Stress state and major-earthquake risk on the southern segment of the Longmen Shan fault zone

The NE-trending Longmen Shan fault zone can be divided into southern, central and northern segments, and the 2008 Wenchuan M8.0 earthquake occurred on the central-northern segments. After the 2008 Wenchuan earthquake, whether a major earthquake can occur on the southern segment of the Longmen Shan fault zone in the near future becomes an important issue receiving much attention. Using the seismic data from the Sichuan Seismic Network, we have calculated and mapped the b-value distribution of the southern segment for the period from 1977 to 2012. We also determined the distribution of the Delta b-values, i. e., difference of b-values of the two periods, 1977 to 2012 and 1977 to 11, May 2008. Additionally, we calculated apparent stresses of M-L >= 3.8 events on/around the studied fault segment since 2007 by digital seismic waves. Our results show that since the 2008 Wenchuan earthquake, the stress level decreased in the Dayi area near the southern end of the Wenchuan aftershock area, while for some portions between Tianquan and Lushan, Luding, and north of Baoxing, stress level increased obviously. It is possible that moderate to major earthquakes will occur between Tianquan and Baoxin due to its relatively high stress level. The southern segment of the Xianshuihe fault zone south of Kangding has low stress level; thus the probability of major earthquakes on that section should be small in the near future.