[S-VC33] Monitoring and assessment of volcanic activities

[SVC33-P05] Bottom topography and the sub-bottom structure revealed by acoustic technology and small ROV for the Okama creter, Zao volcano

*Shintaro Yamasaki¹, Akio Goto², Nobuo Hirano³, Noriyoshi Tsuchiya³, Tetsuya Matsunaka⁴ (1.Disaster Prevention Research Institute, Kyoto University, 2.Center of Northeast Asian Studies, Tohoku University, 3.Graduate school of Environmental Studies, Tohoku University, 4.Institute of Nature and Environmental Technology, Kanazawa University)

Keywords:crater lake, side-scan sonar, sub bottom profiler, sonar, subaquatic landslide, Zao volcano

Significant anomalies relating to volcanic activities have not been observed around its Okama crater lake, Zao volcano for about 70 years. However, a part of lake water surface suddenly got cloudy in 2014, and similar phenomenon was observed again in 2019. Several aquatic investigations for the lake bottom have been conducted, though, the origins of the partial turbidity have not yet clarified. By the way, Yamasaki et al., 2018 discovered a small mound on the lake basin, which is similar to a pyroclastic corn by the fish-finder-equipped remotely operated vessel. Scanned image detected a columnar reflection that was a potential of gas emission, but it is likely to be noise. The possibility to be a topography relating to volcanic activity however remains, because the mound stands alone on plane lake basin of 23 m deep, and there is few fallen rocks or rocks from debris flows around the mound. In addition, the mound is far from the crater wall. In September 2022, the authors’ team thoroughly investigated the lake floor by using various technologies including the sonars for the bottom material analysis and imaging, the side-scan sonar for the bottom horizontal imaging, the sub-bottom profiler for the sub-bottom geological structure analysis and the optic observation by a mini-ROV, called an underwater drone. Those results suggest a subaquatic landslide formed the small mound on the lake basin.