Mount St. Helens の 1980 年 5 月 18 日大噴火による気圧波

抄録

Atmospheric pressure waves generated by the big eruption of Mount St. Helens at 15 32 GMT on May 18, 1980 are recorded with microbarographs at eight stations of Japan Meteorological Agency in Japan of about 7, 000-9, 000 km in great circle distance from the volcano. The wave train is also detected with World Wide Standard Seismograph (long period and vertical component) settled at Matsushiro, Nagano Prefecture. It is the third time since the 1883 Krakatau Eruption and the 1956 Bezymianny Eruption that atmospheric pressure waves caused by volcanic eruptions at long distance are recorded with barograph or micrographs in Japan. The barograms of atmospheric pressure waves begin with increase of atmospheric pressure and distinct waves which give the maximum amplitude of 3-5 minutes of period. The mean of the maximum amplitude of records is 0.10 mb and the mean propagation velocity 308 m/sec. The antipodean wave train (A₂) and the third one (A₃) are not detected. The estimated energy of the eruption by applying the results by HUNT et al. (1960) and WESTON (1961) is at least 10²³ ergs. This estimated energy almost coincides to the energy of the 1956 Bezymianny Eruption estimated by MURAYAMA (1969). It would not be appropriate to directly compare with the kinetic energy of the eruption estimated through the amount of ejecta, but the above-mentioned estimated energy is almost same order of the mechanical energy of the blast according to DECKER R. and DECKER B. (1981). Comparing with records of atmospheric pressure waves detected in the United States, West Germany and the Netherlands, the maximum amplitude of barograms obtained in Japan is less about 10%. This may be caused by prevailing westward winds in the upper atmosphere on the propagation path from Mt. St. Helens to Japan, but the remarkable difference of propagation velocity is not seen.