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This study reassesses the claim of an anomalous geomagnetic diurnal variation two months prior to and northwest of the eventual epicenter of the 2011 Tohoku earthquake, Japan (Mw 9.0). Instead of discussing the ratios of geomagnetic diurnal variation ranges at observation and reference stations, which previous studies have analyzed, the ranges themselves are examined in the present work. For the reassessment, the technique to identify the range of typical variations is improved, leading to more appropriate values of the geomagnetic diurnal variation range than those obtained by previous studies. The variations that were deemed anomalous by previous authors can be explained by coupling of the ranges of three components of the geomagnetic diurnal variation; this coupling effect was not discussed in previous studies. The cause of the high ratio was a combination of low-amplitude quiet-day diurnal solar variations in the geomagnetic field in winter, day-to-day variations, and a weak magnetic disturbance during the period. It is therefore not strictly correct that the observed variations were anomalous and, as such, they cannot be unambiguously related to seismo-magnetic phenomena due to crustal activity prior to the megathrust earthquake. In addition, due to issues with the statistical treatment of samples in previous studies, a more appropriate statistical discussion of the matter is presented.
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