In April 2012, Indonesia's Banda Aceh, the city worst hit by the tsunami that killed nearly 230,000 people on Dec. 26, 2004, received a terrifying reminder of how unprepared it was for the next disaster.

As an magnitude-8.6 quake struck at sea, thousands of residents shunned purpose-built shelters and fled by car and motorcycle, clogging streets with traffic. A network of powerful warning sirens stayed silent.

No wave came. But if it had, the damage would have been "worse than 2004, if it was the same magnitude of tsunami," said Harkunti Rahayu from Indonesia's Bandung Institute of Technology.