Departmental Bulletin Paper 道東地域の地磁気永年変化と三成分絶対測量(続報)

三嶋, 渉  ,  橋本, 武志  ,  茂木, 透  ,  森山, 多加志  ,  仰木, 淳平  ,  森永, 健司

78pp.69 - 82 , 2015-03-19 , 北海道大学大学院理学研究院
Regional to subregional-scale anomaly in the rate of geomagnetic secular changes have been observed in eastern Hokkaido. The origin of such anomalous changes has long been discussed. Based on the total field records, some previous studies suspected the piezomagnetic effect is due to stress loading accompanying subducting plate motion as the major cause. We started three-component absolute measurements at eight continuous magnetic stations in 2009, aiming at revealing the mechanisms of the anomaly. This study reports the results of the absolute measurements up to 2014, as well as the continuous total field records. Although we have performed the absolute measurements only twice or three times at each station, it is readily recognized that the rate of secular change with respect to Memambetsu in horizontal and vertical components differs from site to site. Steeper trends near the southern coast are the characteristics similar to the case in the total field. Declination looks more complicated, presumably showing clockwise rotation in the northern area, while anti-clockwise in the south. Considering the compressional stress region of eastern Hokkaido and existence of highly-magnetized rocks in the coastal area, the piezomagnetic effect is still a candidate for these spatiotemporal variations. Based on the available dataset, we conclude that the anomaly in the total field is not brought by the so-called orientation effect, and that there must be such real changes of tens to hundred km scale, whatever the cause is. It should be the next step in the future works to verify the piezomagnetic hypothesis by comparing the three-component rate of change through a numerical simulation.

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