Landslides DOI 10.1007/s10346-018-1017-x Received: 9 March 2017 Accepted: 15 May 2018 © Springer-Verlag GmbH Germany part of Springer Nature 2018 Shinro Abe I Dinh Van Tien I Do Ngoc Ha I Takashi Hoshide I Tadashi Nishitani I Toyohiko Miyagi Topography and landslides in weathered granitic rock areas—Hai Van landslide in central Vietnam Abstract Many landslides occur every year during heavy rains at the Hai Van Pass and surrounding area in central Vietnam, where granitic rocks are distributed. As is common in granite areas, these landslides often occur as small-scale flow-type and slump-type landslides. However, several horseshoe-shaped loose slopes of widths and lengths of 500 to 800 m, which incorporate these landslides, are observed on slopes across the area. These topogra- phies resemble those formed by past and present large-scale land- slides. The presence of such a topography and the repeated occurrences of landslides within this topography are rare in gran- ite areas, where shallow flow-type landslides are generally fre- quent. To understand the mechanism causing the landslides in the Hai Van region, and as a support for future risk assessment, the factors and processes leading to the formation of such a topography and their relationship with these landslides must be identified and assessed. This study investigated the history of past landslide movement in the Hai Van Pass and surrounding area through observations of drill cores and outcrops, and analysis of the direction of remanent magnetism in the granitic rocks. Mineral compositions, cracks, degrees of weathering, and topographic shapes of the granitic rocks and their relationship to the landslides occurring today were also investigated. The results of the study reveal no variation in the direction of remanent magnetism in the granitic rocks in the region that would indicate disturbance of the ground due to a past large-scale landslide. No evidence of such an event could also be found both in the drill cores and the rock outcrops. Further, results of the analysis of cracks and weathering pattern confirm that the topography of the region is affected by the weathering of the granitic rocks that progresses in concentric circles of various sizes. Thus, it can be concluded that these topographies were not formed by a singular large-scale landslide of the past, but rather by a composite of relatively shallow land- slides occurring on the slope of dome structures unique to granite areas, which are formed by differential weathering and denudation regulated by cracks. Keywords Vietnam . Hai Van landslide . Remanent magnetism . Granitic dome . Weathered granite Introduction Many shallow landslides of either slump or flow type, with widths and lengths of 30 to 200 m, occur every year on granitic rock slopes in the Hai Van Pass and surrounding areas, located 10 km north of the city of Da Nang, Vietnam (Fig. 1). The landslides that occurred behind the Hai Van Train Station during a heavy rain event in November 2007 destroyed railroad tracks and part of the station and led to the temporary suspension of railroad transpor- tation. In this region, there are major roads and railroads connecting north and south Vietnam. Therefore, the damages from these landslides had a profound impact on the transportation and economy of the country. On the slope behind the Hai Van Station, a horseshoe-shaped, gentle-sloped topography of width and length of 800 m and inclination of 10–15° is observed. Many landslides have recently occurred in this topography. Similar topography can be found in multiple locations on the slopes from Hai Van Pass to the coast (Fig. 2), where features usually formed by a slump-type landslide are displayed, such as steep cliffs at the head and on the sides of the landslide, as well as zones of depression along the steep cliffs (e.g., Cruden and Varnes 1992). However, the outlines of these topographies are not very clear, and they may have been formed by a large-scale pre-historic landslide. The occurrences of large-scale landslides in granite areas have been reported in the literature, such as the landslide in the Muonglay area of northern Vietnam (Khanh 2009) and the ancient landslide in the State of Arizona, America (Gootee 2008), but such cases are rare. Landslides in a granite area generally occur as a shallow flow-type landslide, usually triggered by heavy rain events on a slope in which granite has been weathered to sand. An example of this is the landslide in the Hiroshima region that caused 76 casualties in 2014 (Wang et al. 2015). The presence of a horseshoe-shaped gentle-sloped topography is indicative of a large-scale, slump-type landslide, and many recent landslides have occurred within such a topography around the Hai Van Pass. As noted above, such cases are rare in granite area. Therefore, it is important to identify the cause of formation of such topography and the factors that cause landslides in this topography in order to evaluate future risk, develop measures against landslides, and identify the landslide mechanism in granite areas. For convenience, in this paper, we refer to the large-scale horseshoe-shaped gentle-sloped topography as Blarge-scale land- slide topography^ regardless of its cause. Those landslides that are occurring numerously in recent years within these large-scale landslide topographies with widths and lengths of 30 to 200 m, are referred to as Brecent landslides.^ Finally, the large-scale land- slide topography behind the Hai Van Station is referred to as the BHai Van landslide.^ The causes of formation of the large-scale landslide topography observed in the granite area surrounding Hai Van Pass, together with its relationship with recent landslides, are hypothesized to be either one of the following: (1) The large-scale landslide topography is the trace of a past large-scale landslide, and the recent landslides occur on a mass that moved in the past and stopped on the slope. (2) The topography is formed by differential weathering unique to granite, and recent landslides occur within this topography. Landslides Technical Note