[1] We conducted 1-year continuous measurements of in situ compressional wave velocity and attenuation across a distance of 12 m in a vault at the coast of Miura Bay, Japan, using a piezoelectric transducer as the ultrasonic wave source. We detected, for the first time, periodic variations in both velocity and attenuation, with amplitudes of 0.3%, and of 4%, respectively, corresponding to the diurnal and semidiurnal lunar and solar tides. These variations correlate well with the in situ areal strain change due to tidal ocean loading estimated from the strain records about 100 m away from the observational site. Relative minimum velocity and relative maximum attenuation occur at every time of relative maximum areal strain (expansion), suggesting that opening/closure of pores or cracks of in situ rocks is responsible for these periodic variations. The velocity variation shows a remarkable 14-day periodicity corresponding to the spring and neap tides, arising from a nonlinear response of the velocity change to tidal strain change such that the velocity change depends on how slowly the tidal dilatation departs from its peak value.

One‐dimensional electrical conductivity structure in the mid‐mantle of the one‐fourth of the Earth beneath the north Pacific Ocean was obtained by a semi‐global electromagnetic induction study. Electromagnetic response functions estimated from electric field variations measured by submarine cables and geomagnetic field variations obtained by magnetic observatories and long‐term observations sites were inverted into radially symmetric conductivity distribution by taking the distribution of land and ocean at the surface into account. As a most preferred model, a smooth conductivity‐depth profile was obtained with two abrupt increases that possibly correspond to the seismic discontinuities at 410 and 660 km.

The magnetotelluric component of the Mantle Electromagnetic and Tomography (MELT) Experiment measured the electrical resistivity structure of the mantle beneath the fast-spreading southern East Pacific Rise (EPR). The data reveal an asymmetric resistivity structure, with lower resistivity to the west of the ridge. The uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles. Mantle resistivities to the west of the ridge are consistent with a low-melt fraction (about 1 to 2 percent interconnected melt) distributed over a broad region and extending to depths of about 150 kilometers. The asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates.

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