Magmatic fluid pathways in the upper crust: insights from dense magnetotelluric observations around the Kuju Volcanoes, Japan

Abstract

Summary Magnetotelluric (MT) observations have revealed sub-vertical electrical conductors that extend from shallow depths into the mid-crust at various geothermal zones, active volcanoes, and active faults worldwide. These deeply rooted sub-vertical conductors have typically been interpreted to represent entire zones of dedicated fluid transport through the crust. We estimate the high-resolution three-dimensional crustal resistivity structure below the Kuju Volcanoes, Japan, using dense observations from 146 broadband MT measurement sites and 47 telluric measurement sites. The resistivity structure highlights sub-vertical conductors that merge into a deep conductor to the north of the volcanoes, with deep low-frequency earthquakes occurring near the southeastern edge of this sub-vertical conductor at 10–30 km depth. This deep conductor branches into several sub-vertical conductors at 2–10 km depth, coinciding with a shallow zone where tectonic earthquakes rarely occur. The surface expressions of active geothermal areas and past volcanic eruptions are all located above the edges of the conductors at 2–6 km depth. Widespread conductive layers exist around the volcanoes above 2 km depth, and their distribution approximately corresponds to a low-gravity-anomaly zone. We discuss the nature of these sub-vertical conductors, the potential causes of their complex structure, and their relationship to local magmatic fluid transport. These sub-vertical conductors, a shallow clay-rich layer, developed fracture systems, and high-strength solidified magma may all contribute to magmatic fluid transport to the surface at the Kuju Volcanoes. In this study, we add the possibility that the edges of these sub-vertical conductors act as important magmatic fluid pathways.

Three‐Dimensional Electrical Structure and Magma System of the Monogenetic Longgang Volcanic Field, Northeast China, Inferred From Broadband Magnetotelluric Data

“…Fan et al., 1999; J. Liu et al., 2009; Sui et al., 1999; F. S. Yu et al., 2003). As shown in Figure 7, C1 also merges into channel C2 at depths of 40–50 km, such branch structure has been found on other volcanic areas (Aizawa et al., 2022; Comeau et al., 2015). In addition, Figures 5 and 6 also show that the C2 magmatic channel is located near fault F2, indicating that the fault may influence the material migration along the C2 magmatic channel.…”

“…MT signal is not shielded by high-resistivity layers and is sensitive to fluid and thermal materials. It is widely used in volcanic studies and in other fields (Aizawa et al, 2014(Aizawa et al, , 2022Comeau et al, 2015Comeau et al, , 2021Cordell et al, 2020;Gao et al, 2020;Hill et al, 2009Hill et al, , 2015.…”

Tracking supercritical geothermal fluid distribution from continuous seismic monitoring

“…The less intense seismicity in the southern part of the cluster, where resistivity and Vs are lower, signi es a less brittle area. Overlaying our results with the resistivity 21 and S-wave velocity pro les 20 (Figs. 5 and 6) shows that the hypocenters are mostly clustered near boundaries between lower and higher values of resistivity and Vs.…”

“…Given the agreement of our interpretation and previous ones, we interpret the earthquakes as cracking events associated with geothermal uid circulation. Seismic clusters at depths greater than 3 km may re ect crack generation related to an apparent subvertical conduit extending from Shishimuta caldera 21 toward Mt. Kuroiwa and Mt.…”

“…In the Kuju volcanic complex, geophysical eld surveys have been conducted to characterize the deep geothermal reservoir (e.g., [18][19][20][21][22] ), but the connection between the distribution of geothermal uids and the BDT is still unclear. In this case, earthquakes are often located near the BDT ( 3,23,24 ).…”