Imaging structure and geometry of slabs in the greater Alpine area – a P-wave travel-time tomography using AlpArray Seismic Network data

Paffrath, Marcel; Friederich, Wolfgang; Schmid, Stefan M.; Handy, Mark R.; the AlpArray and AlpArray-Swath D Working Group,

doi:https://doi.org/10.5194/se-12-2671-2021

Articles | Volume 12, issue 11

https://doi.org/10.5194/se-12-2671-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

New insights into the tectonic evolution of the Alps and the...

https://doi.org/10.5194/se-12-2671-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 11

Research article

|

25 Nov 2021

Research article |

| 25 Nov 2021

Imaging structure and geometry of slabs in the greater Alpine area – a P-wave travel-time tomography using AlpArray Seismic Network data

Marcel Paffrath, Wolfgang Friederich, Stefan M. Schmid, Mark R. Handy, and the AlpArray and AlpArray-Swath D Working Group

Download

Final revised paper (published on 25 Nov 2021)
Supplement to the final revised paper
Preprint (discussion started on 07 May 2021)

Interactive discussion

Status: closed

RC1:
'Comment on se-2021-58', Anonymous Referee #1, 08 Jun 2021

My comments and questions can be found in the attached document.

Best regards

Citation: https://doi.org/10.5194/se-2021-58-RC1
- CC1: 'Reply on RC1', Marcel Paffrath, 10 Jun 2021
  
  Dear reviewer,
  I shortly want to clarify here an issue regarding the checkerboard resolution test. It seems that we have not communicated clearly enough how the test was conducted. Due to the inability to resolve the crustal structure with teleseismic data, we did not perturbe the uppermost ~100 km with a checkerboard at all. However, we simulated through the a priori crustal (and upper most mantle) model together with the checkerboard perturbation below to examine how smearing of the checkerboard anomalies interferes with the a priori model.
  
  The results in Fig. 10 therefore show the resolved checkerboard pattern together with the (slightly altered) a priori crustal model (which is for the checkerboard test also part of the starting model). Any alteration of the a priori crustal model is therefore caused by smearing or the noise we added. However, this alteration of the crustal model is nearly everywhere way below 1%! I attached a figure showing only the velocity differences relative to the starting model as well as one through the checkerboard model used to calculate synthetic traveltimes.
  We are aware of the possible missinterpretation of the a priori crustal model as artifacts of the checkerboardtest, as we already received feedback from collegues who had the same problem. We will therefore certainly clarify this (e.g. using the provided figures or highlight the boundaries of the a priori model) in the revised manuscript!
  Kind regards,
  Marcel Paffrath
  
  Citation: https://doi.org/10.5194/se-2021-58-CC1
- AC1: 'Reply on RC1', Marcel Paffrath, 19 Jul 2021
  
  Please find our detailed response in the attached file.
  
  Citation: https://doi.org/10.5194/se-2021-58-AC1
RC2:
'Comment on se-2021-58', Anonymous Referee #2, 13 Jun 2021

The comment was uploaded in the form of a supplement: https://se.copernicus.org/preprints/se-2021-58/se-2021-58-RC2-supplement.pdf

Citation: https://doi.org/10.5194/se-2021-58-RC2
- AC2: 'Reply on RC2', Marcel Paffrath, 19 Jul 2021
  
  Please find our detailed response in the attached file.
  
  Citation: https://doi.org/10.5194/se-2021-58-AC2
EC1:
'Comment on se-2021-58', Anne Paul, 26 Jul 2021

Dear authors,
Your replies to the reviewers' questions solve most of the problems and questions raised by the first version of your paper. I therefore encourage the submission of the revised version, which should take into account all reviewers’ comments.
However, I would like to stress the issue of resolution in the uppermost mantle, that is at the transition between the base of the imposed crustal model (at depth< 77.5 km according to l. 214 of your text) and the upper mantle (130 or 150 km depth, that are the minimum depths documented by the checkerboard tests of Figures 8 and 9). Both reviewers asked questions and made comments on this point, in particular on Fig. 10. The changes you propose to apply to Fig. 10 and in the text will certainly be useful. They may however not be sufficient to answer the critical question of the resolution between 80-130 km, which cannot be easily assessed from the depth maps of Figs. 8 and 9 and the single cross-section of Fig. 10. I would suggest showing the results of the checkerboard test(s) on additional sections, for example an E-W section at 45°N, which goes through the high-velocity crustal anomaly of the Ivrea body and the strong low-velocity anomaly of the Po basin. Another section along the EASI line (and the eastern part of the Swath-D array) where your station coverage is densest would also be useful to assess the influence of station density on the resolution at shallow depth. If you consider that these additional sections don’t bring enough interesting information, you can put them in a supplementary information file.
On the same topic, you write in l. 303-304 that the checkerboard anomalies leak vertically to at least 20 km below the crust, that is to ~150 km depth. Is that right everywhere in your study region, or in areas with average station spacing? I mean that you probably have better resolution at shallow depth beneath the dense Swath-D or EASI arrays (see my comment on additional sections above). If this is right (almost) everywhere, I agree with Rev. 1 that the depth slice at 90 km should not be shown and discussed as a result of your tomography, or with much caution. Same remark for the anomalies located at depths shallower than ~150 km, such as for example the “W” and “D” anomalies of Figs. 13a and 14a (see comment 11 by Rev. 1).
With a few additions and clarifications, your manuscript has the potential to provide reliable and important answers on key issues on the structure of the upper mantle beneath the Alps. I look forward to the revised version.
Regards

Citation: https://doi.org/10.5194/se-2021-58-EC1
- AC3: 'Reply on EC1', Marcel Paffrath, 06 Aug 2021
  
  Dear Anne Paul,
  please find our detailed response in the supplement.
  Kind regards,
  Marcel Paffrath
  
  Citation: https://doi.org/10.5194/se-2021-58-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Marcel Paffrath on behalf of the Authors (16 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (18 Aug 2021) by Anne Paul

AR by Marcel Paffrath on behalf of the Authors (31 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (02 Sep 2021) by Anne Paul

Dear authors,

Your paper is almost ready for publication provided that you apply the few minor corrections listed below. Once again, the line numbers refer to the authors’s tracked change manuscript, and not to the last version of the manuscript.

- L. 18 (abstract): “a short western Alpine slab whose surface trace ends about 100 km from the Penninic front”? According to the corresponding part of the discussion l. 521), you rather mean “a short western Alpine slab whose easternmost end is located at about 100 km depth beneath the Penninic front”. Please correct.
- L. 47-55: you cite a number of papers on seismic refraction and reflection surveys, local earthquake tomography studies, etc. but none of these lists is complete. You should then either cite all relevant papers, which are numerous, or as a stop back, start all lists with “e.g.”.
- L. 50: you cite Fry et al. (2010) and Molinari et al. (2015), and not the first paper published on ambient-noise tomography of the Alps, which is Stehly et al. (2009, doi: 10.1111/j.1365-246X.2009.04132.x). You should at least add this one.
- L. 242: you probably mean “data base” rather than “data basis”
- Caption of Fig. 5: “will get visible WHEN (?) analysing resolution capabilities”
- L. 454, 456: rather than “Ivrea zone” (which is a surface outcrop of low-velocity rocks), you probably mean “Ivrea body” (which is the high-velocity, high-density anomalous body at 10 km depth beneath the Ivrea gravity high of the western Alps).
- L. 506-508: I still disagree with your argument on the influence of seismic anisotropy measured from SKS splitting analyses on the low-velocity anomalies. The SKS wave is split in a fast and a slow quasi-S waves when propagating through a transversely isotropic medium with a horizontal symmetry axis. As a consequence, both the fast-velocity and the slow-velocity directions measured from SKS splitting are horizontal. Therefore, the steeply incident teleseismic P-waves may be slower if their propagation plane is perpendicular to the fast directions, but they can be faster if it is parallel to the fast direction. As the orientation of the propagation plane depends on the source-receiver backazimuth and is highly variable, there is no single effect of reduction or increase in P-wave velocity. As analyzed by Bezada et al. (2016), the time delays of teleseismic P-waves are affected by anisotropy with dipping symmetry axes in subduction regions. You should read this paper and be more precise on its outcomes on teleseismic tomography around subductions, and the possible implications on your results.

A. Paul

Hide

AR by Marcel Paffrath on behalf of the Authors (03 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (06 Sep 2021) by Anne Paul

ED: Publish as is (09 Sep 2021) by CharLotte Krawczyk (Executive editor)

AR by Marcel Paffrath on behalf of the Authors (15 Sep 2021)

Short summary

The Alpine mountain belt was formed by the collision of the Eurasian and African plates in the geological past, during which parts of the colliding plates sank into the earth's mantle. Using seismological data from distant earthquakes recorded by the AlpArray Seismic Network, we have derived an image of the current location of these subducted parts in the earth's mantle. Their quantity and spatial distribution is key information needed to understand how the Alpine orogen was formed.