Delineation of shallow volcanic structures from audio-frequency magnetotelluirc data beneath Ulleung Island, East Sea (Sea of Japan)

Abstract As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was also interpreted from the geothermal system perspective, the geochemical compositions of volcanic rocks, and the possibility of the presence of a magma chamber. AMT data were obtained from 25 stations and processed using remote reference techniques. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by the sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions at a frequency range of 10,000–1 Hz. The results showed that the Ulleung volcanic edifice is characterized by three layers of electrical structures as follows: the near-surface, a conductive layer, and a layer with relatively constant resistivity. In addition, the eruption pathway of volcanic materials that formed the Nari caldera and Albong was estimated. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Finally, based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal depths is suggested. In summary, this study presents a geological interpretation of the substructure of Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice.