Sub-instability stress state prior to the 2008 Wenchuan earthquake from temporal and spatial stress evolution

In recent years, the concept of sub-instability stage was proposed from the study of rock experiments. The basic characteristics of this stage are the stress release evolution after the stress peak in comparison with the stress accumulation evolution before the stress peak and the corresponding deformation fields. Stress tensor evolution before the Wenchuan earthquake is investigated by using the method of Cataclastic Analysis based on multiple focal mechanisms, to study whether there is measurable sub-instability stage before strong earthquakes. The results show that the maximum compressive stress axis of the Longmenshan fault zone is near horizontal, the maximum tensile stress axis is near vertical, the stress state is consistent with the subsequent dynamic mechanism of the Wenchuan earthquake and supports the significant dextral strike slip component of the fault in this earthquake. The temporal and spatial evolution of the stressfield is obtained around the focal area by using the mechanism data measured by the Zipingpu Reservoir Network for more than 3 years before the Wenchuan earthquake. It shows that the apparent stress change occurred in June 2007, and a new stress increasing area was added in the northeastern corner of the reservoir area near the main shock. The stress increment distribution over the two periods before and after June 2007 shows that the stresses increase near Wenchuan and Pengxian but decrease in most of the other locations, which indicates that the stress level of the study area is weakened significantly after June 2007. In comparison, the stress increment field at different time points before June 2007 shows that regional stresses increases with time in most areas. The stress changes are in accordance with the stress evolution characteristics of the fault from the steady state to the sub-instability state in rock experiments. With the results of previous studies such as cross-fault measurements, regional seismic activity and borehole strain, we infer that accelerated eastward movement happened in the east part of the Bayankela block in the middle of 2007, and it is directly related to the sub-instability state before the Wenchuan earthquake.