Magma Reservoir and Magmatic Feeding System Beneath Hakone Volcano, Central Japan, Revealed by Highly Resolved Velocity Structure

Abstract

AbstractDelineation of the characteristics of magmatic fluid such as melt and water beneath volcanoes has been challenging. In this study, we obtained a detailed picture of the magma reservoir beneath the felsic caldera of Hakone volcano, central Japan using a velocity model obtained from a dense seismic observation network. In the shallow region of the caldera, a high‐velocity region associated with a solidified magma where many earthquake swarms occur was estimated, and a significantly low‐velocity region exists beneath the lower depth limit of seismicity, deeper than 6 km. The upper part of this low‐velocity region is interpreted as a water‐rich region established by the dehydration from the magma reservoir located beneath it. The temperature in this water‐rich zone was estimated to be above 350°C based on its location below the seismogenic depth. The upper depth of the magma reservoir was determined to be 9 km deep, where the high‐ratio of P‐ to S‐wave velocity was estimated. The volume fraction of melt was estimated as less than 10%, suggesting a highly crystallized mush structure. The low‐velocity zone was estimated beneath the shallow magma reservoir at a depth of 15 km and deep extension of the source region of deep low‐frequency earthquakes. The results could suggest the pathway of magma transport from the deep part of the volcano to this shallow reservoir.