Dependence of Seismic Hazard Assessment on the Observation Time Interval: Insights from a Synthetic Earthquake Catalogue in Southeastern Spain

Abstract. Traditionally Probabilistic Seismic Hazard Assessment (PSHA) is mainly based on historical and instrumental catalogues. The seismic catalogue in SE Spain covers a period of almost 1 kyr. Although this catalog can be considered long, its time span is not enough to record the complete seismic cycle of the slow moving regional faults, for example, the ones which conform the Eastern Betics Shear Zone (EBSZ). To assess whether the seismic hazard at the EBSZ depends on the time interval in which the earthquake catalogue has been recorded, this study performs a PSHA using synthetic seismicity. The synthetic seismicity in the EBSZ consists of a 1 Myr catalogue generated in previous studies using the RSQSim earthquake simulator, applied to the fault system that forms the EBSZ. This catalogue has been divided into ten thousand sub-catalogues of the same duration as the historical and instrumental one and randomly distributed over time. The magnitude-frequency distributions of the synthetic sub-catalogues show significant variability in the maximum reached magnitude, in the slope of the Gutenberg-Richter relationship and in the annual rate of earthquakes. A hundred sub-catalogues have been selected to perform individual PSHA and have been compared with the results derived from historical and instrumental seismicity. Using R-CRISIS software, this study obtains seismic hazard curves for the main cities in the region, showing the estimated return period for different values of Peak Ground Acceleration (PGA). The hazard curves reveal that each sub-catalogue leads to different return period values. The obtained variability ranges from 11 % to 21% for a PGA = 0.04 g, and from 25 % to 58 % for a PGA = 1 g. Our results show that there is a dependence between seismic hazard and the observation time interval in which an earthquake catalogue is recorded.