Acoustic-Gravity Waves from Earthquake Sources

This chapter reviews various observations of very low-frequency, and medium- to high-frequency atmospheric pressure waves propagating directly from earthquake sources, and those from coupled with seismic Rayleigh waves and tsunami waves. The observations and theoretical waveform modeling of the first type of waves with frequencies between 1.2 and 8.3 mHz are described in some detail. It is shown that incorporating a realistic thermal structure in the lower atmosphere up to an altitude of 220 km with seismic source parameters, the low-frequency waves observed after the 1964 Alaskan earthquake, and the 2004 Sumatra-Andaman earthquake are acoustic-gravity waves generated by large-scale, coseismic vertical uplift and subsidence of the sea bottom and associated swelling and depression of the sea surface over the extended source regions of these great thrust earthquakes. Comparison between the recorded and synthetic waveforms provides estimates of the amount and its time constants of the coseismic vertical deformation. A recent, comprehensive observation of the third type of infrasonic waves has been made at the time of the 2003 Off-Tokachi earthquake, Japan, with sensitive microbarographs together with co-located broadband seismographs. A close comparison between the two types of the records clearly indicates that the observed pressure changes are excited by the ground motion excited by the passage of seismic Rayleigh waves. From these observations, the seismic-infrasonic pressure transfer function has been made clear from the spectral ratio of the pressure perturbation to the vertical ground velocity as a function of frequency.