Abstract
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 Qα (Q for P wave) and Qβ (Q for S wave) are estimated graphically. It is shown that Qβ is much larger than Qα in the frequency range 0.1 to 10Hz. The ratio of Qβ to Qα 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.
