Source parameters of the 1933 Long Beach earthquake

Abstract

Regional seismographic network and teleseismic data for the 1933 (M_L = 6.3) Long Beach earthquake sequence have been analyzed. Both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the Newport-Inglewood fault. The focal mechanism had a strike of 315°, dip of 80° to the northeast, and rake of −170°. Relocation of the foreshock-main shock-aftershock sequence using modern events as fixed reference events, shows that the rupture initiated near the Huntington Beach-Newport Beach City boundary and extended unilaterally to the northwest to a distance of 13 to 16 km. The centroidal depth was 10 ± 2 km. The total source duration was 5 sec, and the seismic moment was 5 * 10^(25) dyne-cm, which corresponds to an energy magnitude of M_W = 6.4. The source radius is estimated to have been 6.6 to 7.9 km, which corresponds to a Brune stress drop of 44 to 76 bars. Both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents. When the slip estimate from the seismic moment of 85 to 120 cm is compared with the long-term geological slip rate of 0.1 to 1.0 mm/yr along the Newport-Inglewood fault, the 1933 earthquake has a repeat time on the order of a few thousand years.