Geophysical constraints on the structural evolution and hazards of Masaya Volcano, Nicaragua
Geophysical constraints on the structural evolution and hazards of Masaya Volcano, Nicaragua
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尼加拉瓜马萨亚火山结构演化和灾害的地球物理约束
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发表时间：
2014
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通讯作者：
G. Caravantes
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作者：
G. Caravantes
调查活火山的结构框架对于了解它们过去和现在活动的原因以及对它们未来演变的限制是必不可少的。位场地球物理技术可提供与识别影响火山建筑物演化的结构有关的若干物理参数（密度、传导率、磁化率）的信息。马萨亚火山是位于尼加拉瓜的一个11公里x6.5公里的玄武岩盾状破火山口，是过去不同类型火山活动（包括玄武岩普利尼安火山喷发）的源头。考虑到受马萨亚火山（尼加拉瓜最大的火山）影响的地区人口密度很高，了解其结构框架对火山活动的影响必须是一个高度优先事项。
利用地质和地球物理技术查明了马萨亚玄武岩破火山口内的重要地质特征，并提供了结构与火山活动之间关系的完整图像。在破火山口的西北半部分发现了一个直径约3.5公里的环形断层，它连接了破火山口底部的大多数火山现象（火山孔、飞溅锥、>50米高的火山锥和活跃的山顶区域）。这条断层在历史上充当了岩浆和热液系统的通道。使用重力方法对1-3公里深的细长（NNW-SSE）侵入体进行了表征。环形断层下方的这个体（解释为玄武岩浆库）的空间重合是环形断层SW段喷发物质集中的原因。这占了大部分的火山物质喷发在过去的1.8万年在破火山口，从而有助于部分排空水库，并促进目前可观察到的（使用干涉合成孔径雷达方法）4厘米/年的沉降这部分破火山口地板（SW部分的盖子内的环形故障）。这种塌陷的特征类似于破火山口活板门塌陷的端元模型。一个NNE-SSW 1.5公里的线性裂缝，破火山口和当地地堑之间的连接的证据，已被描述。我们以前已知的（Metaxian 1994年）密集入侵东北破火山口的特点的知识也得到了改善，揭示了它部分地位于破火山口地板和它的体积在以前的研究中被高估。
利用甚低频（VLF）方法，在马萨亚火山山顶地区完成了使用两个正交的VLF波源发射机的地图。该地图提供了关于山顶地区断层的大小、形状和方向的信息，并显示主要断层目前被热液系统占据。使用重力方法，地下传质过程中发生在三个不同的时间尺度（10年）的首脑会议地区已被确定。通过结合甚低频（VLF）、重力和地质技术，发现了一个含水的熔岩管系统。这种结合的方法产生了巨大的潜力，特点断层，空洞，岩脉和其他功能常见的火山环境。
Investigating the structural framework of active volcanoes is essential to understand the reasons for their past and present activity and the constraints on their future evolution. Potential field geophysical techniques can provide information on a number of physical parameters (density, conductivity, magnetic susceptibility) relevant for recognizing structures with influence on the evolution of a volcanic edifice. Masaya Volcano is a 11 km x 6.5 km basaltic shield caldera located in Nicaragua and the source of different types of volcanic activity in the past (including basaltic plinian eruptions). Considering the high population density of the area under the influence of Masaya Volcano (the largest in Nicaragua), understanding the implications of its structural framework for volcanic activity must be a high priority.
Geological and geophysical techniques were used to identify significant geological features within Masaya basaltic caldera and provide a complete picture of the relationship between structures and volcanic activity. A ≈3.5 km diameter ring fault found in the NW half of the Caldera connects most of the volcanic manifestations on the caldera floor (fumaroles, spatter cones, >50 m tall cinder cones and the active summit area). This fault has acted in historical times as a path for magma and the hydrothermal system. An elongated (NNW-SSE) intrusive body 1-3 km deep has been characterized using gravity methods. The spatial coincidence of this body (interpreted as a basaltic magma reservoir) below the ring fault is responsible for the concentration of erupted material in the SW section of the ring fault. This accounts for most of the volcanic material erupted in the last 1.8 ka in the caldera, thus contributing to partially emptying the reservoir and facilitating the current observable (using InSAR methods) 4 cm/yr subsidence of this part of the caldera floor (SW section of the lid inside the ring fault). The characteristics of this collapse resemble the end-member model of caldera trapdoor collapse. A NNE-SSW 1.5 km linear fissure, evidence for the connection between the caldera and the local graben, has been described. Our knowledge of the characteristics of a previously known (Metaxian 1994) dense intrusion NE of the caldera has also been improved, revealing that it partially underlies the caldera floor and its volume had been overestimated in previous studies.
Using the Very Low Frequency (VLF) method, a map using two orthogonal source transmitters of VLF waves has been completed in the summit area of Masaya Volcano. This map provides information on the size, shape and orientation of faults in the summit area and reveals that the main faults are currently occupied by the hydrothermal system. Using gravity methods, subsurface mass transfer processes in the summit area that occur on three different timescales ( 10 years) have been identified. A water-bearing lava tube system has been discovered by a combination of Very Low Frequency (VLF), gravity and geological techniques. This combined approach yields a great potential for characterizing faults, voids, dikes and other features common in volcanic settings.