2008 Mw 7.9 ᄿᕖኬᆀ㟀ࡡ᩷ᒒᵋ㏸ᶅᘟ㸯ᚗὩࡊࡒ㱗㛓ᒜ㏣᩷ᒒᖈ贾 東 （中 国南 京大 学）ᯐ  យ ᪺ ࣬௴  ἖ ᆙ㸝 㟴ኬ ࣬ኬ Ꮥ 㝌㸞Structural model of 2008 Mw 7.9 Wenchuan earthquake
in the rejuvenated Longmen Shan thrust belt, China
Dong Jia (Nanjing Univ., China),Aiming Lin and Zhikun Ren (Graduate School, Shizuoka Univ.)

The Longmen Shan range exhibits a great steep relief between the eastern Tibetan plateau
and the Sichuan basin. The structural configuration of the Longmen Shan may be schematicallydescribed as a southeastward-verging fold-and-thrust belt system, which experienced at least
two major periods of contractional deformation in the Late Triassic and Cenozoic (Burchfiel etal., 1995; Jia et al, 2006). However, GPS data of the last decade indicated that there is nolarge-scale shortening (shortening rates < 3mm/yr.) of the upper crust across the Longmen Shan.
Thereby the rise of the Longmen Shan range was interpreted by lateral eastward flow of ductile
deep crust causing thickening by crustal inflation. Whereas the 12 May, 2008 (Mw=7.9)
Wenchuan earthquake occurred in the rejuvenated Longmen Shan thrust belt and displayed an
ongoing uplift process of the eastern margin of Tibetan plateau. The hypothesis of deep crust
flow model seems to be dramatically challenged by this thrusting event. Our field investigation
discovered that the Wenchuan earthquake ruptures often inherit the pre-existing thrust faults andthe destruction of the buildings and most landslides are limited to the hanging wall of thereactivated thrust faults. Some available focal mechanisms and GPS measurement of coseismic
displacement show prevailing reverse and reverse-oblique kinematics, compatible with a nearly
average NW–SE shortening and with field evidence of active thrusting and oblique thrusting
deformation (Lin et al., 2008, 2009).
The 12 May 2008 Wenchuan earthquake of China occurred in the rejuvenated Longmen
Shan thrust belt and displayed an ongoing uplift process of the eastern margin of Tibetan
plateau. The coseismic surface ruptures exactly follow the pre-existing thrust fault trace and thedestruction of the buildings and most landslides are limited to the hanging wall of thereactivated thrust faults. In this paper, in order to detect any indication of coseismic deformation
linked to subsurface structures, we construct the subsurface fault and fold geometries using
petroleum seismic reflection profiles, as well as constrained by coseismic surface ruptures andseismicity. Our data suggest that the deformation can be divided into two different structuralsegments related to the regional coseismic deformation during the Wenchuan earthquake. In thesouthern segment, two coseismic surface rupture zones accord with two pre-existing thrustfaults (the Yingxiu-Beichuan and Pengguan faults) in the seismic profiles and both coseismic
active thrusts become incorporated into the deep main detachment. This through-going thrustfault connecting directly from the hypocenter to the surface break, but cannot be easily extended
to the northern segment along the Yingxiu-Beichuan thrust fault zone. In contrast, only one
shallow coseismic active thrust fault with oblique-slip occurs as the active passive roof ofimbricate thrust sheets in the northern segment. Our results provide an actual structural model
which shows nearly classic ramp-flat geometry and the wedge structures at the blind front of therejuvenated thrust belt. We emphasize that there is a potential 15~17 km-deep main detachment
associated with this large earthquake in the Longmen Shan belt, and infer that some active
displacements along the 6~9 km-depth shallow detachment have been propagating into theSichuan basin since late Cenozoic.