Magic Apocalypse!!

Heyo! I have been actually getting to a plot for my story and I want my world to be set after modern day society has collapsed. This would give my story a good reason for a bunch of teenagers to be in one house with one adult and one kid. I want it to be at least a decade after the event that happened for the world to become like this.
The problem I have is I want magic to be none existent before this happened and most of humanity to be wiped out. What could be the event that could cause both magic to appear/come back and ruin humanity?

If you need more information just ask.

Tell me more about the magic in this world. Do you want it to be malevolent, as in it might have caused the apocalypse? Or benevolent? Neutral? Can anyone use it, or just a certain group/type of person? Are there magical creatures as well as magic itself?

Neutral as far as the magic goes. Most of the magic comes for the creatures and/or spirits. I have several people that have magic but they have it because they have a bond with a spirit I call a Shade. It very well could cause the apocalypse though.

Okay. So, here's my crazy idea. There are fault lines all over the earth, right? What if something sent out a tremor through the ground that was so strong, it triggered massive global earthquakes? and maybe some of those earthquakes in turned triggered volcanic eruptions? And tsunamis? And major changes in weather patterns? That takes care of the destruction you were talking about.
Now, for the magic. What if the components of the magic were hidden underground this whole time, and the earthquakes uncovered them? Or, what if the earth's magnetic fields plus a healthy does of radioactive fallout (you know those nuclear power plants aren't quake-proof) created a new element, the element of magic? Idk, that part's a little weak, but maybe you'll get some ideas from it.
I guess that also begs the question, what caused the first tremor? Maybe it was some deliberate act of destruction by some war-hungry maniac, or maybe it was a freak accident with a bomb or something, or maybe the whole thing's a mystery. That would be up to you if you like this idea at all lol ^^
Hope some of that helps!

That sounds pretty cool! I had an idea similar to that but I wanted to heard what other people would think of.

Glad you like it! ^^