FAILURE MECHANISM OF LARGE LANDSLIDES TRIGGERED BY EARTHQUAKES IN THE EL SALVADOR FAULT ZONE: THE CASE OF JIBOA LANDSLIDE

In this work, we present the slope stability analysis and the failure mechanisms proposed for the Jiboa landslide, which is one of the greatest co-seismic slope instabilities, triggered during El Salvador earthquake of February 13, 2001, with a magnitude of M_w 6.7.

The seismic events that took place at the beginning of 2001, besides inducing a large number of shallow slope instabilities, triggered large-scale landslides putting hundreds of thousands of cubic meters of material into motion. These large landslides produced the greatest damage in both economic losses and human lives. In the case of the Jiboa landslide, it is characterized by a medium-low slope (20º-25º) with an estimated volume of 12 million m³ and located in an open valley with little thick surface material. It did not seem that seismic amplification (site effect) was the determining factor in its generation, as in the cases of the rest of recorded slope movements during the 2001 earthquakes (e.g. Las Colinas landslide).

The geological and geotechnical field investigation, laboratory tests, as well as the stability analysis for different conditions on the slope, reveal that the failure of the landslide took place in three independent phases. The triggering factor of the landslide is attributed to the existence of very porous and meta-stable volcanic ash intercalations, which collapsed due to the released seismic energy and lost their shear strength parameters (phase 1). This collapse produced a large sudden settlement and, consequently, a steep tension crack was generated in the head of the slope (phase 2). Finally, the movement propagated along a sub-horizontal plane with very low post-failure strength and located in such volcanic ash intercalations (phase 3).

The results of this study can explain also the failure mechanism of the numerous large paleo-landslides, which are rainfall stable, observed along the whole Jiboa river basin.