Seismic Source Characterization by Ionospheric Sounding from GPS Data
Rolland, L.; Lognonné, P.; Kherani, A. E.
Institut de Physique du Globe de Paris

Imaging the terrestrial ionosphere is now possible since the multiplication of measuring points through the installation of numerous GPS receivers. Due to the decreasing earth neutral atmosphere density with altitude, acoustic and gravity waves launched from the ground into the atmosphere are substantially but naturally amplified and then revealed to remote sensing by coupling mechanisms with the ionosphere. Such waves are detectable ponctually in case of large near-surface earthquakes with a current detection threshold at magnitude 6.5 since the 1960s and theoretical works frequency ranges deciphering the origin at ground level of post-seismic waves : in near-field from the pulse at epicentre or in far-field from Rayleigh surface waves or tsunamigenic waves. Most recently, the extraction of Total Electronic Content (TEC) from bifrequency GPS data followed by a proper filtering process of the calculated fluctuations gave the way to a global visualization of the horizontal propagation of coseismic ionospheric disturbances. The Global Postioning System array can be assimilated to a network of space seismometers. Thanks also to the sounding capability of the method above the ocean, the technology is announced as a conclusive complement to the traditional techniques used in seismology.

From now on, after imaging seismic waves in the ionosphere, the challenge is the characterization of the seismic source, where coupling mechanisms between the moving solid earth and its surrounding atmosphere are involved. This will also prove with certainty the seismic origin of waves that can be easily confused to travelling ionospheric disturbances (TIDs) from other origin. The study presented here is based on the TEC variations mapped close to the source and shortly after the Tokachi-Oki earthquake that occured in September 25, 2003 in the northern part of Japan, from data furnished by the japanese GPS Earth Observation Network (GEONET). With more than one thousand continuously receiving stations, installed every 10 km, and a measure every second, this very dense network is capable to observe properly the spatial distribution of strong ionospheric seismic waves. Those are characterized by hectometric wavelengths, and propagate at a horizontal speed of a few kilometers per second.

Such coseismic ionospheric waves are excited by new coupling mechanisms between the top of the neutral atmosphere and the ionosphere, they will be detailed here. The rupture process is modelled in reference to the coseismic displacements retrieved by [Yagi,Y.,2004] concerning the Tokachi-Oki Earthquake. Therefore, a modeling of the horizontal and vertical propagation of acoustic waves generated by three separated sources, and distributed along a single fault, is developed. For physical modeling diverse methods are involved, as normal modes and ray tracing in the atmosphere. The redistribution of charged particules under geomagnetic dependance will be also taken into account. The preliminary results of the subsequent GPS data inversion tests will be presented in the paper.