____________________________________________________________________________www.paper.edu.cn Journal of Asian Earth Sciences 21 (2003) 989–998 www.elsevier.com/locate/jseaes The concealed active tectonics and their characteristics as revealed by drainage density in the North China plain (NCP) Zhujun Hana,*, Lun Wub, Yongkang Rana, Yanlin Yeb aInstitute of Geology, China Seismological Bureau, Beijing 100029, People’s Republic of China bDepartment of Geography, Peking University, Beijing 100871, People’s Republic of China Received 6 July 2001; revised 16 August 2002; accepted 9 October 2002 Abstract Drainage density, defined as river length per unit area, is an important tool to reveal the concealed active tectonics in the Quaternary- covered North China plain (NCP). The distribution of high-drainage density is characteristic of zonation and regionalisation. There are two sets of belts with high-drainage densities, which trend at 45–50 and 315–3208. The different locations and trends of these belts make it possible to divide the NCP into three regions. They are Beijing–Tianjin–Tangshan region in the north, Jizhong–Lubei region in the middle and Xingtai region in the south. Similarly located regions can be defined using contours of deformation data from geodetic-surveys from 1968 to 1982. Belts of high-drainage density are spatially coincident with seismotectonic zones and overlie concealed active faults. These active structures strike at an angle of <158 to that in the Eocene–Oligocene, a discordance, which may be an indication of an evolution of the fault pattern through time in the NCP during the Cenozoic. The current phase has more important consequences regarding seismic hazards in the NCP. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Drainage density; Concealed active tectonics; North China plain 1. Introduction to be a newly established zone, which formed in response to the modern tectonic stress field (Xu et al., 1985, 1996). The North China plain (NCP) is a region famous for the The earthquake geological survey is often an important high frequency of strong earthquakes, especially during the tool for active tectonic research. However, due to the period 1966–1976 (Ding and Lu, 1983)(Fig. 1). The 1966, Quaternary cover in the NCP, this approach cannot be used. Xiatai M7.2 earthquake in the NCP marks the start of a 10 The NCP appears to be a large tectonic depression that has year period from 1966 to 1976 during which time nine subsided regionally since the Neogene. The thickness of earthquakes with M . 7:0 occurred on the Chinese Neogene and Quaternary unconsolidated deposits in the continent (Ma et al., 1982). The earthquakes in the NCP NCP is 1000–1500 m, which hampers the ability to observe during that period delineate a NE-striking seismic zone from active faults at the surface. Also, these faults are newly Tangshan in the north, crossing Hejian, to Xingtai in the established structures and such structurally immature faults south, which is called the Tangshan–Hejian–Xingtai are less connected and more widely oriented than those in a seismotectonic zone (THXSZ) (Yang, 1987; Feng et al., mature zone (Schweig and Ellis, 1994; Guo et al., 2000). 1989). It cuts through the NNE-striking Tertiary-age The THXSZ is mainly recognised from seismic activity. tectonic elements, such as the Jizhong depression and The foreshocks and aftershocks of the 1966 Xingtai M7.2 earthquake, the 1967 Hejian M6.7 earthquake and the 1976 Cangxian platform, and is not spatially coincident with Tangshan M7.8 earthquake form a lineament, which earlier tectonic zones, which are mainly interpreted from extends about 300 km (Xu et al., 1996). The seismological seismic reflection profiles. THXSZ, therefore, is considered record is, however, short compared with the return time of ð : Þ * Corresponding author. Tel.: þ86-10-62009037; fax: þ86-10- large magnitude earthquakes M $ 7 0 in the interior of a 62028617. plate which is expected to be thousands of years (McCalpin E-mail address: [email protected] (Z. Han). and Nishenko, 1996; Ran and Deng, 1999). It is therefore 1367-9120/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(02)00175-X 中国科技论文在线____________________________________________________________________________www.paper.edu.cn 990 Z. Han et al. / Journal of Asian Earth Sciences 21 (2003) 989–998 Fig. 1. Location of the research area. difficult, sometimes impossible, to determine the active describe quantitatively the drainage spatial distribution. A tectonic elements in intraplate areas based on seismological change of drainage density, from high to low, can therefore data alone. be easily identified both numerically and in map form by Drainage analysis may provide clues as to fault activity assigning variable colours to different densities. and evolution that are difficult to obtain by more The coincidence of high-drainage density belts with the conventional geological means. Useful structural infor- concealed active fault zones has been demonstrated by mation is preserved in the drainage associated with active experimental work in the Weihe basin, China (Hou and Han, fault systems as shown by Leeder and Jackson (1993), 1997). The reasons for this relationship may include the Jackson and Leeder (1994), Jackson et al. (1996, 1998), and following: (1) concealed fault zones could provide a passage Hovius (1996). Mathematical simulation of drainage for underground water; small lakes and rivers are usually patterns can also provide useful insights (Tomkin and abundant along fault zones. (2) Clustering, forking or Braun, 1999). These previous studies of drainage patterns changing directions of rivers or streams along a fault zone were, however, carried out in regions where tectonic can produce the high-density drainage. Boundaries of deformation caused relative uplift and subsidence. Previous regions with different distribution features of drainage geomorphology studies in the Weihe basin in China and the densities can also be correlated with concealed fault zones NCP have demonstrated that drainage across plains under- and coincided with the high-density belts. lain by Quaternary deposits was established about 10,000– This paper presents an example using data from digitised 12,000 years ago (Han et al., 1994; Hou and Han, 1997; river locations. These data provide a basis for calculation of Wang et al., 1999). The distribution of drainage in these drainage density and analysis of the relationships between plains is correlated with subtle uplift or subsidence at the the spatial distribution of drainage densities and the ground surface deformation. For example, rivers or streams locations of concealed fault zones in the NCP. In this may cluster, fork or change direction when crossing a fault study we aim to address the following questions: (1) Is the zone (Leeder and Jackson, 1993; Han et al., 1998). Non- spatial distribution of the belts of high-drainage density tectonic factors may also play a role in the establishment of controlled by the locations of concealed fault zones? Are drainage patterns (Jackson and Leeder, 1994). In the NCP regions with high or low drainage densities related to the influence of these factors, such as topography and subsidence or uplift? We address this question, based on the weather influencing drainage patterns are secondary to the locations of concealed Holocene-active faults inferred from impact of deformation at the ground surface (Han et al., shallow seismic reflection profiles (Xiang et al., 1994), and 1994). Since the drainage density is an average value for a geodetic-surveying data. (2) This will determine whether unit area, it enhances the main factor. By assigning a the tectonic regime revealed by the drainage data is threshold value, it reduces the impact of the secondary, non- consistent with that inferred from seismological data in tectonic factors. The drainage density can also be used to the NCP. A Tertiary tectonic framework for the NCP has 中国科技论文在线____________________________________________________________________________www.paper.edu.cn Z. Han et al. / Journal of Asian Earth Sciences 21 (2003) 989–998 991 been established by interpreting thousands of kilometres of in the early 1950s and published in 1956 just prior to a major seismic reflection lines (Xu et al., 1985), but these data are change in drainage in China, which occurred due to an inconsistent with historical earthquakes in the NCP. If a intensification of farming practices in the late 1950s. Since newly formed set of active faults exist in the NCP, it is China is a country with a long history, about 5 ka, the critical to identify the zones of potential seismic sources. anthropogenic modification certainly has existed and had an These youthful zones will be less obvious than along mature effect on the drainage density, but one of the important systems and, if present, will have important consequences principles in Chinese life is to ‘guide the action or drainage for seismic hazard model for the region (Schweig and Ellis, according to its circumstances’. It means that the basic 1994). Differences between the latest active tectonic regime distribution of drainage has not been changed too much by and previous, Tertiary tectonics will be discussed. There- human activity during the long history of China. Four maps fore, the research carried out in this paper could also be were used to digitise the drainage patterns in the NCP, helpful for assessment of potential seismic sources and which covers a region of six latitudinal degrees (E114– analysis of the modern geodynamics in the NCP. 1208) and three longitudinal degrees (N37–408). The digital drainage is shown in Fig. 2. In order to simplify the calculation of drainage density, the NCP was subdivided 2. Drainage density calculation in the NCP into 360 £ 180 cells with each side equal to one minute (10) in length. As the actual lengths of one latitudinal minute (10) Drainage density ðrÞ is defined as the sum of the length of and one longitudinal minute (10) are different, the ratio is all rivers in one square unit area.