抄録
By using P-wave first motions, focal mechanisms of 61 acoustic emission (AE) events in a cylindrical sample of Yugawara andesite were determined during triaxial compression under a confining pressure of 20MPa. The AE events were divided into the following two groups in terms of the space-time distribution of hypocenters. Group-I: the randomly distributed events. Group-II: the events belonging to a swarm activity precursory of the ultimate fracture which formed fracture planes nearly parallel to the loading axis. About two thirds of the events in the group-I show dilatational P-wave first motions at almost all the observation points. The focal mechanism of these events is considered as a sudden closure of pore. On the other hand, almost all the events in the group-II show quadrantial distributions of P-wave first motions which can be reasonably assigned to the double couple sources. The predominant orientations of their two nodal planes are subparallel to the fracture planes and nearly perpendicular to the loading axis. These focal mechanism solutions may suggest that the microfracturings in the swarm region are closely related to the stress-induced tensile cracks whose planes are preferentially oriented parallel to the forthcoming fracture planes.
