Pre-historic secular variation of the geomagnetic field and its application to the archeomagnetic dating

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We studied on the following three subjects to accomplish the purpose of this study.1) Completion of the pre-historic geomagnetic secular variation.Paleomagnetic samples were collected from the stratified sedimentary strata at Oiso hilly area, Minami-Taikoyama I archeological site and five active fault trench excavation sites. The results of the paleomagnetic measurements clarified the brief geomagnetic secular variation of the past 12,000 years.2) Archeomagnetic age dating.It is found that the magnetization of sediments of the excavated ancient rice field are strong and stable enough for archeomagnetic age dating. The dating which we tried in this study will become an important method for the dating of the ancient rice field.Sediments located very close to the active fault plane and suffered from a severe tectonic deformation were often remagnetized parallel to the geomagnetic field direction at the time of the faulting activity. It becomes possible, therefore, to date the paleo-earthquake by comparing the magnetic direction of sediments to the archeomagnetic secular variation curve.3) Age dating of sediments by means of the geomagnetic excursions.It is reported that the geomagnetic excursion occurred several times during the Brunhes Normal Polarity Epoch. Paleomagnetic samples collected from the paleolithic excavation site of Babadan A, and the Nishiyagi excavation site show anomalous magnetic directions at Horizons 19 and 29 of Babadan A site, and at the horizons Stratum IV of the Nishiyagi site. From the similarity of the virtual geomagnetic pole path, the stratum IV of the Nishiyagi site might be correlated to the Ina I Excursion. Horizons 19 and 29 of the Babadan A site should be correlated to two of the following four excursions, that is, the Ina I Excursion, the Ina II Excursion, the Blake Event and Biwa I Event.