Nonstationary Background Seismicity Rate and Evolution of Stress Changes in the Changning Salt Mining and Shale-Gas Hydraulic Fracturing Region, Sichuan Basin, China

Abstract

The Ms 6.0 earthquake in Changning, Sichuan, China, on 17 June 2019 was the largest recorded earthquake in the stable Sichuan basin. It occurred in a complicated region with salt mining and shale gas production. Whether this earthquake is induced raises concerns among the public and the scientific community. Furthermore, the relation between this earthquake and nearby industrial activities has also been of great interest. To address these questions, we estimated the nonstationary background seismicity rate and inverted for spatiotemporal stress changes. The results show that the background rate dramatically increased after hydraulic fracturing (HF) and remained at a high level until the present. Starting in 2005, the study region experienced an accelerating stress increase, and the rates of cumulative modified Coulomb stress changes were approximately 0.11 MPa/yr from January 2005 to January 2015 and 0.24 MPa/yr from January 2015 to December 2018. The 2019 Changning earthquake produced a stress step of 0.32 MPa. A clear difference between seismicity induced by salt mine injection and by HF is documented. Our results suggest that the Changning sequence might have been induced by long-term injection for salt production. Furthermore, the seismicity-stress inversion method provides a tool for using seismicity rate changes as a stress meter to monitor human-induced seismicity.

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.…”

Green, yellow, red, or out of the blue? An assessment of Traffic Light Schemes to mitigate the impact of hydraulic fracturing-induced seismicity

“…Peña-Castro et al, 2020) and the Sichuan Basin (e.g. Jia et al, 2020). We note that these two plays represent the two most extreme cases of HF-IS, with the Sichuan Basin hosting the largest HF-IS events yet recorded, with the Montney shale being the secondlargest, and also showing the most extreme examples of magnitude jumps during injection.…”

“…Lei et al (2019) reported further HF-IS in the same region, focussing on pad N201, which produced an M 5.7 earthquake in December 2018. We note that caution may be required with these events, since recent studies have linked some of the induced seismicity that has occurred in the Sichuan Basin with salt mining processes rather than hydraulic fracturing (Lei et al, 2019b;Jia et al, 2020).…”

“…The M 5.7 December 2018 event was reported to have injured 17 people and caused extensive damage to roughly 400 houses (Lei et al, 2019a), while the M 6 event in June 2019 is reported to have killed 13 people, injured over 200 people, and damaged a large number of buildings (Yi et al, 2019). However, the causative factor for the June 2019 M 6 event is unclear, with several water injection wells associated with salt mining also found in close proximity to the epicentre (Lei et al, 2019b;Jia et al, 2020).…”

Interevent-time distribution and aftershock frequency in non-stationary induced seismicity

“…Modeling non-stationary seismicity brings complications far beyond those of steady state. With sufficient data these have already been addressed successfully by ETAS models [14][15][16]18,20,21 . But since our purpose is to consider the added complication of small catalogs we therefore choose as an alternative the Gamma form (1).…”