Indonesia's tsunami early warning system, widely praised during this week's Indian Ocean-wide alert, has been compromised by thieves and vandals in the country's waters, officials said on Saturday.
The $US130 million ($A124.96 million) system of tidal gauges, buoys and seismic monitors sent warnings to Indonesian authorities on Wednesday after an 8.6-magnitude quake struck off Sumatra island.
Ten people died in the earthquake, mostly elderly people suffering heart attacks from shock. The quake caused little damage.
"We have had problems with theft and vandalism of our system for a while," Indonesian National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho told AFP.
"We got the tsunami warning from a seismograph but because so many of the buoys are destroyed we can't tell how big a potential tsunami would be."
Nugroho said just three of 25 buoys in Indonesian waters were in operation, mostly because of vandalism.
Indonesian authorities are now working with the United States to try to develop deeper-water equipment to prevent theft and vandalism, officials said.
Experts told AFP that the system had functioned well in Aceh province on the northern tip of Sumatra, where 170,000 people were killed and entire towns were flattened by the massive Indian Ocean tsunami in 2004.
They said government agencies sent SMS messages and sounded sirens to warn people to search for higher ground.
But an AFP correspondent in Aceh province said some sirens did not work and that the government did not sound the sirens until 45 minutes after the quake.
Most people began running uphill as soon as they could, remembering the 2004 tsunami that hit Aceh within 15 minutes of the earthquake.