6:15 PM - 7:30 PM
[SVC12-P01] Atmospheric effect on infrasound detectablity at Chichi-jima from Nishino-shima assessed by a Monte Carlo phonon method
Keywords:Volcano, Infrasound, Monitoring, Eruption, Wave propagation
A million phonon particles were transmitted from the ground to the atmosphere in random directions between zero and 90 degrees in the vertical plane cutting from Nishino-shima to Chichi-jima. Ray-tracing calculation (Tahira, 1982) was performed for each particles assuming one dimensional atmospheric structure with the effect of wind advection in the plane. We counted the number of the particles that reached Chichi-jima in the area of the infrasound stations spanning about 1 km, and regarded that the number represented the infrasound energy that reached the stations. Perfect reflection was assumed on the sea surface, but the particles that were trapped in the bottom layer thinner than the scale of the infrasonic wave length were eliminated. The calculation was performed for atmospheric structures from May to December 2014, using the data from radiosonde measurements twice a day by the Japan Meteorology Agency.
The calculated infrasonic energy arrivals were compared with the power of the signal from Nishino-shima detected by the infrasonic array at Chichi-jima. The calculation and the observation showed similar tendencies in general confirming that detection/non-detection of infrasound at Chichi-jima was controlled by atmospheric conditions. However, discrepancies were found in the beginning of May and in the end of June. The calculation shows infrasound was detectable, but it was not detected by the observation. These were the periods with low growth rates of the new island according to the satellite image analysis (Maeno et al., 2014). We conclude that in these periods the non-detection was due to the lack of the infrasonic source at Nishino-shima and not due to the propagation effect.
Although information for the atmospheric structure is limited, this simple method provides a first-order evaluation for the atmospheric effects and improves the interpretation of the infrasonic data at Chichi-jima for Nishino-shima activity.