Of course extant fault lines are involved, but they are probably being lubricated past their sticking points with the extra water/oil being undergrounded. Perhaps it is a good thing and can prevent very large quakes from occurring. Might even have a future use to prevent very large quakes from happening.
If volcanic activity starts there again, anything caused by humans is really no match. Plenty of igneous formations around the basin.
Any recoverable oil is removed prior to injection, but the water is itself a lubricant.
The good Dr. is probably onto something in #8.
To quote hitlery “what difference can it make?”. Unless I missed something, haven’t all these quakes blamed on water injection been just relatively light tremors? Now if these start getting up in the breaking things range, might be something to worry with, until then enjoy the thrill.