The tsunami of March 11, 2011 off Japan lifted this boat far inland (Image: Stephen Vaughan / CC-by-sa 3.0)
When it comes to the warning of an imminent tsunami, everything has to go very fast. For a seaquake near the coast often only minutes remain, maybe a maximum of half an hour, until the tsunami wave arrives. Previous early warning systems determine the danger by means of seismic measurements. However, they often underestimate the magnitude of earthquakes and tsunami - protective measures may therefore be inadequate. Researchers at the German Research Center for Geosciences Potsdam (GFZ) have now developed and tested a more precise method: they use GPS data. In the case of the Tohoku earthquake off Japan in March 2011, this method would have indicated after three minutes the exact quake strength and the height of the tsunami - the Japanese system reacted at that time similarly fast, underestimated the energy of the earthquake but almost 30 times. The Tohoku earthquake on March 11, 2011 was the heaviest ever registered in Japan and the fourth strongest in the world, with a magnitude of 9.0. Over a distance of 400 kilometers, the seam between two tectonic plates broke along the Japangrabens, while the seabed was jerked up in the air. The water above the fault absorbed this energy, forming a wave front that spread rapidly in all directions. When this tsunami reached the coast of Japan about 200 kilometers away, the wave had a height of some 40 meters and penetrated in some places up to five kilometers into the country. More than 15, 000 people died, large parts of the northeastern coast were devastated, and the Fukushima Daiichi nuclear power plant suffered catastrophic cooling system failures.

Danger dramatically underestimated

"The current tsunami early warning systems should issue a warning within the first five to ten minutes after a quake", explain Andreas Hoechner of the GFZ Potsdam and his colleagues. In the case of the Tohoku earthquake, Japan's meteorological authorities issued the first warning message for three prefectures after just three minutes. The experts, however, initially only from a quake of magnitude 7, 9. Only after several hours the actual strength was determined. "In the period of the highest tsunami activity, the risk was therefore dramatically underestimated, the actual seismic moment was almost 30 times higher than reported at the beginning, " the researchers said.

The reason for this inaccuracy lies not in the inability of the experts, but simply in the limits of the previously common system. Here, the quake level and the location of the epicenter are derived from data from seismic networks. However, the consideration of a tsunami origin as a simple point source is usually insufficient to produce an exact tsunami warning, especially at close range, explain Hoechner and his colleagues. Especially in quakes on subduction zones near the coast, one has to consider in which direction and how far a break extends. The resulting deformation of the seabed ultimately determines whether and how a tsunami develops. display

But there is now a better way, as the researchers now prove in their study. "Japan has a very dense network of GPS stations, " says Hoechner. These receivers use the satellite signals from the Global Positioning System to register every movement of the ground to the nearest millimeter. Placed in the vicinity of possible earthquake foci - for example on the seabed - these measuring stations can show in real time how strong and where the subsoil rises during a quake. This, in turn, allows you to use models to calculate the altitude and expected arrival time of a tsunami. So far, however, the approximately 1, 200 GPS receivers of the Japanese GEONET array were not used for tsunami early warning - and similar GPS networks elsewhere. "To our knowledge, there is not a single example of a GPS-based tsunami early warning, " the scientists report.

Accurate warning after only three minutes

To test how powerful such a system would be, Hoechner and his colleagues tested it in the 2011 Tohoku earthquake. For their study, they used only the GPS data registered during the earthquake from the GEONET array and simulated the warning scenario virtually in real time. Out of a total of 1, 200 stations, they chose 50, in another approach, only 20. To calculate wave heights and arrival times of the tsunami, the researchers fed a common tsunami model with the GPS data for the displacement of the seabed.

The result: Just one minute after the quake began, the system provided the information that the magnitude was at least 8.4 and that the seabed had risen six meters. "Thirty seconds later, the displayed magnitude was already at 8.7 and for much of the Japanese coast tsunami heights of more than ten meters were predicted, " the researchers report. After three minutes, the model had already determined the final quake level of 9.0 and predicted a wave height of the tsunami of up to 29 meters. Japan's early warning system, based on traditional methods, was more than one megabyte away and also expected much lower wave heights.

"This shows how fast and precise a GPS-based early warning system would have been in the event of the Tohoku earthquake, " emphasize Hoechner and his colleagues. It is therefore immensely important that the existing GPS stations are integrated into the existing tsunami warning systems. In tsunami-proven Japan, the density of the GPS network is already tight enough to allow for an accurate early warning. As part of the Indonesian-Indonesian tsunami early warning system, the researchers have already installed numerous new GPS receivers in the Indonesian tsunami early warning system since 2004, and have installed data for the local system there deliver. "However, the station density is not high enough to form a GPS-based system, but we are in the process of adding more GPS receivers, " says Hoechner.

Andreas Hoechner (GeoForschungszentrum Potsdam (GFZ)) et al., Natural Hazards and Earth System Sciences - === Nadja Podbregar

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