Anatomy of the fumarole field of Hakone Volcano, Japan: Interpretation of its resistivity structure and inferences for the steaming activity and recent hydrothermal eruption

Abstract

Understanding the underground structures of fumarole fields is essential for deciphering lethal hydrothermal eruptions and elucidating the chemical evolution of hydrothermal fluids en route to surface hot springs and steams. We implemented a controlled-source audio-frequency magnetotellurics (CSAMT) survey in Owakudani, the largest fumarole field on Hakone volcano, Japan. The hydrothermal system lies beneath a caprock structure, detected as a low-resistivity zone in two-dimensional electrical resistivity cross-sections. The caprock formed a plateau-like convexity with a diameter of approximately 500 m, centered on an inactive, forest-covered steaming area west of the 2015 hydrothermal eruption center. The caprock top subsided before the 2015 eruption, possibly because of depressurization of the hydrothermal system. Notably, local but distinctive high-resistivity zones exist within the caprock, especially in the southern part of the survey area, which contains major fumaroles and hot springs. Such zones, along with the high resistivity detected beneath the caprock, can be interpreted as vapor-dominated. Therefore, the fumaroles and hot springs in Owakudani overlie a two-layered vapor-dominated zone, with the upper layer located within the caprock and the lower layer beneath it. Classical works suggested that meteoric water heated by steam from the vapor-dominated zone of the hydrothermal system forms hot springs in the fumarole area. However, the chemical composition of the artificial hot spring—fromed from a steam and meteoric water—suggests simple mixing cannot explain its composition. Therefore, the two-layered vapor-dominated system beneath the fumarolic zone potentially contributes to the chemical differentiation of the hydrothermal fluid.