## 3 The Study Area and Spatial Datasets
### Geospatial Data  
Landslide inventory map for the URRB was constructed from two main sources: (i) historic landslides from the project VAST05.02/14-15 in 2015, which was prepared by [PERSON] et al. [40]; and, (ii) landslide polygons from the State-Funded Landslide Project (SFLP) 2016 [9], a national landslide program that is carrying out in Vietnam. The SFLP project has systematically investigated and collected historic landslides for all northwest mountainous provinces in Vietnam, including the study area. Accordingly, these landslides were mainly interpreted and mapped using aerial photos and field investigations. Detailed descriptions of methods and techniques for obtaining these historic landslides in the SFLP project are present in [9]. As result, a total of 255 historic soil-mixed-boulder slides that occurred during the last two decades were registered for the landslide inventory map (Figure 1). It is noted that many rock falls were excluded out of this research because their falling mechanism are very different when compared to that of the soil-mixed-boulder slides. Analysis of the landslide inventory map showed that these slides occurred due to rainfall during tropical rainstorms [40]. Our statistical analysis of these slides showed that the largest and the smallest landslides are 116627.9 m\({}^{2}\) and 6.2 m\({}^{2}\), respectively, with the mean is 3742.5 m\({}^{2}\) and the standard deviation is 11467.3 m\({}^{2}\). Approximately 9.1% of the landslide inventories are large landslides (>10,000 m\({}^{2}\)), whereas 9.1% of the landslide inventories are medium landslides (1000-10,000 m\({}^{2}\)), and the remaining are landslides less than 1000 m\({}^{2}\). Two examples of landslide photos in the study area are shown in Figure 3.  
Because the rainfall-triggered landslides in this study area occurred due to interactions of various geo-environmental factors, including topography, land cover, lithology, soil type, and river network [9; 66; 67; 9], these factors were selected for this analysis. Digital elevation model (DEM) with resolution of 25 \(\times\) 25 m for the URRB area was constructed using digital topographic maps 1:50,000 scale provided by the Ministry of Natural Resource and Environment of Vietnam. Using this DEM, three morphometric factors, slope, elevation, and aspect, were generated. To build the slope map (Figure 4a), seven categories were used. For the elevation map (Figure 4b), eight categories were considered. These categories were determined using Jenks natural break available in ArcGIS. For the aspect map, nice facing slopes were used (Figure 4c). Land cover map (Figure 4d) at scale of 1:50,000 with nine classes for the URRB area was derived from the project No.02/2012/ HD-HTSP funded by Ministry of Education and Training of Vietnam. The nine classes were obtained through the classification of Landsat 8 OLI imagery in 2013 using ENVI software. Soil type map (Figure 4e) at 1:100,000 scale with 13 soil types for the URRB area was provided by Department of Agriculture and Rural Development of the Lao Cai province. Figure 3: Two photos of landslides in the study area: (**a**) Landslide at the Mong Sen area and (**b**) Landslide at Km 7 Lao Cai. The two photos were taken by [PERSON] in August 2014. Lithological map for the URRB area was constructed based on National Geological and Mineral Resources Maps at scale of 1: 200,000, as provided by the Ministry of Natural Resource and Environment of Vietnam. Our analysis showed that more than 15 formations outcrop in the URRB area (see Figure 2). For this research, the lithological map with seven categories was constructed (Figure 4f) and these categories were separated based on clay composition, weathering characteristics, and material strength [68; 69; 24]. Detailed characteristics of the seven categories could be found in [PERSON], et al. [70].