What happens between eruptions?

What happens between eruptions?

TkalcicMost of the excitement in Iceland is focused on the eruption in Holuhraun, but perhaps the most important effort should be directed towards understanding what is happening beneath Bárðarbunga volcano.  It is particularly important to get an understanding of the processes taking place between eruptions, which is the advance warning of things to come.  At the very beginning of the current activity I pointed our a paper published by  Meredith Nettles and Göran Ekström in  1998. There they addressed some umusual but large earthquakes, which occurred beneath Bárðarbunga from 1976 to 1996.  These earthquakes were size 5 or larger, like the current earthquakes, but there is nevertheless a fundamental difference between these earthquakes.  The current quakes can be explained by motion on vertical faults as the caldera floor sinks down towards the magma reservoir below (“double-couple” earthquakes).  The earthquakes from 1976 to 1996 were on the other hand of totally different type. They are what seismologists term  “highly non-double-couple” quakes, with a strong vertical component.  It is not within the reach of an ordinary geologist to explain all in connection with these complex quakes, but nevertheless I would like to examine the studies, because I believe that perhaps we can find here important information about what may be happening under Bárðarbunga before eruption and between eruptions.  Thus it may help in clarifying what will happen under this volcanic center when this eruption stops. Nettles and Ekström propose that the earthquakes of 1976 to 1996 were caused by upwards pressure, either due to increased magma pressure or vertical crustal motion of a crustal plug beneath a magma reservoir.  In another paper in 2009  Hrvoje Tkalčić,  Gillian R. Foulger and others again discussed these unusual earthquakes.  They explain them as a result of magma flow within the crust between two magma reservoir at a depth of approx. 3.5 km, or flow of magma into a crack.  Their proposed model of the structure beneath Bárðarbunga is shown above.   It seems clear from studies of these large earthquake events that unusual processes occur beneath Bárðarbunga some years or decades before eruptions.  Are such magma flow processes connected to flow of magma up from the mantle and into a shallow reservoir beneath the caldera? Are such quakes perhaps connected to incresing magma pressure in the reservoir and therefore also to elevation or up-doming of the caldera floor?  If the caldera floor is now subsiding because of decreased magma pressure in the reservoir, then it is equally likely that it will be elevated between eruptions in response to increased magma pressure.