Abstract

We describe a multipart study to quantify the potential ground-shaking hazard to Joes Valley Dam, a 58-m-high earthfill dam, posed by mining-induced seismicity (mis) from future underground coal mining, which could approach as close as ∼1 km to the dam. To characterize future mis close to the dam, we studied mis located ∼3–7 km from the dam at the Trail Mountain coal mine. A 12-station local seismic network (11 stations above ground, one below, combining eight triaxial accelerometers and varied velocity sensors) was operated in the Trail Mountain area from late 2000 through mid-2001 for the dual purpose of (1) continuously monitoring and locating mis associated with longwall mining at a depth of 0.5–0.6 km and (2) recording high-quality data to develop ground-motion prediction equations for the shallow mis. (Ground-motion attenuation relationships and moment-tensor results are reported in companion articles.) Utilizing a data set of 1913 earthquakes (M ≤ 2.2), we describe space-time-magnitude distributions of the observed mis and source-mechanism information. The mis was highly correlated with mining activity both in space and time. Most of the better-located events have depths constrained within ±0.6 km of mine level. For the preponderance (98%) of the 1913 located events, only dilatational P-wave first motions were observed, consistent with other evidence for implosive or collapse-type mechanisms associated with coal mining in this region. We assess a probable maximum magnitude of M 3.9 (84th percentile of a cumulative distribution) for potential mis close to Joes Valley Dam based on both the worldwide and regional record of coal-mining-related mis and the local geology and future mining scenarios.

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