The faulting mechanism of the San Fernando Earthquake

Abstract

The large displacement pulse occurring at Pacoima dam approximately 2.5 s after the triggering of the accelerogram is identified as the shear radiation emanating from the point of initial rupture, this interpretation suggesting that the San Fernando, California, earthquake was initiated with massive but localized rupture in the hypocentral region. On the basis of the resulting S-P time at Pacoima dam and teleseismic observations of the reflected phase pP, together with the estimated uncertainties in the local hypocentral determination of Allen et al. (1973), the point of initial rupture is located at 34°27.0′N, 118°24.0′W and h = 13 km. The breakout phases resulting from the rupture of the earth's surface are tentatively identified on the Pacoima dam accelerograms and teleseismic World-Wide Standard Seismograph Network seismograms. For the San Fernando earthquake the breakout phases do not appear to be particularly energetic, but this may only reflect the especially energetic nature of the initial rupture phases. The inferred length of faulting and the estimated origin times of the initial rupture and the breakout phases yield an average rupture velocity, of at least 2.8 km/s. Although they are subject to the uncertainties of the suggested effects of source propagation and of single-station observations, the source parameters of the initial rupture re estimated as follows: source dimension, 6–3 km; stress drop, 350–1400 bars; average slip, 4.6–9.2 m; and seismic moment, 1.7–0.85 × 1026 dynes cm. A 0.8-s time interval after the arrival of S1 contributes 40% of the radiated energy flux at Pacoima dam; the initial rupture event may have generated as much as 80% of the total energy radiated by the San Fernando earthquake. The stress difference accompanying the initial rupture is comparable to an estimate of the north-south compressive stress that is assumed to maintain the continued uplift of the San Gabriel Mountains; the stress drop obtained for the entire faulting process, however, is smaller than either of these by 1–2 orders of magnitude. That the San Fernando earthquake was apparently initiated with massive but localized failure not representative statically or dynamically of the full faulting process most likely reflects a highly nonuniform distribution of strain energy density in the incipient source region. This possibility adds a new dimension to laboratory studies, numerical models, and conventional seismic investigations of the earthquake mechanism.