The following interview took place at the Mt Agung volcanology observation post in Rendang, Karangasem, Bali on Saturday the 14th of October.
Rio Helmi: word has it that yesterday there was the first non-harmonic tremors since the initial level 4 “AWAS” status was set. Initially non-harmonic tremors led to the status to be raised, is that correct?
Devy Kamil Syahabana: Yes, these tremors did happen once then. So, yesterday included, in all, tremors have occurred a total of nine times. There were eight two days ago. (ed: Since this interview took place, there have been another three).
RH: Would you say this a sign of escalation?
DKS: Non-harmonic tremors are a succession of volcanic earthquakes. So one earthquake takes place, and before it is finished another occurs. Physically it is a process of streams of fluid pounding new cracks open, making a staccato “dek-dek-dek-dek” sound. A single earthquake only makes only one “dek”, like that. Before this happened the other day, we recorded 900 volcanic quakes which meant that there was a significant rise in pressure. Excessive pressure needs to be channeled somehow, which causes tremors. However, this does not necessarily manifest on the surface as an eruption. Although there are examples around the world of these “spasmodic bursts” as they are called, preceding an eruption, spasmodic bursts can also occur without manifesting an eruption. There can be thick smoke, for example. Yesterdays spasmodic burst only displayed this kind of thick smoke on the surface.
RH: Some people say that large numbers of closely reoccurring spasmodic bursts are a sign of an imminent eruption. Is this correct?
DKS: No. There are three types of volcanic tremors. The first is are deep tremors of high frequency (non-harmonic). This type of tremor takes place at the stage where cracks are being pounded open, which opens access to the surface. So after this, what are called harmonic tremors will normally occur. When this harmonic tremor occurs, it is a sign that the magma has risen hundreds of meters and is close to the surface – but things don’t always happen in this way, it depends on the type of mountain. It is only after the occurrence of these harmonic tremors that the continuous forms of tremors occur in a process of breaking up the lava plug. After the 1963 eruptions, the remaining magma formed a lava plug due to being crystallized over time, and it thus acted as a stopper. It’s similar to a gassed up bottle of coca-cola after being shaken. In this case the gas is hot, so it is able to rise in the crater. Under normal conditions, there are no emissions of smoke. Last July and August, Gunung Agung did not emit any smoke; this is because it does not have an ‘open system’ where smoke can rise easily. Gunung Agung has a ‘closed system’; it began to smoke due to the increase of inside pressure.
RH: Oh, so this steam was not just from water hitting the hot crater?
DKS: If it was only steam from water there would not be any smoke. If there is smoke, it means there is steam from magma, real ‘magma heating’ like what happened last 7th October, reaching 1500 meters. This high pressure is called ‘volcanic puff’. These ‘puffs’ occur when some of the pressure has led to the manifestation of organic gas, but not yet of solid material.
RH: So continuous tremors can be taken as a sign of an imminent eruption; are there any other symptoms that the public should know?
DKS: Instrumentally, the tremors that are continuously monitored by seismographs etc gather strength to the point of going off the scale, that is to say when they go beyond the capacity of the recording device then it’s imminent.
RH: what about external manifestations?
DKS: Eruptions can happen without apparent prior signs. If we feel it coming, it is because the energy has become extremely powerful. But in cases where the energy is not very strong, we can’t tell that it is about to manifest. There are many volcanoes that appear to be quiet and then erupt all of a sudden. They do not necessarily have to emit smoke before erupting.
RH: So in the case of ordinary people, inside the danger zones (KRB) for example, what are the signs that need to be looked out for?
DKS: Normally those in KRB will first know when they hear the sounds. At this point, the eruption is already beginning to take place. So if possible, one needs to get away as fast and as far as possible. Also, there is often a preliminary eruption that is not so big, a sort of opening eruption, which is then followed by a bigger, higher eruption. Gunung Agung has behaved in this way in the past, but we cannot predict things for sure. Remember in 1963 there were no instruments installed here, we only have anecdotal history. We cannot make very precise predictions (in volcanology). This was shown by the case of St Helens in America where the predictions turned out to be wrong. So it is better to play safe and not to be too confident when making exact predictions as to which way etc it will go. We can be convinced we are on safe territory only to end up being taken by surprise and getting hit by disaster.
As to external signs, I had the experience of being around the Krakatau when it erupted. There was a sound of stones being ground against each other, a kind ‘crack-crack-crunch’sound, like that, and if you happened to be on the mountain you’ll feel it rocking. Well, if this happens and you still have the time to run, you can try, but what with rocks being thrown up and falling in all directions at a very high speed, hundreds of meters per second… Well you would have to be very lucky to survive.
RH: Pak Devy, thank you for your time.