Many earthquakes have been induced by human activities, including wastewater disposal, geothermal production, gas injection and hydraulic fracturing (HF). The maximum magnitude of most induced earthquakes from different areas ranges from 3.0 to 4.0, although some induced earthquakes with magnitudes up to 5.0 have occurred in the Sichuan basin, China. The Ms 6.0 earthquake in Changning, Sichuan, China, on 17 June 2019 was the largest recorded earthquake in the stable Sichuan basin, taking place in a complicated region with salt mining and shale gas production. Whether this earthquake was also induced raises concerns from the public and the scientific community.
If the Ms6.0 earthquake is associated with industrial injection activities, it would be the largest recorded injection-induced earthquake in China and the most destructive induced event in the world.
In a recent paper, LML External Fellow Jiancang Zhuang and colleagues examine this possibility. In the study, they estimate the nonstationary background seismicity rate and invert for spatiotemporal stress changes. Moreover, they detect the time-varying background seismicity rate using an improved version of the space–time epidemic-type aftershock sequence (ETAS) model with a nonstationary background rate. It should be noted that the term “back-ground seismicity” here refers to de-clustered seismicity (the Omori-type aftershocks were removed) resulting from tectonic loading and injection activities, which differs from background seismicity in the traditional sense (only tectonic seismicity). The injection-induced activity is controlled by time-varying factors and thus is non-stationary.
The authors’ results indicate that the background rate dramatically increased after hydraulic fracturing (HF) and remained high until the present. Also, starting in 2005, the 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. Overall, their analysis documents a clear difference between seismicity induced by salt mine injection and by HF, suggesting that the Changning sequence might have been induced by long‐term injection for salt production. The authors also suggest that continuation of such activity may push the causative fault of the Changning sequence to its critical state, after which it may no longer be stable.
The paper is available here.