Of those, 23 were located within about two miles (3.2 km) of high-volume injection wells that pumped more than 150,000 barrels per month of water underground, Frohlich wrote in the Proceedings of the National Academy of Sciences.
I am amazed. As I've said, the whole injection earthquake thing seems to be over. Somehow I can't believe that they have cut down on injection. So most likely, they have abandoned sites with 'rough' faults (lots of little earthquakes before a big one), and have gone to 'smooth' faults (no warning).