A DOUGHNUT-SHAPED PATTERN OF SEISMIC ACTIVITY AND ITS FORMATIVE PROCESS PRECEDING THE TANG- SHAN EARTHQUAKE OF 1976

The spatial-temporal variation of seismicity in the wide region surrounding the epicentre of the 1976 Tangshan earthquake (Ms=7.8) was examined. It has been pointed out that the pattern described as a seismicity gap or a doughnut-shaped pattern preceding the main shock is practically a partial manifestation for the whole pattern composed of seismically quiet epicentral area and three seismicity bands in the surrounding region, that the stress concentration toward the margin of the quiet area and the increase of seismicity are caused by the movement on the fractured zone. Finally, the mechanical process of the formation of the seismic pattern is explained by using the model of inhomogenuity of intensity and stress distribution along the fault zone.