Structure of Paleosecular Variation : Relation to the Core Dynamics

用一种新引进的热退磁器对日本云仙火山和翁竹火山的火山岩进行了实验研究。从云仙获得的Brunhes年代的古长期变化（PSV）的角度标准差（ASD）与日本其他地区的水平相同，但它是较大的老云仙比新云仙。古地磁方法也被应用于一些火山地层学问题，如火山碎屑流侵位温度的估算和熔岩的识别。日本翁竹火山古地磁研究最重要的成果之一是建立了更新世晚期的地磁漂移。由于是在两个安山岩熔岩流中发现的，其剩磁的稳定性远高于沉积物中的剩磁，因此，这些偏移的可靠性很高。其中一个熔岩流的古地磁强度约为现今值的1/10，其虚拟地磁极（VGPs）位于太平洋中部附近。由于在新西兰奥克兰考察中也报道了VGP在中太平洋的位置，因此提出在这次考察中赤道偶极矩占主导地位。为了解释PSV的结构，提出了一个新的统计模型，其中对偏心偶极子施加高斯涨落。这个简单的模型解释了PSV的大部分结构;纬度依赖的ASD和古强度直方图的形状。

Experimental studies with a newly introduced thermal demagnetizer were made on many volcanic rocks from the Unzen Volcano and the Ontake Volcano, Japan. Angular standard deviation(ASD) of the paleosecular variation(PSV) in the Brunhes Chron obtained from Unzen was in the same level as those from other areas of Japan, but it was larger for Older Unzen than Younger Unzen. Paleomagnetic methods were also applied to some volcanostratigrphic problems ; estimation of emplacement temperature of pyroclastic flows and lava identifications. One of the most important results obtained from the paleomagnetic studies of the Ontake Volcano is the establishment of the geomagnetic excursions in the latest Pleistocene in Japan. Reliability of the excursions is very high because they were found from two andesite lava flows, in which stabilities of the remanences are much higher than those from sediments. Paleointensities obtained from one of the two lava flows are abut 1/10 of the present-day value, and the associated virtual geomagnetic poles(VGPs) locate near the central Pacific Ocean. As the VGP positions in the central Pacific were also reported from the Auckland excursion in New Zealand, it was proposed that the equatorial dipole moment becomes dominant during the excursion. To explain the structure of PSV, a new statistical model was developed, in which Gaussian fluctuation was given to an eccentric dipole. This simple model explains most of the structures of PSV ; latitude dependent ASD and the shape of the histograms of the paleointensities.