The physical mechanism of electromagnetic triggering of earthquakes discovered more than 30 years ago is still poorly understood. Recent experimental studies of behavior of acoustic emission from the stressed samples under electric action supposed that the mechanism responsible for increased crack formation may be the Joule heating of pore fluid by applied DC pulses resulted in pore pressure rise and subsequent failure of the samples. Nevertheless, the tests were carried out for DC density of 5–50 A/m2 that is too far from estimated values of 10–7–10–8 A/m2 for field conditions when the DC pulses from artificial pulsed power systems injected into the earth crust through emitting grounded dipole. In order to verify the hypothesis of the thermal mechanism of electromagnetic earthquake triggering we carried out an experimental study of the variations in temperature and pressure of mineralized water in an artificial cavity simulating a crack or pore in the rock under electrical action of various intensities. It was demonstrated that the effect of the fluid pressure in the cavity is observed during applied DC pulses with the current density over 0.01 A/m2 only that is 5–6 orders more than the density of DC pulses applied during field experiments. Thus, the mechanism of Joule heating of pore fluids in rocks cannot explain the electromagnetically triggered seismicity observed under field conditions.