Here's a map of all the mass shootings in America, 1982-2013

You can't extrapolate much from the clustered locations, just because they tend to be the same areas with higher populations (NorCal, SoCal, NE Corridor, Houston-Dallas). There's a good deal more info here courtesy of Mother Jones, and there's additional context and links throughout this paragraph:

More than half of the cases involved school or workplace…