Distribution pattern of coseismic landslides triggered by the 2014 Ludian, Yunnan, China Mw6.1 earthquake: special controlling conditions of local topography

摘要

The distribution of coseismic landslides triggered by the 2014 Ludian, Yunnan, China Mw6.1 earthquake exhibits a particular pattern. They do not concentrate linearly along a fault (e.g., the causative fault) or a tectonic zone, instead mostly appear in the southeast and west of the epicenter of the main shock in a relatively clustering manner, 5-9 km distant. Majority of them occur at the river reaches with cliffs, including the unusually large-scale landslide at the Hongshiyan village, north bank of the Njiulanjiang River. Such a phenomenon implies that local conditions of terrain of the river valley might control the distribution of coseismic landslides induced by the Ludian event, which is of a moderate magnitude. To verify quantitatively this intuitive conjecture, this study proposes a parameter of the relative landslide area ratio, with which a statistics is made for landslide area ratios and local terrain conditions. The results demonstrate that this ratio value is highly correlated with the three terrain variables, i.e., slope angle, local relief, and slope aspect (facing direction of the slope). In the study area, most coseismic landslides occur at loci with slope angle greater than 30A degrees-40A degrees, particularly the river reaches with slope angle exceeding 50A degrees. Localized distribution of the landslides is positively correlated with terrain relief, of which majority lies at places with altitude differences over 400 m. The side slopes facing south and southeast are most prone to occurrence of coseismic landslides, which may be explained by the mainshock process that the seismogenic fault ruptured from northwest to southeast and from deep to shallow. It may also be an account on which there is no coseismic landslide southwest of the epicenter, though there also exist river reaches with steep slopes. It means that the seismic source process can pose a constraint on the control of local terrain conditions on the distribution pattern of earthquake-induced landslides.