Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake

Abstract

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments.

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Fig. 1: Seismotectonic context.
Fig. 2: Multiple-point-source centroid moment tensor (CMT) inversion using OBS waveforms.
Fig. 3: Teleseismic analyses.
Fig. 4: Interpretation of rupture dynamics for the 2016 Romanche earthquake.

Data availability

The facilities of IRIS Data Services, and specifically the IRIS Data Management Center (IRIS-DMC), were used for access to waveforms, related metadata and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience (SAGE) Award of the National Science Foundation under Cooperative Support Agreement EAR-1851048. Additional seismic data were obtained from the GEOFON data centre of the GFZ German Research Centre for Geosciences (GFZ-GEOFON), ORFEUS/EIDA, the INGV Seismological Data Centre, and the RESIF Data Center. RESIF is a national Research Infrastructure, recognized as such by the French Ministry of Higher Education and Research. RESIF is managed by the RESIF Consortium, composed of 18 research institutions and universities in France. RESIF is additionally supported by a public grant overseen by the French National Research Agency (ANR) as part of the Investissements d’Avenir programme (ANR-11-EQPX-0040) and the French Ministry of Ecology, Sustainable Development and Energy. Continuous raw seismic waveform data from the PI-LAB ocean bottom seismometer network53 can be downloaded from IRIS-DMC (network code XS). Continuous raw seismic waveform data from various global seismic networks used for the slip-rate inversion54,55,56,57,58,59,60,61, BP54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81 and Mach cone54,56,57,58,59,60,67,68,69,70,79,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103 analyses are available to download from the data centres named above. Source data are provided with this paper.

Code availability

The ISOLA software used for RMT inversion is available at http://geo.mff.cuni.cz/~jz/for_ISOLAnews/ (accessed June 2020). The Palantiri software used for teleseismic BP analysis is available at https://braunfuss.github.io/Palantiri/ (accessed June 2020). We used the ObsPy package for seismic analysis104. Figures were made using GMT105 and matplotlib106.

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Acknowledgements

We thank the captain and crew of the RV Marcus Langseth and the RRS Discovery, and also the scientific technicians. S.P.H. was funded by the Natural Environment Research Council (NERC) grant NE/K010654/1; C.A.R., N.H. and P.B. acknowledge funding from NERC (NE/M003507/1 and NE/K010654/1) and the European Research Council (GA 638665); J.-M.K. and D.S. were funded by NERC (NE/M004643/1). A.S. and H.S. thank the German Research Foundation (DFG) for funding through an Emmy Noether Young Researcher Grant (no. 276464525). J.Z. was supported by the Czech Science Foundation (GACR-18-06716J). We thank M. Mai for taking the time to provide very useful feedback. T. Craig also provided useful comments.

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Authors

Contributions

S.P.H. managed the study, carried out the mainshock and aftershock source analysis and wrote the manuscript. R.O. computed the teleseismic slip-rate inversion, assisted with the Mach cone analysis and contributed to the manuscript. A.S. and H.S. computed the teleseismic BP images and contributed to the manuscript. C.A.R., N.H. and J.-M.K. conceived the experiment, acquired funding, managed the project and contributed to the manuscript. R.E.A. carried out a preliminary teleseismic slip inversion and contributed to the manuscript. P.B. and D.S. worked on early estimates of source depth, assisted with aftershock detection and relocation and commented on the manuscript. J.Z. assisted with the RMT inversions and contributed to the manuscript. Y.Y. and K.S. assisted with the teleseismic slip-rate inversion and commented on the manuscript.

Corresponding author

Correspondence to Stephen P. Hicks.

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The other authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Rayleigh wave vertical displacement seismograms.

Waveforms are plotted for the Mw 7.1 Romanche mainshock and Mw 5.8 co-located aftershock, for stations shown in Fig. 3b. The top-left box indicates the network code, station name, epicentral distance (Δ), azimuth relative to rupture direction (Φ), and cross-correlation coefficient. Stations N4.N59A and II.EFI are located in the predicted Mach cone.

Extended Data Fig. 2 Predicted Mach cone.

Azimuthal dependence of directivity factor (blue line) for a rupture velocity of 5.7 km/s, period of 10 s, rupture duration of 25 s and Rayleigh wave velocity of 3.4 km/s. The black-dashed line, red dashed lines, and green shaded areas demarcate azimuths where high waveform similarity between the Romanche mainshock and co-located aftershock can be found. These azimuth values correspond well to the locations where high waveform similarity is observed in the data (Fig. 3b).

Supplementary information

Supplementary Information

Supplementary Notes 1–6, Figs. 1–16 and Tables 1 and 2.

Supplementary Data 1

Compressed zip file comprising a QUAKEML-formatted catalogue containing hypocentral locations for the Romanche mainshock and aftershocks, along with arrival times, picks and magnitudes.

Supplementary Data 2

Microsoft Excel compatible spreadsheet containing full source parameters of the single- and multi-point source regional moment tensor (RMT) inversions.

Supplementary Data 3

csv file containing the station locations used for the teleseismic slip inversion.

Supplementary Data 4

csv file containing the station locations of sub-arrays used for teleseismic back projection imaging.

Supplementary Video 1

Movie showing the evolution of maximum semblance (and uncertainty) from teleseismic back projection imaging, demonstrating the back-propagating rupture in map view. The white star is the mainshock epicentre, the black circles show aftershocks.

Source data

Source Data Fig. 3a

Teleseismic slip rate model for plotting Fig.3a.

Source Data Fig. 3b

Back projection semblance over a 2-D geographic area for 2.5-km depth BSL for each time step. This is used for generating Fig. 3b and Supplementary Video 1.

Source Data Fig. 3c

Rayleigh wave cross-correlations for plotting Fig. 3c.

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Hicks, S.P., Okuwaki, R., Steinberg, A. et al. Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake. Nat. Geosci. 13, 647–653 (2020). https://doi.org/10.1038/s41561-020-0619-9

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