THE SEISMIC CYCLE IN SOUTHERN CALIFORNIA - PRECURSOR OR RESPONSE

The seismicity rate (M greater than or equal to 3.0) in southern Calif ornia 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 o ccur after the 1952 Kern County (M7.5) and the 1992 Landers (M7.3) ear thquakes. The sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic mo ment released in the 1952 earthquake. This temporal correlation with t he preceding earthquake suggests that the seismic cycle (lower seismic ity after a major earthquake and higher seismicity before the next maj or earthquake) should be interpreted as a response to the first earthq uake 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 196 0s.