A couple of things to keep in mind...
One, if the NRC thinks for an instant that a power plant is trying to hide something, they will “rip them a new one” through inspections and/or fines. Therefore, plants will generally report things that may not be a problem, but may lead to a problem or not reporting the event may be percieved as “hiding a problem”.
So, when the Emergency Guidelines for the plant say something like, “An earthquake felt in plant with significant magnitude for SUSPECTED structural or equipment damage, then issue an Alert” you can bet they will issue an Alert, EVEN if there is no immediate signs of structual or equipment damage. (BTW, the difference between calling an Unusual event and an Alert is the “suspected damage aspect”.)
The NRC likes to see plants operating with that kind of mindset. The plants that issued their alerts or unusual events have procedures and guidelines to check that everything is good and to get back on line.
As a side note, this is more notification then what you get from other large industrial plants which could pose a risk to its workers and surrounding communities. When the coal plant next door has an over pressure condition in one of it’s boilers, you never hear about it, but in a Nuclear power plant you will.
The alert was issued due to loss of external power. That was a fairly obvious problem.