Mechanisms and Implications of Lower Crust Earthquakes and Shallow Events Beneath Yanzhou Coal Mining Area, North China

Abstract The Yanzhou Coal Mining area in North China has experienced a significant increase in shallow depth (<2 km) seismic activity in recent years, particularly following the 2017 M 4 lower crust earthquake. However, the relationship between deep tectonic earthquakes and shallow seismicity remains poorly understood. This study employs seismic tomography to resolve a high-resolution 3D crustal velocity structure and inverts the source parameters of the 2017 M 4 event (22 km depth) to calculate static Coulomb stress changes. By integrating these analyses, we investigated the mechanical relationship between the lower crustal and shallow seismicity. The 2017 earthquake, which is spatially correlated with a low-velocity anomaly and the westward migration of the hypocenters in the lower crust, suggests that fluid-driven fault activation occurred where deep fluids from the upper mantle led to increased seismicity rates. In contrast, Coulomb stress changes generated by this event are insufficient to induce stress-triggering effects and lack a direct causative relationship with shallow seismic activity. Although our analysis rules out static stress transfer associated with the 2017 M 4 event, the possibility of remote dynamic triggering remains and cannot be definitively excluded. Instead, the statistically significant increase in shallow seismicity correlated temporally with extended subsurface resource extraction and the post-2017 use of new deep-mining methods under conditions of high stress and elevated temperatures, highlighting anthropogenic production behaviors as a primary driver. The study demonstrates that lower crustal seismicity and shallow mining-induced activity operate within distinct mechanical frameworks. Although deep fluids influence tectonic earthquakes by reducing the effective normal stress along the fault plane, shallow seismicity is dominantly controlled by the intensity of coal mine extraction. These findings highlight the necessity of distinguishing between tectonically driven and mining-induced seismic hazards.