Tony, see link.

There was some paper or some discussion that M9 quakes are clustered. John V and Roger would be more up on this. Certainly you don't normally get two M8 quakes in 1 day; I forget the statistics but I think it is one every year or two. Although there may have been 2 M8 quakes distant from each other, 1 in Columbia, in 1905 or 1906, but someone should check that.

I think the article was talking about the interval after 2004. But, the last 2 years have had th M8.8 Chile quake (Maule) in Feb 2010, The Tohoku M9 in March 2011, and these. The only other one that was close is a 7.9 aftershock just after the Tohoku Japan quake. There was a m 8.1 Samoa quake in Sept 2009. So 5 M 8s in 2 1/2 years. More than normal, and energy released far more because of the M9, 8.8, and 8.6.

OK, so there is a cluster; is there an explanation? I think the article had one but I forget what it was.

Chris

• significant quakes by year