Pure Appl. Geophys. 175 (2018), 193–205 Ó 2017 Springer International Publishing AG https://doi.org/10.1007/s00024-017-1691-y Pure and Applied Geophysics The Crustal Structure of the North–South Earthquake Belt in China Revealed from Deep Seismic Soundings and Gravity Data 1 1 2 2 YANG ZHAO, LIANGHUI GUO, LEI SHI, and YONGHUA LI Abstract—The North–South earthquake belt (NSEB) is one of Liupan Mountains, and Longmen Mountains, along the major earthquake regions in China. The studies of crustal the southeastern edge of the Tibetan Plateau to the structure play a great role in understanding tectonic evolution and in evaluating earthquake hazards in this region. However, some Red River fault zone of Yunnan Province (Wang fundamental crustal parameters, especially crustal interface struc- et al. 1976, 2015a). The NSEB presents complicated ture, are not clear in this region. In this paper, we reconstructed the geological features and crustal structure. To the west crustal interface structure around the NSEB based on both the deep seismic sounding (DSS) data and the gravity data. We firstly of the NSEB is the relatively active Tibetan Plateau, reconstructed the crustal structure of crystalline basement (inter- and to the east are the stable Yangtze and Ordos face G), interface between upper and lower crusts (interface C) and blocks. Large variations of Moho depth and velocity Moho in the study area by compiling the results of 38 DSS profiles published previously. Then, we forwardly calculated the gravity structure are presented between the east and west anomalies caused by the interfaces G and C, and then subtracted areas (Liu et al. 1989; Song et al. 1991; Yin et al. them from the complete Bouguer gravity anomalies, yielding the 1998; Guo et al. 2009). The reconstruction of crustal regional gravity anomalies mainly due to the Moho interface. We structure in the NSEB would be beneficial in under- then utilized a lateral-variable density interface inversion technique with constraints of the DSS data to invert the regional anomalies standing tectonic evolution and geodynamics and in for the Moho depth model in the study area. The reliability of our evaluating earthquake hazards in this region. Moho depth model was evaluated by comparing with other Moho The crust of the NSEB is one of the most complex depth models derived from other gravity inversion technique and receiver function analysis. Based on our Moho depth model, we geological structures among the Chinese continent. mapped the crustal apparent density distribution in the study area Many geophysical studies were conducted around the for better understanding the geodynamics around the NSEB. NSEB in the past decades. However, so far the crustal Key words: North–South earthquake belt, crustal structure, interface structure is still not clear in the NSEB area. Moho depth, gravity inversion, deep seismic soundings. Seismic refraction/reflection is the most direct and effective means to detect the crustal structure and interface information with good vertical resolution. Since 1980s, a number of deep seismic soundings 1. Introduction (DSS) profiles were accomplished in this area. Sev- eral crustal models in the Chinese continent were The North–South earthquake belt (NSEB) is the built based on the DSS data (Deng et al. 2011; Stolk main boundary between the east and west parts of et al. 2013). Since some new DSS profiles were China (see Fig. 1). It extends from the western edge conducted in recent years, a refined crustal model for of Ordos’s basin, through western Qinling Mountains, the NSEB should be constructed. Whereas, the reconstruction of crustal interface structure was restricted due to the sparseness of the DSS profiles in the study area. The complete Bouguer gravity 1 Key Laboratory of Geo-detection Ministry of Education anomalies are important data with a considerable and School of Geophysics and Information Technology, China lateral resolution for studying the crustal structure University of Geosciences (Beijing), Beijing 100083, China. E- and tectonics. They contain the gravity effects not mail: [email protected] 2 Institute of Geophysics, China Earthquake Administration, only from many of anomalous bodies and interface Beijing 100081, China. 194 Y. Zhao et al. Pure Appl. Geophys. Figure 1 Left: Map of the NSEB and the adjacent areas with the major tectonic features. The black rectangle outlines the study area. Right: Distribution of the earthquake epicenters (1970–2016) in the NSEB and the adjacent areas, colored according to the magnitude of earthquakes. Blue: 5–7 magnitude earthquakes; green: 7-8 magnitude earthquakes. Red lines indicate block boundaries. KT Kunlun-Qaidam Terrane; YO Yanshan Orogen; OB Ordos Basin; QB Qaidam Basin; ST Songpan-Ganzi Terrane; TP Tibetan Plateau; SB Sichuan Basin; IP Indian Plate; YC Yangtze Craton structure within the crust, but also from construction interface inversion technique based on a lateral- of a Moho interface. Since high-resolution gravity variable density contrast model is required to seek a data cover the whole study area, it is necessary to reliable interface model. Prior information from the separate the gravity anomalies due to the density published literature is necessarily involved as con- interface and subsequently invert them for the inter- straints for better inversion. face depth. The aim of this study is to reconstruct the crustal Anomaly separation is one key technique in structure of the NSEB by combining the DSS and gravity data processing. Many frequency-domain fil- gravity data. We firstly constructed crustal interfaces tering techniques have been proposed for anomaly based on the published DSS data. Then, we separated separation, such as upward continuation (Jacobsen the gravity anomalies caused by the Moho interface 1987), matched filtering (Spector and Grant 1970), by removing the contribution of the basement inter- Wiener filtering (Pawlowski and Hansen 1990), face G and upper/lower crustal interface C. preferential continuation (Pawlowski 1995), and Subsequently, the reconstructed Moho interface was preferential filtering (Guo et al. 2012, 2013). But few applied to establish a variable density contrast model. of them could separate the target gravity anomalies We finally inverted the separated anomalies for Moho well, because of no theoretical relationship between depth by a lateral-variable density interface inversion the source depth of gravity field and anomaly spec- technique with constraints of the DSS data. trum. Hence, anomaly stripping would be an alternative technique for better separation. The con- 2. Data Source ventional technique of interface inversion from gravity data was based on a constant-density contrast 2.1. Deep Seismic Soundings Data model without any prior information (Oldenburg 1974). However, due to the complex crust structure Around the NSEB, many DSS profiles have been and the large scale of the NSEB, the density contrast completed for revealing the crustal structure in the value is not always a constant laterally. Thus, an past 30 years, with a cumulative length of over Vol. 175, (2018) The Crustal Structure of the North–South Earthquake Belt in China Revealed 195 Table 1 Information of 38 DSS profiles around the NSEB Number Profile name Year References 1 Lijiang-Panzhihua 1984 Xiong et al. (1993) 2 Mouding-Xichuang 1984 Yin and Xiong (1992) 3 Zhefang-Binchuan 1982 Bai and Wang (2004) 4 Menglian-Malong 1982 Bai and Wang (2004) 5 Lijiang-Xinshizhen 1984 Cui et al. (1987) 6 Lazha-Changheba 1984 Lu et al. (1989) 7 Eryuan-Jiangchuan 1982 Zhang et al. (2007) 8 Simao-Zhongdian 1987 Lin et al. (1993) 9 Huashixia-Jianyang 1987 Cai et al. (2008) 10 Jinchuan-Tangke 1987 Cui and Chen (1994) 11 Batang-Zizhong 2000 Wang et al. (2003b) 12 Benzilan-Tangke 2000 Wang et al. (2003b) 13 Puer-Luxi 2011 Zhang et al. Figure 2 (2013a, b) The locations of the DSS profiles around the NSEB 14 Zhenkang-Luxi 2010 Pan et al. (2015) 15 Yushu-Gonghe 1998 Wang and Qian 18,000 km. Here, we assembled 38 DSS profiles (2000) around the NSEB for reconstructing the crustal 16 Altyn Tagh - 1987- Wang et al. (2005) Longmenshan 1989 model. Figure 2 shows locations of these profiles, 17 Aba-Shuangliu 1990s Zhu (2008) and Table 1 lists information of each profile. 18 Heishui-Beibei 1980s Song (1994) Although the geographical location of these profiles 19 Amuquhu-Lingtai 1986 Zhou (2005) 20 Maqin-Jingbian 1997 Li et al. (2002) is certainly uneven, the coverage is remarkably good 21 Menyuan-Weinan 1982 Zhang et al. (1985) along the NSEB, especially the regions of intense 22 Wendeng- Alxa left 1990s Wang et al. (2014a) tectonic deformation where earthquakes frequently banner 23 Xiji-Zhongwei 1999 Li et al. (2001) occur. 24 Chengxian-Xiji 1986 Li et al. (1991) 25 Tongwei-Huangling 2012 Li et al. (2014a, b) 26 Kangxian-Shiyan 2013 Li et al. (2015) 2.2. Gravity Data 27 Hezuo-Jingtai 2009 Zhang et al. (2013a, b) We assembled the free-air gravity anomaly 28 Aba-Suining 2010 Jia et al. (2014) around the NSEB from the Earth Gravitational Model 29 Jinchuan-Leshan 2013 Wang et al. (2015b) (EGM) 2008 (Pavlis et al. 2012). The EGM 2008 was 30 Chuxiong-Luoping 2005 Wang et al. (2009) 31 Zhongshan-Zizhi 1999 Wang et al. (2002) produced based on marine, land, air-borne gravity 32 Lincang-Yuxi 2011 Wang et al. (2014b) measurements and GRACE satellite altimetry with 33 Yunxian-Ninglang 2012 Chen (2015) resolution of 50 9 50. Compared to other previous 34 Maerkang-Gulang 2004 Zhang et al. (2008) 35 Aba-Wuqi 2014 Wang et al. (2017) models, the EGM 2008 model made great progress on 36 Maduo-Alxa right 2014 Wang et al. (2017) resolution and precision. After standard terrain and banner Bouguer corrections, we obtained the complete 37 Yanyuan-Mahu 2005 Yang et al. (2011) Bouguer gravity anomalies in the study area with 38 Tangke-Pujiang- 1983 Chen et al.