Almost all EM methods used in resources and deep structure prospecting were began to be used in the monitoring and prediction of earthquake precursors since 1966 Xingtai Ms6.8 earthquakes. The new techniques using satellite observation based on the DEMETER and CSES satellites, and CSELF have been developed during past decade. Some research results are outlined in follows.
Earth resistivity (DC): Prior to earthquakes with a magnitude > 7.0, the ground resistivity at stations adjacent to the epicenter showed typical abnormal decline characteristics. Resistivity is one of the rare parameters with repeatable anomaly characteristics before earthquakes. More than 80 stations currently exist in China.
Magnetotelluric （MT）note: The resistivity decreased at first and then recovered during the 1976 Tangshan Ms7.8 etc are observed. The earthquakes and aftershocks are concentrated near the boundary or conversion zone, in between there exists low resistivity with large rock porosity and weak mechanical property, and high resistivity. The observation using CSELF station DAL shows that apparent resistivity starts increasing in a pulsive manner about three months before Yangbi Ms5.1 earthquake of March 27, 2017 at DAL station (32km epicenter distance) and then decreases to background value 3 days before the quake (Fig.1).
Geomagnetic observation: The relationship between geomagnetic anomalies and seismicity can be divided into two main categories: (1) the magnetic susceptibility and remanent magnetism of rocks under stress will change (‘piezomagnetic effect’) which can be used in the short-term and imminent earthquake prediction. (2) The geomagnetic anomaly is presented by the induction effect due to the change of underground electrical property (‘induced magnetic effect’) which usually occurred several months or longer before quake. Hundreds of geomagnetic stations have been built in China up to now. In case of earthquake emergency, the mobile measuring stations or sites with fluxgate magnetometers will be deployed for emergency observation
Geoelectric field observation: The conventional geoelectrical method, called ‘Tu Di Dian’ played an important role in successful prediction of 1975 Haicheng Ms7.3 earthquake. A revolutionary improvement of the geoelectric field observation has taken place since 1981 VAN method proposed by the Greek scientists and a special instrument was developed. More than 120 stations have been constructed up to now.
EM radiation note: Most stations only observe the electric field or magnetic field. For stronger earthquakes, anomalies generally start earlier, with a large area of measurable anomalies. For example, the EM radiation anomaly range of earthquake of magnitude ≥ 5 it can reach 500 km. This method has lagged far behind the other technologies. However, due to its low costs, it has developed rapidly and more than 200 stations have been built in China.
Satellite EM observation: Since the launch of the first seismo-electromagnetic satellite detector DEMETER on 29 June 2004 by France etc., the research using satellite observation data to study seismo-electromagnetic anomalies has become a new research focus. China’s first seismo-electromagnetic satellite CSES is also launched on February 2, 2018. A satellite observation mainly focuses on the ionospheric EM field, in-situ plasma parameters and high-energy particle flux etc. The anomalies related to earthquakes mainly occurred in a week or a few hours prior to the earthquakes, showing typical short-term and impending time characteristics. The main disadvantage of the satellite observation is that the satellite is situated directly in the ionosphere, so it can easily record interferences in space, which can lead to too many anomalies. The transit time of a single satellite is limited, the orbital interval is large, and large space-time blank areas can exist. A new satellite, named CSES-2, is being developed that will be launched in 2022. This will further improve the coverage and space-time resolution.
The outgoing long-wave radiation (OLR) of infrared detection exhibited a significant increase one month prior to earthquakes. Several ground-based ionospheric detecting technologies have also been applied to study ionospheric vertical ionosonde, very-low-frequency radio waves, Schumann resonance, and GNSS TEC etc. The joint study of ground-based, the satellite EM and infrared anomalies is very promising in the research on lithosphere-atmosphere-ionosphere coupling mechanisms.
Physical experiment: Most resistivity measurements of rocks in the lab at normal temperature or high temperature and pressure suggest that the change of resistivity caused by the fracture of rocks is much larger than the strain induced by seismic deformation. In the experiments, the resistivity generally undergoes a three-stage change prior to final break-down: slow rising, steady or minor perturbation, and final rapid decline. The experiments show that most of the radiated EM waves have the shape of a single pulse or continuous pulse train, but paroxysmal pulses can occur as well. The electrical signal appears earlier than the magnetic signal, with higher frequency and amplitude. Field tests such as large-charge blasting, small-scale blasting, and blasting of limestone mines have shown that the waves of different frequencies appear in different orders. The secondary radiation excited by earthquake waves exists. These phenomena are similar to those observed by seismic stations.
Numeric modelling: The electrokinetic effect of seismo-electromagnetic radiation signal generated by porous media model has received widespread attention in recent years. The electric field accompanying seismic P-wave is not only related to P-wave amplitude and the dynamic electric coupling coefficient, but also related to the pore structure of the medium. There exist three main seismic ionospheric coupling pathways: additional DC electric field coupling, acoustic-gravity wave propagation, and EM wave propagation coupling. The test by the controlled-source signal in the very-low-frequency range of the Earth’s surface showed that the lower the frequency, the smaller the propagation loss in the ionosphere.
Proposals: To obtain more comprehensive anomaly information prior to the earthquake, a detailed three-dimensional (3D) observation and research of multi-parameters must be conducted (Fig. 2).
The stability of earth surface impedance and apparent resistivity parameters calculated from synchronous observation of electric and magnetic fields (such as the MT method) and the judgability of reasonable data are especially helpful to discriminate between the origin of the electric field or magnetic field anomaly from the source area and from the interference of non-seismic factors.
Many seismo-electromagnetic anomalies appear in the alternating EM fields, therefore in the process of building a well-equipped ‘natural laboratory’ the multi-parameter observations in the frequency band of alternating EM field should be addressed. It is recommended to standardize the use of frequency band designation of EM field officially issued in China: very-low-frequency (VLF, 3k to 30 kHz), ultra-low-frequency (ULF, 300 to 3000Hz), super-low- frequency (SLF, 30 to 300 Hz), extremely-low-frequency (ELF, 3 to 30 Hz), and tremendously-low-frequency (TLF, < 3 Hz).
Two biases occur that need to be clarified: (1) In the absence of sufficient analysis of the source and characteristics of anomalies, it is supposed that the anomalies in the data are related to the earthquake. (2) It is generally proposed that many abnormal phenomena are caused by interferences. According to the observation examples, the paper has summarized the ‘four stage’ methods of identifying and extracting anomalies, and achieved reasonable results. In this process, obtaining the effective background field forms the basis, whereas identifying the actual anomaly caused by the earthquake is the key.
Numerous studies have confirmed the effect and relevance of fluids in the crust on the induction or triggering of earthquakes. The EM method has unique advantages in detecting fluids, and partially molten rocks of the lower crust and upper mantle and has become an irreplaceable technology in earthquake prediction. It is necessary to conduct detailed EM surveys that cover the depth of the entire crust and up to the upper mantle in areas with typical seismic activity or in areas that were affected by strong earthquakes.
Zhao G, Zhang X, Cai J, Zhan Y, Ma Q, Tang J, Du X, Han B, Wang L, Chen X, Xiao Q, Sun X, Dong Z, Wang J, Zhang J, Fan Y, Ye T 2022. A review of seismo-electromagnetic research in China. Science China Earth Sciences, https://doi.org/10.1007/s11430-021-9930-5
Science China Earth Sciences