mechanisms of slope failure in volcanic soils during earthquakes

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Data on the location of landslides were collected using SPOT monospectral satellite imagerycollected on 28 January 2001 (figure 3). These data have a spatial resolution of 10m.Additional geographic data on debris flows were collected by field survey. The majority ofthe debris flows was large, and was easily observed on SPOT data.SPATIAL DISTRIBUTION OF LANDSLIDESThe majority of landslides of all types occurred in the Balsamo Cordillera. The ridge itselfwas strongly affected either by rockfall and debris fall type landslides that resulted in seriousblockages due to loss of a section of the road, or, because of landslide debris blocking thecarriageway. Due to the metastable nature of the material that failed it, was not uncommon tofind the road surface buried in approximately 0.1 to 0.2 m of silty sand.Landslide densities in the order 100-200 failures per kilometre of road were observed alongthe Balsamo Ridge between Santa Tecla and the town of Comosagua. The majority of thesefailures were of the rockfall and debris fall varieties, with occasional translational slides.Where rocks of the Balsamo Formation were exposed at crest of the slope larger, rock blockfalls occurred.NDendritic drainage patternformed on terrain dominatedby the Balsamo FormationThe Balsiamo R dgeLas BarriolerasLas ColinasComosaguaFigure 3. SPOT monochromatic image (near infra-red) of the area west of SanSalvador. Landslides appear as white areas on the image.The incidence of debris flows appears to be unrelated to the distance between the epicentreand the site of slope failure. Based on the simple attenuation of seismic energy between thesource and a potential landslide, it would seem logical that a decrease in landslide incidencewould occur with distance. Such a relationship is in fact supported by Bommer [5] who bothobserve that to trigger landslides at a larger epicentral distance, a greater energy release isrequired. Equally therefore, in terms of work done, it would be expected that the size of thelandslide would be related to the energy arriving at the site.
Figure 4 shows a plot of the area of landslide area against the epicentral distance. The area ofthe landslide was measured from SPOT data, and therefore concentrates on the larger failures(smaller landslides, less than about 30 m, being beneath effectively below the resolution ofthe SPOT data). It can be seen that there is no relationship between the epicentral distanceand the size of the landslide. There does however appear to be an upper boundary, thereforewhile there is no simple relationship between the energy arriving at the site, there is clearly apoint where there is insufficient energy to induce further slope deformation.25area of landslide (acres)2015105090 95 100 105 110epicentral distance (km)Figure 4. Graph showing the relationship between the sizeof landslide measured as an area and the distance to theepicentre of the 13 January 2001 earthquake.landslide enlargement (%)16014012010080604020030 35 40 45 50 55 60epicentral distance (km)Figure 5. Graph showing landslide enlargement after the13 February 2001 earthquake related to epicentralBecause of the second large earthquake of 13 th February 2001, many landslides increased involume. Again, dealing only with large debris flows triggered by the earthquake, it can beseen that there is no correlation between the enlargement of a given landslide and the distancebetween the epicentre and the landslide (figure 5). This again tends to suggest that the
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mechanisms of slope failure in volcanic soils during earthquakes

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