It was not like December 2004. Sirens wailed, warnings blared and police moved millions of people away from coastlines around the Indian Ocean as Wednesday's 8.6 magnitude earthquake off northern Indonesia sparked fears of another tsunami.
Damage was light - the quakes were horizontal rather than vertical - and the big waves never came, unlike eight years ago when walls of water roared across the same ocean and ploughed into seaside communities in 13 countries without warning.
"The reports were of people panicking but there was little damage," Eko Budiman, deputy head of the emergency mitigation agency, said.
The alerts and evacuations mean a regional system passed a major test since the tsunami of 2004 that killed 230,000 people, including 170,000 in northern Indonesia alone.
But luck helped avert disaster this time as much as the warning system, especially in Aceh province, where roads were jammed with residents trying to flee and damaged power lines silenced the sirens.
"The simple message is that in any critical condition like this it's impossible to get everyone out in time," said Keith Loveard, chief risk analyst at Jakarta-based security firm Concord Consulting. "The tsunami alert system worked to a degree ... While awareness has improved, it still needs to get better through public education."
Made up of seismographic stations and deep-ocean sensors, the Indian Ocean Tsunami Warning System was activated in June 2006 after an agreement at a UN conference in Japan.
When a quake hits, data is sent to the Pacific Tsunami Warning Centre in Hawaii and the Japan Meteorological Agency, which coordinate with national tsunami centres.
It can take 15-20 minutes for data to be analysed and a tsunami watch to be issued to governments. Using their own data, nations warn citizens in a variety of ways.
Wednesday's earthquake surprised scientists. The biggest earthquakes tend to occur in subduction zones where one plate of the Earth's crust dives under another. This grind produced the 2004 magnitude-9.1 Indian Ocean disaster and the magnitude-9 Japan quake last year.
Scientists say it's rare for strike-slip quakes - Wednesday's was one - in which blocks of rocks slide horizontally past each other, to be this large. Sumatra coast has seen three such quakes since 2004.