9月27日に、長野県と岐阜県の県境にある御嶽山（3067メートル）が噴火しました。この件につき、海外SMC発の専門家コメントをお送りします。

翻訳は迅速さを優先しております。ご利用の際には必ず原文をご確認ください。

NZ SMC発　Prof Shane Cronin

Earth Sciences, Institute of Agriculture and Environment, Massey University

日本には優秀な火山監視システムがありますが、今回は予知できませんでした。普通は噴火前2週間に地殻が不安定になることが多いですが、御嶽山は非常に活発な火山であるため、地下で熱せられた水による地震が日常的におきています。だからといって、何もできなかったともいえません。

一般的には地震の増加が噴火の前兆となりますが、地下水温の上昇、ひび割れ、温泉温度の上昇、ガス放出といった変化がみられることもあります。2012年にニュージーランドのトンガリロ山が噴火した際には、3週間前から地震がみられました。ただし、噴火に結びつかない地震もあります。

人体にとって、噴火による最大の危険は、灰やガスによる窒息でしょう。高温の水蒸気や大きな岩石が降ってくる衝撃も脅威となります。噴火エリアでは、風向きに注意して避難してください。より下流の地域でも風向きに注意する必要があります。

【コメント原文】

"The monitoring of Japanese volcanoes is excellent. There was some two-weeks of slight unrest before the event, but at a frequently active volcano such as Ontake, there is commonly seismicity associated with hydrothermal systems. The eruption was a true 'blue-sky' event in terms of not being preceded by major unrest.

  "We are familiar with similar types of eruption at NZ volcanoes, such as 2007 Ruapehu and 2012 Te Maari/Tongariro. The suddenness of these eruptions is a real challenge for any monitoring technology. We have been very lucky in past situations in NZ to avoid such calamitous situations, although such blue sky eruptions remain a real possibility at both Ruapehu and Tongariro."

 

What warning signs would scientists have expected to see before an eruption of this kind? 

 "Typically, an increase in seismicity – mainly small earthquakes associated with heating up of hydrothermal systems. There may also be changes in hot spring temperatures, gas release, fumaroles and possibly ground surface changes in the vent area (bulging, cracking and similar). In the case of Tongariro 2012, there were around 3 weeks of preceding earthquakes, although their number had decreased by the time of the August 6 sudden eruption. The November 21 eruption was preceded by no seismicity. The challenge, even once earthquakes or other changes are detected, to pinpoint the exact time of an eruption. Such volcanoes present often with signs of unrest, many of these periods of unrest, however don't necessarily lead to an eruption."

 

What were the main hazards faced by people on the mountain at the time? 

"Hazards in this case were fast-moving currents of ash, gas and steam… these would kill by asphyxiation, physical impact of large rocks, and possibly burning. These are similar to the energetic ash currents generated near to Te Maari crater in 2012 at Tongariro. Other dangers are ballistic blocks – which are large rocks (up to metres in diameter) being thrown out like mortar shells from the vent site.

 

 Are there any parallels that can be drawn with New Zealand volcanoes and recreational use of active volcanoes more generally?
 

 "There are direct parallels with Tongariro, as mentioned above, and with Ruapehu. There are many volcanoes around the world with high-level heat flows in the upper summit areas – many in temperate or tropical climates have hydrothermal systems that store heat and super-heated water. Often in such cases, subtle changes in the heat or gas flow from below can lead to the hydrothermal system becoming destabilised. If a cap or sealing plug develops in such systems, water can be super-heated, like within a pressure cooker, once the pressure is too great for the rock cap to hold in – sudden release generates eruptions with very little if any warning. At Tongariro, a small landslide triggered the breaking of such a seal on the hydrothermal system and triggered the August 6 eruption."

 

What advice can be offered to people who find themselves up close to an erupting volcano that might increase their likelihood of survival?

 "Be aware of all hazards when going into an area. If there is an eruption – make your way out of the area as quickly as possible obviously, but take stock of which particular areas are mostly being affected. Many ground-hugging currents will follow a valley pathway – hence avoid the valleys, even those well downstream of an affected area. Note the wind direction and try to evacuate up-wind."