Isomagnetic Chart Deduced from Remanent Magnetization of Widespread Marker Tephras and Geomagnetic Secular Variation in the Japanese Islands

对Aira Tn （AT）、Daisen-Kurayoshi （DKP）、Aso-4、Sanbe-Kisugi （SK）和Ata tephras进行了古磁测量。这些麻黄在日本列岛广泛暴露，形成了上第四纪序列的重要时间标志。AT、DKP、Aso-4、SK和Ata的腐蚀年龄分别估计为约25,000、约50,000、约90,000和约100,000 yBP （Machida & Arai, 1992）。共采集了AT区52个点、DKP区10个点、so-4区2个点、SK区6个点、Ata tephras区2个点的1500份古地磁样品。这些样品相对于70 mt的渐进交变磁场清理具有稳定的剩余磁化强度。根据AT的稳定剩余磁化强度，可以绘制日本岛屿上约25000 yBP的等磁图。等磁图显示，地磁偏角分布在8 ~ 10‰，倾角分布在44 ~ 56‰。其特征是磁偏角比现在的地磁场偏东15‰。在图中，在冈山县和富山县观测到地磁方向异常。它应该是由区域内的非偶极子场引起的。另一方面，从DKP和SK推断的地磁场方向为西风，从Aso-4和Ata推断的地磁场方向为北风。磁化率测量结果表明，同一辐射体的磁化率与辐射源的距离有关。

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.