Building the Crust through Melt Intrusions, Askja, Iceland
Abstract:Persistent seismicity recorded within the Askja segment of the Northern Volcanic Rift Zone of Iceland since 2007 by a network of up to 55 three-component broadband seismometers reveals melt transport in distinct regions at 20 km depth in the mid crust. The bulk of the seismicity occurs beneath the main central volcano but significant seismicity exists in two other, distinct, locations. The cluster located beneath Vadalda, a shield volcano to the east of Askja, experienced a swarm in December 2012 during which more 50% of the total earthquakes in this cluster occurred. Accurate earthquake locations have been generated using hand picked arrival times and cross-correlation techniques in combination with HypoDD. This has revealed that events within the seismic swarm all occurred at a single depth in the crust and migrated ~2.5 km to the southwest over a four week period. We interpret this as a sill intruding laterally into the mid-crust.
The swarm culminated in intense seismicity on the 26th December with over 50 earthquakes in a two hour period. Each of these earthquakes produced identical waveforms and locate in exactly the same place. This implies a repeating mechanism causing the earthquakes. The fault plane solution of the repeating event is well constrained but is an unusual mechanism, consisting of either a vertical or a horizontal fault plane. This is highly unusual for a tectonic mechanism, further suggesting that the movement of melt along a sub-horizontal sill is driving the microseismicity. An earthquake which occurred three days before the final swarm has a similar waveform but is larger in magnitude. We interpret this as the initial event which opened a conduit, with the subsequent repeating earthquakes occurring when the flow rate was sufficiently low to allow cooling of the melt through the glass transition and induce brittle failure.