Earthquakes are produced by slippage quickly unzipping along faults, causing Earth’s vibrations, that is, ground shaking. The impact of the earthquake can become more catastrophic by triggered phenomena, like landslides and tsunamis, as witnessed during the 2018 Palu (Indonesia) earthquake of magnitude 7.5. Generally, the faster the earthquake rupture, the stronger the shaking. The Palu earthquake is among a class of very fast but rare earthquakes whose speed exceeds that of shearing waves in rocks. Theoretically, these so-called “supershear earthquakes” can propagate steadily only if faster than a speed known as Eshelby’s speed. Surprisingly, the Palu earthquake is slower than this limit. How can we explain this unusual speed? Did it affect the triggering of landslides, including submarine landslides that likely contributed to the tsunami? We address these questions through computer simulations, particularly focusing on the possible effect of a “fault damage zone,” a layer of softened rocks surrounding faults and caused by rock fracturing accumulated throughout the past fault activity. We found that, if a damage zone exists around the Palu fault, it can explain the unusual speed of this supershear earthquake and may have had the beneficial effect of reducing the shaking and thus its induced landslide and tsunami hazards in Palu.