Fine Details of Mining-Induced Seismicity at the Trail Mountain Coal Mine Using Modified Hypocentral Relocation Techniques

Master event and double-difference techniques were used to relocate mining-induced seismicity (MIS) at the Trail Mountain Mine, a longwall coal mine in central Utah. Travel-time data were collected by Arabasz et al. (2002) using a surface seismic network with stations at elevations both above and below mine level (because of the topography) and a single in-mine station. Arabasz et al. (2002) only used surface stations above mine level to determine locations. Using this network geometry, they were only able to constrain focal depths for 321 of 1829 events. In contrast, we use all stations, creating a 3D network. Hypocentral locations are improved by implementing a master event methodology to reduce the effects of uncertainties in the velocity structure, though the resulting locations do not correspond with known structures or stratigraphy. The mismatch between the locations and geology is likely due to fracturing of the rock mass by the mining process, thereby decreasing the seismic velocity near mined-out regions. Assuming a 10% velocity decrease places the MIS in the roof of the mine. A double-difference procedure is used to mimic a timevarying velocity structure. The time-varying velocity structure results in locations that approximate the dip of the coal seam. By using all available stations and allowing for a time-varying velocity structure, we find the MIS is located immediately above the coal seam and closely follows the position of the coalface. The epicenters align with the roads along the longwall panel, where stress concentrations are expected during mining.