We have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.

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[1] 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 À or Cl À 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.

The three‐dimensional P‐wave velocity structure of the Iwate volcano, northeastern Japan, is determined to depths of 2 km through an active seismic survey conducted in October 2000. Seismic tomography is applied to approximately 2700 travel‐time data. The most prominent discovery is an existing of column‐like high‐velocity body (Vp > 5.4 km/s) that extends vertically for 2 km beneath the caldera. While the western part of the volcano extending from the caldera is characterized by a moderate‐velocity region (4.8 < Vp < 5.4 km/s), the summit and eastern flank of the volcano are covered with very low‐velocity material (Vp < 4 km/s) which represent relatively younger volcanic edifices. The spatial difference in the velocity structures between the western and eastern parts of the volcano is explained by the evolutionary history of the volcano. And we find that the western structure may give constraints on the volcanic activity in 1998.

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