For those who need a brusher up on Hekla I recommend reading the two posts that is linked below if you need a refresher. The first one is a rather traditional write up about Hekla; the second is a revision of how I interpret Hekla based on Papers by Sturkell and Carmichael respectively.
In short one can say that even if Hekla good at being hard to interpret she is rather consistent in her uplift between the eruptions. So far it has been smooth going from the 2000 eruption up until 2012-01-30.
Back then we had a period of activity in and around the Saurbaer area with small scale tremor, a few minor earthquakes ranging from 0.2M to 0.7M and some microseismic activity. That was then interpreted as hydrothermal activity in the area.
As that calmed down we then got an earthquake of 1.2M at shallow depth in the topmost part of the fissure at 0.1km depth. The focal point of the earthquake was exactly at the 1947 eruption center.
For being Hekla that is a lot of activity in a short time.
A few days ago I looked through the GPS-plots of the network that the Institute of Earth Sciences runs under the supervision of Sigrún Hreinsdóttir. Without the gracious release of that data publicly it would not have been possible to do the plot below. All copyright to the data used is held by the Institute of Earth Sciences and Sigrún Hreinsdóttir respectively.
On the various GPS-plots I noticed that around the first of February a marked deflation started at several of the stations around Hekla. Due to the complexity and sheer number of stations I could not get a clear picture, so I enlisted the help of the master of plotting, GeoLurking. Any mistakes in the interpretation of the plot are entirely mine. GeoLurkings plot is a daily average trend line for the period 02-01 to 02-18.
“This is the daily linear trend based on the interpolated available data from 1 February to 18 February. It was then re-grided with a quadratic poly sheet at 255 x 255 in order to obtain the fill between stations points. Purple is -1.2 mm/day, red is 0.4 mm/day. Green is -0.26 mm/day.” (GeoLurking, 2012)
There is due west of Hekla a clear center of hard deflation (purple), there is also a marked deflation NNW that is deflating rapidly (dark blue). From the last spot there is a pale blue band running over the northern part of Hekla proper.
On the southern part of Hekla central volcano (green) there is a small deflation going on.
Regarding the parts with inflation (red 0,4mm daily) there is no surprise that the area up towards Búrfell is inflating, it has been doing that for a long time. What is though surprising is that this trend continuous even though all of northern Hekla is deflating. This inflation has been interpreted as being a dyke emplacement running from Hekla. That is starting to look a bit less likely with this data.
What is brand new is the well defined inflation hypocenter west of Hekla. This is an unusual spot for activity at Hekla since it would be outside of the regular fissure of Hekla.
The red part seen towards Vatnafjöll is equally surprising since Hekla and Vatnafjöll has never during the last 10 000 years erupted at the same time. When Vatnsfell is active, Hekla goes dormant. And do not make the mistake of believing them being the same volcano, or even volcanic field. Vatnafjöll is erupting magma normal for Iceland, and Hekla is erupting lava associated with subduction volcanoes. So the link is mechanical only, probably some kind of pressure release function. The inflation at Vatnafjöll might therefore be a sign that Hekla is nearing the end of its 1 000 year long cycle of high eruptive pace, but that is highly speculative at this stage.
Hekla is known to have more than one chamber, as many as nine major chambers has been suggested in various papers. Be that as it may, Hekla has complicated innards, and it seems like magma can run freely between the chambers. In any other volcano on Iceland a displacement of this large amount of magma would be associated by continuous large scale harmonic tremor and numerous seismic events and here almost nothing of that has been evident.
From the looks of it a possibility is that Hekla would be more likely to have a flank eruption this time, or even a new fissure opening up in parallel to Hekla. If so, it would be the first time that has been observed by humans. One should though note that the Litla Hekla Fissure is in that direction, and that the inflation center is right on top of yet another fissure. So I guess it is not totally out of the question.
All of this should be read with a large caveat in the back of the head, any interpretation of a volcano is most likely wrong in parts. This is the least reliable volcano on earth, so all of it might end up being wrong.