# Earthquake Risk Probability Evaluation for Najin Lhasa in Southern Tibet
## 1 Introduction  
Najin Lhasa (91.219\({}^{\circ}\) E, 29.659\({}^{\circ}\) N) is located in the Lhasa city vicinity, southern Tibet, and at the intersection of the central Tibet seismic belt and the Himalayan seismic belt, which has strong seismic activity (Figure 1). The site and adjacent approximately 150 km are used as the study area (89.55\({}^{\circ}\)\(\sim\)92.85\({}^{\circ}\) E, 28.20\({}^{\circ}\)\(\sim\)31.13\({}^{\circ}\) N). Associated with strong neetocrionic movement, various faults in this region have been well developed. The major faults are oriented nearly E-W and N-S, followed by NNE-NE and NW faults with obvious differences in tectonic properties (e.g., [1,2]). The E-W-trending faults feature large-scale thrust and reverse strike-slip motions; however, the approximately N-S-, NNE-NE- and NW-trending faults are generally smaller in scale, are often concentrated, and constitute a shear-extensional fault tectonic belt. Early in the lower Quaternary period, the faults were active and have witnessed the occurrence of numerous M > 7 earthquakes (e.g., [3, 4, 5]). The focus of this study is, thus, to investigate the regional earthquake risk probability at Najin Lhasa, Tibet (Figure 1).  
The central Tibet earthquake statistical area is distributed in the northern Tibetan Plateau to the north of the Yarlung-Zangbo suture zone and is dominated by a series of near-E-W-trending compressive fault zones [6,7]. The southern part of the Tibetan Plateau is closer to the collision boundary between the India plate and the Eurasian plate, which has strong tectonic activity (e.g., [8,9]). Relevant studies have shown that the seismic activity period in this area is approximately 50 years (e.g., [10]). Except for a few moderate-depth earthquakes in this area, most earthquakes are shallow. As of December 2014, a total of two earthquakes of magnitudes 8.0 to 8.9 had been recorded, i.e., the Dangxiong (1411-10-08) and Dangxiong north (1951-11-18) M8 earthquakes, as well as five earthquakes of magnitudes 7.0 to 7.9, which feature strong seismic activity in this region (e.g., [6,11,12]).  
The Himalayan earthquake statistical area includes the region between the Yarlung-Zangbo suture zone and the Himalayas, which are part of the Himalayan tectonic fold belt (e.g., [13]). Modern tectonic movement is manifested as compressional movement on the central Himalayan thrust fault, the main boundary thrust fault zone, and the compressional and strike-slip fault activity at the Yarlung-Zangbo plate suture zone (e.g., [14,15]). This area has the highest frequency of earthquake occurrence in China and adjacent countries, and there occurred moderate-depth earthquakes in the western and eastern ends of the area (e.g., [14]). Because most of this area is located close to the country borders, the earthquake records remained are incomplete (e.g., [16]). According to the available data, as of December 2014, a total of three M8 class earthquakes and twenty-one M7 class earthquakes have been recorded including the Nyalam M8 (1833-08-26), Nepal-India border M8.1 (1934-01-15) and Chayu M8.6 (1950-08-15) earthquakes (e.g., [11,17]). Hence, it is necessary to incorporate the data from both two seismogenic areas when evaluating the earthquake risk probability  
Figure 1: Tectonic map. The red rectangular indicates the study area. The background colored in pink indicates the Tibet autonomous region, and the green part indicates the Qinghai province, China. Black lines indicate the boundaries of the earthquake statistical area. The dashed red lines indicate the main faults and terrain boundaries. YZSZ: Yarlung–Zangpo suture zone; BNSZ: Bangong–Nujiang suture zone; JRSZ: Jinsha River suture zone; MBT: Main Boundary Thrust.  
at Najin Lhasa. In this study, we focus to establish a fragility curve that can correctly represent the relation between the earthquake risk probability and the peak horizontal ground motion acceleration to avoid the crippling economic and social consequences caused by earthquakes (e.g., [18; 19; 20]).