There is a study by the Oklahoma Geological Survey that is in the process of being published that reports a possible link. The lead author was a skeptic that fracking could be linked with any seismic activity, but as he studied the January 18th quake in Oklahoma, he does report a likely correlation between between specific fracking operations and the series of small earthquakes in the vicinity. Correlation does not equal causation, but this seismologist started as a skeptic and this is his conclusion about that particular investigation.