採掘域で発生する微小地震の震源過程

抄録

This paper describes a new method for estimating the focal mechanism of microseismic event in mining panel, and also demonstrates a few illustrative results obtained from the analysis of the microseismic events induced by underground coal mining. This method gives formula of the radiation patterns of P, SV and SH waves emitted from a point source at which the major, intermediate and minor principal stresses change simultaneously. The formula imply that there are two potential stress paths during fracture at focal region. One is accompanied by increase of the minor principal stress during fracture process beyond the strength failure. This type of stress path will take place if the elastic resiliance of surrounding rock is greater than the load bearing capability of failed region. On the other hand, second one is characterized by decrease of both the major and minor principal stresses. This type will be undergone in rock adjacent to underground opening when the confining pressure is removed or reduced by enlargement of the opening.
The illustrative analysis showed a few features of microseismic event in an active longwall panel:
(1) At ribside of the panel, microseismic event seems to take place when the ribside losses effective confining pressure.
(2) At mined out area, microseismic event appears to occur under the major principal stress that is normal or parallel to the seam plane depending on the condition of caving behind the coal face.
(3) At the zone adjacent to pre-existing fault, microseismic event seems to be associated with the fault movement.