2003 Volume 56 Issue 3 Pages 281-296
A concealed fault named “the Gifu-Ichinomiya Line” along the eastern margin of the Nobi Plain was stratigraphically reexamined. This fault has been regarded as an active fault concealed by Cenozoic strata of the Nobi Plain, which runs in the NNW-SSE direction, and connects Gifu with Ichinomiya and Inazawa. However, the exact location of the fault has not been well defined. We focused our attention on the vertical position of a pumice bed of 80, 000 to 90, 000 years old that appears in bore-hole records around the site, because the pumice bed is a key to stratigraphic correlation. The correlation of 97 bore-hole records showed a throw of the pumice bed amounting to 5.6m in average. The strike of this throw parallels to the JR-Tokaido Line and runs about 1, 100m to the east of the line. The vertical displacements were also recognized for two gravel beds (G1 and G2) above and under the pumice bed. The throw for the deeper strata tends to be larger. We regard this throw as an active fault that moved at the Nobi earthquake of 8.0 magnitude in 1891 and other earthquakes repeated during the Quaternary.
Recently, Aichi Prefecture carried out stratigraphic examinations of shallow well records and geophysical prospecting and reported that neither fault nor flexure is realized around the presumed site of the fault in question. On the basis of the results, Governmental Earthquake Research Committee concluded the fault to be nonexistent. However, the survey by Aichi Prefecture exploited exclusively the area to the west of the fault disclosed from our stratigraphic work and scarcely covered the site of the fault mentioned above. Therefore, the survey by Aichi Prefecture is meaningless in the examination of the Gifu-Ichinomiya fault, and their conclusion is erroneous.
