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Abstract This paper describes the performance of a seabed benchmark system for measuring ground deformation associated with underwater volcanism. The system is installed in the center of the Aira Caldera, Japan, on the caldera floor 30 m below the sea surface. This is the first attempt to directly measure the expected ground deformation within a flooded caldera in Japan. The system consists of an anchor, a universal joint, a rigid steel shaft, a float, and a deck. A global navigation satellite system antenna array, consisting of a primary antenna and auxiliary antennas, is installed at the top of the moving buoy to monitor the position of the benchmark at the caldera bottom. A post-processing procedure is used to reduce the tilt of the master antenna based on measurements made by the auxiliary antennas. The system was installed in March, 2023, and has since been operating successfully with an acceptable measurement standard deviation. Graphical Abstract

Abstract The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km. Graphical Abstract