KEY POINTS:
Eruption warning measures are being reviewed after criticism that no alarms were activated during a small earthquake associated with this week's eruption.
But already concerns are being raised that adjusting the alarm threshold downward will result in more false alarms - eventually leading to a blasé attitude among mountain users.
The 8.23pm blast on Tuesday triggered two lahars that flowed down the northwest and eastern side of the mountain to the Whangaehu River. Rocks, volcanic debris, ash and water were blown from the crater and landed in a 2km radius - seriously injuring Auckland climber William Pike.
Visitors to the mountain, staying in lodges in the area, have criticised authorities and said they were not alerted to the eruption. A cellphone text message alert to subscribers failed to activate because the earthquake associated with the eruption measured only 2.8 on the Richter scale. The threshold for alerts is set at 3.4.
The earthquake was also too small to activate barrier gates on State Highway 1, at the Desert Rd, or State Highway 49 at Tangiwai.
Authorities have said they will look at making the warning triggers more sensitive.
Skiers at many lodges, including Ngauruhoe and Snowline Ski clubs, have complained about the alarm system's failing to activate and the lack of communication from authorities. Evacuation orders came only when internal managers ordered people from their beds and down the mountain, to rendezvous at the Chateau.
On the other hand, some lodges did not evacuate.
Department of Conservation volcanologist Harry Keys told Radio New Zealand yesterday that there was no way scientists would have reasonably thought an eruption as small as Tuesday night's would cause a lahar.
"Essentially if we can lower the threshold whereby the thing goes to full alarm, we'll be able to cover these small eruptions that produce those hazardous lahars."
But he said tweaking the sensitivities was a fine balance.
"You can go too much in the system. It's like the smoke detector in your house. If you have it set wrong it'll go off every time someone breathes."
Mr Keys said he was surprised at the reaction from members of the ski lodges about not being warned.
"This always mystifies me ... club people in the club lodges are normally the most experienced. All the clubs are in a safe location ... they are five kilometres, four and a half kilometres from the crater. So there needs to be an absolutely huge eruption to threaten them and there's been nothing like that for 10,000 years."
The only potential threat would be ash fall, but that was more of a nuisance than life threatening, he said.
If there was going to be a big eruption, warning signs would be seen for up to 20 years or more.
"You get lots of warning because there's lots of magma coming up through the volcano. It's very difficult anywhere in the world to predict very small eruptions."
Manager of Edge to Edge ski shop Sam Clarkson, who has lived at Whakapapa for 25 years, said he was baffled by criticism of the warning system.
Ski club members should "calm down a bit", he said. "They are completely out of the lahar path. If they adjust the alarm system down too far there will be many false alarms and people will become blasé at that point.
"It's [the warning system] a bloody good system. Yes, it needs fine-tuning and they're improving it all the time. But this was a very small lahar."
He said the event looked worse than it actually was, and the mountain had merely "burped".
The lahar flows ran longer distances than expected because the event occurred at night and therefore debris had skated across the hard ice.
The snow groomer and two climbers on the mountain at the time were merely "unbelievably unlucky", he said.
"Those guys up at the hut were not supposed to be there at all. That hut is not there to be stayed in, it is for data collection and emergencies. DoC highly discourages people from staying there for the very reason of what just happened."
-additional reporting NZPA
