How often and why do earthquakes occur in California?
It depends on where you are talking about. Along the San Andreas fault, a major earthquake occurs about every 100 years. Earthquakes are caused by plates of Earth rubbing together.
As of now, it's been overdue. Scientist said that the earthquake has a 100% chance that will occur before 2037.
Though this is overdue, California has smaller ones almost daily and depending on the location and depth of the quake, some are felt and some are not.
On the lighter side...
California has been waiting for the "Big One for over 60 years or more. I heard about it every time and so did my grandparents did even sense they moved to the area after there would be anything of any significant size. At this point its just a matter of time and the news media just loves to play on that and freak people out.