Authors: Burgmann, R., Sukhatme, J. S., Fielding, E.
The Hayward fault in the San Francisco Bay area, California, slips both episodically, as in the large 1868 M=6.8 earthquake that ruptured the southern 50 km of the fault, and by aseismic creep of the upper few km of the fault. The northern portion of the fault has not slipped coseismically since more than 220 years. As recurrence interval estimates for the Hayward fault are in the range of 180-250 years, this suggests a high earthquake hazard in the urban eastern Bay area. However, it is plausible that the northern Hayward fault may be creeping to greater depths and that it is not capable to produce large earthquake ruptures. We use differential synthetic aperture radar interferometry (InSAR) to measure range changes between the Earth's surface and the ESA ERS spacecrafts, from 1992 to 1997. The data clearly reveal tectonic offset across the northern portion of the fault near Richmond that agree well with surface creep measurements of about 6 mm/yr. We utilize the range-change gradient away from the Hayward fault to constrain the depth of the lower edge and the strike-slip rate of a rectangular dislocation in an elastic half-space. Inversions for best-fitting fault parameters from two interferograms spanning 3.4 and 5.2 years, respectively, suggest that the Hayward fault is aseismically slipping to depths of at least 3 km. Integration of the range change data with sparse, but precise GPS measurements in the region help us better constrain the locking depth and rupture potential of the Hayward fault. InSAR geodesy proves to be a valuable tool to resolve highly detailed images of subsurface slip and elastic strain accumulation.