11. But slower, secondary S waves, the other type of body wave, move particles perpendicular to the direction the wave itself is traveling: up and down or side to side.
12. Surface waves, which travel only at or near Earth’s surface, actually cause the most damage. These waves move slower than body waves but don’t lose their oomph as they travel.
13. While seismic waves are now well understood, science still can’t explain earthquake lights (EQL), multicolored flashes in the sky typically reported before or during many large tremors. EQL sightings date back to at least the fourth century B.C.
14. A 2014 study in Seismological Research Letters suggested that EQL may be the result of positive charges building up along the fault line as stress increases before the big shake.
15. Large earthquakes happen mostly along faults where tectonic plates meet as they move over the planet’s surface. But plenty of smaller earthquakes, most not even felt by humans, occur across the world every day due to detonations, such as nuclear weapons testing or mining, or rising magma linked to volcanic activity.
16. Magma on its way to the surface can fracture crust or expand existing fissures; it’s a process similar to the way wastewater pumped underground during oil and gas extraction operations can cause small-scale seismic events.
17. Non-tectonic temblors are usually magnitude 3 or less on the Richter scale, the famous but now antiquated way to measure a quake’s shake. The Gutenberg-Richter law, however, remains fundamental to both seismology and geophysics. Simply put, the law states that within a region, the bigger the quake, the less likely its occurrence.
18. In January, Scientific Reports published an update to the law, which researchers believe more accurately models risk for catastrophic quakes.
19. And it’s good timing: The planet appears to be experiencing an uptick in high-magnitude quakes. From 1977 to 1999, the world experienced zero seismic events that were magnitude (m) 8.5 or higher. Since 2004, however, we’ve had six big shakes of m8.5 or more.
20. But our fussy planet does go through dips and peaks in seismic activity. From 1950 to 1965, for example, no fewer than seven quakes of m8.5 or more jolted Earth. So don’t retreat to your bunker just yet.