Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-195 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 27 July 2018 c Author(s) 2018. CC BY 4.0 License. 1 Characteristics of surface damage in China during the 25 April 2015 2 Nepal earthquake 3 Zhonghai Wu a Patrick J. Baroshb, Xin Yao a, Yongqiang Xu c 4 5 a Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Natural 6 Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences, 7 Beijing 100081, China 8 b P.J. Barosh and associates, 103 Aaron Avenue, Bristol, RI 02809, USA and Visiting 9 Research Fellow, Chinese Academy of Geological Sciences, Beijing 100081 10 China 11 c China Institute of Geo-Environment Monitoring, Beijing 100081, China 12 13 Abstract: The seismic effects in Nyalam, Gyirong, Tingri and Dinggye counties along 14 the southern border of Tibet were investigated during 2-8 May, 2015, a week after the 15 great Nepal earthquake along the Main Himalaya Thrust. The intensity was VIII in the 16 region and reached IX at two towns on the Nepal border; resulting in the destruction of 17 2,700 buildings, seriously damaging over 40,000 others, while killing 27 people and 18 injuring 856 in this sparsely populated region. The main geologic effects in this steep 19 rugged region are collapses, landslides, rockfalls, and ground fissures; many of which 20 are reactivations of older land slips. These did great damage to the buildings, roads and 21 bridges in the region. Most of the effects are along four incised valleys which are 22 controlled by N-S trending rifts and contain rivers that pass through the Himalaya 23 Mountains and flow into Nepal; at least two of the larger aftershocks occurred along the 24 normal faults. Areas weakened by the earthquake pose post-seismic hazards. Three 25 valleys have the potential for dangerous post-seismic debris flows that could create 26 dangerous dams especially during the monsoon season. Loosened rock and older slides 27 also may fail. In addition, there is an increased seismic hazard along active N-S 28 trending grabens in southern Tibet due to the shift in stress resulting from the thrust 29 movement that caused the Nepal earthquake. NW trending right-lateral strike-slip 30 faults also may be susceptible to movement. The results of the findings are 31 incorporated in some principle recommendations for the repair and reconstruction after 32 the earthquake. 33 34 Key Words: Nepal earthquake, Himalaya Mountains, Seismic hazard, Post-seismic 35 hazards, southern Tibet 36 Corresponding author: Wu Zhong-hai and Patrick J. Barosh, E-mail: [email protected], [email protected] 1 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-195 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 27 July 2018 c Author(s) 2018. CC BY 4.0 License. 37 1. Introduction 38 On 25 April 2015 at 14:11:26 MGT+8 (Beijing Time), a Ms 8.1 (Mw 7.8) great 39 earthquake struck Nepal and adjacent regions. The epicenter was near Pokhara 77 km 40 northwest of the capital of Kathmandu and the hypocenter was at a depth of 10-24 km. 41 Many aftershocks occurred of magnitude 4.5 Mw or greater, of which a Ms 7.5 (Mw7.3) 42 aftershock occurred after 17 days, on 12 May 2015 at 15:05: the epicenter was near the 43 Chinese border 77 km east-northeast of Kathmandu and the hypocenter was at a depth 44 of 12-16 km. According to incomplete statistics, this Nepal great earthquake killed 45 more than 8,800 people and injured more than 23,000. 46 The earthquake occurred on the south slope of the Himalaya Mountains and 47 formed a 120-140 km long, about 80 km wide rupture zone with a dip-slip of 3.5-5.5 m, 48 which shows an expansion from west to east (U.S. Geological Survey National 49 Earthquake Information Center, 2015a, b；IRIS, 2015). The aftershock distribution, the 50 focal mechanism solution and the source rupture inversion suggest that the earthquake 51 was a release of built-up strain along the Main Himalaya Thrust (MHT) fault zone; part 52 of the ongoing process of the Indian Plate underthrusting the Eurasian Plate (Fig. 1). 53 This was a strongest seismic event since the 2005 Pakistan Kashmir Ms 7.8 earthquake, 54 which also occurred along the MHT. This activity may indicate that the seismic activity 55 along the thrust is entering a new active phase. 56 The earthquake affected northern India, Pakistan, Bhutan, and the southern Tibetan 57 region of China. In China the tremors were felt in Xigazê and Lhasa to the north but 58 were strongest in the China-Nepal border area which is only about 40 km (Fig. 1, 2) 59 from the epicenter. The earthquake disaster caused 27 deaths, 856 injured, and 3 60 missing, and extensive damage in China (Fig. 3a). The affected people are about 30 61 thousand and the direct economic loss is more than 33,000 million Yuan (RMB). 62 Fortunately, the border area has a low population density and the earthquake occurred 63 in the afternoon when many were outside, otherwise the casualty and economic loss 64 would be much higher. Due to the rapid response of local government the affected 65 people were soon resettled in southern Tibet (Fig. 3b). 66 In order to quickly know the effects caused by the earthquake and potential future 67 threats to provide the basis for the post-earthquake reconstruction, an emergency 68 seismic hazard investigation group of 12 people that was organized by the Ministry of 69 Land and Resources did a field survey in the hardest hit four counties of Nyalam, 70 Gyirong, Tingri and Dinggye on 2-8 May, a week after the main shock. The group then 71 presented their findings to the local government. This paper is a brief summary of the 72 investigation. 73 2. Seismic-Geological Setting 74 The Tibetan Plateau is well known for its numerous E-W to NW north-dipping 75 thrust faults that facilitated its rise as the India plate collided and was thrust beneath it. 76 Most of the uplift occurred by the Miocene (Dewey et al, 1988; Wu et al., 2008) and 2 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-195 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 27 July 2018 c Author(s) 2018. CC BY 4.0 License. 77 most of the thrusting ceased as the movement evolved and concentrated along fewer 78 strike-slip faults, which remain very active and capable of great earthquakes (Fig. 1). 79 However, thrusting remains dominant in the collision zone at the south edge of Tibet 80 south of the Himalaya Mountains with the continued northward movement of India. 81 Here the greatest activity occurs along the very shallow north-dipping Main Himalaya 82 Thrust (MHT), which gave rise to the Nepal earthquake and has a log history of great 83 earthquakes along its length (Fig. 1). Less generally known are a series of nearly 84 N-S-trending normal faults and grabens to the north of the MHT that complement some 85 of the movement across the MHT. These also are capable of producing significant 86 earthquakes although they are much shorter in length (Wu et al, 2011). This array of 87 active faults plus a set of NW right-lateral strike-slip faults that may aid extension 88 constitute the seismic framework of the region. 89 The China-Nepal border region is located on the south slope of the Himalaya 90 Mountains close to the MHT and contains many active normal faults that control the 91 transverse valleys that lead into Nepal. The high rugged steep landforms and the 92 well-developed incised river valleys in this region further amplify earthquake disasters. 93 Therefore, it is not strange that it was greatly affected by the disastrous Nepal 94 earthquake. 95 3. Seismic Intensity 96 Overall 2,699 houses and one temple were destroyed, 39,981 houses and 242 97 temples seriously damaged, and about 2,600 km of long trunk highway, 263 bridges, 98 and a part of communication, power and water facilities damaged to some degree in the 99 southern Tibetan region affected by the Nepal earthquake. The seismic intensity 100 distribution based on observations of 26 sites of the 10 affected counties (data from 101 China Earthquake Administration), and Combined with our observations of 16 sites to 102 seismic intensity around China-Nepal border is shown in Fig. 2 and listed in Table 1. 103 In different intensity area, the feeling of people, damage of buildings with different 104 materials and structures and damage surface are obvious differences. Only a handful of 105 people in the room felt the earthquake occurred in Lhasa in where the seismic intensity 106 is only III degrees. But in Xigazê city, most of the people of inside and outside of 107 biuldings are obviously felt the earthquake and show the seismic intensity maybe IV 108 degrees at here. The differences of the damage of building and surface have been 109 simply described in table 1 from IX to VI intensity zone. 110 Among the four counties we investigated, Nyalam County is located on the south 111 slope of the Himalaya Mountains, while Gyirong County, Tingri County, and Dinggye 112 County are located north of the Himalaya Mountains.