2003 年 40 巻 1 号 p. 22-31
Rockmagnetic studies were conducted on three boring cores drilled at Bandai Volcano of Fukushima Prefecture. The cores include the debris avalanche deposit of 1888 collapse. Debris avalanche deposits in BD 1 and BD 2 cores, with the thickness of 20m and 80m respectively, show a stable remanent magnetization with fairly constant inclination. It suggests that the 1888 debris avalanche deposit was transported without significant deformation, i.e., slid a distance of 1 to 2km as a block to rest on the area that is BD 1 and BD 2 at present. In the BD 3 core at the center of collapse area, the magnetizations of the samples show appreciable deflected and scattered directions from the geomagnetic inclination, suggesting that both the debris and the underlying volcanic sequence have been severely fractured.
Rockmagnetism was also studied on the hummocky hill of Okinajima debris avalanche, 5km from the crater. Eight samples collected from the outcrop (20m high and 80m wide) showed the concentrated direction of remanent magnetization. It indicates that hummocky hill of the Okinajima debris avalanche (collapse at 80-90ka; Suzuki, 1987) was transported 5km as a sliding block.
Thus, the rockmagnetic method is proved to be useful in the study of transportation and depositional process of mega-block debris avalanches as a huge landslide event.
