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Global frequency of oceanic and continental supershear earthquakes

Nature Geoscience volume 15, pages 942–949 (2022)Cite this article

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Abstract

Earthquakes are supershear when their rupture speed is faster than that of the seismic shear waves produced. These events are rare, but they can be highly destructive owing to the associated strong ground shaking, and understanding why they occur may provide insights into fault mechanics. Only a few supershear earthquakes have been reported previously, most of which were continental. Here we perform a systematic global search for supershear earthquakes by analysing seismic data from all large (Mw ≥ 6.7) shallow strike-slip earthquakes occurring between 2000 and 2020. Based on the rupture speeds determined by slowness-enhanced back-projection, and the identification of Rayleigh Mach waves, we identify four oceanic earthquakes consistent with supershear events. We find that at least 14.0% of large earthquakes during the study period were supershear, with oceanic events occurring as frequently as continental ones. We further observe a wider range of stable rupture speeds during supershear events than predicted by two-dimensional fracture mechanics theory, which we attribute to the presence of fault damage zones or slip obliqueness. The transition to and propagation of supershear earthquakes may be promoted in oceanic settings due to the thicker crustal seismogenic zones and the material contrast at oceanic–continental boundaries.

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Fig. 1: Global supershear earthquakes.
Fig. 2: Spatiotemporal distribution of high-frequency (HF) (0.5–2 Hz) radiations imaged by SEBP.
Fig. 3: Estimated rupture speeds of the supershear earthquakes.
Fig. 4: Evidence of far-field Rayleigh wave Mach cones.

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Data availability

All input data (seismograms and travel times) are available online. The seismic data are provided by the Incorporated Research Institutions for Seismology (IRIS, www.iris.edu) and Observatories & Research Facilities for European Seismology (ORFEUS, www.orfeus-eu.org), and the travel times are provided by the EHB catalogue (www.isc.ac.uk/isc-ehb). BPs of mainshocks and movies, and records and relocation catalogues of teleDD can be found at https://doi.org/10.5281/zenodo.7014024.

Code availability

The MATLAB code of SEBP is available at https://github.com/lsmeng/MUSICBP/tree/SEBP. The Python code of Mach wave analysis is available at https://github.com/lsmeng/MUSICBP/tree/MachWave. All other codes used in this study are available upon request.

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Acknowledgements

We thank J. Fineberg, N. Lapusta, P. Bird, E. Dunham, R. Okuwaki, H. Kehoe, T. Lay and S. Park for valuable comments and discussions. We thank H. Weng for the discussion of the effect of energy ratio on rupture sustainability. We thank B. Wu for the discussion of the BP synthetic test. We thank SRTM 15+ for providing global bathymetry and topography data. H.B, L.M. and L.X were supported by NSF grant no. EAR-1848486 and Leon and Joanne V.C. Knopoff Fund. J.-P.A. was supported by UCAJEDI Investments in the Future project (ANR-15-IDEX-01) managed by the French National Research Agency. H.Z. and L.G. were supported by the Fundamental Research Funds for the Central Universities (WK2080000144). Figures were produced using Generic Mapping Tools (GMT) and MATLAB. The Python software package ObSpy was used for data requesting, waveform filtering and CCs.

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  1. These authors contributed equally: Han Bao and Liuwei Xu.

Authors and Affiliations

  1. Earth, Planetary, and Space Sciences, University of California Los Angeles, Los Angeles, CA, USA

    Han Bao, Liuwei Xu & Lingsen Meng

  2. Observatoire de la Côte d’Azur, Université Côte d’Azur, IRD, CNRS, Valbonne, France

    Jean-Paul Ampuero

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

    Lei Gao & Haijiang Zhang

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Contributions

L.M. conceived and led the study. L.X. and H.B. performed the SEBP and the Rayleigh Mach wave analysis. H.B. and L.X. performed synthetic tests of BP. H.B., L.G. and H.Z. performed the aftershock relocation. H.B. performed rupture speed estimation. H.B. and L.X. designed the figures and tables. H.B. and L.M. wrote the original draft. H.B., L.X., L.M. and J.-P.A. contributed to finalize the manuscript and participated in the interpretation of the results.

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Correspondence to Lingsen Meng.

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Extended data

Extended Data Fig. 1 Synthetic BPs of rupture with realistic slip distribution and constant rupture speeds.

The assumed slip models are displayed on the top of each case. Four cases are shown: ruptures with the slip model of the 2013 Mw 7.3 South Sandwich Island earthquake (Hayes, 2017) and constant rupture speed of 3 km/s (top left) and 5 km/s (top right); ruptures with the slip model of the 2009 Mw 7.3 Caribbean earthquake (Hayes, 2017) and constant rupture speed of 3 km/s (bottom left) and 5 km/s (bottom right). Both the shadowing artifact and the tailing artifact are highlighted. To analyze the shadowing artifact, we plot the moment-time function (green curve) and the gradient of the moment-time function (black curve; dashed lines are the absolute value of the negative gradients of the moment-time function). See Supplementary Information for further discussion.

Extended Data Fig. 2 Statistical relation between the fitted rupture speed and the true rupture speed.

The sample points are the results of all synthetic BP cases (described in Supplementary Text 2): uniform slip with constant rupture speeds (green diamond; 5 cases), realistic slip with constant rupture speeds (blue circles; 45 cases), and abrupt changes in rupture speeds from 2 km/s (orange box; 4 cases) and from 3 km/s (purple box; 3 cases). For each input rupture speed, the mean value and the root-mean-square (RMS) error are shown as a red circle and a red errorbar. The red solid line represents the least-squares linear regression of the mean values. The two dashed lines represent the least-squares linear regression of the mean values plus and minors the RMS errors, Vfit+ and Vfit−, respectively. Therefore, we propose an empirical relation between the fitted rupture speed and the true (input) rupture speed with uncertainties based on the linear regression of the two dashed lines, as shown in the red font at the bottom of the figure. See Supplementary Information for further discussion.

Extended Data Fig. 3 Comparison of synthetic BPs using EGFs with (upper) and without (bottom) coda waves.

(Left) BP power as a function of time for different input rupture speeds. (Middle) Along-strike location and timing of BP radiators. Time is relative to the origin time. Location is the horizontal position relative to the hypocentre, projected along the strike direction. The straight lines indicate the speeds of the input rupture fronts (see the inset legend). (Right) Least-squares linear regressions between the timings and the along-strike distances of the leading BP radiators, defined as the furthest BP radiator at any given time. The fitted rupture speeds are shown in the inset legend. The dashed lines represent the 95% confidence interval (CI) of the fittings. This group of cases illustrate the affect of coda waves on BP imaging and speed estimation. See Supplementary Information for further discussion.

Extended Data Fig. 4 Statistical relation between the fitted rupture speed and the true rupture speed using EGFs without coda waves.

The sample points are the results of all synthetic BP cases (described in Supplementary Text 2): uniform slip with constant rupture speeds (green diamond; 5 cases), realistic slip with constant rupture speeds (blue circles; 45 cases), and abrupt changes in rupture speeds from 2 km/s (orange box; 4 cases) and from 3 km/s (purple box; 3 cases). For each input rupture speed, the mean value and the root-mean-square (RMS) error are shown as a red circle and a red errorbar. The red solid line represents the least-squares linear regression of the mean values. The two dashed lines represent the least-squares linear regression of the mean values plus and minors the RMS errors. The underestimation of the rupture speed is almost ignorable when using EGFs without coda waves. See Supplementary Information for further discussion.

Extended Data Fig. 5 Synthetic tests of the minimum propagation distance of supershear rupture that can be resolved by BP.

All synthetic tests have a subshear segment that is followed by a supershear segment. The rupture propagation distance of the subshear segments are the same, which is 50 km, while that of the supershear segment varies from 25 km to 150 km. See Supplementary Information for further discussion.

Extended Data Fig. 6 The Fig. 3 (in Main Text) with All BP Radiators and BP Powers.

For each event, the BP power is plotted as a function of time, while the BP radiators are plotted as the along-strike location versus time. Time is relative to the origin time. Location is the horizontal position relative to the hypocentre, projected along the strike direction. The tailing artifact and the shadowing artifact are highlighted. See Supplementary Information for further discussion.

Extended Data Fig. 7 Synthetic BPs of uniform rupture with abrupt changes in rupture speeds.

(top) The rupture propagates with Vr = 2 km/s for the first 50 sec then accelerates to Vr = 4 km/s for another 50 sec. (bottom) The rupture propagates with Vr = 2 km/s for the first 50 sec then accelerates to Vr = 6 km/s for another 50 sec. The left panels are the BP power as a function of time. The middle panels are along-strike location and timing of BP radiators. Time is relative to the origin time. Location is the horizontal position relative to the hypocentre, projected along the strike direction. The green line indicates the speed of the input rupture front. The right panels are the least-squares linear regressions after the velocity change between the timings and the along-strike distances of the leading BP radiators, defined as the furthest BP radiator at any given time. See Supplementary Information for further discussion.

Extended Data Fig. 8 SEBP of Multi-arrays of the 2018 Caribbean Earthquakes.

(a) Spatial temporal evolution of the rupture that are imaged by Alaska array (circle) and European network (EU). The symbols are color-coded by time with their size proportional to the normalized BP power. The red stars denote the NEIC epicenters for each of the seven supershear earthquakes. (b) The along-strike distances (relative to the hypocenter) of the HF radiators imaged by SEBP are plotted against the rupture times (with respect to the origin time).

Extended Data Fig. 9 SEBP of Multi-arrays of the 2020 Caribbean Earthquakes.

(a) Spatial temporal evolution of the rupture that are imaged by Alaska array (circle) and European network (EU). The symbols are color-coded by time with their size proportional to the normalized BP power. The red stars denote the NEIC epicenters for each of the seven supershear earthquakes. (b) The along-strike distances (relative to the hypocenter) of the HF radiators imaged by SEBP are plotted against the rupture times (with respect to the origin time).

Supplementary information

Supplementary Information

Supplementary Figs. 1–30, Tables 1–4, Texts 1–4 and references.

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Bao, H., Xu, L., Meng, L. et al. Global frequency of oceanic and continental supershear earthquakes. Nat. Geosci. 15, 942–949 (2022). https://doi.org/10.1038/s41561-022-01055-5

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