Features of focal mechanisms of mining-induced earthquakes: a case study of the Fushun Laohutai coal mine, Liaoning Province

In order to explore the seismogenic mechanism of mining-induced earthquakes, taking for example the Fushun Laohutai coal mine, based on the focal model theory of the double couple point source and by using the P-wave first motion method, the focal mechanisms of 27 stronger mining-induced earthquakes are solved. According to these focal mechanisms, combined with the focal mechanism solution of regional strong earthquakes, composite fault plane solution of natural local small earthquakes and mining-induced earthquake source investigation at the nearest field, the seismogenic conditions of mining-induced earthquakes are analyzed. The authors find that there is appreciable difference in focal mechanism between mining-induced earthquakes in the Laohutai coal mine and natural earthquakes of the region. Thus it is concluded that the secondary stress field produced by unloading is primary stress source inducing mine earthquakes and that unloaded gravity stress field plays a prominent role in inducing the seismogenic process. Two kinds of new explanation of non-double-couple mining-induced earthquakes are advanced: instability of coal pillars and large-scale roof weighting. Ways of reducing mining-induced earthquakes are proposed.