Melt re-injection into giant magma reservoir after giant caldera eruption

Abstract Melt re-injection after a giant caldera eruption was quantitatively investigated for the Kikai Caldera Volcano in Japan, which erupted 7300 years ago (Kikai-Akahoya eruption). A seismic refraction survey was used to derive the P-wave velocity structure. Results revealed that the low-velocity anomaly was located directly beneath the central lava dome of the Kikai Caldera Volcano, allowing us to identify the existence of a giant magma reservoir with a melt fraction of 3–8%. The magma reservoir can be approximated by a trapezoidal shape in this 2D section, with upper and lower bases of 4 and 17 km, respectively, and a height of 4 km (2–6 km depth). Assuming that this magma reservoir is an axial object, its volume would be as large as 390 km3, indicating the large size of the magma reservoir; its total melt volume was calculated to be 21 km3. We therefore propose a melt re-injection model: the magma reservoir for the Kikai-Akahoya eruption was at the same location as the magma reservoir found in this study. Almost all the melt in this reservoir was ejected during the Kikai-Akahoya eruption; the new melt was re-injected into the same magma reservoir. Our estimated melt injection rate was more than 8.2 km3/kyr.