Magnetotelluric observations across the Juan de Fuca subduction system in the EMSLAB project.

A magnetotelluric transect has been carried out near latitude 45degrees N from the Juan de Fuca spreading center, across the subduction zone and volcanic arc, and into the northwestern Basin and Range. Broadband MT soundings (approx 0.01 s to 500 s period) were collected on land with considerable redundancy in site location, and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle. Sixteen long period MT recordings (about 40 s to 10,000 s) on land affirm the broadband responses in their common period range and extend the depths of exploration well beyond a hundred kilometers. On the Juan de Fuca plate offshore, 39 seafloor instruments were deployed at 19 locations. Of these locations, five magnetotelluric soundings plus two geomagnetic variation sites, covering the period range 200 to 10 sup 5 approximately, constitute the ocean bottom segment of our profile.