Model of Soil Deposits - part 1 | part 2 | part 3
Model of basement
To carry out the calculations of ground response during earthquakes, it should be known the depth to the bedrock or similar, with shear wave velocity higher than that of the overlaying deposits [Ekholm & Norberg, 1998].
The basement is the material that can be considered rigid for practical purposes, and which is not overlaying materials with shear wave velocities significantly smaller [Dowrick, 1995] [Ovando & Romo, 1989].
Diverse approaches to define the equivalent rock bed have been established in several parts of the world. It is common to define the bedrock with Vs=700 m/s [Ordaz et al., 1989].
In Managua, results of recent geophysical studies [Ekholm & Norberg, 1998] locate the basement to depths ranging from 2 to 9 meters. On the other hand, Las Sierras Group is commonly identified as the bedrock in a geologic sense. However, it is not so easy to locate and to define the basement in the capital city in what refers to seismic aspects.
Although the materials of the Las Sierras group respond like an unit during seismic shakes, researchers coincide in consider inappropriate to locate the basement in the upper Las Sierras; the firm strata appear starting from 4 or 5 meters deep, but the shear wave velocity increase so slowly with the depth that a marked contrast in velocities doesn't exist with the overlaying soils like to consider those shallow firm layers as basement [Faccioli et al., 1973] [Johansson, 1988].
Recent investigations include studies to assess in a more detailed form the characteristics of the basement under Managua (Project «Managua: Ciudad Más Vulnerable»). From the preliminary results of those studies, an uniform model of upper Las Sierras was elaborated:
Model of the deposit
The pattern adopted to represent the soil profiles in the dynamic analysis is described as a system of n horizontal layers, which has a halfspace as the lower stratum. The thickness h, the density ρ and the shear wave velocity Vs (from which the shear module is determined), characterize each layer.