抄録
The natural remanent magnetization of Pleistocene andesite and scoria samples collected from Usami Volcano Izu Penninsula was examined by paleomagnetic methods and found to be stable. The directions of the stable magnetization of these rocks show significant departure from that of the present geomagnetic field in Japan A hypothesis based on geological field evidence that the strata in the sampled area have been tilted sometime after the formation of the rocks, can account for this fairly well. Using these samples with stable remanences, the intensity of the Pleistocene geomagnetic field (normal and reversed) in Japan has been determined by means of Thellier's metood. Many samples show some irreversible changes in magnetic properties when they are heated above certain critical temperatures. The intensity estimates were obtained from the linear relation of Jn-Jt in the temperature range where irreversible changes did not occur. The intensity of the Pleistocene geomagnetic field determined in the present study is very similar to the present field intensity in Japan (0.46oe), the mean values being 0.61±0.61oe for reversed samples and 0.55±0.16oe for normal ones.
