The volcanic activity is on intimate relation with the variation of the volcanic micro-tremors and the deformation of the local crust in the volcanic area. It may be thought that these phenomena occur in the process of accumulating of magma beneath the crater and erupting of ejecta. The volcanic micro-tremors at Volcano Aso were divided into four classes by Prof. K. Sassa, according to their periods. Expecting that the volcanic activity will be accompanied with some phenomena having longer period than the volcanic micro-tremors of short period found by Prof. K. Sassa, the author has begun the observation of the long-period volcanic micro-tremors at Aso Volcano-logical Laboratory since March 1958, using a long-period horizontal seismometer having T0 = 180sec. h0 = 1, Tg = 100sec., hg = 1 and Vmax = 780. The long-period volcanic micro-tremors were observed before the eruption, June 24, 1958, of which the energy was estimated at nearly 1017 ergs. The nature of these long-period volcanic micro-tremors are: - 1) the period of the long-period volcanic micro-tremors is of 40〜55 seconds, 2) about twenty hours before the eruption the wave-trains are composed of three or four crests and troughs, then about five hours before they are composed of six or seven crests and troughs and finally about one hour before they decay, 3) and it is thought that these long-period volcanic micro-tremors are not induced by the volcanic micro-tremors of shorter periods, but due to the state of magma beneath the crater.
