Results
The response spectra from 58 points within Managua City were analysed, and were plotted with 5% critical damping value, which is a pertinent value from the point of view of the structural engineering [Pérez, 1973].
Then, smoothened spectra for the two probabilities of exceedence of interest were proposed.
Results obtained from the dynamic response show that the natural periods of the analysed deposits oscillate between 0,09 and 0,45; 90% doesn't exceed 0,20 seconds.
The response spectrum at the base rock subjected to the accidental earthquake also presents a peak with a maximum of 0,99g at period 0,62s.
A similar peak can be observed in the study from Faccioli, Santoyo and León (1973). Therefore, that peak can be considered part of the frequencies content of the 1972.12.23 earthquake, from which the design accelerograms were generated.
Since the accidental earthquake is of higher magnitude, it would be expected that the response at the surface under this condition was greater. However, the surface accelerations under the operative earthquake are bigger in practically all the cases. This result is attributed to the characteristics of the frequencies content of the accelerograms generated, and it is not considered to be wrong. Nevertheless, the frequencies content of a real event can be different to the frequencies content of the accelerogram used in this study, even if the real event presented the same parameters of the design earthquake used. Therefore, an earthquake of such a magnitude (accidental case) will not necessarily produce accelerations in surface minor that one of similar magnitude to that of the earthquake of 1972.
The spectrum of the operative earthquake has been smoothened following the curve y=e(-0,623+1 044,87x3) for periods smaller than 0,1 seconds; between 0,1s and 0,4s the acceleration is constant and equal to 1,45g; and for periods greater than 0,4s the curve y=(0,317+0,129/x2)2.
The ordinate of acceleration in the plateau has been fixed in this case, as well as for the spectrum for the accidental earthquake, following the recommendation of Newmark (1979) to draw the smoothened spectra with more or less 90% of the calculated real values.
Curves to smoothen the spectrum for the accidental earthquake are y=(0,698+321,855x3)2 for accelerations smaller to 0,1 seconds; between 0,1s and 0,3s the acceleration is constant and equal to 2g. Finally, for periods greater than 0,3s the accelerations decrease according to the equation y=(-0,156+2,231x)-1. The latter, contrary to the curves previously showed, was not selected to adjust better to the data within the given period intervals, but rather deliberately taking greater acceleration values in the range from 0,3s to 0,56s, for not underestimate the peak at 0,62 seconds.
Average and smoothened response spectra for the operative earthquake
Average and smoothened response spectra for the accidental earthquake
The values of acceleration in that period (0,62s) usually range between 1g and 1,15g in all the sites, except for four, one of those, the ESSO Refinery (Site 1, with a natural period of 0,45s), presents an acceleration of 1,75g; hardly smaller than the maximum spectral acceleration of 1,81g.
The other three sites are part of a group of seven, with maximum spectral accelerations greater than 3g, where there are high probabilities to develop the resonance phenomenon between their own natural period and the predominant period of the earthquakes. The deposits in question are Altagracia (Site 56), Autonomous National University of Nicaragua - RURD (Site 57) and the National Theatre (Site 9), with natural periods of 0,29s, 0,27s and 0,31s, respectively. Meanwhile, the predominant periods of the design earthquakes are 0.27s and 0.3s.
Such singularity fo the response at these sites is not surprising, since already previous studies [Faccioli et. al., 1973] [Johansson, 1988] [Martínez, 1977] [Woodward-Clyde, 1975] show results comparable with those here presented.
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