Further investigations of geomagnetic diurnal variations associated with the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0)

As one of the most promising candidates for short-term earthquake forecasting, the seismo-electromagnetic phenomena have been intensively studied for several decades. Recently, Xu et al. (2013) have reported unusual behaviors of geomagnetic diurnal variations in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To validate this result, further investigations have been applied in this study. Geomagnetic data of 16 years' long term observation have been analyzed using the same method in Xu et al. (2013). Ratios of diurnal variation range between the target station Esashi (ESA) which is about 130 km from the epicenter and the remote reference station Kakioka (KAK) about 300 km distant to the epicenter have been computed. After removing seasonal variations revealed by wavelet transform analysis, the 15-day mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event. This anomaly is unique over a 16-year long background, and further discussions indicate that this anomaly is unlikely caused by strong geomagnetic storms or a statistical fluke. Therefore, this study has provided a strong support to the previous results in Xu et al. (2013).