I had to do one for Japan!
You won't believe it (I didn't) but this part of Japan that ruptured is the North American Plate, down from Alaska.
The earthquake ruptured the big red area. They usually stop at the crooked sections, and at significant junctions.
So the lower section of Japan has a totally different subduction zone, and this is what will affect Tokyo eventually.
These earthquakes usually happen far out, so the shaking isn't that bad, and also they tend to rupture somewhat slower. But when they go, they flap a big piece of crust, and that's what shlooshes out, sort of like holding water in a pizza pan.
People have always expected this type of earthquake, but the interior is subject to super-shear thrust earthquakes (M7), which produce a very high PGV. This got another nuclear plant in Japan, and is probably what hit Christchurch. You should never assume that the tiny sliver of continental crust is stable....