MINING-INDUCED SEISMICITY AND SURFACE GROUND-SHAKING HAZARD IN THE MANTI-LA SAL NATIONAL FOREST, UTAH

The Manti Division of the Manti-La Sal National Forest in east-central Utah encompasses parts of the Wasatch Plateau that are underlain by high-quality coal deposits in gently dipping (< 5°) upper-Cretaceous strata. In the Trail Mountain area, the planned leasing of a major coal tract adjacent to Joes Valley Dam (a 58-m-high earthfill dam) raised a critical question for regulatory agencies responsible for hazard mitigation and resource management. Given the potential hazard of ground shaking from mining-induced seismicity (MIS), how close to the dam should future underground coal mining be allowed? To help answer this question, a joint study was undertaken by the University of Utah, the U.S. Geological Survey, and the U.S. Bureau of Reclamation.

We deployed a 12-station temporary seismic network in 2000–2001 and analyzed data from more than 1,800 mining-induced earthquakes (magnitude ≤ 2.2) located ~3–7 km from the dam and associated with longwall mining at a depth of 0.5–0.6 km in the nearby Trail Mountain mine; 99% of the focal depths were < 1 km. The MIS was highly correlated with mining activity both in space and time, and implosive or collapse-type source mechanisms predominated. High-quality accelerograms from a subset of the MIS enabled the development of new ground-motion prediction relations as a function of magnitude and distance appropriate for the engineering problem at hand. We characterized potential MIS that might accompany future longwall mining in the proposed lease tract and assessed a probable maximum magnitude of 3.9 (84th-percentile of a cumulative distribution) based on (1) seismic monitoring in the Trail Mountain area, (2) the regional and worldwide record of coal-mining-related MIS, and (3) the local geology and future mining scenarios in the proposed lease tract. The determination of an exclusionary distance from the dam for future mining still awaits decision-making by federal and state agencies having regulatory control. Three companion articles describing details and results of this case study were published in the February 2005 issue of the Bulletin of the Seismological Society of America. We acknowledge the contributions of four other co-authors involved in those publications.