Originally Posted By: Pete
This expectation is not based upon unreliable "earthquake prediction" methods. It is actually based on considerable experience with aftershocks of major quakes.

Well, these estimates are based on historical averages, so all they can do is give you an idea of what "typically" happens. It's a rule of thumb, like the Rule of 3's is a survival rule of thumb, that a 9.0 earthquake will typical be followed by one 8.0, ten 7.0's, a hundred 6.0's, etc. The biggest aftershock they have had so far is a 7.9 that occurred shortly after the main quake, and a 7.1 (originally reported as 7.4) earlier in April.

I do worry about Tokyo, however. The earthquake did shift stress onto the fault system close to Tokyo but no one has any idea if that has substantially increased the risk of a megaquake there. Geologists have never found any solid evidence that major quakes can trigger other major quakes--at least on our time scale. Actually, a variation of this understanding is why the Japan quake was so surprising--it caused the rupture of a series of areas of built up stress along the fault, which seismologists say does not normally happen. Normally only an isolated area of stress is released in an earthquake.

Therefore, the observation that stress was transferred southward towards Tokyo doesn't mean that the risk for Tokyo of a megaquake has significantly increased above what it already was. Let's hope that's true. They already have enough to worry about.