The seismic cycle in southern California: Precursor or response?

The seismicity rate (M ≥ 3.0) in southern California shows two cycles with periods of high activity (90 events/year), from 1945–1952 and 1969–1992, and lower activity (60–70 events /year) from 1952–1969 and 1992-present. Abrupt drops in the seismicity rate occur after the 1952 Kern County (M7.5) and the 1992 Landers (M7.3) earthquakes. The sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake. This temporal correlation with the preceding earthquake suggests that the seismic cycle (lower seismicity after a major earthquake and higher seismicity before the next major earthquake) should be interpreted as a response to the first earthquake rather than a precursor to the second. Southern California is now at a rate of seismicity as low as it experienced in the 1950s and 1960s.