姶良Tnテフラの古地磁気方位 Paleomagnetic Directions of the Aira Tn Tephra Deposits
Paleomagnetic measurements have been carried out on the Aira Tn tephra, which erupted about 22, 000-25, 000 yrsBP from Aira caldera in Southern Kyushu. The tephra is distributed widely in Japan, and is the important marker bed in the Quaternary research. We collected about 800 oriented samples at 44 sites of outcrops of the tephra. The stable component of remanent magnetization was isolated from all samples through stepwise alternating field demagnetization up to 70mT. The direction of stable component was obtained using principal component analysis, anchored to the origin of Zijderveld diagram.<br>The VGPs obtained from different sites agree well with one another, which, means that their remanent magnetizations were acquired under the same geomagnetic dipole field (DF) at the time of eruption of the tephra. The pole of the DF can be assumed to be located on the all-mean VGP (Long=104.6°W, Lat=81.1°N, A<sub>95</sub>=1.3, N=44). Declinations and inclinations over the Japanese islands calculated from the pole range between 8°E and 10°E, and between 45° and 55°, respectively. The surveyed area was divided into 10 regions, and the local geomagnetic field (LF) direction of individual regions was calculated from the regional mean VGP. The difference between the DF and the LF direction at each region is less than 3°, except in Okayama Prefecture and Toyama Prefecture. The declination of the LF in Okayama Prefecture is deflected westerly by about 5° from those in the nearby regions, and the inclination of the LF in Toyama Prefecture is steeper by about 7° than those in the nearby regions. These local anomalous directions might be caused by the non-dipole field in the regions.
第四紀研究 34(4), 297-307, 1995-10-31
Japan Association for Quaternary Research