What stopped rupture propagation of the 2008 Wenchuan Earthquake, eastern Tibet Plateau, China

Abstract:

The 2008 Wenchuan earthquake (Mw 7.9) initiated within the southern part of the Yingxiu-Beichuan fault that is the middle-northern segment of the Central Longmen Shan fault zone, eastern Tibet Plateau. The earthquake rupture propagated unilaterally to the northeast over a distance of ~300 km and was accompanied by sinistral-thrust slip within the fault zone. However, the kinematics of the termination of the rupture on its two ends remains poorly constrained. Although surface rupture terminated near Kuangpingzi village in the northeast, the aftershock distribution suggests that the rupture may extend at depth to northeast of Qingchuan along a NE-trending zone. Our field observations suggest that the aftershock zone is likely in accordance with a NE-trending linear active structure at surface. Our field work also shows that the Qingchuan fault did not rupture during the Wenchuan earthquake. Detailed investigations show that the aftershock distribution belt at the northern end stopped at an area between the Qingchuan fault and the North-Qingchuan fault where these two faults bound a 10-to-20-km-wide mountain block. The Qingchuan fault, the North-Qingchuan fault and the block between, which are called as oblique cross-structures, have a trend (E to NEE strike) different from the Yingxiu-Beichuan fault. These pre-existing cross-structures may act as a persistent structural barrier that halt fault propagation. The NEE-striking pre-existing structures appear to be less likely to produce strong magnitude events. Along them, no surface evidence for late Quaternary faulting has been found thus far and historical seismicity shows small events. At the surface, the southern-most part of the Wenchuan earthquake rupture zone is marked by a NE-trending, 50-cm-high uplift near the intersection between the southern segment and the middle-northern segment of the Longmen Shan fault zone. The intersection between these two fault segments is a Qionglai Shan mountain block that trends northerly and is bounded by faults. The Qionglai Shan block may have acted to arrest spread of the rupture to the southwest onto the adjacent southern segment of the Longmen Shan fault zone. The initiation and propagation of rupture are all at fault tips, which means the entire Yingxiu-Beichuan fault ruptured during the 2008 Wenchuan earthquake.