Geomagnetic field changes in response to the 2011 off the Pacific Coast of Tohoku Earthquake and Tsunami

Does the geomagnetic field change in association with Earthquakes? This question was first raised more than 100 years ago, and since that time, theoretical and observational researches have been conducted in order to obtain an answer to this question. Large earthquakes provide an opportunity to examine this problem, because large signals are generally expected in association with large earthquakes. We herein present a brief report on simultaneous measurements of the geomagnetic field in association with the 2011 Tohoku Earthquake (M9.0) and Tsunami by magnetometers at 14 stations operating in Japan. Since the earthquake occurred during a geomagnetic storm, we used time series after correcting the effect of external disturbances and induced fields. A coseismic change of the geomagnetic field was observed at several stations located relatively close to the epicenter. The magnitude of this coseismic change was only about 0.8 nT at one station and much smaller at other stations, which prevented us from quantitatively comparing these observations with predictions based on piezomagnetic theory. More distinct and rapid changes were observed during the hours following the main shock. These changes are supposed to be caused either by motional induction of the tsunami or by ionospheric disturbance. Contrary to the significant observations of these coseismic and postseismic geomagnetic changes in response to the Tohoku Earthquake and Tsunami, we did not observe any clear precursors.