Emerging from the Stress Shadow of the 1992 Mw 7.3 Landers Southern California Earthquake? A Preliminary Assessment

Abstract

Numerous felt earthquakes have occurred in southern California in 2001. The most prominent sequences have been near Big Bear, in the Coso region in eastern California, in the northern Los Angeles basin, and along the San Jacinto Fault (Figure 1). These main shocks have been followed by productive aftershock sequences and in some cases by enhanced microseismicity in adjacent regions, which were recorded by the Caltech-USGS TriNet (Hauksson et al. , 2001). These sequences thus raise the possibility that background seismicity has increased regionally, as has happened in the past. The late 1980's and early 1990's were a particularly active time in southern California, with numerous earthquake sequences. The ML 5.9 1987 Whittier Narrows earthquake (Hauksson et al. , 1988), the ML 4.9 1988 Pasadena earthquake (Jones et al. , 1990), the 1988 and 1990 Upland earthquakes (Hauksson and Jones, 1991), the ML 5.8 1991 Sierra Madre earthquake (Hauksson, 1994), and the Mw 6.7 Northridge earthquake (Scientists, USGS and SCEC, 1994) occurred in the Los Angeles basin. To the east, the Mw 6.0 1986 North Palm Springs, MS 6.6 1987 Superstition Hills (Magistrale et al. , 1989), and the 1992 Mw 6.1, Mw 7.3, and Mw 6.5 Joshua Tree, Landers, and Big Bear earthquakes occurred adjacent to the San Andreas system and within the eastern California shear zone (ECSZ) (Sieh et al. , 1993). Following the 1992 Landers earthquake, seismic activity throughout southern California appeared to drop off dramatically. To quantify the change in seismicity rate, Jones and Hauksson (1997) analyzed the rate of seismicity from 1945 to 1997. They showed that there were periods of high activity, with 90–100 events/yr of M ≥ 3 from 1945–1952 and 1969–1992, and periods of low activity, with 60–70 events/yr of M ≥ …