Recent activity in the region of the Katla volcano. (Icelandic Met Office) A couple of years ago there was a paper suggesting that many of the small quakes are more likely to be due to ice movement, rather than volcanic activity and magma movement. While that may be true of many of the small events that have occurred recently (and bearing in mind that it is the end of the summer, over which temperatures are warmer) the lineation of some of the quakes seem, to me, to be more evidence of more significant activity.
One can look, for example, at the aligned quakes that occurred around Sept 8th, Sept 20th and Sept 21st.
Earthquake activity for Sept 8th, 20th and 21st (Icelandic Met Office).
The question of how seriously to take all this activity is made more difficult to answer by similar levels of earthquake activity that is occurring further west around Hellisheiðarvirkjun. This is the more westerly of the two starred earthquakes on the map below. (The star indicates that the levels are at a magnitude 3 and above).
Location of recent earthquakes in Iceland (Icelandic Met Office)However the activity at Hellisheiðarvirkjun is not totally natural since there is a nearby geothermal plant at Hengill which is in the process of being expanded from 213 MW to 300 MW of electrical power and 400 MW of thermal energy. Although there bave recently been a considerable number of small earthquakes (as today’s map would indicate) in the region, and the larger one today, the history of the region shows that it has been 2,000 years since the last major eruption there. The plant is about a 20-minute drive from Reykjavík.
I wrote about some of the problems of water injection into the stressed rock of a geothermal site, and the earthquakes that can be induced, some time ago. Jón Frímann has noted that water injection is currently taking place at Hengill, and thus that many of the quakes are man made. In the earlier piece that I wrote I noted that earthquakes up to 4.6 in size had been induced around the Geysers in California, but with the high incidence of quakes that they see, this has not been considered a problem. I have quoted Ernie Meyer on the work at the Geysers in California where he has said that the largest quake they have seen there was a 4.6, and that "there has never been a damaging geothermal earthquake anywhere in the world."
Iceland is a little different, given the more active ground movement, and that the water may induce larger scale ground movement if it lubricates too many of the potential failure surfaces. The plant apparently intends to continue injecting the process water after the heat has been removed, so that it can recycle and regain heat before again reaching the wells for extraction. Earthquakes up to a level 3 are expected, and are not considered of concern. Though as Denise-Marie noted there have now been a couple of quakes that are a little more than that.
Hengill drilled wells down to 250 m from 50 to 70 years ago, but then with more advanced technology drilled down into the higher temperature zone that can be found down to 2,500 m.
Difference between Low Temperature and High Temperature geothermal deposits in Iceland (Sverrir Thorhallsson ) The wells at Hellisheiði are High Temperature, and it has been calculated that the average well flows at 35.5 liters/sec with a power production of 8.7 MW.
Apparently even geothermal wells can blow out, and blow-out preventers can fail to work. One of the wells at the Krafla geothermal plant in northern Iceland blew-out, although the environmental consequences are much less, since the escaping fluid is largely steam and water.
Geothermal Blowout at Well KR-4 (GeoThermHydro ) Time to wait and see again, how things progress.
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