Seismic Hazard Analysis - part 1 | part 2 | part 3
Estimate of accelerations
Once defined the exceedence probabilities for the design earthquakes, the return period is determined for each of the probabilities used.
Based on these return periods, the maximum ground accelerations are obtained from the seismic hazard curve.
Seismogenetic sources
Two sources of seismicity had been identified for the western region of the Nicaraguan territory, both consequences of the subduction process.
The first is associated with the regional stresses acting in the faulting area within the Middle-American Trench. The subduction process generates earthquakes starting from approximately 10 km deep, under the oceanic floor; to maximum depths of up to 250 km. Seismicity depth in this area ranges from shallow to intermediate, and generally is of high energy.
The second mechanism is related to the local tension stresses acting within the Nicaraguan Depression, with shallow seismicity (practically superficial in the Managua area), and presenting low or intermediate seismic energy.
Earthquakes originated in the Subduction Zone represent a smaller threat for the Managua city area than those originated in the local fault system [Moore, 1990].
Attenuation equation
The accelerations induced in the ground by an earthquake at a particular site depend mainly on two stochastic values: the magnitude of the earthquake, M, and the hipocentral distance to the earthquake, R [Larsson & Mattson, 1987].
Acceleration associated to a site can be related to the magnitude of an earthquake and its distance to the point where it was originated, by means of attenuation laws.
Such equations are based on different quantities of data or in different quality of measurements made in diverse parts of the world. This is important of mentioning since the soil type and, in general, the geographical location of the earthquakes used to develop the attenuation equations determines its applicability to a given place.
The equation of Campbell (1981) is pointed out as the most suitable for small distances. Escobar and Corea (1998) checked the convenience of using this equation to analyse the effect of earthquakes locally originated in Managua.
