Important lessons from a deadly lahar in New Zealand 60 years ago

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During the course of an eruption, a volcano can produce a variety of hazards, such as lava flows, pyroclastic flows, and lahars (volcanic mudflows). But you might be surprised to learn that latent hazards may exist for years after an eruption ends, and can reappear, seemingly out of nowhere, with deadly consequences—which is what happened in New Zealand 60 years ago.

This week marks the anniversary of New Zealand's worst rail disaster, which occurred on the night of Christmas Eve in 1953 and stemmed from an eruption of Ruapehu Volcano eight years earlier.

The 1945 eruption of Mt. Ruapehu (elevation 2,797 m, or 9,177 ft), on the North Island of New Zealand, produced several lava domes and numerous explosive events at its ice-covered summit. The summit generally hosts a crater lake filled with water 80–300 meters (yards) deep, but the lake was emptied during the 1945 eruptive activity. Over the following years, water slowly refilled the crater. The water was impounded, in part, by tephra (volcanic rock fragments exploded into the air during an eruption including ash and larger particles) deposited around the crater rim in the 1945 eruption.

At 8:00 p.m. on December 24, 1953, this natural dam of tephra and ice abruptly gave way, draining the lake and sending a large flood of water down the Whangaehu River. As the water surged down the valley it picked up soil, rocks and vegetation, creating a dense lahar. Several hours later, the lahar impacted the Whangaehu River bridge near Tangiwai, destroying one of the bridge piers and collapsing a section of the bridge span.

At the foot of the mountain, the No. 626 express train from Wellington had passed through the Tangiwai Station at 10:20 p.m., on schedule to bring its passengers—no doubt many traveling for the holidays—to Auckland. As the train approached the Whangaehu River bridge, the engineer applied the emergency brakes, possibly in response to a motorist who saw that the bridge had just been destroyed and waved his flashlight at the oncoming train.

Unfortunately, the brakes were applied too late, and the engine and first five cars of the train plunged off the end of the bridge into the torrent of the river below. A sixth train car dangled over the end of the bridge, attached to the remaining cars on the track, but soon detached, and fell in. In total, 151 people were killed, with the devastating news reaching the country's population on Christmas morning.

In the following years, the crater lake refilled with water, but was emptied in the eruption of 1995–96. That eruption again deposited tephra on the crater rim, impounding water as the crater lake refilled, much like what happened during the 1945 eruption.

On March 18, 2007, the tephra dam collapsed, creating the largest lahar at Ruapehu in 100 years. This time, however, scientists were prepared.

A system called ERLAWS (New Zealand Department of Conservation's Eastern Ruapehu Lahar Warning System), installed in 2002, immediately detected warning signs of the lahar. Vibration sensors detected the initial failure of the tephra dam, and a webcam imaged the dam breach as lake level sensors tracked the lake drop. A sequence of sensors was triggered downstream, confirming that the lahar was on its way. This information was used to alert emergency managers, who closed nearby roads and bridges. In the end, the lahar caused no casualties and only minor damage.

We can take away two lessons from the 1953 and 2007 lahars on Ruapehu.

First, the lahars illustrate how indirect hazards can remain for years after a volcanic eruption ends. A similar scenario has played out in the Philippines. In the years following the 1991 eruption of Mt. Pinatubo, heavy rains have repeatedly remobilized tephra in river valleys and produced several deadly lahars.

Second, the contrasting outcomes of the 1953 and 2007 lahars demonstrate how innovative technology, such as the ERLAWS system, can help provide a constant, long-term watch on lurking volcanic hazards and directly minimize their risk. For communities near active volcanoes, monitoring systems like ERLAWS provide the gift of peace-of-mind.


Volcano Activity Update

A lava lake within Halema‘uma‘u produced nighttime glow that was visible via HVO's Webcam during the past week. The tilt level was relatively steady, and the lava lake level hovered around 50 m (164 ft) below the rim of the Overlook crater.

On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continued to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō. The active front of the flow was about 6.3 km (3.9 miles) northeast of Pu‘u ‘Ō‘ō on December 26.

There was one earthquake reported felt on the Island of Hawai‘i in the past week. On Saturday, December 21, 2013, at 9:27 p.m., a magnitude-3.2 earthquake occurred 24 km (15 mi) northwest of Kailua-Kona at a depth of 22 km (14 mi).