The initial temblor caused strong shaking lasting more than a minute around Palu, a city of about 400,000 people and the capital of Central Sulawesi province.
Scattered damage was reported, and several hospitals evacuated patients, some with IV drips, outside as a safety measure. No information on casualties was immediately available.
The initial quake was centred 43 kilometres (27 miles) east-southeast of Palu, and the US Geological Survey said it was about 10 kilometres (6 miles) deep. The strongest subsequent quake measured 5.2 magnitude. There was no tsunami risk.
Indonesia is crossed by several seismic faults, and earthquakes and volcanic activity are common.
Many Sulawesi residents are haunted by a magnitude 7.5 earthquake that devastated Palu city in 2018, setting off a 3-meter (10-foot) high tsunami and a phenomenon called liquefaction in which soil collapses into itself. More than 4,000 people were killed, including many who were buried when whole neighbourhoods were swallowed by the falling ground.
In January 2021, a magnitude 6.2 earthquake near the city of Mamuju on Sulawesi island left at least 100 people dead, with thousands sleeping outdoors for days out of fear of aftershocks.
