Tsunami-induced Magnetic Field Records at the Chichijima Island Observation station
Hidee TATEHATA
Abstract
Because seawater conducts electricity, its movement can generate electric fields and currents through its geomagnetic field; these currents induce secondary magnetic fields. The Chichijima geomagnetic observation station (CBI) is located in the Pacific Ocean on Chichijima Island, where the tsunami observation station (Futami tide gauge) is also located. Tsunami and magnetic data can be obtained concurrently on Chichijima Island because the distance between these observation stations is only 1 km. Chichijima Island is therefore suitable as a site for research on tsunami-induced magnetic fields. After investigating about twenty tsunami events in the CBI database and Chichijima Futami tide gauge data from 1995 to 2013, I identified six cases of tsunami-induced phenomena. Of the six events, the signal of one was faint, but the signals of the following five events were clear: the 2011 earthquake and tsunami off the Pacific coast of Tohoku (2011/3/11 M9．0), the Chichijima Island earthquake (2010/12/22 M7．4), the 2010 Chile earthquake (2010/2/27 M8．8), the Kuril Islands tsunami (2007/01/13 M8．2), and the Kuril Islands tsunami (2006/11/15 M7.9). The fact that so many induced magnetic phenomena have been detected at one observation station may attract worldwide attention to the CBI. During periods of low solar activity, the induced magnetic signal can be detected if the amplitude (peak-to-trough distance) of the tsunami is 1 m or more. If a major tsunami with an amplitude of 2 m or more hits Chichijima Island, the induced magnetic fields are definitely detectable. Analysis of the five events with clear signals revealed that the ratio of tsunami amplitude to induced magnetic field amplitude is about 1．0m per nT. The fact that the tsunami-induced perturbation of the magnetic field is detectable 10-20 min before the tsunami waveform arrives is a great advantage from the standpoint of disaster prevention. The magnetic field perturbations induced by tsunamis can therefore serve as a bridge between the sciences of geomagnetism and tsunami disaster prevention.
