Width Distribution of the Surface Ruptures Associated with the Wenchuan Earthquake: Implication for the Setback Zone of the Seismogenic Faults in Postquake Reconstruction

The 12 May 2008 Wenchuan earthquake ( M w 7.9) produced a 240-km-long surface rupture zone along the Beichuan fault and a 72-km-long rupture along the Pengguan fault, which are two of the three subparallel secondary strands of the northeast-striking Longmen Shan thrust fault system in the province of Sichuan, China. Among a total of twenty coseismic surface ruptures surveyed in the field, seventeen are less than 40 m wide, about half of them are 10–30 m wide, and only 3 are over 40–59 m wide. Many buildings sitting directly on the path of the surface ruptures were either completely demolished or severely damaged, including those with a frame or reinforced concrete structure. Other buildings a short distance from the rupture zones were subjected to only minor or no damage. These observations reinforce the use of fault setback zone in the postquake reconstruction of Wenchuan. Considering the crustal shortening of the inverse faulting that triggered the Wenchuan earthquake and uncertainties in width survey of the surface ruptures, we estimate the setback distance of 25 m for the seismogenic faults, which is comparable to the 15–30-m setback distance used in the seismically active regions of California. It is therefore recommended that construction of public buildings and residential structures should not be permitted within the 25-m setback zone of the seismogenic faults in the postquake reconstruction of Wenchuan and its surrounding areas.