Faulting, shallow seismicity and seismic hazard analysis for the Costa Rican Central Valley

Abstract

Faulting, shallow seismicity (0–30km), and seismic hazard of the Costa Rican Central Valley were analyzed. Faults in the study area are oriented northwest or northeast. There is an active fault system in the south flank of the Central Volcanic Ridge and another in the north flank of the Talamanca Ridge. Faults of these systems have generated 15 destructive earthquakes in the area during the last 228 years all of them shallow and their locations show one cluster near the Poas Volcano and another southward the Central Valley. These earthquakes have damaged cities of the Central Valley, two of them destroyed Cartago city and almost 1000 people were killed. Regarding recent seismicity, there are three main seismic sources at the Central Volcanic Ridge: Irazu, Bajo de la Hondura and Poas and other three in the Talamanca Ridge: Puriscal, Los Santos and Pejibaye.A seismic hazard map for the Metropolitan Area of San José has been elaborated, based on local tectonic and seismic information. The area for the hazard computation covers an area of 20×15km2 and includes the zone where the most population and socioeconomic activities are concentrated. The computation analysis are based on areas zones and faults, each one characterized by recurrence parameters, geometry, minimum and maximum magnitude and source depth. A recent local spectral attenuation model, which includes relations for shallow crustal sources and subduction zone earthquakes, has been applied in this study. The seismic hazard results are presented in terms of contour plots of estimated peak ground acceleration (PGA) for bedrock conditions for return period of 50, 100 and 500 years. In the Central Park of San Jose City the following PGA values were found: 0.29g for 50 years, 0.36g for 100 years, and 0.53g for 500 years.