Free Oscillation of the Earth Excited by the Earthquake of Sumbawa Island, Indonesia August 19, 1977

Abstract

The free oscillation of the earth excited by the Earthquake of Sumbawa Island, Indonesia, August 19,1977 with magnitude 7.9 (Ms) was investigated using the records of the silica tube extensometer at Matsushiro, and borehole type strainmeters at Mikkabi and Irako. The main results of this study are summarized as follows. (1) Eigenperiods of 53 spheroidal modes and 23 torsional modes which were determined in this study are in agrement with the results obtained by Anderson and Hart (1976) within about ±0.5%. (2) Phase velocities of Rayleigh waves with periods 200-900 sec and of Love waves with periods 280-900 sec were determined using the periods of free oscillation. The phase velocities of Rayleigh waves with periods less than 300 sec seem slightly higher than calculated values by Anderson and Hart (1976). This discrepancy may be interpreted as the differences of tectonic condition along the paths. (3) The degree number m is determined by a comparison between the vanishing points in the specific spherical surface harmonics and the missing peaks in the observed spectral for the spheroidal modes at Matsushiro. It can be considered that oscillations of 0 or 2 degree number are generated. © 1979, Japan Meteorological Agency. All rights reserved.