COMPARATIVE STUDY OF APPARENT STRESS AT THREE UNDERGORUND MINES

Grant, E. B., Chapman, M.C., Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24061, and IANNACCHIONE, A.T., NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania, elgrant@vt.edu, mcc@vt.edu, aai3@cdc.gov.

This study examines the ratio of radiated seismic energy to seismic moment using data from underground seismic monitoring systems at three mines in sedimentary rock units. The Springfield Pike limestone quarry is an underground room-and-pillar mining operation in Pennsylvania, with overburden of approximately 100 meters. The Willow Creek Mine, in Utah, was a longwall coal-mining operation with overburden in the study area ranging from 750 to 900 meters. The Moonee Colliery is a longwall coal-mining operation near Newcastle, Australia. Overburden at Moonee ranges from 90 to 170 meters. The data were provided by the National Institute for Occupational Safety and Health (NIOSH), Pittsburg Research Laboratory. Data for Springfield Pike consist of uniaxial (horizontal) recordings from underground sensors with a sample rate 1926 Hz. The seismic events at Springfield Pike occurred during a 2 day period prior to a large roof fall. The Willow Creek data are also uniaxial: underground data are horizontal and the surface data are vertical. The recordings at Willow Creek cover a typical day of operation, and involve events occurring above and below the mining horizon, ahead of the advancing longwall. The sample rate at Willow Creek was 500 Hz. The best data set is from the Moonee Colliery, with triaxial recordings and sample rate of 2000 Hz. Data from Moonee cover an 8 day period between two major roof falls. Source receiver distances range from 100 to 500 meters at Springfield Pike, 700 to 1000 meters at Willow Creek and 50 to 150 meters at Moonee. The different mining operations and various failure mechanisms of seismic events in these mines present an opportunity to examine the question of whether or not apparent stress, which is proportional to the ratio of radiated energy to seismic moment, varies appreciably in the mining environment. Preliminary estimates of apparent stress (4x10^-2 MPa) for larger events at Moonee lie in the range of previously published results for tectonic earthquakes and many mining related events. However, preliminary results for Springfield Pike and Willow Creek indicate very low apparent stress ( 2x10^-3 MPa) compared to the previously published results.