Source characteristics of the Loma Prieta, California, Earthquake of October 18, 1989 from global digital seismic data

Abstract

Displacement, velocity and acceleration records of P and SH body waves recorded at teleseismic distances are analysed to determine the static and dynamic source parameters of the Loma Prieta, California earthquake of October 18, 1989 ( m b 6.6, M s 7.1). Three distinct bursts of energy corresponding to three subevents can be recognized in most records. The displacement waveforms indicate that the first subevent contributes negligible moment while the largest releases of moment and energy are controlled by the second and third subevents. The second and third subevents are located north and south of the initial nucleation. A fourth small subevent needed to model later features of the P waveforms suggests that slow slip continued after the major releases of energy occurred. The waveforms are fit with a fault plane solution having strike 130°, dip 65° and slip 140° for all the subevents. The focal depths of the two major subevents are 16 and 12 km, and their asperity radii are 4.0 and 6.0 km, respectively. The seismic moment, M 0 , is 2.2×10 26 dyne-cm. From spectral analysis of teleseismic velocity, the radiated energy, E s , is estimated to be 1.1×10 22 dyne-cm, implying an apparent stress of 15 bars. From the high-frequency level of the teleseismic acceleration spectrum and a rupture area of 440 km 2 , we derive a dynamic stress drop of 51 bars.