Interview by Rio Helmi with PVMBG volcanologist Devy Kamil Syahbana stationed at the Mt Agung observation post in Rendang this morning, 24th October 2017 translated from Indonesian.
RH: According to you Pak Devy what is the situation now? It’s still Extreme Caution (level 4) right?
DKS: Yes indeed. Since the 20th, the seismic activity has come down. We can’t make a judgement on this just yet. Because the trend is still too short, and we can’t draw any conclusions yet. We will wait two more days and then enter all the data in a kind of simulation, seen not only from the seismic perspective but all factors, all aspects. But even if it decreases, the radius of danger zones will not be much different from the old one.
RH: Down to level 3?
DKS: Oh, if we go down to level 3 we have to refer also to 1963. In ’63, on day one and two an 8 km radius was hit. But we don’t know if this is down to 3 or not yet. We must wait for the data of today and tomorrow, and see the results after all the data goes into the so-called ‘event tree’. Only after the ‘event tree’ has assessed by an expert can it be known whether the danger is reduced or not.
RH: The event tree is a computer simulation?
DKS: Well, the event tree is a method that is used in (whole)world. There is also ‘paper’ involved, not just computers. We use this to make informed decisions. One of them is to determine what VEI (Volcanic Explosivity Index) is in the current condition. It must be an expert who analyzes it. For example from the seismology it can be seen as relatively stable, but other factors must be also taken into account. Well from there we can extract a percentage of possibilities for eruption for example. It can also be expected in such a condition that the ‘ramp-up’ follows a certain pattern. Well for that we also refer to the previous one. The time from level 2 to 3 was approximately four to eight days. If we see from all aspects of its activities that is going down, well if we have lowered the level we still have a look at the radius. If you look at the area we also have to see the character of the mountain. For Agung we already have a probable timeline. Even if we lowered it to level 3, that does not mean that it will not erupt, it’s just a question of the percentage or likelihood if it’ll erupt or not. If it’s at level 3 it only means that the likelihood is lower but not improbable at all. Even if the potential danger changes, but the potential danger must be judged in accordance with the character of the mountain itself. In 1963, in one day a complete set of deadly volcanic phenomena occurred.
RH: In one day?
DKS: So on the 19th of February it erupted, and on the same day it issued pyroclastic flows. There was no phreatic onset actually. There was an opening eruption, and it was called a ‘small eruption’. But it’s ‘small’ eruption- but small compared to what? Well it is, compared to Agung’s own potential. Yet when compared with other volcanos this was still larger, it was VEI 3. VEI 3 is larger than the largest eruption of Sinabung. So we must measure it from the potential of Mt Agung itself. Well if all things like that are taken into consideration (in the event tree) along with the evidence, then we can start calculating. Suppose the radius (of danger zone) is down to 8 km, people (layman) see it says: “You know this is the same number with level 2?” But they don’t realize what the condition was when it was raised to 2 (or 3) – the volume of magma then was negligible. When it went up to Awas (4) it was because of seismological considerations, but now there is an estimated volume of 18.5 million cubic meters of magma in the belly of Mount Agung. It’s ‘fat’. This means even though the number of earthquakes is down, don’t forget that the mountain is ‘fat’. The vertical upliftnya of the mountain is up by 6cm! So even though seismic activity is down, we should not equate it to when it was raised to level 2. Before there was no influx of magma, it was still far below, now it’s inside.
RH: And it’s 4 km below the crater, right?
DKS: Yes. Yes so we can create some likely scenarios from that (taking into account all factors).
RH: There seems to be a lot of pressure for you to bring down the danger status? Though I’m sure there must be important considerations why PVMBG hasn’t brought down the level yet.
DKS: Yes, but we have no other motivation (except to act according to our observations).
RH: Thank you for your time.
text and photo ©Rio Helmi