Indonesia is no stranger to destructive earthquakes and tsunamis, but what was surprising about the devastating, magnitude 7.5 quake that shook the island of Sulawesi last Friday was the size of the tsunami that followed. About 30 minutes after the quake, six metre-high (nearly 20ft) waves surged through the coastal resort of Palu, destroying buildings and killing hundreds of people.

Most tsunami-generating earthquakes in this region emerge from movement on the Sunda “megathrust” fault where the Indo-Australian plate dives down underneath the Eurasian plate. Violent vertical motion caused by thrust quakes can displace a huge volume of seawater, setting off a high-speed tsunami wave.

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However, the recent Sulawesi quake occurred on a “strike-slip” fault, meaning the plates lurched horizontally past each other and shouldn’t have displaced much water. So what caused the tsunami? One possibility is that the quake triggered an underwater landslide, which would have displaced water and created a tsunami wave. Alternatively, if the rupture occurred on a steeply sloping region of the seafloor, the horizontal motion could have pushed seawater in front of the slope. And almost certainly the tsunami was magnified by the narrow shape of the bay, with the wave energy focused by the coastline as it rolled towards Palu.