The Elginfield Infrasound Array (ELFO)
Interesting (non-meteoric) Events detected by ELFO
Mysterious Explosions over Lake Huron, August 1, 2008
Around 11pm EDT on July 31st, 2008, residents of Kincardine, Ontario were awakened by two loud, thunderous bangs emanating from somewhere over Lake Huron. The sounds, as reported by residents, were apparently loud enough to rattle windows and objects on walls. Initial guesses as to the source of the “thunder” were (1) a perseid meteor, (2) an accident at a regional nuclear power plant, (3) nearby salt mine blasting (4) supersonic aircraft. ELFO infrasound showed that two blasts were observed at 02:12:30 UT along with a series of inaudible phases (Fig. 4a) arriving from 315-323° (Vapp: 348-353 m/s). These observations (along with all-sky cameras) ruled out a meteor source, as well as operations at the Bruce Nuclear Power Plant, while Goderich salt mine logs eliminated it as a source. The impulses were also observed seismically as ground coupled acoustic waves around South Western Ontario and Northern Michigan (Fig. 4b). From observed back azimuths and particle motions observed at seismic sites, the source of the two blasts was identified as originating from an area in Northern Michigan (45.04±0.12°N, 83.62±0.21°W, 44.905±0.061°N 83.492±0.047°W) with a probable origin time of ~2:59 UT with explosive yields between 2-3 tons on TNT (Whitaker 1995), consistent with quarry-type explosions. Wurtsmith AFB and restricted airspace R-4207 lie near this region, but AF activities ceased in 1993 and local airport radar reported no aircraft in the area at the time. The Alpena open pit calcite mine, however, lies only 18 km NE. If the sounds were blasts from the mine, seismic phases should have arrived at the stations at ~03:01 UT. Such arrivals can be readily identified at several stations. It appears that due to propagation conditions, Kincardine lay near a convergence of sound propagating from explosions at the Alpena mine (a known source of other inaudible signals at ELFO).
Figure 4a (observed blast)
Shown is the location of ELFO and the location of all four events.