4. Other low magnetic anomaly area: Visual observation
during tunnel construction indicates that
low magnetic anomalies on the left-bank side of the
Toga River are likely to have originated in the cataclastic
Funatsu granitic and Funatsu granitic basement
that contained numerous cracks, which were commonly
altered by hydrothermal processes to clay
seams and veins. Where alteration pervades the core
of the rock, white clay is common and the rocks show
low magnetic anomalies (Sugimoto and Sakai, 2013).
The Funatsu granite zone of the Toga region is
characterized by many small faults. The dip of these
faults is the dip slopes with a NW trend, causing one
of the most important predisposing factors for landslides
on the right-bank side. In contrast, on the leftbank
side, the dips of the faults are opposite slopes
and a slope failure region is therefore likely to occur
in the surface layer portion undergoing deterioration.
In addition, intense low magnetic anomalies are
observed within the Oshiba to Kita-mamedani area.
This district, which consists of well-developed fractures
and faults and contains numerous clay veins
formed by hydrothermal alteration, is more likely to
undergo slope failure. Hence, this area has become a
thin-ridged terrain with deep erosion valleys.
4. Other low magnetic anomaly area: Visual observation
during tunnel construction indicates that
low magnetic anomalies on the left-bank side of the
Toga River are likely to have originated in the cataclastic
Funatsu granitic and Funatsu granitic basement
that contained numerous cracks, which were commonly
altered by hydrothermal processes to clay
seams and veins. Where alteration pervades the core
of the rock, white clay is common and the rocks show
low magnetic anomalies (Sugimoto and Sakai, 2013).
The Funatsu granite zone of the Toga region is
characterized by many small faults. The dip of these
faults is the dip slopes with a NW trend, causing one
of the most important predisposing factors for landslides
on the right-bank side. In contrast, on the leftbank
side, the dips of the faults are opposite slopes
and a slope failure region is therefore likely to occur
in the surface layer portion undergoing deterioration.
In addition, intense low magnetic anomalies are
observed within the Oshiba to Kita-mamedani area.
This district, which consists of well-developed fractures
and faults and contains numerous clay veins
formed by hydrothermal alteration, is more likely to
undergo slope failure. Hence, this area has become a
thin-ridged terrain with deep erosion valleys.
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