Spatio-temporal Variation Of The Stress Field In The Central Longmenshan Fault Zone

The NE-trending Longmenshan fault zone is located in the middle of the north-south seismic belt between the Tibetan plateau and the Sichuan basin of the South China block. This fault zone can be divided into south, central, north segments with the boundaries of Yingxiu and Beichuan. On 12 May 2008, the Mw 7.9 Wenchuan earthquake ruptured the central part of Longmenshan, which causing huge casualties and economic losses. The researches on mainshock source rupture propagation show that the focal mechanism of this quake is dominated by thrust with some right-lateral strike-slip component. The primary driving force comes from the accumulation of the strain in the Longmenshan fault zone which is the result of the Bayankala block moving to SEE hampered by the South China block. That the Longmenshan fault zone is able to generate such a great earthquake is attributed to 3 conspicuous factors:First, the Longmenshan is composed of a complex massif with strong strength. Second, the listric-shaped of the fault plane is favorable to energy accumulation. And third, the faults are overall in a locking state with very slow slip rates.Stresses in the Earth’s crust are affected by plate motion and regional geological structure as well as stress changes caused by large earthquakes. Crustal stresses are also the direct driving forces for earthquakes. Regional-scale studies of crustal stress and its spatial and temporal variations can therefore provide insights into earthquake mechanics and tectonic deformation. Stress inversion using focal mechanisms is one of the main lines of thought to determine the stress field.A large number of aftershocks occurred in Longmenshan after the Wenchuan earthquake. It is possible to investigate the dynamics of the area with significant data base and model parameters from the abundant aftershock data and the large-scale fault zone. Many scientists and institutes have acquired the spatial and temporal variations of the stress field and their tectonic significance by inverting the focal mechanism data set. Those results are mainly focused on the major aftershocks with magnitude greater than ML4, which lack constraints for the local stress field.The driving force magnitude of moderate earthquakes is far greater than the small ones, which means that moderate temblors are mainly controlled by the tectonic stress. Relatively, micro earthquake is sensitive to the Coulomb stress change triggered by great earthquakes and the faults nearby. Besides, the number of micro earthquakes is huge while the shortcoming lies in the lack of effective observation numbers due to low signal-to-noise ratio. With the deployment of transportable broadband seismic stations and the research of fine velocity structure of the target area, the quality of focal mechanism and location parameters of the micro earthquakes have been improved greatly. It is necessary to utilize the focal mechanism of micro earthquakes to reveal the fine spatio-temporal variations of the stress field.This thesis first makes use of micro earthquake data recorded by the West Sichuan movable seismic array during January 2007 to October 2008 and inverts the focal mechanisms through P-wave onset polarities and SH/P amplitude ratios. It applies the focal mechanism solutions in the stress inversion with the damped linear stress inversion method to better understand the adjustment and migration of the stress field in the central Longmenshan fault zone.The results of this work are summarized below.(1) Before the Wenchuan earthquake, the focal mechanisms in the central part of Longmenshan are primarily thrust with similar characters indicating that the local stress field is uniform and controlled by the tectonic stress. The stress tensor inversion shows that the maximum principal stress S1 is nearly horizontally, trending in NWW-SEE, and S3 is almost vertical implying that the style of faulting is thrust. Slip tendency study shows that the stress field has a high consistency, implying that the area has the ability to generate great earthquakes. The P-axis direction of macro earthquake focal mechanism in Baoxing area is along fault trend, suggesting that there was abnormal stress state existing in the south of initial rupture area of the Wenchuan quake. The relation between abnormal stress and the occurrence of Wenchuan and Lushan earthquake needs further research.(2) Focal mechanisms of micro earthquakes are dominated by thrust with a considerable proportion of strike-slip and normal faulting. The proportions of different types vary with time, of which the proportion of thrust increases greatly over time while the normal decreases. The spatial distribution of the type of FMSs is closely correlated with local geology and the characters of the deep fault.(3) The stress tensor inversion results reveal that the orientations of the maximum horizontal compressive stress (Sh) shortly after the mainshock in most areas are mainly in EW to NWW-SEE directions, while in the area along Xiaoyudong-to-Lixian aftershock branch it is in NE-SW direction in shallow crust. This implies that the stress field at the early stage after the Wenchuan earthquake was mainly controlled by the dynamic stress change caused by the rupture propagation, and the conspicuous factor determining the SH direction is the characteristics of the rupture plane. Temporal variation of the stress tensors shows that the stress regimes at depth changed from a mixture of reverse and strike-slip faulting to pure reverse, indicating that local Coulomb stress caused by the main shock was released through strike-slip faulting and gradually recoverd to the background stress field. The stress in the shallow subsurface changed following the deep portion with observable time delay.(4) The stress tensors from inversion show that in the deep part of the source area, SH orientations are vague, while the fault regime remained thrust which means that there occurred local tectonic uplift of high-strength rock. The stress anomalies found previously may be the result of the uplift of the Pengguan complex massif at 20km depth. This would cause material change on the NE side of the complex, which may have relation to the formation of the Xiaoyudong-to-Lixian left-lateral strike-slip fault.This work does research on macro earthquake focal mechanism and the stress field of the whole Longmenshan area before Wenchuan earthquake and central segment after, the results provide new evidence and constraint on the dynamics process of Wenchuan and Lushan quake research.