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Early and persistent supershear rupture of the 2018 magnitude 7.5 Palu earthquake

Abstract

The speed at which an earthquake rupture propagates affects its energy balance and ground shaking impact. Dynamic models of supershear earthquakes, which are faster than the speed of shear waves, often start at subshear speed and later run faster than Eshelby’s speed. Here we present robust evidence of an early and persistent supershear rupture at the sub-Eshelby speed of the 2018 magnitude 7.5 Palu, Indonesia, earthquake. Slowness-enhanced back-projection of teleseismic data provides a sharp image of the rupture process, along a path consistent with the surface rupture trace inferred by subpixel correlation of synthetic-aperture radar and satellite optical images. The rupture propagated at a sustained velocity of 4.1 km s–1 from its initiation to its end, despite large fault bends. The persistent supershear speed is further validated by seismological evidence of far-field Rayleigh Mach waves. The unusual features of this earthquake probe the connections between the rupture dynamics and fault structure. An early supershear transition could be promoted by fault roughness near the hypocentre. Steady rupture propagation at a speed unexpected in homogeneous media could result from the presence of a low-velocity damaged fault zone.

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Data availability

The ALOS-2 original data can be obtained from JAXA. Derived pixel offset maps can be obtained from the authors. Copernicus Sentinel images are available at no cost from the Copernicus Open Access Hub (https://scihub.copernicus.eu/). PlanetScope images are available from Planet Labs (https://www.planet.com/). The broadband seismograms are accessed from IRIS (www.iris.edu) data centres for the Australian and Alaskan networks, from ORFEUS (www.orfeus-eu.org) for the Turkish network, from GEONET (www.geonet.org.nz) for the New Zealand network and from Hi-net (http://www.hinet.bosai.go.jp) for the Japan network. The earthquake catalogues are obtained from the USGS NEIC (http://earthquake.usgs.gov). The background topography and bathymetry used in our figures are provided by the NOAA National Center for Environmental Information (https://www.ngdc.noaa.gov/mgg/global/etopo1sources.html). The USGS W-phase solution can be accessed at https://earthquake.usgs.gov/earthquakes/eventpage/us1000h3p4/moment-tensor. The computer code for back-projection is available upon request to L.M.

Additional information

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Acknowledgements

H.B. and L.M. were supported by NSF Earthscope grant no. EAR-1614609, NSF Geophysics grant no. EAR-1723192, and by the Leon and Joanne V.C. Knopoff Foundation. J.-P.A. acknowledges funding from the UCA-JEDI Investments in the Future project managed by the French National Research Agency (ANR, grant no. ANR-15-IDEX-01) and from ANR grant no. ANR-17-CE31-0008-01. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) for the Earth Surface and Interior focus area and NISAR Science Team. The ALOS-2 original data are copyright JAXA and provided under JAXA ALOS RA6 PI projects P3278 and P3360. Sentinel-2 images used in our analysis contain modified Copernicus Sentinel data (2018), processed by the European Space Agency. We thank Planet Labs for access to their PlanetScope imagery. Funding for C.W.D.M. was provided under a NASA Postdoctoral Program fellowship administered by the Universities Space and Research Association through a contract with NASA. H.B. acknowledges that the Python software package ObSpy was used for data requesting, waveform filtering and cross-correlations.

Author information

J.-P.A. conceived and led the study. H.B. and L.M. performed the SEBP. H.B. performed the Rayleigh Mach wave analysis. C.L. and E.J.F. carried out the SAR image analysis. C.W.D.M. conducted the optical image analysis. J.-P.A., H.B., L.M., E.J.F. and C.W.D.M. wrote the paper and participated in the interpretation of the results. T.F. conducted a preliminary back-projection analysis. H.H. performed the aftershocks relocation.

Competing interests

The authors declare no competing interests.

Correspondence to Jean-Paul Ampuero.

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Further reading

Fig. 1: Surface rupture trace and supershear speed of the Palu earthquake.
Fig. 2: Calibration of back-projection based on aftershock data.
Fig. 3: Evidence of a far-field Rayleigh-wave Mach cone.
Fig. 4: Evidence of Rayleigh Mach waves.