CV4
Location: Wright Road, Hollister, CA
Fault: Calaveras
The Calaveras fault intersects Wright Road on the northern outskirts of Hollister, California. Wright Road is a two lane blacktop road, surrounded by agricultural fields. The road is much used by heavy farm equipment and its condition steadily deteriorated until early 2004, when the road was partially repaved across the fault zone. The fault zone was previously detectable as a 10-meter-wide zone of coarsely crumbled asphalt near a couple of farmhouses north of the railroad crossing. There is currently no visible creeping trace of the fault at this site. About a kilometer northwest of the site on Highway 25, are nice examples of fault geomorphology such as sag ponds and a large pressure ridge.
This site exhibits some of the most interesting changes and varieties of creep behavior of all our sites. It appeared to exhibit both steady-state and episodic behavior prior to the 1989 Loma Prieta earthquake (LPE). The LPE, which occurred less than 50 km away, apparently triggered right-lateral slip of 12-14 mm at the site (Galehouse, 1990). This event was followed by a period of no creep from 1989 to late 1994, and then began to undergo steady-state creep with an average rate of ~9.9 mm/yr since 1995. The triggered slip may have been caused by transient, dynamic stress changes from the passing of body or surface waves of the LPE (e.g., Bodin et al., 1994). As noted by Galehouse (2003), the lack of creep during the five years after the earthquake might be understood in terms of static stress changes caused by the earthquake. Reasonberg and Simpson (1992) modeled static stress changes associated with the LPE and showed that the Calaveras fault, near Holister, lies in an area that experienced a relaxation in failure stress (fault patches #114, 115 on Figure). In otherwords, slip (creep) on this section of the fault would be inhibited for a period of time following the earthquake.
References
Bodin, P., R. Bilham, J. Behr, J. Gomberg, and K. Hednut (1994). Slip triggered on the southern California faults by the Landers earthquake sequence, Bull. Seism. Soc. Am. 84, 806-816.
Galehouse, J.S. (1990). Effect of Loma Prieta earthquake on surface slip along the Calaveras fault in the Hollister area, Geophys. Res. Lett. 17, 1219-1222.
Galehouse, J.S., and J.J. Lienkaemper (2003). Inferences drawn from two decades of alinement array measurements of creep on faults in the San Francisco Bay region, Bull. Seis. Soc. Am. 93 (6), 2415-2433.
Reasenberg, P.A., and R.W. Simpson (1992). Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake, Science 255, 1687-1690.
