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首页> 外文期刊>Journal of Volcanology and Geothermal Research >A Preparation Zone For Volcanic Explosions Beneath Naka-dake Crater, Aso Volcano, As Inferred From Magnetotelluric Surveys
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A Preparation Zone For Volcanic Explosions Beneath Naka-dake Crater, Aso Volcano, As Inferred From Magnetotelluric Surveys

机译：根据大地电磁测量推断，阿苏火山中沼岳火山口下方的火山爆炸准备区

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The 1 st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.
机译：阿苏火山中岳的第一火山口是日本最活跃的火山口之一，据了解其火山口的形成具有特征性的活动周期，包括火山口湖的形成，湖水的干and和最后是斯特伦伯利亚型喷发。最近的观察表明，火山喷发活动增加，包括湖水水位下降，泥浆喷发和火山口壁上的炽热红光减少。 Naka-dake火山口周围观测到的地磁场的时间变化也表明，地下岩石的热退磁已在第一个火山口附近的浅层地下区域发生。火山爆炸的作用是释放从岩浆或火山流体中传递的能量。地下电阻率的测量是一种有前途的方法，可用于调查火山建筑物的浅层结构（来自各种来源的能量聚集在其中）以及调查岩浆和火山流体的行为。我们分别于2004年和2005年在中岳（Naka-dake）火山口周围进行了音频大地电磁测量，以确定深度约1 km的详细电气结构。这项研究的主要目的是确定特定的地下结构，该结构可将能量存储起来，作为火山爆发的准备区。将二维反演应用于整个陨石坑的四个剖面，揭示了在第一个陨石坑及其周围区域下方几百米深度处的强导电带。相比之下，我们在第四弹坑下方的浅层深度中没有发现如此引人注目的导体，而第四弹坑一直没有活动了70年，而是找到了一个相对电阻性的物体。大地电磁阻抗张量的旋转不变分布与反演结果一致。这种不寻常的浅层结构可能反映出存在向火山口底部提供高温火山气体的路径。我们提出，在第一火山口下方确定的导体上部主要由热液蚀变带组成，该区域既是顶盖岩浆供应的上升流的顶盖，又是从火山口湖渗入的底板。在第四陨石坑下面发现的相对电阻体代表着合并的岩浆。这些结果表明，活动火山口下的浅层导体与控制中岳内火山活动的机制的一个组成部分密切相关。

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