Led by Prof. Yang Hongfeng of the Earth System Science Programme, a research team conducted comprehensive study of the 2019 earthquake sequence in Weiyuan Shale Gas Field in the southern Sichuan Basin of China. It was found that on 25 February 2019, the earthquake occurred at around 1 km below the surface, an extremely shallow depth where earthquakes do not generally happen. There is also evidence that the earthquake was induced by the shale gas production nearby. The findings not only advance our understanding of earthquake physics, but also hold important implications for seismic hazards in shale gas fields. The research article has been published online in Seismological Research Letters on 19 August 2020.
To extract oil and gas from low-permeability shale rocks, hydraulic fracturing (also termed fracking), a technique of injecting high-pressure fluid to create fractures in rocks, is commonly used to increase permeability within the reservoir. Shale gas production in the Rong County of Sichuan has shot up since 2012.
The research team found that two foreshocks occurred at depths of around 2.7 km, coinciding with the fracking depth. Before the main shock, fracking activities were going on at nearby injection wells. The spatial and temporal correlations suggest that these two earthquakes were induced by fracking, possibly through increasing pore pressure on the fault and thus leading to fault slip.
Using high-quality seismic reflection data, Professor Yang and collaborators from Hohai University identified the source faults of these earthquakes. The two foreshocks ruptured the detachment fault dated Silurian, within the shale layer. The main shock occurred on the Molin fault, a geologically mapped west-dipping reverse fault with a depth extent of around 1.5 km. The very shallow portion of a fault at depths of less than 2 km is usually considered aseismic. However, the Molin fault is exceptional, marking a rare record of extremely shallow rupture.