The low frequency tremor 23rd August 2014
The seismic signal that raised the Aviation Colour Code to red
The figure shows averaged real time seismic amplitude (RSAM) at the seismic station on Dyngjuháls (dyn) 23rd and 24th August (enlarge).
The amplitude is shown in three frequency bands (0,5-1 Hz; 1-2 Hz; 2-4 Hz).
A low frequency tremor pulse is clearly seen just before noon 23rd August. This tremor (red line) was interpreted as signs of magma having arrived at the surface under the glacier and prompted IMO to raise the aviation colour code from orange to red. The tremor lasted a few hours and then gradually decreased during the afternoon.
No other corraborating signals of magma reaching the surface were observed on other monitoring systems. Observations from the Coast Guard aircraft, TF-SIF, as well as electrical conductivity measurements from IMO's hydrological monitoring network, showed no signs of activity.
The scientists' conclusion was that magma did not arrive at the surface under the glacier that day; no sub-glacial eruption had begun.
The figure also shows that seismicity drastically increased during morning of 24th August while the low frequency tremor remained low. The blue line reflects the seismic activity during the time the dyke was quickly advancing northward, as well as increased activity that morning.
