Rayleigh Waves Recorded by the Volumetric Strainmeters at the Time of a Rather Small Volcanic Eruption.

Abstract

Volumetric strainmeters operated in the Kanto and Tokai districts, south-west of Tokyo, recorded a ground motion of simple waveforms in association with the November 16, 1987, eruption of Izu-Oshima volcano. The waves were identified as Rayleigh waves by their propagation speed, particle motion and their attenuation property with distance. The radiation pattern of the Rayleigh waves was uniform in all directions from the source. This suggests that the waves were excited by a vertical single force operating at the source. A magma falling in the vent is presumably a cause of the vertical single force which generated an M3.9 earthquake ten and several seconds prior to the visible eruption and was responsible for the observed Rayleigh waves. The potential energy of the Rayleigh waves was calculated to be 6×105 J from the strain observations. The kinetic energy could be assessed independently of the potential energy from observations by seismographs. Both are almost equal as the theory predicts. The total energy, 106 J, carried by the Rayleigh waves corresponds to that of an ordinary M1 tectonic earthquake.