Dynamic stress drop and rupture dynamics of the October 15, 1979 Imperial Valley, California, earthquake

Abstract

Abstract A dynamic faulting model is constructed to determine the dynamic stress drop and relative strength distribution for the October 15, 1989 Imperial Valley earthquake, and to study the dynamics of rupture propagation for this event. Reproduction of the kinematic quantities of the strike-slip rate, strike-slip final offset, rupture time, and rupture duration, bu varying the dynamic parameters of dynamic stress drop and yield strength, is attempted. The fault plane is discretized in 1.5 km square grids. The model incorporates first-order corrections for the effect of the free surface and material property heterogeneity on the calculation of rupture at depth. The strike-slip particle velocities, final offsets, and rupture durations of the kinematic model are matched within about 40% everywhere below about 5 km, except in the hypocentral region and in a high displacement region 20 km north of the epicenter. The strike-slip rate of the dynamic model is half and rupture duration double those of the kinematic model in the epicentral region. The rupture durations and strike-slip final offsets are two thirds those of the kinematic model in a high-stress drop region beneath the El Centro crossing array. Two slip concentrations correspond to stress drop concentrations, one of about 80 bar at 20 km from the epicenter and at 8 km depth, and the other of about 70 bar at 30 km from the epicenter and 10 km depth. Most of the dynamic stress drop occurs between the depths of 9 and 11 km, averaging about 60 bar. Stress drops in the 7–9 km range vary between 25 and 35 bar, while stress drops in the 5–7 km depth range vary between 10 and 15 bar. This depth dependence of stress drop was a consistent feature across nearly 20 km of the fault. Stress drop appeared to be well correlated with rock type; negative stress drop was found in the sediment regions and the regions at depth, while positive stress drop was found in the metasediment regions between the depths of 5 and 11 km. Yield strength and rupture velocity are inversely related; high yield strength is found in regions of low rupture velocity and vice versa. Rupture was found to terminate at the northern end of the fault by collision with a high strength barrier. A region of supersonic rupture velocity was found to occur in a region of high stress drop and low fracture strength. The kinematic rupture velocity at the northern end of the fault was found to be unreasonably low; results show that this region reaches its peak stress before the kinematic break time. Yield strength increased with depth in the seismogenetic zone until the base of the zone was reached, below which it tapered off slightly