  Recently several cases of observations of unipolar magnetic field pulses
associated with earthquakes at different points (California, Italy, Peru) have
been recorded. The paper attempts to model unipolar magnetic field pulses based
on one mechanism that should be omnipresent for all measurement points, namely,
the magnetic field diffusion through a conductive medium. The structure of
magnetic fields supported by electric current sources is thoroughly modelled.
The source of electric current is considered as an elongated volume of finite
cross-section being immersed in a conductive medium. To model the unipolarity
feature of the observed pulses prior to and at the earthquake main shock, the
electric current of the source is of impulse form. Special attention is paid to
the differences in the pulse structure (as amplitude envelope and the pulse
width) that are measured by various magnetometers (fluxgate or search-coil). An
analysis and comparison with recorded magnetic field pulse characteristics
reveal that the observed unipolar pulses may have a common genesis, an electric
current source within a conductive medium such as the earth crust.
