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

龙泉附近的艾拉火山口壁经常发生大规模的泥石流。但由于陡峭危险的悬崖，没有对详细的地质演代进行调查。所有岩心钻探都是在1995年进行的。1996年对岩心的岩性和裂缝进行了描述。研究结果总结如下：1。从高到低依次为吉野褐煤组、Kekura组、玄武岩屑凝灰岩、龙神津安山岩。吉野烟灰岩、玄武岩凝灰岩和琉球水安山岩中多节理面发育。因此，由于裂缝孔隙度高，这些岩石是良好的含水层。Kekura地层渗透率低。吉野高原平缓地向火山口壁相反的方向倾斜。地下水通常从启仓地层的顶部流向吉野。在强降雨的情况下，地下水向墙的方向溢出。然后，由于溢出的水发生泥石流。

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.