The Morlet wavelet transform is commonly utilized to understand amplitude distributions in the frequency domain. To compare amplitude distributions at two different stations, the wavelet coherence as a normalized numerical index is employed. The wavelet coherence at two stations away from an epicenter often approaches 1 and their relative phase presents synchronous variations suggesting that geomagnetic fields are dominated by changes in the ionosphere and/or magnetosphere. When a station located near an epicenter is added to the analytical process, the small wavelet coherence at the period of approximately 0.5 day appears and disappears suddenly 40 days before and few days after earthquakes, respectively. Analytical results are in agreement with ratio changes of daily variation ranges at two stations during earthquakes reported in previous studies and shed light on location estimation of epicenters for forthcoming events using geomagnetic intensity fields.
