We thank the Reviewer for this detailed review, helpful comments, and nice words. Please find our point-to-point response to the Reviewer’s comments in the attached pdf. 

RC2: ‘Comment on nhess-2021-339’, Anonymous Referee #2, 16 Jan 2022 

First, I apologize about the long delay in returning this review.

In this paper, the authors use data from a permanent seismic network to explore the seismic signals generated by an extreme precipitation event in the south of France. The data provide insights into the flood generated on one of the highly affected rivers and identify a swarm of small earthquakes apparently triggered by the storm. I found this an interesting and nice-presented paper. Destructive floods like the one described here are indeed difficult to observe using traditional methods, and this paper demonstrates the potential of seismic methods for flood observation. As well, it’s not so often that major flash floods occur within such a conveniently laid out seismic network, so it’s great to see this opportunity exploited. Overall, I found this an enjoyable paper to read, and I have no major concerns to raise, just some relatively minor comments about clarifications and presentation.   

Our response: We thank the Reviewer for the helpful review and nice words. Please find below our replies to the Reviewer’s comments.

How were the thresholds for the peaks selected? It would be nice to indicate both the start and end of each peak in figure 2 – you’ve selected a set of points for each peak in fig. 2G, but we can’t tell exactly what those correspond to in the time series. It could also be interesting to identify on the waveform and power plots where the breaks in slope in fig. 2G are. Especially with the different units, it’s a little difficult to compare to panel A. Otherwise, I really like figure 2!

Our response: Thank you for this comment. We determine the thresholds manually; they delimit the values in seismic power when the seismic power strongly and rapidly increases. We will add the following sentence (in italics) to the manuscript (Lines 91-92):

“Their start and end times are marked in color in Figure 2, and the seismic power thresholds used to define the maxima are shown in Figure B3. We determine the thresholds manually; they delimit the values in seismic power when the seismic power strongly and rapidly increases.”

We thank the Reviewer for this helpful suggestion. We added the peak end times as vertical dashed lines to Figure 2 and Figure B5:

Figure 2. Analysis of continuous seismic signals recorded during storm Alex. Seismic power (PSD) averaged for 1-20 Hz and recorded at stations A. SPIF, B. BELV, and C. TURF. The results of the runoff simulation are marked in yellow (CN60), light green (CN70), and green (CN80), where CN denotes three different basin saturation scenarios: CN70 (moderate saturation), CN60, and CN50 (rather dry conditions). Seismic power is smoothed with a moving time window of 30 min and the runoff is calculated with a 5-min time step. D. Vertical ground velocity recorded at the SPIF station filtered in 1-50 Hz. E. Peak frequency calculated for each 200 s segment. Peak frequency and corresponding time segment are marked in the same color. F. Backazimuth (smoothed over three 30-min consecutive time windows) calculated at the SPIF station averaged over 3-8 Hz and its standard deviations (dashed lines). G. Seismic power in the 2-10 Hz frequency band versus seismic power in 10-45 Hz at SPIF station. All results are shown from 07:00 UTC October 2 to 07:00 UTC October 3, 2020.

Figure B5. SPIF station seismic data analysis and meteorological data. A. Vertical ground velocity recorded filtered in 1-50 Hz. B. Peak frequency calculated for each 200 s segment. C. Rainfall measured by the rain gauge at located at Saint-Martin Vésubie. This is the closest rain gauge to the SPIF station located at the distance of 1.9 km. The measurement stopped when the instrument was destroyed. Lightning in the distance <15 km from the SPIF station. Each circle represents a lightning strike, the larger and the darker the circle the closer the lightening. D. Seismic power calculated in windows of 200s. Peak frequency, corresponding time segment, and seismic power (PSD) are marked in the same color.

The detection of an earthquake swarm related to the storm is quite interesting. This is not my expertise, so I can’t properly comment on this aspect, but I found the discussion clear and reasonable. I do wonder if you’ve tried calculating dv/v over this time period? Maybe this would provide some insight into the state of the subsurface in the months after the storm?

Our response: Thank you for this suggestion. We agree with the Reviewer that measurements of relative seismic velocity changes (dv/v) could provide additional insights into the state of the subsurface before, during, and after the storm. Such analysis is beyond the scope of this short communication but can be performed in future studies.

We will add one sentence on this topic in the Discussion section (end of section 4.2):

“In future studies, measurements of relative seismic velocity changes (dv/v, e.g., Brenguier et al., 2008; Illien et al., 2022) could provide additional insights into the state of the subsurface before, during, and after the storm.”

Line 54: this statement makes it sound like there was higher rainfall in 1997, but I guess 1997 was just when the record starts. This should be made clear, as it’s a big difference.

Our response: Thank you for pointing this out. We will change this phrase to:

“Although heavy rainfalls occur regularly in autumn in the Mediterranean region, the storm Alex maximum daily rainfall was the highest since the beginning of the rainfall measurements in 1997. The continuous regional rainfall COMEPHORE database used in this study started in 1997, Figure 1C.”

Line 68: “particularly adequate” is a strange combination. Particularly suitable?

Our response: We will change this phrase to particularly suitable.

Lines 79-80: this is basically a repeat of lines 58-60

Our response: Thank you for noticing it, we will remove the repetition from Lines 79-80.

Line 156: Roth et al., 2016 could be cited here

Our response: We will add the citation of Roth et al., 2016 to this sentence.

Line 161: it’s not clear how are proposing that a slope failure would cause a sustained change in the peak frequency? I could imagine a slope failure that increases the local river noise (due to sediment input, geometry changes, roughness, etc) in a more distant segment, but I’d think that the failures themselves would produce much more punctuated signals.

Our response: We agree with the Reviewer that a slope failure would cause punctuated signals. However, failures of adjacent river banks could deliver coarse-grained material into the river which can possibly generate lower frequencies (e.g., Burtin et al., 2011; Farin et al. 2019).

We will modify this part of the manuscript as follows:

“Therefore, the observed drop in the peak frequency (down to 4 Hz) that temporally correlates with the occurrence of the second seismic power maximum at the SPIF station (Figure 2A, E) can potentially be generated by a more distant source or by coarser grains, originating from e.g., slope failures, since they might generate lower frequencies (e.g., Burtin et al., 2011; Farin et al., 2019).”

Line 183: you haven’t been mentioning times throughout this paragraph, so specifying night here sounds a bit funny – like there’s something diurnal that makes the time of day matter, which I guess is not the intention.

Our response: We will change the phrase from “At night,” to “Next,”.

Lines 199-205: Could the lack of peaks also be influenced by the larger sampling area of the TURF station? Because of its farther distance, it will have a similar sensitivity to a pretty long stretch of channel, which should smooth out moving peaks.

Our response: Thank you for this pertinent comment. We agree with the Reviewer that the lack of peaks can also be related to the sensitivity of the TURF station that samples a more extended river segment.

We will add the following sentences in Lines 199-205:

“The absence of the two main maxima on the TURF station can be related to a lack of sensitivity of this station to the bedload transport due to its large distance from the river (~6 km). Farther distance means stronger geometrical attenuation at higher frequencies versus lower frequencies, and thus lower sensitivity to bedload compared to water flow (Gimbert et al., 2014). Also, this station samples a longer river segment because of its farther distance, which could smooth out moving peaks.”

Line 237: stick with “central”, not “middle”

Our response: Thank you, we will use “central” in this paragraph.

Fig. 1: the precipitation colors on top of the Google earth colors make this a bit busy. It is ok – we can see the precip pattern, but it might look much nicer with a hillshade for the background. In panel D, the labels on the waveform y-axes are too crowded. Maybe you could make panel C shorter to give panel D a little more space? 

Our response: Thank you for this comment. We shifted panels C and D in Figure 1 to the right, and we also shifted panel C to the top of the figure to give panel D a bit more space.

Fig. 2: It would be helpful to have the frequency range used for A-C in the caption

Our response: We added the frequency range to the caption (see above).  

References:

Brenguier F., Campillo, M., Hadziioannou, C., Shapiro, N. M., Nadeau, R. M., Larose, E. (2008), Postseismic relaxation along the San Andreas fault at Parkfield from continuous seismological observations. Science, 321(5895):1478-81, https://doi.org/10.1126/science.1160943

Burtin, A., Cattin, R., Bollinger, L., Vergne, J., Steer, P., Robert, A., Findling, N., Tiberi, C.,(2011), Towards the hydrologic and bed load monitoring from high-frequency seismic noise in a braided river: The “torrent de St Pierre”, French Alps. Journal of Hydrology, 408, 1–2, 43-53, https://doi.org/10.1016/j.jhydrol.2011.07.014

Illien, L., Sens-Schönfelder, C., Andermann, C., Marc, O., Cook, K. L., Adhikari, L. B., & Hovius, N. (2022). Seismic velocity recovery in the subsurface: Transient damage and groundwater drainage following the 2015 Gorkha earthquake, Nepal. Journal of Geophysical Research: Solid Earth, 127, e2021JB023402. https://doi.org/10.1029/2021JB023402

Farin, M., Mangeney, A., de Rosny, J., Toussaint, R., &Trinh, P.-T. (2019), Relations between the characteristics of granular column collapses and resultant high-frequency seismic signals. Journal of Geophysical Research: Earth Surface, 124, 2987-3021, https://doi.org/10.1029/2019JF005258¨