The Concealed Active Tectonics and Their Characteristics As Revealed by Drainage Density in the North China Plain (NCP)

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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

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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. Each river, represented on about 1:1.267, this simplification produce a region longer the topographic maps, is included in the analysis; the width along the latitudinal direction than the longitudinal direc- of the active river channel is not incorporated into the tion. As the enlargement along the latitudinal direction is calculations. The width of a river is also an important generally carried out, it should not change the basic patterns parameter for characterising the river morphology, but it of drainage densities. After calculating the drainage density, changes downstream and is difficult to measure from the same shortening ratio along the latitudinal direction is topographic maps. Drainage densities are calculated as applied to the map again. In order to keep a gentle boundary follows: and further reduce the influence of non-tectonic factors, the window area (S) is defined as 90 £ 90. The value of density in 1 X r ¼ LS the window is assigned to the cell at the centre of the S i i window. The unit of drainage density, that is the cumulative length per area, is degree(8)/degree(8)2, which in many cases in which, S is the area of the window, LS is the length of the i increase the differences in drainage density. river i in the window. For GIS, river i is a discrete line and By applying the special model of calculating drainage expressed by a series of co-ordinates ðx ; ; y ; Þ: The length i j i j density to GIS, it is easy to acquire the values for all cells between ðx ; ;y ; Þ and ðx ; ; y ; Þ is: i j i j i jþ1 i jþ1 (360 £ 180) in the NCP. The values of drainage density for qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 all cells range from 0 to 56 degree(8)/degree (8) . When each l ¼ ðx ; 2 x ; Þ þðy ; 2 y ; Þ j i jþ1 i j i jþ1 i j value from 0 to 56 is represented by one colour, the spatial and the length of river i is: distribution of drainage densities was difficult to interpret. It was, therefore, necessary to subdivide values (0–56) into Xn six groups (1–6) (Table 1). The numerical range for each L ¼ l i j group is not constant. The range of values for each group j¼1 was based on area considerations with the higher values in which n is the number of river i co-ordinates in the unit. covering smaller areas. The decrease in area size with As the calculation of drainage density is based on the vector higher groups was kept smooth. As the size of the area for (line) data model, it is impossible to deal directly with data each drainage density decreases with increases in its value, from natural lakes, which are planar. However, the the range of values for each group increases generally with development of a lake may indicate subsidence of an area. the drainage densities. The area characterised by drainage Sometimes, a lake occupies the centre of a fault-controlled density of 0–15 is about half of the study region, so groups 1 depression (Han et al., 1995). It is therefore necessary to and 2 have values of 0–5 and 6–15, respectively. Groups 1 account for lakes in the calculation of drainage density, in and 2 are defined as low-drainage densities. Ranks 3 and 4 order to study the impact of vertical tectonic movement of have the same value ranges, but the area of group 3 is larger the Quaternary plain. We simplify lakes as a set of lines and than that of group 4. From group 4 above, the range of the lakes or ponds can appear as areas with high-drainage values for each group increases (Table 1). The values for densities. group 3 or above are referred to as high-drainage densities. Topographic maps with a scale of 1:500,000 are used for The drainage densities are illustrated in Fig. 3. this study (Topographic Bureau, the Headquarters of the Based on the spatial variance of high-drainage density General Staff, China PLA, 1956). The maps were compiled belts, the NCP can be divided into three regions (Fig. 3), 中国科技论文在线______www.paper.edu.cn

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Fig. 2. Drainage in the North China plain, digitised from topographic maps of 1:500,000 (Topographic Bureau, the Headquarters of the General Staff, China PLA, 1956). which are the Beijing–Tianjin–Tangshan region (BTTR) surface deformation, although some non-tectonic factors, in the north (I), Jizhong–Lubei region (JLR) in the such as groundwater pumping, also affect the geodetic- middle (II) and Xingtai region (XR) in the south (III). surveying results (Ying et al., 1986). For example, the The BTTR is crossed by some (about four to five) high- intensive subsidence in Tianjin was partly caused by rapid drainage density belts, trending northeast, and about the pumping of groundwater (Fig. 4). Another important factor same number of belts trending northwest. Fig. 3 also is the occurrence of the 1976 Tangshan M7.8 earthquake, demonstrates that the BTTR is dominated by high- not far from Tianjin. According to contours of surface drainage density. The belts in the BTTR truncate or cross deformation in Fig. 4, the NCP can be divided into as ‘two each other and define some small square blocks. The JLR subsiding regions and one uplifting corridor’ (Gui, 1986). is dominated by low-drainage density (the drainage The two subsiding regions in the north and the south density groups 1 and 2), which is in contrast to the correspond to the BTTR and XR discussed above, which are BTTR. Three narrow, northeast-trending high-drainage dominated by high-drainage density. However, the two density belts developed in the JLR divide this region into boundaries in Fig. 4 are not precisely in the same locations elongated blocks, trending in the northeast direction. The as those in Fig. 3. As to the southern boundaries in Figs. 3 dimensions of blocks, surrounded by high-drainage and 4, the difference in the locations is not significant. The density belts in the JLR, are generally larger than that significant difference between the northern boundaries in of blocks in the BTTR. The drainage density in the JLR Figs. 3 and 4 is probably due to the intensive pumping of changes less rapidly than in the BTTR. The XR in the groundwater in Tianjin megalopolis during the 1970s and south is somewhat similar to the BTTR, dominated by the occurrence of the 1976 Tangshan M7.8 earthquake to the high-drainage density. As this area is of limited extent, north of Tianjin, which pushed the boundary of the subsiding further discussion of the XR will not be included here. region southward. However, it is clear that the uplifted As water is generally abundant in low terrain, drainage is corridor in the middle is about in the same location as the easily developed in these regions. Drainage densities in the JLR with the low-drainage density. The compaction of BTTR, the JLR and the XR indicate that the BTTR and the XR subsided, and the JLR was uplifted during the Holocene. Table 1 The subdivision of the NCP, as determined from the The relationship between drainage density values and groups analysis of drainage density, is partly consistent with geodetic-surveying data from 1968 to 1982, during which Value range 0–5 6–15 16–20 21–25 26–35 36–56 two cycles of levelling were carried out (Fig. 4)(Gui, 1986). Value increment 6 10 5 5 10 20 Group 1 2 3 4 5 6 Tectonic movements provide the most significant control on 中国科技论文在线______www.paper.edu.cn

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Fig. 3. The distribution of drainage density groups.

Quaternary sediments also influences the geodetic data, but hamper the transfer of water and may inhibit drainage the maximum depth of Quaternary sediments across the development. As to the influences of weather or old whole region in the NCP is about 400 m and distributed very topography, their features may be secondary (Han et al., locally (Xu, personal communication). It is generally only 1994). Their significance can be reduced by the large 0–100 m in depth, so the effect from the compaction of window dimension used above in calculating the drainage Quaternary sediments on the geodetic data is very limited. density. Such a window dimension may, however, optimise The spatial relationship between the uplifting and subsiding the impact of regional faulting, or uplift, and subsidence regions in Fig. 4, as demonstrated from the geodetic- patterns on drainage densities. surveying data, is the same as between the low-drainage It seems reasonable to correlate the high-drainage density and high-drainage density regions in Fig. 3. density with tectonics. The Holocene-active Nankou– If we only consider the distribution of the high-drainage Sunhe fault ( in Fig. 5), strikes NW–SE and is covered densities (equal to, and above group 3), Fig. 3 is also by Quaternary deposits in the vicinity of Beijing. It was characterised by numerous belts, even though some are found and described using shallow electro-magnetic long, short, narrow, or wide. All belts interpreted from Fig. 3 surveys, borehole drilling and excavation of trenches are shown in Fig. 5. Their trends can be grouped into two (Xiang et al., 1994) and is coincident with a belt of high- sets, one at an azimuth of 45–508 and another at 315–3208 drainage density. As to the famous Tangshan–Hejian– Fig. 5 inset. Xingtai seismic zone (THXST) ( in Fig. 5), although no Research to identify concealed active faults in the NCP is active faults have been detected, a clear high-drainage-belt very limited, and has only occurred northwest of Beijing exists from Tangshan, through Hejian and into Xingtai. (Xiang et al., 1994). However, active faults or seismic zones Furthermore, the belt is not continuous and comprises correlate with the high-drainage density belts in Fig. 5.The several high-drainage density segments. These segments same conclusion was also made by Hou and Han (1997). provide a basis for the study of the geometry and structure Conversely, fault-gouge in the old, inactive faults might of THXST. The discontinuous nature of THXST, as 中国科技论文在线______www.paper.edu.cn

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Fig. 4. The contour of geodetic-survey data in NCP for 14 years from 1968 to 1982 (modiﬁed from Gui (1986)). determined from high-drainage densities, analogous to the estimate the magnitude and epicentre of the earthquake. early stages of a rock deformation experiment, may also The accuracy of such work may be as good as the show that the zone is newly established in a similar manner distance between a series of towns. In addition to the as described by Schweig and Ellis (1994). The Huanghekou THXST, it seems that earthquakes are also relatively fault is also an active fault zone (From Xu, 2000; personal concentrated along the boundary between the BTTR and communication) ( in Fig. 5), which also corresponds to a the JLR. Most of the strong earthquakes in the NCP are belt of high-drainage density. concentrated in the high-drainage density belts or their The epicentres of the historical and instrumental vicinity, but some high-drainage density belts indicate no earthquakes with magnitudes larger than 5.0 are rep- strong earthquakes over the past 2 ka. It seems contrary resented in Fig. 5. From BC231 to AD1998, 121 to the fundamental assumption that the drainage has earthquakes with M $ 5:0 occurred in the region of developed within <10 ka as a result of tectonics. There E114–1208 longitude and N37–408 latitude. A complete are two types of tectonic movements along a fault, stick seismological catalogue covering more than 2000 years is slip and creep slip. The stick slip can cause a strong available in China due to its long history. The earthquake, but the creep slip releases the tectonic energy characterisation of the earthquakes prior to the introduc- without earthquakes, or with minor earthquakes. Liu et al. tion of seismographs was achieved using historical (2000) showed that crustal shortening in the Andes and records. Every county in China (East China) has its USA were 30–40 mm/a based on GPS measurements, own complete historical record for important events but no more than 12 mm/a as determined from geological spanning more than 2000 years. For example, if a and seismological data. Earthquake activity releases only disastrous earthquake occurred, the historical record part of the tectonic energy in the crust. Besides, if we would include how many people died, how many houses consider the return interval of strong earthquakes in the collapsed and how many villages felt it. Based on the intraplates as 1–10 ka with an average of 5 ka (Kanamori records, the isoseismical intensity can be drawn to and Brodsky, 2001), the earthquake records over the last 中国科技论文在线______www.paper.edu.cn

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Fig. 5. High-drainage density belts and their relationships to strong earthquakes ðM $ 5:0Þ and some known faults or seismic zone; (Inset) the predominant trends of high-drainage density belts.

2 ka are still within one cycle. So it is reasonable high-drainage density have been determined. The trends that some high-density belts may still correspond to of all faults in Fig. 6 were measured. If one fault shows active faults although there were few strong earthquakes a significant change in strike, it was divided into another recorded in these areas to date. segment with a different strike. The dominant trends of Tectonic movements in the NCP during the Cenozoic normal fault systems for the Eocene–Oligocene are 30– are characteristic of two periods (Ding and Lu, 1983; Xu 35 and 300–3058(Fig. 6 inset), while the high-drainage et al., 1985). The NCP was affected by fault-controlled density belts trend at 45–50 and 315–3208 (Fig. 5 inset). basin-range tectonics during the Eocene–Oligocene. The difference between the strike of the Eocene– During the Neogene, the NCP formed a regional Oligocene active faults and the trends of the belts of depression, which has persisted to the Quaternary. Due the high-drainage density are also, for example, reflected to the large number of seismic reflection profiles in the by the famous Taihangshan Range frontal fault zone NCP, the tectonics in the Eocene–Oligocene is well (TRFFZ). The TRFFZ played an important role in known (Fig. 6), but knowledge of the later period is the evolution of the NCP during the Eocene–Oligocene poor, and is limited by the resolution of seismic by controlling the formation of the Jizhong depression reflections on the very top sediment layer. Comparison (Fig. 6). However, there is not a continuous high- of the tectonics during the Eocene–Oligocene (Fig. 6) drainage density belt along the fault zone (Fig. 5), and with the distribution of belts of high-drainage density even the trends of some short, high-drainage density (Fig. 5), shows a variance between them. It is clear that segments were diverted from the surface trace of the the difference between the strikes of the Eocene– TRFFZ. This observation is consistent with the absence Oligocene active faults (Fig. 6) and the trends of belts of major earthquakes of M $ 7:0 along the TRFFZ of high-drainage density is systematic. Given the where only some moderate earthquakes have occurred in uncertainty in the data, statistics for the trends of the historical record. The high-drainage density belts the Eocene–Oligocene active faults and the belts of may represent the latest active events in the NCP, 中国科技论文在线______www.paper.edu.cn

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Fig. 6. The tectonic framework of the NCP in Eocene–Oligocene (modified from Xu et al. (1985)); (Inset) the predominant trends of fault zones. which are different from the previous deformation of However non-tectonic factors, such as the weather and the Eocene–Oligocene. Therefore, the strikes of late- the old natural topography, also have an effect on drainage Quaternary faults deviate by <158 to the earlier evolution and distribution. Based on previous geomorpho- structures. logic studies on the NCP, we accept the conclusion that compared to the tectonic effects; the influence of non- tectonic factors on the drainage is secondary. In this case, 3. Discussion since the drainage density in a window area is averaged, the effects of the tectonics may be enhanced and the influence of Although some disastrous earthquakes, such as the 1966 the secondary, non-tectonic factors reduced. It is therefore Xingtai M7.2 earthquake and the 1976 Tangshan M7.8 reasonable to correlate the high-drainage density belts with earthquake occurred in the NCP, our knowledge of the the tectonics. active tectonics of the NCP is poor. The main reason for this The results of drainage density calculations show that the is that the NCP is a wide plain covered with Quaternary distribution of high-drainage density is characteristic deposits. It is very difficult to detect any active tectonics of not only zonation (belts), but also regionalisation. from the geological survey. This paper tries to address The high-drainage density belts show a good the question using drainage density calculations and relationship with some concealed Holocene-active faults analysis. The drainage in the NCP was established in the and earthquake-concentrated (seismotectonic) zones. The Holocene, beginning about 10–12 ka ago. The distribution low-drainage density and high-drainage density regions also of drainage in the Quaternary-covered plain is closely indicate a close relationship to the uplifted and subsiding correlated to the active fault zones and uplift or subsidence regions as demonstrated by the geodetic-surveying results of the ground, created by the Holocene active tectonics. The from 1968 to 1982, although the pumping of groundwater boundaries of regions with different drainage densities can pushed the boundary between the BTTR and the JLR be correlated with concealed fault zones and distinguished southward. by the belts of high-drainage density. The uplifted regions, The high-drainage density belts in the NCP also show an where water can easily flow away, have a lower drainage apparent variance to the strike of Eocene–Oligocene density while the subsided regions, where water accumu- structures, which was mainly acquired from the interpret- lates, have a higher drainage density. ation of seismic reflection profiles and confirmed by borehole 中国科技论文在线______www.paper.edu.cn

Z. Han et al. / Journal of Asian Earth Sciences 21 (2003) 989–998 997 data. It seems that the latest active deformation is not a Quaternary) there is an average 158 difference in the continuation of the previous deformation, but may instead strike directions. This new phase has important reflect a change in the tectonic conditions. There has been an consequences for the study of seismic hazards. evolution of the fault pattern through time, so that at two widely spaced epochs (Eocene–Oligocene and late Qua- ternary) there is an average difference of 158 in overall strike. Acknowledgements Thus seismic hazard could not be evaluated properly if the determination of potential seismic sources were based on the This study is supported by China high-priority basic Eocene–Oligocene structures in the NCP. Some fault zones science planning project (No. 95-13-01-05) and China with a long evolutionary history are usually clear geomorphic features, but closer relationships exist between the newly Earthquake Science Foundation (No. 197062). The established fault zones and strong earthquakes, even though authors are thankful to Drs Andy Nicol and McVerry the latter structures often have no topographic expression. Graeme for English writing, Dr Peizhen Zhang for his Although the calculation and analysis of the drainage encouragement, Prof. Xu Jie for his discussion about the density in the NCP provides some interesting results, there latest active framework in NCP and Prof. Xiang Hongfa are some uncertainties in our calculation of the drainage for shearing his ideas about active tectonics in the density. Specifically, simplification in the calculation of the vicinity of Beijing. drainage density causes a slight change in the actual length of rivers. Since the change occurs in every cell, we assume that such simplification does not affect the general pattern of the drainage density distribution. Of course, in order to References make the results more reliable, a more precise calculation model is still required. However, the major problem may Ding, G., Lu, Y., 1983. Discussion of the basic features about neotectonic come from the effect of human activity, especially for a appearance in North China platform. Earthquake Science in North country like China with a long history. If possible, the old China 1, 1–9. topographic maps should be used. Feng, R., Huang, G., Zheng, S., Wang, J., Yan, H., Zhang, R., 1989. Crust tectonics and earthquakes in North China. Acta Geologica Sinica 63, 111–124. Gui, K., 1986. The crust movement and surface deformation in North China 4. Conclusions plain. Geodetic Survey 4, 24–34. Guo, S., Xiang, H., Zhou, R., Xu, X., Dong, X., Zhang, W., 2000. The drainage density spatial distribution shows a close Longling–Lancang fault zone in southwest Yunnan, China. Chinese relationship to the current tectonics in the NCP. Based on Science Bulletin 45, 376–379. Han, M., Hou, J., Zhao, J., Zhu, S., 1994. The method about the usage of the calculation of the drainage density and the analysis of its drainage to study the concealed tectonic movement in Holocene. In: distribution in the NCP, some interesting results can be Committee of Seismo-geology, China Seismological Society (Ed.), summarized as follows: Research on Active Faults in China. Seismological Press, Beijing, pp. 291–295. 1. The distribution of high-drainage density in the NCP is Han, Z., Xu, J., Guo, S., Yang, Z., Xiang, H., Wu, D., 1995. Research on tectonic features and dynamics in the Northwestern Yunnan extensional characteristic of not only zonation, but also regionalisa- region. Earthquake Research in China 9 (2). tion, which outlines a tectonic pattern of the NCP for the Han, Z., Zhang, P., Wu, L., Hou, J., 1998. Characters about the modern latest phase of the Cenozoic. movement of North Qilianshan block. In: Department of Geology, 2. The differences in the trends, continuity and structure Peking University (Ed.), Collection of International Conference on of the high-drainage density belts make it possible to Geological Science in Peking University. Seismological Press, divide the NCP into three regions. They are the BTTR Beijing. Hou, J., Han, M., 1997. A morphometric method to determine with high-drainage density in the north, the JLR with neotectonic activity of the Weihe basin in Northwestern China. low-drainage density in the middle and the XR with Episodes 20, 95–99. high-drainage density in the south. Similar regionali- Hovius, N., 1996. Regular spacing of drainage outlets from linear mountain sation also appears in the contours of geodetic-survey belts. Basin Research 8, 29–44. data from 1968 to 1982 for the NCP, which show Jackson, J., Leeder, M., 1994. Drainage systems and the development of normal faults: an example from Pleasant Valley, Nevada. Journal of two regions of subsidence separated by an uplifted Structural Geology 16, 1041–1059. region. Jackson, J., Norris, R.J., Youngson, J., 1996. The structural evolution of 3. There are two sets of high-drainage density belts, one active fault and fold systems in central Otago, New Zealand, evidence striking 45–508 and another striking 315–3208, which revealed by drainage patterns. Journal of Structural Geology 18, are parallel to the concealed Holocene-active faults 217–234. Jackson, J., Van Dissen, R., Berryman, K., 1998. Tilting of active folds and and seismotectonic zone. There has been an evolution faults in the Manawatu region, New Zealand: evidence from surface of the fault patterns through time, so that at two drainage patterns. New Zealand Journal of Geology and Geophysics 41, widely spaced epochs (Eocene–Oligocene and late 377–385. 中国科技论文在线______www.paper.edu.cn

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Kanamori, H., Brodsky, E., 2001. The physics of earthquakes. Physics Tomkin, J.H., Braun, J., 1999. Simple models of drainage reorganisation on Today June, 34–40. a tectonically active ridge system. New Zealand Journal of Geology and Leeder, M., Jackson, J., 1993. The interaction between normal faulting Geophysics 42, 1–10. and drainage in active extensional basins, with examples from the Wang, R., Guo, L., Han, M., Ye, Y., 1999. Research on concealed active western United States and central Greece. Basin Research 5, tectonics in Holocene in Tangshan and Xingtai seismic-regions. 79–102. Geodetic Survey 16, 2–7. Liu, M., Yang, Y., Steun, S., Zhu, Y., Engeln, J., 2000. Crustal shortening in Xiang, H., Fang, Z., Jia, S., Zhang, W., Li, R., 1994. Research on Concealed the Andes: why do GPS rates differ from geological rates? Geophysics Fault and its Application in Engineering, Seismological Press, Beijing, Research Letters 27, 3005–3008. pp. 68–102. Ma, Z., Fu, Z., Zhang, C.., Wang, C., Zhang, G., Liu, D., 1982. The Nine Xu, J., Hong, H., Zhao, G., 1985. Evolution and mechanic features of the Major Earthquakes in China (1966–1976), Seismological Press, Cenozoic rift basin in North China plain. The Research on Modern Beijing, pp. 1–5. Crust Movement 1, 26–40. McCalpin, J., Nishenko, S.P., 1996. Holocene paleoseismicity, temporal Xu, J., Han, Z., Wang, C., Niu, L., 1996. Preliminary study on two newly- clustering and probabilities of future large ðM . 7Þ earthquakes on the established seismotectonic zones in north and southwest China. Wasatch fault zone, Utah. Journal of Geophysics Research 101, Earthquake Research in China 10, 393–401. 6233–6253. Yang, L., 1987. Xingtai–Hejian–Tangshan strong-earthquake zone is a Ran, Y., Deng, Q., 1999. History, status quo and development tendency of seismotectonic-rupture zone. Earthquake Research in China 3, paleoseismology. Chinese Science Bulletin 45, 12–20. 61–66. Schweig, E.S., Ellis, M.A., 1994. Reconciling short recurrence intervals Ying, S., Shen, Y., Guo, L., 1986. The modern tectonic with minor deformation in the New Madrid seismic zone. Science 264, movement surrounding Bohei Sea. Earthquake Research in China 1306–1311. 2, 27–35.