the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2023
Probabilistic Seismic Hazard Assessment of Sweden
Abstract. Assessing seismic hazard in stable continental regions (SCR) such as Sweden poses unique challenges compared to active seismic regions. With diffuse seismicity, low seismicity rate, few large magnitude earthquakes and little strong motion data, estimating recurrence parameters and determining appropriate attenuation relationships is challenging. This study presents a probabilistic seismic hazard assessment of Sweden based on a recent earthquake catalogue which includes a large number of events with magnitudes ranging from 5.9 to -1.4, enabling recurrence parameters to be calculated for more source areas than in previous studies, and with less uncertainty. Recent ground motion models developed specifically for stable continental regions, including Fennoscandia, are used in logic trees accounting for their uncertainty and the hazard is calculated using the OpenQuake engine. The results are presented in the form of mean peak ground acceleration (PGA) maps at 475 and 2500 year return periods and hazard curves for four seismically active areas in Sweden. We find the highest hazard in the northernmost part of the country, in the post-glacial fault province. This is in contrast to previous studies, which have not considered the high seismic activity on the post-glacial faults. We also find relatively high hazard along the northeast coast and in southwestern Sweden, whereas the southeast and the mountain region to the northwest have low hazard. For a 475 year return period we estimate the highest PGAs to be 0.04–0.05 g, in the far north, and for a 2500 year return period it is 0.1–0.15 g in the same area. Significant uncertainties remain to be addressed in regards to the SCR seismicity in Sweden, including the homogenization of small local magnitudes with large moment magnitudes, the occurrence of large events in areas with little prior seismicity and the uncertainties surrounding the potential for large earthquakes on the post-glacial faults in northern Fennoscandia.
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RC1: 'Comment on nhess-2023-213', Anonymous Referee #1, 03 Jan 2024
The manuscript presents a seismic hazard analysis for Sweden, a low-seismicity country in northern Europe. It includes information about seismicity, geology, the available earthquake catalog, and post-glacial faults there. The preprocessing of the earthquake catalog is explained in detail. The ground-motion logic tree is perused. As outcomes, the manuscript presents two seismic hazard maps for Sweden, with mean estimates for peak ground acceleration corresponding to return periods of 475 and 2500 years, and new hazard curves for four plus one sites. The previous seismic hazard analyses in the country are reviewed and the new results are compared to the new ones. The new hazard maps are also compared to the European Seismic Hazard Map 2020 (ESHM20). My main suggestion is to strengthen the results and align with the previous work for Sweden over 20 years ago by augmenting disaggregation. The line-by-line suggestions mainly deal with lesser issues.
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RC2: 'Comment on nhess-2023-213', Ilaria Mosca, 12 Feb 2024
The aim of the paper NHESS-2023-213 “Probabilistic Seismic Hazard Assessment of Sweden” is to present seismic hazard estimates (hazard maps and hazard curves for selected sites) for Sweden using the probabilistic seismic hazard analysis (PSHA). This country is characterized by low levels of seismicity and therefore the time length of the earthquake observations, which span a few hundreds of years in the best case, is much shorter than the seismic cycle of large earthquakes, which is of the order of thousands of years in low seismicity regions (e.g. Stein et al. 2015). Using sparse and limited sets of data represents a challenge to fully capture the epistemic uncertainties in a national seismic hazard model. In this context, the aim of this paper is of primary importance for seismic hazard analysis. However, there are some inaccuracies in the manuscript (e.g., the description of the steps for PSHA) and more explanations to justify the decisions taken by the authors to develop the seismic hazard model for Sweden are required. Furthermore, the English language seems to be quite poor in some paragraphs. Although I provide below some editorial comments on wording and sentences, I would suggest a significant revision in terms of the language throughout the manuscript.
Here I list the main technical and editorial points.
- A discussion on the uncertainty in the parameters of the earthquake catalog is not mentioned at all. What are the uncertainties in the epicentral locations and the magnitude? Are they accounted for in the estimation of the recurrence parameters?
- The authors do not mention at all the focal mechanisms of the earthquakes in Sweden and Fennoscandia. Are there any focal mechanisms known for earthquakes that occurred in the region? Similarly, what is the hypocentral depth, together with the associated uncertainty, of the earthquakes in the final catalog built for this work?
- The discussion on the magnitude homogenization and assessment of the completeness thresholds (Section 4.1.3) in the catalog is difficult to follow and lacks crucial information. Is the ML(HEL) used for all events in the final catalog, including those from NORSAR, NNSN, and SNSN? If not, the description of how ML(HEL) was estimated is unnecessary. What are the equations used to convert ML into Mw? Are they applied to all the data in the final catalog? For the assessment of the completeness threshold(s), from which year is the catalog complete for Mc = 2 Mw? Furthermore, is a single Mc value used for the calculation of the recurrence parameters? Why did the authors not use the completeness thresholds for Fennoscandia estimated in ESHM20 or ESHM13?
- The authors should explain better how they defined a Mmax distribution between 6.3 and 7.5 (I assume this is Mw, isn’t it?)? In analogue regions, there are no examples of 7.5 Mw earthquakes, so the authors should justify better the 7.5 Mw value.
- If I have understood correctly, the authors have defined new TSZs and ASZs from the ESHM20. If this is the case, why did the authors use the TSZs and ASZs from ESHM20?
- Is a single source model considered for the PSHA of Sweden? Alternative source models would account for different interpretations of the mapped tectonic structures, large-scale deformation, regional stress field, and observed seismicity in Sweden and Fennoscandia. It would ensure to capture the epistemic variability in the behavior and location of seismogenic structures and their correlation with seismicity. Did the authors consider to use of the zoneless (zone-free, smoothed) models (see Beauval et al. 2006; Zechar and Jordan 2010 for more details) approach as an alternative seismic model? This was included in the ESHM20 model and other national seismic hazard models, such as Germany (Grünthal et al. 2018) and France (Drouet et al. 2020).
- How were the weights in the ground motion logic tree decided? Are there any available ground motion recordings for instrumental earthquakes in Sweden and Fennoscandia? If so, it would be useful to compare them with the predictions from the selected ground motion models. This comparison can be used to assign the weights for the ground motion models in the logic tree, together with expert judgments due to the limited ground motion dataset in the region.
- Why was a minimum magnitude of 4.5 Mw selected for the hazard calculations? The minimum magnitude (Mmin) in a hazard calculation is defined as the threshold for potentially damaging earthquakes (e.g. Bommer and Crowley 2017). This parameter is usually defined between 4 and 5 Mw for PSHA. In the PSHA for the UK, it was set to 4.0 Mw because it includes the probability that the impulsive nature of small earthquakes and their high-frequency content could be potentially causing damage (Mosca et al., 2022). I would think that due to the low levels of seismicity in Sweden, this may be appropriate also for this country.
- Section discussion (Section 6 here) should not repeat what was already written previously. It should emphasize the main result, highlight the strengths and limitations of the study, provide the interpretation of the results in the context of regional hazard and eventually give future research directions. For example, Subsection 6.2 “Comparison with previous studies” should be part of Section Results.
- An acronym should be explained only when it is mentioned the first time in the manuscript. ML and Mw are not explained when they are used for the first time in Section 2.
- All the geographical names mentioned in the text should be indicated on a map because not all the readers are familiar with the geography, geology and tectonics of Sweden.
Data sets
Earthquake catalogue data - SNSN Swedish national seismic network https://doi.org/10.18159/SNSN
Earthquake catalogue data - FENCAT Institute of Seismology, University of Helsinki https://www.seismo.helsinki.fi/EQ-search/query.php
Earthquake catalogue data - FNSN The Institute of Seismology, University of Helsinki https://doi.org/10.14470/UR044600
Earthquake catalogue data - NNSN Department of Earth Science, University of Bergen http://seismo.geo.uib.no/nnsn/#/
Earthquake catalogue data - GEUS The Geological Survey of Denmark and Greenland https://www.geus.dk/natur-og-klima/jordskaelv-og-seismologi/registrerede-jordskaelv-i-danmark
Earthquake catalogue data - Estonian seismic network Geological Survey of Estonia https://www.egt.ee/en/fields-activity-and-objectives/geology-and-environment/seismic-research-and-monitoring
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