Once again Bob has been declared dead, and proving everyone wrong.
Earlier today I noticed a marked drop in the harmonic tremor plot. This was interpreted over the internet to be a sign of Bobs death (again), the feeder tube being blocked up through collapse, and quite a few other theories.
What is actually happening (most likely) is that the high harmonic tremoring is gas being released from the magma chamber(s) that reside under the Tanganasoga central volcano and the adjacent Tinganar fissure. As the pressure builds up, it reaches a critical level, and start to push fresh material and gas through the feeder tube to Bob south of La Restinga.
This has happened numerous times by now, actually so often that one can predict what will happen. What usually follows now is that Bob has an increase in activity starting about 5 to 15 hours after the decrease in harmonic tremoring.
The reason for so many making this error in judgment is that they miss the critical point. Bob is not a volcano. It is a volcanic vent far out on a feeder tube (probably and old magma tube left over from an old fissure eruption). The same types of old lava tubes can be found around the entire island, both above ground, and out under the water. So, judging Bob from what is recorded at CHIE station is not a good idea. Anything showing up there is related to the magma-chamber under Tanganasoga and Tinganar. So, there is first of all a time-lapse between the tremoring and Bob due to travel speed from the chamber to the vent of Bob. Second of all, Bob is a safety valve from the chamber per see. And as such has distinctly different traits than a normal volcanic eruption.
Being that as it may, Bob has now matured as a volcano, and found her rhythm. And the current behavior will probably continue for quite some time. If nothing drastic happens we can expect Bob to stay in action for at least another month.
I often get questions about the possibility of a new eruptive vent opening up at Tanganasoga. I see this as highly unlikely as long as Bob is active, this due to Bobs function as a safety pressure relief valve.
What would then be the likely signs of a Tanganasoga eruption on the way? Well first of all that Bob disappears. If Bob had been quiet for a few days, and we still see high tremoring, and a return of numerous earthquakes, then I would suspect that the feeder tube leading to Bob had been blocked, and that the risk is increasing of an eruption at Tanganasoga due to pressure build up. One should though remember that a lot of other possibilities could be at least as probable. New Bob even closer to land, an eruption close to La Restinga (not so likely due to the tube probably not going under the town, or another of the old lava tubes replacing Bob as relief valve.
I just want to say that you should not trust any rumors about Bobs death. And I would also like to say that we really should need new bathymetric imaging of Bobs volcanic cone. We need to know how close she is to the surface. There are too many rumors right now.
The tremoring has picked up again, to levels not seen for over a month. Earlier today we had signal clipping for period, so do not be surprised if the signal is rescaled again.
As I wrote yesterday that activity at Bob would pick, so has happened. It seems that Bob today is mainly producing a lot of tephra or grained pumice. So far I have not seen any pyroclasts bobbing about, and no steam. But the water is highly discolored (large stain), and there seems to be unusually large amounts of gas coming up to.
And this time the tremoring is increasing while Bob is having an episode of high activity. My guess is that this is due to new material having arrived, this is surmised on the activity of low frequency harmonics on several stations indicating a deep source. So, I would definitly say that Bob has time left before closing shop.