What if an earthquake isn't as big as the predicted BIG ONE?
Here's the scenario. An earthquake strikes your area. It's a big quake, maybe a 5.8 to 6.2. It causes some damage and injuries. But, all of the predictions of the overdue earthquake are for a magnitude 7.0 to 7.4 rupture.
In this scenario you'll likely hear predictions that the quake is a foreshock and the BIG ONE is imminent. And you'll hear people claim that the fault released pressure so the BIG ONE isn't likely to strike very soon.
These are the two general theories of thought about how "big" earthquakes--that aren't as big as the BIG ONE that is expected (or usually seen as "over due" for the fault)--affect the potential for the BIG ONE to strike.
First, some people believe that an earthquake of a higher magnitude will release stress on the fault and delay the occurrence of a larger earthquake. So, if a 6 happens instead of the 7, they believe the "Big One" will happen later, rather than sooner, because some stress on the fault was released from the magnitude 6 quake. This seems to be the more accepted and prevailing thought among geologists and seismologists.
The second theory isn't as hopeful. In this theory it's claimed that an earthquake of lesser magnitude than the expected BIG ONE can actually increase strain on the fault, or nearby faults, and may even lead to a bigger than expected earthquake. For these people, the Big One has an increased chance of happening sooner rather than later.
A Simple Weight-Pressure Model
Here's a simplistic comparison. Think of balancing a large weight on multiple points of contact, like little pillars that hold the weight up. Several pillars are actually holding the weight and there are more that are lower down and hold very little or none of the weight. The weight's bottom is uneven so it exerts different pressure on different contact points.
When one point breaks away it increases the stress on the other points. When enough points break at the same time a large rupture occurs and the weight settles down onto lower points of contact.
In our overly simplistic model, let's say that the 7.0 rupture expected would break 10 points of contact, leaving the weight to settle more evenly on lower points of contact. However, the 6.0 rupture only breaks 7 contact points, which leaves more pressure on the 3 remaining points that would have needed to break for the rupture to be a 7.0 magnitude.
In the first line of thinking, the next rupture would be smaller because the weight doesn't have as far to fall before it finds a settling point on other contact points. In other words, the 6.0 rupture released some stress.
However the second theory is a little more dire. With the 6.0 rupture being smaller than expected, the result is a lot of pressure on 3 points and when those break the weight may fall further and break through more of the lower contact points and cause a larger rupture than predicted.
Since it’s all theory, that is we can’t actually go down into the earth and observe or test our theories, the reality is probably “it depends.” The real problem is thinking all faults fit into one or the other theory. People have this mistaken idea that things need to be one way or the other.
There are probably earthquake zones that fall onto one or even both sides of thought. Certainly there seems to be historic evidence that some quakes appeared to release pressure and delayed a big one. But there are others where a large quake strikes and (relatively) soon after a larger quake strikes. The problem is we don’t know which faults or seismic zones are which.
Personally, I think for some faults a significant quake will release pressure, but in other fault locations it may increase the stresses at the fault. This doesn’t mean a larger than normal earthquake should be expected anytime, it just means we don’t know.
The safest option is to always be prepared for the larger earthquake, even if the fault just experienced a big quake.
The Dangerous Viewpoint
One of the more dangerous scenarios is in the months and years after a large earthquake strikes. People in the affected area may become more casual about earthquake preparedness, believing another Big One won’t happen in their lifetime. They may think they survived the Big One and they don’t need to worry about another.
And, they may be right.
Or they may be deadly wrong.
In my post The Longest Earthquake Observed I mention how some seismologists believe the 7.3 Sumatra earthquake that happened in 2002 may have been a foreshock to the 2004 Indian Ocean earthquake that had a magnitude 9.1-9.3.
I'm not saying to expect that large of an earthquake, rather I'm just pointing out that it doesn't have to be very long between big earthquakes.