Well there were c.1300 earthquakes in Iceland during April 2012. If you want to know more, read on dear reader….
Every month the Iceland Meteorological Office (IMO) publishes a brief report (see http://www.vedur.is/um-vi/frettir/nr/2484), so what’s below is extracted from this and supplemented with my own comments and views. The image below is taken from the report and copyright resides with Veðurstofa Íslands (IMO).
Around 500 of the earthquakes occurred in and around the newish geothermal power station (Hellisheiði) that lies to the East of the capital Reykjavík, and which supplies the capital with hot water etc. These earthquakes are caused when cold water is pumped back deep into the geothermal field, which is necessary to replenish the hot water and steam that is extracted. It’s actually a rather neat way of accelerating the natural cycling that takes place anyway in such geothermal systems. And sustainable, until the heat source diminishes. The earthquakes are triggered by ‘thermal cracking’ and the movement of small faults and fractures which are lubricated by the incoming fluids. Some of the earthquakes have been a tad large (M3.5) which has shaken the residents in the nearby town of Hveragerði who are naturally anxious because they were deeply affected by the May 2008 M6.3 earthquake. But most are fortunately small and cause just a modest tremble.
Katla (also known as Mýrdalsjökull, as ‘jökull’ means glacier, and a glacier covers Katla).
Well it would take me a few pages to tell what I think of Katla, and what I know about her. But the period of seismic unrest that kicked off in July 2011 with the modest but significant bridge-busting glacial outburst flood, tailed off towards the end of 2011 but continued through to March 2012. Arguably it is still continuing, albeit in a diminshed and more fluctuating form. See http://www.geos.ed.ac.uk/research/geohazard/KatlaEQ.html
But Katla is still ticking over, with 135 earthquakes in April, and with another small flood that did no damage but merely pointed to the sudden release of a pulse of meltwater – most likely either a body of water had been accumulating for while and that was stored in a specific location before being released, or a sudden burst of thermal energy from the bedrock that rapidly melted ice at the glacier base and which escaped immediately. No one knows which, as instrumentation cannot resolve at the scale necessary to indicate exactly what is going on under 400-700 m of ice.
In my opinion Katla is currently ticking over like an idling car. And although she is as carefully monitored as she can be (and the Icelanders are excellent at this, as proven during the 2010 Eyjafjallajökull eruptions). Past substantial eruptions, like 1918, are preceded by large earthquakes. So it would be very surprising if there was a large eruption without precursory large earthquakes again. For now, there is nothing to suggest that Katla is preparing for a substantial eruption. But this is not an exact Science, so surprises can happen!
In Part 2 I’ll say a bit about earthquakes elsewhere in Iceland and how these link to large scale structures such as the transform fault systems. And on the interesting earthquake swarm that’s currently occurring west of the spectacular subglacial basaltic table mountain of Herðubreið. Of which I attach a picture taken from the Askja volcano, below.