Survivors and mourners gathered at mass graves in Indonesia, Thailand and other places along the Indian Ocean that were devastated by the 2004 tsunami — the deadliest in the world.

Christmas in 2004, a 9.1 magnitude earthquake triggered tsunami waves that killed some 230,000 people across a dozen countries.

Understanding of the Sumatran subduction zone and its hazards has increased since the 2004 Indian Ocean earthquake and tsunami.