[S-VC26] Active Volcanism

[SVC26-17] Shallow hydrothermal system beneath Numanotaira Crater, Adatara volcano, Japan, as inferred from audio-frequency magnetotelluric surveys

*Hiromasa Kawada¹, Wataru Kanda¹, Masahiro Ichiki², Hiroki Nagaike¹, Tatsuki Ishigo¹ (1.Tokyo institute of technology, 2.Tohoku university)

Keywords:Adatara volcano, Magnetotellurics, Resistivity structure, Hydrothermal system

The Numanotaira Crater of Adatara volcano, located in central Fukushima Prefecture, has repeatedly experienced phreatic eruptions in the past. Recently, although no eruption occurred from, hot mud effusion and increased fumarolic activities were observed from 1996 to 2003. Intensive observations conducted during this active period confirmed significant crustal deformation and changes in total magnetic force and gravity (Yamamoto et al., 2008), indicating the formation of an active hydrothermal system beneath the crater. However, as no exploration for the subsurface structure has been conducted so far, it remains unclear where and on what scale the hydrothermal system is developed. In this study, an audio-frequency magnetotelluric (AMT) survey was conducted in and around the crater from August 25 to September 3, 2023. There are 25 observation sites on the Numanotaira Crater floor and its surroundings: Observations of five components of the electromagnetic field were conducted at 8 sites, mainly on the crater floor , and observations of only two components of the electric fields were conducted due to the steep topography. One of the sites was located 2.8 km west of the crater, the data was continuously acquired during the whole period and used as a reference site for remote reference processing. The MT response function was estimated using nighttime data(10 hours from 8:00 p.m. to 6:00 p.m.), but at some two-component sites, data from about 1 hour during the daytime was used to estimate the MT response function. As a result, we obtained good quality MT response function in general between 1 and 10,000 Hz for 10 sites and between 3 and 10,000 Hz for 14 sites. A three-dimensional resistivity structure was inferred from MT response functions using the inversion code FEMTIC (Usui, 2015; Usui et al., 2017).The obtained resistivity structure model revealed a characteristic structure in which a large low resistivity region extending from the deep southwest toward the Numanotaira crater floor branches off near the surface layer.

In this presentation, we will discuss the relationship between the inferred shallow resistivity structure and various sources of volcanic activities estimated in previous studies.