Abstract
In the present paper the underground structure peculiar to Japan and its neighborhood is dealt with, based on the seismic activity in Japan and neighborhood, especially taking account of the fact that each of the seismic activity area and the seismic inactivity area shows in the subterranean space its respective space, and adding the authors' investigational result to the fruits of researches hitherto.
Main results of the present paper are:
(1) The seismic activity area was divided into 3 zones: the shallow earthquake zone (particularly the existence of “a thick shallow earthquake zone”), the joint earthquake zone and the deep earthquake zone.
(2) A concept that the seismic activity zone can be regarded as “the crust” has been proposed.
(3) From the discussion of the travel time residual and the area of abnormal distribution of seismic intensities, the values of Low V and Low Q of the seismic inactivity area were obtained, and in particular the stronger point of that property was found at a comparatively small depth in the underground on the continental side along the volcanic front.
(4) The mantle flow in Japan and neighborhood is assumed to be composed of its main flow (along the deep earthquake zone) which flows descending from the Pacific side to the continental side and from east to west, of a weak flow northward in the offing of Tosa and in Enshu-nada, and of flows eastward and southward from the direction of the Sea of Japan.
(5) In general, where the mantle flow ascends, there exist Low V and Low Q and the area is related to volcanic activities, and where the flow descends, the area is related to the seismic activity area.
(6) The joint earthquake zone is an area of the strongest seismic activity, and is closely related with large-scale earthquakes.
