Seismic hazard scenarios for the city of San José,Costa Rica: Evaluation of critical ruptures on nearby faults

Abstract

Costa Rica has several active shallow (depth <15 km and Mw >5.3) crustal faults near towns with high seismic hazard. We present three critical deterministic hazard scenarios in San José city for the Belo Horizonte, Cipreses, and Río Azul fault systems, with Mw 6.2, 6.3, and 6.6, and closest distances to the rupture surfaces of 5.0 km, 4.3 km, and 5.2 km, respectively (from the capital center). We estimate accelerations in rock and soil conditions using adapted amplification factors. The results show Peak Ground Acceleration values reaching 0.6–0.8 g in San José and the Gran Área Metropolitana of Costa Rica, including soil conditions, the scenarios for the Cipreses and Río Azul faults being the most hazardous. We also estimated the response spectra for specific locations, comparing them with those spectra based on the seismic code for Costa Rica. We found that the code spectra were exceeded for rigid and medium soils in short structural periods, but they were conservative for softer soils. Finally, to highlight the relevance of our scenarios, we estimated the deformation using InSAR techniques and a potential fault reactivation assessment by analyzing slip and dilation stress tendencies. The small deformation observed (<3 mm/year) compared with nearby deformation in the Aguacaliente-Navarro fault systems (over 1 cm/year) could indicate either gradual stress accumulation or that scarce energy is being released seismically. Furthermore, analyses of the stress tendencies in the 3 faults indicate a high potential fault reactivation in a short-medium term hazard. San José has not suffered severe earthquake destruction in the last 500 years, although the occurrence of an earthquake such as the scenarios modeled could cause significant losses given the high exposure and vulnerability of the city. Our results provide a further step in seismic risk assessment for San José aimed at the development of mitigation measures.