the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jun 2024
A Holocene alpine seismic chronicle from Lake Aiguebelette (NW French Alps)
Abstract. Lake sediments are valuable archives and can help construct a chronology of event deposits induced by seismic events. Such a chronology can be used to better understand the recurrence times between seismic events over longer periods than those covered by historical seismicity catalogs. However, only a few studies in lake palaeoseismology have focused on areas with moderate seismicity. This study aims to improve the catalog of paleoseismological archives in the front of the Western Alps. In this part, new multi-proxy data from the sedimentary archives of Lake Aiguebelette (France) allow the identification of 32 homogenites (thickness ≥ 0.5 cm) interpreted as of coseismic origin over the Holocene. An age model based on short-lived radionuclides, paleomagnetic data and radiocarbon ages constrains the chronology of sedimentary deposits in the deep basin of Lake Aiguebelette.
Among these homogenites, several were deposited at time intervals compatible with historical seismic events. To correlate the historical seismic events likely to have generated the event deposits identified in the sedimentary sequences of the deep basin of Lake Aiguebelette, the Earthquake Sensitivity Threshold Index (ESTI) method is used. Historical seismicity catalogs with uncertainties about intensities and epicenter coordinates for earthquakes make correlations to event deposits difficult. To better understand which seismic events may have been archived, a relative comparison was conducted between the pseudospectral acceleration (PSA) values calculated for each event in the FCAT-17 seismic catalog and for two distinct frequencies.
Based on this PSA approach, for higher frequencies (5 Hz), the contribution of nearby and moderate events is significantly stronger than that of strong and distant events in the lake sequence of Aiguebelette. Thus, the chronicle established based on the event deposits archived in Lake Aiguebelette sediment is interpreted as representative of local events (epicentral distance to the lake < 50 km). Recurrence intervals between the deposition of event layers do not follow a specific distribution (log-normal, Weibull, gamma or exponential) but might be a combination of several distributions. This suggests possible coexistence of several processes over the Holocene, impacting the evolution of the seismicity in this area.
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Status: open (until 15 Jul 2024)
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RC1: 'Comment on nhess-2024-83', Katleen Wils, 28 Jun 2024
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The manuscript by Banjan et al. shows an interesting lake paleoseismic study in a very poorly known (from a paleoseismic point of view) region, showing the complexity of such settings. While showing its value, this study also underscores the need for more lacustrine studies in order to definitely validate or reject some of the proposed hypotheses. The paper itself is written in a clear way and is particularly well illustrated. I do have the impression that some of the available literature on Alpine lakes as well as other lakes with similar issues around the world have not been included, as I indicated throughout the annotated manuscript attached. Additionally, while most of the techniques and concepts applied in this paper are established and validated in lake paleoseismic studies around the world, I have some questions with regards to some of the specific approaches and interpretations here. I list my main concerns below, and added some smaller remarks in the attached file. Nevertheless, I do believe all the necessary data is presented in the paper, and that it could still become significant once these concerns are addressed.
- Although log-ratios are often used for interpreting XRF data, it is advisable to use centered log-ratios (as also explain by the reference cited here in the methodology) - especially if you base your core-to-core correlations partly on the absolute abundance of a certain element in the core. It is not clear if you used this approach, as the single element XRF values are not plotted in figure 6 (as was mentioned in the text).
- While the full age model is shown in the manuscript, it is not entirely clear how it was constructed. This should be placed more upfront. Additionally, integrating such a large number of dating techniques for the long core (especially also the varve counts, which is not clear how you incorporated these), using the very simplistic clam model is probably not the ideal way. I strongly advice to use Bacon instead, which also does not require to manually define outliers.
- You seem to add a high confidence level for seismic triggering to any event deposit thicker than 2 cm, even though not all of them actually are, even in historical times. It is not clear how this is incorporated in the discussion, and if you left out these deposits for the section on recurrence statistics. And if not, how does this affect your results, as there could likely be more of those event deposits non-seismically triggered?
- Why do you consider PSA values? It is extremely difficult to determine threshold values based on quantitative ground motion parameters, as so far only intensity data was used and is available. Although I strongly encourage to move away from intensity, I do question why PSA is used instead of for example PGA or PGV. Considering the high uncertainties, why not start with an IPE test and evaluate local intensities at the lake rather than just epicentral intensities? And when you then move to PSA, why specifically 0.5 and 5 Hz? Are there any indications as to why these would be more relevant than others? As far as I understand, low-frequency shaking is amplified in sediments, so the conclusion that is made here on 5 Hz being more relevant seems questionable and not supported by other literature. Additionally, what do you consider as site effects (e.g. Vs30 values)? These should be used as input for the GMPEs I assume, and would also strongly impact the outcomes.
- I agree that historical earthquake catalogs are often incomplete. However, using this as the only argument to discard the potential for the Basel earthquake to have resulted in an event deposit seems questionable. Even more so considering the ESTI approach does not rule out a deposit at all. Are there any other indications as to why you do not consider this earthquake as a likely trigger? If not, I don't see an objective reason to discard it.
- With distance, high-frequency shaking is consistently attenuated faster than low-frequency shaking. So the fact that PSA values are lower for 5 Hz for distant earthquakes compared to closer earthquakes, and not so much for 0.5 Hz, does not really present a solid argument as to why nearby events would more likely trigger the event deposits in Lake Aiguebelette. This entirely depends on which frequency content lake sediments respond to (related to my previous point). Of course, the SA5 would become attenuated faster than the SA0.5, so there will always be a stronger difference with distance for PSA5.
- You do not discuss on why some event deposits consist only of homogenites, and other homogenites plus turbidites. This clearly involves different depositional processes, and might help further distinguish seismic from non-seismic deposits? A seismic seiche without turbidite seems to rather implausible, in my opinion.
- Throughout the paper, you never consider the process of surficial remobilization for turbidite deposition. Although the thicker event deposits might still relate to slope failures, especially the thinner ones might rather be attributed to this surficial process. The fact that event deposit material originates from within the lake only strengthens the possibility for surficial remobilization. It would be beneficial to elaborate on this, as in this particular case, sedimentation rate would not be relevant for turbidity current generation and thus further advocating for a seismically quiet period in stage 2.
- For the recurrence statistics calculations, it would make more sense to focus only on the long core to avoid having a change of recording sensitivity in the recent period that is covered by one of the pilot cores.
- How do your periods of seismic quiescence and activity relate to other Alpine regions? For example, also in Carinthia, some clustering of events and quiet periods have been proposed.
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