Seismic Activity Accompanying the 1977-1978 Eruption of Usu Volcano, Japan

Abstract

   For the purpose of investigating the mechanism of the 1977-1978 eruption of Usu volcano, Hokkaido, the observational results of the earthquakes, deformations and eruptions are analyzed and discussed in connection with energetics and statistics.
   The seismic energy and some kinds of the eruption energies discharged during the eruption are approximately estimated. The total seismic energy, 9.6×10²⁰ erg, is comparable to the kinetic energy of ejecta by major pumice eruptions. Daily discharge rates of seismic energy are found to decay exponentially with time separately for the two periods. The data on earthquakes and deformations indicate a proportional relation between seismic energy and upheavals at the summit crater, throughout the period of activity. This relation suggests that the earthquakes and upheavals result from the same source.
   In the eruption period of the latter stage of activity, the seismic activity is complementary to the eruptivity. This means that not only the earthquakes and deformations but also eruptions originate in the same energy source. On the assumption that the sum of the seismic and the deformation energy is quantitatively complementary to the mechanical energy of eruptions, the deformation energy is estimated at about nine times as much as the seismic energy. Moreover, on the basis of the relations among these energies, the exponential decay processes of discharge rates of seismic energy suggest that the magma does work on its surroundings through its volume increase without material and energy supplies from the depths, and the work is converted into the forms of seismic energy and deformation energy in periods without eruptions.
   As for the relations between earthquake magnitude and frequency, the magnitude (or log-amplitude) vs log-cumulative frequency curves show the upward convex type throughout the activity, and this indicates that the earthquake swarm has the upper bound of M4.3, which is far smaller than is expected from the general relation between magnitude and cumulative frequency of tectonic earthquakes. In the seismic activity since 1979 the magnitude vs log-frequency curves show the peculiar type with a peak of frequency, which is explained by the superposition of two different types of distribution belonging to the exponential type and peak type.
   Time intervals between successive earthquakes are analyzed, and it is found that earthquake occurrence is not random in time and even large earthquakes have a tendency to cluster within two days. In the period regarded as stationary in seismic activity, the events of earthquakes clustering within two days are defined, and time intervals between the two successive events are analyzed. The results suggest that the occurrence of the events with a seismic energy sum of 1.4×10¹⁸ erg or more has a periodicity with intervals of 9-12 days.