Theoretical & Computational Seismological Mesh Page

Atmosphere - Solid Earth coupling

After strong earthquakes, the ground displacement due to seismic waves induces a pressure wave in the atmosphere. This atmospheric perturbation propagates upward and is remarkably well detected at high altitude (150-300 km) by ionospheric measurements, because of a strong amplification while propagating upwards. Other “solid Earth” events such as volcanic eruptions, nuclear explosions, tsunamis, may produce similar signal, while atmospheric turbulence may be a continuous source of excitation for the free oscillation of the Earth.

In some sense, this is just an extension of seismology to the atmosphere; it is indeed possible to extend techniques such as normal-modes computation and summation to a full system including the Earth and the atmosphere. We are now working on adapting Spectral-Element method to atmospheric waves propagation. This would allow studying the effect of wind, lateral variations of the atmosphere, thermal dissipation and interaction with the ionosphere in a more realistic way.


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Figure 1. Solid Earth atmosphere coupling at teleseismic distances. Propagation of the signal with time, adapted from Calais and Minster [1995]. Data show the vertical displacement in France after the Izmit earthquake recorded on a seismometer (SSB station, Geoscope, France) and by the Francourville Doppler sounder, at an altitude of about 170 km. The refraction due to vertical variations of the sound speed, as well as horizontal winds, shifts by 30–45 km the ray from its surface location.

Figure 2. (Left panel) Domains of existence of acoustic and gravity waves as a function of frequency and angular order. Green dots are the discrete spectrum of solid Earth free spheroidal modes. Seismic wave couple with upward-propagating acoustic waves for frequencies higher than the acoustic cut-off frequency, here ?a = 3.68 mHz. (Right panel) Spheroidal solid normal modes used for the summation. This figure represent energy (v?U, where ? is the density and U the vertical displacement) for fundamental solid spheroidal modes with angular orders l = 2 to 200, as a function of radius. Two regimes are found: when ? < ?a, the atmospheric part of the mode is trapped and decreases exponentially with altitude. At higher frequencies in contrary, the energy propagates upward. Viscous attenuation appears above 100 km of altitude (radius > 6470 km).