The 2008 Wenchuan, 2013 Lushan and 2017 Jiuzhaigou earthquakes, Sichuan, in the last more than one thousand years of rupture history of the eastern margin of the Bayan Har block

By taking the active tectonic belt along the eastern margin of the Bayan Har block as the study region, this paper first examines the earthquake history over the last 1 000 years and the temporal variation in strong earthquake activity, and determines the temporal positions in this history of the most-recent three major events, the 2008 M_S8.0 Wenchuan, 2013 M_S7.0 Lushan and 2017 M_S7.0 Jiuzhaigou earthquakes. Then, based on mapping historically ruptured areas the author identifies the seismic gaps along main active fault zones in and near the study region. The result mainly shows that the study region has experienced three stages of a strong-earthquake cycle during the last about 1 000 years, including the last half of the quiet period before 1630, the transitional period from 1630 to 1878, and the strong-event occurrence period since 1879. The last two periods together have a total duration of several hundred of years, and probably represent such a seismicity process in which the study active tectonic belt went through from the pre-release episode （between 1630 and 1878） to the main release episode （since 1879）, following an earlier quiescence period of probably 2 000 to 3 000 years in length for strain accumulation. Accelerating strain energy release appears obviously in the main release episode. The three events of the 2008 M_S8.0 in Wenchuan, 2013 M_S7.0 in Lushan and 2017 M_S7.0 in Jiuzhaigou, Sichuan, are the most-recent ones occurred in the main release episode. However, it still remains unknown whether the main release episode has closed or not yet after the occurrences of the latest three events. Several seismic gaps are identified and mapped along some segments of the major active fault zones in and near the study region, which have had no ruptures of major earthquakes for long time. So, enough attention should be paid to the seismic potentials for major earthquakes occurring on these seismic gaps in the not far future.