The idea of an invasion on the west coast was VERY REAL. There were several ships attacked off of the CA coast:
http://www.militarymuseum.org/Emedio.html
Submarine attacks on the West coast are one thing.
Bringing the men, equipment, supplies, and ships across the Pacific and landing on the West coast againts well defended beaches and sustaining them against bitter opposition is another.
Remember, Japan did not have enough merchant ships to feed itself, let alone sustain an invasion of the United States.
The below is about Hawaii, not the West coast, but the point remains:
http://www.combinedfleet.com/pearlops.htm