A series of early-warning buoys deployed after a 2004 tsunami devastated parts of Indonesia's western Sumatra island were not working when a huge quake rocked the same area on Wednesday, an official said.
But a tsunami was not generated and there have been no reports of casualties or major damage, with life largely returning to normal in affected areas on Thursday.
However, the national disaster agency said the process of confirming that a tsunami had not occurred was hindered because none of the country's 22 early-warning buoys were working.
The early part of the warning process ran smoothly, with a tsunami alert quickly sent out to communities across Sumatra, which led to sirens sounding and people heading to higher ground.
But without the working buoys, which can detect changes in water movement and relay the data back to officials, it took authorities around three hours to confirm that destructive waves had not been generated and to call off the alert.
"The tsunami buoys have been damaged by vandalism, and a there is a lack of funds for maintenance," disaster agency spokesman Sutopo Purwo Nugroho told reporters.
"This situation makes it difficult to confirm whether a tsunami has occurred or not," he added.
The buoys were part of a sophisticated, multi-million-dollar warning system, constructed with help of foreign donors following the 2004 tsunami that occurred when a magnitude 9.1 earthquake struck off Sumatra.
It sent a huge tsunami barrelling into Aceh province, on Sumatra's northern tip, and to countries around the Indian Ocean, leaving around 220,000 people dead.
A series of strong aftershocks continued to rock Sumatra Thursday but the Meteorology, Climate and Geophysics Agency, which monitors earthquakes, urged people to remain calm.
"Based on data from these aftershocks we do not believe there will be an earthquake of greater strength," the agency said in a statement.
Calm had returned to the city of Padang in western Sumatra, where the quake had been felt strongly.
A team of military personnel and search and rescue officials were also dispatched Thursday to the remote Mentawai Islands, the closest land to the epicentre, to check on communities that had not been contacted since the quake.