Spatial and Temporal Stress Variations before and after the 2008 Wenchuan M_w 7.9 Earthquake and its Implications: A Study based on Borehole Stress Data

In situ stress measurement data was analyzed to estimate the temporal and spatial stress variations at shallow depths in the Longmenshan fault zone (LMSF), prior to and following the 2008 Wenchuan earthquake (WCEQ). Analysis of the stress field related to fault strength and behavior is useful for understanding geodynamic processes and conducting hazard assessments. The shallow stress changes after the WCEQ show clear along-strike variations. Degrees of stress orientation rotations have a negative correlation with the horizontal principal stress ratios and the WCEQ apparently reduced the magnitude difference between horizontal principal stresses. Taking stress magnitudes and orientation distribution relative to the fault strike into account, we propose an intermediate-strength of LMSF, with a friction coefficient generally constrained between 0.35 and 0.6. In addition, high-pressure fluids in the fault zone reduce the effective normal stress and to a certain degree weaken the fault strength. The accumulated stress over a certain period following release of the WCEQ indicates the start of another earthquake cycle. The changing crustal stress field makes the LMSF stable or slipping optimally during geodynamic processes. The segmentation feature of the shallow crustal stress field in the LMSF may imply a different tectonic loading and seismic release processes along the fault. The southwestern section to the epicenter of the WCEQ favors the occurrence of future earthquakes, as high μ_m in a state of critical failure was present in this area, which indicates that the Wenchuan and Lushan earthquakes did not release the accumulated stress to a sufficient extent there.