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Clear electric field variations have been observed during the passage of seismic wave. The most notable feature is circular polarization of electric field and it has been interpreted as reflecting circular motion of ions in groundwater under the Earth's magnetic field. We have called such a mechanism 'seismic dynamo effect' but more convincing evidence is required to support this mechanism. Here we show strong evidence in terms of transfer function relating the electric field to the ground velocity due to seismic wave. We estimate transfer functions for electric field and ground velocity records for five aftershocks after the occurrence of M 6.9 earthquake. Then some peaks in the transfer function are found at specific frequencies where the resonance of ion motion in groundwater with the Earth's magnetic field is expected. This result clearly supports the mechanism, which provides a method for in-situ estimation of ion contents in groundwater.

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