Latin America is no stranger to earthquakes and records reveal seismic activity in Quito (Ecuador) dating back to the 1587 Guayllabamba earthquake. The urban centres built on deep alluvial basins have the potential to amplify the seismic waves thereby increasing the intensity of ground shaking. The rapid urbanisation in these cities only adds to the seismic risk of the built environment and human casualties. One of the objectives of this project was to build seismic hazard and risk models incorporating the local site effects that can be useful for engineers and policymakers in making an informed decision.
GEM scientists developed site response models for three urban centres, namely, Cali, Colombia; Quito, Ecuador; and Santiago, Dominican Republic, based on geotechnical data provided by local experts. GEM used the available methods to combine in a probabilistic framework, the seismic sources (such as faults), ground motion models and the local site response models to compute the intensity of ground shaking at the surface.
The end product includes 1) hazard maps - showing the level of ground motion at a certain probability of exceedance in the given time frame, 2) hazard curves and 3) uniform hazard spectra.
The results of site response analyses show in some cases (e.g. in Cali) the de-amplification of motion, whereas in others - like in Quito and Santiago - increased levels of shaking. In some zones in each of these cities, soil softening and hardening, as well as resonance was captured in the site response models. At some locations, the use of site-specific information in hazard calculation produces estimates that are nearly twice that of ground motion values at bedrock. Further, the study highlights the prominent non-linear behaviour in soft soils at high levels of ground shaking which can be captured only through site specific amplification models and their impact on seismic hazard and risk.
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The map shows peak ground acceleration values corresponding to 10% probability of exceedance in 50 years.