Physical stratigraphy in relation to collapse mechanism at the Aira caldera wall in Ryugamizu area, Kagoshima Prefecture

在龙上水附近的空拉破火山口壁经常发生大型泥石流。但由于其陡峭危险的悬崖，详细的地质演替没有调查。1995年进行了所有岩心钻探。1996年描述了岩心的岩性和裂缝。研究结果总结如下：1.从大到小依次为吉野熔结凝灰岩、柯仓组、玄武质碎屑凝灰岩和龙上水安山岩.吉野熔结凝灰岩、玄武质凝灰岩和龙上水安山岩中节理面发育。这些岩石裂隙孔隙度高，是良好的含水层. Kekura组渗透率低.吉野高原相对于火山口壁向相反方向倾斜。地下水通常在Kekura组的顶面流向吉野。6.遇强降雨，地下水向墙方向溢流。然后由于溢出水而发生泥石流。

Large debris flows frequently occur at the Aira caldera wall near Ryugamizu. But detailed geological succession is not surveyed because of their steep dangerous cliffs. All core drilling was carried out in 1995. The lithology and cracks of the cores are described in 1996. Results of the study are summarized as follows.1. There are Yoshino ignimbrite, Kekura formation, basaltic lappilli tuff and Ryugamizu andesite in descending order.2. Many joint planes are well developed in Yoshino ignimbrite, basaltic tuff and Ryugamizu andesite. Therefore these rocks are good aquifer because of high crack porosities.3. Kekura formation has low permiability.4. Yoshino plateau gently dips to the oposite direction against the caldera wall.5. Underground water usually flows on the top planes of Kekura formation to Yoshino.6. In case of heavy rainfall, underground water overflows to the direction of wall. Then debris flows occur due to overflow water.