Any military artillary gunfire ranges in the area? We get the same kind of booms here in south Orange County from Navy ships doing live fire practice on an island just off the coast. Depending on the wind and atmospheric conditions, they can be fairly loud.
Also the Marines at Camp Pendleton do artillery practice that can be heard up to 50 miles away.
http://www.marines.mil/unit/basecamppendleton/Pages/Information/noiseAdvisory.aspx