Quake triggers alarm

A lahar warning system on Mt Ruapehu had its first test this week when an earthquake under the crater lake triggered an alert.

The system, installed four years ago, aims to prevent a repeat of the Tangiwai disaster of Christmas Eve 1953.

A lahar - the word is Indonesian - is a rapid flow of rocks, mud and water from a volcano.

They can occur when the mountain is erupting, or when the crater lake's wall collapses.

A wall collapse caused the lahar which destroyed the Tangiwai bridge near Waiouru in 1951, sweeping away the overnight express train and killing 151 passengers.

Eruptions have caused lahar 13 times since 1945. The most sustained eruption, in 1995, generated 30 lahar.

"Lahars are a significant natural hazard," scientist Dr Harry Keys says. "They can affect every valley on the mountain."

Scientists are predicting a wall-break lahar soon because the water level in the crater lake is rising.

"It could happen this summer, but it's more likely to be next summer," Dr Keys said.

On Wednesday, an earthquake under the crater triggered the Eastern Ruapehu Lahar Alarm and Warning System for the first time.

A water-level sensor at the crater lake was damaged, suggesting a small eruption and lahar may have occurred.

Scientist could not confirm this because bad weather prevented them flying over the mountain.

Signs of a lahar or eruption would include blackened or discoloured snow.

Dr Keys, advisory scientist to the Department of Conservation, said the alarm trigger had been a good test of the system and had prompted a meeting yesterday of agencies to refine their emergency response to a lahar.

GNS Science is monitoring the mountain closely for further signs of seismic activity, saying five small earthquakes since Wednesday night could be a sign of Ruapehu "waking up".