关键词: 诱发地震, 三维构造模型, 盐矿涌水, 地震发震机制, 地震危险性

Abstract:

Human-induced seismicity associated with large-scale industrial resource extraction has attracted increasing public and scientific attention because of its potential to damage infrastructure. Previous studies have mainly focused on the spatiotemporal evolution of seismicity, surface deformation monitoring, and geomechanical stress perturbations induced by fluid injection. However, the integrated effects of complex geological settings, localized fault structures, and lithological heterogeneity on induced seismic responses remain insufficiently constrained.

In this study, we investigated recent seismic swarms associated with water injection and subsequent brine extraction at the Huai'an anhydrous glauberite salt mine in the Subei Depression, eastern China. Solution mining in such seismically sensitive settings involves the continuous circulation of fluids that dissolve target evaporite formations, thereby perturbing the local stress-strain equilibrium. To characterize the geological framework of the mining area, we integrated lithological and structural stratigraphic data from the Zhaoji sub-depression with regional seismic velocity profiles and electrical resistivity structures to construct a detailed three-dimensional geological model. High-resolution DEM data, drilling records, and three-dimensional electrical and velocity structures were used to extract key geophysical parameters, including resistivity and S-wave velocity, and to characterize the continuous distribution of strata. These data were then processed using discrete smooth interpolation in GOCAD. Drilling data provided vertical stratigraphic constraints, whereas microseismicity and three-dimensional electrical structures constrained the lateral and deep extent of the model.

The resulting three-dimensional structural model clearly delineates the depression geometry of the mining area. Cross-sections show that Member 4 of the Funing Formation reaches its maximum thickness in the depression center and thins outward, defining a pronounced depression structure extending from the Quaternary strata to the pre-Cretaceous basement. Using a high-precision microseismic catalog relocated with the double-difference method since 2019, we further analyzed the spatial distribution of earthquake swarms across different sedimentary units and delineated the three-dimensional extent of water inflow within the salt mining area. We then examined the distinct causal mechanisms of these swarms and their relationship with dynamic groundwater migration.

The relocated earthquake catalog reveals two distinct seismic clusters with different seismogenic mechanisms. Cluster 1 is located within the Funing Formation beneath the mining zone and consists of diffusely distributed, low-magnitude microseismic events concentrated in Member 4, where the target salt and anhydrous glauberite layers are extensively developed. These localized events likely reflect distributed, mechanically weak fracturing caused by local strain perturbations and immediate stress relaxation associated with repeated water injection and salt dissolution. In contrast, events within the underlying Members 1-3 of the Funing Formation have relatively larger magnitudes and are dominated by thrust-type focal mechanisms. This pattern indicates localized stress concentration caused by downward fluid infiltration and partial dissolution of salt layers, with mudstone barriers locally trapping elevated pore pressure.

Cluster 2, located to the north near Gaojiayan Town within the Taizhou Formation, is associated with natural groundwater inflow. The microseismic events in this cluster are aligned with pre-existing secondary faults and localized tectonic fractures characterized by relatively weak mechanical cementation. This spatial correspondence indicates that fluid migration pathways and the mechanical properties of fault zones exert primary control on the propagation of induced seismicity.

Overall, the systematically observed seismicity in the Huai'an region is dominated by small-to moderate-magnitude events confined to specific rheological stratigraphic units or localized fracture zones, suggesting a relatively low likelihood of future large-scale destructive earthquakes. Nevertheless, our results demonstrate that transient pore-pressure variations, fluid migration, lithological heterogeneity, and localized reactivation of secondary faults jointly control the spatial distribution of induced seismicity in actively exploited salt mines. This study provides a physically informed framework for assessing seismic hazards related to industrial fluid injection in the complex Subei Depression and extends the application of three-dimensional geological modeling to the analysis and prediction of engineering-induced geological hazards.

Key words: induced seismicity, three-dimensional geological modeling, fluid injection, seismogenic mechanism, seismic hazard