...most services here are underground, at least for the past 40 years or so. I guess the mast requirement in California has something to do with earthquake issues?

Not really the mast/earthquake issue, but maybe a little insight:

Overhead service in SoCA is pretty much limited to existing urban areas and rural areas. Anything in a new area is underground. Utilities are in a continuous process of replacing OH sytems and putting them UG. Even a OH service replacement in most areas will be required to have the capability for future underground feed, or might be required to change from OH to a UG lateral to an existing pole. The idea is to NOT do what was done in a couple of areas in the mid/late '70s, when UG started to become the norm.

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Our conditions and history are different here than in most other areas. We don't have the issues of ice on OH lines, and lightning is a rarity. OH utilities and service were a natural for us when the place built up like a wildfire. Nobody really realized the magnitude of what was to come. By the time the smoke cleared, there was an OH infrastructure. Great, until someone looked up.

Putting the power and other utilities UG in OH districts is more a matter (besides funding) of disruption to roads and the like than the physical relocation itself.
The effect is somewhat like putting a shovel into an anthill.
There's no heaves or frost line to contend with, so minimum cover requirements up to 35KV can be as little as 30".