Subsurface creep on the Hayward fault, Fremont, California

Surface creep on the Hayward fault at Fremont has been sustained for the past century at 8.2 +/- 0.1 mm/year. Two surface creepmeters record a similar mean creep rate in the past 3 years, but the data show significant irregularity despite being anchored at depths of 10-28 m. The serendipitous piercing of the subsurface fault by one of the creepmeter anchors, whose deflection at depth has been regularly monitored with a biaxial inclinometer system, reveals two fault traces dipping at approximate to 70 degrees. At 21 m depth the Hayward fault slips dextrally at 8.1 +/- 0.4 mm/yr and at approximate to 7 m depth a subsidiary fault slips bidirectionally adding noise to the surface creep data. Surface creep on this segment of the Hayward fault was arrested by stress changes following the 1989 Loma Prieta earthquake, but resumed in 1994. Tilt of the borehole, and a similar borehole east of the fault, is 3 orders of magnitude too large to be driven by uniform creep extending to depths of several km, and we conclude that irregular slip on the fault occurs in the uppermost few hundred meters. Cumulative dilatational strain since 1868 near segments of the fault NW of the creepmeter may exceed 200 mu strain, unless absorbed by undocumented slip off the main fault trace.