Abstract
<p>Changes in the electromagnetic (EM) field after an earthquake rupture but before the arrival of seismic waves (“early EM signals”) have sometimes been reported. Quantitative evaluations are necessary to clarify whether the observed phenomena are accounted for by known theories and to assess whether the phenomenon can be applied to earthquake early warning. Therefore, analytical expressions for the magnetic field generated by an impulsive line-current are derived for a conductive half-space model, and for a two-layer model; the somewhat simpler situation of a conductive whole-space is also considered. By analyzing the expressions obtained for the generated EM field, some expected features of the early EM signals are discussed. First, I verify that an early EM signal arrives before the seismic waves unless conductivity is relatively high. Second, I show that early EM signals are well approximated by the whole-space model when the source is near the ground surface, but not when it is at depth. Third, I show that the expected amplitudes of early EM signals are within the detection limits of commonly used EM sensors, provided that ground conductivity is not very high and that the source current is sufficiently intense. However, this does not mean that the EM signals are easily distinguishable, because detector sensitivity does not account for additive noise or false positive detections.</p>
Highlights
Seismic fault slip mainly generates ground motion, but electromagnetic (EM) variations have been reported during and prior to some earthquakes [e.g., Eleman 1966, Iyemori et al 1996, Honkura et al 2002, Honkura et al 2004, Abdul Azeez et al 2009, Honkura et al 2009, Okubo et al 2011, Gao et al 2014]
Discussion and numerical examples This section considers three points based on the analytical expressions derived for early EM signals
In this paper, I have derived analytical expressions for the time-varying EM field generated by an impulsive line-current in both a two-layer and whole-space model
Summary
Seismic fault slip mainly generates ground motion, but electromagnetic (EM) variations have been reported during and prior to some earthquakes [e.g., Eleman 1966, Iyemori et al 1996, Honkura et al 2002, Honkura et al 2004, Abdul Azeez et al 2009, Honkura et al 2009, Okubo et al 2011, Gao et al 2014]. Electric currents can be induced by the motion of conductive crustal material in the ambient geomagnetic field [e.g., Gershenzon et al 1993, Yamazaki 2012, Gao et al 2014]. The whole-space model (Figure 2b) is somewhat unrealistic; solving Maxwell’s equations in the whole-space model is considerably easier than for the half-space model For this reason, I consider the whole-space model throughout the following, so that any situations in which the simplified model is valid can be understood. When the electric current density I is given as a function of location (x, z) and time t, the electric and magnetic fields, E and B respectively, are determined via Maxwell’s equations: EARLY EM SIGNALS BY LINE CURRENTS d.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
