Source parameters of the volcanic eruption earthquakes at Mount Tokachi, Hokkaido, Japan, and a magma ascending model

The source parameters of the eruption earthquakes observed at Mount Tokachi in 1989 are estimated on the basis of the single force model. By fitting the theoretical source spectra with the observed ones, the peak amplitude of force and the source duration were estimated to be 10⁷-10¹⁰ N, and 0.5-0.8 s, respectively. To give a physical explanation of the empirical relations of source parameters, a magma ascending model is presented as a source of eruption earthquakes; a spherical batch of gas-rich magma vertically migrates from a deep-seated magma reservoir to the surface and is cooled by heat conduction. When the magma reaches the ground surface, it erupts and causes an eruption earthquake. Comparing the empirical relations derived from seismic analysis with the magma ascending model, it is concluded that the radius of an ascending batch of magma plays an important role in the generation of eruption earthquakes. -from Author