A Dislocation Model of the 1994 Northridge, California, Earthquake Determined From Strong Ground Motions

Abstract

A preliminary rupture model of the 1994 Northridge, California earthquake, determined from strong motion waveform inversion and analysis, is presented. The fault rupture plane chosen is based on the distributions of aftershocks and teleseismic surface-wave and bodywave point-source solutions. The fault strikes 122°, dips 42°, and has a slip vector of 109°. The average slip is about 1.2 meters over the rupture area and the peak slip reaches nearly 4 meters. Our estimate of the seismic moment is 1.2 ± 0.2 x 10^(26) dyne-cm. The area of rupture is small relative to the aftershock dimensions and is approximately 14 km along strike (west-northwest from the hypocenter) and nearly 20 km in the updip direction. There is little indication of slip shallower than about 7 km. The up-dip, near-source strong-motion velocity waveforms show two distinct, large S-wave arrivals 2-3 sec apart (as do the teleseismic P waves), indicating separate source subevents. An along strike (west-northwest) subevent separation of about 8 km is most consistent with the observation that the two main arrivals are separated more in time to the south and southeast (about 4.5 sec at Stone Canyon Reservoir and Sherman Oaks, for example), than at northern azimuths. The interpretation of secondary arrivals observed at more distant stations to the south and southeast (e.g., Santa Monica) is more tenuous, since several of the aftershocks recorded there indicate later arrivals as well. However, a secondary source contribution is expected based on our model of the closer stations. After placing these constraints on the general nature of the rupture, we predict the characteristics of the long-period (1-10 sec) ground velocities over a grid of stations covering the near-source region. This exercise provides a basis for separating the effects of source radiation (dominated by radiation pattern and directivity) from the complex waveform modifications due to wave-propagation and site response.