Readers ask: What is the probability of a magnitude-7 or higher aftershock in Japan? Could the Japan quake lead to other quakes across the globe? Are we having more earthquakes than before? Why are we seeing such a dramatic increase in seismic activity globally?
Science answers: From past quakes, an estimate of the likely largest aftershock can be made. A magnitude 7—far, far smaller than a 9—sounds reasonably likely. In recent years, seismologists have realized that the biggest quakes can trigger more quakes thousands of kilometers away, but these are typically small and often located at hot springs or volcanic areas already prone to small quakes. Global seismicity has not been going up in the long term; high-profile events like Haiti and Japan make it look as if it is. Quakes can trigger eruptions from nearby volcanoes; nothing from this one so far.
Globally, seismic energy has been released over the years at a fairly constant rate, with the inevitable random fluctuations. Part of the reason we have a sense of greater seismic activity is the notoriety of recent quakes. Haiti was not a big deal seismically speaking, but because of its location, it killed hundreds of thousands and captured world attention. There is a record of quakes in historical and geologic times preserved in sediments on faults or deposited by tsunamis, but it is not complete enough to address global seismicity trends.