I suspect shutting down the power lines is one of those ideas that sounds better in theory than it would be in actual practice.

To be effective, you would need shut down the grid every time high winds are predicted, and keep it shut down till the winds subside. But in effect, that would mean shutting down much of daily life for a huge area and population. The electric grid powers hospitals, traffic signals, home medical equipment, cash machines, refrigeration and freezers at your local supermarket, gas pumps at your local service station.......the list goes on and on.

Given the extent of the grid, putting all those lines underground would be hugely expensive, and possibly technically impractical.

A better solution might be selective upgrades and preventative maintanece, in areas where fires are most likely to be started by the lines. Simply keeping brush and undergrowth cut back along the lines might go a long way. Or raising the lines higher above the ground in places such as ridge tops where winds might be expecially intense.

As hikermor has noted, fires and earthquakes are just part of life in Socal. Kind of like tornadoes in Oklahoma.