The magnitude 9.1 quake ruptured a 900-mile stretch of fault line where the Indian and Australian tectonic plates meet. It was a powerful megathrust quake, occurring where a heavy ocean plate slips under a lighter continental plate.

The quake caused the ocean floor to suddenly rise by as much as 40 meters, triggering a massive tsunami. Within 20 minutes of the earthquake, the first of several 100-foot waves hit the shoreline of Banda Aceh, killing more than 100,000 people and pounding the city into rubble. Then, in succession, tsunami waves rolled over coastlines in Thailand, India, and Sri Lanka, killing tens of thousands more. Eight hours later and 5,000 miles from its Asian epicenter, the tsunami claimed its final casualties on the coast of South Africa. In all, nearly 230,000 people were killed, making it one of the deadliest disasters in modern history.

Since the 2004 tsunami, governments and aid groups have prioritized disaster risk reduction and preparedness. Only three weeks after the tsunami, representatives of 168 nations agreed to the Hyogo Framework for Action, which paved the way for global cooperation for disaster risk reduction. Since then, ocean floor earthquake sensors have been installed to trigger early warnings, and many local communities have been trained in evacuation and disaster response.

Major earthquakes and tsunamis in August and September 2018 have tested Indonesia’s ability to respond and recover. Then, in December 2018, Anak Krakatau volcano’s ongoing eruptions in the Sunda Strait caused undersea landslides that triggered a tsunami that struck beaches in both Sumatra and Java. With no warning triggered by the volcanic activity, more than 400 people died. Now, the Indonesian government is working to add volcano sensors to its warning systems.