Inelastic triggering of the 2013 MW 6.6 Lushan earthquake by the 2008 MW 7.9 Wenchuan earthquake

Stress changes due to the co-seismic slip on the source fault of the 2008 MW 7.9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake, seismicity shows progressively increasing in a wide region. However, the south segment of the Longmenshan Fault did not show any significant change in seismicity, where positive Coulomb failure stress change (ΔCFS) was estimated under the elastic half-space model. Under such a background, the 2013 MW 6.6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0.2~0.4 bar, a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M 6.2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area, which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake, which ruptures the entire gap, may occur in the near future if there are no other unknown factors.