I suspect if the prediction of America reduced to third world status and especially California were to come true, whilst I don’t think America would become that poor I have a feeling a declining economy might affect many other things. If California were to be the worst thing, expect Hollywood to decline real badly and possibly having some studios be bought by Mexicans, Indians and Chinese.
(Something like Warner Bros being on its way to be an Indian owned company under Tata Warner I think.)
Great American cities might still exist but not in forms we’d expect it to be. Detroit might and should serve as an omen in that it was this badly affected by the recession and got stray dogs running about. If California and much of America in general were to economically decline, it would quadruple many of the problems Detroit already had.
Perhaps Mexico might be poised to get back those states as they used to belong to it. But in time, any Californian would be considered Mexican citizens by default and happenstance. One might except Los Angeles to go from one of the most prominent American cities to one of those Mexican cities.
(If this badly affected by earthquakes, unless if some people move away to safer places or by sheer luck, Los Angeles would be reduced to rubbles then countryside.)
If an earthquake were to hit America real badly, it would be Detroit magnified really.