Among electric field variations supposed to be associated with earthquakes, electric field variations coincident with the passage of seismic waves have been well documented and interpreted mostly in terms of the electrokinetic effect. Here we present two examples of electric field variations obtained in association with small artificial earthquakes caused by blasting and three examples for aftershocks of two large earthquakes of magnitude 6.9 and 7.2, respectively. The electric field turned out to be circularly polarized in some cases, whereas linearly polarized cases were also seen. Since it is unclear whether such a peculiar behavior is understood in terms of existing models, we propose another mechanism to explain circular polarization; here we call this mechanism as "seismic dynamo effect," which would be regarded as an extended model of the so-called induction effect. In our model we consider ions motion in pores filled with water in the ground, which is driven by ground motion in the Earth's magnetic field. With this model we show that circular polarization of electric field is realized in association with resonance between the frequency of ground velocity due to seismic wave and the cyclotron frequency of ions, such as HCO~-_3 or Cl~- contained in pores, for the Earth's magnetic field at the observation site. Ions with positive charge, such as Na~+ , also seem to be responsible for circular polarization of electric field with rotation direction opposite to that for ions with negative charge. We also show that in this model the magnitude of electric field can be estimated in terms of the number density of ions.
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