For the last couple days, I've recently discovered the whole "skyquake" phenomenon. I, along with the rest of the population, are probably curious about this as well. I have come across many theories about what they could possibly be and I would like to share them with you.
Has anyone ever seen Neon Genesis Evangelion? In the show, there are Angels that attack humanity in order to kill them and erase original sin. There are some similarities that I have found that don't necessarily prove angels, but make you think. In order to combat the Angels, Evas are used. Evas are very large and man made human lifeforms covered in armor, essentially giant human robots. With the scale of the angels and evas being so big, I say that these noises could be these creatures. Also, I've seen pictures of sunrises or sunsets int the shape of crosses made from bright light. In NGE, when an angel dies, A giant light in the shape of a cross blazes across the sky. Another thing that I have happened to come across is a video of an unusual "star" in the shape of a sphere. The star then looked like it was "kicking" from the inside. Another reference to NGE when Eva unit 1 was fighting an angel whose shadow was a sphere while the actual angel was the shadow on the ground. Eva unit 1 was swallowed by the angel and broke free of the angel by kicking and punching it's way out.
I know, I know, I'm crazy, but hear me out. There are two alien species that I know of that are friendly to humans: The Grays and The Nordics. Now, there are also a lot of other alien species that do not like us, at all. What if, the aliens are fighting a war right above our heads. A war that we know nothing about. If so, the Nordics, the Grays, and even maybe some other alien species could be fighting to protect the earth from uninvited invaders. Perhaps it has nothing to do with a war, but something to do with aliens could be possible.
3. Inter dimensional travel/ Alternate universes colliding with our own.