Bedrock structure related to active faults in Lijiang basin, Yunnan, China, and its seismic microzoning

1996年丽江7.0级地震给丽江盆地带来了严重的破坏。虽然其发震断裂被认为是位于盆地西缘的雪山断裂，但破坏最严重的区域位于距离盆地边缘1.5 ~ 2 km的地方，在南部穿过盆地的丽江—建川断裂带附近，破坏程度变化显著。作为微分区问题，开展了重力测量和地质研究，揭示了与活动断裂相关的基岩构造。在丽江盆地、毗邻的腊市盆地和河清盆地南部建立了三维重力基底模型。每个盆地到基岩的深度都在2公里以上。通过对卫星图像的分析，绘制了现场检查的活动断层图。利用GPS观测监测了与断层相关的地壳运动。在滇北区域构造控制下的拉分构造基础上，探讨了盆地构造及其成因。破坏的异常分布与断层相关的陡峭起伏的基岩构造（盆地边缘效应和/或聚焦效应）和沉积物中断裂带的地质条件有关。

The 1996 Lijiang Earthquake of M7.0 brought serious damage to the Lijiang basin. Although its seismogenic fault is considered to be the Xueshan fault lying along the western edge of the basin, the most severely damaged zone was located 1.5-2 km far from the basin edge, and there was a remarkable change in degree of damage near along the Lijiang-Jianchuan fault zone, which crosses the basin in the southern part. As an issue of microzoning, gravity survey and geologic study were conducted to reveal the bedrock structure associated active faults. 3D gravity basement was modeled for Lijiang basin, adjacent Lashi basin, and Heqing basin ranging south. The depth to bedrock reaches more than 2km for each basin. Satelite images were analyzed to map active faults which were checked in the field. GPS observation was carried out to monitor the crustal movement associated the faults. The basins structure and its genesis were discussed on the basis of pull-apart structure controlled by the regional tectonics in northern Yunnan. The anomalous distribution of damage should be attributed to the fault-related bedrock configuration with steep relief (basin-edge effect and/or focusing effect), and to the geological condition of fault zone in the sediments.