Tag Archives: tsunami warning

Earthquake acoustics can indicate if a massive tsunami is imminent

As bad as earthquakes can be, and we’ve recently had our fair share of earthquakes around the world, the tsunamis they generate can be even worse.

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When an earthquake has a significant effect in a body of water, it displaces large quantities of water – and it is that displacement which causes the huge waves which we call tsunamis. They are fundamentally different than usual waves; rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason they are often referred to as tidal waves.

But computer simulations conducted by geophysicists from Stanford revealed that sound waves in the ocean produced by the earthquake reach land tens of minutes before the tsunami, and if correctly interpreted, they can offer a tsunami warning.

Although we are very good at detecting all earthquakes that take places on our planet, including those that take place underwater, tsunami warnings are still rather lackluster. It takes a lot of time after the earthquake before they can tell if a tsunami will be created, and information about its size is very hard to estimate.

Discovering the signal

Interestingly enough, finding the signal came as somewhat of a surprise. They were modelling the signal from an epicenter in the Japan Trench, a subduction zone about 40 miles east of Tohoku, the northeastern region of Japan’s larger island.

japan trench

They used both known geologic and tectonic features of the trench, and used the cluster of supercomputers at Stanford’s Center for Computational Earth and Environmental Science (CEES) to simulate how the tremors moved through the crust and ocean. They applied the model to some documented tsunamis, and it was retroactively able to predict the seafloor uplift which is directly connected to tsunami height.

“We’ve found that there’s a strong correlation between the amplitude of the sound waves and the tsunami wave heights,” Dunham said. “Sound waves propagate through water 10 times faster than the tsunami waves, so we can have knowledge of what’s happening a hundred miles offshore within minutes of an earthquake occurring. We could know whether a tsunami is coming, how large it will be and when it will arrive.”

Even though they created the model only for the Japan Trench, they believe they can apply it to tsunami-generating areas throughout the world, though the parameters of the signal are greatly dependent on the geology of the zone.

“The ideal situation would be to analyze lots of measurements from major events and eventually be able to say, ‘this is the signal’,” said Kozdon, who is now an assistant professor of applied mathematics at the Naval Postgraduate School. “Fortunately, these catastrophic earthquakes don’t happen frequently, but we can input these site specific characteristics into computer models – such as those made possible with the CEES cluster – in the hopes of identifying acoustic signatures that indicates whether or not an earthquake has generated a large tsunami.”

Geophysicists also pointed out that identifying the tsunami signal doesn’t profie a complete warning system – underwater sound detectors called hydrophones (as opposed to those used on land which are called geophones) would need to be deployed on the seafloor or on floating buoys to detect the signal, which then has to be analized – both of which can be pretty costly. Then there’s also the delicate matter of the degree of certainty which has to be reached before alarming the people.

Via Stanford

Small hilltop city becomes refuge for earthquake and tsunami survivors

In the aftermath of the earthquake that violently struck Japan and the tsunamis it generated, the small, industrial city of Hitachi emerged unscathed from what Prime Minister Naoto Kan has described as Japan’s “worst crisis since the Second World War”, making it somewhat of a refuge for the hordes of refugees that flooded from many parts of the country.

How the city managed to escape unharmed from the seismic disaster is credited to its topography – basically the city was built on some a higher ground. The Friday temblor, which ravaged the entire country and created several aftershocks and tsunamis is estimated to have killed at least 10.000 people, and will probably cause additional deaths. In Hitachi, however, things are very different.

“We were so lucky really. I haven’t heard of any serious injuries amongst people I know,” says a security guard keeping watch outside the Hitachi Futo shipping company’s lot in the city’s harbor.

As aftershocks and tsunamis continue to rattle the area, more and more people continue to move to this lucky town – several thousands of refugees are now literally at the gates of the city, waiting and hoping for shelter, and a steady roof above their heads.

Israel, in danger of being hit by tsunamis

tsunami3Dr. Beverly Goodman of the Leon H. Charney School of Marine Sciences at the University of Haifa was doing some research on the ancient port and shipwrecks of the place, when she stumbled upon information that led her to this conclusion.

“There is a likely chance of tsunami waves reaching the shores of Israel. Tsunami events in the Mediterranean do occur less frequently than in the Pacific Ocean, but our findings reveal a moderate rate of recurrence.”

“We expected to find the remains of ships, but were surprised to reveal unusual geological layers the likes of which we had never seen in the region before. We began underwater drilling assuming that these are simply local layers related to the construction of the port. However, we discovered that they are spread along the entire area and realized that we had found something major”

What they did is they drilled at various depths and proceeded to date the layers they found, using two methods: everybody’s friend carbon-14 dating and OSL (optically stimulated luminescence). They found evidence of tsunamis in 1500 BC, 100-200 CE, 500-600 CE, and 1100-1200 CE. There is still much we still have to learn about tsunamis, especially as more and more areas seem vulnerable to their threat. However, this exact need may be what gives researchers the push they need to find out methods of protection.

Innovative method improves tsunami warning systems

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In case you don’t know, a tsunami is a series of waves created when a body of water, such as an ocean, is rapidly displaced. Earthquakes, mass movements above or below water, volcanic eruptions and other underwater explosions, landslides, underwater earthquakes, large meteoroid or asteroid impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami. A tsunami can be unobservable, but it can just as well be devastating. Recent disasters showed how unprepared we were for them, so scientists have been trying to do something to improve the warning systems.

Studies conducted by NASA on tsunamis revealed an innovative method which could achieve that goal, and a potentially groundbreaking theory on the source of the 2004 tsunami in the Indian Ocean. In one study, published last fall in Geophysical Research Letters, researcher Y. Tony Song of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., demonstrated that real-time data from NASA’s network of global positioning system (GPS) stations can detect ground motions preceding tsunamis and reliably estimate a tsunami’s destructive potential within minutes, well before it reaches coastal areas. Conventional methods rely on estimates of an earthquake’s magnitude to determine whether a large tsunami will be generated but earthquake magnitude is not always a good thing to rely on.

This method estimates the energy that an undersea earthquake transfers to the ocean and generates data from coastal GPS stations near the epicenter. With the data, ocean floor displacements caused by the earthquake can be inferred.

“Tsunamis can travel as fast as jet planes, so rapid assessment following quakes is vital to mitigate their hazard,” said Ichiro Fukumori, a JPL oceanographer not involved in the study. “Song and his colleagues have demonstrated that GPS technology can help improve both the speed and accuracy of such analyses.”

To test his method, Song examined three historical tsunamis using well documented data: Alaska in 1964; the Indian Ocean in 2004; and Nias Island, Indonesia in 2005. He was able to replicate all three. Co-author C.K. Shum of Ohio State University said the study suggests horizontal faulting motions play a much more important role in tsunami generation than previously believed.

“If this is found to be true for other tsunamis, we may have to revise some early views on how tsunamis are formed and where mega tsunamis are likely to happen in the future,” he said.

Hopefully this method is going to do better and disasters like the one in 2004 will be avoided [edit: it didn’t work out so fine, in 2012, a massive earthquake hit Japan, generating a massive tsunami that caused significant damage].