Review of unprecedented ULF electromagnetic anomalous emissions possibly related to the Wenchuan <i>M</i><sub>S</sub> = 8.0 earthquake, on 12 May 2008

Abstract

Abstract. This work presents ground based ultra low frequency (ULF) electromagnetic field measurements in the frequency range 0.1–10 Hz from January 2007 to December 2008. In this time period a strong earthquake series hits the Wenchuan region with a main shock of magnitude MS = 8.0 on 12 May 2008. The Hebei ULF electromagnetic observation network includes eight observation stations in north China and the observation system named E-EM is employed to record the electric potential difference between two electrodes with an analog automatic real-time continuous pen recorder. First, weak electric signals appeared on 11 October 2007 at Ningjin station, most of which are with relative long periods ~0.4–3 s and unequal amplitudes ~0.5–20 mm. Then, similar signals appeared at Gaobeidian station at the end of October. Abnormal behavior with various time intervals appeared randomly and not every day. At the beginning of April 2008, one and a half months before the Wenchuan MS = 8.0 earthquake, the anomalies were gradually subject to an intensive increase mainly in Gaobeidian SN direction and Ningjin EW direction. The abnormal behavior appeared almost every day and the amplitudes of electric signals, with short periods of ~0.1–0.3 s, enhanced to ~3–30 mm. Qingxian station started to record marginally high frequency signals in SN and EW components in the middle of April. On 9 May, 3 days before the main shock, the amplitude of high frequency information increased sharply at the same time in two components at Gaobeidian station and the maximum amplitude was up to 70 mm, i.e. 1.3 mV m−1 for the electric field. This situation did not stop until 17 May, 5 days after the main event. However, this kind of climax phenomena did not happen at Ningjin station and Qingxian station. Then weak anomalous information lasted about four months again, and strong signals appeared again for a short time before several powerful aftershocks. It is the first time that an abnormity with so large an amplitude and so long a duration time in the observation history of this network though several strong earthquakes were recorded. Furthermore, no obvious interferences have been found during this period. So this event is possibly related to this shock although all these three stations are more than 1300 km away from the Wenchuan earthquake epicenter.

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

“…4b) The SN information lasted till the end of September 2008 except for high emissions appearing before several powerful aftershocks. It is the first time that the abnormality is with such a large amplitude and such a long duration in the observation history of this network although several strong EQs were recorded before (Li et al, 2013).…”

“…Furthermore, the comparison between the observation distance reported by Li et al (2013) (D = 1440 km) and the length of the Wenchuan earthquake main rupture L = ∼ 150 km (Zhang et al, 2009a) indicates that the length of the dipole source is not negligible. So in this paper, based on the work of Key (2009), a three-layer (Earth-air-ionosphere) physical model as well as a two-layer (Earth-air) model, containing a finite length dipole current source co-located along the fault and beneath the Earth, is introduced in Sect.…”

“…More details of the observation system can be found in Zhuang et al (2005) and Li et al (2013). The system measures electrical signals and a DJ-1 recorder is employed to record the potential difference between two electrodes (SN: south-north and EW: east-west).…”

“…A more notable example reported by Li et al (2013) is the Wenchuan M S 8.0 earthquake on 12 May 2008, a typical mid-crust, which resulted in great devastation and 69 000 deaths. This earthquake was preceded by more than 1 month of increasing anomalous ULF emissions with a climax starting on 9 May, 3 days before the Wenchuan main shock (f = 0.1-10 Hz, D = 1440 km, A = 1.3 mV m −1 ).…”

“…In Sect. 4, using assumed parameters, the simulation results for the case of the Wenchuan earthquake reported by Li et al (2013) are presented. Discussion and conclusions are given in Sects.…”