【摘要】 使用国家地震台网固定台站和区域流动地震台站资料,通过对最长达18年记录开展的剪切波分裂分析,得到青藏高原东缘构造域(包括龙门山断裂带)的上地壳各向异性空间分布和时间变化特征.通过快剪切波(快波)偏振和慢剪切波(慢波)时间延迟参数,获得了快波偏振的分区分布特征和汶川地震前后不同分区统计特征的时间变化.剪切波分裂参数受到应力场和断裂构造的影响,块体边界或断裂附近的快波偏振特征比块体内部更为复杂,导致一些分区显示两个优势方向.在汶川MS8.0地震前后,可以观察到一些台站或区域的剪切波分裂参数的变化,既有快波偏振方向的改变,也有慢波时间延迟的变化.地震前后有明显变化的台站,基本符合两种情况:一是震中距较近;二是处于大的断裂或构造边界附近.研究发现,汶川地震前后,龙门山断裂带区域的慢波时间延迟的降幅显著大于周边区域;龙门山断裂带北段的慢波时间延迟降幅,大于龙门山断裂带中段,大于龙门山断裂带南端与鲜水河断裂、安宁河断裂的交汇区,这反映了地壳应力及介质物性状态的变化.通过分析剪切波分裂参数,推断出龙门山断裂带域及周边区域的主压应力分布.本文同时还分析了芦山地震前后剪切波分裂特征的变化.研究认为,利用地震剪切波特性监测应力变化,进而可应用于地震应力预测研究.需要注意是,不同的构造部位对应力变化有不同的响应.更多还原

【Abstract】 Spatial distribution and temporal changes of seismic anisotropy in the upper crust are revealed in the eastern margin of the Tibetan Plateau, including the Longmenshan fault zone, where the MS8.0 Wenchuan earthquake occurred on May 12, 2008, by analyzing seismic data recorded by long-running seismic stations in national permanent and regional portable seismic networks, with a recording period of up to 18 years. The resulting shear-wave splitting parameters, including the fast shear-wave polarization orientation(i.e., fast orientation) and the slow shear-wave time delay(i.e., time delay, also named splitting time) are measured to characterize the spatial distribution of crustal anisotropy and its variation before and after the Wenchuan earthquake in different subzones. The splitting parameters are influenced by both the stress field and fault structure, and consequently, some subzones show two predominant fast orientations. In addition, relative to the interior of tectonic blocks, more complicated polarization characteristics are observed along the margins of the subzones and in the vicinity of the faults. Changes in both the fast orientation and the splitting time in some subzones or at some stations are observed before and after the Wenchuan earthquake. Stations demonstrating such changes are either located near the epicenter, or are adjacent to faults or tectonic boundaries. The results suggest that relative to the surrounding area, the Longmenshan fault zone shows a greater reduction in the observed time delay after the earthquake. The reduction in time delay is the greatest in the northern section of the fault zone and reduces toward the south, and the smallest reduction is found in the southern end of the Longmenshan fault where the Xianshuihe and Anninghe faults join, reflecting lateral variations in crustal stress and physical properties. The splitting parameters are used to infer the distribution of the principal compressive stress in the Longmenshan fault zone and surrounding areas. Changes of the splitting parameters after the MS7.0 Lushan earthquake are also analyzed in this study. This study suggests two important phenomena.(1) The time delays before a large earthquake can be statistically larger than those after the earthquake, indicating a decrease in stress after the earthquake or an increase prior to the earthquake.(2) After a large earthquake, the splitting parameters show different patterns in different subzones, and changes of time delays along the seismogenic fault are more significant than the other areas. Shorter time delays indicate lower stress levels. Because the splitting parameters are sensitive to stress change, the latter can be monitored by the characteristics of seismic shear-wave splitting, a technique that can be further applied to the research on earthquake stress-forecasting. Note, different tectonic position is differently response to the stress change.更多还原