Abstract: Effects of the Loma Prieta Earthquake on Creep Rates at Certain Sites on the San Andreas, Hayward, and Calaveras Faults

Jon S. Galehouse

Measurement sites along the San Andreas fault that were not creeping before the Loma Prieta earthquake are still not creeping. One site, however, just south of San Juan Bautista has been creeping at about 14 mm/yr since the Loma Prieta earthquake. This is about twice as fast as the rate that the U.S. Geological Survey had been measuring at this site prior to the quake.

Creep at most of the measurement sites along the Hayward fault slowed for several years following the Loma Prieta earthquake. Most of these sites have now resumed creeping at close to their overall long-term average of about 4.5-5 mm/yr. However, the southernmost part of the creeping fault in southern Fremont has had no right-lateral creep at two sites since the Loma Prieta earthquake. This is the same part of the fault that the U.S. Geological Survey had determined was creeping at about 8-9 mm/yr before the quake.

The Loma Prieta earthquake triggered more than 1 cm of coseismic slip at two sites on the Calaveras fault in the Hollister area that had been creeping at about 7 and 12 mm/yr for the previous several decades. Since the triggered slip, both sites have been creeping much more slowly than before the quake.

It appears that changes in regional stress can cause changes in creep rates that can persist for years. All the rate changes mentioned above are consistent with calculated static stress changes on these faults due to the Loma Prieta earthquake.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California