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 shootings (12 and 20, respectively); the other 30 cases took place in locations including shopping malls, restaurants, and religious and government buildings. Forty four of the killers were white males. Only one of them was a woman. (See Goleta, Calif., in 2006.) The average age of the killers was 35, though the youngest among them was a mere 11 years old. (See Jonesboro, Ark., in 1998.) A majority were mentally troubled—and many displayed signs of it before setting out to kill. Explore the map for further details—we do not consider it to be all-inclusive, but based on the criteria we used we believe that we’ve produced the most comprehensive rundown available on this particular type of violence. (Mass shootings represent only a sliver of America’s overall gun violence.) For a timeline listing all the cases on the map, including photos of the killers, jump to page 2. For the stories of the 151 shooting rampage victims of 2012, click here, and for all of MoJo’s year-long investigation into gun laws and mass shootings, click here.
There’s also more here and here. Can easily get very political about this, and will avoid doing so for the purposes of this post (which is primarily to show the map and the links available), but this whole thing is a lot to take in. For example: 194 children have been shot since Newtown. That doesn’t seem like a lot since Newtown was 18 months ago and 9 billion people live on the planet, but still.
The killers in all the cases from 1982-2012 had a total of 143 weapons, 48 of which would be outlawed by 2013’s Assault Weapons Ban. Most were high-capacity magazines, as you’d probably expect.