International Space Science Institute - Europlanet Workshop on “Planetary Atmospheric Electricity” Bern, Switzerland, 23 – 27 July 2007

The Earth’s Surface/Subsurface Electrical Conductivity. M. Uyeshima 1, 1Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo, Tokyo, 113-0032 Japan, e-mail: [email protected].

In this presentation, I want to introduce the meth- instruments, data processing methods and for- ods to determine surface/subsurface electrical conduc- ward/inversion schemes, we can now determine detai- tivity by using natural electromagnetic(EM) distur- led crustal resistivity structures. Recently, even 3-D bances and review fundamental aspects of electrical interpretaions have been feasible. conductivity in the crust. From here, I want to show some examples of the EM disturbances of iosnosphere/magnetosphere 2-D/3-D electrical conductivity structures from the MT origin and with various frequency range propagate method. Then, as a case study, I want to estimate spa- down to the inner earth. Due to the Joule dissipation, tial distribution of water content from the resistivity the disturbance loses its energy in the propagation. As structure beneath back-arc area of the Tohoku district, the frequency is lower, the deeper part the distrubance NE Japan (Ogawa et al., 2001). This example will help can reach. Assuming that average electrical conductivi- you to recognize what determines the electrical ty of the earth is 100 Ohm-m, skin depth is 50 m for the conductivity structure. As the first step, temperature VLF band (10kHz), 1.6km for the ELF band (10Hz), structure was determined from thermal parameters such and 160km for the ULF band (1,000s). Thus EM fields as surface heat flow, thermal conductivity and crustal from VLF to ULF bands on the earth’s surface contain heat generation. Based on this temperature structure, information from the crust (30-100km thick) to the we can estimate whole rock resistivity versus depth uppermost mantle. If telluric current is induced in the dependences for various water contents, by referring earth due to EM disturbances of the external origin, laboratory experiment results compiled by Kariya & magnetic field is generated proportionally to the cur- Shankland (1983) (for dry rocks) and Nesbitt (1993) rent intensity. Then subsurface resistivity can be esti- (for crustal fluids). Then we estimated whole rock re- mated by determining the ratio between intesities of sistivities for Hashin & Shtrikman (1962)'s perfectly electric and magnetic fields (i.e. impedance) on the connected(HSc) and isolated(HSi) bounds. For the HSi earth’s surface. This is the fundamental principle of the model, whole rock resistivity takes almost the same magnetotelluric (MT) sounding method. In the conven- value as that of dry rocks unless water content becomes tional MT, plain source field exitation is assumed. In more than 10 %, and more than 90 % water content is its measurements, two component horizontal electric necessary almost throughout the crust to explain the and magnetic fields are measured. Then complex 2 observationally determined resistivity range: from 0.1 times 2 impedance tensor in frequency range is estima- to 10k Ohm.m. On the other hand, for the HSc model, ted. Frequency dependence of the complex impedance water content range from 0.01% to several % can ex- is inverted to yield the electrical conductivity structure. plain the 2-D resistivity cross section. In the early stage of the MT method, 1-dimensional (1- Ogawa et al., 2001's 2-D resistivity cross section D) layer structure was assumed. In the 1-D case, dia- contains three remarkable low resistivity portions in the gonal elements of the impedance are zero and two off- upper crust beneath three tectonic faults in the area diagonal elements are equivalent. In the 2-D case, in (Kita-Yuri thrust fault, Senya fault and Kitakami Lo- general, all elements of the impedance tensor are non- wland West Boundary fault). Depths of these low resis- zero. But if the tensor is rotated to the strike direction tivity portions range 10-20 km and resistivity is as low of the 2-D structure , diagnoal elements again becomes as 1-10 Ohm.m. In order to explain this low resistivity, zero, whereas two off-diagonal elements are not equi- interstitial water should be connected and its content is valent. This is due to separation of Maxwell equations estimated as 0.5-5 %. This estimation is not inconsis- into H- and E-polarization modes in 2-D case. In the tent with the water content estimation only from tomo- H- and E-polarization modes, (Hx,Ey,Ez) and graphic results for P and S seismic wave velocity per- (Ex,Hy,Hz) exists, respectively, where x is assumed to turbations (Matsubara et al., 2004). be parallel to the 2-D strike. In the E-polarization mode, vertical magnetic field appears due to horizontal gra- References dient of the induced current along the strike direction. Z. Hashin and S. Shtrikman, 1962, J. App. Phys., 33, 3125-3131. Since the vertical magnetic field posseses information K.A. Kariya & T.J. Shankland, 1983, Geophysics,48, 52-61. for lateral variation of the electical conductivity, we M. Matsubara et al., 2004, Tectonophysics, 388, 33-45. measure it in addition to previous four horizontal EM B.E. Nesbitt, 1993, J. Geophys. Res., 98, 4301-4310. components. Owing to significant improvements in Y. Ogawa et al., 2001, Geophys. Res. Lett., 28, 3741-3744.