A 7.2 magnitude earthquake struck Rosario García González’s home in Baja California on a spring afternoon in 2010. González, an elder of the native Cucapah community, later recounted the scene. Notable to scientists: When the earthquake cracked the surface, it triggered the dust cloud, like a car racing on a bush landscape
But it looks like the car went the wrong way.
Earthquakes often crack the surface moving in one direction, like the tear of a piece of paper. But according to González, a cloud of dust from the ongoing earthquake is rushing towards where the temblor came from ̵1; the exact opposite direction scientists expected.
This first-hand witnessing story of an earthquake running upstream touched scientists. Orlando Teran, who at the time was doing a Ph.D. at the Ensenada Center for Scientific Research and Education, called the “spectacular” description. But exactly what happened that day is yet to be confirmed, as seismic evidence cannot verify what González had seen.
Now, an international team of researchers has finally captured one of these boomerang quakes in great detail, documenting the temblor racing in one direction and then reversing the way it did. happening.
This 7.1-magnitude quake started deep underground, during an earthquake in the Atlantic seabed, a little more than 650 miles off the coast of Liberia, in western Africa. It darted east and up, then turned around and back along the upper part of the fault with astonishing speed – so fast that it caused a geological version of an explosion of sounds.
The intense shaking from an earthquake is usually focused in the direction in which the temblor moves. But a boomerang earthquake, or a “back-propagating rupture,” in scientific terms, can spread violent shaking over a larger area. It’s not yet clear how common boomerang earthquakes are – and how many move at such great speed. But the new study, published today in the journal Natural geoscience, This is an important step towards untangle the complex physics behind these events and understanding their potential dangers.
“Studies like this help us understand how earthquakes in the past can break, how earthquakes in the past can break up,” said Kasey Aderhold, a seismologist with the Seismological Research Foundation. the future is likely to fail and that has to do with the potential impact on faults near densely populated areas ”.
A kick to the ground
The latest boomerang is recorded near the mid-ocean ridge in the Atlantic, where the South American and African tectonic plates slowly separate little by little. In the spring of 2016, scientists placed 39 seismographs near the mountainside to collect the rumble of distant earthquakes in an attempt to visualize the foundation of the tectonic plate.
A few months later, a magnitude 7.1 earthquake struck. Stephen Hicks, an earthquake seismologist at Imperial College London and the first author of the new study, said the temblor struck a nearby bug in a place known as the Romanche Fault Area.
The seismometer team faithfully recorded the ground vibrations in a series of squeals, including what appeared to be a pair of pulses. Intriguingly, Hicks and his colleagues took a closer look, determining what were the earthquake’s two steps. By examining the position of the epicenter and the energy released by each rumbling phase, the team connected geological points: The earthquake initially headed east, but then turned back west. . “This is an odd kind of configuration to watch,” he said.
The team is still not sure if the earthquake has actually been reversed. So Hicks reached out to Ryo Okuwaki of the University of Tsukuba in Japan, who was looking for faint echoes of the event that other seismometers around the world encountered. In just a few days, an analysis of these distant traces came up with the answer: The earthquake was likely to have caused shock.
Deeper computer modeling suggests that the earthquake may have started deep underground, plunging east until it reached the mid-ocean ridge. There, it comes back and runs through the upper part of the error. This second leg of the temblor moves remarkably rapidly, at a speed known as super-speed. The quake unpacked the surface at about 11,000 miles an hour fast enough to dart from New York to London in 18.5 minutes. This is so fast that seismic waves piling up like a Mach cone form from pressure waves as an aircraft flies at supersonic speeds. The focused cone of waves from an ultrasonic earthquake can further amplify the temblor’s destructive power.
A boomerang beam
Understanding when and why these boomerang events occur is critical to coping with the range of earthquake hazards that exist. Vibration from an earthquake can be concentrated near one end of the bug, in the direction in which the temblor is moving, similar to muting the tone of the horn as a train rushes. “Like the Doppler effect,” said seismologist Lingsen Meng of the University of California, Los Angeles, who was not part of the study team. While this focused vibration is generally thought to occur in one direction, the boomerang can focus the vibrations in opposite areas. And if it was super sonic, the clatter could have been even worse.
But at least one big question remains: How often does this happen?
A boomerang earthquake at supersonic speed, as the team observed in the Atlantic, can be a rather rare breed. “To the best of my knowledge, this is the first time it has been reported,” said Geophysicist Yoshihiro Kaneko of GNS Science in New Zealand, who was not part of the research team.
But more widespread evidence of boomerang earthquakes is on the rise. These reverse-tracked events have been studied in computer models as well as simulated in laboratory experiments. “The theory says it’s there, but it’s quite hard to see that [in the real world], ”Says geophysicist Louisa Brotherson, a doctoral researcher at Liverpool University in the UK who simulates earthquakes in the lab.
Seismologist Jean-Paul Ampuero of Côte d’Azur University in France said boomerang fractures have been observed in slow earthquakes that do not occur with strong tremors but progress slowly over days or even months. Recently, he identified backward propagating earthquakes in computer simulations.
There have also been hints of these events for other earthquakes. Some scientists have argued that the 9.0-Richter Tohoku earthquake that occurred in Japan in 2011, the most powerful earthquake in the country’s recorded history, may have caused some faults, Meng noted. Kaneko added that the 2016 earthquake that rocked Kumamoto appeared to have broken in a similar process. For that event, the initial shake triggered two other earthquakes in a series of events, one of which ran backwards to partially overlap the original breakpoint.
“This may be actually more common than we think,” said Kaneko.
These boomerangs can be obscured by common methods used to analyze earthquakes, based on the assumption that a temblor plunges in one direction. “Of course we weren’t looking for it, we didn’t expect it to exist,” Ampuero said. With respect to earthquakes, however, it seems, complexity may be the norm rather than the exception.
As Hicks puts it: “The more we are looking at earthquakes, of course the more we see strange things.”