As stated in Geofish Clarified, it started in Ohio. They have had a few M5+ earthquakes due to injection. As the shale gas goldrush went 'hockey stick', there was a great demand for injection, with a huge increase in price. It was only when they injected under a city, that they finally wised up and stopped it.
Then it was fun to watch things migrate down the paths of the megathrusts, into the other states. Many more earthquakes later, it seems to be calming down, as more bans are instituted.
I have a feeling that even Oklahoma and Texas now have a clue, since huge earthquakes were starting to bubble under big cities. An M5 or 6 can do a number on the older sections of a city. I'm now beginning to think they are following the Main Rule:
Do not inject into the fractured Precambrian.
Time will tell. :)