Since AFAIK they were designed to withstand a 8.2 earthquake, they pretty much exceeded expectation. And we can reasonably count on the standards being upgraded significantly after this.
I like a lot the comparison of nuclear energy with airplane safety: when you look at the statistics, it's a lot safer. But when it fails, it makes the headlines.
The richter scale is pretty meaningless in that regard because it measures the total energy released by a quake, not the destructive power at a given point. To use it for a design spec, you'd have to specify the depth of the quake, the length and frequency of the shockwaves and, most importantly, the distance from the epicenter.
I'd be very interested to learn what those specs actually said. If it really was a Richter scale value, it would logically have to be at point blank, which this quake wasn't. If so, the energy that hit the reactors may well have been considerably less than what they were designed for,
I like a lot the comparison of nuclear energy with airplane safety: when you look at the statistics, it's a lot safer. But when it fails, it makes the headlines.