A sequence of seismic event triggering in longwall coal mines based on the aftershock cascade theory

Abstract

Seismic events associated with longwall coal mining have not been comprehensively understood, in particular the spatial-temporal relationship between mining-induced seismic events. Current studies on longwall mining-induced seismicity have proposed various spatial-temporal relationships between seismic events, but they may not explicitly follow the Gutenberg–Richter (GR) law for seismic magnitude distribution. This study applies a modified GR law to describe longwall mining-induced seismicity by considering the spatial-temporal distance between events. The closest event pair for each seismic event, which is the most probable source to trigger this event, is determined based on the spatial-temporal distance using the nearest neighbour method. A threshold based on the spatial-temporal distance is set via the trial-and-error method, enabling seismic events to be classified into triggering events and non-triggering events. Two groups of seismic events from the classification are further tested and proved to be valid by the temporal Bi-test and spatial Ripley's K function. The temporal Bi-test and spatial Ripley's K function demonstrate a greater tendency for clustering among triggering events and more randomness among non-triggering events. Our analysis of seismic events associated with longwall mining reveals that triggering events account for 60 % of all seismic events, making up a significantly higher percentage than that in earthquake seismology. The event family tree analysis suggests that a single mining-induced seismic event could generate up to five generations in the event triggering catalogue, and the average moment magnitude between each generation decays exponentially. We also find that the triggering between high-energy events is non-local, manifested as the propagation of discontinuities from different ends of the same fault. In addition, high-energy events may not necessarily be triggered by their closest precedent event but by the combined effects of mining activities and discontinuities. This study provides significant implications for the relationship between seismic events in mining engineering.