Fig. 2. Source mechanism of impulsive events.

(A) Vertical-channel seismograms (4 to 40-Hz band-pass filter) of impulsive events at three representative locations (Fig. 1A, white diamonds) recorded at various sampling points on the focal sphere. (B) Three hypothetical source mechanisms compared with observed first motion polarities. Open circles indicate downward first motions, which represent the dilatational zones. The sampling points are based on the impulsive event located at the coordinates (45°58′12.00″N, 129°59′24.00″W). Compressional quadrants are colored. All focal mechanisms are shown using the lower hemisphere projection.