Pump large amounts of fluids — such as "produced water" from fracked oil and gas wells — down an injection well and into an existing fault. It happened at the Rocky Mountain Arsenal, causing earthquakes in and around Denver back in the l960's. Geologists are concerned it could begin happening again, given the huge amount of shale gas and oil production.
For one, they have proof that the number of medium-sized earthquakes has increased in recent years:
A new study from the USGS links this to the huge upsurge in fracking. (Again, it's not the fracking that's the problem, but the injection wells.) Lead author Bill Ellsworth calls for seismic monitoring to avoid disasters. Haven't yet heard a response from the industry.