Slope unit-based distribution analysis of landslides triggered by the April 20, 2013, Ms 7.0 Lushan earthquake

Abstract

On April 20, 2013, a series of earthquakes with a maximum Ms = 7.0 hit central Sichuan, China. Thousands of landslides were triggered. One thousand two hundred eighty-nine earthquake-induced landslides were visually interpreted on Geographic Information System (GIS). Study area was divided into 1745 slope units through watershed segmentation of the ridgeline and valley line. This study was to investigate correlations between landslide and geologic, geomorphologic, and earthquake factors using two indexes. Landslide–area ratio (LAR), defined as the percentage of the area affected by landslides, was only 0.58 %. However, landslide number percentage (LR), ratio of slope units with landslides on all slope units, was 33.97 %. Results showed that LAR and LR had a positive relationship with slope gradients. Landslides occurred not only on steep slopes but also over a large distribution of gentle slopes. Gradients of all 1745 slope units were normally distributed, and so was that of 438 slope units with landslide occurrence, which might indicate a threshold state of slope units due to long-term and intensive modern tectonization in this area. The seismogenic fault had an strong impact on the spatial distribution of landslides: LAR and LR had a strong positive correlation with distance from the main seismogenic fault and the peak ground acceleration (PGA); they were higher on hanging wall than on footwall; landslide occurrences were concentrated slope units facing towards east, southeast, and south, coinciding with the trend of the seismogenic fault. Besides, slope composed of Triassic and Cretaceous were more susceptible to landslides.