I noticed the same thing last year when I was stranded in Barcelona during the Icelandic volcanic eruption. Then, people were looking for ridesharing, free rooms, news, etc. under the hashtag #ashtag. Not as severe as an earthquake, but a similar mechanism blocking twitter's utility.
Then the problem wasn't spam, but #ashtag retweet avalanches from some well-followed celebrities. The most egregious example I remember: this guy Paulo Coelho in Brazil, retweeted through pages and pages of search results (the source: https://twitter.com/#!/paulocoelho/status/12399786645). I'm pretty sure Justin Bieber said something too, so that was it for #ashtag.
This would probably have been easier to deal with than spam, because people weren't actively trying to game them. Twitter just needed to aggregate some information and make it blockable, e.g. "don't show me (re-)tweets with this text anymore". As a sometimes-user, I still don't see a straightforward way to do something like this in the clients I use. So now the spam angle is not a surprise at all.
Then the problem wasn't spam, but #ashtag retweet avalanches from some well-followed celebrities. The most egregious example I remember: this guy Paulo Coelho in Brazil, retweeted through pages and pages of search results (the source: https://twitter.com/#!/paulocoelho/status/12399786645). I'm pretty sure Justin Bieber said something too, so that was it for #ashtag.
This would probably have been easier to deal with than spam, because people weren't actively trying to game them. Twitter just needed to aggregate some information and make it blockable, e.g. "don't show me (re-)tweets with this text anymore". As a sometimes-user, I still don't see a straightforward way to do something like this in the clients I use. So now the spam angle is not a surprise at all.