We are not talking about "causing" earthquakes, we are talking about "triggering" earthquakes that about to happen anyway - but will now happen several days or weeks earlier than without the triggering agent. The causes of earthquakes determine the maximum earthquake possible, not the trigger. The trigger merely is the "straw that breaks the camel's back". The strain necessary for large quakes is provided by plate tectonics. To understand triggering you must not confuse trigger with cause.