diberry - 9:39 pm on May 3, 2013 (gmt 0)
It's off about 10% a day since, but it's not only Google, it's across the board and it's not "normal peaks and valleys", it'll be down 20% then spike at an odd time for a couple hours and then drop off again, and again, it's not only Google referrals that have been totally odd in the pattern of "below average" then "random spike" since. It's all of the big three and I seriously doubt they're all just happening to change something that causes the site to rank better or worse in unison, so for at least one niche it looks like there's some difference in search patterns/behavior of searchers since the Boston events.
I think the way people search, surf and find new stuff is changing in general, and what you're seeing might a case where that's especially noticeable. What I'm talking about is:
People aren't just defaulting to Google for all info. Even my mom knows you go to Yelp for info on local places, Wikipedia when you're curious about howler monkeys, and [news site of choice] when you want news. The engines are getting bypassed completely on increasing numbers of, well, "queries" for lack of a better term.
And then of course what people are looking for online changes dramatically when there's big news. It used to be it just sucked them offline and they watched TV. More and more, they're staying online, but they're not doing the things they normally do, or if they saw something at work and thought they'd look at it again later, they may forget. That kind of stuff.