The September 2018 earthquake Palu, Indonesia, which generated a devastating tsunami that killed over 2,000 people, ruptured at supershear speeds, report two papers published online this week in Nature Geoscience.

The magnitude 7.5 Palu earthquake on 28 September 2018 was located on a site where two crustal blocks slide horizontally past each other (a strike-slip fault). The speed at which earthquake faults rupture controls the behaviours of the seismic waves generated and consequently the associated hazards. Usually, rupture speeds are around 70-90% of the speed at which shear-waves (those in which movement is perpendicular to the wave’s direction) travel through the Earth’s crust; ruptures at faster speeds are rare.

In two separate papers, Anne Socquet and colleagues, and Jean-Paul Ampuero and co-authors examined satellite imagery and seismic records of the Palu earthquake. They find that the geological fault ruptured and propagated along 180 kilometres at a supershear speed of 4.1 kilometres per second. The authors show that the damaged rocks around the fault helped to lock in and maintain the propagating rupture at this speed.

In an accompanying News & Views article, Martin Mai comments that “whether or not the supershear rupture aggravated the tsunami excitation remains to be studied, but it certainly affected the near-fault shaking levels. Such compounded tsunami and shaking hazard in marine strike-slip environments is largely neglected in standard seismic hazard assessment.