有珠山自然電位観測2006

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We conducted self-potential (SP) surveys on Usu volcano since July to December 2006.The compiled SP map reveals positive anomalies around Ko-Usu lava dome and at the foot ofthe volcano, and a negative anomaly on the top of O-Usu lava dome. The SP profile on thesummit caldera shows the same pattern of 1985’s. However, the peak-to-peak variation ofthe SP value is different: the 1985’s of that shows 1000 mV while the 2006’s shows 1400 mV.Topographic effect is clearly shown along the southwestern foot of the volcano, in whichcoefficient is about -2.5 mV/m. The SP profile corrected of the topographic effect reveals alarge and extensive positive anomaly over +600mV and several local positive anomalies over+1000 mV on the summit caldera. The corrected SP also reveals that a positive anomaly atthe south part of Ko-Usu lava dome and a negative anomaly at the O-Usu lava dome do notexist. The revealed positive SP anomalies are likely to be affected by an extensive alteredlayer, located beneath the summit caldera. The largest positive anomaly is shown on theridge of Usu-Shinzan cryptodome after topographic correction. This anomaly is notregarded to be formed by hydrothermal upwelling, because any indications of fumarolic andgeothermal activities have not been observed on the ridge of the volcano. The SP anomalymay be affected by intruded material into the volcano. The SP amplitude in the northwestern part of Nishiyama is very small in spite of rugged topography. An extensive lowresistivity layer (< 10 ohm-m) located in the shallow part in the northwestern of Nishiyamaprobably shields the SP variation.
北海道大学大学院理学研究院自然史科学部門（地球物理学）の論文
2007-03-15

有珠山自然電位観測2006

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