Xiaowei Chen

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

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 Mu2009≥u20093.5 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 Mu2009≥u20093 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.

Coulomb Stress Interactions during the Mw 5.8 Pawnee Sequence

ABSTRACT We investigate the stress interaction between the Watchorn, Labette, and Sooner Lake fault systems and the effect of precursory activities on the 3 September 2016 M w xa05.8 Pawnee earthquake. We obtain fault‐plane solutions for earthquakes with sufficient azimuthal coverage using the HASH algorithm, and then perform coulomb stress analysis on both seismogenic faults and individual nodal planes. Our results found that the three M w ≥3.0 foreshocks exerted a cumulative coulomb stress change increase of 0.68–1.98 bars at the mainshock hypocenter and also promoted failure for most aftershocks within 2xa0km of the mainshock. The coulomb stress change of 5 bars exerted by the mainshock also promoted failure for most aftershocks within the conjugate fault system. The results suggest that earthquake interaction should be fully considered in hazard assessment for induced seismicity.

Preface to the Focus Section on the 3 September 2016 Pawnee, Oklahoma, Earthquake

The M wxa05.8 Pawnee earthquake occurred at 12:02 UTC on 3 September 2016, local time at 6:02 a.m. on Saturday. It was widely felt throughout Oklahoma and neighboring states. The earthquake occurred near the junction of the two previously mapped faults: Watchorn fault and Labette fault. However, the actual source fault was the previously unmapped Sooner Lake fault (also known as the Pawnee fault). This is the largest earthquake since the 1950s in the instrumental history in Oklahoma. Many recent studies suggest that most earthquakes in Oklahoma since 2009 are induced by wastewater injection (e.g., Ellsworth, 2013; Keranen etxa0al. , 2014; Walsh and Zoback, 2015), and the Pawnee earthquake is potentially the largest injection‐induced earthquake that has occurred so far (Yeck etxa0al. , 2016).nnThis focus section of Seismological Research Letters includes 10 original research papers that provide a complete view of the stress evolution leading to the mainshock inferred from foreshock activities and coupled poroelastic modeling; coseismic stress changes and deformations from both seismological and geodetic observations; dynamic hydrological responses at far‐field; and liquefaction observations from geoelectrical and surface mapping.nnWang etxa0al. (2017) and Kroll etxa0al. (2017) investigated hydrological responses due to the M xa05 earthquakes in Oklahoma. Among them, Wang etxa0al. (2017) observed groundwater level changes over distances greater than 150xa0km from the epicenter. The model that is most consistent with observation is aquifer recharge due to enhanced crustal permeability produced by seismic waves. Their simulation suggests that the source of recharge was high‐pressure chambers near the responding wells, which became hydraulically connected to the well by enhanced permeability during the earthquake. Kroll etxa0al. (2017) investigated how the poroelastic properties of the Arbuckle group change laterally and …

Absence of remote earthquake triggering within the Coso and Salton Sea geothermal production fields

Geothermal areas are long recognized to be susceptible to remote earthquake triggering, probably due to the high seismicity rates and presence of geothermal fluids. However, anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields. Here we examine the remote triggering phenomena in the Coso geothermal field and its surrounding areas to assess possible anthropogenic effects. We find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas. Similar observation is also found in the Salton Sea geothermal field. We hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures. To test this hypothesis, we analyze the pore-pressure-driven earthquake swarms, and they are found to occur outside or on the periphery of the geothermal production field. Therefore, our results suggest that the geothermal operation has changed the subsurface fracture network, and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields.

Temporal Correlation Between Seismic Moment and Injection Volume for an Induced Earthquake Sequence in Central Oklahoma

The rapidly increased earthquake rate in the central United States has been linked with wastewater injection. While the overall understanding appears clear at large scales, the interaction between injection and faulting at smaller scales within individual sequences is still not clear. For an earthquake sequence in central Oklahoma, we conduct finer scale analysis of the spatiotemporal evolution of seismicity, and pore pressure modeling. The pore pressure modeling suggests that nearby wells show much stronger correlation with earthquake sequence evolution. Detailed temporal analysis found correlation between earthquake rate, seismic moment and injection rates from wells in close proximity. However, the observed maximum magnitude (Mmax) is about one order of magnitude smaller than expected based on a theoretical relationship between Mmax and cumulative volume. This discrepancy may point toward additional parameters, such as fault size and stress, which influence Mmax. The lower Mmax is consistent with the truncated Gutenberg-Richter distribution observed from matched-filter detected catalog. Overall, the detailed observations suggest that it is possible to resolve relationships between individual disposal wells and induced earthquake sequences.

Locations of Injection-Induced Earthquakes in Oklahoma Controlled by Crustal Structures

In recent years, many small- to moderate-size earthquakes occurred in central northern Oklahoma, likely associated with wastewater injection. The most recent one is the M5.8 Pawnee earthquake occurred on 3 September 2016. It is still not clear what controls the locations of these injection-induced earthquakes. Here we conduct 2-D Pg wave tomography with anisotropy to image seismogenic structures in this region, using more than 10xa0years of Pg arrivals recorded by 81 seismic stations. Our high-resolution Pg wave tomography shows two high-velocity zones with northwest fast direction alternated by low-velocity zones with northeast fast direction. The two dominant anisotropy directions are consistent with conjugated microfractures from surface faults and two nodal planes from focal mechanisms of moderate-size earthquakes. Most moderate-size (Mxa0>xa04) earthquakes occurred either close to the boundaries between high- and low-seismic velocity zones or within the high-velocity zones, suggesting that they are associated with geological boundaries of different basement rock properties or with strong material properties in the upper crust. We suggest that although these moderate-size earthquakes were induced by fluid injection, their spatial locations were likely controlled by local geologic structures.