This was a very tiny earthquake, although it was felt. All nuclear plants are supposed to have some seismic detection capability, but nobody says it has to work! In fact, there have been much larger earthquakes beside nuclear plants, and they weren't detected either! You can safely say that 90% of these accelerometer systems don't work.
They are run by people who have no clue what they are for, or what they do, or how important they are. Probably, Diablo Canyon has the only working system in North America.
I was involved in doing this for the old company. I was so frustrated with the internal stupidity, that I finally put in 2 external seismometers now part of the Polaris Network. So, when there is an earthquake, we will have a ground motion recording.