Tsunami prediction
Killer waves are born away from the coast. There they are several centimeters high, but they move at the speed of a jet plane. Near the coast, in shallow water, the waves slow down and grow, sometimes reaching a height of tens of meters, and turn into a tsunami that destroys everything in its path and takes away human lives. In 2011, Japan was struck by a magnitude 9 earthquake off the east coast of Honshu, which generated an unprecedented tsunami - a wave more than 40 meters high. Sixteen thousand people became its victims.
Early warning services underestimated the strength of the tremors, so they could not predict the power of the phenomenon. Modern warning systems usually use data on seismic vibrations in order to estimate the parameters of impending waves by their strength. There are also more direct and costly methods, such as deploying a network of buoys with radio transmitters in the ocean.
People's lives literally require more reliable predictions. A new principle for creating warning systems was proposed by scientists from University College London, the Alan Turing Institute and two Japanese universities. Its essence is the use of global positioning satellites (GPS) for forecasting. The work was accomplished by a team led by Dr. Ryuichi Kanai and supervised by Professor Serge Guillas.
The authors analyzed the atmospheric phenomena during the 2011 earthquake and found that the ocean waves generated by the earthquake caused the appearance of sound waves in the atmosphere. They propagated upward, in seven minutes they reached the 300-kilometer height of the ionosphere and caused the mixing of its plasma. This changed the dynamics of the processes of production and recombination of ions in it. The increase in recombination reduced the concentration of free electrons, which means it modified the transmission of radio waves that satellites send through the ionosphere to GPS ground stations. At certain frequencies, radio waves have changed their speed or direction of propagation.
Scientists have proposed a statistical method that allows, by the characteristics of radio waves, to determine the distribution of electron density over the region of tsunami origin. Deviations in satellite signals, even if they are unevenly distributed in space, can be reliably processed in ten minutes. The method does not require spatially uniform measurements. Data on the distribution of electrons and the laws of plasma physics make it possible to extrapolate the results of measurements and estimate electron concentrations over regions where no measurements have been made.
Geophysicists have shown that in 2011 the method could give an accurate warning ten minutes before the arrival of waves to the coast of Japan. This would save many lives. The method is universal, it can be used to warn of a tsunami caused not only by earthquakes, but also by underwater landslides or volcanic eruptions.
For the operation of the Japanese GPS warning system, there is no need for information from all 1200 ground stations in the country, which receive and analyze the waves of the international satellite constellation, distorted by the atmosphere. Only data from six dozen stations is enough. This feature will allow cost-effective deployment of early warning systems even in poor countries, such as the Pacific island nations.
Now scientists are working on increasing the spatial detail and amplitude accuracy of the method. If, as it happens, the tsunami comes very quickly, the method is still useful for predicting the second and third strikes of the elements and determining the end time of the tsunami.
Image source: pixabay.com
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