Fault creep and microseismicity on the Hayward fault, California: Implications for asperity size

The Hayward fault is documented to undergo significant creep, with some patches accommodating 50% or more of the long-term fault displacement. In spite of this, the fault has also experienced moderate to large earthquakes. By comparing the patterns of microseismicity observed on the fault with models of fault zone creep, we can investigate the long-term displacement/deformation history of the fault in terms of the relative roles of aseismic creep, fault slip accommodated through microseismicity, and strain accumulation (slip deficit). We find that microseismicity on the Hayward fault produces a negligible percentage of the seismic moment dissipated on the fault. Combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault. For small asperities associated with repeating earthquakes on the Hayward fault, the rupture areas of these asperities range from 20 to 60 m².