the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jun 2025
Paleoseismic history of the intermountain Rieti Basin (Central Apennines, Italy)
Abstract. From the paleoseismological and seismotectonic point of view, the intermountain basins of the Central Apennines of Italy are one of the most studied areas worldwide. Within this context, however, the Rieti Basin, bounded at its sides by active faults and with its peculiar rhombohedral shape, is a relatively overlooked area, and its most recent paleoseismological studies date back to the ‘90s. This is a key area both for completing the paleoseismological history of this sector of the chain and for understanding how the present-day extensional regime is accommodated, through time by the faults bounding the basin. With this aim in mind, we excavated 17 paleoseismological trenches along the normal faults bordering the Rieti Basin (Central Apennines, Italy) and unveiled 15 paleoearthquakes that ruptured the faults during the last ca. 20 kyr.
Our analysis of the paleoearthquake succession along the basin-bounding faults suggests that a temporal clustering of rupturing events characterizes the basin with a maximum credible earthquake of Mw 6.5, consistently within this sector of the Central Apennines. These results suggest that for the Rieti Basin, stress transfer among surrounding faults can be ascribed as one of the processes behind the temporal clustering of earthquakes.
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RC1: 'Comment on egusphere-2025-2531', Gerald P. Roberts, 01 Jul 2025
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Reviewer’s report on Livio et al. “Paleoseismic history of the intermountain Rieti Basin (Central Apennines, Italy)” (Prof. Gerald Roberts - 1st July 2025)
The manuscript presents new insights regarding the paleoseismic history of the Rieti Basin in Central Italy, derived from the excavation of 17 paleoseismic trenches.
These results will be incredibly valuable, both locally for the seismic assessment of the Apennines, but also in a general sense to aid the understanding of active faults worldwide. I was particularly impressed by the number of trenches, the number of AMS dates, and the detailed trench logs and photos. The descriptions of the trenches and the stratigraphy was also very detailed. The maps of the sites were also of high quality. The observations are put into context of the Rieti basin faults in a clear way. As such, I suggest accepting this manuscript with minor changes.
My comments as follows:
Overall, I was left wondering about how these observations fit in with the regional context of extension across the Apennines. Would you like to add some text on this? For example, how does the timing and magnitude of slip you have observed relate to that on other neighbouring faults? For example, were other faults on the SW flank of the Apennines also slipping when the Rieti basin experienced fault slip? I think it would be a missed opportunity not to say something about the regional context of the slip you have documented.
The text from lines 560 to 630 is well written and clear. However, reporting the ages in “BCE” means readers need to convert the ages in years BP. Can you also add the ages as “years BP”? This would make it much easier for most readers.
Please try to provide the amount slip for each of the events, even if this is a minimum estimate. Your approach is common amongst palaeoseismic trenching papers, where the magnitude of slip is commonly challenging to interpret, and it is common for nothing to be reported (I know you do report it for two of the events). However, it would be helpful to know what you think about the slip for all the events. If you have no constraints on the slip (e.g. because dated material just fills a fissure) state this. If you have some idea (e.g. colluvial wedge has a vertical extent of X cm), please state the value of X. If you can identify piercing points please state the exact value of offset (e.g. vertical offset, and some estimate of the fault dip with error bars). I feel it is better for you to say what you have observed about possible offsets per event rather than someone trying to interpret this from your trench logs at a later date – it is you who have the best chance of producing the best estimates, so it is a shame not to provide this.
Figure 15b is good for seeing the spatial pattern of clustering, but the text on line 635 implies a pattern of temporal clustering, which I feel is not adequately displayed on Figure 15b. Can you provide a figure that shows the temporal pattern of clustering?
I also think that somewhere in your manuscript you need to define exactly what you mean by a “cluster” (e.g. a spatial cluster? Or temporal cluster where the implied slip rate exceeds the long-term slip-rate, or both? Or some other way of defining it?). You mention this word “cluster” on Line 634, yet you do not define what you mean by this. Also, if these are temporal clusters, as implied by the text on lines 635-640 (e.g. Line 640 mentions a period of quiescence”, implying the slip is clustered in time), how do you define this? For example, how do you assess the relative weightings of the (i) average recurrence interval over the long-term, (ii) the intervals defined by the preferred ages of the events (e.g. the aperiodicity), whilst considering (iii) the uncertainties defined by the analytical uncertainties on the 14C ages and the "brackets" they define? In other words, if you are going to talk about clustering, you need to add some text about how you define clustering in your particular case.
Line 502 – It might be better to start the discussion with a paragraph that states the main findings, and then go on to set the scene for the coming discussion by outlining the main points that will be covered. The reader will then know what is to come next. Starting with disclaimers is a little dull and off-putting. Instead start with positive outcomes.
Line 521 – you say it is “impossible” to find suitable sites, but what you really mean is that it is challenging and beyond your current capabilities. Please change the text.
Line 535 – The text “underwent slope recession during the Last Glacial, are now very limited” is very vague. Imagine the reader who is not familiar with the Apennines; will they understand what this means? Re-phrase to describe the precise interplay between tectonic slip-rates and the change in erosion rate that occurred at the end of the last glacial maximum, perhaps even stating values in mm/yr, because such values are available in the literature (e.g. Tucker et al. 2011, JGR), and it is your duty to point this out to readers who do not know the Apennines.
Line 545 – you imply that studying scarps is challenging due to human effects, but then show you can get results. So, don’t you think you are overstating the effect of humans on ruining scarps? This will be confusing and possibly misleading for readers unfamiliar with the Apennines. If one does a good job of picking sites one can avoid human effects. This is nothing new and you show it very well with your work. Even in sites with less forest and a lower value for the population density through time one still has to work hard to find suitable sites. So, please clarify on this point. We don’t want people being put-off from working on the faults in the Apennines because they gain the false impression that it is impossible to get results due to human degradation of the sites.
Line 555 – Please state some values in mm/yr for slip-rates on the faults you mention to prove your claim that they are “similar”.
Line 565 and Figure 15 – It is not very easy to move between the “black-boxes” on Figure 15 and the events and ages on Table 2. Please improve this. In fact, I found Figure 15a quite poor and hard to understand. I had to make my own figure to try to understand the timings of events using the data in the table. I don’t think the “fading colours” are clear in their meaning, e.g. purple to white and dark-green to white, as you have not defined what “white” means. Please use solid symbols (something like error bars on a cross-plot?). I also think the “red” blocks showing the 14C dates are unclear in their meaning, so please use a symbol with the analytical error bars and the calibrated age as a data point (so a data point and error bars). Overall, this Figure is your key result and its present design makes it hard for the reader to extract the results – as I say I had to try to make my own figure. Please improve this because this is what will improve the citation of your paper. If people don’t understand the figures and hence the paper they will not cite it.
Line 650 – I agree they look like release faults. However, you don’t need structural inheritance to form these. If there is evidence for pre-existing structures in this specific example, please tell us what the evidence is. If there is no evidence, don’t claim this.
Line 655 – Have you conducted a stress transfer calculation, or are you speculating? It might be worth doing a stress transfer calculation to make sure you are correct. After all, the value of stress on the receiver fault is influenced by the dip and strike of the fault, and the depth considered, as well as its position relative to the ruptured fault.
Line 658 – You say they are not compatible with a single phase of extension, but they are compatible if you believe they are release faults (which you say a few lines earlier). I suggest removing the “hardly compatible” phrase as it contradicts your earlier claim of release faults (also this is incorrect English – it is either compatible or incompatible – “hardly compatible” is not correct English).
Line 660 – why not cite a paper on the 1983 Borah peak ruptures where ruptures fanned out at the NW end onto the Willow Creek Hills at a relatively high angle to the main fault. This supports what you claim.
Line 654 – change the word “descent” - incorrect English. “originate”?
On Figures where you show the “vertical offset”, e.g. Figure 3, please state what feature is offset. I presume it is the topographic slope, so I also presume that this is the vertical offset across the topographic scarp formed after the slopes stabilised. However, I don’t think you have stated this, so I am not sure. Would you like to comment on how the magnitude of slip in the trenches relates to the “vertical offset” on the profile?
On Figures the photos are in places “greyed-out” and covered by an interpretation. Is it possible to provide the un-interpreted photos as well, perhaps in an Appendix?
On Figure 7 please change the text “unit is truncated, to the top, by an erosive surface – Bedrock” That is obviously incorrect English, and does not state the field relationship precisely or correctly. It is not "to the top".
On Figure 9, there is mention of slickensides on unit 12 – do you have any measurements of the slip vector to show if these faults are dip-slip or oblique slip?
The graphics on the inset ERT plot for Figure 11 are poorly quality (pixellated) – can you improve the resolution of this figure?
Figure 13 is hard to visualize the context of the image. Can you provide a context photo to show the aspect of the slope?
Overall, I enjoyed the paper and found it quite inspiring. Well done.
Professor Gerald Roberts, 1st July 2025
Citation: https://doi.org/10.5194/egusphere-2025-2531-RC1 -
RC2: 'Comment on egusphere-2025-2531', Gordon Woo, 14 Jul 2025
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This is a very detailed study which plugs a number of significant gaps in knowledge. To their credit, the authors have themselves identified limitations of their work. One of the most crucial seismic hazard parameters is the maximum magnitude. The authors conclude as follows:
Our results indicate that the maximum credible earthquake in the Rieti Basin is in the order of magnitude Mw 6.5, which is
consistent with the general setting of the Central Apennines. Given the resolution of chronological constraints obtainable
with radiocarbon dating techniques in paleoseismic trenches, we cannot disentangle the occurrence of a single earthquake as
compared to multiple earthquakes occurring over a short time interval (like the 2016 seismic sequence). Additionally,
paleoseismic data inherently focus on surface-rupturing earthquakes, thus aliasing smaller seismic events, which however
could have caused significant damage.Given the ambiguity over the release of seismic energy in one large event, or a sequence of lesser events, it would be helpful if the authors could provide more substantive discussion over the maximum magnitude in the Rieti Basin, In particular, could the maximum magnitude be as high as 6.7 or 6.8?
Citation: https://doi.org/10.5194/egusphere-2025-2531-RC2 -
RC3: 'Comment on egusphere-2025-2531', Anonymous Referee #3, 15 Jul 2025
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This manuscript presents a thorough paleoseismological investigation of the Rieti Basin in the Central Apennines. Through an extensive trenching involving 17 excavation sites along active fault segments, the authors successfully identified 15 paleoearthquakes during the last ca. 20 kyr. The scope and resolution of the work are impressive, demonstrating a significant investment in fieldwork, stratigraphic analysis, and chronological interpretation. While the study is of high quality, addressing a few specific issues could further enhance its clarity and impact.
- Figure 2 contains only three subfigures, but the caption refers to a subfigure “d)”, which appears to be an error and should be corrected. In addition, the caption mentions a star symbol, but it is not visible in Figure 2a; the authors should ensure that all referenced symbols are clearly displayed. Lastly, in Figure 2b, the GPR17 and GPR18 survey lines are difficult to distinguish.
- On page 13, line 220, Figure 3d does not include the ERT_14 section. Therefore, the sentence “Figure 3d shows the ERT_14 section (see location in Fig. 2b);” should be corrected to “Figure 3 shows the ERT_14 section (see location in Fig. 2b);”.
- Please consider indicating the fault in Figure 3d (ERT_15 result) to help clarify the relationship between resistivity anomalies and fault structures. Additionally, the main text does not provide a detailed interpretation of this figure. The absolute error associated with the Wenner–Schlumberger array is smaller than that of the dipole–dipole array (Fig. 13d). Could this be interpreted as an indication that the Wenner–Schlumberger result is more reliable, or might it instead suggest that the inversion result is overfitting the observed data?
- On page 16, “CAM_ERT_02” should be corrected to “CAM_ERT_17” to ensure consistency with the context.
- The fault zone in ERT17 (Figure 4) shows a weaker resistivity contrast than in ERT14 (Figure 3). It would be helpful if the authors could clarify whether this difference arises from subsurface lithological variations, differences in survey configuration, or other influencing factors, as this would offer useful guidance for ERT applications in fault zone imaging.
Citation: https://doi.org/10.5194/egusphere-2025-2531-RC3
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