Volcanic Micro-seisms : Discussion on the Origin

Abstract

Nature and origin of volcanic micro-seisms (micro-tremor or harmonic tremor) are to be attached great importance for the prediction of eruption of volcanoes, especially of Strombolian and Hawaiian type. Ten years ago, OMER. G.C. advocated the origin of volcanic micro-seisms observed at various volcanoes as the natural period of longitudinal vibrations of horizontal elastic layer composing the outer superficial layer of the volcanoes. However, his adopted materials, being the base of the calculation, jumbled the observed data obtained by the seismographs of various characteristics and at various distance from the origin. Therefore, we gathered the seismograms observed by the seismographs of the same frequency characteristics and registered very near the source (not more than 600m). The frequency distribution of micro-seisms were constructed for the volcanoes Kilauea, Nyiragongo, Mihara. Aso and Sakura-zima. The results show that the predominating period of the micro-seisms becomes shorter as the volcano becomes acidic. The author concluded that the volcanic micro-seisms are generated by the free logitudinal vibration of the viscous lava column having the effective length of 500m. The natural period for each volcanoes as calculated by verifing the viscosity of the column agreed satisfactorily with the observed predominating period of volcanic micro-seisms.