Source mechanisms of Usu volcano, Japan, earthquakes and their tectonic implications

Abstract

An intensive earthquake swarm and remarkable surface deformation due to dacitic doming have taken place at the Usu volcano in southwestern Hokkaido, Japan, since August, 1977. A major U-shaped fault has formed in the summit crater, and the amount of upheaval reached ∼ 180 m for ∼ 5 years. In order to reveal the quantitative relation between this doming deformation and the associated earthquakes, the mechanisms of the latter have been studied. For this purpose, broadband and wide dynamic range seismic observations were performed in a near-field region of the Usu volcano for the period from June, 1979 to July, 1980. Long-period seismometers and a digitally recording accelerometer system were used. For relatively large earthquakes, the data from the digital recording system permit very accurate calculations of ground displacements by employing the recursive-filter method. Source parameters such as seismic moment, average dislocation, stress drop and fault dimensions for five earthquake families have been determined from a comparison of the observed ground displacements with synthetic ones. Synthetic displacements were computed using Haskell's formulation for relatively deep earthquakes. The discrete-wavenumber (DW) method was used for relatively shallow earthquakes. The stress drop varies widely among earthquakes in the same earthquake family. The cumulative seismic deformation caused by the large earthquakes is almost consistent with the observed surface deformation. It is suggested that the large earthquakes are caused by stick-slip motions at the two sides of the U-shaped fault.