古地磁気・考古地磁気編年による年代推定

概要

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Past geomagnetic field direction is recorded as the remanent magnetization in sediments and baked clays. There are several kinds of mechanism to acquire the remanent magnetization. Thermoremanent magnetization (TRM) is the most popular and reliable remanent as the record of the ancient geomagnetic field. There reported more than one thousand archeomagnetic measurements on the baked clays remained in the excavated archeological sites in Japan. Detrital or depositional remanent magnetization (DRM) is the another important mechanism of recording the past geomagnetic field. Although the intensity of DRM is not so strong as compare to TRM, it is possible to trace the time variation of the geomagnetic field direction by taking samples stratigraphically from sedimentary strata. There is a mechanism of magnetization called shock remanent magnetization (SRM), which is acquired when shock and vibration are applied on sediments. Magnetized grains are rotated by the shock to a line their remanent magnetization parallel to the ambient geomagnetic field. Various variations of the geomagnetic field such as the secular variations, excursions and reversals recorded as the pemanent magnetization can be available for the age dating of the late Quaternary. Age of the baked clays of ancient kilns, hearths, fireplaces and fired dwelling pits can be dated by the archeomagnetic method by using the geomagnetic secular variation curve for the recent 2,000 years which is clarified by the archeomagnetic studies in the southwestern Japan. The precise geomagnetic secular variation in the period before 2,000 years BP back to 10,000 years BP is now being established so that it will become possible to date the sediments and baked clays of the period by the paleo- and archeomagnetic methods in near future. Several geomagnetic excursions observed in the Brunhes normal polarity epoch can good time markers for the middle and late Pleistocene. There reported the successful trials of paleomagnetic dating from the Babadan A, the early paleolithic site, in Hurukawa City, Miyagi Prefecture and the Nishiyagi excavated site in Akashi City, Hyogo Prefecture. The records of the geomagnetic excursions were found at the both sites. The age of the lower horizons of the sediments of Babadan A site were estimated to be 110 ky of 170 ky BP, and that of the Nishiyagi site to be 50 to 60 ky BP. There observed by the investigation of the trench excavating method of the active faults that sediments locating the fault plane were severely distorted or were squeezed to flow at the time of the past earthquake movement. Samples obtained from such sediments found to show a good convergent directions of remanent magnetization despite of the great distortion of the strata. This fact indicates that the sediments were remagnetized by the mechanism of SRM acquisition and changed their magnetic direction parallel to the geomagnetic field at the time when the earthquake occurred. So that the remagnetized direction can be used to date the age of the paleo-earthquake by correlating to the archeo-secular variation of the geomagnetic field.
日本地質学会の論文
1988-02-25

著者

広岡公夫
富山大学理学部

古地磁気・考古地磁気編年による年代推定

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