IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_2C-P16

Date/Time: July 24 Poster

A ray-tracing study for wind effects in middle-distance infrasound propagation
Makiko Iwakuni1 , Mie Ichihara2 , Giorgio Lacanna3 , Minoru Takeo2 , Masato Iguchi4 , Maurizio Ripepe313Japan Weather Association, Japan, 2 Earthquake Research Institute, University of Tokyo, Japan,
Earth Science Department, University of Florence, Republic of Italy, 4 Disaster Prevention ResearchInstitute, Kyoto University, Japan
E-mail: iwakuni_01@jwa.or.jp

Infrasound has become an important component of observation for volcanic activity. At present, infrasoundobservations for volcanoes are concentrated in two distinct scales: close to the volcano in less than 10 km, orin hundreds or thousands of kilometers away. Observations and studies of infrasound in the middle-distances arevery few. Infrasound signals induced by explosive eruptions of Sakurajima are often observed clearly at the dense
network of well-calibrated infrasound sensors, which Earthquake Research Institute installed around Kirishima
volcano, about 40 km to NNE of Sakurajima volcano. The variations are considered to be caused by changes in
the atmospheric structures and possibly in the radiation patterns of infrasound generation. This study focuses on
the wind effect to understand the middle-distance infrasound propagation variations.
We have been analyzing more than one thousand signals of Sakurajima explosions from July, 2011 till now.
Temporal stations were installed in various distances and directions from Sakurajima. The variation of waves among
the network is numerically investigated using a finite differential method, in 2-D with a correction of geometricalspreading and taking account of the topography and the profile of atmospheric temperature (Lacanna et al., 2012,AGU). Observed variations are easily explained by the effect of topography. On the other hand it is hard to explain
observed variations in winter by 2-D simulation even considering large variable in atmospheric temperature. To
evaluate the degree of wind effect, we tried to simulate the ray-paths by using two kinds of models. One istaking account of the wind effect as a moving medium and the other is taking account of the wind effect as themodification of temperature (medium traveling waves is static). In close-distance station both models showed similar
ray-paths. On the other hand, two models sometimes showed different ray-paths in middle-distance stations. Next,
although infrasound waveforms observed in the north and east directions were sometimes very similar regardlessof distances, signals recorded at a station 43 km to SSW were quite different and much weaker than those atsimilar distances in NNE, the stations at Kirishima. Sound propagation is increased by wind toward the down-wind
direction and inverse layers of effective sound speed are formed. These inverse layers were frequently formed in
the direction of Kirishima but rarely to the south during the analyzed period. The inverse layers prevent upward
propagation of infrasound and confine waves to increase the observed amplitudes. When the inverse layers were
not clear or lower than Kirishima peaks, the wave amplitudes were distinctly reduced behind the peaks. In this way,
wind effect is significant in the middle-distances. In order to obtain quantitative information of the source, we needatmospheric data with better resolutions in time and space.

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