著者

岡崎 紀俊 田中 和夫 三品 正明

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.35, no.4, pp.375-388, 1990-12-28

被引用文献数

1

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Surveys of magnetic total intensity and gravity were conducted to reveal the underground structure of Medake, one of the central cones in the cadera of Akita-Komaga-take Volcano, from which about 1.42 million m^3 of lavas flowed in the period of 1970-1971 eruption. Magnetic and gravity anomalies obtained show some characteristic features suggesting the subsurface structure. Long wave length magnetic anomalies show that Me-dake is composed of uniformly magnetized volcanic rocks, lavas and scoriae, On the other hand, short ones found at the summit of Me-dake indicate there is a body with reversal or weaker magnetization than the surroundings. The distribution of the Bouguer anomalies is characterized by a narrow area of higher values of anomalies at the summit and a trend of higher values in northwest of Me-dake and lower ones in southeast. The latter suggests the basement structure of the caldera. The former implies an exislence of more dense intrusive rocks under the ground surface. Both centers of short wave length magnetic anomaly and of the narrow area of higher value of gravity locate at the same place about 100 m northeast of the 1970 crater on the summit. Judging from the topographic features, this place is one of the old craters. To interpret magnetic and gravity anomalies, numerical analyses for demagnetization model and for more dense intrusion model with various shapes were carried out independently. Finally, magnetic anomalies can be expained by a demagnetized vertical pentagonal prism, of which size is 80 m in N-S, 150 m in E-W and 300 m in thickness and of which top is at the depth of 5-10 m. Gravity anomalies can be interpreted by a more dense vertical rectanglar prism, of which size is 30 m×110 m×300 m and the top is at 15 m deep. The zones of high ground temperature and fumaroles are found at the surroundings of the 1970 crater and the area of the higher gravity anomalies. The fumarolic area preceding the 1970 eruption was found at north of the crater. We can infer the processes of the 1970 eruption by using the interpreting subsurface model of Me-dake and the distribution of fumaroles. At the first stage of eruptive activity in 1970, magma intruded into northeast of the 1970 crater. At the second, most of magma passed through the vent conncted with the crater and flowed out. Small quantity of magma remained in the narrow part at northeast of the crater make the present magnetic and gravity anomalies and fumaroles.

著者

三品 正明 立花 憲司 三浦 哲

出版者

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) = Department of Natural History Sciences (Geophysics), Graduate School of Science, Hokkaido University

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

no.72, pp.287-298, 2009-03

被引用文献数

1

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Borehole strainmeters installed in the coastal area of Miyagi and Fukushima prefectures recorded the remarkable postseismic strain change following earthquake off Fukushima Prefecture on July 19, 2008 (M 6.9). Using exponential function, we simulated the postseismic strain change and obtained time constant and amplitude of the postseismic change. Time constant was 57 h. The amplitude ratio to the amplitude of the co-seismic change was 0.9 (KNK: Kinkazan), 1.1 (EN3: Enoshima) and 1.6 (MH2: Mihodo), respectively. This means almost the same amount of slip as the main shock occurred on the fault plane after the shock. We also analyzed data at an off Fukushima Prefecture earthquake on October 31, 2003 (M 6.8). Time constant of the postseismic change was 40 h. The amplitude ratio was 0.5 (KNK), 0.6 (EN3) and 1.5 (MH2), respectively. This earthquake also had similar feature as the 2008 event. These characteristics of postseismic slips are considered to relate the distribution of the coupling coefficient between Pacific plate and continental plate. In off Fukushima area, the coupling coefficient may be weaker around the epicenter area of the 2008 earthquake than around the 2003 event.