The previous state record quake -- 5.2 magnitude -- occurred in the same area in 1952. Prior to that, there was another five-something back in the twenties. Same area.
Point being there is an extensive earthquake history in the area, going back well beyond the start of injection wells.
Probably looking at something around a 250kTn energy release to generate Richter 5.6 seismic waves. That doesn’t account for the total energy release where much of it might be absorbed as heat in the soil mechanics in the media.