<p>In deep mining process involving multi-layered, thick, and hard overlying strata, frequent strong seismic events pose a serious threat to both underground production and surface construction. Understanding the fracture behavior of these thick and hard overlying strata is essential for the effective control and prevention of mine earthquakes. This study utilizes a combination of theoretical analysis, numerical simulation, and microseismic monitoring to investigate the composite fracturing of multiple key strata and the mechanisms behind mine earthquakes. A mechanical model is developed to analyze the composite effects of strata, with criteria for identifying composite key strata derived, and a discrimination method for key strata that considers the composite effect is developed based on Timoshenko beam theory. The disk-based discontinuous deformation analysis (DDDA) method is used to simulate the entire process of movement and deformation process of the key strata. Results indicate that the primary zone of strong mine earthquakes in the 63_upper06 working face at the Dongtan Coal Mine includes the low-position inferior key stratum, the overlying composite key stratum, and the interval between these two strata. The composite key stratum plays a controlling role in the formation of the overlying strata structure and stress evolution. Vertical stress transmitted upward from the low-position inferior key stratum significantly increases the stress level at the ends of the composite key stratum. During the process of overlying strata movement, the composite key stratum develops a large-scale suspended-roof structure. Micro-fractures form and coalesce at its ends and mid-span positions, ultimately leading to structural failure, causing the composite key stratum to break and triggering strong mine earthquakes. These research findings can guide the prevention and control of mine earthquakes in deep mining environments.</p>