The Electromagnetic Phenomena Before the M_s8.0 Wenchuan Earthquake

Abstract

We processed the observational data of geo‐electric and geo‐magnetic fields around the epicenter of the Ms8.0 Wenchuan earthquake of 2008 in Sichuan Province using the maximum entropy spectrum and other methods, and studied the EM field changes related to the earthquake. The following understandings are got: (1) Significant EM phenomena including self‐potential changes, increased power spectrum density (PSD) of the geoelectric field, and waveform distortion of the geo‐magnetic field diurnal variation appeared along the northeastern edge of the Qinghai‐Tibet Plateau before the Ms8.0 Wenchuan earthquake. Especially, the PSD of the low frequency components with periods less than about 3 hours of the geo‐electric field were increased by more than 3 orders of magnitude. (2) Within about 500 km from the epicenter the PSD of the low frequency components of the geo‐electric and geo‐magnetic fields increased obviously. In particular, at Chengdu station which was 35km from the epicenter, the PSD of the geo‐electric field were increased by 4 orders of magnitude. (3) The spatial and temporal distributions of the EM phenomena and the great earthquakes occurred in the Qinghai‐Tibet Plateau, i.e. the Wenchuan and the Ms7.3 Yutian, Xinjiang, earthquakes have a tectonic relevance in a large spatial range. The mechanisms of the EM phenomena are discussed, as a result, it could be believed that in the processes of both the development of the numerous micro cracks in the underground media before earthquakes and the directional arrangement of the cracks, such influencing factors as the “electro‐mechanical transformation function” and the underground water migration and so on, induced the electric and magnetic changes. The EM phenomena before the Wenchuan earthquake were complex in temporal, spatial and intensity evolutions, and the possible reasons for the complex evolutions are the action of the local medium under the EM stations and the EM radiations excited by the focal preparation process.

Shifting Correlation Between Earthquakes and Electromagnetic Signals: A Case Study of the 2013 Minxian–Zhangxian M L 6.5 (M W 6.1) Earthquake in Gansu, China

“…Moreover, the correlation of the high-frequency component was not significant, especially preseismically. This is in agreement with the results of FAN et al (2010), FUJINAWA et al (1998) and PARK et al (1993 who performed studies on the frequency of electromagnetic anomalies.…”

“…The correlation coefficient at 0.01 Hz was 0.5 and 0.7, respectively. The lead time of the pre-seismic anomaly was approximately similar to that in studies of the Wenchuan, Lushan, and Izu Island earthquake by use of different methods (UYEDA et al 2002;MA et al 2013;FAN et al 2010). Moreover, the correlation of the high-frequency component was not significant, especially preseismically.…”

“…In fact, the SEMS may not be obviously abnormal signals because of low-intensity and/or mixing with the background field and interfering noise. This may be why no SEMS anomalies have been observed before and after some earthquakes, and why some abnormal electromagnetic signals cannot be related to the corresponding earthquakes (ORIHARA et al 2012;FAN 2010;HAN et al 2014;HATTORI et al 2012). Although many methods have been used to investigate and extract SEMS on the basis of ground and/or ionosphere measurements, the correlation between earthquakes and electromagnetic signals is still difficult to quantify solely on the basis of electromagnetic anomalies before and after a single event, not to mention the general features of complex and inconstant SEMS.…”

“…FAN et al 2010), time-frequency analysis (EFTAXIAS et al 2001), wavelet transform (ZHANG et al 2013;XIE et al 2013;HAN et al 2011), and principle-component analysis (UYEDA et al 2002;HAN et al 2009), have been used. The occurrence time, frequency band, and propagation path of SEMS have been studied (FUJINAWA et al 1998).…”

Earthquake-related Electric Field Changes Observed in the Ionosphere and Ground

“…Since the 1950s, the former Soviet Union, Japan, the United States, China, and other countries have carried out the ground electromagnetic observations and reported the electromagnetic precursory phenomena in the time and frequency domains before earthquakes (Varotsos et al 1986, Varotsos and Lazaridou 1991, Michel and Zlotnicki 1998, Zlotnicki et al 2001, Uyeda et al 2002, Telesca et al 2005, Enomoto et al 2006, Telesca and Hattori 2007, Nagamoto et al 2008, Tang et al 2008, Fan et al 2010, Liu et al 2011. Furthermore, a lot of loading experiments and theoretical studies have been made on the mechanism of generation and propagation of the electromagnetic signals (Ogawa et al 1985, Cress et al 1987, Yamada et al 1989, Enomoto and Hashimoto 1992, Varotsos et al 1998, Huang and Ikeya 1998, Adler et al 1999, Ishido and Pritchett 1999, Huang 2005.…”