On Sept. 28, a powerful earthquake struck the Indonesian island of Sulawesi, triggering a tsunami that devastated the provincial capital, Palu. The two events together killed more than 2,200 people in the region.

Although Indonesia is one of the most seismically active countries in the world, the Palu tsunami came as a surprise to geophysicists. A tsunami occurs when a quake on the seabed abruptly pushes water upward, producing a dangerously tall wave. Usually the culprit is a megathrust earthquake, as one tectonic plate slides, or subducts, beneath another. The tsunami that hit Sumatra in 2004, causing 230,000 deaths, was generated by a megathrust quake.

In contrast, the tsunami in September was caused by what is known as a strike-slip quake. These occur at seismic faults where two tectonic plates are sliding past each other. The ground motion in such quakes is mostly horizontal — in Sulawesi, the rocks on either side of the fault lurched past each other by more than 10 feet — and rarely produce tsunamis. But the Palu earthquake caused an underwater landslide, which produced a small tsunami that grew as it swept up a narrowing bay.

And, it turns out, something even more unusual was at work, according to two papers published in the journal Nature Geoscience on Monday. As the fault ruptured, the leading edge of the rupture tore through the crust much faster than usual, perhaps magnifying the shaking that led to the underwater landslide. Such behavior has been predicted in theory, but had not been conclusively documented in nature.