LANDSLIDE DISTRIBUTION BEFORE AND THE AFTER THE 2015 GORKHA EARTHQUAKES IN CENTRAL NEPAL: RELATIONSHIPS WITH DIP SLOPES AND VILLAGES

The distributions of landslides in Nepal before and after the April-May, 2015 Gorkha earthquakes have been mapped on Google Earth images and draped on top of dip slope and escarpment slope maps developed by calculating the topographic slope aspects and geological bedding intersection angles (TOBIA analysis). For landslides documented throughout Nepal before the April-May earthquakes, visual examination shows a clear strong spatial relationship among existing landslides and dip slopes, particularly those that are near or at critical threshold angles (30-40 degrees). This correlation is supported by statistical relationships. Remapping of a local area in central Nepal after the April-May earthquakes shows that post-earthquake landslides are also strongly correlated with threshold dip slopes. It is also noteworthy that more than 60% of the villages in this area are located on near-threshold or threshold dip slopes. Presumably this preference for village locations is a function of relatively smooth topographic surfaces on dip slopes, and the presence of soils thick enough for terracing and agriculture. The close association between dip slopes and landslides, however, makes the village location situation dangerous, particularly during times of intense monsoon rainfall when threshold dip slopes are at greatest risk of failure.