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Dike intrusion simulation in 3D stress field using discrete element method

Eisuke Fujita

Abstract

Recent sensitive monitoring enables us to detect the small precursors of subsurface magma migration like volcanic earthquakes and crustal deformation, and to issue the warning on the forthcoming volcanic eruption. In this study, we develop a numerical simulation of dike migration using the discrete element method. Volcanic crust is modeled by 30,000,000 discrete elements of 10 meters radius, for 10km x 10km x 10km area. After the gravity packing, we assign magmatic area with initial excess pressure, and formulate magma migration 3D stress field. Numerical simulation results suggest that the threshold to decide eruption / failed eruption is the excess pressure, about 10 times of lithostatic pressure. In this model, a volcanic earthquake is modeled by the breakage of connection between elements. The stress concentration and opening are remarkable in the boundary area between the magma and the surrounding crust, indicating that earthquakes are occurring in this vicinity. The stress drop is related to the predominant frequency of the earthquake with small and high frequencies. It is noted that at the beginning of the dike penetration, low frequencies tend to predominate at depth, but high frequencies tend to predominate as they ascend. In the case of the failed eruption due to insufficient initial excess pressure, it has been confirmed that an earthquake with a predominant low frequency will continue to occur by repeating stress concentration and descent at almost the same point in the depths.