[S-VC26] Active Volcanism

[SVC26-15] Magma re-injection after the 7.3 ka Kikai-Akahoya eruption of the Kikai submarine caldera volcano

*Nobukazu Seama^1,3, Akihiro Nagaya¹, Gou Fujie², Satoshi Shimizu³, Reina NAKAOKA^1,3, Satoru Tanaka², Yojiro Yamamoto², Hiroki Miyamachi⁴, Hiroko Sugioka^3,1, Shuichi Kodaira^2,3 (1.Department of Planetology, Graduate School of Science, Kobe University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Kobe Ocean-Bottom Exploration Center (KOBEC), Kobe University, 4.Graduate School of Science and Engineering, Kagoshima University)

Keywords:Magma re-injection, 7.3 ka Kikai-Akahoya eruption, Velocity anomaly

We image the current magma reservoir to study how the magma re-injected after its last eruption. We focus this study on the Kikai submarine caldera volcano, south of Kyushu Island, SW Japan which had its last large eruption at 7300 years ago, known as the 7.3 ka Kikai-Akahoya (K-Ah) eruption. The volume of the K-Ah eruption products beneath the seafloor was estimated using high-resolution seismic reflection surveys with stratigraphic and chemical analyses of the submarine deposits (Shimizu et al., 2024). Their results indicate that the total bulk volume of the K-Ah eruption is 332-457 km³ (133-183 km³ in DRE; Dense Rock Equivalent), suggesting that the K-Ah eruption is probably the largest eruption of the Holocene. The current magma reservoir of the Kikai caldera volcano was imaged by investigating the current velocity structure using seismic refraction surveys conducted with 39 seafloor seismometers and air-gun arrays along a 175 km survey line across the volcano (Nagaya et al., in prep.). The 2-D P-wave velocity structure was estimated just below this survey line by first-arrival tomography analysis. The low velocity anomaly, as a difference from the background velocity structure, is indicated below the volcano at depths of 2-12 km. This low velocity anomaly was used to estimate the size of the magma reservoir and the total amount of melt present. The result suggests that the magma reservoir is located immediately beneath the volcano's caldera at a depth of 2-6 km in a trapezoidal shape with an upper base of 5 km and a lower base of 18 km. The average melt fraction of the magma reservoir is 5% and the total melt volume is estimated to be 24 km³. Moreover, the post-caldera lava dome in the caldera is recognized with a volume of more than 32 km³ (Tatsumi et al., 2018) and is estimated to have formed after 3900 years ago based on the rock composition (Hamada et al., 2023). All this occurred after the 7.3 kyr K-Ah eruption with its total bulk volume of 133-183 km³ in DRE, leading us to propose a magma reinjection model.