Zusammenfassung
We have mapped microearthquakes caused by magma migration preceding
and during the flank and summit eruptions in March-May 2010 of Eyjafjallajökull
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
Fimmvörðuháls 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 deg 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.
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