https://en.m.wikipedia.org/wiki/Diablo_Canyon_earthquake_vul...
Also note, the linked page states there are four "nearby" faults -- it doesn't state the distance to them, but included is the San Andreas fault, and I'm relatively confident we know it's capable of producing >= 7.5 magnitude earthquakes.
I don't know, nor claim to know, much of anything about how susceptible [or not] this plant is to seismic events -- and I'm generally quite pro-nuclear -- but it's worth noting that there are other seismic concerns here, and potentially some questionable incentives for PG&E who is the one attesting to the capabilities of the area & this specific plant.
According to USGS seismologist, Jeanne L. Hardebeck, the Shoreline Fault has potential to trigger an earthquake of 6.4–6.8 magnitude
Earthquake sizes are usually expressed via Richter scale or Moment Magnitude Scale values, which both attempt to quantify the total energy released by the quake. These scales do NOT quantify the force experienced at any particular location, which depends heavily on distance from the epicenter and local geology. For that kind of quantity, you would use a seismic intensity scale (https://en.m.wikipedia.org/wiki/Seismic_intensity_scales).
So it's wrong to say that a quake "...would not be 8 when it reaches Diablo Canyon" because the value '8' is an MMS or Richter value. Instead, you could say that "...the damage potential would be reduced by the distance to Diablo Canyon."
If they're based on historical data, on the worst known past earthquake/s in the region, then they may be effectively black-swan blind. Eg, they may be failing to take into account that the worst past events always exceeded the prior worst event before it, and therefore prior worst events were not, and are not, a reliable measure for future ones.
Building earthquake-proof nuclear reactors requires, at minimum, taking the worst past earthquake nearby, and assuming the next will be worse yet, and building for that one (or the one after, etc).
Or better yet, just don't build nuclear plants on top of fault lines, regardless of what historical data shows about it.
It doesn't speak well to their competence, but "inspect and maintain 1,000s of miles of line" is different than "inspect and maintain 1,000 items in one place."
Moreover, it sounds suitably silly to be used for such silly nonsense.