Does frac cause earthquakes?
It's not a simple answer. You're pumping water and sand into rock with the stated goal of breaking the rock, forcing cracks open then keeping those fractures open with pumped sand. That in and of itself is creating earthquakes. These are all, of course, very very low magnitude events and not what people are concerned with.
The bigger question, and the better question is, are hydraulic fracturing operations a contributing factor in these earthquakes? Wells are often drilled across small faults that cut through the target reservoir, and larger faulys can demark boundaries between sets of wells. It's entirely possible, especially with the larger faults near the toe or heel of wells that the completion operations could alter the state of residual stress on the fault and cause the fault to slip.
Operators routinely deploy both surface seismometers and downhole geophones to monitor fluid injection activities.
The more likely contributor to earthquakes is disposal operations. This includes water that flows back from a well after the frac is completed and water routinely produced alongside oil. Produced water in older wells and fields can be as high as 10:1 or even greater and (if the water can be easily disposed of) remain profitable. These disposal wells can have greater impact since the injections take place continuously (or nearly so) for extended periods of time.