Possible precursory signals in damage zone foreshocks

Foreshocks may provide a precursory signal of an impending earthquake, but their role in nucleation of the main shock is unclear. One way to further our understanding of foreshock failure mechanisms is to determine where they occur in the fault zone. However, earthquake locations commonly include uncertainties large enough to allow rupture on either the main fault interface or on subsidiary fractures within a surrounding damage zone. Here we obtain precise earthquake locations, with similar to 10m uncertainty, for foreshocks and aftershocks of an M-w 5.0 near Prague, OK, USA. Repeating earthquakes imply that some precursory slow slip occurred before the main shock. In addition, we show that foreshocks initially rupture faults and fractures throughout the 300m thick fault damage zone and later localize onto a narrower zone (<100m thick) nearer the main shock hypocenter. Focal mechanisms corroborate that foreshocks occur in the surrounding damage zone as well as on the main shock rupture interface. These results highlight that earthquake nucleation is most likely a complex feedback between frictional failure processes on the fault interface and deformation in the surrounding damaged rock, rather than simply nucleation on a single surface.