Crustal structure beneath Aso Caldera,Southwest Japan,as derived from receiver function analysis |
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Authors: | Yuki Abe Takahiro Ohkura Takuo Shibutani Kazuro Hirahara Mamoru Kato |
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Institution: | 1. Aso Volcanological Laboratory, Graduate School of Science, Kyoto University, Minamiaso, Aso, Kumamoto, 869-1404, Japan;2. Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011, Japan;3. Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Sakyo, Kyoto, 606-8224, Japan;4. Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto, 606-8501, Japan |
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Abstract: | Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano. |
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Keywords: | Aso Caldera huge pyroclastic eruption deep crustal structure receiver function genetic algorithm inversion |
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