The Norwegian Fireball of June 7th, 2006
Local Seismic Observations
As the airwaves (sound) produced by the fireball propagated to the ground they were intense enough to couple with the Earth’s surface, causing the ground to move slightly. This movement was recorded by three seismic instruments located nearby. At Tromso (TRO) and Kautokeino (KTK1) this appears as a dispersed pulse, with hints of possibly the three peaks seen in the Swedish IRF data. At the NORSAR seismic array (ARCES), a 25 element seismometer array laid out in a circlular pattern just over 1 km in diameter, a complex wavetrain was also detected from the fireball. A single pulse with low frequency content (1 – 6 Hz) may be associated specifically with the airwave arrival. Similar to the infrasound, beamforming procedures can be used on this seismic arrival as it passed over the array, doing this a back azimuth of 258.6 deg. is found.
Using the azimuth intersections from the Swedish (IRF) infrasound data a location of 68.860 oN, 22.204 oE is found (Burst 1 – IRF intersections) just over the Norway-Finland border. The azimuth intersection from the long range CTBTO infrasound data places this position further to the NW at 69.782 oN, 24.246 oW (Burst1 – CTBTO). An average between these two positions places the terminal point of the fireball to the north of the KTK1 seismic station. A separate method that uses the arrival times of the seismic observations (and the initial observation at Kiruna) along with a reconstructed atmosphere model from June 7th, allows a 4D location for the terminal point to be found in a similar way in which earthquakes are located in the solid Earth. This position lies at 69.203oN, 22.396oW at an altitude of 43 km occurring at ~00:06:28 UT.