ULF geomagnetic changes possibly associated with the 2008 Iwate-Miyagi Nairiku earthquake

摘要

Anomalous ULF geomagnetic field changes associated with earthquake is one of the most convincing and promising phenomena due to deeper skin depth. Since ULF signals associated with large earthquakes are weak, effective signal discrimination methods should be required. In this study, we investigate ULF geomagnetic changes possibly associated with the 2008 Iwate-Miyagi Nairiku earthquake based on spectral density ratio analysis, geomagnetic transfer function analysis, fractal analysis, and direction finding analysis. Geomagnetic data observed at Esashi, where the epicentral distance is about 47 km and Kakioka, the distance is about 317 km, and as a reference station have been analyzed. Wavelet transform have been performed for the spectral density analysis instead of FFT method. Before the earthquake, the variation of spectral density ratio, Sz/Sx and Sz/Sy, at the nearest station of Esashi exhibits an apparent increase from the trend. On the contrary, there are no corresponding significant changes at a remote station of Kakioka. The level of peak is beyond the 3 σ and its duration is 3 days. The lead time is about 3–4 weeks before the earthquake. At the periods from 3 to 105 sec, similar anomalous changes occurred. Results of transfer function analysis are similar. Fractal analysis and direction finding analysis have been also applied. The obtained fractal dimension and direction of arrival of ULF waves show the possible relation to the earthquake. These facts suggest the anomalous change is a possible candidate of earthquake-related ULF magnetic change.