Multiple detection of a "rapid" slow slip event before the March, 2011 Tohoku earthquake from regional and global ionospheric, ground GPS and oceanic DART observations

Seismic and tsunami waves caused by the 11 March 2011 Tohoku mega-quake generated Pacific-wide ionospheric signals from the coupling between ground/sea/atmosphere/ionosphère. High-quality observations of the ionospheric signals have been performed using the dense GPS network located in Japan, and less dense networks in Hawaii and Chile. In Hawaii and possibly in Chili, co-located GPS-airglow measurements of the ionospheric response to the tsunami have been performed for the first time. The Hawaii observations indicate that the first ionospheric waves are observed approximately one hour before the arrival time of the tsunami. Long period ionospheric perturbations on GPS signals close to the epicenter emerge also prior the quake occurence while GPS ground data analyzed in a stacked way reveal a slow and oscillating ground vertical displacement prior the earthquake. Finally, the analysis of several DART measurements indicates a low-frequency signal in the hour preceding the tsunami's arrival, with again a timing corresponding to the same event. With the exception of the Hawaii precursor signal, all the other signals have small amplitude, with low signal to noise ratio. However, the modeling of the propagation delay of atmospheric gravity waves shows that the timing of both the ionospheric signals (in Hawaii and Japan) and of the GPS ground deformation and DART water level coincide, suggesting that all these signals are very likely associated to a common source, associated to a small amplitude and long period vertical displacement near the Tohoku epicenter. Further modeling of the generated gravity wave (in both the ocean and atmosphere) are also done, in order to compare the amplitude of the modeled ionospheric signal with the observation and to confirm the coherency of the amplitude of signals from the ground to the ionosphere. We propose that these low-frequency signals are associated with a Slow Slip Event possibly associated to the initiation of the mega-quake and that the monitoring of atmospheric waves in the ionosphere might open a new temporal window in the observation of tectonic deformation with typical periods of a few hours, between the rapid seismic rupture and the slow slip deformation, classically monitored by seismometers and GPS respectively.