Simulation Of Earthquake-Induced Electromagnetic Wave Field Due To The Electrokinetic Effect

Electromagnetic (EM) signals have been observed in earlier researches during earthquakes. These signals accompany seismic waves. However, the reason for the existence of such coseismic EM signals is unclear. The electrokinetic effect corresponding to the electric double layer is one of the mechanisms for the coupling between the seismic and EM fields. When a seismic wave propagates in a rock, a relative fluid-solid flow is induced. Such relative flow drives the excess ions in the pore fluid to cause EM field. In this paper, the EM fields generated by an earthquake due to the electrokinetic effect is studied. The earthquake is modeled by a fault slip.When the wavelength considered and the receiver-to-source distance are larger than the characteristic length of the fault, the fault can be taken as a double couple point source. The properties of the seismoelectromagnetic wave fields induced by a double couple source in an infinite porous medium is investigated. Based on the Pride's equations coupling the seismic and EM waves, the author derive the Green's functions of the solid displacement, the electric field and the magnetic field due to a single point source. Furthermore, these Green's functions are extended to cater for the moment tensor sources. Then the analytical expressions of the solid displacement, the electric and magnetic fields in the frequency-space domain excited by a double couple source are derived. The waveforms of the displacement, the electric and magnetic fields due to a double couple source are then calculated at earthquake frequencies. In these waveforms, there is an electric field accompanying a P wave as well as electric and magnetic fields accompanying an S wave. It is found that the electric field accompanying the S wave is smaller than that accompanying the P wave. It is proved that the S wave has a weaker capacity than the P wave in inducing an electric field at earthquake frequencies in a porous medium which is not dynamically compatible or nearly dynamically compatible. In the waveforms, there is also an independently propagating EM wave, which has a much higher speed than the seismic waves, and reaches the observation point immediately after the source originates. However, it has a weaker amplitude than the EM field accompanying a seismic wave. It is also found that the seismoelectromagnetic field depends on the observation orientation. There are orientations, in which the displacement induced by the P wave is not seeable while the electric field accompanying the P wave is apparent. And there are orientations, in which the displacement induced by the S wave is apparent while the magnetic field accompanying the S wave is not observable. It is noticed that the compatibility of the elastic parameters of the porous medium affects the electric field accompanying the P wave. And on the basis of the seismoelectric coupling equations, it is proved that when the parameters satisfy the dynamically compatible condition, the P wave does not induce any electric field.The seismoelectromagnetic wave fields generated by a double couple in a horizontally-layered geological model are also studied. On the basis of the seismoelectric coupling equations, the stress-displacement-EM discontinuity vectors for the moment tensor sources are derived. And then the author extend the algorithm which is used by previous researchers to calculate the seismoelectromagnetic wave fields due to a point source in layered media to cater for moment tensor sources. Therefore, a simulation of the seismoelectromagnetic wave fields generated by a fault with arbitrary strike, dip and slip rake is allowable. As examples, the wave fields generated by a double couple source in a half-space and multi-layered formations are calculated, respectively. Due to the existence of the free surface, both the P and S waves cause coseismic electric and magnetic disturbances when they arrive at the receivers near the surface. There are also electric field and horizontal magnetic field accompanying a Rayleigh wave. It is found that the coseismic electric and magnetic fields have a close connection to the corresponding seismic fields. The waveforms of the coseismic electromagnetic fields and those of the seismic fields are similar, but of different phases. It is also found that the amplitude of the coseismic electric field decreases when either the salinity or the viscosity increase, and the coseismic magnetic field decreases when the viscosity increases. In the waveforms, there is also a kind of critically-refracted EM wave which reaches the receiving point earlier than the seismic wave. Such an EM wave can be converted from a seismic or EM wave incident on the free surface. Its amplitude is much smaller than the coseismic EM field, and decreases when either the salinity or the viscosity increases. The result shows that the seismoelectric and seismomagnetic signals due to the electrokinetic effect are measurable.When the wavelength considered and the receiver-to-source distance are close to or smaller than the characteristic length of the fault, the fault can not be taken as a point source. In this situation, the fault is discretized to a series of subfaults, each of which can be treated as a point source. The wave fields generated by the whole fault are a stacking of those generated by these subfaults. The displacements, the electric and magnetic fields generated by a strike-slip and a dip-slip fault are simulated, respectively. The result shows that the fault generates notable displacement and EM disturbances in the vicinity of the fault. The seismic and EM fields have clear directivity. The seismic, electric and magnetic fields in front of the fault rupturing direction are much larger than those in the opposite direction. For a dip-slip fault, the seismic and EM signals on the hanging wall are stronger than those on the foot wall. The influences of the cover layer and the source burial depth on the seismoelectromagnetic wave fields are investigated in the present paper. It is shown that when a cover layer is present, the seismic, electric and magnetic fields are amplified because of the multi-reflections and -refractions of the seismic waves in the cover layer. The source burial depth also exerts an impact on the amplitudes of the displacement, and the electromagnetic fields. Thinner burial depth makes stronger seismoelectromagnetic disturbance on the ground.The result also shows that for either a strike-slip fault or a dip-slip fault, there are permanent displacements and remnant electric fields but no remnant magnetic field near the fault after the fault rupture stops and the seismic wave propagates away. Such a remnant electric field damps with time and it is sensitive to the receiving depth. When the receiving depth increases the horizontal component of the remnant electric field increases, while the vertical component decreases. There is no horizontal remnant electric field right at the free surface. Both the coseismic oscillatory component and the postseismic decaying component of the vertical electric field are 2 orders of magnitude larger than those of the horizontal electric field. However, the horizontal and vertical electric fields become on the same order at a certain depth.The coseismic EM fields calculated from a point source model and a finite fault model have been investigated in this paper. Some suggestions are made for the measurement of the EM fields during earthquakes, i.e. avoiding the dynamically compatible or nearly dynamically compatible stratum, choosing a stratum with low fluid salinity and low fluid viscosity, and taking the vertical electric field into account.