A resistivity cross section to depths exceeding 200 km has been derived from magnetotelluric observations along a profile near latitude 45øN from the Juan de Fuca spreading center, across the coastal subduction complex, the High Cascades volcanic arc, and into the back-arc Deschutes Basin region. In this two-dimensional interpretation, emphasis was placed on data approximating the transverse magnetic mode since these data are relatively robust to common departures from the two-dimensional assumption. The vertical magnetic field, however, has been very valuable in defining structure of the offshore sediments, of the oceanic asthenosphere and below the arc volcanics of the Westem and High Cascades. The transverse electric data on land suffer a variety of three-dimensional effects, making their interpretation very difficult. In contrast, the greater uniformity of upper crustal conditions on the seafloor allowed a good fit to both modes of the impedance plus the vertical magnetic field at least down to 104 s' Important components of . While a number of aspects of the subsurface resistivity can be deduced straight from inspection of the data, computer simulation of the observations helps to quantify more rigorously the permissible or required structures. In this paper, we derive a two-dimensional resistivity model, with a presumed northsouth strike, by trial-and-error fitting of our MT measurements with a finite element forward modeling algorithm. Constraints on model geometry from independent geological or geophysical investigations have been incorporated where justified. Our paper is divided into three major sections. First, we explain the approach toward two-dimensional modeling. On land, the transverse magnetic (TM) impedance functions are emphasized because theory and experience show that they are more robust to plausible three-dimensional effects in the region than are the vertical magnetic field or, especially, the transverse electric (TE) impedance. Upper crustal structure on the seafloor appears to be rhuch simpler than on land, however, and both modes of impedance plus the vertical field are fit fairly well. Second, the model cross section is described and its goodness of fit to the data demonstrated. Model uniqueness is investigated by perturbing certain features of the model and assessing the increase in misfit.

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.

Abstract. In April-June 1989, seafloor magnetotelluric data across and along the leading edge of the Tahiti hotspot were obtained. The magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects, and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced. The corrected response functions are consistent with a two-dimensional structure. Inversion of the data shows a slightly higher conductivity (relative to a reference site located away from the hotspot) down to 130 km depth beneath the active area southeast of Tahiti underlain by a more resistive structure. There is a suggestion for a change in conductivity in the 400-450 kln depth range, which is consistent with elevated temperatures. This result is consistent with a mantle plume of limited extent (less than 150 km radius) located near the leading edge of the Tahiti hotspot. The rnagnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area, as would be required by a superswell model.

On March 14, 1979 a sizeable earthquake (Ms = 7.6, Richter scale) occurred on the continental shelf adjacent to S.W. Mexico, near Petatlan in the state of Guerrero. This earthquake generated a small tsunami that was recorded in deep water, 1000 km away, thus providing for the first time a glance at a tsunami traveling in the open ocean. The same sea floor pressure record displays conspicuous signals associated with vertical sea floor motions generated at the passage of the first Rayleigh seismic wave, R1. Seismic and tsunami travel velocities are in

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