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Deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric

Nature Geoscience volume 11, pages 696–700 (2018)Cite this article

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Abstract

Analysis of deep subduction-zone earthquakes, those at depths greater than 60 km, reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths. Over the past five decades, it has been observed that a large fraction of deep earthquakes has non-double-couple (non-DC) seismic radiation patterns. In contrast, shallow earthquakes tend to have DC radiation patterns due to mechanisms of shear faulting. These observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes. Here we show that the observed global distribution of non-DC deep earthquakes could be caused by shear faulting mechanisms, but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs. For intermediate-depth earthquakes (~60–300 km), we found a large shear-wave anisotropy of ~25%, possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface, which provides new supporting evidence for the metamorphic dehydration reactions in slabs. However, at deep-focus depths (>300 km), the putative metastable phase-change mechanism alone cannot explain the seismic anisotropy. Instead, our results and those from recent experiments suggest materials such as magnesite, or perhaps carbonatite melt, may play a role in generating deep-focus earthquakes.

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Fig. 1: Inverted TTI anisotropy symmetry axes (coloured arrowheads) with respect to slab geometry.
Fig. 2: Apparent non-DC events caused by shear dislocations in anisotropic media.
Fig. 3: Summary of inverted anisotropy parameters.
Fig. 4: Schematic diagram of the origin of anisotropy.

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Acknowledgements

We thank T. Lay for his suggestion to investigate the influence of the slab velocity anomaly on the radiation pattern. We also thank K. Burke, H. Zhou and J. Suppe for discussions, manuscript reading and comments. We appreciate the CMT data without which this work is impossible. We appreciate J. Wu for providing help on the tomography models. This work is supported by NSF EAR-1621878.

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Authors and Affiliations

  1. Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA

    Jiaxuan Li, Yingcai Zheng, Leon Thomsen, Thomas J. Lapen & Xinding Fang

  2. Department of Ocean Science and Engineering, South University of Science and Technology of China, Shenzhen, Guangdong, China

    Xinding Fang

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  1. Jiaxuan Li
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Contributions

Y.Z. started this research by analysing the CMT database to investigate anisotropy for global slabs using non-DC deep earthquakes. He also contributed to all aspects of the research, including manuscript writing. L.T. contributed to the analysis of anisotropy and earthquake radiation patterns. J.L. did the inversion of anisotropy and analysed and ruled out the effect of slab heterogeneity as a major factor in producing non-DC radiation. X.F. did the 3D elastic anisotropic FD modelling for the slab heterogeneity influence on earthquake radiation patterns. T.L. contributed to the petrological causes of the anisotropy.

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Correspondence to Yingcai Zheng.

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Li, J., Zheng, Y., Thomsen, L. et al. Deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric. Nature Geosci 11, 696–700 (2018). https://doi.org/10.1038/s41561-018-0188-3

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