Almost all EM methods used in prospecting for resources and deep structures began to be used in monitoring and forecasting earthquake precursors since 1966 Xingtai Ms6.8 earthquakes. New satellite observation techniques based on the DEMETER and CSES, and CSELF satellites have been developed over the last decade. Some search results are shown below.
Earth Resistivity (DC): Prior to earthquakes of magnitude >7.0, ground resistivity at stations adjacent to the epicenter showed typical characteristics of anomalous decline. Resistivity is one of the few parameters with characteristics of repeatable anomalies before earthquakes. More than 80 stations currently exist in China.
Magnetotelluric （MT）observation: Resistivity first decreased and then recovered during the 1976 Tangshan Ms7.8 etc are observed. Earthquakes and aftershocks are concentrated near the boundary or conversion zone, in between there is low resistivity with high rock porosity and low mechanical property, and high resistivity. Observation using the CSELF DAL station shows that the apparent resistivity begins to increase in a pulsed fashion about three months before Yangbi MsEarthquake of 5.1 of March 27, 2017 at the DAL station (distance from the epicenter of 32 km) then decreases to the background value 3 days before the earthquake (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 earthquake prediction. short-term and imminent earth. (2) The geomagnetic anomaly is exhibited by the induction effect due to the change in underground electrical property (“induced magnetic effect”) which usually occurred several months or more before the earthquake. Hundreds of geomagnetic stations have been built in China so far. In the event of a seismic emergency, mobile measuring stations or sites equipped with fluxgate magnetometers will be deployed for emergency observation
Observation of the geoelectric field: The conventional geoelectrical method, called ‘Tu Di Dian’ played an important role in the successful prediction of 1975 Haicheng Ms7.3 earthquake. A revolutionary improvement in the observation of the geoelectric field has taken place since 1981. The VAN method proposed by Greek scientists and a special instrument has been developed. Over 120 stations have been built to date.
Electromagnetic radiation observation: Most stations observe only the electric field or the magnetic field. For stronger earthquakes, anomalies usually start earlier, with a large area of measurable anomalies. For example, the range of EM radiation anomalies from an earthquake of magnitude ≥ 5 can reach 500 km. This method is far behind other technologies. However, due to its low cost, it developed rapidly and more than 200 stations were built in China.
Electromagnetic observation by satellite: Since the launch of the first seismo-electromagnetic satellite detector DEMETER on June 29, 2004 by France etc., research using satellite observation data to study seismo-electromagnetic anomalies has become a new area of research. China’s first CSES seismo-electromagnetic satellite is also launched on February 2, 2018. Satellite observation mainly focuses on ionospheric electromagnetic field, in situ plasma parameters and high-energy particle flux, etc. Earthquake-related anomalies mostly occurred within a week. or a few hours before earthquakes, showing typical short-term and imminent temporal characteristics. The main disadvantage of satellite observation is that the satellite is located directly in the ionosphere, so it can easily register interference 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 voids may exist. A new satellite, named CSES-2, is under development and will be launched in 2022. This will further improve coverage and spatio-temporal resolution.
Outgoing long-wave radiation (OLR) from infrared detection showed a significant increase one month before the earthquakes. Several ground-based ionospheric detection technologies have also been applied to study vertical ionospheric ionosonde, very low frequency radio waves, Schumann resonance and GNSS TEC, etc. The joint study of anomalies on the ground, the EM satellite and the infrared is very promising. in research on lithosphere-atmosphere-ionosphere coupling mechanisms.
Physical experience: Most measurements of resistivity of rocks in the laboratory at normal temperature or at elevated temperature and pressure suggest that the change in resistivity caused by the fracture of rocks is much larger than the deformation induced by seismic strain. In experiments, resistivity typically undergoes a change in three stages before final decay: slow increase, steady or minor disturbance, and final rapid decline. Experiments show that most radiated electromagnetic waves are in the form of a single pulse or a continuous train of pulses, but paroxysmal pulses can also occur. The electrical signal appears earlier than the magnetic signal, with higher frequency and amplitude. Field tests such as large-load blasting, small-scale blasting, and limestone mine blasting have shown that waves of different frequencies appear in different orders. Secondary radiation excited by seismic waves exists. These phenomena are similar to those observed by seismic stations.
Numerical modeling : The electrokinetic effect of the seismo-electromagnetic radiation signal generated by the porous medium model has received wide attention in recent years. The electric field accompanying the seismic P wave is not only related to the amplitude of the P wave and the dynamic electrical coupling coefficient, but also related to the porous structure of the medium. There are three main pathways of seismic ionospheric coupling: supplemental DC electric field coupling, acoustic and gravity wave propagation, and electromagnetic wave propagation coupling. Testing by the controlled source signal in the very low frequency range of the Earth’s surface has shown that the lower the frequency, the lower the propagation loss in the ionosphere.
Proposals: To obtain more comprehensive information about the anomalies before the earthquake, detailed three-dimensional (3D) observation and multi-parameter research should be conducted (Fig. 2).
The stability of the earth’s surface impedance and apparent resistivity parameters calculated from synchronous observation of electric and magnetic fields (such as the MT method) and the ability to judge reasonable data are particularly helpful in distinguishing the origin of the electric field or source magnetic field anomaly and interference from non-seismic factors.
Many seismo-electromagnetic anomalies appear in alternating EM fields, therefore, in the process of building a well-equipped “natural laboratory”, multiparameter observations in the frequency band of the alternating EM field must be taken into account. It is recommended to standardize the use of the EM field frequency band designation officially issued in China: very low frequency (VLF, 3k to 30 kHz), ultra low frequency (ULF, 300 to 3000 Hz), super low frequency . frequency (SLF, 30 to 300 Hz), extremely low frequency (ELF, 3 to 30 Hz) and extremely low frequency (TLF,
Two biases appear that need to be clarified: (1) In the absence of sufficient analysis of the source and characteristics of the anomalies, it is assumed that the anomalies in the data are related to the earthquake. (2) It is generally accepted that many abnormal phenomena are caused by interference. According to the observation examples, the paper summarized the “four-step” methods of identifying and extracting anomalies and achieved reasonable results. In this process, obtaining the effective background field forms the basis, while identifying the actual anomaly caused by the earthquake is the key.
Many studies have confirmed the effect and importance of crustal fluids 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 entire depth of the crust and up to the upper mantle in areas of typical seismic activity or in areas that have been affected by strong earthquakes.
See the article:
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 Earth Sciences in China, https://doi.org/10.1007/s11430-021-9930-5
Science China Earth Sciences