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Quantitative evaluation of temporal changes in subaqueous hydrothermal activity in active crater lakes during unrest: Time-series analyses of lake chemistry

Akihiko Terada 1, Muga Yaguchi2, Takeshi Ohba 3

  • Affiliations: 1Volcano and Earthquake Research Division, Multidisciplinary Resilience Research Center, Institute of Integrated Research, Institute of Science Tokyo; 2Meteorological Research Institute, Japan Meteorological Agency; 3Department of Chemistry, Faculty of Science, Tokai University

  • Presentation type: Talk

  • Presentation time: Friday 09:15 - 09:30, Room S150

  • Programme No: 6.8.4

  • Theme 6 > Session 8

Abstract

Regular sampling of lake water has been performed at many volcanoes to evaluate the state of volcanic activity. However, it is not clear whether the absolute concentrations or rate of changes in concentrations are more suitable for such evaluations. In this study, we show that temporal changes in concentrations of an element in lake water are described by a simple differential equation. The response time (63%) to change in the chemical concentration in lake water ranges between 20--1,000 day for the volcanoes in Japan, meaning it can take a long time to evaluate volcanic activity based on the absolute chemical concentration. In order to evaluate changes in volcanic activity in a shorter time period, we developed a simple numerical method to calculate temporal changes in the bulk hydrothermal fluid injected from fumaroles and hot springs at the lake bottom. We applied our method to Yugama crater lake at Kusatsu--Shirane volcano, Japan, and quantitatively evaluated temporal changes in the hydrothermal input from 1964 to 2020. As a result, we detected changes in concentrations of several elements in the bulk hydrothermal input that were associated with unrest including the phreatic eruption and earthquake swarms. The future concentration in the lake water can be predicted from the time-series of lake concentrations. Comparing the predicted concentration curve with the concentration obtained from lake water samples, it is possible to quickly evaluate whether the concentration of the bulk hydrothermal input has increased, decreased, or remained constant.