Abstract: The depth distribution of aftershocks of the 1996 Lijiang M_S7.0 earthquake is strongly time-dependent. Events occurring shortly after the main shock had deeper focal depths, and as the time going on, the focal depth of the aftershocks became shallower and shallower, i.e. the cutoff depth of seismicity became shallower and shallower with time. As we know, the lowermost events occur around the depth of brittle-plastic transition, and this depth depends on strain rate. The postseismic deformation model inferred from GPS data show that the major contribution of postseismic strain release comes from the lower layer of the crust. These results suggest that significant afterslip is related to viscous relaxation of lower layer. We estimated the lower bound of the strain rate according to Marone's et al.(1991)afterslip model and the slip data observed at the surface of Xianshuihe Fault. The results show that the strain rate is high after the main shock, and decreases gradually with time. We calculated the strength profile of middle crust based on flow law of wet quartz, estimated strain rate, temperature profile determined using the heat flow data at Lijiang, as well as crustal structure based on P wave velocity. By comparing the cutoff depth of seismicity and the brittle-plastic transition depth of the middle crust, we found the two depths are consistent to each other. We suggest the temporary existence of deeper small events after main shock and the depth distribution of aftershock is due to the changing brittle-plastic transition of the middle crust corresponding to strain rate variation from high to lower values after the main shock, and this kind of change is the manifestation of rheology of the middle crust.

Key words: focal depth, aftershock, brittle-plastic transition, rheology, Lijiang M_S7.0 earthquake

摘要： 1996年丽江M_S 7.0地震的余震深度分布明显具有时间依赖性，主震发生后短时间内余震震源深度较深，随着时间的延续，余震震源深度变得越来越浅。余震的这种深度分布受地壳脆塑性转化带深度控制，而脆塑性转化带的深度变化与地震前后断层的应变速率有关。由震后GPS地表变形数据得到的地表变形模型表明，震后地表变形主要来自地壳深部，震后滑动与地壳深部弹性松弛有关。根据鲜水河断层地表的滑动数据和按Marone’s（1991）给出的方程确定的震后滑动模型，估计的应变速率显示，主震发生后应变速率较高，随时间延续，应变速率逐渐下降。基于地壳P波速度结构和利用热流数据估计的丽江地区地壳温度，采用含水石英的塑性流变参数，估计了中地壳脆塑性转化带深度随震后应变速率的变化。结果表明，主震震源深度与余震深度分布下限与中地壳脆塑性转化带的深度随时间变化趋势一致。由于断层的震后快速滑动致使断层带深部具有很高的应变速率，高应变速率引起断层脆塑性转化带深度下移，主震之后短时间内发生了较深的余震；随着震后时间的延续，断层逐渐进入蠕变阶段，断层滑动速率逐渐减小，地壳应变速率逐渐降低，断层脆塑性转化带也逐渐恢复到间震期的深度，相应余震深度随之变化。因此，余震分布的深度变化是中地壳流变结构和脆塑性转化带深度变化的直接反映。

关键词: 震源深度, 余震, 脆塑性转化带, 流变, 丽江M_S7.0地震