Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Nat. Hazards Earth Syst. Sci. Discuss., 3, 367–399, 2015 www.nat-hazards-earth-syst-sci-discuss.net/3/367/2015/ doi:10.5194/nhessd-3-367-2015 NHESSD © Author(s) 2015. CC Attribution 3.0 License. 3, 367–399, 2015 This discussion paper is/has been under review for the journal Natural Hazards and Earth Why the 2014 Ludian, System Sciences (NHESS). Please refer to the corresponding final paper in NHESS if available. Yunnan, China Ms 6.5 earthquake triggered Why the 2014 Ludian, Yunnan, China an unusually large landslide? M 6.5 earthquake triggered an unusually s Z. F. Chang et al. large landslide? Title Page Z. F. Chang1, X. L. Chen2, X. W. An1, and J. W. Cui1 Abstract Introduction 1Earthquake Administration of Yunnan Province, Kunming, 650041, China 2Key Lab of Active Tectonics and Volcano, Institute of Geology, China Earthquake Conclusions References Administration, Beijing, 100029, China Tables Figures Received: 24 November 2014 – Accepted: 10 December 2014 – Published: 13 January 2015 Correspondence to: X. L. Chen ([email protected]) J I Published by Copernicus Publications on behalf of the European Geosciences Union. J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 367 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract NHESSD The 3 August 2014 Ludian, China Ms 6.5 earthquake has spawned a mass of se- vere landslides. Of them the biggest occurred at Hongshiyan near the epicenter, which 3, 367–399, 2015 has 1200 × 104 m3, clogging the Niulanjiang River, and creating a large dammed lake. 5 Post-event field investigations yield detailed data on following aspects: rock structure Why the 2014 Ludian, of landslides, lithology, and geometry of the dam, composition and grain sizes of debris Yunnan, China Ms 6.5 avalanches. Based on these data, this work further analyzes the geology and topog- earthquake triggered raphy of the Hongshiyan area, and explores the mechanism for occurrence of such an an unusually large unusual big landslide at this place. Our analysis suggests the following conditions are landslide? 10 responsible for this catastrophic event: (1) during the Ms 6.5 earthquake, the special terrain and site conditions led to abnormally strong ground shake. (2) Hongshiyan lies Z. F. Chang et al. nearby an active fault, where intense crustal deformation resulted in rock fractures and weathering. (3) Intense incision on the river increased topographic relief with steep slopes and scarps. (4) Combined structures, including unloading fissures, high-angle Title Page 15 joints and low-angle beds along the river as well as upper-tough and lower-soft struc- Abstract Introduction ture on the slopes. It is the joint functions of these conditions that triggered such seldom seen landslides during a moderated-sized earthquake. Conclusions References Tables Figures 1 Introduction J I At 16.30 p.m. local time on 3 August 2014, an earthquake of magnitude M 6.5 struck s J I 20 Ludian county, Yunnan Province, southwest China. The epicenter was located at 27.10◦ N, 103.33◦ E, the Longtou town in this county, with a focal depth 12 km. (http: Back Close //www.cea.gov.cn/publish/dizhenj/468/553/100821/index.html). This event was felt in Full Screen / Esc more than ten cities or counties in Yunnan, Sichuan, and Guizhou province, resulting in IX intensity at the meisoseismal area. As for 8 August 2014, it killed 617 people and Printer-friendly Version 25 112 missing (http://www.cea.gov.cn/publish/dizhenj/468/553/100821/index.html). Interactive Discussion 368 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | This shock occurred in a rugged mountainous area, which has spawned very severe landslides including rock falls, avalanche, and debris flow. Field investigations show NHESSD that these landslides are of scales from tens to several ten thousands 104 m3 in vol- 3, 367–399, 2015 ume. In particular, a huge landslide happened at Hongshiyan, on the right bank of the 5 Niulanjiang River in the southern most heavily affected region. Coupled with the front of an old landside on the left bank, this new avalanche body clogged the Niulanjiang Why the 2014 Ludian, 4 3 River, creating a 120 m-high, 1200 × 10 m dam, which made the river level rise at Yunnan, China Ms 6.5 least 30 m and water area increase to 3 times that of the normal case, posing severe earthquake triggered threat on residents as well as hydropower stations both upstream and downstream the an unusually large 10 river. landslide? Although earthquake-induced landslides are common in southwest China, it is sel- Z. F. Chang et al. dom seen that a moderate quake like the 2014 Ludian Ms 6.5 could have produced such a large-scale severe land slumps. The objective of this paper is to explore the mechanism of this unusual event. Our analysis is based on the data from field obser- Title Page 15 vations, and focuses on the geologic conditions of the Hongshiyan area where a huge dam on the river was created by the landslides. Abstract Introduction Conclusions References 2 Geologic setting Tables Figures The Ludian Ms 6.5 event took place in northeastern Yunnan Province of China, be- tween the southeastern margin of the Tibetan plateau and South China block (Fig. 1). J I 20 This area is characterized by intense neotectonic movements, numerous active faults J I and frequent major earthquakes (Allen et al., 1989; Song et al., 2002; Zhang et al., 2004; Xu et al., 2005; Guo et al., 2013). Field investigations suggested that the Ludian Back Close quake was generated by the southwest segment of the NE-trending thrust-dominating Full Screen / Esc Zhaotong-Lianfeng fault that lies at the east flank of the Sichuan-Yunnan block (Fig. 1). 25 This fault zone consists of two faults: the Lianfeng fault and Zhaotong-Ludian fault, Printer-friendly Version dominated by thrusting (Wen et al., 2013), which serves as the south boundary of the secondary active Daliangshan block (Fig. 1). In this block, there exists also the nearly Interactive Discussion 369 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | northerly trending Daliangshan fault dominated by left slip with thrusting motion (Shen et al., 2000; Zhou et al., 2003; Zhang et al., 2005; He et al., 2008). Since Cenozoic NHESSD time, the neotectonic movement in the Daliangshan block is expressed by left-slip and 3, 367–399, 2015 thrust on the Daliangshan fault as well as shortening of nearly NS-trending folds, which 5 is about 10.9km ± 1.6 km (Chen et al., 2008). The Zhaotong-Lianfeng fault aforemen- tioned lies just at the southeast edge of the Daliangshan block, accommodating the Why the 2014 Ludian, southeastward motion of this block (Li et al., 1975, 1993; Kan et al., 1977; Tappon- Yunnan, China Ms 6.5 nier P. et al., 1986; Armijo et al., 1989; Zhong et al., 1996; Hodeges et al., 2001; Zhang earthquake triggered et al., 2003; Xu et al., 2003). Nearby the epicenter of the Ludian event, there exists an unusually large 10 the major branch of the Zhaotong-Lianfeng fault zone: the Zhaotong-Ludian fault and landslide? its NW-trending branch-Xiaohe fault. The Zhaotong-Ludian fault comprises three NE striking faults, from east to west which are the Sayuhe fault (F1), Zhaotong-Ludian fault Z. F. Chang et al. (F2), Longshu fault (F3), and Xiaohe fault (F4) (Fig. 2). These phenomena show a com- plicated structure of this area, where NE- and NW trending faults intersect each other, Title Page 15 and NE ones are predominant. Except the Cretaceous System, strata from Sinian through Quaternary are exposed Abstract Introduction in the Ludian area. Of them, the pre-Mesozoic formations are most widely distributed, Conclusions References which are dominated by sandstone, shale, limestone and dolomitite, containing many joints and fissures. Mesozoic strata are present in the northeast and southeast of the Tables Figures 20 seismic area, which are continental debris accumulation dominated by sandstone and mudstone. Few Cenozoic strata are scattered in the intermountain basins, such as J I the Ludian basin, which are made up of fluviolacustrine clay rock and sandy gravel (Table 1). J I Geographically, the Ludian seismic area is situated in the northeastern Yunnan- Back Close 25 Guizhou plateau. Before Miocene time, it was a relatively intact and uniform peneplain. Full Screen / Esc Since the end of Miocene, due to many times of tectonic movements this area was in- tensively uplifted and suffered from erosion and dissolution. Consequently, the original Printer-friendly Version peneplain was reformed into steep mountainous landscape with many intermountain lowlands (basins). As one of major rivers in southwest China, the Niulanjiang River Interactive Discussion 370 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | flows from southeast to northwest through the Ludian seismic area, with incision depth up to 1200–3300 m forming V-shaped valleys. On either bank of this river, the slope is NHESSD usually very steep. Almost 80 % of the slopes steeper than 40◦ concentrate along ei- 3, 367–399, 2015 ther bank of the Niulanjiang River. While in the north and east of the seismic area, the 5 terrain are relatively gentle, dominated by middle-height and low hills with steepness generally less than 20◦ (Fig. 3). Why the 2014 Ludian, Yunnan, China Ms 6.5 earthquake triggered 3 Seismic dam at Houngshiyan an unusually large landslide? The Ludian Ms 6.5 earthquake has triggered a huge landside on the right bank of the ◦ ◦ Niulanjiang River nearby the Hongshiyan village (27.035 N, 103.397 E). Meanwhile it Z. F. Chang et al. 10 also partly reactivated the old landslide on the opposite left bank, part of which thrust down into the river.