During the last 48 hours Hekla has suffered an increase in rapid strain movements. On the images below you see two types of motion. The first type that is creating a sinusoidal movement (wave-like) is a phenomenon called earth-tides. They are normal and do not influence even the most agitated volcano. The second type of motion is rapid motion of strain changes as the mountain makes small trembles. Those small trembles are actually rapid mountain strain changes.
Rapid mountain strain changes are normally seen as a warning sign of unrest in an active volcanic system. However, I have not found any data stating that Hekla exhibits this pattern before an eruption. The only reference is Páll Einarsson stating the previous instance as the reason behind the July 2011 Hekla alert from IMO (Icelandic Met Office). Hekla then suffered an episode like this that ended with a 2.2 earthquake in Hekla proper.
Also, before the 1991 there was a set of shallow earthquakes at Haukadalur the week before onset of eruption. And during the last couple of weeks there has been another pattern of similar earthquakes, but at a slightly greater depth.
This increase in rapid strain movements might be interpreted as increased unrest in the volcanic plumbing of Hekla. But we still lack the tell-tale earthquakes at Hekla proper that normally start 30 to 60 minutes before onset of eruption. The earthquake will be in Hekla proper, or within a couple of kilometers.
Please, keep the question coming for our GPS Q&A with Professor Sigrún Hreinsdottir!