Teleseismic P waves at the AlpArray seismic network: wave fronts, absolute travel times and travel-time residuals

Paffrath, Marcel; Friederich, Wolfgang; the AlpArray and AlpArray-SWATH D Working Groups,

doi:https://doi.org/10.5194/se-12-1635-2021

Articles | Volume 12, issue 7

https://doi.org/10.5194/se-12-1635-2021

© 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-12-1635-2021

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

Articles | Volume 12, issue 7

Research article

|

20 Jul 2021

Research article |

| 20 Jul 2021

Teleseismic P waves at the AlpArray seismic network: wave fronts, absolute travel times and travel-time residuals

Marcel Paffrath, Wolfgang Friederich, and the AlpArray and AlpArray-SWATH D Working Groups

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Final revised paper (published on 20 Jul 2021)
Preprint (discussion started on 10 Dec 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'Comment on se-2020-189', Michael Behm, 02 Jan 2021
- SC3: 'Reply on SC1', Marcel Paffrath, 07 Jan 2021
SC2: 'Comment on se-2020-189', Anne Paul, 05 Jan 2021
- SC4: 'Reply on SC2', Marcel Paffrath, 07 Jan 2021
RC1: 'Review report on SE-2020-189', Anonymous Referee #1, 11 Jan 2021
- AC1: 'Reply on RC1', Marcel Paffrath, 20 Jan 2021
RC2: 'Review Paffrath et al', Anonymous Referee #2, 28 Jan 2021
- AC2: 'Reply on RC2', Marcel Paffrath, 23 Feb 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Marcel Paffrath on behalf of the Authors (10 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Mar 2021) by CharLotte Krawczyk

RR by Anonymous Referee #2 (13 Apr 2021)

Suggestions for revision or reasons for rejection

General Comments:

In the revised version of their manuscript, Paffrath and co-authors considered all my previously raised concerns. The calculation of crustal corrections and the presentation of the travel-time residuals have been greatly improved. The new Figure A2 suggests that the updated/improved model used for crustal corrections seems much more complete and realistic compared to the one presented in the previous version of the manuscript. I am still in doubts, however, if the current quality of text and English is sufficient for immediate publication in Solid Earth. There are still many flaws in style and punctuation, a lot of repetitions and partly superfluous statements and conjunctives. The manuscript would also benefit from improvements in terms of structure. For instance, the discussion (about 3 pages) is mainly an extensive conclusion and, on the other hand, the results presented in section 5 (>9 pages) already include interpretations I would have expected in the discussion (e.g. section 5.4). I have provided a list with selected examples (and suggestions how to improve) as well as additional minor comments below.

One final comment regarding the interpretation of the residuals: In section 5 the authors provide long and detailed descriptions of observations (L, A, W, C, E in Fig. 10), but the interpretation of these observations (especially in section 6) is extremely short and vague. The main conclusion seems that the expected 3-D mantle structure is “complex” (which is not a surprise based on previous studies). Anomalies W and A are associated with known slabs, but the interpretation of C and E is extremely vague and short in section 6 (see previous comment on structure). Also, I have to admit that I do not fully understand how the authors defined the outlines of anomalies W, C, E in Figure 10 and elsewhere. Especially for C and E it is not clear to me at all. On the other hand, the delays (positive residuals) in the western Alps are not discussed/mentioned at all in section 6, although potentially important in terms of the discussion about slab break-off in this part. I completely understand that this will be part of the author’s follow-up tomography study, but I am wondering if all the details in section 5 are really needed for the rather vague interpretation of the residuals.

Detail comments:

- l. 12: “that already indicate” -> “that indicate”

- l. 35: “… below the array.” add a reference, e.g. Aki et al 1977

- l. 36: “Imagining … ” -> “Assuming” or “Considering”?

- l. 40: Why not add a reference in this paragraph, others described the trade-off between array aperture und resolved depth range before, e.g. Sandoval et al 2003 & 2004 (GJI), etc.

- l. 61: “forbidding” -> “unfeasible”?

- l. 73: -> Rowe et al doesn’t really relate to teleseismic: -> “In case of similar waveforms, e.g. from earthquake clusters or planar waves of teleseismic wave fronts, one can improve traveltime measurements by … ”

- l. 89: -> “… that already indicate the approximate location of high and low velocity anomalies in the upper mantle prior to any tomographic inversion”

- l. 99: -> ground motions

- l. 115: -> “Because oceanic microseismic noise … was only possible for strong earthquakes of magnitudes >XX.”

- l. 131: “… capability of algorithms applied to different characteristic functions to resolve … required for high-resolution traveltime tomography. We will summarize the most… ”

- l. 134: “characteristic function picking algorithms” -> Rephrase! See above.

- l. 148: “The crucial question is, how to obtain” -> Rephrase!

- l. 151: “read” -> “determine”

- l. 162: “…propagate into” -> “does not affect”

- l. 172: In this paragraph you should add the reference to Figure 3 to give an overview on the procedure before starting with the detailed description of the individual steps.

- l. 178: -> “... which was originally designed….”

- l. 188: what is the “calculation window”?

- l. 191: We select the moving time window…” Meaning of this sentence is still not clear to me… Try to rephrase?

- l. 208: -> “Fig 5a…” The order is wrong! Figure 3 and 4 are not mentioned in the text yet. Therfore Figure 5 should be Figure 3 otherwise mention Figure 3 and 4 before.

- l. 208: The last sentence of this paragraph doesn’t make much sense to me. You mean “To resolve the fine-scale mantle structure below the Alps, it is crucial to reduce the uncertainties of the onsets using additional constraints provided by the high station density of the AlpArray network”?

- l. 210: -> “By visual inspection of selected examples, we validated that the large uncertainties…”

- l. 212: “is hidden in the site-specific noise”?

- l. 213: “Another limitation…”

- l. 217: -> “Although too uncertain to be used for tomographic inversion, the AIC onsets turned out to be more precise than onsets predicted with standard 1-D earth models”.

- l. 233: Meaning of last sentence of paragraph not clear. “Yield” in terms of what? More picks?

- l. 244: -> “... Alps, within the shortest possible distance to all other stations to minimize …”?

- l. 253: -> “… is ≥0.8 (Fig. 3).” Delete the following sentence.

- l. 257 -> “… can be determined precisely either automatically or manually. In our case, we applied the automatic picking procedure of section 3.2 to determine the onset on the beam trace.”

- l. 264 -> “shown in Figure 8” Wrong order as before… Figures 6 and 7 have not been mentioned yet…

- l. 271: -> “Figure 5c demonstrates… makes them insufficient”

- l. 318: -> “… only about 10% of the total number…”

- l. 320: It is not explained what you are doing with class-4 picks. Since they don’t have an upper error-bound (only a lower bound) these picks should be rejected for tomography. This should be mentioned.

- Figure caption 7: “are identifiable” -> “are characterized by”

- l 331: “… in one dataset are not present…”

- l. 377: -> “A closer examination of traveltime residuals shown in Figure 9b and 9d reveals that…”

- l. 400: “influence of incident angle is small” -> but doesn’t Figure 9b and 9c show otherwise?

- Figure caption 10: “small crosses mark station residuals mostly independent of variations in backazimuth”

- Figure caption 10: “Tectonic map” -> “Tectonic units”; provide a proper reference for Handy 20XX if possible, otherwise Handy personal communication?

- l. 421: -> “The resulting vertically stacked traveltime differences (…) between our crustal 3-D model and the crustal 1-D model of Diehl et al. 20XX can be found…” You should add something to this paragraph such as “Crustal contributions to traveltime residuals related to Ivrea body or sedimentary basins (Po etc) are in the order of XX seconds and comparable to the crustal corrections derived by Waldhauser et al. 2002 and therefore significant when compared to the potential mantel signals and need to be removed…” Otherwise the aim/purpose of Figure A2 is not clear.

- l. 423: -> “The most striking features of the stacked traveltime residuals after crustal correction are the…”

- l. 428: “Defining a separate negative anomaly…” -> this sentence is odd and unclear and needs to be rephrased.

- l. 432: This section and the following ones seem better suited for the discussion (see my general comments)…

- l. 437: “…move around…” -> rephrase! E.g. “location of anomalies varies depending on azimuth” or something?

- l. 440: “… we imagine” -> “we consider”?

- l. 449: “with laterally moving imprint” -> Try to rephrase! “with locations depending on azimuth of incoming wavefield” or something?

- l. 456: “… can not…“ -> “cannot”

- l. 463: “The high velocity anomaly building (C)…” -> meaning unclear. You mean the anomaly labeled “C”? rephrase

- l. 466: How are these outlines defined (see my general?

- l. 472: “Up to now not much was said about…” odd, please rephrase…

- l. 474: “using the techniques described above” -> ”using the same procedures as for the high-frequency data (including azimuthal binning, crustal corrections, etc.)” Then you can remove the repetition around line 489 and 490.

- l. 475: “We find that the obtained maps differ systematically…” -> Add a reference to Figure 12 showing the residuals for the low-frequency data. Otherwise it is not clear what you are refereeing to.

- l. 492: “less strong” -> “smaller”? Can you quantify this? E.g. in terms of seconds or in %. Is the difference between the two datasets still significant compared to the mean errors or your picks?

- l. 496 and elsewhere: “disparity” -> Not sure, if this is the right term. I would prefer “difference”

- l. 521: -> “Sophisticated, automatic single-channel picking approaches apparently do not achieve the targeted accuracy for teleseismic traveltimes.”

- l. 530: “These are absolutely essential” -> repetition, please delete sentence… “To overcome this problem…”

- l. 531: -> “It proved to be sufficient…”

- l. 539: -> “into the error estimation”

- l. 539: -> “the higher …, the better”

- l. 540: -> “The definition nicely includes…” -> try to rephrase

- l. 544: -> “… conspicuous bumps should appear…” odd expression, rephrase!

- l. 553: what is “high-reaching fast upper mantle”? You mean “Upper mantle at shallow depths with fast seismic velocities”?

- l. 554: -> “The strike of the anomaly A correlates with the strike of the Apenninic mountain chain…”

- l. 555: -> what is “slab-like” material? Why not “lithospheric slab”? I think there is evidence for its existence from other studies…

- l. 584: -> “The AlpArray network proved to…”; “high correlation” in terms of what?

- Figure caption A2: Should be improved. If I understand correctly you show the difference between your updated 3-D crustal model (compiled from different crustal models) with the 1-D crustal model of Diehl et al 2009 for a vertical plane wave? It shows the potential crustal contribution to the teleseismic residuals which needs to be corrected for. This is not clear from the current caption.

- l. 632: Somewhere here you might also acknowledge the contributions and comments of editors and reviewers of your manuscript…

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ED: Publish subject to minor revisions (review by editor) (16 Apr 2021) by CharLotte Krawczyk

AR by Marcel Paffrath on behalf of the Authors (22 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (02 May 2021) by CharLotte Krawczyk

ED: Publish as is (02 May 2021) by CharLotte Krawczyk (Executive editor)

AR by Marcel Paffrath on behalf of the Authors (05 May 2021)

Short summary

Using the AlpArray seismic network, we have determined highly accurate travel times of P waves from over 370 major global earthquakes between 2015 and 2019, which shall be used for a tomography of the mantle beneath the greater Alpine region. Comparing with theoretical travel times of a standard reference earth model, we receive very stable patterns of travel-time differences across the network which provide evidence of varying subduction behaviour along the strike of the Alpine orogen.