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

Tex M KubackiKeith D Koper

Kristine L PankowMichael K McCarter

Seismic Monitoringat 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

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

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

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

Sequence of Events

Average Events per Hour

-37.0000000000001-27.2500000000002-17.4999999999999-7.75000000000002 2 11.750

5

10

15

20

25

0

0.5

1

1.5

2

2.5

August CollapseMarch Bump

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

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

Reference

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