The 2019 MW 5.7 Changning earthquake, Sichuan Basin, China – a shallow doublet with different faulting styles

Abstract

The increased seismic activity of the last ~10 years in Changning county of Sichuan Province comprised just small (mostly ML < 5.0) injection-induced earthquakes. The MW 5.7 earthquake of June 17, 2019 is the largest event ever reported there. Moment tensor of the mainshock was remarkably dominated by a compensated linear vector dipole. We resolve its fine structure showing it was a doublet, allowing approximation by a thrust- and strike-slip subevent. The mainshock nucleated as thrust faulting, which (together with the largest aftershocks) can be linked with previously known reverse faults, favorably oriented to regional stress field. Contrarily, the strike-slip segment of the mainshock, less favorably oriented, was probably facilitated by elevated pore pressure due to previous injections. Shallow active strike-slip faulting, not yet mapped in the region, is a new feature, important for future hazard assessment.

InSAR Evidence Indicates a Link Between Fluid Injection for Salt Mining and the 2019 Changning (China) Earthquake Sequence

“…On the other hand, the region also contains active fluid injection for solution mining of salt (Figure 1b), which is spatially much closer to the sequence and has induced earthquakes during past operations (e.g., Sun et al, 2017). Accordingly, the sequence could be potentially associated with salt mining, which was also suggested by three recent seismological studies (Jia et al, 2020; Lei et al, 2019b; Liu & Zahradník, 2020). Moreover, Lei et al (2019b) regarded that the 2018 Ms 5.7 earthquake in the shale gas block (Figure 1b) could also bring the Changning sequence closer to failure.…”

“…Within this context, the Changning earthquake sequence struck the southwest of the Sichuan basin for more than 2 weeks (Figure 1b). The sequence began with a Ms 6.0 earthquake on 17 June 2019 and comprised five M5 + events and ~600 minor earthquakes (Lei et al, 2019b; Liu & Zahradník, 2020; Yi et al, 2019), which has caused 13 deaths and injured more than 200 people. The spatial proximity of the sequence to HF operations in the Changning shale gas play (<10 km) raised an extensive controversy over a potential relationship with shale gas exploration.…”

“…Focal depths and mechanisms of earthquake are key references to aid in determining whether they are induced by some fluid injection operations. Three published studies have used seismological data to relocate the Changning earthquake sequence and also to invert for focal mechanism solutions of some large earthquakes in different time periods: 17 to 22 June (Yi et al, 2019), 17 June to 30 August (Lei et al, 2019b), and 17 June to 4 August (Liu & Zahradník, 2020). However, their relocation and inversion results have large disparities (supporting information Table S1).…”

“…In addition, the causative fault illuminated by the relocated seismicity is approximately vertical (Lei et al, 2019b; Yi et al, 2019), which contradicts with the low‐angle thrust faults indicated by most of the focal mechanisms (Table S1). Liu and Zahradník (2020) argued that the largest event nucleated as thrust faulting then ruptured a strike‐slip fault. These poorly determined parameters are unfavorable to be used as evidence to investigate the cause of the Changning earthquake sequence.…”

Injection‐Induced Earthquakes on Complex Fault Zones of the Raton Basin Illuminated by Machine‐Learning Phase Picker and Dense Nodal Array

“…Recent M > 5 induced earthquakes are mostly associated with fluid injection or extraction such as waste‐water injection in Oklahoma, USA (e.g., Keranen et al, 2014), geothermal production in Pohang, Korea (e.g., Grigoli et al, 2018), and hydraulic fracturing or salt mine exploration in Sichuan, China (e.g., Lei et al, 2019; Liu & Zahradník, 2020; Meng et al, 2019). In the Raton Basin, waste‐water injection related to coal‐bed methane production is thought to be the dominant driver of increased seismicity over the past two decades (Nakai, Weingarten, et al, 2017; Rubinstein et al, 2014).…”