The fireball image below which I posted in the astrophotography section triggered a discussion, which rather belong here.
This fireball has turned out to be quite interesting. A thundering sound was heard over a large area, including the area where a recent quick clay slide destroyed a number of homes and killed 10 people, so it caused quite a bit of concern and media attention. Norwegian seismologists also recorded the event at a nearby infrasound array, and given the camera recordings we had from our meteor network, I could quickly point out that the source was up in the air rather than in the ground.
I also soon realised that the trajectory ended pretty much right over the infrasound station. Knowing that it would be a far shot, I asked the seismologists nevertheless to go through the seismic data as well and look for something out of the ordinary 1 to 3 minutes after the initial infrasound signal. And guess what, precisely two minutes after the infrasound a weak, but clear, seismic signal shows up in multiple sensors in the array. Trying various dark flight simulations, we found that the timing matches with an impact of a meteorite between ~100g and 1 kg, which would hit the ground at speeds between 55 and 70 m/s. We know that the luminous path ended at ~20 km altitude, and the sonic transition point must have been at 16 km or below (12 - 16 km). We also assume an pre-atmospheric mass of 5 to 10 kg (at a speed close to 15 km/s giving a peak luminosity of -8 or -9 matching the video evidence well), and the expected surviving mass would be between 0.5 and 2 kg (probably not in one piece). So, could the seismic signal really be a meteorite impact? To my knowledge, it would be the first time a fall is documented with video, infrasound data and seismic data. The main concern is that the source of the seismic signal is a few km off the expected fall location, but still within the range of possibilities given the uncertainties in the data from the cameras and infrasound detectors, wind conditions and fragmentation. We know that the seismic signal is real and very close to the sensors, and we don't have any good explanations other than that something hit the ground. We've considered a falling tree, but it would likely give a different signal, or frost expansion in the ground, but this was the only signal that night, and frost expansion should trigger a series of signals. And animals, not even a moose, could create a single signal like this.
Usually a fireball dropping at most a couple of kg over a forested area would not be worthwhile organising a search for. But in this case we have a highly accurate candidate position triangulated from multiple sensors up to a few 100 meters away. The trouble is that it's winter. It's like looking for a wallet lost in the woods after it has snowed. But who wouldn't give it a try anyway? So we brought skis and gave it a try today. It's in an easily accessible, rural area 300 masl. We were 5 people including one of the seismologists. Ok, it was pretty futile, but interesting nevertheless. We found that the snow wasn't very deep, quite shallow in the densest parts of the forest, so it seems totally credible that a falling meteorite would penetrate the snow and cause a seismic signal. Also, the fallen trees in the area were covered by old snow, so they must have fallen before the fireball event. But there were animal tracks all over the place, as well as cavities in the snow surface caused by lumps of snow that had fallen off the trees, and there was no way we could check how deep every hole went. So we need to try again in spring.
Images from the seismic spot, how to find a meteorite here?:
National TV followed our little expedition giving us a couple of minutes of fame in the prime time news today, here interviewing the seismologist: