Earthquake lightning concentrated on the rock slides

Recent laboratory experiments show that the flashes of light seen during earthquakes come from rocks that fracture during landslides caused by ground shaking.

Written by Melissa L. Weber, science writer (@ Melwriter)

Citation: Weber, ML, 2021, Earthquake Lightning Anchored in Rock Slides, Tumbled, http://doi.org/10.32858/temblor.155

Sometimes, when the ground shook during an earthquake, witnesses reported seeing flashes of light, called earthquake lightning (EQL). These bright lights can illuminate an entire slope and take many different shapes, from diffuse glow to vertical stripes. They are even sometimes mistaken by eyewitnesses for volcanic eruptions, according to historical records.

Scientists recently conducted experiments in the laboratory to show that landslides caused by Earth’s shaking could be responsible for at least some of these flashes. The experiments were led by physicist Yuji Inomoto. He and his team tested whether the earthquake lightning observed in Japan could be explained by the cracking of various types of rocks as they slid, scraped and shattered during landslides.

Although the rocks appear to be solid, they contain minerals and even gases that are bound together by chemical bonds. When rocks are fractured during landslides, they can release gases and electrons, causing a visible flash of light called photoemission. Quartz, a mineral found in granite, can generate an electrical charge under pressure, called the piezoelectric effect, which has been used to explain EQL in the past, where rocks collide with each other during a landslide. But EQL has been observed in places that do not contain granite, leading some to question the cause of these flashes. Inomoto said he believes unexplained mysteries must be challenged and solved.

In a recent study published in Earth, Planets and Space, Enomoto and colleagues present a set of their laboratory experiments, which are designed to mimic the conditions different rocks undergo during a landslide. They were able to photograph the light from some rocks in their experiments.

Light emission from granite sample during experiment. Credit: Tsuneaki Yamabe

A variety of rocks in Japan

Japan’s geology is complex. Several active faults cut across the landscape and earthquakes are common. Enomoto and his research team surveyed historical Japanese earthquake archives and identified five earthquakes with numerous eyewitness accounts of EQL, which are alleged to have been caused by landslides.

Scientists collected rocks from earthquake-caused landslide sites and smashed them up in the laboratory using an instrument that measures energy release during impact. They did this to simulate the bouncing of rocks on a hill and colliding with each other during a landslide. Using a digital camera, they photographed the light emitted during the collision. Thanks to the spectrophotometer, they were also able to take temperature measurements.

The test results showed that light emission processes, or flashes of light, differ for different rock types. Granite samples produced the brightest light emission. Other rock types produced flashes of red or orange of varying intensity. Some rocks did not show any photoreceptors.

It’s not surprising that this kind of impact study generates an observable light emission, said David Lockner, a geophysicist with the U.S. Geological Survey. “Connecting individual earthquakes has always been a more difficult proposition,” he said.

Optical images of two types of rocks used in the study. Credit: Modified from Enomoto et al. (2020), CC-BY4.0

It can light up hot rocks

Light emission is also affected by temperature rise due to friction during collision. In some cases, the flammable gases present in some rocks may lead to light emission. According to Inomoto and colleagues, the electrons that are stimulated by temperature are most excited when rocks are broken, and ionizing particles may be in the atmosphere.

The intense heat may not seem like an obvious occurrence during a landslide, but a study published last year in the journal Nature discussed research by a team led by Runqiu Huang of Chengdu University of Technology in China on “a massive landslide in China”. Their study showed that the landslide caused by the Wenchuan earthquake in 2008 with a magnitude of 8.2 degrees generated enough heat to vaporize some sliding materials. Scientists at Chengdu University concluded that temperatures at the boundary between the slide and the intact slope reached at least 1,562 degrees Fahrenheit (850 degrees Celsius).

The Enomoto group was able to estimate the effect temperature using the light emitted from the granite. Inomoto said the recorded temperature was around 816 degrees Fahrenheit (1500 degrees Celsius).

Inomoto said he believes all causes of seismic lightning are due to the physical and chemical processes occurring in the focal area of ​​the earthquake and subsequent landslides. “However, whether or not EQL is monitored depends on the geological, geographic, meteorological and ionospheric conditions,” he said.

Not all seismic lightning cases can be explained by this study. Enomoto and his team are now trying to solve a mysterious phenomenon that occurred in the 1995 Kobe earthquake. Eyewitnesses reported that the sky was bright in the west, which turned bluish purple, white, then red, while it should have remained dark. Mysteries like these will continue to drive research as we seek logical explanations for events that cannot be easily explained, including seismic lightning.

Further reading

Enomoto, Y., Yamabe, T., Mizuhara, K., Sugiura, S., & Kondo, H. (2020). Laboratory examination of earthquake lightning from landslides of Earth, planets and space, 72 (1), 1-8.

Hu, W., Huang, R., McSaveney, M., Yao, L., Xu, Q., Feng, M., & Zhang, X. (2019). Superheated steam, hot carbon dioxide, and dynamic recrystallization from frictional heat lubricated the giant landslide: a field and guide. Earth and Planetary Sciences Letters, 510, 85-93.

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