Background Stress State Before the 2008 Wenchuan Earthquake and the Dynamics of the Longmen Shan Thrust Beltстатья
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Дата последнего поиска статьи во внешних источниках: 26 декабря 2018 г.
Аннотация:A stress reconstruction was performed based on
focal mechanisms around the Longmen Shan region prior to the
2008 Ms 8.0 Wenchuan earthquake using a newly developed
algorithm (known as MCA). The method determines the stress
tensor, including principal axes orientations, and quantitative stress
values, such as the effective confining pressure and maximum shear
stress. The results of the MCA application using data recorded by
the regional network from 1989 to April 2008 show the background
stress state around the Longmen Shan belt before the Wenchuan
earthquake. The characteristics of the stress orientation reveal that
the Longmen Shan region is primarily under the eastward extrusion
of the eastern Tibetan plateau. Non-uniform quantitative stress
distributions show low stress levels in the upper crust of the middle
Longmen Shan segment, which is consistent with the observed
high-angle reverse faulting associated with the 2008 Wenchuan
earthquake. In contrast, other study areas, such as the Bayankela
block and the NW strip extending to the Sichuan basin, show high
stress intensity. This feature coincides with heterogeneity in the
wave speed image of the upper crust in this region, which shows
high S-wave speed in the high stress areas and comparatively low
S-wave speed in low stress areas. Deformation features across the
Longmen Shan belt with the slow rates of convergence determined
by GPS and the distribution of surface deformation rates also are in
keeping with our stress results. We propose a dynamic model in
which sloping uplift under the Longmen Shan, which partly
counteracts the pushing force from the eastern plateau, causes the
low-quantitative stresses in the upper crust beneath the Longmen
Shan. The decreasing gravitational potential energy beneath the
Longmen Shan leads to earthquake thrust faulting and plays an
important role in the geodynamics of the area that results from
ductile thickening of the deep crust behind the Sichuan basin,
creating a narrow, steep margin.