I think that crack was made by the explosions heard in the very first episodes. AND they were not man made. AND they cause the zombies... AKA the mole people! Yes I knew it!!
The mole people are invading LA and the world. That is why the "zombies" are ugly, have copious amounts of zits and pimples, and are different sizes. People in the world are different sizes so it could reason that mole people are different sizes as well. Also they carry an infectious virus in their saliva that makes normal people into stupid cannibals and since their mole-world was running out of food they decided to BOOAAM! and take over the Earth. That could also explain why Skittles said that some of the special kinds are blind and have a good sense of smell... that describes... A MOLE!! or... Mole People! And the one with the markings is their Mole Leader! Plus the "haze" could be an underground vapor that the mole people are used too and makes normal people into zombies. That is my theory: Mole People are invading LA and their infectious saliva and haze turns normal people into the zombies. Yea!