The violence of shaking from an earthquake is usually focused in the direction the quake is traveling.But a boomerang quake may spread the intense shaking across a wider zone.The new study, published today in the journal Nature Geoscience,is a major step toward clearing up the complex physics behind boomerang quakes and understanding their potential damage.
The latest boomerang was recorded near the mid-ocean ridge in the Atlantic in 2016.By examining the position of the epicenter(震中)and the energy released by each shaking phase,Stephen Hicks,an earthquake seismologist at the Imperial College London and first author of the new study,identified what seemed to be two steps of the quake:The quake initially headed eastward,but then turned back west. "This was weird,"he says.This second leg of the quake moved remarkably quickly, at so-called supershear speeds.The quake unzipped the surface at an estimated 11,000 miles per hour—fast enough to dart from New York to London in 18.5 minutes,so fast that it caused the geologic version of a sonic boom.
How often does this happen?A boomerang quake at supershear speeds,as Hicks and his team observed in the Atlantic,may be a fairly rare type.But wider evidence of boomerang quakes is mounting.These back-tracking events have been studied in computer models as well as simulated in lab experiments."The theory says that it's there,but it's quite difficult to see that [in the real world],"says Hicks.The 2016 earthquake in Kumamoto also seems to have ruptured(破裂)in a similar process.For that event,the initial shake triggered two other quakes,one of which raced backward to partially overlap the initial break.
Understanding when and why these boomerang events happen is vital to grappling with risks earthquakes present."Studies like this help us understand how past earthquakes ruptured,how future earthquakes may rupture,and how that relates to the potential impact for faults(断层)near populated areas,"says Hicks.
These boomerangs maybe obscured by conventional approaches used to analyze quakes,which are based on an assumption that a quake rushes in one direction. "Naturally we're not looking for it,we don't expect it to exist,"Hicks says.Yet for earthquakes,it seems,complexities might be the norm rather than the exception.As Hicks puts it:"The more and more we look at earthquakes in more detail,of course we see stranger things."
