Seismic tomography for subsurface structures imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan

We present a seismic tomography study of the subsurface structure beneath Hachijojima Island, one of the volcanic fronts in the Izu-Bonin Arc, Japan. Seismic observations were conducted over two 7-month periods in 2019 and 2021, utilizing 55 densely installed stations on the island. During these periods, a total of 179 local earthquakes were recorded — 119 in 2019 and 60 in 2021 — resulting in 4671 P-wave arrival times and 3927 S-wave arrival times. The 3-D tomography, derived using the double-difference technique, revealed a shallow low-velocity region between the island’s two main volcanoes, Nishiyama and Higashiyama, suggesting the presence of volcanic sediments near the surface. Additionally, a high-velocity anomaly was identified at a depth of 4–5 km, extending vertically from deeper regions beneath Nishiyama. This feature is interpreted as a magma pathway from past volcanic activity, with high P-wave velocities and elevated Vp/Vs ratios indicating possible fluid presence. At greater depths, low P-wave velocity perturbations and elevated Vp/Vs ratios suggest a magmatic plumbing system comprising a mid-crustal magma chamber at approximately 8–12 km depth and lateral magmatic pathways at 10–20 km depth. Furthermore, a distinct zone characterized by reduced P-wave velocity and increased Vp/Vs is interpreted as a shallow magma chamber with H₂O-saturated magma accumulation. These findings provide valuable insights into the subsurface magmatic processes beneath Hachijojima Island, which are crucial for improving volcanic hazard assessment.