Because of the finite density of the atmosphere, a tsunami wave does not stop at the surface of the sea, but induces a displacement of the atmosphere, in the form of a gravitational wave accompanying the tsunami during its propagation. The volumetric energy density of this upward continuation of the tsunami decreases with height, but because the atmosphere rarefies even faster, the amplitude of the resulting vibration will actually increase with height. A tsunami wave of amplitude 10 cm at the surface of the ocean will reach 1 km at the base of the ionosphere at an altitude of 150 km. This fascinating proposition was initially suggested by Peltier and Hines (1976) and confirmed by Artru et al. (2005) during the 2001 Peruvian tsunami. The detection methodology uses dense arrays of GPS receivers, because large-scale fluctuations of the ionosphere affect the propagation of the electromagnetic waves from the GPS satellites, thus distorting the signals recorded at the receivers. Occhipinti et al. (2006) have successfully modeled such records quantitatively and have shown that other space-based techniques involving reflection at the bottom of the ionosphere (e.g., over-the-horizon radar) could be useful for remote detection of a tsunami on the high seas without the need to instrument the ocean basin itself. The speed of propagation of the atmospheric gravity wave, however, is very low and presents an even greater complication than that described above for acoustic propagation in the ocean’s SOFAR channel.
Conclusion: Novel and potentially useful approaches to the estimation of earthquake magnitude and tsunami detection are emerging. Some of these approaches could become operational in the not-too-distant future with proper support for research and testing.