A new study published in Earth, Planets and Space sheds new light on the electromagnetic anomalies occurring before large earthquakes. The research supports the hypothesis that fault rupture progresses just before an earthquake, and the invading gas is charged and forms a large current, causing various electromagnetic anomalies.
It has been documented over hundreds of years that various electromagnetic anomalies occur a few weeks before the occurrence of a large earthquake. These electromagnetic anomalies are variations that appear in telluric current, geomagnetism, electromagnetic waves, etc. before the earthquake, authors of the new study say.
Although there are various models to explain the mechanism, the large current generated at the source was not fully explained.
For example, many researchers thought that the stress applied to the fault produced an electric current, but the stress applied to the fault takes place over hundreds or thousands of years before the occurrence of the earthquake.
It is a common belief among seismologists that it is impossible for the stress to suddenly increase and generate a large current just before the earthquake, and therefore the mechanism had not yet been explained.
To resolve this mystery, Shinshu University and Genesis Research Institute, Inc. conducted a joint research project on earthquake-preceding phenomena under the leadership of Professor Emeritus of Shinshu University, Dr. Yuji Enomoto. The research group made the following hypothesis and conducted laboratory experiments on indoor rock fracture and gas-electric interactions to solve the mystery of electromagnetic anomalies.
In the area that is at the epicenter of a seismic fault, a fault-valve forms before the next earthquake occurs. It is believed that dense layers in the crust are formed over time. The fluid, including some gases such as water that springs up from the vicinity is trapped by the fault valve and stays there. When the shear stress applied to the fault or the pressure of the stagnant reserved fluid reaches criticality, the fault valve cracks, the high-pressure fluid rises along the fault, and the pressure gradually decreases.
As the pressure decreases, carbon dioxide or methane that are now dissolved in the fluid are degassed at once, expanding in volume and expanding the cracks. The model considers the fault becomes fragile and the rupture accelerates, leading to an earthquake. The gas becomes electrified in the process. That is, it is charged with electricity. The trapped electrons in the defects are suddenly released due to the thermal stimulus and attached to gas molecules. Because it is negatively charged, a current is generated as the gas moves.
In the lab, several types of rock, including granite, gabbro, quartz diorite and basalt were tested. A simple estimation found that there is a high possibility that a large current will be generated immediately before the earthquake, depending on the earthquake magnitude.
This supports the above-mentioned hypothesis that fault rupture progresses just before an earthquake, and the invading gas is charged and forms a large current, causing various electromagnetic anomalies. In the future, the group plan to carry out field observations to verify this model.
"Laboratory investigation of coupled electrical interaction of fracturing rock with gases" - Yuji Enomoto et al. - Earth, Planets and Space - April 15, 2021 - http://dx.
In the coupled electric interaction of rock fractures and gas invasion, that is, when gases interact with newly created crack surfaces, the unpaired electrons within the rock crystal defects are thermally stimulated, released into the crack due to the temperature rise at the crack tip via plastic work, and attached to ambient gas molecules to electrify them in a negative state. Using a working hypothesis that this mechanism is the source mechanism of seismo-electromagnetic phenomena, we conducted laboratory experiments in which rocks were fractured with pressurized N2, CO2, CH4, and hot water vapour. Fractures were induced by a flat-ended indenter equipped with a flow channel, which was loaded against blocks of quartz diorite, gabbro, basalt, and granite. Fracture-induced negatively electrified gas currents at ~ 25 °C and ~ 160 °C were successfully measured for ~ ≥ 100 μs after full development of the crack. The peak electric currents were as high as 0.05–3 μA, depending on the rock species and interaction area of fractured rock and gas and to a lesser extent on the gas species and temperature. The peak current from fracturing granite, which showed higher γ-ray activity, was at least 10 times higher than that from fracturing gabbro, quartz diorite, and basalt. The results supported the validity of the present working hypothesis, that coupled interaction of fracturing rock with deep Earth gases during quasi-static rupture of rocks in the focal zone of a fault might play an important role in the generation of pre- and co-seismic electromagnetic phenomena.
Featured image credit: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
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