Abstract
The Pawnee M5.8 earthquake is the largest event in Oklahoma instrument recorded history. It occurred near the edge of active seismic zones, similar to other M5+ earthquakes since 2011. It ruptured a previously unmapped fault and triggered aftershocks along a complex conjugate fault system. With a high-resolution earthquake catalog, we observe propagating foreshocks leading to the mainshock within 0.5 km distance, suggesting existence of precursory aseismic slip. At approximately 100 days before the mainshock, two M ≥ 3.5 earthquakes occurred along a mapped fault that is conjugate to the mainshock fault. At about 40 days before, two earthquakes clusters started, with one M3 earthquake occurred two days before the mainshock. The three M ≥ 3 foreshocks all produced positive Coulomb stress at the mainshock hypocenter. These foreshock activities within the conjugate fault system are near-instantaneously responding to variations in injection rates at 95% confidence. The short time delay between injection and seismicity differs from both the hypothetical expected time scale of diffusion process and the long time delay observed in this region prior to 2016, suggesting a possible role of elastic stress transfer and critical stress state of the fault. Our results suggest that the Pawnee earthquake is a result of interplay among injection, tectonic faults, and foreshocks.
Highlights
A well-established injection experiment in the 1960s demonstrates that human can influence earthquake occurrence by pumping water in the subsurface[1, 2]
The earthquake occurred near the intersection of three fault traces (hereinafter referred as the Pawnee “triple-junction”: PTJ, see Fig. 4, with two mapped faults (Watchorn Fault and a segement of the Labette Fault) and the unmapped Sooner Lake Fault (SLF) where the mainshock ruptured and most early aftershocks occurred)
The final catalog has a magnitude of completeness (Mc) of 2.0, and reveals several interesting observations
Summary
A well-established injection experiment in the 1960s demonstrates that human can influence earthquake occurrence by pumping water in the subsurface[1, 2]. The larger spatial scale of pressure propagation makes it more challenging to understand the mechanism driving earthquake occurrence patterns, especially for large damaging earthquakes, as the expected pressure increase would be much smaller due to the spatial decay[8, 10] While it could be explained by critically stressed fault being re-activated by small amplitude of stress perturbation (e.g., kPa), the roles of earthquake-to-earthquake interaction and aseismic slip are often omitted. The aseismic slip started on a shallow normal fault following a rapid increase in injection rate two years before the M5 events occurred Both of these observations suggest that aseismic slip initiates within highly pressurized zones close to injection zones, and triggers subsequent seismic slip. As most potentially induced sequences occur as earthquake “swarms”, the role of foreshocks are often not discussed, and all earthquakes are assumed to be triggered by pore pressure
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