Everyone has been pointing to the explosion Michael heard in his classroom as possibly being the point at which the ground ruptured in Inglewood. It's possible but I do agree that there likely would have been early reports of earthquakes and what not, despite the fact that the bigger news of people acting batshit crazy was taking place. I can't overlook the fact that perhaps there were reports of earthquakes but we just didn't hear about them. As far as I know, earthquakes are pretty common place in L.A., right? So perhaps the survivors did experience earthquakes, but the more pressing matter of zombies has been occupying thier mind. It doesn't seem so unlikely that the minute more survivors learn about the cracks that people could start saying, "Yeah, come to think of it, I did feel tremors."
We DO HAVE ONE DOCUMENTED CASE FROM THE MAIN CHARACTER in episode 2-1, May 8th, the day of the outbreak. I documented it in the time line long ago. Michael take guard duty and puts his back to the door. He notes that he feels vibrations and dismisses them as distant explosions. It could have been more earthquakes, possibly on a more severe and news worthy level.
As to what caused them or what is in the Haze, I have no clue. I did like Triggernator's post but we also have to suspend a certain amount of disbelief. Although I think some research into the hardy bacteria that survive in harsh climates are worth reading into. There are some amazing things to be found in deep sea thermal vents, though I don't imagine any bacteria can thrive in magma. I really did put a lot of stock into the alien theory, especially with the groups of zombies acting in a military fashion, but it seems that is falling apart as a solid theory since there were no badass spaceships to be found at ground zero.