Supershear shock front contribution to the tsunami from the 2018 $\mathbf M_{\mathbf w}$ 7.5 Palu, Indonesia earthquake

TL;DR

This study demonstrates that supershear rupture speeds during the 2018 Palu earthquake contributed to tsunami generation through Mach fronts interacting with local bathymetry, explaining the unexpected tsunami despite a strike-slip fault.

Contribution

It provides evidence that supershear rupture speeds can generate tsunamis in strike-slip earthquakes, highlighting a new mechanism for tsunami formation.

Findings

Supershear rupture speed confirmed by GPS data.

Simulations reproduce observed tsunami motions.

Mach fronts contributed to tsunami generation.

Abstract

Hazardous tsunamis are known to be generated predominantly at subduction zones. However, the 2018 Mw 7.5 Sulawesi (Indonesia) earthquake on a strike-slip fault generated a tsunami that devastated the city of Palu. The mechanism by which this tsunami originated from such an earthquake has been debated. Here we present near-field ground motion (GPS) data confirming that the earthquake attained supershear speed, i.e., a rupture speed greater than the shear wave speed of the host medium. We study the effect of this supershear rupture on tsunami generation by coupling the ground motion to a one-dimensional nonlinear shallow-water wave model accounting for both time-dependent bathymetric displacement and velocity. With the local bathymetric profile of Palu bay around a tidal station, our simulations reproduce the tsunami arrival and motions observed by CCTV cameras. We conclude that Mach (shock) fronts, generated by the supershear speed, interacted with the bathymetry and contributed to the tsunami.