A better anticipation of tsunamis thanks to GPS satellites

When an earthquake occurs, the disturbances produced by the rupture of the earth’s crust propagate in all directions, including through the atmosphere to its highest layers, including the ionosphere. If this earthquake is underwater, the movement of the rupture of the ocean floor can trigger a tsunami. The largest tsunamis are generated by submarine ruptures in subduction zones. However, it has been observed that a rupture in the most superficial part of the subduction zone often produces a tsunami that is much larger than expected for the magnitude of the earthquake.
Described for the first time in 1972, these tsunami earthquakes (or “tsunami-earthquakes”) are earthquakes that generate a tsunami that far exceeds the estimate calculated using seismometer data. This type of tsunami is often underestimated by current tsunami warning systems, even though they are among the most deadly.

In this study, Fabio Manta and Giovanni Occhipinti, from the IPGP, and their colleagues from EOS, introduce a new technique capable of transforming the oscillations of the ionised layers of the upper atmosphere, detected by GPS satellite, into an estimate of the volume of water displaced during the genesis of a tsunami. In particular, they are studying two earthquakes of magnitude Mw 7.8 (one a tsunami earthquake and the other not) that occurred along the Sunda fault, off the coast of Sumatra. These two earthquakes were virtually identical and were estimated by conventional warning systems to pose no risk of tsunami, even though one of them generated a deadly tsunami that killed 400 people.
By applying their method to the data from these two earthquakes, the scientists have demonstrated for the first time that ionospheric observations from satellite positioning systems (or GNSS) can be used to assess the tsunamigenic potential of earthquakes as early as 8 minutes after the main tremor. They thus open up a new avenue for tsunami risk warning and estimation systems, exploiting the high density of GPS sensors already orbiting our planet.

This study is part of the IPGP’s work, supported by CNES, to explore new ways of probing the Earth’s atmosphere and increasing the number of observables capable of improving tsunami warning.