Abstract: Two-Dimensional Principal Component Analysis (2DPCA) has been performed to examine ionospheric Total Electron Content (TEC) data in order to detect TEC precursor for a deep earthquake occurred at 02: 59:42 on 14 August, 2012 (UTC) (M_w = 7.7) with its depth at 625.9 km near Poronaysk, Russia. The examined TEC data were during the time period from 00:00 on 09 August to 02:50 on 14 August, 2012 (UTC) which were 5 days before the earthquake because the ionospheric precursors usually revealed in such time period. A ionospheric TEC precursor with the more large principal eigenvalues of 2DPCA have been localized around the epicenter during the time period from 02:25-02:30 on 14 August, 2012 (UTC) and the duration time was at least 5 min. Ionizing radiation Radon gas release should be a possibility to cause the anomalous TEC fluctuation, e.g., electron density variation. If a mount of radiation radon which caused the TEC anomalies was trapped in the asthenosphere, then the asthenosphere should be very thick with deep bottom in this zone which belongs to the subduction zone between the Pacific plate and the North America. A zone that was previously unknown to be the subduction zone could now be identified as through earthquake-related TEC anomaly. Therefore in this study, not only TEC anomaly could be detected but also new plate boundary with subduction zone is possible to be found. However, if this very deep earthquake does indeed record inter-plate seismicity, then the asthenosphere in this region must have a large thickness.