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The impact of earthquakes can be severely aggravated by cascading secondary hazards. The 2018 M-w 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 V-S and Eshelby's speed root 2V(S), 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.