Abstract: The Daliangshan sub-block is a boundary region among the Bayan Har block, the Sichuan-Yunnan block and the South China block. It hosts four major fault systems:The southwest to south trending Xianshuihe-Zemuhe Fault zone in the west, the Longmenshan fault zone is the northern boundary, the Zhaotong-Lianfeng fault zone in the south, and the NS-trending Mabian-Yanjin fault zone in the east. This study focused on focal mechanisms and the regional stress field of the Daliangshan sub-block to help understand the earthquake preparation process, tectonic deformation and seismic stress interaction in this area. We collected broadband waveform records from the Sichuan Seismic Network and used multiple 1-D velocity models to determine the focal mechanisms of moderate and large earthquakes(M_L ≥ 3.5)in the Daliangshan sub-block by using the CAP method. Results for 276 earthquakes from Jan 2010 to Aug 2016 show that the earthquakes are dominated by strike-slip and trust faulting, very few events have normal faulting and the mixed type. We then derived the regional distribution of the stress field through a damp linear inversion(DRSSI)using the focal mechanisms obtained in this study. Inversion results for the spatial pattern of the stress field in the block suggest that the entire region is predominantly under strike-slip and trust faulting regimes, largely consistent with the focal mechanisms. The direction of maximum compression axes is NW-NWW, and part of the area is slightly rotated, which is consistent with the GPS velocity field. Combining geodynamic background, this work suggests that because the Sichuan-Yunnan block is moving to SE and the Tibetan plateau to SE-E along major strike-slip faults, the stress field of the Daliangshan sub-block and its adjacent regions is controlled jointly by the Bayan Har block, the Sichuan-Yunnan block and the South China block.

Key words: Daliangshan sub-block, focal mechanism, regional stress field, Cut-And-Paste method(CAP), Damp regional-scale stress inversion(DRSSI)

摘要： 基于Cut-And-Paste （CAP）全波形拟合反演震源机制解方法和阻尼线性逆推法（DRSSI），采用分区域选取地壳速度模型的形式，计算得到大凉山次级块体及邻区（101.5°～104.5°E，26.5°～30.5°N）震源机制解和区域应力场。结合构造地质、GPS观测数据等已有研究资料综合分析认为，大凉山次级块体及邻区震源机制解主要以走滑和逆冲机制为主，其空间分布特征与地质构造活动性质较吻合；区域应力场方向与GPS速度场运动方向基本一致，优势方向为NW-SE和NWW-SEE向，整个区域主要以走滑和逆冲性质为应力特征。震源机制解和区域应力空间分布特征表明大凉山次级块体在川滇块体、巴颜喀拉块体和华南块体三者联合作用的影响下形成了现今的应力格局，川滇块体相对向SE滑移以及青藏高原物质的E、SE向逃逸为大凉山次级块体及其周围地区强烈变形及大尺度位移的主要动力来源。

关键词: 大凉山次级块体, 震源机制解, 区域应力场, CAP方法, 线性阻尼法