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Detections of ultralow velocity zones in high-velocity lowermost mantle linked to subducted slabs

Nature Geoscience volume 17, pages 332–339 (2024)Cite this article

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Abstract

Ultralow velocity zones revealed by seismological observations at the core–mantle boundary lend clues to the physico-chemical characteristics and dynamic evolution of the Earth’s deep interior. Ultralow velocity zones have been primarily detected within and at the edges of the large low-velocity provinces. Ultralow velocity zones in high-velocity lowermost mantle have also been reported, but global assessment has been limited by data coverage. Here we use geophysical observations of the seismic phase SKKKP and its B-caustic diffractions (SKKKP waves beyond the B-caustic distance) for 60 large and deep-focus events recorded in North America, Europe and China to detect ultralow velocity zones at the core–mantle boundary. We analyse and simulate the extended SKKKP B-caustic diffractions with different velocity anomalies in the mantle and outer core. In addition to ultralow velocity zones around the two large low-velocity provinces beneath the mid-Pacific and Africa, our results support ultralow velocity zones in previously under-explored high-velocity lowermost mantle regions, including Central America, Alaska, Greenland and West and Central Asia. We suggest our evidence for ultralow velocity zones in high-velocity lowermost mantle is consistent with the presence of partially molten subducted oceanic crust at the deep lower mantle.

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Fig. 1: Basic information of SKKKP waves.
Fig. 2: Three examples of seismograms that show SKKKP and its B-caustic diffractions along with P′P′.
Fig. 3: Synthetic seismograms for 2D ULVZ models at four CMB interaction regions of SKKKP.
Fig. 4: Sampling regions of SKKKP B-caustic diffractions and dynamic processes of ULVZ evolution.

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Data availability

Most waveform data were downloaded using ObsPy68 from the Incorporated Research Institutions for Seismology Data Management Center (http://service.iris.edu), the Southern California Earthquake Data Center (https://scedc.caltech.edu), the GEOFON data centre of the GFZ German Research Centre for Geosciences (http://geofon.gfz-potsdam.de), the Italian National Institute of Geophysics and Volcanology (http://webservices.ingv.it) and the following networks (described by their network codes): GR (https://doi.org/10.25928/mbx6-hr74), RN (https://doi.org/10.7914/SN/RN), SX (https://doi.org/10.7914/SN/SX), TH (https://doi.org/10.7914/SN/TH), NO (http://fdsn.org/networks/detail/NO), NS (http://fdsn.org/networks/detail/NS), KO (https://doi.org/10.7914/SN/KO). Other waveform data were downloaded under license from the International Earthquake Science Data Center (https://doi.org/10.11998/IESDC). Earthquake information in Supplementary Tables 1–4 was obtained from the ISC earthquake catalogue search engine (http://www.isc.ac.uk/iscbulletin/search/catalogue). Source parameters of the three events in Fig. 2 were obtained from the GCMT catalogue (https://www.globalcmt.org). Tomographic models used in this study are accessible through the references provided29,30,31,32,33. The source data for Figs. 1–4 and Extended Data Figs. 1–8, information on event–station pairs and waveform data from International Earthquake Science Data Center in this study can be accessed at https://doi.org/10.6084/m9.figshare.24465991. Source data are provided with this paper.

Code availability

The synthetic seismograms were generated by the open source AxiSEM (http://seis.earth.ox.ac.uk/axisem) and SEM-DSM hybrid method (https://github.com/wenbowu-geo/SEM_DSM_hybrid). The finite-frequency sensitivity kernels were calculated using open source MC Kernel (http://seismology.github.io/mc_kernel). The waveform analyses were done using the ObSpy68. Figures were created using GMT (https://www.generic-mapping-tools.org). Specific data processing and figure plotting scripts are available from the corresponding author upon request.

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Acknowledgements

All maps in this paper were produced using GMT developed by P. Wessel and W.H.F. Smith. We thank T.T. Cao for helping with the cartoon in Fig. 4. This study is supported by National Natural Science Foundation of China (42030311 received by S.N., 42274077 received by B.Z., 42394114 received by M.H.), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant number XDB41000000 received by B.Z.).

Author information

Authors and Affiliations

  1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China

    Yulong Su & Daoyuan Sun

  2. State Key Laboratory of Geodesy and Earth’s Dynamics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan, China

    Sidao Ni, Baolong Zhang, Yulin Chen, Heping Sun, Mingqiang Hou & Xiaoming Cui

  3. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China

    Sidao Ni, Baolong Zhang, Yulin Chen, Heping Sun, Mingqiang Hou & Xiaoming Cui

  4. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

    Wenbo Wu

  5. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Mingming Li

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  1. Yulong Su
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Contributions

S.N. conceived the study. Y.S. analysed the data, prepared the figures and drafted the paper under the supervision of S.N. and B.Z. Y.S. and W.W. performed the synthetics. Y.S. and Y.C. calculated the sensitivity kernels. M.L. contributed to geodynamical interpretations and paper preparation. D.S. and W.W. provided guidance on seismological results. H.S., M.H. and X.C. contributed to the discussion of conclusions. All authors discussed the results and contributed to writing the paper.

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Correspondence to Sidao Ni.

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Extended data

Extended Data Fig. 1 Distribution of events and stations used in this study.

A total of 143 events (stars) with a magnitude of more than Mb 6.0 and a depth of more than 300 km recorded by broad-band and short-period stations (triangles) were used for SKKKP B-caustic diffractions analyses. The events that show SKKKP or its B-caustic diffractions are marked with orange stars, while the events that do not show these seismic waves are marked with cyan stars. The red points and purple points indicate the theoretical core entry point and core exit point at B-caustic distance for event-station pairs with SKKKP B-caustic diffractions. The contours are drawn for reference at the –0.5% S-wave velocity level from tomographic model S40RTS at a depth of 2,850 km.

Source data

Extended Data Fig. 2 Observations of SKKKP and its B-caustic diffractions.

a-c, Record sections display vertical component seismograms in the frequency band 0.5–1.2 Hz for three deep-focus events. a, The 20130514 event, magnitude Mb 6.2, depth 611.7 km. b, The 20131001 event, magnitude Mb 6.3, depth 578.4 km. c, The 20140721 event, magnitude Mb 6.4, depth 614.0 km. The dashed blue lines show the theoretical travel time curves of SKKKP and its B-caustic diffractions calculated with the AK135 model using a constant slowness of –4.37 s per deg. The maps above the record sections show the locations of events (orange stars) and stations (grey triangles).

Source data

Extended Data Fig. 3 Record sections and particle motions of SKKKP and its B-caustic diffractions from the Southern California Seismic Network.

a,c,e,g,i, Record sections show vertical component seismograms in the frequency band 0.5–1.2 Hz for five deep-focus events at different epicentral distance ranges. b,d,f,h,j, Particle motions at one of the stations of the record section on the left with a high signal-to-noise ratio.

Source data

Extended Data Fig. 4 Vespagrams and beamforming results of SKKKP and its B-caustic diffractions.

a,c,e,g,i, Fourth-root vespagrams of the seismograms shown in Extended Data Fig. 3. The black crosses indicate the theoretical arrival times and slownesses for SKKKP or its B-caustic diffractions calculated with the AK135 model. b,d,f,h,j, Beamforming results of the seismograms shown in Extended Data Fig. 3. The black crosses indicate the theoretical slownesses and back azimuths for SKKKP or its B-caustic diffractions calculated with AK135 model.

Source data

Extended Data Fig. 5 Map and SEM model set-up for testing the effects of slab structure anomalies.

a, The large map shows the locations of the 20070716 event (Mb 6.2, depth 358.2 km, orange star), real stations (grey triangles) and added stations (light-yellow triangles) that were used for the synthetic test. The red lines represent the epicentral distance contours. The small map displays the magnified area of the blue box in the large map. The solid black lines represent the depth contours of the slab. b, The cross-shaped slice (A-A′, B-B′, C-C′ and D-D′ in a) of the 6° (longitude) × 6° (latitude) × 670 km (depth) SEM box (the dashed black box in a). c-e, The depth profiles of P-wave velocity along the slab downdip direction (A-A′ in a). The slab has a thickness of 120 km and the black lines in c-e show the velocity anomaly of the slab. The inset in c shows the location of earthquakes at different depths in synthetic tests that use the same OC LVZ model.

Source data

Extended Data Fig. 6 Distance profiles for 1D model and 3D slab models with different focal depths.

a, 1D synthetics with AK135 model. b-f, 3D synthetics generated by varying the depth of the earthquake while using the same OC LVZ model in Extended Data Fig. 5c. The source depths are 348 km (b), 358 km (c), 368 km (d), 378 km (e) and 388 km (f). The insets in a-f show the amplitude of SKKKP versus epicentral distance. All the waveforms are filtered with 0.5–1.0 Hz.

Source data

Extended Data Fig. 7 Synthetic seismograms for 1D model and 2D ULVZ models with different P-wave velocity variations on the receiver side.

a, AK135 model. b-f, 2D sinusoid-shaped ULVZ models with a thickness of 30 km, a width of 60 km, and a P-wave velocity increase of 6% (b), a P-wave velocity reduction of 6% (c), a P-wave velocity reduction of 8% (d), a P-wave velocity reduction of 12% (e), a P-wave velocity reduction of 20% (f). All the waveforms are aligned with the theoretical arrival times of SKKKP and its B-caustic diffractions (red lines) and filtered with 0.5–1.0 Hz.

Source data

Extended Data Fig. 8 Sampling regions of SKKKP B-caustic diffractions.

a-c, Locations of the sampling regions on different tomographic models. The background models display S-wave velocity tomographic model GyPSuM (a), SEMUCB-WM1 (b) and P-wave velocity tomographic model MIT-P08 (c). Yellow lines indicate the potential sampling regions of SKKKP B-caustic diffractions at the CMB, and their lengths represent the extension lengths. The purple points represent the theoretical core exit point of SKKKP at B-caustic distance for event-station pairs.

Source data

Extended Data Table 1 Parameters of 1D anomalous velocity models near the CMB region
Full size table
Extended Data Table 2 Extension lengths of SKKKP B-caustic diffractions for 2D ULVZ models with different parameters on the receiver side
Full size table

Supplementary information

Supplementary Information

Supplementary Texts 1–3, Figs. 1–24 and Tables 1–7.

Source data

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Su, Y., Ni, S., Zhang, B. et al. Detections of ultralow velocity zones in high-velocity lowermost mantle linked to subducted slabs. Nat. Geosci. 17, 332–339 (2024). https://doi.org/10.1038/s41561-024-01394-5

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