Conrad Lindholm, - Norwegian Seismic Array (NORSAR), P.O.
Hilmar Bungum,

Central America is located on the brim of the Caribbean plate with the major population centres near or within the seismically active volcanic chain which is running parallel to the coast.Tectonic earthquakes, both smaller local ones and larger ones from the Pacific subduction zone, occur with regular intervals, and while both types of earthquakes have different characteristics the damage potentials of both types are significant. Through a 10-year regional cooperation project between Norwegian and Central American institutions three microzonation studies for larger cities in Panama, Costa Rica and Nicaragua were successfully conducted.The study for San Jose, Costa Rica, was based on a seismic hazard study where ground motions were estimated using a probabilistic approach with ground excitation stemming from area sources and well as active faults for which the seismic activity potentials had been quantified. The resulting local hazard maps and associated equal-probability spectra served as one layer in the microzonation. The local geology and soil conditions were mapped in detail, and seismic amplification as function of location and frequency was quantified using a combination of empirical and analythical methods. A reasonable correlation between empirically defined amplification and the geology was found. In San Jose, where population growth and demands on housing is high, the microzonation maps offer an important support for the city administration and its technical departments in defining how the city should grow and develop. The results will moreover be used in updates of the Costa Rican national building code.