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Another day, another eldfjallavakt, part of our ongoing series keeping up with the serious situation inside Iceland's largest active volcano, Bárðarbunga.

The big news for the day is quite simple: the pattern has changed, and it's sending the system ringing. More below the fold.

Volcanic earthquakes, as a general rule, don't get as large as tectonic quakes. The largest volcanic quake ever recorded in the cascades, for example, was a 5.5. Normally the maximum ranges from 4.5 to 5.5.

So the fact that Bárðarbunga just experienced a 4.8 right on its caldera rim is no small matter. Needless to say, it's the strongest quake of this whole episode.

Had this been even a lone quake it'd still be significant. But it's not. In the past 16 hours, a series of strong quakes has kept occurring:

So what's happening? What's changed?

Note where all the strong quakes have been: in the exact same spot, right on the rim of the caldera itself.

The long conduit that's been breaking its way northeast each day with a magma flow rate equivalent to the flow of a good-sized river has also changed. It's entirely stopped it's northeast motion, but the quakes have not stopped. Instead they seem to be enlarging the channel at its endpoint.

What does all this mean?

Tremendous amounts of magma have flowed into the side channel; in five days, the rate was greater than the Heimaey eruption released in five months, even though it has no outlet. Yet the pressure in the system stubbornly refused to drop. This constant pressure was interpreted to be, in a way, a good thing in terms of meaning that the eruption wouldn't take place imminently. But it was a very concerning sign that the system has such tremendous magma reserves that it could sustain such a flow for so long with no signs of a pressure drop.

Now, at long last, we're seeing that.  GPS meters show that  subsidence is now starting to take hold over the caldera. As the pressure drops, it no longer can support the weight of rock and ice over it, and this begins to collapse downward. However, there is not at this point evidence of a sigkatli forming over it.

What does this all mean?

Earlier today, seismologists stated that they did not feel that the smaller (but still large) quakes on the caldera rim were a sign of an imminent eruption. Due to the late hour, nobody has yet commented on the new powerful quake, but I imagine that despite its strength, it will be interpreted likewise, as it doesn't imply a channel reaching the surface. Rather, it seems that the key takeways are that:

1) at least there are some limits to how fast magma can flow into this disturbingly abundant system;

2) the conduit is continuing to fill up with magma at an alarming rate, but it is doing so by enlarging at its end, not by tunnelling further to the northeast; and

3) we're entering a new phase, and we're yet to see where it will lead.

There's relatively little more news on the preparation front. The transportation office has announced that they've secured equipment ready to rip the road beds off the northern bridges in short notice to reduce the stresses on their columns, as even the low end of flood predictions could easily take out all three bridges. They will only do this on two of the bridges; one of them is considered too close to the glacier (and not important enough), and they will just let it wash away entirely should the volcano erupt.

As for the 10% of Iceland that has become a closed zone, the state police have announced an exemption for ranchers to go out and retrieve their sheep (sheep here graze in the highlands and the mountains until the fall roundup).

As a final note, in the last part of the series, there was a bit of a discussion on people's attitudes here toward an eruption, whether people were looking forward to it like some travel website had suggested. My response follows below:

A coworker at lunch today said (twice) he was "skíthræddur" (scared shitless) about it when I asked. Nobody disagreed.

Don't get me wrong, I'm sure some people are looking forward to it, just like some people look forward to hurricanes and the like. And as we all know, it's in every young Icelander's job description to take a picture of themself standing in front of an erupting volcano looking all töff to set as their Facebook profile picture.  ;) Plus, getting a laugh out of foreign reporters struggling to pronounce Icelandic place names is always fun.  But a general statement like "Icelanders are looking forward to it" seems so far off base, I don't know where to start.

Even if you ignore the danger aspect, it's already costing money and it hasn't even erupted yet. I feel bad for tour operators who live in the region, their businesses are all about giving tours to places like Askja, Kverkfjall, Lofthellir, and all the other cool geological features in the closed region... I mean, their business just basically got closed. Ranchers are already having to take risks going out there to bring their sheep in early, and could lose some of them. Even a jökulhlaup on the lower end of the predictions would wipe out tens of millions of dollars worth of bridges in the north and disconnect the north from the northeast, and even a small eruption would keep a large chunk of tourists away from the country.

Here we're just talking about the most minimal of possible events - why would anyone look forward to it? On the far other end of the scale, you're talking about another Móðaharðindin, biblical scale floods, poisoned water supplies, the loss of half our power supply and aluminum industry, the complete disappearance of our tourism industry, and on and on. How fun.  :Þ

Like an approaching hurricane, it can be exciting. But like a hurricane, the reality is not so fun.

Who knows whether it's going to stop or where it's all going to lead. But for now, at least, the rumbling continues.

Update, 9:00 august 22: Páll Einarsson, a geophysicist, talks more about the situation. He states, as I assumed, that the quake in the caldera does not mean anything about magma on the way to the surface, despite its shallow depth. He feel that the cessation of lengthening of the intrusion that's been boring its way to the northeast is a good thing. Now that the flow rate has been reduced, even though the rate of quakes hasn't, he says  the conduit will begin to cool and harden, and while it contains a massive flow, it's only about a meter wide, which will help it cool faster. If the flow would stop entirely, it would only take a few days to harden in its current state. But if the tunnel widens, the time for it to harden increases exponentially.

He says that the movement of magma in this event is reminding him a lot of the leadup to the Krafla Fires, which were a long but relatively slow eruptive event from 1975 to 1984 that reshaped a good chunk of the northern part of the country, including creating brand new rivers and lakes. Krafla created over 20 such intrusions, allowing them to learn a great deal about how they behave.

Honestly, another Krafla would be fine by me. Bárðarbunga is certainly capable of a lot worse  ;) That said, I don't know if his comparison to Krafla was talking about strength or just to magma movement behavior - my read was that he was mainly referring just to latter. I'd also welcome the possibility of this intrusion hardening without any new ones being created  ;)  I guess we'll see!

Update 10:30, 23 august: As I've already pretty well covered the preparations, and the pattern hasn't changed significantly from last night, I'm not going to write a new diary today. As noted previously, the pattern is now different: the steady earthquake activity has declined in the magma intrustion, while rarer but more powerful equations keep happening over the primary magma chamber due to subsidence. This pattern is continuing, with even further declines in the intrusion and quite regular larger quakes over the caldera. I interpret this new pattern as a good thing, but we'll see!

Update 11:30, 23 august: Or, maybe not...

Too soon to say.

Update 11:40, 23 august: Damn.

Back on the move to the north... Gotta check the depth...

I hope we get an update from the Met Office about this before I head out to the Menningarnótt celebrations  :(

Update 12:30, 23 august: Damn.

I need to head out. No update from the met office, but it keeps expanding  :(

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