I firmly believe that Natural Gas drilling is causing earth quakes.
I think that Natural Gas collects in large amounts along the edge of the North American Craton Plate. By drilling and fracking in these locations is starting to make the NA Craton plate unstable resulting in earth quakes in areas that do not normally experience them.
USGS: Recent Earthquakes ‘Almost Certainly Manmade’
Report implicates oil and natural gas drilling, aka fracking