Does a Damaged‐Fault Zone Mitigate the Near‐Field Impact of Supershear Earthquakes?—Application to the 2018 Mw 7.5 Palu, Indonesia, Earthquake.

The impact of earthquakes can be severely aggravated by cascading secondary hazards. The 2018 Mw 7.5 Palu, Indonesia, earthquake led to devastating tsunamis and landslides, while triggered submarine landslides possibly contributed substantially to generate the tsunami. The rupture was supershear over most of its length, but its speed was unexpectedly slow for a supershear event, between the S wave velocity VS and Eshelby's speed 2VS, an unstable speed range in conventional theory. Here, we investigate whether dynamic rupture models including a low‐velocity fault zone can reproduce such a steady supershear rupture with a relatively low speed. We then examine numerically how this peculiar feature of the Palu earthquake could have affected the near‐field ground motion and thus the secondary hazards. Our findings suggest that the presence of a low‐velocity fault zone can explain the unexpected rupture speed and may have mitigated the near‐field ground motion and the induced landslides in Palu. Plain Language Summary: 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. Key Points: The unexpected supershear but sub‐Eshelby rupture speed of the 2018 Palu earthquake can be explained by a fault damage zoneThe reduction of rupture speed by a fault damage zone mitigates the near‐field ground motion and landslide hazardFault zone waves could have amplified ground motions, but not enough to compensate for the mitigation effect of reduced rupture speed [ABSTRACT FROM AUTHOR]