mechanisms of slope failure in volcanic soils during earthquakes

During January and February 2001 the Central American republic of El Salvador was shakenby a succession of strong earthquakes. Two significant shocks occurred which showedmarkedly different characteristics. The first of these two events had an epicentre within thesubducted slab of the Coccos plate at a depth of approximately 60 km, while the second had afocal depth of between 10 and 15 km. The mechanisms of these two events were notablydifferent. The former was an extensional event, the latter was almost pure strike-slipdeformation. The main earthquake of 13 January generated a peak horizontal groundacceleration in a North-South direction of 1.1 g in the town of La Libertad approximately 10km from the projected fault rupture. The second large event of 13 February generated a peakhorizontal ground acceleration of 0.41g recorded at Zacatecoluca approximately 9 km fromthe projected fault rupture.Numerous landslides were triggered by the main earthquake, many of which expanded due tothe second large earthquake of 13 February. Numerous large landslides which can beclassified as debris flows (Varnes [1], Dikau [2]) affected the Balsamo Cordillera. Themajority of these slope failures occurred in natural vegetated slopes in areas underlain byolder volcanic rocks.GEOLOGYThe geology of San Salvador is entirely volcanic (Schmidt-Thomé [3]). Three broadgeological units can be identified which are the Balsamo Formation; the CuscatalanFormation and the San Salvador Formation. Of these three geological formations, the mostimportant to this study are the Pliocene-Pleistocene rocks of the Balsamo Formation and aHolocene member of the San Salvador formation called the Tierra Blanca.Figure 1. The rocks of the Balsamo Formation exposed in the head of thelandslide at Las ColinasThe Balsamo formation is composed dominantly of felsic igneous rocks. These often forminterlayered rhyolites, dacites and pyroclastic deposits and occasional palaeosols. Field