I think it surprised scientists, too, which is why it got big headlines. This region, well, if Earth were a ball of yarn, this would be a BIG tangle. Seismic events like these help them untangle it.
Of course, there's probably also concern because of all the radioactive water stored at the wrecked Fukushima plant, but those storage barrels can likely handle another big one. The original power plant, though, and the fuel assemblies in Units 1 and 2? I haven't read anything about a new sea wall there, either.