Ionospheric influence on the seismo-telluric current related to electromagnetic signals observed before the Wenchuan <i>M</i><sub><i>S</i></sub> 8.0 earthquake

Abstract

Abstract. A three-layer (Earth-air-ionosphere) physical model, as well as a two-layer (Earth-air) model, is employed in this paper to investigate the ionospheric effect on the wave fields for a finite length dipole current source co-located at a hypocenter depth and along the main fault of an earthquake when the distance between the epicenter and an observing station is up to 1000 km or even more. The results show that all electrical fields are free of ionospheric effects for different frequencies in a relative short range, e.g., ∼ 300 km for f = 1 Hz, implying the ionospheric influence on electromagnetic fields can be neglected within this range, which becomes smaller as the frequency increases. However, the ionosphere can give a constructive interference to the waves passing through and make them decay slowly when an observation is out of this range; moreover, the ionospheric effect can be up to 1-2 orders of magnitude of the electrical fields. For a ground-based observable 1.3 mV m −1 electric signal at f = 1 Hz 1440 km away from the Wenchuan M S 8.0 earthquake, the expected seismo-telluric current magnitude for the Earth-air-ionosphere model is of 5.0 × 10 7 A, 1 magnitude smaller than the current value of 3.7 × 10 8 A obtained by the Earth-air model free of ionospheric effects. This indicates that the ionosphere facilitates the electromagnetic wave propagation, as if the detectability of the system were improved effectively and it is easier to record a signal even for stations located at distances beyond their detectability thresholds. Furthermore, the radiating patterns of the electrical field components |E x | and |E y | are complementary to each other, although any two-dimensional (2-D) power distribution of these components shows strong power areas as well as weak ones, which is advantageous to register a signal if the observing system is designed to measure both of them instead of only one.

Indications of Ground-based Electromagnetic Observations to A Possible Lithosphere–Atmosphere–Ionosphere Electromagnetic Coupling before the 12 May 2008 Wenchuan MS 8.0 Earthquake

“…Many researchers have investigated the EM effects since the occurrence of the Wenchuan MS 8.0 EQ on 12 May 2008. These studies were mainly based on the ground-based ultra low frequency / extremely low frequency (ULF-ELF) EM data [32,[34][35][36][37][38][39][40][41] and DC-ULF geomagnetic data [42][43][44][45][46][47][48][49][50][51].…”

“…Li et al [32,40,41] reported the outstanding electrical emissions which occurred at the Hebei ULF (0.1-10 Hz) EM observing network in northern China during the Wenchuan M S 8.0 EQ. The Hebei EM observing network was established during 1980s around the area of Beijing in northern China after the occurrence of the Tangshan M S 7.8 EQ on 28 July 1976.…”

Anomalous phenomena in DC–ULF geomagnetic daily variation registered three days before the 12 May 2008 Wenchuan <i>M</i>_S 8.0 earthquake

“…Lu J et al (2016) have reported that there are a 6.7% negative apparent resistivity anomaly at Pixian station (CD station here in Figure 2) before the Wenchuan EQ and coseismic changes of -0.61% at this station. suggested that the main rupture developed rapidly over the three days before the main shock, producing a strong seismo-telluric current that propagated mainly along the Longmenshan fault; Li M et al (2016) have similarly interpreted the observable 1.3 mV·m -1 electrical field at f = 1 Hz at Gaobeidian station, 1440 km away from the Wenchuan epicenter, using an ionospheric influence physical model. The three-layer model employed here, in our analysis, has also been proposed as a partial explanation of the geomagnetic daily variation observed.…”

“…The top medium (ionosphere: z < -100 km) is characterized by electrical properties μ -1 , ε -1 and σ -1 (10 -5 S·m -1 ). More details can be found in Li M et al (2016).…”

“…According to the actual observations, we set the x-direction to be along the Longmenshan fault of north 30° east (Xu, 2009); the surface coordinate system can be seen in Figure 6. We also use parameters utilized by Li M et al (2016), the dipole length L = 150 km, the Earth conductivity σ 1 =10 -3 S·m -1 , depth d = 19 km of the Wen- We present electromagnetic wave fields for the Wenchuan source and this is done in the ground plane region -1,000 km < x < 1,000 km and -1,000 km < y < 1,000 km in order to visualize the 2-D distribution of the wave power surrounding the electrical source. Figure 8b displays the 2-D power distributions of the vertical magnetic field components |B z | after making a logarithmic calculation on the Earth's surface.…”