Crustal magma pathway beneath Aso caldera inferred from three‐dimensional electrical resistivity structure

Abstract

At Naka‐dake cone, Aso caldera, Japan, volcanic activity is raised cyclically, an example of which was a phreatomagmatic eruption in September 2015. Using a three‐dimensional model of electrical resistivity, we identify a magma pathway from a series of northward dipping conductive anomalies in the upper crust beneath the caldera. Our resistivity model was created from magnetotelluric measurements conducted in November–December 2015; thus, it provides the latest information about magma reservoir geometry beneath the caldera. The center of the conductive anomalies shifts from the north of Naka‐dake at depths >10 km toward Naka‐dake, along with a decrease in anomaly depths. The melt fraction is estimated at 13–15% at ~2 km depth. Moreover, these anomalies are spatially correlated with the locations of earthquake clusters, which are distributed within resistive blocks on the conductive anomalies in the northwest of Naka‐dake but distributed at the resistive sides of resistivity boundaries in the northeast.

Three‐Dimensional Electrical Resistivity Distribution Beneath the Beppu–Shimabara Graben With a Focus on Aso Caldera, Southwest Japan Subduction Zone

“…These results reflect the distribution and migration path of magma in the upper crust beneath the caldera. C1 is deeper and located more to the west than a similar northward‐dipping conductive anomaly extending to a depth of ~10 km in our previous 3‐D model, which was obtained by inverting MT data of a period range 0.005–2380 s at 55 sites around Aso caldera in 2015 (Hata et al, ). Our 3‐D model in the present study clearly reveals the magma pathway leading to the active volcano from a depth of ~17 km (see the horizontal cross‐sections in Figures ).…”

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

“…The magnetotelluric (MT) method was employed to obtain a 3‐D electrical resistivity model for the upper and lower crustal structures of the entire Aso caldera region. The MT method is sensitive to even a few percent of fluids in the host rock (e.g., aqueous fluids and/or melt), especially in the case of interconnected fluids (e.g., Hyndman & Shearer, ), and the method is optimal for clearly detecting magma reservoirs (e.g., Hata et al, ). We conducted two MT surveys across Aso caldera in November–December 2015 and November–December 2016 to characterize the volcanic structure and characteristics of the region.…”

The 2016 Kumamoto Mw = 7.0 Earthquake: A Significant Event in a Fault–Volcano System

“…Lin et al () suggest that the termination of the coseismic rupture near the Aso volcano is due to the presence of a localized magma chamber. The structure of the Aso volcano has been investigated by magnetotelluric (Hata et al, ) and gravity surveys (Miyakawa et al, ). These studies show high‐conductivity and low‐gravity anomalies beneath the caldera of the Aso volcano, which may be related to the existence of a magma chamber.…”