Paleomagnetism of Widespread Marker Tephras and High Resolution Magnetostratigraphy

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Paleomagnetic measurements have been carried out on the Kikai-Akahoya tephra, which erupted at about 7,300yrBP from Kikai caldera in Southern Kyusyu. The tephra is distributed widespreadly in Japan, and is the important marker bed in the Quaternary research. We collected about 600 oriented samples at 30 sites of outcrops of the tephra. The stable component of remanent magnetization was isolated from the samples of 19 sites through stepwise alternating field demagnetization up to 70mT. The direction of the stable component was obtained using principal component analysis, anchored to the origin of Zijderveld diagram.The directions of co-ignimbrite ash-fall samples are identical to those of welded tuff samples in the Kikai-Akahoya tephra. It is therefore likeky that DRMs of the ash samples are as reliable as TRMs in the welded tuff samples, and that the DRMs and TRMs represent the geomagnetic field direction at the time of the Kikai-Akahoya eruptin.The VGPs obtained from different sites agree well with one another. It 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.=38.1°W, Lat.=85.5°N, A_<95>=2.9, N=19).The present and the previous paleomagnetic studies regarding widespread tephra (Nakajima and Fujii, 1995 ; Fujii et al., 2002) yield the exact geomagnetic fields at the times the tephras were deposited, based on the paleomagnetic results from multiple sites for individual tephras. It can be expected that GSV in the Quaternary would be comple-tely elucidated by accumulating those data with the accuracy.