ALBERT

Description: The University of Utah Seismograph Stations (UUSS) earthquake catalogs for the Utah and Yellowstone National Park regions contain two types of size measurements: local magnitude (M (sub L) ) and coda magnitude (M (sub C) ), which is calibrated against M (sub L) . From 1962 through 1993, UUSS calculated M (sub L) values for southern and central Intermountain Seismic Belt earthquakes using maximum peak-to-peak (p-p) amplitudes on paper records from one to five Wood-Anderson (W-A) seismographs in Utah. For M (sub L) determinations of earthquakes since 1994, UUSS has utilized synthetic W-A seismograms from U.S. National Seismic Network and UUSS broadband digital telemetry stations in the region, which numbered 23 by the end of our study period on 30 June 2002. This change has greatly increased the percentage of earthquakes for which M (sub L) can be determined. It is now possible to determine M (sub L) for all M〉 or =3 earthquakes in the Utah and Yellowstone regions and earthquakes as small as M〈1 in some areas. To maintain continuity in the magnitudes in the UUSS earthquake catalogs, we determined empirical M (sub L) station corrections that minimize differences between M (sub L) s calculated from paper and synthetic W-A records. Application of these station corrections, in combination with distance corrections from Richter (1958) which have been in use at UUSS since 1962, produces M (sub L) values that do not show any significant distance dependence. M (sub L) determinations for the Utah and Yellowstone regions for 1981-2002 using our station corrections and Richter's distance corrections have provided a reliable data set for recalibrating the M (sub C) scales for these regions. Our revised M (sub L) values are consistent with available moment magnitude determinations for Intermountain Seismic Belt earthquakes. To facilitate automatic M (sub L) measurements, we analyzed the distribution of the times of maximum p-p amplitudes in synthetic W-A records. A 30-sec time window for maximum amplitudes, beginning 5 sec before the predicted Sg time, encompasses 95% of the maximum p-p amplitudes. In our judgment, this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events.

Description: 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 approximately 1 km to the dam. To characterize future MIS close to the dam, we studied MIS located approximately 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〈 or =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 + or -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.