The Pawnee earthquake as a result of the interplay among injection, faults and foreshocks

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.

Multiscale Analysis of Spatiotemporal Relationship Between Injection and Seismicity in Oklahoma

“…This difference could be an indication of different triggering processes involved in different types of sequences, for example, some sequences maybe dominated by earthquake‐earthquake triggering, while other sequences are dominated by fluid‐induced stress triggering (Chen et al, ; Schoenball & Ellsworth, ). For the four M 5 earthquake sequences: the Fairview sequence has statistically significant diffusive migration both before and after aftershock removal; however, the direction is unclear: before removing aftershocks, it exhibits unilateral migration toward southwest; after removing aftershocks, it exhibits bilateral migration pattern. The Pawnee mainshock fault itself does not clearly exhibit migration, but the conjugate mapped fault where most of the foreshock activities occur exhibit diffusive migration, consistent with observation of injection‐driven foreshock sequences in Chen et al (); the Cushing and Prague sequences do not exhibit significant migration pattern.…”

Wastewater Disposal and the Earthquake Sequences During 2016 Near Fairview, Pawnee, and Cushing, Oklahoma

“…We also note that significant poroelastic coupling may arise from injection rate variability (Segall & Lu, ); and, although this coupling evidently influenced the timing of earthquakes in the Pawnee sequence (Barbour et al, ), the corresponding stress change amounts to a small perturbation added to the much larger increase in effective stress, of the order of a seismic stress drop (McGarr, ), due to injection of wastewater into the fault zone. Similar remarks apply to the effects of Coulomb stress transfer from one earthquake to another within the Pawnee sequence (Chen et al, ; Pennington & Chen, ).…”

Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

“…Source complexity has been observed in many natural earthquakes and attributed to heterogeneity of fault zone stress and/or material properties (e.g., Abercrombie, ; Uchide & Imanishi, ). A high degree of small‐scale fault zone heterogeneity is pervasive in Oklahoma earthquake sequences as delineated by complex spatiotemporal seismicity evolution (Chen et al, ; Schoenball & Ellsworth, ), spatial variability of focal mechanisms and stress drops (Pennington & Chen, ; Wu et al, ), and complex earthquake interaction and triggering processes (Chen et al, ; Qin et al, ; Sumy et al, ). In the Guthrie fault zone, small earthquakes can be interpreted as the breaking of small patches of the fault as they reach critical stress state, and the stress heterogeneities along the fault zone prevent the generation of large events.…”