| Research Abstract |
Palcomgnetic measurements have been carried out on the Aira Tn (AT), Daisen-Kurayoshi (DKP), Aso-4, Sanbe-Kisugi (SK), and Ata tephras. These tephras are exposed in a widespread area of the Japanese islands, forming the important time-marker in the upper Quaternary sequences. The cruption ages of AT,DKP,Aso-4, SK and Ata were estimated to be about 25,000, about 50,000, about 90,000, and about 100,000 yBP,respectively (Machida & Arai, 1992). A total of 1,500 paleomagnetic samples were collected from 52 sites of AT,10 sites of DKP,2 sites of Aso-4,6 sites of SK,2 sites of Ata tephras. These samples have stable remanent magnetization with respect to the progressive alternating field cleaning up to 70 mT.
The isomagnetic chart for about 25,000 yBP over the Japanese islands can be drawn on the basis of the stable remanent magnetization of AT.The isomagnetic chart shows that the geomagnetic declination and inclination is distributed in the range from 8ﾟ E to 10ﾟ E,and 44ﾟ to 56ﾟ, respectively. The feature is that the declination is more easterly by 15ﾟ than that of the present geomagnetic field. In the chart anomalous geomagnetic direction is observed in Okayama Prefecture and in Toyama Prefecture. It should be caused by the non-dipole field in the regions.
On the other hand, the geomagnetic fields deduced from DKP and SK show westerly direction, and those from Aso-4 and Ata northerly direction.
The result of the magnetic susceptibility measurements on the tephras reveals that the susceptibility of the same tephra is depend on the distance from the source.
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