Intraplate earthquakes in North China - Intraplate Earthquakes - page 103

the 660 km transition zone. Convection in this mantle wedge could have contributed to the
Cenozoic lithospheric thinning, volcanism, and associated extensional basins in the eastern
NCB (Liu et al ., 2004 ; Zhu et al ., 2009 ) .
Accordingly, the lithosphere under the eastern NCB is abnormally thin, in some places
reaching

60-80 km (Chen et al ., 2009 ; Huang et al ., 2009 ) . It thickens westward, reaching
more than 200 km under the Ordos Plateau (Chen et al ., 2009 ) . The thickening occurs
abruptly along the foothills of the Taihangshan mountains, correlating to the western edge
of the stagnant subducting slabs. Whereas the western part of the NCB avoided thermal
thinning for the most part, the lithosphere under the rift systems around the Ordos Block
may be thinned to 80 km (Chen et al ., 2009 ) .
The variation of crustal thickness generally mimics that of the lithosphere. It is more
than 40 km thick in the western NCB, and thins to 32-26 km in the eastern NCB (Ma,
1989 ) . The Ordos Plateau has a thin Cenozoic cover; in many places the Mesozoic strata,
which are a few kilometers thick, are exposed. These strata are flat-lying except near the
margins of the Ordos Plateau, indicating the tectonic stability of the western NCB since
at least the Mesozoic time (Zhang et al ., 2007 ) . In contrast, the eastern NCB is covered
by thick Cenozoic sediments, on top of widespread extensional basins, the largest one
being the basin system centered around Bohai Bay (Allen et al ., 1998 ; Li et al ., 2012 )
( Figure 5.2 ) . In these regions the upper crust is crosscut by a complex system of listric
normal faults associated with these extensional basins; they change to strike-slip faults in
the middle-lower crust where most of the destructive large earthquakes in the NCB initiated
(Xu et al ., 2002b ) .

5.2.3 Major seismogenic faults

Seismicity in the western NCB is concentrated within the circum-Ordos rift systems
( Figure 5.2 ) . These rifts developed mainly in the Neogene, related to the Indo-Asian
collision and the northeastern expansion of the Tibetan Plateau (Ye et al ., 1987 ; Zhang
et al ., 1998 ) . Along the northwestern edge of the Ordos Block is the Yinchuan rift, which
connects with the Hetao rift along the northern side of the Ordos. On the southern side of
the Ordos Block is the Weihe rift, which produced the deadly 1556 Huaxian earthquake.
Along the southeastern edge of the Ordos Block, the Weihe rift connects with the northeast-
trending Shanxi rift, which is a dextral shear extension zone consisting of more than 10
discontinuously distributed fault-depression basins controlled by normal strike-slip faults
or normal faults (Deng et al ., 2003 ) . Together, these two rifts have hosted more than 36
large (M > 6.5) earthquakes since 1303 (Liu et al ., 2007 ) . Seismicity within the Ordos
Plateau is minor, similar to seismicity in other stable cratons (Mooney et al ., 2012 ) .
In the eastern NCB the most prominent fault is the Tanlu fault system, which was
developed during the Mesozoic collision between the South and North China blocks (Li,
1994 ; Yin and Nie, 1996 ) . Its Cenozoic development may be related to the opening of Bohai
Bay during the Paleogene back-arc extension (Ren et al ., 2002 ) . GPS surveys shows that
the fault is extensional, accommodating east-west extension (Shen et al ., 2000 ) . Although