susana thrust, a fault along which older rocks are known to override Quatemary alluvium. Within the quadrangle, disruption of the ground surface in response to the earthquake of February 9, 1971 was confined mainly to a zone roughly 2 miles wide along the eastern boundary of the quadrangle (a line roughly coincident with Balboa Boulevard) from Rinaldi Street on the south, north to about San Fernando Pass.

Across most of the southeast quarter of the Oat Mountain quadrangle, the complexly imbricate surface zone of the Santa Susana thrust trends due east. In general, the sense of movement is from north to south. Furthermore, subsurface data indicate that this thrust is folded along an east-west axis. West of Bee Canyon no element of this thrust appears to have moved during the recent earthquake. East of Bee Canyon tectonic and/or lurching motion appears locally to have been coincident with or closely parallel to the sole of the Santa Susana thrust zone as previously mapped and may represent a local deflection of energy along existing planes of weakness. Where the base of the Santa Susana thrust crosses the road to St. Vincent de Paul Camp (locality 1) left-lateral movement offset a line on the pavement about 1 foot (30 cm), with no visible vertical component of movement. To the west of this road the trace of ground movement turns northwest into Bee Canyon, athwart the zone of the Santa Susana thrust, and appears to be absorbed along the strike of the bedding in the sedimentary rocks of the north wall of Bee Canyon. Northeast of St. Vincent De Paul Camp road, ground breakage follows the trace of the sole of the Santa Susana thrust (see photo, page 83) along a discontinuous and locally ambiguous series of breaks as far as the large road cut on Balboa Boulevard north of Van Norman Lake (localities 2 and 3).

In low-lying areas, surface cracks appear to be largely the result of lurching or settling of alluvium. In hilly areas, effects range from minor lurching of fills and soils to intensely shattered soil, landslides and rock falls. The most severe effects are principally confined to the square mile surrounding the mouth of Bee Canyon. Here, soil and weathered rock debris on ridge crests lies in cracked and jumbled disorder as though it had been heaved, and the flanks of most ridges are scarred by shallow landslides. In the Bee Canyon and Cascade Oil Field area, rocky ridges, road cuts, and the steep cliff along the north side of the canyon yielded rock falls.