Acoustic-gravity waves from the source region of the 2011 great Tohoku earthquake (Mw=9.0)

Abstract

Atmospheric pressure waves were recorded within 5 h after the 2011 great Tohoku earthquake (Mw = 9.0) by sensitive microbarographs at four regional stations and eight International Monitoring System stations at distances up to 6700 km. While its apparent phase velocity between the regional stations is 341 m/s, the global stations indicate weak dispersive wave trains with low frequencies between 1.6 and 4.8 mHz, propagating with an average phase velocity around 364 m/s. The low-frequency waves may be interpreted as acoustic-gravity waves excited by upheaval and depression of the sea surface in the source region due to coseismic uplift and subsidence of the sea bottom during this great thrust earthquake. Assuming the source dimension and the average coseismic vertical displacements of the sea surface, with reference to tsunami observations, we calculate synthetic waveforms for some of the far-field stations by incorporating a standard sound velocity structure in the atmosphere up to an altitude of 220 km. The synthetics provide reasonable explanations for the general features of the observed waveforms, suggesting possible ranges for the source parameters generating these acoustic-gravity waves. Our analysis suggests that the average initial upheaval of the sea surface in the central zones of the source region may exceed 4–6 m and that the risetime of the coseismic deformation may be in the range between 3 and 4 min. In the eastern narrow zone adjacent to the Japan Trench, the deformation has significantly higher initial amplitude and shorter risetime.