Bimodal distribution of creep event amplitudes on the San Andreas fault, California

Abstract

EPISODIC fault creep, at several instrument sites along the San Andreas and associated faults in central California consists of a few small and large slip events per year generally superimposed on a background of gradual yielding at low rates^1–3. Most of the events are aseismic, but a few minor displacement steps have occured in association with local earthquakes¹². After removal of earthquake steps, event lists for several sites include significant numbers of small events about an order or magnitude below the typical 1–4-mm amplitude range for large events^{1, 3}. Recent experimental rock-deformation results demonstrate that under biaxial loading some rocks show episodic slip on pre-cut surfaces^9,10. It is not yet clear how the laboratory and field observations are related, but the data presented here indicate that episodic fault creep in nature may be more complex than previously realised. In light of the laboratory results, it is more important than ever to consider all the details of the field data concerning fault creep.