June 4, 2009 The Mw 7.9 Wenchuan earthquake occurred at 2:28 PM LT (6:28 UTC) on 12 May 2008, striking Wenchuan, Beichuan, and Qingchuan counties, located in the northwestern part of the Sichuan province. This earthquake was the most devastating earthquake in China in the past three decades. As of 29 September 2008, 69,227 deaths had been confirmed, with 374,643 people injured and 17,823 people missing. The maximum meizoseismal intensity (MMI) reached XI around Yingxiu Town, Wenchuan County, and Beichuan Town, Beichuan County, where 80% of homes were destroyed during the quake.
Field investigations by Xu et al. reveal that the Mw 7.9 Wenchuan earthquake ruptured an imbricated thrust fault system, activating different faults of the system at the same time. The earthquake produced a 240-km-long rupture along the Beichuan fault. The rupture is characterized by oblique faulting, mixing reverse motion and horizontal motion. An additional 72-km-long rupture, along the Hanwang segment, part of the Pengguan fault, is characterized by almost pure reverse faulting.
The average vertical offset of the surface rupture zone along the Beichuan rupture zone reaches 3 to 4 meters. Along the 135-kilometer-long northern section, a maximum vertical offset of 6.5 plus or minus 0.5 meters has been measured at Beichuan Town, and a maximum right-lateral offset of 4.9 plus or minus 0.5 meters at Pingtong Town. The maximum vertical offset on the 105-kilometer-long southern section is 6.2 plus or minus 0.5 meters at Shenxi Village, near Yingxiu Town.
A maximum vertical offset of 3.5 meters was measured along the Hanwang rupture zone. A 3-D model for rupture geometry shows that the Beichuan and Hanwang rupture zones appear to merge at depth. The total crustal shortening accommodated by the Wenchuan earthquake may reach 8.5 meters, and the total vertical uplift may reach 7.5 meters. The oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of the Tibetan Plateau is transformed into crustal shortening and uplift along its eastern margin that is responsible for the growth of high topography in the region.
This research was published by Xiwei Xu et al., (Institute of Geology, China Earthquake Administration, Beijing) in the June 2009 issue of Geology (Pages 515-518)
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