Local Sq response functions from EMSLAB data

Abstract

Magnetic signals from six seafloor stations on the Juan de Fuca plate and one land station are examined for information content about the structure of the regional lithosphere and mantle. Single‐site Z/H responses for the first four Sq harmonics are calculated from 24 quiet days. Two corrections are applied to the data: (1) to the usual strictly Pm+1m source morphology, we also include the contribution of the Pm+3m spherical harmonic to the magnetic potential of Sq; and (2) the coast effect is removed by use of two‐dimensional model calculations with a simulated nonuniform source field. The model enables us to separate the anomalous vertical magnetic field attributable to the lateral conductance change from the normal vertical magnetic field in the absence of lateral conductance changes. The normal vertical field is calculated from the vertical conductivity distribution and the particular form of the source field. The sum of both normal and anomalous fields reproduces the observations. The anomalous part has been used to correct the observed vertical to east magnetic component ratio for the effect of the land‐ocean transition. The corrected ratio has been interpreted with Schmucker's “equivalent conductor” method, yielding estimates of the impedance at Sq frequencies. These estimates are used as an independent check on magnetotelluric (MT) impedances in the Sq frequency range, in particular for the purpose of identifying the local static distortion of the electric field. No significant distortion seems detectable at the outermost seafloor stations. The Sq and MT impedances are consistent with a single model.