Characteristics and implications of the stress state in the Longmen Shan fault zone, eastern margin of the Tibetan Plateau

摘要

Using stress data measured in 16 boreholes along the strike of the Longmen Shan fault zone by hydraulic fracturing from 2008 to 2012 after the Wenchuan earthquake and before the Lushan earthquake, we characterize the contemporary stress state in the Longmen Shan thrust belt along the eastern margin of the Tibetan Plateau to understand the implications of in-situ stress for fault activity. The stress regimes are generally conducive to reverse faulting and partly to strike-slip faulting characterized by sigma(H) > sigma(h)> sigma(v) and sigma(H) > sigma(v) > sigma(h), indicating that the regional stress field is definitely dominated by the maximum horizontal stress. The fracture impression results reveal that the maximum horizontal principal stresses are predominantly NE in the northern segment of the Longmen Shan fault zone and NW in the southern segment, postulating a preliminary understanding of the coupling between the shallow crustal stress field and lower crustal flow. According to Coulomb frictional failure criteria, horizontal principal stresses can be predicted as functions of rock density, rho, frictional coefficient, mu, depth, H, and water level, H-W, in frictional equilibrium. The influence of H-W on critical stresses is discussed, and the decrease in the stress values corresponds to an increase in the water level. The depth profiles of the stress magnitudes in different segments are illustrated, indicating that the stress values are relatively higher in the southern and northern segments and lower in the middle segment. The stress state in the southern segment, specifically, near the epicenter of the Lushan earthquake, favors the occurrence of earthquakes. Under the stress state in the northern segment, the Longmen Shan fault might be the optimally oriented failure plane, assuming that the plane is critically stressed. This finding may imply that the northern segment of the Longmen Shan fault is likely to be active when the stress builds up sufficiently to destroy the frictional equilibrium. (C) 2015 Published by Elsevier B.V.