Many aftershocks (3.5≤M≤7.1) of the Mid-Japan Sea Earthquake (May 26, 1983, M=7.7JMA) were recorded at Fukaura by velocity type strong-motion seismometers. P-wave spectra of these aftershocks were obtained by band-pass filters and converted to the displacement spectra at a reference distance of 35km.
The spectra thus obtained were added to the seismic spectral data of other regions previously obtained by the same method. From the total spectra, scaling of seismic spectra was produced by earthquakes with the magnitude range of 0_??_M_??_7.5. The seismic spectra of small or micro-earthquakes have only one peak, but most spectra of large earthquakes have two peaks at low- and high-frequencies. The low- and high-frequency spectral peaks vary with different slopes of f-3 and f-9, respectively for various magnitudes.
For the large earthquakes, the low- and high-frequency spectral peaks correspond to the coherent movement of main fault and small ruptures, respectively. Clear separation of the observed two spectral peaks for a large earthquake implies that the dimension of small rupture element is not arbitrary but limited.
