Tsunami forecasting in real time

NASA'S GPS-based technology predicts the probability, strength of tidal waves

NASA scientists have demonstrated a prototype tsunami prediction system that can quickly and accurately determine if an earthquake has spawned a tsunami and then estimate the size of the resulting wave.

A research team led by Y. Tony Song of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., used data from the agency’s Global Differential Global Positioning System network to successfully predict the size of the tsunami caused by the magnitude 8.8 earthquake in Chile on Feb. 27. The JPL-managed network combines global and regional data from hundreds of GPS sites and estimates their positions in real time.

Song said the system can detect ground motions as small as a few centimeters. By comparison, commercial GPS only has an accuracy of several meters. “That’s a big difference,” he said.

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During the Feb. 27 earthquake, ground motion was recorded by the network’s stations in Santiago, Chile, about 146 miles from the earthquake’s epicenter. This data was made available to Song’s team within minutes of the event, allowing him to determine the motions on the seafloor.

Based on the GDGPS data, Song calculated the tsunami’s source energy and ranked it as a moderate 4.8 on the system’s 10-point scale. This conclusion was based on ground motion data indicating that the slip of the fault transferred a relatively small amount of kinetic energy to the ocean.

According to NASA, Song’s team concluded that the Chilean earthquake, the fifth largest ever recorded, would generate a moderate, local tsunami unlikely to cause significant effects outside of Chile. Agency officials said the GPS-based prediction was later confirmed by sea surface height measurements from the joint NASA/French Space Agency Jason-1 and Jason-2 altimetry satellites.

Published in 2007, Song’s prediction method estimates the energy an undersea earthquake transfers to the sea, which is what -- when it's strong enough -- creates a tsunami. The method relies on data from coastal GPS stations near an earthquake’s epicenter and information about the local continental slope. The continental slope is a measure of how steeply the ocean floor descends from the edge of the continental shelf to the ocean bottom.

NASA officials said that conventional tsunami warning systems rely on estimates of an earthquake’s location, depth and magnitude to determine if large waves are generated. But previous tsunamis have demonstrated that earthquake magnitude is not a reliable indicator of tsunami size. Older models presume that a tsunami’s power is generated by the amount of vertical displacement of the sea floor. Song’s theory indicates that horizontal motions of a faulting continental slope also contribute to a tsunami’s power by transferring kinetic energy to the sea. “Earthquake magnitude is not a good indicator of tsunami size,” he said.

The Chilean earthquake also served as a test of Song’s method. “We think the technology is almost there,” he said. In the future, upgrades to the U.S. GPS satellite constellation will greatly improve the system’s capabilities, but he added that it is “almost good enough” with the existing satellite infrastructure.

Another key need is to increase the density of the GPS ground receivers. But Song said that the United States does not have the resources or the jurisdiction to monitor the entire global ocean. “We need international collaboration,” to provide the necessary density, he said.

However, cost is not an issue because GPS receivers are relatively inexpensive. Song said that it is easy to place the monitors around the coasts of the world’s oceans. The only challenge is getting nations to join the United States.


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