[H-DS08] Landslides and related phenomena

[HDS08-P05] Distribution and characteristics of the unconsolidated volcanic deposits on the Aso ignimbrite for the risk assessment of slope failure in volcanic region

*Nobuo Geshi¹, Hideo Hoshizumi¹, Yasuo Miyabuchi², Daisaku Kawabata³ (1.Research Institute of Earthquake and Volcano Geology, The National Institute of Advanced Industrial Science and Technology, 2.Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, 3.Research Institute of Geology and Geoinformation, The National Institute of Advanced Industrial Science and Technology)

Keywords:volcano, pyroclastic fall deposit, unconsolidated deposit, Aso

Blankets of unconsolidated volcanic soil that mainly consist of pyroclastic fall deposits and secondary aerial dust deposits have caused slope failures triggered by earthquakes and heavy rain precipitation in volcanic fields. Therefore, the distribution of these blankets of unconsolidated volcanic soil is fundamental information for the evaluation of the risk of slope failure in volcanic areas. The assessment of the risk of slope failure of these blanket layers is difficult from the classic geological maps since these geological maps remove these blanket layers to show the underlying basements. Here, we try the mapping of the distribution of the unconsolidated volcanic blanket layers developing on the plateau of welded tuff of Aso Caldera in the central Kyushu area, for the evaluation of the potential risk of slope failure of the unconsolidated volcanic blanket layers.
The ignimbrite plateau that mainly consists of the welded tuff of Aso-4 ignimbrite distributes on the eastern side of the Aso caldera. The slope of the ignimbrite plateau is covered by the unconsolidated aerial deposit mainly consisting of the airfall pyroclastic material. We clarified the distribution of the thickness of the aerial volcanic deposits using the field survey and borehole data. The results show that the total thickness of the unconsolidated volcanic deposits is maximum near the caldera rim at the western margin of the plateau and decreases towards the east with the distance from the caldera. As most of the airfall units distribute toward the east from the central cones of the caldera, the thicknesses of the aerial deposits decrease to the north and south in the plateau.
The aerial deposits consist of several units. The shallowest part of the aerial deposits consists of dark-colored soil with humus. The lower portion of the aerial deposits consists of "loam" with a yellowish-brown color. In these loam deposits, there are numerous pumice fall layers and volcanic sand layers. These aerial deposits are affected by argillation. The hardness of these deposits increases with the depth from the surface. In an outcrop, the volcanic sand layers that are formed by phreatomagmatic eruptions have higher hardness than the other deposits. The weathered pumice fall deposits have extremely low hardness with the progress of argellation. Reflecting these contrasts of the hardness among the deposits, the blanket layers of the aerial volcanic deposits have a stratified structure with different hardness.
As shown by the geological evidence in many outcrops, slope failure of the blanket layers on the ignimbrite plateau may occur either at the boundary between the underlying consolidated welded tuff and the volcanic ash layer or at the boundary between the units with different mechanical properties within the blanket layers. Therefore, it is necessary to consider the distribution of the unconsolidated volcanic deposits and their units as well as the topography and dip of these deposits for the assessment of the risk of slope failure of volcanic ash layers.