青藏高原东南部地震b值时空演化及其对区域应力场特征的启示

区域应力分布特征是地震危险性评价的重要指标之一。采用青藏高原东南部1970—2019年的地震目录数据，使用最大似然法对该区进行地震b值时空扫描，得到该区b值时空分布特征；利用地震b值与应力的负相关关系，分析青藏高原东南部主要断裂带上的应力分布及变化。研究结果显示：1）在鲜水河—小江断裂系统的中部，大凉山断裂带附近区域的b值比安宁河—则木河断裂带附近区域低，推测大凉山断裂带承担较多的应力，可能是未来地震风险较高的断裂带；2）研究区浅层（0~20 km）比深层（20~40 km）的b值高，这是与浅层区域围压低、岩石倾向脆性破裂，而深层区域围压高、岩石倾向韧性变形的特征相一致的；3）汶川地震发生前后，震源及周边区域的b值经历了降低—升高—降低的过程，揭示了区域内应力的累积—释放—累积过程；地震震级越大，震前b值降低趋势持续越久，该地震的发生对b值的影响范围越大、b值波动越明显，距震中越近的区域的b值在地震前后的波动越明显；4）龙门山断裂带及其附近地区b值较低，推测该区域应力较大。

The b-value spatiotemporal evolution in southeastern Tibetan Plateau and its implications on regional stress field characteristics

Stress field is an important factor to assess regional seismic risk. We collected the seismic catalogue data from 1970 to 2019 in southeastern Tibetan Plateau, used the maximum likelihood method to calculate regional b values, and then obtained the spatial and temporal distribution of b values of this region. Based on the negative correlation between seismic b value and stress, we analyzed the stress distribution and variation on the major fault zones in southeastern Tibetan Plateau. The results are as follows. 1) In the center of Xianshuihe-Xiaojiang fault system, the b value of Daliangshan fault zone is lower than that of Anninghe-Zemuhe fault zone. This indicates that the stress on Daliangshan fault zone is greater and its seismic risk is relatively high in the future. 2) The b values in shallow layer (0-20 km) are higher than those of deep layer (20-40 km), which is consistent with the characteristics that confining pressure is low and rock tends to brittle fracture in the shallow, while confining pressure is high and rock tends to ductile deformation in the deep. 3) Before and after the 2008 Wenchuan earthquake, b value of epicenter area had a decrease-rise-decrease process, and this showed the accumulation-release-accumulation process of regional stress. The larger the magnitude of earthquake, the longer the decreasing trend of b value before the earthquake, the greater the impact of earthquake on b value. The closer to the epicenter, the greater the decrease of b value. 4) The b value of Longmenshan fault zone is relatively low at present and this shows that Longmenshan fault zone is accumulating stress.