On the August day that an earthquake hit near the South Sandwich Islands in the South Atlantic last year, scientists knew something strange was happening.
“Within perhaps 20 to 25 minutes or so, we realized something odd was going on,” said Stuart Weinstein, deputy director of the Pacific Tsunami Warning Center, which is run by the National Oceanic and Atmospheric Administration.
Algorithms that characterize earthquakes were failing, he said. A tsunami was triggered that would send waves traveling around the world.
Now, new research suggests a hidden earthquake -- larger than initial estimates -- sent that tsunami from the islands just east of South America’s southern tip all the way to Alaska.
The hidden earthquake, which rumbled beneath the remote ice-covered islands, initially went unnoticed because it was part of a complex series of temblors, according to a study published Tuesday in the journal Geophysical Research Letters.
Tsunami watchers on that day were on alert after the 7.5-magnitude earthquake ruptured about 29 miles below the earth’s surface. But a deep earthquake that small is unlikely to produce a powerful tsunami, so they were surprised at the size of waves it generated.
“We were aware of the tsunami right away,” said Summer Ohlendorf, a science officer at the U.S. National Tsunami Warning Center, who was not part of the new research. “It was a little larger than would have been expected based on the earthquake parameters.”
The new study says that there were actually five earthquakes over several minutes, including a much shallower 8.2-magnitude earthquake that early reports did not identify and that likely caused the tsunami. The analysis could help spur improvements to earthquake warning systems so they better account for unusual and complex events.
The earthquake produced waves that had amplitudes of more than 2 feet at the nearest tidal gauge, before they spread across the world with smaller impacts.
“It was powerful enough that it propagated around the Southern Ocean and around all the major ocean basins,” Weinstein said. “It was recorded by sea level stations worldwide.”
He said there aren’t hard and fast rules about when a tsunami can cause inundation because it depends so much about the nature of the coastline affected. But when waves exceed 1 meter (3 feet) that’s when “you have to worry about inundation threat,” he said.
The seismic signals of all these events interfered with one another because they happened in close succession, said Zhe Jia, a seismologist at the California Institute of Technology and the lead author of the study. The initial earthquake masked the more powerful, slower earthquake, which was the third of the series.
“The third one is very special. It’s silent,” he said “A huge, hidden, slow event like the third sub-event could lead to a significant underestimate of tsunamis.”
The South Sandwich Islands, a British overseas territory, are a series of small volcano-formed land masses that are more than 1,000 miles from the southern tip of South America. About half of the islands’ landscape is permanently covered in ice.
No one lives on the islands permanently and they feature no hotels, but researchers and tourists can visit and even get married there, according to its territorial government. You’d have more penguins and seals than people as wedding guests.
The earthquake gave the islands quite a shaking.
“If the same earthquake occurred in densely populated regions … it’s probably more devastating,” Jia said.
The island does not feature equipment to measure seismic events, researchers said.
“It’s in a pretty remote location, it’s pretty hard to study earthquakes like that. There’s not a lot of data available to look at them,” said Jeremy Maurer, an assistant professor of geoscience at the Missouri University of Science and Technology.
To examine what happened in detail, the Caltech researchers built a new algorithm that was able to break down the earthquake into five different quake events.
The researchers found that the largest earthquake — which played out over more than 3 minutes and represented about 70 percent of the overall energy released — had been hidden by the other seismic waves.
When the researchers looked at longer wavelengths, which indicate a slower moving earthquake, they were able to see more clearly what had happened.
The difference between an 8.2-magnitude earthquake and a 7.5-magnitude earthquake is substantial, with about 10 times as much energy released in the former?, according to a U.S. Geological Survey magnitude calculator.
“That’s a big difference,” Weinstein said.
Shallower earthquakes also have more tsunami potential.
“The deeper the earthquake, the less effect it has on the surface,” he said
Weinstein, who was not involved in the research, said the new analysis was comprehensive and valuable for understanding more precisely what happened. Jia said future earthquake and tsunami monitoring systems should be adjusted to more accurately process unusually complex earthquakes.
Weinstein added that the research bolsters arguments for establishing an earthquake monitoring system in the South Atlantic, which is something under consideration by a group of experts within the United Nations’ Intergovernmental Oceanographic Commission.
“We also need to beef up the amount of equipment we have,” Weinstein said. “We need more seismic information” from the South Sandwich Islands, including stations to take sea level measurements and buoys that measure pressure changes as tsunamis pass from the ocean floor.