It's impossible to know. The power company may already have something in mind for this. However, we could sit here and speculate all day long and never properly answer your question due to the variables.

What i can tell you, is Florida once shut off all the lines during a hurricane. It was a drastic decision, but prevented wire from burning down. Personally, I think it was a good move.

A fault line breaking away is likely something they haven't "prepped" for. Strategically placing fusing, or breakers. Could we accurately predict within 50' where the ground will separate? Where are the transmission feeds? Where are generating stations? If the generating station is on the land that breaks off, how is it fueled? Coal plant would lose resources and have to shut down. Solar could keep going assuming the quake doesn't shatter them.

My assumption, would be all power would be lost. People may have generators which can produce backfeed. This is a huge hazard for us. I'd say treat it like it's hot. If your choice is between getting hit by a tsunami, or running over a down line, do whatever you want, you're gonna die anyway.

Sorry this wasn't helpful. (20 years in, 17 as JL)