Abstract
Aftershocks of the Tokachi Earthquake of 1968 and other shocks in Northeast Honshu and Hokkaido regions, recorded by the broad-band large dynamic range seismograph at Urakawa, were studied. The following is concluded from the observation and analyses:
(1)I A delay in arrival time of high-frequency energy both in P and S, compared with low-frequency, was seen in comparatively distant Tokachi aftershocks of Δ=1.5°∼3.5° (Fig.3).
(2) Such delay time becomes longer with increasing epicentral distance (Fig.4).
(3) Earthquakes, which showed this effect of pseudo dispersion, occurred in the mantle as well as in the crust.
(4) Two distant but deep aftershocks in the delayed group showed no such effect (Fig.6).
(5) Earthquakes which gave the pseudo-dispersion effect also showed large attenuation of high frequency energy without exception.
(6) The region in which earthquakes with this effect occurred is limited to areas off the Pacific coast of Northeast Honshu at some distance from the coast. Other earthquakes of the same order in epicentral distance but occurring in other parts showed normal arrival and abundant high-frequency energy (Fig.9).
These observed facts seem to suggest that there is off the Pacific coast of Northeast Honshu an unusual region in the crust and the uppermost part of the mantle. It is suspected that some scatterers in the region cause the high-frequency energy to make a detour. The ordinary attenuation of Q-type is also larger in this region. This unusual region is located at the northernmost part of the Japan trench, where it bends to the east and continues as far as the Kuril trench, and the focal mechanism in this region seems different from that in surrounding regions. This region may possibly have some relation to the large-scale tectonics currently in discussion, but in a very complicated way.
