If the shear stress decreases, a fault is more likely to rupture and cause an earthquake. There are hundreds of wells in Oklahoma actively injecting high-pressure fluids into the earth, and the locations of these wells show a strong correlation with earthquake epicenters. However, this fact is not entirely reliable in proving the cause of earthquakes, since Oklahoma has a high spatial density of wells as it is. The increase in earthquakes in this area alone, however, is enough evidence that something is changing underground to generate such high seismic activity in