**3. General characteristics of the seismicity in the region**

About 100 earthquakes with magnitude over 4.7 have occurred in the region, since the Wudu, Wen County magnitude 7.0 earthquake which occurred in 186 BC. 18 Earthquakes with a magnitude over 6.0 and 7 earthquakes with a magnitude over 7.0 have occurred since 186 BC (Fig.9, Table.1). The magnitudes of Serial number 1-7 are determined according to the historical earthquake description of local chronicles and field survey of historical earthquakes. As there were no instrumental records, the value is not accurate. The distribution of seismic activity has obvious features of zoning (Fig.10). The area can be divided into three division zones and 8 subdivisions (Table.2).

Fig. 10. Historical earthquakes in northwestern Sichuan (from Hu Xingping et al 2008 modified)

Fig. 10. Historical earthquakes in northwestern Sichuan (from Hu Xingping et al 2008

modified)


Notes: the magnitude of Serial number 1-7 is according to the description of annals of local history

Table 1. Strong earthquake catalogue of northwestern Sichuan (Ms≥6.0)


Table 2. The seismic activity zoning
