Nearly Instantaneous Tsunamis Following the Mw 7.5 2018 Palu Earthquake

The tsunami observations produced by the 2018 magnitude 7.5 Palu strike‐slip earthquake challenged the traditional basis underlying tsunami hazard assessments and early warning systems. We analyzed an extraordinary collection of 38 amateur and closed circuit television videos to show that the Palu tsunamis devastated widely separated coastal areas around Palu Bay within a few minutes after the mainshock and included wave periods shorter than 100 s missed by the local tide station. Although rupture models based on teleseismic and geodetic data predict up to 5‐m tsunami runups, they cannot explain the higher surveyed runups nor the tsunami waveforms reconstructed from video footage, suggesting either these underestimate actual seafloor deformation and/or that non‐tectonic sources were involved. Post‐tsunami coastline surveys combined with video evidence and modeled tsunami travel times suggest that submarine landslides contributed to tsunami generation. The video‐based observations have broad implications for tsunami hazard assessments, early warning systems, and risk‐reduction planning. Plain Language Summary Tsunami hazard assessment is routinely based on assessing the impacts of long‐period waves generated by vertical seafloor motions reaching the coast tens of minutes after the earthquake in typical subduction‐zone environments. This view is inadequate for assessing hazard associated with strike‐slip earthquakes such as the magnitude 7.5 2018 Palu earthquake, which resulted in tsunami effects much larger than would normally be associated with horizontal fault motion. From an extraordinary collection of 38 amateur and closed circuit television videos we estimated tsunami arrival times, amplitudes, and wave periods at different locations around Palu Bay, where the most damaging waves were reported. We found that the Palu tsunamis devastated widely separated coastal areas within a few minutes after the mainshock and included unusually short wave periods, which cannot be explained by the earthquake fault slip alone. Post‐tsunami surveys show changes in the coastline, and this combined with video footage provides potential locations of submarine landslides as tsunami sources that would match the arrival times of the waves. Our results emphasize the importance of estimating tsunami hazards along coastlines bordering strike‐slip fault systems and have broad implications for considering shorter‐period nearly instantaneous tsunamis in hazard mitigation and tsunam