Recent lab experiments show that flashes of light seen during earthquakes come from rocks that fracture during landslides caused by shaking ground.
By Melissa L. Weber, science writer (@Melwriter)
Citation: Weber, M. L., 2021, Earthquake lightning is grounded in rock slides, Temblor, http://doi.org/10.32858/temblor.155
Sometimes, when the ground shakes during an earthquake, witnesses report seeing flashes of light, so-called earthquake lightning (EQL). These bright lights can illuminate an entire hillslope and take several different forms, from a diffuse glow to vertical streaks. They are sometimes even mistaken by eyewitnesses for volcanic eruptions, according to historic records.
Scientists have recently conducted experiments in a lab to show that landslides caused by shaking ground can account for at least some of these flashes. The experiments were led by physicist Yuji Enomoto. He and his team tested whether earthquake lightning observed in Japan might be explained by the fracturing of various rock types as they slide, scrape and crash during landslides.
Although rocks look solid, they include minerals and even gases held together by chemical bonds. As rocks break apart during landslides, they can release gases and electrons, creating a visible light flash called photoemission. Quartz, a mineral found in granite, can generate an electric charge under stress, called a piezoelectric effect, which has been used to explain EQL in the past, as rocks slam into one another during a landslide. But EQL has been observed in places without granite, which leads some to wonder about the cause of these flashes. Enomoto said he believes that unexplained mysteries should be challenged and solved.
In a recent study published in Earth, Planets and Space, Enomoto and colleagues presented their set of laboratory experiments, which were designed to mimic the conditions different rocks experience during a landslide. They were able to photograph light from some of the rocks in their experiments.
Japan’s wide variety of rocks
Japan’s geology is complex; many active faults cut through the landscape and earthquakes are common. Enomoto and his research team surveyed the Japan Historical Earthquake Archives and identified five earthquakes with numerous eyewitness accounts of EQL, purportedly caused by landslides.
The scientists collected rocks from the locations of these earthquake-induced landslides and smashed them in a laboratory using an instrument that measures the release of energy during an impact. They did this to simulate rocks bouncing down a hillside and colliding with one another during a landslide. With a digital camera, they photographed light emitted during the collisions. They were also able to take heat measurements thanks to a spectroradiometer.
Their test results showed that photoemissions, or light flashes, varied with rock type. Samples of granite produced the brightest photoemissions. Other rock types generated flashes of red or orange of varying intensities. Some rocks showed no photoemission.
It is unsurprising that this kind of impact study would generate observable light emission, said David Lockner, a geophysicist at the U.S. Geological Survey. “Making the link to individual earthquakes has always been a more difficult proposition,” he said.
Hot rocks can make light
Photoemission is also affected by the increase in temperature due to friction during impact. In some cases, flammable gases contained in some rocks may create photoemission. Electrons that are stimulated by temperature are further excited when the rock breaks, perhaps ionizing molecules in the atmosphere, according to Enomoto and his coauthors.
Extreme heat may not seem like an obvious occurrence during a landslide, but a study reported last year in Nature discussed research by a team led by Runqiu Huang at Chengdu University of Technology in China on a “gargantuan landslide in China.” Their study showed that a landslide induced by the 2008 magnitude-8.2 Wenchuan earthquake generated enough heat to vaporize some of the sliding material. The scientists at Chengdu University concluded that temperatures at the boundary between the slide and the intact slope reached at least 1562 degrees Fahrenheit (850 degrees Celsius).
Enomoto’s group was able to estimate the temperature of the impact using the light emitted by the granite. Enomoto said the temperature recorded was about 816 degrees Fahrenheit (1500 degrees Celsius).
Enomoto said he believes that all causes of earthquake lightning are due to physicochemical processes occurring in the focal zone of the earthquake and subsequent landslides. “However, whether or not the EQL is witnessed depends on geological, geographical, meteorological and ionospheric conditions,” he said.
Not all earthquake lightning occurrences can be explained by this study. Enomoto and his team are now trying to solve a mysterious phenomenon that happened in the 1995 Kobe earthquake. Witnesses reported a brightened sky in the west, which turned bluish purple, white and then red, when it should have still been dark. Mysteries like these will continue to drive research as we seek logical explanations for occurrences that can’t be easily explained, including earthquake lightning.
Enomoto, Y., Yamabe, T., Mizuhara, K., Sugiura, S., & Kondo, H. (2020). Laboratory investigation of earthquake lightning due to landslide. 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 CO2 and dynamic recrystallization from frictional heat jointly lubricated a giant landslide: Field and experimental evidence. Earth and Planetary Science Letters, 510, 85-93.
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