The concealed active tectonics and their characteristics as revealed by drainage density in the North China plain (NCP)
Introduction
The North China plain (NCP) is a region famous for the high frequency of strong earthquakes, especially during the period 1966–1976 (Ding and Lu, 1983) (Fig. 1). The 1966, Xiatai M7.2 earthquake in the NCP marks the start of a 10 year period from 1966 to 1976 during which time nine earthquakes with M>7.0 occurred on the Chinese continent (Ma et al., 1982). The earthquakes in the NCP during that period delineate a NE-striking seismic zone from Tangshan in the north, crossing Hejian, to Xingtai in the south, which is called the Tangshan–Hejian–Xingtai seismotectonic zone (THXSZ) (Yang, 1987, Feng et al., 1989). It cuts through the NNE-striking Tertiary-age tectonic elements, such as the Jizhong depression and Cangxian platform, and is not spatially coincident with earlier tectonic zones, which are mainly interpreted from seismic reflection profiles. THXSZ, therefore, is considered to be a newly established zone, which formed in response to the modern tectonic stress field (Xu et al., 1985, Xu et al., 1996).
The earthquake geological survey is often an important tool for active tectonic research. However, due to the Quaternary cover in the NCP, this approach cannot be used. The NCP appears to be a large tectonic depression that has subsided regionally since the Neogene. The thickness of Neogene and Quaternary unconsolidated deposits in the NCP is 1000–1500 m, which hampers the ability to observe active faults at the surface. Also, these faults are newly established structures and such structurally immature faults are less connected and more widely oriented than those in a mature zone (Schweig and Ellis, 1994, Guo et al., 2000). The THXSZ is mainly recognised from seismic activity. The foreshocks and aftershocks of the 1966 Xingtai M7.2 earthquake, the 1967 Hejian M6.7 earthquake and the 1976 Tangshan M7.8 earthquake form a lineament, which extends about 300 km (Xu et al., 1996). The seismological record is, however, short compared with the return time of large magnitude earthquakes (M≥7.0) in the interior of a plate which is expected to be thousands of years (McCalpin and Nishenko, 1996, Ran and Deng, 1999). It is therefore difficult, sometimes impossible, to determine the active tectonic elements in intraplate areas based on seismological data alone.
Drainage analysis may provide clues as to fault activity and evolution that are difficult to obtain by more conventional geological means. Useful structural information is preserved in the drainage associated with active fault systems as shown by Leeder and Jackson, 1993, Jackson and Leeder, 1994, Jackson et al., 1996, Jackson et al., 1998, Hovius, 1996. Mathematical simulation of drainage patterns can also provide useful insights (Tomkin and Braun, 1999). These previous studies of drainage patterns were, however, carried out in regions where tectonic deformation caused relative uplift and subsidence. Previous geomorphology studies in the Weihe basin in China and the NCP have demonstrated that drainage across plains underlain by Quaternary deposits was established about 10,000–12,000 years ago (Han et al., 1994, Hou and Han, 1997, Wang et al., 1999). The distribution of drainage in these plains is correlated with subtle uplift or subsidence at the ground surface deformation. For example, rivers or streams may cluster, fork or change direction when crossing a fault zone (Leeder and Jackson, 1993, Han et al., 1998). Non-tectonic factors may also play a role in the establishment of drainage patterns (Jackson and Leeder, 1994). In the NCP the influence of these factors, such as topography and weather influencing drainage patterns are secondary to the impact of deformation at the ground surface (Han et al., 1994). Since the drainage density is an average value for a unit area, it enhances the main factor. By assigning a threshold value, it reduces the impact of the secondary, non-tectonic factors. The drainage density can also be used to describe quantitatively the drainage spatial distribution. A change of drainage density, from high to low, can therefore be easily identified both numerically and in map form by assigning variable colours to different densities.
The coincidence of high-drainage density belts with the concealed active fault zones has been demonstrated by experimental work in the Weihe basin, China (Hou and Han, 1997). The reasons for this relationship may include the following: (1) concealed fault zones could provide a passage for underground water; small lakes and rivers are usually abundant along fault zones. (2) Clustering, forking or changing directions of rivers or streams along a fault zone can produce the high-density drainage. Boundaries of regions with different distribution features of drainage densities can also be correlated with concealed fault zones and coincided with the high-density belts.
This paper presents an example using data from digitised river locations. These data provide a basis for calculation of drainage density and analysis of the relationships between the spatial distribution of drainage densities and the locations of concealed fault zones in the NCP. In this study we aim to address the following questions: (1) Is the spatial distribution of the belts of high-drainage density controlled by the locations of concealed fault zones? Are regions with high or low drainage densities related to subsidence or uplift? We address this question, based on the locations of concealed Holocene-active faults inferred from shallow seismic reflection profiles (Xiang et al., 1994), and geodetic-surveying data. (2) This will determine whether the tectonic regime revealed by the drainage data is consistent with that inferred from seismological data in the NCP. A Tertiary tectonic framework for the NCP has been established by interpreting thousands of kilometres of seismic reflection lines (Xu et al., 1985), but these data are inconsistent with historical earthquakes in the NCP. If a newly formed set of active faults exist in the NCP, it is critical to identify the zones of potential seismic sources. These youthful zones will be less obvious than along mature systems and, if present, will have important consequences for seismic hazard model for the region (Schweig and Ellis, 1994). Differences between the latest active tectonic regime and previous, Tertiary tectonics will be discussed. Therefore, the research carried out in this paper could also be helpful for assessment of potential seismic sources and analysis of the modern geodynamics in the NCP.
Section snippets
Drainage density calculation in the NCP
Drainage density (ρ) is defined as the sum of the length of all rivers in one square unit area. Each river, represented on the topographic maps, is included in the analysis; the width of the active river channel is not incorporated into the calculations. The width of a river is also an important parameter for characterising the river morphology, but it changes downstream and is difficult to measure from topographic maps. Drainage densities are calculated as follows:in which, S is the
Discussion
Although some disastrous earthquakes, such as the 1966 Xingtai M7.2 earthquake and the 1976 Tangshan M7.8 earthquake occurred in the NCP, our knowledge of the active tectonics of the NCP is poor. The main reason for this is that the NCP is a wide plain covered with Quaternary deposits. It is very difficult to detect any active tectonics from the geological survey. This paper tries to address the question using drainage density calculations and analysis. The drainage in the NCP was established
Conclusions
The drainage density spatial distribution shows a close relationship to the current tectonics in the NCP. Based on the calculation of the drainage density and the analysis of its distribution in the NCP, some interesting results can be summarized as follows:
- 1.
The distribution of high-drainage density in the NCP is characteristic of not only zonation, but also regionalisation, which outlines a tectonic pattern of the NCP for the latest phase of the Cenozoic.
- 2.
The differences in the trends,
Acknowledgements
This study is supported by China high-priority basic science planning project (No. 95-13-01-05) and China Earthquake Science Foundation (No. 197062). The authors are thankful to Drs Andy Nicol and McVerry Graeme for English writing, Dr Peizhen Zhang for his encouragement, Prof. Xu Jie for his discussion about the latest active framework in NCP and Prof. Xiang Hongfa for shearing his ideas about active tectonics in the vicinity of Beijing.
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