The mission link between the Hayward and Calaveras Faults

The left stepover between the Calaveras and Hayward faults gives rise to compressive strain that is accommodated by vertical deformation, producing the highest hills bordering the east side of San Francisco Bay (Monument Peak to Mission Peak). The projection of the active trace of the Hayward fault is separated 3.5 km from the junction of the northern and southern segments of the Calaveras fault at Calaveras reservoir. For pure strike slip on the Hayward and Calaveras faults, the stepover region is compressed in area at a rate given by the separation times the long‐term slip rate of 9 mm/yr on the Hayward fault. Assuming a seismogenic depth of 10 km, the horizontal compression accumulated over 150 years is equivalent to a dip‐slip seismic moment of 2.8 × 10 18 N m. This moment, if released in a single event, would generate an earthquake of magnitude 6.3 due to dip slip alone. Dip slip may occur on buried faults throughout the stepover region. The base of the southwest face of Monument Peak suggests a buried thrust or reverse fault dipping northeast. The prominent scarp on Mission Peak has the same strike and sense of slip. Current microearthquakes between the Calaveras and Hayward faults align parallel to the Mission fault, but show only horizontal slip on a vertical fault plane. A dipping fault is needed to satisfy the geometric constraint of the convergence. Therefore, the Mission fault zone must consist of two or more fault surfaces inclined at different dip angles, as is observed in the San Andreas fault zone in the Loma Prieta area. Horizontal slip is building up stress on the inclined fault, which is currently locked. Growing compressive stress will increasingly impede horizontal slip until a thrust or reverse‐slip event occurs, allowing then more horizontal slip to be transferred from the Calaveras to the Hayward fault. The dip‐slip fault might be the unstable element governing slip in the larger system.