The initial blast alone from 100 Mt (Tsar Bomba) could completely take out San Francisco, the Silicon Valley, the North Bay, and the East Bay. Neighboring areas remotely close to the destruction zone would also undergo substantial collateral damage. Further, the nuclear fall out would cause massively more carnage just to be sure.

If a 100 Mt bomb hit the Los Angeles area, the nuclear fallout would contaminate the water supply for those who happened to survive the initial blast. The fallout would also directly sicken everybody near the area. Anybody who has spent time in L.A. knows the traffic is mind-numbing stagnant 24-hours of the day. If you add a 100 Mt bomb into the mix, commerce would be completely inoperable and the entire region would be rather dangerous for years. In short, there would no longer be a Los Angeles.
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If you're reading this, it's too late.