Landslides triggered by earthquakes and their relations with faults and mountain slope geometry: an example from Ecuador

A large number of landslides occurred during two seismic events (respectively, 6.9 and 6.1 Msw) on 5 March 1987 in the Ecuadorian Andes. These landslides have been mapped, digitized, and coregistered with topography at 1:50,000 scale. Geometry of coseismic and Holocene faulting has been assessed integrating field and geophysical data. Landslide distribution and Holocene tectonic features have been compared with earthquake foci, geological deposits, slope area, inclination and orientation, and vegetation cover. The macro-seismic field deduced by landslide distribution is ellipse-shaped with the major axis striking NNE and coinciding with the trend of the coseismic and Holocene faults. Landslide distribution also shows a correlation with respect to the dip-direction of these faults and the orientation of mountain slopes. Slopes parallel to the coseismic fault planes, sloping towards WNW, are almost unaffected by landsliding, while this increases on the slopes at high angle to the fault plane and lying along its strike. If these results could be confirmed in other areas, a method of reconstruction of geometry of seismogenetic faults from topographic effects appears feasible.