Tsunami genesis of strike-slip earthquakes revealed in the 2018 Indonesian Palu event

The devastating tsunami after the 2018 Indonesian Sulawesi-Palu strike-slip earthquake was a surprise because strike-slip faulting is a known phenomenon of primarily lateral movement of land, while tsunamis are believed to be caused by vertical movements of seafloor or landslides. Here we demonstrated how the strike-slip faulting could create enough oceanic kinetic energy in the lateral directions that might have powered the tsunami. We constructed three earthquake inversion solutions from seismographs, satellite radar and optical imagery, and used an open-source ocean circulation model to replicate the tsunami. Our experiments revealed three new results: (1) the main force of the tsunami could be created by the horizontal displacement of deeper-water slopes along the Makassar coast, evidenced by the Pantoloan tide-gauge record that has two distinguished crests, propagating southward into the Palu Bay, twice higher than the local resonance waves; (2) the Mamuju tide-gauge recorded two types of tsunamis: the “early arrival” tsunami originated from nearby coast and the “later arrival” larger tsunami originated from the Palu Bay; and (3) the eyewitness account of the whirlpool circulation in the Palu Bay could be better explained by the horizontal strike-slip forcing of the two involved tectonic plates. Our findings suggest that the tsunami formation mechanism of strike-slip earthquakes is more complex than previously thought.