Abstract: Comparing with earlier seismic data, alternation between seismically active and quiescent periods with longer time scale for Xianshuihe faults, west and east zones of Sichuan-Yunnan crustal block, Songpan-Longmenshan faults, and whole Sichuan-Yunnan area was studied. The results indicated that there was obvious alternation between seismically active and quiescent periods with longer time scale in north section of east zone of Sichuan-Yunnan block (Xianshuihe faults), Songpan-Longmenshan faults, and middle and south section of west zone of Sichuan-Yunnan block (Honghe faults), there was a decrease in moderate earthquakes but a few strong earthquakes M≥7 in middle and south section of east zone of Sichuan-Yunnan block (Anninghe-Zemuhe-Xiaojiang faults) and north section of west zone of Sichuan-Yunnan block (Jinshajiang fault) during the quiescent periods of above faults. Therefore, there were complicated stage characteristics for alternation between seismically active and quiescent periods in whole Sichuan-Yunnan area: strong seismic active periods accompanied with many moderate earthquakes and sudden seismic active periods in which a few strong earthquakes suddenly occurred alternately appear. It should be noticed that strong seismic active period accompany with many moderate earthquakes from the end of 19th century in Sichuan-Yunnan area has lasted beyond 100 years, there has been a long time quiescence up to now, and a sudden seismic active period may come. From distribution of sudden strong earthquake of last sudden seismic active period in Sichuan-Yunnan area, it could be inferred that next sudden strong earthquake probably will occur along some faults with strike NS or along some convergent zones of faults with strike NS and other faults. The mechanism of the above-mentioned statistical phenomena was briefly discussed in this paper.

Key words: Langer time scale, Alternation between seismically active and quiescent periods, Sichuan-Yunnan area