Sounds to me like the 869 earthquake in the same area could have been as big as 2011. The surface wave magnitude scale is not good for quakes this large: but will no seismometers maybe this does not mean anything. Go to the link and then check the different quakes. Here is a relevant quote:

"The earthquake had an estimated magnitude of 8.6 on the surface wave magnitude scale. The tsunami caused widespread flooding of the Sendai plain, with sand deposits being found up to 4 kilometres (2.5 mi) from the coast."

So, I don't think the nuclear safety people can say this was unexpected: one of my favorite hypotheses is: "If it happened, it must be possible"

The problem for California is we don't have a long historical record. I've been involved in work on giant anticlines very far offshore southern California. These are very young and the trend is >200 km-long. Can there be a M8 thrust earthquake offshore California? We don't know.

I think there must be permanent subsidence of large parts of the plains and that is why they are still flooded from 2011. Alaska 1964 had large vertical motions with huge areas uplifted or subsided...by maybe up to 2 m, with much more uplift near a secondary surface fault. But, the loading between quakes has reversed much of the motion in Alaska. Where there are very flat plains, I suspect the net long-term motions would be subsidence.

Chris

• Japan earthquake history