Subsurface Creep and Geometry of the Hayward-Calaveras Stepover

The San Francisco Bay Area has not experienced a major earthquake beneath and urban center since 1906. The Hayward fault is the most populated fault in the area, and 140 years after its last rupture, also has the greatest risk associated with it. A contiguous subsurface stepover connecting the Hayward and the Calaveras faults appears to directly transfer slip between the two faults, and may affect earthquake rupture scenarios on both faults. Although the Hayward fault is partially locked for much of its trace, the southernmost fault creeps up to 9 mm/yr, equal to its long-term slip rate. At the proposed juncture region, the southern Calaveras fault also exhibits a surface creep rate close to total slip rate of 15mm/yr. Although, creep on the northern Calaveras is poorly constrained, its geologic slip rate is about 6 mm/yr, suggesting direct slip transfer from the southern Calaveras to the Hayward fault. Relocated seismicity outlines an eastward-dipping Hayward fault which appears to dip into and merge with the central Calaveras fault at depth. Additionally, characteristic repeating earthquakes through the stepover indicate that subsurface creep occurs between the two faults. Using both continuous GPS and updated GPS campaign data, we re-evaluate slip on Bay Area faults, and map slip through the Hayward-Calaveras stepover using a contiguous fault model. We use background seismicity and repeating earthquakes to constrain the geometry of the stepover, and invert GPS and InSAR data for slip. We interpret our map of creep transfer between the Hayward and Calaveras faults for its implications for seismic rupture through the stepover and seismic hazard in the Bay Area.