ON RELATIVITY OF SEISMICITY BETWEEN NORTH CHINA AND JAPANESE ISLANDS AND THEIR TECTONIC ENVIRONMENT

Abstract

From the earthquake records of about 2, 000 years, it can be seen that North China, Korea, and Japanese Islands are associated with seismicity. During this period there are several seismically active periods of about 200 years, and each period can be subdivided into several seismically active episodes. In 1966, the seventh episode of the present active period was initiated and four strong earthquakes (M_S≥7.0) have occurred in North China. Before or after these events, noticeable anomalies emerged at several places, which are called "regional stress windows." Eight "windows" (including six of seismicity, one of crustal deformation, and one of radon) are discovered. To our surprise, remarkable anomalies appeared at all the windows before the Japan Sea (or Akita-Oki) earthquake of 1983. It does not seem to be a mere coincidence. For a unified explanation of the phenomena of the episode and "windows, " the author presented the multiple stress concentration hypothesis in 1979. The locally stress-concentrated points would develop either into an earthquake focus or into a "window." There seems to be a close tectonic relation between the eastern China and the island arc system. In terms of geological and geophysical data, this region can be divided into several laths (long and narrow pieces of plate), stretching along a co-latitudinal direction. One of them is the North China-Korea-Honshu, Japan lath, which is a seismically active lath with large continental earthquakes. The configuration of the lath appears to be closely related to the geodynamic process.