Preliminary seismological and geological studies of the San Fernando, California, earthquake of February 9 1971

Abstract

The San Fernando earthquake was the largest earthquake to occur in the metropolitan Los Angeles area in more than 50 years. It has tentatively been assigned a magnitude, M_L of 6.6, a focal depth of 13.0 km, and an epicentral location about 12 km east of Newhall, California, at 34°24.0'N, 118°23.7'W (Figure 1), but these figures undoubtedly will be modified as further data become available. Although the focal depth is not as well defined as the epicenter, it is consistent with other observations suggesting thrusting on a fault plane dipping north about 45 ° and breaking the surface in the Sylmar-San Fernando area (Figure 1). It should be emphasized that the hypocenter of the main shock represents only the point of initial rupture. Breaking, presumably, then propagated southward and upward from this point, so that the main geological and engineering effects were observed farther south where the fault was shallower and the displacement greater. The location of the main shock is based on readings from permanent stations of the Caltech network, as well as the U. S. Geological Survey station at Point Mugu (SBLG) and the California Department of Water Resources stations at Pyramid (PYR) and Cedar Springs (CSP). Portable Caltech seismographs were installed in the epicentral area as early as 3 hr following the main shock, and, within a few days, there were at least 30 portable units in the region operated by various groups and agencies.