Powerful eruptions of large volcanoes were recognized as seismic sources capable of exciting wavetrains with very unusual characteristics. The eruption of Mount Pinatubo generated a strong seismic signal that was recorded on very-long-period seismometers around the world. Widmer and ZürnJ[1992] studied waves excited by both the Mount Pinatubo and El Chichon eruptions. In each case they observed a bi-chromatic excitation of Rayleigh waves, with one of the frequencies common to the two eruptions. They interpreted the frequency band that was excited by both eruptions as the fundamental gravitational mode of the atmosphere. They attributed the other band to the fundamental acoustic mode of the atmosphere, which might vary in frequency due to its sensitivity to atmospheric temperature. Widmer and Zürn [1992] also observed the direct air wave from the El Chichon eruption at great-circle distances approaching 100. Kanamori and Mori [1992] studied the signal from Mount Pinatubo and also concluded that it was due to Rayleigh waves excited in two narrow frequency bands. They attributed the excitation to coupling of acoustic and gravitational modes of the atmosphere to the ground in the near-source region.
Kanamori et al. [1993] studied the seismic waves and tsunami generated by the 1984 Tori Shima event in the incipient back-arc basin behind the Bonin arc. This earthquake had a body wave and surface wave magnitude of only 5.5, yet it generated a tsunami of nearly 1.5 m at a distance of 150 km. The long period data are most consistent with a compensated linear vector dipole mechanism with a vertical tension axis. Kanamori et al. [1993]Jinterpreted the source to be a combination of magma injection and hydrofacture caused by supercritically heated water.