The earthquake that started either the pre-eruptive run up, or a new eruption.
On the seventeenth of June a new phase of increased earthquakes was observed by commentators on this blog. The level of seismicity was both larger and more numerous than during the previous 3 months. The location of these earthquakes corresponded with the place where the first earthquakes happened during the run up to the previous eruption.
As I have said before, when new magma comes up from the deep via the mantle plume we would see earthquakes at this spot. On the twenty-second of June low frequency harmonic tremor started at two distinct frequencies (0,29 and 0,59Hz) that previously has proved to be related with renewed activity. The frequencies are believed to be a sign of new magma arriving from the deep.
At 21.23 hours on the twenty-fourth of June (yesterday evening) an earthquake of magnitude 3.1 occurred, and within minutes the harmonic tremor had increased in sufficient numbers to herald movement of magma up the magmatic conduit system. After that there has been 49 earthquakes ranging from 2M to 3.8M, which is a rather good sized earthquake swarm for an already active volcanic system.
It has been noted that the earthquakes are not at the exact spot where they were during the previous eruptive phase. The answer is rather simple, the previous earthquake loci have by now gone above the solidus point and is not any longer brittle enough; therefore the renewed pressure breaks the adjacent rock. In other words, the magma chamber has matured and evolved.
As the pressure started to build up from the recently arrived new magmatic material from deep it started to move towards the old conduit. This conduit is by now fairly blocked by solidified magma at the opening.
A renewed eruption will start somewhere along the old conduit leading south from the Tanganasoga volcanic system towards the old eruptive vents that build up the edifices of Bob south of La Restinga. The question is if the new eruption will take place at the original pillow lava and pumice cone of Bob, or up at the new vent slightly closer to La Restinga. Another option is that the old vents by now are to plugged for the lava to be able to escape that way. Then the most likely place is somewhere along the route of the old conduit. And that means closer to the town of La Restinga, or even on land close to or inside the town.
Currently I would say that there is a high risk of an eruption starting, or that it already has started. Visible signs of the eruptions should be coming within the next 3 days judged on previous behavior. The eruption will most likely not be larger than the previous one due to the pressure being lower, but if it happens closer to shore, or even on land, the effects might be more dramatic for the residents.
My judgment would have been to immediately evacuate La Restinga until the eruptive locus has been confirmed.
IGN and Pevolca has declined to comment on this.
Update:
Regarding why I wrote that unusual and stark warning directed to the residents of La Restinga. I want to clarify why.
1. From the current signs and previous behaviour of the Tanganasoga volcanic system I judge that there is a 2/3 risk for a new eruption soon.
2. I am fairly certain that Bob and the newer vent on the ridge above Bob towards La Restinga is congested by solidified lava and not viable any longer as eruptive vent.
3. During the end of the previous eruptive phase there was anomalous gas ventings closer to La Restinga, and 2 earthquakes located close to the port of La Restinga (one was inside the actual port).
4. We know that the conduit leads from Bob towards a magma chamber under Tanganasoga. The usual behavious for volcanoes of this type is propagating vents erupting ever closer to the central part of the volcano.
5. From all of this I judge it to be about a 50 percent risk that the new eruption would be sufficiently much closer to La Restinga to cause risk for the inhabitants, or even on land close to or inside La Restinga. One should remember that there is a scoria cone 100 meters outside of La Restinga.
6. The current harmonic tremor episode is of the broadband type that is normally associated with sudden heavy degassing caused by the earthquakes. This can in a few eruptions increase the likelyhood of a non benign type of eruption. Ie, that the next eruptive phase may be having a more vigorous start than the previous due to a lot of gas pressure building up.
7. IGN and Pevolca is not the most warningsome agencies on the planet, to put it mildly.
All of this put together gave me a bit of a judgement call crisis. Based on my consciense, my experience, and so forth I decided that someone should issue the warning that IGN and Pevolca most likely will not give. So, I gave a warning that I myself would adhere to. I personally would not be inside of La Restinga right now. I would rather have someone say I am an idiot and should shut up, or even in the end try to sue me, than feeling that I through being quiet was part in people dying.
I would though like to say that those who so far have questioned me (privately) have been quite correct in doing so, specifically since I was in a hurry when writing and therefore a bit vague.
Pevolca has notified that there will be a statement after their meating this afternoon, and also commented that the afternoon today was the fastest possible oportunity for the meeting regarding the current new activity.
Update 2
People have noticed down in the comments that the harmonic tremor is higher in frequency this time and look different. That is because it does.
Click on the image for a bigger view. Do not be alarmed by the text, what is stated there is about volcanoes that have had a long repose period.
The earthquakes have knocked off a lot of gas from the lava, and this causes something called a Broadband signal. These are normaly associated with more vigorous eruptions. The text in the image states the limit here as being a VEI-3 eruption following a broadband signal. But that is for a volcano that has been dormant for a while. At Tanganasoga the plumbing system is still in large parts open, so the pressure can not build up sufficiently to produce a VEI-3 eruption, and that most likely gives the broadband signal for a smaller possible event.
CARL
