Principal results of the Portoviejo (Ecuador) seismic microzonation project and importance for seismic hazard mitigation

Abstract The 2016 Mw7.8 Pedernales earthquake caused extensive damage along the central Ecuadorian coastline, resulting in 661 casualties, 4859 injured and the collapse of numerous buildings. The city of Portoviejo (Ecuador), located 140 km from the epicenter, was one of the most severely impacted municipalities with approximately 10% of its buildings damaged and 40 reinforced concrete buildings in the city center collapsed. A seismic microzonation study was conducted in Portoviejo to better understand the damage distribution and guide reconstruction efforts and future engineering projects. Based on a seismic hazard assessment and through equivalent linear analyses of multiple soil profiles integrating geological, geotechnical, and geophysical investigations, the study revealed that local conditions significantly influenced seismic ground motion. Specifically, the low Vs30 values, in the valley generally between 225 and 180 m/s, and even lower in some areas, played a critical role. Additionally, these conditions, associated with fluviomarine sandy deposits, contributed to induced effects such as soil liquefaction, which was a key factor in structural damage and infrastructure failure. These findings were used as primary criteria for delineating seismic microzones in the flat areas of the valley. The technical results of the seismic microzonation study, including response spectra for seismic microzones and soil liquefaction potential assessment, have been incorporated into municipal codes. While landslide hazard in Portoviejo is generally low, the risk could increase under extreme events. This integration has enhanced the city’s understanding of its natural environment and contributed to seismic risk mitigation efforts introducing site-specific parameters in the local building code.