Effect of the Seismic Source on the Probabilistic Hazard Assessment in a Heritage City in Mexico: A Case Study of Morelia

Mexico is a geographical region prone to strong earthquakes from transform faults, interplate and intraplate faults, and shallow crustal earthquakes. The historic center of Morelia, the capital of the state of Michoacán, with more than 200 historic buildings, is a UNESCO World Heritage Site. The city is in the central region of Mexico and is frequently hit by earthquakes. This study evaluates the seismic hazard of Morelia using a probabilistic approach that incorporates information from a seismic network composed of 11 instruments installed in the city. To assess the expected soil amplifications, in addition to the seismic records, information from soil boring/sampling was also analyzed to evaluate transfer functions using both sources of information. The expected soil amplification when using spectral ratios, based on the information of the seismic network, or transfer functions using the properties of soil strata were similar. Moreover, a clear influence of the seismic source was observed on the expected spectral demands, which is relevant for the expected performance of the existing buildings and bridges, particularly for the city’s historic center. Disaggregation of the seismic hazard showed that earthquakes originating in intraplate and crustal seismic sources would put buildings in the city center at greater risk. Moreover, based on soil properties and using response and Fourier spectra, the study proposes seismic zoning and design spectra for Morelia. Finally, this study shows that intraplate earthquakes, which originate in various regions of the world, despite being less frequent than interplate events, can cause significant damage to buildings of limited height and residential structures. The evaluation of the expected seismic intensity, for the ground and structural periods, in the different areas of the city will be the basis for continuing with a seismic vulnerability assessment of Morelia and thereby quantifying the seismic risk for several return periods. The identified zones with the highest risk may be the subject of more refined studies that lead to preventive actions to mitigate the risk.