震源近くでの長周期地震波の観測 (2)

抄録

A temporary observation was carried out at Izu peninsula using a wide band (0.02-50Hz) seismograph system of portable type. Seismic waves observed at short focal distances (Δ≈10Km) were analysed for studying the source characteristics. The wide band seismograph system consisted of two electromagnetic vertical seismometers with natural periods of 1Hz and 0.2Hz, electrical R-C integrating circuits and a long term data recorder. Seven seismograms with good signal to noise ratio up to long period range were analysed by using an analogue type band-pass filter.
The displacement spectra thus evaluated were found not to have any flat portion at lower frequency, showing convex and concave shapes, corresponding frequencies varying systematically with magnitude of earthquake. Speaking generally, the frequencies corresponding to the concave and convex decreased with increasing magnitude in an observed range from 2.7 to 4.0. From this fact, the peculiar spectral structure may be closely related to some of the source parameters, that is, the portion of the spectral structure lower than the concave frequency represents the perpetual displacement caused by dislocation as usual, but the convex must arise from some resonance of the source region caused perhaps by relaxation process.