ULF electric and magnetic anomalies accompanying the cracking of rock sample

Abstract

The anomalies of electric-magnetic field and self-potential before earthquakes are important precursory phenomena. A simulating experiment study on the variations in ultra-low frequency (ULF) magnetic field and self-potential during rock cracking was carried out in a magnetic field-free space. The results revealing in detail the whole process of the occurrences of electric and magnetic anomalies are significant for understanding the microscopic mechanism of ULF electric and magnetic signals. The experiment indicated that at the initial stage the slow changes in strain, self-potential and magnetic field with small amounts appeared firstly near the source of initial cracking, and then extended as the crack developed on. In the time domain, the self-potential anomaly emerged first and ULF magnetic field changes arose then. The shape of the ULF electric and magnetic anomaly varied obviously in early-, mid-and late-term of the test. The authors attributed the pulse-like changes of self-potential to the generation and movement of the accumulated electric charges during the cracking caused by charge separation on the crack tips within the sample. While the magnetic pulses of shorter-period at the last stage of the test, may be induced by instantaneous electric current of the accumulated charge during the cracking acceleration. The technical method and the observational results of this experiment are given in detail and the microscopic mechanism of electric and magnetic precursors before earthquake are discussed in the present paper as well.

New Result of the Experiment on Self‐Potential Change of Rocks Under Biaxial Compression

“…2, 3). It confirms the phenomenon previously observed in the expansion experiment [6] . It is noticed that these two kinds of variations emerged not only in the quartz-bearing granite but also in no-quartz-bearing gabbro that implies the variety of SP variation mechanism.…”

“…It was found that in the rock fracture experiment in the magnetic and electric field-free space [6] , many pulse-like SP signals similar to that reported by VAN occurred under the swell force by the acting of the expanding agent. It is plausible to suppose that this kind of signal seems related with the process of the stress accumulation-sudden release, i.e.…”

“…We realize from above-mentioned phenomena that the SP anomaly found in the experiment by means of the expanding agent [6] might be also caused by the stress accumulation-sudden decrease, i.e. caused by the inhomogeneous extension of the crack under the action of swell force.…”

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

“…Laboratory experiments by Qian et al (1996Qian et al ( , 2003 and Hao et al (2003) presented that electromagnetic signals are always recorded when rock samples are subjected to dynamic stresses. Shorter-period electromagnetic pulses appearing at the last stage of the experiment may be induced by instantaneous electric current of accumulated charges during the stress accelerating.…”

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

“…Abundant indoor and/ or outdoor experiments and numerical simulations have been conducted to verify the existence of SEMS phenomena (HUANG et al 1998;KUO et al 2014;ZHAO et al 2009;ENOMOTO et al 2012;REN et al 2012;POTIRAKIS et al 2012;HUANG 2011a). For example, radiation of ultra-low-frequency (ULF) electromagnetic signals was observed in the early, middle, and late stages of a rock-fracture experiment (HAO et al 2003). At the end of last century, the Greek physicists proposed the VAN method for earthquake monitoring on the basis of seismic electrical signals (SES).…”