THE CHARACTERISTICS OF SEISMICITY BELTS BEFORE STRONG EARTHQUAKES WITH MAGNITUDE M≥6 IN SICHUAN

In the present paper we compare the concept of seismicity belt before strong earthquakes with that of seismic zone, and of the earthquake-zoned transfer. We investigate the background seismicity belt and the microseismic active belt before M≥6 earthquakes. By background seismicity belt we mean a kind of seismicity belt in which M≥4 earthquakes occur during a period of over 10 months to several years before a strong earthquake. A microseismic seismicity belt is another kind of seismicity belt in which weak earthquakes occur during a period of several months before a strong earthquake.Background seismicity belts appeared before three earthquakes with magnitude M≥7 and the Yanyuan—Ninglang earthquake with magnitude 6.9. Of these, two perpendicular background seismicity belts appeared before the Luhuo earthquake with magnitude 7.9 and the Songpan—Pingwu earthquake with magnitude 7.2. There was only one background seismicity belt before the Yongshan earthquake of magnitude 7.1 and the Yanyuan—Ning Lang earthquake with magnitude 6.9. There occurred micro-seismic active belts before seven out of the eight M≥6 earthquakes. Strong earthquakes often occur near the intersection of two belts or one of the two ends of a belt or in the gap within a belt.In this paper, we analyse the statistic relation between the microseismic active belt paremeters (length, or width, or appearance duration etc.) and the magnitude of the strong earthquake that followed, using data of the seven earthquakes before which microseismic active belts appeared. The study shows that the magnitude M and duration ΔT and area S of a microseismic active belt with confidence Level =0.05 and 0.10 respectively are linearly related. The empirical formulas obtained by least squares method are  M=3.401gS—3.91±1.3  M=1.841g△T+6.01±1.3  where, the unit of S is 10³ km² and the unit of △T is one month.Finally, we also study the relations between the seismicity belt and the tectonics or the focal mechanism of the strong earthquake that followed, and discuss the role of seismicity belt in earthquake prediction studies.