The subject is once more topical due to an unusual cluster of earthquakes around the world. Phil Plait writes:
We continue to be asked by many people throughout the world if earthquakes are on the increase. Although it may seem that we are having more earthquakes, earthquakes of magnitude 7.0 or greater have remained fairly constant.
A partial explanation may lie in the fact that in the last twenty years, we have definitely had an increase in the number of earthquakes we have been able to locate each year. This is because of the tremendous increase in the number of seismograph stations in the world and the many improvements in global communications. In 1931, there were about 350 stations operating in the world; today, there are more than 8,000 stations and the data now comes in rapidly from these stations by electronic mail, internet and satellite. This increase in the number of stations and the more timely receipt of data has allowed us and other seismological centers to locate earthquakes more rapidly and to locate many small earthquakes which were undetected in earlier years. The NEIC now locates about 20,000 earthquakes each year or approximately 50 per day. Also, because of the improvements in communications and the increased interest in the environment and natural disasters, the public now learns about more earthquakes.
You need to look at the statistics, and not by coincidence the United States Geological Survey provides them. When you look at the chart, you see that there is 1 quake per year somewhere on Earth that’s magnitude 8 or more. There are 15 between 7.0 and 7.9 every year, or on average about one every three weeks. Mag 6? 134 per year, or 2-3 per week. Mag 5: 1300 per year, or about 4 per day.
Right away, you can see that there are going to be decent-sized earthquakes somewhere on Earth all the time. And while on average you get mag 7 quake every few weeks, in reality the distribution is random. Getting two of them within a few days of each other is not only not surprising, statistically speaking it’s expected!
It’s unusual to get a quake centered in Virginia, but it’s not that odd. They’re rare for sure, but there was a bigger one in 1897. Colorado has had its share, too. Every state in the union has quakes; I remember one in Michigan when I was an undergrad at Ann Arbor. So in and of itself, having an earthquake anywhere in the US is not necessarily suspicious. Again, a chart on that USGS page shows that we should expect 50-70 mag 5 quakes a year in the U.S., so having two even on the same day is not all that unusual
Earthquake average frequency
Plait notes that the reason for this perceived increase in earthquakes is simply a function of pattern-seeking humans living on a geologically active planet:
That's it. Perhaps we can bury this "severe earthquakes are becoming more frequent" meme once and for all. There is an interesting coda however. Plait continues:
So what is the deal then?
What the science is telling me is pretty simple: what we have here is simply a restless planet coupled with our all-too-human nature of correlating events if they happen close in time or place. The latter isn’t surprising; it’s an evolutionary advantage to be able to pin an effect to a cause ("Hmmm, that rustle in the trees is probably a tiger. I’d better run."). That ability can be fooled, and get us in trouble as it might in the case of the apparently-clustered earthquakes, but in general it’s better to be able to put the horse before the cart than otherwise.
Plait is a sceptic, so would not endorse any theistic readings of this data. I need to state this up front. What I would say as a theist (as others have done elsewhere) is that earthquakes and the inevitable suffering they cause are a trade-off of having a habitable planet. It is hardly unreasonable to ask a non-believer who asks why God doesn't stop earthquakes to design a planet without plate tectonics that would support complex life.
And having a restless planet is a consequence of having a habitable one. Earthquakes and other tectonic events are a major threat to humans, but they are the trade-off of having a thin crust floating on a magma ocean. We may owe our existence to that fact; volcanoes built up our continents and helped create our atmosphere, and the liquid inner bits of our planet are what generate our magnetic field that protects us from the solar wind. Mars doesn’t have that, and over a few billion years the Sun eroded away that planet’s atmosphere. Continental drift helped drive evolution (separating species and forcing them to adapt to new environments), and hey, here we are.