ON SOME CHARACTERISTICS OF SOURCE SPECTRA OF HOKKAIDO-KURIL EARTHQUAKES OBSERVED AT TSUKUBA STATION

抄録

A spectral analysis is made of records at Tsukuba station. The earth-quakes analyzed are located in Hokkaido and the Kuril Island. Almost all of their ray paths are in the seismic dipping zone of the island arc. The Q_α (Q for P wave) and Q_β (Q for S wave) are estimated graphically. It is shown that Q_β is much larger than Q_α in the frequency range 0.1 to 10Hz. The ratio of Q_β to Q_α is as large as 10. This is attributed to the weak attenuation of S waves in the dipping seismic zone. Comparison with the records at Urakawa, Sendai and Tsukuba indicates that short period waves are propagated without significant attenuation together with long period waves. This is also attributed to the weak attenuation of short period seismic waves in the dipping seismic zone. It is suggested that the earthquakes have spectral peaks at their source.