Abstract
Brecciated rocks of several centimeters to several meters in size are distributed on the surface and underground of a deep-seated landslide that occurred in 2005 at the Tsukabaru site in the Mimikawa basin of Miyazaki Prefecture, Japan. A breccia-predominant formation (BPF) composed of breccia and a fine-grained matrix is distributed above the fresh basement rock. We carried out detailed core logging and field observation of the BPF, classifying areas into five grades based on the brecciation process. From investigation of the grain size distribution and assuming a continuous distribution of the BPF, we conclude that BPFs are composed of crushed fine-grained breccia distributed parallel to the slope. BPFs are formed not by faulting due to a lack of sliding plane and clay formation but by the concentration of gravitational strain. As evidence of this, basement rock was crushed along the deformation zone where the gravitational stress is concentrated underground of the unstable slope. That slope resulted from rapid river erosion, and several blocks were formed with river erosion. A deep-seated landslide occurred in 2005 along the boundary between fresh basement rock and the BPF. It is reasonable to assume that BPF is the precursor of the deep-seated landslide and significant tectonic features should be evaluated to elucidate the formation mechanism and make predictions of deep-seated landslides.
