A magnitude 7.5 earthquake hit the city of Palu in Sulawesi, Indonesia on 28 September 2018 at 10:02:43 (coordinated universal time). It was followed a few minutes later by a 4–7-m-high tsunami. Palu is situated in a narrow pull-apart basin surrounded by high mountains of up to 2,000 m altitude. This morphology has been created by a releasing bend in the Palu-Koro fault, a rapidly moving left-lateral strike-slip fault. Here we present observations derived from optical and radar satellite imagery that constrain the ground surface displacements associated with the earthquake in great detail. Mapping of the main rupture and associated secondary structures shows that the slip initiated on a structurally complex and previously unknown fault to the north, extended southwards over 180 km and passed through two major releasing bends. The 30 km section of the rupture south of Palu city is extremely linear, and slightly offset from the mapped geological fault at the surface. This part of the rupture accommodates a large and smooth surface slip of 4–7 m, with no shallow slip deficit. Almost no aftershock seismicity was recorded from this section of the fault. As these characteristics are similar to those from known supershear segments, we conclude that the Palu earthquake probably ruptured this segment at supershear velocities.
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Synthetic aperture radar data were processed using GMTSAR software, freely available from https://topex.ucsd.edu/gmtsar/. Optical satellite images were mosaicked using the Geospatial Data Abstraction Library (http://gdal.org) and then correlated using the COSI-Corr software package available at www.tectonics.caltech.edu/slip_history/spot_coseis/download_software.html. Deformation generated by a static earthquake source was modelled using Okada57 equations implemented in the disloc program (www.physics.hmc.edu/GL/disloc/disloc.c).
The data sets generated during the current study (displacement fields from Landsat-8, Sentinel-2 and WorldView image correlation and from the ALOS-2 interferogram, as well as the static slip distribution) are available from the corresponding author upon request. Raw satellite optical imagery was made freely available by ESA (Sentinel-2, https://scihub.copernicus.eu/dhus/#/home), USGS (Landsat-8, https://earthexplorer.usgs.gov/) and DigitalGlobe (WorldView, www.digitalglobe.com/opendata/indonesia-earthquake-tsunami/). Raw ALOS-2 data availability is restricted to PI investigation at www.eorc.jaxa.jp/ALOS/en/aw3d30/data/index.htm.
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Our first thoughts go to the population of Sulawesi and to the victims of this earthquake. Satellite optical imagery has been made freely available by ESA (Sentinel-2, https://scihub.copernicus.eu/dhus/#/home), USGS (Landsat-8, https://earthexplorer.usgs.gov/) and DigitalGlobe (WorldView, www.digitalglobe.com/opendata/indonesia-earthquake-tsunami/). We thank JAXA for providing ALOS-2 data under PI investigation no. 3328 (ALOS 6th RA) and the AW3D30 digital elevation model. We also thank the contributors to GMT and GMTSAR opensource software.
The authors declare no competing interests.
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Socquet, A., Hollingsworth, J., Pathier, E. et al. Evidence of supershear during the 2018 magnitude 7.5 Palu earthquake from space geodesy. Nat. Geosci. 12, 192–199 (2019). https://doi.org/10.1038/s41561-018-0296-0
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