Stress Shadow on the Southwest Portion of the Longmen Shan Fault Impacted the 2008 Wenchuan Earthquake Rupture

The 2008 Wenchuan earthquake (Mw7.9) unilaterally ruptured the Longmen Shan fault (LMSF) along eastern Tibet. The earthquake rupture propagated about 270km northeastward, whereas it propagated only about 20km southwestward along the strike of the fault. Although a significant attention has been paid to the question of predominantly unilateral propagation, the primary reasons for this type of propagation remain unclear. In this research, we examined the change of Coulomb stress along the LMSF caused by the historical earthquakes near and on the LMSF from 1725 to 2008. We found that the 14 preceding large earthquakes (M6.5) on the Xianshui He fault cast a stress shadow on the SW segment of the LMSF, which was not activated by the 2008 Wenchuan earthquake rupture. The 1970 Dayi earthquake on the SW segment of the LMSF contributed significantly to this stress shadow. Compared with the segment of the 2008 Wenchuan earthquake rupture, this stress shadow caused strong stress contrasts of -214, -22, and -80kPa in the seismic gap of the very SW segment of the LMSF, the rupture zone of the 2013 Lushan earthquake, and the seismic gap between the 2008 Wenchuan and 2013 Lushan earthquake ruptures, respectively. Stress contrasts in these values were consistent with tectonic loading over 165, 17, and 62years, respectively, by integrating an interseismic stress-loading rate of 1.3kPa/a. We proposed that this stress shadow might have created a barrier at the SW segment, preventing the earthquake rupture propagating southwestward.