Generation of the high-frequency wave for intermediate-depth earthquakes in and around Hokkaido region is estimated from the source spectra in the frequency range from 1 to 10Hz. The average stress drop Δσ that is calculated from the high-frequency flat level of acceleration source spectra is about 30MPa for the down-dip-extension type earthquakes while it is about 8MPa for the subduction type earthquakes. Variation of Ads of the down-dip-extension type earthquakes is meaningfully smaller than that of the subduction type earthquakes. As for the down-dip-extension type earthquakes, Aaof large earthquakes coincides with the upper level of those of small earthquakes.
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IKEDA et al. (2002) studied this question.
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