Why study Southern California?

A complex web of active faults slice through southern California, making it a fascinating target for seismic and geologic study. Not only does the great San Andreas fault transit this region, but here it takes an enormous 40° bend between the Cajon and Tejon Passes (click on the map below to find those places). One of California’s largest historical events, the great 1857 shock, ruptured 300 km of the San Andreas fault from Cajon Pass in the southeast to 80 km north of Tejon Pass. To the southwest of the bend, ribbons of secondary strike slip fault conceal hidden or ‘blind’ thrust faults at depth, along which the 1971 M=6.7 San Fernando and 1994 M=6.7 Northridge events struck. To the northeast of the bend, another band of strike-slip and normal faults extend into the Mojave Desert, and are collectively termed the Eastern California shear zone. The 1992 M=7.3 Landers and 1999 M=7.1 Hector Mine shocks occurred there.

---

What are we up to?

Our work has focused on several aspects of the earthquake process in southern California (click on the map to see the complete references). Grant Marshall and Ross Stein created three 35mm slides that summarize the effects the 1994 Northridge earthquake. Ross Stein and Tom Hanks reassessed the catalog of earthquakes to see if there is a ‘seismic moment deficit’ that will soon be filled by more vigorous earthquake activity; instead they found a deficit neither for the rate of large shocks nor for the seismic moment (Stein and Hanks, 1998). Tom Parsons and Ross Stein studied how the earthquakes interact, one triggering another. In several papers, we have examined the San Fernando, Northridge, Landers, Homestead Valley, Joshua Tree, Big Bear, and Hector Mine shocks. Chuck Wicks and Wayne Thatcher studied the Coso volcanic field to track the ground movement caused by inflation of a magma chamber beneath the geothermal field (Wicks et al. 2001). Gerald Bawden, Wayne Thatcher, Ross Stein and Ken Hudnut used InSAR imagery and GPS data to identify and remove the large seasonal and long-term deformation of the earth’s surface caused by pumping for water and oil (Bawden et al. 2001). This work reveals the component of deformation that must be accommodated by the mysterious but threatening blind thrust faults in greater Los Angeles.

Because the region is heavily populated and seismically active, there is a wealth of monitoring networks in place, such as the seismic TerraScope and the Southern California Seismic Network, and the Southern California Integrated GPS Network. In addition, a series of major seismic experiments such as LARSE I and II, and the collaboration of institutions participating in the Southern California Earthquake Center, provide one of the world’s best laboratories to study earthquake occurrence.

We study the San Francisco Bay area from a number of perspectives, all focused on our efforts to understand earthquake occurrence in this heavily populated urban corridor. Some of our investigations have examined the fault slip and crustal deformation that preceded, accompanied, and followed the Great 1906 earthquake. Other studies have looked at the deformation associated with the 1989 Loma Prieta shock [Marshal and Stein, 1996]. These help us understand how the earthquake changed the conditions for future fault failure in the Bay area. Other studies have concentrated on the transfer of stress from one earthquake to another [Parsons et al., 1999]. We have looked closely at the effect of 1988 foreshocks on the 1989 Loma Prieta shock [Perfettini et al., 1999], and we have also looked at how the 1989 inhibit future earthquakes on the southern Hayward fault, and promoted them on the offshore San Gregorio fault.

Several studies identified the 27 June 1988 M=5.3 and 8 August 1989 M=5.4 Lake Elsman earthquakes as rare events that struck within 5 km of the future Loma Prieta rupture plane, and within 11 km from the 17 October 1989 M=7.0 Loma Prieta hypocenter. After both Lake Elsman earthquakes, the USGS and California State Office of Emergency Services issued a joint advisory of a heightened probability of M=6.5 shocks during the succeeding 5 days. The advisory was partly motivated by the observation that the two Lake Elsman events were among the three largest shocks to occur anywhere along the extent of the 1906 San Andreas rupture since 1914. Here we attempt to calculate the effect of the Lake Elsman shocks on the future Loma Prieta rupture.

We seek answers to the question, Did the Lake Elsman events hasten the occurrence of the Loma Prieta shock, influence the site of its nucleation, or its distribution of earthquake slip?

We find that the Lake Elsman events did not bring the future Loma Prieta hypocentral zone closer to failure. Instead, they are calculated to have unclamped the Loma Prieta rupture surface at the site where the greatest slip subsequently occurred in the Loma Prieta earthquake. This association between the sites of peak unclamping and slip suggests that the Lake Elsman events did indeed influence the Loma Prieta rupture process. Unclamping the fault would have locally lowered the resistance to sliding, an effect that could have been enhanced if the lowered normal stress permitted fluid infusion into the unclamped part of the fault. The Lake Elsman-Loma Prieta result is similar to that for the 1987 M=6.2 Elmore Ranch-M=6.7 Superstition Hills earthquakes, suggesting that foreshocks might influence the distribution of mainshock slip rather than the site of mainshock nucleation.

Read this online paper:

H. Perfettini, R. S. Stein, R. W. Simpson and M. Cocco,

Stress Transfer by the 1988-89 M=5.3, 5.4 Lake Elsman Foreshocks to the Loma Prieta Fault: Unclamping at the Site of Peak Mainshock Slip, J. Geophys. Res., 104, pp. 20,169-20,182, 1999.

[Online article][Printable article (6.74 Mb)]