High stress drop events in the Victoria, Baja California earthquake swarm of 1978 March

Abstract

The Victoria earthquake swarm of 1978 March with magnitudes up to ML= 4.8 occurred near the northern end of the Cerro Prieto fault in northern Baja California, Mexico. Accurate epicentre locations of a number of earthquakes in the swarm reveal that the activity concentrated in a zone of about 6 km radius (projected on the horizontal), with earthquakes occuring mostly at depths of around 12 km. A composite fault plane solution prepared with data from the larger earthquakes of the swarm indicates right‐lateral strike‐slip motion along a vertical plane extending parallel to the Cerro Prieto fault. Seismic moment, source radius and stress drop are calculated from strong motion records and digital seismograph records obtained at short epicentral distances, in most cases less than 10 km. From calculated displacement spectra and Brune's model, stress drops between 1 bar and ∼ 1 kbar were estimated for earthquakes with seismic moments in the range 1.4 × 1019 ‐ 2.9 × 1023 dyne cm. Eighteen events (of 44 studied here) had stress drops higher than 90 bar, a result in striking contrast to lower stress drop results from previous distant‐station studies of earthquakes in the region. Results of comparing near‐source and large distance spectral shapes for two earthquakes in the Victoria swarm region indicate that scattering and attenuation are a likely cause of the severe reduction of high‐frequency energy (clearly shown by the larger distance spectra), and thus of the resulting lower stress‐drop determinations at large distances. If these events are characteristic of other large events in the region then the low stress‐drop determinations of Thatcher, and Thatcher & Hanks for this region may be in doubt. Very high peak horizontal ground accelerations (up to 0.6 g) were measured from the near‐source records which were characterized by short‐duration bursts of high‐frequency energy. It is suggested here that these high peak accelerations resulted from a combined effect of high stress drop and sediment amplification. Using a crustal model derived from McMechan & Mooney, a sedimentary amplification factor of 3.4 is estimated and used as a correction in calculation of the earthquake source parameters. Four other high stress drop (up to 2.5 kbar) earthquakes in this area were also analysed. The high stress‐drop values obtained from seismic spectra are corroborated by using the rms (root mean square) acceleration formulation introduced by Hanks. Copyright © 1984, Wiley Blackwell. All rights reserved