Just over a week ago, the Indonesian volcano Anak Krakatau blew its top, losing about two-thirds of its height.

Most of this 150m cubic metres of rock is thought to have slid into the sea in one go, generating a tsunami that killed more than 400 people. Tsunami warning systems are in place around Indonesia, but they are tailored to earthquake tsunamis, triggering only if an earthquake and large wave are detected.

“Recognising a landslide tsunami is much harder. The time between detection and the tsunami coming ashore is likely to be very short, so it is hard to make the system effective,” says Dave Petley, a landslide expert at the University of Sheffield.

Volcano flank collapses are common and can be gigantic: prehistoric landslides from the Canary Islands have been over 100bn cubic metres. They probably didn’t slide in one go, but there will still have been very large tsunami waves locally.

All active volcanoes near the sea – of which there are many – present a potential landslide tsunami risk. Sadly it is too late for Anak Krakatau, but installing movement sensors on other vulnerable volcanoes, and monitoring with satellite, could help give warning and save lives in future.