How earthquakes work, and how science makes us safer

(Reuters)

The very powerful earthquake that hit Japan today was even more powerful than everybody first thought. US Geological Survey seismologists upgraded it to a magnitude 9.0—making it the most powerful earthquake to ever hit Japan. It's not unusual for scientists to revise their calculations on the strength of an earthquake, reports New Scientist. In fact, we're likely to see more recalculation on this quake.

To find out more about the science happening behind the scenes of this disaster, I spoke this morning with geophysicist Brian Shiro, who works out of the NOAA Pacific Tsunami Warning Center in Ewa Beach, Hawaii. He gave me some good background information on the science of seismology and plate tectonics, and the changing technology that's making people safer in the face of powerful natural forces.

Maggie Koerth-Baker: My first thought, when I heard about this earthquake, was to wonder whether it had any connection to the recent earthquake in Christchurch, New Zealand. They're both part of the Pacific Rim, could what happens on one fault line affect what happens on another?

Brian Shiro: It's doubtful that there's a connection to the New Zealand earthquake, but the question isn't unreasonable. There's actually a lot that seismologists have been learning about the ways that earthquakes could possibly affect one another. If you'd asked this five years ago, they'd have said, "No way." But now, particularly after studying the Sumatra earthquake and tsunami [from 2004], there's been a lot of people looking into that question. The idea of one earthquake triggering another seems to have some legs to it. It's not a crazy idea. But the jury is still out on how often it happens.

MKB: How is the Pacific Rim set up? In the maps from grade school, it looks like one, continuous gap that circles the Pacific Ocean, but I'm assuming there's more to it than that.

BS: It's a bunch of distinct faults that, taken as a whole, create this so-called Ring of Fire where most earthquakes and volcanoes and other seismic activity originates. I like to think of the Earth like hard boiled egg. If you crack it a little bit, that's like the plates. The plate boundaries are riding on mushy mantle. People also use the baseball analogy, which is maybe even better because of the stitching. You can see the seam on a baseball where it's connected all the way around, but you can also see the stitching. The Pacific Rim is like that. Even though the fault systems are distinct, it forms a continuous band. It's a series of subduction zones that mark areas in the Pacific Ocean where the Pacific plate is sliding underneath another one.

MKB: Why are there so many more earthquakes and volcanoes in this region. Is there something that makes the seismic activity in the Pacific Rim different?

BS: It's a matter of numbers really. The Pacific Rim covers a huge portion of the Earth. Really, there's just that much more opportunity for things to happen. Look at the various seas and oceans. The Mediterranean, for instance, has one subduction zone. The Atlantic has none. The Indian Ocean has two. The Pacific has on the order of 10.

MKB: What is the magnitude rating of an earthquake actually measuring?

BS: Seismologists stopped using Richter Scale 20-30 years ago. That was based on ground displacement. The public still uses the name because it is so embedded in consciousness. Today, we still use a logarithmic scale like the Richter Scale, but now it's based on the amount of energy that the earthquake released. Bigger magnitude means more energy released. And that's really related to the type of rupture in the fault.

MKB: There seems to have been a lot of confusion this morning about how big the tsunami waves would be by the time they hit places like Hawaii and the American West Coast. What factors make that difficult to predict? I think that, to a lot of laypeople, it seems like it must be a fairly simple distance/force physics problem. What complicates it?

BS: You have to separate this out into two questions: Time, and how big it will be. Those are two very different things. Estimating when a tsunami wave will reach a place is a simple thing to do from fundamental physics. You can do it with a pen and paper, and a calculator. The key thing is that you have to know the depth of the water, which we do pretty well throughout the ocean. Then you can predict time with great deal of accuracy. We've been doing that for several decades.

But knowing the size of the wave—or even if the wave will still exist after traveling a certain distance—that's another thing. There are lots of nonlinear effects. It's not trivial. This is serious fluid dynamics. Until the last 5 years, maybe, it wasn't possible to make those predictions in reasonable amount of time. Today, we have the computing power to do it in a matter of seconds. We're advancing because of computer speed.

But there are still conflicting estimations, and those have to do with different computer models asking the same question. There's a lot of variation between models on methodology and assumptions—for example fault orientation and magnitude go into it. It all boils down to conservation of energy, in the end. That's what the numbers are crunching. And that gets particularly difficult when you reach the coast. Out in the open ocean this isn't a big deal. But when you get down to the nitty gritty of a coastline, then you have to take into account a lot of nonlinear effects like reflecting or refracting. That's where small differences in the models can lead to a big difference in the end.

In this case, though, the models were right on for Hawaii, which means that they'll likely be good for California, as well.

MKB: Cory Doctorow, one of our editors at BoingBoing, moved to California not long after the Loma Prieta earthquake and remembers people telling him that nobody really knew, until it happened, how well earthquake-proof building designs would work. Not sure if this is outside your area, but can you tell me a little about how researchers test those designs before nature tests them?

BS: It is outside my area, but I can tell you a little about it. They do computer simulations, they also do shake tables that can shake the models and see what happens. Also, especially in last few years, they've installed instruments in structures like bridges and skyscrapers. Seismometers and accelerometers are both a lot cheaper now. There's an accelerometer in every iPhone. So now they can basically implant them in a building as you build it. That helps you really understand how the earth moves.

One other thing: The shaking that a building experiences really depends on the type of ground it's sitting on. Bedrock won't shake as much as landfill or alluvial soil. That's why sometimes one neighborhood may be OK and in the next neighborhood over everything topples. An earthquake can liquefy soil but it can't liquefy solid rock.

MKB: How well do these designs hold up when the time comes? From the little I've seen this morning, it's sounding like most of the deaths in Japan were tsunami related, rather than earthquake related. Is that a win for engineering? [NOTE: After further research, it's not clear at this time whether the earthquake or the tsunami has been responsible for more deaths than the other. We probably won't know the answer to that for a long time.]

BS: If that's true then that's really interesting. It says a lot about the engineering. Japan has the most sophisticated earthquake and tsunami warning system anywhere. They have hundreds of seismic stations throughout Japan and they're all connected to an earthquake early warning system, like what California is currently developing. When these centers first detect a seismic wave, they can send a signal out ahead of the wave to dams, power plants, and subways—places that need to shut down to avoid damage. Before the earthquake even hits they can be shut down.

[NOTE: This is actually why Japan, a country with 30% nuclear-powered electricity, is only having problems with two of its nuclear power plants today. The early warning system shut down 15 nuclear plants for safety. 11 of those are already back online, operating as normal. The two you've been hearing about today were those nearest the epicenter. —MKB]

MKB: Is it possible to engineer tsunami safety the way we engineer earthquake safety?

BS: There are. In fact, Japan again leads the way on this. They have these huge metal gates that can close off rivers, which are conduits for tsunamis traveling inland. The tsunami early warning system triggers the gates, the gates crash down, and it saves a lot of lives. There are a number of cabled offshore buoys and detectors—that gives people a few minutes notice. There's also a lot of proposals out there, too. I'm not aware of any serious efforts to implement this, but building huge artificial reefs could help because those are one of the natural protections against tsunamis. As are mangrove forests. Putting up those barriers is possible, but it would take a lot of money.

You can read about some other aspects of the science of the Sendai Earthquake on LiveScience.com

Loading...