An Extremely Shallow M w 4.1 Thrust Earthquake in the Eastern Sichuan Basin (China) Likely Triggered by Unloading During Infrastructure Construction

Triggered or induced earthquakes have been widely reported as resulting from various human activities, yet seismicity triggered by small‐scale infrastructure construction is rare. Here, we report on an investigation of an extremely shallow M w 4.1 earthquake which occurred on 11 August 2016 in the Sichuan Basin (China), a region with historically low seismicity. Our seismic waveform analyses indicate an almost pure thrust focal mechanism at a centroid depth of ~1 km. Furthermore, 18 Sentinel‐1 synthetic aperture radar interferograms, stacked to obtain subcentimeter accuracy, reveal up to 3 cm line‐of‐sight deformation which overlaps with an automotive testing site constructed in 2014. Removal of 10 m of a surface rock layer during the construction may have produced an unloading effect and resulted in up to 0.11 MPa Coulomb stress changes on a blind fault, larger than the 0.01 MPa threshold typically invoked in studies of tectonic earthquakes. However, the delayed triggering still requires further investigation. While induced earthquakes in the shallow part of the crust have been observed globally, a remaining knowledge gap is whether small‐scale infrastructure construction can trigger earthquakes or not. We investigate these questions on an extremely shallow M w 4.1 earthquake in the eastern Sichuan Basin (China). To determine earthquake source parameters, we combine seismological data with satellite imaging. We find this thrust event ruptured at the depth of ~1 km, located just beneath an automotive testing site constructed in 2014. Crucially, we note the testing site construction involved removal of a large volume of surface rock. We further analyze the stress changes produced by the unloading of the surface rock. We find a possible triggering relationship between the infrastructure construction and the occurrence of this shallow earthquake. Our results thus shed new light on the causes of shallow earthquakes associated with small‐scale infrastructure construction. The 11 August 2016 M w 4.1 Dianjiang earthquake was an extremely shallow thrust event (~1 km) Stacking of Sentinel‐1A SAR interferograms with subcentimeter resolution reveals LOS deformation of up to 3 cm produced by the earthquake The earthquake was likely triggered by the unloading effect after surface rock layer was removed during infrastructure construction