How feasible are natural gas generators or diesel generators after a major earthquake?

If the tanks aren't cracked (or destroyed) by debris, they should function well enough. The obvious limitation is capacity.

They are all well and good however, remember, you can't recreate the electrical grid that was there before the earthquakes.

Is NG fuel cell power generation a better alternative than diesel generators?

Should cities in California start planning for an big earthquake?

What modifications are required to connect a 10 mw diesel generator to the grid for emergency power?

This is a little tricky but it is done all the time. Basically, the generator must be running at the same voltage, frequency, and phase as the grid before connecting them together. This is something that must be done by an electronic controller. If you try to connect without this controller there could be a big bang. Now, most solar power systems have this capability. I'm sure there is a controller you could buy. The question is: what kind of shape is the grid in after an earthquake? I think you meant a 10 KW diesel generator (I wish I had one but they are a little too expensive for me to invest in.) . This should be enough to power your house and forget about the grid. The only thing you could add to that is a battery backup which requires an inverter and a controller. Did you know that back in the '30s a lot or rural farms lived off of systems like this? This was why the government Rural Electrification Project program was started and is still in effect today.