Experimental study and active tectonics on the Zhangjiakou-Penglai fault zone across North China

Accepted Manuscript Experimental study and active tectonics on the Zhangjiakou-Penglai Fault Zone across North China Lingli Guo, Sanzhong Li, Yanhui Suo, Yuntao Ji, Liming Dai, Shan Yu, Xin Liu, Ian.D. Somerville PII: DOI: Reference:

S1367-9120(15)00194-7 http://dx.doi.org/10.1016/j.jseaes.2015.03.045 JAES 2325

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

21 January 2015 26 March 2015 29 March 2015

Please cite this article as: Guo, L., Li, S., Suo, Y., Ji, Y., Dai, L., Yu, S., Liu, X., Somerville, Ian.D., Experimental study and active tectonics on the Zhangjiakou-Penglai Fault Zone across North China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.03.045

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Experimental study and active tectonics on the

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Zhangjiakou-Penglai Fault Zone across North China

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Lingli Guo1,2, Sanzhong Li1,2*, Yanhui Suo1,2, Yuntao Ji3, Liming Dai1,2, Shan Yu 1,2,

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Xin Liu1,2 Ian. D. Somerville4

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1. College of Marine Geosciences, Ocean University of China, Qingdao, China,

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266100

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2. Key Lab of Submarine Geosciences and Exploration Techniques, Ministry of

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Education, Qingdao, China, 266100

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3.The state key laboratory of Earthquake Dynamics, Institute of Geology, China

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Earthquake Administration, Beijing, China, 100029

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4. UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4,

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Ireland

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Corresponding author: Sanzhong Li

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College of Marine Geosciences, Ocean University of China,

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No. 238, Songling Road, 266100, Qingdao,

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Shandong Province, China

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Email: [email protected]

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Tel: +86-532-66781971 (office)

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1

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Abstract: The Zhangjiakou-Penglai Fault Zone (ZPFZ), as a large-scale

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WNW-trending active fault zone with frequent seismic activity, is an area prone to

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moderate to intense earthquakes across North China. A detailed analysis of the ZPFZ

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has been carried out based on earthquake activity, deep structural response, dynamic

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mechanism and physical simulation by conducting an en échelon fault model

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experiment. The results show that the ZPFZ is a lithospheric active fault zone that has

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controlled some small en échelon Quaternary pull-apart basins and has triggered

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many earthquakes as seismogenic structures. The earthquake location and intensity

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distribution by the experimental simulation fit well with those of actual earthquakes.

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The earthquake distribution is controlled obviously by the en échelon structural

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properties. The intensity of seismic activity is strong in the middle and weak at the

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western and eastern ends of the fault zone, migrating from the southeast to the

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northwest along the entire fault zone. This WNW-trending fault zone in North China,

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is closely related to the surrounding plate activities. The ZPFZ is not only dominated

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by the collision of the Indian Plate to the Eurasian Plate, but is also influenced by the

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subduction of the Pacific Plate. It depends on the comprehensive reflection of two

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geodynamic settings from the western collision of the Indian to the Eurasian plates

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and the eastern subduction of the Pacific Ocean Plate to the Eurasian Plate.

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Key words: Zhangjiakou-Penglai Fault Zone; seismic activity; en échelon fault

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experiment; en échelon fault interaction; plate boundary 2

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1. Introduction

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There are complex and various types of structures in East China under a

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long-lived evolutionary process. Besides the Mesozoic NNE- and NE-trending faults,

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many Cenozoic NW- and WNW-trending faults developed in East China, which

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include normal, reverse and strike-slip faults (Li et al, 1983a; 1983b; Li, 1992).

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Compared to the continuous and large-scale NNE- or NE- trending faults systems, the

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NW- and WNW-trending fault systems are less conspicuous and diffuse in East China

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and consequently are less well known and underappreciated until now. However, their

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close correlation with earthquakes, ore deposits formation, and hydrology has

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attracted the attention of many researchers (Chen, 1978; 1984; Shao, 1980; Xu, 1982;

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Hu et al, 2005).

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The Zhangjiakou-Penglai Fault Zone (ZPFZ) is considered to be one of the

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large-scale and active tectonic zones that control the distribution of earthquakes in

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North China (Ma, 1987; Xu et al., 1998a, 1998b; Wang et al., 2004; Cao et al., 2013;

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Suo et al., 2013). It extends in a NW-SE-trending direction for over a distance of

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about 700 km (Fig. 1), which starts west of Zhangjiakou City, passes through Huailai,

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Nankou, Shunyi, Sanhe and Tianjin, and ends at Penglai and Yantai. There are two

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major active tectonic zones in the Bohai Bay in the Cenozoic, one is the

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Zhangjiakou-Penglai Fault Zone, and the other is the NNE-trending Tanlu Fault Zone

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(Xu et al, 1997; 1998a and 1998b; Gao et al, 2001; Deng, 2001; 2002; 2006; Zhu et al,

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2006; Li et al, 2009). Many large earthquakes frequently occurred along their

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secondary faults (Fig.1). More than 26 earthquakes with magnitude (M) larger than 3

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6.0 have occurred in this region during the past 2000 years, of which one event is M=

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8.0 and six events are larger than M= 7.0 to 7.9. In 1679, an earthquake of M= 8.0

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occurred in the Sanhe County of Beijing, which is the largest one among the known

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historical earthquakes in this region. The 1976 Tangshan earthquake (M= 7.8), located

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150 km east of Beijing, totally destroyed the city of Tangshan with a population of

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one million and killed about 240,000 people. It was perhaps the most destructive

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earthquake in the world recorded in human history. Therefore, the ZPFZ is a much

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more significant and important fault system than the other fault zones in the region.

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The ZPFZ controls a series of small Quaternary basins with rhombus shape and

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left stepping en échelon distribution of secondary faults. According to field

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observations, the onland segment of the ZPFZ is composed of the WNW-trending and

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nearly EW-trending faults (You et al., 2008), and constitutes a flower-like structure

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with normal faults and sinistral strike-slip faults. The transition of the northward- to

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southward-flowing rivers to the eastward flowing at the southern piedmont of the

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Yanshan Mountain shows that sinistral horizontal dislocation existed in this fault zone

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(Ding and Lu, 1983). NW-trending source fault plane is a property of sinistral

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strike-slipping (Xu et al., 1998). The three subsidence centres in the Neogene Bohai

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Bay Basin (called the Wuqing, the Qikou and Bozhong depressions) distributes

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inclinedly along this WNW-trending fault zone (Xu et al., 1985). Those previous

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researches proved that the ZPFZ is a Neogene sinistral strike-slipping en échelon

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structural zones. The deep gravity anomaly in the area revealed that it is a single main

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fracture at depth and splays into multiple-branched faults at shallow depth following 4

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the growth faulting (Suo et al., 2012; Zhang et al., 1994). It did not develop a unified

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single cross-cutting fault in the shallow crust, but form more than 20 NW-trending

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discrete secondary fault zones accompanying with a series of the small Quaternary

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pull-apart basins. Therefore, it is overall a NW-trending lithospheric-scale en échelon

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fault zone.

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En échelon faults are a common fault combination in the crust. The interactions

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among en échelon faults play an important role in the formation and development of

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the fault zone, which has obviously controlled the seismic activity and the

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propagation of earthquake rupture. As a kind of typical and important seismogenic

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structures, the en échelon fault model has attracted much attention by many

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researchers in seismology, geology and other research fields in recent years. En

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échelon fault pattern can be observed in a wide range of length scales. They can be

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some 20 km long in the San Andreas Fault (Segall and Pollard, 1980), while in

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mining-induced normal faults observed in the South Africa gold mines (Gay and

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Ortlepp, 1979), en échelon faults measured in centimeters can also be found. The

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observed en échelon cracks are even smaller in rock samples stressed in laboratory

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(Ewy and Cook, 1990; Saimoto et al., 2003). Seismologic evidence indicates that

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some earthquakes tend to cluster near en échelon faults (Segall and Pollard, 1980;

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Sibson, 1985). Geological evidence indicates that some basins and ranges can be

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formed in jogs (Aydin and Schultz, 1990; Zachariasen and Sieh, 1995). Therefore,

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considerable attention has been given to the problems of deformation, failure process

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and stability of en échelon fault (Bomblakis, 1973; Ma et al.,1986; Swanson, 1990; 5

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Olson and Pollard, 1991; Smith, 1992; Thomas and Pollard, 1993; Wong and Chau

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1997; Talwani, 1999; Wong et al. 2001; Wong et al. 2002; De Joussineau et al., 2003;

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Barker, 2005; Cembrano et al., 2005; Smith and Yamauchi, 2005; Wong et al. 2006;

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Soliva et al., 2010; Guo et al., 2013; 2014a; 2014b;). The deformation and failure

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characteristics have been analyzed from the field observatory data (Sibson, 1985;

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Aydin and Schultz, 1990; Wesnousky, 2006). In addition, using a number of numerical

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methods, such as finite difference methods (Harris and Day, 1993, 1999), finite

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element methods (Liu et al., 1998; Tang et al., 2001; De Joussineau et al., 2003;

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Palusznya and Matthäib, 2009; Wang et al., 2011; 2013), and boundary element

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methods (Aydin and Schultz, 1990; Olson and Pollard, 1991; Du and Aydin, 1993,

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1995; Ohlmacher and Berendsen, 2005), and a variety of experimental techniques,

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such as acoustic emission techniques (Jiang et al., 2002; Ma et al., 2008b; Liu et al.,

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2008), optical methods (De Joussineau et al., 2003; Chen et al., 2005; Wong et al.

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2006; Ma et al., 2008a, 2010; Soliva et al., 2010; Guo et al., 2013; Yin et al., 2014),

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and infrared thermal imaging systems (Liu et al., 2007; Ma et al., 2007, 2008a, 2010;

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Ren et al., 2013), intensive studies have been conducted to investigate the

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deformation, failure, precursor, and stability of en échelon faults. These investigations

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are especially important for us to obtain a full understanding of en échelon fault

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interactions and various complex seismic or geological phenomena.

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The ZPFZ manifests itself as a combination of en échelon structures. A detailed

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investigation of the crustal architecture and seismotectonics of this region is very

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important for the understanding of the physics of intracontinental earthquakes and for 6

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the assessment and mitigation of earthquake hazards. This paper comprises a detailed

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analysis of the ZPFZ in earthquake activities, deep-seated structures, kinematics, and

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physical simulation on conducting the en échelon faults model experiment. Then this

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paper will compare the physical simulation results with the actual earthquake situation

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to further reveal the causative mechanism and factors of earthquakes controlled by the

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fault zone.

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2. Regional geological setting

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The present study area is located in the intersection of the Taihangshan and the

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Yanshan uplift regions. The North China Plain in the central part of the study area is a

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large-scale epicontinental basin being characterized by alternative uplift and

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depression zones in space (Fig. 1). To the western and northwestern parts are the

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Taihangshan uplift region with some small-scale intermountain basins To the

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northeastern part is the relatively stable Yanshan uplift region with its major

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E–W-trending structures (Li, 1981; Ye et al., 1987; Liu, 1987). Many

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NE–SW-striking active faults developed in the North China Plain and the

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Taihangshan uplift region. In the two regions, all the structures and mountain ranges

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are NE–SW-trending. In our study area, the WNW-trending ZPFZ is the most active

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one that contains a majority of large earthquakes (Fig. 1).

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In terms of the regional tectonic background, the ZPFZ cuts across some

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secondary tectonic units with different geological time. These tectonic units are the

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North China Craton basement, the Mesozoic Yanshan Orogenic Belt, the Cenozoic

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Bohai Bay Basin and the Tanlu Fault Zone from northwest to southeast (Li et al., 7

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2012a; 2012b; 2012c; 2013). From the landscape, the ZPFZ traverses the Cenozoic

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Yanshan and Taihangshan uplift belts and the North China Plain from west to east.

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The tectonic framework and geophysical fields are obviously different on both sides

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of this fault zone which is dominated by NNE- to NE-trending faults on the southwest

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side and ENE – or EW-trending faults on the northeast side (Zheng et al., 1981; Feng,

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1988; Gao et al., 2001). The ZPFZ developed in the Archean and Paleoproterozoic

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basement as a ductile shear belt. However the brittle fault zone was observed in the

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Mesozoic. In the Paleogene, only the middle segment of the fault zone was active, and

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as a transtensional fault zone in the Bohai Bay Basin, modulating the different

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structural patterns and extension of the northeast and middle sections of the basin.

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Since the Neogene, i.e. the neotectonic period, the fault zone extended and propagated

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toward northwestward and southeastward, and developed into a unified fault zone and

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seismotectonic zone. This fault zone has played a critical role in the sedimentary,

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magmatic activity and structures on both sides at different evolution stages. It is not

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only an important tectonic zone in the Bohai Bay Basin, but also a critical tectonic

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transfer zone distributed within the Cenozoic rift basins in the Yellow Sea and the

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East China Sea regions (Xu et al., 1994; Hao et al., 2003). The ZPFZ is the most

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active seismic zone that contains a majority of large-scale earthquakes across North

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China. It has important implications for tectonic division, regional structural evolution

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and seismic activity.

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Most of the study area is popularly covered by Quaternary sediments. The

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Archean and Proterozoic basement is less exposed, and the Jurassic and Cretaceous 8

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strata with middle-late Jurassic and early Cretaceous granites and granodiorites are

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locally exhumed.

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The ZPFZ is composed of a lot of WNW- and ESE-trending left stepping en

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échelon short fractures (Fig. 1). The fault zone intersects with the NNE- and

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NE-trending Yanqing-Weixian Fault (F2), the Zijingguan Fault (F3), the Taihangshan

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Fault Zone (F4), the Xiadian Fault Zone (F5), the Cangdong Fault Zone (F6), the

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Yingkou - Weifang Fault Zone (F8), as a Bohai segment of the Tanlu Fault Zone, and

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the Huanghekou- Liaocheng-Lankao Fault Zone (F7), the Penglai-Zhaoyuan Fault

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(F9), the Taocun-Dongdoushan Fault (F10) along the strike direction, respectively. It

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has a complex relationship with other WNW-trending faults and NE-trending faults,

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showing a distinct fault segmentation. From WNW to ESE, this fault zone can be

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subdivided into four segments: the Zhangbei-Huailai segment and the Beijing-Tianjin

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segment on land, and the Bohai segment and the Penglai-Weihai segment at sea (Xu

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et al., 1994, 1998a and 1998b).

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3 Seismic geological characteristics in the ZPFZ

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3.1. Seismic activity

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The Zhangjiakou-Penglai Fault Zone, is a huge WNW-trending active fault zone

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with frequent seismic activity, being dominated by moderate to strong earthquakes

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across North China (Fu et al., 2000; Diao et al., 2006). The focal mechanism analysis

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shows that most of these earthquakes are controlled by strike-slip faulting (Ma et al,

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2004), which is closely related to the kinetics of this fault and depends on the

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differential movement between the north and the south tectonic blocks (Xu et al., 9

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1994).

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Most of the earthquakes on the fault zone, especially the M>6 earthquakes, are

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not distributed uniformly along the fault zone, but instead tend to cluster at the

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intersections of the WNW-trending and the NE-trending fault zones. For instance, the

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Zhangbei M=6.2 Earthquake in 1998 occurred at the intersection point of the fault

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zone to the Daihai- Huangqihai Rift Belt (Xu et al., 1998). There were four occasions

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of M= 6 to 6.9 earthquakes in the Zhangjiakou-Yanqing region which is at the

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intersection segment between the ZPFZ and the Shanxi Rift Belt. The Beijing-Sanhe

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segment, at the intersection of the ZPFZ to the Huangzhuang- Gaoliying and Xiadian

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fault zones, has recorded three earthquakes of M= 6 to 6.9 and one earthquake of

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M=8.0. The Tangshan M=7.8 earthquake in 1976 occurred at the intersection point

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between the ZPFZ and the Cangdong Fault Zone. The M= 6 to 6.9 earthquake and

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three M=7 to 7.9 earthquakes occurred in the central Bohai Bay segment, at the

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intersection of the ZPFZ and the Huanghekou-Miaoxibei Fault Zone. The large-scale

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one in the Bohai Bay is the M=7.4 magnitude earthquake in 1969. The Penglai-Yantai

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segment at the intersection of the ZPFZ to the Taoshan-Dongdoushan Fault Zone,

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yielding one M= 6 to 6.9 earthquake and one M=7.0 earthquake.

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The focal mechanism solutions at the ZPFZ mainly focus on two types:

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NE-trending dextral strike-slip and WNW-trending left-lateral strike-slip seismogenic

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faults. There ware different habits and activities at different strong earthquake

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sections along the ZPFZ. Sinistral offset is dominated at the northwest section of the

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seismogenic fault, such as the 1998 Zhangbei Earthquake and the 1720 Huailai 10

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Earthquake. Dextral offset is dominated at the middle and southeast parts, such as the

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1976 Tangshan Earthquake and the 1969 Bohai Sea Earthquake. The northwestern

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segment is propagated to the northwest since Neogene so that the NW-trending fault

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had stronger activity. The unified tectonic motion released and the activity decreased

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at the middle and southeast segments due to the more strong activity induced by

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intersection of NE-trending faults.

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3.2. Deep-seated tectonic respones

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The ZPFZ, as an active sinistral strike-slip fault zone in the late Quaternary, is

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not a continuous fault on the Earth’s surface, but consists of more than 20 discrete

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secondary WNW-trending faults (Fig. 1). The geophysical fields of gravity and

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aeromagnetics in North China show that an obvious WNW-trending disturbance is

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distributed along the ZPFZ and extends southeastward into Bohai Bay and finally to

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the north of the Jiaodong Peninsula (Gao et al., 2001).

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Based on the deep artificial seismic exploration, the Moho at this region is

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obvious, and the deep-seated fault extends to the Moho at depth, indicating that it is

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a crustal- or lithospheric-scale fault. which caused the uneven distribution of the

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substances and density on both sides of this fault zone, and controlled or separated the

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structures of the entire crust in the area (Suo et al., 2013; Zhang et al, 1998; Liu and

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Liu, 1984).

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The depth and shape of the Moho under the fault zone and adjacent areas have

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been obtained according to deep seismic reflection combined with gravity data

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inversion and travel-time tomography (Zhang et al, 1994; Yu et al., 2003; Huang and 11

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Zhao, 2004; Qi et al., 2006; Lei et al. 2008). The seismic surface of Moho depths (Fig.

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2) shows that the ZPFZ is obviously controlled by the Moho and crustal thickness,

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and reveals that the ZPFZ is overall a NW-trending lithospheric-scale en échelon fault

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zone (Liu and Liu, 1982).

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The deep-level anomaly revealed that the single NW-trending main fault at depth

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splits into a number of NW- or WNW-trending faults towards the shallow levels; it

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does not form a unified single cross-cutting fault at the Earth’s surface. The

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arrangement of the Quaternary depocentres also shows that a series of NW-trending

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Quaternary pull-apart basins were formed with the corresponding shallow

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left-stepping sinistral faults.

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3.3. Dynamic mechanism

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Generally, the Daxinganling-Taihang Mountains-Wuling Mountain gravity

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gradient zone in East China is an important anomaly belt in gravity, magnetics and

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geology from the surface to deeper levels. This belt is the eastern boundary of a

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triangular topography within the East Asia Continent, being related inevitably to the

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indentation of the Indian Plate to the Eurasian Plate (Li et al., 2010; Zhao, 2009). The

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ZPFZ in the North China Plain is consistent with the NW- or WNW-trending fault

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zones at the East Asia continental margin, which shows that it is closely related to the

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subduction of the Pacific Plate.

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The tectonic stress field at the onland segment of the ZPFZ is relatively simple

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and mainly strike-slip type (Zhang et al., 2009). Its second principal stress axis is

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substantially upright, and the maximum and minimum principal stress axes are nearly 12

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horizontal. The tectonic stress is dominated by the horizontal stress. Therefore, the

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WNW-trending fault is impossible to link with the Quaternary deep mantle plumes

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which is associated with the Pacific Ocean subduction. This dynamic background is

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related inevitably to the activities of the surrounding plate boundaries. The stress axis

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directions showed a clockwise rotation from northwest-directed to southeast-directed,

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indicating that it has a differential rotation between the adjacent blocks and results in

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a sinistral strike slipping faulting along the WNW-trending fault zone during the

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processes of intraplate response to the forces from plate boundaries.

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According to the stress source and tectonic stress field analysis, it can be divided

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into the eastern stress domain including the NNE-oriented stress field and

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south-oriented stress field, and the western stress domain including the Xinjiang stress

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field and the Tibet Plateau stress field by the Daxinganling- Taihang Mountains-

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Wuling Mountain Gravity Gradient Zone (Li et al. 2010; 2011). The former is mainly

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controlled by the combined effect of subduction of the Pacific Plate and the Philippine

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Plate, showing the ENE-WSW-directed extrusion. The latter is mainly affected by the

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collision of the Indian Plate to the Eurasian Plate, showing the nearly NS- and NNE-

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directed extrusion. The ZPFZ straddled across both stress domains, so it is possibly

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controlled by the combined geodynamic effects from the India and Pacific plates.

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Neogene rifting in the western part of North China was significantly intensive

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than that in the eastern part. The Okinawa Trough and other back-arc regions since

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Neogene is strongly influenced by the rifting caused by the western Pacific Ocean

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subduction. However, the western part is away from the western Pacific subduction 13

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zone. Therefore, during the neotectonic period, the eastern segment of the ZPFZ is

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affected by the ENE-WSW-oriented subduction of the Pacific Plate, but the western

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segment is controlled mainly by the NS- to NNE-oriented extrusion due to indentation

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of the India Plate to the Eurasian Plate.

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GPS observations show that the whole of the North China Plain is moving

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eastward and the south side of the ZPFZ, has a motion rate of 2 ~ 4 mm/a faster than

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the north side (Fig. 3). It reveals that the ZPFZ as the northern border of the North

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China Plain has an obvious sinistral strike-slipping offset (Zhang et al., 2002; 2005;

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Niu et al., 2005). The fault is a transpressional property according to the GPS motion

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rates and strain rates. The western segment shows sinistral strike-slip faulting and the

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eastern segment displays mainly compressive characteristics. The fault zone as a

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whole is under compression based on the strain rates.

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4. Physical simulation on en échelon faults

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From a seismic activity perspective, the ZPFZ has strong seismic activity and

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triggered many earthquakes in historical times. Earthquake distribution is controlled

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obviously by the en échelon fault zone. The ZPFZ as a lithosphere-scale fault zone

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from deep tectonics underwent compression to develop an en échelon fault zone

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dominated by the collision of the Indian Plate with the Eurasian Plate and was

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influenced by the subduction kinematics of the Pacific Plate . The simplified model

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has been established according to the structural feature of the ZPFZ. An en échelon

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fault simulation experiment has been conducted to study the earthquake propagation

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and to analyze the seismic features and stress field, prior to discussing and evaluating 14

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the earthquake mechanism and the accompanying hazards.

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4.1. Experimental design

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Experimental samples of size 300 mm × 200 mm × 50 mm were made of the

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Fangshan granodiorite close to the ZPFZ. This type of rock belongs to a massive

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structure and is of medium-fine grained equigranular texture. Granodiorite was

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chosen as a fundamental analog material for earthquake simulation due to its rigid

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physical properties since 1980. The Fangshan Granodiorite with stable properties and

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low grade metamorphism, can be on behalf of the basement material in this area.

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The two en échelon faults of 30° from the long axis direction were cut vertically

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in the middle of the sample surface using a thin circle diamond blade of 50 mm

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diameter. The pre-existing faults were filled with plaster. The bridge width was 30

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mm and the overlapping length of the two faults along the strike direction was 30 mm,

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which was a compressional en échelon structure with a strong interaction between the

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two faults. The tested sample and acoustic emission (AE) sensor’ distribution are

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illustrated in Fig. 4a. A total of 16 AE sensors were placed on both sample surfaces

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for monitoring the deformation and the development of micro-fractures. After the

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experiment, a 3D AE location analysis was carried out with a newly developed 3D

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AE location procedure (Liu et al., 2007). The deformation of the front surface was

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measured by the DSCM system (digital speckle correlation method) for the stress

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field analysis during the fault propagation process. The dotted square area in Fig. 4a is

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the observation area of the DSCM system. The DSCM system, based on the analysis

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of the digital white-light images, can achieve a higher spatial resolution from the 15

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whole strain field of the sample (Zhou and Goodson, 2001; Ma et al., 2002; 2004;

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Zhang et al., 2003; Zhao and Ma, 2009).

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A two-way horizontal compressor with a servo control system was used in this

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study. The control parameters and loading rate could be freely switched during testing.

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In this experiment, the left and bottom sides had been fixed, the right and top sides

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were free ends. The loading was first synchronously applied to the samples, with a

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loading rate of 0.02 MPa per second. When a stress was applied in the two directions

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of up to 5 MPa, the loading along the Y direction was then stopped and maintained at

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5 MPa. At the same time, the loading applied along the X direction was switched to

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the displacement rate of 0.5 µm per second until the experiment finished. Fig. 4b

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shows the variation of differential stress with time.

341

4.2. Experimental results

342

The AE activity as shown in Fig. 4c indicates that the sample begins to deform

343

and coordinates with the load, and the AE events occur initially in the periphery of the

344

two en échelon faults. The AE activity increases quickly and tends to become dense

345

gradually overall with the increase of the differential stress. Before the peak stress, the

346

AE increases dramatically over a short time period. The AE distribution showed

347

strong localized clusters at the jog spatially. The differential stress has declined from

348

its peak value, and the coalescence of the jog occurs with the coordinating role of the

349

two faults.

350

Fig. 5 shows the failure process of the en échelon faults. The stress-time curve

351

can be roughly divided into four stages based on its slope by Points A - D in Fig. 4b. 16

352

In Fig. 5 as the stress-time graph in Fig. 4b, Points A - D in the latter correspond to

353

parts a - o of the former. The sample image, the x direction displacement (Dx), the

354

maximum shear strain (εxy) and the distribution of the AE locations at each stage has

355

been discussed in Fig. 5.

356

At the initial stage, the differential stress at point A is at a low loading level, and

357

the value of Dx and εxy change diminutively and are distributed uniformly across the

358

sample surface. The phenomenon of displacement and strain concentration is

359

not obvious. There is no micro-fracture because the AE event does not occur at this

360

stage.

361

The stage from Point A to Point B phases is elastic deformation stage. The x

362

direction displacement at the outside of the jog increases with differential stress

363

loading and the bridge area is compressive (Fig. 5f). As shown in Fig. 5j, the zones of

364

maximum shear strain concentration are localized in the vicinities of the tips of the

365

pre-existing faults, and coincided with the locations where cracks marked as white

366

patches were observed in the sample shown in Fig. 5b. The AE events with

367

lesser magnitude are concentrated basically inside the jog (Fig. 5m), showing a strain

368

increasing process of the jog. The activity of AE events during this period is related to

369

the formation of a macroscopic crack and extends to the outside of the jog with the

370

corresponding directions of maximum shear strains shown in Fig. 5j.

371

During the weakening stage before the instability (from B to C), the compression

372

of the jog is more obvious, and the relative displacement on both sides is greater with

373

continued differential stress loading, as shown in Fig. 5g. The zones of the maximum 17

374

shear strain concentration extended to the outside of the jog along the tips of the

375

pre-existing faults. The primary direction of the large AE distribution changes to the

376

horizontal direction approximately (Fig. 5n) with the corresponding directions of

377

maximum shear strain concentration shown in Fig. 5k. This pattern coincides with the

378

main crack occurring during the instability (Fig. 5c), indicating the formation of the

379

weakening belt of the crack.

380

The stage from point C to D showed the propagation of the local micro-cracks at

381

the vicinities of the jog. The AE spatial distribution shows a strong localization during

382

this period. After the peak stress and before the instability of the jog, the differential

383

stress decreases with time. Owing to the interaction between two pre-existing faults is

384

obvious at the jog, a lot of small AE events occurred in a short time period and

385

developed a wide weakened zone first, and then the localization occurs at the centre

386

of the zone and the macroscopic crack developed subsequently (Figs. 5h and 5l).

387

There are several local crack propagations and energy release to prepare for the

388

coalescence of the jog at this stage. An obvious localization zone with a lot of AE

389

events is formed at the centre of the weakened zone. Multiple secondary cracks

390

appeared at the zones of maximum shear strain concentration. Those secondary cracks

391

connected each other to the macroscopic crack weakened zone. The intensive AE

392

events with the propagation of the secondary cracks showed that the synergistic

393

effects of en échelon faults played an important role in an earthquake process.

394

5. Seismicity analysis of the ZPFZ based on experimental results

395

By comparing the M - t contrast diagram of the earthquake catalogue of the 18

396

ZPFZ and the AE location from experimental simulation (Fig. 6), it can be seen that

397

AE activity similar with the earthquake activity along the fault zone. As mentioned

398

above, the ZPFZ is affected by the activity of the surrounding blocks. The

399

tectonic conditions are more complex than those in the experimental simulation, so

400

that there is a certain bias in distribution direction between the AE location and the

401

earthquake activity.

402

Earthquake distribution on the ZPFZ (Fig. 6a) showed that the intensity of

403

seismic activity is strong in the middle and weak at the west and east end of the fault

404

zone. The AE location distribution (Figs. 5m-p, Fig. 6b) shows that the distribution of

405

AE events is significantly affected by the interaction of en échelon faults. The outer

406

edge of the pre-existing faults with weak fault interaction appeared less AE events

407

with less energy, and the large AE events distributed at the jog with strong interaction.

408

It is in line with earthquakes recording along this fault zone.

409

The locations of earthquake distribution migrated from southeast to northwest

410

along the entire fault zone shown in Fig. 6a. The AE location distribution (Figs. 5m-p,

411

Fig. 6b) shows that fault interaction is weak at the initial loading stage and the AE

412

occurred mainly in the lower right side of en échelon faults. The AE events migrated

413

gradually toward the strong interaction en échelon fault zone with the stress

414

concentration and continually to the upper échelon faults, which is consistent with the

415

earthquake migration process of the ZPFZ.

416

The simulation experiment results show that the coordination of two faults

417

controls the distribution and intensity of earthquake activity. The cracking region (i.e. 19

418

jog) is sensitive to the change of regional stress fields. Its major reason is that the

419

fractured region has a lower strength and is more easily deformed or ruptured than the

420

other regions when the differential stress changes a little. This provides a potential

421

explanation for phenomena of "sensitive site" in seismicity (Zhang and Sun, 1985;

422

Wang, 1986; Jiang, et al, 1989). It is the main reason why earthquakes often occur at

423

the junction of different faults with different direction.

424

The strong earthquake focal depths of the ZPFZ are mainly 10-25 km at depth

425

and generally in the lower crust, The source dislocation obtained form focal

426

mechanism and macro earthquake damage was mostly steep-dipping fracture

427

dislocation, and the shovel-shaped planar fractures or fault zones in the shallow crust

428

are mainly normal faults, and only cut through a depth of 5-10 km. The deep-seated

429

buried fault is consistent with the attitude of the focal depth and source dislocation, so

430

deep-seated en échelon faults control the in this fault zone.

431

6. Conclusions

432 433

Based on the above-mentioned study, the overall understanding of the ZPFZ can be summarized by the following three points.

434

(1) The Zhangjiakou-Penglai Fault Zone, as a huge WNW-trending active fault

435

zone, controlled some en échelon Quaternary pull-apart basins and triggered many

436

earthquakes as seismogenic structures. It is also a WNW-trending lithosphere-scale

437

fault zone at depth.

438

(2) The Zhangjiakou-Penglai Fault Zone in North China, is closely related to the

439

surrounding plate activities, not only dominated by the collision of the Indian Plate 20

440

into the Eurasian Plate, but also influenced by the subduction of the Pacific Plate. It

441

depends on the comprehensive reflection of two kinematic backgrounds.

442

(3) Earthquake location and intensity distribution revealed by experimental

443

simulation match well with those of actual earthquakes. Earthquake distribution is

444

controlled obviously by the en échelon faults. The intensity of seismic activity is

445

strong in the middle and weak on both sides, migrating from southeast to northwest

446

along the entire fault zone.

447

Acknowledgments

448

We appreciate Prof. Peixun Liu for his helpful comments and AE analysis

449

software. This research was fund by the National Science Foundation of China (NSFC)

450

for Distinguished Young Scientists (Grant Nos. 41325009) and key project of NSFC

451

(Grant No. 41190072) and China Geological Bureau project (Grant No.

452

1212011120099).

453 454

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Wong, R.H.C., Lin, P., Tang, C.A., 2006. Experimental and numerical study on

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splitting failure of brittle solids containing single pore under uniaxial

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compression. Mechanics of Materials, 38(1), 142-159.

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Zhao, D.P., 2009. Multiscale seismic tomography and mantle dynamics. Gondwana Research, 15, 297-323.

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34

779

Figure Captions

780

Fig. 1 Large earthquake distribution along the Zhangjiakou-Penglai Fault Zone. NW-

781

trending faults: F1-Zhangjikou-Penglai Fault Zone; Secondary faults: F1-1-Zhangjikou

782

Fault, F1-2-Ximalin Fault,F1-3-Nankou-Sunhe Fault, F1-4-Baodi-Ninghe Fault, F1-5-

783

Langfang Fault, F1-6-Shaxi Fault, F1-7-Shanan Fault, F1-8-Chengbei Fault. NE-trending

784

faults: F2-Yanqing-Weixian Fault, F3-Zijingguan Fault, F4-Taihangshan Fault,

785

F5-Xiadian Fault, F6-Cangdong Fault, F7-Huanghekou-Liaocheng-Lankao Fault

786

Zone, F8-Yingkou-Weifang Fault Zone, F9-Penglai-Zhaoyuan Fault, F10-Taocun-

787

Dongdoushan

788

Zhangjiakou-Penglai Fault Zone and its adjacent areas (revised after Zhang et al.,

789

1994)

790

Fig. 3 The present-day intra-blocks' movement of the Zhangjiakou-Penglai Fault Zone

791

and its adjacent areas(revised after Ma et al., 1989) and Horizontal component of

792

plates and crustal blocks movement around China by GPS measurement (revised after

793

Zhang et al., 2002).

794

Fig. 4 Tested sample and AE sensor’ distribution and the variation of differential

795

stress with time. a. Tested sample and AE sensor’ distribution. The dotted area with

796

line is the observation area of the DSCM system. b. the variation of differential stress

797

with time. c. the M - t diagram of the AE events.

798

Fig. 5 The failure process of the en échelon fault pattern. a-d. The granodiorite

799

specimen image captured by CCD camera at points A-D; e-h. The diagrams of x

fault.Fig.

2

Seismic

surfaces

of

Moho

depths

along the

35

800

direction displacement (Dx) at points A-D; i-l. The diagrams of maximum shear strain

801

(εxy) at points A-D; m-o. The distribution diagrams of the AE location at A-B, B-C,

802

C-D stages.

803

Fig. 6 The earthquake distribution and M-t diagram of the ZPFZ and the experimental

804

AE results. a. Earthquake distribution on the Zhangjiakou-Penglai Fault Zone; b. The

805

M- t diagram of the earthquake catalogue of the Zhangjiakou- Penglai Fault Zone; c.

806

The M- t diagram of the AE location from experimental simulation (Some AE events

807

with energy magnitude less than 10 have been removed, greater than 10 AE events are

808

only shown in this diagram).

36

*Highlights (for review)

    

The ZPFZ controlled en échelon Quaternary pull-apart basins; This lithospheric fault zone was seismogenic structure. The experiment match well with actual earthquakes; The seismic intensity is strong in the middle and weak at the two fault tips. The ZPFZ is dominated by the combined effect of two geodynamic settings.