The background of historical and modern seismic activities of the occurrence of the 2008 M_S8. Wenchuan, Sichuan, earthquake

In order to know what is the seismicity background on which the M_S 8. 0 Wenchuan, Sichuan, earthquake of May 12, 2008, occurred, we investigate the activities of strong and large earthquakes in the last 1000 to 2000 years before the Wenchuan earthquake, and the background seismicity during the last 20 years before the earthquake, in the middle segment of the North- South Seismic Belt and its near surroundings, based on a systematic analysis of historical and modern seismic data. Our study shows that: (1) No any M≥7 event had occurred on the Longmenshan fault zone at least in the last 1100 to 1700 years before 2008, suggesting that relative to other active fault zones (or sections) on the north and south, a seismic gap had formed along the Longmenshan fault zone for a long time before 2008, in which the M_S 8. 0 Wenchuan earthquake occurred. (2) Since the 17th century, a sequence of 12 M_S6. 5 to 8. 0 earthquakes have occurred along the eastern Bayan Har Block boundary consisting of the larger part of Longmenshan fault zone, the Minjiang and Huya fault zones in northern Sichuan, as well as the Wenxian-Wudu fault zone in southern Gansu, which represents a near 400 years lasting process of seismic strain energy release with gradual accelerating. The 2008 M_S 8. 0 Wenchuan earthquake is one of two great earthquakes occurred already in this process. (3) During the last 20 years before the Wenchuan earthquake, no quiescence of background seismicity emerged along the middle and southern sections of the Longmenshan fault zone. Instead, the background seismicity there was somewhat higher than that on the Wenxian-Wudu fault zone of southern Gansu, where an M_S 8. 0 earthquake happened in 1879. (4) The size of the 2008 Wenchuan earthquake is much greater than that of the biggest historical event on the Longmenshan fault zone. This proves that potential seismic hazards along those large-scale active fault zones with relatively low slip-rates are not possible to be assessed correctly from the historical earthquake record that is available only for the past several hundreds or 1 to 2 thousand years.