# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 2 Seismo-Electromagnetic Phenomena in the Lithosphere  
The seismo-electromagnetic phenomena in the lithosphere are commonly recorded by passive ground-based observations of electric fields and/or magnetic fields [18]. Early seismo-electromagnetic studies measured perturbations of the electric potential difference between two electrodes buried in the ground. The case study of an M5.6 earthquake (depth = 17 km) was reported by [PERSON] [19]. The observation carbon rod electrodes were set at an epicentral distance of 3 km. They found that potential differences changed clearly prior to the earthquake. This kind of signal was known as the seismic electric signal (SES). The most famous SES may be the one reported from Greece by [PERSON], [PERSON], and [PERSON] (VAN) [20; 21; 22]. They observed anomalous electric signals on both short (10-200 m) and long (1-3 km) dipoles with electric field amplitudes of up to 20 \(\mu\)V/m and durations of a few minutes prior to several moderate-large earthquakes [21]. [PERSON] et al. [22] issued some predictions of earthquakes since 1990 using the SES. These predictions soon drew lots of attention and aroused active debates (see details in Vol. 23 Issue 11, _Geophysical Research Letters_, 1996). In addition to seismic electric signals in Greece, there are some examples of SES observed in various earthquake-prone areas worldwide [23]. Recently, the natural time analysis of geoelectric field and seismicity changes were combined to study the precursory SES activity [24; 25]. Magnetic field changes associated with earthquakes were also observed. One of the most promising candidates is the measurement of ultralow frequency (ULF) electromagnetic phenomena (less than 1 Hz), because of their deeper skin depths [12; 14; 15]. To date, now, a large number of ULF electromagnetic phenomena associated with earthquakes have been reported [10; 11; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. One of the most well-known ULF seismo-magnetic phenomena was the magnetic field changes related to the Ms7.1 Loma Prieta earthquake that happened on 18 October 1989, reported by [PERSON] et al. [40]. They found that the amplitude of ULF magnetic fields, especially at the frequency of 0.01 Hz, began to increase significantly about two weeks before the main shock. Due to the limitations of observation equipment, only the averaged amplitude of the magnetic field in the horizontal component was registered, which made it very difficult to analyze detailed waveforms for further study. Other case studies of the 1988 Spitak earthquake (M6.9) and the 1993 Guam earthquake (M 8.0) reported by the Russian group [41] and the Japanese group [42], respectively, also showed potential changes in the magnetic field before earthquakes. [PERSON] et al. [43] analyzed the geomagnetic data associated with the 1997 Kagoshima earthquakes (M6.5 and M6.3). By employing several reference stations, they eliminated the influence of global ionospheric disturbances and detected anomalous changes in the magnetic field, possibly related to the two large earthquakes. The 2000 Izu earthquake swarm was another important case for seismo-electromagnetic study, as there were both electric and magnetic measurements in the vicinity of the epicenters. By analyzing these data simultaneously, [PERSON] et al. [44] reported very robust results for seismo-electromagnetic phenomena. The ratio of electric potential between the two different observation dipoles at the frequency of 0.01 Hz has shown clear anomalous changes about two months before the earthquake swarm. These anomalies were unique against a 3-year-long background. The principal component analysis (PCA) of geomagnetic data also showed anomalies before the swarm.