I saw an interesting article yesterday that had colored maps showing where the borders between baseball team allegiances. For example, people in Hartford CT are Yankee fans in the majority while just north are Red Sox fans. The dividing line between them is called the "Munson-Nixon" line, for Thurman Munson and Trot Nixon.
The dividing line between Dodger fans and Giants fans is just north of San Luis Obispo. That's probably as close as you could locate that line between Northern and Southern California, which I propose we call the "Mucci Line."
(The fan data was based on analysis of Facebook data).