Geophysical Constraints on the Structural Evolution and Hazards of Masaya Volcano, Nicaragua

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