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First ionospheric images of the seismic fault slip on the example of the Tohoku‐oki earthquake

1Hz GPS measurements from the Japanese GPS network GEONET allowed to retrieve information on the seismic fault of the great M9.0 Tohoku‐oki earthquake from the ionosphere total electron content (TEC) measurements. The first arrival of the TEC perturbation was registered 464 seconds after the earthquake ∼140 km on the east from the epicenter. Within next 45 seconds the distribution of ionospheric points imaged a rectangular area (37.39 ‐ 39.28°N; 142.8 – 143.73°E), which coincides with the area of the coseismic crustal uplift. From this source region, the coseismic ionospheric perturbation further propagated at 1.3‐1.5 km/s. Such velocity values are 30‐40% higher than previously reported for acoustic waves. It is likely that we observed shock‐acoustic waves propagating at supersonic speed and having blown all the electrons available between the ground and the height of detection. This fact is coherent with registration of the first arrival of perturbation 464 sec after the earthquake that is, generally speaking, too short time for a regular acoustic wave to reach the ionosphere. Our findings show that the real‐time GPS monitoring of seismo‐active areas could inform about the parameters of coseismic crustal displacements and can be, subsequently, used for short‐term tsunami warnings. In the case of the 03/11/2011 earthquake, the first ionosphere perturbations were registered ∼17 minutes before the tsunami arrived on the east coast of Honshu.