Science

The magnetic anomalies preceding the earthquakes finally explained?

If seismology is a relatively well understood field today, certain gray areas still persist. This is the case with seismomagnetism, i.e. the emergence of underground magnetic anomalies preceding the occurrence of an earthquake. Known for several years by seismologists, this phenomenon remained unexplained. But researchers may finally have found an explanation.

In recent years, seismologists have detected brief and subtle anomalies in underground electric fields leading to an earthquake, sometimes occurring as early as a few weeks before the earthquake. It’s tempting to think that these electromagnetic bursts could be used to predict when an earthquake will occur. Until now, however, the cause of these bursts was not certain.

Interactions between rocks and gas: they produce large electrical currents

New research suggests the key lies in gases that get trapped in what’s called a fault valve and can build up before an earthquake. These impermeable layers of rock can slide through a fault, effectively creating a portal that blocks the flow of groundwater.

When the fault valve eventually cracks and the pressure drops, carbon dioxide or methane dissolved in the trapped water is released, increasing in volume and spreading the cracks in the fault. As the gas emerges, it is also electrified, with electrons released from cracked surfaces attaching to gas molecules and generating a current as they travel upward.

The results supported the validity of the present working hypothesis, that the coupled interaction of fracturing rock with deep Earth gases during the quasi-static fracture of rocks in the focal zone of a fault could play a role. important role in the generation of seismic electromagnetic pre- and co-phenomena », Write the researchers in their article.

A hypothesis supported by experience

Using a custom experimental protocol, the team was able to test the reactions of quartz diorite, gabbro, basalt, and fine-grained granite in small-scale earthquake-like simulations. They showed that electrified gas currents could indeed be linked to the fracture of the rock.

The researchers reproduced in the laboratory the interactions between rocks and gases (top: quartz diorite + CO2; bottom: gabbro / basalt + CO2 / N2 / CH4), and observed the emergence of electric current producing a detectable magnetism. © Yuji Enomoto et al. 2021

The type of rock makes a difference, the researchers found. Rocks, including granite, have lattice faults that capture unpaired electrons over time by natural radiation rising below the surface, which leads to a larger current. And the type of flaw also seems to have an effect. The study confirms previous research by the same scientists on seismic electromagnetism, showing how carbon dioxide released from a seismic fault could be electrified and produce magnetic fields.

Other hypotheses on electromagnetic bursts include the idea that the rocks themselves could become semiconductors under sufficient stress and with sufficient heat, while other experts do not think these strange bursts are predictors. at all.

As a result of this laboratory experiment, it might be possible to detect the electrical signal accompanying an earthquake by observing the telluric potential / current induced in a conductor, such as a steel water pipe buried underground. Such an approach is currently being tested in the field on models. », Conclude the researchers.

Sources: Earth, Planets and Space

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