What can we say about the short-term seismic future for Japan and the region?
When we look at other magnitude 9s, they have many aftershocks. The average magnitude 9 has one magnitude 8. And there was one magnitude 8 that was hidden in the main earthquake data. They will have 10 magnitude 7s, 100 magnitude 6s and so on. That's the typical sequence.
After the Sumatran earthquake there was a magnitude 8.7 earthquake roughly a month or so later, and there have been many other magnitude 8s south of the Sumatran earthquake. In a sense the entire zone kind of lit up for years. So there's no scientific reason to say that this thing is over. Just on average, something like this magnitude 9 would be the biggest and you wouldn't see another one somewhere else. But we just don't know.
What are some of the big questions that still need to be answered about this region?
Does this zone primarily slip in earthquakes, or does it primarily slip in a slower process—either through steady creep or occasionally large, slower earthquake that may occur over a period of days or weeks? And we just don't know the answer to that.
I think many people were assuming since there hadn't been activity over the last 1,000 years that it must be slipping steadily or in slow earthquakes. But now that we've had this large event, it's pretty obvious that that view was wrong.
It also calls into question our approach to protecting ourselves from earthquakes. Often we characterize our earthquake hazard with some kind of probabilistic map. We prioritize how we respond based on these maps. Unfortunately, recently a lot of our most important earthquakes have been in areas that were low-hazard areas. And it to me is an interesting statistics problem. It is like trying to predict where and how large the next war will be, or where and how large the next epidemic will be. Right now what we use is like a heart attack model of risk. Our problem is more like a bird flu kind of problem.
Is there any way that engineers and seismologists can prepare for this sort of event—a massive earthquake and continued large aftershocks?
Really large earthquakes are so rare that there's no way that engineers have been able to say that they know what the ground shaking is like to know how to build for it. This is the first time we're going to have ground recordings from a magnitude 9, so we're all very interested in it.
But, to be honest, I'm a little concerned now that because we'll finally have ground recordings, people will say: "Now we know what a magnitude 9 looks like." But just because you've seen one, you haven't seen them all. Most of the rupture was a long distance from land, so it was far offshore. But in my opinion that distance really spared Japan a lot of damage.
It looks like part of the fault that's been locked for 20 years is closer to land. That part would be much closer to the buildings of Japan—between Sendai south to the Boso Peninsula.
From our perspective, one of the really interesting parts of this sequence was that the Japanese have—just in the last two years—begun operating a real-time alerting system that alerts people when shaking has commenced somewhere and is on its way to them. And the system actually worked in this earthquake. For Sendai, the warning was in the neighborhood of 10 seconds; for Tokyo it was more like a minute.
The idea that there's some new tool out there that would give you a heads-up—and some idea of what's about to happen—really is a new thing.
*Update (4/7/11): The magnitude of this earthquake was downgraded midday by the U.S. Geological Society from 7.4 to 7.1. This paragraph was changed after publication to reflect that change.