Intraplate earthquakes in North China - Intraplate Earthquakes - page 99

ongoing Indo-Asian continental collision; most of them are concentrated along the roughly
E-W-trending fault systems resulting from the collision.
Seismicity in eastern China is weak today relative to western China, but the long historic
records in eastern China show abundant large earthquakes, especially in the North China
Block (NCB) ( Figure 5.1 ) . Here the fault systems are more complex: in the North China
Plain and the coastal regions, the fault systems are mostly NE and NEE trending, owing
their origin to subduction of the Pacific plate under the Eurasian plate (Deng et al ., 2002 ;
Zhang et al ., 2003 ) ; further to the west, seismicity is concentrated in the rift fault zones
around the Ordos Block.
The NCB is a geological province including the Ordos Plateau and the surrounding
rift systems, the North China Plain, and the coastal regions. The cause of earthquakes in
the NCB is uncertain. These earthquakes are clearly intraplate events, because the NCB
is located in the interior of the Eurasian plate, within the Archean Sino-Korean craton,
and thousands of kilometers away from plate boundaries. North China, being the cradle
of the Chinese civilization, has the most complete historic records of earthquakes. Today,
North China is one of the most densely populated regions in China with vital economic
and cultural centers; hence, understanding earthquake hazards here is of great importance.
In past decades, intensive geological and geophysical studies in North China have greatly
refined the geological history and earth structure, and extensive Global Positioning Sys-
tems (GPS) measurements have delineated crustal kinematics. In this chapter, we briefly
summarize the tectonic background of the North China region, discuss the main features
of active tectonics, and describe historic and instrumentally recorded earthquakes. We
highlight the complex spatiotemporal patterns of large earthquakes in North China, and
discuss their implications for earthquake hazard assessment in North China and other
mid-continents.
5.2 Tectonic background
The geologically defined North China Block is part of the Archean Sino-Korean craton; in
China it is also referred to as the North China Craton (NCC). From west to east, the NCB
includes the Ordos Plateau, the North China Plain, and the coastal regions ( Figure 5.2 ) . The
Ordos Plateau is a relic of the NCC, with thick lithosphere and little internal deformation
through the Cenozoic, hence is also referred to as the Ordos Block. Its margins are bounded
by a system of rifts developed in the late Cenozoic, perhaps as a consequence of the Indo-
Asian collision (Xu et al ., 1993 ; Zhang et al ., 1998 ) . These rifts include the Weihe rift and
the Shanxi rift on the southern and eastern margins of the Ordos Plateau, respectively. These
rifts are structurally connected and perhaps formed together; in China they are sometimes
collectively referred to as the Fenwei rift system. This is a major seismic zone in North
China ( Figure 5.2 ) .
The North China Plain and the coastal regions are the part of the Sino-Korean cra-
ton where the cratonic root was destroyed by thermal rejuvenation in the Mesozoic; the
process produced widespread extension and volcanism during the late Mesozoic and early