mechanisms of slope failure in volcanic soils during earthquakes

developed into debris flows. Evidence suggests that the process of 'fluidisation' of the slidingmass was associated with the collapse of the soil structure of the Tierra Blanca observed inthe laboratory.The occurrence of debris flows appears to be controlled, at least partly, by the geologicalconditions. The majority of this type of slope failure had an origin where the Tierra Blancawas deposited on top of the older, stronger and less permeable Balsamo Formation. In theabsence of strong motion data, it is impossible to rule out significant differences in theseismic response of the two different geological units. However, it seems likely that theprincipal control was hydrogeological. The frictional strength of the Tierra Blanca at peak,and even residual, strength conditions is not consistent with the long runout conditions ofmany of the landslides observed in the field and on satellite imagery.The use of geological and geomorphological observations can be used as a 'first order'method of identifying future hazards.ACKNOWLEDGEMENTSThe authors wish to acknowledge the receipt of funding from the Natural EnvironmentResearch Council, UK (NER/A/S/00030) and the Royal Academy of Engineering. Theassistance of colleagues from PRISMA (especially Herman Rosa) and the University ofCentral America was invaluable during firledwork.REFERENCES1. Varnes, D. J. Slope movement type and processes. In: R. L. Schuster & R. J. Krizek,(eds), Landslides: analysis and control: 11-33, Washington. Transportation ResearchBoard Special Report 176.2. Dikau, R., Brunsden, D., Schrott, L. & Ibsen, M. L. Introduction. In Dikau, R., Brunsden,D., Schrott, L. & Ibsen, M. L. (Eds), Landslide Recognition: Identification, Movementand Causes. 1996, pp251, Wiley, Chichester, ISBN 0-471-96477-8.3. Schmidt-Thomé M. The geology in the San Salvador area (El Salvador, CentralAmerica), a basis for city development and planning. Geol. Jb. 1975; 13: 207-228.4. Hoek, E. The strength of rock and rock masses. News Journal, International Society ofRock Mechanics, 1994, 2, 4-16.5. Bommer JJ, Rodríguez CE. Earthquake- induced landslides in Central America.Engineering Geology 2002, 63(3/4).6. Murphy, W., Petley, D.N., Bommer, J.J. & Mankelow, J. M.. Uncertainty in groundmotion estimates for the evaluation of slope stability during earthquakes. QuarterlyJournal of Engineering Geology and Hydrogeology, 2002. In press.7. Bommer JJ, Rolo R, Mitroulia A, Berdousis P. Geotechnical properties and seismic slopestability of volcanic soils. In: Proceedings of Twelfth European Conference onEarthquake Engineering; UK: London, Paper No. 695, 2002.8. Mavrommati, Z. C. Seismic behaviour of slopes in an undersaturated volcanic soil. M.Sc.Dissertation, 2000. Imperial College London.9. Holzer, T. L., Youd, T.L. & Hanks, T.C. Dynamics of Liquefaction During the 1987Superstition Hills, California, Earthquake. Science, 1989,Vol. 244, pp. 56-59.