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Fig.1 Location map of the Yurihara Area, Northeast Japan
The area covers the Yurihara Plateau and Kisakata Area, north of Chokai volcano. The rectangle shows the area of an aeromagnetic survey conducted by Geological Survey of Japan (GSJ) in 1990.
The Yurihara Plateau is situated 10 - 20 km north of Chokai volcano (Figure 1). It is bounded by the north-south trending Nikaho Hill to the west and the Koyoshi River lowland to the east. Chokai volcano faces the Sea of Japan to the west, and its latest eruption occurred in 1974. The enclosed part with solid lines in Figure 1 shows the area of 1990 aeromagnetic survey. Our aeromagnetic survey covers the Yurihara Plateau and adjoining areas, including offshore areas.
A: Kisakata Oil Field, B: Kotaki and Kamihama Oil Field, C: Innai Oil Field, D: Nishi-Oguni Gas Field, E:Katsurazaka Oil Field, F: Yurihara Oil and Gas Field; A1: Kisakata Anticlines, A2: Nikaho Anticlines, A3: Yurihara Anticlines, A4: Yashima Aniticlinorium; S1: Shirayuki River Syncline, S2: Nishime Syncline, S3: Shinjyo Syncline; F1: Active fault west of Otake, F2: Kotaki Reverse Fault, F3: Nikaho Thrust Faults, F4: Kamaga-dai Fault, F5: Oyachi Fault, F6: Toritame Faults.
The Yurihara Plateau is widely but thinly covered with the Quaternary Yurihara Debris Avalanche Deposits (YDAD) (Ozawa et al., 1982; Ozawa et al., 1987; Ozawa et al.., 1988) (Fig. 2). Younger Kisakata Debris Avalanche Deposits (KDAD) is distributed in the southern half of the Yurihara Plateau and Kisakata area west of the Nikaho Hill. These debris avalanche deposits comprise chiefly of andesite blocks and volcanic ash and sand. Radiocarbon dating data (Ozawa et al., 1982) shows that the latter debris avalanche deposits were formed around 3, 000 years ago.
Fig.3 Regional aeromagnetic anomaly map of the Yurihara Area, Northeast Japan (Okuma et al., 1995)
The map was originally compiled by NEDO (1982). The datum plane is 2,400 m (8,000 ft) above sea level. Contour interval is 10 nT.
Fig.4 High-resolution aeromagnetic anomaly map of the Yurihara Area, Northeast Japan (Okuma et al., 1995)
The datum plane is 920 m (3,000 ft) above sea level. Knotted lines show flight paths. See also Fig.3.
Most high-frequency anomalies in the high-resolution aeromagnetic anomaly map (Fig. 4) correspond to surface and/or shallow-depth origins: magnetic highs in the Yurihara Plateau and the Kisakata Area, in the center and western parts of the map, correspond to the distribution of the Kisakata Debris Avalanche Deposits (KDAD), which are younger debris avalanche deposits from Chokai volcano in the plateau. Andesite blocks in the Yurihara Plateau have strong natural remanent magnetizations (NRM) in addition to high magnetic susceptibilities. The direction of the NRMs of the blocks have been confirmed to be randomly oriented by a rock magnetic study (Okuma et al., 1998). This result shows that the corresponding magnetic anomalies are caused mainly by the magnetic susceptibilities of the andesite blocks of the KDAD.
Fig.5 Chokai volcano (behind) and the Kisakata Debris Avalanche Deposits (front)
Taken from the Yurihara Plateau north of Chokai volcano.
Debris avalanches are one of dangerous volcanic hazards which spread further from the causative volcanoes. Generally, high-frequency magnetic highs show a good correlation with the distribution of the KDAD, but magnetic highs extend further northward in the Yurihara Plateau without any outcrops of the KDAD (Fig. 4). This area is covered by older debris avalanche deposit, called Yurihara Debris Avalanche Deposits (YDAD), but the classification of the deposits is not sufficient. The extension of the magnetic highs indicate magnetic parts of the YDAD, which may belong to a different group. In conclusion, the high-resolution aeromagnetic anomaly map (Fig. 4) is useful for mapping the distribution of magnetic debris avalanche deposits in the Yurihara Area.
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