Features of the Electromagnetic Field of Lithospheric Sources

One of the key problems in the search for electromagnetic precursors of earthquakes is the possibility of separating magnetospheric and seismogenic disturbances. This paper presents the results of using a model that enables us to calculate the ultra-low-frequency (ULF) fields on the Earth’s surface created by a linear horizontal current of finite length. This model simulates the occurrence of mechano-electric transformers during a shift along a fault zone at the final stage of the earthquake preparation. The calculations show several characteristics of the field of the underground source in comparison with the field of ionospheric disturbances. If the vertical component \({{B}_{z}}\) of the magnetic field of an ionospheric disturbance is small compared to the horizontal component \({{{\mathbf{B}}}_{ \bot }}\), then for an underground source \(\left| {{{B}_{z}}} \right| > \left| {{{{\mathbf{B}}}_{ \bot }}} \right|\) in the vicinity of the source. For ionospheric sources, this apparent impedance (i.e., the \({{{{\mu }_{0}}\left| {{{{\mathbf{E}}}_{ \bot }}} \right|} \mathord{\left/ {\vphantom {{{{\mu }_{0}}\left| {{{{\mathbf{E}}}_{ \bot }}} \right|} {\left| {{{{\mathbf{B}}}_{ \bot }}} \right|}}} \right. \kern-0em} {\left| {{{{\mathbf{B}}}_{ \bot }}} \right|}}\) ratio) coincides with the impedance of the Earth’s surface Z_g, while the impedance of disturbances created by the lithospheric source may exceed Z_g, up to order of magnitude in the source vicinity. An underground current source can create a vertical electric field \({{E}_{z}}\) of significant magnitude. This is due to the vertical current continuity at the Earth–atmosphere interface, which acts as a powerful “amplifier” with a coefficient determined by the ratio of the complex conductivities of the Earth’s crust and air. Calculations have shown that these ideas are incorrect. The vertical component \({{E}_{z}}\) on the Earth’s surface is of the same order of magnitude as the transverse component \({{{\mathbf{E}}}_{ \bot }}\). There have been suggestions to use short-baseline gradient measurements to reduce the contribution of large-scale ionospheric disturbances. The calculation of the field structure has revealed that amplitude-phase gradients in the vicinity of an underground source are highly variable and may provide ambiguous results.