“Cross-Correlation Detection of Seismic Events Related to the Crandall Canyon Mine Collapse”

1. “Cross-Correlation Detection of Seismic Events Related to the Crandall Canyon Mine Collapse” Tex M Kubacki Keith D Koper Kristine L Pankow Michael K McCarter

2. Seismic Monitoring at Crandall Canyon: • 10 March, 2007 - M 1.8 event recorded - Heavy damage reported - North pillars abandoned • 6 August, 2007 - M 3.9 event recorded - Six workers trapped - Nearest station ~20 km - Temporary array deployed • 16 August, 2007 - M 1.6 event recorded - Three rescuers killed

3. Detecting Seismic Events • Goal: detect and locate enough small after-shocks to delineate the pattern of collapse • 55 August events detected via routine UUSS procedures (STA/LTA, &c) • MSHA logbooks suggest many more • Events form two well-correlated clusters • Continuous data scanned using 55 templates • Similar locations & mechanisms • July 26 – August 19 & February 20 – March 18

5. Cross-Correlation Detection • Waveform-similarities used to detect 1,328 new events, many not detectable by visible inspection • Detection threshold lowered from M 1.6 to M -0.5 using only regional stations (> 20 km away) • Temporary array used for verification (and depths) • Relative locations determined via double-difference inversion of p-wave lag times M -1.0 Event Invisible to Traditional Detectors Detection Confirmed at Temporary Station

6. Technical Points • Conservative correlation coefficient threshold of 0.5 selected based on analysis of noise content and temporary array verification • Magnitudes determined via amplitude scaling factors • All templates and continuous data filtered between 0.5 and 5.0 Hz

7. Sequence of Events

11. Average Events per Hour

12. Summary • August Collapse • 1,328 Detected & locatable events • Distinct linear & planar clusters to the east & west • Linear clusters striking ~210° • Planar cluster dipping ~10° to the north • March Bump • 357 Detected & locatable events • Clustering to the north & south of mining activity

14. Conclusions • Cross-correlation techniques • 2 Order-of-magnitude improvement over STL/LTA • High-quality detections without in-mine arrays • ~1,300 more events than previously known • Help to delineate shifting stresses during collapse • May help rescue workers avoid areas prone to heavy seismicity, should future collapses occur • Difficult to conclude from single case study whether collapse could have been predicted

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