1986 年伊豆大島噴火後の航空磁気測量成果とその地球電磁気学的考察; B 火口付近に推定される熱消磁構造

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Hydrographic Department, M.S.A. Japan conducted an airborne magnetic survey on Osima Island by an aircraft, YS-11, to investigate magnetic structures and magnetic changes accompanied by volcanic activity after the eruption on Nov. 21, 1986. In airborne magnetic surveys, total intensities were measured by a proton precession magnetometer every two seconds. Positions just over the island were determined using video image and those over sea were measured by the VLF/OMEGA positioning system. Positioning error over the island was supposed to be less than 100 m in usual condition and the survey line spaces range from 300 m to 800 m. Detailed magnetic anomaly map at the altitude of 3000 ft was compiled based on above survey data. Using above survey data, three-dimensional magnetic analysis was conducted by Talwani's method. Bulk mean intensity of the magnetization of the edifice of Osima amouts to 12 A/m reflecting a basaltic volcano. Direction of the magnetization vector varies depending on the method of approximation of a trend field in anomaly analysis. Under the condition that the bias level as well as the magnetization vector is calculated in an analysis simultaneously, direction of the magnetization vector becomes Dec=12°E and Inc=64°, respectively, where declination of 12°E is more close to the normal field direction than values calculated by the planar trend assumption (VACQUIER and UYEDA, 1967 ; KODAMA and UYEDA, 1978). Magnetic anomaly caused by the central cone of Mt. Mihara and its adjacent area is derived by subtracting the calculated anomaly field by the Osima edifice from the observed one, on the assumption that the Osima edifice is magnetized in present field direction. From above analysis, magnetic anomaly caused by themal demagnetization has been discovered just over the fissure of B-crater, created by eruption on Nov. 21, 1986. Amplitude of anomaly caused by thermal demagnetization amounts to 300 nT and the width of demagnetized part, which is running N60°W from the vicinity of B-crater, is ranging from 300 m to 400 m in horizontal distance. Residual field obtained above does not show apparent anomaly field originated from Mt. Mihara, which may imply that Mt. Mihara is thermaly demagnetized in 1∿10^<-1> A/m order. Two-dimensional anomaly analyses on two magnetic profiles in NE-SW direction show that volcanic basement is extending from the submarine Habu volcanic spur to the area beneath the edifice of Osima in NW direction. Such a two-dimensional magnetic structure is significant to understand a discrepancy of magnetization direction obtained by three-dimensional model analysis from present field one. The total intensity field derived from the airborne magnetic survey in December 1986 was compared with that in September 1964, which was compiled by UTASHIRO et al. (1972) to obtain magnetic change for these 22 years. This comparison shows a drastic magnetic decrease amounting more than 1000 nT in the vicinity of B-crater. The decreased area is consistent with the demagnetized area inferred from magnetic analysis, however, it's amount is much larger than one of the latter demagnetization source. Results of repeated aeromagnetic surveys, which were conducted from December 24, 1986 to January 19, 1987 to detect a magnetic change accompanied by volcanic activity, are also reported together with some discussion of error estimate and model calculation of thermal demagnetization.
1988-06-30

1986 年伊豆大島噴火後の航空磁気測量成果とその地球電磁気学的考察; B 火口付近に推定される熱消磁構造

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