Testing Earthquake Nucleation Length Scale with Pawnee Aftershocks

Abstract

The interpretation of precursory seismicity can depend on a critical nucleation length scale h*, yet h* is largely unconstrained in the seismogenic crust. To estimate h* and associated earthquake nucleation processes at 2–7 km depths in Oklahoma, we studied seismic activity occurring prior to nine M 2.5–3.0 earthquakes that are aftershocks of the 3 September 2016 M 5.8 Pawnee, Oklahoma, earthquake. Four of the nine M 2.5–3.0 aftershocks studied did not have detectable seismicity within a 2 km radius of their hypocenters in the preceding 16 hr time windows. For the other five events, which did exhibit foreshock sequences, we estimated the static stress changes associated with each event of each sequence based on precise earthquake relocations and magnitude estimates. By carefully examining the spatiotemporal characteristics, we found all five of these M 2.5–3.0 aftershocks, and 70% of our studied events were plausibly triggered via static stress transfer from nearby earthquakes occurring hours to seconds earlier, consistent with the cascade nucleation model and a small h* in this region. The smallest earthquakes we could quantitatively study were M −1.5 events, which likely have 1–2 m rupture dimensions. The existence of these small events also supports a small nucleation length scale h*≤1 m, consistent with laboratory estimates. However, our observations cannot rule out more complicated earthquake initiation processes involving interactions between foreshocks and slow slip. Questions also remain as to whether aftershocks initiate differently from more isolated earthquakes.