An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

使用大地电磁法探索了日本东北部 Azumayama 火山下方的电阻率模型，以探测岩浆/热液流体的分布。东山是最令人担忧的活火山之一，能够产生由 2011 年东北冲地震引发的潜在喷发。三维电阻率模型揭示了海平面以下 3-15 公里 (bsl) 深处的导电岩浆储层 (< 3 Ωm)。对于岩浆储层，电阻率的 67% 和 90% 置信区间分别为 0.2-5 Ωm 和 0.02-70 Ωm。我们假设英安岩熔体 + 岩石位于 4 km bsl 的浅层深度，而安山岩熔体 + 岩石位于更深的 9 km bsl。电阻率的置信区间无法通过使用 4 km bsl 深度的英安岩熔体 + 岩石条件来解释。这表明在岩浆储层的浅部，导电性很强的热液流体与英安岩熔体和岩石共存。对于 9 公里 bsl 的深度，电阻率的 67% 置信区间被解释为水饱和（8.0 重量%）安山质熔体-基性岩复合体，熔体体积分数大于 4%，而横波速度需要流体和/或该深度处 6–7 体积 % 的熔体体积分数。考虑到 6-7% 的流体和/或熔体体积分数，同样需要导电水相来解释 67% 的电阻率置信区间的宽范围。根据大地测量数据确定的 Mogi 暴胀源位于 2.7 或 3.7 公里 bsl 深度的导电岩浆储层顶部边界附近的电阻侧。假设膨胀源区的电阻率高于导电岩浆储层电阻率置信区间的上限，且源区由热液+岩石组成，则源区的电阻率用热液解释体积分数低于 5 体积 %，这是喷发中的渗透阈值孔隙率。这表明渗透阈值表征了膨胀源区域。