Piezoelectricity as a mechanism on generation of electromagnetic precursors before earthquakes

Abstract

SUMMARY To produce the electromagnetic (E&M) precursors of an earthquake, the existence of electric field due to stress-induced charges on the ground surface or in shallow depths of upper crust inside the fault zone is a basic condition. Here, we consider the piezoelectric effect or the elastic–electric coupling as a major mechanism on generating such an electric field. A 1-D model based on the elastic mechanics and electromagnetic Maxwell equations is built up to formulate the relationship between electric field and slip as well as stress on a fault before an earthquake. From the model, we may estimate the low-bound values of stress and slip to yield the critical electric field, Ec, for generation of electromagnetic signals. The normal and shear stresses on the fault planes for three faulting types are constructed. The normal stress is stronger than the shear stress to result in piezoelectricity. The depth ranges for yielding an average normal stress being able to generate Ec are similar for thrust and strike-slip faults and deep for normal faults. The possibility of generating Ec is almost the same for thrust and strike-slip faults and low for normal faults. The pre-earthquake slip could be related to nucleation phases or microfractures. The possible occurrence time of E&M signals may be several 10 min to few hours before impending earthquakes. The major factor in yielding a piezoelectric field to generate the TEC anomalies before an earthquake is the existence of fault gouges composed mainly of clays. A thick gouge layer with low electric resistivity and a piezoelectric coupling coefficient ≥0.67 × 10−12 coul  nt–1 is an important condition for yielding piezoelectricity.