Generalized ray models of the San Fernando Earthquake

The exact Cagniard-de Hoop solutions for a point dislocation in half-space are used to construct models of the strong ground motion observed during the February 9, 1971 San Fernando earthquake (M_L = 6.4). By summing point dislocations distributed over the fault plane, three-dimensional models of a finite fault located in a half-space are constructed to study the ground motions observed at JPL (Pasadena), Palmdale, Lake Hughes, and Pacoima Dam. Since the duration of faulting is comparable to the travel times for various wave types, very complex interference of these arrivals makes a detailed interpretation of these wave forms difficult. By investigating the motion due to small sections of the fault, it is possible to understand how various wave types interfere to produce the motion due to the total fault. Rayleigh waves as well as S to P head waves are shown to be important effects of the free surface. Near-field source effects are also quite dramatic. Strong directivity is required to explain the difference in amplitudes seen between stations to the north and stations to the south. Faulting appears to have begun north of Pacoima at a depth of 13 km. The rupture velocity, which is near 2.8 km/sec in the hypocentral region, appears to slow to 1.8 km/sec at a depth of 5 km. Displacements on the deeper sections of the fault are about 2.5 m. Fault offsets become very small at depths near 4 km and then grow again to 5 m near the surface rupture. The large velocity pulse seen at Pacoima is a far-field shear wave which is enhanced by directivity. Peak accelerations at Pacoima are probably associated with the large shallow faulting. The total moment is 1.4 × 10^(26) ergs.