A Signal in Giant Earthquakes That Could Save Lives

By Anonymous

Trilobites

The full power of the biggest temblors could be determined in as little as 10 to 15 seconds after they begin, a new study finds, and long before it ends.

ImageA Signal in Giant Earthquakes That Could Save Lives
Rescue workers searched Balaroa village after a 7.5-magnitude earthquake struck Palu, Indonesia, on Oct. 6.CreditCreditUlet Ifansasti/Getty Images

Seismologists have never had a better understanding of earthquakes. But tragedy after tragedy shows that quakes still surprise and shock people with their mercurial behavior. Precise predictions of when and where quakes will occur, and how deadly they may be, are not yet possible. If, however, researchers could chronicle how quakes grow, they might be able to better forecast how powerful they will become.

The mightiest quakes are far from instantaneous. They can last minutes, which makes them less like a single subterranean blast and more like a series of explosions moving outward. A new study, published on Wednesday in Science Advances, explains that the outward journey of these explosions differs depending on the power of the quake.

That means that the final magnitude of a quake could be determined in as little as 10 to 15 seconds after it begins, and long before it ends.

[Like the Science Times page on Facebook. | Sign up for the Science Times newsletter.]

A single-digit leap in earthquake magnitude means that 32 times more energy is being released. Many factors determine the hazard level of a quake, but small increases in magnitude can make the difference between merely damaging and catastrophic. If final earthquake magnitudes could be ascertained early on, it would give rapid, more precise warnings to populations yet to be shaken.

Diego Melgar, an assistant professor of seismology at the University of Oregon, explained that this connection was not what he and the paper’s other co-author, Gavin P. Hayes of the United States Geological Survey, were originally looking for. Instead, they had been gathering data from quake databases to make the most accurate simulations of the most powerful quakes.

“And along the way we just stumbled upon something interesting,” Dr. Melgar said: a key moment in time that frames an earthquake’s future.

The team took a close look at 3,000 earthquakes recorded by the agency’s seismometers. The data captured by these sensors can show the energy release of an earthquake over time far from the source. The researchers also dug through 30 quakes’ worth of GPS station data, where an antenna bolted to the ground tracks the development of the rupture close to the earthquake.

Building on earlier work, the team described how large earthquakes evolve. Immediately after they start, they grow chaotically for a few seconds, a pandemonium that lasts longer for more prolonged quakes. The rupture then organizes itself — for reasons that are for now unclear — into something resembling a pulse, a ring-shaped area that moves outward from the source of the quake over time.

This pulse ring denotes where the rock is breaking or slipping. A thinner pulse is less likely to keep growing into a bigger event, whereas a thicker pulse is more likely to do so. The team argues that because of these differences, the dimensions can be used to determine the quake’s final magnitude mid-rupture.

ImageA Signal in Giant Earthquakes That Could Save Lives
Workers inspected a collapsed road in Anchorage, Alaska, after a 7.0-magnitude earthquake in November.CreditMike Dinneen/Associated Press

Since the 1980s, seismologists have been debating if such a feat is possible. Some said that final magnitudes could be calculated right at the quake’s birth, while others suspected that seismologists would have to wait for the rupture to terminate. Others, like Dr. Melgar and Dr. Hayes, fall somewhere in between.

Stephen Hicks, a seismologist at Imperial College London who was not involved with the research, said the data suggested that the correlation between rupture evolution and final magnitude was not a coincidence. The way in which big quakes accelerate may be a recurrent feature among the overarching chaos.

Men-Andrin Meier, a seismologist at the California Institute of Technology, said that his own research also showed it was possible to determine final magnitudes mid-quake. But he differs from Dr. Melgar and Dr. Hayes on how soon into the rupture the final magnitude can be calculated. Their new paper places it at around 10 seconds, but Dr. Meier says that this depends on magnitude and can vary wildly.

One limitation of the new model is that it assumes an average earthquake behavior. In reality, “any individual earthquake still has a personality,” Dr. Melgar said. The evolution of certain earthquakes may not fit with expected patterns, making mid-rupture calculations of final magnitudes more difficult or, in some cases, easier.

Dr. Melgar also acknowledges that powerful quakes, especially those above magnitude 8.5, are rarer than their weaker counterparts. More data on big temblors, from simulations or real events, is required to shore up this model.

“It’s a good speculative idea, we just need to fill it in before we can have a lot of confidence in it,” said John Vidale, a professor of seismology at the University of Southern California.

Recent reporting on earthquakes