Complex Source Behaviors and Spatiotemporal Evolution of Seismicity During the 2015–2016 Earthquake Sequence in Cushing, Oklahoma

Abstract

AbstractIn September and October 2015, three M4+ earthquakes occurred as a sequence along a fault northwest of the Cushing city, Oklahoma, followed by another M5 earthquake in November 2016. While previous studies have shown that moderate‐size earthquakes in Oklahoma are likely induced by wastewater injections, it is still not clear what controls the rupture process and spatiotemporal evolutions of seismicity during individual sequences. In this study, we investigated the rupture process of these four M4‐5 events in 2015–2016 with finite fault model (FFM) inversions, and computed the static stress changes during this sequence. We found that the rupture processes of four M4‐5 earthquakes were very complex, and each of them had several subevents with different rupture directivities. The 2016 M5 earthquake started near the region where three M4+ events initiated, but the majority of the slip occurred a few kilometers away in the northeast direction. In comparison, the 2015 M4.3 event mainly ruptured toward the southwest direction. Due to data limitation and inversion uncertainties, we were unable to constrain the rupture directivities for the other two M4+ events. The foreshocks 3 days before the first M4+ earthquake in 2015 occurred in a region of positive shear stress changes caused by previous earthquakes in 2014–2015 on unmapped faults several kilometers to the south. Our results suggest small‐scale heterogeneity in controlling complex seismicity and rupture patterns in the 2015–2016 Cushing sequence, and possible triggering of this sequence by a small stress perturbation on order of a few kilopascals.