I was actually in Palo Alto at SRI when the 89 quake hit.
It knocked out power, gas, water, etc. for quite a while afterwards. The destruction of that double-stacked highway was horrendous. That quake was a 7.9.
If the same thing hit LA, I’d say that devastation will be especially bad. My first guess is the expressway flyovers would be taken out nearly everywhere.
Nothing compared to the supervolcano under Yellowstone.
Loma Prieta was a 6.9 magnitude.