龙门山断裂带可分为南段、中段和北段,2008年汶川M8.0级地震发生在该断裂带中-北段. 龙门山断裂带南段是否存在发生强震的危险性倍受关注. 利用1977—2012年四川区域地震台网资料,获得了龙门山断裂带南段的地震活动性参数b值图像以及汶川地震前、后b值的差值Δb图像. 同时,根据宽频带数字地震波形资料,计算了2007年以来南段及附近区域M_L≥3.8级地震的视应力. 结果表明,2008年汶川地震后,龙门山断裂带南段天全—芦山、泸定和宝兴北部等区域应力增强,而靠近汶川余震区南端的大邑地区应力水平降低. 天全至宝兴段应力水平相对较高,具有发生中-强地震的条件. 鲜水河断裂带康定以南段应力水平低,短期内发生强震的可能性较小.

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 Δb-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.