| 04 Dec 2025
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.
This manuscript explores how the length of the observation time window of earthquake catalogues influences probabilistic seismic hazard assessment (PSHA) results, using a 1-Myr physics-based synthetic catalogue for the Eastern Betic Shear Zone (EBSZ). The topic is timely and relevant for slow-deforming fault systems where historical and instrumental records span only a fraction of the seismic cycle. However, several key methodological aspects require clarification and stronger justification, particularly regarding the characterization and validation of the synthetic seismicity and the transparency of fault parameters used in the simulations, detailed below.
Fault parameters and slip rates
The manuscript relies heavily on fault-based modelling, yet essential fault parameters are not explicitly reported. It would be important to:
this information would be helpful for readers to validate the physical plausibility of the synthetic seismicity and the resulting hazard estimates.
Validation of the synthetic seismicity
The manuscript states that the synthetic catalogue reproduces the main features of observed seismicity, but quantitative validation is limited. How is the credibility of the RSQSim outcomes validated beyond qualitative agreement? Are synthetic and historical catalogues statistically comparable in terms of rates, magnitude distributions, and moment release? The cumulative seismic moment analysis is useful but should be more clearly integrated into the validation framework rather than presented mainly as a discussion element.
Maximum magnitude
Accessibility of key references
Several critical methodological choices (e.g., ZESIS parameters, Mmax estimation, declustering rules) rely on references written only in Spanish.
Interpretation of higher hazard from synthetic catalogues
The manuscript concludes that synthetic catalogues lead to systematically higher hazard than historical or ZESIS-based assessments. This interpretation should be refined. That is, historical catalogues do not necessarily always underestimate hazard; sampling a rare large event from a long-recurrence fault could also lead to overestimation. Alternative explanations should be discussed more explicitly, including catalogue incompleteness, overestimated slip rates or coupling in the synthetic model, and the absence of secondary faults or aseismic deformation.
Epistemic uncertainty
The manuscript states that epistemic uncertainties are common to all synthetic sub-catalogues, but fixing epistemic uncertainty does not eliminate it. The authors should clarify how epistemic uncertainties related to fault geometry, slip rates, coupling, and GMPE selection are treated, and how they differ from variability associated with the observation time window.
Thus, I suggest this manuscript can be published after a major revision.
Chung-Han Chan, National Central University, Taiwan, January, 2026.