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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS012 Poster presentation 2208
Sensitivity of geomagnetic transfer functions to stagnant-SLAB-type
electrical conductivity heterogeneity
Dr. Hisayoshi Shimizu
Earthquake Research Institute University of Tokyo IAGA
Yuta Baba, Takao Koyama, Kiyoshi Baba, Hisashi Utada
Electrical conductivity structure in the Earth obtained by using observed geomagnetic and geoelectric
field provides important and independent information to discuss the dynamics and material in the Earth.
First 3D semi-global mantle conductivity model in the northern Pacific obtained by Koyama (2001) was
examined together with the seismic velocity by Fukao et al. (2003) in the same region, and Fukao et al.
(2004) pointed out that temperature anomaly cannot be the cause of anomalously high conductive
region beneath Philippine Sea in 350-550 km depth. Koyama et al. (2006) estimated using the Nernst-
Einstein relationship (Karato, 1990) with standard geotherm model (Ito and Katsura, 1989) and the
diffusivity of hydrogen in wadsleyite at high pressure (Hae et al., 2006) that the high electrical
conductivity in the area can be explained if 0.3 wt. per cent of water exist in the region. The estimate is
an important constraint to discuss the dynamics in the mantle. Although observationally determined
electrical conductivity structures have already been employed for further interpretations, the estimates
have much room to improve the credibility and accuracy. Koyama (2001) employed GDS (Geomagnetic
Deep Sounding) and magnetotelluric responses in the period range of 5-15 days in the 3D inversion.
The fit of model output with observed data seems fine, but more geoelectromagnetic information may
be utilized to improve the credibility of the conductivity model. In this study, we are going to seek the
plausibility to employ the ratio of horizontal geomagnetic components between geomagnetic stations
(horizontal transfer function; HTF hereafter), which has sensitivity to 3D heterogeneities, as new
information to constrain the conductivity structure. We estimated HTFs at stations in the north-western
Pacific in reference to the horizontal component at Kakioka, and compared them with the HTFs
calculated by using three dimensional conductivity model by Koyama (2001) as a trial. Observed and
modeled HTFs have large differences, more than the estimation error of the observed HTFs, at some
stations. HTF can be a promising candidate to be included in further inversion on conductivity
structures. We are going to present the results of the sensitivity test of HTF and GDS responses against
synthetic three dimensional conductivity structures. Preliminary calculations supposing 1000km scale
checkerboard type lateral conductivity heterogeneities, having an order of magnitude contrast, at 350-
850km depth embedded in one-dimensional reference model (Utada et al. 2003) imply that the
signature due to lateral heterogeneities are larger than the estimation error of HTF and GDS responses
at stations in geomagnetic mid latitudes in the period range of 5-60 days. Discussions on the sensitivity
in with respect to the size and depth of the heterogeneity, conductivity contrast, and period of the
response functions are presented in the paper.
Keywords: electrical conductivity, response functions