The Variation of Waveform and Analysis of Composition for the Geoelectrical Field before Moderate and Strong Earthquakes in Qinghai‐Tibetan Plateau Regions

Abstract

Site‐selective phenomenon exists in the normal waveforms of the geoelectrical fields of Qinghai‐Tibetan Plateau and its adjacent areas: rock structure of the sites, cracks and fissure water, tectonic activity and other factors affect the normal waveforms of the geoelectrical field; a large lake will contribute to the occurrence of TGF‐A waveforms nearby, TGF‐B waveforms will mostly occur in the areas where there are thick Quaternary sediments, and high water content and good water permeability of rocks, while the geoelectrical fields with no diurnal variations are prone to appear in the bedrock mountains where there are intense tectonic activities. Before moderate or strong earthquakes in the region, short‐term and impending anomaly of the geoelectrical field has a cluster phenomenon, in which there is a temporal synchronization or a few days' difference, while spatially it can be distributed near several faults in the region, manifested as a discrete phenomenon. This phenomenon of temporal clustering and spatial scattering is the short‐term and impending precursor characteristics of the geoelectrical field in the region. The Σ – Δ summation method is introduced to analyze the usual stability of spontaneous field through the observational data of the geoelectrical field, which indicate that before a moderate or strong earthquake, background value jumps of part of geoelectrical fields are a transformation phenomenon of spontaneous field; the concept and calculation method of TA, which is the sum of the first 10 order tidal harmonic amplitudes divided by the sum of the first 100 order harmonic amplitudes, are established to show quantitatively that the tidal effect of waveforms of TGF‐A, TGF‐B and non‐diurnal variation is successively reduced; the first order difference method of ΔE is adopted to analyze the higher frequency electromagnetic components in the minute order geoelectrical field monitoring data, usually the high‐frequency jumps show a certain degree of randomness and limitedness. By using three different methods of analysis, the features of spontaneous field, the telluric field and high‐frequency electromagnetic components are analyzed from the composition of the geoelectrical field, which provides a theoretical and methodological basis for the physical analysis of the geoelectrical field.

Correlation between abnormal trends in the spontaneous fields of tectonic plates and strong seismicities

“…For example, on the Qinghai-Tibet plateau, where tectonic activities are intense, various forms of short-impending anomalies could always be detected at more than ten remote E SP or E T stations before and after major earthquakes in recent years. The occurrence time of these anomalies exhibited quasi-synchronization (Tan et al 2010(Tan et al , 2012. In recent years, studies have shown that morphological differences and time quasi-synchronization were also present in the regional trend anomalies of E SP and DE SP (Tan et al 2014), as shown in Fig.…”

“…Figure 2e shows that during 2008-2009, the EW and NE long polar distances at Yanyuan station exhibited distinctive diurnal waveforms. E SP , E T , and the adjacent Lugu lake station exhibited quasi-synchronization (Tan et al 2010(Tan et al , 2011(Tan et al , 2012(Tan et al , 2014. However, the day correlation coefficients of long and short distances data in all directions were lower than 0.3.…”

“…When formula (2) is applied for calculation, the relatively stable main composition of E T , namely the effect of tidal geoelectric field, is eliminated (Tan et al 2012):…”

“…A possible explanation is that there were spectrum and other differences between the pre-seismic SESs and the contemporary seismic signals despite the lack of evidence. Thus, the application of long and short polar distances is mainly to explore the short-impending SESs from the seismic sources, nearfield variation of geoelectric fields, tidal waves, and distortions, which can be regarded as ''interference'', despite the existence of these variations (Huang and Liu 2006;Tan et al 2010Tan et al , 2011Tan et al , 2012Tan et al , 2014. Figure 2e shows that during 2008-2009, the EW and NE long polar distances at Yanyuan station exhibited distinctive diurnal waveforms.…”

Features of Regional Variations of the Spontaneous Field

“…Zhao and Lu (2003) pointed out that there existed a correlation between the diurnal variation of apparent resistivity and solid tides; Huang et al (2005; believed that when extracting relatively weak seismic electromagnetic signals from the complex electromagnetic environment, it was necessary to explore a physical analysis method combining math, signal processing and seismic electromagnetic physical process; in the years between 2006, Huang and Liu (2006, Zhang et al (2007) analyzed the spectral characteristics of the solid tidal phenomena of geoelectric field and the variations before earthquakes. Based on the research results on space Sq current by Xu (1992) and Zhao et al (2008), and those on solid tidal waves of geoelectric field by Huang et al (2010a), Tan et al (2010) classified the tidal wave of geoelectric field into two categories of TGF-A and TGF-B, he explored the preliminary principles of physical resolution for spontaneous field and telluric field (Tan et al, 2012).…”