Imaging the Magmatic Plumbing System Using Seismic Tomography Beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan

This study presents a seismic tomography analysis of the magmatic plumbing system beneath the Hachijojima Island, a populated volcanic island in Japan. The island hosts two volcanoes, Nishiyama and Higashiyama, with Nishiyama considered to be the active volcano. The seismic data were collected from seismic observation which was conducted over two separate 7-month periods in 2019 and 2021, utilizing a dense network of 55 seismic stations installed on the island. During the observation period, a total of 179 local earthquakes were recorded, with 119 occurring in 2019 and 60 in 2021. The earthquake events were predominantly located approximately 20-30 km northwest of the island, rather than directly beneath it. These recorded earthquakes provided 4642 P-wave arrival times and 3927 S-wave arrival times, which were subsequently analyzed using the Double Difference (DD) Tomography method to derive the subsurface velocity structure.

The seismic tomography analysis employed a two-step DD Tomography approach. It aims to construct a robust initial reference velocity model and obtain a better resolution at shallower region beneath Nishiyama. The first step utilized a coarser and uniform grid size to generate a 3D velocity model, which was then utilized as the initial model for the second step of DD tomography inversion with finer grid size beneath Nishiyama.

The 3-D tomography results revealed a high-velocity anomaly region at approximately 4 km depth, extending vertically from the deeper area beneath Nishiyama. This suggests the presence of a potential pathway through which magma from past volcanic activity may have migrated. This high-velocity region is characterized by high P-wave velocities, low S-wave velocities, and high Vp/Vs ratios, potentially indicative of the existence of fluid in this area. Furthermore, the Vp perturbation image clearly visualized a magmatic plumbing system to a depth of approximately 20 km in the deeper, northwestern region of the island. The hypocenters which are predominantly located in this zone appear to be associated with the long-distance lateral magma transport. This region, situated in the middle to lower crust at depths of 10-20 km, is driven by the regional tectonic conditions within the deeper crust.