Magnetic induction effects caused by an underwater earthquake

• Smagin V.P.

  Vladivostok State University of Economics and Service
  Vladivostok. Russia

• Semkin S.V.

  Vladivostok State University of Economics and Service
  Vladivostok. Russia

• Savchenko V.N.

  Far Eastern Federal University
  Vladivostok. Russia

An analytical and numerical study of magnetic inductive effects caused by hydroacoustic waves arising from underwater earthquakes is carried out. Sound waves propagating in sea water lead to the generation of electric currents due to the action of the Lorentz force (ponderomotive force) in the geomagnetic field. A magnetic field induced by such currents is considered in the case when a sound wave propagates over long distances from a source in an oceanic waveguide and an essentially important role is played by the waveguide character of its motion in the ocean. The case in which the observation of an acoustic wave field and the magnetic field effects induced by it occur in the vicinity of a nonstationary seismoacoustic point source of frequency that is turned on at a certain instant of time has been studied in detail. Such a source can serve as a model of an underwater eruption or an earthquake with a localized focus. The magnetic field generated by sound waves of both stationary and non-stationary sources can be affected by several factors, including the influence of the bottom and the free surface, as well as the vertical stratification of the marine environment, both on the sound field itself and on the magnetic field induced by it. Calculations of the induced magnetic field were carried out in the quasistatic and magnetoinductive approximations (which takes into account the phenomenon of self-induction of the magnetic field), which in some situations seem unjustified. The space-time structure of the induction effects of the magnetic field for several models of acoustic waves excited by earthquakes is calculated and analyzed in detail, among which the simplest model is a point source in an infinite homogeneous oceanic medium. In the case of a non-stationary source of acoustic waves, calculations have shown that the dynamics of the variation with time of the magnetic field can be divided into two stages-transition and stationary regimes. The amplitude of the induced magnetic field in the stationary mode falls off in inverse proportion to the square of the distance to the source (while the amplitude of the oscillations of acoustic pressure and velocity decreases more slowly - inversely proportional to the distance). In a number of cases, there has been established the existence of a magnetic precursor for the arrival of an acoustic wave. Taking into account the self-induction in the magnetoinduction approximation in the majority of the cases considered leads to a decrease in the amplitude of the field oscillations in the steady-state regime in comparison with the quasistatic solution.

Keywords: earthquake, ocean, magnetic induction, quasi-static approximation, self-induction.