Using microearthquakes to track repeated magma intrusions beneath the Eyjafjallajkull stratovolcano, Iceland

We have mapped microearthquakes caused by magma migration preceding and during the flank and summit eruptions in March-May 2010 of Eyjafjallajkull stratovolcano in Iceland using a Coalescence Microseismic Mapping technique. Spatial and temporal clustering of >5,000 microearthquakes under the eastern flank of the volcano illuminates several northeast-southwest striking sub-vertical dikes at 2-6 km b.s.l., emplaced before the Fimmvruhls flank eruption in March. This intense precursory seismicity had a lateral extent of ∼6 km east-west and ∼3 km north-south. A sequence of 386 microearthquakes during the summit eruption, refined by double-difference relative relocation, defines a sub-linear trend inclined ∼5-10 from vertical extending from the upper mantle at ∼30 km depth to the summit crater. This sequence includes two major clusters at ∼19 km and ∼24 km b.s.l., each containing >100 earthquakes. All microearthquakes display characteristics of brittle fracture, with several subsets of events exhibiting closely similar waveforms within clusters. This suggests similar, repetitive source processes. The deeper clusters may be caused by fracturing solidified magma plugs that form constrictions in an otherwise aseismic melt conduit. Or they may occur at exit points from melt pockets, in which case they indicate positions of magma storage at depth. The seismicity deeper than 10 km only starts three weeks after the onset of the summit eruption, after which the largest clusters occur at progressively greater depths. This temporal pattern may result from pressure release at shallow levels in the magmatic plumbing system progressively feeding down to mobilize deeper melt pockets.