Local slab penetration into lower mantle controls deep-focus seismicity and Changbaishan volcanism in northeast China

Domain: earth_science
Published: 2025-03-21
Editorial outcome: major_revision

Abstract
Intraplate volcanism in the Changbaishan region of northeast China is underlain by a deep subduction-related earthquake cluster in the mantle transition zone, but the underlying physical connections between them remain elusive. Here we use teleseismic double-difference tomography to obtain high-resolution images of the subducting Pacific slab to provide better constraints on slab morphology and potential relationships with volcanism and deep-focus seismicity. Our results reveal a narrow slab tongue penetrating the lower mantle, flanked to the north and south by slabs stagnating in the mantle transition zone. This geometry creates a gap for sub-slab hot materials to ascend, providing a deep mantle source for Changbaishan volcanism. Additionally, the slab tongue is warped, which spatially coincides with a cluster of deep earthquakes, implying that an active deformation zone causes the clustered deep earthquakes. Our findings demonstrate that Changbaishan volcanism and deep-focus earthquakes are fundamentally controlled by the subducting slab’s dynamic morphology beneath northeast China.

Introduction
Cenozoic volcanoes are widely distributed in northeast China1, with the active Changbaishan volcanic field (CVF) being the largest among them. The CVF is well known for its Millennium Eruption at Changbai/Paektu Mountain (AD 946-947)2,3, one of the biggest in recorded history4.

The CVF is an intraplate volcanic field located ~1500 km away from the Japan Trench and 1200 km behind the volcanic arc (Fig. 1). The subducted Pacific Plate extends beneath this region in the mantle transition zone (MTZ) out to about 120°E5. Basaltic melts originating from the CVF have ocean island basalt (OIB) affinity with an enriched (EM1) component3,6. Analysis of major and trace elements suggests that these melts are derived from small-fraction partial melting of a garnet-bearing mantle source7,8. The lithosphere beneath the CVF, as well as other Neogene volcanic fields in eastern China, has a thickness of less than 100 km9, thin enough to allow a small fraction of melting of enriched upwelling mantle without requiring particularly high source temperatures. Source mantle potential temperatures are estimated to be 1300–1400 °C10, which is similar to or slightly higher than average mantle values. Water contents of primitive magmas are variously argued to be <1 wt%7 or 1.2–1.8 wt%10, significantly lower than arc magmas. This implies a mantle source region with H2O concentrations ranging between 350 and 550 ppm10, which could correspond to a variety of mantle sources without requiring significant H2O sourced from a subducting slab11.

The formation mechanism of the CVF is currently a subject of intense debate. One widely discussed mechanism, dubbed the “big mantle wedge” (BMW) model12, proposes that mantle melting is triggered by deep dehydration of the subducting slab13 and/or upwelling of hydrous material within the mantle wedge14. However, this mechanism fails to explain why late Cenozoic volcanism (<1.5 Ma) is not widely distributed along the strike of the subduction zone, but rather concentrated in a specific region1. An alternative formation mechanism suggests that sub-slab hot materials upwell through a gap in the slab to supply magma for the Changbaishan volcano15. Recent seismic tomography studies provide for the existence of such a slab gap16,17. Nevertheless, it remains unclear how this slab gap in the MTZ is formed15,17.

In addition to the extensive intraplate volcanism and magmatism, another prominent geological feature in NE China is the occurrence of deep-focus earthquakes (300–650 km)18. Specifically, there exists an elongated cluster of deep-focus earthquakes in the MTZ ~300 km eastward from the CVF at depths ranging from 500 to 600 km (Fig. 1). The isolated nature of this cluster remains enigmatic; the potential mechanisms of deep-focus earthquake generation, including dehydration embrittlement19, transformational faulting of olivine20 and adiabatic shear instability18, are all dominantly temperature-controlled, and the thermal and chemical structure of the slab should be almost constant along-strike beneath East Asia21, so why should deep-focus earthquakes be localized here?

The spatial correlation between the CVF and deep-focus earthquakes raises the possibility that the two could be related. Zhao & Tian13 proposed that the frequent occurrence of deep-focus earthquakes near the CVF could be attributed to a locally more hydrated slab, where dehydration leads to brittle failure in the slab19 and enhanced volatile-flux melting beneath the CVF. However, based on the subduction zone thermal model in this region21, dehydration reactions of various hydrous minerals in the slab are most likely intersected at depths shallower than ~300 km22. In addition, this hypothesis contradicts the inferred presence of a metastable olivine wedge (MOW) in the core of the slab23,24, which indicates a minimal amount of water within it25. The connection between the CVF and deep-focus earthquakes, therefore, remains tenuous.

Interactions between the subducting slab and its surroundings are likely to influence the distribution of deep-focus earthquakes in the MTZ. Fukao and Obayashi26 conducted an analysis on the correlation between deep-focus earthquakes and slab morphology, suggesting that slabs stagnating in the MTZ are associated with deep-focus earthquakes limited to depths shallower than ∼620 km, while slabs penetrating into the more viscous lower mantle have events deeper than this threshold. In the Tonga subduction zone, intense seismicity has been attributed to the collision between a relic slab and the subducted Pacific slab27–29. More generally, high densities of deep-focus earthquakes are commonly observed in slab regions undergoing folding or more complex macro-scale deformation (e.g., ref. 30), and could be related to localized regions of high deviatoric stress31, otherwise sudden release of strain in the form of earthquakes would be prohibited32.

To better understand the origins of the slab gap in the MTZ and deep-focus earthquakes beneath NE China, as well as their potential links, we have constructed a high-resolution seismic velocity model for the subducting slab within and surrounding the MTZ to better characterize its morphology.

Results
In this study, we employed the teleseismic double-difference tomography method33 to determine the mantle structure beneath northeast China (see Methods). Our final tomographic model clearly shows westward-dipping high Vp anomalies that correlate well with the relocated earthquake hypocenters and Slab 2.0 model34 (Supplementary Fig. 2), which can be interpreted as the subducting slab. Figure 2 shows cross sections of the Vp model along profiles that are nearly perpendicular to the trench. To the north (Profile A) and to the south (Profile F) of the cluster of deep-focus seismicity, the deep slab is flat and stagnant in the MTZ. Through the cluster of deep-focus seismicity (Profiles C and D), the high-velocity anomaly continues through the “660 km” discontinuity and down to ~800 km. At the margins of the cluster (Profiles B-C and D-E), there is a transition from slab penetration through “660 km” to stagnancy within the MTZ. Deep earthquakes within the MTZ primarily concentrate along Profile D. Synthetic test with a characteristic velocity model shows these features can be well-resolved (Supplementary Fig. 8).

The local penetration of the slab into the lower mantle can also be illustrated by a horizontal slice through the velocity image at a depth of 760 km, 100 km beneath the nominal “660 km” discontinuity (Fig. 3). This slice clearly depicts a high-velocity zone with the Changbaishan volcano situated around its southwestern corner and the cluster of deep seismicity (hypocenters at 500–600 km depth) aligned along its east edge (Fig. 3). Figure 4 presents a 3D iso-surface view of the imaged high-velocity anomaly. It is evident that this anomaly, interpreted as the subducting slab, extends through the “660 km” discontinuity to nearly 800 km at latitudes ranging from ~42oN to ~46oN, while remaining stagnant on both sides.

Methods
Teleseismic double-difference tomography for the mantle structure in northeast China

The teleseismic double-difference tomography method utilizes both regional and global seismic stations to better resolve seismic velocity structures within and around seismically active source zones33,53. We assembled three sets of P-wave arrival-time data, including the selected ISC-EHB Bulletin dataset comprising ~770,000 P-wave arrival times from 3158 events occurring in proximity to the study region59, the selected China Digital Seismic Network dataset consisting of ~60,000 P-wave arrival times from 889 events, and the NorthEast China Extended SeiSmic Array (NECESSArray) dataset encompassing 11,000 P-wave arrival times from 145 events from September 2009 to August 201115. Overall, we gathered a total of 850,000 P-wave arrival times from 4192 events recorded by ~10,426 stations (Fig. 1 and Supplementary Fig. 1). From these absolute arrival times, a total of 3,500,000 P-wave differential arrival times are constructed with an average inter-event distance of 140 km.

The teleseismic double-difference tomography method employs a nested regional-global gridding scheme for inversion. In the target study area, the grid spacing is finer, with an interval of 1o in longitude, an interval of 1.5o in latitude, and varying intervals in depth. Within the MTZ, the grid spacing is set at 50 km in depth, which is finer than used in previously published velocity models. We utilize the LLNL_G3Dv360 as our initial global velocity model in three dimensions. The grid intervals of the global model are set to be 5o in both longitude and latitude while ranging from 55 to 225 km in depth. The checkerboard model resolution test and the model restoration test demonstrate that the model is well-resolved for depths from 160 km down to 1200 km, particularly within the MTZ (refer to Supplementary Information).

Furthermore, we also test tomographic inversions with different initial crustal models and global models, and the final tomographic models are shown to be robust (Supplementary Figs. 9, 10).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "In this study, P-wave arrival times from three datasets (ISC-EHB、CDSN and NECESSArray) have been compiled to\ninvestigate the mantle structure of northeast China by using teleseismic double-difference tomography. The main finding of\nthis paper is that the pacific plate has been subducted into the lower mantle beneath the Changbaishan volcanic region\nwhereas it stagnates in the mantle transition zone nearby. This makes the link of the slab geometry and its control on deep-\nfocus seismicity and intraplate volcanism. The tomographic results are good and generally reliable. However,\nthe detailed structure of the subducting Pacific slab resolved by this study is quite similar to previous tomographic results (e.g., Tao et al.,\n2018) as shown in the supplementary materials of Figures S10 and S11, which makes me consider the novelty of the\nresults. Furthermore, this is an over-interpretation of the seismic imaging with lack of geodynamics background. I think other\nevidences are required to support the hypothesis proposed by authors.\nA more detailed list of my thoughts follows below.\n1. The initial regional model is formed by combing CRUST1.0 model in the crust and AK135 model in the mantle. Because\nthe 3D velocity model in the crust is used, it is necessary to introduce the raytracing method and provide some examples to\nshow the reliability of this method to accurately calculate travel times in the 3D initial model. It confused me why the anomaly\nin the crust is not prominent in the tomographic model (Figure 2) when a 3D CRUST1.0 model was used as an initial model.\nFor teleseismic tomography, the crustal structures are generally not well constrained because the nearly vertical ray path in\nthe crustal depth. This means that the initial 3D velocity model in the crust is hardly modified by inversion and the inversion\nmodel in the crust is supposed to be nearly the same as the CRUST1.0 model.\n2. The global LLNL_G3Dv3 model (Simmons et al., 2012) was used as the initial model in three dimensions. I think the\ninversion model and the initial LLNL_G3Dv3 model should be compared to show which part of the model is modified by\nusing the updated datasets and tomographic method during inversion. I am wondering why the authors not conduct\ntomographic inversion using the global average 1D AK135 model as the initial model. At least this kind of tomographic\nresults is supposed to be exhibited to show the reliability of the used datasets and tomographic method.\n3. The authors suggest that a gap exists within the mantle transition zone due to the complex geometry of the subducting\nPacific slab, which facilitate upward migration of sub-slab hot materials and cause the volcanism in the Changbaishan\nregion. The seismic images from tomographic studies mainly reflect the state of present mantle structure. The massive basalt\neruptions in the Changbaishan volcanic regions can be traced back to ~16 Ma (Liu et al., 2015), during which the geometry\nand position of the pacific slab was significantly different from present seismic images, so other scenarios are supposed to\nbe examined. The Tianchi volcanism underwent a multi-stage evolution. The model proposed by the authors may be\nassociated with the latest volcanism. However, is there any evidence to support the existence of thermal anomaly in the\nlower mantle beneath the Changbaishan volcanic region, considering that the cold subducted slabs continuously recycled\nback to the core-mantle boundary beneath this region.\n4. Lines 122-124. I think this idea was firstly proposed by Chen and Brudzinski (2001) through jointly analyzing results of\nseismic tomography and fault plane solutions to investigate the nature of subducted lithosphere and deep earthquakes\nbeneath the Tonga back-arc. This paper should be properly cited.\n5. Lines 236-241. Many geodynamic models were proposed to investigate the evolution and dynamics of the subducted\nPacific slab other than Wu et al (2023). A few much more influential works are not cited. For example, a recent geodynamic\nmodeling of Lin et al (2024) shows that the continental lithosphere in northeast China is tectonically controlled by the\nsubduction of Pacific slab, which plays an important role in leading to the migration of volcanism in this region since 28 Ma.\n6. Lines 255-259. The slab reference frame should be defined to make the introduction of focal mechanisms of deep\nearthquakes more clear.\n7. Lines 292-293. Some evidences have been reported recently to support slab deformation at depths of mantle transition\nzone of the northwestern Pacific subduction systems, such as Guo et al (2021) and Montagner et al (2021).", "reviewer_3": "Gao et al. conducted a tomographic study on the East Asia region using the teleseismic double-difference tomography\nmethod and discovered a slab tongue beneath the MTZ where the Pacific plate is subducting. The author suggests that the\ndeep earthquakes occurring in the East Asia region are related to this feature. This article is highly interesting and is sure to\nattract the attention of colleagues studying the East Asia region. It is suitable for publication in Nature Communications after\nmajor revisions. The corresponding author, Prof. Zhang, is highly reputable in the field of seismic tomography, so I have no\nadditional comments on the tomography methods or the robustness of the model. I have only several suggestions for the\ninterpretation of the model that the authors might consider to enhance the overall narrative.\n1. In Figure 2, profiles C, D, and E all seem to show that there are also some remnant slabs in the MTZ above the slab\ntongue, which appears more like a double-layered high-velocity anomaly. This feature is not reflected in the schematic\ndiagrams in Figures 4 and 5. Would this omission affect the overall framework of the story?\n2. In Zhang et al.(2019, Nature Communication), a similar feature (double-layered high-velocity anomaly or slab tongue) was\nobserved in the Izu-Bonin region, as seen in profile A-B in Figure 2 of Zhang et al. (2019). Does this double-layered high-\nvelocity anomaly also represent a slab tongue similar to that in the present study? Are the deep earthquakes in the Izu-Bonin\nregion also related to the slab tongue? The 2015 Bonin earthquake, studied in Zhang (2019), is thought to be controlled by\nslab tearing. If a slab tongue exists in the East Asia region, then slab tearing must also be present. Why are deep\nearthquakes distributed along the slab tear, similar to the Izu-Bonin region, not observed in East Asia? Is the double-layered\nhigh-velocity anomaly or slab tongue observed in the East Asia and Izu-Bonin regions generated by the same tectonic\nmechanism? If the mechanisms behind the deep earthquakes in these two regions can be unified, it would have significant\nimplications for the universality of the model.\n3. Figure 3 presents the velocity distribution at 760 km and compares it with the depth of 660 km obtained from receiver\nfunction studies. Why not directly use the velocity distribution at 660 km for comparison? From the 760 km velocity\ndistribution, it seems that this slab tongue extends from 47.5 degrees to 39-38 degrees, meaning that the southern boundary\nof this slab tongue should be around 38.5 degrees. Can we consider that intraplate volcanism such as Jingpohu, Longgang,\nChangbaishan, and those south of 40 degrees are all controlled by this slab tongue? Why are deep earthquakes within the\n38.5-40.5 degree range much fewer than those to the north?\n4. The model of Tang et al. (2014) shows the slab gap. Chen et al. (2017) attributed the slab gap to an artifact of teleseismic\ndata. How can the authors exclude this? Is there any slab tongue found in the two tomography models?\n5. What is the depth range of earthquakes affected by this slab tongue? Does it only influence earthquakes below 500 km?\nWill it also affect earthquakes around 400 km?\n6. Lines 55 & 57: The “NE China” and “Northeast China” should match with each other.\n7. Lines 83 & 84: The water content is not the criteria to discriminate the OIB and EM1, the high water content can also\nappear in EM1 mantle.\n8. Lines 108-116: Very interesting. I want to remind the author that there may also be significant amounts of water and\nvolatiles in the subducting oceanic crust, so this model doesn't necessarily contradict the model of the metastable olivine\nwedge (with low water content within the slab). I checked Line 114, and the reference 'Jing et al.' should be 'Jiang et al’.\n9. Lines 231-234: I checked the article by Han et al. (2024), and they only detected localized vertical flow, which does not\ndirectly indicate the presence of an upwelling penetrating the slab. The citation here could potentially mislead readers.\n10. Lines 236-240: I didn't understand what this section is trying to convey. What is the connection between the time points\nof 20 Ma and 5 Ma and the slab gap? How did you arrive at this conclusion: 'Therefore, the linkage of the slab gap in the\nMTZ with the Changbaishan volcanism is also supported by the timing of volcanism and slab stagnation.'?\" The Pacific slab\nhas started to be stagnant in the MTZ since ~20 Ma, which is earlier than the initiation of the Changbaishan volcanism\n(~5Ma). So, the chronology of volcanism does not directly support the slab gap model, nor does it directly rule out the big\nmantle wedge model.\n11. Lines 254-258: Based on Figure 3, it doesn't seem that this pattern is apparent (In the center of the penetrating section of\nthe slab, most of the focal mechanisms suggest along-strike extension. At the edges of the penetrating slab, slab-normal\nextension is observed)."}, "author_response": "Response to Reviewers\nReviewer #2:\nIn this study, P-wave arrival times from three datasets (ISC-EHB、CDSN and\nNECESSArray) have been compiled to investigate the mantle structure of northeast\nChina by using teleseismic double-difference tomography. The main finding of this\npaper is that the pacific plate has been subducted into the lower mantle beneath the\nChangbaishan volcanic region whereas it stagnates in the mantle transition zone nearby.\nThis makes the link of the slab geometry and its control on deep-focus seismicity and\nintraplate volcanism. The tomographic results are good and generally reliable. However,\nthe detailed structure of the subducting Pacific slab resolved by this study is quite\nsimilar to previous tomographic results (e.g., Tao et al., 2018) as shown in the\nsupplementary materials of Figures S10 and S11, which makes me consider the novelty\nof the results. Furthermore, this is an over-interpretation of the seismic imaging with\nlack of geodynamics background. I think other evidences are required to support the\nhypothesis proposed by authors.\nResponse: We sincerely appreciate your constructive comments, which are helpful for\nimproving our manuscript. We agree with you on the fact that our model is consistent\nwith main features of the FWEA18 model (Tao et al., 2018), which was determined by\nthe full waveform inversion. This actually proves the reliability of our velocity model,\nwhich is determined by a different tomographic method and different datasets. However,\nTao et al. (2018) did not discuss the important slab morphology change in the MTZ and\nits link with the formation of the slab gap introduced by Tang et al. (2014) and the deep\nseismicity cluster to the east of the Changbaishan volcano. Actually, after Tang et al.\n(2014) proposed the slab gap model responsible for the formation of the Changbaishan\nvolcano, it is actually questioned by some studies. For example, Lei et al. (2018)\nquestioned the existence of the slab gap proposed by Tang et al. (2014) (Lei, J., Zhao,\nD., Xu, Y., Fan, Q., Mi, Q., Du, M., & Lu, M. (2018). Is there a gap in the stagnant\nPacific slab in the mantle transition zone under the Changbaishan volcano?. Acta\nPetrologica Sinica, 34(1), 13-22). They questioned the slab gap could be an artifact\nintroduced by the teleseismic tomography method used by Tang et al. (2014) (Chen, C.,\nZhao, D., Tian, Y., Wu, S., Hasegawa, A., Lei, J., ... & Kang, I. B. (2017). Mantle\ntransition zone, stagnant slab and intraplate volcanism in Northeast Asia. Geophysical\nJournal International, 209(1), 68-85). Therefore, it is still not clear what exactly caused\nthe formation of the CVF: big mantle wedge model or slab gap model. Through our\nindependent study, we actually showed the slab gap indeed exists in the MTZ.\nHowever, Tang et al. (2014) did not provide the mechanism why there exists a slab gap\nbeneath the CVF. Our study for the first time proposed that the slab morphology\ntransition from slab stagnation to slab penetration to slab stagnation from north to south\nis the cause for the formation of the slab gap. Moreover, for the first time we also link\nthe isolated deep seismicity cluster with this slab morphology change in the MTZ.\nThis also provides an outstanding example that the local slab deformation is actually the\nfundamental cause for the deep-focus seismicity, a long-time outstanding geoscience\nmystery to be answered. We think these new findings from our study will make great\ncontributions to the understanding of intraplate volcanism and deep-focus seismicity.\nIt is true that the full waveform inversion could provide more detailed velocity\nstructures. However, many studies have shown that it could also produce artifacts in\nthe velocity model, making it difficult to discern true or false features. As seen in the\nFWEA18 model, although there are more detailed features, they are complex and\nchallenging for reasonable geological interpretation, and the slab morphology change\nin the MTZ is not clear. In comparison, the slab morphology change and thus the slab\ntearing features are very distinct in our model, which are first reported by our study.\nTherefore, although our model bears some similar features to the FWEA18 model, it\ndoes not necessarily mean that our study is not novel. We should emphasize again that\nthe first finding of the slab morphology change in the MTZ and its linkage to both\nChangbaishan volcanism and the nearby deep seismicity cluster makes our study\nunique and novel.\nBased on the seismic images obtained in our study, we only interpret the slab\nmorphology change in the MTZ and propose it causes the slab gap in the MTZ. These\nfeatures are clear by themselves and we do not think we overinterpret them. We actually\nuse an existing geodynamic modeling study to explain why there exists a local slab\npenetration beneath the CVF, as follows:\n“The reason for localized slab penetration may also be related to slab shape. The dip\nof the subducting Pacific plate undergoes a rotation from north-northwestward to west-\nnorthwestward around 45°N, accompanied by a corresponding rotation in the\norientation of the trench, resulting in the formation of a subduction cusp (Fig. 1).\nGeodynamic modeling results indicate slab penetration is favored near to this\nsubduction cusp where the slab dip angle is the smallest and the trench retreat velocity\nis slowest (Zhao et al., 2021). Away from the cusp, the velocity of trench retreat is\nhigher and the slab is more likely to flatten and stagnate within the MTZ (Zhao et al.,\n2021).”\nWe agree that it would be more supportive if a three-dimensional geodynamic modeling\ncould be done for east Asia to show this slab morphology change, but we think this\nwould need extensive work, and is beyond the scope of this study. Similar to our\nprevious work on Izu-Bonin that imaged slab overturn in the MTZ (Zhang et al., 2019,\nNC), recently Li et al. (2024) is able to simulate this phenomenon by geodynamic\nmodeling (Liu et al. (2024). Slab segmentation and stacking in mantle transition zone\ncontrols disparate surface and lower mantle subducting rates and complex slab\nmorphology. Geophysical Research Letters, 51(17), e2024GL110202). We hope after\nour velocity images are published, some groups can also conduct geodynamic modeling\nto simulate features that we imaged, which will be very useful for understanding the\ndynamics in the region.\nFor other comments, we have made point to point responses, as follows.\nA more detailed list of my thoughts follows below.\n1. The initial regional model is formed by combing CRUST1.0 model in the crust and\nAK135 model in the mantle. Because the 3D velocity model in the crust is used, it is\nnecessary to introduce the raytracing method and provide some examples to show the\nreliability of this method to accurately calculate travel times in the 3D initial model. It\nconfused me why the anomaly in the crust is not prominent in the tomographic model\n(Figure 2) when a 3D CRUST1.0 model was used as an initial model. For teleseismic\ntomography, the crustal structures are generally not well constrained because the nearly\nvertical ray path in the crustal depth. This means that the initial 3D velocity model in\nthe crust is hardly modified by inversion and the inversion model in the crust is\nsupposed to be nearly the same as the CRUST1.0 model.\nResponse: Thank you for your comment. The raytracing method employed in this study\nis the pseudo-bending ray tracing method widely used in many tomographic studies.\nWe have added the relevant information in the Supplementary Materials as follows:\n“For the inversion, we use the multiscale teleseismic double-difference (DD)\ntomography method of Pesicek et al. (2014), which is a global version of the DD seismic\ntomography method (tomoDD) (Zhang and Thurber, 2003, 2006). It utilizes the pseudo-\nbending raytracing algorithm (Um and Thurber, 1987) to determine the ray paths and\ncompute the travel times between events and stations within a spherical Earth model.\nThe pseudo-bending raytracing method is very efficient and has been widely used in\nseismic tomography studies at various scales (e.g., Zhao et al., 1992; Chiarabba et al.,\n2000; Rawlingson et al., 2010).”\nAs for the velocity anomalies shown in Figure 2, they are inverted P-wave velocity\nperturbations relative to the initial model in the study region, which is the CRUST1.0\nmodel for the upper 60 km combined with the AK135 model for the mantle. Please note\nthat the teleseismic double-difference tomography method used in this study is different\nfrom the conventional teleseismic tomography method in that it uses arrival times from\nevents in the study region recorded by the global stations. In addition, it also uses arrival\ntimes from regional events recorded by regional stations. Therefore, the crust can be\nresolved to some extent. Response Figure 1 shows depth slices of the inverted model in\nthe shallower depths (0-60 km). It is true the crustal model may not be well resolved\ndue to the sparse crustal seismicity in the region. In the supplementary materials, we\nhave compared three cases with different velocity models in the upper 60 km: the\nAK135 model, the CRUST1.0 model, and the FWEA18 model. As seen in\nSupplementary Figure 8, the velocity features around the slab are very similar to each\nother, indicating shallower velocity structures affect little the mantle velocity anomalies.\nThis is because the teleseismic DD tomography method used in this study can use\ndifferential times to resolve velocities around the slab seismicity and is very little\naffected by shallower velocity models.\nResponse Figure 1 The inverted Vp model at shallower depths. Velocity perturbations are\ncalculated with respect to the average velocity at each depth. The black dashed lines depict\nboundaries between major tectonic blocks, the red triangles represent volcanoes, and the black dots\nare relocated events.\n2. The global LLNL_G3Dv3 model (Simmons et al., 2012) was used as the initial model\nin three dimensions. I think the inversion model and the initial LLNL_G3Dv3 model\nshould be compared to show which part of the model is modified by using the updated\ndatasets and tomographic method during inversion. I am wondering why the authors\nnot conduct tomographic inversion using the global average 1D AK135 model as the\ninitial model. At least this kind of tomographic results is supposed to be exhibited to\nshow the reliability of the used datasets and tomographic method.\nResponse: Thank you for your suggestion. Sorry for not describing our method more\nclearly. To clarify, our model is parameterized by nested grids with coarser global grid\noutside the study region and finer regional grid for the study region. Within the study\nregion, the initial model is the combination of the CRUST1.0 model (Laske et al., 2013)\nin the upper 60 km and AK135 model (Kennett et al., 1995) in the mantle. Outside the\nstudy region, the initial global model is the 3D LLNL_G3Dv3 model (Simmons et al.,\n2012). The reason for doing this is to use the global model to better take care of the\nvelocity heterogeneities globally, so that the seismic rays outside the study region are\naccurate. For comparison, we also conducted another tomographic inversion by using\nthe 1D AK135 model as the initial global model (Supplementary Figure 9). As we can\nsee, the velocity features are similar to those shown in Figure 2, indicating the initial\nglobal model has little influence on the final inverted regional velocity model. We have\nincluded this figure as Supplementary Figure 9 and revised the Supplementary\nMaterials as follows:\n“Due to the utilization of both regional and teleseismic stations in the inversion,\nthe inversion model in our study is parameterized by nested grids with finer regional\ngrid for the study region, and coarser global grid outside the study region. The initial\nregional model is formed by combining CRUST1.0 model (Laske et al., 2013) in the\nupper 60 km and AK135 model (Kennett et al., 1995) in the mantle. ...... For the global\nmodel, we use the 3D LLNL_G3Dv3 model (Simmons et al., 2012) as the initial global\nvelocity model in order to better constrain seismic ray paths outside the regional\nmodel…...”\nSupplementary Figure 9 Comparison of velocity models along profiles A ~ F from inversions with\ndifferent initial global models. (Left) Inversion with the LLNL_G3Dv3 model (Simmons et al., 2012)\nas the initial global model. (Right) Inversion with the AK135 model (Kennett et al., 1995) as the\ninitial global model. The gray dots are relocated earthquakes. The dashed black lines represent 410\nand 660 km discontinuities.\n3. The authors suggest that a gap exists within the mantle transition zone due to the\ncomplex geometry of the subducting Pacific slab, which facilitate upward migration of\nsub-slab hot materials and cause the volcanism in the Changbaishan region. The seismic\nimages from tomographic studies mainly reflect the state of present mantle structure.\nThe massive basalt eruptions in the Changbaishan volcanic regions can be traced back\nto ~16 Ma (Liu et al., 2015), during which the geometry and position of the pacific slab\nwas significantly different from present seismic images, so other scenarios are supposed\nto be examined. The Tianchi volcanism underwent a multi-stage evolution. The model\nproposed by the authors may be associated with the latest volcanism. However, is there\nany evidence to support the existence of thermal anomaly in the lower mantle beneath\nthe Changbaishan volcanic region, considering that the cold subducted slabs\ncontinuously recycled back to the core-mantle boundary beneath this region.\nResponse: Following your suggestion, we have revised the related part substantially,\nas follows:\n“It is important to recognize that the length and position of the Pacific slab, and\nvolcanic activity in the CVF have changed through time. The Pacific slab probably only\nreached the MTZ at ~20 Ma (Wu et al., 2022) or ~15 Ma (Lin et al., 2024). The earliest\nmassive basalt eruptions in the CVF can be traced back to ~16 Ma (Liu et al., 2015b),\nprobably predating the presence of a stagnant slab (Lin et al., 2024). At ~15 Ma, the\nPacific slab started to stagnate in the MTZ and the sub-horizontal slab continued to\nadvance westward (Lin et al., 2024). We propose that the formation of the slab gap and\nassociated mantle upwelling may be associated with the shield forming stage of CVF\nvolcanism from ~5-1 Ma (Liu et al., 2015b; Ward et al., 2021; Zhang et al., 2018). We\nnote that this interpretation is not mutually exclusive of other mechanisms of mantle\nupwelling, such as lithospheric dripping beneath the Songliao Basin (Lin et al., 2024).”\nGeodynamic modeling of Wu et al. (2022) has shown the Izanagi plate has been totally\nfallen into the lower mantle since ~10-20 Ma. As shown in the temperature model of\nWu et al. (2022), this could trigger the upwelling of hot lower mantle materials. It has\nbeen found from receiver function imaging that there exist double 660 interfaces (Ai\net al., 2003) beneath northeast China, which could be formed due to combined effects of\npost-spinel and post-garnet phase transitions with upwelling hot lower mantle materials\n(Liu et al., 2018). Therefore, the existence of thermal anomaly in the lower mantle\nbeneath the Changbaishan volcanic region can be supported by geodynamic modelling\nand the existence of double 660 km interfaces. We have included the follow text in the\nrevised manuscript:\n“The geodynamic modelling has shown that Izanagi plate has been totally fallen into\nthe lower mantle since ~10-20 Ma in east Asia, which can trigger the upwelling of hot\nlower mantle materials (Wu et al., 2022). This is also supported by the existence of\ndouble 660 km interfaces beneath northeast China (Ai et al., 2003), which could be\nformed due to combined effects of post-spinel and post-garnet phase transitions with\nupwelling hot lower mantle materials (Liu et al., 2018).”\nAi, Y., Zheng, T., Xu, W., He, Y., & Dong, D. A complex 660 km discontinuity beneath northeast\nChina. Earth and Planetary Science Letters 212, 63-71 (2003).\nWu, B., Wang, Y. & Huang, J. Dynamics of the Subducted Izanagi-Pacific Plates Since the\nMesozoic and Its Implications for the Formation of Big Mantle Wedge Beneath Eastern Asia.\nFront. Earth Sci. 10, 829163 (2022).\nLiu, H., Wang, W., Jia, X., Leng, W., Wu, Z., & Sun, D. The combined effects of post-spinel and\npost-garnet phase transitions on mantle plume dynamics. Earth and Planetary Science Letters\n496, 80-88 (2018).\n4. Lines 122-124. I think this idea was firstly proposed by Chen and Brudzinski (2001)\nthrough jointly analyzing results of seismic tomography and fault plane solutions to\ninvestigate the nature of subducted lithosphere and deep earthquakes beneath the Tonga\nback-arc. This paper should be properly cited.\nResponse: Thank you for recommending this reference. Sorry for not citing this\nreference. In addition, we also cited Brudzinski and Chen (2005), which suggested the\nearthquakes in the slab could be due to localized sources of stress. In the revised manuscript,\nwe have cited both references:\n“In the Tonga subduction zone, intense seismicity has been attributed to collision\nbetween a relic slab and the subducted Pacific slab (Chen and Brudzinski, 2001;\nRichards et al., 2011; Liu et al., 2021).”\n“More generally, high densities of deep-focus earthquakes are commonly\nobserved in slab regions undergoing folding or more complex macro-scale deformation\n(e.g., Myhill, 2013), and could be related to localized regions of high deviatoric stress\n(Brudzinski and Chen, 2005), otherwise sudden release of strain in the form of\nearthquakes would be prohibited (Scholtz, 1990).”\nBrudzinski, M. R., & Chen, W. P. Earthquakes and strain in subhorizontal slabs. Journal of\nGeophysical Research: Solid Earth 110, B08303 (2005).Chen, W. P., & Brudzinski, M. R.\nEvidence for a large-scale remnant of subducted lithosphere beneath Fiji. Science 292, 2475-\n2479 (2001).\nScholz, C. H. Mechanics of Earthquakes and Faulting, 439 pp., Cambridge Univ. Press,\nNew York (1990).\n5. Lines 236-241. Many geodynamic models were proposed to investigate the evolution\nand dynamics of the subducted Pacific slab other than Wu et al (2023). A few much\nmore influential works are not cited. For example, a recent geodynamic modeling of\nLin et al (2024) shows that the continental lithosphere in northeast China is tectonically\ncontrolled by the subduction of Pacific slab, which plays an important role in leading\nto the migration of volcanism in this region since 28 Ma.\nResponse: Sorry for missing Lin et al. (2024) paper, which was published later than\nthe original submission date for this manuscript. Thanks for your suggestion. Indeed,\nthis is an important reference for our manuscript. Now this paper is cited as follows:\n“It is important to recognize that the length and position of the Pacific slab, and\nvolcanic activity in the CVF have changed through time. The Pacific slab probably only\nreached the MTZ at ~20 Ma (Wu et al., 2022) or ~15 Ma (Lin et al., 2024). The earliest\nmassive basalt eruptions in the CVF can be traced back to ~16 Ma (Liu et al., 2015b),\nprobably predating the presence of a stagnant slab (Lin et al., 2024). At ~15 Ma, the\nPacific slab started to stagnate in the MTZ and the sub-horizontal slab continued to\nadvance westward (Lin et al., 2024). We propose that the formation of the slab gap and\nassociated mantle upwelling may be associated with the shield forming stage of CVF\nvolcanism from ~5-1 Ma (Liu et al., 2015b; Ward et al., 2021; Zhang et al., 2018). We\nnote that this interpretation is not mutually exclusive of other mechanisms of mantle\nupwelling, such as lithospheric dripping beneath the Songliao Basin (Lin et al., 2024).”\nLin, F., Qi, L., Zhang, N. & Guo, Z. An ongoing lithospheric dripping process beneath\nnortheast China and its impact on intraplate volcanism. Geology 52, 435–440 (2024).\nLiu, J. Q., Chen, S. S., Guo, W. F., Sun, Q., Zhang, M., and Guo, Z. Research advances\nin the Mt. Changbai volcano. Bull. Mineral. Pet. Geochem 34, 710–723 (2015).\n6. Lines 255-259. The slab reference frame should be defined to make the introduction\nof focal mechanisms of deep earthquakes more clear.\nResponse: To avoid confusion, we have deleted the term “the slab reference frame”.\nThe revised sentences are as follows:\n“In the center of the penetrating section of slab (Fig. 2, Profile D), most of the focal\nmechanisms suggest along-strike extension and down-dip compression (i.e., strike-slip\nmotion in the plane of the slab). At the edges of the penetrating slab segment (Profiles\nC and E), slab-normal extension and down-dip compression is seen (i.e., thrust faulting\nin the plane of the slab), similar to shallower events (Zhao and Ohtani, 2009; Gan et\nal., 2015).”\n7. Lines 292-293. Some evidences have been reported recently to support slab\ndeformation at depths of mantle transition zone of the northwestern Pacific subduction\nsystems, such as Guo et al (2021) and Montagner et al (2021).\nResponse: Thank you for recommending these references. We are sorry for having\nmissed these relevant papers. Accordingly, we have revised the manuscript to include\nthem:\n“The region of slab penetration exhibits strong approximately NS-oriented azimuthal\nanisotropy at depths of 500 and 800 km (Montagner et al., 2021) and no radial\nanisotropy in the MTZ (Guo and Zhou, 2021), consistent with the orientation of\nfractures formed for local slab penetration through the 660-km interface.”\nGuo, Z. & Zhou, Y. Stagnant Slabs and Their Return Flows From Finite‐Frequency\nTomography of the 410‐km and 660‐km Discontinuities. JGR Solid Earth 126,\ne2020JB021099 (2021).\nMontagner, J.-P., Burgos, G., Capdeville, Y., Beucler, E. & Mocquet, A. The mantle\ntransition zone dynamics as revealed through seismic anisotropy. Tectonophysics 821,\n229133 (2021).\nReviewer #3:\nGao et al. conducted a tomographic study on the East Asia region using the teleseismic\ndouble-difference tomography method and discovered a slab tongue beneath the MTZ\nwhere the Pacific plate is subducting. The author suggests that the deep earthquakes\noccurring in the East Asia region are related to this feature. This article is highly\ninteresting and is sure to attract the attention of colleagues studying the East Asia region.\nIt is suitable for publication in Nature Communications after major revisions. The\ncorresponding author, Prof. Zhang, is highly reputable in the field of seismic\ntomography, so I have no additional comments on the tomography methods or the\nrobustness of the model. I have only several suggestions for the interpretation of the\nmodel that the authors might consider to enhance the overall narrative.\nResponse: Thanks for encouraging and constructive comments, which help us to\nimprove our manuscript. We hope the revised manuscript has better clarified some\nunclear issues raised by you.\n1. In Figure 2, profiles C, D, and E all seem to show that there are also some remnant\nslabs in the MTZ above the slab tongue, which appears more like a double-layered high-\nvelocity anomaly. This feature is not reflected in the schematic diagrams in Figures 4\nand 5. Would this omission affect the overall framework of the story?\nResponse: Thanks for catching this interesting feature for us. By the checkerboard and\ncharacteristic model tests, the region that this feature appears is relatively well resolved.\nNot including it in the diagram would not affect the story presented in this study. Again\nfor the fully understanding this phenomenon would need complex geodynamic\nmodelling. We have included some discussions on this feature and its connection with\nthe Izu-Bonin slab feature in the revised manuscript, as follows:\n“One interesting feature of our tomographic images along Profiles C and D is that the\nhigh velocity anomaly bifurcates at around 600 km depth, with a 300-500 km long high\nvelocity “limb” lying flat along the “660 km” discontinuity, and a longer “limb”\npenetrating into the lower mantle. This feature appears to be well-resolved (see\nSupplementary Materials). Given the high viscosity of subducting slabs, we feel that\nthis is unlikely to be “splitting” of the slab, and may instead represent one contiguous\nslab and a stranded slab fragment. This feature could represent the footprint of the slab\nmorphology change from slab stagnation within and penetration through the MTZ. This\nhigh velocity bifurcation differs from the features we observed in the Izu-Bonin\nsubduction zone, where some high velocity anomalies are observed in the uppermost\nlower mantle. Those anomalies probably represent slow sinking of “stagnant” slab\nmaterials through the 660 km interface, which is not a perfect barrier to convection\n(Čížková & Bina, 2019).”\n2.\n2.1 In Zhang et al.(2019, Nature Communication), a similar feature (double-layered\nhigh-velocity anomaly or slab tongue) was observed in the Izu-Bonin region, as seen in\nprofile A-B in Figure 2 of Zhang et al. (2019). Does this double-layered high-\nvelocity anomaly also represent a slab tongue similar to that in the present study?\nAre the deep earthquakes in the Izu-Bonin region also related to the slab tongue?\nResponse: Thanks for also mentioning high-velocity anomaly beneath the flat-lying\nslab along the profiles a and b in the Izu-Bonin region in our previous study (Zhang et\nal., 2019). The feature seen in Zhang et al. (2019) is different from the one in the present\nstudy. The high-velocity anomaly beneath the flat-lying slab in the Izu-Bonin region\nrepresents part of stagnant slab materials being slowly penetrated through the 660 km\ndiscontinuity. We have revised the text accordingly:\n“This high velocity bifurcation differs from the features we observed in the Izu-Bonin\nsubduction zone, where some high velocity anomalies are observed in the uppermost\nlower mantle. Those anomalies probably represent slow sinking of “stagnant” slab\nmaterials through the 660 km interface, which is not a perfect barrier to convection\n(Čížková & Bina, 2019).”\n2.2 The 2015 Bonin earthquake, studied in Zhang (2019), is thought to be controlled by\nslab tearing. If a slab tongue exists in the East Asia region, then slab tearing must also\nbe present. Why are deep earthquakes distributed along the slab tear, similar to the Izu-\nBonin region, not observed in East Asia? Is the double-layered high-velocity anomaly\nor slab tongue observed in the East Asia and Izu-Bonin regions generated by the same\ntectonic mechanism? If the mechanisms behind the deep earthquakes in these two\nregions can be unified, it would have significant implications for the universality of the\nmodel.\nResponse: Zhang et al. (2019) found that the 2015 Bonin earthquake occurred “<30 km\nsouth of the tear in the slab, near the fold hinge where the slab rotates from near-vertical\nto overturned”. Therefore, the 2015 M7.9 earthquake did not occur along the slab tear,\nbut some distance away near the fold hinge, where the strain is supposed to be large.\nWe agree with you that slab tearing should also occur when the slab morphology\nchanges in the MTZ. As seen in Figure 4, deep seismicity did not occur along the slab\ntears, but along the warping region of the slab tongue where the deformation is the\ngreatest. In this sense, the mechanisms producing deep seismicity in both regions are\nsimilar, which are controlled by local slab deformation. The reason that the deep\nseismicity did occur along slab tear is most likely because it is too hot to have brittle\ndeformation. We have reenforced this argument in the revised manuscript, as follows:\n“It is noted that deep seismicity clustering is only seen along the warping region of the\nslab tongue, and not along slab tears created when slab transits from stagnation above\nto penetration through the 660 km discontinuity and then reverses. This is also the case\nobserved in the Izu-Bonin subduction zone (Zhang et al., 2019). This may be because\nshear heating and conductive heating near to the slab tears render temperatures too\nhigh for brittle deformation to occur.”\n3. Figure 3 presents the velocity distribution at 760 km and compares it with the depth\nof 660 km obtained from receiver function studies. Why not directly use the velocity\ndistribution at 660 km for comparison? From the 760 km velocity distribution, it seems\nthat this slab tongue extends from 47.5 degrees to 39-38 degrees, meaning that the\nsouthern boundary of this slab tongue should be around 38.5 degrees. Can we consider\nthat intraplate volcanism such as Jingpohu, Longgang, Changbaishan, and those south\nof 40 degrees are all controlled by this slab tongue? Why are deep earthquakes within\nthe 38.5-40.5 degree range much fewer than those to the north?\nResponse: For comparison the local slab penetration region with the 660 km\ndiscontinuity depression region imaged by receiver functions, it is better to use the 760-\nkm velocity depth slice than the 660 km slice. In this way, because slabs are stagnant\nin the MTZ outside the slab penetration region, the velocity in the 760 km depth beneath\nthe slab penetration region should be higher, as shown in Figure 3. For the slab\npenetration region through the 660 km discontinuity, the interface should be depressed.\nThe fact that the depressed region is in accordance with the area where the slab\npenetrated through the 660-km discontinuity supports the reliability of our velocity\nmodel.\nIn Figure 3, the blue rectangle indicates an estimate of the main slab penetrated region\nat a depth of 760 km, which extends from 46.3 degrees to 42 degrees in latitude. The\nreason that within the 38.5-40.5 degree range there are much fewer deep earthquakes\nthan those to the north is likely because the local strain rates induced by slab\ndeformation are smaller in this region that is farther away from the main slab\npenetration area.\nBased on the spatial correlation between the slab penetration region and volcanoes of\nJingpohu, Longgang, Changbaishan, and those south of 40 degrees, we think the\nintraplate volcanism in this region is partly controlled by this slab tongue. We have\nmade this point clear in the manuscript.\n“Additionally, our findings indicate that other nearby volcanoes such as Jingpohu\nand Longgang are also situated around this same slab gap, suggesting a shared origin\n(Figs. 3-5).”\n4. The model of Tang et al. (2014) shows the slab gap. Chen et al. (2017) attributed the\nslab gap to an artifact of teleseismic data. How can the authors exclude this? Is there\nany slab tongue found in the two tomography models?\nResponse: We should emphasize that the teleseismic tomography method used in our\nstudy is totally different from that illustrated in Chen et al. (2017). The teleseismic\ndouble-difference tomography method adopted in our study uses differential times from\nregional events recorded by global seismic stations, instead of travel times from\nteleseismic events recorded by regional stations. In addition, we did not use relative\ntravel time residuals determined by subtracting average delay times in the study region.\nTherefore, our teleseismic tomography method will not create artifacts shown in Chen\net al. (2017). Actually, the tomography method used in Tang et al. (2014) is also\ndifferent from that in Chen et al. (2017). Tang et al. (2014) used finite-frequency\ntomography method, which employs Born-Fréchet kernels to account for effects of off-\nraypath structures on traveltime data. The data they used in the inversion are differential\ntravel times between station pairs which are measured by a cross-correlation based\nmethod. Consequently, both the data processing procedure and the tomography method\nused by Tang et al. (2014) are different from those applied by Chen et al. (2017).\nMoreover, two independent seismic tomography studies, Tao et al. (2018) and Ma et al.\n(2018), also vaguely imaged the local slab penetration into the lower mantle, consistent\nwith our velocity images. As a consequence, the slab gap or slab tongue for the\nsubducting Pacific plate is real beneath the CVF. We have made this point clearer in\nthe Supplementary Materials, as follows:\n“For the inversion, we use the multiscale teleseismic double-difference (DD)\ntomography method of Pesicek et al. (2014), which is a global version of the DD seismic\ntomography method (tomoDD) (Zhang and Thurber, 2003, 2006). It utilizes the pseudo-\nbending raytracing algorithm (Um and Thurber, 1987) to determine the ray paths and\ncompute the travel times between events and stations within a spherical Earth model.\nThe pseudo-bending raytracing method is very efficient and has been widely used in\nseismic tomography studies at various scales (e.g., Zhao et al., 1992; Chiarabba et al.,\n2000; Rawlingson et al., 2010).\nCompared to the conventional teleseismic tomography method, as described in Chen et\nal. (2017), the teleseismic DD tomography method uses both absolute event-station\narrival times and differential arrival times from pairs of regional events observed on\nglobal seismic stations to simultaneously determine event locations and seismic velocity\nmodels. This is in contrast to the conventional teleseismic tomography method that uses\narrival times from teleseismic events recorded on regional seismic stations to determine\nvelocity structure beneath stations. As a result, teleseismic DD tomography method is\nnot subject to potential artifacts introduced by conventional teleseismic tomography\ndescribed in Chen et al. (2017).”\n5. What is the depth range of earthquakes affected by this slab tongue? Does it only\ninfluence earthquakes below 500 km? Will it also affect earthquakes around 400 km?\nResponse: The depth range of earthquakes affected by this slab tongue depends on the\ndistribution of high strain rates induced by local slab deformation related to the slab\nmorphology change in the MTZ. Based on cross sections of seismicity shown in Figure\n1 and Figure S14, the slab tongue mainly affects earthquakes deeper than ~400 km. We\nhave added one sentence in the revised manuscript to state this fact, as follows:\n“From cross sections of seismicity shown in Figure 1 and Figure S14, the local slab\npenetration beneath the CVF mainly affects the seismicity deeper than ~400 km based\non the fact that the slab seismicity is distributed above ~400 km outside the slab\npenetration region.”\n6. Lines 55 & 57: The “NE China” and “Northeast China” should match with each other.\nResponse: Thank you for pointing this out. The relevant sentences have been corrected.\n7. Lines 83 & 84: The water content is not the criteria to discriminate the OIB and EM1,\nthe high water content can also appear in EM1 mantle.\nResponse: Thanks for pointing this out. We have revised this sentence accordingly:\n“This implies a mantle source region with H O concentrations ranging between 350-\n2\n550 ppm (Kuritani et al., 2019), which could correspond to a variety of mantle sources\nwithout requiring significant H O sourced from a subducting slab (Jamtveit et al.,\n2\n2001).”\n8. Lines 108-116: Very interesting. I want to remind the author that there may also be\nsignificant amounts of water and volatiles in the subducting oceanic crust, so this model\ndoesn't necessarily contradict the model of the metastable olivine wedge (with low\nwater content within the slab). I checked Line 114, and the reference 'Jing et al.' should\nbe 'Jiang et al’.\nResponse: Based on the thermal model for the Pacific plate subduction in east Asia,\ndehydration reactions of various hydrous minerals in the subducting oceanic crust and\nmantle would have been finished at depth of ~300 km (Hacker et al., 2003). This is the\nreason why we think this is a contradiction. We have slightly revised the text to make\nthis point clearer.\n“However, based on the subduction zone thermal model in this region (Syracuse et al.,\n2010), dehydration reactions of various hydrous minerals in the slab are most likely\nintersected at depths shallower than ~300 km (Hacker et al., 2003). In addition, this\nhypothesis contradicts the inferred presence of a metastable olivine wedge (MOW) in\nthe core of the slab (Jiang et al., 2008; Shen and Zhan, 2020), which indicates a\nminimal amount of water within it (Green et al., 2010).”\nThe typo “'Jing et al.'” has been changed to “Jiang et al.”\n9. Lines 231-234: I checked the article by Han et al. (2024), and they only detected\nlocalized vertical flow, which does not directly indicate the presence of an upwelling\npenetrating the slab. The citation here could potentially mislead readers.\nResponse: Sorry for not making this point clear. If there exists a slab gap and hot mantle\nmaterials from lower mantle upwell through this gap, the upwelling “plume” should be\nlocalized above the slab gap, instead of widespread in the big mantle wedge. Seismic\nanisotropy study of Han et al. (2024) found localized vertical flow, consistent with the\nexistence of the slab gap and its role on forming the CVF. We have slightly revised the\nmanuscript to make this point clearer, as follows:\n“Recent seismic anisotropy analysis further supports the idea of mantle flow migrating\nupwards through the slab by revealing a localized region of vertical flow directly\nbeneath Changbaishan volcano (Han et al., 2024).”\n10. Lines 236-240: I didn't understand what this section is trying to convey. What is the\nconnection between the time points of 20 Ma and 5 Ma and the slab gap? How did you\narrive at this conclusion: 'Therefore, the linkage of the slab gap in the MTZ with the\nChangbaishan volcanism is also supported by the timing of volcanism and slab\nstagnation.'?\" The Pacific slab has started to be stagnant in the MTZ since ~20 Ma,\nwhich is earlier than the initiation of the Changbaishan volcanism (~5Ma). So, the\nchronology of volcanism does not directly support the slab gap model, nor does it\ndirectly rule out the big mantle wedge model.\nResponse: Following the suggestion of reviewer #2, we have included a recent\ngeodynamic modelling of Lin et al. (2024) in the revised manuscript. As a result, we\nhave revised the part accordingly to make the argument more logic.\n“It is important to recognize that the length and position of the Pacific slab, and\nvolcanic activity in the CVF have changed through time. The Pacific slab probably only\nreached the MTZ at ~20 Ma (Wu et al., 2022) or ~15 Ma (Lin et al., 2024). The earliest\nmassive basalt eruptions in the CVF can be traced back to ~16 Ma (Liu et al., 2015b),\nprobably predating the presence of a stagnant slab (Lin et al., 2024). At ~15 Ma, the\nPacific slab started to stagnate in the MTZ and the sub-horizontal slab continued to\nadvance westward (Lin et al., 2024). We propose that the formation of the slab gap and\nassociated mantle upwelling may be associated with the shield forming stage of CVF\nvolcanism from ~5-1 Ma (Liu et al., 2015b; Ward et al., 2021; Zhang et al., 2018). We\nnote that this interpretation is not mutually exclusive of other mechanisms of mantle\nupwelling, such as lithospheric dripping beneath the Songliao Basin (Lin et al., 2024).”\n11. Lines 254-258: Based on Figure 3, it doesn't seem that this pattern is apparent (In\nthe center of the penetrating section of the slab, most of the focal mechanisms suggest\nalong-strike extension. At the edges of the penetrating slab, slab-normal extension is\nobserved).\nResponse: We acknowledge that the distribution of focal mechanisms is not very clear\ndue to the overlapping of different types of mechanisms on Figure 3. To clarify, we\nhave revised Supplementary Figure 15 to also plot the histogram of normal fault, thrust\nfault and strike-slip fault mechanisms along latitude. The related text is as follows:\n“Focal mechanisms from the Global Centroid Moment tensor catalog indicate down-\ndip compression and a mixture of slab-normal and along-strike extension (Gan et al.,\n2015; Supplementary Figure 15). In the center of the penetrating section of slab (Fig.\n2, Profile D), most of the focal mechanisms suggest along-strike extension and down-\ndip compression (i.e., strike-slip motion in the plane of the slab). At the edges of the\npenetrating slab segment (Profiles C and E), slab-normal extension and down-dip\ncompression is seen (i.e., thrust faulting in the plane of the slab), similar to shallower\nevents (Zhao and Ohtani, 2009; Gan et al., 2015).”\nSupplementary Figure 15 Distribution of deep-focus earthquakes and their focal mechanisms in the\nmantle transition zone in northeast China. (a) Map view of deep-focus earthquakes, denoted as\nbeachballs with colors representing the source depth. Each earthquake is indexed with the first letter\nT representing thrust-fault focal mechanism, N representing normal-fault focal mechanism, and S\nrepresenting strike-fault focal mechanism, respectively. Slab penetration region is marked a blue\ndashed line. (b) Histogram of normal fault, thrust fault and strike-slip fault mechanisms along\nlatitude."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed most of my concerns about the original manuscript. I suggest the manuscript to be published\nafter minor revisions.\nTo avoid misleading, I suggest the authors add a figure in the supplementary materials to show tomographic results of\nvertical profiles relative to the average velocity model as shown in Response Figure 1. Noting that Figure 2 shows any\nstructural information in the crust.", "reviewer_3": "The authors have revised the manusript carefully accroding to my comments, I have no more questions."}, "author_response": ""}]

Decision letter