As shown by the magnetic anomaly map in Fig.
4, the boundary between the low and high magnetic
anomaly values is located in a line passing from Kitamamedani
to the Iwabuchi district, where Funatsu
granitic rocks are on the northwest side of the boundary
and early Neogene volcanic rocks are on the
southeast side. This boundary corresponds to a normal
fault trending north-northeast of the eastside moving
downward (Figs. 1 and 2); the linear magnetic anomalies
are concordant with the trend of the inferred fault
location in the estimation of faults and the geological
boundary. The fresh Funatsu granitic rocks in the
northern study area show high magnetic susceptibility
as observed in the core samples (Fig. 5). Conversely,
the Funatsu granitic rocks of the Oshiba district have
low magnetic susceptibility due to hydrothermal alteration.
However, there is no significant difference in
the susceptibilities of the Funatsu granite and the
cataclastic granite, indicating that cataclasis does not
change the susceptibility.
Hence, the intensity of the magnetic anomaly reflects
the degree of alteration of the Funatsu granitic
rocks. Low magnetic susceptibility is likely produced
by hydrothermal alteration, weathering, and/or grain
size reduction caused by disruption due to landslides.
Therefore, the strength of the magnetic anomaly reflects
the occurrence and history of landslides and the
degree of deterioration of the Funatsu granitic rocks.
The magnetic anomaly is also compared with the
regional geology of the A-A' cross section shown in
Fig. 2(a), (b). The thicker the deposition layer of colluvium
is, the weaker the magnetic field observed.
The observed smallest magnetic field is found at the
maximum thickness of the deposition. The Iwaine
formation region consists of ash breccia curd and andesite,
causing a strong magnetic field. In this area,
there is a positive correlation between the magnetic
field and the layer thickness, and there is a correlation
with the layer thickness and the layer changes in the
landslide colluvium. Ikeda et al. (1973) reported that
the magnetic field changes with the geological structure.
Here, we have shown a positive relationship between
the magnetic field and geological structure in
igneous-rock-distributed areas