Associations between temporal velocity changes and induced seismicity

Abstract

A new technique to image temporal velocity changes is proposed and applied to mining‐induced microseismic data recorded at Falconbridge's Strathcona Mine, Sudbury, Canada. The motivation for this work is to back analyze temporal changes in the velocity structure, which may shed light upon observed correlations between seismicity and velocity structure. The sequential imaging method involves cross‐correlating waveforms of event doublets and computing the relative event location from the time delays. Time delays, which are corrected for the travel time delay due to the differences in locations, are then used to compute images of temporal velocity differences between the recording periods of the doublets. Controlled blast data was used to examine the accuracy of relative locations, which was found to be 2.8 m compared to 13.5 m for the absolute locations. Corrected blast doublet time delays were inverted to test the sensitivity of the method, which confirmed the occurrence of no significant velocity changes. A case study of imaging velocity differences with event doublets recorded at different times, demonstrated a correlation between velocity decreases and both the location of induced microseismic events and a mN 2.6 tremor. The velocity decreases were attributed to fracture unclamping due to stress field rotations associated either with mining or the tremor. The events were found to locate in zones of anomalously high velocity in the static velocity image, corroborating similar observations made in both mining‐induced and natural seismicity studies.