Seismic Hazard Analysis - part 1 | part 2 | part 3
Determination of magnitudes
Once obtained the maximum accelerations from figure 5 (0,29g for the operative earthquake and 0,56g for the accidental case) and, using a hipocentral distance of 5 km, from Campbell (1981) attenuation equation, values of magnitude of 6,2 and 7,8 are deduced for the operative and accidental earthquakes, respectively.
It is assumed that the seismic history of a region gives a key of what can be expected in the future; that the same pattern will
continue.
When analysing the seismic history of the city (table to the right) and the studies that estimate the highest magnitudes that can be expected (table below), the possibility that a seismic event with magnitude 7,8 occur in the fault system of Managua is discarded.
So it was decided to take as accidental earthquake an event with magnitude of 6,75, following the arguments presented by Montero (1990) on the possible maximum earthquake along the Central American quaternary volcanic chain, where the observed event of highest magnitude was Ms = 6,8, occurred in southwest Guatemala in 1930.
Notice that a value higher than the one proposed by Montero himself was taken, however likely to occur, as for the type of associated faulting.
Estimate of the predominant period
The ground structure at the site could amplify the movements transmitted to the surface, as a consequence of resonance of the
natural period of the soil deposit with the predominant period of the earthquake. Also, the worst damages will take place if the
predominant period of the earthquake corresponds to the fundamental period (or resonance period) of the structures.
The frequency content, especially in the epicentral area, is presumed to be a function of the mechanism that originated the earthquake [Dowrick, 1995].
The illustration shows the tendency of the predominant period toward highest values as the distance increases from the epicentre can be seen, this because the high frequency movements are filtered, fading out. This phenomenon was studied by Housner in 1959.
Estimate of the duration
The relationship between the duration of the strong movement and the magnitude of the earthquake was proposed by Housner in
1965 and then confirmed by Donovan in 1974. The duration of the earthquakes used in this study is of 16 seconds,
approximately the same duration of the Managua earthquake of 1972.