Oral Presentation

Aseismic slip at sub-seismogenic speeds can substantially moderate the earthquake potential and thus earthquake hazard of lithospheric faults. Field measurements of surface creep, space-geodetic observations, and the distribution of repeating and low-frequency earthquakes provide an increasingly detailed picture of the kinematics of slow fault slip in space and time. In California, aseismic slip is evident on many faults and plays an important role through the interseismic, preseismic and postseismic phases of the earthquake cycle. Aseismic slip during the period between large ruptures occurs on poorly coupled fault sections, both as steady fault creep and episodic slow slip events. The Hayward Fault shows evidence of a decades-long acceleration of aseismic slip, suggesting that aseismic slip slowly expanded into previously locked zones. This redistribution suggests interseismic asperity erosion, a progressive loss of locked fault area, which may be characteristic of partially coupled faults. This finding challenges the notion of stable asperities and implies that seismic hazard on partially coupled faults is dynamically evolving. To better understand the underlying dynamics and constitutive properties, we can probe the response of faults to changes in external stress conditions. The response of the partially coupled San Andreas Fault near Parkfield to stress changes from earthquakes, tides and hydrological surface loads helps illuminate the spatially variable fault rheology and thus fundamental physics of aseismic slip. Given that both the Hayward Fault and the Parkfield segment are near the end of their average recurrence intervals, and given their rich records of historical observations, intensifying research on these two natural laboratories is timely.