A Shallow Shock: The 25 February 2019 ML 4.9 Earthquake in the Weiyuan Shale Gas Field in Sichuan, China

Earthquakes rarely occur at extremely shallow depths, for example, less than 2 km. Even for induced earthquakes that are typically shallower than tectonic events, only very small ones have been reported in such depths. The M-L 4.9 earthquake (M-w 4.3) that struck the Rongxian County, Sichuan, China on 25 February 2019 was an extremely shallow event. Seismological and geodetic data constrained the mainshock depth at similar to 1 km with a thrust-faulting mechanism, consistent with the Molin fault orienting northwest. Two foreshocks with magnitudes larger than 4 occurred on an unmapped fault striking northeast, right next to an injection well where hydraulic fracturing (HF) was conducted. The focal depths of the two foreshocks were at similar to 2.7 km, coinciding with the depth of HF. Coulomb failure stresses of the two foreshocks on the Molin fault was similar to 3 kPa, smaller than typical static triggering threshold (10 kPa), and thus their triggering effects were mild. As the fault was hydraulically sealed from HF, we suggested that the M-L 4.9 earthquake was possibly triggered by nearby HF activities through poroelastic stress transfer. Such findings held significant implications for shale gas development by considering seismic hazard associated with shallow faults.