Microseismicity and creeping faults: Hints from modeling the Hayward fault, California (USA)

Creeping segments of strike-slip faults are often characterized by high rates of microseismicity on or near the fault. This microseismicity releases only a small fraction of the slip occurring on the fault and the majority of the accumulating elastic strain is released either through aseismic creep or in rare large events. Distinguishing between creeping or non-creeping patches on faults and determining the resulting accumulated slip deficit is important in assessing the seismic hazard associated with a fault. Unfortunately, surface creep data alone are insufficient to constrain the creep at depth on the fault. Here we analyze the possibility of using microseismicity as a further constraint. An analysis of the accumulation of Coulomb stress associated with the fault creep indicates that the transition from creeping regions to locked patches has the potential to affect the local seismicity pattern. Precise relative relocations of the microseismicity of the Hayward fault [1] [F. Waldhauser, W.L. Ellsworth, Fault structure and mechanics of the Hayward Fault, California, from double-difference earthquake locations, J. Geophys. Res. 107(3), doi:10.1029/2000JB000084, 2002.] indicate that a fraction of the events repeat, indicating recurrent ruptures of the same small patch. A comparison of the creeping pattern resulting from a Finite Element deformation Model with this precisely relocated microseismicity indicates that the non-repeating earthquakes mainly occur in the transitional zones from creeping to locked patches, while recurrent (repeating) earthquakes cluster in high creep-rate regions. Building from this observation, we have developed an analysis approach to better define patterns of creep, and thus the slip deficit, on the Hayward fault. Additionally this creep rate and its spatial pattern on the fault vary as a function of time after the system is loaded by earthquakes on the locked patches.