Harmonic Excitation of Mantle Raleigh Waves by the 1991 Eruption of Mount Pinatubo, Philippines

An unusually long (at least two hours) seismic wave train having periods of about 230 sec was recorded at many worldwide seismic stations during the major eruption of Mount Pinatubo in the Philippines on June 15, 1991. This wave train exhibits two sharp spectral peaks at 228 and 270 sec. The group velocity, phase velocity, and the particle motion of this wave train indicate that it is a Rayleigh wave. The most probable xcitation mechanism is acoustic oupling of atmospheric oscillations that were set off by continuous thermal energy flux from the volcano. The two spectral peaks correspond to the characteristic periods of acoustic and gravity modes of the Earth's atmosphere. The magnitude of the vertical single force equivalent o the acoustic oupling is 1.6x1017 dynes over a frequency band of 0.003 to 0.01 Hz. The results suggest the possibility of using acoustically coupled Rayleigh waves for detection, characterization a d quantification f volcanic eruptions. Acoustic oupling of the atmosphere and the solid Earth provides a unique seismic murce with long duration.