A new approach for earthquake precursors analysis based on anomalous variations of electron concentration in the ionosphere is proposed. We concentrated on variations of total electron content (TEC) of the ionosphere integrated over the earthquake region and critical frequency of the F2-layer (foF2) measured by ionosonde network before the catastrophic Tohoku earthquake on March 11, 2011 near the east coast of Japan. The proposed approach is based on representation of TEC and foF2 as a time series and its further analysis using the Seasonal-Trend decomposition with LOESS (STL) algorithm. The simultaneous anomalous variations of extrapolated values of TEC and foF2 over the area of the Tohoku earthquake epicenter, were found. These variations cannot be attributed to external influences, such as diurnal variations, solar or geomagnetic activities. The algorithm revealed that all the major aftershocks of the Tohoku earthquake were also preceded by anomalous variations of TEC and foF2.