Supershear shock front contributions to the Palu bay tsunami following the Mw 7.5 Sulawesi earthquake

The 2018 Sulawesi strike-slip earthquake generated an unexpected tsunami in the nearby Palu bay with deadly consequences. The underlying mechanisms that caused this have been much debated since such strike-slip earthquakes are not known to generate large waves. However, a notable feature of this earthquake is that it went supershear, i.e., the rupture attained a speed greater than the shear wave speed of the host medium. Such ruptures have been shown to produce two shock (or Mach) fronts, corresponding to an exceedance of shear and Rayleigh wave speeds, that carry significant particle velocities to large distances. In this study we confirm that the Sulawesi earthquake attained supershear speed by providing the first evidence of a supershear rupture recorded by a high-rate GPS station. We then couple the corresponding ground motion produced by the such effects to a non-linear shallow water wave model that accounts explicitly for the time-dependent bathymetric velocity. By considering the local bathymetric profile of the Palu bay around the Pantoloan harbor tidal gauge, we have been able to reproduce the primary motions of the tsunami as observed by CCTV/social media camera footage. This suggests that the Mach fronts may have combined with the shallow bay geometry to contribute to the timing of the tsunami.