廣岡 伸治

© 2017 The Institute of Electrical Engineers of Japan. Recent studies have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, time-spatial analysis of GDV has been applied in this study. Geomagnetic data of long term observations at 17 stations in Japan have been analyzed using the same method in Han et al. 2015. Ratios of diurnal variation range between the target station and the reference station KAK 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 in both ESA and MIZ stations in the Tohoku Region. Similar results could not be found in other regions of Japan. Spatial distributions of the ratios show a good agreement between the location of the anomalies and the epicenter of Mw 9.0 earthquake. These time-spatial results are consistent with independent results obtained from other observations such as radon density, seismicity, and GPS displacements, which suggest the geomagnetic data might be useful in earthquake monitoring and disaster mitigation.

Xu et al. (2013) and Han et al. (2015) have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, temporal-spatial analyses of GDV have been applied in this study. Geomagnetic data of long-term observations at 17 stations in Japan have been analyzed using the same method in Han et al. (2015). Ratios of diurnal variation range between the reference station KAK and the target stations have been computed. After removing seasonal variations, the 15-day backward running mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event at both ESA and MIZ stations in the Tohoku Region. Locations of anomalies in spatial distribution show a good correlation with the epicenter of the Mw 9.0 earthquake. These spatiotemporal results are consistent with those obtained from other independent observations such as groundwater level and GPS displacements. The coupling of multiple pre-earthquake phenomena may help to understand the preparation process of a mega earthquake in the subduction zone. (C) 2016 Published by Elsevier Ltd.

A numerical simulation has been performed to evaluate the performance of ionospheric tomography using the residual minimization training neural network (RMTNN) method. The results indicate that reconstruction with high precision is possible when the standard deviation of the noise is about 2.5% or less of the average value of the observed data (slant total electron content [STEC]). Moreover, in the daytime, when the value of STEC becomes large, the signal to noise ratio (SNR) increases and the reconstruction accuracy becomes high; at night when the SNR falls, conversely, it becomes low. The results of detectability tests show that the RMTNN method has a good performance at about F-layer height with shape and peak intensity reconstruction. In conclusion, the newly developed RMTNN ionospheric tomography is effective in reconstructing the 3D electron density distribution from realistic STEC data in the daytime, and is able to estimate images around the F-layer.