Crustal seismic anisotropy and compressive stress in the eastern margin of the Tibetan Plateau and the influence of the <italic>M</italic><sub>S</sub>8.0 Wenchuan earthquake

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 M S8.0 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 M S7.0 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.