抄録
The focal mechanism of the Kita-Mino earthquake of August 19, 1961, is investigated in relation to an active (Quaternary) fault, the Hatogayu-Koike fault, running in N35°E direction along the Uchinami River near the border region of Fukui and Gifu prefectures in central Japan.
The re-located hypocenter is situated at the northeast end of the Hatogayu-Koike fault and is shallow in depth. The re-analysis of the first motion data of Japanese stations yields a fault plane solution of the "quadrant type, " while in a previous work, the P-wave first motion data of this earthquake was explained only as the "cone-type." The spatial distribution. of aftershocks is concentrated on the area along the Hatogayu-Koike fault. The observed trend of the static vertical displacements changes near tbe fault. These facts lead to a conclusion that the Kita-Mino earthquake of 1961 was a result of right-lateral and reverse faulting of the Hatogayu-Koike fault. A comparison of the synthesized seismograms calculated for a semi-infinite medium with the long-period seismograms recorded at Abuyama, about 170 km distant to the southwest from the epicenter, yields rupture velocity and rise time estimates.
Focal parameters thus obtained are as follows; origin time 1961, August 19, 5h, 33m, 32.8sec (GMT); epicenter 36°05′N, 136°44′E; focal depth 2 km; fault plane, dip direction N55°W, dip angle 60°; slip direction of the hanging wall 130°(right lateral and reverse); fault length 12km; fault width 10km; average dislocation 2.5m; rise time 2sec; rupture velocity 3km/sec; seismic moment 0.9×1026 dyne·cm; stress drop 160 bars; strain energy released 1.7×1022 ergs.
