Abstract
Surface waves emitted after large earthquakes are known to induce atmospheric infrasonic waves detectable at ionospheric heights using a variety of techniques, such as high frequency (HF) Doppler, global positioning system (GPS), and recently over-the-horizon (OTH) radar. The HF Doppler and OTH radar are particularly sensitive to the ionospheric signature of Rayleigh waves and are used here to show ionospheric perturbations consistent with the propagation of Rayleigh waves related to 28 and 10 events, with a magnitude larger than 6.2, detected by HF Doppler and OTH radar respectively. A transfer function is introduced to convert the ionospheric measurement into the correspondent ground displacement in order to compare it with classic seismometers. The ground vertical displacement, measured at the ground by seismometers, and measured at the ionospheric altitude by HF Doppler and OTH radar, is used here to compute surface wave magnitude. The ionospheric surface wave magnitude (Msiono) proposed here introduces a new way to characterize earthquakes observing the signature of surface Rayleigh waves in the ionosphere. This work proves that ionospheric observations are useful seismological data to better cover the Earth and to explore the seismology of the Solar system bodies observing the ionosphere of other planets.
Highlights
This work has been inspired by the picture of Charles F
Charles Richter’s intuition that the maximum amplitude in the far-field, 150 km away from the epicenter, was related to seismic surface waves, pushed Gutenberg & Richter[2] to the forward development of the surface wave magnitude (Ms, eq 1) in order to generalize the magnitude estimation to seismic events measured over the entire Earth at teleseismic distances from the epicenter
The choice of the filtered band is only indicative to highlight that the ionospheric signature of Rayleigh wave is coherent from the Brunt-Väisälä frequency and until 60 mHz
Summary
Event location Costa Rica Mexico Turkey Volcano Islands Iceland Japan Vanautu Island Vanautu Island El Salvador Molucca Passage China Vanuatu Island Loyalty Island Colombia date/time 1999-08-20 10:02:21 1999-09-30 16:31:14 1999-11-12 16:57:21 2000-03-28 11:00:20 2000-06-17 15:40:43 2000-07-30 12:25:47 2000-10-04 16:58:45 2000-01-09 16:49:29 2001-01-13 17:33:31 2001-02-24 07:23:49 2001-11-14 09:26:12 2002-01-02 17:22:50 2004-01-03 16:23:20 2004-11-15 09:06:56. This effect becomes less evident at higher frequency (above 40 mHz), and, in particular for the Doppler sounder, the estimated Msiono matches perfectly the Msseismo from a single seismometer This effect is related to the limit of the atmospheric and ionospheric models that strongly affect the transfer function and could be reduced and better understood using global scale observations with several ionospheric sounding networks. No-relation between the mean discrepancy dM and the reflection altitude (see S7–S12) strongly support the hypothesis of one-to-one coupling between the neutral and plasma This surprising result opens terrific perspective in seismology: first of all, the ionospheric measurement by Doppler sounder and OTH radar could be included in the seismic database as GPS has been included in the last decade[25,26,27,28] performing a better coverage of the planet. Introducing the ionospheric magnitude Msiono we wish to improve the seismic coverage on the Earth and extend the magnitude estimation to the entire Solar system and beyond
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