Normally, when there is subduction, you might think that the coast might rise up. The subduction does raise the volcanoes in the back of Japan, but apparently drags down this part of the coast. The mere fact that it is productive lowland shows that this has been going on for millions of years. That's how the land changes, a metre at a time with big earthquakes. They will have to fill in the land and rebuild, thus creating a hazard for another earthquake.
It's the same when you look at the lowlands around Hamilton, and you can predict what will happen with the next earthquake.