mechanisms of slope failure in volcanic soils during earthquakes

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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
examination indicates that the rock masses formed from rocks of this part of the successionshow layers of medium thickness, which are dominantly horizontal. Discontinuities showmedium spaced. Making use of Geological Strength Index (GSI) proposed by Hoek [4], theserock masses could be split into two categories of materials. The first category is thecompetent rocks with GSI between 50 and 60. These are normally rhyolites and dacites andshow low degrees of alteration. The second group of rocks is substantially weaker showingGSI values of 20-30 and generally consists of weak pyroclastic ashfall deposits with veryclosely spaced discontinuities. Material properties of these two groups of materials aregenerally moderately to extremely STRONG, while the latter group is dominated bymaterials which are WEAK. Additionally, palaeosols formed from tropical weathering ofvolcanic materials has resulted in the formation of local aquitards. Figure 1 shows rocks ofthe Balsamo formation exposed in the rear scarp of the landslide at Las Colinas. Figure 2shows a typical type of slope failure in the Tierra Blanca.Figure 2. The Tierra Blanca exposed in aroad cutting showing characteristic rock/debris fall type failureDEBRIS FLOWS TRIGGERED BY THE EARTHQUAKEThe earthquake of 13, January 2001 resulted in the initiation of a significant number of debrisflows. The most infamous of these was the landslide at Las Colinas. However, this was notthe only such failure; debris flows occurred extensively throughout the Balsamo Cordillera.The question of whether such landslides were the results of strong shaking, topographicamplification or some related site effect was investigated.
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mechanisms of slope failure in volcanic soils during earthquakes

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