Earthquake Monitoring in Southern California for Seventy-Seven Years (1932–2008)

Abstract

The Southern California Seismic Network (SCSN) has produced the SCSN earthquake catalog from 1932 to the present, a period of more than 77 yrs. This catalog consists of phase picks, hypocenters, and magnitudes. We present the history of the SCSN and the evolution of the catalog, to facilitate user understanding of its limitations and strengths. Hypocenters and magnitudes have improved in quality with time, as the number of stations has increased gradually from 7 to ~400 and the data acquisition and measuring procedures have become more sophisticated. The magnitude of completeness (M_c) of the network has improved from M_c ~3.25 in the early years to M_c ~1.8 at present, or better in the most densely instrumented areas. Mainshock–aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470,000 events. The earthquake frequency-size distribution has an average b-value of ~1.0, with M≥6.0 events occurring approximately every 3 yrs. The three largest earthquakes recorded were 1952 M_w 7.5 Kern County, 1992 M_w 7.3 Landers, and 1999 M_w 7.1 Hector Mine sequences, and the three most damaging earthquakes were the 1933 M_w 6.4 Long Beach, 1971 M_w 6.7 San Fernando, and 1994 M_w 6.7 Northridge earthquakes. All of these events ruptured slow-slipping faults, located away from the main plate boundary fault, the San Andreas fault. Their aftershock sequences constitute about a third of the events in the catalog. The fast slipping southern San Andreas fault is relatively quiet at the microseismic level and has not had an M>6 earthquake since 1932. In contrast, the slower San Jacinto fault has the highest level of seismicity, including several M>6 events. Thus, the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation.