So do you think looking at this, the thunder is centered on some very low frequency below the mic's response and what it gets, and what we hear/feel is the harmonics of that?
I suppose you might be far enough away to have dissipation and absorption of higher frequencies by air become a factor as well.
I need to do a capture of the nearby train. Those are some low frequencies too. You can see them miles before you hear. Probably have to wait to winter though. Too much wildlife noise for now.