In the 1970s, Li Ping et al. (1975) and Kan Rongju et al.(1977, 1980) put forward the concept of the Sichuan-Yunnan rhombic block bounded by the Xianshuihe fault zone, Anninghe fault zone, Zemuhe fault zone and Xiaojiang fault zone as its northeast boundary and the massive Honghe fault zone as its southwest boundary from the perspective of a seismically geological background and tectonic movement, focal mechanism of strong earthquakes and earthquake surface rupture zone. The Sichuan-Yunnan rhombic block, located along the southeastern margin of Qinghai-Tibetan Plateau with the most intensive eastward extrusion, is a highly deformed area where the Indian plate collides with the Eurasian plate. This region has complex geological structures, dense active faults, with high-frequency and intense earthquake activities, which is one of most important strong earthquake zones in China.
Many strong earthquakes occurred along the arc-shaped fault belt at the eastern boundary of Sichuan-Yunnan rhombic block in history (Wen Xueze, 2000).There are dynamic factors behind a large number of strong earthquakes along the eastern boundary of the Sichuan-Yunnan rhombic block.The eastward movement of the central Qinghai-Tibetan Plateau and the southeastward movement of the central and southern part of the Qinghai-Tibetan Plateau causes lateral extrusion sliding of the Sichuan-Yunnan rhombic block, which results in left-lateral shear deformation of the eastern boundary (Wang Qiang et al., 2001). Earthquakes occur frequently along the eastern boundary of the Sichuan-Yunnan rhombic block, where small earthquakes are frequent at present.
Accurate seismic location results are of great significance for the study of active tectonics and seismogeological background in this region. In this study, the large arc-shaped fault belt and nearby earthquakes are accurately located by the multi-step location method, and a more accurate epicentral distribution map and depth profile are obtained, providing fundamental data for the study of seismic activities and seismogenic structure in this region (Long F. et al., 2015).1 SEISMOGEOLOGICAL BACKGROUND 1.1 The Xianshuihe Fault Zone
The Xianshuihe fault zone is located on the eastern edge of the Qinghai-Tibetan Plateau, on the northern section of the eastern boundary of the Sichuan-Yunnan rhombic block, which is also the boundary between the Sichuan-Yunnan block and the Sichuan-Qinghai block. This fault zone, starting from Garzê in the north and extending in the NW-SE direction to the south of Moxi, passing through Zhuwo, Luhuo, Dawu, Qianning and Kangding, is a large-scale left-lateral strike-slip fault belt consisting of seven faults including the Luhuo fault, Dawu fault, Ganning fault, Kangding fault, Yalahe fault, Zheduotang fault and Moxi fault. The Xianshuihe fault is about 350km long with intense and high-frequency seismic activities (Luo Zhuoli et al., 1987; Li Tiantiao, 1998; Pan Mao et al., 1994; Qian Hong, 1987; Qian Hong et al., 1990). According to historical records, since 1700, a total of 8 earthquakes with M≥7.0 have occurred along the Xianshuihe fault belt, which are respectively the Kangding M7.0 earthquake on August 1, 1725, the Kangding-Moxi M7 3/4 earthquake on June 1, 1786, the Luhuo M7.5 earthquake on December 8, 1816, the Qianning M7 1/4 earthquake in Dawu on August 29, 1893, the Dawu M7.0 earthquake on August 30, 1904, the Luhuo-Dawu M7.2 earthquake on March 24, 1923, the Zheduoshan M7.5 earthquake in Kangding on April 14, 1955 and the Luhuo M7.6 earthquake on February 6, 1973 (Sun Chengmin, 2010; Editorial Group of the Data Compilation of Earthquakes in Sichuan, 1980).1.2 The Anninghe Fault Zone
The Anninghe fault zone, located in the middle section of the eastern boundary of the Sichuan-Yunnan rhombic block, is connected with the southern end of the Xianshuihe fault zone, which starts in the north from Tianwan of Shimian County. This presents a very complex spatial pinnate distribution with the Xianshuihe fault near the Tianwan-Xinmin region, forming images of dense surface rupture, as it runs southward through Mianning, Xichang, Dechang and Miyi to the bank of the Jinsha River in Panzhihua, and the fault zone as a whole spreads in nearly a SN direction (Chen Fubin et al., 1989; Tang Rongchang et al., 1989, 1992). The Anninghe fault, with a length of about 150km, is a left-lateral strike-slip fault and shows strong compression characteristics (Zhang Yueqiao et al., 2004). GPS survey results and studies on focal mechanism near Mianning show that the Anninghe fault is an active fault with both strike-slip and thrust movements (Zhang Peizhen et al., 2003; Lü Jiangning et al., 2003; Long Sisheng et al., 2003).
The Anninghe fault zone is an important pleistosiesmic zone in western China, which is an intense active fault zone, where many strong and destructive earthquakes have occurred in history (Wen Xueze et al., 2000; Zhou Rongjun et al., 2001; Xu Xiwei et al., 2003a, 2003b; Ran Yongkang et al., 2008; Cheng Jianwu et al., 2010). The Anninghe fault zone shows obvious segmentation features, and the seismic gap in the northern segment and the stress-locked zone in the southern segment indicate potential risk of strong earthquakes along the fault zone (Wen Xueze, 2000; Wen Xueze et al., 2007; Yi Guixi et al., 2004).1.3 The Zemuhe Fault Zone
The Zemuhe fault zone is connected with the southern segment of the Anninghe fault zone, with an overall trend of 330° and a dip angle of the rupture surface above 60°, leaning NE or SW. It starts from Xining Town in the northwest, runs SE through Xichang, Puge and Ningnan, and intersects with the Xiaojiang fault near Qiaojia, with a total length of about 120km.
Since 1536, there have been 6 earthquakes with M≥6.0 along the Anninghe and Zemuhe fault zones. Five of them occurred in the south of Mianning, in the southern segment of the Anninghe fault, and one occurred in the Shimian-Mianning section, in the north of Mianning and on the middle-north segment of the Anninghe fault. Earthquakes with a magnitude over 7.0 are the Xichang M7.5 earthquake on March 19, 1536 and the Xichang-Puge M7.5 earthquake on September 12, 1850 (Sichuan Provincial Chronicle Compilation Committee, 1998; Sun Chengmin, 2010).2 DATA 2.1 Observed Data from Seismic Stations
Seismic data used in this paper comes from a total of 147 seismic stations, including fixed broadband seismic stations in Sichuan, local digital seismic stations, some reservoir seismic stations, mobile observation stations and shared stations in neighboring provinces and cities. As the southern part of the study area is located at the junction of Sichuan and Yunnan, if only observation data from Sichuan network was used, the accuracy of location results will be seriously affected due to the large gap angle of station distribution. Therefore, some stations from the Yunnan seismic network are added to provide better coverage of stations, so as to improve location accuracy. Stations used for seismic location are shown in Fig. 1.Abundant data sources and extensive spatial distribution of seismic stations give the southern segment of the Xianshuihe fault zone-Anninghe-Zemuhe fault zone better monitoring ability; meanwhile, those nearer stations can effectively restrain the source location.
A total of 7, 787 earthquakes from January 1 to December 31, 2010 are collected in the study area. The Wadati curve (Fig. 2) shows low dispersed degree of seismic phase travel time, indicating that arrival time of most earthquakes are correctly read. Seismic phase observation records with major deviations are corrected to obtain final observations of a higher quality.
Affected by the collision between the Indian and Eurasian plates, the geological structure and crustal structure of the Sichuan-Yunnan rhombic block are very complex. The crustal thickness changes significantly, and the velocity structure shows strong lateral variations (Wang Qiang et al., 2001). In order to improve the accuracy of the seismic location, a velocity model suitable for this region should be adopted.
In this paper, with the western Sichuan model given by Zhao Zhu et al. (1987) as the initial model, location program Hypo2000 is used for seismic location in the study area. And in the subsequent locating process, local one-dimensional velocity structure is inverted.3 RESEARCH METHOD AND PROCESS
In this paper, the multi-step locating method is adopted to relocate earthquakes in the study area (Long Feng et al., 2015). The locating process is as follows: firstly, based on the western Sichuan model given by Zhao Zhu et al.(1987), Hypo2000 is used to preliminarily determine the 7, 787 earthquakes (Fig. 3), considering the influence of structure on travel time at a longer distance, full weight is given to seismic stations within 80km from the epicenter, while the weight of stations within 80km-150km is given as an attenuation of the cosine function. At the same time, source location and station azimuth are obtained (Klein, 2002). Observation reports of earthquake events recorded by at least 6 seismic stations and with azimuth angle clearance less than 120° are selected to invert a new one-dimensional velocity structure (Table 1) and station correction in the study area (Kissling, 1988; Kissling et al., 1994, 1995). The results of the velocity structure inversion show that there exists a low velocity layer in the upper crust near the boundary of Sichuan-Yunnan rhombic block, which is similar to the geophysical exploration results obtained by Cui Zuozhou et al. (1992). The spatial distribution map of station correction (Fig. 4) shows that there are significant differences between station correction in the South China block and the Qinghai-Tibetan Plateau, which should be taken into account fully in the calculation of travel time.
The new velocity model and station correction are then substituted into Hypo2000 for relocating, and location results of 6, 981 earthquakes are obtained (Fig. 5).Finally, earthquakes with a station azimuth clearance less than 180° are selected for the double-difference location (Waldhauser F., 2000). With a 10km search radius, the selection rate of P and S seismic phase is above 90%, and finally the double-difference location results of 2, 794 earthquakes and epicenter distribution with precise locations are obtained (Fig. 6). The east-west error is 514m, the south-north error 502m, vertical error 1.1km, travel time residual 0.044s.
It can be seen from the epicenter distribution map after initial locating that the location results are relatively discrete (Fig. 3), and the Hypo2000 locating results tend to be concentrated (Fig. 5). Error analysis of Hypo2000 locating (Fig. 7) shows that horizontal error for most earthquakes can be controlled within 3km, vertical error within 5km, and travel time residual within 0.3s, indicating that the locating accuracy is acceptable. The HypoDD locating results are more concentrated, and epicenters of earthquakes are more clearly distributed in bands along the fault belt (Fig. 6).
According to spatial distribution of precise locating results of earthquakes, the earthquakes in the south Dawu-Qiaojia section along the eastern boundary of the Sichuan-Yunnan rhombic block are mainly distributed along the southeast segment of the Xianshuihe fault zone, Sanchakou-Shimian region, Daliangshan fault and south segment of the Zemuhe fault.
Earthquakes on the southeast segment of the Xianshuihe fault are relatively dense and distributed well along the fault zone. The most recent strong earthquake is the Kangding M6.3 earthquake that occurred in the south-central part of Xianshuihe fault zone at 04:55 p.m. on November 22, 2014. Small earthquakes occur frequently in the northern part of the Anninghe fault zone, especially in the Shimian region at the end of the fault zone, that is, the intersection of the south segment of the Xianshuihe fault and the Anninghe fault zone and the intersection of the Xiaojinhe fault zone and the Daliangshan fault zone in the east, earthquakes are rather dense.
Seismic gaps in the northern segment of the Zemuhe fault zone continues to exist. Earthquakes along the Zemuhe fault zone are mainly concentrated on the Puge-Ningnan section along the southern segment. Seismic distribution along the Daliangshan fault zone is uniform.4.2 Analysis of Depth Profile
According to the epicenter distribution obtained by precise locating and structural tracing, four profiles are divided in different parts of each fault zone along the eastern boundary of the Sichuan-Yunnan rhombic block (Fig. 6), and the distribution of focal depths at each profile is as below (Fig. 8).
Profile A-A′ runs along fault strike, extending in the SE-NW direction from nearby south of Dawu along the southern segment of the Xianshuihe fault zone to the vicinity of Shimian. Profile A-A′ shows that small earthquakes are relatively dense from the southern segment of the Xianshuihe fault zone to the vicinity of Shimian, with obvious segmentation characteristics, and most earthquake clusters occur within 15km. However, earthquakes near Shimian are obviously divided into shallow and deep groups in depth, reflecting complex local tectonic process.
Profile B-B′, passing through Kangding, which is perpendicular to profile A-A′, shows that the entire Xianshuihe fault zone and its branch fault (Shade fault) as a whole presents a nearly vertical fault plane, while earthquakes in Songpan-Garzê block on the right side of the profile are relatively scattered.
From Jiulong, depth distribution in profile C-C′, perpendicular to profile A-A′, shows complex interaction between faults at the intersections of the south end of the Xianshuihe fault with the Anninghe fault and Daliangshan fault. According to the distribution pattern, the Anninghe fault seems to be W-trending, while earthquakes between the Anninghe fault and Daliangshan fault are rather deep, and their tendency is not obvious, which may be caused by the collision between the Qinghai-Tibetan block and South China block. From depth distribution, earthquakes near Shimian are obviously divided into shallow and deep groups in depth. Earthquakes are mainly concentrated at a depth of 0km-15km in the upper crust and 20km-35km in the brittle deformation layer in the middle-lower crust, and earthquakes are rare at depths of 15km-20km. Based on experimental results under high temperature and pressure, Zhu Ailan et al. (2005) calculated crustal strength in the western Sichuan region and deduced that granite at depths of about 14km-19km was in a state of plastic flow. The thickness and position of the plastic flow layer is consistent with the less-shock layer obtained by precise locating.
Profile D-D′ stretches nearly EW across the Anninghe-Zemuhe fault and the Daliangshan fault from nearby Xichang. Profile D-D′ is located in the seismic gap of the Anninghe-Zemuhe fault, so there are few earthquakes. In general, the Daliangshan fault tends to be relatively vertical.5 CONCLUSION AND DISCUSSION
The multi-step locating method is adopted to precisely locate earthquakes in the south Dawu-Qiaojia region along the eastern boundary of the Sichuan-Yunnan rhombic block in Sichuan Province, the accuracy of the source location is significantly improved and the epicenter distribution is consistent with the linear distribution of the fault zone.
The one-dimensional velocity structure in the study area obtained using VELEST inversion shows that there exists a low velocity layer in the upper crust near the boundary of the Sichuan-Yunnan rhombic block, which is similar to the geophysical exploration results obtained by Cui Zuozhuo et al.(1992).
Epicenters are well distributed along the Xianshuihe fault zone, and seismic distribution in the southeast segment is relatively dense. Small earthquakes occur frequently in the northern part of the Anninghe fault zone, especially at the intersection of the south segment of the Xianshuihe fault and the Anninghe fault zone and the intersection of the Xiaojinhe fault zone and Daliangshan fault zone in the east. Earthquakes along the Zemuhe fault zone are mainly concentrated in the Puge-Ningnan section in the southern segment. The seismic gap in the northern segment of the Zemuhe fault zone continues to exist. Seismic distribution along the Daliangshan fault zone is uniform.
According to epicenter distribution after precise locating and structural tracing, 4 profiles are divided to discuss the relationship between focal depth and active structures.The profile along the strike of the Xianshuihe fault zone shows obvious segmentation characteristics of seismic activity, and most earthquake clusters occur within 15km.The profile, passing Kangding, which is perpendicular to the Xianshuihe fault zone, shows that the Xianshihe fault zone and Shade fault as a whole, presents a nearly vertical fault plane, while earthquakes in the Songpan-Garzê block are relatively scattered. From Kowloon, the profile perpendicular to the Xianshihe fault zone shows complex interaction between faults at the intersections of southern end of the Xianshuihe fault with the Anninghe fault and Daliangshan fault. Earthquakes near Shimian are obviously divided into deep and shallow groups in depth, and earthquakes are rare at depths of 15km-20km, which is consistent with the view of thickness and position of plastic flow layer at depths of 14km-19km proposed by Zhu Ailan et al.(2005). The profile stretching nearly EW across the Anninghe-Zemuhe fault and the Daliangshan fault from nearby Xichang shows that the Daliangshan fault tends to be relatively vertical.
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