The majority of earthquakes located within the state of California are not caused by humans. There is a large fault that runs the coast and through the land. It is a slip fault that drags with one plate moving north and the other south. The pressure builds until it snaps and an earthquake occurs. This would happen even if we didn't exist.
No, we are not the cause of the Earthquakes in California, because there is nothing that little humans running around on the surface of the earth could do that would have that big of an impact. We have recorded earthquakes for as long as we can remember. They happened long before industrialization.
There might be an occasional tremor or shifting due to removing natural resources. Oil extraction, fracking, mining, etc.might cause the occasional stir, but certainly not a full-blown earthquake. Earthquakes happen when plates below the surface of the earth rub against each other. This has been going on since the earth had plates floating on a molten surface.
No we are not the cause of the Earthquakes nature is and I dont think that there is anything really we can do to control what the weather is going to do it has a mind of its own. We should take care of our envinorment and help in anyway we can to prevent pollution.