Triggered slip on the Calaveras Fault during the magnitude 7.1 Loma Prieta, California, Earthquake

Abstract

After the magnitude (M) 7.1 Loma Prieta earthquake on the San Andreas fault we inspected selected sites along the Calaveras fault for evidence of recent surface displacement. In two areas along the Calaveras fault we documented recent right‐lateral offsets of cultural features by at least 5 mm within zones of recognized historical creep. The areas are in the city of Hollister and at Highway 152 near San Felipe Lake, located approximately 25 km southeast and 18 km northeast, respectively, of the nearest part of the San Andreas rupture zone. On the basis of geologic evidence the times of the displacement events are constrained to within days or hours of the Loma Prieta mainshock. We conclude that this earthquake on the San Andreas fault triggered surface rupture along at least a 17‐km‐long segment of the Calaveras fault. These geologic observations extend evidence of triggered slip from instrument stations within this zone of Calaveras fault rupture.

Triggered aseismic fault slip from nearby earthquakes, static or dynamic effect?

“…Besides afterslip and perturbation of creep rate, the occurrence of an earthquake also can produce “triggered aseismic fault slip,” which is a form of fault creep coinciding closely in time with a large nearby event while being distinct spatially from the primary rupture [ Sylvester , 1986]. It was first observed after the 1968 Borrego Mountain earthquake [ Allen et al , 1972], and subsequently detected after the 1979 Imperial Valley [ Sieh , 1982; Fuis , 1982], the 1981 Westmorland [ Sharp et al , 1986a], the 1986 North Palm Springs [ Sharp et al , 1986b; Williams et al , 1988], the 1987 Elmore Ranch and Superstition Hills [ Hudnut and Clark , 1989; McGill et al , 1989; Sharp , 1989], and the 1989 Loma Prieta earthquakes [ Galehouse , 1990; McClellan and Hay , 1990] as well as the 1992 Landers event sequence [ Bodin et al , 1994]. …”

The Loma Prieta, California, earthquake of October 17, 1989: Aftershocks and postseismic effects

“…Fortunately, we were able to photograph them before they were obliterated by rainfall that occurred that afternoon. The cracks had to have formed sometime after the last previous rainfall on 29 September 1989 (McClellan andHay, 1990) and probably formed shortly after the LPEQ (see the following discussion).…”

“…For example, such slip occurred after the 1968 Borrego Mountain earthquake (Allen and others, 1972), the 1979 Imperial Valley earthquake (Fuis, 1982;Bilham and Williams, 1985), the 1981 Westmorland earthquake (Sharp and others, 1986a), the 1983 Coalinga earthquake (Mavko and others, 1985;Schulz and others, 1987), the 1984 Morgan Hill earthquake , the 1986 Tres Pinos earthquake (Simpson and others, 1988), the 1986 North Palm Springs earthquake (Fagerson and others, 1986;others, 1986b, Wesson andothers, 1986;others, 1986, 1988), the 1987 Superstition Hills earthquake (Sharp, 1989;Hudnut and Clark, 1989;McGill and others, 1989), and the 1989 Loma Prieta earthquake (Behr and others, 1990;Galehouse, 1990;McClellan and Hay, 1990).…”

“…It is more likely that shaking from the LPEQ triggered slip on the Calaveras fault at the exact time of the earthquake or within a few minutes after it. Additional evidence that slip on the Calaveras fault was triggered by the LPEQ comes from field observations made by McClellan and Hay (1990), who observed fresh en echelon cracks and offset cultural features that indicated at least 5 mm of movement along 17 km of the Calaveras fault from the city of Hollister northwest to Highway 152. Table 2 shows a comparison of Calaveras fault creep rates in the Hollister area determined by USGS creepmeters and San Francisco State University (SFSU) theodolite measurements.…”

Slip triggered on southern California faults by the 1992 Joshua Tree, Landers, and Big Bear earthquakes

“…Subsequent examples included the 1979 Imperial Valley earthquake (Sieh, 1982;Fuis, 1982), the 1981 Westmoreland earthquake (Sharp et al, 1986a), the North Palm Springs earthquake (Williams et al, 1988;Sharp and et ai., 1986b), and the 1987 Superstition Hills earthquake (McGill and et al, 1989;Hudnut and Clark, 1989). Triggered slip has also been observed from several recent central California earthquakes: the 1983 Coalinga earthquake (Mavko et al, 1985;Schulz et al, 1989), the 1984 Morgan Hill earthquake (Schulz, 1985), the 1986 Tres Pinos earthquake (Simpson et al, 1988), and the 1989 Loma Prieta earthquake (Galehouse, 1990;McClellan and Hay, 1990 Triggered slip is of interest for several reasons. First, there seem to be several seemingly inconsistent proeesses at work in the phenomenon that require reconciliation.…”