Earthquake-Induced reactivation of large landslides: Vera-Cruz Landslide, El Salvador

El Salvador has suffered several destructive earthquakes during the past 100 year, causing severe damage and a great social alarm fundamentally associated with induced-landslides. The losses by the landslides exceed that directly caused by the earthquake itself. For example, in January 13 and February 13, 2001, two earthquakes (Mw 7.6 and Mw 6.6 respectively) triggered at least 10000 landslides, killing more than 800 people, damaging many roads and burying villages. The triggered landslides were of different types, ranging from rockfalls and relatively shallow slides, to large deep-seated landslides, being the latter the most damaging. Most of the landslides in El Salvador are concentrated in the central part of the country where recent volcanic, unconsolidated pyroclastic deposits exist, those being prone to large seismic amplification due to their special geotechnical characteristics. Landslides generally occur during earthquakes or in a short time after the seismic event. Besides, the reactivation of large landslides which has been triggered by previous earthquakes is common. In this work, a very large paleo-landslide (Vera-Cruz landslide) located also in the highest landslide concentration area of El Salvador is identified and mapped. The objective is the study of the relationship between this paleo-landslide (triggering or reactivation) and four large earthquakes, occurred between 1982 and 2001, through Newmark coseismic displacement analyses. Geotechnical properties and static factor of safety were established by performing a limit equilibrium back-analysis for a non-circular failure surface. Then the critical acceleration is obtained, using the geometry of the slope prior to the landslide. The peak ground acceleration of the site was estimated using four ground motion prediction equations established for Central America, for both volcanic arc and subduction zone. Finally, the Newmark displacement considering the influence of local amplification effects is estimated using four different empirical relationships proposed for volcanic areas. The results of the study ​​indicate that the Vera-Cruz landslide could have been triggered first by the 1982 (Mw 7.3) earthquake and subsequently reactivated by the January 13, 2001 (Mw 7.7) and/or February 13, 2001 (Mw 6.6) earthquakes. The result of this work can help in refining the study and prediction of earthquakes triggering paleo-landslides in the area, being that useful for evaluation and mitigation of coseismic landslide hazard in the region.