Studies on the mechanism and countermeasure of natural disaster caused by mud volcano and mud diapir

泥火山的活动被认为是由地下深处产生的异常水压引起的，对铁路隧道、地下放射性废物处理设施等地下建筑造成了严重的问题。研究泥火山地下结构及其异常积水形成机制对泥火山设施选址和安全评价具有重要意义。泥火山下180米深处曾发生过岩崩等严重事故。挖掘150米长的路段花了10多年时间。在日本中部新泻地区第三纪沉积盆地中发现了4座陆相泥火山，所有泥火山均沿地形盆地边缘分布，位于山区NE-SW走向的峰顶，沿背斜翼分布。盆地内地质构造受到严重扰动。已灭泥火山暴露在新建道路的边坡上，可观察到泥火山内部的通风口结构。该喷口直径30m，由泥角砾岩和鳞状网状粘土组成，被认为是由水力压裂和泥岩与地下水之间的水-岩相互作用产生的。泥火山喷出的地下水为高盐水，电导率为15mS/cm， O同位素比值为1.2‰，O同位素比值<18> / <16>。此外，镜质组反射率为1.5 ~ 1.9%，这在近地表暴露的沉积岩中是不可能的。因此，可以假设这些喷发的物质是从大约4000米深的地下深处引入的。利用CSA-MT物探对地下构造进行了调查，获得了地表至800m深度的电阻率剖面。在泥火山地下600m深、200m厚、800m径处发现了高矿化度导致的小于1Ωm的盘状低电阻率带。此外，低电阻率带沿盆地边缘延伸至地表，构造类似于泥火山活跃的火山口。少

Activity of mud volcano is thought to be caused by an abnormal water pressure generated in deep underground and make a serious problem for underground constructions such as railway tunnel, underground facility for radwaste and so on. It is important to investigate the underground structure of a mud volcano and the mechanism of abnormal water formation for site selection and safety assessment of such facilities.Serious trouble such as tunnel wall collapse due to the rock swelling has happened 180m deep under mud volcanoes. It took more than 10 years to excavate the section of 150m long. 4 terrestrial mud volcanoes were found in the Tertiary sedimentary basin in Niigata, central JapanAll the mud volcanoes are distributed along the rim of the topographic basin that is located at the NE-SW trending crest of mountainous area and distributed along the wing of anticline. Geological structure inside basin is heavily disturbed.The extinct mud volcano is exposed in the side-slope of newly constr … More ucted road and the internal vent structure of mud volcano can be observed. The vent is 30m in diameter and is consisted of mud breccia and scaly network clay that is thought to be generated by hydro-fracturing and the following water-rock interaction between mudstone and groundwater.Groundwater erupted from mud volcano is highly saline with electric conductivity of 15mS/cm and high ^<18>O/^<16>O isotope ratio of 1.2‰. Also, the vitrinite reflectance is 1.5 to 1.9% that is not expected in the sedimentary rocks exposed near ground surface. As a result, it is assumed that these erupted materials were introduced from the deep underground about 4000m deep.CSA-MT geophysical exploration was carried out to survey the underground structure and obtained the profile of electrical resistivity from the surface to 800m in depth. It is found that the disk-shaped low resistivity zone less than 1Ωm due to the high salinity content is identified in underground 600m deep, 200m thick and 800m in diameter under the mud volcanoes. Moreover, low resistivity zone is continuing to the ground surface along the rim of the basin structure similar to volcanic caldera where mud volcanoes are active. Less