Low Frequency (0{middle dot}1-6{middle dot}0 cpd) Electromagnetic Study of Deep Mantle Electrical Conductivity Beneath the Hawaiian Islands

Abstract

Low frequency (0.1-6-0 cpd), naturally occurring electromagnetic .,:ids on Oahu, Hawaii are used to compute E over B response functions that are complex functions of frequency containing information about the electrical conductivity structure at depth. Smoothly varying estimates of the response, unbiased by random electric field noise, are constructed with the aid of polynomials in frequency. Remarkably small standard deviations, for the smoothed response, between 1 and 3 per cent, result from using 22 months of hourly electric field data accurate to 1 pV m-I.The island effect is found to be essentially frequency independent with zero phase shifts for frequencies less than 6 cpd. This enables us to estimate the mantle response that is consistent, at these frequencies, with a conductivity model dependent only on depth. A horizontally layered model is constructed by the inversion method of Schmucker and the layered profile is smoothed by the Backus-Gilbert method that yields locally smoothed conductivity values varying from a near surface value of 0.08mhom-' to 0*8mhorn-' at 700km depth as in the Parker profile. In addition there appears to be a definite zone of high conductivity in the depth range 330-380 km with a locally smoothed conductivity value of 0-8+0.1 mho m-' for a smoothing kernel of 30 km spread. The layer conductivity model and the island effect quite accurately reproduce all the observed complex response functions. on the electrical conductivity as determined from the ionospheric generated signals.The oceanic generated signals will be treated in a separate paper.A major obstacle in the interpretation of island fields is the complication caused by the island conductivity structure. Fields measured at sea-floor sites distant from topographic features such as ridges, islands or continents are less complicated by topography effects. However, islands are limited in size and have known offshore

Conductivity of the subcontinental upper mantle: an analysis using quiet-day geomagnetic records of North America.

“…Some published estimates of the Earth conductivity exist for the depth range of our report. The five that summarize such estimates are : LAHIRI and PRICE , 1939;BANKS, 1969 ;LARSEN, 1975 ;ANDERSSEN et al, 1979 ;and SCHMUCKER , 1979. They are shown in Fig.…”

Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

“…While its mere existence is questionable given the data inconsistency discussed previously, very low resistivities at depths less than 300 km have also been interpreted beneath the East Pacific Rise spreading center [Filloux, 1982] and beneath Hawaii, a site of presumed hot spot activity [Larsen, 1975]. These depths appear too shallow to be correlated with the olivene-spinel phase transition [Akimoto and Fujisawa, 1965] If mantle resistivities down to 400 km depth remain in the range 15-20 ohm m however, as the apparent resistivity alone suggests, then no such deep melting is implied.…”

Strategies for improved global electromagnetic response estimates.

“…The compilation could be presented in the form of a map of the equivalent currents, which should prove extremely valuable in determining which observatories, if any, would be likely to yield worthwhile response estimates at the intermediate periods. If sufficient data were available, it might be possible to devise a means of correcting the observations for the effects of the surface layer, as Larsen has done in Hawaii (LARSEN, 1975).…”