Seismic ground motion of sedimentary valleys - example La Molina, Lima, Peru

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J. Zahradnik
F. Hron


Strong motion accelerograms recorded at two sites in Lima, Peru, during the earthquake of November 9, 1974, exhibit serious dissimilarities although the sites have nearly the same epicentral distance. The two sites are the Instituto Geofisico del Peru in central Lima and the La Molina sediment-filled valley on the periphery of the city. The anomalously strong and prolonged ground motion at the La Molina site seems to be explained by a combined effect of the complex topography of the bedrock and the presence of low-velocity subsurface sediments. In contrast to an intuitive feeling, a strong velocity contrast along the whole sediment-bedrock interface is not necessary. Because severe earthquake effects in La Molina are of site origin, they should be expected to repeat in the future. As indicated by synthetic accelerograms, the anomaly refers to large areas of the La Molina valley and not only to the immediate vicinity of the recording point. For purposes of seismic microzoning and land-use planning, two microzones in the studied part of the valley will probably be appropriate.

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Zahradnik, J., & Hron, F. (1987). Seismic ground motion of sedimentary valleys - example La Molina, Lima, Peru. Journal of Geophysics, 62(1), 31-37. Retrieved from
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