Why was it a suprise?
Why was the Mw=9 earthquake in the Tohoku area a surprise?
The issue is that the maximum earthquake scenario is generally unknown for most places, whether they be in California, Sumatra, Japan or Cascadia. This is because historical records are generally too short to include the full range of earthquakes that have occurred. This was the case in Japan, where the Japan trench earthquakes were thought to run in the M=8.4 range based on historical records. Even though those records are long in Japan, they aren’t long enough to guarantee the safety of something critical like a nuclear plant. It turns out there probably was a similar event to the March 11 event in the year 869, and another large one in 1611, but that wasn’t well known when the plants were built, and still we do not know much about them. The same is true for most fault systems of the world. The construction standards are set for a ‘maximum credible earthquake” which in many regions is little more than a guess. That may sound harsh, but these educated guesses are made all the time by expert opinion, and are rarely made on the basis of detailed field investigations, which is what are needed. One solution to this is simple paleoseismology, which we now know how to do to get records 10,000 years long in some cases. This wasn’t known in the 70’s, but it is now and is a very cheap way to directly answer the key questions, and remove much of the guesswork.
In Cascadia, the famous “orphan tsunami” in 1700AD was most likely not launched by the largest earthquake produced by the Cascadia Subduction zone. That earthquake, viewed through the lens of 10,000 years of past events, looked to be pretty average. Two much larger events may have occurred ~ 5800 years and 8800 years ago. So going back in time to the previous few earthquakes is not enough. In Cascadia, more than ten earthquake cycles are needed to establish an MCE, maybe more. Clearly the two largest events in Cascadia are the 11th and 16th events back in time, 5800 and 8800 years ago. These events must be quite a bit larger than the others, and are recorded as “large” at numerous sites, including those with no modern sediment supply at all, so are independent of climate, catchment basin size, recurrence time or other issues.
Are the nuclear plants in California and elsewhere safe from the maximum earthquake that could occur on nearby faults? I don’t think anyone knows. From the geology side, we certainly don’t know what the maximum credible earthquake is beyond an educated guess, and even if we did, just knowing earthquake “size” is only part of the story; how does the ground respond, what are the peak accelerations, would it generate a tsunami or a landslide generated tsunami? What other cascading failures could conceivably occur? Like an airplane crash, it’s not usually a single catastrophic failure, but some small unanticipated failure that leads to a chain of problems that the engineers did not anticipate. In Japan, the containment vessels were not breached by the earthquake or the waves, it was the lack of power for the cooling pumps that let to the problems. It’s not easy to think through all the possible failure cascades and design for them. The loss of the Challenger was a good example of lots of engineers and lots of oversight, but they just forgot that o-rings get brittle when cold, and then there was the pressure to launch on time. Nuke plants and airplanes are enormously complicated systems, and it’s not easy to think through every possible problem, but both are truly “failure is not an option” situations that we just need to admit we’re not that good at, and take a lot more care.