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Author(s): Satoshi Ide (corresponding author) [1]; Gregory C. Beroza [2]; David R. Shelly [2]; Takahiko Uchide [1] Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic [...]

To better quantify how injection, prior seismicity, and fault properties control rupture growth and propagation of induced earthquakes, we perform a finite‐fault slip inversion on a Mw 4.0 earthquake that occurred in April 2015, the largest earthquake in an induced sequence near Guthrie, Oklahoma. The slip inversion reveals a rupture with slip patches that are anti‐correlated to the locations of prior seismicity. The prior seismicity driven by low pore pressure changes and static stress changes occurred on weaker portions of the fault, while the Mw 4.0 earthquake likely ruptured relatively stronger portions of the fault. To resolve if pore pressure changes or the initial underlying stress distribution and fault strength controlled the final slip distribution of the Guthrie Mw 4.0 earthquake, we compare strike‐slip events of similar magnitude from tectonically active regions and previously inactive regions. Earthquakes on reactivated faults exhibit different slip distributions than active regions, they have more prominent and well separated slip patches, a behavior often associated with faults of lower fault maturity. Pore pressure shows little effect on the distributions. These observations suggest that the initial underlying stress distribution and fault strength of reactivated faults in low deformation regions is the primary controlling factor of the slip distribution with pore pressure perturbations and earthquake interactions being secondary. Therefore, Guthrie Mw 4.0 earthquakes slip distribution was enhanced by pore‐pressure perturbations and earthquake interactions by creating an optimal stress state for its failure, but the slip distribution itself is controlled by its fault's initial stress and strength state. Plain Language Summary: Understanding how the rupture growth of earthquakes on reactivated faults differs from actively deforming tectonic ones is important to the proper estimation of hazard. To better understand the factors that control the rupture and eventual size of earthquakes in Oklahoma, we examine the rupture process of a Mw 4.0 earthquake that occurred near Guthrie, Oklahoma. Using seismic data, we calculate the slip pattern for the event and find a majority of slip occurs on four distinct slip patches that are outlined by past seismicity triggered by pore pressure changes from nearby injection wells. The slip patches that failed during this rupture likely represent the strongest portions of the fault that were pushed to a critical state through both pore pressure and past seismicity. Comparing the ruptures of earthquakes in low and high deformation rate regions, those in low deformation rate regions have different slip characteristics with the most significant being isolated high slip regions. This suggests that reactivated faults are immature and produce more heterogeneous slip distributions. This indicates that the fault properties such as initial stress and strength state play the largest role in controlling the slip distribution on reactivated faults and in the Guthrie Mw 4.0 earthquake. Key Points: A finite‐fault slip model is obtained for a Mw 4.0 induced earthquake near Guthrie, OklahomaBoth past seismicity and injection affect the observed heterogeneous slip pattern of the Mw 4.0 earthquakeIn the context of past studies, reactivated faults exhibit rupture behavior associated with immature faults [ABSTRACT FROM AUTHOR]

A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone. [ABSTRACT FROM AUTHOR]