the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-4870', Chung-Han Chan, 11 Jan 2026
- AC2: 'Responses to referee #1', Elena Pascual-Sánchez, 03 Mar 2026
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RC2: 'Comment on egusphere-2025-4870', Francesco Visini, 20 Jan 2026
The manuscript is well written and supported with informative figures. The methods employed are applicable, appropriately applied and adequately described. I have only some minor/moderate suggestions prior to publication.
General comment on the use of synthetic sub-catalogues
The manuscript states that 100 sub-catalogues of 1 kyr duration are randomly extracted from a 1 Myr synthetic catalogue and used for PSHA. However, I would like to suggest clarifying more explicitly that each sub-catalogue represents only one possible realization of earthquake occurrence over a limited time window, and not the long-term “true” seismic hazard of the system. As currently written, the text may give the misleading impression that each sub-catalogue is equally representative of the long-term hazard, whereas they should instead be interpreted as stochastic realizations sampling different portions of the seismic cycle.
Slip-rate input and data origin.
The mean slip rate of the fault system (1.5 ± 0.3 mm/yr; Echeverría et al., 2013) is a key input of the synthetic model and therefore strongly controls the long-term seismic moment release and earthquake productivity. However, its origin is not discussed. I would like to suggest clarifying how these slip rates were derived (e.g., paleoseismological trenching, geomorphology, geodesy) and over which temporal window they are averaged. This information would be important for assessing the consistency between the geological input and the simulated seismicity.
Magnitude–frequency distributions of sub-catalogues
It may be useful to include a figure showing the variability of the magnitude–frequency distributions (MFDs) of the individual sub-catalogues compared with those derived from the historical/instrumental catalogue and from the ZESIS database. This would help the reader to visually assess how representative the synthetic sub-catalogues are with respect to observed seismicity and to the long-term model.
Spatial variability of hazard curves
In Figures 8 and 9, the hazard curves and return periods differ significantly between cities. It is not clear why such variability is expected if all sites belong to the same tectonic domain (same seismogenic area). If the cities are located within the same source zone, the observed differences may simply reflect source discretization effects (e.g., edge vs. central locations). I would like to suggest discussing this point more explicitly. Alternatively, if some cities are located outside the main study area, it may be worth questioning whether including peripheral sites is necessary.
Interpretation of epistemic vs aleatory uncertainty
The manuscript states that the use of multiple sub-catalogues allows the evaluation of epistemic uncertainty. However, since all sub-catalogues are generated from the same synthetic model with identical fault geometry and slip-rate inputs, epistemic uncertainty is common to all of them. I would like to suggest that the variability observed among sub-catalogues mainly reflects aleatory variability associated with earthquake occurrence. In practice, the study is quantifying the dispersion of hazard estimates around the long-term mean due to temporal sampling effects.
Complex ruptures and maximum magnitudes
The manuscript suggests that complex ruptures are more common than previously thought and associates the largest magnitudes (Mw ≈ 7.2) with such events. However, it is not entirely clear from the figures whether “complex” refers to multi-segment/multi-fault ruptures or simply to large slip values on individual faults. I would like to suggest clarifying this distinction. Moreover, it may be useful to explore whether the large magnitudes are controlled primarily by rupture geometry or by slip distribution, and whether this can be verified within the simulation framework.
Interpretation of Table 3 and justification of the ensemble size
The justification of the use of 100 sub-catalogues based on the similarity of the Gutenberg–Richter parameters in Table 3 is not entirely clear. I would like to suggest explaining more explicitly how this comparison demonstrates that the ensemble provides a statistically stable estimate of the long-term seismicity model.
Synthetic vs observed hazard levels
The manuscript concludes that hazard derived from historical and instrumental catalogues is less conservative than that obtained from synthetic seismicity and attributes this to the limited sampling of large earthquakes. While this is a plausible interpretation, it implicitly assumes that the synthetic model represents the “true” long-term hazard. I would like to suggest treating this assumption more cautiously. Historical catalogues do not necessarily always underestimate hazard: the sampling of a rare large event on a long-recurrence fault could also lead to an overestimation (assuming the average recurrence represents the “true” hazard). Alternative explanations could be discussed, including catalogue incompleteness, uncertainties in slip-rate estimates or fault coupling in the synthetic model, and the possible absence of secondary faults or aseismic deformation.
Role of very large magnitudes in hazard
The manuscript highlights a significant contribution of magnitudes larger than Mw 7.2. However, in my experience (mainly in Italy, so this may not be fully transferable), such events are not expected to dominate hazard at return periods of 500–2000 years and may be more relevant for risk analysis than for classical PSHA. I would like to suggest further discussing this point.
Validation/comparison with observations of the synthetic catalogue
The manuscript states that the synthetic catalogue reproduces the main features of observed seismicity, but quantitative validation is limited. I would like to suggest discussing whether synthetic and historical catalogues are comparable in terms of rates, magnitude distributions, and cumulative moment release. The cumulative seismic moment analysis is useful, but I would encourage integrating it with a direct MFD comparison.
Best Regards
Citation: https://doi.org/10.5194/egusphere-2025-4870-RC2 - AC1: 'Responses to referee #2', Elena Pascual-Sánchez, 02 Mar 2026
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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.