On August 24, the San Francisco Bay Area was struck by a magnitude 6.0 earthquake centred on the Napa Valley. More than 100 people were injured and extensive damage caused to buildings, in what was the largest quake in the Bay Area for 25 years.
There was a strange phenomenon during this earthquake. Many people reported seeing mysterious flashes of light in the sky, and many wondered whether these were flashes of lightning, electrical transformers blowing up, or even UFOs. Twitter was abuzz with the reports, including “Everybody felt the earthquake but I’m the only one who saw the blue flashes in the sky??” said one Napa Valley resident. Another person tweeted, “Saw flashes of light that looked like lightning right after earthquake, then huge green flash.”
By coincidence, a scientist who has investigated other cases of earthquake lights is based in Palo Alto, south of San Francisco. Friedemann Freund claimed that the Napa Valley lights originated in stresses in rocks deep underground. When these rocks are stressed by seismic waves from earthquakes, they release electrical charges resulting in currents passing toward the surface and forming a “plasma-like state” that create displays of earthquake lights. And his theory is partly backed up by laboratory experiments that showed electrical fields can be generated in certain types of rock by squeezing them.
Earthquake lights have been reported in the past from many parts of the world, sometimes resembling bluish flames, lightning strikes, or floating balls of light — similar to the phenomenon of ball lightning sometimes seen during intense thunderstorms. There is even hope that earthquake lights, or the electrical currents in the ground, could lead to a breakthrough in predicting earthquakes. However, this phenomenon is rare and only occurs in less than an estimated 0.5 percent of earthquakes worldwide, so it remains of very limited use.