Reports of hydraulic fracturing causing felt earthquakes are extremely rare. However, wastewater produced by wells that were hydraulic fractured can cause “induced” earthquakes when it is injected into deep wastewater wells.
Wastewater disposal wells operate for longer durations and inject much more fluid than the hydraulic fracturing operations. Wastewater injection can raise pressure levels in the rock formation over much longer periods of time and over larger areas than hydraulic fracturing does. Hence, wastewater injection is much more likely to induce earthquakes than hydraulic fracturing.
Most wastewater injection wells are not associated with felt earthquakes. A combination of many factors is necessary for injection to induce felt earthquakes.