Seismicity is the term for the size and number of earth tremors or quakes that occur in a region. Induced seismicity is where the seismic activity is triggered by human activities such as mining, the filling of reservoirs or indeed hydraulic fracturing.
A 2013 study led by ReFINE showed that hydraulic fracturing for shale gas is known to cause induced seismicity. In the case of fracking, seismicity is caused when fluids used in the fracturing process "lubricate" existing faults, allowing movement to occur. However, the precise mechanisms by which this happens are not fully understood, nor is the frequency and maximum magnitude of induced seismicity that could occur.