Hydrothermal characteristics of the Mienhua submarine volcano in the southernmost Okinawa trough

The Mienhua submarine volcano (MHV) is located in the southernmost Okinawa Trough and exhibits vigorous hydrothermal activity. This paper presents a detailed volcanic morphological analysis of the MHV, which has not been fully explored or discussed in terms of its hydrothermal characteristics and their relationship to hydrothermal activities. The MHV is situated at a water depth of 1370 m and has a width of 2.2 km and relief height of 220 m. The MHV is an asymmetric volcano composed of two summits in the center, rugged mounds in the west, and flat regions in the east. Two hydrothermal vents, Devil Chimney and Witch Mound were discovered through acoustic flares and a high backscatter intensity in the eastern flank. Based on three repeated mappings, no noticeable depth changes were observed at the acoustic flare zone compared to the area of the rugged mounds and two summits. In addition, a sediment core collected in a high backscatter intensity patch displayed low magnetic susceptibility, which could be influenced by the high methane levels in hydrothermal fluid that flows through sediments. An acoustically transparent zone adjoining active flares was observed in the sub-bottom profiles in the southeastern flank of the MHV, suggesting that the morphological and volcanic features are mainly concentrated in the southeast. Based on the seabed characteristics and the distribution of sills and reflectors in the multichannel seismic profiles, we inferred that the MHV is an off-axis vent that has experienced at least two stages of morphological development.