To say all fracturing causes earthquakes is a little misleading. Yes, they are mapped by the "tiny" earthquakes. The energy in those "earthquakes" is around -3 (typical range is -4.0 to -2.0) on the richter scale. That is the same as dropping a liter of water 20 cm. So, if you have ever dropped your water bottle, you too have caused an earthquake of similar size. The question is can they cause bigger, felt or damaging events. So far, the answer from the studies regarding hydraulic fracturing is no. Does that mean you can hydraulicly fracture anywhere? The studies also say no to that. Major faults should be avoided, minor ones don't matter from a seismic perspective. However, if the created fractures extend into an existing fault, the energy that was supposed to make a new fracture network to increase the gas/oil flow is lost, which make the fracture treatment not effective from a financial standpoint. The oil or gas companies have a financial incentive to not treat in areas that have that type of faulting. As to waste injection wells, according to the USGS there are 150,000 Class II injection wells, 40,000 of which are used for oil and gas operations. These have been around for decades. There have been several papers written that link seismic activity to some of these wells. There is a better solution than injection large volumes of waste water into these disposale wells. More and more companies are recycling the water to re-use it for fracturing and also treating the water to make it OK for drinking or surface disposal. It also depends on what shale basin you are in as to what the flowback water contains. Some areas are definitely worse than others.
