Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

The upper boundary of the mantle transition zone, known as the “410-km discontinuity”, is attributed to the phase transformation of the mineral olivine (α) to wadsleyite (β olivine). Here we present observations of triplicated P-waves from dense seismic arrays that constrain the structure of the subducting Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least ≈20% lower than in the ambient mantle and an apparent thickness of ≈20 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored.


Web links to the author's journal account have been redacted from the decision letters as indicated to maintain confidentiality This manuscript has been previously reviewed at another Nature Portfolio journal. This document only contains reviewer comments and rebuttal letters for versions considered at Communications
17th Feb 23 Dear Dr Li, Please allow me to apologise for the delay in sending a decision on your manuscript titled "Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity". It has now been seen by our reviewers, whose comments appear below. In light of their advice I am delighted to say that we are happy, in principle, to publish a suitably revised version in Communications Earth & Environment under the open access CC BY license (Creative Commons Attribution v4.0 International License).
We therefore invite you to revise your paper one last time to address the remaining concerns of our reviewers. At the same time we ask that you edit your manuscript to comply with our format requirements and to maximise the accessibility and therefore the impact of your work.

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Please use the following link to submit the above items: [link redacted] ** This url links to your confidential home page and associated information about manuscripts you may have submitted or be reviewing for us. If you wish to forward this email to co-authors, please delete the link to your homepage first ** We hope to hear from you within two weeks; please let us know if you need more time. The authors analyzed waveform triplication data to study velocity structure around the 410km discontinuity within the subducting slab beneath northwest Pacific. They found the P waveforms from an intermediate depth earthquake and two shallow earthquakes recorded by broadband stations in northeast China have multiple arrivals, which cannot be explained by a regular 1-D velocity model. They used a 2-D finite difference method to model the waveforms and a 1-D waveform inversion method to obtain velocity structures at the turning depths of the P waves. They found that the observed waveform complication is caused by an ultra-low-velocity layer within the subducting Pacific slab. The ultra-low-velocity layer is ~ 20 km thick and has a Pwave velocity reduction of ~17%. The layer extends ~300-800 km around the P-wave propagation direction with a width greater than 200 km. They speculate that the ultra-low velocity layer could be related to metastable -olivine (e.g., poirierite) with reduced grain size, which was discovered recently by mineral physics studies. I think the seismic observations here are very interesting and could have significant implications to our understanding of the mineral physics and dynamics of subducted slabs. Overall, the manuscript is well written, and the seismic results are robust, therefore it is suitable to be published in Communication Earth & Environment with a minor revision. My detailed comments and questions for the authors are listed below.
1. Line 21: "Where temperatures are lower than 1000°C inside the harzburgitic…" need references 2. Line 40: "~15-km resolution scale…" I think the Fresnel zone is associated with a specific frequency. 3. Lines 45-46, Figure 1 caption: "… blue for S1S1' and red for S2S2'…" I couldn't find the letters S1 and S2 in Figure 1(a). 4. Lines 49, Figure 1 caption: "…solid white curves are ray paths…" , 1-D ray paths based on the iasp91 model? 5. Line 106: "…band-pass filtered between 0.02 and 1 Hz…" The dominant period of the waveforms shown in Figure 3 is ~5 s. I am wondering why there are no high frequency (~0.2-1.0 Hz). 6. Line 192: "… the Fresnel zone of a P-wave (~0. 5 Hz) is 15-20 km…" since the dominant period is ~5 s (0.2 Hz), I am wondering whether it is appropriate to use 0.5 Hz to compute the Fresnel zone.
Reviewer #2 (Remarks to the Author): I am pleased to read the revised manuscript of Li and coauthors. The authors have made a good effort to answer my comments and suggestions, and those of the other reviewers. I think their revisions have really strengthened the paper. I therefore recommend publication of the paper.

Response to the editor and reviewers [Manuscript COMMSENV-22-1326-T]
Dear Editor Dr. Aslin, Thank you for your time and expertise with our manuscript. We appreciate the positive feedback from the reviewers. We have further improved our manuscript according to the constructive comments from Reviewer #1. We have also carefully followed the Editorial Requests Table to edit our manuscript to comply with the policy and formatting requirements of Communications Earth & Environment. In the followings, the comments from the reviewers are written in black and our responses in red.

Reviewer #1 (Remarks to the Author):
The authors analyzed waveform triplication data to study velocity structure around the 410-km discontinuity within the subducting slab beneath northwest Pacific. They found the P waveforms from an intermediate depth earthquake and two shallow earthquakes recorded by broadband stations in northeast China have multiple arrivals, which cannot be explained by a regular 1-D velocity model. They used a 2-D finite difference method to model the waveforms and a 1-D waveform inversion method to obtain velocity structures at the turning depths of the P waves. They found that the observed waveform complication is caused by an ultra-low-velocity layer within the subducting Pacific slab. The ultra-low-velocity layer is ~ 20 km thick and has a P-wave velocity reduction of ~17%. The layer extends ~300-800 km around the P-wave propagation direction with a width greater than 200 km. They speculate that the ultra-low velocity layer could be related to metastable-olivine (e.g., poirierite) with reduced grain size, which was discovered recently by mineral physics studies. I think the seismic observations here are very interesting and could have significant implications to our understanding of the mineral physics and dynamics of subducted slabs. Overall, the manuscript is well written, and the seismic results are robust, therefore it is suitable to be published in Communication Earth & Environment with a minor revision. My detailed comments and questions for the authors are listed below.
1. Line 21: "Where temperatures are lower than 1000°C inside the harzburgitic…" need references A1: Thanks for this important comment. We admit that previously we did not clearly distinguish between the two different concepts of the temperature near the plate and its cold center. This time, we clarified this: "Near cold subducting slabs (e.g., with a temperature lower than 1000°C), the 410-km can be elevated due to the positive Clapeyron slope of the α-β phase transition 8 . Inside the harzburgitic layer of the slab (e.g., with a temperature lower than 500-600°C near its center), the nucleation and growth mechanismis inhibited 9,10 ." Reference #8 is for the temperature of 1000°C and references #9-10 are for the temperature of 500-600°C.
We also added a sentence justify that the temperature of the subducting Kuril slab is sufficiently cold to inhibit the nucleation and growth mechanism: "Given the discovery of the metastable olivine in the nearby subducting slab near Honshu (Shen and Zhan, 2020), and the similar thermal parameters of the Honshu and Kuril arcs (Syracuse et al., 2010), the subducting Kuril slab is sufficiently cold to inhibit the nucleation and growth mechanism." 2. Line 40: "~15-km resolution scale…" I think the Fresnel zone is associated with a specific frequency.