LAUSR

We investigated observational data from 20 geomagnetic observatories at longitudes of 130°E to 150°E between 2009 and 2012 in order to detect possible anomalous changes in the geomagnetic solar quiet daily (Sq) variation related to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) that occurred on March 11, 2011, in Japan. We selected the five quietest days in every month and analyzed three-component magnetic data with 1-min resolution using the natural orthogonal component method to identify Sq variations from observations, and performed spherical harmonic analysis (SHA) to separate the internal and the external equivalent Sq current systems of J e and J i. We found that J e and J i were distorted near the epicenter on two geomagnetic quiet days, January 23 and 27, 2011. This was about 2 months before the earthquake. We then employed backward computation on the external and internal parts of the SHA model geomagnetic fields, B e and B i, to determine the possible anomalous magnetic field variations at each station. The results showed significant differences in daily variation for all components on January 23 and 27, 2011. On the other days, we could not recognize any apparent anomalous behaviors apart from known secular, seasonal, and solar activity dependence. A possible connection between this anomaly and the very strong Tohoku earthquake, which followed only 1.5 months after the geomagnetic anomaly, is of extreme interest for research on earthquake precursors.