ロングバレーカルデラ地域の地磁気全磁力差に見られる年周変化について

Abstract

米国ロングバレーカルデラにおいて1999年から2001年にかけて地磁気全磁力の連続観測を実施た。観測された全磁力単純差には顕著な年周変化が認められた。原因として，地下数m程度の地中温度変化に伴う岩石の磁化変化が有力であることが明らかにされ，1次元熱拡散モデルに基づいて補正を行った。更に，地磁気3成分を用いた外部起源擾乱の補正を行い，地殻活動起源の異常磁場変動の検出を試みた。

We deployed some proton magnetometers in Long Valley caldera in order to detect anomalous geomagnetic changes of tectonic or volcanic origin during the period of 1991-2001. Distinct annual variations can be seen in the geomagnetic differences. The most plausible explanation for the annual variation is the re(de)magnetization of shallow rocks due to the temperature change. We made a correction of the annual changes assuming the 1-D thermal diffusion theory. In addition, reduction of external disturbances using three component geomagnetic data has been applied to the records. It is unlikely that there was any anomalous geomagnetic change related to the Hector Mine earthquake (Oct. 16, 1999), which was the most distinct event during the observation period. However, there still remains a possibility that long-term trends represent anomalous changes due to the volcanic activity of the area.