4:20 PM - 4:35 PM
[MTT38-04] Propagation of infrasound emitted by volcanic eruptions of Mount Aso and Moto-Shirane
Keywords:Infrasound, Propagation, Volcanic Eruption
We have successfully observed infrasound signals that originate the volcanic eruptions of the Mount Aso on Oct. 6, 2016, and Moto-Shirane on Jan. 23, 2018. For example, in the case of the Mount Aso (erupted time at 1:46:37 JST), the time of arrival at Kuroshio-cho (~180 km from the crater) was 1:55:26 JST, indicating that the travel time is ~530 s. Assuming that acoustic wave propagates horizontally with the sound speed of 340 m/s, the distance from Kuroshio-cho to the crater is calculated to be ~180 km, which corresponds to the actual distance. In contrast, the time of arrival at Kochi University of Technology (~250 km from the crater) was 2:01:23 JST. If we assume horizontal propagation, the distance is calculated to be ~310 km. The difference between the calculated and actual distances, ~60 km, is large, implying that the acoustic wave did not propagate horizontally. Similar results were obtained at the CTBTO observation point, IS30, in Isumi for Mount Moto-Shirane eruption as well.
In our study, we built a 2D model to describe sound propagation in the atmosphere. For simplicity, the model ignores wind field and reflection on the ground as a first step. We consider the propagation in the troposphere (from 0 to 10 km) and the stratosphere (from 10 km to 50 km). Given the vertical temperature profiles and the dispersion relation ω=csk, we calculate the ray paths of the sound by the Hamilton equation, where ω is the angular frequency, cs is the sound speed, and k is the wave number. For both eruptions, we found that some of the observed sound cannot be explained by a straight horizontal ray path. In this presentation, we compare the travel time with the observations and discuss results for the two cases for Mount Aso and Moto-Shirane eruptions.