Creep on the Hayward fault at Fremont is erratic due to the occurrence of occasional creep events with amplitudes of up to 4 mm. Since December 1993 creep has averaged 7.8 mm/year at the ruined Gallegos Winery in Fremont. Creep is measured every 10 minutes by a displacement transducer connected to a buried 30 m invar rod that crosses the fault obliquely at 30 degrees. The monuments to which the rod is attached penetrate 28 m beneath the surface and include an inclinometer system to measure the lateral displacement of the surface monument relative to its base. The westernmost of these monuments is vertical but pierces the fault at 21.5 m depth, indicating that the surface fault dips at 85 degrees to the SW. The offset of the borehole proceeds at the same rate as creep on the surface fault, which at this location has displaced the foundation of the ruined 1886 winery by more than 90 cm.

Above: 30 Nov 1993 to 1 Oct 1998 dextral creep data (CFW1) from Fremont, California. Soil motions and near-surface temperature effects cause the ragged appearance of the record at weekly and monthly periods. The sudden accelerations, with durations of several days to a week, evident in early 1996 and 1997, and mid-1997 and late-1998 are creep events occurring in the uppermost 100 m of the fault. If these and smaller episodic creep events are removed, the non-episodic creep rate is closer to 5 mm/year. The long term surface creep on the Hayward fault, here and elsewhere, is caused by creep occurring at depths of up to 7 km.

References

Bilham, R., and P. Bodin, Fault zone connectivity: slip rates on faults in the San Francisco Bay Area, California, Science, 258, 281-284, 1992.

Bilham, R. and S. Whitehead, Subsurface creep on the Hayard Fault, Fremont,California, Geophys. Res. Lett., 24(11), 1307-1310, 1 June 1997

Bilham, R. Measurements of Surface Stability of Engineered Geodetic Control Points, in The Global Positioning System for the Earth Sciences, 202-210, Nat. acad. Press Washington D.C. 1997 pp. 239.