Probabilistic seismic hazard analysis in La Paz-Los Cabos, Mexico: The importance of active Quaternary fault segments

A study of Probabilistic Seismic Hazard Analysis (PSHA) in the southern part of the Baja California Peninsula, Mexico is presented. In this study, the contribution of Quaternary fault segments are analyzed including two faults that have been considered potentially active by some authors, but inactive by others, namely, La Paz and San José faults. Therefore, different scenarios were tested to compare the hazard estimation with the contribution of these faults and finally a logic tree was proposed to add the epistemic uncertainties. In addition, three dams situated around the study area were chosen: La Buena Mujer, La Palma and Santa Ines. The peak ground acceleration values (PGA) were compared for 50, 100 and 200 years at 10, 5 and 2% of exceedance. The combinations of periods with percentages of levels of exceedance were used as reference for different degrees of hazard assessments. The preferred model is presented in a classical PSHA logic tree. Contrary to expectations, it seems that it is not so important to include all the seismic sources as La Paz and San José faults in hazard assessment of engineering design because in this region the characteristic faults are not sensitive for lower rates. However, this is only a mere artifact of the arbitrary decision of using return periods as a degree of protection. The present results show that in the case of essential facilities 2% probability of exceedance in 200 years is the best for this specific region. It is necessary to perform paleoseismic studies in this region, to know if these geological faults are active because many important structures are located close to the studied faults.