Distribution and Characteristics of Debris Flows Occurred in the Cretaceous Hofu Granitic Region, Yamaguchi Prefecture

Abstract

 Many debris flows were caused in northern Hofu and central Yamaguchi Cities, Yamaguchi Prefecture by a heavy rain, named the “Heavy Rain in Chugoku and Northern Kyushu of July in 2009” after the Japan Meteorological Agency. Late Cretaceous Hofu granitic body is widely distributed in the debris flow area. Fourteen people were killed in Hofu City by the debris flow. Debris flows were occurred also in northern Hofu City by a heavy rain in 1993, and killed four people. In this report, we describe characteristics and distribution of the 2009 debris flow in detail, and then compare the distribution to that of the 1993 debris flow. Source areas of the 2009 debris flow amount to 524, 478 of which occurred in the Hofu granitic region. From rock facies, the granitic body is tentatively classified into coarse-, medium- and fine-grained biotite granites, and granodiorite. Number of the source area per unit area is the most in the coarse-grained biotite granitic region, with the maximum being calculated to be 5.5/km². The number in the other types of granites is estimated to be less than about one-half of that in the coarse-grained granite. On the other hand, Source areas of the 1993 debris flow amount to 130, 109 of which occurred in the region of medium- and coarse-grained biotite granites. The relationship between the amount of topographic relief of 1 km meshes and the location of the source area indicates that both the 2009 and 1993 debris flows concentrated on the mesh of the elevation interval 201-250 m. Based on the analysis of a summit-level map, the histogram of the 2009 and 1993 debris-flow number have the modes in the elevation interval of 201-250 m and 101-150 m, respectively.