# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 5 Possible Mechanisms of Seismo-Electromagnetic Phenomena  
To understand the mechanisms of seismo-electromagnetic phenomena in the lithosphere, various experiments and simulations have been conducted [171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 181]. The mechanisms proposed for the generation of electromagnetic signals include: electrokinetic effects [182; 183], piezo-electric effects [184], piezomagnetic effects [185], triboelectric effects [186], induction effects [187; 188], microfracture effects [89; 189], and P-holes [190; 191; 192; 193]. For most of these models, electromagnetic signals are more or less related to stress or strain changes. The numerical simulation of coupled seismic and electromagnetic waves in multilayered porous media were developed to explain the coseismic electromagnetic signals [194; 195]. However, field observations can hardly record strong, clear co-rupture signals [11]. The selectivity phenomenon of SES might explain why some electromagnetic anomalies are observed before earthquakes and others are not [175]. One of the most studied issues in the physics of earthquake precursors is the role of seismic-induced radon exhalation in the generation of electromagnetic disturbances. It has been proposed that the enhancement of the total rock surface due to failure would increase the emissions of radon and other gases from grains and facilitate migration. This would be in agreement with the enhancement of radon concentration observed in aftershocks and some laboratory experiments [196] which showed the growth of radon emissions of granites under compressional stress. Although a number of studies have discussed possible mechanisms of pre-earthquake disturbances in the atmosphere [129; 192], the generation process is still not well understood. Most hypotheses were based on the propagation of EM waves, which originated from the ground due to certain mechanisms such as piezo-electric effects [80], the triboelectric effect [186], and positive holes (P-holes) [190]. [PERSON] and [PERSON] [129] discussed a model based on the emissions of radioactive gases or metallic ions. [PERSON] and [PERSON] [197] proposed a unified concept for the pre-earthquake atmospheric disturbances, named the \"Lithosphere-Atmosphere-Ionosphere Coupling (LAIC)\" model. According to the LAIC model, radon can generate local ionization in the lower layers of the atmosphere that can facilitate water vapor condensation, leading to the release of latent heat exhalation (that could explain thermal fluctuations) and local variation of the conductivity, thereby impacting the electric circuit over the earthquake preparation zone and generating the observed seismo-associated ionospheric anomalies. [PERSON] et al. [198] argued that enhanced ionization in the lower atmosphere would increase the vertical cur rent flow in fair weather, which might help understand earthquake-cloud coupling [199] through the global atmospheric electric circuit [200]. [PERSON] et al. [201; 202] performed some numerical simulations and showed ionosphere plasma bubbles and density variations induced by pre-earthquake rock currents and associated surface charges. Other mechanisms including the atmospheric gravity waves (AGWs) triggered by gas releases or thermal anomalies [203] and electric current of seismic origin due to the convective transfer of charged soil aerosols [204; 205] can also explain pre-earthquake disturbances in the atmosphere to some extent [206]. The coseismal lower atmospheric and ionospheric perturbance triggered by seismic waves through acoustic and/or gravity waves was verified for the M9.0 Tohoku earthquake [207]. Some researchers present a potential physics-based research approach that takes into account the interaction among observation, data processing methodology, and physical model [208]. The coupling between the lithosphere and atmosphere requires multiple instruments installed on and near the Earth's surface and space satellites.