Surface Rupture of the 2008 Wenchuan, China, Earthquake in the Qingping Stepover Determined from Geomorphologic Surveying and Excavation, and Its Tectonic Implications

The Wenchuan M-w 7.9 earthquake of 12 May 2008 caused the rupture of the Longmen Shan thrust belt, which bounds the eastern margin of the Tibetan plateau, and generated a very complex surface rupture. The Beichuan-Yingxiu fault (BYF) was the main seismogenic fault and formed two distinctively different surface rupture zones separated by the Qingping and Gaochuan stepovers. Real-time kinematic (RTK) surveying of alluvial terrace sequences indicates that terraces T1-T3 and river floodplain T0 have the same vertical displacement associated with the Wenchuan earthquake. Trench excavation and optical stimulated luminescence (OSL) dating of alluvial deposits indicate that only the deformation of the Wenchuan earthquake was recorded in the Qingping stepover since at least similar to 20 ka. Other deformations since similar to 20 ka probably occurred in other places or did not reach the surface. This can be meaningful to analyze the completeness of paleoearthquakes in trench excavation on thrust faults. The width of the Qingping stepover is similar to 1 km and it is not strong enough to form a barrier to arrest the propagation of rupture in the 2008 Wenchuan earthquake. The Gaochuan stepover is similar to 7 km wide and it is a barrier to separate the two surface rupture zones associated with the Wenchuan earthquake along the BYF. The empirical relationship of rupture displacement and stepover dimensions on strike-slip faults is probably suitable for the large thrust fault zone with a strike-slip component. This may be beneficial to the study of cascade rupture sections and seismic hazard assessment on large thrust faults.