Study On Coseismic Magnetic Field Induced By Rotation Of Coil-type Magnetometer

A large number of observational data have shown that electromagnetic disturbances occur during earthquakes.There are mainly two types of electromagnetic signals observed: early electromagnetic waves arriving earlier than the seismic waves and coseismic electromagnetic fields synchronized with the seismic waves.The former is of great significance for understanding the physical processes of earthquakes and earthquake early warning,while the latter has important potentials in resource prospecting due to its carried near-surface information,and has recently received more and more attentions.However,the generation mechanism of seismic electromagnetic field has not been studied clearly.Researchers have proposed several possible mechanisms for coseismic electromagnetic fields,such as electrokinetic effect,motional induction effect,piezoelectric effect,piezomagnetic effect and so on.In addition,in many cases,the induction magnetometers are used to observe the magnetic field,that is,a magnetic rod wound by a coil.When the seismic wave passes by,it causes the magnetometers to rotate,resulting in the change of magnetic flux of the geomagnetic field within the coil,thereby generating the rotation-induced magnetic field(RIMF).Such signal appears as a coseismic disturbance on the magnetic field record,and may be superimposed on the electrokinetic effect and other mechanisms.However,compared to the magnetic field signal generated by the electrokinetic effect or the motional induction effect,the RIMF is only caused by the rotation of the magnetometers and has nothing to do with the electrical properties of the underground medium.It is an unwanted signal,which may contaminate the useful magnetic signals generated by the other mechanisms.Therefore,if we want to extract useful information from the total observed magnetic field,we must remove this RIMF.Based on the propagation of plane wave theory,the previous researchers gave a method to calculate the RIMF,but this method is suitable for teleseismic situations,and large errors will occur in the case of small epicenter distance.Therefore,in order to accurately simulate the RIMF during the earthquake,this study establishes a semianalytical method to simulate the rotation motion and the RIMF excited by the seismic source based on the horizontally-layered model.Without the introduction of plane wave assumption,the displacement and the rotation components in the horizontally-layered are obtained by calculating the theoretical seismograms.Then,the expression of the RIMF is obtained by introducing previous results to the relationship between the RIMF and the rotation motion of the particle.Through numerical calculation,the full waveform response of the RIMF excited by the earthquake can be obtained.In addition,based on the displacement formula caused by the Rayleigh wave in the half-space model,we derive the expression of the RIMF induced by the Rayleigh wave.We first analyze the characteristics of the RIMF generated by the PSV-mode and SH-mode in the cylindrical coordinate system,and then conduct numerical simulations to study the characteristics of the RIMF excited by the explosive source,the SH-wave source and the fault slip source.The results show that the seismic waves for the PSV-mode mainly cause radial and vertical RIMF,while the SH mode waves mainly generate tangential RIMF.We also apply the method adopted in this study to interpret the coseismic magnetic field signal of the M7 Jiuzhaigou earthquake in 2017.The theoretically predicted RIMF can explain the observation data well,indicating that the coseismic magnetic field observed in this earthquake is mainly caused by the rotation of the magnetometers.The calculation method of the RIMF proposed in this study will help to predict the RIMF induced by the earthquake more accurately,so as to better extract useful signals from the magnetic field,which is of great significance to the interpretation of seismic electromagnetic data in the future.