A tsunami caught experts by surprise. Now, they're starting to understand why.
S E E D E D C O N T E N T
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.
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Just another way of knowing when Mother Nature is not happy.
Not especially. Earthquakes are pretty much a normal part of existence for the Earth. I mean, we're riding around on plates of solid ground sitting on top of molten magma. Things are going to shift about a bit.
Agreed. My comment was meant as more of tongue in cheek.
I live approx 150 miles from the New Madrid Fault Line. That's the one that caused the Mississippi River to flow backwards about 200 years ago. Anyway, at least once a year they do a Shake, Cover, and Drop exercise here in Arkansas and Mississippi. I think they do it in Missouri, too
A few years back, a quake hit Virginia that I felt. My patient and receptionist also felt it. My assistant sitting 2 feet away thought we were all crazy and couldn't figure out why I'd stopped working.
It seemed pretty minor at first, but did some damage to the Washington Monument, IIRC, as well as to some old farmhouses south of us, closer to the epicenter.
I was using the tiller in the garden and did not notice it, but the dogs barked at the bushes and carried on
I was about to ask the patient to please lie still and stop fidgeting, then noticed that she wasn't fidgeting, but her eyes were wide open.
My sister-in-law is an xray tech in West Virginia. She was doing a contrast CT, and the IV lines started swinging back and forth.
I experienced 2 earthquakes when I lived in Alaska. The first one was in my dorm room. The second one was when we got married and were living in an apartment close to the fault line. Woke me so I reached over and woke Mr Giggles especially when I heard the dishes rattle in the cupboard. He told me to go back to sleep
My Aunt and Uncle were living in Anchorage during the 1964 Great Alaskan Earthquake. When I lived in Fairbanks we'd get little shakes but nothing big.
I experienced several during the four years I lived in the Philippines when I was in the Navy. I remember one time I was standing outside the medical clinic I worked at and the ground beneath me started swaying back and forth. I thought it was me and was starting to get a little freaked out, while my buddy who was outside with me just ignored it and took in stride. He had already been there for a year or so and was used to them.
I believe it. You had a very active volcano there. I believe that volcano erupted when I was still in Alaska. Everyone had to be evacuated from Clark (?) AFB.