  The strong geomagnetic influences on mechanical properties of crystals due to
their exposure to ultralow crossed magnetic fields, the Earth's field (approx.
50 microTesla) and the AC field (approx. 3 microTesla) in the electron
paramagnetic resonance scheme, are discussed. Resonance relaxation
displacements of dislocations in NaCl crystals are found both for a harmonic
pump field and for a pulse AC field of resonance duration approx. 0.5 microsec.
Resonant changes have been also detected in the microhardness of ZnO,
triglycine sulfate, and potassium acid phthalate crystals after their exposure
in the same EPR scheme. The both effects manifest new strongly anisotropic
properties. In particular, the frequency of the resonance is very sensitive to
the mutual orientation of the sample and the Earth's field. Physical mechanisms
and practical significance of the phenomenon are discussed.
