Have you ever heard of "earthquake lights"? I've spent a good chunk of my life in shake-happy coastal California and this phenomenon is news to me—but, for centuries, people have reported seeing a wide variety of illuminations just slightly before and during major tremblers. The origin of these glows have consistently baffled scientists—and no doubt freaked the hell out of eyewitnesses—but a new study seems to have found an explanation (one that doesn't involve supernatural forces).
Nature reports on the extensive research led by Robert Thériault, a geologist at Quebec's Ministry of Natural Resources in Quebec City, Canada. He and his team embarked on a fascinating deep-dive into firsthand reports from 65 big quakes dating back to the 1600s; as it turns out, a whopping 63 of these took place on vertical geological faults. Apparently, this orientation allows for electric charges sparked by grinding rocks to travel upward to the earth's surface. Once they hit the atmosphere, they get all kinds of luminous.
It all starts with defects in a rock, where oxygen atoms inside a mineral's chemical structure are missing an electron. When the stress of an earthquake hits the rock, it breaks chemical bonds involved in these defects, creating holes of positive electrical charge. These 'p holes' flow can vertically through the fault to the surface, triggering strong local electric fields that can generate light.
A YouTube search for the phenomenon—so, grains of salt and all that—pulls up a handful of examples everywhere from Christchurch, New Zealand to Sichuan, China.
Maybe if more folks are aware of what to look for, the terrestrial radiance could be used as an early warning that the earth is beginning to move. As I'm writing this from famously Big One-prone San Francisco, you better believe that I will be keeping my eyes out for strange flashes in the sky. [Nature]
Lead image is a composite via Library of Congress & NASA