  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.
