On 28 September 2018, a shallow, large earthquake struck in the neck of the Minahasa Peninsula, Indonesia, with its epicentre located in the mountainous Donggala Regency, Central Sulawesi. This wasn't at all out of the ordinary: Indonesia is a nation of islands born from very complex and deadly plate tectonics.
Sulawesi is right in the path of a triple junction, where three major plates – the Australian, Philippine, and Sunda – converge. The island is stitched together from island arcs and continental fragments mashed together by colliding plates, plus volcanoes birthed by subduction. The area is a mess of thrust and strike-slip faults all trying to accommodate the motions of the major plates, plus microplates resulting from all the tectonic chaos.
The magnitude 7.5 quake was located 77 km (48 mi) away from the provincial capital Palu and was felt as far away as Samarinda on East Kalimantan and also in Tawau, Malaysia. This event was preceded by a sequence of foreshocks, the largest of which was a magnitude 6.1 tremor that occurred earlier that day.