The 2011 Tohoku Tsunami observed by an array of ocean bottom electro-magnetometers

Abstract:

Previous studies have claimed that EM sensors can be used as a type of tsunami sensor. In fact, recent studies have reported the observation of EM field variations possibly caused by devastating tsunamis including the 2011 Tohoku Tsunami. During the period from June 2010 to November 2011, we deployed a small array consisting of 4 ocean bottom electro-magnetometers (OBEMs) in the northwestern Pacific basin, about 1,000 km to the east from the epicenter, to explore the mantle structure. This array successfully recorded EM signals associated with the 2011 Tohoku Tsunami.
We found that the observed EM signal propagated in the array as a wave with a phase velocity approximately equal to that of an oceanic long wave. Also, we estimated the motional impedance and tipper corresponding to the propagation direction at each site from the major components of observed EM signals and found that they were approximately equal to the phase velocities of the tsunami propagation as suggested by a simple theory. Besides, ocean bottom seismograms and tilt data show that observed EM variations were not produced by instrumental shaking. These results gave us a clear proof that observed EM signals were generated by the motional induction of the 2011 Tohoku Tsunami and therefore can be used to estimate tsunami parameters such as wave height, arrival time, and propagation direction.
Then, we estimated the tsunami propagation direction from observed tsunami-induced EM fields at four seafloor sites through several methods (both single station and array analyses). Resulting frequency dependence of the propagation direction was found consistent with a source model of the 2011 Tohoku earthquake and tsunami. We also estimated the tsunami height from EM data recorded at each site, and obtained the tsunami amplitude in the array to be approximately 1 m. The same set of parameters (propagation direction and wave height) was calculated from EM signals simulated by using a tsunami flow model with a linear long-wave approximation. Resulting parameters showed a reasonable agreement with those of the tsunami simulation, which indicates the accuracy of an OBEM or OBEM array used for tsunami detection.