Obviously, this is a poor example. It was a magnitude 3.1, so no one felt it, and it was 100 miles north of San Francisco.
The last one anybody felt was on Feb 21st at 11:01 AM, and the first (non-automated) tweet was at 11:21 AM (http://twitter.com/indiaknight/statuses/1234962301). This example is similarly bad, since it didn't really affect anyone in a big way. However, for instant reactions to notable events, Twitter can't be beat. If today's earthquake had been "the big one," you can bet twitter would have been flooded with information, missing people reports, locations for aid, firsthand accounts and so on.
Yes. I was at my desk a few weeks ago and felt something. Couldn't tell if it was my neighbor or a quake, so I searched twitter. Got the answer instantly.
I also got stuck in crazy traffic on the 405 recently. I searched twitter and learned which side streets to take. Google's map program doesn't seem to cover side streets.
It doesn't replace Google for me, but it does replace it for certain searches.
http://www.google.com/search?q=earthquake+san+francisco
the most recent earthquake it tells me about is 3 hours ago (as of this writing).
The time, place and links to maps are all right there at the top of the serp where I expect.
If I do the same query on twitter:
http://search.twitter.com/search?q=earthquake+san+francisco
The most recent result is 19 hours old and then I have to start clicking tinyurls in people's tweets to find more info.