Three‐Dimensional Electrical Resistivity Modeling to Elucidate the Crustal Magma Supply System Beneath Aso Caldera, Japan

Abstract

AbstractA magma supply system is identified in the upper crust beneath Aso caldera, Japan, appearing as a conductive anomaly via three‐dimensional electrical resistivity modeling. Aso caldera contains numerous postcaldera volcanoes, with Naka‐dake cone recently experiencing eruptions at the first crater, including an explosive eruption that ejected volcanic ash 11,000 m into the air in October 2016. The magma supply system imaged by the resistivity model consists of four key components: a magma reservoir at ~17 km depth that corresponds to the Conrad discontinuity, a magma‐filled crack‐propagation zone that extends obliquely from the magma reservoir, another magma reservoir centered at 6 km depth, and an upright crack‐intrusion zone at 2–4 km depth. The shape of the conductive anomaly is consistent with the regional stress regime, with the locations of the low‐velocity anomalies and a deformation source found beneath the caldera. This anomaly is therefore defined as the magma supply system, which feeds magma to the first crater of the Naka‐dake eruptions. The averaged melt fractions of the upper magma reservoir are estimated to be 3%–4% for 89.3 km3 of the reservoir and 96%–99% for 6.3 km3, with resistivity thresholds of <40 and <1 Ωm, respectively. Furthermore, the resistivity model detects three conductive anomalies in Aso caldera that are associated with an aqueous fluid‐flow system in the region. The resistivity model also suggests that the resistivity distribution can play an important role in characterizing the stress regime based on the distribution of earthquake clusters in the upper crust beneath the caldera.