Abstract
In the Karasu River Valley of Gunma Prefecture, central Japan, the authors have discovered an active mega-landslide, 2.8km wide and 1.5km long. This landslide occurred during the late Pleistocene before deposition of the Aira-Tanzawa tephra, and is reactivated at the present. We call the landslide the Yudonosan Mega-landslide. The landslide mass and its bed rocks are composed of andesitic tuff breccia and welded tuff of the late Miocene to Pliocene. In the upper portion of the landslide mass, horsts and grabens are observed to be arranged in echelon pattern, which was formed by ancient landslide cracks. However, compared with the strata of surrounding areas, the strata in the lower portion of the landslide is sheared into pieces and their bedding planes are disturbed. Furthermore, there are tension cracks in the upper portion, and irrigation facilities and flood control measures are now collapsing in the lower portion. These facts suggest that the landslide is reactived now. There is a large possibility that the Yudonosan Mega-landslide was reactivated at least twice in the past. One reactivation occurred about 10,000yrs BP and the other occurred between 1108 and 1783 AD. It has become clear that these two reactivations at the same times as liquefaction in the tephra layer of the landslide and surrounding areas. From this it can be inferred that earthquakes triggered the reactivations of those ancient landslides. However, since the present reactivation has no clear relation to any earthquakes, it should be studied in detail in further. In addition, there is a possibility that this mega-landslide might have dammed up the Karasu River in the past. Therefore, further studies on the damming will give us a considerable amount of useful information for predicting disasters. The disaster prevention measures against the Yudonosan Mega-landslide, which is on a geological scale in both time and space, should be considered in all aspects, providing landslide prevention measures and informing the inhabitants about the details of the landslide. However, since prevention measures for mega-landslide have not been implemented yet, it is essential to reveal the entirety of the landslide as it exists, and to offer the area's inhabitants a great deal of information about the landslide. This can be done through detailed investigations independent of prevention measures.
