How long has the sensor system been around, and how much better have we gotten?
We have been around since 1949, and then for the first several years of existence of the tsunami warning system it was just to warn Hawaii. But then 1960 happened, which was the big Chile earthquakethe 9.5. The tsunami that occurred from that event across the ocean killed people as far away as Japan. So then all the countries of the Pacific countries came together under the framework of the United Nations and formed what is now know as the Pacific Tsunami Warning System.
Throughout most of the history of the tsunami warning system, the warnings were based purely on the earthquake informationour best guess for the magnitude, location and depth. Especially back in the older days of the system, that was about as good as you could do. But then later on we started installing sea level gauges all around the Pacific Ocean. Pretty much every port and harbor in the world has one or two of these. These are normally strapped onto the piers and they measure the height of the ocean. We issue a warning after a big earthquake, and we can predict what the tsunami arrival times should be because tsunami speed is directly controlled by the depth of the water through which it travels. So even decade ago it was easy to say that we expect the tsunami to arrive in this place at this time.
Within the past five years, tsunami forecast modeling has really come to the forefront. So now what we can do is not only predict when a tsunami going to arrive placeswe can predict the height of the tsunami. You have to take in the physics of the water and the equations of motion. There are a lot of assumptions that go into a model like this, but that area is rapidly improving and has active research. As a tsunami gets close to shore a lot of non-linear stuff starts to happen to the waves, which means dispersion and diffraction. That stuff is really hard to model. And if you want to make your model better you want the measurement of the tsunami out in the ocean where it is not affected by that stuff.
So there was this program called The DART Program, which has been around since the late 90s. As soon as [the 2004] tsunami happened and people saw the devastation, Congress authorized money for improving the tsunami warning system. The DART program got a big shot in the arm and ramped up tremendously. They have been installing these deep ocean buoys; there are 39 U.S.-owned ones and there are 10 that are owned by other countries. The buoy has nothing to do with the measurement; the buoy just floats there and has an antenna on it that relays data to a satellite. The actual science happens down on the ocean. There is a package down on the sea floor, which is measuring the pressure of the water above it. So it measures the pressure, and if the pressure grows, that means the height grew. It is always monitoring so it knows the natural variation of the sea level based on the normal wind waves. It can subtract that off. When a tsunami comes, it can trigger itself into this more active mode where it starts sampling more frequently and transmitting more data more frequently, and we get that information here.
Are most of these sensors right near the most dangerous tsunami areas?
You could think of these things being between what you care about and where the tsunami is likely to come from. But it is better to locate them closer to the source so you get information sooner. That's why they [are] kind of parallel to coast lines in the Pacific, in the so-called Ring of Fire, where the subduction zones are.
So you said there was a lot of research happening in the tsunami sector, are there any new systems in development of technology that will be deployed?
There are proposals for using GPS, where you can use little sensors on lots of little buoys that are floating around, thousands of them, cheap ones, with little GPS receivers and you can detect the tsunami as it travels that way. But no one's tried it on a large scale. There are some promising ideas for using radar that look from towers across the horizon. The idea is that you're on a coastline in Seattle or somewhere like that, and look out along your coast and measure the tsunami a hundred miles out, or however far you can see, before it reaches you.
There are ideas for making satellite observations of altimetry. There are a couple of instruments up there now that can do that, like the Jason Instrument on the Topex/Poseidon satellite. On the 2004 tsunami, well it just happened to be in the right place at the right time to measure the tsunami from space using altimetry-that's radar altimetry in that case-which is really cool. But if you're going to rely on something like that operationally, you need a constellation of satellites to cover the world all the time. Research has shown that that sort of thing may only work for really big tsunamis like that, so it might not be a silver bullet.
If [tsunamis] happened all the time it would make sense to put a bunch of satellites in orbit to monitor them. But, if the satellites only purpose was this it would be kind of hard to justify, but if the satellites had other purposes, maybe it wouldn't be.
What determines tsunami direction, and how intense it is going to be in a certain place?
A tsunami is not like dropping a pebble into water and seeing the ripples go out equally in all directions. It's very directional. It is highly dependent on what disturbed the water to begin with, the earthquake in this case. It more or less goes perpendicular with the fault rupture of the earthquake.
So just imagine you are holding a globe and are looking down at the Pacific Ocean. Imagine the tsunami spreading outward across the ocean. The energy is most concentrated where it started because it's this big pulse of energy put into the waterbasically the water gets picked up and dropped, and gravity takes over and pulls in back down. As the tsunami spreads out [to] more and more area, the conservation of energy means that it is spread out over more volume. So even if no energy is lost, you are still going to get less amplitude as it is spreading out.
Say an underwater earthquake (like the one off Japan) happenstake us through what happens from there.
The underwater earthquake happens and the seismic waves spread out. Seismic waves are the fastest way to get information out from the earthquake. They travel through the earth and they reach the first seismic stations, which are almost without exception on land. Seismic data reaches the seismic stations and we get it from all over the world here.
There you kind of sit there and wait for the data because even though the seismic waves are fast it still takes finite time to travel to other places. Once we have enough measurementsyou need at least four measurements to triangulatewe find the earthquake location, depth, and origin time. We can run our magnitude calculators and say, "Ok process the data, seismic data, and give me an estimate on how big the magnitude is." The one that we [use], the gold standard, is a moment magnitude.
We use a software and send a tsunami warning. It has a couple of questions like "Are you sure you want to do this?" Yes. Boom. And it goes. Then we get more refined estimates in location, particularly of the depth. The depth of the earthquake is kind of hard to constrain. But the more data we get, the easier it gets. The depth is really important from tsunami point of view because say you have an 8.5 earthquake right underneath the surface 10 kilometers deep, which is considered very shallow, that is going to make a huge tsunami. But if you make that 8.5 earthquake 100 km deep, it may not make a tsunami at all, or it may just be a really small one.
By this time, 30 minutes after the earthquake, our attention goes to the water. We start looking at the sea level gauges. Sometimes we have to wait a little while for the tsunami to reach those gauges. The tsunami travels fast but not nearly as fast as seismic waves. The speed depends on the depth of the water. [In] the deepest part of the ocean it is going 600 miles per hour. That's fast, but it's still slow enough that it will take maybe 30 minutes [to] one hour to reach gauges because they are just spread apart.
Our policy is that we put out a supplement message at least every hour and if there is new information we put it in there. If we have measurements for gauges it will go in there. If we have estimated a forecast arrival height based on the models, we will out that in there too. The estimated arrivals times are also in there. We update that as time goes on and if we refine the earthquake location even a little bit we might change it in there too.
In Hawaii, [for example], we are on conference with the local and state officials updating them on whatever information we have, and [in the case of the February earthquake that happened in Chile] they made the call rather early on to evacuate. They weren't going to take any chances.
I think it went pretty well [in that case]. It was a great test of the tsunami system from what we do all the way down to mobilizing the people, and that is pretty rare. We don't really do that much in this country. In Japan, they actually do end-to-end tasks where the people evacuate. We don't really do that here.