system
Given the following post, respond as if you're commenting on Reddit:
human
Can a seismically active area be "overdue" for an earthquake?

I live in the beautiful pacific northwest. Growing up we had small to moderate sized earthquakes at least once a year, but the last one I experienced was a largish one in 2004. Some people say we are due for "the big one." Is that how it works?
gpt
There isn't really a TL;DR answer to this question as this is a fundamental question in the study of active faults and earthquakes. As best we understand it, there is some fundamental and physically based limit to the strength of any particular fault. Once a sufficient amount of strain has built up, this fault will then fail and there will be an earthquake.

To think about this more, lets consider a totally idealized case. Here are some of the main components to this idealized scenario: 1) There is one fault in our system, 2) This fault is completely homogeneous along its length so it's perfectly straight with no variation in orientation and has the same exact rock types on either side and down the length of the fault plane, 3) The loading rate (the motion of the two blocks on either side of the fault with respect to each other) does not change with time or along the length of the fault, 4) The fault has a set amount of strain it can accommodate before it fails in an earthquake, 5) When the fault fails, all of the strain that was accommodated is released in the earthquake, 6) The fault length and properties (how much strain it can accommodate, etc) do not change with time or after earthquakes. Now, in this idealized scenario, we would expect a predictable cycle of earthquakes to happen on this fault, so in this idealized place, we could unequivocally say when a "big one" will occur, presuming we know exactly how long it was since the last earthquake occurred.

As you might have guessed, almost all of those assumptions are violated in real systems. On human timescales, the far field loading rates likely are not changing, but there almost always multiple faults within a system which may accommodate different amounts of strain at different times. Additionally, when you have an earthquake on one fault, it may add or decrease stress/strain on another fault. Faults almost always contain complexities, gaps, different rock types, changes in orientation with respect to the movement of the sides of the fault, etc so that determining how much strain various parts of a fault can accommodate before it fails is challenging, to say the least. In many areas, we also don't have particularly long records of earthquakes, so coming up with chronologies for faults is hugely important to be able to make reasonable assumptions about recurrence intervals.

All that being said, it does seem that some large faults do have relatively (emphasis on relatively) repeatable earthquake scenarios. Really though, what is repeatable is how much energy a fault or fault system needs to dissipate through earthquakes. The San Andreas is a perfect example. On the basis of histories of earthquakes going back to ~1600 in the San Francisco Bay area, this section of the fault system can accommodate ~4 meters of displacement before it has to begin dissipating this via earthquakes. The weird thing about the SFBA segment of the San Andreas system is that it seems to do this in one of two ways. Way one is through a "cluster" of relatively large magnitude (6.5-7.5) earthquakes, where almost all of the faults in the system fail over the course of a few decades like what seems to have happened between ~1690-1760. Way two is one, giant earthquake that ruptures only one or two faults, but does so over a huge area, i.e. the 1906 earthquake. Behavior like this is why it's fundamentally hard to say whether a "big one" is eminent. For the SFBA, we know that the region has accommodated about ~3.5 meters of strain so we're getting close to what appears to be the threshold, but we can't say with absolute certainty how that strain will be released (decades of big earthquakes or one giant earthquake) or when it will be released. 

There are similar complications in the records for the Cascadia subduction zone, the main seismic source for the pacific northwest. There is some evidence that there is a periodic aspect to the failure of the subduction zone, but determining where we are in the seismic cycle can be extremely difficult.

EDIT: Not that this verbose answer needed more words, but the Cascadia subduction zone is also problematic because in terms of thinking about earthquakes because it seems to experience a lot of what we call "slow-slip events", "episodic tremor and slip" or "episodic creep", basically where measurable amounts of strain is released in things that kind of look like earthquakes, but last for extended periods of time (hours-weeks) thus don't generate seismic waves that people can feel. How these fit into the total slip budget and then determining how that relates to where we are in the earthquake cycle add an extra layer of complication.