Landslides
Landslides triggered by the earthquake were observed during ground reconnaissance involving vehicular traverses along both the Lima-to-Ica segment of the Pan-American Highway (north-south oriented) and the east-west oriented Highway 024A. The Highway 024A traverse originated near the coast close to Pisco and continued east along the highway roughly 110 km inland to an elevation of approximately 4,500 m. It is estimated that in total the earthquake triggered thousands of landslides, with a significant majority of these being disrupted landslides including rock falls, rock slides, soil falls, soil avalanches, and disrupted soil slides (see Keefer 1984 for detailed descriptions of these categories). Disrupted slope failures occurred in both natural and altered (e.g. road cuts) terrain. Reports by the highway department indicate that rockfalls occurred over a vast region stretching as far a north as the Department (State) of Chachapoyas, at approximately 700 km from the fault plane (see newspaper insert in Spanish). As noted earlier, many liquefaction-induced lateral spreads were also observed, mostly along the coast in the epicentral region. The following paragraphs discuss several noteworthy landslides.
Figure 4.1 shows a complex soil avalanche thought to be triggered by liquefaction-induced soil softening or ground movement near the toe of the slope. The landslide occurred near Jahuay along the Pan-American Highway, which was partially blocked at this location after the earthquake. This feature is discussed in the Liquefaction Section (Section 3.2).
The massive rockfall shown in Figure 4.2 occurred along Highway 024A roughly 44 km from the fault plane at an elevation of approximately 500 m. The rockslide, which fully closed the highway for 3 days, was in the process of being removed during the visit to the site on August 23, 2007. Highway personnel estimated that 8,000 m3 of material was removed prior to this date. The total volume of rockfall debris is estimated to be over 20,000 m3. The rockfall involved blocky rock with closely spaced, and unfavorably oriented, joints. Geologic maps indicate that rocks at the site are Lower Cretaceous monzonites. Most of the slide debris consisted of blocks ranging between 0.2 and 2.0 m in diameter, though several 3.5 m blocks were also observed on the roadway. After filling the roadway, debris spilled over a cliff located to the right (south) of the road, ultimately blocking a water canal. During the visit, debris, including a number of massive (several meters in diameter) rock blocks, were being blasted to reinstate flow in the canal. It is worth noting that away from the oversteepend rockcuts, the rocks weather to gentle slopes (estimated 35 to 50 degree), which showed little if any evidence of instability during the earthquake.
Figure 4.3 shows the upper portion of one of two adjacent rockslides occurring in natural terrain along Highway 024A at an elevation of approximately 2250 m, and about 65 km from the fault plane. The landslide debris cone was 40 m wide at its base and extended to an estimated height of 150 m. The rock slope consisted of moderately fractured and jointed igneous rock. Rock slide debris, consisting of blocks as large as 3 to 4 m in diameter, spilled into a major river below restricting (but not blocking) the flow of water.
The disrupted soil/rock slide shown in Figure 4.4 was encountered at an elevation of 2020 m along Highway 024A. The closest distance to the fault plane is estimated at 65 km. A significant quantity of slide debris completely covered the road surface after the earthquake, closing off the highway at this location. Debris was removed to allow passage of one lane of traffic prior to the reconnaissance team visit to the site. The slope consisted of sedimentary deposits of large (up to 1 m), subrounded igneous rocks in a silty sandy soil matrix. Other nearby stable road cuts in this material were inclined at 60 to 65 degrees. In contrast to many of the rockslides seen in the region, this disrupted soil/rock slide had significantly more run out, and ultimately spilled over the road surface and into the canyon to the right (north) of the road. It is worth noting that a similar failure occurred in this material across the canyon in natural terrain.
References
Keefer, D. K., 1984, Landslides caused by earthquakes: Geological Society of American Bulletin, v. 95, p. 406-421
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Figure 4.1 Soil avalanche along the Pan-American Highway (8/21/07 4:53PM, S13.7102 W76.2183). |
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Figure 4.2 Rockslide along Highway 024A (08/23/07 9:42AM, S13.7106 W75.8401).
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Figure 4.3 Rockslides in natural terrain (08/23/07 5:53PM, S13.6061 W75.3969).
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Figure 4.4 Disrupted soil/rock slide along Highway 024A (08/23/07 12:26PM, S13.6103 W75.4159). |
